HEUICOPITER ELECTRO-OPTICAL 53 STEM DISPLAY

The Effects of CRT Display Size, System Gamma-Function, and Terrain Type on_Pilots' Required• p e -n,-Piot

• spla minance

i-AaronLym'n/Rlchard M. ohnson.Ild Paul AGade

0 MANPOWER AND EDUCATIONAL SYSTEMS TECHNICAL AREA

U. S. ArmyResearch Institute for the Behavioral and Social Sciences

Marcah 1SS

iL " Approved for public release; distribution unlimited.

,"f/-i.- SO 013A'-'--A. k

U. S.ARMY RESEARCH INSTITUTEFOR THE BEHAVIORAL AND SOCIAL SCIENCESA Field Operating Agency under the Jurisdiction of the

Deputy Chief of Staff for Personnel

I FRANKLIN A. HARTJOSEPH ZEIDNER Colonel, US ArmyTechnical Director Commander

NOTICES

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AM. The findings in ;his report are not to be Construed as an Official Oallarsment Of the Army position,unless so designated by Other authoriiged doaepaie.

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1. REPORTF MmeRl 12 OTCEION NO. 3. RCCIPEYT-S CATALOG NUMBER

Technical Report 441 9 e 99 _____________

4. TITLE (and Subile) S. TYPE OF REPORT & PERIOD COVERED

* HELICOPTER ELECTRO-OTCLSSE IPA REQUIRE-MENTS: 1. THE EFFECTS OF CRT DISPLAY SIZE, _____________

SYSTEM GAMMA FUNCTION, AND TERRAIN TYPE ON PILOTS a. PERFORMING ONG. REPORT NMBNERREQUIRED DISPLAY LUMINANCE

7. AUTHOR(e) 8. CONTRACT OR GRANT "UMBER(&)

Aaron Hyman, Richard M. Johnson, and Paul A. Gade

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and Social Sciences (PERI-OK) 2Q162722A7655001 Eisenhower Avenue, Alexandria, VA 22333

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II. SUPPLEMENTARY NOTES

IS. KEY WORDS (Contiue cii reeres, aide it aaesary OWd 5dmiUIf by weock umbec)

Nap-of-the-earth (NOE) flightNight flyingNight vision

Twenty-four Army helicopter pilots viewed videotaped segments of low-level

additional ambient illumination, these pilots were asked to set cockpit moni-tor luminance at the lowest level that they judged would permit successfulflight control over the terrain being overflown. Each pilot adjusted

DD, W3 S'IOU O ? DV 6 ISMDSOETSUNCLASSIFIED

ISCCUMITY CLASSIFICATION OF TNIS PACE (11fer Daes Entura(,

mill

...... ......... , ...... ., . . - , .. . . . . . .. . . , , . _ 1 _ . . .4 "

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UNCLASSIFIEDMM V iMLAWitCrATION OF THIS PAG(kM. Data EAfm

20. luminance levels for eight different display conditions formed by thefactorial combination of display size, type of terrain, and object-luminanceto display-luminance transfer function (system gamma function). .Results showthat pilots used lower luminance settings when viewing the larger of the twodisplay sizes presented. They also used significantly lower luminance settingswhen viewing wooded terrain, with the system gamma function modified toprovide benhancedO contrast in the luminance range of interest, as againstan unmodified system gamma function. The pilots' subjective impressions agreedwith their measured settings. This report discusses the impact of theseresults on the specification of display requirements for a low-light-leveltelevision system for aiding night NOE flight.

MeC TABVw mounc ed

Justif ic .to- . . -

Dist. e l

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Ir O- ~

Tehinica Reprt 441

HELICOPTER ELECTRO-OPTICAL SYSTEM DISPLAYREQUIREMENTS: 1.

The Effects of CRT Display Size, System GammaFunction, and Terrain Type on Pilots' Required

Display Luminance

Aaron Hyman, Richard M. Johnson, and Paul A. Gade

Halim Ozkaptan, Team Chief

Submitted by:James D. Baker, Chief

MANPOWER AND EDUCATIONAL SYSTEMS TECHNICAL AREA

Approved by:

Edgar M. Johnson, Acting DirectorORGANIZATIONS AND SYSTEMSRESEARCH LABORATORY

U.S. ARMY RESEARCH INSTITUTE FOR THE BEHAVIORAL AND SOCIAL SCIENCES5001 Eisenhower Avenue, Alexandria, Virginia 22333

Off ice, Deputy Chief of Staff for PersonnelDepartment of the Army

March 19N0

Army Project Number Heicopter Display Requirements20162722AM6

Approred for public rems distribution unlirdted.

ARI Research Reports and Technical Reports are intended for sponsors ofR&D tasks and for other research and military agencies. Any findings readyfor implementation at the time of publication are presented in the last partof the Brief. Upon completion of a major phase of the task, formal recom-mendations for official action normally are conveyed to appropriate militaryagencies by briefing or Disposition Form.

i v

p .4 4

FORMEORD

Under daylight nap-of-the-earth (NOE) flight conditions, little time isavailable for the correct detection of obstacles and targets. The helicopterpilot's detection and reaction time is greatly reduced when NOR flight oust beperformed at night. Previous ARI research on NOE has addressed various fac-tors involved in obstacle detection, recognition, and avoidance, such as pilotresponse time as a function of aircraft velocity, obstacle shape, and distancesand accuracy of judgments based on the perception of absolute distances. Thisreport is the first of three reports on experiments designed to determine thebehavioral requirements for a helicopter electro-optical display system foruse in night NOE flight. The experiment described here is specifically con-cerned with determining the effects on display requirements of display size,system goma function (a contrast/brightness function), and type of terrainoverflown.

This experiment resulted from an in-house, technology-based research ef-fort begun under the direction of Dr. Aaron Hyman. It is responsive to therequirements of Army Project 2Q162722A765. The results of this experimentare also responsive to Human Resources Need 77-311 from the Deputy Chief ofStaff for Plans and Operations.

h S PH ZEIDHN'kehnical Director

V

-~~~TT 7R '.>.Z. .

HELICOPTER ELECTRO-OPTICAL SYSTEM DISPLAY REQUIREMENTS: I- THE EFFETSOF CRT DISPLAY SIZE, SYSTD GAMMA FUNCTION, AND TERRAIN TYPE ON PILOTS'REQUIRED DISPLAY LUMINANCE

BRIEF

Requirement:

To reduce the hazards of flying in high-threat environments, the Army hasemphasized low-level flying and night operations. In nap-of-the-earth (NOE)flight, the aviator stays as close to the ground as vegetation and other ob-stacles will permit. This is a stressful task, and the problems associatedwith using this tactic during daylight hours are intensified at night. Fornighttime NOE operations to be successful, usable visual aids need to be de-veloped, and the specification of their related display parameters is a neces-sary first step.

Procedure:

Twenty-four Army rotary wing pilots viewed videotaped segments of low-level and NOR helicopter flights, presented on television monitors designedto simulate a low-light-level television display system for helicopters. Theywere asked to set the display luminance at the lowest level that they judgedwould permit successful flight over the terrain. Each subject adjusted theluminance level for eight different display conditions derived from the com-bination of two different display sizes (13- or 26-cm CRTs viewed at 69 cm),two types of terrain (wooded or semi-arid with sparse vegetation), and twodifferent system gamma functions (normal contrast or enhancement of contrastin darker portion of display luminance range). Participants were also askedfor their subjective impressions of the various display conditions.

Findings:

Pilots were able to use significantly lower luminance settings with the26-cm display than they were with the 13-cm display. They also used signifi-cantly lower settings when viewing videotapes of wooded terrain presented ondisplays with enhanced contrast in the darker areas (i.e., system gamma mod-ification). The pilots' subjective impressions and indicated preferencesagreed with their measured settings. Pilots preferred a 26-cm over a 13-cmdisplay.

vii

HG03mW PAN s AA v UM

Utilization of Findings:

Pilots are able to use larger displays at lower luminance levels. Thefact that the utility of modifying the system gamma functions was terrain-specific opens up the possibility of developing optimal object-luminance todisplay-luminance transfer functions for different types of terrain. However,before specific recommendations for display luminance requirements can bemade, dark-adaptation losses with larger but dimmer displays need to be de-termined so that an objective evaluation of the utility of low-luminancedisplays can be established.

viii

HELICOPTER ELECTRO-OPTICAL SYSTEM DISPLAY REQUIREMENTS: I. THE EFFECTS

OF CRT DISPLAY SIZE, SYSTEM GAMMA FUNCTION, AND TERRAIN TYPE ON PILOTS'REQUIRED DISPLAY LUMINANCE

CONTENTS

Page

INTRODUCTION ... . ... ..... .. . . . . .

METHOD Faclit . . . . . . . . . . . . . . . . . . . . . . . . . 5

Research Facility . .. .. .. .... . . .. . .. 5

Participants . . . . . . . . . . . . . . . . . . . .8

Procedure . . . . . • . . . . . . . . . .* * * * * * * • * * • * • * 11

RESULTS AND DISCUSSION ...... . . . . .... ................ . 11

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . 12

REFERENCES . . . . . . . . . ....... .............................. 15

LIST OF TABLES

Table 1. Mean highliqht luminance settings (in footlamberts)for display size, system gamma function, and terraintype ... . . . . . . . . . . . . . .. . . 13

2. Means pooled across conditions for mean highlightluminance settings listed in table I . . . . . . . * . . . . . 13

LIST OF FIGURES

Figure 1. Functional relations illustrating advantage inmodifying the system gamma function when luminancelevel of television display is decreased . * . . . . . . . . 3

2. System gamma functions used in present research . . . ... 4

3. Schematic presentation of optical layout utilizingfilmchain projector for simulating a full-colorwindscreen display and correlated LLLTV cockpitmonitor . .. . .. . * . . . . . . . 7

4. Schematic presentation showing arrangement ofcomponents in the optical probe . . . . . . . . .. 9

5. Schematic presentation showing the approach used for ob-taining translational movement of the simulated helicopter • 10

ix

* .14.

HELICOPTER ELECTRO-OPTICAL SYSTEM DISPLAY REQUIREMENTS: I. THEEFFECTS OF CRT DISPLAY SIZE, SYSTEM GAMMA FUNCTION, AND

TERRAIN TYPE ON PILOTS' REQUIRED DISPLAY LUMINANCE

INTRODUCTION

Technological advances in enemy visual and sensor surveillance systemsthreaten combat survivability of the Army helicopter. As a countermeasure,the Army has emphasized using low-level flying and night operations. Morespecifically, nap-of-the-earth (NOE) flight is the tactic of choice for heli-copters in this high-threat environment (U.S. Army, 1976; Abbey & Carson,1978). In NOE flight, the aviator stays as close to the earth's surface asvegetation and obstacles permit. Thus the use of natural terrain featuresas a mask increases the probability of avoiding detection by the enemy.

Performing NOE flight is a difficult and fatiguing task for the pilot,and the margin for pilot error is extremely small. Since NOE flight is muchmore difficult at night, effective night vision aids and displays are importantto successful nighttime NOE flight.

This research was directed toward obtaining data to aid in specifyingdisplay parameters for a low-light-level television (LLLTV) system as a visualaid in night NOE flight. An LLLTV camera is an image-intensifying devicecapable of responding to low-intensity radiation in the visible and near in-frared regions of the electromagnetic spectrum. The radiation is first con-verted to a highly amplified electrical signal and then reconverted to a raster-scan visual display on a cathode ray tube (CRT). A suitably mounted LLLTVcamera on a helicopter and an accompanying display of the output on a monitorin the cockpit can provide the pilot with a real-time television picture ofthe terrain in front of the helicopter. But for safe and effective use ofsuch an LLLTV display, optimal display parameters need to be specified. Be-fore the specific parameters to be investigated in this research effort wereselected, several germane factors were analyzed.

Rectangular TV displays usually have a 3-by-4 aspect ratio. Thus a CRTdisplay that is 15.6 cm high is 20.8 cm wide; and its diagonal is 26 cm; andit is characteristically described in terms of its maximum linear dimension,namely, the diagonal: a 26-cm display. Similarly, a rectangular CRT dis-play 7.8 cm high and 10.4 cm wide is called a 13-cm display. When the latterdisplay is viewed from 69 cm, its height subtends approximately 388 arc min-utes. If the viewer has the normal visual acuity of 20/20 Snellen (i.e, 2arc minutes per line pair or 1 arc minute per TV line) and the display has aresolution of 400 TV lines per picture height, there is good matching of eyeand display resolution.

Visual acuity, however, depends partly on display luminance level (Shlaer,19371 Riggs, 19651 Kaufman & Christensen, 1972). For example, when the lumi-nance of the background is about 0.2 footlambert (fL) and the target has highcontrast, acuity is approximately 20/20 Snellen; when background luminance is

1

........................... ..

about 0.015 fL, acuity is approximately 20/40 Snellen (i.e., 4 arc minutesper line pair). In the latter case, retaining the resolution match betweendisplay and the eye now requires a 26-cm CRT with 400 TV lines per pictureheight resolution.

There is an advantage in flying with a dim TV display at night, becauseas visual dark adaptation increases, the pilot is better able to see the ex-ternal environment when looking through the windscreen. Research by Baker(1953, 1963), also described by Bartlett (1965) and by Brown and Mueller(1965), shows that dark adaptation recovers at a very rapid rate during thefirst 1/2 second after an adapting luminance is turned off. The questionis, can one set cockpit TV displays for night flying so they are brightenough to permit acceptable form perception when flying "heads-down" (i.e.,looking at the instrument panel) and yet dim enough to allow rapid acquisi-tion of the visual world when flying "heads-up" (i.e., looking through thewindscreen)?

Another variable to be considered when using a dim display is the object-luminance to display-luminance transfer function, usually called system gammafunction (where gamma is the slope of the curve describing the object to dis-play luminance functional relation when the coordinates are in log units).The advantage in having a nonlinear gamma function with dim displays arisesfrom the fact that the minimum perceptible luminance difference (i.e., con-trast discrimination) is a nonlinear function of background luminance (Mueller,1951; Brown & Mueller, 1965; Kaufman & Christensen, 1972). For a given ratioof luminances, contrast discrimination is poorer for the lower portion of theassociated log-luminance range when the display is dim, whereas contrast dis-crimination is almost linear for a comparable log-luminance range in a brightdisplay. In Figure 1, the upper drawing shows the perceptual nonlinearityin contrast discrimination for dim display luminance levels as compared tobright display luminance levels. The lower drawing in Figure 1 shows howthis can be compensated for, by modifying the system gamma function. Curve Ashows an object-luminance to display-luminance transfer function useful witha bright TV-generated display. Curve B shows an object-luminance to display-luminance transfer function useful with a dim TV-generated display. It isdesigned to counteract the nonlinearity in contrast perception at dim luminancelevels, as demonstrated by the curve in the upper panel. "Gamma" is the termused to designate the slope of the object-luminance to display-luminance trans-fer function, for displays originating from TV cameras, when coordinates areplotted in log units.

Thus, for a given range of input signals, to obtain a relatively uniformperceptual discrimination of contrast such as can be achieved with a bright dis-play and the transfer function defined by curve A, the transfer function definedby curve B is needed for a dim display. To evaluate this, two representativesystem gamma functions have been selected for comparison in this research.These are shown in Figure 2. Curve N in Figure 2 describes the system gammafunction identified in this report as "normal." Curve M describes the systemgamma function identified in this report as "modified." Photometric measure-ments were made of the display face from input obtained using an EIA Logarith-mic Reflectance Chart. Three different levels of display highlight luminancewere used. Obtained logarithmic values were then averaged, and the sets ofdata were shifted vertically to represent the case where highlight luminanceis 0.2 fL

2

V,._ , J+

HDinm BrightRegion [Region]

BACKGROUND LUMINANCE (Log fL)

'A

-,O b

0

z

0

Is 4

W"LTIVE OBJE.CT LUMINANCE (Log fL)

Figure 1. Functional relations illustrating advantage in modifyingthe system gamma function when luminance level oftelevision display is decreased.

3

-0.8-

S-1.0-

-1.4 M0 1.4

' -1.6

04

.. -2.0-

-2.2-.

0.15 0.30 0.45 0.60 0.75 0.90 1.05 1.20 1.35

Relative Luminance of Bars on EIA LogrithmicReflectance Chart, in Log fL

Figure 2. system galmma functions used in present research.

4

Based on the findings of other investigators (e.g., Baker, 1953, 1963;Mueller, 1951; Shlaer, 1937), it was estimated that a TV display having ahighlight luminance of about 0.05 fL might provide a pilot with an acceptablepresentation with which to perform night NOE flights and also have an accep-tably low impact on dark adaptation. The data in the literature are reportedin trolands; in converting them to footlamberts, it was assumed that the ob-server would have a pupil diameter of 5 mm and that retinal integration timewould be 0.1 second.

A special simulation facility was developed, and three display parameterswere selected for initial investigation. The first parameter was CRT displaysize. Because of limited panel space in the helicopter cockpit, the smallestCRT that can be safely used by a pilot must be selected. Therefore CRT dis-plays with an aspect ratio of 3:4 and diagonals of 13 cm and 26 cm were com-pared. With reference to real-world visual angles and rate of closure onobstacles, these CRT displays, when viewed at about 69 cm, represented abouta 6x and 3x minification respectively.

The second parameter investigated was display luminance. Because thepilot or copilot may periodically have to look through the windscreen, the CRTdisplay must be dim enough to optimize dark adaptation and bright enough topermit adequate form perception when viewing the cockpit display.

The electro-optical system gamma function (i.e., the transfer function ofinput luminance to CRT display luminance) was the third parameter studied. Thegamma function can be readily manipulated electronically in a TV system; suchmanipulation, through local contrast variation, can enhance the visibility ofselected features (e.g., trees and green foliage).

The specific objective in studying the effects of the above parametersis to provide data for developing some critical display requirements for ahelicopter-mounted LLLTV system.

METHOD

Research Facility

A flexible NOE visual flight simulation facility was developed at the U.S.Army Research Institute for the Behavioral and Social Sciences (ARI) to providevarious modes of presentation of stimulus materials to the participant pilots.The three configurations used in the present series of research efforts aredescribed below.

Configuration I. A 16-mm filmchain projector (Bell and Howell model 652),1

together with an optical projection system, was used to present the partici-pant with both a full-color windscreen display, which simulated real-worldvisual angles, and a panel CRT display of the same scene but minified and inmonochrome. The windscreen display, which could be presented at various lumi-nance levels, was projected on a rear-projection screen and viewed through a

ICommercial names are used in this report for purposes of clarity only and donot represent endorsement by the Department of the Army.

5

'--.

Fresnel lens that placed the image at optical infinity for the observer. Atelevision camera (GBC model CTC 6000), mounted in line with the 16-am pro-jector, was used to generate a simulated LLLTV display to the participants.The use of a beam splitter suitably placed in the optical train provided theparticipant simultaneously with the simulated windscreen display and the cor-related LLLTV display. The size, luminance level, and system gamma functionof the simulated LLLTV display could be adjusted by the experimenter. Aschematic view of this system is shown in Figure 3.

In Figure 3, panel A is a plan view of the arrangement in which the simu-lated TV camera is fixed with respect to the aircraft. For this configura-tion, the TV camera must have its vertical sweep reversed. Panel B is a planview of the arrangement in which the simulated TV camera can be moved in azi-muth and elevation with respect to the aircraft coordinate axes. Because ofthe relatively low screen luminance and the need for using a small lens aper-ture to obtain depth of field, a silicon-intensified TV camera (SIT) (GBCmodel NVC 100) was used here. Panel C shows a side view of the participant'swork station, which is the same for either of the above arrangements. Inpanel A, focal length of the projection lens is 25 mm, faces of the beam-splitter are 50 mm by 50 mm, and the relay lens system used for convergingthe rays to form an image on the TV camera tube comprises an Erfle eyepieceof 37 mm focal length plus a Barlow lens of minus 44 mm focal length. TheFresnel lens shown in panel C is 38 cm in diameter and has 49 grooves per cmand a focal length of 32 cm. It images the projection screen (i.e., windscreendisplay) at optical infinity. Also shown is the two-mirror arrangement forpresenting a virtual image of the cockpit monitor at 69 cm from the observer.

Configuration II. The windscreen stimulus materials in this configura-tion were presented by means of a CONRAC RQA 17 television monitor instead ofa rear-projection screen. The resultant monochrome windscreen display was col-limated and provided the observer with a 36-by-48-degree field of view (FOV)presented at real-world angular subtense. The observer looked at this displaythrough goggles mounted on a light-tight viewing hood. Display luminance wasvaried by one of two methods. With the first method, only the experimentercontrolled the luminance of the display, and the luminance level was discretelyvaried by inserting neutral density filters into the goggles. With the secondmethod, the goggles were fitted with a fixed and a rotating polaroid filterthat permitted the subject to continuously vary the luminance level; the set-ting could also be remotely monitored by the experimenter. Stimulus materialcould be generated from an on-line TV camera or from videotape recordings playedback on a SONY videotape recorder (model VO 1800). Directly below the viewinghood was another CRT monitor simulating a cockpit panel display viewed at 69 cmand presenting the same material as that generated for the windscreen display.The size of the panel display could be varied by the experimenter, and its lumi-nance level was controlled by the participant, who set the angular position ofthe rotating member of a pair of polaroid filters mounted in front of the simu-lated panel area. The panel display luminance level was continuously adjustableand was remotely monitored by the experimenter. The participant's work stationenvironment was an enclosed, dark-walled, sound-attenuating chamber.

Configuration III. The participant's environment was the same as thatused in Configuration II. The source for the stimulus material, however, wasa 1.22 m by 1.83 m, three-dimensional, full-color terrain model designed to

6

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CINE MA ICiINE h MROFILM To' N U Ix v M

C CTMIRRORMONITOR

FRESNEL

REAR-

PROJECTIONSCREEN

Z-MIRROR

riqure 3. Schematic presentation of optical layout utilizing filskchainprojector for simulating a full-color windscreen display anicorrelated LLLTY cockpit monitor.

447

simulate partially wooded terrain at a 1:300 scale. It was used in conjunc-tion with an optical probe mounted on a TV camera (GBC model NVC 10U) thathad a SIT camera tube to provide the video output. The probe (see Figure 4),with its symmetrical optical system and telecentric aperture stop, was de-signed to have unit magnification, a 60-degree circular instantaneous FOV,good color correction, and very little distortion. A scanning mirror, rotat-able on two orthogonal axes and located at the cntrance pupil of the probe,was used to set the pitch and the heading of the simulated aircraft. Rollangle and any needed derotation was obtained by rotating a Pechan prism aboutthe optic axis. (Rotational servo controls had not then been incorporated inthe probe, so that for a given simulated flight, the pitch, heading, and rollsettings remained fixed.)

When viewing directly through the probe, with entrance pupil set at about0.5 mm and probe focus set for an object 26 cm from the entrance pupil, the20/30 line of the Snellen eye chart (suitably reduced) was resolved for chartplacements anywhere in the FOV. Depth of field for this 20/30 resolution wasfrom 20 cm to infinity (i.e., for a 1:300 scale, from 60 m simulated to in-finity), and at 5 cm (i.e., at 15 m simulated for a 1:300 scale) resolutionwas about 20/70 Snellen. When the probe was interfaced with the TV camera,the display monitor provided the observer with a 42-degree horizontal and a31-degree vertical real-world FOV, and resolution was degraded to about 20/90Snellen (equivalent to about 400 TV lines per picture height).

The participant controlled the probe altitude (from 0 to 61 m simulated)and forward groundspeed (from 0 to 45 knots simulated). To obtain the 3 de-grees of translational freedom (continuously varied), the position of theterrain model (which could be moved in X and Y), and the position of the probe(which could be moved in Z) were controlled through an INTER DATA 90 computerand associated peripherals and electromechanical equipment. The computer wasalso used to record and display the participant's altitude, airspeed, flightpath, and number of crashes during simulated helicopter runs over the terrainmodel. A schematic diagram of the probe is presented in Figure 4, and the re-lationship between probe and terrain model is shown in Figure 5.

Direct visual viewing of the display, with unit magnification, was ob-tained by removing the TV camera shown in Figures 4 and 5 and placing theobserver's eye at the exit pupil of the probe (i.e., behind relay lens 3).Image focusing was accomplished through small axial movement of relay lens 2.The objective and relay lens 3 are alike, each being an Erfle eyepiece of20 mm focal length; and relay lenses 1 and 2 are each Schneider-KreuznachComponon lenses of 50 nu focal length. Field-of-view is 60 degrees and circular.

As shown in Figure 5, the 3 degrees of translation are controlled throughservo drives. In the present series of investigations, simulated movement ofthe helicopter was simplified to closed loop control of translation only.

Participants

The research participants were 24 rated Army helicopter pilots who volun-teered to serve in the study. All participants had normal or corrected normalvision.

~8

TV CAMERA TUBE

> RELAY LENS 3

7? DEROTATION PRISM

- - RELAY LENS 2

- - RELAY LENS 1

- - APERTURE STOP

< > OBJECTIVE

CYAW-AND-PITCH MIRROR

SPITCH

YAW

Figure 4.* Schematic presentation showing arrangement of componentsin the optical probe.

9

-~~~~~~4 -. I .-- -4"

3-D TERRAIN MODEL

OPTICAL PROBE

zTV CAMERA

Figure 5. Schematic presentation showing the approach used f or obtainingI

translational movement of the simulated helicopter.

10

Procedure

Prior to the start of data collection, all participants were shown a simu-lated windscreen display of NOE flight in conjunction with a correlated instru-ment panel CRT presentation. The Configuration I apparatus (see Figure 3,panels A and C) was used for this demonstration. The windscreen view waspresented at approximately full-moon luminance levels (scene highlight lumi-nance of 0.01 to 0.02 fL). While the participants viewed this display, theexperimenter brought to their attention the angular minification present withthe 13-cm CRT monitor (6x minification) and the 26-cm CRT monitor (3x minifi-cation) in comparison to the real-world visual angles as seen in the windscreenview. The experimenter also explained the potentially detrimental effect of abright CRT display on dark adaptation.

After the familiarization demonstrations, CRT display luminance was varied,so that the participants could judge the dimmest setting they could use for thedisplays and still feel they could fly safely using such a system.

In the formal experiment, the Configuration II facility was used, and onlythe panel display was presented. The participants viewed two 10-minute seg-ments of NOE flight, recorded on videotape. One segment was filmed at theHunter-Liggett, Calif., ntilitary reservation, and the other segment was filmednear Fort Rucker, Ala. The tapes were viewed by each participant pilot on the13-cm and the 26-cm CRT displays. They could attenuate the display luminanceby rotating one of a pair of polaroid filters while the other remained fixed.This permitted scene highlight variation of from 0.01 fL to 0.20 fL. Partici-pants also viewed each display with a normal system gamma function or with amodified system gamma function designed to enhance detail in the luminancerange of terrain features such as trees and other green foliage. Thus, eachsubject made judgments in eight different conditions: 13- or 26-cm CRT dis-play, Hunter-Liggett or Fort Rucker terrain, and normal or modified displaysystem gamma function.

The pilots were instructed to view each display as though it were beingused to perform NOE flight at night. They were asked to make the display asdim as possible without sacrificing any information necessary to conduct a safeand effective mission. Each participant made six judgments with each displaycondition, rotating the polaroid filter in alternate directions for successivejudgments to minimize positional cues in setting the display luminance. Atthe end of the experiment, the participants were fully debriefed, their ques-tions were answered, and any of their observations concerning the displayswere recorded. Each participant was asked to indicate a preference for eitherthe 13-cm or the 26-cm display and to comment on the desirability and utilityof having a system gaimma control.

RESULTS AND DISCUSSION

A 2 x 2 x 2 factorial ANOVA with repeated measures on all factors wasused to evaluate'the effects of CRT size, system gamma function modification,and type of terrain on display luminance levels used by pilots to maximizevisual dark adaptation when flying with an LLLTV system. The independentvariables were CRT size (13 cm vs. 26 cm), system gamma function (normal vs.

- 11

modified), and type of terrain (semi-arid as at Hunter-Liggett vs. heavilywooded as at Fort Rucker). The dependent measure was the mean of the sixluminance judgments (in fL) made by each of the 24 pilots for each of theeight display conditions.

The overall mean luminance judgments made by the 24 pilots for each dis-play condition are presented in Table 1, and some pooled means for these dataare presented in Table 2. As indicated in these tables, the pilots felt thatthey could tolerate a 29% dimmer display when using the 26-cm as compared tothe 13-cm CRT monitor (F (1,23) = 40.58, p < .01). It can also be seen thatwith a normal system gamma function, pilots required a brighter (i.e., higherluminance) display when flying over heavily wooded terrain than when flyingover the semi-arid terrain of Hunter-Liggett (F (1,23) = 60.89, P < .01).A significant effect for system gamma function modification was also obtained(F (1,23) = 51.03, 2 < .01) however, this effect was mainly manifested in aterrain by contrast interaction (F (1,23) = 40.58, p < .01).

Table 1 shows that system gamma function modification had little if anyeffect for semi-arid terrain, but allowed for a 29% dimmer display duringflights over heavily wooded terrain. No other interactions were statisticallysignificant. Also, when a 26-cm display size was used in conjunction with amodified system gamma function, mean display highlight luminance, selected bypilots as adequate for night flying, was 0.06 fL or less. This comparesfavorably with the extrapolation of 0.05 fL obtained from previous research(e.g., Baker, 1953, 1963; Mueller, 1951; Shlaer, 1937).

During debriefing, 92% of the pilots felt that they would be more com-fortable using the 26-cm display, 4% preferred the 13-cm display, and the re-mainder had no preference. All the pilots felt that the modified system gammafunction would be of value when flying over heavily wooded terrain.

CONCLUSIONS

Two general conclusions can be drawn from the results of the present study.First, both the empirical and subjective data from the pilots support the useof a 26-cm CRT display when using an airborne LLLTV system. When using the26-cm display as opposed to the 13-cm CRT display, pilots could tolerate adimmer display, thus reducing the adverse effects of display-viewing on visualdark adaptation. In addition, the pilots felt more comfortable when using the26-cm display. Preference was usually expressed in terms of a feeling of havinga larger real-world field of view, even though the 26-cm and the 13-cm displaysboth presented the same real-world field of view. The "larger," subjectivevisual FOV provided by the 26-cm display gave pilots greater visual comfort,perhaps because the 26-cm display reduced the restriction on scanning eyemovements.

12

......

Table 1

Mean Highlight Luminance Settings (in Footlamberts) for DisplaySize, System Gamma Function, and Terrain Type

System gama function

Normal ModifiedHeavily Heavily

Semi-arid wooded Semi-arid woodedDisplay size terrain terrain terrain terrain

13 cm 0.066 0.119 0.066 0.093

26 cm 0.046 0.096 0.044 0.060

Table 2

Means Pooled Across Conditions for Mean HighlightLuminance Settings Listed in Table I

Mean settingIndependent variable (in footlamberts)

Display size

13 cm 0.08626 cm 0.061

System gamma function

Normal 0.082

Modified 0.066

Terrain type

Semi-arid 0.056Heavily wooded 0.092

13

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BEEN

A second general conclusion is that the modified system gaimna functionmay be useful when flying over heavily wooded terrain. The pilots thoughtthat the fine structure of the forest canopy was more easily discriminablewith this modification. Furthermore, they adjusted their displays to a lowerluminance when using the modified system gama function. They were apparentlyable to discern the needed detail in the heavily wooded areas, even thoughthey were using a lower display luminance. The lack of differences with systemgamma function modification when flying over semi-arid terrain is not surprising,since this terrain is comprised of high-contrast elements for either of the twogamma functions used. This finding is important because it indicates that theeffect of system gamma function modification is terrain-specific and opens upthe possibility of developing optimal real-world-luminance to display-luminancetransfer functions for other characteristic terrain types.

A third conclusion, although tentative, is that pilots flying in moonlightcan set their cockpit CRT display at a luminance level high enough to providethem with an adequate cockpit flight display and yet low enough to permit themto have relatively rapid visual acquisition when subsequently viewing directlythrough the windscreen (i.e., adequate visual dark adaptation for heads-upviewing).

14

." *.

REFERENCES

Abbey, C. W., & Carson, F. L. Low, lower, lowest. United States Army AviationDigest, 1978, 24(4), 24-27.

Baker, H. D. The instantaneous threshold and early dark adaptation. Journalof the Optical Society of America, 1953, 43, 798-803.

Baker, H. D. Initial stages of dark and light adaptation. Journal of theOptical Society of America, 1963, 53, 98-103.

Bartlett, N. R. Dark adaptation and light adaptation. In C. H. Graham (Ed.),Vision and visual perception. New York: John Wiley and Sons, 1965.

Brown, J. L., & Mueller, C. G. Brightness discrimination and brightness con-trast. In C. H. Graham (Ed.), Vision and visual perception. New York:John Wiley and Sons, 1965.

Kaufman, J. E., & Christensen, J. F. IES lighting handbook (5th ed.). NewYork: Illuminating Engineering Society, 1972, Section 3.

Mueller, C. G. Frequency of seeing functions for intensity discrimination atvarious levels of adapting intensity. Journal of General Physiology,1951, 34, 463-474.

Riggs, L. A. Visual acuity. In C. H. Graham (Ed.), Vision and visual percep-tion. New York: John Wiley and Sons, 1965.

Shlaer, S. The relation between visual acuity and illumination. Journal ofGeneral Physiology, 1937, 21, 165-188.

U.S. Army, Rotary Wing Night Flight, United States Army TC1-28, February 1976.

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DISTRIBUTION

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