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Incorrect lighting can cause glare, eye strain,
headaches, impairment of vision or accidents. It is
also one of the main areas of energy wastage. Good
lighting can improve efficiency at work, enhance
leisure activities, and can even save lives. Ensuring
correct lighting is, therefore, a heavy responsibility
for managers and local authorities.
A Light Introduction
Why Measure Light ?
In modern industrial society we spend most of our
time in buildings lighted partly through windows
but largely by artificial illumination. On the road
we may also be totally dependent on vehicle- or
street-lamps for our safety. Thus, most of the things
we see, both by night and by day, are lit artificially.
Other important applications of photometry (the
science of measuring light) range from use by
standards institutes when calibrating reference
sources, to routine measurements in research and
quality control of light sources.
What is Light ?
Light is energy in the form of electromagnetic
waves. In contrast to sound and vibration, which is
mechanical vibration, electromagnetic waves do
not need a medium to travel through. They can pass
through many solids, liquids or gases but are most
efficiently transmitted through a vacuum, in which
there is nothing to absorb their energy.
The many different forms of electromagnetic
radiation are arranged in order of magnitude of
their frequencies or wavelengths. This arrangement
is known as the Electromagnetic Spectrum.
We are continuously exposed to electromagnetic
radiation. At the long wave (low frequency) end of
this spectrum there are electromagnetic waves used
for radio communications, with wavelengths
ranging from tens of kilometres down to a few
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millimetres. At the other (high frequency) end of
the electromagnetic spectrum there are X-rays and
gamma rays which have wavelengths that are very
small indeed. Only a small part of this radiant
energy can be perceived by the human eye. This is
what we call light.
Light makes up the small region of wavelengths
from 380 to 780 nanometres (10-9 m). The eye is
able to detect different wavelengths in this range of
radiation and gives the sensation of colour. Blue
and violet correspond to the short wavelengths, red
to the long, and yellow and green to the middle of
the visible range.
The V(λ) curve is of fundamental importance in
measuring light since it has been internationally
agreed to base all light measurements on this
sensitivity curve whatever the level of
illumination.
Vision and the V(λ) curve
Light entering the eye through the cornea and the
lens creates an image on the retina which senses
the light and transmits the information to the brain
through the nerves. The retina has two kinds of
light sensitive receptors: cones and rods. The
cones are used for photopic vision: seeing in
conditions of high illumination. There are three
types of cones each of which covers a different
spectral range. This is what makes colour vision
possible. The rods are more sensitive than the
cones and thus make scotopic vision (seeing in the
dark) possible. Since the rods have no facility for
spectral discrimination scotopic vision is without
colour.
The accuracy with which an instrument complies
with this curve in terms of its sensitivity to light of
different colours is known as its spectral match.
The sensitivity of the human eye is not uniform
over the visible range but varies with the
wavelength of the light. When adapted to high
illumination levels the maximum sensitivity of the
eye is determined by the cones and occurs at a
wavelength (λ) of 555 nm. When the rod system
only is utilised however (in the dark) the
maximum sensitivity corresponds to λ= 510 nm.
The range in between, over which both rods and
cones are used, is called mesotopic vision.
Two sensitivity curves are internationally
recognised:
(1) The spectral luminous efficacy for photopic
(cone or light-adapted) vision or V(λ) curve
and
(2) The spectral luminous efficacy for scotopic
(rod or darkadapted) vision.
Natural Sources of Light
Sunlight
This is by far the most powerful light source
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affecting the earth. Mankind has always been
dependent on the sun. It is essential to the
provision of warmth and food and is the world's
most important source of light.
“Man-made” Sources
Electric lamps may be divided into two main
groups: incandescent and discharge. Discharge
lamps may be further divided according to
whether the gas is contained at low or high
pressure.
Moonlight
The moon shines by reflecting sunlight. The
distance between the sun and the moon and the
low reflectance of the moon's surface both
contribute to its luminance being over half a
million times less than that of the sun.
The light from an incandescent lamp is generated
by bringing a filament to a high temperature. This
is achieved due to the heating action of the electric
current passed through it when the lamp is
switched on. Before any light is produced the
temperature of the filament must rise above600oC.
Lightning
Lightning is a meteorological phenomenon. It is
the visible electric discharge between clouds or
cloud and ground. This effect arises from the
accumulation of electrical charges in the
formation of clouds which can suddenly be
released in a spark-type discharge to earth.
Filament materials should have a high melting
point, low vapour pressure, high strength, high
ductility, as well as suitable radiation and
electrical resistance characteristics. Due to a very
favourable combination of these desirable
properties tungsten is used for nearly all lamp
filaments.
Aurora Borealis (Northern Lights) and
Aurora Australis (Southern Lights)
These consist of hazy horizontal patches or bands
of greenish light on which white, pink, or red
streamers are sometimes superimposed. They are
caused by electron streams spiralling into the
atmosphere, primarily at polar latitudes.
Bioluminescence
This is the name given to the emission of light by
living organisms. Several species of plant and
animal are able to manufacture compounds which
give out light when exposed to oxygen.
Discharge lamps have no filament, but produce
light by the excitation of gas contained between
two electrodes. In a low-pressure mercury-vapour
fluorescent lamp electrodes are sealed into the
ends of a tubular bulb. When a current is passed
through the gas mixture, predominantly ultraviolet
radiation is produced. For this reason the inner
surface of the tube is often coated with fluorescent
powders (generally phosphor crystals) which
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transform this U.V. radiation into (visible) light.
Spectra of Light Sources
distribution in the energy they emit.
The radiant-flux or electromagnetic-power spectra
of different light sources varies considerably. A
tungsten-filament (incandescent) lamp, for
example, emits most of its radiant energy in the
infrared region of the electromagnetic spectrum.
This is obviously inefficient in terms of the
conversion of electrical energy into light.
Incandescent lamps are, however, cheap and easy
to work with.
In the following section we will look at some of
the theory behind photometry.
Most of the energy radiated by a fluorescent lamp,
on the other hand, is emitted as visible light. This
gives fluorescent lamps a relatively high efficacy
and good colour rendering properties. They have a
long life compared to incandescent lamps but are
more expensive and more complicated
electronically.
Some fluorescent lamps are monochromatic: they
emit light at just one wavelength or spectral line.
The light emitted by a more typical fluorescent
tube consists of several prominent spectral lines.
Daylight consists of a much more even spread of
wavelengths. Lamp manufacturers often aim to
make fluorescent lamps which reproduce this
Some Light Theory: An Overview
This drawing should provide a useful summary of some of the basic concepts in lighting and help
illustrate how they relate to each other. A more detailed discussion of these concepts is given in the
following pages.
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the peak of the V(λ) curve. At this wavelength one
watt of radiation is said to produce a luminous
flux of 683 lumens. This “maximum possible
conversion from radiant flux to visual sengation”
has been given the symbol Km and is used as a
scaling factor in the calculation of luminous flux
in order to give the result in units of lumens/watt.
Luminous flux
In the visible range, radiant flux is considered to
have a luminous flux, $, which is a measure of the
visual response.
The luminous flux from a light source is found by
weighting the radiant flux, P(λ), at each
wavelength by the CIE standardised sensitivity
curve for photopic vision, V(λ), and then
integrating over the whole visible range.
Typical values for the luminous flux of some
common light sources are shown in the diagram.
By comparing the luminous flux to the electrical
power consumed, sources can be classified
according to their Luminous Efficacy.
If, for example, a source is consuming electrical
power at a rate of 60 watts to produce a luminous
flux of 5001m, the luminous efficacy, η, of this
source is 500/60 = 8,31m/W.
Similarly we find that a typical efficacy of a
tungsten lamp is:
η = 1000/100 = 10lm/W
and that of a fluorescent tube is:
η = 3000/40 = 751m/W.
In the case of discharge lamps efficacy increases
with the power consumption of the lamp.
Although the low pressure sodium lamp type has a
very high efficacy its poor colour-rendering ability
makes it less desirable for use on roads. In some
countries the use of the high-pressure type is
therefore recommended for safety reasons.
The eye is most sensitive to radiation at 555 nm,
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Luminous Intensity
The luminous intensity, I, is a measure of the light
output in a given direction. It is the basic
parameter in photometry and is defined as the
luminous flux from a source in a specified
direction, inside a small solid angle.
Examples of Luminous Intensity
A table or polar diagram of the intensity
distribution around a light source can be built up
from luminous intensity measurements. The
diagram gives a good graphical representation of
the light distribution whereas a table is more
useful if further calculations are to be made. In the
diagram the dotted isocandela curve represents the
lamp alone whilst the solid curve shows the effect
of using a reflector or "shade".
The solid angle, ω, is expressed in terms of
steradians and is a way of describing the
‘broadness’ or ‘narrowness’ of cones and
pyramids relative to a sphere. The apexes of these
cones and pyramids can be imagined as being at
the centre of a sphere, with the bases making up
part of the surface area of that sphere. The biggest
solid angle possible is the sphere itself. This has a
volume of 4π or 12, 5664 steradians. A beam of
light from a lighthouse, on the other hand, has a
solid angle of less than a 1/20 of a steradian.
I
In the case of luminaires the light distribution
varies with the shape of the light source itself and
the design of the fixture. A polar diagram for a
luminaire is usually plotted in terms of candela
per 1000 lumens of lamp flux so the power of the
lamp used can be varied without affecting the
validity of the diagram. The type of lamp must,
however, remain the same.


In theory the equation only applies to a point
source since, by definition, a solid angle must
have a point at its apex. In practice, however,
results obtained using this equation are
satisfactory provided only that the dimensions of
the source are small compared with the distance
from which it is observed.
The diagrams and tables for luminaires with
asymmetrical light distributions must provide
information on the intensity in at least two planes.
These are normally the vertical plane through the
longitudinal axis of the luminaire and the plane at
right-angles to that, known as the transverse plane.
The unit for luminous intensity is the candela (cd).
A source radiating 1 1m through a solid angle of 1
sr, gives an intensity in that direction of 1 cd.
In one commonly used intensity-distribution
measurement-technique the luminaire is rotated
about two axes while the photocell remains
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stationary. If a more comprehensive survey of the
intensity distribution is required then the
luminaire may be kept stationary whilst the
photocell is moved over a hemispherical path.
the surface due to the angle of incidence.
Light striking a surface at large angles will result
in a smaller illuminance than light striking the
surface at small angles because less of the
luminous flux is intercepted by that area. The
maximum illuminance will be obtained when the
angle of incidence is zero i.e. when the surface is
perpendicular to the beam direction.
Luminous-intensity distribution diagrams are
particularly important when it comes to finding
the most suitable luminaire for a particular
application.
If there is more than one source, the resulting
illuminance is the sum of the contributions from
each source.
Illuminance
When light emitted from a source strikes a surface
that surface will be illuminated.
Examples of llluminance
The Illuminance, E, is thus defined as the
luminous flux incident on a surface per unit area
of that surface.
E
The range of illuminances encountered in daily
life varies by a factor of more than a million. A
few approximate values are shown in the diagram.
A
A
In many circumstances the illuminance on a
horizontal surface, En, does not correspond very
well with the visibility of the thing which has to
be seen. In the case of facial recognition (in street
at night, for example) a much better parameter is
the semi-cylindrical illuminance, ESC. This is
simply because the light falling on a semi-cylinder
corresponds better to that which falls on the body
of a pedestrian. If we now consider what
illuminance we should measure to see if a road
surface is properly lit and, once again, we take
into account the the obstacles on the road and the
directionality of the light, we find that
hemispherical illuminance, EHS. is a much better
When light of 1 lumen falls on a surface of 1 m2
the illuminance of that surface is 1 lux (lx).
The illuminance on a particular surface is related
to the intensity of the light source through the
formula:
E
I
cos v
d2
where I is the intensity of the source in the
direction of the surface and d is the distance
between the source and the surface. The factor
cos v accounts for the reduction in light striking
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measure of the visibility of objects in the road or
of the kerb than horizontal illuminance. Once
again, the explanation is simple: the visibility of
these objects depends to quite a large extent on
light falling on their vertical surfaces; and this
light is only measured (or given the correct
weighting) in the case of hemispherical
illuminance measurements.
material and is read from a table of values. If the
surface is diffuse then ß can be replaced with ρ,
the diffuse reflection coefficient for the material.
A typical luminance for a piece of white paper
under an illuminance of 500 lux is thus 130cd/m2.
The eye can detect luminances from as little as
one millionth of a cd/m2 up to a maximum of one
million cd/m2.The upper limit is determined by
the luminance required to damage the retina. The
reason that our eyes are so easily damaged by
looking at the sun is explained when we see that
its luminance is 1000 times greater than this
maximum level.
Luminance
When a part of the incident light striking a surface
is reflected the human eye will observe that
surface as a light source. The brightness observed
is called the Luminance, L, and is defined as
intensity per unit apparent area of light source.
The apparent area, A', is the area the source seems
to have as seen by the observer. Thus,
L
IU
A
Contrast on a Document
Contrast plays an essential role in vision. The
process of seeing is made up of a succession of
individual viewing actions, each of which consist
of the perception of one or several details against
their background. To distinguish these details
there has to be a certain difference between their
luminance and the luminance of the background.
This distinction is expressed by the luminance
contrast, C:
where A’ tends to 0.
For a plane surface the apparent area can be found
from the equation: A’ = Acos u, where A is the
actual area of the source, and u the angle between
the normal to the surface and the direction of
observation, IU is the luminous intensity in that
direction.
C
Alternatively, the luminance of a surface can be
calculated from the formula
L
E
Ld  Lb
Lb
where Ld is the luminance of the detail and Lb the
background luminance (i.e. that of the paper).

where ß is the luminance factor of the surface
The luminance of a surface varies with the angle
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of the incident light and the observation angle.
When these two angles are equal the contrast will
normally be at a minimum and reflection of light
off the document is most likely to be a problem.
This makes it difficult or impossible to distinguish
parts of the text. The extent to which this
phenomenon will affect the visibility of the text is
often not easy to evaluate since it depends not
only on the lighting but also on the type of
document and the reflection characteristics of the
detail. The contrast of the document will generally
decrease the more glossy the paper and will also
vary depending on whether ink, pencil, type, or
print is used.
shown. A luminance transducer and contrast
standard are mounted in a carriage on a radius arm
so that they can be positioned accurately. The
contrast meter calculates the contrast from the
luminance measurements made at each point; the
results can then be entered directly onto a map of
the standard work surface.
In order that the contrast of documents can be
considered during the design stage of lighting
systems in schools, offices, and libraries etc. the
CIE has introduced the Contrast Rendering
Factor, CRF. This is defined as the ratio of the
contrast of a task under the installed lighting, C, to
its contrast under diffuse sphere illumination, GREF
In order to make repeatable measurements of the
contrast in different locations and under a variety
of lighting conditions, it is necessary to use an
object which has well specified and consistent
reflectance characteristics over a wide range of
illuminance and viewing angles.
Accurate values for contrast rendition are also
most easily obtained using a contrast standard.
The acronym CRFR is often used to indicate that a
reference standard has been used for the
measurement.
A contrast standard is shown in the diagram. The
greatest contrast will occur under side lighting; at
places where reflections occur, the luminance of
the dark contract-taskis practically equal to that of
the light contrast-task: there is almost no contrast
at all.
For this purpose a contrast standard has been
designed. A contrast standard consists of a dark
field and a light field simulating character and
background respectively.
Measurement of the luminance of these fields in a
particular location in the work area can be
performed with a luminance transducer. The
contrast can then be calculated in
accordance
with the formula shown on the previous page. In
the diagram a system for measuring contrast is
Contrast on VDU Screens
Many parameters influence the readability of a
Visual Display Unit (VDU) screen. The luminance,
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dots (pixels) ~3 cm2 are generated on the screen.
The character luminance, Lc, at each point is
measured by pointing the photocell at the dot-field
or cursor (from the normal viewing distance); the
background luminance, Lb, is then measured
without the dot-fields or cursor on the screen.
contrast, dimensions, sharpness, and the colour of
the characters all have a significant effect. The
most important of these is, per haps, the contrast
between the characters on the screen and that of
the background and it is this simple ratio that is
most commonly used to define CS, the contrast on
the screens of visual display units:
Glare
L
Cs  d
Lb
Annoyance, discomfort, or loss in visual
performance and visibility caused by a luminance
within the visual field which is greater than the
luminance to which the eyes are adapted is called
glare. Despite many different causes of glare most
occurrences can be classified as either
discomfort- or disability glare.
Ld is the luminance of the detail on the screen (a
character, for example) and Lb is the luminance of
the background.
The measurement of contrast on the screen of a
VDU is similar to that on a horizontal work
surface. The same measurement cell and contrast
meter can be used but measurements are made
directly from the screen and not from the contrast
standard. Note that the contrast must be measured
from the typist's position and the measurement
cell pointed towards the area on the screen which
is to be evaluated. The person recording the
contrast conditions at the workstation simulates
the body shadow of the typist by sitting in the
appropriate
position
whilst
taking
the
measurements.
Discomfort Glare
Discomfort glare is annoying but does not
necessarily prevent the performance of the visual
task in question. It is often attributed to the eye's
tendency to fix upon the brightest spots within the
field of view and is thus related to phototropism
(the way in which plants are drawn towards light).
The degree of discomfort produced by a luminaire
is dependent upon four main parameters: the
luminance of the source, the size of the source, the
angle between the source and the observer's line
of sight, the adaption level of the observer's eye.
Disability Glare
Disability glare prevents the performance of the
visual task and can be caused in three different
ways:
1) Scatter of light in the lens of the eye
producing a veiling luminance on the retina
2) Insufficient time for the eye to adapt to the
new vastly different illuminance
3) After- or ghost images. Most of us experience
this strange phenomenon after
having our
photograph taken, during night-time driving,
or on encountering reflections of the sun on a
bright summer day. The photochemical
processes essential to our vision are
temporarily disturbed due to the eye becoming
“overloaded” with light. The brain becomes
confused and we continue to see a succession
of images of the bright object or reflection,
alternatively positive and negative of irregular
strength
and of decreasing frequency. Full
vision is normally restored within 5 or 10
minutes of the event.
Direct measurement of the contrast of something
as small as a character on a VDU screen can be
acheived with an illuminance meter with focus
and 1/3º acceptance angle. With instruments of
minimum acceptance angle 2º, uniform fields of
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There are many different ways of avoiding
disability glare. You may shade lamps, pull
curtains or blinds over windows or reduce
reflectance by using matt materials and by
positioning desks, terminals and light-sources
carefully. In certain circumstances, such as
playing outdoor sports on a sunny day you have to
make do with shading your eyes with a pair of sun
glasses or a visor. Card players and desk-clerks
used to wear visors to shade their eyes from the
light-source they were using.
detector directly onto the screen. These values,
along with the luminance of the background
(including reflections), Lb, are an important aid to
defining the most suitable setting for the screen
luminances.
Assessing the character-contrast on keyboards and
the ratio of mean luminance between the
manuscript and the screen are also particularly
important. The eyes of a typist may have to focus
alternately on the manuscript and screen several
thousand times a day. If this ratio is greater than a
factor of 5 eye strain may well result. In the
proposed ISO standard DP9241 a maximum ratio
of 1 to 10 is recommended.
Some Light Work
Measuring On VDU Screens
Luminance distribution and contrast conditions at
the workplace can be entered into a
measurement-report form like the one shown in
the diagram.
A measurement report should be made for each
work station in the room, for one or more typical
ambient lighting situations. Together these reports
form a good base for the planning of
improvements.
The measurement results recorded here are the
contrast conditions at a word processing terminal.
The lighting system consisted of a ceiling
luminaire mounted directly above the work place
and gave rise to the reflection marked on the
screen. The contrast of the 9 fixed dot-fields are
given and should be supplemented by a minimum
contrast for the whole screen, as shown.
When all the measurements have been recorded in
this way the report form can be studied and used
as a guide to what problems exist at the
work-station. Recommendations can then be made
as to how visual conditions may be improved and
once these have been implemented the analysis
can be repeated to check the improvement.
L*C
and
screen-generated
L*B — the
luminances---should be measured in a dark room
or, if this is not possible, by placing the luminance
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Under normal road-lighting conditions, values of
semi-cylindrical
illuminance,
ESC,
and
hemispherical illuminance, EHS. provide a better
assessment of the visibility of three dimensional
objects than horizontal illuminance, EH,
measurements.
The CIE have proposed typical semi-cylindrical,
hemispherical, and horizontal illuminance values
required to enable pedestrians to assess the
attitude of other persons from at least 4m away.
Sport Lighting and Stage Lighting
Sport lighting and stage lighting installations are
designed mainly on the basis of what illuminance
levels are required by actors, players, their
audience, and sometimes television cameras.
With sports lighting the main visual tasks for both
players and spectators are both vertical and
three-dimensional: following movements of the
players and the ball. The illuminance of the
playing surface and its markings is usually of
secondary importance.
Lighting of Residential Areas
Not surprisingly visibility of 3-dimensional
objects---the most important parameter in sport
and stage lighting---is more closely correlated to
illuminance on a semi-cylindrical surface than that
measured on a flat or hemi-spherical surface.
Night-time lighting in urban areas must fulfil
several requirements. Motorists need to be able to
see people or objects on the road and kerb.
Cyclists and pedestrians also have to make sure
that their path is not obstructed.
This can be partly explained by the fact that the
semi-cylindrical-adaptor surface is more similar in
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shape to that of a standing person. The visibility
of both are influenced to a far greater extent by
side and/or front lighting than by vertical lighting.
This is part of the reasoning behind the
introduction of semi-cylindrical illuminance, ESC,
as a better measure of the quality of stage, sport,
and television lighting.
Road Lighting
The safety and comfort of a road user deteriorates considerably with the onset of darkness. Studies in
many countries and by many different institutions have shown that road lighting can help to reduce
night-time accidents by more than 30%. A reduction by this amount will usually be more than sufficient to
offset installation and running costs of good road lighting.
Lighting Criteria
Whilst driving, our visual performance or — “how well we can see” — is determined, to a large extent,
by the speed at which we are travelling, the level of lighting, and the reflectance of the road surface. To
take all these things into account road-lighting specifications should be in terms of the average luminance
of a roads surface at between 60 to 160 meters in front of the driver. Road-lighting standards state that
average road-surface luminances should vary from: 0,5 cd/m2 for roads with traffic of very low density
and limited speed; to 2 cd/m2 for roads where density and speed of traffic are high.
Lighting Criteria
Best indicator of
Visual performance
Recommended value
Level
Uniformity
Glare
Average surface Luminance Overall uniformity Threshold increment TI
Lav
Uo
0.5-2 cd/m2
0.4
10-20%
Two other parameters which are essential in assessing visibility on the roads at night are the uniformity of
the light and the degree of disability glare due to the lighting.
To ensure not only a good average but also a certain minimum visual performance at all locations on the
road, lighting specifications normally include a minimum value for overall uniformity, UO. This is the
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ratio between the minimum and the average road-surface luminance.
The degree of disability glare due to road lighting is normally assessed by calculating a value known as
the thresh old increment (TI). Threshold increment values are only valid if the average road-surface
luminance lies between 0.05 and 5 cd/m2.
Adequate luminance uniformity is important for both visual comfort-and visual performance.
Investigations have revealed that both the overall uniformity ratio, Uo, and the longitudinal uniformity
ratio, Ui, have to be within certain limits in order to specify adequate road lighting. The longitudinal
uniformity ratio, UI (Lmin / Lmax measured along the middle of a traffic lane) should not be below 0, 7 on
motorways if an uncomfortable impression of patchiness is to be avoided, whereas the overall uniformity
Uo (Lmin/Lav) should nowhere be below 0, 4 so as to ensure good visual performance.
Measurements on roads
Practical measurements from a motorway from the driyer's eye point are illustrated. The measurement
field angle is 1ºand the observation angle is 3º to the road surface
The level-recorder was synchronized with the car's speed (~100 mm/km). Many of the lamps were not
working due either to failure or to money saving measures. This can be seen on the recording by the
general low levels and the rapid fall of luminance in many places.
The projection of the measurement field on the road surface forms an ellipse which in this case has a
length of 8 m and a width of 0.44 m. Compared with the distance between lamp posts (25m) the
resolution is fairly good. The sidelights only were used during the measurements. A typical set-up is
shown on the previous page.
Since perfect spectral and cosine weighting is not
possible, a third requirement---for accurate
measurements whatever the spectral and spatial
distribution
of
the
light---is
detailed
documentation of the cosine and spectral
sensitivity of the instrument. The influence of
temperature on the optics should also be well
documented.
Designing Light Measuring Instruments
Some of the most essential features of a light
measuring instrument, or photometer, are shown
in the diagram opposite and the block diagram on
the following page. The
transducer (optics, filters, and silicon detector)
ensures that the signal (current) it sends to the
amplifier is proportional to the light being
measured.
An amplifier is necessary to ensure that very low
light powers can be accurately determined by the
transducer. The fact that the offset of any
amplifier varies with time means that automatic
zero drift compensation saves the user a lot of
time and reduces the opportunity for error.
This implies also that a transducer for illuminance
measurements must have a weighting as close as
is possible to
a) standardised human vision, V(λ)
b) angular cosine response
The transducer itself should have a wide dynamic
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range, and stable output with regard to
temperature, for example. Again, some change in
sensitivity can be expected with a change in
temperature. This too should be well documented
so that the user can check to see at what
temperatures some sort of extra compensation has
to be made.
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