AD-A254.983

U.S. Army Research Institute

for the Behavioral and Social Sciences

Research Report 1622

Selective Factors Affecting Rotary Wing

Aviator Performance With Symbology

Superimposed on Night Vision Goggles

John W. Ruffner, Monty G. Grubb,

and David B. Hamilton

Anacapa Sciences, Inc.

SDTIC_

ELECTE

SAUG2 7 1992 .D A

'i•2-23834

July 1992

Approyed for ps*t r*Wes; d&wlgbon unis W d.

I2
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

EDGAR M. JOHNSON MICHAEL D. SHALER

Technical Director COL, AR

Commanding

Research accomplished under contract

for the Department of the Army Accesion For

NTIS CRA&I

Anacapa Sciences, Inc. DTIC TAB 13

Unamiouaced 0

Justification

Technical review by B y ................. .. ..........................

N. Joan Blackwell Dist. ibLtioi I

Gabriel P. Intano Avaabiiiv Cov•cc

Robert H. Wright

Dist

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the U.S. Arny Research Institute for the Behavioral and Social Sciences.

NOTE: The findings in this report ae not to be construed as an official Depaitnent of the Anny posiion,

unle so designated by other audtzed documnents
Forp, ADDrok ea

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1. AGENCY USE ONLY (Leave WIank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

1992, July Interim Feb 91 - Dec 91

4. TITLE AND SUBTITLE S. FUNDING NUMBERS

Selective Factors Affecting Rotary Wing Aviator MDA903-87-C-0523

Performance With Symbology Superimposed on Night 62785A

Vision Goggles 790

6. AUTHOR(S) 1211

Ruffner, John W.; Grubb, Monty G.; and C03

Hamilton, David B.

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

REPORT NUMBER

Anacapa Sciences, Inc.

P.O. Box 428 AS1690-349-91

Fort Rucker, AL 36362-5000

9. SPONSORING /MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/ MONITORING

U.S. Army Research Institute for the Behavioral and AGENCY REPORT NUMBER

Social Sciences

ATTN: PERI-IR ARI Research Report 1622

5001 Eisenhower Avenue

Alexandria, VA 22333-5600 "11. SUPPLEMEN•TARY NOTES

Contracting Officer's Representative, Charles A. Gainer.

"12a. DISTRIBUTION/ AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Approved for public release;

distribution is unlimited.

"13. ABSTRACT (Maximum 200 words)

This report presents the findings of a review of the literature on night vision

goggles (NVGs), head-up displays (HUDs), and helmet-mounted displays (HMDs). The

review was conducted to identify factors affecting the performance of pilots using

the NVG-HMD system that superimposes symbology on the NVG image. The perceptual

and attentional problems associated with using NVGs, HUDs, and HMDs are well documented in the literature but are not well understood. The literature suggests that

use of the NVG-HMD system is likely to result in several perceptual and attentional

problems, most notably errors in distance estimation, inappropriate division of

attention, and spatial disorientation. A coordinated program of research on the

NVG-HMD system should be undertaken using laboratory simulation devices, flight simulators, and operational aircraft. Priority should be given to investigating

the effects of variables that are identified as important in the literature review,

and of practical significance to the Army and that have not been determined by

design decisions.

14. SUBJECT TERMS IS. NUMBER OF PAGES

Attentional switching Helmet-mounted display 69

Perceptual judgment Symbology 16. PRICE CODE

Head-up display Night vision goggles -- 17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

OF REPORT OF THIS PAGE OF ABSTRACT

Unclassified Unclassified Unclassified Unlimited

NSN 7540-01-280-5500 Standard Form 298 (Rev 2-89)

i-0• br ANS Std in-ia
Research Report 1622

Selective Factors Affecting Rotary Wing Aviator

Performance With Symbology Superimposed

on Night Vision Goggles

John W. Ruffner, Monty G. Grubb,

and David B. Hamilton

Anacapa Sciences, Inc.

Field Unit at Fort Rucker, Alabama

Charles A. Gainer, Chief

Training Systems Research Division

Jack H. Hiller, Director

U.S. Army Research Institute for the Behavioral and Social Sciences

5001 Eisenhower Avenue, Alexandria, Virginia 22333-5600

Office, Deputy Chief of Staff for Personnel

Department of the Army

July 1992

Army Project Number Human Performance Effecilveneas

2Q162785A790 and Simulation

Approved for pubfl relaese; diavbutw is unlirmied.

Ii
ACKNOWLEDGMENTS

The authors wish to express their appreciation to D. Michael

McAnulty, Anacapa Sciences, Inc., and Dennis K. Leedom and Robert

H. Wright, U.S. Army Research Institute Fort Rucker Field Unit,

who reviewed and critiqued earlier versions of this report.

vi
FOREWORD

The U.S. Army Research Institute Fort Rucker Field Unit is

an operational unit of the U.S. Army Research Institute for the

Behavioral and Social Sciences (ARI) and provides research support in aircrew training to the U.S. Army Aviation Center, Fort

Rucker, Alabama. Research is conducted in-house and augmented by

contract support as required. This report documents work performed for the MANPRINT Division under Task 1211, "Improving Crew

and Team-Level Performance in Aviation and Ground Operations" and

Work Unit 1211-C03 "Crew Performance Instrument Design" in support of an in-house research program on the Aviator Night Vision

Imaging System Head-up Display (ANVIS-HUD).

During low-level night vision goggle (NVG) flight, it is

difficult and potentially unsafe for aviators to divert their

attention from the external scene to obtain critical flight

information from cockpit instruments. The ANVIS-HUD, which

superimposes flight symbology on the view obtained through the

NVGs, has been proposed as a solution to this problem. The Army

is engaged in an accelerated system test, acquisition, and fielding program to acquire the ANVIS-HUD for the UH-60A/L, OH-58A/C,

UH-1V, AH-IF, and CH-47D aircraft. At a fully funded level, up

to 600 units of the ANVIS-HUD system will be acquired over the

next 5 years, representing an investment of approximately $150

million. However, many questions remain about the perceptual and

attentional problems associated with using the ANVIS-HUD.

This report presents the findings of a literature review

that identifies factors affecting the performance of pilots using

the ANVIS-HUD system. The report concludes that superimposing

symbology on the NVG is likely to result in perceptual and attentional problems that warrant investigation and recommends that a

coordinated program of research be undertaken that addresses

these problems.

It should be noted that the contractor conducting this

review did not have access to some of the recent Army tests of

proprietary prototype concepts for ANVIS-HUDs. Therefore, no

reference to this testing is included in this report.

Technical Director

v
SELECTIVE FACTORS AFFECTING ROTARY WING AVIATOR PERFORMANCE

WITH SYMBOLOGY SUPERIMPOSED ON NIGHT VISION GOGGLES

EXECUTIVE SUMMARY

Requirement:

During low-level night vision goggle (NVG) flight, it is

difficult and potentially unsafe for pilots to divert their

attention from the external scene to obtain critical flight

information from cockpit instruments. One solution to this

problem is to superimpose flight symbology on the view obtained

through the NVGs, thus creating an NVG-Helmet-Mounted Display

(HMD) system. The NVG-HMD may reduce workload by giving pilots

access to flight information while they maintain visual contact

with the external scene.

Although the NVG-HMD has several benefits, it may distract

pilots' attention from the tasks of obstacle detection, recognition, and avoidance and interfere with NVG visual scanning patterns. Further, the NVG-HMD may contribute to errors in pilots'

judgments of distance, altitude, and closure rates and to the

tendency to become spatially disoriented.

Although some information is available on pilots' performance capabilities and limitations with NVGs, head-up displays

(HUDs), and HMDs, little is known about the potential effects of

adding symbology to the NVG. Further, there has been no attempt

to integrate the available research findings on NVGs, HUDs, and

HMDs to guide research on the NVG-HMD system. Therefore, the

U.S. Army Research Institute Fort Rucker Field Unit requested

that Anacapa Sciences, Inc., review the literature on the NVG-HMD

system, identify the factors that affect the performance of

pilots using the system, and identify the research issues.

Procedure:

Researchers conducted a review of the literature on the use

of NVGs, HUDs, and HMDs. The purpose of the review was to summarize relevant findings in these areas and to identify issues that

should be investigated in a program of research on the NVG-HMD

system.

Findings:

The results of the literature review indicate that many of

the perceptual and attentional problems associated with using

NVGs, HMDs, and HUDs are documented but are not well understood.

vii
With few research results available on HMDs and NVG-HMD system,

little is known about the perceptual, attentional, and

performance consequences of these devices.

NVGs are limited by low resolution and a narrow field of

view (FOV). Pilots using NVGs can overestimate distances and

underestimate closure rates, become spatially disoriented, and

experience high levels of fatigue and workload. The performance

capabilities and limitations of pilots using NVGs must be considered a part of evaluating the effects of adding an HMD. In addition, many factors affecting pilot performance with NVGs may

interact with factors affecting pilot performance with HUDs and

HMDs.

When pilots view the external scene through a collimated

HUD, there is a tendency to overaccommodate to the symbology,

which appears to be closer than the real world scene. Pilots

have difficulty attending to both the HUD symbology and the real

world scene and must switch their attention back and forth between the two. This may be due to cognitive influences on accommodation or to a requirement to process information in a serial

rather than parallel manner. The ability to divide attention

between two activities presented on the same display is likely to

improve with practice.

When viewing both HUD symbology and a real world scene or a

synthetic image, there is a tendency to fixate on one source of

information. Thus, pilots experience difficulty detecting unexpected events in the symbology or the external scene, especially when the workload is heavy. The variables that affect

attentional fixation are not well understood and should be investigated. However, it appears that attentional and cognitive

factors are equally or more important than sensory and perceptual

factors.

Many findings from HUD research, such as the difficulty

pilots have dividing their attention between symbology and the

external scene, may generalize to HMDs. However, the effects of

many characteristics unique to HMDs (e.g., proximity to the

pilot's eyes, attachment to the pilot's head and presence in the

pilot's field of vision, and the potential for monocular or

binocular symbology and imagery) should be investigated.

Informal evaluations of prototype NVG-HMD systems have

reported that use of the NVG-HMD reduces workload and increases

the safety of NVG flight. Most of the problems reported in the

evaluations were attributed to improper fit, adjustment, or

operation of the NVG-HMD system. Perceptual and attentional

problems with the NVG-HMD were cited indirectly, possibly because

few perceptual or attentional problems existed or because their

effects were small relative to fit, adjustment, or operational

problems.

viii
A research program that investigates the effects of the NVGHMD on distance estimation, event detection, and spatial orientation is recommended. The program should be a coordinated effort

using laboratory simulation devices, flight simulators, and

operational aircraft. Priority should be given to investigating

the effects of variables that are identified as important in the

literature review and of practical significance to the Army and

that have not been determined by design decisions.

Utilization of Findings:

The results of the literature review should form a basis for

planning and conducting a systematic program of research on the

NVG-HMD system.

ix
SELECTIVE FACTORS AFFECTING ROTARY WING AVIATOR PERFORMANCE

WITH SYMBOLOGY SUPERIMPOSED ON NIGHT VISION GOGGLES

CONTENTS

Page

INTRODUCTION ........................... I

objective .. .. .. .. .... . . . .. .. .. .... ... .2

Organization of the Report. . . . . . .......... 2

PHYSIOLOGICAL OPTICS.............. . ......... . . . 3

Light, Lenses, and Acuity ................ 3

Visual Receptors ...... . . ............. 4

Modes of Vision . . . . . . . . . . . . . . . . . . 5

Visual Angle . . . . . . . . . . . . . . . . . . . . 5

Field of View . . . . . . . . .. ........ . . . 5

Perception of Distance and ept 6

Modes of Presentation...........7

NIGHT VISION GOGGLES ................... 8

System Characteristics t..... . 8

NVG Factors That Degrade Vision . . . . . ... . . . . 10

Illusions and Misperceptions .... .......... . . . 13

Spatial Disorientation .. .... .. ... 14

HEAD-UP DISPLAYS .o. .o. ... o. .. .. ..... o.... 15

HUD Characteristics . ..... ...... ......... 16

Attentional Issues. . . . o.. .. .. ....... . . . . 18

Spatial Disorientation . .. o .. o . 22

Physiological Basis for HUD Design .............. 23

HELMET-MOUNTED DISPLAYS oL. . o ................ . 24

Perceptual Aspects of HMDs.. . . . . . . . . . . . . 25

Divided Attention .o . . . . . . o . . . . . . . . . . 28

Spati••Disorientation 28

NVG-HMD Characteristics .o . . . . . .. . ... o . . . 28

Evaluations of Prototype NVG-HMD Systems .......... .. 30

PERCEPTUAL AND ATTENTIONAL ISSUES .... .............. 33

Perceptual Judgment Errors. . . . ......... . . 34

Attentional Switching.... . . . . . . . . . . . . 35

CONCLUSIONS . . . . . . . . o . . . . .............. 35

xi
CONTENTS (Continued)

Page

RECOMMENDATIONS FOR RESEARCH . ............. . 37

Research Issues . . . .................... 37

Independent Variables . . . .... . . . . . . . . . . . 38

Dependent Variables . . . . . . . . . . . . . . . . .. 39

Research Apparatus ............ ................... .. 44

Research Approach ................... ........... 46

REFERENCES . . ........................... 49

LIST OF TABLES

Table 1. Nighttime Illusions and Misperceptions ..... 14

2. Independent Variables for NVG/HMD Research:

NVG Characteristics ...... ............... .. 39

3. Independent Variables for NVG-HMD Research:

HMD Symbology Characteristics ... .......... .. 40

4. Independent Variables for NVG-HMD Research:

Pilot Characteristics ...... .............. .. 41

5. Independent Variables for NVG-HMD Research:

Environmental/Task Characteristics ....... .. 41

6. Importance of Variables for NVG-HMD Research . 42

7. Dependent Variables for NVG-HMD Research . . .. 43

8. Recommendations for Using Research Apparatuses

in an NVG-HMD Research Program . . . . . . . . . 47

LIST OF FIGURES

Figure 1. Night vision goggles .................. ...... 9

2. Schematic representation of an NVG image

intensifier tube. . . .......... . . . . 9

3. Example of a head-up display. ....... .... 16

4. NVG-HMD visual perspective .c..i.... ...... 29

5. NVG-HMD laboratory simulation apparatus . . . 45

xii
GLOSSARY OF ACRONYMS AND ABBREVIATIONS

AN/AVS Army Navy/Aviator Vision System

AN/PVS Army Navy/Pilot Vision System

ANVIS Aviator Night Vision Imaging System

CRT Cathode Ray Tube

FLIR Forward Looking Infrared

FOV Field of View

H Horizontal

HDD Head-Down Display

HMD Helmet-Hounted Display

HUD Head-Up Display

m Meter

MS Millisecond

NASA-TLX National Aeronautics and Space Administration Task Load

Index

NOE Nap-of-the-Earth

NVG Night Vision Goggles

PNVS Pilot's Night Vision System

s Second

STRATA Simulator Training Research Advanced Testbed for

Aviation

SWAT Subjective Workload Assessment Technique

TRS Training Research Simulator

V Vertical

VFR Visual Flight Rule

xiii
SELECTIVE FACTORS AFFECTING ROTARY WING AVIATOR

PERFORMANCE WITH SYMBOLOGY SUPERIMPOSED

ON NIGHT VISION GOGGLES

Introduction

Army doctrine requires that helicopter pilots perform

many night missions at terrain flight altitudes using night

vision goggles (NVGs), a binocular night vision device that

amplifies available light. Night terrain flight with NVGs,

especially in the nap-of-the-earth (NOE) mode, is very

demanding and often results in a high level of individual

workload that is fatiguing. During night NOE flight with

NVGs, it is difficult and potentially unsafe for aviators to

divert their attention from the external scene to get flight

information from cockpit instruments (Department of the Army,

1988; Simmons, Kimball, & Hamilton, 1985).

The most recent version of the NVGs requires the pilot

to view cockpit instruments by looking under or around the

goggles. However, reading instruments that differ in

illumination and optical distance from the external scene is

time-consuming and difficult. Therefore, pilots often must

rely on verbal information from the copilots about the

aircraft status while maintaining their attention on the

external scene. This procedure requires a high level of

cooperation and coordination by flight crews and increases

crew workload.

To address this problem, the U.S. Army is considering

superimposing flight symbology (e.g., altitude, heading,

airspeed) on the NVG image. This combination is similar to

the head-up display (HUD) that projects symbolic information

on a combiner glass allowing the pilot to view both the

symbology and the external scene simultaneously. The

superimposed imagery and symbology will be displayed on a

surface mounted to the pilot's helmet, thus creating a type

of helmet-mounted display (HMD). This system is referred to

as the NVG-HMD in this report. 1

The NVG-HMD system has several potential benefits. The

pilots can access critical flight information without

iAlthough the popular literature sometimes refers to the

system as the NVG-HUD, the term NVG-HMD is more descriptive

and, therefore, is used in this report.

1
redirecting their gaze from the NVG image of the real world 2

scene to the instrument panel, and overall workload may be

decreased. However, the NVG-HMD symbology may

inappropriately distract the pilots' attention from obstacle

detection, recognition, and avoidance, and may interfere with

proper visual scanning patterns. The NVG-HMD symbology may

also affect the pilots' judgments of distance, altitude, and

closure rates and increase their tendency to become spatially

disoriented.

The NVG-HMD system was developed from several

technological advancements, most notably NVGs, HUDs, and

HMDs. Although a considerable amount of information is

available on pilots' performance capabilities and limitations

with NVGs, HUDs, and HMDs, little is known about the

potential effects of adding symbology to the NVGs.

Furthermore, the available research findings on NVGs, HUDs,

and HMDs have not been integrated to guide research on the

NVG-HMD system. Therefore, the U.S. Army Research Institute

Field Unit at Fort Rucker, Alabama, requested that Anacapa

Sciences, Inc., review the literature pertaining to the NVGHMD system, identify the factors that affect the performance

of pilots using the system, and identify the research issues.

This report summarizes the results of the literature review.

Organization of the Report

This report is organized into eight sections. The next

section provides background information on physiological

optics for readers who do not have a background in visual

perception. The third section describes the NVGs and

discusses relevant research. The fourth section discusses

research on HUDs that is important for understanding the

NVG-HMD system. The fifth section discusses research on HMDs

and summarizes the results of research on three prototype

NVG-HMD systems. The sixth section summarizes relevant

research on perceptual judgement and attentional switching.

The seventh section presents the conclusions drawn from the

literature review. The final section proposes

recommendations for research.

2 Throughout this report, the term real world is used to

differentiate the actual external, outside-the-cockpit visual

scene from that depicted by an image of the real world scene

on a panel-mounted display, HUD, or HMD.

2
Physiological Optics

A basic understanding of physiological optics and

associated perceptual phenomena is necessary for

understanding the capabilities and limitations of the NVG-HMD

system. Therefore, selected topics in physiological optics

and visual perception are discussed briefly in this section.

The overview is taken primarily from TC 1-204 Night Flight

Techniques and Procedures (Department of the Army, 1988) and

Coren and Ward (1989).

Light. Lenses. and Acuity

Humans are sensitive to a small part of the

electromagnetic spectrum, ranging from approximately 400

nanometers (violet) to 700 nanometers (red). Three important

characteristics of light are illumination, luminance, and

reflectance. Illumination is the amount of light that

strikes a surface from a source. Luminance is the amount of

light per unit area reflected from a surface. Reflectance is

the ratio of the amount of light reflected from the surface

to the amount of light reaching the surface.

Contrast is a measure of the difference in luminance

between an object and its background. Contrast increases

when the difference in luminance between an object and its

background increases. Contrast sensitivity refers to the

perceptual ability to distinguish adjacent areas that differ

in luminance.

Light entering the eye is bent, or refracted, by the

cornea and the lens and falls on the retina, a membrane

covering the inner surface of the eye. The cornea performs

most of the refraction. However, an additional amount of

refraction is required to focus light rays on the retina that

emanate from objects at different distances. Accommodation

is the process by which the lens changes shape to focus light

on the retina. The closer the object, the more the lens has

to accommodate to refract light.

The refracting power of the lens is measured in

diopters, which is the reciprocal in meters (m) of the lens'

focal length. A lens that has a focal length of 1.0 m has a

diopter value of 1.0. A lens that has a focal length less

than 1.0 m has a diopter value greater than 1.0. Conversely,

a lens that has a focal length greater than 1.0 m has a

diopter value less than 1.0.

3
The accommodative capacity of the lens may be

insufficient to bring an object into focus. This results in

different types of refractive or focusing errors. An

individual who is farsighted (hypermetropic) can see far

objects clearly but near objects are blurred on the retina,

resulting in a loss of acuity (i.e., the ability to detect

fine detail). In comparison, an individual who is

nearsighted (myopic) can see near objects clearly but far

objects are blurred. An individual who is slightly myopic

may experience a decrease in acuity at night, which is known

as night myopia.

When an individual looks at a visual scene that lacks

detail, there is a tendency for accommodation to relax to an

intermediate or resting level. In most individuals, this

distance is approximately 1 m. Dark focus is a similar

situation that can occur under low levels of illumination.

There are differences in the nearest and farthest point to

which an individual can focus and the resting level of

accommodation.

Visual Receptors

The retina contains two types of light-sensitive cells:

rods and cones. Cones concentrate in the central part of the

retina called the fovea. They operate under relatively high

levels of illumination and mediate higher levels of visual

acuity than the rods. Cones respond differently to different

wavelengths of light and mediate our perception of color.

The color that we perceive depends, in large part, on the

distribution of wavelengths in the light source and the

characteristics of the reflecting surface. Light sources

containing only a small number of wavelengths are called

monochromatic.

The rods concentrate in the periphery of the retina,

operate under relatively low levels of illumination, respond

to changes in luminance but not wavelength, and play an

important role in motion detection. Rod-mediated vision has

poorer acuity than cone-mediated vision. During darkness,

visual acuity decreases as the level of illumination

decreases and rods mediate visual perception.

The visual field is the area in space from which visual

information stimulates the two eyes. Visual information from

the left visual field stimulates the right half of each

retina and primarily projects to the right hemisphere of the

brain. Visual information from the right visual field

stimulates the left half of each retina and primarily

4
projects to the left hemisphere of the brain. A number of

cross-hemisphere connections, however, provide some

representation of the ipsilateral visual field in each

hemisphere.

Modes of Vision

Different ambient light conditions bring about three

modes of vision: photopic, mesopic, and scotopic. Photopic

vision functions under high levels of illumination during

which the cones are operating. High visual acuity and color

sensitivity are characteristic of photopic vision. Mesopic

vision functions at moderate light levels (e.g., dawn, dusk,

and full moonlight). Both the rods and cones operate during

mesopic vision. Scotopic vision functions under low light

levels. During scotopic vision, the cones become

ineffective, resulting in poor acuity and loss of color

vision. NVG viewing involves both mesopic and scotopic

vision.

During dark adaptation, the eyes increase their

sensitivity to low levels of illumination. Dark adaptation

results from the shift from photopic, or cone-mediated

vision, to scotopic, or rod-mediated vision. Most of the

increase in sensitivity occurs within the first 30 minutes of

dark adaptation, although additional increases in sensitivity

occur up to 45 minutes.

Visual Angle

Visual angle is a measure of the size of an object's

image that is projected on the retina. The object viewed is

said to subtend a given visual angle which is usually

expressed in degrees and minutes or milliradians. 3 As the

object moves closer, the visual angle increases; conversely,

as the object moves farther away, the visual angle decreases.

Field of View

Field of view (FOV) refers to the size of the visual

area that can be seen with the human eye or with a sensor at

one time. During normal viewing conditions, each eye has a

3There are 3600 in a circle, 60 minutes in 10, and 60 seconds

in I minute. One degree equals approximately 17.5

milliradians.

5
FOV of about 1200 vertical (V) x 1500 horizontal (H) . With

both eyes, the FOV increases to about 1200 V x 2000 H, with a

fair amount of overlap (Verona & Rash, 1989). The FOVs of

NVGs, HUDs, and HMDs are smaller than the FOV of the unaided

eye. For example, the NVGs provide a circular FOV of 400,

whereas the FOVs of some HUDs may be as small as 120 H. High

detail form vision is restricted to about 10 at the central

fovea. The more peripheral visual fields are sensitive to

visual flow from self-motion, object motions, and flashes.

Perception of Distance and Depth

Several sources of information or cues in the visual

environment influence our judgment of distance and depth. The cues can be classified as monocular, those requiring only

one eye to be effective, and binocular, those requiring both

eyes. Examples of pictorial monocular cues (i.e., those

available in pictures, drawings, and photographs) are

interposition, relative size, linear perspective, motion

perspective, texture gradient, shadowing, relative

brightness, aerial perspective, and relative motion.

Accommodation is another monocular depth cue. Because

the lens in the eye can focus only at one distance at a time,

some objects are focused while others are blurred. Thus,

objects in focus are judged to be at a different distance

than blurred objects. The cue is described as ancillary and

is not thought to be very powerful by most researchers.

However, some researchers (e.g., Iavecchia, Iavecchia, &

Roscoe, 1988; Roscoe, 1984; 1985; 1987a; 1987b) regard it as

a potential major accident causal factor.

Binocular cues. Certain characteristics of human vision

exist because our two eyes have a horizontal separation of

about 65 millimeters (mm). Thus, the eyes receive slightly

different images. This provides the basis for the depth cue

of binocular disparity or stereopsis.

Another binocular depth cue is convergence. Convergence

is the turning inward of the eyes on their vertical axis.

Because the best visual acuity is obtained when an image is

on the fovea, our eyes move to bring the image to the fovea.

When an object is near, both eyes must rotate inward to

accomplish this for both foveas. The eyes converge when

looking at a near object and diverge when looking at a far

object. There is evidence that the mechanisms for

convergence and accommodation are closely linked and together

they provide accurate depth information.

6
Modes of Presentation

Natural viewing involves the simultaneous use of both

eyes, with each eye receiving a slightly different image.

NVG, HUD, and HMD display systems use modes of presenting

visual information that differ from natural viewing

conditions. NVGs present binocular images on two displays

that are mounted to the helmet only a few centimeters from

the eyes. HUDs present symbolic information on a combiner

glass located a few feet in front of the pilot. HMDs present

different combinations of monocular or binocular symbology or

imagery on a combiner glass located only a few centimeters

from the pilot's eyes. For example, the AH-64A Helmet

Display Unit presents both imagery and symbology to the

pilot's right eye. The NVG-HMD presents imagery to both eyes

and symbology to one eye. All the display devices collimate

the images to optical infinity (i.e., greater than 50 m).

Collimation. Collimation is a procedure by which light

rays are adjusted so near objects, such as NVG, HUD, or HMD

displays, are located at optical infinity. Collimation

eliminates the need for the pilots to reaccommodate as they

shift their gaze back and forth between symbology and the

real world scene because both are located at the same

apparent distance.

Binocular rivalry. In normal binocular viewing, the

brain fuses the images from the two eyes into one common

percept. If the images are not markedly different, the brain

uses the disparity between the two eyes' images as a cue for

depth. However, when the images are markedly different, such

as with the AH-64A Helmet Display Unit or the NVG-HMD,

binocular rivalry occurs. One image becomes perceptually

dominant while the other is suppressed, with alternations of

dominance and suppression lasting between 1 - 4 seconds (s)

occurring between the two eyes or between different parts of

the left and right eye images.

The factors affecting dominance and suppression are

complex; however, an image with high contrast and much detail

will usually dominate (longer and more frequent periods of

domination) an image with low contrast and little detail.

Although individuals may prefer one eye over the other for

better acuity or performance on sighting tasks during

monocular viewing (Porac & Coren, 1976; 1986), total

dominance of one eye for extended periods is rare (Arditi,

1986; Beaton, 1985).

7
Night Vision Goggles

NVGs are binocular night vision devices that increase

nighttime visual capabilities by enhancing available light.

This section is divided into four subsections. The first

subsection summarizes the characteristics of the NVG system

and is based primarily on the literature of Brickner,

(1989b), Price and McLean (1985), and Verona and Rash (1989).

The second subsection discusses NVG factors that degrade

vision. Subsections three and four discuss illusions and

misperceptions that often occur when wearing NVGs, especially

the tendency to become spatially disoriented.

System Characteristics

The NVG system consists of two image intensifier tubes

and the hardware required to mount and position the tubes on

the pilot's helmet (see Figure 1). A lens on the far end of

the NVG intensifier tube (the objective lens) focuses light

on a sensor. Another lens on the near end (the ocular lens)

allows pilots to see the viewing screen at infinity focus and

to adjust for individual differences in refractive error. A

diopter setting in the near lens can be adjusted from +2

to - 6 diopters.

The objective lens focuses light entering the

intensifier tube onto a photocathode which is sensitive to

both visible and near infrared radiation (see Figure 2). The

light striking the photocathode causes a release of electrons

in proportion to the amount of light. The released electrons

are multiplied in a microchannel plate of small glass tubes

before striking a phosphor screen. The number and velocity

of the electrons striking the phosphor screen determine the

amount of light produced by the system. The result amplifies

the intensity of the image. The display image on the viewing

screen is collimated to infinity.

Each intensifier has an automatic gain control that

adjusts the tube's sensitivity to differences in ambient

illumination levels. The automatic gain control protects the

electron multiplier from extensive firing and damage from

exposure to high light levels.

The first NVGs used by Army pilots, the Army Navy/Pilot

Vision System (AN/PVS-5), were originally designed for ground

vehicle operators. The AN/PVS-5 uses second-generation

image-intensifier tubes and includes a full faceplate that

obscures most of peripheral vision. Compared to natural

8
Figure 1. Night vision goggles.

SCENE PHOTOCATHODE VIEWING SCREEN

MICROCHANNEL PLATE

2>F IMAGE

FIBEROPTIC TWISTOC L R EN

OBJECTIVE LENS "- PHOSPHOR SCREEN LN

Figure 2. Schematic representation of an NVG image

intensifier tube.

9
viewing conditions, the system has limited resolution and

FOV. Under optimal conditions (i.e., high brightness and

contrast), the best visual acuity obtainable is 20/50.

Resolution with the AN/PVS-5 is best in the center of the FOV

and decreases in the periphery. When the AN/PVS-5 is

properly fitted close to the eyes, it provides a circular 400

FOV. The image viewed through the AN/PVS-5 has the same 1:1

magnification as when viewing with the unaided eye.

A modified version of the AN/PVS-5 was developed using a

cutaway faceplate that obscured much less peripheral vision

than the full faceplate. This change allowed the NVGs to be

mounted to the helmet and flipped up when not in use.

However, if the NVG tubes are moved away from the eyes to

look under them, the circular 400 FOV decreases and the pilot

loses the periphery of the image.

The Army has recently developed a new generation of

NVGs, the Army Navy/Aviator Vision System (AN/AVS-6), known

as the Aviator Night Vision Imaging System (ANVIS). The

ANVIS, illustrated in Figure 1, uses third-generation

intensifier tubes and operates in a similar manner to the

AN/PVS-5. It has the same 400 FOV as the AN/PVS-5 but is

more sensitive to red and near-infrared light that is

characteristic of nighttime illumination. The ANVIS has

greater sensitivity, slightly improved central and peripheral

resolution, and weighs less than the AN/PVS-5. Although the

improved resolution of the ANVIS provides 20/40 acuity under

ideal conditions, acuity usually decreases under field

conditions. Tredici and Miller (1985) found that acuity

under starlight conditions dropped to less than 20/80 with

the ANVIS.

Although both monocular and binocular cues are available

for judging distances with unaided vision, the cues available

for judging distances with the NVGs are primarily monocular.

According to Wiley (1989), NVGs essentially eliminate

stereopsis, causing NVG depth perception to be similar to

monocular vision. However, having two independent images

improves perceived brightness.and contrast and reduces visual

noise (Verona & Rash, 1989).

NVG Factors That Degrade Vision

NVGs degrade pilots' visual capabilities, most notably

acuity and depth perception. Six commonly cited factors that

degrade visual capabilities are intensifier tube

deficiencies, inappropriate focusing, instrument myopia,

night myopia, degraded visual cues, and limited FOV.

10
Although the following paragraphs discuss the individual

influence of the factors, they may interact to degrade

pilots' visual capabilities in ways that are not well

understood.

Intensifier tube deficiencies. The NVG light

intensifier tubes lose their sensitivity to light as they

age. Because the tubes can be replaced independently, one

failed tube can be replaced before the other, resulting in

one tube being brighter. Moreover, the brightness is not

adjustable. Differences in brightness between the two tubes

can cause depth and movement illusions or suppress the image

from one of the eyes. This, in turn, may result in a

monocular rather than a binocular image. Suppression of one

of the images can also occur if the tubes are misaligned

(Brickner, 1989b).

Inappropriate focusing. Each NVG tube can be focused

independently. However, failure to focLs the NVGs properly

for each eye individually and then for the two eyes together

may result in the eyes accommodating to a nearer distance

than infinity. The resulting overaccommodation (focusing too

close) can cause eyestrain or blurred vision.

Instrument myopia. Pilots usually perceive objects seen

through NVGs as being farther away or smaller than they

actually are. Brickner (1989b) attributes this misperception

to instrument myopia, which is the persistent state of

overaccommodation while looking through an optical instrument

(Hennessy, 1975). The reasons for instrument myopia are not

completely understood. It may result from the tendency of

the lens of the eye to assume an intermediate resting state

without an adequate stimulus for accommodation.

When an individual overaccommodates during instrument

myopia, a negative diopter setting on the NVGs may be

required to focus the image. This over ccommodation

indicates a misperception of depth that may result in

inaccurate judgment of the size and distance of objects in

the visual scene (Brickner, 1989b).

Roscoe (1985) noted that optically and sensor-generated

computer-animated displays that present synthetic images of

the world produce systematic errors in size and distance

judgments. Objects are usually judged to be smaller and

farther away than they actually are and must be magnified for

objects to appear at the correct distance. Roscoe's research

suggest that pilots using NVGs are likely to overestimate the

distance of objects.

11
Night myopia. A visual phenomenon related to instrument

myopia is night myopia. Night myopia is a condition in which

an individual with normal day vision experiences myopia at

night. Leibowitz and Owens (1975) described night myopia as

the tendency to overaccommodate for distant objects as

luminance is decreased. The overaccommodation results from

the lens returning to a resting or dark focus position. The

resulting accommodation is a compromise between the

individual's idiosyncratic resting focus and that required by

the stimulus. Accommodation becomes progressively biased

toward the dark focus as decreased luminance degrades the

adequacy of the accommodative stimulus.

Degraded visual cues. Even when pilots focus NVGs

properly, estimating the size and distances of objects

accurately is difficult because NVGs degrade the monocular

visual cues normally available for estimating size and

distances. The NVG image lacks variations in color and

reduces variation in shade, detail, and texture, which are

important perspective distance cues. During daylight,

objects with poorer detail or texture and objects whose

normal colors appear washed out and bluish-gray are perceived

as being farther than objects with finer texture and vivid

color. The loss of image resolution is considered one of the

major factors that causes errors in distance estimation.

Limited field of view. The 40* circular FOV provided by

the NVGs reduces the number of objects visible at one time.

With fewer objects visible, pilots are less able to use the

relative size of known objects as distance cues. Pilots are

less likely to see as much object overlap, or interposition,

as with a wider FOV. Weintraub (1987) proposed that a narrow

FOV produces a framing effect in which objects inside the

frame are perceived as smaller than their actual size.

With a narrow FOV, the pilot also loses some of the

streaming effect (motion perspective) from objects passing

out of sight in the periphery. Streaming is considered an

important cue for spatial orientation and for perceiving

speed of movement and closure rates. It is most effective at

off-angles to the direction of motion and least evident in

the direction of motion. By attending to the external scene

straight ahead, the pilot may overestimate the distance of

objects and underestimate closure rate (Brickner, 1989b).

Roscoe (1984) required student observers to adjust the

magnification of a periscope with a 300 square FOV so objects

viewed through the periscope were the same apparent size as

when viewed directly. On the average, the observers

magnified the periscope image approximately 1.2 times.

12
Roscoe's results suggest that errors in judging distances

with a limited FOV may occur because of the loss of

foreground texture, an important cue for estimating distance.

Foyle and Kaiser (1991) obtained distance estimates from

four Army helicopter pilots under seven different viewing

conditions: day unaided, day unaided with 400 circular FOV,

night unaided, night unaided with 400 circular FOV, AN/PVS-5,

ANVIS, and the AH-64A pilot's night vision system (PNVS), which

has a 4 0 ° H by 30 * V FOV. Two of the four pilots overestimated

distances and two underestimated distances across all viewing

conditions. The researchers' results did not support the

commonly reported finding that distances are overestimated with

NVGs.

The size of the NVG FOV has also been found to affect the

accuracy of aviators' judgements of ground speed. For example,

Armstrong, Hofmann, Sanders, Stone, and Bowen (1975) found

aviators' ground speed judgments with NVGs were closer to the

judgments obtained with the unaided eye when the NVG FOV was

600 rather than 400.

Illusions and Misperceptions

The factors discussed above, both alone and in

combination, may result in several types of misperceptions when

using NVGs. Many of these problems have been reported during

unaided night flight and are described in TC 1-204, Night

Flight Techniques and Procedures (Department of the Army,

1988). Seven misperceptions that occur during normal night

flight and that may be increased by the use of NVGs are

summarized in Table 1.

Little research literature is available that documents

the frequency of illusions and misperceptions with NVGs.

Crowley (1991) conducted a survey of 221 pilots with NVG

experience and identified several perceptual problems

associated with using NVGs. The most frequently reported

problems were (a) undetected aircraft drift (18%),

(b) illusory aircraft drift (14%), (c) faulty height judgment

(16%), and (d) disorientation (12%). However, the influence

that NVGs have on these percentages cannot be determined

because the researchers did not measure the incidence of the

illusions during unaided night flight.

13
Table 1

Nighttime Illusions and Misperceptions

Illusion/

misperception Description

Autokinesis An illusion of movement caused by

staring at a stationary light source

in a dark environment

Illusory motion An illusion of movement brought

about by lights believed to be

stationary (e.g., another aircraft)

beginning to move

Height misperception Judging the aircraft to be higher

than it actually is above the

te-:rain

Fixation Stopping a normal scan pattern and

directing attention to an outside

object or on an instrument display

Size-distance Interpreting a light source that is

increasing or decreasing in size as

approaching or retreating

Reversible perspective Judging another aircraft to be

moving away when actually

approaching

Confusion of lights Confusing ground lights with stars

or with lights from another aircraft

Spatial Disorientation

Aviators using NVGs are likely to experience spatial

disorientation. Spatial disorientation occurs when a pilot

has an erroneous sense of the aircraft's position relative to

the ground (Gillingham, 1990). It is thought to result from

a conflict between vestibular cues (as the pilot experiences

the motion of the aircraft) and visual rues (as the world

visually appears to be moving). Because of the limited NVG

FOV, pilots must use frequent head and eye movements to scan

the external environment. Adopting a proper scanning pattern

14
is very important for effective use of NVGs (e.g., Department

of the Army, 1988); improper scanning has been cited as a

major contributor to NVG accidents among Army aviators (U.S.

Army Safety Center, 1991). If the head is moved too rapidly

while scanning, spatial disorientation may result (Brickner,

1989b; Price & McLean, 1985). Pilots using NVGs are

cautioned to rotate their heads and to move their eyes slowly

in a continuous scan pattern to avoid disorientation.

In addition to rapid head movement while wearing NVGs,

the loss of object detail and the restricted NVG FOV may

cause spatial disorientation. Spatial disorientation may be

more likely when critical details of the visual scene are

lost or when the pilot transitions from visual to instrument

flight conditions and has not begun a full instrument scan

(Vyrnwy-Jones, 1988).

Head-Up Displays

The HUD is a collimated display in the pilot's forward

FOV that projects flight symbols on a combiner glass between

the aviator and the aircraft windshield. The HUD

simultaneously prcjects onto the retina both the distant real

world scene and the superimposed flight symbology. An

example of a HUD is shown in Figure 3. Because the NVG-HMD

system can be considered a HUD that is attached to the NVGs,

the characteristics of HUDs are discussed and research on

pilot performance with HUDs is reviewed before discussing

HMDs.

HUDs were developed in the 1950s as an aid to pilots of

high-performance fixed wing aircraft flying under visual

flight rule (VFR) conditions. The first HUD-type displays

were crude World II gun and bombing sights and a few aids for

making approaches. However, the potential application of

HUDs to other mission phases and to rotary wing aircraft

subsequently led to a variety of design formats (Egan &

Goodson, 1978).

Section 3.5 of MIL-D-81641 (AS) (Department of Defense,

1972) contains the design specifications for HUDs. According

to Egan and Goodson, most of the specifications are based on

expert opinion rather than empirical research. The

specifications serve a purpose in setting standards for

symbology but fall short in providing data to support the

standards.

15
35 00 01

115 252

152GS "---. 1O50VS

180

Figure 3. Example of a head-up

display.

During the past 10 years, a great deal of research has

been conducted on the effectiveness of HUDs. The impetus for

some of this research has been the high incidence of pilots

flying fully functional HUD-equipped aircraft into the ground

(Haber, 1987). Much of the research has focused on object

detection and avoidance, distance estimation, flight path

control at low altitudes, and recognizing and recovering from

unusual attitudes at high altitudes.

The following subsections describe the optical

characteristics of the HUD, attentional and cognitive factors

that affect performance with HUDs, and HUD design features

important for spatial orientation. The subsections also

address HUD symbology selection and format issues. The

section concludes with a brief discussion of the need to

design HUDs to be consistent with pilots' visual and

cognitive capabilities. Detailed reviews of HUD research can

be found in Egan and Goodson (1978) and Larish and Wickens

(1991).

HUD Characteristics

A pilot's ability to use a HUD effectively is dependent

on several characteristics of the HUD. Two characteristics

that are relevant to the NVG-HMD system are the size of the

HUD FOV and the collimation of the HUD symbology.

Field of view. The FOVs of HUDs are relatively small,

ranging from about 120 to 180 (Brickner & Foyle, 1990). To

16
see both the HUD symbology and the real world scene, a

pilot's attention must be directed through the HUD's

restricted FOV. Several research efforts studied the effect

of FOV size on aircraft control and target detection.

The advantages of a wide FOV were reported in a series

of three experiments (Venturino & Wells, 1990; Wells,

Veriturino, & Osgood, 1988, 1989). For example, Wells,

Venturino, and Osgood (1989) investigated target detection

and spatial awareness using helmets with FOVs ranging from

200 H x 200 V to 1200 H x 600 V. Decreasing the size of the

FOV significantly decreased the percentage of targets hit and

significantly increased the time required to detect targets.

The optimal FOV size depended on the task. Less difficult

tasks could be performed effectively with a 200 H FOV while

more difficult tasks required a 600 H FOV.

Brickner and Foyle (1990) investigated the effects of

different size sensor FOVs (250, 400, and 550 H) on pilot

performance. One experime-t used a head-down display (HDD)

and the other used a HUD. The HDD and the HUD formats

differed only in their placement relative to the subject's

line of sight. In both experiments, eight subjects flew a

simulated helicopter through a slalom course. The display

consisted of a takeoff site, course entrance and exits,

horizontally striped pylons, and a ground grid. The subjects

were instructed to take the most efficient path through the

course and minimize the number of pylon hits and gate

(opening between pylon) misses. Hits were defined as

striking a pylon and misses were defined as missing a gate.

The most efficient course was defined as one that minimized

the average turn distance around the pylons.

Significantly fewer hits, misses, and smaller average

turn distances were found with larger FOVs than with smaller

FOVs. The researchers concluded that the subjects perceived

the sensor display as the entire world rather than as a

window into the world. That is, they did not keep objects in

memory that had passed out of the field of view. Thus,

narrow FOVs may increase memory demands for objects outside

the FOV.

Display collimation. HUDs are collimated so the

symbology, located physically about a meter from a pilot's

eyes, is located at optical infinity. Hull, Gill, and Roscoe

(1982) reported that accommodation to a real world scene was

closer to infinity than accommodation to the same scene

represented in a collimated color photograph. Iavecchia,

Iavecchia, and Roscoe (1988) found that, when subjects viewed

a distant scene directly and through a HUD, accommodation

17
shifted inward significantly (about +1 diopter) under the HUD

viewing condition. Iavecchia et al. suggested that pilots

perceive real world objects as more distant than they

actually are when shifts in accommodation are brought about

by viewing HUD symbology. Roscoe (1987a, 1987b) suggested

that pilots cannot process HUD information and the real world

scene simultaneously because the HUD symbology causes

overaccommodation.

The results of Hull et al. (1982) and Iavecchia et al.

(1988) suggest that either visual or cognitive depth cues

cause overaccommodation when viewing HUD symbology. Norman

and Ehrlich (1986) proposed that accommodation may be

affected by the awareness that the HUD is physically nearby.

Pilots know that they do not monitor symbols or characters at

distances beyond arm's reach, certainly not as far as real

world features.

Debate continues over the operational significance of

pilot overaccommodation (Hale, 1990). Roscoe (1987a, 1987b)

suggests that overaccommodation causes accidents. Other

researchers (e.g., Newman, 1987; Weintraub, 1987) do not

judge the problem to be as serious as Roscoe does.

Attentional Issues

Pilots often fixate on HUD symbology and ignore critical

information in the real world scene (Brickner, 1989b;

Fischer, Haines, & Price, 1980; Larish & Wickens, 1991).

Thus, the effects of attentional switching on pilots'

performance with HUDs (e.g., Fischer, 1979) must be

considered as a potentially significant safety issue in

adding symbology to NVGs.

Parallel versus serial processing. One objective of

placing HUD symbology at the same focal distance as the real

world scene is to facilitate parallel processing. However,

the intrinsically different nature of the information

presented on the HUD may contribute to the tendency for

pilots to perceive the HUD as separate information (Fischer,

1979) and thus disrupt parallel processing.

When symbology is superimposed on the real world scene,

a complex display is created whose movement differs from the

real world scene alone (Larish & Wickens, 1991). As the real

world scene moves across the visual field, the HUD symbology

remains in the same location and is detached from the

landscape. This enhances the perception that the pilot is

looking through the display. Thus, pilots may have

18
difficulty attending to the HUD and the real world scene

simultaneously because they are perceived as different

sources of information.

The results of several experiments (e.g., Brickner,

1989a; Fischer et al., 1980; Foyle, Sanford, & McCann, 1991;

Larish & Wickens, 1991; Neisser & Becklen, 1975; Weintraub,

Haines, & Randle, 1985) support the contention that HUD

symbology and the real world scene may not be processed in

parallel. Neisser and Becklen (1975) had student subjects

watch a video display that superimposed two unrelated

activities (i.e., bounce-passing a basketball and clapping

hands). When one of the activities was defined as more

important, the subjects failed to notice unusual events in

the less important activity. A difference in the nature of

two visual sources may be enough to interfere with parallel

processing even though the sources are located in the same

focal plane.

Fischer et al. (1980) used a simulation of a fixed wing

aircraft's approach to a runway to examine pilot performance

with a HUD. Eight experienced pilots each made an approach

in a full scale Boeing 727 simulator with and without a HUD.

Long reaction times and failure to detect an unexpected

airplane on the runway suggested that the pilots had

difficulty processing the HUD and external scene in parallel.

Several pilots reported that they fixated on the HUD

symbology to the exclusion of the external scene.

Weintraub et al. (1985) measured the time required for

eight subjects to shift attention from symbolic displays to a

collimated slide of a runway. Five of the subjects were

pilots. One display was a collimated HUD and another was an

HDD located 100 below the subject's line of sight at an

optical distance of 3 ft. The HUD-to-runway reaction time

was 86 milliseconds (ms) shorter than the HDD-to-runway

reaction time. The researchers considered this difference to

be functionally insignificant; however, three of the pilots

failed to notice an unexpected stationary aircraft on the

runway.

In both the Fischer et al. (1980) and the Weintraub et

al. (1985) experiments, at least half of the pilot subjects

(2 of 4 in Fischer et al. and 3 of 5 in Weintraub et al.) did

not notice an aircraft on the runway. Fischer et al.

suggested there is a tendency for serial processing to occur

and for the pilots to attend more to the symbology than to

the outside scene because "there is much more immediately

perceivable change going on, calling for more attention" (p.

17). Despite degraded obstacle detection with the HUD in

19
both experiments, the pilots preferred to fly with HUD and

believed that it improved their performance.

Brickner (1989a) presented subjects with a dynamic

simulation of a helicopter flight through a slalom course

containing ground texture cues. The subjects were instructed

to maintain a specific altitude while reducing pylon hits and

gate misses. Brickner found that providing the subjects with

altitude information on a HUD and not in the visual scene

decreased altitude errors but significantly increased both

pylon hits and gate misses. He interpreted his findings as

demonstrating that, even though the HUD and the external

scene were at the same physical and focal distance, they

competed for the subject's attention and could not be

processed in parallel. He suggested that the HUD produced a

figure-ground relationship between the symbology and the

simulated external scene that degraded time-sharing ability.

In a follow-on to Brickner's (1989a) experiment, Foyle

et al. (1991) used a similar dynamic helicopter flight

simulation to examine the effects of providing pictorial

altitude information in the visual scene. The display

allowed the subjects to estimate altitude directly from the

visual scene cues (e.g., relative height of buildings)

instead of having to read a digital HUD. Subjects maintained

comparable altitude control in both the HUD condition and the

pictorial display condition. However, when the subjects used

only the HUD for altitude information, they made greater

flight path errors than when they used the pictorial display.

Foyle et al. argued that the information in the HUD and the

synthetic external scene is perceptually segregated and

requires serial processing. In comparison, the integration

of the information available from the HUD into the external

scene allowed the pilot to process the altitude and flight

path information in parallel.

Attentional fixation and cognitive capture. Attentional

fixation and cognitive capture are terms that have been used

more or less interchangeably in the literature. Attentional

fixation occurs when an observer attends to a specific

element or area in the visual field for an inappropriately

long time instead of scanning the entire field. The area may

be a confusing or critical element in the external visual

scene or a changing indicator in the symbology.

In many circumstances, there may be good reason for

fixating longer than normal on one particular element in the

visual scene. However, pilots may not realize how long they

have focused attention on one item and may not notice other

critical events in the visual field. As Brickner (1989b)

20
noted, "The danger is that the pilot might 'forget' to look

at the world and spend most of the time watching flight

symbology, thereby ignoring obstacles and other crucial

information" (p. 19).

Larish and Wickens (1991) investigated attentional

fixation by presenting expected and unexpected events on the

HUD and in a synthetic visual scene. The researchers used 20

instrument-rated pilots in a dynamic computer simulation of

an approach to a landing. Task workload was manipulated by

using two different levels of turbulence.

The pilots' reaction times to unexpected events

presented on the display (a wind-shear warning) and in the

synthetic visual scene (another aircraft on the runway) were

slower when using a HUD than when using an HDD, especially

during periods of high task workload. Larish and Wickens

(1991) found no practical performance advantage for the HUD

over the HDD. They concluded that the advantages of the

conventional HUD may be related to its information content

and collimation rather than to its physical location. These

results were similar to those of Weintraub, Haines, & Randle

(1984) and Weintraub et al. (1985).

Allocation of attentional resources. Wickens (1984) has

proposed a multiple-resource theory of attention and timesharing that is useful for explaining many of the findings

discussed above. The theory states there are different

attentional resources with unique properties rather than one

pool of limited resources. Tasks that share resources are

more likely to interfere with each other. Three dichotomous

dimensions are used to define the resources: (a) two stagedefined processing resources (early vs. late processing),

(b) two modality-defined encoding resources (auditory vs.

visual encoding), and (c) two processing code resources

(spatial vs. verbal). When two tasks demand common resources

on any of the three dimensions, they are likely to interfere

with each other. This may take the form of less efficient

time-sharing, decreased performance as task difficulty

increases, or decreased performance on one task as resources

are directed to the other task.

Thus, Wickens' theory can explain the failure of pilots

to process the external scene and the HUD symbology in

parallel. Both tasks involve early processes (perceptual and

central processing) rather than late processes (selection and

execution of responses) and involve the visual modality.

However, some symbols (e.g., vertical altitude tape) require

spatial processing, while others (e.g., digital airspeed)

require verbal processing. In comparison, the visual scene

21
primarily requires spatial processing. According to Wickens'

(1984) theory, the requirement to monitor the HUD and the

external scene simultaneously should result in a degradation

in information processing capability. The degradation is

especially likely when the task requires a high level of

workload.

Effects of practice. In the experiments discused above,

the time that subjects practiced dividing attention between

superimposed images was relatively small, ranging from a few

minutes (Neisser & Becklen, 1975) to 2 hours (Fischer et al.,

1980). However, these experiments did not report data on

performance at the beginning or end of the practice trials or

following different amounts of practice.

In a follow-on to the Neisser and Becklen (1975)

experiment, Becklen and Cervone (1983) examined the effects

of practice on subjects' ability to divide attention between

two superimposed visual scenes. Increasing the subjects'

practice from 30 to 60 s significantly increased the

percentage of subjects who noticed unexpected events in the

visual scene. Stoffregen and Becklen (1989) found that

subjects noticed significantly more events in two

superimposed visual tasks after 2 days of practice.

Wickens (1984) reviewed several experiments studying the

effects of practice on time-sharing skills. He concluded

that efficient time-sharing performance results from the

combination of both automated processing of component tasks

and a true time-sharing skill. The time-sharing skill

involves developing optimal display sampling and response

selection strategies and learning to integrate the

information from different tasks. However, the extent that a

time-sharing skill acquired in one environment can transfer

to other environments is not well understood.

The research suggests that a pilot's ability to divide

attention between HUD symbology and the external scene

improves with practice. Newman (1987) proposed that a lack

of experience using HUDs and HMDs was the major obstacle to

their effective use. Therefore, the effect of practice on a

pilot's ability to use the NVG-HMD effectively is an

important issue that should be further investigated.

Spatial Disorientation

When external visual cues are degraded, such as in

limited visibility, pilots get pitch and roll information

from the aircraft attitude indicator to maintain correct

22
spatial orientation. Therefore, an unambiguous display of

pitch and roll attitude information is important when an

aircraft is in an unusual attitude. Unusual attitudes can

result from factors such as turbulence or distraction of

attention. The HUD displays attitude information to keep

pilots properly oriented while they look outside the aircraft

and to help them recover safely from an unusual attitude.

Research cited in Deaton, Barnes, Kern, and Wright

(1990) indicated that 30% of pilots flying with conventional

HUD systems reported an increase in spatial disorientation

when they used HUDs. Concern about HUD attitude displays has

increased interest in improving the communication of attitude

information and unusual attitude recognition in high

performance fixed wing aircraft (e.g., Deaton et al., 1990;

Dudfield, 1991; Ercoline, Gillingham, Greene, & Previc, 1989;

Osgood & Venturino, 1990; Weinstein & Ercoline, 1991; Zenyuh,

Reising, McClain, Barbato, & Hartsock, 1987). The

researchers used simulated HUDs and low fidelity static and

dynamic simulations to investigate the effects of different

variables on reaction time and control error.

Several HUD design features have been identified that

improve a pilot's spatial orientation and facilitate the

recognition of unusual attitudes. Examples of the features

are directional arrows, color coding, and angled pitch ladder

lines. Although recovery from attitudes such as inverted

flight may not be a serious problem for helicopter pilots,

the design of the HUD attitude indicator display is

important. Simmons, Lees, and Kimball (1978) observed that

attitude information accounted for 35 - 45% of the

information that a helicopter pilot needs during instrument

flight.

Physiological Basis for HUD Design

The research suggests that pilots process information

from the HUD and the external visual scene in a serial rather

than a parallel manner. Previc (1989) suggested that

attentional resources may be used more efficiently and some

amount of parallel processing may be possible if engineers

designed HUD symbology to be consistent with the natural

perceptual capabilities of the visual system. He recommended

that HUDs should (a) be consistent with the division of

attention between near and far space, (b) exploit the global

perceptual capabilities of the visual system that require

fewer attentional resources, (c) use symbology cues that are

important in elementary figure-ground separation and that are

preattentive (i.e., before the commitment of attentional

23
resources), and (d) apply a clear and unambiguous frame of

reference for depicting the movement of the aircraft with

respect to the horizon.

Previc (1989) suggested that the HUD information should

be arranged in quadrants, with airspeed and altitude

information located in the upper left and upper right

quadrants, navigation and weapons information located in the

lower left and lower right quadrants, heading information

located in the lower center, and attitude information located

in the middle of the display. All the information in each

quadrant should be related to the same function to allow it

to be processed during a single glance. Finally, he

suggested that the upper right half of the visual field is

better for processing information about distant objects,

while the lower left half of the visual field is better for

processing information about near objects.

Helmet-Mounted Displays

HMDs have some of the same properties as HUDs and much

of the research discussed in the section on HUDs is relevant

to HMDs. Although many of the results from HUD research may

generalize to HMDs, there are at least three notable

differences between HUDs and HMDs that need to be taken into

account.

First, HMDs are located a few centimeters from the

pilot's eyes, whereas HUDs are located about a meter away.

Second, because HMDs attach to the helmet and follow the

pilot's head, they are always in the pilot's FOV. Third, the

pilot views HUD symbology and the real world scene

binocularly; the HMD can display several combinations of

monocular or binocular symbology and imagery. These

differences add a level of complexity to HMD research that is

not present in HUD research.

The remainder of this section is organized into five

subsections. The first subsection discusses several issues

in visual perception that are important for understanding

pilot performance with HMDs. Issues in divided attention and

spatial disorientation are briefly addressed in the second

and third subsections. The fourth subsection describes the

basic characteristics of the NVG-HMD system. The final

subsection discusses three evaluations of prototype NVG-HMD

systems.

24
Perceptual ASpectS of HMDs

Four aspects of visual perception that are important for

an understanding of aviator performance with HMDs are

accommodation and convergence, binocular rivalry, distance

estimation, and FOV. These are addressed briefly in the

following paragraphs.

Accommodation and convergence. How the eye responds to

HMD symbology and imagery is of critical concern to the

design of the HMD. Four HMD configurations are likely:

binocular imagery and symbology, monocular symbology and

imagery presented to different eyes, monocular symbology and

imagery presented to the same eye (e.g., the AH-64A Helmet

Display Unit), and binocular imagery and monocular symbology

(e.g., the NVG-HMD).

McLean and Smith (1987) reviewed research on the effects

of monocular and binocular viewing of HMD imagery and

symbology. They concluded there was evidence that night

vision device imagery should be binocular but that additional

research was needed to determine whether symbology should be

monocular or binocular.

Moffitt (1989) examined convergence and accommodation to

different HMD configurations with varying scene backgrounds

and attentional instructions. Two subjects viewed a

collimated slide of symbology superimposed on collimated

slides of either clouds or mountains. The subjects were

required to attend first to the symbology and then to shift

their attention from the symbology to the external scene.

Viewing monocular symbology (one eye was blocked or

occluded 4 ) produced less accurate convergence and

accommodation than viewing binocular symbology. Viewing the

scenery decreased the effect.

Binocular rivalry. Binocular rivalry may occur when a

display presents different stimuli to each eye. For example,

the AH-64A Helmet Display Unit displays Forward Looking

Infrared (FLIR) imagery and symbology only to the pilot's

right eye. It may also occur when symbology is presented on

one of the two tubes of the NVG system, as is planned for the

NVG-HMD system.

4This occurs when a pilot viewing monocular imagery and

symbology eliminates binocular rivalry by closing the other

eye (e.g., the AH-64 Helmet Display Unit).

25
Little research has been devoted to studying pilots'

ability to control attention during tasks involving dichoptic

viewing (i.e., substantially different input to each eye).

Neisser and Becklen (1975) found that subjects were less

accurate at getting information from two scenes when the

scenes were presented to separate eyes than when the scenes

were presented to both eyes. Kimchi, Rubin, Gopher, and Raij

(1989) found that simple target detection was equivalent

under dichoptic and binocular viewing. However, in a followon experiment, Gopher, Grunwald, Straucher, and Kimchi (1990)

found performance decreased on tracking and letter

classification tasks under dichoptic viewing conditions.

Brickner (1989b) proposed that the binocular rivalry

created when symbology is superimposed on the NVG image of

only one eye creates two potential problems. First, if the

symbology presentation in front of the intensifier tube is

too bright, the NVG's automatic gain control changes the gain

on the intensifier tube, producing different display

brightnesses and different levels of dark adaptation in the

two eyes. The resulting brightness difference is known to

cause illusions of motion in depth. Second, the differences

in the images may interfere with their fusion. One of the

images may be suppressed (possibly without the pilot's

awareness), resulting in a loss of contrast sensitivity and a

decrease in the signal to noise ratio.

Distance estimation. As is the case with HUDs, pilots

frequently report that objects appear farther away when

viewed through an HMD. For example, 65% of the AH-64A pilots

surveyed by Hale and Piccione (1990) reported this effect

when viewing FLIR imagery through the AH-64's Helmet Display

Unit. However, none of the pilots reported experiencing the

problem when looking through the symbology at the real world.

In comparison, Bennett and Hart (1987) found that

objects appeared closer to AH-64A pilots when viewed through

a FLIR than with the unaided eye, especially when the FLIR

imagery was bright. This contradicts the results of Roscoe

(1984), Iavecchia et al. (1988), and Hale and Piccione

(1990).

The extent that the superimposed symbology of an HMD may

contribute to the misperception of distance reported with

HMDs used with night vision devices like NVGs and FLIR

sensors is unclear. It is likely that the sharp-edged, high

brightness symbology elements will have some effect on NVG

image perceptions. NVGs and FLIRs alone are generally

considered to cause distances to be overestimated. The

limited data of Foyle and Kaiser (1991), however, indicate

26
substantial individual differences in perceptual accuracy

(both overestimation and underestimation of distance) for

both direct vision and viewing through NVGs and FLIRs.

Because of the proximity of the HMD display and the

pilot's eye, it is unlikely that the HMD display provides an

effective stimulus for overaccommodation. Therefore, if

overaccommodation is occurring, it must be caused by other

factors. Most likely, poor distance estimation results from

a complex interaction of factors such as image and symbology

size and brightness, symbology location, FOV, and monocular

and binocular presentation modes.

Field of view. The effects of FOV size on pilot

performance discussed previously indicated that aircraft

control and obstacle avoidance performance improved with

larger FOVs. If magnification is used with a consequent

smaller FOV, however, detection and identification for

objects in the FOV will be improved (i.e., they will be seen

farther away), while the probability of detection and

identification will be reduced due to objects being beyond

the FOV. These findings can probably be generalized to HMDs.

However, when determining the potential benefit of larger

FOVs for HMDs, the resulting increase in helmet weight must

be considered.

Pilots fly most fixed wing aircraft at considerably

higher altitudes and faster airspeeds than rotary wing

aircraft. The high gravitational forces involved in

maneuvering and ejecting can harm pilots wearing helmets

weighing more than 3.5 lb (Tatro & Taylor, 1989). Therefore,

the HMD FOV in fixed wing aircraft is kept small to reduce

weight. Furthermore, because fixed wing aircraft are flown

at higher altitudes than rotary wing aircraft, the size of

the HMD FOV may not be as critical. For example, some

experiments involving fixed wing aircraft showed satisfactory

pilot performance with FOVs as small as 120 (Arbak, 1989).

Rotary wing pilots also experience high gravitational

forces during certain tactical maneuvers. However, rotary

wing pilots need a wide FOV to detect and avoid obstacles in

different directions during missions flown at low altitudes

(Simmons et al., 1985). Research is needed to determine the

optimal tradeoff between FOV size and factors such as

resolution and helmet weight for rotary wing pilots.

27
Divided Attention

Most of the research on the effects of superimposed

symbology on aviators' attention has been performed with HUDs

and was summarized in the preceding section. Aviators also

find it difficult to attend simultaneously to collimated HMD

symbology and either real world scenes or collimated imagery

(Crowley, 1991; Hale, 1990). However, because of several

characteristics unique to HMDs (e.g., the proximity of the

HMD image and symbology, monocular vs. binocular imagery and

symbology, slaving of imagery and symbology to head

movement), attentional effects are difficult to separate from

perceptual effects. Whatever the cause, pilots logically

must lose some amount of time switching attention between the

imagery and symbology when using HMDs.

As discussed before, helicopter pilots wearing NVGs must

use systematic scanning patterns to compensate for the

limited NVG FOV. Effective scanning techniques are acquired

through training and experience and may be resistant to

change. Superimposing symbology on the NVG image may

interfere with previously acquired scanning patterns.

Therefore, this problem warrants investigation.

Spatial Disorie t

Because the HMD is attached to the head, spatial

disorientation is likely to occur when a pilot looks in a

different direction than the one in which the aircraft is

moving; this is known as off-axis viewing. Another factor

that may contribute to spatial disorientation is that the

attitude indicator is referenced to the airframe rather than

to the pilot's gaze. The frequency of pilot spatial

disorientation increases with a narrow FOV, as with the AH64A Helmet Display Unit and the NVG-HMD, because the narrow

FOV eliminates many peripheral cues that allow the pilot to

resolve the sensory conflict (Hart & Brickner, 1989).

NVG-HMD Characteristics

HMD symbology for the NVG is created by taking data from

onboard aircraft systems, converting any analog signals to a

digital format, generating the symbology, and projecting the

symbology from a miniature cathode ray tube (CRT) onto a

semitransparent combiner lens. Depending on the design approach, the combiner lens is mounted either in front of or

behind one of the NVG intensifier tubes. The pilot sees what

appears to be a single image with superimposed symbology.

28
Figure 4 shows the visual images provided the two eyes by the

NVG-HMD. The NVG-HMD image is a combination overlay of the

visual perspective and symbology. Actual symbology in the

typical NVG-HMD will appear brighter rather than darker, as

depicted in Figure 4. The NVG-HMD allows pilots to select

different symbol sets during different phases of flight

(e.g., hovering, cruise) and to display separate symbol sets

to the pilot and copilot.

The NVG-HMD symbology must have enough brightness and

color contrast to be distinguished from the NVG external

scene image. Both the pilot and the copilot can

independently adjust the brightness of the NVG-HMD symbology

or turn off the symbology .

If the combiner lens is positioned in front of the

photocathode, the symbology will be the same monochromatic

green color as the NVG image. In this configuration, the

brightness of the symbology can cause the brightness of the

Figure 4. NVG-HMD visual perspective.

29
NVG image to decrease. In doing so, the goggle reduces its

sensitivity, resulting in a dark image and reduced quality of

the external scene view (Brickner, 1989b). Under these

conditions, objects in the external scene will have less

contrast and be more difficult to detect.

In comparison, if the combiner lens is positioned in

back of the photocathode, the brightness and color of the

symbology are independent of the NVG display. In this case,

the brightness of the NVG image does not affect the

brightness of the symbology and the symbology remains if an

NVG intensifier tube fails.

Both the NVG external scene image and the symbology are

collimated CRT images. Both images have the same focal

distance and theoretically require the same amount of

accommodation. Thus, there should be no response time delays

attributable to reaccommodation when the aviator switches

attention between the symbology and the external scene.

As discussed previously, McLean and Smith (1987),

Moffitt (1989), and Brickner (1989b) noted that pilots

perceive HMD symbology as being closer to their eyes than the

external scene image. This may occur with NVG-HMDs even

though both the external scene image and the symbology are

projected at the same focal distance and are seen through the

same optical system. Presently, the accommodation of a pilot

wearing an NVG-HMD cannot be measured; therefore, it is

difficult to determine if differences in accommodation occur.

Evaluations of Prototype NVG-HMD Systems

Except for the AH-64A Helmet Display Unit, little

empirical research has been reported on pilot performance

using operational HMD systems in rotary wing aircraft. The

U.S. Army, the U.S. Air Force, and the U.S. Marines reported

informal evaluations of prototype NVG-HMD systems in rotary

wing aircraft. The following paragraphs describe the results

of the evaluations.

U.._m. The U.S. Army developed and informally

evaluated a prototype NVG-HMD for rotary wing aircraft known

as the micro-HUD (Simmons et al., 1985). Ten helicopter

pilots wore the micro-HUD during flights in the UH-I

aircraft. The micro-HUD presented a different set of symbols

to each eye by using small mirrors to reflect the information

directly to the retina. Both the symbol generators and the

mirrors were mounted on a pair of safety glasses worn between

the aviator's eyes and the NVGs. Digital information

30
representing altitude, trim, airspeed, and heading was

presented to the left eye at the 3, 6, 9, and 12 o'clock

positions respectively. A fixed aircraft position reticle

and a moving horizon line were presented to the right eye in

the center of the visual field.

The micro-HUD produced very poor quality symbology.

Vibrations from the helicopter or slight movements of the

glasses often caused the symbols to disappear. The

brightness of the symbology was barely enough to be seen

against the NVG image. Also, the pilots had difficulty

monitoring the NVG external scene and the numeric symbols

simultaneously or viewing two symbols at the same time.

Simmons et al. (1985) did not report performance data on

the effectiveness of the NVG micro-HUD. Subjective

evaluations by the pilots suggested that the micro-HUD

provided sufficient information for helicopter pilots to fly

a g7ound controlled approach with the NVG system turned off.

In addition, the pilots generally considered the micro-HUD to

be a labor saving device during instrument-dependent

maneuvers. Therefore, the researchers considered the microHUD to be useful.

The researchers suggested improving the micro-HUD design

by attaching the combiner lens directly to the NVG and

changing the image generator to stabilize the symbology.

These changes were apparently incorporated in more recent

NVG-HMD prototype systems. Since the development of the

early prototype NVG-HMD tested by Simmons et al. (1985), the

design of the NVG-HMD device and the quality of the symbology

have been significantly improved.

u-S. Air Force. Runyon (1985) described the results of

a U.S. Air Force test of the feasibility of using an NVG-HMD

system in helicopter special operations and for combat rescue

missions. The NVG-HMD used an eyepiece attached to the end

of one of the NVG intensifier tubes. A fiber optic cable

connected to a CRT enclosed in an electronics box transmitted

flight and navigation symbology to the NVG intensifier tube.

Thirteen pilots subjectively evaluated the NVG-HMD during

several mission segments.

Runyon (1985) cited the following conclusions from the

test results.

* Using the NVG-HMD was feasible for all phases of

special operations and combat rescue missions; it

reduced pilot workload and made night operations

safer.

31
"* Using the NVG-HMD significantly reduced the need for

intercom call-outs (e.g., altitude and airspeed) from

other crewmembers.

"* Most of the pilots preferred to use the vertical

altitude tape instead of the vertical velocity scale.

"* The HMD decreased the visibility of the NVG image even

when the HMD intensity level was just above the

absolute minimum, especially when using the ANVIS

(i.e., more sensitive) NVGs.

"* Except at high HMD intensity levels, HMD symbology was

difficult to read when superimposed on bright areas

(lack of contrast).

"* The center of the FOV was cluttered by symbology.

"* The numbers on the HMD were too small to be read

quickly.

"* Disconnecting the fiber optic bundle from the NVG

slowed egress.

"* Some pilots experienced eyestrain when shifting their

attention between the symbology and the NVG image.

"* The pilots judged that 1 hour of ground training and

two low-level flights were required to train

previously NVG qualified pilots.

Runyon reported that the change from binocular to monocular

vision may be the source for the eyestrain, preventing

simultaneous viewing of the visual scene and the HMD

symbology.

All 13 pilots judged that the NVG-HMD could reduce pilot

workload and make night operations safer but that the NVG-HMD

did not expand night flying capabilities. As a result of the

test, the Air Force (a) moved the symbology from the center

of the HMD to cover approximately two-thirds of the 400 NVG

FOV, (b) increased the size of the numbers, and (c) placed a

master caution light indicator on the HMD.

U-S. Marine Corps. More recently, the U.S. Marine Corps

completed an evaluation of three prototype NVG-HMD systems

using a variety of rotary wing aircraft (U.S. Marine

Helicopter Squadron One, 1989). The NVG-HMD systems provided

symbolic flight and navigation information. Eighty

32
helicopter pilots representing the four armed services

observed NVG-HUDs on demonstration flights. After the

flights, the pilots completed a questionnaire describing the

difficulties of using the NVG-HMD, the experiences of

discomfort, the workload, and the potential of the NVG-HMD

for enhancing mission performance.

Six highly experienced pilots used the NVG-HMD systems

in more extensive evaluations during four terrain flight

tasks under four levels of ambient light. The four tasks

were (a) terrain flight over desert, (b) single and

multiaircraft confined area landings in heavy woods,

(c) shipboard landings, and (d) low level overwater

operations. The ambient light levels ranged from a full moon

and clear skies to no moon and overcast skies. The pilots

performed the tasks once while wearing NVGs alone and again

while wearing the NVG-HMDs.

The pilots used a variation of the Cooper-Harper scale

(Cooper & Harper, 1969) to rate their workload during each

task. Workload ratings were higher in all conditions when

the NVG was used without the HMD, especially under low light

conditions. However, no tests of statistical significance

were reported.

The Marine Corps reached the following conclusions:

"* The addition of the HMD to the NVG made flying

considerably safer than flying with the NVG alone,

primarily as a result of a reduction in crew workload.

"* Using the NVG-HMD eliminated many of the

inside/outside scan transitions otherwise required by

pilots to access flight status information.

"* The NVG-HMD is most useful when flying in environments

that require the pilot's total attention to be outside

the cockpit, such as terrain flight and ship landings.

"* Using the NVG-HMD reduced the workload of the pilot

not on the controls during terrain flight navigation

and ship landings.

Perceptual and Attentional Issues

The literature on the NVGs, HUDs, and HMDs indicates

that two issues stand out as most important for evaluating

the effect of adding symbology to the NVGs: perceptual

judgment errors and attentional switching. These issues have

33
been discussed together in the sections on NVGs, HUDs, and

HMDs. However, their importance for NVG-HMD research

warrants that they be addressed separately in this section.

Perceptual Judgment Errors

In research that uses perceptual judgments through

NVG-HMDs as criterion measures, it is important to understand

the perceptual errors that characterize direct vision as well

as vision through different types of imaging systems. For

example, research by Gilinsky (1955) and Palmer, Mitchell,

and Pettit (1979) indicated that judgments of equivalence in

size and angular subtense of objects located at different

distances varied significantly from the true state both with

direct vision and through television imaging systems.

Galanter and Galanter (1973) and Wright (1966) found that

distance estimation in the field to or from aircraft with the

unaided eye is typically inaccurate but that training may be

able to improve perceptual accuracy. In addition, the

results of their research suggest that the specific

conditions of viewing influence bias and accuracy.

Substantial errors in aviators' judgments of altitude with

direct vision were reported by Armstrong et al. (1975) for

Army aviators and by Ungs and Sangal (1990) for Coast Guard

aviators.

The literature reviewed in this report suggests that

common perceptual judgment errors with NVGs are

overestimation of distance and underestimation of closure

rates; however, there is considerable variability in the

accuracy (e.g. Foyle & Kaiser, 1991). The effects of

NVG-based vision without symbology on perceptual judgments

are poorly understood at present, with only a few studies on

a few types of perceptual judgments. The effects of

symbology on perception through NVGs remain an issue on which

objective performance data are completely lacking. Research

on the effects of symbology on perceptual judgments through

NVGs will require comparisons with the same types of

judgments through NVGs without symbology, with direct vision,

and with measures of the actual state.

The effects of NVG-HMD systems on the accuracy of

perceptual judgments are an unknown factor in their safety.

Symbology is added to NVGs in part for the purpose of

compensating for their perceptual judgment limitations.

Symbology also has the potential of adversely affecting

perceptual judgments with the NVG image viewed through it.

Assessing the effects of symbology on perceptual judgments

with NVGs is difficult, however, because of the substantial

34
errors and pilot variability in these judgments with both

direct vision and with NVGs.

Attentional Switching

The ability of pilots to time share or switch attention

among the image and symbolic cues of the NVG-HMDs is a

critical factor in their safety and operational

effectiveness. However, this ability is not well understood.

The selective attention research reviewed by Posner (1982)

provides much of the basis of understanding that does exist,

but it is general. Specific consequences for NVG-HMDs are

uncertain in many respects.

HUDs were designed originally on the presumption that a

common image-symbology focus would provide simultaneous

perception of both types of cues. The research reviewed in

this report and in Morey and Simon (1991) increasingly casts

doubt about the validity of this presumption and suggests

that symbology and the real world scene may be processed in a

parallel manner rather than in a serial manner. Furthermore,

there is some evidence that totally exclusionary switching of

attention between image and symbology may exist, in which

critical image cues may remain completely undetected when

attention is on symbology and vice versa (e.g., Fischer et

al., 1980; Foyle, et al., 1991; Larish & Wickens, 1991;

Weintraub et al., 1984; 1985).

Although switching attention between symbology and

imagery may occur, an aviator's ability to divide attention

between the two information sources is likely to improve with

extensive practice. The effects of practice on subjects'

ability to divide attention between two tasks has been

demonstrated in laboratory research (e.g., Becklen & Cervone,

1983; Stoffregen & Becklen, 1989; Wickens, 1984) but has

little support in operational settings. In addition, there

is some evidence that increasing the conformal

characteristics of symbology (i.e., making symbols analogous

representations of their real world counterparts) may make it

easier for pilots to process symbology and imagery in

parallel (Larish & Wickens, 1991).

Conclusions

The results of the review of the literature on NVGs,

HUDs, and HMDs indicate that many of the perceptual and

attentional problems associated with using these devices,

though documented, are not well understood. With relatively

35
little research available on HMDs and NVG-HMDs, little is

known about the perceptual and performance consequences of

using these devices. The major conclusions that can be drawn

from the literature review are discussed in the following

paragraphs.

" NVGs provide helicopter pilots with enhanced visual

capabilities compared to unaided night vision but are

limited by low resolution and a narrow FOV. Critical

flight information can be obtained from the aircraft's

instruments by looking under or around the NVGs or

from another crewmember. However, this may result in

an increase in individual and crew workload.

" Pilots using NVGs experience certain types of

perceptual errors: they tend to overestimate

distances and underestimate closure rates, become

spatially disoriented, and experience high levels of

fatigue and workload. Thus, the performance

capabilities and limitations of pilots using unaided

vision and using NVGs alone should be examined before

attempting to evaluate the effects of adding

symbology. In addition, factors affecting pilot

performance with NVGs may interact with factors

affecting pilot performance with HUDs and HMDs.

" When pilots view the external scene through a

collimated HUD, they tend to overaccommodate when

viewing the symbology, even though the symbology and

the external scene are at the same focal distance.

The symbology appears to be closer than the real world

scene. Pilots have difficulty attending to both the

HUD symbology and the real world scene and must switch

their attention between the two. This may be due to

cognitive influences on depth perception or to a

requirement to process information in a serial rather

than a parallel manner. The ability to divide

attention between two activities presented on the same

display is likely to improve with practice.

" When viewing both HUD symbology and a real world scene

or a synthetic image, there is a tendency to fixate on

one source of information. Thus, pilots experience

difficulty detecting unexpected events in the

symbology or the external scene, especially when under

stress or high levels of workload. The variables that

affect attentional fixation or cognitive capture are

not well understood and should be investigated.

However, attentional and cognitive factors appear to

be equal to or more important than sensory and

perceptual factors.

36
" Many findings from HUD research may generalize to

HMDs. The difficulty pilots have dividing attention

between symbology and the external scene is an

example. However, characteristics unique to the HMD

(e.g., proximity to the pilot's eyes, slaving to the

pilot's head, and the potential for monocular or

binocular symbology and imagery) should be

investigated.

" Superimposing symbology on the NVGs has the potential

for decreasing pilot and crewmember workload and

increasing the safety of NVG flight. Most of the

problems reported in evaluations of prototype systems

were attributed to improper fit, adjustment, or

operation of the NVG-HMD system. Perceptual and

attentional problems with the NVG-HMD were cited

indirectly. This may be because few perceptual or

attentional problems existed or because their effects

were small relative to fit, adjustment, or operational

problems.

Recommendations for Research

In the evaluations of prototype NVG-HMD systems,

helicopter pilots preferred using the NVG-HMD system to using

the NVGs alone and judged that the NVG-HMD reduced workload.

However, the evaluations provided little data to evaluate the

effectiveness of the NVG-HMD. Additional research is needed

to evaluate the factors affecting pilot performance with the

NVG-HMD and to identify individual differences in perceptual

and attentional abilities. This section discusses the

primary research issues and suggests a research approach for

investigating the issues.

Research Issues

The review of NVG, HUD, and HMD literature suggests

there are three issues that should be investigated in a

program of NVG-HMD research.

"* Will adding symbology to the NVG affect pilots'

judgments of distance, altitude, and closure rates

compared to using NVGs alone?

"* Will adding symbology to the NVG distract attention

from critical events in the external scene or inside

the cockpit?

37
* Will adding HMD symbology decrease or increase pilots'

spatial disorientation?

In addition, individual differences may play a role in each

of these issues and in the selection of pilots for NVG-HMD

missions. The research should also identify the

effectiveness of training as a solution to these problems.

Independent Variables

The literature suggests that several independent

variables may affect pilot performance using the NVG-HMD.

The variables can be organized into four categories: NVG

characteristics, HMD symbology characteristics, pilot

characteristics, and task/environmental characteristics.

Research has indicated that the NVG variables listed in

Table 2 are likely to interact with the HMD symbology, pilot,

and environmental/task variables. However, these variables

are established by NVG design, so they can only be

manipulated when comparing different versions of the NVGs.

Some of the variables listed in Table 3 (e.g., size and

brightness) may also be established by design specifications,

but they can be manipulated for experimental purposes. As

discussed previously, existing design specifications for

symbology are based on expert opinion rather than empirical

research. Therefore, all the variables identified in Table 3

should be considered in a program of research. However,

priority should be given to variables that are not yet

determined by design specifications.

The variables identified in Table 4 are important

because they represent individual differences that may

moderate how effectively pilots can use the NVG-HMD system.

Some of these variables (e.g., flight and NVG experience) may

be obtained from existing records but other variables (e.g.,

sensory or cognitive ability, subjective workload) will have

to be measured before, after, or during an experimental

session.

The variables identified in Table 5 represent influences

external to the NVG-HMD system and the user that may affect

its use. All these variables can occur over a large range of

values. In initial experiments, the researcher should select

representative and extreme values to determine the

relationship between these variables and the performance

38
Table 2

Independent Variables for NVG-HMD Research: NVG

Characteristics

Variable Description

Resolution The spatial modulation (contrast) per

unit visual angle that can be seen

with the NVGs

Lag The temporal rate of change in

contrast that can be seen with the

NVGs

Field of view The size of the maximum visual area

that can be seen with the NVGs

Brightness The amount of light emitted from the

screen

Contrast The ratio of the brightest and darkest

area of the entire screen, or parts of

it

Display color The colors provided by the display

measures. Subsequent experiments can use intermediate values

to refine the variable-performance relationships.

The variables, identified in Table 2 through Table 5,

are not exhaustive and are provided to guide research. It is

not possible to investigate the effects of all the variables

in a program of research. Therefore, the authors' judgments

of the relative importance of the variables are presented in

Table 6. Variables rated as high should be given priority in

a program of research.

Dependent Variables

Performance measures. The effects of the NVG-HMD system

on pilot performance can be measured in several ways,

depending on the research issue. Table 7 suggests

performance measures for each of the three research issues

39
Table 3

Independent Variables for NVG-HMD Research: HMD Symbology

Characteristics

Variable Description

Size The height and width of the

characters and figures

Location The placement of the symbology

Field of view The area within the NVG image that

is occupied by the symbology

Residual field of view The area within the NVG FOV that

lies interior to the symbology FOV,

excluding centrally located

symbology

Density The number of symbols on the display

at one time

Format The digital or analog form of the

symbology

Brightness The intensity of the symbology

Contrast The ratio of the symbology

brightness and the NVG image

brightness

Color The wavelength of the symbology

Movement The displacement of the symbols over

time

Rate of change The relative frequency of symbol

change

identified in Table 6. With two exceptions, the variables in

Table 7 are measures of time and accuracy of response. In

addition to collecting time and accuracy measures, it is

advisable to examine the manner in which pilots trade off

time and accuracy in determining their response strategies.

40
Table 4

Independent Variables for NVG-HMD Research: Pilot

Characteristics

Variable Description

Sensory/perceptual The detection, organizatior, and

interpretation of visual information

Cognitive Information processing and the

allocation and division of attention

Eye dominance The tendency for one eye to dominate

visual perception

Flight experience The amount and type of rotary wing

experience

NVG experience The amount and type of NVG experience

Training/practice The amount and type of instruction to

improve performance on the task

Fatigue Extended performance and exposure to

difficult environmental conditions

Subjective workload The pilot's perception of task demands

Table 5

Independent Variables for NVG-HMD Research: Environmental/

Task Characteristics

Variable Description

Ambient illumination The amount of light available in the

environment, whether from natural or

cultural light sourceScene content The amount and type of detail in the

external scene

Scene movement The amount, type, and rate of object

displacement in the external scene

Task workload The mental and physical demands of the

task

41
Table 6

Importance of Variables for NVG-HMD Research

Research issue

Distance Event Spatial

Variable judgment detection orientation

NVG characteristics

Resolution High Medium Medium

Field of view High Medium High

Brightness Medium Medium Medium

Contrast High Medium - edium

Display color Medium Medium Medium

HMD symbology characteristics

Size Medium Medium Medium

Location Medium High High

Field of view High High Medium

Residual field of view High High High

Density Medium High Medium

Format Medium High Medium

Brightness High High High

Contrast Medium High Medium

Color Medium High Medium

Movement Medium High High

Rate of change Medium High Medium

Pilot characteristics

Sensory/perceptual High High High

Cognitive Medium High Medium

Eye dominance High High Medium

Flight experience Medium Medium Medium

NVG experience High High High Training/practice High High High

Fatigue High High High

Subjective workload High High High

Environment/task characteristics

Ambient illumination High High High

Scene content High High Medium

Scene movement Medium High High

Task workload High High High

42
Table 7

Dependent Variables for NVG-HMD Research

Research

issue Dependent variables

Distance Accuracy of distance judgments

judgment Accuracy of altitude judgments

Accuracy of closure rate judgments

Accommodation and convergence

Event Time to detect critical scene events

detection Time to detect critical symbology events

Accuracy of recall of scene events

Accuracy of recall of symbology events

Accuracy of control movements

Scanning patterns (eye movements)

Spatial Accuracy of recall of aircraft attitude

orientation Time to recognize unusual attitude

Time to recover from unusual attitude

Accuracy of control movements

Workload measures. One of the reported advantages of

adding symbology to the NVG is that it reduces pilot

workload. The effects of the NVG-HMD on pilot workload are

not well understood and should be investigated. Therefore,

research on the factors affecting pilot performance with the

NVG-HMD should include measures of pilot workload.

Two types of workload measures are appropriate:

subjective and physiological. Subjective workload measures

commonly used in aviation research are the National

Aeronautics and Space Administration Task Load Index (NASATLX) (Hart & Staveland, 1987), the modified Cooper-Harper

Scale (Cooper & Harper, 1969), and the Subjective Workload

Assessment Technique (SWAT) (Reid, Shingledecker, &

Eggemeier, 1981). Heart rate variability has generally been

found to be the most reliable physiological measure of

workload (Hicks & Wierwille, 1979; Meister, 1985).

43
Research Apparatus

The effects of the variables identified in Table 2

through Table 5 can be examined using three types of research

apparatuses: laboratory simulation devices, flight

simulators, and operational aircraft. The following

paragraphs discuss the characteristics, advantages, and

disadvantages of each type of apparatus.

Laboratory simulation devices. Laboratory simulation

devices are suitable for investigating attentional (event

detection) problems with the NVG-HMD. Two general paradigms

for studying divided attention in laboratory simulations are

passive observer and active controller.

In the passive observer paradigm (e.g., Brickner &

Staveland, 1989), the subject observes prerecorded video

footage of helicopter flights from videodiscs or videotapes.

The subject monitors and responds to critical events in the

external scene (e.g., an aircraft crossing one's flight path)

and in the symbology (e.g., airspeed going out of tolerance).

This paradigm permits the researcher to study the subject's

ability to divide attention between two visual tasks.

Performance measures are reaction times to expected and

unexpected external scene and symbology events. In addition,

the researcher can study visual scanning patterns by

recording eye movements with a head-mounted eye tracker. The

principal advantages of the passive observer paradigm are

control of the external scene and symbology events and the

realism of the scene. The principal disadvantage is the

subject's passive rather than active role.

Figure 5 illustrates an apparatus suggested by the work

of Neisser and Becklen (1975) that can be used with the

passive observer paradigm to simulate NVG-HMD viewing

conditions (i.e., monochrome imagery presented to both eyes

and superimposed symbology presented to the left or right

eye). The device uses a videotape/videodisc player for

generating the visual scenes and a personal computer for

superimposing symbology and recording responses.

In the active controller paradigm (e.g., Brickner &

Foyle, 1990), the subject controls critical parameters of a

simulated aircraft such as airspeed and altitude while

following a specified flight path. This paradigm permits the

researcher to study the subject's ability to divide attention

between a psychomotor control task and one or more visual

search tasks. Generation of the dynamic visual scene,

presentation of symbology, and recording of responses are

performed by a graphics workstation computer. Performance

44
v-t- I 779

V ideo Plapeor Spliter Compuer

(Terrain Scnes) L* (HMDSym,

SMonitor Mo nitor ,

Observer

Figure 5. NVG-HMD laboratory simulation apparatus.

measures are flight path deviations and reaction times to

expected and unexpected external scene and symbology events.

As with the passive controller paradigm, the researcher can

measure eye movements.

The principal advantage of this paradigm is the

similarity of the task situation to the operational

environment. The principal disadvantages are the lack of

control over the external scene and symbology events and the

relatively low realism of the visual imagery.

Flight simulator. A more realistic representation of

the flight environment is provided by a flight simulator than

in the laboratory simulation. A flight simulator also

permits greater control in a safe environment over the

conditions of the experiment and the variables under

investigation than does an operational aircraft. The

researcher can initiate and terminate maneuvers at any point

and is able to study the pilot's behavior in either an active

or passive role. A flight simulator is suitable for

investigating attentional and spatial disorientation problems

with the NVG-HMD.

Operational aircraft. The operational aircraft is the

target environment of the NVG-HMD system and has the lowest

45
degree of experimental control. The aircraft is most

appropriate for studying the effects of adding symbology to

the NVG on distance, altitude, and closure rate judgments.

However, the flight tasks and the range of conditions under

which pilot performance can be evaluated are limited by

safety, operational considerations, and available terrain.

The aircraft is better suited for observational than

controlled research; therefore, it should be used to

supplement laboratory and flight simulator devices in an

NVG-HMD research program.

Research Approach

This subsection recommends a research program for

investigating performance issues with the NVG-HMD

coordinating the use of the three types of research

apparatuses. Recommendations are made in Table 8 and are

discussed in the following paragraphs for using each

apparatus to investigate specific research issues. The

recommendations reflect the availability of the required

personnel and equipment, the cost of implementing the

research approach, safety considerations, and the required

level of realism.

Research should begin with a series of experiments to

investigate the effects of the NVG-HMD on the pilot's ability

to divide attention between external scene events and

symbology events. The initial experiments should use the

passive observer paradigm and a laboratory simulation device

like the one illustrated in Figure 5. The independent

variables should be (a) identified as important in the

literature, (b) of practical significance to the Army, and

(c) not already determined by design decisions. Two

variables that clearly meet these criteria are amount of

practice and eye dominance.

The research program should then be extended to

examining problems in divided attention and spatial

orientation with the NVG-HMD using a flight simulator. This

research should concentrate on the pilot in the active

controller role. A near-term goal of this research should be

to investigate the effects of variables such as the amount of

practice and symbol density that may have an impact on

NVG-HMD training and operational use. A far-term goal should

be to investigate the effects of variables such as symbology

format and location for which design changes may be

appropriate.

46
Table 8

Recommendations for Using Research Apparatuses in an NVG-HMD

Research Program

Research issue

Research Distance Event Spatial

apparatus judgment detection orientation

Laboratory

simulation X

Flight

simulator X X

Aircraft X

Two flight simulators, the Training Research Simulator

(TRS) and the Simulator Training Research Advanced Testbed

for Aviation (STRATA), are scheduled to be available soon to

ARIARDA and should be considered for this research. The TRS

is a UH-I instrument simulator equipped with a visual system

that presents computer-generated imagery on front and side

window CRTs. Symbology can be superimposed on the visual

scene presented on the CRTs and eye movements can be measured

by a head-mounted eye tracker. Unfortunately, the realism

with which NVG conditions are simulated is limited and

simulating viewing conditions specific to an HMD is not very

realistic.

The STRATA is an advanced research simulator that

integrates visual scene information and symbology on an HMD

to create a virtual world for the pilot. The simulator has

several features that make it attractive for NVG-HMD

research, including advanced display and performance

measurement capabilities. It can be programmed to simulate

nighttime visibility conditions and can track pilots' head

and eye movements. In addition, the STRATA can be used to

investigate NVG-HMD monocular and binocular design issues

such as eye dominance and binocular rivalry.

Laboratory simulation and flight simulators use twodimensional collimated CRT displays. The displays do not

provide an adequate representation of the pilot's visual

47
world for investigating distance estimation problems with the

NVG-HMD. Therefore, a portion of the research program should

be devoted to investigating these problems using a

combination of ground-based static displays and moving

operational aircraft. For example, a pilot's ability to

judge the distances of ground targets with the NVG-HMD could

be assessed by placing the pilot in a tower overlooking a

firing range that contains a variety of targets at known

distances. However, a pilot's ability to judge closure rate

with the NVG-HMD at realistic altitudes, which is the most

critical aspect of distance estimation during low level

helicopter flight, should be assessed using a moving

operational aircraft.

Regardless of the apparatus used, the research program

should evaluate the NVG-HMD for a variety of flight tasks.

The tasks chosen should be ones in which the pilot needs

aircraft status information while maintaining attention

outside the aircraft. Examples of such tasks are terrain

flight takeoff, terrain flight approach, hovering in- and

out-of-ground effect, masking and unmasking, maneuvering in

confined areas, and NOE flight. For instance, the NVG-HMD

may be beneficial to pilots during a terrain flight approach

to a confined area, but it may interfere with their ability

to detect obstacles during NOE flight.

48
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