Effect of varying contrast ratio and brightness nonuniformity over human attention and tunneling aspects in aviation

INTERNATIONAL JOURNAL OF ELECTRONICS AND
International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
– 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME

ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 3, Issue 2, July- September (2012), pp. 400-412
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EFFECT OF VARYING CONTRAST RATIO AND BRIGHTNESS NON-
UNIFORMITY OVER HUMAN ATTENTION AND TUNNELING
ASPECTS IN AVIATION

Vinod Karar, SmarajitGhosh*
CSIR-Central Scientific Instruments Organisation, Chandigarh
*
Thapar University, Patiala
vkarar@rediffmail.com

ABSTRACT
The prime role of head-up displays (HUDs) is to provide primary flight, navigation, aircraft and guidance
information to the pilot in his forward field of view on see-through glasses with optical filter properties known
as beam combiners (BC) which facilitates the pilot in excellent control of an aircraft activities through the
concurrent scan of instrument data as well as the outside scene. Even though the HUDs are known to improve
the flight performance, there are perceptual and cognitive issues associated with its use. This paper puts forward
the results of an experimental study carried on human aspects in form of attention capture and tunneling due to
HUD use under the environmental conditions where ambient brightness is changing and the HUD display screen
presents a non-uniform brightness across the display screen.
The pilots tend to have division of near and far domain attentional resources resulting due to the attention or
cognitive capture as established in this study and also established in studies carried worldwide. HUDs may
cause decrease in pilot’s situation awareness (SA) in tasks that require constant monitoring of information in the
environment which may demand split second decisions at times. It has been established through the study
carried out in this work that there is a distinct relation of HUD display brightness, contrast ratio (CR) and the
non-uniformity in HUD display brightness with the attention capture and tunneling. The study conducted based
on participant’s response to varying contrast ratios on HUD resulting due to the non-uniform brightness of HUD
display under the simulated conditions shows that the non-uniformity in the HUD display brightness causes the
tunneling effect as evident from the study results which clearly suggests that the viewer is forced to pay his
attention more either on the HUD events which is the flight symbology, or the outside view. The pilot is not able
to maintain the balance between the two events which are equally important. He or she is presented with
differential brightness, variation of which can be significant to the extent that it distracts the pilot significantly
such that his performance in observing HUD and the outside events simultaneously is affected.

Keywords: Head-up display, Attention Capture, Tunneling, Situational Awareness, Clutter, Ambient Brightness,
Display Brightness, Differential Brightness, Brightness Non-uniformity, Contrast ratio, Beam Combiner

I. INTRODUCTION
The traditional aircraft cockpit contains a host of display systems with vital flight information like
airspeed, artificial horizon, navigation, radar display, altitude, angle of attack, etc. displayed in different formats
on separate instruments panels in the cockpit display suite. Such kind of cockpit puts forward the requirement of
pilot needing to look around at various instruments panels to know any of the flight parameters forcing him/her
to split his/her attention between the outside world and the cockpit displays. These displays along with the
conventional Head down displays (HDD) put stress on the pilot to cope with the continual eye adjustments due
to the requirement of varying focus, changing brightness etc. required which may result in longer reaction times,
fatigue and reduced efficiency. The performance comparison of the two situations: while the pilot is flying using
HDD and while flying with a head-up display (HUD) yields that indeed use of HUD is a better choice [1]. This
is not only of concern in modern passenger aircraft where the lives of hundreds of passengers depend on pilot
decisions but also for modern fighter aircrafts also where the pilot cannot afford to divert his attention from the

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target ahead. To enable the pilot view all the crucial flight information on these displays without having to divert
attention, the display systems like HUD and helmet-mounted display (HMD) have been developed which
presents the information and data collected from various instruments/panels to the pilot in a defined format.
While the HUDs are of immense use for critical tasks like bad weather take-off and landing, they are also of
immense value in other precision flying tasks such as cargo drops in remote areas, formation flying, refuelling
for transport aircraft etc. Its use primarily in a fighter aircraft is to provide target data for weapons, guidance and
flight data in order to facilitate the pilot maintain complete awareness of the situation with respect to all the
flight-critical parameters without having to look towards other panels and instrument cluster present in the
cockpit[2-7].

II. SITUATIONAL AWARENESS ENHANCEMENT DUE TO HUD
The HUD displays the composite view of the symbology or Forward Looking Infra-Red (FLIR) sensor
video superimposed on the outside scene to the pilot’s forward vision. This composite view is collimated, i.e., it
is focused at infinity enabling pilot see the image as overlaid and fixed on the outside world scene. As the image
is focused at infinity, the collimated symbology appears to be in exact register and in the same focal plane as the
outside view. This feature allows a fighter pilot aim weapons onto the targets.
There are several factors of the HUD symbology which are very important to maximize the benefits of
HUD. This includes redundancy of symbol characteristics, minimum clutter, brightness level adjustment for
optimum contrast under all the possible illumination surroundings, update rate for usability of the display in
dynamic situation, flicker and symbols lag, basic analogue and conformal symbol set, symbology design symbol
set, symbology hierarchy such that higher priority symbology clearly and unambiguously overwrites lower
priority symbols, outside world view to correlate with the real world as seen by the pilot etc. to study the effect
of each of these a modeling approach is adopted where a simulated environment is created to study the
individual effect of these parameters [8-10]. The optimization of brightness and contrast ratio plays a major role
in maximizing the benefits of HUD. Another aspect of brightness parameters regarded as brightness non-
uniformity of the HUD display play a significant role in determining HUD performance. It refers to the variation
of the symbol brightness throughout the HUD display area. The effect of these three parameters on attention
capture and the tunneling effect due to HUD have been covered extensively in this work[11].

III. EFFECT OF HUMAN FACTORS ASPECTS OF HUD USAGE ON ATTENTION
CAPTURE AND TUNNELING
Information processing and capture by the pilot in HUDs is significantly dependent on attention as pilots
can distinguish events in a situation in a better way if their attention is focused on the event area. Now since
attention is a resource with limited capacity, under some situations, a single task may capture all of the pilot’s
attention. If the pilot focuses attention in this way then he/she will filter out unattended information and may not
notice vital information. In context of the HUD, the attention capture refers to the inefficient attentional
switching from HUD to primary task of flying under varying conditions resulting in overlooked outside targets,
late responses to external events, irregular switching times to switch from HUD to external visual processing
and vice-versa. Attentional tunneling is only apparent when performance degradation is established as a function
of eccentricity resulting in difficulty in switching attention between objects. This indicates that the forceful
nature of the HUD image restrains the recognition of other significant events which might lead to uncertain
situations [2-7, 12-15].
The collimated symbology is optically very close to infinity which appears to be much closer to the
.
pilot than the outside view. This occurs when the scene is viewed directly or is relayed as a thermal image on
the HUD. When it is relayed as thermal image with symbology superimposed on it, the symbol and scene image
are identical in colour and focal distance. The perplexing observation is a consequence of the brightness and
contrast of the display and the manner they move over the scene, which are powerful cues that indicate the
relative proximity of the symbols. This perplexity is further increased as the information is predicted to be at
less distance than the outside view seen through HUD. Studies show that the pilots find it hard to understand the
two forms of image information simultaneously and thus are forced to switch attention between the two. Such
kind of attentional focusing is experienced not only during reading the numbers and digits but is also
experienced with conformal symbology. This also results in reduction of probability of seeing a hazard in poor
visibility conditions or at night. However, the collimated HUD imagery does not induce the eyes to adopt a
comparably distant focus even during the day mode operation. The very existence of the virtual imagery, and the
HUD beam combiner along with its frame, may also make the eyes to focus inappropriately between the near
and far states which may make pilot misapprehend the size of real world things and their distance[10, 16].

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IV. RELEVANCE OF HUD DISPLAY BRIGHTNESS, CONTRAST &NON-UNIFORM
BRIGHTNESS ON ATTENTION CAPTURE AND TUNNELING EFFECT

The HUD functionality is defined in two modes namely day and night modes corresponding to the pure
stroke mode of symbology and the stroke-in-raster vertical flyback mode respectively. The stroke mode
symbology is utilized during the day mode operation for obtaining maximum brightness with the dynamic
contrast range. The absolute brightness range on the display device is usually four orders of magnitude, i.e.,
10,000:1, to span the ambient brightness conditions ranging from bright sunlight to very low light conditions.
The factors affecting the visibility of HUD display as well as the outside scene on and through the HUD beam
combiners (BC) are HUD image brightness corresponding to the display symbology, FLIR videos or the outside
view reflected or emitted from the BC, sunlight and skylight scattered diffusely from the screen that combines
with HUD image and lowers feature contrast, sunlight and skylight reflected from outer glass surfaces that
causes shine reducing the contrast further, and the ambient light that decides the adjustment of the pilot’s eye.
The symbol brightness needed for adequate contrast against the HUD display background through BC
glasses is one of the main factors that affect HUD image readability. Literature suggests that an image contrast
of at least 20% contrast is required to see the image even against the bright clouds. The intra-ocular glare can
reduce the apparent contrast of HUD imagery substantially when the background is very bright occurring in
clear air when flying towards the sun or when the sun is within about 30° of the aircraft nose. On the other hand,
during night or twilight conditions, brightness of the display must be reduced considerably to let the pilot
maintain a minimum photopic adjustment. Automatic brightness adjustment systems can be employed to match
display brightness to the prevailing ambient lighting conditions.
The display, the ambient and the relative brightness between the two contributes significantly in
affecting the perception of the image presented on a HUD during daylight. Although the eye acclimatizes to the
brightness of the HUD display, a wider, brighter skylight can have an overriding effect. The range of brightness
variations that carry the display information must fall within the distinctive dynamic range of spatial brightness
variations that the pilot can differentiate, i.e., between subjective black and subjective white which is dictated by
a combination of the ambient and the display field.
The brightness non-uniformity of HUD display occurs due to several factors namely non-uniformity of
CRT phosphor, improper functioning of video and blanking section, improper coating on HUD folding mirror
responsible for folding the CRT image towards the BC, improper coating on the BC glasses resulting in non-
uniform and wavelength variable reflections, and improper overlapping of primary and secondary BC
glasses[10, 17].

V. EXPERIMENT

It has been observed that the display brightness plays a key role in effecting pilot’s event detection
capability. A set of experiment was conducted under varying ambient brightness conditions to understand the
effects of ambient brightness, contrast ratio and varying HUD brightness non-uniformity on the capability of
pilot to detect changes in events taking place on HUD and outside environment.
By ambient brightness we mean available light in an environment. The contrast ratio is a property of a
display system, we define contrast ratio as:

ℎ + ℎ
=
ℎ

In the experimental setup (figure 1 and 2), contrast ratio ranges from 1 to 18 were simulated. The brightness
non-uniformity for four different cases across the HUD combiners i.e. 1:1, 1:1.15, 1:1.29, and 1:1.47 were taken
into consideration under ambient brightness ranges from 20cd/m2 to 40,000cd/m2. Experiment was focused on
how user would respond to events on the HUD display and in the environment, that is, the outside scene when
attention was modulated through ambient, HUD display brightness and brightness non-uniformity, thus varying
the contrast ratio across the HUD display area.

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Figure 1: Experimental Set Up

The experimental setup consisted of head-up display system mounted on Cockpit mockup display simulator
along with seat adjustment mechanism, HUD signal simulator, Projector setup coupled with the background
simulation PC, Light source, Light diffuser, Photometer, and a TV monitor. A light source capable of simulating
light brightness of more than 85,000cd/m2 along with the light diffuser were used to simulate ambient lighting
for generating brightness from 20cd/m2 to 40,000cd/m2.

Figure 2: HUD symbology as seen through the beam combiner

The experiments were carried out with the participation of 22 people comprising of 12 males and 10
females in the age group of 24 to 32 years, all with engineering/technology subjects as their academic
background. The participants were asked to carry out two tasks: First, to report any detected changes in the
right, middle and the left portion of the upper and the lower half of the HUD display seen on the HUD BC;
Second, to report any changes observed in the outside view and scene. Though the option of adaptive brightness
control was available but the same was disabled to make the control manual so that the participant could be
tested on single contrast setting. The participants first participated in training session on the setup to familiarize
them with the task and the setup. Further, the experiments were conducted in the morning and the afternoon
sessions to eliminate the effect of the fatigue factor on the experimental results. During the experimentation, sort
of changes that were used were: shape/objects/character that appeared and disappeared from the background
image as well as on the HUD display, changed status and location with changes taking place between scenes.

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The participants were also asked to answer a set of questions based on their observation during the course of
experimentation which not only required them to make spatial and directional judgments but also the tally of the
observed changes and objects. Each symbology page scene limited to two questions with next symbology page
automatically displayed once the set of question based on one set were answered which amounted to participants
required to tell changes observed by them in a scene followed by responding to the associated query. The
participants were not told about the brightness non-uniformity of the HUD display. One set of result was totally
out of the trend hence not considered [18].
The results obtained are as shown in the figures given below.

Figure 3: Comparison of HUD event detection with Outside event detection at ambient brightness
40,000cd/m2:Effects of Display Brightness Non uniformity

Figure 4: Comparison of HUD event detection with Outside event detection at ambient brightness 30,000cd/m2:
Effects of Display Brightness Non uniformity

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405




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Figure 9: Comparison of HUD event detection with Outside event detection at ambient brightness 500cd/m2:


407



Figure 12: Comparison of HUD event detection with Outsideevent detection at ambient brightness 20cd/m2:

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VI. RESULTS AND DISCUSSIONS
One area in which HUDs are particularly useful is when visibility of the outside environment is poor, such
as rain or fog, because high-quality HUD images, typically collimated in the aviation domain, make the pilot
focus further out, which reduces visual accommodation problems. But there are human factor issues of attention
capture and tunneling with the use of head-up displays. The pilots using HUDs experience inefficient attentional
switching from HUDto primary task and vice-versa. [15, 19-21]

The study related to the problem of attention capture and tunneling effect due to absolute, relative and non-
uniformity of display brightness was undertaken as it was observed during the course of HUD testing that
though specification for HUD display brightness non-uniformity allowed is 1: 1.5, but it resulted in definite
amount of stress on the pilot while viewing bothHUD displayas well as outside view simultaneously. Hence it
was hypothesized that there is a definite relation between the HUD display brightness, contrast ratio against the
variable ambient brightness, and brightness non-uniformity with the attentional tunneling due to the usage of
HUD. During testing of various sets of HUD units, the display brightness non-uniformity was observed on the
HUD display primarily due to inaccuracies of coating made on the BC and folding mirror glasses though
brightness non-uniformity of CRT also contributed but was negligible. The testing of these sets revealed that
these inaccuracies causes variation of the contrast ratio across the HUD display area resulting in distraction to
the pilot to the extent that he or she finds it difficult to notice changes and distinguish between symbols and
objects in the display area apart from facing difficulty in noticing the outside events. Thebrightness non-
uniformity will not only cause differential brightness of symbols across the HUD display area but it will also
result in variable transmission through the HUD. The resultant differential contrast over a smaller area of HUD
display forces the pilot to divert his attention frequently between symbols which may result in missing the
outside events. It may make the pilot lost in the HUD display events. The reason is obvious, by the time pilot
becomes accustomed to one brightness level, he needs to focus his attention towards other display area on other
symbols which may be lesser or greater in the brightness levels to the one which he or she has already scanned.
The experiments conducted to simulate these conditions under variable ambient lighting conditions confirmed
the hypothesis and quite interesting results were obtained. The experiments were confined only to the day mode
of stroke symbology. Then following table summarizes the results shown in the graphs in figures 3 to 12.

Table 1: Summary of the experimental results

Ambient brightness Contrast ratio HUD event Outside event Variation in HUD Variation in outside
(fL) range due to HUD detection success detection success event detection event detection
brightness non- rate range rate range (%) range due to HUD range due to HUD
uniformity for (%) brightness non- brightness non-
single brightness uniformity (%) uniformity (%)
setting
40,000 1.0425 - 1.2375 54- 64 99 - 97 0 -4 0 -1
30,000 1.034- 1.3166 54 - 66 98 - 96 0 -3 0-1
20,000 1.051 - 1.475 54 - 73 98 - 95 0- 6 0-2
10,000 1.034 - 1.95 54 - 81 98 - 95 0-7 0-2
5,000 1.068 - 2.90 55 - 90 98 - 92 0-6 0-3
1,000 1.068 - 5.5 55 - 95 98 - 83 0-8 0-5
500 1.136 - 10.00 59- 98 98 - 75 0-8 0-5
100 1.068 - 10.00 56- 98 98 - 73 0-9 0-6
50 1.136 - 15.2 59 - 99 95 - 69 0-4 0-6
20 1.70 - 18.0 76 - 99 95 - 64 1-8 1-7

As discussed above, brightness non-uniformity has the effect of differential brightness across the HUD
display screen which effectively results in differential contrast ratio across the display screen resultant because
of the patches of non-uniformity in the HUD brightness. This result in good brightness at some places, and
grading of reduced brightness on HUD combiner at other places resulting in diverted attention of the viewer on
the HUD as well as on the outside events. This causes significant amount of attention capture and tunneled
attention. The brightness non-uniformity was simulated with a BC which had definite patches of brightness non-
uniformity areas corresponding to resultant CR values of 1:1, 1: 1.15, 1: 1.29 and 1:1.47 with viewer presented
with the display across these patches and with outside scene also changed throughout on the HUD background.

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The figures of comparative data presented in table suggests that under the ambient lighting levels of 30,000
cd/m2 and 40,000cd/m2 which are quite brightness levels, the success rate of HUD event detection varied from
54% to 66% as the CR across the HUD display screen was varied from 1.034 to 1.3166. This variation was the
result of brightness control of the HUD display as well as due to contribution of beam combiner in causing non-
uniform brightness of the HUD display. The outside scene detection varied very little from 99% to 96% for the
same CR range. These outcomes were the result of lower contrast. The reason for lower CR is very high range
of ambient brightness which limited the display contrast range.Lower and differential contrast ratio resulted in
lower HUD event detection while high ambient brightness was the reason for excellent outside scene detection
which eased the viewer’s view. Here, for the same HUD brightness setting, the available symbol brightness was
reduced over the scattered area of HUDBCdue to the brightness non-uniformity which resulted in variation of
success rate of HUD event and the outside event detection across the HUD display screen. For example, when
ambient brightness was 40,000cd/m2, at display brightness corresponding to CR of 1.2375, the CR at other
places were 1.2065, 1.184, and 1.161 resulting in HUD and the outside event detection success percentage of 64
and 97, 63 and 97, 61 and 97, and 60 and 98 respectively. For same brightness setting, the differential brightness
across the HUD display screen caused variation in HUD event detection ranging from 0% to 4% though there
was no significant variation (0% to 1%) in the outside event detection. These results were due to the fact that the
ambient brightness was high and CR was low. Due to this, viewer could not focus properly on HUD events
while his attention on outside events was significant as the ambient brightness levels were very high.
The experiment carried with ambient lighting of 20,000cd/m2, 10,000cd/m2, and 5,000cd/m2 resulted in
HUD event detectionranging from 54% to 90% as the CR was varied from 1.034to 2.90 obtained by varying
HUD display brightness and due to contribution of BC in causing non-uniform brightness. However, again the
outside scene detection changed very little with success percentage varying from 98% to 92%. Here also, the
higher ambient brightness limited the contrast range, but the CR was good enough to cause marked
improvement in the HUD event detection. Dueto higher ambient brightness and relatively lower CR, outside
scene detection success rate was again on the higher side as expected. For same brightness setting, the
differential brightness across the HUD screen caused variation in HUD event detection ranging from 0%to 7%
whereas there was no significant variation (0% to 3%) in the outside event detection. This indicated that the
brightness non-uniformity of display was more noticeable as CR improved due to reduced ambient brightness
varying the degree of attention capture on the HUD events across the HUD screen. The reason for larger
variation in HUD event detection success rate was due to the fact that the viewer got distracted due to large
variation of the HUD display brightness and the resultant variation of the CR across the display. Theygot more
engaged in outside events as the background was well lit thereby not getting affected by the variable brightness
of the HUD display as far as outside scene detection is concerned. It was also noticed that as the ambient
brightness went down, the optimum values of CR helped the viewer to focus on HUD display as well as on the
outside scene simultaneously.
During the experimentation carried with reduced ambient lighting levels of 1,000cd/m2, 500cd/m2,
100cd/m2, HUD event detection varied from 55% to 98% with CR varying from 1.068 to 10.00 which resulted
due to manual variation of HUD brightness and with contribution from the HUD brightness non-uniformity. In
this case, outside scene detection changed significantly ranging 98% to 73%. It was noticed that the reduced
ambient brightness improved contrast ratio, which on one hand helped the viewer to focus on HUD events more
effectively but on the other hand he lost his attention on the outside scene detection resulting in reduced outside
scene detection. Here the percentage change in the HUD event detection due to the differential brightness across
the HUD display screenwas in the range from 0% to 9% and from 0% to 6% for the outside event detection. It
was significantly more than what we have observed for higher ambient lighting conditions. This again was due
to the fact that the under reduced ambient lighting, the differential brightness was more noticeable and hence
distracted the viewer and hence more tunneling.
For ambient lighting of 50cd/m2, the event detection in HUD display varied from 59% to 99% as the CR
was varied from 1.136 to 15.2 resultant due to manual variation of HUD display brightness and due to
contribution of BC in causing non-uniform brightness. The outside scene detection in this case varied from 95%
to 69%. The percentage change due to the differential brightness across the HUD display screen for the same
brightness setting in the detection success on HUD event was from 0% to 4% and 0% to 6% for the outside
event detection. The difference in HUD event detection success rate at single brightness setting at various places
of HUD display was minimal except for the very low CR value where it was 4% for obvious reason of
noticeable brightness variation across the screen due to lower HUD display brightness. The bigger difference of
0% to 6% in outside scene detection was due to the fact that higher CR across the display screen under low
ambient lighting improved the HUD event detection at the cost of reduced outside event detection. The viewer
got focused on the HUD display as the CR was very high for lower ambient brightness conditions.
Forambient lighting of 20cd/m2, the percentage change due to the differential brightness across the HUD
display screen for the same brightness setting in the HUD event detection was 1 - 8% and 1 - 7%for the outside
event detection. Here it is quite apparent that since the ambient brightness was very low, smaller variation in

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HUD display brightness drastically changed the CR hence the effect on HUD event detection also changes
rapidly. This variation became lesser at higher CR values as then the CR was high enough at all the locations.
Variation in outside scene detection was more at higher CR values as at those values in range of 12 or above, the
brightness non-uniformity caused large changes in CR variation throughout the screen making outside scene
detection varying in large range. It clearly indicated that the lesser the ambient brightness the bigger is the
impact of the non-uniformity in the HUD brightness on the both the events indicating severe effect of attention
capture. This also happened as the display also got reflected from the BC glasses as the outside scene beyond
beam combiners was with very low lighting which not only reduced the visibility through BC glasses but also
added to the reduction in outside scene event detection.

CONCLUSION
The above results suggest that the HUD symbol brightness, contrast ratio and brightness non-uniformity of
HUD display plays a definite role in attention capture and tunneling due to HUD usage. The pilot tends to pay
more attention towards the HUD display and slightly loses focus on the outside scene when the contrast ratio is
more than 4.0. The high contrasts capture most of the pilot’s attention which reduces optimal allocation of focus
on both, HUD as well as the outside events. This phenomenon happens when the ambient lighting is lesser.
When the contrast ratio is less than 1.5 and the ambient is very bright, the pilot gets engaged more towards the
outside events as brighter ambient grabs most of his attention though HUD event detection improves as he
approaches contrast ratio of 1.5. For the same CR under less bright ambient lighting, pilot gets better
distribution of his focus to both events than during the brighter ambient conditions. Though, in the darker
ambient, the reflection from the BC glasses adds to the confusion and further deteriorates the attention capture
distribution. The best tradeoff performance is obtained at a contrast ratio of 2.5 -4 which produces the optimum
attention capture distribution at all the ambient brightness levels though the absolute brightness level of the
HUD display and the ambient lighting significantly affects the attention capture. Brighter HUD display makes
the salience of the changes against the background which in turn could distract the pilot and capture their
attention and, therefore, increase response times to aircraft events. Alternately, it could be said that high contrast
ratio would benefit display event more at the cost of aircraft eventdetection when compared to the case of lower
contrast ratios. Therefore, a midlevel contrast ratio of 2.5-4 gave the best results.
The non-uniformity of the HUD display results in differential brightness across the HUD display screen
which has the effect of the variable contrast ratio across the screen. This further adds to the confusion as the
pilot has now to look differential brightness over a smaller area at the same time. This has been verified in the
study results. At higher ambient brightness, the non-uniform brightness causes more degradation in the HUD
event detection as compared to the outside events. Atlow ambient lighting conditions, the degradation in both
the events is significant with HUD event detection variation pattern being more. At lower brightness, the
prominence of the symbol brightness variation significantly forces the pilot to get engaged in HUD event sand
in the process he loses focus on the outside events significantly.

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