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Elements of lighting Design752

LUMINOUS FLUX (Fig.1)

It is the amount of luminous energy emitted in the space by a source in a period oftime. The luminous flux is identified by the symbol Φ, and is measured in lumens (lm).

A lumen is equal to the luminous flux emitted within a unit solid angle from a spot

source subtended at the centre of a sphere having a luminous intensity of 1 candela

in all directions. In the International System, the measuring unit for solid angle is thesteradian (sr), which gives the following relation: 1 lm =1 cd x sr.

As the luminous flux is the time rate of light emitted by a source, it must be conside-

red as power from the dimensional point of view as it is energy divided by the unit of

time. An interesting extension of the concept of luminous flux as power is the conceptof luminous efficiency.

Luminous efficiency is the ratio of the luminous flux emitted by a light source to the

power input of the source. Through this value it is possible to assess the energy

saving provided by one lamp compared to another.

Luminous flux (Fig. 1)

   E   F   F   I   C   I   E   N   C   Y

   l   m   /   W

   L   A   M   P

   T   Y   P   E

   P   O   W   E   R

    W   F   L   U   X

   l  m

LUMINOUS INTENSITY (Fig.2)

Luminous intensity is the amount of light (l) emitted by a spot source which pro-

pagates in a given direction. This intensity is defined as the flux ratio Φ  emittedin any specified direction in a unit solid angle cone ω, which gives l=dΦ /dω. It is

the fundamental physical quantity in the International System and is measured in

candelas (cd). The XVI General Conference for Weights and Measurements in 1979

established that the intensity of 1 cd is equal to the intensity of a source that emits - ina solid angle of 1 sr - the frequency and power monochromatic radiation Φ=1/683 W.

A standard international eyesight, defined by ClE, is used to determine the maximum

relative visibility value for radiations at a 555 nm wavelength. This value corresponds

to that of the source under consideration, which therefore has 1 Im.

Luminous intensity (Fig. 2-1)

100

200

300

400

500

600

700

800

0 ϒ 15 ϒ    15 ϒ 

30 ϒ 

45 ϒ 

60 ϒ 

30 ϒ 

45 ϒ 

60 ϒ 

90 ϒ    90 ϒ 

BZ1

BZ2BZ3

BZ4

BZ5

15 ϒ 

45 ϒ 

60 ϒ 

75 ϒ 

90 ϒ 

15 ϒ 30 ϒ    30 ϒ 

90 ϒ 

0 ϒ 

45 ϒ 

60 ϒ 

75 ϒ 

50

100

150

200

250

300

BZ6

BZ8

BZ9

BZ10

BZ7

Luminous intensity (Fig. 2-2)

a 2 rad=tot

a =1rad

a =1

BZ CLASSIFICATION (Fig. 3)

The BZ method defines project parameters to obtain a greater precision in calcula-tions as compared to the standard method. In particular, this method classifies fixtu-

res according to 10 standard distributions of luminous intensity, i.e. 10 increasingly

wide polar curves that can be represented by a simple mathematical formula. At this

point, the fixture is given a BZ classification.The higher the BZ label, the wider thelight beam and the mounting spacing that would ensure correct uniformityBZ1 1 x cos4 α BZ2 2 x cos3 α  BZ3 3 x cos2 α BZ4 4 x cos1.5 α  BZ5 5 x cos α  BZ6 6 x (1+2 cosα)BZ7 7 x (2+cos α) BZ8 8 BZ9 9 x (1+sen α)BZ10 10 x sen α

Classification diagram (Fig. 3-1) Classification diagram (Fig. 3-2)

1cos   =

LUMINANCE (Fig. 4)If the light source is greater than a point, its size becomes relevant and the above

definition of luminous intensity can no longer be applied. We must therefore introduce

a new concept which determines the amount of light energy that is emitted either bylight sources or by reflection surfaces.

This photometric quantity is the luminance (L), which is defined as the ratio of the

source luminous intensity in the direction of an observer to the emitting surface as

seen by the same observer (or apparent surface).The unit of measurement is cd/ sqm. The fundamental relation is given by:

L=dI α/ dA x cos α

Where 1 is the candlepower at the angle α; is the source area, cos α is the cosine of

the angle formed by the observer’s eye and the normal to the source.

LUMINOUS EFFICIENCY (Fig. 7)

Luminous efficiency is the ratio of the total luminous flux emitted by the lamps to the

total flux used by the fixture

  Φ un=

Φ tot

Since luminous efficiency is a ratio between two homogeneous quantities, it is non-

dimensional and is generally expressed as a percentage value. For fixture classifica-tion, luminous efficiency is divided into lower (ni) and upper (ns).

Illuminance (Fig. 4)

Illuminamento (Fig. 5)

  Midday sun 16x109 cd/m2 

Sunset 6x106 cd/m2 

Blue sky 8000 cd/m2

  Cloudy sky 2000 cd/m2 

Lawn 800 cd/m2

  Snowy plane 3,2x104 cd/m2

  Tallow candle 5000 cd/m2 

NC 60W clear bulb 5x106 cd/m2 

FL 18W 4000 cd/m2

  JM 70W 1,5x107 cd/m2

ILLUMINANCE VALUES

ILLUMINANCE (Fig. 5)

The concept of illuminance is critical in illumination design. This value is useful to

determine he amount of light that is emitted by a source and is present on a surface.The illuminance (E) is the density of the luminous flux incident on a surface:

  dΦ  Lm

E= lux=  dA sqm

where dΦ is the luminous flux incident on the surface, dA is the surface area struckby the flux. The measuring unit of illuminance is lux (lx), which is dimensionallyexpressed as cd/sqm

  ILLUMINANCE VALUES

  Sunshine, blue sky 100.000lx Cloudy sky 10.000lx

  Starry sky without moon 10-4lx

  Average street lighting 5-30lx

  Minimum light for pedestrians

  to avoid obstacles 0.2-1lx

  Well-lit house 100-200lx

  Commercial conc. 200-3000lx

  Offices and sc. 300-2000lx

115 ϒ 

105 ϒ 

95 ϒ    95 ϒ 

105 ϒ 

115 ϒ 

85 ϒ 

75 ϒ 

65 ϒ 

55 ϒ 

45 ϒ 

35 ϒ 

25 ϒ 15 ϒ 5 ϒ 5 ϒ 15 ϒ 25 ϒ 35 ϒ 

45 ϒ 

55 ϒ 

65 ϒ 

75 ϒ 

85 ϒ 

420

350

280

210

140

70

cd/km

1 lumen

POINT-TO-POINT METHOD (Fig. 6)

The method used to determine the horizontal illuminance at a specific site is com-

monly called “point-to-point” method. Its formula is:  Ip x Klm x cos3 α Ep = where:  h2 Ep  = illuminance at a site (in lux)Ip  = candlepower referred to 1000 lm, at the relevant siteKlm = the luminous flux of the lampcos3 α  = cube of the cosine of the angle between normal to the fixture and relevant siteesameh2 = the distance between the source and calculation plane

metodo punto-punto (Fig.6)

40

80

120

160

200

Ip

h

P

Rendimento (Fig. 7)

TECHNICAL DATA

COLOUR TEMPERATUREColour temperature is defined as a balanced mixture of various colours. By this defi-nition, the colour temperature of a lamp, measured in Kelvin, is extremely importantfor the installation of a luminaire. The temperature of a lamp can be regarded as aquality criterion of choice, just as the flux is the quantity criterion. The table on theright lists some examples of the luminous output of various sources:

- Stearic candle flame 1800 K

- Incandescent lamp 2700 K- WHITE fluorescent lamp 3500 K

- Sun at sunset 3500 K - 4000 K- COOLWHITE fluorescent lamp 3000 K

- Sun at noon in Summer 5500 K- Clear sky 6500 K

- DAYLIGHT fluorescent lamp 6000 K - 6500 K

  ALO 300 5000 17MBF 125 6300 50FL comp. 24 1800 75FL tubolare 36 3350 93JM 2000 180000 90SAP-T 400 48000 120SBP 90 13500 150

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ILLUMINANCE CALCULATION USING THE CIE METHOD (Fig.13)

We will first calculate the K index of a room, where “a” and “b” are the sides and

hu is the height of the fixtures above the working plane

  a x bK =

hu x (a+b)

The number of fixtures required for a specific lighting installation is calculated with

the following formula:  Em x (axb)napp =

Cu x Cm x Φ 

Where Em isthe required average illuminance in Iux, Cm is the maintenance factor

(new installation = 1), Φ  is the flux emitted by the lamp(s) in lumen. The utilisationcoefficient Cu is found on the table in Fig. 6-2. Locate the row corresponding to the

K room index, and the column of the total reflection factors of the room walls.

Example: To illuminate the following room:

a = 7m, b = 5m,

h = 3m, hp.l. = 0.80m, with 350 lux on a new installation; the fixture used is: art601Disanlens 2x36W.

The reflection factors are: ceiling = 0.7; frieze = 0.7; walls = 0.3; working plane =

0.1 so the column (as shown in Fig. 13-2) is the blue column 7731. The K room

coefficient is therefore:- 

hu = h - hp.l. = 3 - 0.8 = 2.20m

K = (7 x 5) / (2.20 x (7 + 5)) = 1.3 (red row)

then Cu = 0.45 (yellow rectangle).

The number of the fixtures is found to be:napp = 350 x (7 x 5) / (0.45 x 1 x 6900) = 4

LUMINANCE CHART (Fig. 14)

This chart is used to determine the direct glare produced by each fixture. Luminance

values for the two curves are plotted in relation to an observer looking to the fixture

from an angle of 45° to 85°. Values are represented on a logarithmic scale. Limitcurves border the area in which the luminance of the fixture cannot be considered as

glare. Each curve is referred to an average illuminance value on the working plane,

and is divided into five CIE quality classes: if the luminance curve son the left side of

the limit curves, glare is considered as acceptable. On table nr. 15 you will find the

prospectus concerning glare limitations, indicating when and where to use a fixture

with one, or another, quality classification (UNI 12464).

“DISTRIBUTION CURVES” floodlight (Fig. 12)As a floodlight beam is narrower than that of the above fixtures, polar coordinates

do not provide sufficiently detailed values. Therefore, the distribution curve is better

represented with Cartesian co-ordinates.

753Elements of lighting Design TECHNICAL DATA

Light emitted from a light fixture can be represented by a graphic system called“distribution curves”. These are the union of points joining the various luminous

intensities emitted by a light source in every direction in space and making up the

“photometric solid”. By intersecting this solid with a number of planes, one can obtain

“distribution curves”. When these planes are described through polar coordinateswhose centres correspond to the center of the fixture, one obtains “polar distribution

curves”. These planes can also be made to rotate around an axis so as to explore

the photometric solid under every angle. According to the axis used for rotation, there

are different systems of planes determined by CIE standards. An alternative modeof representing distribution curves would be substituting the polar description with a

description using the Cartesian coordinates. With this system, the narrow beam cur-

ves are more readable and this system is generally used in representing the luminousintensity of floodlights. In this diagram, the values of the angles are positioned along

the x-coordinate, with zero in the middle of the graph, while the values of intensity are

positioned along the ordinate. The two planes normally represented are the transver-

sal and the longitudinal ones, which in the CIE system correspond respectively to the

C0-C180 (continuous line) plane and the C90-C270 (broken line) plane.

Ceiling lamp “DISTRIBUTION CURVES” (Fig. 8)All measurements of the luminous intensity emitted by a fixture in any direction pro-

duce the “photometric solid”. Normally, information on the photometric solid is only

given with reference to two vertical orthogonal planes crossing the optical centre ofthe fixture. The values of the luminous intensity (referred to 1000 lm) that are plotted

on a plane are called “distribution curves”. For indoor and street lighting fixtures,

these distribution curves are represented with polar coordinates. Photometric data

for indoor fixtures according to the applicable UTE and DIN 5040 classification is

available on request.

ISOLUX DIAGRAM (Fig. 9)This is composed of a number of lines connecting all the points on a surface at whichilluminance is the same. The lighting fixture is assumed to be mounted at 1 m heightwith a 1 klm reference lamp. The co-ordinates d/h and l/h express the relationshipbetween the road width (l), the distance between two poles (d) and the height ofthe poles (h).

SOCANDELA DIAGRAM (Fig. 10)Isocandela diagrams result from the projection on a plane of candlepowers of a

given photometric solid having the same value. They are therefore the connectionlines of all points on a plane having the same candlepower.

ILLUMINANCE DIAGRAM (Fig. 11)

The illuminance diagram is used to facilitate the choice of the fixture for urban

decoration i.e. to illuminate underways, open areas: gardens and especially roads.

Illuminance values in lux are given on the Y ax is, the distance from the light source is

given on the X axis. Unlike other charts, which are presented with relative reference

values (i.e. normalised installation height and luminous flux values), this chart shows

absolute values, the mounting height is real and the flux is the flux that is actually

emitted by the lamp. In this way, data shown are ready to be used.

75 ϒ 

65 ϒ 

55 ϒ 

45 ϒ 

35 ϒ 

85 ϒ 75 ϒ 

65 ϒ 

55 ϒ 

45 ϒ 

35 ϒ 25 ϒ 

15 ϒ 5 ϒ 5 ϒ  15 ϒ  25 ϒ   35 ϒ 

45 ϒ 

55 ϒ 

65 ϒ 

75 ϒ 

85 ϒ 105 ϒ 

95 ϒ 

85 ϒ 

75 ϒ 

65 ϒ 

55 ϒ 

45 ϒ 

35 ϒ 

115 ϒ 

35

70

105

175

140

25 ϒ  15 ϒ    25 ϒ 5 ϒ    5 ϒ   15 ϒ cd/Klm

105 ϒ 

95 ϒ 

85 ϒ 

75 ϒ 

65 ϒ 

55 ϒ 

45 ϒ 

35 ϒ 

115 ϒ 

125 ϒ 

35

70

105

luxm

4

3.5

3 108

79

60

Ø 8.56

Ø 9.99

Ø 11.42

-60 ϒ 

-40 ϒ 

-20 ϒ 

0 ϒ 

20 ϒ 

40 ϒ 

60 ϒ 

-60 ϒ -40 ϒ -20 ϒ 0 ϒ 20 ϒ 40 ϒ 60 ϒ 

400

150

isocandela curves (Fig. 10)

-10

12

I/hd/h43

21

0d

h

I

60%

40%

20%

m 1 2 3 4 5

135

120

105

90

75

60

45

30

15

lux

300

400

200

100

80 ϒ 60 ϒ 40 ϒ 20 ϒ 0 ϒ 20 ϒ 40 ϒ 60 ϒ 80 ϒ 

cd/klm

coefficiente utilizzatore

lato marciapiedelato strada

posizionamentocentro luminoso

rapporto larghez-za strada-altezza

h

Room dim. (Fig.13-1)

b

Zh

aX

hu

hpl

YK 8873 7773 7753 7731 5551 5511 3311 0000

0.6 0.45 0.42 0.34 0.28 0.31 0.24 0.23 0.21

0.8 0.53 0.49 0.41 0.34 0.37 0.29 0.28 0.261.0 0.59 0.55 0.47 0.40 0.41 0.34 0.33 0.30

1.3 0.65 0.61 0.53 0.45 0.46 0.39 0.38 0.35

1.5 0.69 0.65 0.58 0.49 0.50 0.43 0.41 0.38

2.0 0.76 0.71 0.65 0.55 0.55 0.49 0.47 0.43

2.5 0.80 0.75 0.69 0.59 0.58 0.53 0.51 0.463.0 0.83 0.78 0.73 0.62 0.61 0.56 0.53 0.49

4.0 0.85 0.80 0.76 0.65 0.63 0.59 0.55 0.50

5.0 0.88 0.83 0.79 0.67 0.65 0.61 0.58 0.52

Example of a CIE table (Fig.13-2)

Reflection values (as a percentage)

taken from the illuminance handbook

6 6 14 12 48 14 16 12

14 10 22 18 52 28 32 22

34 26 40 36 60 44 48 30

52 44 62 54 68 58 62 40

66 60 72 72 80 72 74 48

864

3

2

1

85

75

65

55

4510

22 3 4 5 6 8 2 3 4 5 6 8 2 3 410

310

4

Classe

A(1.15)

B (1.5)

C (1.85)

D (2.2)

E (2.55)

2 00 0 1 00 0 5 00 <30 0

2 00 0 1 00 0 5 00 <30 0

2 00 0 1 00 0 5 00 <30 0

2 00 0 1 00 0 5 00 <30 0

2 00 0 1 00 0 5 00 <30 0

Illuminamento [lx]

   I   N   F   O   R   M   A   T   I   O   N

   R

   E   C   O   M   M   E   N   D   E   D

Indirect light

output

luminousintensity cd/kIm

outputangles(degrees)

distribution

curve

(cd/klm)

lengthwiseplane

installationheight in m

light diameter on the working

plane (expressed in m)

ceiling lamp distr. curves (Fig. 8)

Isolux diagram (Fig. 9)

spacing-to

height ratio

Illuminance space betweenone isolux and the next

illuminance chart (Fig. 11)

positioning h

fixture

distance in m.

illuminance curve

distributioncurve (cd/klm)

crosswide

plane

distribution

curve (cd/klm)

lengthwise plane

output angles

(degrees)luminous int.cd/kIm

Floodlight distr. curves (Fig. 12)

qualityclasses

Illuminance

levels

shielding

angle

longitudinal

curve

transversal

curve

QualityClassification type of visual duty or activity

  A very difficult visual duty 

visual duty requiring high visualB performances

visual duty requiring normalC visual performance

 visual duty requiring fair visual

D performances 

for interiors where people areE located in specific working

positions but who also movefrom one area to another tocarry out duties requiring fairvisual performances.