Lighting for work: A review of visual and biological effects

Abstract

With the detection in 2002 of a novel photoreceptor cell in the eye, the biological effects that light has can be better understood. From the research on the biological effects of lighting, it is evident that the rules governing the design of good and healthy lighting installations are, to a certain degree, different from the conventionally held rules. We demonstrate that it can be beneficial to be able to adapt both the level and the colour of the lighting. Not only the light on the visual task, but also that entering the eye determines the overall quality of lighting. In a working environment, not only are the advantages in terms of health and wellbeing important for the workers themselves, they also lead to better work performance, fewer errors, better safety, and lower absenteeism.

Full text

Lighting for work:
visual and biological effects
Ir.W.J.M. van Bommel
Ir. G.J. van den Beld
April 2004
Philips Lighting, The Netherlands

Authors
Ir.W. J. M. van Bommel and Ir. G. J. van den Beld
The two authors, the former with a university degree in physics and the latter with a degree in electrical
engineering, joined Philips Lighting in the early 1970’s. They have since been involved in fundamental
lighting application research in many different application areas whilst occupying a number of different
positions within the company. They also actively participate in the work of the International
Commission on Illumination (CIE) and of the European Standardisation Technical Committee for
Light and Lighting (CEN TC 169), and have individually presented papers at many different national
and international Congresses. Their papers have also been published in many renowned lighting
journals.
Wout van Bommel has recently been elected president of CIE for the period 2003-2007.
Gerrit van den Beld is the Dutch representative of CIE Division 6: “Photobiology and Photochemistry”
and is a member of the Board of the Dutch Foundation: “Lighting and Health” that promotes a wider
medical-scientific knowledge of the influence of light on human beings.
“Lighting for Work: visual and biological effects” (April 2004) updates and extends the previous article
“Industrial lighting and productivity” by the same authors (August 2002) with the latest findings on the
biological effects of lighting on health and well being. For detailed information on lighting and industrial
productivity, please refer to pages 6 to 8 of the August 2002 article.
Contents
Summary
Introduction
Three types of photoreceptor cells in the eye
Lighting and visual effects
Visual performance
Visual environment
Vision-related quality aspects of lighting installations
Lighting and biological effects
Light and body rhythms
Lighting, alertness, mood, and stress
Health-related quality aspects of lighting installations
Conclusion

Lighting for work:
visual and biological effects
Summary
With the detection in 2002 of a novel photoreceptor cell in the eye, the biological effects that light
has on human beings can be better understood. The spectral sensitivity of the novel cell type has in
the meantime been studied and shows that bluish light has biologically a larger activating effect
than does reddish light.
A large number of research projects that compare the effects on health, well-being and alertness as a
result of people working under different lighting conditions have been carried out. The results, as
summarised in this article, show that good lighting indeed has important beneficial effects, not
only visually but also biologically. From the research on the biological effects of lighting, it is
evident that the rules governing the design of good and healthy lighting installations are, to a
certain degree, different from the conventionally held rules. We demonstrate that it can be
beneficial to be able to adapt both the level and the colour of the lighting. Not only the light on the
visual task, but also that entering the eye determines the overall quality of lighting.
In a working environment, not only are the advantages in terms of health and well-being important
for the workers themselves, they also lead to better work performance, fewer errors, better safety,
and lower absenteeism. An example from an industrial environment demonstrates that by
changing the lighting from 300 to 500 lux may easily increase the overall productivity by 8 per
cent.

The visual effects of lighting have been studied for more than 500 years. Leonardo da Vinci (1452-1519)
described ideas about “street lighting”. Christiaan Huygens (1629–1695) formulated the wave theory of
light, while Sir Isaac Newton (1642-1727) developed the corpuscular theory of light. Johann Wolfgang
Goethe (1749-1832) analysed the colour effects and aspects of lighting.
With the introduction of gaslight and electric light in the early-to-mid 1800s, the study of visual lighting
effects was directed more and more towards practical lighting application research.
As regards the mechanism of visual effects, as early as 1722 the Dutchman Antony van Leeuwenhoek noted
the presence of “rod and cone cells” in the retina. Their existence was confirmed as “the light sensitive
photoreceptors” in 1834 by the German Gottfried Treviranus. This discovery opened the way to the
understanding of many of the visual lighting effects already described and to a more concrete investigation
into the visual effects of lighting, the goal being to design more effective lighting installations.
For more than 150 years, scientists considered rods and cones to be the only photoreceptor cells in the eye.
Seen in this historic context, it is sensational that in 2002 David Berson et al. [1] of the Brown University
(USA) detected a novel, third type of photoreceptor in the retina of mammals. This novel photoreceptor is a
“missing link” in describing the mechanism of biological effects as controlled by light and darkness. That
lighting has important biological effects has been the subject of extensive studies in the biological and
medical scientific world during the past twenty-five years. From this, we have learned that the effects of
good lighting extend much further than visual effects only: the biological effects mean that good lighting
has a positive influence on health, well-being, alertness, and even on sleep quality [2], [3], [4], [5]. At the
same time, it means that the lighting parameters with which good lighting can be described need to be
revised.
This article first describes the mechanism of both visual and biological effects based on the three
photoreceptors in the eye. Subsequent sections deal with lighting and visual effects, and lighting and
biological effects. The first of these sections concludes with a summary of the “vision-related” lighting
quality aspects, while the second concludes with a discussion of “health-related” lighting quality aspects.
INTRODUCTION
2PHILIPS
Lighting for work: visual and biological effects

THREE TYPES OF PHOTORECEPTOR CELLS
IN THE EYE
The photoreceptor cells in the retina of the eye, the cones and rods, regulate the visual effects. When light
reaches these cells, a complex chemical reaction occurs. The chemical that is formed (activated rhodopsin)
creates electrical impulses in the nerve that connects the photoreceptor cells with the back of the brain
(visual cortex). In the visual cortex of the brain the electrical impulses are interpreted as “vision”.
Figure 1 shows the nerve connection between cones and rods in the eye and the visual cortex of the brain.
The rods operate in extremely low-level light situations (scotopic vision) and do not permit colour vision.
The cone system is responsible for sharpness and detail and colour vision. For all indoor lighting situations,
the cones are to a very large extent decisive.
The sensitivity of the cone and rod systems varies with varying wavelength of light, and thus with varying
colour of light. This is illustrated in Figure 2, where the spectral eye sensitivity curves Vλfor the cone system
and V’λfor the rod system are given. The Vλcurve for the cone system is the basis for all lighting units such
as lumen, lux and candela. It is called the photopic system. As can be seen from the Vλcurve, the eye is not
very sensitive to extreme blue and extreme red light, and has its maximum sensitivity for green-yellow light.
PHILIPS 3
Fig.1 Visual and biological
pathways in the brain:nerve
connections between the
retina of the eye,with its cones
and rods,and the visual cortex
on the one hand (in red) and
between the retina,with the
novel photoreceptor cell,and
the suprachiasmatic nucleus
(SNC) and the pineal gland
(in blue).
Fig.2 Spectral eye sensitivity
curves,Vλfor the cone system
(photopic vision:solid line) and
V’λfor the rod system
(dotted line).
visual
cortex
pineal gland
SNC
retina
100
% 75
50
25
0
400 500 600 700 800 nm
λ
V'λVλ

It should be noted that different light colours can be obtained by different mixtures of wavelengths.
White light consists of such a mixture. It is evident that the (visual) efficacy of a light source is very much
determined by the spectral eye sensitivity and the wavelengths that are incorporated in its light.
The novel photoreceptor cell type in the retina of the eye detected by David Berson et al. [1] in 2002
regulates the biological effects1. When light reaches these cells, a complex chemical reaction occurs (here
involving the photo pigment melanopsin [6]), again producing electrical impulses. These cells have their
“own” nerve connections to, amongst others, locations in the brain called the suprachiasmatic nucleus
(SNC), which is the biological clock of the brain, and the pineal gland. Figure 1 shows the nerve
connection between the novel photoreceptor cells in the eye and these locations in the brain.
The sensitivity of this novel photoreceptor cell also varies for different wavelengths of light, and thus for
different colours of light. On the basis of the biological factor “melatonin suppression”, Brainard [7] was
already able to determine the spectral “biological action” curve2. This curve is given in Figure 3, together
with the visual eye sensitivity curve of cones.
4
Lighting for work: visual and biological effects
Fig.3 Spectral biological action
curve (based on melatonin
suppression),in blue,
(source:Brainard [7]),
and the visual eye sensitivity
curve, in red.
By comparing the two curves it is immediately evident that the biological sensitivity for different
wavelengths of light is quite different from the visual sensitivity. Where the maximum visual sensitivity lies
in the yellow-green wavelength region, the maximum biological sensitivity lies in the blue region of the
spectrum. These phenomena have an important meaning for the specification of healthy lighting.
PHILIPS
1Probably,the rods and cones do play a certain role in this respect as well.
2As will be discussed further on in this article, one of the biological effects of light is the suppression of the hormone melatonin.Probably many other biological factors regulated
by lighting will have an action spectrum similar to that determined on the basis of melatonin suppression.
100
% 75
50
25
0
400 500 600 700 800 nm
λ

Visual performance
Lighting for work covers a wide range of different working interiors and tasks: from offices and small
workshops to huge factory halls, and from reading, writing and PC working tasks to fine precision work or
heavy industrial tasks.
The lighting quality should always be high enough to guarantee sufficient visual performance for the tasks
concerned. However, a person’s actual visual performance depends upon not only the quality of the lighting
but also upon his or her own “seeing abilities”. In this respect, age is an important criterion, since lighting
requirements increase with age. Figure 4 gives the relative amount of light required for reading a well-
printed book, as a function of age. This research was carried out with test persons wearing, if required, the
correct reading glasses. It is evident from this curve that the age effect is extremely severe. One of the many
reasons for this age effect is the deterioration of the transmittance of the eyes’ lenses: the lenses gradually
turn yellowish (see Figure 5). This deterioration means that the ageing lens has a lower transmittance.
It also means that less and less bluish light is transmitted. The ageing eye sees a less-blue world.
5
Fig.4 Relation between age and
relative amount of light required
for reading good print
(source:Fortuin [8]).
Fig.5 Lens transmittance for
various age categories.Values
are expressed as a percentage
of the 560 nm point for the new
born (source:adapted from
Brainard et al.[9]).
LIGHTING AND VISUAL EFFECTS
PHILIPS
300 350 400 450 500 550 600
0
20
40
60
80
100
wavelength (nm)
% transmittance
new born
20-29 years
60-69 years
0 10203040506070
0
2
4
6
8
10
12
14
age (years)
ight requirement
Figure 6 serves as an illustration of the many research results pertaining to the influence of lighting quality
on visual performance. It gives the relative visual performance as a function of lighting level for different
visual task difficulties: one for a moderately difficult task (e.g. office work or general machine work in an
industrial environment) and another for a difficult task (e.g. colour inspection work or fine assembly work).
All tasks show a clear increase in visual performance with increased lighting quality – in this example the
lighting level. In the graph, the required lighting levels (EN) for industrial environments, as in many cases
specified in the European Norm for the lighting of work places [10], are indicated.

6
Lighting for work: visual and biological effects
The graph shows that the requirements laid down in the European Norm are, in fact, well-suited to the
younger persons. However, the visual performance of the older workers is considerably lower. Luckily,
compensation with a higher lighting level is completely possible for the moderately difficult task. In
practice, this calls for adaptable lighting on top of the lighting required by the “EN Standard” for those
moments that daylight is not sufficient to give the higher lighting levels needed for the older workers.
Of course, an improvement in visual performance yields, in its turn, an improvement in sustained work
performance, reflected in a higher output and in a lower number of errors. The extent to which good-
quality lighting enhances work performance depends on the visual component of the task. A task with an
important visual component will benefit more from good seeing conditions than a task with a less
important visual component.
Visual environment
Besides its effect on visual performance, lighting can also have a powerful influence on atmosphere and the
visual impression of the workplace. Properly designed, the overall working environment can have a
stimulating effect on the people working within it [12]. Today, a lot of emphasis is placed on the layout and
interior design of the workplace. Good lighting can strengthen the interior design, but poorly-designed
lighting can diminish or even “destroy” the effect of the interior design.
One aspect that is important in this respect is a controlled brightness of the surfaces that form the physical
limits of the space, such as walls, floor and ceiling. The brightness of these surfaces determine to a large
extent how the total space is experienced. Another factor is a proper limitation of glare and undesirable light
reflections. Glare is the sensation produced by brightness levels within the visual field that are considerably
greater than the brightness to which the eyes are adapted. Owing to limitations in the adaptation properties
of the eye, abrupt changes in brightness may lead to reduced visual performance and to visual stress and
discomfort.
PHILIPS
Fig.6 Relation between
relative visual performance
(in %) and lighting level (in lux).
Continuous blue line:young
persons;broken red line: older
persons (source:CIE [11]).
EN:lighting levels specified in
the European Norm.
100 300 500 1000 3000
20
30
40
50
60
70
80
CE
level (lux)
visual performance (%)
100 300 500 1000 3000
20
30
40
50
60
70
80
CE
level(lux)
visual performance (%)
Moderately
difficult task
Difficult task

7
The colour properties of the light should also receive considerable attention. The lighting should permit the
“real” colours to be seen. Proper colour rendering of the human skin is especially important, since lighting
that makes the skin look pale and unhealthy often leads to complaints. Also, the colour appearance of the
light itself plays a role in providing the space with an atmosphere. It may even have an emotional influence.
For example, a somewhat bluish-white light gives a cool impression that is often experienced as
businesslike, while reddish-white light gives a warm impression that may be experienced as cosy and
relaxing.
Finally, daylight contribution to the interior is another very important factor determining the quality of the
working environment. Fortunately, in many cases daylight penetrates the building for at least several hours
each day, considerably increasing the overall lighting levels. But daylight not only facilitates the visual
performance of the visual task by contributing to the lighting on that task; because of its dynamic, varying
character in both intensity and colour, it also contributes greatly, if properly controlled (e.g. by proper
window and sun-shielding design), to a good working environment. The dynamic changes in daylight have
a positive influence on mood and stimulation. An extensive study under office conditions has shown that
people prefer artificial lighting in addition to the normal daylighting present in an office environment:
average 800 lux on top of the prevailing daylight contribution [13].
Vision-related quality aspects of lighting installations
Most national and international recommendations and standards specify lighting quality figures for the
majority of the visual quality aspects mentioned above, and for a wide variety of interiors and activities.
Table 1 lists the visual quality aspects together with the most important parameter for each aspect as used in
the European Norm for the lighting of workplaces.
It should be noted that the colour appearance of the light itself is not specified in the European Norm.
The reason for this is that so far the colour appearance is seen as a matter of psychology and aesthetics and
is considered to be natural.
PHILIPS
Table 1 Visual quality aspects
of lighting installations with
their quality parameters as
specified in the European
Norm for the lighting of
workplaces [10].
Visual quality aspect Quality parameter
Lighting level Average illuminance level,Eav
Spatial distribution ⎧Uniformity:Emin / Eav
⎨
Glare restriction:UGR⎩
Colour rendering Ra

As an illustration of what quality is required in different situations, Tables 2 and 3 give the required values
specified in the European Norm for an office and for an industrial environment (the chemical, plastics and
rubber industries)3. These requirements are values that meet the needs of visual performance and visual
comfort for workplaces for the majority of the workforce. However, as discussed above, the age effect is so
important that adaptable lighting on top of the “EN Standard lighting” is needed for those moments when
daylight is not sufficient to give the higher lighting levels that are required for the ageing eye.
3 Offices
Ref.no. Type of interior,task or activity ¯
Em UGRL Ra Remarks
3.1 Filing,copying, etc. 300 19 80
3.2 Writing,typing,reading, 500 19 80 DSE-work:
data processing see clause 4.11.
3.3 Technical drawing 750 16 80
3.4 CAD workstations 500 19 80 DSE-work:
see clause 4.11.
3.5 Conference and meeting rooms 500 19 80 Lighting should be
controllable.
3.6 Reception desk 300 22 80
3.7 Archives 200 25 80
2.5 Chemical,plastics and rubber industry
Ref.no. Type of interior,task or activity ¯
Em UGRL Ra Remarks
2.5.1 Remotely-operated processing 50 - 20 Safety colours
installations shall be recognisable
2.5.2 Processing installations with limited 150 28 40
manual intervention
2.5.3 Constantly-manned workplaces 300 25 80
in processing installations
2.5.4 Precision measuring rooms,laboratories 500 19 80
2.5.5 Pharmaceutical production 500 22 80
2.5.6 Tyre production 500 22 80
2.5.7 Colour inspection 1000 16 90 TCP ≥4000 K.
2.5.8 Cutting,finishing,inspection 750 19 80
8
Lighting for work: visual and biological effects
Table 2 Lighting requirements
for offices
(source:EN 12 464 [10]).
Table 3 Lighting requirements
for the chemical,plastics and
rubber industries
(source:EN 12 464 [10]).
3The values specified for the average illuminance are “maintained illuminances”:viz. values below which the average illuminance on the specified surface is never allowed to fall.
The value specified for uniformity on the task is always the same:Emin / Eav ≥0.7.
PHILIPS

9
Fig.7 Double plot
(2 x 24 hours.) of typical daily
rhythms of body temperature,
melatonin,cor tisol,and
alertness in humans for a
natural 24-hour light/dark
cycle.
The beneficial effect of (day)light has been well known since ancient times, an example being heliotherapy,
or the treatment of disease by exposure of the body to the sun’s rays. Light therapy for dealing with health
problems was popular until the 1930s, after which time the introduction of penicillin led to
pharmaceuticals taking the leading role. Over the last 20 to 30 years, however, the appreciation of light as
an important contributor to health and well-being has been revived, thanks to various findings in biological
and medical research.
We normally think of the eye as an organ for vision, but due to the discovery of additional nerve
connections from recently-detected novel photoreceptor cells in the eye to the brain, it is now understood
how light also mediates and controls a large number of biochemical processes in the human body.
The most important findings are related to the control of the biological clock and to the regulation of some
important hormones through regular light-dark rhythms. This in turn means that lighting has a large
influence on health, well-being and alertness.
Light and body rhythms
Light sends signals via the novel photoreceptor cells and a separate nerve system to our biological clock,
which in turn regulates the circadian (daily) and circannual (seasonal) rhythms of a large variety of bodily
processes. Figure 7 illustrates some typical rhythms in human beings. The figure shows only a few examples:
body temperature, alertness, and the hormones cortisol and melatonin
PHILIPS
LIGHTING AND BIOLOGICAL EFFECTS
6 121824 121824 66
cortisol
melatonin
alertness
body temp.
The hormones cortisol (“stress hormone”) and melatonin (“sleep hormone”) play an important role in
governing alertness and sleep. Cortisol, amongst others, increases blood sugar to give the body energy and
enhances the immune system. However, when cortisol levels are too high over an extended period, the
system becomes exhausted and inefficient. Cortisol levels increase in the morning and prepare the body for
the coming day’s activity. They remain at a sufficiently high level over the course of the bright day, falling
finally to a minimum at midnight. The level of the sleep hormone melatonin drops in the morning,
reducing sleepiness. It normally rises again when it becomes dark, permitting healthy sleep (also because
cortisol is then at its minimum level). For good health, it is of importance that these rhythms are not
disrupted too much. In case of a disruption of the rhythm, bright light in the morning helps restoring the
normal rhythm.

10
Lighting for work: visual and biological effects
PHILIPS
In a natural setting, light, especially morning light, synchronises the internal body clock to the earth’s 24-
hour light-dark rotational cycle. Without the regular 24-hour light-dark cycle, the internal clock would be
free-running with, for humans, an average period of about 24 hours and 15 to 30 minutes. This would, as a
consequence, produce ever-greater day-to-day deviations in our body temperature, cortisol and melatonin
levels from those set by the environmental clock time [14]. This deharmonisation in the absence of the
“normal” light-dark rhythm would result in an incorrect rhythm of alertness and sleepiness, ultimately
leading to alertness during the dark hours and sleepiness during the bright hours. In fact, the same
symptoms, and for the same reasons are associated with jet lag after travelling over several time zones [15].
Rotating shift workers also experience the same symptoms for a couple of days after each shift change, again
for the same reason [16].
Lighting, alertness, mood,and stress
A wealth of research projects that compare the effects of health, well-being and alertness as a result of people
working under different lighting conditions have already been carried out. In this article we will discuss
only a limited but typical number of these.
Küller and Wetterberg [17] studied the brain-wave pattern (EEG) of people in a laboratory made to look
like an office environment, once with a relatively high lighting level (1700 lux) and once with a relatively
low lighting level (450 lux). The composition of the EEGs exhibit a pronounced difference: the higher
lighting level results in fewer delta waves (the delta activity of an EEG being an indicator of sleepiness),
meaning that bright light has an alerting influence on the central nervous system (see Figure 8).
Many investigations into the effects of light on alertness and mood have been carried out under night-shift
conditions, because here the effects to be expected would be strongest. Figure 9 shows the effect of two
lighting regimes on arousal as a function of time at work for shift-workers [15]. A decline in arousal over the
night occurs for both regimes, but the high-light regime always results in a significantly increased arousal
level and thus better alertness and mood.
100
75
50
25
450
1700 450
1700
left hemisphere
of brain
right hemisphere
of brain
EEG delta activity (%)
024 68
-13
-9
-5
-1
2800
250
time (hrs. after midnight)
arousal level
Fig.8 Delta activity in the EEG
of office workers under lighting
levels of 450 lux and 1700 lux
(source:Küller and Wetterberg
[17]).
Fig.9 Alertness and mood
expressed as arousal level for
lighting levels of 250 lux and
2800 lux,as a function of
working hours after midnight
(source:Boyce et al. [18]).

11PHILIPS
Other studies show that the use of higher lighting levels to cope with fatigue results in the subjects indeed
staying alert longer [19], [20], [21].
Studies of stress levels and complaints in people working indoors have been made by comparing a group of
people working solely under artificial light with a group working under a combination of artificial light and
daylight [22]. As can be seen from Figure 10, in January, when daylight penetration is not sufficient to
make a substantial contribution to the lighting level, there is hardly any difference between the two groups.
But in May, when daylight really contributes, the group benefiting from daylight has a considerably lower
stress complaint level. Another study shows that bright artificial light in interiors in winter has a positive
effect on mood and vitality [23].
Fig.10 Stress complaint levels
(with statistical spread) in a
group of workers working
either under artificial light only
or under a combination of
artificial and daylight
(source:Kerkhof [23]).
Some, but few people experience headaches because of the light ripple caused by the 50 Hz power supply of
fluorescent lamps operated on magnetic ballasts. Fluorescent lamps running on modern, high-frequency
electronic ballasts operate at around 30 kHz and thus do not exhibit this flicker or ripple phenomenon. In a
comparison, it has been found that the occurrence of headache is, indeed, significantly lower when
electronic ballasts are used [24]. Küller and Laike [25] measured the EEG of persons working in an office
environment under respectively magnetic (50 Hz) and high-frequency fluorescent lighting. At the same
time, they also measured the speed and errors made in a proof-reading task. Figure 11 shows that the
reciprocal value of the alpha activity of the EEG, and therefore the brain arousal (“stress”), is higher with the
50 Hz operated lighting. The working speed is slightly higher, but the errors are dramatically higher (more
than double). The combined effect means that it is wise, from both the well-being and productivity points
of view, to use high-frequency fluorescent lighting instead of magnetic 50 Hz lighting to limit brain arousal
or stress.
Fig.11 Brain arousal measured
as the reciprocal value of the
alpha activity of EEGs in
persons in offices under 50 Hz
and under high-frequency HF
(30 kHz) fluorescent lighting.
The working speed and errors
of a proof-reading task are also
given (graph adapted from:
Küller and Laike [25]).Subject
group:high flicker sensitivity.
0,2
0,1
0
-0,1
-0,2
Stress Level
January May artificial light only
artificial light + daylight
100%
50%
0%
100%
50%
0%
100%
50%
0%
HF 50 Hz HF 50 Hz HF 50 Hz
brain arousal speed errors

12 PHILIPS
Lighting for work: visual and biological effects
Health-related quality aspects of lighting installations
The visual-quality aspects of lighting installations as listed in an earlier section, i.e. lighting level, spatial
distribution of light and colour rendering, have to be refined and extended if we want to arrive at truly
“good and healthy” lighting installations.
The biological effect of light is not steered directly by the illuminance on the working plane, but by light
entering the eye. Studies are under way to see how this difference between “visual lighting level on the task”
and “biological lighting levels” can be accounted for [26]4.
As has been illustrated, especially because of the effects due to ageing eyes, the lighting level has to be
adaptable.
Daylight by its nature is dynamic in its intensity. There are indications that a variable lighting condition has
a positive effect on the activation state of people in an office environment [28]. Where the benefits of the
dynamics of daylight intensity are not sufficiently available, dynamic artificial light can be advantageous.
Two complete new aspects relate to the timing and duration of the lighting. Visually, of course, light is only
needed when and for as long as one “views”. Biologically, however, the time when the light (or darkness) is
received and its duration plays an essential role, as is evident from the rhythm graph of Figure 7.
We have always realised that the colour of light itself has an emotional meaning, and is therefore important
for the atmosphere of a space. But we now also understand that the spectrum and thus the colour of light
has an important biological meaning. As was shown in the section on the novel photoreceptor cell, bluish,
cool light has biologically a larger effect than warmer coloured reddish light (Figure 3). The daylight
situations of the photographs shown in Figure 12 produce not only a different emotional feeling but also
have a different biological effect.
Fig.12 Ambient colour early in
the morning and early in the
evening in Paris.
4Very recent research indicates that light on the upper and lower part of the retina has different importance as far as the resulting biological effect is concerned [27].This suggests
that also the spatial distribution of light is important from a “health” point of view.

13PHILIPS
The bluish morning light has biologically an activating (alerting) effect, while the red sky in the early
evening has a relaxing effect. In a working environment, both activating and relaxing moments are
required. The colour and lighting level of the artificial lighting together may help to create these moments.
Studies on the preferred colour of light in an office environment have shown that there is no trend in
preference between individuals in this respect: everyone has their own personal preference (Figure 13).
Fig.13 Colour preference of
artificial light in an office with
(daylight) windows,expressed
as correlated colour
temperature of the light Tk for
different ages and for men and
women (source:Tenner [29]).
Table 4 Vision- and health-
related quality aspects of
lighting installations
Table 4 summarises the vision- (from Table 1) and health-related lighting quality aspects that together
determine “good and healthy” lighting.
Lighting quality aspects
Vision related Health related
(adaptable) lighting level on the task (adaptable) lighting level in the eye
spatial distribution spatial distribution
colour rendering (adaptable) colour appearance
timing
duration
Tk(K)
5000
4500
4000
3500
3000
2500
women
men
20 30 40 50 60
age

14 PHILIPS
Lighting for work: visual and biological effects
CONCLUSION
Thanks to the recent discovery of a novel photoreceptor in the eye, we are now much better able to
understand why the benefits of good lighting at work, taking into account both the visual effects and the
biological effects (viz. health, well-being and alertness), are so important. Apart from the health and well-
being advantages for the workers themselves, good lighting also leads to better work performance (speed),
fewer errors and rejects, better safety, fewer accidents, and lower absenteeism. The overall effect of all this is
better productivity.
For an industrial environment (moderately difficult visual task), we investigated the possible resulting total
productivity increase as a result of improved lighting level [30]. Table 5 provides a summary of the results.
Improvement of lighting level Productivity increase
From 300 to 500 lux 8 %
From 300 to 2000 lux 20 %
To confirm the results, we are carrying out real-life productivity investigations in a number of industrial
environments where the lighting has recently been renovated. Realising the importance of the biological
component in the productivity increase, we believe that similar figures can also be obtained in an office
environment.
By putting our advice for flexible and adaptable lighting levels and colours into practice, such productivity
advantages will become even more impressive.
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Table 5 Increase in
productivity in the metal-
working industry with a
moderately difficult visual task
as the combined effect of
increased work performance,
errors/ rejects reduction and
accident reduction (source:
van Bommel et al.[30]).

15PHILIPS
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