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

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976– 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME

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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, [email protected]

ABSTRACTThe prime role of head-up displays (HUDs) is to provide primary flight, navigation, aircraft and guidanceinformation to the pilot in his forward field of view on see-through glasses with optical filter properties knownas beam combiners (BC) which facilitates the pilot in excellent control of an aircraft activities through theconcurrent scan of instrument data as well as the outside scene. Even though the HUDs are known to improvethe flight performance, there are perceptual and cognitive issues associated with its use. This paper puts forwardthe results of an experimental study carried on human aspects in form of attention capture and tunneling due toHUD use under the environmental conditions where ambient brightness is changing and the HUD display screenpresents 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 orcognitive capture as established in this study and also established in studies carried worldwide. HUDs maycause decrease in pilot’s situation awareness (SA) in tasks that require constant monitoring of information in theenvironment which may demand split second decisions at times. It has been established through the studycarried out in this work that there is a distinct relation of HUD display brightness, contrast ratio (CR) and thenon-uniformity in HUD display brightness with the attention capture and tunneling. The study conducted basedon participant’s response to varying contrast ratios on HUD resulting due to the non-uniform brightness of HUDdisplay under the simulated conditions shows that the non-uniformity in the HUD display brightness causes thetunneling effect as evident from the study results which clearly suggests that the viewer is forced to pay hisattention more either on the HUD events which is the flight symbology, or the outside view. The pilot is not ableto maintain the balance between the two events which are equally important. He or she is presented withdifferential brightness, variation of which can be significant to the extent that it distracts the pilot significantlysuch 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. INTRODUCTIONThe 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 formatson separate instruments panels in the cockpit display suite. Such kind of cockpit puts forward the requirement ofpilot needing to look around at various instruments panels to know any of the flight parameters forcing him/herto split his/her attention between the outside world and the cockpit displays. These displays along with theconventional Head down displays (HDD) put stress on the pilot to cope with the continual eye adjustments dueto 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 usingHDD and while flying with a head-up display (HUD) yields that indeed use of HUD is a better choice [1]. Thisis not only of concern in modern passenger aircraft where the lives of hundreds of passengers depend on pilotdecisions but also for modern fighter aircrafts also where the pilot cannot afford to divert his attention from the

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ISSN 0976 – 6464(Print)ISSN 0976 – 6472(Online)Volume 3, Issue 2, July- September (2012), pp. 400-412© IAEME: www.iaeme.com/ijecet.htmlJournal Impact Factor (2012): 3.5930 (Calculated by GISI)www.jifactor.com

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target ahead. To enable the pilot view all the crucial flight information on these displays without having to divertattention, the display systems like HUD and helmet-mounted display (HMD) have been developed whichpresents 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 ofimmense value in other precision flying tasks such as cargo drops in remote areas, formation flying, refuellingfor transport aircraft etc. Its use primarily in a fighter aircraft is to provide target data for weapons, guidance andflight data in order to facilitate the pilot maintain complete awareness of the situation with respect to all theflight-critical parameters without having to look towards other panels and instrument cluster present in thecockpit[2-7].

II. SITUATIONAL AWARENESS ENHANCEMENT DUE TO HUDThe 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., itis focused at infinity enabling pilot see the image as overlaid and fixed on the outside world scene. As the imageis focused at infinity, the collimated symbology appears to be in exact register and in the same focal plane as theoutside 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 ofHUD. This includes redundancy of symbol characteristics, minimum clutter, brightness level adjustment foroptimum contrast under all the possible illumination surroundings, update rate for usability of the display indynamic situation, flicker and symbols lag, basic analogue and conformal symbol set, symbology design symbolset, symbology hierarchy such that higher priority symbology clearly and unambiguously overwrites lowerpriority symbols, outside world view to correlate with the real world as seen by the pilot etc. to study the effectof each of these a modeling approach is adopted where a simulated environment is created to study theindividual effect of these parameters [8-10]. The optimization of brightness and contrast ratio plays a major rolein 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 variationof the symbol brightness throughout the HUD display area. The effect of these three parameters on attentioncapture 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 ATTENTIONCAPTURE AND TUNNELING

Information processing and capture by the pilot in HUDs is significantly dependent on attention as pilotscan distinguish events in a situation in a better way if their attention is focused on the event area. Now sinceattention is a resource with limited capacity, under some situations, a single task may capture all of the pilot’sattention. If the pilot focuses attention in this way then he/she will filter out unattended information and may notnotice vital information. In context of the HUD, the attention capture refers to the inefficient attentionalswitching 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 processingand vice-versa. Attentional tunneling is only apparent when performance degradation is established as a functionof eccentricity resulting in difficulty in switching attention between objects. This indicates that the forcefulnature of the HUD image restrains the recognition of other significant events which might lead to uncertainsituations [2-7, 12-15].

The collimated symbology is optically very close to infinity which appears to be much closer to thepilot than the outside view. This occurs when the scene is viewed directly or is relayed as a thermal image onthe HUD. When it is relayed as thermal image with symbology superimposed on it, the symbol and scene imageare identical in colour and focal distance. The perplexing observation is a consequence of the brightness andcontrast of the display and the manner they move over the scene, which are powerful cues that indicate therelative proximity of the symbols. This perplexity is further increased as the information is predicted to be atless distance than the outside view seen through HUD. Studies show that the pilots find it hard to understand thetwo forms of image information simultaneously and thus are forced to switch attention between the two. Suchkind of attentional focusing is experienced not only during reading the numbers and digits but is alsoexperienced with conformal symbology. This also results in reduction of probability of seeing a hazard in poorvisibility conditions or at night. However, the collimated HUD imagery does not induce the eyes to adopt acomparably distant focus even during the day mode operation. The very existence of the virtual imagery, and theHUD beam combiner along with its frame, may also make the eyes to focus inappropriately between the nearand 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-UNIFORMBRIGHTNESS ON ATTENTION CAPTURE AND TUNNELING EFFECT

The HUD functionality is defined in two modes namely day and night modes corresponding to the purestroke mode of symbology and the stroke-in-raster vertical flyback mode respectively. The stroke modesymbology is utilized during the day mode operation for obtaining maximum brightness with the dynamiccontrast 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 beamcombiners (BC) are HUD image brightness corresponding to the display symbology, FLIR videos or the outsideview reflected or emitted from the BC, sunlight and skylight scattered diffusely from the screen that combineswith HUD image and lowers feature contrast, sunlight and skylight reflected from outer glass surfaces thatcauses 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 BCglasses is one of the main factors that affect HUD image readability. Literature suggests that an image contrastof at least 20% contrast is required to see the image even against the bright clouds. The intra-ocular glare canreduce the apparent contrast of HUD imagery substantially when the background is very bright occurring inclear 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 pilotmaintain a minimum photopic adjustment. Automatic brightness adjustment systems can be employed to matchdisplay brightness to the prevailing ambient lighting conditions.

The display, the ambient and the relative brightness between the two contributes significantly inaffecting the perception of the image presented on a HUD during daylight. Although the eye acclimatizes to thebrightness of the HUD display, a wider, brighter skylight can have an overriding effect. The range of brightnessvariations that carry the display information must fall within the distinctive dynamic range of spatial brightnessvariations that the pilot can differentiate, i.e., between subjective black and subjective white which is dictated bya combination of the ambient and the display field.

The brightness non-uniformity of HUD display occurs due to several factors namely non-uniformity ofCRT phosphor, improper functioning of video and blanking section, improper coating on HUD folding mirrorresponsible 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 BCglasses[10, 17].

V. EXPERIMENT

It has been observed that the display brightness plays a key role in effecting pilot’s event detectioncapability. A set of experiment was conducted under varying ambient brightness conditions to understand theeffects of ambient brightness, contrast ratio and varying HUD brightness non-uniformity on the capability ofpilot 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 adisplay system, we define contrast ratio as:

= ℎ + ℎℎIn 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 takeninto consideration under ambient brightness ranges from 20cd/m2 to 40,000cd/m2. Experiment was focused onhow user would respond to events on the HUD display and in the environment, that is, the outside scene whenattention was modulated through ambient, HUD display brightness and brightness non-uniformity, thus varyingthe 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 simulatoralong with seat adjustment mechanism, HUD signal simulator, Projector setup coupled with the backgroundsimulation PC, Light source, Light diffuser, Photometer, and a TV monitor. A light source capable of simulatinglight brightness of more than 85,000cd/m2 along with the light diffuser were used to simulate ambient lightingfor 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 10females in the age group of 24 to 32 years, all with engineering/technology subjects as their academicbackground. The participants were asked to carry out two tasks: First, to report any detected changes in theright, 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 brightnesscontrol was available but the same was disabled to make the control manual so that the participant could betested on single contrast setting. The participants first participated in training session on the setup to familiarizethem with the task and the setup. Further, the experiments were conducted in the morning and the afternoonsessions to eliminate the effect of the fatigue factor on the experimental results. During the experimentation, sortof changes that were used were: shape/objects/character that appeared and disappeared from the backgroundimage 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 ofexperimentation which not only required them to make spatial and directional judgments but also the tally of theobserved changes and objects. Each symbology page scene limited to two questions with next symbology pageautomatically displayed once the set of question based on one set were answered which amounted to participantsrequired to tell changes observed by them in a scene followed by responding to the associated query. Theparticipants were not told about the brightness non-uniformity of the HUD display. One set of result was totallyout 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 brightness40,000cd/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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Figure 9: Comparison of HUD event detection with Outside event detection at ambient brightness 500cd/m2:Effects of Display Brightness Non uniformity



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Figure 12: Comparison of HUD event detection with Outsideevent detection at ambient brightness 20cd/m2:Effects of Display Brightness Non uniformity


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VI. RESULTS AND DISCUSSIONSOne 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 pilotfocus further out, which reduces visual accommodation problems. But there are human factor issues of attentioncapture and tunneling with the use of head-up displays. The pilots using HUDs experience inefficient attentionalswitching 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 thatthough specification for HUD display brightness non-uniformity allowed is 1: 1.5, but it resulted in definiteamount of stress on the pilot while viewing bothHUD displayas well as outside view simultaneously. Hence itwas hypothesized that there is a definite relation between the HUD display brightness, contrast ratio against thevariable ambient brightness, and brightness non-uniformity with the attentional tunneling due to the usage ofHUD. During testing of various sets of HUD units, the display brightness non-uniformity was observed on theHUD display primarily due to inaccuracies of coating made on the BC and folding mirror glasses thoughbrightness non-uniformity of CRT also contributed but was negligible. The testing of these sets revealed thatthese inaccuracies causes variation of the contrast ratio across the HUD display area resulting in distraction tothe pilot to the extent that he or she finds it difficult to notice changes and distinguish between symbols andobjects 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 alsoresult in variable transmission through the HUD. The resultant differential contrast over a smaller area of HUDdisplay forces the pilot to divert his attention frequently between symbols which may result in missing theoutside events. It may make the pilot lost in the HUD display events. The reason is obvious, by the time pilotbecomes accustomed to one brightness level, he needs to focus his attention towards other display area on othersymbols 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 confirmedthe hypothesis and quite interesting results were obtained. The experiments were confined only to the day modeof 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(fL)

Contrast ratiorange due to HUDbrightness non-uniformity forsingle brightnesssetting

HUD eventdetection successrate range(%)

Outside eventdetection successrate range (%)

Variation in HUDevent detectionrange due to HUDbrightness non-uniformity (%)

Variation in outsideevent detectionrange due to HUDbrightness non-uniformity (%)

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 HUDdisplay screen which effectively results in differential contrast ratio across the display screen resultant becauseof the patches of non-uniformity in the HUD brightness. This result in good brightness at some places, andgrading of reduced brightness on HUD combiner at other places resulting in diverted attention of the viewer onthe HUD as well as on the outside events. This causes significant amount of attention capture and tunneledattention. 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 presentedwith 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 30,000cd/m2 and 40,000cd/m2 which are quite brightness levels, the success rate of HUD event detection varied from54% to 66% as the CR across the HUD display screen was varied from 1.034 to 1.3166. This variation was theresult 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 thesame CR range. These outcomes were the result of lower contrast. The reason for lower CR is very high rangeof ambient brightness which limited the display contrast range.Lower and differential contrast ratio resulted inlower HUD event detection while high ambient brightness was the reason for excellent outside scene detectionwhich eased the viewer’s view. Here, for the same HUD brightness setting, the available symbol brightness wasreduced over the scattered area of HUDBCdue to the brightness non-uniformity which resulted in variation ofsuccess rate of HUD event and the outside event detection across the HUD display screen. For example, whenambient brightness was 40,000cd/m2, at display brightness corresponding to CR of 1.2375, the CR at otherplaces were 1.2065, 1.184, and 1.161 resulting in HUD and the outside event detection success percentage of 64and 97, 63 and 97, 61 and 97, and 60 and 98 respectively. For same brightness setting, the differential brightnessacross the HUD display screen caused variation in HUD event detection ranging from 0% to 4% though therewas no significant variation (0% to 1%) in the outside event detection. These results were due to the fact that theambient brightness was high and CR was low. Due to this, viewer could not focus properly on HUD eventswhile 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 inHUD event detectionranging from 54% to 90% as the CR was varied from 1.034to 2.90 obtained by varyingHUD display brightness and due to contribution of BC in causing non-uniform brightness. However, again theoutside scene detection changed very little with success percentage varying from 98% to 92%. Here also, thehigher ambient brightness limited the contrast range, but the CR was good enough to cause markedimprovement in the HUD event detection. Dueto higher ambient brightness and relatively lower CR, outsidescene detection success rate was again on the higher side as expected. For same brightness setting, thedifferential 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 thebrightness non-uniformity of display was more noticeable as CR improved due to reduced ambient brightnessvarying the degree of attention capture on the HUD events across the HUD screen. The reason for largervariation in HUD event detection success rate was due to the fact that the viewer got distracted due to largevariation of the HUD display brightness and the resultant variation of the CR across the display. Theygot moreengaged in outside events as the background was well lit thereby not getting affected by the variable brightnessof the HUD display as far as outside scene detection is concerned. It was also noticed that as the ambientbrightness went down, the optimum values of CR helped the viewer to focus on HUD display as well as on theoutside 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 resulteddue to manual variation of HUD brightness and with contribution from the HUD brightness non-uniformity. Inthis case, outside scene detection changed significantly ranging 98% to 73%. It was noticed that the reducedambient brightness improved contrast ratio, which on one hand helped the viewer to focus on HUD events moreeffectively but on the other hand he lost his attention on the outside scene detection resulting in reduced outsidescene detection. Here the percentage change in the HUD event detection due to the differential brightness acrossthe HUD display screenwas in the range from 0% to 9% and from 0% to 6% for the outside event detection. Itwas significantly more than what we have observed for higher ambient lighting conditions. This again was dueto the fact that the under reduced ambient lighting, the differential brightness was more noticeable and hencedistracted 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 CRwas varied from 1.136 to 15.2 resultant due to manual variation of HUD display brightness and due tocontribution 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 samebrightness setting in the detection success on HUD event was from 0% to 4% and 0% to 6% for the outsideevent detection. The difference in HUD event detection success rate at single brightness setting at various placesof HUD display was minimal except for the very low CR value where it was 4% for obvious reason ofnoticeable brightness variation across the screen due to lower HUD display brightness. The bigger difference of0% to 6% in outside scene detection was due to the fact that higher CR across the display screen under lowambient lighting improved the HUD event detection at the cost of reduced outside event detection. The viewergot 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 HUDdisplay screen for the same brightness setting in the HUD event detection was 1 - 8% and 1 - 7%for the outsideevent 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 changesrapidly. 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, thebrightness non-uniformity caused large changes in CR variation throughout the screen making outside scenedetection varying in large range. It clearly indicated that the lesser the ambient brightness the bigger is theimpact of the non-uniformity in the HUD brightness on the both the events indicating severe effect of attentioncapture. This also happened as the display also got reflected from the BC glasses as the outside scene beyondbeam combiners was with very low lighting which not only reduced the visibility through BC glasses but alsoadded to the reduction in outside scene event detection.

CONCLUSIONThe 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 paymore attention towards the HUD display and slightly loses focus on the outside scene when the contrast ratio ismore than 4.0. The high contrasts capture most of the pilot’s attention which reduces optimal allocation of focuson 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 theoutside events as brighter ambient grabs most of his attention though HUD event detection improves as heapproaches contrast ratio of 1.5. For the same CR under less bright ambient lighting, pilot gets betterdistribution of his focus to both events than during the brighter ambient conditions. Though, in the darkerambient, the reflection from the BC glasses adds to the confusion and further deteriorates the attention capturedistribution. The best tradeoff performance is obtained at a contrast ratio of 2.5 -4 which produces the optimumattention capture distribution at all the ambient brightness levels though the absolute brightness level of theHUD display and the ambient lighting significantly affects the attention capture. Brighter HUD display makesthe salience of the changes against the background which in turn could distract the pilot and capture theirattention and, therefore, increase response times to aircraft events. Alternately, it could be said that high contrastratio would benefit display event more at the cost of aircraft eventdetection when compared to the case of lowercontrast 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 screenwhich has the effect of the variable contrast ratio across the screen. This further adds to the confusion as thepilot has now to look differential brightness over a smaller area at the same time. This has been verified in thestudy results. At higher ambient brightness, the non-uniform brightness causes more degradation in the HUDevent detection as compared to the outside events. Atlow ambient lighting conditions, the degradation in boththe events is significant with HUD event detection variation pattern being more. At lower brightness, theprominence of the symbol brightness variation significantly forces the pilot to get engaged in HUD event sandin the process he loses focus on the outside events significantly.

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