Basic Dayl. Arch

8/13/2019 Basic Dayl. Arch

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TECHNICAL UNIVERSITY OF BUDAPESTFaculty of Architecture

DEPARTMENT OF BUILDING E NERGETICS AND SERVICE SYSTEM

Andrs T. MajorosPh.D., Dr. Habil .

BASIC DAYLIGHTINGFOR ARCHITECTS

1998


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CONTENTS pagePREFACE ................................................ ..................................... 3SYMBOLS ................................................ .................................... 4VISUAL ENVIRONMEN AND LIGHTING .............................. 5

ELEMENTS OF LIGHTING .......................................................... 6The characteristics of light ............................... 6The quality of human vision ............................... 7The principles of using light ............................... 8

THE REQUIREMENT OF LIGHTING ............................................. 12THE SOURCE OF NATURAL LIGHT ............................................. 13

The characteristic features of sunlight .................. 14The characteristic features of diffuse sky light .... 18The characteristic features of the environment .... 24

OPENINGS ................................................... .................................. 27Sidelights .......................................................... 30

Windows ............................................ 32Transparent walls ............................... 32

Toplights ....................................................... ... 32Linear toplights ............................................ 33

Sawtooth type toplights ................. 33Horizontal type toplights ................. 34Monitor type toplights ................. 35Pitched type toplights ................. 36

Spot-like toplights .............................. 37Dome type toplights ................. 38Pyramid type toplights ................. 38Prism type toplights ................. 39

THE UTILIZATION OF NATURAL LIGHT ............................... 40The effect of the location of the opening ................. 40The effect of the slope and orientation ................. 41The effect of the structure of openings ................. 43

Transparent surfaces ................. 43Structural obstruction ................. 46Reflecting surfaces ................. 46

The effect of the interior ............................................. 49THE DAYLIGHTING OF INTERIOR ............................................ 51

Lighting systems ............................................. 51The quantitative characteristics of daylighting .... 51Uniformity in space ............................................ 55Permanency in time ............................................ 60Glare ................................................... .................... 63The direction of light and the effect of shadows .... 63Colour appearance ............................................. 64Colour rendering ............................................. 64

THE DESIGN OF NATURAL LIGHT ............................................. 65Designing illuminance ............................................. 65The restriction of glare ............................................. 71

Design the direction of light and shadow .................. 72Good colour appearance ............................................. 72Good colour rendering ............................................. 72

NEW TECHNOLOGIES ....................................................... ... 73METHODS ........................................................... ........................... 76

Window sizing ............................................................ 76example .............................................. 88

Dimensioning of toplight ............................................. . 89example ............................................... 92

Grn method ....................................................... ..... 92examples .............................................. 97

INDEX ................................................... .................................... 99


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PREFACE

Lighting is one of the most important requirements of interiors, as the visible environment is an

essential condition of our activity. Although nowadays there are two possibility to meet thedemands - with natural or artificial lighting - the two options are not equivalent.Being able to design the artificial lighting of interiors for permanent quantity and quality isundoubtedly favourable, however features of daylighting are advantageous in the followingrespects:.- Quality of daylight is best, in spite of the fact, that it changes permanently. It is best first of all,

because human vision developed by natural lighting.- Its quantity enables it to provide larger and preferred illuminance than that of artificial lighting

during daytime in a considerable part of the year. We may enjoy an illuminance of athousand lux or larger in rooms where level of artificial illuminance is only a couple of hundredlux.

- Daylighting works with renewable energy, daylighting is the most friendly available usage ofthe Sun and sky radiation.

- The luminous efficacy of daylight is very good, only the best artificial light sources cancompete with it.

- Well designed daylighting able to meet required illuminance in 80-90% of the daylight hours,so give a possibility to save a considerable part of energy consumption of artificial lighting.

- Daylighting is more than simple lighting: making the environment visible. It ensures more orless connection with the external environment, external radiation. We need this connection.While the light spectrum of external radiation ensures vision, other parts of it provide ourbodies with further information to react. The best known of these connections is the visualconnection to the exterior.

- The continuous changing of quantity of daylighting is supposed to be advantageous becauseof its stimulating effect.

The effect of adequate or poor daylighting of a dwelling on its price illustrates the above factsvery well.

Designing of daylighting is completely the job of architects. Designing of daylighting needs a lotof expertise. The present note intent to introduce the reader to the essential characters ofdaylighting:

- the sources and features of natural light,- different openings and their characteristics,- the characteristics of daylighting by different openings,- the connection between external condition and internal lighting,- how requirements may be met,- the method of design for the most common cases.


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SYMBOLS

a width of the room (m)

A relative area of the window ( %) A Rw required relative area of the window

( %)b depth of the room (m)d length of lights way through the

opening (m)e daylight factor (%)e av average value of daylight factor along

the working plane (%)e min minimal value of daylight factor along

the working plane (%)e P daylight factor at point P (%)e R required average daylight factor on

the working plane (%)e Rmin required minimal daylight factor on

the working plane (%)Ee external illuminance, i.e. illuminance

from the sky on a horizontalunobstructed surface (lx)

Ee D design extenal illuminance ( lx)ER required average illuminance on the

working plane (lx)Es illuminance from the Sun on a

horizontal unobstructed surface (lx)F area of opening through which the

light penetrates to interior (m 2)h headroom (m)K luminous efficacy ( lm/m 2)L luminance (cd/m 2)Lz luminance of the zenith (cd/m 2)Lq luminance of sky at q above horizon

(cd/m 2)m high of opening (m) p width of window or toplight (m)q height of window or length of toplight

(m)r height of working plane above the

floor (m)t length of time during a day (h)S wR required area of window (m 2)S TR required area of toplight (m 2)T length of time during a year (h/year)Tc colour temperature (K)

(alpha) orientation of opening ( 0)

azimuth ( 0) s azimuth of the Sun ( 0)

(gamma) horizontal angle ofobstruction ( 0) (delta) slope of opening ( 0) (epsilon) vertical angle of obstruction ( 0) (theta) angle of high above

horizon ( 0)s sunheight ( 0) o (eta) efficiency of the opening (-) (kappa) angular distance of particular

point from the Sun ( 0)

(lambda) wavelength (nm) (rho) reflection factor (-)c reflection factor of ceiling (-) f average reflection factor

of floor (-)

g reflection factor of ground (-)o average reflection factor of

obstruction (-)

tw reflection factor of toplight's

wall (-)w average reflection factor of

walls (-)

(sigma) angular distance of zenith fromthe Sun ( 0)

(tau) transmission factor for parallellight (-)

diff transmission factor for parallellight (-)

(phi) incident angle of light ( 0)

(phi) luminous flux (lm)e radiant flux (W/m 2)ext luminous flux irradiated on

transparent surface of theopening from exterior (lm)

in t luminous flux entering theinterior through opening (lm)

(psi) uniformity of illuminance along (-)


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THE VISUAL ENVIRONMENT AND LIGHTING

Human life has a very close connection to the visual or visible environment, it depends on it. Weget nearly 90% of our sensory information by vision, our activity is also connected more or lessto vision.

The visual environment is a three dimensional pattern of brightnesses and colours of oursurroundings. It has two components: a passive component i.e. the natural or artificialenvironment without light; and an active component: light itself. Lighting makes the environmentvisible.

The visual environment is a result of the multiplication-like relationship between the passiveenvironment and active light. It follows from the foregoing that-a given environment may be a different visual environment depending on lighting,-good lighting itself does not necessarily produce an optimum visual environment.

In everyday practice, lighting may be divided into natural and artificial lighting on the basis of thelight that has been used.The distinction may be justified by differences in the characteristics, tools and working of thetwo kinds of lighting.

Although the visual environment may come into being as a result of natural and/or artificiallighting, the quality of interiors has so far been mainly determined by natural lighting. Thereason for this is not only that our wakefulness is connected to daylight, but our vision has beendeveloped by natural light, so natural light is our base of comparison from the point of view of

vision.Moreover, natural lighting is much more than lighting alone /e.g. artificial lighting/, because itprovides a connection between the interior and the exterior. This connection is partly a visualconnection and partly a connection to global radiation. These connections are limited, but theysatisfy very important psychological and physiological needs.Consequently, natural lighting is a determining factor of the quality of interiors.

The passive part of the visual environment in the interior is the room without lighting. Thecharacteristics of a given building are determined-by lighting i.e. by the quantity and quality of light used-and by the relative size and position of luminaries in comparison to the surfaces of the room.

Consequently, natural and artificial lighting differ greatly for the following reasons:While the quantity and quality of light of artificial lighting is comparatively permanent, thesecharacteristics of natural light keep changing.While artificial lightsources and luminaries are inside the room and their dimensions are muchsmaller than that of the room, the natural lightsource and "luminaire" are outside the room andtheir dimensions are far greater than that of the interior.

As a consequence, the particularities of natural and artificial lighting are very different.

The two kinds of lighting also differ because natural lighting is designed by an architect, andartificial lighting is designed by an expert of artificial lighting.


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ELEMENTS OF LIGHTING

The object of lighting is to create an appropriate visual environment. In order to do this, we

should be acquainted with the characteristics of light, the special quality of vision as well as withthe characteristics and the principles of using light.

The characteristics of light

Light /e / is the visible part of the electromagnetic spectrum between the wavelengthsof =380...780nm. Every wavelength corresponds to a given colour. Colours at smallerwavelengths are called cool (colours like purple and blue), colours at longer wavelengths arecalled warm colours (like orange and red).

Fig. 1. The quality of white light.

The visual environment is a result of so called white light . Light is called white light if theintensity of radiation is nearly the same at every wavelength. Every non coloured light is whitelight.

The quality of white light may be different depending on the ratio of the different colours. Sowhite light containing a lot of blue is called cool white, white light containing a lot of red is calledwarm white.The quality of white light may be given with help of its spectral distribution curve e ( ).The quality of white light may be characterized in practice, simple, with the help of colourtemperature. Colour temperature Tc in Kelvin of a given light is the temperature of the blackbody, at which the sprectal distribution of its radiation is nearly the same as that of the givenlight.Light is always a part of some electromagnetic radiation. Light is characterized from the point ofview of energetics by the luminous efficacy K, which is the ratio of the sense of light /luminousflux/ and the electromagnetic radiation belonging to the light. Its unit is lm/W.


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The quality of human vision

Human eyes can see nearly half of a sphere, but we are able to perceive exactly only arelatively small part of it, at the axis of our field of view. The rest provides background

information.

Fig. 2. The field of view.

The sensitivity of the human eye depends on the wavelength /colour/ of the perceived lightaccording to the spectral luminous efficiency, V

.

Fig. 3. The sensitivity of the human eye.

It follows that light seen by the eye, as a physical effect, is not the same as the sense of light.The sense is important from the point of view of vision, therefore what is essential from the point


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Illuminance E, whose unit is lux /lx/, is the luminous flux collected by a surface. This is an effectthe surface is exposed to. A given surface may be brighter or darker depending on itsilluminance, but how it is seen is not determined by illuminance alone. The illuminance of aletter and that of its surroundings on a page of a book are practically the same, and still they areseen differently.

Fig. 5. Illuminance.

In the case of a given luminous flux, the greater the angle of incidence, the smaller theilluminance.Illuminances from different sources may add up.

Surfaces change the characteristics of light in the following way:- Every surface reflects a part of the luminous flux, and it transmits another part of theluminous flux if the surface is transparent.- Surfaces change the direction of light ### in most cases they disperse it.- Coloured surfaces change the quality of light.

The ratio of reflected luminous flux is characterized with the reflection factor .

The ratio of transmitted luminous flux is characterized with the transmission factor .

Fig. 6. The quality of reflection and transmission.


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Light dispersion may be characterised with the help of I luminous intensity , which is a luminousflux emitted in a solid angle. Its unit is candela /cd/.

Fig. 7. Luminous intensity.

The change of the direction of light depends on the quality and roughness of the surface.The reflection of mirrors, shiny and mat surfaces, or the transmission of normal glass, sandblown and milky glass differ greatly.

Fig. 8. The spread of light.

As the illustration shows- in the cases of mirror and normal glass, point " R " may only be seen from direction

"B ".- in cases of shiny and sand blown glass, point " R " is darker from direction " A ", than " B ",

that is to say the brightness of point " R " depends on the direction of looking.- in cases of mat surface and milky glass, the brightness of point " R " is constant, it does

not depend on the direction of viewing, because the virtual size of surface " R " and the


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luminous flux radiated in the given direction change in the same way.The observer usually sees a surface of the field of view at an angle, whose virtual size / A*/ issmaller than in reality. The observer percieves the luminous intensity I* from the surface. Whatone sees is the luminance L , whose unit is cd/m2.

Fig. 9. Luminance.

What we see is the luminance and the colour of surfaces. The greater the illuminance and thegreater the reflection of a surface, the greater its illuminance, namely

L = * E

In other words, what we see is a result of an architectural quality / / and an effect of thequantity of lighting / E /

The level of adaptation, i.e. the accuracy of vision depends on the average luminance of thefield of view, which may be influenced by architecture / / and lighting / E/.

Fig. 10. The variation of visual ability with luminance of field of view.


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THE REQUIREMENTS OF LIGHTING

The goal of lighting is to produce an adequate visual environment. An environment is adequateif it ensures visual comfort, and if it serves the visual tasks required by the function of the room.Visual comfort is a condition of the mind which means satisfaction with the visual environment.

An interior space meets these requirements if its parts may be seen well without any difficulty.Of course, visual comfort may be ensured to different degrees and at different levels. Visualcomfort is connected to the whole visible sphere.Like thermal and acoustic comfort, visual comfort is a part of complex comfort.Serving the visual task required by the room's function means that lighting makes the details ofthe reference plane visible correctly, quickly and free from discomfort. Of course, theserequirements may vary greatly in practice.This requirement is connected to the reference plane that is to say to a well defined, given partof the environment.

It follows from what has gone before, that lighting has to provide visual comfort at all times, andadditional requirements beyond this have to be met only in cases of specifically definedfunctions.

The following characteristics are generally specified for providing suitable visual environment ineveryday practice:

-average illuminance on the working plane,-uniformity of illuminance on the working plane,-ratios of luminances in the room,

-the allowable level of glare,-the direction of the light and the effect of shadows-colour temperature Tc and-colour rendering.

It goes without saying, different levels or values of practical requirements are connected to thedifferent functions.

The question "How far can the requirements of lighting be met with natural lighting?" may beanswered only with full knowledge of the natural light-source and the circumstances determiningthe changes of natural lighting.


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THE SOURCE OF NATURAL LIGHT

The Sun is the source of natural lighting. Its light arrives into the interior space directly orindirectly, scattered in the atmosphere, and reflected on the natural or artificial environment.The function of the Sun is similar to that of an incandescent lamp, a fluorescent tube, etc. inartificial lighting.

The luminair of natural lighting is the exterior space which lets the Sun's light into the interiorspace by transmitting, scattering or reflecting it. This includes the sky, as well as the natural andartificial external environments. The function of this external environment is very similar to aluminair without light-source in artificial lighting.

The "luminair" of natural lighting of interior spaces is basically the exterior space, which allowsnatural light into the room, namely the part of the exterior which the room can "see", which may

be seen from the room.Consequently the Sun, the sky, the surface of the earth, the plants, another building and so onmay serve as a "natural luminair". The function of these elements in daylighting may vary widelyfrom time to time and from one case to another.

Fig. 11. Different natural luminairs.

In one extreme alternative there is no obstruction in front of the opening, and daylightingcomes from the Sun, the sky and the ground.

Another extreme alternative is a built up area, where the sky and the ground may not be seenfrom the room, so daylight is a result of lightreflected from the buildings standing opposite.Usually, the Sun, the sky, natural obstructions /plants, the terrain/ and artificial obstructions/buildings, constructions/ contribute to a different degree to the natural lighting of interiors. Thisdegree keeps changing partly because the Sun moves and the cloud cover of the sky changes,and partly because the plants' foliage and the ground's reflection change with the seasons (e.g.due to snow cover).


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Although the above mentioned parts of exterior space may let light into the room in any ratio,their function in changing the quality and quantity of natural lighting varies according to thefollowing:

Natural lighting is based on sunshine, which reaches the Earth's atmosphere in amounts thatchange seasonally according to the Sun-Earth distance. The average value of the specificpower of this nearly parallel radiation is 1377 W/m 2, and the colour temperature of the light is5760 K. The average value does not change considerably during the year.The Sun, as the light-source determines the essential characteristics of the available naturallight, namely the length of the days and its seasonal changes, as well as the character ofchanging during the day. These characteristics are closely connected to the Earth's movementand to the angle of the Earth's axis. Because of these, the essential characteristics of thenatural light-source depend on geographical location.

In the quantitative characterisation of the natural light-source or "luminair", the total lightirradiated on the surface of the Earth is irrelevant from the point of view of daylighting, as

interiors can only use a very small portion of it. What is relevant is how much of this is availableat a given geographical location. Accordingly, it is illuminance on an unobstructed horizontal surface that can be used tocharacterise the quantity of the natural ### luminair ### .

The characteristic features of sunl ight

Direct sun radiation may reach the surface of the Earth through the atmosphere if clouds do notobstruct it. One part of the sun's radiation is absorbed passing through the atmosphere.Direct sunlight is characterised with

- its always changing direction,- its probability,- the illuminance (the luminous flux collected by a unit of

surface) it creates on an unobstructed horizontal surface,- its colour temperature Tc (the temperature of the black body,

at which the spectral distribution of its radiation is nearly thesame as that of the given light) and

- its luminous efficacy K,(is the ratio of rated input of lamp andluminous flux it produces)

in the following way:

The direction of the Sun's radiation may be characterised with the angle of s azimuth and ssunheight .The direction of sunshine keeps changing in the course of the days and the year. The dailychange of these two angles may be given with the help of the so called sun-path curve . Eachsun-path curve refers to two days of the year /except the shortest and the longest day/.

Although, the sun-path changes daily, the changing of sunshine during the year may be wellillustrated with the help of two curves per month, that is to say with 11 curves. This group ofcurves is the so called sunpath diagram .


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Fig. 12. Sunpath diagram.

(this example refers to latitude 48 0, the Northern hemisphere,)

Of course, different sun path diagrams refer to different geographical places.

The probability of sunshine is the function of the clouds to be expected, so it also depends onthe geographical location. It may be well characterised with the length of time of sunny hoursexpected at 50% probability. This is the so-called probable value of sunshine.The representation of the 50% probability of sunshine in one hour intervals connected to the

sunpath diagram gives a good survey of sunshine during the year.


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Fig. 13. The expected length of sunshine

( this example refers to Budapest, Hungary, - latitude 48 0, Northern hemisphere-.)

The values of illuminance created by the Sun on an unobstructed horizontal surface - Es - maybe expected between 0 and approximately 50 000 lx, depending mainly on the height of the Sun

and the cloud cover.The probable values of Es during the year can be illustrated well with the help of the so-calledisopleth . Its curves show the point of time when Es exceeds certain values at 50% probability.The area inside the curve shows the period of the year in which external illuminance Es isgreater than the value indicated by the curve.


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Fig. 14. The point of time when illuminance from the Sun ( E s ) exceeds certain values.(this example refers to Hungary,- latitude 48 0, Northern hemisphere-.)

The colour temperature Tc of direct sunlight is about 3000K when the Sun is near the horizon,and approximately 5800K when it is near the zenith.

The luminous efficacy K of sunlight depends on the height of the Sun. Its value starts at noughtat the horizon, and it uniformly increases until s = 20 o. Over this angle, its value is nearly 100

lm/W.The use of direct sunlight for natural lighting is limited by the following circumstances:Sunshine may only be expected in part of the period between sunrise and sunset, because ofthe obstruction of clouds. For example this portion is between 20 and 70% in Europe.Because of the continuous movement of the Sun, insolation of an interior depends on therelative position of the Sun and the opening. For example, during a 14-hour clear day the periodof insolation is

- 12 hours, through a window oriented to the equator,- 7 hours, through a window oriented east or west,- 2 hours, through a window oriented opposite to the equator,- 14 hours, through a horizontal opening.

Direct sunlight illuminates only a part of the room in any case. There is a sharp dividing linebetween the insolated part and the rest of the room. The insolated area is highly illuminated incontrast to the rest of the room. For this reason, the illuminance of the interior is very irregularand the very bright surfaces may cause glare.If the Sun is visible from the room and it is in the field of view, it causes glare.Because of the undesirable effects of direct sunlight on the visual environment, insolation maybe permitted only in some cases.

At places of work, where the use of the room is defined, insolation is not be permitted duringworking hours.If the use of the room is undefined, e.g. in a corridor or a living-room, insolation may be allowedfrom time to time, or permanently.


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After all, the use of direct sunlight for the natural lighting of interior spaces is rather limited. In aconsiderable portion of cases we have to provide protection against the discomforting effects ofinsolation.

The characteristic features of dif fuse sky light.

The atmosphere of the Earth is similar to a more or less translucent surface of a hemispherewhich is transilluminated by nearly parallel sunlight. The central point of this hemisphere isalways the room which it illuminates. Considering that the dimension of the atmosphere is muchgreater than the dimension of a room /their ratio is about 1:1000/, every single point of the roommay be considered the central point of the hemisphere. The transparency of every element ofthe surface of the hemisphere changes continually.

Fig. 15. Sky model.

One extreme possible sky condition is a totally clear, cloudless sky. This corresponds to atransparent hemisphere that disperses light only to a small degree.

Another extreme possible sky condition is a uniformly overcast sky which corresponds to atranslucent hemisphere that disperses light to the highest degree.

All other sky conditions may be regarded as situations between these two extremes. Either thecloud cover gets thinner and thinner, or parts of the sky are clear and other parts are overcast.Fog is another condition of the sky, when the clouds extend to the ground.While sunshine passes through the atmosphere

- it is scattered on its elements and- its spectral distribution changes.

These effects depend very much on the condition of the sky.If the sky is absolutely clear, scattering is relatively small, the quality of light differs from the lightof the Sun, this is what ### blue sky ### refers to.


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If the sky is overcast, scattering is very strong, the quality of sky light is different from that of theSun: it is colder, but the difference is not very great, ### grey sky ### describes this condition.

Fig. 16. The effect of the atmosphere on radiation f e .

The luminous flux (is the part of radiant light that produce a visual impression) reaching thesurface of the Earth from the sky depends very much on the height of the Sun s and on thecondition of the sky. As the atmosphere changes according to the laws of meteorological-statistics, the characteristics of the diffuse light of the sky changes accordingly. The amount ofnatural light potentially available at a given geographical place and at a given time of the yearchanges from year to year. Consequently, it can only be characterised with a probability value.

Fig. 17. Changes of luminous flux of the sky on a certain day of the year throughout a great

many years.

This value may be one of the extreme values or the value which has a 50% probability, thislatter being the so called expected value. These characteristics are basically lighting-meteorological characteristics.The sky, as a luminaire may be described

- with its luminance distribution or with the help of the illuminance created by the sky onan unobstructed horizontal surface,

- with the length of time during which light may be used,

- with the colour temperature of light Tc and


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- with the luminous efficiency of light K as follows.

From the point of view of quality, the sky, as a luminaire may be accurately characterised withits luminance distribution. This can be used to calculate its effect inside a room.

There are two problems about this manner of characterisation. On the one hand, there areinnumerable possible distributions and only three of them can be described by a mathematicalformula; on the other hand, the values of luminance are statistical variables, so its values with agiven probability may be a result of measurements over several years. Such a collection of datahas only been started at a few locations of the Earth. These automatic sky scanners collect dataof 145 elements of the sky every few minutes.

In spite of this, this manner of characterisation is very important, because it enables us to definetwo extreme and an intermediate sky condition as follows:

- the luminance distribution of a uniformly overcast sky is

L L z

=+1 2

3sin

- the luminance distribution of a foggy sky is

L = constant


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- the luminance distribution of a clearsky is

L Le e

e z

=

+ +

+ +

10 32

0 91 10 0

0 0 91 10 0

3 2

3 2

,sin

( , ,45cos )

,274( , ,45cos )

where, Lz is the luminance of

zenith, is the angle of the heightabove the horizon of the investigatedpoint, is the angular distance of theparticular point from the Sun, and is the zenith distance from the Sun.

These functions give only ratios, not absolute values. They show that- the zenith is three times brighter than the horizon if the sky is overcast,- the brightness of the sky is the same independent of direction in foggy weather,- the brightest parts of the sky are around the Sun and at the opposite side; the horizon

may be brighter than the zenith if the sky is clear.

In addition, it is true that the luminance of the sky depends on the height of the Sun both whenthe sky is overcast and when the weather is foggy. It is also true that the sky is darker in themorning than at noon and it is lighter in summer than in winter.These distribution functions are important because two extreme and an intermediate skycondition may be determined with their help.If we know the luminance distribution of the sky - L( , ), the illuminance on an unobstructedhorizontal surface, Ee may be calculated.


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Fig. 18. Illuminance from the sky.

The sky, as a luminaire may be characterised as a first approximation with the effect that isproportional to the lighting of the interior space This effect is the illuminance on an unobstructedhorizontal surface E

e, the so-called external illuminance .

This manner of characterisation is not as exact as the former one, because it does not take intoaccount which part of the sky the effect originates from. In this way, the same externalilluminance may be created by an overcast or a foggy sky, while the horizon is darker when thesky is overcast than when it is foggy. Consequently, the illumination of the interior will bedifferent in these cases.

In spite of this, this manner of characterisation is widely used because it is simple, and it needsone datum instead of 145. Its inexactitude is negligible compared to this advantage; and it canbe corrected to satisfy practical needs.

The reliable values of the external illuminance Ee may be provided by several /15-20/ years ### collection. Although no measured values of illuminance are available, values of the externalilluminance may be determined with good approximation from radiant meteorological data.

This is a considerable practical advantage, because the values of radiant energy have beenrecorded in a number of countries for decades.

The daily changes of Ee , the external illuminance, is similar to that of a distorted sinus curve.


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Fig. 19. Changes of external illuminance throughout a great many years.

The amplitude of distorted sinus curve is in direct proportion of the sun high at noon. So thesmallest curve belongs to the shortest day and the largest curve belongs to the longest day of

the year. As above figure shows the real value of external illuminance at a given time of a given day ofthe year may vary in a very wide range. Values of external illuminance may be occurred in arange of t expected value about 50%.

The expected value of Ee , the external illuminance may be well illustrated with the help of anisopleth. Its curves show the points of time when Ee exceeds certain values at a 50%probability. The area inside a curve shows the period of the year in which external illuminanceEe is greater than the value denoted by the curve.

Fig. 20. Point of time when illuminance from the sky ( Ee ) exceeds certain values.(this example refers to Hungary - latitude 48 0, Northern hemisphere-.)


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The duration curve tells us what portion of the year we may expect various Ee illuminancevalues.

Fig. 21. The duration curve of illuminance from the sky ( Ee ).(this example refers to Hungary,- latitude 48 0, Northern hemisphere-.)

Sky light is available for 4400 hours a year in such a way that the length of time betweensunrise and sunset changes in the course of the year.

The quality of sky light first of all depends on the condition of the atmosphere.The colour temperature of the light Tc of an overcast sky changes between 4500 and 7000 K.The colour temperature of the light Tc of a clear sky may be 10 000.-40 000 K.In intermediate sky conditions the colour temperatures may be expected between the twoextreme values.

The luminous efficacy K of sky light depends less on the height of the Sun. Its value variesbetween 115 and 130 lm/W.

Further, the following characterise the sky as a luminaire from the point of view of illuminatinginterior spaces as well:

- The sky surrounds buildings like a huge hemisphere, so the use of its light may onlybe reduced by some external obstruction in daytime.

- The illuminance created in interior spaces by diffuse sky light is generally free fromunbearable inequality, from a sharp dividing line and consequently it does not cause

glare.- The sky, when seen from the interior space, only rarely causes unpleasant glare.- Skylight is available continuously and safely during the day.

Based on the above, the sky plays a decisive role in natural lighting.

The characteristic features of the environment

The natural and artificial environment may play a role in the illumination of interior spaces byreflecting the light of the Sun and of the sky. In this way, they play a passive role. The quantityand the periodic change of light reflected from the environment are basically determined by thelight of the Sun and the sky. The reflection of the environment has a secondary effect on the


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amount of available light. The colour of the surfaces of the environment may modify the qualityof reflected light to a large degree.

The natural and artificial environment seen from the interior space may be considered asobstruction or ground . It is an obstruction if it hinders the direct illumination of the working planeby the Sun or by the sky.

Whether a given element of the external environment is an obstruction or ground can bedetermined in relation to a given point of the working plane of a given room. For example, thesame building may be obstruction for a point of a room while it is ground for another point of theroom or for another room.

The effect of obstruction on illumination is twofold: it partly hinders the effect of part of the skyand it may preclude the possibility of insolation part of the time. On the other hand, it may reflectlight from another part of the sky into the room.

Fig. 22. The effect of obstruction on lighting.

As a result of geometrical position, a given external surface may present a different obstruction

viewed from different points of the room .

An obstruction of a given point may be characterised- with the solid angle at which it is seen or approximately with

the ( 1+ 2) horizontal and vertical angles which form itsboundary viewed from the point and

- with its average reflection factor o .


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Fig. 23. The characteristics of obstruction.

The effect of the ground on natural illumination is of secondary importance, because its lightmay only reach the working plane after repeated reflections. It may be characterised with itsreflection.


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OPENINGS

Natural light enters the room through transparent surfaces. Constructions which involvetransparent surfaces are called opening s.

An opening is always a constructional part of the building and in addition to its function indaylighting, it has to meet a number of other specifications. It follows that, although its mainpurpose is daylighting, its form is influenced by other essential considerations.

An opening, as a unit of building construction, may be a window, a window in the roof, atransparent wall or a ceiling as well as some other transparent envelope of space.

The constructional elements of openings are divided into three parts from the point of view oflighting, namely

- transparent surfaces,- constructional obstructions and- light-reflecting surfaces.

Openings are characterised from the point of view of natural lighting by the following:- their location,- their nominal dimensions,- their angle of slope and orientation,- the way of light transmission,- their character of distributing light in space,- their efficiency andtheir ageing.

In addition to this, the possibility of visual connection is a very important quality.

An opening may be either in a wall or in the roof.

The nominal dimension of an opening is the dimension of the surface of the wall, the ceiling orsome other envelope of the interior which involves the opening as a constructional unit. In side-lighting, the size of the transparent surface is not much smaller than the nominal opening, intop-lighting, however, the size of the transparent surface may be far smaller than that of theopening.

The slope of opening is the angle between the transparent surface and the horizontal plane,the orientation of the opening is the angle between the projection of the normal vector of thetransparent surface to North clockwise.


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Fig. 24. The characteristics of natural lights.


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The way of light is transmitted depends on the quality of the transparent surface, which may betransparent, like normal clear glass or more or less translucent, like mat glass.

Fig. 25. The effect of the quality of transparent materials on the way of light.

The distribution of light in space can be characterised with the illuminance distribution in thedirection of typical change on the working plane. It is determined by the location andconstruction of the opening.

The efficiency of the opening o is the ratio of the luminous flux entering the interior and theluminous flux irradiated on transparent surface/s/ of the opening. It characterises theeffectiveness of the resultant transparency of the opening.

Fig. 26. The efficiency of openings.

The surfaces of openings gather dirt. This effect is in direct proportion to the pollution of theenvironment and time, but it also depends on the form of the structure of the opening.Furthermore, there are materials whose transparency and/or reflectancy change over time.

These jointly result in the ageing of openings and in reducing their efficiency.


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The pollution of openings disperses light a bit on the originally clear glass, which thus gets moreor less translucent.

Fig. 27. The reduction of transparency because of dirt.

Sidelights

The transparent surfaces of sidelight s are almost always vertical, their slope is = 90 o, theirorientation may be optional.Location limits the possibilities of their forms. Sidelights may be windows, doors, or fixedtransparent parts of walls.

Their transparent surfaces may be transparent or translucent glass /clean-, milky-, sand blown -,ornament glass/, glass block, profile glass and so on.

The construction of windows is essentially determined by their other functions beyond lighting./thermal insulation, noise insulation, visual connection, etc./

The lighting characteristics of sidelights may be very different. They are influenced by- quality of glass,- the number of envelopes,- the pollution of the glass,- their location, form and relative size,- their construction,

- the thickness of the wall surrounding them and the manner of connection,- their orientation. An important feature of the illuminance distribution of side-lighting is that it rapidly decreases aswe move away from the window.In side-lighting, the degree of possible insolation depends on the orientation of the window.


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Fig. 28. The characteristics of illumination by sidelighting.


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Windows

The efficiency of illumination depends mainly on the glazing, the number of its envelopes, theconstruction, the thickness of the wall surrounding the window and on pollution.The efficiency of ordinary, clean 2-5 m 2 double glazed windows set in walls of 35 cm is 0.4...0.5.

A reduction of efficiency may be expected because of greater wall thickness or pollution.Commonly, reduction due to thicker walls may be about 10% and due to pollution it mayamount to 30% .The effect of various constructions on the above mentioned efficiency is about 15...20 %.

The efficiency of windows increase only to a small degree with increased size.

Transparent walls

The approximate values of the efficiency of transparent walls in clean condition:- in single clear glass approx. 0.7- in double clear glass approx. 0.6- in glass block approx. 0.3

Pollution may reduce these values by 25-30%.

Toplights

Windows in the roof ( toplights ) are characterised by the fact that they are located above theplane of the ceiling.

Theoretically the size of windows in the roof is only limited by the size of the ceiling.

The slope of the transparent surface may vary between 0 and 90 o at will, its orientation isoptional.

The transparent surface may be clear or more or less translucent.

The form and construction of windows in the roof may vary greatly. These determine both theirlight distribution and their efficiency.They may be classified into two groups, namely the group of line-like and the group of non-linelike openings.Line-like windows ( linear toplights ) in the roof are characterised by the following:

- the hole on the ceiling is rectangular, and its length is several times greater than itswidth,

- their typical vertical section is constant along its length.

The non linelike group ( spot-like toplights ) is characterised by the fact that the hole in theceiling is a square, a circle, a symmetrical polygon or some other similar form.

The above similarities in geometrical features of windows in the roof (over and above similaritiesin architecture and construction,) result in similar characteristics of lighting.


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Linear toplights

Fig. 29. Illuminance distribution by linear toplightings.

The most common type of linear toplights grouped on the basis of similar geometrical form are:- sawtooth ,- horizontal- monitor,- pitched.

Similar geometrical shapes mean that their typical sections are similar to a certain extent, and -as a consequence - this makes lighting provided by them similar.They usually provide natural lighting for the interior placed in several, parallel rows. As aconsequence, toplights may obstruct each other, thus limiting the direct effect of the sky. Inmonitor, horizontal and pitched types the obstruction is mutual.

Sawtooth type toplights

They are characterised by a typical sawtooth section.They have one transparent surface, whose slope is theoretically optional, but in practice, it isgenerally no greater than 45 o. The usual material for its transparent surface is clean glass orglass that disperses light to a small degree. In slopes smaller than 90 o, safety-or wired glass isused.In sawtooth toplighting, illuminance distribution by the typical section is asymmetrical. The formsof reflecting surfaces do not affect the character of distribution significantly. Illuminance at aright angle to the above-mentioned direction changes to a small degree.


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Fig. 30. Illuminance distribution by sawtooth lightings.

Depending on glazing, their efficiency is the following:- at a 90 o slope 0.1...0.2- at a 60 o slope 0.2...0.25- at a 30 o slope 0.3...0.4

It is an advantage of sawtooth type toplighting that insolation may be eliminated with opposite tothe equator orientation and an angle of slope equal to the highest possible angle of the Sun atthe given geographical location. This advantage is usually exploited in practice.

Horizontal type toplights

Formally, they are characterised by curved transparent surfaces, which can practically "see" thewhole sky.Their transparent surface is a clear or more or less translucent artificial material, occasionallysafety glass.In horizontal toplighting, illuminance distribution by the typical section is symmetrical.


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Fig. 31. Illuminance distribution horizontal lighting.

Illuminance at a right angle to the above-mentioned direction changes to a small degree.

The quality of lighting is influenced more by the characteristics of the transparent surface thanby its form.Their efficiency is between 0.25 and 0.45 depending on the transmission factor of thetransparent surface.In the case of clear transparent surfaces insolation may occur. It is very difficult to provideprotection against it, as orientation cannot be changed.In the case of transparent or mat surfaces, direct sunlight penetrates the interior without castingsharp shadows.

Monitor type toplights

They are characterised by two transparent surfaces joined by a non transparent structure.The angle of the slope of the transparent surfaces is usually greater than 60 o. They may besymmetrical or asymmetrical.The material of the vertical transparent surfaces is usually normal clear glass, in other slopeswired glass or safety glass.Illuminance distribution by the typical section is symmetrical in symmetrical monitor lighting andasymmetrical in asymmetrical. Illuminance at a right angle to the above-mentioned directionchanges to a small degree.


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Fig. 32. Illuminance distribution by monitor lightings.

Depending on their form and glazing, their efficiency is the following:- in symmetrical, vertical glazing 0.1.. 0.2- in asymmetrical, vertical glazing 0.15..0.2- at other slopes the expected value is greater.

Insolation can occur, and it cannot be eliminated by orientation. Protection against insolationhas to be taken care of separately.

Pitched type toplights

They are characterised by two sloped symmetrical transparent surfaces that can "see"essentially the whole sky.The angle of the slope of their transparent surfaces are generally about 45 o.The material of the transparent surfaces are usually wired glass or, in some cases, safetyglass.

Illuminance distribution by the typical section is symmetrical in pitched lighting. Illuminance at aright angle to the above-mentioned direction changes to a small degree.


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Fig. 33. Illuminance distribution by pitched lighting.

Their efficiency depends mainly on the glazing, and it may be expected between 0.3 and 0.4.Insolation can occur in pitched toplighting. Protection against it is rather difficult, it cannot besolved with the help of orientation.

Spot-like toplights

The most common forms of spot-like toplights are- dome,- pyramid and- prism.

Spot-like toplights are located in the roof along a net. Their effect on one another and mutualobstruction is negligible.

Fig. 34. Illuminance distribution by spot toplight.Dome type toplights


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They are characterised by the fact that their prefabricated transparent surface as a unit abuts onthe usually circular or quadrate superstructure.The material of their transparent surface is clear or more or less translucent artificial material.Illuminance distribution is turning-symmetrical in circle plan toplights, and it has multiple axes ofsymmetry in square plan toplights.

Fig. 35. Illuminance distribution by dome toplighting.

Their lighting characteristics are mainly determined by the transparency of the dome, and by theform and reflectancy of the building surrounding it.Their efficiency is expected between 0.2 and 0.4, depending on the above mentioned.In clear dome toplights, insolation can occur limited by the well effect /ratio of depth and area ofthe opening/. Protection against insolation has to be taken care of separately.In translucent dome toplights direct sunlight penetrates into the interior in diffuse form.

Pyramid type toplights

They are characterised by a pyramidal form, which is made up of four transparent coincidenttriangles.

The angle of the slope of transparent surfaces is usually 45 o.The material of their transparent surfaces is wired glass as a rule.Their illuminance distribution is symmetrical to several axes.


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Fig. 36. Illuminance distribution by pyramid lighting.

Their lighting characteristics are determined by the transparency of glazing, as well as by thegeometrical ratios and reflectancy of the building construction surrounding it.Their efficiency is 0.25...0.35 depending on the before mentioned: the greater the angle ofslope, the worse the efficiency.If wired glass is used, insolation is restricted only by the well effect.

Prism type toplights

They are similar to sawtooth toplights in form, the only difference being that their plane isquadratic rather than rectangular.They are similar to sawtooth toplights in their construction as well.Illuminance distribution in one direction is similar to that of sawtooth and different in the otherdirection.

Fig. 37. Illuminance distribution by prism lighting.

Their efficiency is worse than that of similar sawtooth toplights.Insolation may be eliminated with the help of orientation opposite the equator.


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THE UTILIZATION OF NATURAL LIGHT

The utilization of natural light is daylighting interior spaces.

The quantitative and qualitative features of daylighting interiors is influenced by direct sunlight,diffuse skylight, the external environment and the architectural design of the interior.Direct sunlight, the diffuse light of the sky, the natural and artificial external environments arepotentials. They surround the architecturally created interior space.When planning daylighting, the interior space has to be fitted into the given exterior in a way

that it should have natural lighting that meets existing requirements. This can be achieved if weknow, on the one hand, the characteristics of the given external environment and, on the otherhand, we should know how the form and the characteristics of the interior space affect naturallighting. We should also be aware of what requirements natural lighting can be realisticallyexpected to meet.Daylighting is influenced by the following specific features of the interior:

- the location of the opening,- the slope and orientation of the opening,- the construction of the opening and- the size and reflections of the opening.

The effect of the location of the opening

The essential goal of illuminating the interior is to provide adequate illuminance on the workingplane. Most commonly, the plane of the tabletop is used as a working plane, located horizontallyat a height of about 85 cms above the floor. When examining the efficacy of the opening, thisfact has to be taken into account.The relative position of the opening to the working plane may vary depending on which part ofthe envelope of the room it is located in.In the overwhelming majority of cases, when the interior space is bounded by walls and ceiling,the opening is part of the walls or the ceiling. It is called side-lighting or top-lighting,respectively. In cases, when there is no sharp borderline between the walls and the ceiling, thisclassification may prove inexact.Openings are accordingly classified into sidelights or toplights. Sidelights are usually windows.Illuminance of a given point of the working plane depends on the location of the openingbecause of the following:

- Openings of the same size may be seen bigger or smaller depending on their relativeposition and on where they are viewed from. The greater the surface of the openinglooks from a point, the greater portion of the sky /the external environment/ illuminates

the point, and the greater the illuminance of the point.- The illuminance of a surface depends on the incident angle of light. The greater theangle of incidence, the smaller the illuminance.

The result of these effects is that an opening provides maximum illuminance for a point of theworking plane if it is just above it.

It follows from the foregoing that there is a great difference between the efficacy of side-lightingand that of top-lighting. Top-lighting is 3..5 times more effective than side-lighting.

Furthermore, it is the place and the orientation of the opening that determines which parts of theexternal environment may be regarded as obstruction and which parts as ground.

The effect of slope and orientation


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It depends on the slope and the orientation of an opening to what degree the various elementsof a given exterior space (e.g. the Sun, the sky, the natural and the artificial environment)contribute to the natural lighting of the interior.

The slope and orientation of an opening jointly determine the hemisphere of the externalenvironment that may be seen from the interior. This is what illuminates the room directly.The sunpath curves belonging to the shortest and longest days determine which part of the skythe Sun moves in during the year. The slope and the orientation of an opening determine whichpart of the sky may be seen from the interior. These two together determine whether and howlong the interior is insolated.The smaller the slope of an opening, the longer the expected duration of insolation.Insolation is largest in horizontal openings and smallest in vertical openings /at a givenorientation/.Insolation is the largest in equator facing orientation /on the Northern hemisphere =180 o onthe Southern hemisphere =0 o / and smallest in orientation opposite the equator. The durationof insolation decreases gradually /at a given slope/ as the orientation moves away from theequator.

The slope and the orientation of an opening determine the part of the sky that, in the absence ofobstructions, contributes to the illumination of the interior. The effect of the slope is moreimportant in this respect.In horizontal openings / =0 o/, the whole hemisphere, in vertical/ =90 o/ openings, only half of the hemisphere may contribute to the illumination of the interior.The effect of a diffuse sky light decreases as the angle of the slope increases.

The effect of the sky is influenced by orientation because its diffuse radiation is asymmetrical.This asymmetry may be calculated on the basis of annual radiant energies of different verticaldirection. The degree of asymmetry for the average value may be expected between -20 and+50 %.The diffuse light of the sky is greatest in equator facing orientation. There is no great differencebetween opposite to equator, easterly and westerly orientations, all their values are below theaverage value.


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Fig. 38. The effect of the orientation and slope of opening on the selection of the activehemisphere.

(this example refers to latitude 48 o, the Northern hemisphere.)

The orientation and the location within the interior of an opening determine which parts of theexternal environment act as obstruction.

The degree to which ground influences the daylighting of an interior depends on both the slopeand the orientation of the opening. Its slope determines how much ground the interior "can see".The more ground it ### can see ### , the greater effect ground will have on daylighting aninterior. The effect of the ground is maximum in vertical transparent surfaces


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/ =90 o /. There is no effect of the ground if the transparent surface is horizontal.Finally, the pollution of transparent surfaces also depends on their angles of slope. The smallerthe angle of slope, the greater amount of dirt the opening gathers in a given time, and the moreits transparency and efficacy is reduced.

The effect of the structur e of openings

The structure of an opening connects a single or multi-layered transparent surface to theenveloping (wall or ceiling) of the building in such a way that it becomes part of the buildingarchitecturally and structurally.

Fig. 39. The effects of structure on utilizable light.

Openings can only be evaluated together with the structure enveloping them. Whencharacterising sidelights, the connecting wall should be considered; while toplights should becharacterised in relation to the ceiling.

Transparent surfaces

From the point of view of illumination, the transparent surface is the most important part of theopening. It is generally characterised by the following:

- the number of envelopes,- the transparency of the envelope and- the manner the envelope spreads light.

Number of envelope is determined by energy requirement of heating. If its demand is smallsingle envelope is enough, if its considerable two or more envelopes are needed.

Transparency may be characterised with the transmission factor ( the ratio of transmittedluminous lux to the incident one). The value of the transmission factor depends on

- the quality and thickness of the material and the quality of itssurface

- the angle of incidence of light .


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Fig. 40. Glass transmittance for parallel light.

When sunlight is parallel, the transmission factor of various materials can be calculated by the// function.Clear glass is the most commonly used material. Its transmission factor is almost constantbetween the incident angles of 0 o and 60 o. It gradually decreases as the angle increases. It is 0for parallel light. / =90 o/.Between 2 and 6 mm, the effect of thickness may be ignored in normal clear glass, and the

value of may be obtained from the given curve. A transparent surface transmits diffuse, multi directional light from different directions at differentintensity. The transmission factor for diffuse light is the weighted average calculated on thebasis of incident angles, intensity and // .

The calculating the effects of the sky, the ground and obstructions, the transmission factor ofdiffuse light has to be taken into account. The value of diff depends on the following featuresof the above:

- the proportion of the sky and the ground the transparent surface ### can see ### - the luminance distribution of the ### visible ### hemisphere,

the condition of the sky / L=f( , )/, and finally,the transmission factor of the ground / g /.

The measured values of diff in normal clear glass may be obtained from the given diagram.


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Fig. 41. Glass transmittance for diffuse light.

The resultant transmission factor of several envelopes may be calculated as a product of thetransmission factors of the envelopes. The value obtained this way is smaller than the actualvalue.

A transparent surface may be clear or translucent from the point of view of light dispersion.


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Light passes through clear surfaces without changing direction. Consequently, such surfaces donot distort the picture of the sphere behind them. Translucent surfaces disperse light. As aconsequence, the picture of the sphere behind such surfaces is fuzzy or it may not be seen.The greater the light dispersion of the material, the fuzzier the picture is behind it.

Materials dispersing light following Lambert ### s law have the greatest dispersion, e.g. milkyglass. The light dispersion of glass sand blown on one side is smaller, all the same, it satisfiespractical demands. It is unnecessary to sand blow both sides of the glass.

A material may be translucent- because the material itself is opaque, like milky glass,- because its surface is not smooth /mat/, for example sand

blown glass,- or it is structurally heterogeneous, for example ribbed glass,

glass brick, ornament glass, cell plastic.

The transparency of translucent materials vary in a very wide range.Trancparency greatly depends on thickness if the material itself is opaque.The thickness of the material does not greatly influence transparency, if light dispersion is dueto the surface.

The value of the transmission factor depends on which side light falls on a glass ofheterogeneous structure e.g. on glass sand blown on one side or on U profile glass. In one sidemat glass the transmission factor is greater if light is received on the mat side !

The transmission factor differs for parallel and diffuse light in translucent materials, too.Unfortunately, only one value of the transmission factor is known even in these widely usedmaterials, and it is not clear what kind of light that value refers to.

If a translucent surface is required, only one envelope of the construction needs to be mat. In acombination of clear and sand blown glass, the sketched order of envelopes is the mostadvantageous.

Structural obstruct ions

Transparent surfaces are held by some structures or frames that fix them to the wall, ceiling orto some other envelope of the building. These structures are not transparent. They reduce theefficacy of the opening to a greater or lesser degree by obstructing part of the hole.The usual relative size of the structural obstruction of openings are as follows:

- single paned, timber window 25%- double paned, timber window 35%- single paned, metal window 20%- double paned, metal window 30%

Reflecting surfaces

The third structural part of openings is made up of those non transparent surfaces that reflectpart of the light entering the interior from the exterior. The effect of these surfaces on theutilization of light depends on the geometrical ratio of the opening, on the reflection factor of thesurfaces, and on the manner of reflection of the surface, in the following way:


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The rays of light that do not fall on the reflecting surfaces of the opening enter the interiorunchanged. But the rays of light that reflect on these surfaces lose their intensity and changetheir direction. The fact that the surfaces absorb part of the light is a loss, the change ofdirection of light causes further losses.

The greater the distance between the external and the internal side of the opening d , thegreater the loss of the luminous flux reflected on its surfaces.The smaller the cross-section of opening F, the greater the loss of the luminous flux reflected onits surfaces.The darker the surfaces, the greater the loss of the luminous flux reflected on its surfaces.The more mat the surfaces, the greater the loss of the luminous flux reflected on its surfaces.

The F/d ratio and the form are important when designing openings. The greater the F/d ratio,the smaller the loss of the luminous flux. The F/d ratio is only constant in some cases.

These effects can be summed up by the so called "well efficiency". The values of this efficiency

are known for simple designs and for constant F/d ratio.


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Fig. 42. The way of light through the openings.


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The effect of the interior

Only a part of the luminous flux entering the room through the opening arrives directly at theworking plane. The rest reaches the working plane only after one or more reflections. This is

why the ratio of the interior space, the reflection of its surfaces and the manner of reflection playa great part in its daylighting.

The degree to which a given surface contributes to illuminating the working plane depends onits position relative to the reference plane and to the opening. A given point is illuminated by ahemisphere. The elements of this hemisphere contribute to the illumination of the point to thedegree that they are visible from the point.

The contribution of surfaces to the illumination of an interior depends on how many reflectionsnatural light needs to reach the working plane. Every reflection reduces considerably theamount of the luminous flux available.Consequently, the effects of the walls, the ceiling and the floor will differ considerably depending

on whether side or top-lighting is used.

In side-lighting, the effect of the two side walls and the back wall is primarily important, as theycan reflect part of the light entering through the opening and reaching the working plane afterone reflection. The effect of the ceiling is smaller, as it cannot "see" the sky, so light from theceiling illuminating the working plane has already been reflected on the ground or on othersurfaces. The effect of the floor is usually negligible, because - on the one hand - it is coveredby furniture, and - on the other hand - light reflected from it may only reach the working planeafter several reflections. The wall by the window plays a secondary role, too, because light fromit has already been reflected at least once.

In top-lighting, the effect of the wall may be important if it is relatively near the opening, and if it

is well illuminated, otherwise it plays a secondary part. The ceiling reflects light onto the workingplane that has already been reflected on the floor or the walls, so its effect is secondary too.Usually interreflection between the floor, the ceiling and the working plane is important as asecondary effect. In this case, furniture also modifies reflection off the floor.

To sum it up, the effect of internal surfaces on daylighting is greater in side-lighting, than in top-lighting.


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Fig. 43. Direct and indirect illuminance on the working plane.


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THE DAYLIGHTING OF INTERIORS

Lighting systems

There are three systems of daylighting depending on which part of the envelope the light entersthe interior, namely

- side lighting,- top lighting and- combined lighting.

In combined lighting , there are openings both on the wall and the ceiling.In an interior where the envelope is not definitely divided into walls and ceiling, e.g. archedenvelopes, side lighting and top lighting may be divided in the following way:If the opening is lower than 2.5m, the system may be considered side lighting, otherwise it is top

lighting.

The illuminance of the reference plane may be direct or indirect. The ratio of light enteringdirectly through the opening and of indirect light reflected form the walls and the ceiling dependson the lighting system, on the location of the opening and on the surfaces of the envelope.

In side lighting, illumination comes from the side, and this is why the illuminance of the workplane near the window is mostly provided by direct lighting. As we move away from the window,the value of direct illumination decreases rapidly, so the relative proportion of the hardlychanging indirect component increases. The portion of indirect illuminance is he same or 2-3times larger than the direct one at the back wall.

In top lighting, the reference plane is illuminated directly. The proportion of indirect illuminationdoes not normally exceed 25%.

In combined lighting, the ratio of the direct and indirect components of illuminance is betweenthe above-mentioned two extreme values.

The quantitative characteristi cs of daylighting

The illumination of the interior is quantitatively characterised with the help of illuminance on thereference plane. The reference plane is the most important plane in the interior from the point of

view of vision, which follows from the function of the interior. The reference or work plane is afictitious horizontal plane at the height of a table top /at 0.85 m/ or at the level of the floor. Ofcourse it may be any other plane, too, for example the vertical plane of a wall at an exhibition.

The illuminance of the reference plane may be characterised with:- the distribution of illuminance along the plane or- the distribution of the average values of illuminance along a given direction, along the

direction of typical changing or- the average value and uniformity ratio of illuminance on the reference plane.

It is spectacular to characterise illuminance with its distribution along a plane, which is mainlyused to demonstrate illuminance at a point in time.


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By characterising illuminance with the distribution of average values, we can obtain sufficientinformation on illumination at a given time, and most of the practical problems can be studied inthis way.It is simplest to characterise illuminance with its average values and its uniformity ratio. This canhelp set and meet practical requirements.

Illuminance at a point of a surface of the interior Ei (internal illuminance ) is in direct proportion toilluminance at the same place and the same time measured on an unobstructed horizontalsurface Ee called external illuminance according to the following equation:

Ei = [e/100] *Ee

where e is the so called daylight factor in percentage.

Fig. 44. External and internal illuminance.

As external illuminance keeps changing, Ei/t/ internal illuminance has to change too, so

Ei/t/=[e/100] *Ee /t/


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Fig. 45. The change of external and internal illuminance.

In this equation, the architectural design of the internal and external environments isrepresented in the daylight factor e , while the sky as a luminaire is represented by Ei/t/ .

Since illuminance of the interior Ei is a multiplication-like result of the architectural design of theinterior in a given environment /

e/, and the effect of the sky / E

e/, it cannot be determined how

much these components contribute to the result.

Daylight factor e is essentially the efficiency of utilising natural skylight for illuminating theinterior.

Daylight factor e tells us how far the built interior /the walls, the ceiling and the structure of theopening/ as well as external obstructions restrict the potentially available illuminance, that is tosay e is 100% in the absence of a room or any obstruction.

Daylight factor e characterises all the effects of the interior and the exterior on the illuminance ofa given point of the interior. This way, daylight factor e is a function of:

- the place of the reference point,-the measurements of the interior,

- the reflection of interior surfaces,- the location, size and structure of the opening,- the location, size and the reflection of external obstructions,- the reflection of the ground.

At a given point of time, namely in an unchanged external environment / Ee = constant/, thedaylight factor changes from one point to another as illuminance of the interior changes fromone point to the next, i.e. the daylight factor changes according to the distribution of theilluminance of the interior.


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This means, that a given distribution curve of illuminance Ee (lx) is the distribution curve ofdaylight factor e (%) on a different scale.

Fig. 46. The connection between illuminance and daylight factor.

Consequently, the daylight factor can have the following values:- e av average value,- e min minimal value,- e min /e av = y uniformity ratio

which are in direct ratio to the respective values of illuminance.In the long run, the daylight factor characterises natural illumination in an indirect way /its unit is

% and not lux/.

Although the value of the daylight factor may be considered constant only as a firstapproximation, it is a very useful idea for characterising daylighting of the interior.

External illuminance Ee varies between nought and a maximum, both during the day andthroughout the year. As a result, the internal illuminance Ei changes with Ee .

So illuminance at a given point of the interior may have an endless number of values throughouta year, but none of them can characterise natural illumination alone.However, every value of Ei may be connected either to an Ee or to a period of time, and thesetwo values together give some characterisation, so it may be said that:

- Ei is the illuminance of a given point if Ee is the external illuminance,- the illuminance of a given point is equal to or greater than Ei if the external

illuminance equals at least Ee , - the illuminance at a given point is at least Ei for the period of time during a day or

during the year when external illuminance is greater than EeThus, a value of illuminance of the interior provides some useful information about naturallighting only in conjunction with another data /the external illuminance, or the length of time/.


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Fig. 47. The connection between external illuminance and the period of time.

Natural lighting may be characterised in two ways:Directly, with the help of the following features of illuminance Ei - its distribution along the plane or- its distribution along the direction of typical changing or- its average value and uniformity ratio on the reference plane.

There is a given external illuminance and /or length of time belonging to these features in eachcase. These values constantly change over time.

Indirectly , with the help of the following features of the daylight factor e - its distribution along the plane or- its distribution along the direction of typical changing or- its average value and uniformity ratio on the reference plane.

It is expedient to characterise daylight indirectly, with the help of the daylight factor every timewe want to illustrate the spatial character of daylighting interiors. These characteristic values donot change.

Uniformity in space

The uniformity of illumination in space is characterised by the inequality of illuminance on theworking plane (part of the room the visual task refers to, normally a horizontal plane 0.85 mabove the floor).Illuminance at given points of the interior has two components, direct illuminance coming fromthe external environment and indirect illuminance reflected from the surfaces of the interior.

Although, the ratio of direct and indirect illuminance on the working plane changes from onepoint to another depending on the lighting system, and the two components of illuminance maybe nearly equal, still the character of the distribution of illuminance is essentially determined bydirect lighting.

Direct illuminance at various points on the working plane mainly depends on the portion of thesky ### visible ### from the points and on the average angle of incidence of skylight on thepoints. Illuminances are in direct proportion to the size of the ### visible ### sky and the cosinesof the angle of incidence of light.


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Fig. 48. The connection between uniformity of illuminance and system of daylighting.

The size of the ### visible ### sky and the incident angle of light alike depend on where theopening is located in the envelope of the room, so direct illumination depends on the lightingsystem.

The spatial variations of daylight can be characterised with:- the distribution of daylight factor e /b/ along the typical direction

or- the average value of the daylight factor e av and e min /e av on

the reference plane.


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In side lighting, the points of the working plane are illuminated by decreasing portions of the skyat increasing angles of incidence as we move away from the window. The distribution ofilluminance changes accordingly.In side lighting illuminance necessarily shows great inequality.The measurements of the interior, mainly its height, limits the degree to which the variation ofilluminance of side lighting can be influenced by the location and size of the window.The architectural parameters effecting natural illumination in side lighting are the following (indecreasing order of importance):

- the depth of the room,- the size of the window,- the height of the room,- the reflection of the walls,- the width of the window and- the reflection of the ceiling.

It is instructive to examine the effect of the location of the window and of its various parts.

Fig 49. Effect of position of window on distribution of illuminance.

The effect of the part of the window which is below the working plane is negligible from the pointof view of illuminating the plane, as light through that part of the window is reduced by multiplereflections and is obstructed by furniture.

The part of the window near the ceiling provides illumination mainly for the back area of theroom, while the part of the window next to the parapet illuminates the area near the window.


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A window placed high in the wall illuminates space more uniformly than a window placed low. Astripe window provides more uniform illumination parallel to the window than a divided window.This effect of the structure of the window on the illumination of the working plane diminishes aswe move away from the window and it becomes negligible at the back wall.

Fig. 50. Illuminance distribution by strip and divided window.

In top lighting it is the number of toplights, the height of the interior and the design of the toplightthat determines how uniformly a toplight illuminates space.

In toplights of equal area and identical structure, the more toplights, the more uniform theilluminance.In toplights of identical number, structure and arrangement, the higher the interior, the moreuniform the illuminance.The design of toplights determine light distribution, which in turn determines the uniformity ofilluminance.Top lighting can meet any practical requirement of uniform illuminance.


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Fig. 51. Effect of placing of top lights on distribution of illuminance.

In combined lighting, the uniformity of illuminance is determined by side lighting and top lighting jointly. Illuminance provided by such lighting may be considered a combination of illuminance byside lighting and top lighting.

Fig. 52. Components of illuminance in combined lighting.

Combined lighting can meet any practical requirement of uniform illuminance.


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Permanency in ti me

Natural illumination necessarily keeps changing as a result of the continuous changing of the

Sun's position in the sky and of sky conditions.The illuminance of the interior is always in direct proportion to the intensity of skylight / E e / andto the sunlight's intensity / Es / if either the interior or an external obstruction is insolated.

Fig. 53. Illuminance from the Sun and the sky on horizontal surface.

The changes of natural lighting over time may be investigated in two respects: daily changes orchanges over some expected annual period. Daily changes in the illuminance of a given point ofthe interior may be followed with the help of the daily changes of illuminances provided bydiffuse skylight / Ee / and direct sunlight / Es /. Both changes may be characterised by theexpected daily values and these values are different for the 365 days of the year.

Illuminance at a point of the interior on a given day of the year changes in direct ratio to a curvewhich is between the curves of the daily change of Ee /t/ and Ee /t/+Es /t/ for the same day .

Disregarding the usually undesirable and occasional effect of sunlight, changes in theilluminance of the interior follow the changes of external illuminance Ee /t/ and its variation

corresponds to the variations of sunlight. Insolation increases this variation.Daily changes in the illuminance of the interior may be followed with the help of the typicaldistribution curve of the daylight factor e /b/ and that of the expected daily change of Ee /t/ .

During a given day, the expected values of illuminance of the interior are between 0 andEimax /b/ . There is a different curve of Ei/b/ belonging to each value of Ee . At time t*, when theexpected value of external illuminance is Ee *, the distribution of illuminance corresponds to theEi/b/* curve.


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The illuminance of point " P " changes between 0 and Epmax during the day, and its value is Ep * at time t*.

Fig. 54. The connection between external and internal illuminance.

Practical questions of daylighting are more often connected to its yearly change, which refers tothe illuminance of the interior and to the expected length of time, namely

- how large will illuminance be during a given period of the year or- for which periods of the year can a given value of illuminance of the interior be

expected.

These questions may refer to single points of the interior, or to the average of illuminance or tothe typical distribution of illuminance.These questions may be answered with the help of the typical distribution curve of the daylightfactor e /b/ and of the duration curve of external illuminance Ee /T/. ( see next page)

At point " P ", where the value of the daylight factor is e p - the value of illuminance is at least EiP =[e p /100] *Ee ** for T**

length of time in the year- E iP *** illuminance may be expected for T*** length of time in

the year, as [ 100/ e P ]*EiP = E e ***

The connection between illuminance and length of time is similar in average illuminance andtypical distribution.

When investigating the changes of illuminance over time in this way, we must keep in mind thatthe changes of natural light follow the laws of meteorological-statistics.

So, according to the laws of meteorological statistics, the characteristics of diffuse sky lightEe /t/ and Ee /T/ and of sunlight Es /t/ are average values of several years, to be expected with aprobability of 50%.


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Fig. 55. The connection between illuminance of a given interior and the length of time of

illuminance.


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Glare

Glare is the most important problem of visual comfort and of quality of illumination

Documents

Basic Dayl. Arch