HEUICOPITER ELECTRO-OPTICAL 3 5 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. Army

Research 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 INSTITUTE

FOR THE BEHAVIORAL AND SOCIAL SCIENCES

A Field Operating Agency under the Jurisdiction of the

Deputy Chief of Staff for Personnel

I FRANKLIN A. HART

JOSEPH ZEIDNER Colonel, US Army

Technical Director Commander

NOTICES

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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 REQUIREMENTS: 1. THE EFFECTS OF CRT DISPLAY SIZE, _____________

SYSTEM GAMMA FUNCTION, AND TERRAIN TYPE ON PILOTS a. PERFORMING ONG. REPORT NMBNER

REQUIRED DISPLAY LUMINANCE

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

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

S. PERFORMING ORGANIZATION NAMC AND ADDRESS 10. PROGRAM ELEMENT. PROJECT. TASK

U.S. Army Research Institute for the Behavioral AREA A WORK UNIT NUMBERS

and Social Sciences (PERI-OK) 2Q162722A765

5001 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) flight

Night flying

Night vision

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

additional ambient illumination, these pilots were asked to set cockpit monitor luminance at the lowest level that they judged would permit successful

flight control over the terrain being overflown. Each pilot adjusted

DD, S'IOU W3 O ? DV 6 ISMDSOETSUNCLASSIFIED

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

-

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MM iMLAWitCrATION V OF THIS PAG(kM. Data EAfm

20. luminance levels for eight different display conditions formed by the

factorial combination of display size, type of terrain, and object-luminance

to display-luminance transfer function (system gamma function). .Results show

that pilots used lower luminance settings when viewing the larger of the two

display sizes presented. They also used significantly lower luminance settings

when viewing wooded terrain, with the system gamma function modified to

provide benhancedO contrast in the luminance range of interest, as against

an unmodified system gamma function. The pilots' subjective impressions agreed

with their measured settings. This report discusses the impact of these

results on the specification of display requirements for a low-light-level

television system for aiding night NOE flight.

MeC TAB

Vw mounc ed

Justif ic .to- . . -

Dist. e l

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Ir O- ~
Tehinica Reprt 441

HELICOPTER ELECTRO-OPTICAL SYSTEM DISPLAY

REQUIREMENTS: 1.

The Effects of CRT Display Size, System Gamma

Function, 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 Director

ORGANIZATIONS AND SYSTEMS

RESEARCH LABORATORY

U.S. ARMY RESEARCH INSTITUTE FOR THE BEHAVIORAL AND SOCIAL SCIENCES

5001 Eisenhower Avenue, Alexandria, Virginia 22333

Off ice, Deputy Chief of Staff for Personnel

Department of the Army

March 19N0

Army Project Number Heicopter Display Requirements 20162722AM6

Approred for public rems distribution unlirdted.
ARI Research Reports and Technical Reports are intended for sponsors of

R&D tasks and for other research and military agencies. Any findings ready

for implementation at the time of publication are presented in the last part

of the Brief. Upon completion of a major phase of the task, formal recommendations for official action normally are conveyed to appropriate military

agencies by briefing or Disposition Form.

i v

p .4 4
FORMEORD

Under daylight nap-of-the-earth (NOE) flight conditions, little time is

available for the correct detection of obstacles and targets. The helicopter

pilot's detection and reaction time is greatly reduced when NOR flight oust be

performed at night. Previous ARI research on NOE has addressed various factors involved in obstacle detection, recognition, and avoidance, such as pilot

response time as a function of aircraft velocity, obstacle shape, and distances

and accuracy of judgments based on the perception of absolute distances. This

report is the first of three reports on experiments designed to determine the

behavioral requirements for a helicopter electro-optical display system for

use in night NOE flight. The experiment described here is specifically concerned with determining the effects on display requirements of display size,

system goma function (a contrast/brightness function), and type of terrain

overflown.

This experiment resulted from an in-house, technology-based research effort begun under the direction of Dr. Aaron Hyman. It is responsive to the

requirements of Army Project 2Q162722A765. The results of this experiment

are also responsive to Human Resources Need 77-311 from the Deputy Chief of

Staff for Plans and Operations.

h S PH ZEIDHN'k

ehnical Director

V

-~~~TT '.>.Z. . 7R 
HELICOPTER ELECTRO-OPTICAL SYSTEM DISPLAY REQUIREMENTS: I- THE EFFETS

OF 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 has

emphasized 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 obstacles will permit. This is a stressful task, and the problems associated

with using this tactic during daylight hours are intensified at night. For

nighttime NOE operations to be successful, usable visual aids need to be developed, and the specification of their related display parameters is a necessary first step.

Procedure:

Twenty-four Army rotary wing pilots viewed videotaped segments of lowlevel and NOR helicopter flights, presented on television monitors designed

to simulate a low-light-level television display system for helicopters. They

were asked to set the display luminance at the lowest level that they judged

would permit successful flight over the terrain. Each subject adjusted the

luminance level for eight different display conditions derived from the combination 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 two

different system gamma functions (normal contrast or enhancement of contrast

in darker portion of display luminance range). Participants were also asked

for their subjective impressions of the various display conditions.

Findings:

Pilots were able to use significantly lower luminance settings with the

26-cm display than they were with the 13-cm display. They also used significantly lower settings when viewing videotapes of wooded terrain presented on

displays with enhanced contrast in the darker areas (i.e., system gamma modification). The pilots' subjective impressions and indicated preferences

agreed with their measured settings. Pilots preferred a 26-cm over a 13-cm

display.

vii

HG03mW PAN AA s v UM
Utilization of Findings:

Pilots are able to use larger displays at lower luminance levels. The

fact that the utility of modifying the system gamma functions was terrainspecific opens up the possibility of developing optimal object-luminance to

display-luminance transfer functions for different types of terrain. However,

before specific recommendations for display luminance requirements can be

made, dark-adaptation losses with larger but dimmer displays need to be determined so that an objective evaluation of the utility of low-luminance

displays 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 terrain

type . . ... . . . . . . . . . . . .. . . 13

2. Means pooled across conditions for mean highlight

luminance settings listed in table I . . . . . . . * . . . . . 13

LIST OF FIGURES

Figure 1. Functional relations illustrating advantage in

modifying the system gamma function when luminance

level of television display is decreased . * . . . . . . . . 3

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

3. Schematic presentation of optical layout utilizing

filmchain projector for simulating a full-color

windscreen display and correlated LLLTV cockpit

monitor . .. . .. . * . . . . . . . 7

4. Schematic presentation showing arrangement of

components in the optical probe . . . . . . . . .. 9

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

ix

* .14.
HELICOPTER ELECTRO-OPTICAL SYSTEM DISPLAY REQUIREMENTS: I. THE

EFFECTS OF CRT DISPLAY SIZE, SYSTEM GAMMA FUNCTION, AND

TERRAIN TYPE ON PILOTS' REQUIRED DISPLAY LUMINANCE

INTRODUCTION

Technological advances in enemy visual and sensor surveillance systems

threaten combat survivability of the Army helicopter. As a countermeasure,

the Army has emphasized using low-level flying and night operations. More

specifically, nap-of-the-earth (NOE) flight is the tactic of choice for helicopters 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 as

vegetation and obstacles permit. Thus the use of natural terrain features

as 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 much

more difficult at night, effective night vision aids and displays are important

to successful nighttime NOE flight.

This research was directed toward obtaining data to aid in specifying

display parameters for a low-light-level television (LLLTV) system as a visual

aid in night NOE flight. An LLLTV camera is an image-intensifying device

capable of responding to low-intensity radiation in the visible and near infrared regions of the electromagnetic spectrum. The radiation is first converted to a highly amplified electrical signal and then reconverted to a rasterscan visual display on a cathode ray tube (CRT). A suitably mounted LLLTV

camera on a helicopter and an accompanying display of the output on a monitor

in the cockpit can provide the pilot with a real-time television picture of

the terrain in front of the helicopter. But for safe and effective use of

such an LLLTV display, optimal display parameters need to be specified. Before the specific parameters to be investigated in this research effort were

selected, several germane factors were analyzed.

Rectangular TV displays usually have a 3-by-4 aspect ratio. Thus a CRT

display that is 15.6 cm high is 20.8 cm wide; and its diagonal is 26 cm; and

it is characteristically described in terms of its maximum linear dimension,

namely, the diagonal: a 26-cm display. Similarly, a rectangular CRT display 7.8 cm high and 10.4 cm wide is called a 13-cm display. When the latter

display is viewed from 69 cm, its height subtends approximately 388 arc minutes. If the viewer has the normal visual acuity of 20/20 Snellen (i.e, 2

arc minutes per line pair or 1 arc minute per TV line) and the display has a

resolution of 400 TV lines per picture height, there is good matching of eye

and display resolution.

Visual acuity, however, depends partly on display luminance level (Shlaer,

19371 Riggs, 19651 Kaufman & Christensen, 1972). For example, when the luminance of the background is about 0.2 footlambert (fL) and the target has high

contrast, 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 minutes

per line pair). In the latter case, retaining the resolution match between

display and the eye now requires a 26-cm CRT with 400 TV lines per picture

height resolution.

There is an advantage in flying with a dim TV display at night, because

as visual dark adaptation increases, the pilot is better able to see the external 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 the

first 1/2 second after an adapting luminance is turned off. The question

is, can one set cockpit TV displays for night flying so they are bright

enough to permit acceptable form perception when flying "heads-down" (i.e.,

looking at the instrument panel) and yet dim enough to allow rapid acquisition of the visual world when flying "heads-up" (i.e., looking through the

windscreen)?

Another variable to be considered when using a dim display is the objectluminance to display-luminance transfer function, usually called system gamma

function (where gamma is the slope of the curve describing the object to display luminance functional relation when the coordinates are in log units).

The advantage in having a nonlinear gamma function with dim displays arises

from the fact that the minimum perceptible luminance difference (i.e., contrast discrimination) is a nonlinear function of background luminance (Mueller,

1951; Brown & Mueller, 1965; Kaufman & Christensen, 1972). For a given ratio

of luminances, contrast discrimination is poorer for the lower portion of the

associated log-luminance range when the display is dim, whereas contrast discrimination is almost linear for a comparable log-luminance range in a bright

display. In Figure 1, the upper drawing shows the perceptual nonlinearity

in contrast discrimination for dim display luminance levels as compared to

bright display luminance levels. The lower drawing in Figure 1 shows how

this can be compensated for, by modifying the system gamma function. Curve A

shows an object-luminance to display-luminance transfer function useful with

a bright TV-generated display. Curve B shows an object-luminance to displayluminance transfer function useful with a dim TV-generated display. It is

designed to counteract the nonlinearity in contrast perception at dim luminance

levels, as demonstrated by the curve in the upper panel. "Gamma" is the term

used to designate the slope of the object-luminance to display-luminance transfer function, for displays originating from TV cameras, when coordinates are

plotted in log units.

Thus, for a given range of input signals, to obtain a relatively uniform

perceptual discrimination of contrast such as can be achieved with a bright display and the transfer function defined by curve A, the transfer function defined

by curve B is needed for a dim display. To evaluate this, two representative

system gamma functions have been selected for comparison in this research.

These are shown in Figure 2. Curve N in Figure 2 describes the system gamma

function identified in this report as "normal." Curve M describes the system

gamma function identified in this report as "modified." Photometric measurements were made of the display face from input obtained using an EIA Logarithmic Reflectance Chart. Three different levels of display highlight luminance

were used. Obtained logarithmic values were then averaged, and the sets of

data were shifted vertically to represent the case where highlight luminance

is 0.2 fL

2

V,._ , J+
HDinm Bright

Region [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 modifying

the system gamma function when luminance level of

television display is decreased.

3
-0.8-

S-1.0-

-1.4 M

0 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 Logrithmic

Reflectance 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 a

highlight luminance of about 0.05 fL might provide a pilot with an acceptable

presentation with which to perform night NOE flights and also have an acceptably low impact on dark adaptation. The data in the literature are reported

in trolands; in converting them to footlamberts, it was assumed that the observer would have a pupil diameter of 5 mm and that retinal integration time

would be 0.1 second.

A special simulation facility was developed, and three display parameters

were selected for initial investigation. The first parameter was CRT display

size. Because of limited panel space in the helicopter cockpit, the smallest

CRT that can be safely used by a pilot must be selected. Therefore CRT displays with an aspect ratio of 3:4 and diagonals of 13 cm and 26 cm were compared. With reference to real-world visual angles and rate of closure on

obstacles, these CRT displays, when viewed at about 69 cm, represented about

a 6x and 3x minification respectively.

The second parameter investigated was display luminance. Because the

pilot or copilot may periodically have to look through the windscreen, the CRT

display must be dim enough to optimize dark adaptation and bright enough to

permit adequate form perception when viewing the cockpit display.

The electro-optical system gamma function (i.e., the transfer function of

input luminance to CRT display luminance) was the third parameter studied. The

gamma function can be readily manipulated electronically in a TV system; such

manipulation, through local contrast variation, can enhance the visibility of

selected features (e.g., trees and green foliage).

The specific objective in studying the effects of the above parameters

is to provide data for developing some critical display requirements for a

helicopter-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 provide

various modes of presentation of stimulus materials to the participant pilots.

The three configurations used in the present series of research efforts are

described 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 participant with both a full-color windscreen display, which simulated real-world

visual angles, and a panel CRT display of the same scene but minified and in

monochrome. The windscreen display, which could be presented at various luminance 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 do

not represent endorsement by the Department of the Army.

5

'--.
Fresnel lens that placed the image at optical infinity for the observer. A

television camera (GBC model CTC 6000), mounted in line with the 16-am projector, was used to generate a simulated LLLTV display to the participants.

The use of a beam splitter suitably placed in the optical train provided the

participant simultaneously with the simulated windscreen display and the correlated LLLTV display. The size, luminance level, and system gamma function

of the simulated LLLTV display could be adjusted by the experimenter. A

schematic view of this system is shown in Figure 3.

In Figure 3, panel A is a plan view of the arrangement in which the simulated TV camera is fixed with respect to the aircraft. For this configuration, the TV camera must have its vertical sweep reversed. Panel B is a plan

view of the arrangement in which the simulated TV camera can be moved in azimuth and elevation with respect to the aircraft coordinate axes. Because of

the relatively low screen luminance and the need for using a small lens aperture to obtain depth of field, a silicon-intensified TV camera (SIT) (GBC

model NVC 100) was used here. Panel C shows a side view of the participant's

work station, which is the same for either of the above arrangements. In

panel A, focal length of the projection lens is 25 mm, faces of the beamsplitter are 50 mm by 50 mm, and the relay lens system used for converging

the rays to form an image on the TV camera tube comprises an Erfle eyepiece

of 37 mm focal length plus a Barlow lens of minus 44 mm focal length. The

Fresnel lens shown in panel C is 38 cm in diameter and has 49 grooves per cm

and a focal length of 32 cm. It images the projection screen (i.e., windscreen

display) at optical infinity. Also shown is the two-mirror arrangement for

presenting a virtual image of the cockpit monitor at 69 cm from the observer.

Configuration II. The windscreen stimulus materials in this configuration were presented by means of a CONRAC RQA 17 television monitor instead of

a rear-projection screen. The resultant monochrome windscreen display was collimated 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 display

through goggles mounted on a light-tight viewing hood. Display luminance was

varied by one of two methods. With the first method, only the experimenter

controlled the luminance of the display, and the luminance level was discretely

varied by inserting neutral density filters into the goggles. With the second

method, the goggles were fitted with a fixed and a rotating polaroid filter

that permitted the subject to continuously vary the luminance level; the setting could also be remotely monitored by the experimenter. Stimulus material

could be generated from an on-line TV camera or from videotape recordings played

back on a SONY videotape recorder (model VO 1800). Directly below the viewing

hood was another CRT monitor simulating a cockpit panel display viewed at 69 cm

and 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 luminance level was controlled by the participant, who set the angular position of

the rotating member of a pair of polaroid filters mounted in front of the simulated panel area. The panel display luminance level was continuously adjustable

and was remotely monitored by the experimenter. The participant's work station

environment was an enclosed, dark-walled, sound-attenuating chamber.

Configuration III. The participant's environment was the same as that

used in Configuration II. The source for the stimulus material, however, was

a 1.22 m by 1.83 m, three-dimensional, full-color terrain model designed to

6

,. .. .,
CINE MA ICiINE h MRO

FILM To' N U Ix M v 

C CTMIRROR

MONITOR

FRESNEL

REARPROJECTION

SCREEN 

Z -MIRROR

riqure 3. Schematic presentation of optical layout utilizing filskchain

projector for simulating a full-color windscreen display ani

correlated LLLTY cockpit monitor.

447
simulate partially wooded terrain at a 1:300 scale. It was used in conjunction with an optical probe mounted on a TV camera (GBC model NVC 10U) that

had a SIT camera tube to provide the video output. The probe (see Figure 4),

with its symmetrical optical system and telecentric aperture stop, was designed to have unit magnification, a 60-degree circular instantaneous FOV,

good color correction, and very little distortion. A scanning mirror, rotatable 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. Roll

angle and any needed derotation was obtained by rotating a Pechan prism about

the optic axis. (Rotational servo controls had not then been incorporated in

the probe, so that for a given simulated flight, the pitch, heading, and roll

settings remained fixed.)

When viewing directly through the probe, with entrance pupil set at about

0.5 mm and probe focus set for an object 26 cm from the entrance pupil, the

20/30 line of the Snellen eye chart (suitably reduced) was resolved for chart

placements anywhere in the FOV. Depth of field for this 20/30 resolution was

from 20 cm to infinity (i.e., for a 1:300 scale, from 60 m simulated to infinity), and at 5 cm (i.e., at 15 m simulated for a 1:300 scale) resolution

was 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 a

31-degree vertical real-world FOV, and resolution was degraded to about 20/90

Snellen (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 degrees of translational freedom (continuously varied), the position of the

terrain 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 computer

and associated peripherals and electromechanical equipment. The computer was

also used to record and display the participant's altitude, airspeed, flight

path, and number of crashes during simulated helicopter runs over the terrain

model. A schematic diagram of the probe is presented in Figure 4, and the relationship between probe and terrain model is shown in Figure 5.

Direct visual viewing of the display, with unit magnification, was obtained by removing the TV camera shown in Figures 4 and 5 and placing the

observer'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 of

20 mm focal length; and relay lenses 1 and 2 are each Schneider-Kreuznach

Componon 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 through

servo drives. In the present series of investigations, simulated movement of

the helicopter was simplified to closed loop control of translation only.

Participants

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

vision.

~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 components

in the optical probe.

9

-~~~~~~4 -. .-- -4" I 
3-D TERRAIN MODEL

OPTICAL PROBE

z

TV 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 simulated windscreen display of NOE flight in conjunction with a correlated instrument panel CRT presentation. The Configuration I apparatus (see Figure 3,

panels A and C) was used for this demonstration. The windscreen view was

presented at approximately full-moon luminance levels (scene highlight luminance of 0.01 to 0.02 fL). While the participants viewed this display, the

experimenter brought to their attention the angular minification present with

the 13-cm CRT monitor (6x minification) and the 26-cm CRT monitor (3x minification) in comparison to the real-world visual angles as seen in the windscreen

view. The experimenter also explained the potentially detrimental effect of a

bright 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 the

displays and still feel they could fly safely using such a system.

In the formal experiment, the Configuration II facility was used, and only

the panel display was presented. The participants viewed two 10-minute segments of NOE flight, recorded on videotape. One segment was filmed at the

Hunter-Liggett, Calif., ntilitary reservation, and the other segment was filmed

near Fort Rucker, Ala. The tapes were viewed by each participant pilot on the

13-cm and the 26-cm CRT displays. They could attenuate the display luminance

by 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. Participants also viewed each display with a normal system gamma function or with a

modified system gamma function designed to enhance detail in the luminance

range of terrain features such as trees and other green foliage. Thus, each

subject made judgments in eight different conditions: 13- or 26-cm CRT display, Hunter-Liggett or Fort Rucker terrain, and normal or modified display

system gamma function.

The pilots were instructed to view each display as though it were being

used to perform NOE flight at night. They were asked to make the display as

dim as possible without sacrificing any information necessary to conduct a safe

and effective mission. Each participant made six judgments with each display

condition, rotating the polaroid filter in alternate directions for successive

judgments to minimize positional cues in setting the display luminance. At

the end of the experiment, the participants were fully debriefed, their questions were answered, and any of their observations concerning the displays

were recorded. Each participant was asked to indicate a preference for either

the 13-cm or the 26-cm display and to comment on the desirability and utility

of having a system gaimma control.

RESULTS AND DISCUSSION

A 2 x 2 x 2 factorial ANOVA with repeated measures on all factors was

used to evaluate'the effects of CRT size, system gamma function modification,

and type of terrain on display luminance levels used by pilots to maximize

visual dark adaptation when flying with an LLLTV system. The independent

variables 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. heavily

wooded as at Fort Rucker). The dependent measure was the mean of the six

luminance judgments (in fL) made by each of the 24 pilots for each of the

eight display conditions.

The overall mean luminance judgments made by the 24 pilots for each display condition are presented in Table 1, and some pooled means for these data

are presented in Table 2. As indicated in these tables, the pilots felt that

they could tolerate a 29% dimmer display when using the 26-cm as compared to

the 13-cm CRT monitor (F (1,23) = 40.58, p < .01). It can also be seen that

with a normal system gamma function, pilots required a brighter (i.e., higher

luminance) display when flying over heavily wooded terrain than when flying

over 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 a

terrain by contrast interaction (F (1,23) = 40.58, p < .01).

Table 1 shows that system gamma function modification had little if any

effect for semi-arid terrain, but allowed for a 29% dimmer display during

flights over heavily wooded terrain. No other interactions were statistically

significant. Also, when a 26-cm display size was used in conjunction with a

modified system gamma function, mean display highlight luminance, selected by

pilots as adequate for night flying, was 0.06 fL or less. This compares

favorably 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 comfortable using the 26-cm display, 4% preferred the 13-cm display, and the remainder had no preference. All the pilots felt that the modified system gamma

function 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 use

of a 26-cm CRT display when using an airborne LLLTV system. When using the

26-cm display as opposed to the 13-cm CRT display, pilots could tolerate a

dimmer display, thus reducing the adverse effects of display-viewing on visual

dark adaptation. In addition, the pilots felt more comfortable when using the

26-cm display. Preference was usually expressed in terms of a feeling of having

a larger real-world field of view, even though the 26-cm and the 13-cm displays

both presented the same real-world field of view. The "larger," subjective

visual FOV provided by the 26-cm display gave pilots greater visual comfort,

perhaps because the 26-cm display reduced the restriction on scanning eye

movements.

12

......
Table 1

Mean Highlight Luminance Settings (in Footlamberts) for Display

Size, System Gamma Function, and Terrain Type

System gama function

Normal Modified

Heavily Heavily

Semi-arid wooded Semi-arid wooded

Display 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 Highlight

Luminance Settings Listed in Table I

Mean setting

Independent variable (in footlamberts)

Display size

13 cm 0.086

26 cm 0.061

System gamma function

Normal 0.082

Modified 0.066

Terrain type

Semi-arid 0.056

Heavily wooded 0.092

13

- ...........

BEEN
A second general conclusion is that the modified system gaimna function

may be useful when flying over heavily wooded terrain. The pilots thought

that the fine structure of the forest canopy was more easily discriminable

with this modification. Furthermore, they adjusted their displays to a lower

luminance when using the modified system gama function. They were apparently

able to discern the needed detail in the heavily wooded areas, even though

they were using a lower display luminance. The lack of differences with system

gamma function modification when flying over semi-arid terrain is not surprising,

since this terrain is comprised of high-contrast elements for either of the two

gamma functions used. This finding is important because it indicates that the

effect of system gamma function modification is terrain-specific and opens up

the possibility of developing optimal real-world-luminance to display-luminance

transfer functions for other characteristic terrain types.

A third conclusion, although tentative, is that pilots flying in moonlight

can set their cockpit CRT display at a luminance level high enough to provide

them with an adequate cockpit flight display and yet low enough to permit them

to have relatively rapid visual acquisition when subsequently viewing directly

through the windscreen (i.e., adequate visual dark adaptation for heads-up

viewing).

14

." *.
REFERENCES

Abbey, C. W., & Carson, F. L. Low, lower, lowest. United States Army Aviation

Digest, 1978, 24(4), 24-27.

Baker, H. D. The instantaneous threshold and early dark adaptation. Journal

of the Optical Society of America, 1953, 43, 798-803.

Baker, H. D. Initial stages of dark and light adaptation. Journal of the

Optical 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 contrast. 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.). New

York: Illuminating Engineering Society, 1972, Section 3.

Mueller, C. G. Frequency of seeing functions for intensity discrimination at

various 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 perception. New York: John Wiley and Sons, 1965.

Shlaer, S. The relation between visual acuity and illumination. Journal of

General Physiology, 1937, 21, 165-188.

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

15

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