On the cause of disability glare and its dependence on glare angle

OPTOMETRY
REVIEW
1
On the cause of disability glare and its dependence
on glare angle, age and ocular pigmentation
Clin Exp Optom 2003; 86: 6: 363-370
Johannes J Vos PhD
TNO Human Factors, Emeritus
Soesterberg, The Netherlands
Submitted: 17 March 2003
Accepted for publication: 19June 2003
Background In the 1920s and 1930s, disability glare was a topic of great interest in the
Commission Internationale de 1’Eclairage (CIE). The Second World War prevented
agreement being reached on a standard to quantify disability glare but the Stiles-Holladay
formula was widely accepted as such. In 1983, CIE started a new effort to develop a CIE
standard making use of research data published in the post-war years.
Methods: A committee was formed that agreed that new data and insights justified an
extension of the angular domain of a disability glare formula and allowed introduction
of an age factor and allowance for ocular pigmentation.
Results Three disability glare equations were formulated, each for an appropriately
restricted angular domain. The most general, the CIE General Disability Glare equation, covers the full angular range between 0.1 degrees and 100 degrees but for optometrists the CIE Age-adjusted Stiles-Holladay Disability Glare equation, with validity domain
between one degree and 30 degrees, will often suffice.
Conclusions: Disability glare is due to intraocular scatter and obeys, in the one-degree
to 30degree angular domain, albeit with great individual spread, the Age-adjusted StilesHolladay equation: (Lveil /Eglare )Ageadjusted Sulce-Holladay = 10 (1 t [Age/70]*) .1/e2.Quantitative examples are given of the manifestation of disability glare, particularly in traffic.
Key words: age, disability glare, drivers’ vision, ocular scatter, the older driver
When there is a strong light source in the
field of view, it appears as though a veil of
light has been thrown over the world outside. Close to the light source, we may be
almost completelyblinded but further away,
visual performance can also be notably
hampered. This experience, well known to
drivers, is usually called glare or, to be more
precise, disability glare. Disability glare is
the subject of this review paper. There are
other types of glare-discomfort glare and
dazzling glare-that are of a different nature. Discomfort glare is the visual annoyance produced by distraction due to light
sources off the line of sight, while dazzling
glare is the hindrance of vision by very
bright visual scenes such as a sunny beach,
presumably due to pupillary spasm by overcontraction. These other types of glare will
not be considered as they are outside the
scope of this review.
The stimulus for this survey is the publication of the CIE report Number 146CIE equations for disability glare,’ which
rounds off almost a century of scientific
discussion and research on this subject.
That report is a dry summary of results,
while this review has a more personal
Clinical and Experimental Optometry 86.6 November 2003
363
touch as it concludes a history of my own
involvement stretching over many decades. It sets out to describe how we came
to understand the nature and cause of disability glare and how more recent work
has allowed us to develop improved equations to quantify it. Optometrists are concerned with disability glare because older
patients frequently complain about glare.
These complaints arise because the scatter of light by the eye increases with age.
The recent work of our CIE committee has
enabled us to quantify the age dependency
of disability glare.
Disability glare Vos
This is the second review I have written
in connection with the appearance of that
CIE report. The first, Reflections on
Glare,2was written for illuminating engineers who are interested in the new CIE
formulas and their application to the
design of lighting. Optometrists and ophthalmologists are the other main group
concerned with the subject but their concern is more to understand the visual
handicap of disability glare, particularly
how it increases with age.
HISTORY UNTIL 1965
The phenomenon of (disability) glare has
been known since time immemorial but
the origin of the masking light veil became the subject of scientific interest only
in the early 19th century. GoetheSdevotes
some pages to subjective halos in his
Farbenlehre and explains them in terms
of a 'conflict between mover and moved',
like a stone (mover) thrown in the water
(moved) causes a wreath of waves that
spreads over the surface. In a free translation in modern terms, this could read: in
the nervous system of the retina, the stimulus of the bright light causes a disturbance
that, like waves on the water, spreads in all
directions, gradually fading away. With this
in mind, Goethe can be regarded as the
first in a long row of investigators who
thought of glare as an essentially nervous
process. In contrast, 13 years later,
Purkinje4 emphatically ascribed the veiling appearance to scattering in the ocular
media. Neither Goethe nor Purkinje did
experiments to support their statements
but by mentioning their names, we recognise that they were among the first to
observe and describe the phenomenon
that we now call disability glare.
As a starting point for further scientific
research, one can best consider a paper
written in 1852 by Helmholtz5 in which
more or less casually, he mentions two
possibilities: a nervous and a physical explanation. According to Helmholtz, scattering processes certainly occur and only
further investigationswould show the role
that nervous processes might play.
It took about 70 years before substantial experimental investigations really got
started, thanks to the development of the
so-called equivalent background technique by Cobb,6 in 1911. The masking
effect on the visibility of objects was compared with the masking effect on the same
objects by a veiling background and
hence, disability glare could be quantified. Of the many investigators who used
this technique, we mention only the p i e
neers H~lladay,~,"
Stilesg and later Stiles
and Crawford.'O Their combined results
were brought by Stiles" to the 1939 CIE
meeting in Scheveningen and resulted in
the now almost classic Stiles-Holladay
disability glare formula for a point glare
source:
with LWthe equivalent veiling.background,
now in cd/m2; Eglmthe illuminance at the
eye by the glare source, now in lux; and 8
the angular distance between the line of
sight and the glare source, in degrees. For
extended glare sources, this formula
should be integrated over the angular
aperture of the glare source.
This formula has since been widely used,
even though-probably due to the outbreak of the Second World War-it was
never officially adopted by the CIE. Undoubtedly, it derived its success from its
astonishing simplicity and transparency.
Its most obvious feature is the proportionality between Legand Eglm,which cannot be read other than as a clear sign that
disability glare is an optical phenomenon
due to light scattering in the eye media
and not a result of some neural process.
Nevertheless, this obvious interpretation
did not win a clear victory for three
reasons:
some investigators reported small but
not insignificant deviations from strict
proportionality, leaving some room for
doubt
the discovery of inhibitive neural networks in the retina,I2which gave some
support to a Goethe-style interpretation'%"
theoretical workI5J6on ocular light scatter did not manage to produce a satisfactory explanation of the 1/02 angular dependency.
The situation changed when carefully
controlled experiments,17in which the influences of variations in pupil size and of
eye movements were eliminated, provided
no indication of deviations from proportionality between Leg and Eglareover five
intensity decades. Even more important
were our analytic experiments, in which
we produced direct and quantitative evidence on the amount of forward light
scatter in the cornea, crystalline lens and
ocular fundus and that these three components together could satisfactorily explain all of Leq according to the StilesHolladay equation.
The idea that cornea, lens and fundus
would be the main sources of stray light is
hardly surprising. That we can examine
the cornea and lens with the slitlamp technique (Figure la) is due to their light back
scattering properties. In contrast, the eye
chambers and to a lesser degree the vitreous are optically virtually empty. In a similar way, we can examine the ocular fundus by ophthalmoscopy due to the light it
scatters back (Figure lb). One should be
careful though when drawing quantitative
conclusions about the scattering properties
towards the retina, because forward scatter
is only loosely related to back scatter and
depends heavily on particle size and refractive index. Therefore, a more direct determination of the forward scatter components of disability glare was welcome.
The cornea is unique in the sense that
it is the only contribuent that lies in front
of the iris. Therefore, corneal share should
be distinguishable by the shadow of the
iris it casts on the retina. Normally, we do
not observe this shadow because, with a
fully exposed cornea the penumbra is expanded and the transition to the deep
shadow is very gradual. Moreover, it has
to be observed in the far periphery of the
field of view and that is difficult. Therefore, we enhanced the visibility of the iris
shadow on the retina by using a very narrow glare beam that projects a sharp
shadow of the iris (Figure 2) and by making small eye movements enhancing the
peripheral visibility. It proved to be possible not only to make the iris shadow visible but also to quantify the luminance
jump by photometry,I8that is, by equating
Clinical and Experimental Optometry 86.6 November 2003
364
Disability glare Vos
Figure la. Obvious sources of entoptic stray light: the cornea and
Figure lb. Obvious sources of entoptic stray light: the ocular
fundus in ophthalmoscopy (courtesy: Lighting Research and
Technology)
crystalline lens as photographed by slitlamp technique
centric
\\
Image of the
///
Figure 2. Narrowing and polarising the glare beam produces a
Figure 3. Changing the pupil entrance of the glare beam from
sharp iris shadow and makes the fundus component of entoptic
scattervisible due to its brush structure (courtesy: LightingResearch
centric to eccentric changes the angle of incidence of the stray
light from the anterior eye media but not that from the fundus
(courtesy:Lighting Research and Technology)
and Technology)
it to an ectoptic artificial luminancejump.
The corneal share turned out to be about
30 per cent of the total glare veil, virtually
independent of the glare angle.
The fundus component has a special
characteristic: it consists of sideward scattered light against virtually forward components for the cornea and lens. Scatter
theory teaches that this sideward scatter
should be highly polarised, just like
sideward scatter of sunlight by the sky. If
so, one should expect a brush-like halo
when the glare beam is polarised (Figure 2).
As a matter of fact this happens but again,
the graduality of the transition between
darker and brighter parts of the brushes
makes its observation difficult. This handicap could be overcome by slowly rotating
Clinical and Experimental Optometry 86.6 November 2003
365
the plane of polarisation, which causes the
brush pattern to rotate. Again, it proved
possible to measureIg the luminance
modulation by photometry. The result was
that the fundus makes up about 40 per
cent of the total glare veil. We should add
that this 40 per cent is approximately
halved, when it comes to scattering from
the fovea towards the periphery. The most
Disability glare Vos
obvious explanation of this is the thinning
of the retina in the foveal region: the
foveal pit. It tells us that the fundus scattering is at least partly due to the scattering in the microscopic neural structures
of the retina and not solely to scattering
in the deeper lying retinal pigment sheet
and choroid.
There is a third way to distinguish between the components of entoptic light
scatter and that is by making use of the
Stiles-Crawfordeffect, that light entering
the eye via the centre of the pupil is about
five times more effective than light entering via the border of the pupil. It is due to
made narrow, as described above, we can
vary the location of the entrance of the
glare beam through the pupil and, consequently, the angle of incidence of the stray
light on the retina. This holds for the stray
light evoked in cornea and lens but not
for that coming from the image of the
glare source at the fundus (Figure 3). If
all stray light comes from cornea and lens,
we would expect a reduction of the glare
veil by the full factor five, and if all stray
light stems from the fundus, we would
expect no reduction at all. As could be
expected on the basis of the just mentioned experiments, the experimentz0
yielded a reduction somewhere in the middle, pointing to a share from cornea and
lens together of about 60 per cent in the
veiling luminance.
The picture that emerges from these
three experiments is clear: the masking effect in disability glare is typically an optical
effect, and the contributions of the cornea,
lens and fundus to the stray light veil are
roughly equal; roughly, because these experiments could not claim two digits of
precision. Moreover, most of them were
done for only one subject, myself, then in
my early 30s. It would have been interesting to have these experiments repeated
with other subjects of other ages. Curiously,
these experiments marked the end of virtually all stray light versus inhibition discussions and apparently nobody has felt the
need to repeat these experiments or even
to refer to them.
-
+
4.0
2.0
-+
1.0
-
L
s0
a,
0
Q:
'
0.5
t+
I
I
I
I
I
I
I
Figure 4. The age dependence of the coefficient k in the Stiles-Holladay equation according
to measurements of JJspeert and colleagues*'(courtesy: Lighting Research and Technology)
1"
10"
1'
lo'
1"
10"
loo"
8
Generalised full range glare equation
llllll experimental spread
6
-x
k
"E 4
B
-e!
urn
2
h
.3
v
0
-0
0
green-blue eyes, 80 y
light blue eyes, 35 y
brown eyes, 35 y
nonCaucasian, 35 y
-2
-4
-4
-3
-2
-1
0
1
2
log %egr
Figure 5. The full range angular dependence of Ld/Em for four subjects of different age
and eye colour (courtesy: Lighting Research and Technology)
Clinical and Experimental Optometry 86.6 November 2003
366
Disability glare Vos
DEVELOPMENTS SINCE 1965
This was roughly the situation when CIE
asked me to head a committee to update
the old and widely used Stiles-Holladay
equation. We could safely leave behind us
the stray light versus neural inhibition discussion and concentrate on two main issues: the influence of age (most subjects,
so far, were young adults) and the extension to a wider angular range than the
approximately one-degree to 30-degree
domain, for which the Stiles-Holladay
equation was well verified. Good luck was
with us as, after a few decades of little research interest in glare, new investigations
were underway that could provide the
required new data. In particular, I should
mention the Amsterdam research group
headed by Van den Berg, which provided
new data on three aspects: age dependence, dependence on ocular pigmentation
and angular dependency in the larger
glare angle domain.
That disability glare increases with age
had been known for a long time but reliable data on its age dependency were
scarce. The data obtained by the Amsterdam group could fill that gap (Figure 4).
These data, determined for the glare
angle range between three degrees and 25
degrees, show a distinct average age
dependency that can be welldescribed by
an age multiplication factor:
AF = 1 + (Age/70)4
One should add that the data show a
very sizable individual spread that, from
our point of view, was like a blessing in disguise, because this spread obviated all hairsplitting on details of the postulated u p
dating of the original Stiles-Holladay
disability glare formula.
As for the glare angle dependency, it
was evident from the onset that the 1/02
decrease in disability glare could apply
to neither very small nor very large angles. For 8 approaching zero, when the
target is in the direct neighbourhood of
the glare source, the glare veil should coincide with what is normally called the
point spread function. Instead of going
to infinity according 1/V, the relation
should flatten off towards a maximum at
8 = 0 degrees. Fortunately, reliable point
spread data were available. However,
these data are confined to a very small
angular distance of a few minutes of arc.
For the angular range between the point
spread domain and the conventional
glare range-say between one degree and
30 degrees-useful data could be obtained from colour contrast experiments,22in which ocular stray light happens to act as an artifact. I n the
conventional glare angle region, there is
clear evidence from the body of experimental data that the 1/02 angular dependency becomes about l/8’ around
one degree. A reasoning similar to that
for the very small glare angle domain
applies to the opposite, very large angle
scatter direction, when the glare source
appears in the far periphery of the field
of view. The main body of this peripheral
stray light was attributed by Van den Berg,
IJspeert and WaardZ3to the diffuse transmittance of the anterior parts of the ocular wall, the iris and sclera. One particular aspect of this is that large angle glare
is substantially stronger in lightly
pigmented eyes than in dark eyes. As a
matter of fact, the new data obtained in
the population study of the Amsterdam
group provided evidence of a less steep
angular dependence, from about 30 degrees. One can say that the Stiles-Holladay
1/e2 approach is nothing but a good
average in the middle of the glare angle
domain.
All these new data, together with theoretical considerations on ocular scatter,
provided sufficiently reliable evidence to
construct an extension of the StilesHolladay equation over the full zero- to
1OOdegreeangular range.24Moreover, the
data obtained by the Amsterdam group on
the influence of age and ocular pigmentation enabled the inclusion of those
effects in that extended Stiles-Holladay
course (Figure 5).
It is worthwhile to pause here to study
Figure 5 in more detail. First, it shows, as
a hatched slanting bar, the range of the
experimental data in the Amsterdam2’
study on 129 subjects, varying in age between 20 and 80 years. For comparison, it
Clinical and Experimental Optometry 86.6 November 2003
367
shows as a thin straight line, the StilesHolladay equation, here drawn over a
much wider angular range than was s u p
posed to be valid. It approximately coincides, as it should, with the lower border
of the hatched bar and markedly deviates
from the 1/e2 Stiles-Holladay slope, for
small and for large glare angles. Next,
there are four partly-overlapping curves,
which are the result of our new analysis.
Three of them are for 35-year-olds with
three levels of eye pigmentation. They
really fan out only in the large angle range,
where the lightest eyes have the highest
position, that is, most disability glare.
Finally, there is one curve for an 80-yearold, which is typically characterised by a
higher glare level beyond about one degree but at the cost of a relative lowering
of the curve in the point spread domain.
In view of these general features, it was
not too difficult to express their course in
mathematical form, just to enable illuminating engineers to easily perform their
calculationson glare in a manifold of lighting situations, such as interior lighting,
roadway lighting or tunnel entrance lighting. CIE adopted the proposal of our committee’ to define three disability glare
equations, each of them valid in a special
glare angle domain.
The most general version, the CIE General Disability Glare Equation, valid in the
glare angle domain 0.1 degrees < 0 < 100
degrees, reads:
in which 8 is in degrees, Lvei,in cd/m2 and
Eglarein lux. Note the switch from Leg in
Equation (1) to LVeilin Equation (3), reflecting the new insight that the veil is
more than a computational entity and is a
light veil due to entoptic scatter. One easily recognises in Equation (3) the indicated features, such as the increased steep
ness on 8 at small angles, the age
dependency in the middle angular region
and the dependence on ocular pigmentation, p (ranging from p = 0 for black eyes
to p = 1.2 for very light eyes) at very large
glare angles. Note that we left out the
Disability glare Vos
Nominal Nominal
detection breaking
distance time
Young adults
64 m
2.3 sec
83 years old
45 m
1.6 sec
------I
*
70 years old
52 m
1.9 sec
-
R
d
Figure 6. lhffic situation with two motor bikes on approaching courses. Can the left rider
detect the obstacle (for example, a pedestrian) when he is blinded by the undipped headlight
of the oncoming rider?
Table 1. Nominal detection distance and
braking time for a crossing pedestrian,while
blinded by an undipped approaching
motorbike
region 8 less than 0.1 degrees, which is the
typical point spread region and therefore
highly dependent on pupil size.
Equation (3) is unnecessary complex for
most purposes and was simplified to the
CIE Small Angle Disability Glare Equation:
with I the headlight intensity, EglWe
= I/Rz
and the luminance of the traffk obstacle
to be seen, we obtain Lobst=p I/Dz when p
is the reflection factor of the obstacle, say
the pedestrian's raincoat. Now the contrast
C by which the obstacle stands out against
the veiling glare luminance is:
(LVd/Eglare)rmdl
angle
+
= 10/Bs+ (5/02).(1 [Age/62.5I4) (4)
valid in the restricted angular range
0.1 degrees < 8 < 30 degrees, which therefore could leave out the terms with the
ocular pigmentation factor p.
As optometrists will seldom be confronted with sight problems due to glare
below one degree, a third equation was
presented, called the CIE Age-Adjusted
Stiles-HolladayDisability Glare Equation:
[Lveil/Eglare
=
Ageadjusted StilcsHolladay
lO(1 + [Age/70]4).l/8z
(5)
which is valid in the conventional glare
angle domain between one degree and 30
degrees. One easily recognises the old
Stiles-Holladay equation, now complemented by the age factor in Equation (2).
For ages under, say, 35 years, the change in
the old Stiles-Holladay equation comes
down to only six per cent-negligible in
view of the spread, even between normal
subjects, of the same age. Only beyond 70
years does the effect of age become really
significant: a factor of two at 70, a factor of
three at 83 years old. That cannot be a big
surprise to optometrists,but at least we can
now quantlfy the glare handicap of the aged
and illuminatingengineers no longer need
be tempted to tune their design to the original Stiles-Holladay equation, which is valid
only for young adults. These practical consequences are discussed in the next section.
In the preceding sections, wavelength
as a parameter is not mentioned. The reason is simple: all evidencez5is that glare is
independent of wavelength-something
that can be understood from the fact that
light scattering in the eye is predominantly
due to scattering at structures that are
large with respect to wavelength.
C = Lobst: Lwi1
= p I/D2 : (101/R2.[1+{Age/70141/8d~2)
so that
P
Ddctection
GLAREINTRAFFIC
I started this paper by introducing glare
as a phenomenon well known in traffic. I
will now return to that theme, by discussing a few more specific examples. In each
of these cases, it will be obvious that the
aged driver will be prone to more visual
problems than the young driver.
The first traffic situation is that of two
approaching motorbikes-for simplicity
reasons not cars, which have two headlights, one to the driver's left, which would
complicate calculations. The light scatter
veil in the eye may impede distinguishing
obstacles in your own lane such asjoggers
or wombats (Figure 6).
The glare angle 8, = d/R, with R their
mutual distance and d their lateral distance, so €Idegrcea = (180/n) d/R. Further,
Clinical and Experimental Optometry 86.6 November 2003
368
=
d'd 10 C (1 + [Age/70l4) (6)
(l8'/')
When we substitute not unrealistic values of 25 per cent both for p, the reflection factor of the obstacle to be seen, and
for C, the minimum contrast for detection
of a small object, and assume a lateral lane
distance of five metres, then:
Ddetection
-
90
d(1 + [Age/70I4)
m
(7)
In this formula we can easily see the influence of age on the detection distance
of a traffic obstacle under conditions of
glare due to non-dipping headlights: when
for young adults the detection distance is
90 metres, it has decreased to 64 metres
at age 70 and to 52 metres at age 83.
Disability glare Vos
Figure 7. Black wall effect for a tunnel entrance contrastingwith a bright sky.
Cars in front vanish from sight, when entering the tunnel.
shown to be an optical effect due to light
scatter, we must add all other types of light
scatter to the intraocular light scatter
encompassed in the (age-adapted) StilesHolladay formula. The most important
sources of extraocular light scatter are a
dirty or scratched windshield and the atmosphere. A survey test on realistic values
for atmospheric and windshield light scatter indicated that doubling the original
Stiles-Holladay coefficient of 10 is normal.27Of course, the atmosphere may be
completely transparent and the windscreen may be freshly cleaned and free of
scratches but one should introduce a
nominal allowance for scattering by the
atmosphere and windscreen glass. Substituting this rough estimation of a nominal
extraocular scatter component in Equation ( 5 ) leads to:
Ddeteelion
Figure 8. Providing traffic lights with background shields strongly reduces
glare by the surrounding sky
Converted to seconds breaking time, this
means 3.2 seconds for the young adult,
against 2.3 seconds for the 70-year-old and
1.9 seconds for the 83-year-old.Combined
with the steadily increasing reaction time
with age, these figures may typify the risks
of glare for the older driver (Table 1).
However, this is not the whole story.
First, contrast sensitivity typically decreases
with age,26so that the value of C to be substituted in Equation (6) might be even
higher for the aged. That is not completely
certain because the decrease in contrast
sensitivity might be due partly to the increase in ocular scatter already brought
into account. Therefore, to avoid being
over-pessimistic, we will keep to Equation
(6). Then, because disability glare was
Clinical.and Experimental Optometry 86.6 November 2003
369
-
90
m
d(2+[Age/70I4)
(8)
which leads to the following revised nominal values for the detection distance and
braking time, which are worse (Table 1).
The obvious remedy for this problem is
dipping the headlights, which means reducing the illuminance in the direction
of the driver of the oncoming vehicle without reducing the intensity in the forward
direction where obstacles should be detected. The lighting industry has managed
to cope reasonably well with this delicate
tuning problem although even dipped
headlights, in particular with heavily
loaded cars, can still be a source of disability glare.
A second example concerns tunnel entrance lighting. When a tunnel entrance
contrastswith the sky-as is usually the case
with river undercrossings-the high sky
luminance casts a glare veil over the tunnel entrance, which can easily mask preceding cars that have already entered the
relatively dark tunnel. Curiously, the net
effect can be that the tunnel entrance, even
though it is more luminous (in cd/m2) by
the stray light veil, is effectively obscured
(which, literally, means darkened) by the
contrast with the bright sky and the nonvisibility of objects in the tunnel. The a p
proaching driver then views a dark wall
Disability glare Vos
instead of a tunnel entrance and this black
wall illusion may lead to a shock reaction
in panicking drivers. As competing with
natural outdoor luminances is hardly feasible, the obvious remedy is to provide tunnel entrances with a light-transition zone,
in which the admission of sky light is gmdually tempered and that is the way modem
tunnels are usually built (Figure 7).
The key question is: which entrance
light level is needed to avoid the black wall
effect? Obviously, the outcome will be
higher for aged than for young drivers. We
have presented a quantitative analysis of
these illumination problems at the 1983
CIE meeting in Amsterdamz7and therefore, will repeat here only the main outcome in terms of the tunnel entrance stray
light luminance LcnvMce,
expressed as a percentage of the surrounding sky luminance
Llty. For a slightly nebulous atmosphere
(visual range five kilometres) one can calculate Lenuance
= 0.25 L,Qfor young drivers
= 0.34 L,kyforthe aged ones.
and LenvMce
The meaning of these percentages is that
they set a criterion for the artificial light
level in the tunnel entrance to compensate for the veiling glare light levels and
to render the cars in the tunnel visible.
Tuning tunnel entrance lighting to the
older drivers, rather than to the youngsters, requires about 33 per cent more
tunnel entrance lighting.
A final example concerns the irradiation
of traffic lights by the surrounding bright
sky. The mechanism is the same as with the
tunnel entrance: light from the surrounding sky is scattered in the eye and veils the
object to be seen, in this case the traffic
lights. In more extreme situations-usually
more readily associated with glare-it may
be the setting sun that produces the scatter veil and so completely impedes traffic
light recognition. We will not pursue further here the quantitative elaboration and
mention only that the best solution here is
to mount background shields around the
traffic lights (Figure8),according to Dutch
lighting standards,**which recommend
three times the width of the traffic lights.
Wider shields add little.
Because the angular width of these background shields at nominal observation distances is in the order of one degree or less,
it is the age-independent 1/03 component
that dominates Equation (4) and so here
correctly reading the traffic sign is not
especially an age-related problem.
CONCLUSION
The main purpose of the CIE study-and
of reporting about it in this journal-was
to introduce age dependence into the
Stiles-Holladay disability glare equation.
The age-adapted version, now accepted by
CIE as a standard, shows that disability
glare rapidly increases beyond the age of
60 years : it doubles around 70 and triples
at 83; of course, with large individual variations. Calculations indicate that the visual
handicap due to disability glare in traffic
and many other situations may be much
more pronounced in the aged than in
young adults.
REFERENCES
1. Commission Internationale de I’Eclairage
CIE. CIE equations for disability glare. CIE
report #146. Vienna: CIE; 2002.
2. VosJ. Reflections on glare. Light& Techno1
2003; 35: 163-176.
3. Goethe JW.Zur Farbenlehre I, Tiibingen,
1810, Ch.8. Deutsch:TaschenbVerlag, 1963.
4. Purkinje J. Beobachtungen und Versuche
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Author’s address:
Dr Johannes J Vos
TNO Human Factors, Emeritus
Kampweg 5
3769 DE Soesterberg
THE NETHERLANDS