HOW FULLY FUNCTIONAL GAS STATION SIGNS HELP DRIVING CUSTOMERS AT NIGHT?
Visual Target Detection Models for Civil Twilight and Night Driving Conditions
Transcript of Visual Target Detection Models for Civil Twilight and Night Driving Conditions
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Visual Target Detection Models for Civil Twilight and Night Driving Conditions
Helmut T. Zwahlen, Ph.D.
Russ Professor
Human Factors and Ergonomics Laboratory
Department of Industrial and Manufacturing Systems Engineering
Ohio University, Athens Ohio 45701-2979
(740) 593-1550
Thomas Schnell, Ph.D.
Assistant Professor
Cognitive Human Factors Laboratory
Department of Industrial Engineering
The University of Iowa
Iowa City, Iowa 52242-1527
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ABSTRACT
A luminance contrast based computer visibility model is discussed in this paper and compared with the
civil twilight method which has recently been introduced. The civil twilight method attempts to predict the
visibility of ordinary objects (reflectance 3% to 79%, average size) using only the headlamp illuminance
at the target. It is suggested by the authors that the one-factor approach used by the civil twilight method
is insufficient to satisfactorily address target visibility in the field. Developers of more advanced visibility
models generally attempt to design their models based on the current state of the visibility research and
with enough capability to obtain a reasonable degree of realism. The authors consider the level of the
benchmark illuminance (3.2 lx) used in the civil twilight method to be too high, leading to very short
detection distances for pedestrians (refer to literature review) under automobile headlamp illumination at
night. The developers of the civil twilight method claim that the 3.2 lx visibility benchmark is based on
systematic visual observations made by astronomers over a century ago. The use of the civil twilight
method for pedestrian detection under automobile headlamp illumination at night is strongly discouraged
by the authors of this paper, because the method may be misused by forensic experts if there is a
need to produce arbitrarily short pedestrian detection distances, irrespective of factors including the
clothing reflectance, contrast, pedestrian size, windshield transmittance, atmospheric transmissivity, etc.
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INTRODUCTION
The detection of targets under low light conditions has been of great interest to the scientific visibility
community in the recent past. A rather large body of research is available, providing the scientific
framework for target detection and target visibility, both from a theoretical and empirical point of view.
The list of visibility research that investigated human visual performance is too long to be completely
covered within the scope of this paper. It is, however, generally accepted that Blackwell [1][2][3] was a
major contributor of visual performance data forming the basis of our understanding of the capabilities
and limitations of the human visual system. More or less elaborate visibility models have been
developed in recent years based on such empirical visual performance data. Visibility models have
become an invaluable tool for predicting the visibility of targets under a wide range of viewing conditions.
Recent visibility models include the TARDEC model (army model) [4][5][6][7], the CIE (Committee
Internationale de LEclairage) model [8], the PCDETECT model [9], Adrians model [10], and computer
based visibility models developed by the authors [11][12][13][14]. These models are fairly elaborate and
were generally designed based on recent visibility research. The aim generally is to provide a visibility
model that is sufficiently precise (near true target value) and sufficiently accurate (small dispersion) in its
predictions under selected (possibly wide) range of input parameters. Another approach to target
visibility predictions was taken by Owens et al [15] and Andre and Owens [16] with their civil twilight
method. On the surface, their approach appears to provide a simple, holistic benchmark method to
address the issue of target visibility under automobile low-beam illumination at night. However, their
method has only 1 input parameter, namely the illumination at a point ahead of an automobile at a
selected height, on a plane normal to the illumination axis. It should be evident to readers who are
familiar with the basics of human visual performance at low light levels, that a one factor approach is
insufficient address target visibility in the field. It seems that the use of illumination at the target as a
visibility benchmark is a poor choice anyway. A better choice would be to use the target luminance,
which at least would implicitly account for target reflectance. That would, of course require exact
knowledge of the target reflectance properties. The civil twilight method was deliberately kept simple by
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Owens et al. [15] and Andre and Owens [16], so that the only instrument needed to conduct
measurements in the field would be a illuminance meter.
The members of the visibility research community should rightfully be concerned about the
proliferation of general, holistic approaches such as the civil twilight method. Such a holistic approach to
a specialized field of research seems to be an attempt to undermine and reverse recent advances in
visibility modeling. Human visual performance is not a field that should be trivialized by one-size-fits all
benchmarks such as the civil twilight method. In light of the vast scientific knowledge that was gathered
to date by many visibility researchers, it is hard to understand why Andre and Owens [16] pose the
general question of how much light do we need to see ? The answer is, it depends. It depends on the
visual target characteristics, on the background characteristics, on characteristics of the light source, on
environmental characteristics such as atmospheric transmissivity, ambient illumination, and on a
multitude of observer characteristics including (but not limited to) age, luminance and color contrast
sensitivity, adaptation level, probability of detection, arousal, expectancy and retinal eccentricity [17]. In
dynamic settings, the list of independent variables may be significantly expanded. It should be noted that
even the most advanced visibility model couldn't guarantee that a given observer would really be able to
detect a target exactly as predicted by the model all the time. In real world settings there may be
unknown physical factors such as dirt on the headlamps or the windshield, transient glare, or unknown
cognitive factors such as focus of attention, distraction, workload, etc. that could potentially influence the
visibility of a target. Although the influence of many of these factors can be modeled, it is not always
sure that their presence is known in a specific case. While even the most advanced current visibility
models are not able to explain all of the variability, it is fairly self evident that they provide far more
precise (near true target value) and accurate (small dispersion) predictions for a specific visibility
situation than a simple model such as the civil twilight method.
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Statement of the Problem
Luminance contrast based driver visibility models represent an important cornerstone in the design of
automobile lighting systems, the design of retro-reflective sheeting materials and pavement markings,
the design of traffic signals, etc. Visibility models are also often used by forensics experts in litigation
cases involving automobile accidents at night. There are a number of relatively advanced and higher
fidelity visibility models [4][5][6][7][8][9][10][11][12][13][14] that are based on the recent and/or present
state of the art in visibility research. These models consider many of the relevant factors of influence.
Thanks to being packaged in computer software, most of these models can be used by specialists with
relative ease.
Owens at al. [15] and Andre and Owens [16] suggest a method by which almost anyone, non-
experts and experts, could categorize a visual target seen under automobile headlamp illumination into
the visible or not visible category, simply by using an illuminance meter. It is our opinion that visibility
modeling should remain the domain of visibility researchers only. Non-experts should consult experts if
they seek a simple answer to the difficult question of how much light do we need to see. Serious
visibility experts know that the answer to that question depends on many issues, non-experts on the
other hand may not be aware of this. The civil twilight method attempts to determine the detection
distance of a target seen at a selected height under automobile headlamp illumination by using the
amount of headlamp illuminance at the target, on a plane normal to the illumination axis as the only
driving factor.
In addition, the level of the benchmark illuminance (3.2 lx) chosen by Owens et al [15] is
considered to be too high, leading to very short detection distances (refer to literature review) under
automobile headlamp illumination at night. Also, the civil twilight method proposed by Owens et al. [15]
does not consider two of the major independent variables, namely target size and reflectance. There is a
need to discuss the advantages, disadvantages, and the structure of visual target detection models as
they relate to dawn, dusk, and night driving conditions.
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REVIEW OF THE TECHNICAL LITERATURE
Owens, Francis, and Leibowitz [15] proposed a simple, functional approach, the civil twilight method,
that according to their claims, allows for quantification of the nighttime motorist visibility under
automobile illumination. The reader should refer to [18] for an additional discussion of the civil twilight
method [15].
The civil twilight method proposed by Owens et al. [15] determines the visibility distance of a
target seen (at a selected height) under automobile headlamp illumination by locating the distance
ahead of the vehicle at which the illuminance provided by the headlamps equals the twilight illuminance
ETD[lx] as indicated by Equation (1)
TD = CD0.3
(1)
where TD is the twilight distance in feet, CD is the luminous intensity in candelas of the (Cyclops)
headlamp in the direction of the target, and 0.3 is the twilight illumination in footcandles (3.2 lx). It should
be noted, that Owens et al. [15] do not consider the two headlamps to be laterally separated but rather
to be both located in the center of the vehicle (cyclops geometry). Owens et al. [15] only refer to the
targets as being ordinary objects, implying that the target reflectance may vary from R=3% to R=79%.
Simply claiming that a visibility benchmark method works for all ordinary objects is vague and
misleading, and it appears that such a method makes very little use of the current state of the art in
visibility research.
Andre and Owens [16] state that they have tested the predictions of their civil twilight method in
the field. They used an automobile (1990 Buick Skylark) with aligned headlamps and simply measured
the illumination from the headlamps at specific grid-points ahead of the automobile at a height of 12,
35, and 59. Andre and Owens [16] found that the distances at which an illuminance of 3.2 lx was
measured agreed rather well with the twilight plateaus provided by their civil twilight method. It should be
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noted that calling such measurements a test of predictions provided by a visibility model is highly
misleading. Andre and Owens [16] simply established the iso-illuminance distances at selected grid
points and vertical heights, nothing more. Their measurements prove nothing, whatsoever, about the
visual performance of human observers under specific conditions. One may agree that such
measurements are useful for checking the accuracy of physical, photometrical quantities. A meaningful
validation, however, would have to include a wide range of observers whose task is to detect a wide
range of targets under a wide range of conditions. Such an experiment would be bound to disclose the
shortcomings of the civil twilight method because this method is unable to explain the large variation that
would be found in such a field experiment.
Other target detection refernces that are discussed in detail in [18] are Hazlett, and Allen [19],
Shinar [20], Chrysler et al. [21], Blomberg et al. [22], Olson and Sivak [23], Strickland, Ward, and Allen
[24], Austin, Klassen, and Vanstrum [25], Zwahlen and Schnell [26], Blackwell [1], Blackwell and
Blackwell [2]. Target detection distances from some of the above references are tabulated in Table 1.
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Basic Issues Regarding Visibility Modeling
In general, models of processes are often developed to demonstrate a concept, to facilitate the
understanding of a concept or process and to describe, in an abstract form, a process the way we think
it works. Sometimes, models are used to predict the behavior of a system without disturbing the system
itself. While approximations are sometimes used in engineering, one should be aware that for such
approximations to be useful, they must provide fairly accurate (small dispersion) and precise (near true
target value) results, they must be validated, and they should be accepted by the majority of the
scientists and engineers. The reader should note that the human performance part of the civil twilight
method has not been validated in the field. Certainly, it would be relatively simple to come up with test
cases involving the factors that are neglected by the civil twilight method to demonstrate that the civil
twilight method may provide completely inaccurate detection distance predictions. Figure 1a illustrates a
basic fact of model building. In order to increase the degree of realism obtained with a model, one has to
increase the complexity and the completeness. It should be noted that Owens et al. [15] claim that the
civil twilight method implicitly accounts for a number of factors by virtue of the generality of the civil
twilight method. The civil twilight method is supposed to encompass the visibility of all targets with an
imaginary average reflectance, an average size, an average background luminance, etc. It is, however,
quite evident that the civil twilight method with its rigid 3.2 lx benchmark value is unable to explain the
large variation of detection distances that are observed for different targets illuminated under different
conditions, and observed by different observers. In model building, one aims at determining the main
effects and as many interaction effects as possible, in order to obtain a high correlation between the
predicted and observed values. Usually, the model with the highest correlation (R2) should be selected
for use until a better model is found.
The visibility research community is called upon to conduct the research needed to build even
more complete models. There should be no doubt that todays state of the art visibility models far
exceed the capability of the civil twilight method in explaining the variability found in the visual detection
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of targets. By no means, however, should one expect a visibility model ever to be free of variation and to
be able to provide precise pinpoint predictions of target visibility. It is well known that human visual
performance follows psychometric functions such as the ones established by Blackwell [1][2]. There will
always be a less than perfect fit (R2
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computers it is fairly easy to obtain the results of target detection models almost instantaneously, thus
further reducing the need for a simple approach such as the civil twilight method.
The Civil Twilight Method, Old Science Taken Out of Context
Owens et al. [15] and Andre and Owens [16] state that their 3.2 lx visibility benchmark is based on
systematic visual observations made by astronomers over a century ago. The simple fact that some
research is over 100 years old does not necessarily lend more credence to its adequacy, especially not
if the research is taken out of context 100 years later. Astronomers back then and today are generally
more concerned about the visibility of astronomical objects rather than target visibility under automobile
illumination. During WWII, the office of scientific research and development established the Tiffany
foundation to further the state of the knowledge in visibility research. A large scale study funded by the
Tiffany foundation was conducted by Blackwell [1], based on the need of the US Navy to learn more
about the threshold contrast of the human eye. This large and relatively expensive study would not have
been commissioned, if the designers and visibility scientists at the time had felt that they already had a
handle on target visibility with the civil twilight observation data provided by astronomers. If these
researchers already felt they needed more adequate data to better understand target visibility it would
appear that more recent data provided by Blackwell [1][2] and other visibility researchers supersedes
observations made by astronomers over 100 years ago. This is especially true since modern
psychophysical experiments were conducted under controlled conditions, allowing for the isolation of
main effects and interaction effects. The human factors and Ergonomics Laboratory at Ohio University,
Athens, Ohio, used extensive visibility field research data to allow the use of laboratory visibility
research data [1][2] in models that predict target visibility under specific real-world conditions (calibration
of model). No psychophysical target detection field research was ever published to determine the validity
of the civil twilight method.
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LUMINANCE AND ILLUMINANCE MESUREMENTS DURING CIVIL TWILIGHT ON AN OVERCAST
EVENING AND ON A CLEAR EVENING
It was hypothesized by the authors of this paper that the ambient illuminance and target luminances
during civil twilight are highly dependent on weather conditions and on the direction in which the
measurements are performed. The measurements reported in this section should provide the reader
with an idea of the range of available ambient natural illuminance and its directionality during the civil
twilight period on an overcast evening and on a clear evening. In addition, luminances were measured
on a gray target and on a white target to obtain the range of luminances as a function of time and
direction during the civil twilight period. It should be noted, that Owens et al. [15] chose an illuminance of
3.2 lx, obtained at the end of the civil twilight on a clear day, as their benchmark illuminance ETDin the
civil twilight method. An overcast evening was chosen for part of the measurements reported herein to
demonstrate that the twilight illuminances can be considerably lower than the value of 3.2 lx reported by
Owens et al. [15] under such conditions. The clear evening measurements were conducted to confirm
that one would obtain twilight illumination values that are considerably higher than the ones measured
during an overcast evening.
Twilight is defined as the transition period from day to night (or night to day) when the sky is not
completely dark. It is technically defined as the period of time beginning (or ending) when the center of
the (refracted) sun is lower than a given elevation. Civil twilight corresponds to the sun being between 0
and 6 below the horizon, nautical twilight to 6 and 12, and astronomical twilight to 12 and 18.
Measurement Site, Setup, and Method
The measurements were conducted on the beach parking lot of the Strouds Run State Park outside
Athens, Ohio (Longitude:82o
, 06', 20" W, Latitude:39o
, 20', 14"N) during the civil twilight on February 3,
1998 (overcast evening) and on March 26, 1998 (clear evening). The measurement cycle was repeated
from 30 minutes prior to the beginning of the evening civil twilight to 30 minutes after the end of the
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evening civil twilight. Care was taken to ensure that virtually no alternative (other than natural) sources
of illumination were present. The gray target had a reflectance of R=0.108 and on the white target had a
reflectance of R=0.92. Both targets were made of posterboard 0.6m x 0.6m. The gray target was spray
painted with a blend of Krylon Satin Black spray paint and Krylon Satin White spray paint. The white
target was left untreated. For the luminance measurements, the targets (see Figure 2) were positioned
in 8 directions: North, Northeast, East, Southeast, South, Southwest, West, and Northwest. The
measurements consisted of repeated cycles of one full revolution of luminance measurements (white
and gray target in each position as indicated in Figure 2) followed by one full revolution of illuminance
measurements including one measurement taken skywards (straight up). The targets were removed for
illuminance measurements. At the end of the illuminance measurement cycle, the Pritchard 1980A was
tilted 90oup to obtain the sky illuminance.
Results of the Luminance and Illuminance Measurements
Figure 3, Figure 4, and Figure 5 summarize the luminance and illuminance values measured on the
overcast evening (February 3, 1998) and on a clear evening (March 26, 1998) at the Strouds run parking
lot site. The measured luminance values (overcast condition) for the white target with a reflectance of
R=0.92 are shown in Figure 3a. In spite of the diffuse overcast sky conditions, a small directionality
effect was found as evidenced by the spread of the luminance curves in Figure 3a. At 6:19pm (end of
the overcast evening civil twilight on February 3, 1998), the white target provided an average luminance
of 0.012 cd/m2. Figure 3b shows the measured luminance values (overcast evening) for the gray target
with a reflectance of R=0.108. At 6:19pm (end of the evening civil twilight), the gray target provided an
average luminance of 0.0012 cd/m2.
Figure 4a shows the luminance values that were measured during the evening civil twilight of
March 26, 1998 (clear evening) on the white target. Again, a substantial directionality effect is observed
and the average luminance of the white target at the end of the civil twilight is 0.52 cd/m2. The clear
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evening civil twilight luminance values of the gray target are shown in Figure 4b. The average luminance
of the gray target at the end of the civil twilight is 0.042 cd/m2.
The ambient illuminances that were measured on February 3, 1998 (overcast evening) at the
beach parking lot of the Strouds Run State park are shown in Figure 5a. Again, a directionality effect
was found. At 6:19pm (end of the evening civil twilight) the average illuminance measured was 0.08 lx.
Figure 5b shows the ambient illuminance [lx] as a function of time with direction as parameter, during the
evening civil twilight of March 26, 1998 (clear sky). As in the overcast evening data (Figure 5a), there
was a fairly large directionality effect. The average illuminance over all measured directions at the end of
the civil twilight was 1.7 lx, which is still 46.9% lower than the 3.2 lx stated by Owens et al. [15].
It appears that the series of measurements conducted under clear sky conditions come much
closer to the 3.2 lx used by the civil twilight method. Benchmarks should not be based on highly variable
phenomena but rather on highly reproducible phenomena. For the specific case of target visibility
modeling the authors recommend against the use of benchmarks, whatsoever. However, the authors
recommend the use of more precise, calibrated visibility models that produce unbiased estimates and
account for as much of the variation as the current state of research permits. Looking at Figure 5 one
can see that each illuminance line is a well behaved, monotonously decreasing function over time.
Therefore, selecting an end of civil twilight illuminance of 3.2 lx is definitely arbitrary since the measured
illuminances at the end of the civil twilight ranged from 0.06 lx (overcast sky) to 3.2 lx (clear sky). The
luminances measured on the targets at the end of the civil twilight period ranged from 0.0009 cd/m2
(gray target, overcast sky) to 1.2 cd/m2 (white target, clear sky). The measured luminance range
practically covers the entire mesopic range. The practice of using the end of civil twilight illuminance as
benchmark is highly questionable, since the photometric conditions associated with the end of civil
twilight appear to span the entire mesopic range.
Figure 6a shows twilight distance plateaus for an automobile using Ford Taurus low-beam
headlamps and a target (size and reflectance are not considered by twilight distance method) with an
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assumed vertical target center at 0.53m above ground. The plateaus were constructed for various
threshold twilight illuminances. Using a twilight illuminance of 3.2 lx, it can be seen that the twilight
distance is about 43m, if the target is placed 1.5m to the right of the longitudinal vehicle axis. A twilight
illuminance of 1.6 lx allows the twilight distance to increase to 60m, if the target is located about 2.2m to
the right of the longitudinal vehicle axis. Decreasing the twilight illuminance to 0.8 lx allows the twilight
distance to increase to 87m, if the target is located about 3.5m to the right of the longitudinal vehicle
axis. A twilight distance of over 100m can be obtained if the twilight illuminance is assumed to be only
0.4 lx and the target is located 2.5m to the right of the longitudinal vehicle axis. Figure 6a basically
illustrates that an arbitrary selection of the twilight illuminance will result in an equally arbitrary twilight
distance. The illuminance measurements have clearly shown, that an illuminance of 3.2 lx seems quite
high. Choosing a twilight illumination of 3.2 lx will consequently lead to relatively short twilight distances
as indicated by Figure 6a. Again, it seems that the twilight distance method is a completely inadequate
approach to solving a complex visual detection modeling problem.
Comparison of Detection Distances Obtained Using the Civil Twilight Method With Detection
Distances Obtained Using a State of the Art Computer Model
It was previously mentioned that the civil twilight method neglects a number of factors that are known to
significantly affect the visibility of a target. This section demonstrates the effect of the angular target size,
the target reflectance, and the driver age upon the detection distance of selected targets. The obtained
detection distances are compared to the corresponding twilight distances.
A proprietary computer based visibility model was used by the authors to calculate the detection
distances for the selected targets. This computer visibility model is based on the Blackwell human
threshold contrast database [1]. The age function [11] used in the computer visibility model is based on
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data provided by Blackwell for 156 normal observers of various ages [2]. The headlamps, the observer,
and the targets are accurately accounted for in 3D space.
Figure 6b illustrates the setup used for the calculations of the detection distance of diffuse targets
located on the right road shoulder. The center of the targets are located 0.53 m above the road surface.
To demonstrate the effect of the target size it is assumed that the target size is increased along the
vertical dimension from 0.13m up to 1.06m. Two target reflectances R1=0.3 and R2=0.1 are used to
demonstrate the effect of target reflectance. The driver eye location and the headlamp location are valid
for a 50 percentile adult in an average car [36]. In computing the visibility of a given target, the
proprietary computer based visibility model first determines the luminance of the background along the
longitudinal target axis by using a road surface reflectance matrix (used asphalt in this case). It should
be noted, that only the luminance of the pavement was used as a target background. It was assumed,
that the target did not extend into the horizon sky. Candlepower matrices are used for each headlamp
separately to determine the illuminance at the target. Then the computer model determines the
luminance of the target along the longitudinal target axis, using the reflectance of the diffuse target. With
the target luminance LT and background luminance LB, the computer model determines the actual
contrast along the longitudinal target axis using Equation (2).
CACT =LT L B
L B(2)
Then, the computer visibility model compares the actual contrast with the threshold contrast CTH
determined from Blackwells human threshold contrast database [1]. The reader is referred to [14] for a
detailed description of the interpolation method used to determine CTH as a function of the visual angle
subtended by the target and the available background Luminance LB. In a final step, the computer
visibility model applies a number of contrast multipliers to account for the observer age [2][11] (25 years
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and 65 years used in present paper), probability of detection (P=99.93%1used in this paper), exposure
time [37] (0.65 seconds used in this paper), and a field factor of 10 to account for the difference between
Blackwells trained observers in the laboratory [1] and normal observers driving in an automobile. The
detection distance is given by the longitudinal location at which the actual contrast CACT
equals the
adjusted threshold contrast CTH.
Figure 7 clearly illustrates the effect of Target Size and Target Reflectance on the detection
distance using the proprietary computer based visibility model. Figure 7a shows that young observers
(25 years) can detect the dark achromatic targets with a reflectance of R=0.1 at a detection distance of
30.0m, 36.0m, 39.3m, and 41.5m for a target height of 0.13m, 0.26m, 0.53m, and 1.06m, respectively.
All of these distances are shorter than the twilight distance (headlamp illumination equals 3.2 lx, 0.53m
above ground) of 53m. Figure 7b shows that young observers (25 years) can detect the light achromatic
targets with a reflectance of R=0.3 at a distance of 62.5m, 77.5m, 87.0m, and 93.0m for a target height
of 0.13m, 0.26m, 0.53m, and 1.06m, respectively. All of these distances are longer than the twilight
distance (headlamp illumination equals 3.2 lx, 0.53m above ground) of 53m. Old observers (Figure 7b)
detect the 1.06m high target with a reflectance of R=0.3 at a distance of only 77.5m which equates to a
loss in the visibility distance of 16.6% over the young observers. Figure 7 provides evidence that the civil
twilight distance method is not only unable to account for target size, but also fails to predict the severe
reduction in detection distance when using dark, diffuse target materials. Also, the civil twilight distance
method does not consider the factors of contrast and observer age, to name a few.
1Probability of Detection of 99.93% (3.2) means that in 9,993 out of 10,000 observations the target is
detected
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Summary, Discussion, and Conclusion
This paper investigated and identified the shortcomings of the civil twilight method proposed by Owens
et al. [15] and Andre and Owens [16] as a simple functional approach to determine the visibility distance
of an ordinary object under automobile illumination at night. The civil twilight method attempts to
determine the detection distance of a target seen under automobile headlamp illumination at night by
using the amount of illuminance provided by the headlamps as the only benchmark factor. The civil
twilight method completely neglects target size and reflectance. In other words, for the civil twilight
method it does not matter whether the target is a dark clad pedestrian or a large light colored farm
animal. Other factors that are neglected by the civil twilight method, in spite of scientific evidence
indicating them to be important for visbility modeling are driver age, windshield transmittance, glare,
atmospheric transmissivity, and exposure time, just to name a few.
A series of luminance and illuminance measurements were conducted during the evening civil
twilight of February 3, 1998 (overcast evening) and March 26, 1998 (clear evening). A strong
dependency of the measured illuminance on the prevailing weather condition and a directionality effect
were found. The measured illuminance of 0.08 lx at the end of the civil twilight during the overcast
evening is 40 times lower than the 3.2 lx cited by Owens et al. [15]. The measured illuminance of 1.7 lx
at the end of the civil twilight during the clear evening is still 1.9 times lower than the 3.2 lx cited by
Owens et al. [15].
It is the opinion of the authors of this paper that the civil twilight method as a whole appears to be
inadequate and insensitive, and should not be used at all. Owens et al. [15] feel that the value of their
method lies in its simplicity rather than in its accuracy. It is the opinion of the authors of this paper that
proposing a simple, non-validated approach such as the civil twilight method to solve target detection
problems under automobile headlamp illumination at night, is highly questionable and the results of the
civil twilight method may be misleading at best. There really is no need for a simple target detection
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algorithm like the civil twilight method, since advanced target detection/visibility/legibility models are
available as computer programs that can be used by specialists. At this point in time, no claim should be
made that the civil twilight method is an accepted, validated, adequate, scientific method to determine
the visibility of targets.
The use of the civil twilight method for pedestrian detection under automobile headlamp
illumination at night is strongly discouraged by the authors of this paper, because the method may be
misused by forensics experts if there is a need to produce arbitrarily short pedestrian detection
distances, irrespective of factors including the clothing reflectance, contrast, pedestrian size, windshield
transmittance, atmospheric transmissivity, etc. Advanced visibility models that are calibrated with field
data and which consider a multitude of factors provide predictions that are more precise (near true target
value) and more accurate (small dispersion) than predictions made by the civil twilight method.
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0.001
0.01
0.1
1
10
100
5:30 5:35 5:40 5:45 5:50 5:55 6:00 6:05 6:10 6:15 6:20 6:25 6:30 6:35 6:40 6:45
Time Begin
Luminance[cd/m^2]
SE
S
SW
W
NW
N
NE
E
Sunset: 5:52pm
End of Civil Twilight : 6:19pm
Luminance of a white target taken in
different directions, 100% overcast.
Strouds Run State Park, Athens, Ohio.
Lat: 39:18:58N, Long: 82:05:42W
Civil Twilight begins: 5:52pm, ends: 6:19pm.
a. White Target, R=0.92
0.0001
0.001
0.01
0.1
1
10
5:30 5:35 5:40 5:45 5:50 5:55 6:00 6:05 6:10 6:15 6:20 6:25 6:30 6:35 6:40 6:45
Time Begin
Luminance[cd/m
^2]
SE
S
SW
W
NW
N
NE
ESunset: 5:52pm
End of Civil Twilight: 6:19pm
Luminance of a gray target taken in
different directions, 100% overcast.
Strouds Run State Park, Athens, Ohio.
Lat: 39:18:58N, Long: 82:05:42W
Civil Twilight begins: 5:52pm, ends: 6:19pm.
b. Gray Target, R=0.108
Luminance Measurements Conducted on the Beach Parking Lot of the Strouds Run State Park Outside Athens, Ohio(Longitude:82
o06 20" W, Latitude:39
o20 14"N) During the Evening Civil Twilight on February 3, 1998
Figure 3. Overcast Evening: Target Luminance [cd/m2] as a Function of Time with Direction as
Parameter, During Evening Civil Twilight
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0.0001
0.001
0.01
0.1
1
10
100
1000
6:15 6:20 6:25 6:30 6:35 6:40 6:45 6:50 6:55 7:00 7:05 7:10 7:15 7:20 7:25 7:30 7:35 7:40
Time at Beginning of Measurements
Luminance[cd/m^2]
SE White
S White
SW White
W White
NW White
N White
NE White
E WhiteBeginning of Civil Tw ilight, 6:46
End of Civil Tw ilight, 7:13
Luminance of w hite square target in
diff erent directions during Civil Tw ilight.
26 March 1998. Clear sky. Strouds Run, Athens, OH.
Latitude: 39:18:58N, Longitude: 82:05:42W
a. White Target, R=0.92
0.0001
0.001
0.01
0.1
1
10
100
1000
6:15 6:20 6:25 6:30 6:35 6:40 6:45 6:50 6:55 7:00 7:05 7:10 7:15 7:20 7:25 7:30 7:35 7:40
Time at Beginning of Measurements
Luminance
[cd/m^2]
SE Gray
S Gray
SW Gray
W Gray
NW Gray
N Gray
NE Gray
E GrayBeginning of Civil Tw ilight, 6:46
End of Civil Tw ilight, 7:13
Luminance of gray square target in diff erent
directions dur ing Civil Twilight.
26 March 1998. Clear sky. Strouds Run, Athens, OH.
Latitude: 39:18:58N, Longitude: 82:05:42W
b. Gray Target, R=0.108Luminance Measurements Conducted on the Beach Parking Lot of the Strouds Run State Park Outside Athens, Ohio(Longitude:82
o06 20" W, Latitude:39
o20 14"N) During the Evening Civil Twilight on March 26, 1998
Figure 4. Clear Evening: Target Luminance [cd/m2] as a Function of Time with Direction as Parameter,
During Evening Civil Twilight
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0.001
0.01
0.1
1
10
100
1000
5:40 5:45 5:50 5:55 6:00 6:05 6:10 6:15 6:20 6:25 6:30 6:35 6:40 6:45
Time Begin
Illuminance[lux]
SE
S
SW
W
NWN
NE
E
Up
3.2lx
Illuminance taken in different directions, 100% overcast.
Strouds Run State Park, Athens, Ohio.
Lat: 39:18:58N, Long: 82:05:42W
Civil Twilight begins: 5:52pm, ends: 6:19pm.
3.2lx at 6:01pm
when
measuring NE
3.2lx at 6:04pm
when
measuring up
0.08lx at end
of
civil twilight
a. Overcast Evening, February 3, 1998
0.001
0.01
0.1
1
10
100
1000
6:35 6:40 6:45 6:50 6:55 7:00 7:05 7:10 7:15 7:20 7:25 7:30 7:35 7:40 7:45 7:50
Time at Beginning of Measurements
Illuminanc
e[lux]
SE
S
SW
W
NW
N
NE
E
Up
3.2 lx
Illuminance in different directions during Civil Twilight.
26 March 1998. Clear sky. Strouds Run, Athens, OH.
Latitude: 39:18:58N Longitude: 82:05:42W
Civil twilight begins 6:46pm, ends 7:13pm
Stated value of illuminance
at the end of Civil Twilight, 3.2 lux.
3.2lx at 7:06pmwhen
measuring
East
3.2lx at 7:13pm
when
measuring
West
b. Clear Evening, March 26, 1998Illuminance Measurements Conducted on the Beach Parking Lot of the Strouds Run State Park Outside Athens, Ohio(Longitude:82o06 20" W, Latitude:39o20 14"N) During the Evening Civil Twilight on February 3, 1998
Figure 5. Illuminance [lx] as a Function of Time with Direction as Parameter, During Evening Civil
Twilight
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a. Twilight Distance Plateaus for Ford Taurus Low-Beam Headlamps, Assumed Vertical Target Centeris 0.53m above Ground
Area enclosed by each plateau receives an illuminance at or above the indicated value in lux
=Origin
Driver
LongitudinalOffset2.05m
Longitudinal Dista nce to Target
DriverLateral
Offset0.32m
Target Heightto Center 0.53m
HeadlampHeight0.607m
DriverEye Height
1.16m
Lane Width3.65m(12ft)
1/2 Lane Width
1.82m(6ft)
1/2 Lane Width
1.82m(6ft)
Headlamp
Separation1.11m(4ft)
Vertical TargetDimension
1.06m0.53m0.26m
0.13m
Diffuse TargetLocated on RightRoad Shoulder
Reflectance10% and 30%Width 0.13m
b. Setup used for the Calculation of the Detection Distance of Diffuse, Achromatic Targets
Figure 6. Iso Twilight Distance Plateaus for Various Twilight Illuminations and Setup Used in Detection
Distance Calculations
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0.001
0.01
0.1
1
10
0 5 10 15 20 25 30 35 40 45 50 55 60
Distance [m]
Contrast
Cth for 1.06m high targetCth for 0.53m high targetCth for 0.26m high targetCth for 0.13m high targetCact
Object with center located along right edge line (1.82m from car
center) and 0.53 m above ground.
Reflectance = 10%, H6054 low beams, Young Observer 25 years
Probability of Detection P=99.93%
Exposure Time 0.65seconds (85th percentile eye fixation duration)
Field Factor = 10
Twilight Distance
= 53m
(3.2 lux at target)
30.0m 36.0m
39.3m
41.5m
a. Reflectance R=0.1
0.01
0.1
1
10
100
0 10 20 30 40 50 60 70 80 90 100 110 120
Distance [m]
Contra
st
Cth for 1.06m high target, 65 yearsCth for 1.06m high target, 25 yearsCth for 0.53m high target, 25 yearsCth for 0.26m high target, 25 yearsCth for 0.13m high target, 25 yearsCact
Twilight Distance
= 53m
(3.2 lux at target)
Object with center located along right edge line (1.82m from car center)
and 0.53 m above ground.
Reflectance = 30%, H6054 low beams, Probability of Detection 99.93%
Exposure Time 0.65seconds (85th percentile eye fixation duration)
Field Factor = 10
62.5m 77.5m 87.0m
93.0m
b. Reflectance of R=0.3
Figure 7. Actual Contrast and Threshold Contrast as a Function of Longitudinal Distance for Diffuse,
Achromatic Targets Ranging in Height from 0.13m to 1.06m, Target Center 0.53m above
Ground, As Seen Against Used Asphalt, Reflectance R=0.1 and R=0.3
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Table 1. Comparison of Average Target Detection Distances Reported by Various Researchers
Note: Entries are sorted by increasing detection distance, entries shown in white on black indicate oncoming car glareconditions. NV=Not Visible
Researcher(s) Method Observer Age Beam Type Target TypeTarget
Reflectance/AppearanceTarget Size
Average
Detection
Distance [m]Owens et al. Twilight Distance Not considered Europe low beams Any target, 43.3" above ground Not considered Not considered NV
Olson and Sivak Field, Approaching Glare source young Low beams Human figure on Left Dark Adult human 18
Olson and Sivak Field, Approaching Glare source young Low beams Human figure on right Dark Adult human 24Owens et al. Twilight Distance Not considered US low beams Any target, 43.3" above ground Not considered Not considered 29
Leibowitz and Owens not stated Not considered Low beams Pedestrian in dark clothing Not specified Not specified 34
Leibowitz and Owens not stated Not considered High beams Pedestrian in white clothing Not specified Not specified 34Hazlett and Allen Field, Blood Alcohol Content = 0 Young Low beams Simulated pedestrian Black cloth, 9% .3m x .3m x 1.2m 34
Olson and Sivak Field, Approaching Glare source young Low beams Human figure on Left Light Adult human 37
Olson and Sivak Field, Approaching Glare source young High beams Human figure on right Dark Adult human 37Olson and Sivak Field, Approaching Glare source young High beams Human figure on Left Dark Adult human 37
Hazlett and Allen Field, Blood Alcohol Content = 0 Young Low beams Simulated pedestrian Gray cloth, 16% .3m x .3m x 1.2m 37
Owens et al. Twilight Distance Not considered Europe low beams Any target, 27" above ground Not considered Not considered 44
Leibowitz and Owens not stated Not considered High beams Pedestrian in dark clothing Not specified Not specified 49Olson and Sivak Field, Approaching Glare source young Low beams Human figure on right Light Adult human 49
Owens et al. Twilight Distance Not considered US low beams Any target, 27" above ground Not considered Not considered 55
Chrysler Field, rural test track 53-75 Low beams Small road hazard 31.5% Reflectance .17m x .33m 73Olson and Sivak Field, Approaching Glare source young High beams Human figure on right Light Adult human 73
Olson and Sivak Field, Approaching Glare source young High beams Human figure on Left Light Adult human 73
Owens et al. Twilight Distance Not considered US low beams Any target on ground level Not considered Not considered 86Chrysler Field, rural test track 53-75 Low beams Child mannequin 37.8% Reflectance 1.06m tall 88
Owens et al. Twilight Distance Not considered Europe low beams Any target on ground level Not considered Not considered 91
Bloomberg et al Field, dark test course Not specified Low beams Child, jacket Gray Garmet size 8 94
Chrysler Field, rural test track 19-25 Low beams Small road hazard 31.5% Reflectance .17m x .33m 96
Shinar Field, rural roads, target not expected 20-58 Low beams Pedestrian Khaki, 5% Reflectance Adult human 101Shinar Field, rural roads, target not expected 20-58 Low beams Pedestrian Khaki, 70% Reflectance Adult human 105
Bloomberg et al Field, dark test course Not specified Low beams Adult, Coverall Gray Garmet size , large 110Bloomberg et al Field, dark test course Not specified Low beams Child jacket White Garmet size 8 114
Owens et al. Twilight Distance Not considered US high beams Any target, 43.3" above ground Not considered Not considered 125
Chrysler Field, rural test track 19-25 Low beams Child mannequin 37.8% Reflectance 1.06m tall 125Owens et al. Twilight Distance Not considered US high beams Any target, 27" above ground Not considered Not considered 137
Bloomberg et al Field, dark test course Not specified Low beams Human adult figure target Gray Not specified 143
Owens et al. Twilight Distance Not considered US high beams Any target on ground level Not considered Not considered 143
Owens et al. Twilight Distance Not considered Europe High beams Any target on ground level Not considered Not considered 146Owens et al. Twilight Distance Not considered Europe High beams Any target, 27" above ground Not considered Not considered 146
Owens et al. Twilight Distance Not considered Europe High beams Any target, 43.3" above ground Not considered Not considered 146
Hazlett and Allen Field, Blood Alcohol Content = 0 Young Low beams Simulated pedestrian White cloth, 75% .3m x .3m x 1.2m 149
Shinar
Field, rural roads, target expected within2km, laterally located either on center, left
or right 20-58 Low beams Pedestrian Khaki, 5% Reflectance Adult human 150
Shinar
Field, rural roads, target expected at exactlongitudinal distance, laterally located
either on center, left or right 20-58 Low beams Pedestrian Khaki, 5% Reflectance Adult human 152
Shinar
Field, rural roads, target walking away
from static car 20-58 Low beams Pedestrian Khaki, 70% Reflectance Adult human 160
Shinar
Field, rural roads, target walking away
from static car 20-58 Low beams Pedestrian Khaki, 5% Reflectance Adult human 165
Shinar
Field, rural roads, target expected within
2km, laterally located either on center, leftor right 20-58 Low beams Pedestrian Khaki, 70% Reflectance Adult human 175
Bloomberg et al Field, dark test course Not specified Low beams Adult, Jacket White Garmet size 44 178
Shinar
Field, rural roads, target expected at exact
longitudinal distance, laterally located
either on center, left or right 20-58 Low beams Pedestrian Khaki, 70% Reflectance Adult human 185Bloomberg et al Field, dark test course Not specified Low beams Adult, Trousers White Garmet size , large 219
Bloomberg et al Field, dark test course Not specified Low beams Human adult figure target White Not specified 257
Bloomberg et al Field, dark test course Not specified Low beams Adult, Coverall White Garmet size , large 275
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Table 2. Comparison of visibility Model Components
Component present in Visibility Method
Civil
TwilightMethod
Ohio
University
Visibility
Model
Component present in Visibility
Method
Civil
TwilightMethod
Ohio
University
Visibility
Model
Candlepower beam pattern yes yes Observer threshold contrast no yes
individually defined with 6 dof no yes Threshold illumination at the eye no
yes, based on
Blackwell
1946 data
Observer location with 6 dof no yes Adaptation of observer no
yes, based on
Blackwell
1946 data
Windshield transmission no yes Age-background luminance interaction no
yes, based on
data from
Blackwell
1946 and 1971
Atmosperic transmissivity no yes Target eye fixation duration no
yes, Ohio
University eye
scanning
research
Above horizon scene background
luminanceno yes Probability of detection no yes
Road surface luminance (for pavement
markings)no yes Disability glare no yes
4 dimensional coefficient of retro-
reflection matrices for micro-prismatic
retro-reflectors
no yes Contrast polarity no yes
2 dimensional coefficient of retro-
reflection matrices for beaded materialsno yes Effects of color contrast no no
2 dimensional coefficient of retro-reflectance matrices for pavement
markings and road surfaces
no yesEffects of non-uniform background
luminanceno no
Reflectance of diffuse reflectors no yes Observer attention/arousal no no
Target location and orientation in 6 dofno, 3 dof
onlyyes
Calibration with target and task
specific psychometric visibility field
data
no yes
Target size no yesValidation with target and task specific
psychometric visibility field datano yes
Model usable for legibility of text no yes
Peripheral detection no noObserver age no yes
dof = degrees of freedom
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