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Road and Public Space Lighting Workshop
15
BASIC LIGHTING CONCEPTS, TERMS, UNITS, ABBREVIATIONS & RELATIONSHIPS
This Session follows through Figure 1.1 above from the production of light in the source to it
reaching the eye of the road user, elaborating the basic concepts of photometry. A ready
understanding of these concepts is essential for the designer in order to appreciate the
performance requirements of road and public space lighting, in terms of light technical
parameters, as set out in AS/NZS 1158.0, AS/NZS 1158.1.1 Clause 2.5 and in 3.1 Clause 2.5
and the computation requirements, as set out in AS/NZS 1158.2.
The components of Figure 1.1 to be considered are:
The light source or lamp which produces light from the electrical input
The lamp used in road and public space lighting
The luminaire (light fitting) containing the lamp and which directs light to the surface
of interest, e.g. part of the roadway; onto a person
llluminance or light on a surface - see the performance requirements of Category P
lighting in AS/NZS 1158.3.1
Luminance or the brightness of the surface of interest as seen by an observer - see
the performance requirements of Category V lighting in AS/NZS 1158.1.1
Reflectance or reflecting properties of the surface, particularly that of the
carriageway.
Reference: AS 3665 (1989) Simplified lighting terms
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This is the spectral power distribution of a tri-phosphor fluorescent lamp together with the
spectral sensitivity of the eye.
The Basic Concepts in Detail
Light source
The total light flux (F) available from a light source is given in lumens (Im); note the radiant
energy has been evaluated with respect to the sensitivity of the light adapted eye to the spectral
composition of the radiation produced by the source using the CIE λ function.
Two major influences have resulted in colour of light being specifically referred to in the 1158
Standards:
(i) Recent interest in the practical effect of the changing sensitivity of the eye to light
radiation as the light level falls; a shift of peak sensitivity towards to a shorter
wavelength suggests that a lamp emitting light with a large yellow component will be
over-valued in effective lumens compared to one emitting 'white' light.
(ii) The increasing desire to use white light in lighting schemes, with the availability of
metal halide and improved fluorescent lamps and LED sources.
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Until any universal definitive statement by the CIE the initial lamp lumens, as conventionally
evaluated and stated by the lamp manufacturer (using the V λ function), shall be used except
as here stated. No re-rating lumen multipliers other than 1.0 shall be used except that if used
in sub-categories P4 and P5, the lamp lumens for HPS lamps shall be derated to 0.75 and
those of LPS lamps to 0.5 of the quoted value.
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Lamps
The lamps used in road and public space lighting are discharge lamps, although there are two
basic modes of operation of conventional lamps:
(i) Discharge - through an ionised gas, needs high voltage starting and a ballast to control the
lamp current. These lamps have high efficacy, good lumen maintenance, long life.
(ii) Incandescent - heating tungsten wire, current controlled by resistance. Although the light is
white lamp' efficacy is low (~ 20 Im/ W) and lumen maintenance and life are both relatively
poor.
Three basic types of discharge lamps are used in public lighting:
(i) Sodium
• Low pressure; harsh yellow, efficacy ~200 lm/W; not now favoured
• High pressure (HPS); soft yellow, efficacy ~ 130 lm/W; the economic lamp of choice for traffic routes
(ii) Mercury
• Low pressure (fluorescent); white, now with good colour rendering and efficacy ~ 85
lm/W; used on local roads, the older less efficient tubular fluorescent lamps (2x20W or
1x40W) have been replaced by MPM (BOW) or are being replaced by the newer
tubular (2x14W) and compact lamps (42W)
• High pressure (HPM); white, moderate colour rendering, efficacy ~ 60 lm/W; becoming
obsolescent for use on traffic routes but used on local roads
(iii) Metal Halide (MH)
• High pressure; white, good colour rendering , efficacy ~100 lm/W; used mainly in city
and urban centres.
(iv) Solid State Lighting (LEDs)
• White, good colour rendering a range of CCT, luminaire efficacy about 120 lm/W long
lifetime, will have widespread use in P Category schemes and increasing use in V
Category schemes.
Ballast losses
The actual watts consumed by the lamp and its control gear is greater than nominal lamp
rating, e.g. 250W HPS uses 273W; BOW HPM uses 96W.
Lamp characteristics - Colour of the light emitted by a lamp
There are two distinct aspects, the rendition of colour in the lit space and the colour
appearance of the light source itself.
The apparent colour appearance of the light is indicated by the correlated colour
temperature (CCT), given in degrees Kelvin.
The Colour Rendering Index (CRI)
The Lamp luminous efficacy
Colour temperature
The first aspect of the colour properties of a light source is the correlated colour
temperature (CCT) given in °K (degrees Kelvin) and indicates the apparent colour
appearance of the light source. The CCT value is that of the perfect source to which the
particular light source is correlated by its chromaticity.
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The value of CCT for electric light sources ranges from about 2000 for HPS, to 3000
(incandescent), through to 6500 ('daylight' fluorescent). MH and fluorescent lamps are made in
a range of CCT and may be given the designations 'warm white', 'cool white', 'daylight' etc.
as shown in Table 1.1. The difference in appearance between sources of different CCT will be
not be readily apparent in isolation, except that sources of very high CCT will tend to look
'clinical'. However where sources of different CCT become mixed in a lighting scheme the
differences in colour appearance will be marked.
Table 1.1 - Lamp Colour Appearance - Correlated Colour Temperature CCT
CCT Class CCT
Warm CCT ≤ 3 300K
Intermediate 3 300K < CCT ≤ 5 300K
Cool 5 300K < CCT
Colour rendering
The rendition of colour by a lamp is specified by the CIE colour rendering index (CRI or Ra)
and is a measure of the light source to faithfully render the appearance a gamut of colours
spanning the colour space. The index has a maximum value of 100 which means a light
source with this value is as effective in colour rendering is as the correlated perfect light
source. The scale was designed so that a, then, 'standard' single phosphor fluorescent lamp
had a CRI value of 50, a high pressure mercury lamp will have a CRI of about 60. Metal
halide lamps and multi-phosphor fluorescent lamps have CRI values in the 80 to 90+ range
and that of high pressure sodium will be about 30.
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The huge range in lamps that is now available has resulted in the problem of differences in
appearance of coloured surfaces when illuminated by different lamps. The broader more continuous
the SPD of the lamp the better is the colour rendering of the lamp.
It has become necessary to classify lamps according to their colour rendering properties so
that lamps with poorer colour rendering such as Sodium lamps are not used where colour
appearance is important.
The CIE defines the Colour Rendering Index of a source as a measure of the degree to which the
perceived colours of an agreed set of Munsell tiles illuminated by the source agrees with the
perceived colours when illuminated by a reference source. The measure of the degree of
difference between colours of a Munsell tile when illuminated by the reference source and by the
test source is taken as the difference in their chromaticities as represented in the 1964 UCS
colour space.
The set of Munsell tiles comprises eight tiles which form a hue circle. They are:
• 7.5R 6/4 Light greyish red
• 5R 6/4 Dark greyish red
• 5GR 6/8 Strong yellow green
• 2.5G 6/4 Moderate yellowish green
• 1OBG 6/4 Lightish bluish green
• 5PB 6/8 Light blue
• 2.5P 6/8 Light violet
• 1OP 6/8 Light reddish purple
The reference illuminant is taken to be the black body radiator which has the closest
chromaticity coordinates to the test source, provided the test source has a CCT of 4000K or
less.
If the test source has a higher CCT then one of the phases of daylight is taken as the
reference illuminant. Again, the phase of daylight with chromaticity coordinates closest to the
test source.
After calculation of the colour difference for each tile (DE) a Special Colour Rendering Index is
formed:
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Data for lamps may give the CIE CRI group rather than the numerical value of CRI as given in
the Table 1.2 below:
Table 1.2 - General Colour Rendering Index Ra or CRI and Associated Colour Rendering Group
Colour Rendering Group Ra 1A Ra ≥ 90 18 80 ≤ Ra < 90
2 60 ≤ Ra <80
3 40 ≤ Ra < 60 4 20 ≤ Ra < 40
The use of lamps with high CRI will make the environment more colourful, providing a range
of colours are present with fairly saturated hues. Sources of the same CRI but different CCT
will not show up colours the same - sources of low CCT will tend to enhance reds and of high
CCT will tend to enhance blues.
AS/NZS 1158.1.1 Clause 2.8 and 3.1 Clause 2.7 state - The choice of light source should
be based on an analysis of all the factors relevant to the particular lighting scheme -
aesthetics and environmental, together with lamp mortality and lumen depreciation, cost,
energy use, etc. Compatible with the operational and economic requirements of the
lighting scheme, in general, the type of light source used should have the highest colour
rendering index (CR/) possible.
Applying this recommendation in practice results in white light generally finding
application in major urban centres and locations where aesthetics are important (MH/LED)
and on local roads (HPM/ fluorescent), whereas the generality of traffic routes will be lit by
HPS.
Applying this recommendation in practice results in white light generally finding application in major urban centres and locations where aesthetics are important (MH) and on local roads
(HPM/ fluorescent), whereas the generality of traffic routes will be lit by HPS.
Lamp Efficacy
This defined as:
Electrical Power in
Lamp Efficacy Lm/W
Luminous Flux
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Summary Table of Lamps
Lamp Type Luminous Efficacy (lm/W)
Colour Temp (K)
Colour Rendering Index(Ra)
Quartz Halogen 19 3000 100
Fluorescent 36W/PA
96 4000 80
Low Pressure 200 1700 --------
High pressure
Sodium 112 2000 30
Metal Halide 85 4000 80
LED Luminaire 120-150 3500-6500 70-80
Luminaire
The luminaire directs light in a complex light distribution (RL file) towards the surface of interest with directional intensities (I) in candelas (cd). The RL file is normally expressed in cd per 1000 lamp lumens and in the C, y angular coordinate system as shown Figure 1.2.
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The light distribution for a road lighting luminaire is such that light is emitted at high 𝛶 angles as possible to direct light along the roadway to give long spacings whilst controlling glare to the road user; these principles are demonstrated by Figure 1.3.
This results in the optimum light distribution, for economic luminaire spacing plus reasonable glare control, as employed for the semi-cutoff luminaire which finds general application for Category V lighting.
The driver's eyes are shielded by the roof of the car from the direct light from a luminaire
emitted at angles less than 𝛶 max (taken as -70°), therefore glare control is applied to the light
intensity distribution at 𝛶 angles greater than 70°, the light distribution runback.
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Figure 1.3 - The General Principles Controlling the Light Intensity Distribution of a Luminaire for Category V Lighting
Figure 1.4 shows the actual light distribution of a semi-cutoff Category V luminaire with a 250W HPS lamp. The dashed line shows the distribution, in elevation, along the road; note
the 𝛶 -angle is at 72.5°, for an aeroscreen luminaire this angle will be somewhat less. The full line shows the distribution, in azimuth, across the road; note that the maximum intensity is at C = 10°. This is the tow-in of the luminaire and is advantageous, especially for the lighting of wide roads, to achieve uniformity.
The total light output ratio (LOR) of this luminaire is 0.83, i.e. 83% of the light from the lamp is emitted by the luminaire, with almost all being downward (DLOR = 0.82). The higher the DLOR plus an optimum distribution the more efficient the luminaire will be in lighting the roadway. Some luminaire, aeroscreen in particular, will have DLOR somewhat less than 0.7.
A traditional luminaire cannot have a LOR much in excess of 0.80 since light is absorbed within the luminaire at reflecting surfaces, including at the emitting face. The LOR is not applicable for LED luminaires as the light source is not photometered separately. The luminaire as a whole is photometered and the intensities expresses in absolute photometry (in cd).
The light intensity distribution in the vertical plane parallel to the road edge ( the C0
plane), showing the general shape of the light intensity distribution; note the elevation of maximum intensity and the runback.
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350 0 10 20 30 40
-- = Gamma angle 72.5° DLOR = 0.82 -·- = C angle 10° ULOR = 0.01
Figure 1.4 - The Light Distribution of a Semi-Cutoff Category V Luminaire
A luminaire should be used which has a high IP rating in order to maximise the length of the maintenance cycle; currently IP = 6x for most general service luminaires. In addition the luminaire must comply with AS/NZS 60598.2.3 and may comply with SA/SNZ TS 1158.6 to ensure mechanical and electrical integrity and long service life.
llluminance
The illumination, or illumination as measure or specified, of a surface at a point is the light flux per unit area (E), in lumens per square metre, abbreviated to lux (Ix).
The basic photometric relation is:
E = I cosθ2
where θ is the angle of incidence of the light at the surface; this equation holds for any orientation of the surface, eg. horizontal or vertical.
This equation can be expressed as:
E = I cosθ3/ H2
(horizontal) or E = I cosθ sin2
θ / H2 (vertical)
where H is the luminaire mounting height.
The inverse square and cosine laws of illuminance effectively limit luminaire spacing
since the illumination of a point on a surface falls off sharply with the distance of it from
the light source. These laws influence the light distribution of a road lighting luminaire,
with the peak intensity being at a high 𝛶 angle to project light as far down the road as is
effective to maximise spacing without creating glare; see Figure 1.3.
llluminance is used solely in the performance specification of Category P lighting;
illuminance may be specified on a vertical plane in some situations, as well as on the
horizontal one, where lighting is installed as a crime deterrent. llluminance is specified for
some Category V lighting situations, e.g. intersections.
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Luminance The lumininance, or brightness as measure or specified, of a surface at a point of a
surface, for given directions of incidence and view, is intensity per unit area (L), in
candelas per square metre (cd/m2).
The basic photometric relation is:
L = E ρ / π
where ρ is the reflectance at the point for given directions of incidence and view. That directions of incidence and view are referred to implies that, firstly, an observer at a given
position, that views the bright surface from an angle of view at is implicit in the notion of
luminance (this is not so with regard to horizontal illuminance) and, secondly, that the
reflecting property of the road surface is complex, as illustrated in Figure 1.5.
(a) Smooth textured surface,e.g. R3. R4 & NZN4 (b) Rough textured surface,e.g. R1. R2 & NZN2
Figure 1.5 - The Appearance of a Light Patch on the Carriageway from a Single Luminaire for Two Different Road Surfaces
The light patches formed from the reflected light from the luminaire are elongated
towards the observer, i.e. the oncoming motorist. The head of the patch is indicative
of reflection from a matt surface whereas the elongation is indicative of mirror-like
reflections from the polished facets of the aggregate material of the surface. It can be
seen that the elongation is greatest for the smoother surface; if the road is very wet
from rain the reflecting surface will be, effectively, a water one and all reflections will
appear as long narrow streaks.
Luminance is used in the performance specification of Category V lighting for the straight road elements and in order to calculate luminance the complex reflectances
of the carriageway surface must be known.
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Reflectance
The reflectance is the reflection from a surface as a fraction of the incident light for
given angles of incidence and view. The reflectances of a road surface for a relevant range of angles of incidence and view are given in terms of the angular coordinate
system C, 𝛶, β (RS file), shown in Figure 1.6.
Angle α, the angle of view of vehicle driver, is taken as constant since it is small and
varies only over a small range for the carriageway area involved in the luminance calculation procedure.
Figure 1.6 - The CIE C,𝛶,β Angular Coordinate System Defining the Reflectance at a Point P
There are a series standard CIE road surfaces representative of the various surface textures for which there are reflectance tables; see Table 1.3.
The Table gives the physical description of the make-up of each surface of the series, so that the correct reflectance table can be matched to the surface.
These reflectance tables are included in the computer calculation software supplied with AS 1158.2; the default design surface in Australia is R3, an asphaltic concrete surface, unless known to the contrary, e.g. cement concrete.
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Table 1.3 The CIE Standard Surfaces and Allied Physical Structure
* The reflectance tables for these standard surfaces are taken from CIE 66 **The reflectance tables for these standard surfaces are given in AS/NZS 1158.2
CLASS DESCRIPTION
R1*
(a) Asphaltic road surfaces with at least 15% of artificial brightener
or with at least 30% of very bright anorthosites (arclyte,
labradorite or similar)
(b) Surface dressings with chippings where over 80% of the road
surface is covered and where chippings exist for a great man y
artificial brighteners or for I00% of very bright anorthosites
(c) Concrete road surfaces
R2* & NZR2**
(a) Surface dressings with harsh texture and with normal aggregates
(b) Asphaltic surface with 10% to 15% of artificial brighteners in
the mixture
(c) Coarse and harsh asphaltic concrete rich in gravel (>60%) and
with gravel sizes up to or greater than 10 mm
(d) Mastic asphalt (Gussasphalt) after dressing in new condition
R3*
(a) Asphaltic concrete (cold. mastic asphalt) with gravel sizes
up to 10 mm but with harsh texture (sandpaper)
( b) Surface dressings with coarse texture but polished
R4* & NZN4**
(a) Mastic asphalt (Gussasphalt) after some months of use
(b) Road surfaces with rather smooth or polished texture
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Notes
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