Proceeding of IMECE2004:
International Mechanical Engineering Congress and Exposition
November 14-19, 2004, Anaheim, CA, USA
IMECE2004-60345
ENGINEERING TECHNIQUES FOR THE FORENSIC ANALYSIS OF VISIBILITY CONDITIONS
Scott Kimbrough
MRA Forensic Sciences
ABSTRACT
Many times forensic engineers are asked to evaluate the
visibility of objects involved in accidents. For example they
might be asked to determine if a pedestrian should have been
visible to the driver of an oncoming car. The visibility of an
object depends on the relative size of the object in the visual
field, the level of ambient lighting, the contrast between the
object and its surroundings, and whether glare sources are
present. Equations and results based upon empirical studies can
be used to access visibility using these factors. This paper
outlines the basic tools of visibility analysis and presents a case
study to illustrate their application.
INTRODUCTION
Several books and articles have been written on the subject of
visibility analysis in a forensic setting [1,2,11]. There are also
many books and articles that discuss the techniques of
designing effective lighting for vehicles and highways [3,4,5].
However, the author has found that these sources do not
integrate the required field techniques, human factors, and
analysis needed to perform many common forensic
assignments. The goal of this paper is to combine information
from the cited sources with descriptions of field techniques.
The usefulness of this combination will then be demonstrated
by examining a case study that involves determining the
visibility of an object that was struck by a driver cutting across
an unlit parking lot on a rainy night.
LIGHT METERS
Conducting field visibility analysis requires having two basic
instruments. One instrument, called an illumination meter, is
for measuring the light landing on a surface. The other
instrument, called a luminance meter, is for measuring the
“brightness” of a surface, or the luminance of a surface. The
first instrument measures in units of foot-candles or lux. The
second instrument measures in units of foot-lamberts or
candela-per-square meter. These meters have filters in them
that weigh the light being measured according to wavelength.
Their weighing filters are designed to emulate the sensitivity of
the human eye. These meters are not the same as photographic
light meters, which that weigh the light according to the
sensitivity of photographic film.
An excellent handbook on the terminology and physics of light
and light measurement can be obtained at no cost from
www.intl-light.com/handbook/ [9].
It is important to pick instruments with the proper ranges.
Forensics work requires measurements at relatively low
lighting levels and many light meters are designed for the
higher lighting levels of concern to lighting engineers and
designers. An illumination meter for forensics work should be
able to measure down to .1 foot-candles with 10% accuracy. A
luminance meter for forensics work should be able to measure
down to .001 foot-lamberts with 10% accuracy. This
investigator used an Extech 407026 illumination meter and a
Minolta LS100 luminance meter to take the measurements
presented in the case study. Both meters had recently been
calibrated, and annual calibration of meters is recommended to
withstand possible challenges to their accuracy. These
instruments and similar instruments can be obtained from
various sources. To get started in the search, a good selection
can be found at Davis Instruments (www.davis.com) or
International Light (www.intl-light.com). Also, a general
internet search will uncover other sources. In terms of prices,
whereas an adequate calibrated illumination meter can be
purchased for about $400, an adequate calibrated luminance
meter will cost about $3,000.
HUMAN FACTORS OF VISIBILITY
Visibility refers to how conspicuous an object is to the human
mind. Therefore, one must study human factors to learn how
the physical variables associated with light, those that can be
measured by instruments or calculated, are transformed into the
sensations of sight in the human mind. The measurements taken
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during the field experiments or the results of lighting
calculations need to be interpreted by the knowledge gained in
human factors studies, such as those outlined in [1,2,12]. It is
beyond the scope of this paper to give a comprehensive outline
of the major results in this field; the reader is directed to [12] to
obtain an excellent overview of the subject.
The results of Blackwell [6,7,12] are a cornerstone of visibility
analysis. In the 1950’s Blackwell established in a series of
human factors experiments that the visibility of an object
depends upon:
The size of the object in the visual field.
The ambient level of luminance.
The contrast between the object and its immediate background.
Even though Blackwell’s work was conducted around 50 years
ago, it so effectively captured the basic working of the human
visual perception system in regards to spatial thresholds that it
remains valid today. According to Blackwell’s results, given
the relative size of an object in the visual field of the observer,
there is a basic relationship between the ambient luminance
level and the contrast require for the object to be deemed
readily visible. This relationship between ambient luminance
and contrast can be captured by a curve drawn in the plane
whose coordinates are the ambient luminance and the contrast.
This curve separates the space into regions with “higher”
visibility and “lower” visibility. The curve specifies the
combinations of ambient lighting and contrast at which 50% of
the human test subjects were able to detect the object when it
was flashed before their eyes.
An example of such a “Blackwell Chart” is shown on Figure 1,
which was extracted from [5]. Notice that the greater the
ambient luminance, the greater the contrast required for the
object to be readily visible.
Magnitude of Difference in Luminance (foot-lamberts)
Contrast Required vs. Ambient Luminance
100.0
10.0
1.0
0.1
0.0
0.0
0.0
0.1
1.0
10.0
100.0
1000.0 10000.0
Ambient Luminance (foot-lamberts)
Figure 1: Blackwell Chart
In a somewhat maddening manner, contrast is defined in
various alternate ways in visibility literature. One must be
careful to check what definition is being used. The definition
used in creating Figure 1 is the absolute value of the difference
between the luminance of the object of concern and its
immediate background. But, depending on the definition being
used, other forms of Blackwell Charts can be encountered. An
equation for the curve on the chart can be found in [8].
It is worth discussing the role of the ambient level of luminance
on the Blackwell Chart. The ambient level of luminance
establishes the light adaptation of the observer, assuming the
observer has had opportunity to adapt. Among the things that
occur when the ambient lighting level changes is the size of the
pupils of the eyes change. The size of the pupils changes as the
luminance of the objects upon which the eyes are cast changes.
However, since the eye is constantly scanning the visual field in
front of the viewer, and since the eye’s adaptation mechanisms
have their own time constants, the ambient level of luminance
should be thought of as a time-weighted scan-weighted
quantity. In some situations, the eyes scan rapidly and
uniformly enough that the area-weighted luminance of the
objects in the general field of view of the observer can be used
as the ambient luminance level. It is also known that the eyes
tend to be drawn to the brightest objects in the visual field, so
this can also be an important factor in some cases; for example
the eyes tend to concentrate on the lighted road in front of the
vehicle when driving at night, so in this case the ambient
luminance should be taken as the average luminance of the road
where it is lighted by the headlights. In the forensics arena, it is
often desirable to give the benefit of the doubt to anyone whose
actions are being scrutinized, by choosing an ambient
luminance favorable to their case, from the range of reasonable
ambient luminances.
After measuring the ambient luminance and measuring the
difference between the luminance of the object and its
immediate background, one goes to Figure 1 to see if the point
defined by the two measurements lies above or below the
curve. The curve itself is often referred to as the visibility
threshold. If the point lies above the visibility threshold the
object is deemed to be “visible” and if the point lies below the
curve it is deemed to be less “visible” the farther below the
curve it falls.
Obviously, visibility varies continuously and gradually with
lighting conditions, and the visibility of an object does not
instantaneously change just because conditions cross the
visibility threshold on the Blackwell Chart. The visibility
threshold was established as the boundary at which 50% of the
participants in Blackwell’s human factor experiments reported
seeing disks, with various combinations of contrast and ambient
luminance, flashed before their eyes for 1/5 of a second. Even
though Blackwell’s studies are based upon 1/5 of a second that
is generally adequate, because the detection power of an
adapted eye is generally independent of exposure time,
exposure times greater than .1 second [12].
Through the years, Blackwell and other researchers [12,13]
have proposed refinements to Blackwell’s basic results. The
outcome of these efforts has been the adoption of a series of
multipliers that capture the effects of the sizes of objects in the
visual field, the ages of the observers, the levels of expectancy,
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and confidence levels. Using these multipliers one proceeds as
follows:
1. Given the ambient luminance, one goes to the Blackwell
Chart and finds the contrast required according to the visibility
threshold curve. Let the obtained value be called the “nominal
required contrast for visibility” (nrcv).
2. Then one multiplies the nrcv by a series of multipliers.
The first multiplier takes into account the size of the object
being studied, where size is described as the subtended angle of
the object. Table 1 gives this multiplier.
Subtended angle in
minutes
1
2
4
10
15
20
30
40
60 or greater
Size
multiplier
15.1
3.66
1.0
.32
.213
.17
.127
.105
.086
Table 1.
The second multiplier takes into account the age of the viewer.
Table 2 gives this multiplier.
Age of the viewer
20 – 42 years
42 - 64 years
64 – 80 years
Multiplier
K = 1 + .00795(age-20)
K = 1.175 + .0289(age-42)
K = 1.811 + .1873(age-64)
Table 2.
The third multiplier takes into account the fact the participants
in the laboratory tests were expecting to see the disks flashed in
front of their eyes, whereas in the real word the viewer may not
expect to see the object of concern. This multiplier has been
determined to be 6.67.
The forth multiplier adjusts for the fact that the threshold curve
on the Blackwell Chart is based upon 50% of the participants
detecting the test disks. This final multiplier can be used if one
wants to know the visibility threshold under which 99% of the
participants would detect the disk. This multiplier has been
determined to be 1.98 by looking at the statistics of the original
test results.
For example, say the ambient luminance level is 1 foot-lambert.
Then the Blackwell Chart indicates that the nrcv is .25 footlamberts. But, suppose the object of interest is 1 foot in
diameter and is 100 feet away; then it subtends an angle of tan1
(1/100), which is 34 minutes. Using Table 1, by linear
interpolation, one obtains a size multiplier of .12. Further,
suppose that the viewer is 42 years old, then the age multiplier
is 1.175. In addition, it the observer did not expect to see the
object, then a multiplier of 6.67 is applied.
If we want to know what level of contrast is required so that
50% of unexpectant 42 year old viewers would detect the
object, that would be nrcv times the multipliers listed, or
.25 foot-lamberts x.12x1.175x6.67 = .24 foot-lamberts.
Further, if one wants to know the level required for 98% of
same category of viewers to detect the object then this level is
multiplied by 1.98, to yield .48 foot-lamberts.
Another situation often arises in visibility problems and that is
when glare producing light sources are in the field of view of
the viewer. Glare produces a so-called veiling luminance that
effectively raises the ambient light level. As a consequence, per
the Blackwell Chart, this raises the nrcv, meaning that a higher
contrast will be required to establish an object is visible. The
equation used to determine the veiling luminance is
Lveil = kEi/i2
(foot-lamberts)
(1
Where the summation is over the glare sources, and
k = 28.4(1+(age/66.4)4)
Ei = the illumination in foot-candle from source i.
i = the angle in degrees between the line of sight of the
viewer and the source i.
Note, this equation is restricted to situations in which the angles
i are all greater than 2 degrees.
Once the veiling luminance is calculated, it is added to the
ambient luminance and the sum is treated as the effective
ambient luminance.
CASE STUDY
Recall that the case study looks at a situation where the driver
of a vehicle cutting across an unlit parking lot at night on a
rainy night strikes an object. Accordingly, the analysis began
by conducting experiments at night at the accident site. It was
important to wait for a rainy evening with an overcast sky,
because the light from headlights scatters forward from wet
road surfaces. This causes the roadway to appear darker to the
driver of the vehicle and creates greater contrast between
objects lighted by the headlights and the darker background. It
was also important that the ground be wet a certain time before
hand, since the optical properties of surfaces such as asphalt
and concrete change according to the length of time they have
been wet [8]. If rain is very infrequent at an accident site being
studied, then it may be necessary to use a water truck or other
means to wet the ground.
One of the answers sought in the case study was whether the
driver of a vehicle that struck an object in a parking lot should
have been able to see the object in time to stop or maneuver
around it, if he was paying attention to where he was going.
There was a dispute, because the driver of the vehicle
contended that he was paying attention to where he was going,
but the object he struck was essentially invisible. On the other
hand, the owner of the parking lot contended that the driver
must not have been paying attention and the object was readily
visible.
The analysis of the subject situation had two basic thrusts. One
was to conduct field experiments during which actual
measurements would be taken using the illumination meter and
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the luminance meter and the other was to supply the
information necessary to build an analytical model of the
situation. The model would be used to look at variations of the
situations studied in the field experiments. The analytical model
would be calibrated by the results from the field studies.
Field Experiments
A rainy night was chosen to match the conditions that existed
the night of the accident. It was noted that about 300 feet
behind the object that was struck was a 30-foot high streetlight
with a relatively bright lamp (luminare) on it. This light added
to the ambient luminance and potentially could have produced
some glare.
One of the first tasks was to determine the reflectance of the
object that was struck. In order to do this, the test vehicle was
placed in a position where its low beams illuminated the object
from about 25 feet away. The goal was to choose a distance at
which significant levels of light would be measured, so the
signal to noise ratio of the measurements would be good. First,
the illumination meter was used to measure the amount of light
being cast on the object. Then, the luminance meter was used to
measure the luminance of the object. With these measurements
in hand, the reflectance of the object was determined as the
ratio of the reflected light (i.e., the luminance) to the incident
light (i.e., the illuminance).
The vehicle involved in the accident was taken to the accident
site and positioned 50 feet away from and directed towards the
object that was struck. The 50 foot mark was chosen because
given the speed limit in the parking lot, 10 mph, if the driver
detected the sign at 50 feet he should have been able to avoid
striking it.
Next, this investigator sat in the test vehicle and used the
luminance meter to measure the luminance of the object that
was struck and its immediate surrounding. The luminance
meter was also used to determine the general level of
luminance in the broader field of view of the driver. The
luminance of the patch of asphalt generally lighted by the
headlights was a large factor in choosing the appropriate
ambient luminance level. That is because it was generally the
brightest region in the field of view, and as mentioned above,
the eyes tend to be attracted to the brighter features in the field
of view and since they spend the most time scanning this region
it is dominant in establishing the light-adaptation level of the
eye.
The following measurements were recorded:
Illumination on the object from the headlights – 6.2
foot-candles
Luminance of the object - .65 foot-lamberts
Luminance of immediate surrounding of object - .03
foot-lamberts
Luminance of general field of view - .2 foot-lamberts
Further measurements were made to determine the glare
potential. The illumination meter was used to measure the
illumination from the one major glare source, the streetlight.
The measurement was taken by aiming the sensor of the
illumination meter towards the lamp of the distant streetlight,
with the sensor positioned near this investigator’s eyes in the
vehicle. This streetlight was the major source of illumination in
the parking lot so the vast majority of the light was from the
streetlight. The ground reflection of the streetlight was also
consider but turned out to have negligible intensity. It was also
necessary to measure the angle between the expected line of
sight of the viewer (towards the object) and the streetlight lamp.
This angle was measured with a Suunto Tandem, but could
have been measured with a transit or other means. .
The following measurements were obtained
Illumination from streetlight – .2 foot-candles
Angle between the line of sight to the object and the
streetlight lamp – 6 degrees.
In order to provide a basis for building an analytical model, it
was necessary to measure the light patterns from the headlights
of the vehicle. It would have been possible to use generic data
on the light patterns of typical low beam headlights, since they
are standardized to some extent [10]. However, whenever
possible, it is better to actually measure the patterns of the
headlights in question because that will capture the effects of
the particular alignment of the headlights, and even though
headlights must meet certain standards on their minimum and
maximum intensities in certain directs, a lot of latitude remains
in their optical performance.
The light patterns of the headlights were measured by placing
the subject vehicle in a large dark garage on a flat level floor. A
large flat sheet of foam-core board with a grid drawn it was
setup on the floor, about 20 feet in front of the headlight to be
mapped. The board was aligned to be perpendicular to the floor
of the garage as well as the longitudinal axis of the vehicle. It is
important to set up the board a distance away from the
headlight of at least 8 times the maximum dimension of the
headlight reflector, so that the light from the headlight arriving
at the board behaves essentially like it originates from a point
source. Only one headlight is tested at a time; the other
headlight is covered and all other lights in the garage are turned
off. The origin of the board was placed in line with a line that
originated at the center of the headlight being tested and ran
parallel to the longitudinal axis of the vehicle and the ground.
The illumination meter was moved from intersection point to
intersection point on the grid drawn on the board and readings
were taken of the incident light.
This was a laborious process that took several hours. The
spacing of the grid on the board was relatively fine, 3 inches
both horizontal and vertical; so over 500 light measurements
were taken per headlight. Due to practical limits on the size of
the board and the amount of time that could be allocated, only
the central regions of the light patterns were measured. But this
was adequate, because the light intensity falls off sharply
outside of this central region.
The raw measurement data needed to be input to a computer
program to convert the positions on the rectangular grid drawn
on the board into the angular coordinates required. Also, the
reading taken had to be converted into candela by multiplying
by the reading by the radius squared. The radius being the
distance in feet between the position the readings were taken
and center of the headlight being tested. The results of these
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efforts are shown on Figures 2 and 3 below, which are called
isocandela plots.
Candela PLot for Left Headlight
Now the proper multipliers need to be applied to determine the
corrected visibility threshold. The object had a rough diameter
of 2 feet, and it was 57 feet away from the driver (who sat 7
feet behind the front of the vehicle). Therefore the subtended
angle was about 2 degrees, which according to Table 1,
warrants a multiplier of .086. The age of the driver was 46
years, which according to Table 2, warrants a multiplier of
1.29. The driver did not expect the sign, so the multiplier of
6.67 is warranted. Finally, is was desired to determine the
threshold at which 99% of viewers would observe the object, so
a final multiplier of 1.98 will be applied.
10
Vertical Angle
5
1515.4
5884.88797.8
111701254
.20.7.2
13167
7341.3
0
Lveil = kEi/i2 = 35(.2)/62 = .2 foot-lamberts
This calculated veiling luminance is then added to the ambient
luminance to obtain an effective ambient luminance of .4 footlamberts. This number is then taken to the Blackwell Chart to
determine the corresponding nrcv, which turns out to be .15
foot-lamberts.
44
28 2971.9
.3
1515.4
Applying the multipliers, the final corrected visibility threshold
is
.15 foot-lamberts x .086 x 1.29 x 6.67 x 1.98
-5
-10
-5
0
5
= .22 foot-lamberts
10
Horizontal Angle
This corrected threshold is to be compared to the measured
contrast, which was
Figure 2
.65 foot-lamberts - .03 foot-lamberts = .62 foot-lamberts
Candela Plot for Right Headlight
Therefore, the measured contrast is almost 3 times the contrast
required for 99% of same aged, unexpected viewers, to detect
the object.
10
9.2
558
2802.5
0
1409.2
9
5.
83
5597
75
12 69.
.9
2
.5
6982
11162.5
Vertical Angle
5
4195.8
1409.2
-5
-10
-5
0
5
10
Horizontal Angle
Figure 3.
Analysis of field measurements
The analysis began by preparing to use the Blackwell Chart.
In the field experiments it was determined the ambient
luminance was .2 foot-lamberts. However, we must add to this
the veiling luminance, which is, according to Equation 1,
Analytical Model
Because it was not practical to conduct field studies at every
conceivable position of the vehicle, an analytic model was
created from the information gathered in the field studies. This
model could then be used to predict what conditions would
have been over a broad range of possible conditions.
The light patterns experimentally obtained for the headlights
can be used to calculate the amount of illumination cast upon
the object and its immediate surrounding. In order to do this
one must determine the horizontal angle and vertical angle at
which the object would appear relative to each headlight. Then
from the corresponding isocandela diagram for each headlight
one determines the candela projected into that direction. The
foot-candles at the incident surface are then calculated by
dividing the candela by the radius squared, where the radius is
the distance in feet between the object and the center of the
headlight being considered.
In this example, the object was 50 feet away, directly in front of
the vehicle, and about 1 foot higher than the center of the
headlights. The headlight centers were 48 inches apart and 32
inches off the ground. Using this information, the horizontal
angle from the headlights can be calculated as + tan-1(2/50) = +
2.3 degrees, and the vertical angles can be calculated as
tan-1(1/50) = 1.1 degrees. Referring to the isocandela diagrams
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obtained for the headlights, one finds that the left headlight
would cast about 13,000 candela in the direction of the object
and right headlight would cast about 2,500 candela in the
direction of the object. Candela are converted to foot-candles
by dividing by the distance from the source to the object
squared and then multiplying by the cosine of the angle
between the object surface normal and the line between the
source and the object, which in this case is almost 1. Therefore
the foot-candles cast upon the object is
(15,500)/502 = 6.2 foot-candles.
The reflectivity of the object was 11%, therefore the luminance
of the object will be
6.2 x .11 =. 68 foot-lamberts.
In this case, the immediate background behind the object, from
the driver’s point of view, was a relatively long distance away.
The driver’s head was about 1 foot above the object and 6 feet
behind the headlights, so the line of sight from the driver to the
subject object intersects the ground about 260 feet away from
the vehicle. Given the low reflectance of the wet ground, this
will create negligible luminance. The ground at this distance
would be receiving some light from the streetlight, which
probably explains why a luminance of .03 was measured in the
field. Therefore, in the model, the luminance of the immediate
surrounding in the object was set to .03.
There is one more factor to be included in the analytical model.
That is the transmission efficiency of the windshield. Typical
transmission efficiency is about 90%. Therefore the luminance
of the object when viewed inside the vehicle would be about .9
x .68 = .61.
Therefore the contrast predicted by the analytic model is close
to .61 - .03 = .58 foot-lamberts, which compares to the .62 footlamberts obtained from field measurements.
The value of having the model is that it can be used to explore
what the visibility conditions would be when the vehicle is in
other positions of interest or along trajectories.
QUESTIONS AND ANSWERS
The case study dealt only with luminance contrast, aren’t there
other important factors involved in determining visibility?
Yes, some other important factors are: 1) color, 2) degree of
adaptation of the observer, and 3) position of the object in the
field of view of the observer.
The case study involved visibility of object at low lighting
levels, and under these circumstances luminance contrast
dominates color difference and color difference can be
neglected. At higher light levels, color difference can greatly
enhance visibility of objects.
Also, the case study assumes that the driver had been in the
vehicle long enough that his eyes had essentially adapted to the
ambient lighting levels. On the other hand, if the driver had just
left a bright office a higher contrast threshold would be
warranted.
The further away the image of an object falls from the fovea
area of the retina the greater the contrast threshold required for
visibility (except at very low levels of lighting). The fovea is
an area of the retina where there is a great concentration of
receptors, and it drives the central field of vision. The farther
images fall away from the fovea area of the retinal (becoming
peripheral images) the greater the luminance contrast required
for them to be readily visible.
The Blackwell Chart presented in the paper is based upon
experiments where the image fell upon the fovea. That is why
the result of the case study was stated with the qualifier that the
object would have been visible if the driver had looked in its
direction (i.e., cast the image of the object on the fovea region
of his retina). Keep in mind that a driver has a duty to look
where he going.
If the reader is investigating an incident that occurred at higher
light levels, where color is more important, or where the object
of interest could have legitimately been in the observer’s
peripheral visual field, then a more complex approach is
required. Sometimes it is still possible to use the basic
Blackwell Chart, but to apply additional multipliers. Look to
[12] for information on how visibility thresholds vary under
different conditions than those studied here.
It was also explained above, that once the time of presentation
of an object is greater than .1 second, its visibility does not
improve significantly at longer durations of presentation. That
is why it was permissible to use Blackwell’s results even
though his test objects were only presented to the test subjects
for 1/5 of a second. Interestingly, for small sized objects,
rapidly presented, the product of the area of the object times the
luminance contrast of the object times the presentation time is a
constant at the threshold of visibility, for a fixed level of
ambient luminance [12].
CONCLUSIONS
An array of engineering techniques were presented and their
use was illustrated by a case study. It was shown that the object
struck by the driver should have been clearly visible to him had
he been looking towards it as he approached it.
Bear in mind that the conclusion in the case study was based
upon the fact the actual measured luminance contrast was about
3 times the calculated visibility threshold luminance contrast.
This permitted a strong statement to be made about the
visibility of the object. If on the other hand, the ratio of
measured contrast to calculated threshold contrast turned out to
be closer to 1, then the conclusion would be less certain and
weaker.
It has been this investigator’s experience that the Blackwell
Chart method agrees well with his own perceptions of
visibility. By the time the expectancy multiplier (6.67) and the
99% multiplier (1.98) are applied to the luminance contrast
from the Blackwell chart, an object that can pass the resulting
criterion will indeed be readily visible. At the other end of
spectrum, if when the measured luminance contrast and
ambient luminance are plotted on the Blackwell Chart they
indicate that an object is not readily visible, it will not be.
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REFERENCES
1. Forensic Aspects of Driver Perception and Response , P.
Olsen and E. Farber, ISBN 1-930056-32-X, Lawyers & Judges
Publishing Company.
2. Forensic Aspects of Vision and Highway Safety, M. Allen,
B. Abrams, A. Ginsburg, and L. Weintraub, ISBN 0-91387524-4, Lawyers & Judges Publishing Company.
3. Motor Vehicle Lighting, SAE Publication PT-60, ISBN 156091-753-9
4. Advances in Lighting Technology, SAE Publication PT-98,
ISBN 0-7680-1296-1
5. IES Lighting Handbook, Illuminating Engineering Society
6. R.H. Blackwell, “The Problem of Specifying the Quantity
and Quality of Illumination”, Illuminating Engineering, Vol.
49, No. 2, February 954
7. O.M. Blackwell and R.H. Blackwell, “Visual Performance
Data for 156 Normal Observers of Various Ages”, Journal of
Illuminating Engineering Society, Vol. 1, No. 1, October 1971.
8. Bhise, V., Farber, E., Saunby, C., Troell, G., Walunas, J., and
Bernstein, A., Modeling Vision with Headlights in a Systems
Context, SAE 770238
9. Light Measurement Handbook, A. Ryer, ISBN 0-9658356-93, www.intl-light.com
10. SAE J1383, Performance Requirements for Motor Vehicle
Headlamps.
11. E. Phillips, T. Khatua, G. Kost, and R. Piziali, “Vision and
Visibility in Vehicular Accident Reconstruction”, SAE 900369.
12. Human Factors in Lighting 2nd Ed., Peter R. Boyce, 2003,
ISBN 0-7484-0950
13. Adrian, W., Visibility of Targets: Model for Calculation,
Light Res. Technol., 1989, 21, 181-188
7
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