Table Of ContentLIGHTING PROBLEMS
IN HIGHWAY T R A F F IC
Proceedings of a symposium held at the Wenner-Gren Center,
Stockholm Sweden, October 1962
Edited by
ERIK INGELSTAM
The Royal Institute of Technology,
Stockholm, Sweden
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OPENING ADDRESS
PROFESSOR Y. ZOTTERMAN
Vice-President of The Swedish Road Safety Council
WE have gathered here to discuss for some days lighting problems in high
way traffic. This includes a very complicated system of problems of greatest
importance for road safety during dark hours, which in this country may
extend over most of the day in wintertime.
Man, like the higher animals, rely, with regard to their reactions to light
and to optical signals, on the photoceptive apparatus of our eyes and the
neural mechanisms connected with them. The properties of this system
have developed during the passage of a million years. During this long
period of evolution our visual system has been adapted to our environment.
In a primitive life the change from daylight via dusk to night takes at
least three-quarters of an hour. It is interesting to note that complete dark
adaptation takes about 45 min. Now man recently has developed the habit
of rushing about in the night at highspeed on our roads focusing bright
headlights into the dark adapted eyes of other fellows also driving at high
speed, blinding them for several seconds, during which period they may
travel fully blinded 50-100 m.
The problems we meet in night traffic are manifold, but they all depend,
in the last hand, on the peculiar properties of our visual organs, and
can only be solved by adapting our lighting technically to the physiological
properties of our eyes. For that reason a conference of this kind, where
physicists, technologists, psychologists and physiologists meet, should
provide the right forum for discussing the lighting problems in highway
traffic. We are confident that you here will be able to define the principal
ways to solve these problems which are most important for the safety of
man in traffic.
The Swedish Road Safety Council extends to you a hearty welcome. We
are delighted to see that so many famous experts in different fields of
lighting research have accepted our invitation to attend this meeting.
1
THE GLARE-EFFECT FROM THE STANDPOINT OF
PHYSIOLOGICAL OPTICS
HERBERT SCHOBER
Institut für medizinische Optik der Universität München
THE information capacity of the human eye is dependent on physical,
physiological and psychological factors. The most important physical
moment is given by the scattering of light in the optical media passed by
the rays, i.e. the cornea, the eye-lens and the vitreous body of the eye bulb
and also—a cause which is easily forgotten—in the glasses of goggles or
correction spectacles. It is well known that persons wearing eye glasses and
also older people are much more sensitive to glare effects than no-glass-
wearers and younger people. The interpretation of these connections is not
difficult. The amount of glare-sensitivity is caused by loss of photometrical
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3
4 HERBERT SCHOBER
contrast, also by physical moments. If we look at Fig. 1 we can see that the
probability of eye-defects is increasing with the age of life. All lesions on
the cornea causing permanent stigmas become centres of diffraction figures
or stray-light centres. A similar connection can be found in the eye-lens, in
the vitreous body and in the nervous and static fibres of the retina. An
increase of glare-effects in the middle or higher age of life shows the ophthal
mologist very often the beginning of several eye-diseases such as cataracts,
glaucoma, iritis, etc. Although most of the phenomena described here are
pathologic ones and caused by curable or incurable eye-diseases, these facts
must also be well-known to the non-physician. Especially in this case it
should not be forgotten that an increasing glare-sensitivity is given in higher
ages.
Concerning the diminution of contrast at eye-glass-wearers we can find
two causes. The first one is given by the scattering of light in the glasses.
All damage in the surfaces of these glasses and all soiling effect a very
remarkable increase of glare.
If an eye is not or act well corrected the images on the retina are more or
less unsharp. The image figure of a single object point is larger than in
normal cases and influences therefore as a diminution of contrast the
image figures of the neighbour points. By adding the physical and the
medical causes we get an interesting result for glass-wearers: if a person has
a higher glare-sensitivity, or if the recovering times after disappearing of
glare sources is considerably higher than in normal cases, we should send
this person to an ophthalmologist who is well acquainted with such
problems. He can find out the causes and can prescribe the best eye-glasses.
He should also give recommendations, that such a person should drive
carefully at night and especially at twilight, i.e. in the evening after sunset
or in the morning. It should not be forgotten that during these hours all con
trast change. In daylight the sky is clear and the highway is dark. At night
we see the road much clearer than the sky. Therefore there must always exist
a moment when the contrast changes and another short moment when it
is nearly zero. This situation can cause some optical illusions, as is seen by
Fig. 2, where the direction of the flying birds changes between day and
night.
The same requirements are given for these drivers, if the contrasts in
daylight decrease, namely in foggy weather. It is important to make public
information about the above-mentioned facts, as more and more people
with glare-defects are occupied as drivers. Also road-engineers should
consider this important problem. We should do our best to make all street
borders well recognizable in all illumination conditions. Since a few years,
black roads with white border-markations, possibly even with reflecting
pillars, were constructed in Europe. All drivers recognize this advantageous
fact, when they once have to change from such a modern street to an older
THE GLARE-EFFECT FROM THE STANDPOINT OF PHYSIOLOGICAL OPTICS
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6 HERBERT SCHOBER
one. Only in this connection can the very often discussed question with re
gard to illumination techniques be understood, whether the light or dark
glare effects should be preferred. In the same connection we should under
stand that a chessboard at the surface of a road is always bad. Such a
chessboard is very often caused by repairs of a clearer road with black
material. It gives wrong information and means—expressed in terms of the
information theory—an increasing of the optical noise.
FIG. 3. Generalized Weber-Fechner's law: visibility of relative luminance
differences at different luminance levels,
The physiological causes for glare-effects are less clear. The physiologists
differ between absolute glare, relative glare and adaptation glare. Absolute
glare occurs only when the average luminance in the visual field is higher
than about 20,000 cd/m2. This is, for instance (Fig. 3), the sunny street
but also the road at noon-time, when the sky is overcast. In this case,
sunglasses will be the only help. Its absorption might be 50 per cent. There
is only one requirement: sunglasses should not destroy the visual acuity.
They should be adjusted to the refraction power of the wearer. Therefore
all sunglasses for a driver should be chosen as carefully as other
correction glasses.
In the last few years a lot has been published about sunglasses. If we
realize the above-mentioned facts, such glasses will mostly be used for
persons with anomalous refraction, where the filtering effect is reached by
coating. The couches of these glassses fix very well so that there is no danger
of destruction. If no optical effect is required, also well-chosen blown glasses
THE GLARE-EFFECT FROM THE STANDPOINT OF PHYSIOLOGICAL OPTICS 7
or plastics can be used. Polarization filters avoid the absolute glare and a
considerable part of the refraction glare on sunny and wet streets.
The relative glare is produced by luminance-irregulations in the visual
field. It represents undoubtedly the most frequent form, but its control is
the most complicated one. Goggles do not help here at all and it is an
inexcusable mistake to recommend some kind of goggles for driving at
night. If you do so, you decrease not only the luminance in the field of
glare source but also the luminance in the visual field. Also glasses with a
variable or discontinuous absorption are useless in practice, because they
presume static relations, which we do not have in a moving car. It would
go too far to discuss all relationships of relative glare. This is not necessary,
as all these questions will be discussed in other papers. I would like to
bring only one fact to your attention which is a real medical one, namely
that the individual sensitivity for relative glare is higher in those cases
where turbidities of eye-medium and certain retinal defects are in question.
It is possible that in such pathological cases the relative glare can be
influenced by certain drugs. Exact information cannot be made yet as
to all drug effects, but there is no doubt that we can reach very good
corrections with Privin and other similar substances. As to the normal eye
such investigations had only unimportant or no results at all.
0.5 0.7 1.0 1.4 2 3 5 7 10 H 20
THRESHOLD SIZE (MINUTES)
FIG. 4. Dependence of the contrast threshold from the size of visual objects
(M. Luckish, Light, Vision and Seeing, Nostrand new edit. 1944).
8 HERBERT SCHOBER
As we can see by Fig. 4 there exists a very strong connection between the
size of a visual task and its relative contrast to the surrounding field.
Hyperbolic borderlines separate the visible world of visual task from the
invisible world. The border lines change with the luminance in the visual
field. This fact results from the law of Weber-Fechner. The higher the
luminance level the larger the area of visual world. By all amelioration of
the luminance level in the visual field we gain many visible objects which
are invisible at a worth luminance level. By this reason a luminance level in
the visual field is required as high as possible. Therefore all street illumina
tions are better than the use of headlights on an illuminated road. Figure 4
also demonstrates another result. If a visual task is invisible we can make
it visible by enlarging the size of the objects or by increasing the contrast
to the surrounding field* As we cannot influence the size of the visual
objects in night driving we can only ameliorate the contrast. That can be
done on the objects. A very good example is the white helmet of the motor
cycle drivers. Moreover, all better eye-correction, all cleaning of eye
glasses and wind-screens—I guess—should have a much higher influence
on the contrast in all the work we try to do with high costs in special
illuminations, etc.
350 400 450 500 800 (pm)
Absorption rate of eye glasses
FIG. 5. Spectral absorption factors of yellow glasses for television and traffic
goggles in foggy weather.
THE GLARE-EFFECT FROM THE STANDPOINT OF PHYSIOLOGICAL OPTICS 9
Very often we are told that contrast sensitivity increases in yellow light
or by wearing yellow eye-glasses. That may be true from the psychological
standpoint. But there is no objective sign for a physiological influence. The
physiological contrast function is independent from the colour of light.
Yellow eye-glasses give us the impression of sunny weather and a higher
luminance than we indeed have. Therefore we have a very good result with
yellow eye-glasses for persons whose eyes must be corrected but who
normally dô not want to wear eye-glasses, i.e. young ladies, etc. But yellow
eye-glasses have also some disadvantages. They absorb mostly all shorter
wavelengths in the visible spectrum (Fig. 5). By wearing such glasses we
cannot see objects in blue or blue-green colour if the contrast is only a
colour contrast and not also a contrast of luminance.' Factories have
developed also yellow glasses which do not absorb all short waves, i.e. the
"Samina" of the Balzer's factory in Liechtenstein or the Rodenstock
"Telecolor". Such glasses are much better than the other ones.
Something similar can be said of the adaptation-glare, which is produced
—contrary to the relative glare—because of considerable temporal varia
tions of luminance. We will find here high individual differences especially
with regard to the readaptation during glare-period when they close one
eye, and by this they are much less sensitive against adaption-glare. Other
people, whose relative figure is rather the same, do not succeed, because
the after-images—produced in the closed eye—predominate and miscarry
the result.
Relative glare and adaptation glare are especially attacked by light-
technics. Everything must be done to keep the number of glare sources as
low as possible and to avoid considerable ^regulations in the visual field
on local and temporal arrangement. The good screening of light-sources,
the widening of streets, glare-screening as well as illumination of subways
have to be seriously considered.
Whereas the physical and physiological side of the glare-process can be
measured by relatively satisfactory methods, it is impossible with regard
to the psychological one. The psychological side concerns especially the
subjective glare-impression and the subjective decrease of the information
capacity of the visual sense. The American psychologist Blackwell and his
research team have published in some papers in Illumination Engineering in
the last years some experiments to find out a way of measuring such
influences. His publications have caused a sensation all over the world, but
of course have sometimes also been contradicted. A final opinion will
only be possible if very extensive material is checked.
As the experiments and researches of Prof. Blackwell and his co-operators
are not known everywhere, it seems to be necessary to give some remarks
about his idea.
The visual system is, as Prof. Blackwell assumes, a real information-
