Naval Submarine

NSMRL REPORT 1176

Medical Research Laboratory

30 DECEMBER 1991

EFFECT ON TARGET DETECTION OF ISOLATING

THE TARGET BEARING AREA ON A WATERFALL DISPLAY

WITH A NEUTRAL FILTER

by

S.M. Luria

Joseph Di Vita

and

Kelly Johnson

Released by:

R. G. Walter, CAPT, DC, USN

Commanding Officer

Naval Submarine Medical Research Laboratory

Approved for public release; distribution unlimited
EFFECT ON TARGET DETECTION OF ISOLATING

THE TARGET BEARING AREA ON A WATERFALL DISPLAY

WITH A NEUTRAL FILTER

S.M. Luria

Joseph DiVita

and

Kelly Johnson

Naval Submarine Medical Research Laboratory

Report 1176

Naval Medical Research and Development Command

Research Work Unit 65856N M0100.001-5003

Approved and released by

R. G. Walter, CAPT, DC, USN

Commanding Officer

Approved for public release; distribution unlimited
SUMMARY PAGE

THE PROBLEM

To improve target detection on a waterfall display by isolating and highlighting the target bearing with a neutral density filter.

THE FINDINGS

Although the performance of 9 out of 12 subjects was improved using the filter, the difference was not statistically significant.

APPLICATION

This simple method of delineating the target bearing does not reliably improve performance with waterfall displays.

ADMINISTRATIVE INFORMATION

This investigation was conducted under Naval Medical Research and

Development Command Research Work Unit 65856N M0100.001-5003, Enhanced performance with visual sonar displays. The views expressed in this

report are those of the authors and do not reflect the official policy or position

of the Department of the Navy, Department of Defense, or the U.S. Government. It was approved for publication on 30 December 1991 and has been

designated as Naval Submarine Medical Research Laboratory Report 1176.

ii
ABSTRACT

DiVita and Hanna (1987) have shown that target detection on a waterfall display

suffers, and performance is improved if the pixels of the target bearing are displayed

in red without, however, adding any additional target information. The reason appears to be that the operator has difficulty in keeping the relevant bearing isolated.

We tested the effect on target detection of isolating a target bearing with a neutral

density filter on the face of the CRT which dimmed the surround slightly but kept a

narrow area about three pixels wide around the target bearing at full luminance. Although this improved the performance of 9 out of 12 subjects, the difference was not

statistically significant.

iii
Effect on target detection of isolating the target bearing area

on a waterfall display with a neutral filter

INTRODUCTION

Signals from passive sonar systems

are typically presented as multiplechannel, time-history displays in which

bearing is represented along the horizontal axis and time along the vertical axis.

Signal strength is encoded as pixel intensity. The information from every

bearing at any given moment is seen

along a horizontal line, and this information is updated at regular intervals:

the current information is displayed on

the top line, and the older information

drops to the next lower line, and so on.

This produces a display which appears

to scroll downward, and it is, therefore,

called a "waterfall" display. Random

ocean noise appears as a random dot

display ("snow" is a common description), and a target appears as a discrete

track. If the target remains at the same

bearing over time, it produces a vertical

line through the display.

When the signal strength of the target is not much greater than that of the

background, the target is, of course,

difficult to detect. But DiVita and

Hanna (1987) found that target detection was much worse than would be

expected on the basis of what is known

about the ability of the visual system to

detect faint stimuli. They argued that

this was the result of an inability to

isolate and follow a given target bearing. They found that encoding the

target bearing by color improved target

detection, because the observers no

longer confused information presented

on adjacent bearings with that

presented at the target bearing.

Since it is not readily possible to have

the computer "highlight" a target bearing on current sonar displays, Luria,

DiVita, and Shim (1990) tested the effect

of delineating a target bearing with a

straight-edge. The assumption was that

if an operator suspected the presence of

a target at a given location on the display, he could simply hold a straight- edge

along that bearing and that might make

the target track more visible. Target

detection was not improved, however,

for seven observers.

One problem with this procedure was

that the presence of the straight-edge

completely blocked out part of the display. We have, therefore, investigated a

different procedure in which a neutral

density filter with a narrow slit is placed

over the display. The filter transmitted

25% of the light. This appears to dim

the background only slightly and allows

a narrow part of the display around one

of the bearings to be seen at full

luminance. The bearing of interest is,

thus, in effect isolated.

METHOD

Subjects

Nine staff members and three other

individuals volunteered to participate.

Only one had had experience as a subject with these displays.

1
Apparatus

The apparatus and display have been

described in a previous study (Luria,

DiVita, and Shim, 1990). A VAX 730, a

Ramtek 9400 graphics display generator, and a Matsushita standard

phosphor color monitor (1024 by 2080

pixels; 100 pixels to the inch) were used

to simulate the waterfall display. The

color of the pixels was green. The

ambient illumination on the screen

was 0.8 cd/m2 from fluorescent lamps

covered with neutral density filters

located behind and above the observer.

The Display

The display simulated one depressionelevation sector of a spherical array

passive broadband short-term averaging

display. There were 60 columns and 128

rows. At a viewing distance of about

two feet, the display subtended about 9

deg visual angle in height and 5 deg

visual angle in width. Targets were

presented at a signal strength of-5 dB.

The target bearing (column) was always

in the same location in the middle of the

display, and a white cross-hair was

placed directly above it.

A narrow area, about three columns

of pixels wide around the target column

was delineated by a sheet of neutral

density filter placed on the face of the

CRT. Its density was 0.6, transmitting

25% of the light. A vertical slit in the

center of the filter, 9 deg long and 1 mm

wide (about three pixels wide), allowed

the target bearing to be seen at full intensity while dimming the rest of the

display. The subject viewed the display

with his head in a chin and forehead

rest.

Procedure

Each subject had 12 experimental

sessions, six with the filter to delineate

the target bearing and six without it, in

an "ABBA" design. Half the subjects

observed first without the filter; half

observed first with it. Thus, a subject

might have three sessions with the filter,

then six without it, and finally three

more with the filter for the total of 12

sessions.

One session consisted of these events.

The display scrolled at a rate of about 10

lines/sec until the complete block of data

(128 lines) was presented. It then

stopped and remained visible. The subjects had an unlimited amount of time to

decide if a target was present, although

they could respond while the display

was still scrolling. After the response

was entered on the computer keyboard,

the display again began scrolling, and a

new block of data began to appear. This

was repeated 100 times in a session.

The probability that a target was present on any trial was 50%, although the

subject did not know this. At the end of

the 100 trials in each session, the computer reported how many correct target

identifications the subject had made.

There were three separate sets of practice sesssions. The subjects were

permitted as many practice sessions as

they wished until there was no further

improvement in their performance and

until they felt confident that they could

make the judgment. They were given

feedback during the practice sessions, but

not during the experimental sessions.

For example, assume that the subject

observed first with,::•ut the filter. The

subject was given practice prior to the

2
Table I

Mean percentage (and standard deviations)

of correct target identifications

Subjecct

SL

No Filter

67.7 (5.61)

WA 59.3 (5.85)

SG 67.3 (8.02)

MP 74.8 (2.86)

JJ 69.2 (5.38)

TR 66.0 (3.52)

KK 56.8 (1.47)

JD 70.7 (3.33)

TL 73.0 (3.16)

AD 70.3 (4.31)

MC 76.2 (5.16)

JM 78.5 (5.61)

Mean 69.2 (4.52)

first experimental sessions. After the

initial three experimental sessions

without the filter, he or she was then

given further practice with the filter

prior to the experimental sessions with

the filter. After the six experimental

sessions with the filter, the subject was

allowed further practice without the

filter prior to the final three sessions

without the filter.

RESULTS AND COMMENT

Table 1 shows the mean percentage of

correct identifications during the 600

trials with the filter and during the 600

Filter

76.0 (6.57)

68.7 (4.50)

69.3 (5.82)

69.3 (4.27)

71.5 (9.18)

63.5 (5.50)

67.8 (3.25)

74.3 (3.98)

74.8 (5.23)

71.2 (0.75)

76.5 (4.32)

76.7 (2.80)

71.6 (4.68)

trials without the filter for each subject.

Nine of the 12 subjects identified more

targets with the filter in place than

without it. This was not statistically

significant according to the Wilcoxon

Matched-Pairs Signed-Ranks Test. A

paired t-test also fell short of significance (T = 1.74, df = 11, p < .11), with

the differences between the means equal

to 2.4 and the standard error of the

mean for all the data equal to 1.43.

These results were, nevertheless, a

noticeable improvement over the previous attempt to use a straight-edge, in

which only three of seven subjects

3
showed an improvement (Luria, DiVita,

and Shim, 1990).

Table 1 also gives the standard deviations of the correct identifications for the

six runs in each condition. There is little

difference in the variability of the performance with and without the filter.

Although the filter may have tended

somewhat to improve the number of

correct responses, it did not reduce the

variability of the performance.

We conclude that these results support the contention of DiVita and Hanna

(1987) that isolating the target bearing

enhances the ability of the viewer to

detect the presence of a target at that

bearing. Our premise was that if an

operator suspected the presence of a target, he could improve his confidence as

to what he saw by aligning the filter

over the suspected track. Although the

trend of the results was in the predicted

direction, the present method did not,

however, produce an operationally useful improvement in performance.

The reason for the failure of the

present method to be more effective

seems clear when the two methods of

highlighting the display are compared.

It is immediately apparent to every

observer that displaying the target bearing in red is a much more effective

method of isolating the target bearing.

The filter narrowed the area of interest

to a column only about three pixels

wide. This appears to be better than our

previous method with the straight-edge,

but it still apparently introduced

enough confusion to significantly

degrade performance compared to the

red highlighting.

REFERENCES

DiVita, Joseph and Hanna, Thomas

(1987). Human efficiency for visual

detection of targets on CRT displays

using a two-level multiple-channel timehistory format. NSMRL Report No.

1101. Groton, CT: Naval Submarine

Medical Research Laboratory.

Luria, S.M., DiVita, J., and Shim, M.

(1990). Target detection on a waterfall

display with the target bearing

delineated with a straight edge.

NSMRL Report No. 1160. Groton, CT:

Naval Submarine Medical Research

Laboratory.

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Effect on target detection of isolating the target bearing area on a waterfall display with a neutral filter 12. PERSONAL AUTHOR(S)

Luria, S. M., DiVita, J., and Johnson, K.

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FIELD GROUP SUB-GROUP Target detection; waterfall display

19. ABSTRACT (Continue on reverse if necessary and identify by block number)

DiVita and Hanna (1987) have shown that target detection on a waterfall display suffers, and

performance is improved if the pixels of the target bearing are displayed in red, without,

however, adding any additional target information. The reason appears to be that the

operator has difficulty in keeping the relevant bearing isolated. We tested the effect on target

detection of isolating a target bearing with a neutral density filter on the face of the CRT,

which dimmed the surround slightly but kept a narrow area about three pixels wide around

the target bearing at full luminance. Although this improved the performance of 9 out of 12

subjects, the difference was not statistically significant.

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