Naval Submarine Medical Research Laboratory

NSMRL Report 1168 19 April 1991

A COMPARISON OF SIMULATED PARALLAX AND SINGLE-STATION RANGE

AIDS TO NAVIGATION: FINAL REPORT

by

Kevin Laxar and S. M. Luna

Naval Submarine Medical Research Laboratory

and

Marc B Mandler

U.S. Coast Guard Research and Development Center

Released by:

R.G. Walter, CAPT, DC, USN

Commanding Officer

Naval Submanne Medical Research Laboratory

Approved for Public Release; Distribution Unlimited
A COMPARISON OF SIMULATED PARALLAX AND

SINGLE-STATION RANGE

AIDS TO NAVIGATION: FINAL REPORT

by

Kevin Laxar and S. M. Luria

Naval Submarine Medical Research Laboratory

Groton, CT

and

Marc B. Mandler

U. S. Coast Guard Research and Development Center

Groton, CT

NAVAL SUBMARINE MEDICAL RESEARCH LABORATORY

REPORT NO. 1168

U.S. Coast Guard Research and Development Center

Contract No. MIPR Z51100-9-0002

Naval Medical Research and Development Command

Approved and Released by:

R. G. WALTER, CAPT, DC, USN

Commanding Officer

Naval Submarine Medical Research Laboratory

Approved for public release; distribution unlimited
SUMMARY PAGE

THE PROBLEM

To compare the navigation performance of observers using parallax (two-station) range

lights with three types of single-station range display systems proposed by the U.S. Coast

Guard.

FINDINGS

The ability of observers to detect deviation from range axis and motion across range axis

was determined for seven types of simulated range display systems. When compared

with parallax range systems, the single-station ranges could provide comparable navigational sensitivity under certain conditions, but were characterized by greater uncertainty

on the part of the observers.

APPLICATION

These findings describe the navigational sensitivity

range display systems, and permit the evaluation of

ges with single-station ranges.

afforded by current and proposed

implications of replacing parallax ranADMINISTRATIVE INFORMATION

This study was conducted at the Naval Submarine Medical Research Laboratory under

Contract No. MIPR Z51100-9-0002 with the U. S. Coast Guard Research and Development Center, Groton, CT. The manuscript was submitted for review on 12 December

1990, approved for publication on 19 April 1991, and designated as NSMRL Report No.

1168.

ii
Abstract

An appropriately designed parallax (two-station) range allows a mariner to accurately determine a range line--the correct path to steer his vessel--at great distances. Less expensive alternatives to parallax ranges are desirable, and many ideas, principally single-station ranges, have

been proposed. However, the mariners' abilities to establish range lines with them have not been

measured. The present work has quantified the navigational sensitivity afforded by parallax ranges and three different types of single-station range display systems. The single-station ranges

used (a) temporal characteristics, (b) spatial representation, or (c) color changes of the signal to

represent changes in lateral position. Range systems were simulated either opto-mechanically or

on a high resolution computer display system.

The mariners' ability to determine both lateral position in a channel and direction of motion

across a channel was assessed psychophysically for each range system. Performance with

single-station range systems was compared with that obtained with parallax ranges. This allowed us to evaluate the implications of replacing parallax ranges with the single-station ranges.

All three single-station range types showed the potential for providing navigational sensitivity

as good as, or better than, the parallax displays under certain range conditions. However, each

of the single-station ranges has a set of variables associated with it that could markedly affect the

sensitivity. In general, when compared with parallax ranges, single-station ranges were characterized by greater uncertainty on the part of the observer. Under optimal conditions, the frequency encoded range provided the best sensitivity of the three single-station displays, followed by

the color-coded beacon, and the sequentially flashing beacon. Under operational conditions,

however, the sequentially flashing beacon would likely prove superior.

iii
Table of Contents

SUMMARY PAGE ............................................ 1i

ABSTRACT ....................................... iii

INTRODUCTION .................................... 1

PARALLAX RANGES .................................. 2

M ethod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

ROTATING BEAMS SINGLE-STATION RANGE ................... 8

M ethod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

FREQUENCY ENCODED SINGLE-STATION RANGE ............... 11

M ethod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

CHROMATIC SINGLE-STATION RANGE ..................... 13

M ethod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

R esults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

DISCUSSION ................................................. 21

Static Thresholds ................................ 21

Dynamic Thresholds .............................. 23

APPLICATION ..................................... 26

Static Thresholds ................................ 26

Dynamic Thresholds .............................. 27

CONCLUSIONS .................................... 31

SUM M ARY ....................................... 32

REFERENCES ..................................... 32

iv
A COMPARISON OF SIMULATED PARALLAX AND SINGLE-STATION RANGE

AIDS TO NAVIGATION: FINAL REPORT

INTRODUCTION

The U.S. Coast Guard presently employs a

visual method, the parallax, or two-station,

range beacon, to indicate to a vessel's

operator the correct path or "range" to follow

along such navigation channels as approaches

to harbors and within rivers. For nighttime

use, this consists of a pair of lights positioned

on the range axis with the farther light higher

than the nearer one (Figure 1). Vertical alignment of the lights indicates that the vessel is

positioned on the range's longitudinal centerline, or range axis, and any deviation from

this course is readily apparent. The range

lights therefore provide a means of accurately

judging position in the channel as well as

speed and direction of motion perpendicular

to the channel. Mariners have great confidence steering their vessels when using a

range display because of the constant feedback given about position in the channel.

I RANGE AXIS

W

Although effective and easy to use, such

two-station aids are expensive, since the more

remote range light is typically located on

shore, requiring the purchase, construction,

and maintenance of this site. A single-station

range indicator--that is, a device located at

one site--has therefore long been desired.

Several such devices have been tested and

have had limited success (Ciccolella, 1958).

The present report examines the navigational

effectiveness of several alternative range indicators.

In this study we compared visual performance with four types of parallax ranges and

three types of single-station ranges under

similar conditions. The single-station ranges

evaluated had either been proposed, built, or

tested by others, or are a variation of current

hardware. We determined how well each type

of range display provided the necessary information for navigating a channel; specifically,

we examined how well observers could judge

FRONT REAR (HIGHER)

RANGE RANGE

LIGHT LIGHT

POSITION OF THE OBSERVER

Fig. 1. Top view of a parallax range. W: channel width; Y: distance of observer from range axis; 0: horizontal

component of the angle between the lights.

1
(a) when they were on and off the range axis,

and (b) when they were moving toward and

away from the range axis. Measurements

were made at different lateral positions in the

channel to map the sensitivity of the range system across the width of the channel. The objectives were to determine which range

systems provide information adequate for

navigation and to provide guidance to the engineer designing range systems.

This, the final report in a series, summarizes the research we conducted on each of

the different types of range displays, and compares their effectiveness. Further details about

the experiments can be found in the reports

referenced in the beginning of each of the following sections on the various simulated display types studied.

PARALLAX RANGES

Our baseline performance was the observers' ability using parallax ranges to judge

their motion toward or away from the range

axis (dynamic simulations), and whether they

were on or off the range axis (static simulations). A comprehensive report on these experiments can be found in Laxar & Mandler,

1989.

Method

Observers

In all experiments, volunteers ranging in

age from 23 to 59 years participated. All had

20/25 or better visual acuity, with spectacle

correction if required. In addition, observers

participating in experiments requiring color

perception were screened for normal color

vision. Most were experienced psychophysical observers. In these parallax experiments,

13 observers participated in the dynamic

simulations, and four of them also participated in the static simulations.

Apparatus

The range configurations were simulated

on a Ramtek 9400 high resolution color display system driven by a DEC VAX minicomputer. Observers responded using an auxiliary

key pad.

Displays

Four types of parallax range indicator

lights, listed below, were simulated dynamically. The first two types are in use. The latter

two have been proposed as alternatives.

Two-point fixed. This range display

consisted of two lights that are always on and

are vertically aligned when viewed from the

center of the channel. The lights were 0.6 arc

min in diameter and separated by 4.0 arc min

when aligned (Figure 2A). When viewed from

off center, they were not vertically aligned,

and the misalignment increased with increasing distance from the center of the channel.

Two-point flashing. A second display was similar to the above except that the

two lights flashed continuously. The upper

light was on for 3.0 sec and off for 3.0 sec,

while the lower light was on for 0.3 sec and

off for 0.7 sec (Figure 2B).

Extended source. This range display consisted of two bars of light, 0.3 arc

min x 6.0 arc min, oriented vertically with no

separation between them and always on (Figure 2C). As with the spots of light, they are in

vertical alignment only when seen from the

center of the channel.

Path indicator. The fourth type of

display consisted of a column of lights (Figure 2D). The center light, larger than others,

2
A

FIXED

41[

FLASHING

EOUAL INT 6

0

0

QKFI

C

Fig. 2. Four parallax range light configurations.

was in alignment with the column only when

viewed from the center of the channel. This

type of display, oriented horizontally, is typically used as a glide slope indicator on aircraft

carriers. We oriented it vertically so that

lateral position rather than elevation was indicated. Unlike the device used on aircraft carriers which shows five discrete elevations, our

display provided a continuous change in

lateral position to determine if this enhanced

display improved performance. If implemented, it might be constructed as a single-station range device using Fresnel lenses as on

aircraft carriers.

For the static experiments, only the twopoint and extended source ranges were simulated.

Procedure

Observers were seated 6 meters from the

computer monitor and dark adapted for 5 min.

The monitor screen subtended visual angles of

2.40 high x 3.30 wide and was uniformly illuminated to 0.003 cd/m2, equivalent to the

night sky with a partial moon. The white

stimuli, at a luminance of 100 cd/m2, were

centered on the screen. The luminance level

was imposed by hardware constraints. The

testing room was otherwise dark.

Static thresholds. These experiments were similar to the visual acuity experiments of Westheimer and McKee (1977b).

The static thresholds, here and throughout this

study, were measured with the method of constant stimuli. In separate experiments, either

the two-point or the extended source range

3

0

D

PATH INDICATOR

RIGHT OF J. ON LLEFT OF j.
was presented with the lower light in one of

nine positions up to 37.1 arc sec (0.62 arc

min) right or left of the upper light. The

stimulus positions were chosen to encompass

the range whose extreme values could easily

be judged by the observers as being off axis.

The stimuli were presented in random order

once every 4 sec for 0.2 sec. The observer

pressed one of two buttons on the keypad to

indicate a left or right relative position of the

lower light. Each position was presented randomly 30 times in two 270-trial sessions that

lasted 18 min each, and the computer

recorded each response.

Dynamic thresholds. These

thresholds were measured with the method of

limits throughout this study. For each trial, a

pair of range lights was displayed in a configuration corresponding to a view from some

distance off the range axis. After a variable

foreperiod (1 to 5 sec), the bottom light began

to move slowly to the right or left, simulating

a vessel's motion across the channel. As soon

as the observer could correctly judge the rightleft direction of motion, he/she pressed a button corresponding to that direction. When the

correct button was pressed, the angular distance which had been traversed by the lower

light was recorded by the computer. The direction of motion toward and away from the

centerline was randomized; half of the trials

were toward the centerline and half were

away. Trials were separated by a 2-sec interval; errors were recorded and those trials

rerun later in the session. Figure 3 shows an

example of this procedure for a two-point

range.

Eleven starting positions were chosen randomly, up to 6.2 arc min right and left of center. This simulated situations in which the

mariner was off the centerline by different

amounts when first viewing the display, and it

A

B

C

BUTTON

PUSH

t@

Fig. 3. An example of the experimental procedure for

motion threshold (dynamic) experiments. A: Start

position; B: lower light in motion; C: position when

observer detects that lower light has moved. The angular

distance moved is recorded by computer.

allowed us to calculate how much of a change

in distance from centerline is required before

a change in the display can be detected. The

lower light moved at 9.3 arc sec/sec. For typical channel configurations, this corresponded

to a speed of 2.6 to 11.5 knots across the channel. This was so imperceptibly slow that judgments were based on the position of the lights

at some time after the start of the motion.

4
Performance was measured in a single experimental session. This consisted first of 42

practice trials. Next, one trial at each starting

position was presented in random order in

both left and right directions of motion. This

was repeated over three blocks. The session

thus comprised 66 trials, and lasted about 50

min.

Results

Static Thresholds

Data from the four observers were combined, and probit analyses were conducted on

the 2160 trials from both the two-point and ex.

tended source range configurations. With

chance performance represented by the 50%

probability level and certainty represented by

100%, a probability level of 95% correct

responses was chosen for the practical purposes of this study. With the two-point range,

observers could judge when they were off the

range axis by 30.7 arc sec (0.51 arc min).

With the extended source configuration, the

mean accuracy was 33.2 arc sec (0.55 arc

min). The difference between the two range

configurations was not significant, 1 (3) =

0.80, p > .10. Additional practice and a less

conservative criterion probability level would

likely have made the performance of these observers approach the 5 to 10 arc sec acuity

found by Westheimer and McKee (1977b).

Dynamic Thresholds

Figure 4 shows the average thresholds for

detecting motion both to the left and right of

start position for the four range displays.

Threshold is the average deviation from start

position required by the observers to correctly

judge the direction of motion for that range.

_-

0

0

C0

I2.0-

1.8-

1.6-

1.4--

,.r.. I I I I I I I I

-8 -6 -4 -2 0 2 4 6 8

LEFT RIGHT

START POSITION (arc min)

Fig. 4. Motion thresholds for four parallax range light configurations. Note: In all

references to thresholds, lower thresholds indicate greater sensitivity, or better

performance.

5

TWO- POINT

FLASHING

TWO -POINT

FIXED

PATH

INDICATOR

EXTENDED

SOURCE

\, \ 

°

1 03
A repeated measures analysis of variance

(ANOVA) was computed on the deviations

for the following factors: 4 Range Indicator

Configurations x 2 Directions of Motion (to

the right or left) x 11 Start Positions x 13 Subjects. Thresholds vary significantly with

range configuration, F (3,36) = 3.46, p < .05.

A Newman-Keuls test showed a significant

difference between only the extended source

and the two-point flashing range configurations (p <.05), however.

The effect of start position was also significant. Thresholds are smallest for start positions at or near the range axis (start position

of 0.0) and increase as the start position distance increases left or right from center. This

means that observers can easily determine

whether they are moving toward or away

from the range axis when near the axis, but

they require a greater change in lateral position to correctly judge their direction of motion when farther from the range axis.

The right-left direction of motion effect

was not significant. However, a significant interaction was found between direction of motion and start position, F (10,120) = 15.36, p

<.001. This interaction defines the direction

of relative motion effect (DRM, toward or

away from range axis), which was significant

in separate ANOVAs for all four range configurations. This DRM effect indicates that

thresholds for judging motion toward the

range axis are different from thresholds for

motion away from the range axis. Figure 5

shows an example of these results for the twopoint fixed range. Observers were better at

judging changes when the direction of relative

motion was toward the range axis than when

TWO - POINT FIXED

L.)

00

0

I

V)

I

I.-

2.0-

1.8--

1.6--

1.4-

1.2-

1.0-

-E I I I 1 1

5-6 -4 -2 0 2

LEFT

1 I

4 6

RIGHT

8

START POSITION (arc min)

Fig. 5. Thresholds for relative motion toward and away from the range axis for the

two-point fixed range.

6

AWAY
it was away, by an average of 0.31 arc min.

Results for the other types of parallax displays

were comparable.

Four of the 13 observers had extensive experience in making fine perceptual judgments.

To determine if such experience had any effect on motion thresholds, we compared their

performance with that of the inexperienced

group. The experienced observers had

thresholds averaging 0.9 arc min more sensitive than the inexperienced group, a significant difference, 0(3) = 6.63, . < .01.

Errors--that is, when the observer

responded with the wrong direction of motion-

-were analyzed in a corresponding manner to

that for motion thresholds. Table 1, mean

error rates for the four range configurations,

shows that the two-point flashing range

produced almost twice as many errors as the

other configurations. A four-way ANOVA

showed a significant effect on errors for range

configuration, E (3,36) = 3.44,12 < .05. A

Newman-Keuls test showed that the two-point

flashing range was significantly different from

the other three configurations, p < .05, which

were not significantly different from each

other.

The effect of start position was also significant, F (10,120) = 3.68, U < .001. The

error data for all range configurations combined are shown in Figure 6. As with judgment of motion, best performance was near

the on-axis position and became increasingly

poorer with distance off axis. Interestingly,

direction of relative motion toward or away

from the range axis had no effect on error

rate, in contrast to the significant effect it had

on judgment of motion.

20

0

U)

0

w

-8 -6 -4 -2 0 2

LEFT

START POSITION

4 6

RIGHT

(arc min)

8

Fig. 6. Mean percent errors for four parallax range light

configurations combined.

Table 1

Mean Error Percentages by Parallax Range Configuration

Direction of Relative Motion

Range Configuration Toward Away Mean

Two-point fixed 9.5 12.4 11.1

Two-point flashing 17.1 18.0 17.5*

Extended source 6.7 10.3 8.6

Path indicator 4.4 13.3 9.2

All 11.6

*Significantly different from all others, which were not significantly different from each other.

7

I

I
ROTATING BEAMS SINGLE-STATION

RANGE

This proposed range indicator displays a

horizontal triplet of lights which appear to

flash simultaneously when viewed from the

channel centerline; when the vessel is to the

right of centerline the right light would appear

to flash first, and when to the left of centerline, the left light would appear to flash first

(Brown, 1982). This asynchrony would alert

the mariner that the vessel was off course and

in which direction. The course could then be

altered until the lights were again flashing

simultaneously.

To design such a beacon, the smallest interval at which most viewers can perceive temporal order with reasonable reliability must be

determined. Earlier studies found intervals

ranging from as little as 3 msec (Westheimer

& McKee, 1977a) to 30 msec (Lichtenstein,

1961) for binocular viewing, depending on

the stimuli used. To approximate point source

lights under night viewing conditions, the following experiment was conducted (Luria,

1990a) to simulate this single-station range indicator.

Method

Apparatus

The three flashing lights were produced by

three cylinders with apertures, rotating about

separate light sources. Figure 7 illustrates the

operation of the apparatus. To simulate angles

off the centerline, movable apertures were

placed in front of the beams, rather than rotating the apparatus or moving the observer. At

the viewing distance of 6.1 m, the lights were

0.78 deg (47 arc min) apart and subtended

0.01 deg (0.6 arc min) visual angle. The lights

were flashed at a rate of once every two

seconds (0.5 Hz). Their luminance was 230

cd/mi2, and the flash duration was about 50

msec. This experiment and those following

were conducted in a room with barely enough

illumination to see the large objects in the

room.

Procedure

Ten observers were given several practice

sessions prior to the start of the study, and two

minutes of adaptation to the ambient illumination before each session.

Static thresholds. The observer

viewed the set of lights at either 0 deg (centerline) or at various viewing angles. The magnitude of the angle needed for a correct

judgment of the temporal order (left light first

versus right light first) was measured. A given

angle of view was set and the flashing lights

exposed until the observer made a judgment.

The lights were occluded while a new angle

of view was set, and so on.

Dynamic thresholds. The minimal

amount of change in the viewing angle of the

flashing lights that the observers could perceive was measured. Starting with randomly

varied viewing angles of 0 (simultaneity), 1,

2, 4, or 6 deg to the right or left of centerline,

the difference threshold was measured for

both increasing and decreasing viewing

angles. For each trial, the display was exposed

and the viewing angle remained constant for a

random foreperiod of 5 to 10 sec, after which

the angle was changed at the rate of 5

deg/min. The observer reported when a

change in the flash pattern was detected and

whether the change was toward more or less

simultaneity. Incorrect responses were not

recorded, but the trial was repeated at some

random time later in the session.

8
A

(

B

'7

SIMULTANEOUS

I• i _,I Ir

I I I

"ON CENTERLINE'

V

OI5

C

1ST 2uo 3RD

LEFT OCA ,

I I I

"LEFT OF CHANNEL. MOVE EIGHT'

3RD 2ND 1ST

-0IH- ' -OF 9- - O -- .

'RIGHT OF CHANNEL. MOVE LEFT'IOD

Fig. 7. Operation of the sequential beacons. The left beacon, with two beams, rotates counterclockwise at a given speed.

The center beacon, with one beam, rotates clockwise at twice that speed. The right beacon, with two beams, rotates

clockwise at the same speed as the left beacon. All three beacons therefore flash at the same rate. A: When viewed from

the range axis, the three beacons flash simultaneously. B: When viewed from left of range axis, the left beacon is seen

first; the center is seen later, but before the right beacon, since the center beacon is rotating more rapidly. The interval

between sequential flashes increases with distance from range axis. C: When viewed from right of range axis, the right

beacon is seen first, followed by the center and then the left. Obs: Observer position.

9

)

(

oBs " OBS
Results

Static Thresholds

Mean thresholds were calculated to determine the viewing angle at which the observers

correctly identified the left-right direction of

temporal order. A probit analysis was used to

compute the 95% correct threshold. This

resulted in a mean temporal interval of 8.4

msec between the flashes of the left and middie beacons, or 42.7 arc min (SD = 21.4 arc

min) of angle from the centerline position.

To further study this type of range display,

several parameters were varied. Thresholds

were measured using only two flashing lights

rather than three. Thresholds were not significantly different, although the variability

with the two lights was greater. Again using

just two lights, no significant differences in

thresholds were found when the lights were

separated by only 16 arc min of visual angle

rather than the original 47 arc min.

Thresholds were significantly worse, however, when the luminance of the lights was

decreased in three steps from the original

level of 230 cd/m2 to 0.65 cd/m2 (Luria,

1990b).

Using the three-light display, performance

was measured when the display was flashed at

twice the flash rate (once per second) and at

half the flash rate (once every four seconds)

with that presented in the previous experiments (once every two seconds). The temporal interval threshold to identify temporal

order remained constant at about 5.6 msec for

all flash rates, but the angular deviation from

centerline at which the observers could perceive non-simultaneity decreased proportionally as flash rate decreased. When flashed at the

slowest rate, sensitivity was doubled in comparison with the figures given above, a substantial improvement in performance.

An additional experiment showed that temporal interval threshold, and therefore viewing

angle, decreased when the lights were

defocused by putting lenses up to +2 diopters

in front of the observers' eyes (Luria &

Newacheck, 1991). Performance improved

nearly twofold with the blurred image.

Dynamic Thresholds

Figure 8 shows the mean difference

thresholds both in terms of the change in the

viewing angle and in the temporal interval for

each of the five starting positions. The standard deviations of these values were on the

order of 35 arc min. The data show means for

only nine observers, as one observer found it

too difficult to do the task at the 4 and 6 deg

conditions.

As the angle of the starting position from

the centerline increased (and, therefore, the

magnitude of the temporal interval between

flashes increased), it generally became more

difficult for the observers to detect a change

in the flash pattern. The effect of start position

was highly significant according to the Friedman Analysis of Variance by Ranks, (Q r =

11.93, p <.01). The difference between

toward and away from simultaneity was not

significant.

The curves are, of course, not monotonic.

There is a drop in the thresholds around 1 and

2 deg after which the thresholds rise continuously. One explanation seems evident.

There is a range of perceptual simultaneity,

temporal intervals around simultaneity which

the observer cannot discriminate. When this

range is exceeded the observer can detect nonsimultaneity, which for most observers occurs

at a viewing angle of between 1 and 2 deg. If

the starting position is 1 deg off center, the

resulting temporal interval is typically too

small for the observer to detect. However,

10
only a small increase in temporal interval is required to detect that the lights are no longer

simultaneous. If the starting position is simultaneity, then a larger change is required to exceed the range of perceptual simultaneity. If

the starting position is 2 deg off center, this is

typically just outside the range of perceptual

simultaneity. Thus only a small decrease in

temporal interval results in the observer readily reporting simultaneity. A much larger

change is required if the temporal interval is

increasing.

FREQUENCY ENCODED SINGLESTATION RANGE

This proposed flickering or flashing light

range display would indicate lateral position

in the channel by varying the flash frequency,

combined with chromatic information to indicate left or right side (Figure 9). When on the

130-

E

0

X

,J

0

z

I~iJ

"II--

U.

z

i~J

IAIi.

C.

120 --

110-

00-4

90--

80--

70-+

I I I 0 1 2

centerline, the navigator would see a steady

light. As the vessel moved off thecenterline,

the navigator would see the light start to flash

on and off, increasing in frequency with distance from the centerline. Moving to the right

could be signalled by a flashing red light, and

moving to the left by a flashing green light.

The range centerline position could be indicated by a steady white light. The color

aspect, however, was not simulated in these

experiments.

The basic question is, how well can observers discriminate the frequency of a flashing light? Earlier studies (Brown, 1959;

Gebhard, Mowbray, & Byham, 1955;

Mandler, 1984; Mobray & Gebhard, 1955)

found that over the range of 1 to 20 Hz, the

difference threshold, Af, was a monotonically

increasing function of frequency, but results

varied widely in the range of 0.01 to 2.4 Hz,

depending on stimulus size and experimental

-26

-24

-22

-20 E

-18

-16

-14

1

6 4

4

START POSITION (deg)

Fig. 8. Rotating beams difference thresholds in minutes of arc of viewing angle and temporal interval, as a function of

start position in degrees of off-center viewing. Thresholds are shown for changes toward and away from simultaneity.

11

/X

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X"P

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I
CHANNEL

EDGE

GREEN

TIME

Iz

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TIME

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CHANNEL

EDGE

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RED

TIME

U)Illfff L§z J~

TIME

Y___L

TIME

FREQUENCY ENCODED RANGE

Fig. 9. Frequency encoded range. When on the centerline, the observer sees a steady light. As the observer moves away

from the centerline, the light is seen as flashing, with the frequency of flash increasing with distance from the centerline.

A color code, such as shown, would be additionally required to indicate center, right, or left side of channel. The color

aspect was not simulated in these experiments.

procedure. None used a point source of light

on a dark background nor measured difference thresholds of a constantly flashing

light as it slowly changed frequency, as would

be the case with a flashing range indicator

when a vessel traveled across the width of the

range. The following experiment was therefore conducted (Laxar & Luria, 1990).

Method

Apparatus

The light source was a diffused white beam

that subtended a visual angle of 1.9 arc min at

the 6 m viewing distance. Its steady-state 2 luminance was 41 cd/mi. The 50% duty

cycle of the light was modulated by a rotating

half-sector disk mounted on a rheostat-controlled electric motor. By adjusting the speed of

the motor, the light could be made to flicker at

the desired frequency, which was calibrated

by a Strobotac (General Radio Corp.). Five

base frequencies were used: 0.5, 1.0, 2.0, 4.0,

and 6.7 Hz.

Procedure

The observer sat in a dimly illuminated

room and binocularly viewed the apparatus,

12

I

I
N

<]

.7

.6-

.5.

.4-

.3-

.2

.I

7

0

CUMULATIVE jnds

4 7 II 18

1 I I

I 1 1FT I I I I I

I 2 3 4 5

FREQUENCY (Hz)

241

I

6 7

Fig. 10. Frequency difference thresholds (Af) by base

frequency for four observers. Error bars show standard

deviations. Cumulative jnds are given for 0 Hz to 6.7 Hz.

which was set to one of the five base frequencies. The frequency was then slowly increased

or decreased, at the rate of approximately 1

Hz in 30 sec, until the observer'correctly

reported "faster" or "slower," and the change

in frequency was recorded. A minimum of

three such thresholds was determined for both

faster and slower flicker rates at each base frequency. Four observers participated. Only

thresholds for changes in frequency, simulating a vessel's motion across a channel, were

measured as it was assumed that position on

centerline would be displayed as a light constantly on. The distance from the centerline at

which the light appeared to flash would be

determined by the angle through which the

steady light was displayed and the distance

the observer was from it.

• = !

1 Saturation refers to the purity of a color stimulus, that is, its richness or the degree to which it differs from

white. Two other aspects of a color stimulus are hue and brightness.

13

Results

The mean faster and slower frequency difference thresholds for all observers at each

base frequency were calculated. Since the two

thresholds were similar, their mean was calculated, and these difference thresholds (Af) and

their standard deviations are shown as a function of base frequency in Figure 10. Difference thresholds increase nearly linearly as

base frequency increased. The standard deviations also increase at the higher base frequencies. The results show that the observer's

sensitivity to changes in frequency decreases

as the frequency of the flashing light increases. This would mean that the mariner's

sensitivity to lateral motion decreases, and

performance becomes poorer, as the vessel approaches the edge of the channel.

The mean difference threshold can be

termed a just noticeable difference (nd) in frequency. The number of jnds were summed up

within the range of 0 to 6.7 Hz, resulting in 24

discriminable steps. The cumulative jnds are

given by base frequency in Figure 10.

CHROMATIC SINGLE-STATION RANGE

The final type of proposed range aid investigated would display a beam of light that

varied in color according to lateral position in

the channel, as shown in Figure 11. At the

left edge of the channel, the observer would

see a saturated1 green light. Approaching the

range axis, the green light would gradually

desaturate until a white light was seen on the

centerline. With further movement toward the

right edge of the channel, the light would

gradually become pink, then a highly
saturated red. Comparison of the variable

light with an adjacent standard white light

could provide a reference for judging the

saturation of the variable light and thus the

ship's position relative to the range axis. This

range display would be similar to a sector

light in providing a chromatic code. A sector

light, however, provides crude information

about lateral position in the channel. The

proposed light was studied to determine if a

continuously varying chromatic signal would

improve lateral sensitivity. The results might

also apply to the design of a graded sector

beacon, as well as to a continuously varying

beacon, by specifying the optimal number of

step gradations displayed across the channel.

Studies of saturation discrimination have

found that there are about 20 discriminable

steps in color purity between white and red,

and between white and green (Jones &

Lowry, 1926; Martin, Warburton, & Morgan,

1933), considerably more than, for example,

between white and yellow. Other studies, such

as those of Priest and Brickwedde (1938) and

z

z

Jameson and Hurvich (1955), measured the

minimal amount of spectral light added to

white that is just detectable. These studies all

concluded that saturation discrimination is

relatively good in the red and green areas of

the spectrum and poor in the yellow. All of

the studies of saturation discrimination, however, have tested the ability to perceive differences between two static, unchanging

colors. None has investigated observers' sensitivity to a gradual change in a light's saturation. In previous reports, however, we have

shown that it can be far more difficult to

detect a gradual change in a moving stimulus

(Laxar & Mandler, 1989) or a flickering

stimulus (Laxar & Luria, 1990a) than to perceive that two unchanging stimuli are different. We therefore conducted two

experiments (Laxar & Luria, 1990b). In the

first, we measured observers' ability to discriminate small differences in color from a

white stimulus, in order to assess sensitivity to

position on or off the range centerline (static

thresholds). In the second, we measured observers' ability to judge gradual changes in

CHANNEL POSITION

000

i

0

GREEN 4" WHITE

Fig. 11. Chromatic

range. When on the

centerline a steady white

light is seen. As one

moves to the right the

light changes gradually

to pink and then to deep

red. Similarly,

movement toward the

left of the channel

centerline results in a

change toward green.

@0@

RED

14
LS~

L

G r

NO r

B i '

PI

VD~

SH

D P B ND G L LS

M

6M 0

ifISc 

color, in order to assess sensitivity to motion

across the range (dynamic thresholds).

Method

Apparatus

The stimuli were produced by a two-channel optical system, one channel providing the

variable red-to-white-to-green stimulus, the

other providing the fixed white reference light

(Figure 12). In the variable channel, light

from a tungsten source was passed through a

blocking filter, a polarizing filter, and a

polarizing dichroic filter. As the dichroic filter

was rotated 900 in its plane, the colors it transmitted gradually changed from red to white

and then to green, as given in Table 2 and

shown in Figure 13. The white reference light

was produced by setting a dichroic filter to the

Fig. 12. Diagram of the

chromatic range apparatus.

The light from each tungsten

bulb (LS) goes through a

collimating lens (L), ground

glass (G), a neutral density

filter (ND) when required, a

blocking filter (P), and a

dichroic polarizer (D or VD).

The variable stimulus goes

through a shutter (SH). The

fixed reference stimulus is

reflected by a mirror (M).

Both light beams pass

through their respective

apertures in a screen (SC) 6

m from the observer. The

apertures are vertically

aligned, with the variable

stimulus under the reference

stimulus.

mean neutral (not-red, not-green) point of 12

observers. This value was determined

psychophysically prior to the start of the experiment. The reference light's color temperature was 33380K. The stimuli were calibrated

with a PR-703A/PC Spot SpectraScan fast

spectral scanner (Photo Research Div.,

Kollmorgen Corp.).

Light from the two channels passed

through a vertical pair of apertures separated

by a visual angle of 8.2 arc min on center at

the 6 m viewing distance. The reference light,

when used, illuminated the upper aperture.

Two aperture sizes, subtending 1.0 and 3.5

arc min., were tested. Two luminance levels,

given in Table 2, were tested as well. The

lower luminance was achieved by placing a

1.0 neutral density filter in each channel.

15
Table 2

Stimuli for Chromatic Range Experiment

CIE Chromaticity Luminance

Coordinates (cd/m )

Dominant Excitation

Color Wavelength Purity x y Low High

Red 603 nm 98% .630 .367 218 2184

White 578 nm 68% .439 .454 288 2878

Green 518 nm 48% .195 .565 542 5424

.800

0

"X

Fig. 13. The CIEP Chromaticity Diagram showing the locus of stimuli displayed by the

beam displayed only the white reference stimulus.

Procedure

The observer sat with his/her chin in a chinrest, and adapted for several minutes to the

dim ambient illumination. After a brief practice session, the static thresholds were always

measured first, with the dynamic thresholds

.800

variable light beam. The fixed

measured in subsequent sessions. Data were

collected and analyzed in terms of the angular

setting in degrees of the polarizing dichroic filter, and later converted to dominant

wavelength in nanometers (nm) for reporting

16
purposes. Most observers had served in some

of the previous experiments.

Conditions. Eight conditions were

measured, combinations of the following factors: presence and absence of reference light,

large and small apertures, and high and low

luminance level. The order in which the conditions were presented was counterbalanced

across the eight observers.

Static thresholds. Static thresholds

were measured using the method of constant

stimuli. The observer was presented with a

stimulus for 1 sec, and then he/she responded "red," "green," or "neither." For each of the

eight conditions, there were usually eight

stimuli, covering the range from one that appeared consistently red to one that appeared

consistently green for that observer. Each

stimulus was presented five times. The experiment was run in a single session lasting about

an hour.

Dynamic thresholds. The method

of limits was used to measure the dynamic

thresholds. Five starting points were used:

518 nm--the extreme green end of the range,

537 nm--moderately greenish, 578 nm--the

white stimulus the same as the reference light,

597 nm--moderately reddish, and 603 nm--the

extreme red end of the range. The stimulus

was exposed, and after a random foreperiod of

0 to 5 sec, the experimenter rotated the variable dichroic filter at one-half degree per

second (0.5 nm/sec average). The observer

was required to judge when the color of the

light appeared to change, and in which direction, and then respond "more green," "less

green," "more red," or "less red," at which

point the variable stimulus was extinguished.

At the two extreme starting points, of course,

the direction of change could only be in the

"less" direction, resulting in a total of eight

combinations of starting point and direction of

color change. These eight thresholds,

measured in random order for each condition,

were as follows: 518 nm to Less Green, 537

nm to More Green, 537 nm to Less Green,

578 nm toward Green, 578 nm toward Red,

597 nm to Less Red, 597 nm to More Red,

and 603 nm to Less Red. Typically three to

five measures were taken to determine each

threshold, depending on the variability of the

observer.

The observers were told when their judgment was in error. The errors were recorded,

but those data were not averaged in the results

for that threshold; those trials were rerun later

in the session. No adjustment to the data was

made for observers' guesses that happened to

be correct, so the results may be biased slightly towards overestimating sensitivity. Each

condition was run in a separate session, lasting about one-half hour each.

Results

Static Thresholds

Probit analysis was used to determine the

95% response point for the red and green

thresholds for each observer under each condition. These thresholds were the points at

which the observer changed his/her response

95% of the time from neither red nor green to

either just noticeably red or just noticeably

green. Thresholds varied widely, both across

observers as well as within observers, over the

eight conditions. Some lights that were called

"red" by some observers were consistently

called "green" by others. For the eight observers, the mean threshold across all conditions for a green response was a light with a

dominant wavelength of 575 nm (SD = 6).

For a red response, the wavelength was 585

nm (SW = 5).

17
Mean red and green thresholds for the

three main pairs of experimental conditions

are shown in Figure 14. Absence of reference

light, and lights of higher luminance level and

larger size, all tended to shift both red and

green thresholds in the long-wavelength direction. It was hypothesized that the presence of

a white reference light would make observers

more sensitive to small changes toward reddish or greenish. To test this, as well as the effects of luminance level and aperture size,

repeated measures ANOVAs were computed

on the threshold data for the following factors: 2 Conditions of Reference Light (with

and without) x 2 Luminance Levels x 2

586 -

584 -

A

z

I--

582 -

580 -

578 -

No

REF.

LIGHT

576 -

574 --

572 -

-R

BRIGHT

REF.

LIGHT

G-

-G

Aperture Sizes x 8 Subjects. A separate

ANOVA was computed for the red and the

green thresholds; the results are shown in

Table 3. In both cases, the effect of Reference Light was not significant, and

Luminance Level was significant. Aperture

Size was significant for the green thresholds

only.

A similar ANOVA was computed for the differences between the red and green

thresholds. The only significance found was

for the effect of Reference Light, E (1, 7) =

8.45, p <.05. Examination of Figure 14

shows, however, that, contrary to expectations, the presence of the reference light

LARGE

DiM

-R

SMALL

-G

-G

CONDITION

Fig. 14. Red and green static sthresholds for the three experimental parameters (left to right): absence or presence of

reference light, luminance level, and aperture size. Mean data for eight observers.

18
Table 3

Summary of ANOVAs for Red and Green Static Thresholds

Threshold Source df F Probability

Red Reference Light 1,7 0.23 ---

Luminance Level 1,7 50.46 p < .001

Aperture Size 1,7 4.97 ---

Green Reference Light 1,7 2.07 ---

Luminance Level 1,7 77.52 p <.001

Aperture Size 1,7 37.45 p < .001

increased the difference between the red and

green thresholds. The variables that were

manipulated in this experiment therefore had

no apparent effect on reducing the red-green

threshold differences that would indicate improved sensitivity.

To get an approximation of the neutral (notred, not-green) point, we calculated the midpoint of the red and green thresholds for each

observer by condition. Averaged over all conditions, mean neutral points for the observers

spanned a wide range, from 574 nm (,S =

3.5) to 588 nm (SD = 3). The overall mean

midpoint was 580 nm (51) = 6), slightly redder than the 578 nm wavelength of the reference light determined prior to the

experiments. A corresponding ANOVA on

these data yielded results comparable to those

of the red and green thresholds.

Dynamic Thresholds

A three-way repeated-measures ANOVA,

corresponding to those for the static data, was

computed for each of the eight dynamic

thresholds measured. The presence of the reference light, as with the static thresholds,

never produced a significant effect. In

general, the brighter stimuli and the larger

stimuli produced smaller thresholds and improved performance, although the effects

were only significant at the longerwavelength thresholds. The ANOVA results

for these factors are summarized in Table 4.

The dynamic thresholds for the large bright

stimuli are plotted on the CIE Chromaticity

Diagram in Figure 15 in terms of starting

wavelength from which the just noticeable difference was measured. The arrows on the

diagram show the overall means across all

eight experimental conditions. Except for the

537 nm to Less Green threshold, the

thresholds are larger for the small dim stimuli

than for the larger brighter stimuli. In addition, thresholds are generally larger for

thresholds in the green region than in the red.

The effects of stimulus size, luminance, and

chromaticity found here are in keeping with

results found earlier by other methodologies

(Brown, 1957; MacAdam, 1942; Wyszecki &

Fielder, 1971).

Error percentages by condition were calculated for each observer. The mean errors for

each threshold are presented in Table 4. Observers made the largest number of errors

(22.4%) on the 518 nm (extreme green) to

Less Green threshold. Though they could

19
x

.600

.500

.400

.300

*

'U

'U

4,

jP

'I,

Q

AP

Jf

'U

"I

4,

*

'4' 'UQ.

4,

A,

'I) /

/

*

'U'U

£7

A?

4, A,

a, Ed,

"'U

4, '7

4,

A,

Ed,

/ 4'

a,

A?

404N

P

46

.200 .300 .400 .500 .600

"X

Fig. 15. Dynamic thresholds for the larger brighter stimuli plotted on the 1931 CIE Chromaticity Diagram, labeled in

terms of dominant wavelength of starting point. Arrows indicate mean thresholds for all eight stimulus conditions. Mean

data for eight observers.

Table 4

Summary of ANOVAs and Mean Errors for Chromatic Dynamic Thresholds

Factor

Threshold

518 nm to Less Green

537 nm to More Green

537 nm to Less Green

578 nm toward Green

578 nm toward Red

597 nm to Less Red

597 nm to More Red

603 nm to Less Red

Luminance Level

E n

10.3

157.3

.05

.001

Note. In all cases, df = 1,7.

20

Aperture Size

6.9

Mean

Errors (%)

22.4

9.3

1.6

1.2

0.0

0.6

1.6

9.3

.05

.01

.05

.005

13.7

11.8

26.6

I
perceive a change, they could not tell whether

it was toward more green or less green, since

it was not apparent to the observers that this

was the extreme green stimulus. Not quite as

difficult, but still producing substantial errors,

was the similar threshold at the extreme red

stimulus (603 nm to Less Red), and the 537

nm to More Green threshold. Some of these

errors may be attributable to the slower

change in wavelength at the extreme ends of

the scale as the polarized dichroic filter was

rotated, relative to the change at the middle of

its range. The slower change at the red and

green ends may have placed an additional

memory or perceptual burden on the observer,

making the error rate higher. ANOVAs corresponding to the previous ones were computed on the error percentage data for these

three thresholds. The only factor found significant was Luminance Level for the 537 nm

to More Green threshold, E(1, 7) = 9.9,1p <

.05. Surprisingly, the brighter stimulus

produced 11.7% errors, whereas the dimmer

stimulus produced only 6.9%. Perhaps the observers made fewer errors with the dim

stimuli because they were acting more conservatively in making more difficult judgments.

DISCUSSION

Observer sensitivity for judging position in

the channel depends on the type of range display, the starting point in the channel, the

direction of motion, and the experience of the

observer. The results thus far have been

presented in terms of angular measures of sensitivity or just noticeable differences. To relate the obtained thresholds to accuracy of

navigation it is necessary to convert these

measures to distances in a given channel. The

Commandant, U.S. Coast Guard (1980), has

specified optimal limits for parallax range configurations, represented by a lateral sensitivity

factor, K. This is a proportion of the channel

width that the observer must laterally depart

from the range axis to make it apparent that

the two range lights are no longer on the same

bearing line, that is, are no longer in vertical

alignment. Lateral sensitivity is based on the

length and width of the range and the placement of the range lights, and is defined by the

following equation:

K = (0/A) / ( Y/W ) (1)

where 0 is the horizontal angular separation

between the two range lights, A is the vertical

angular separation, Y is the lateral distance of

the observer from the range axis, and W is the

width of the range or navigation channel

(Commandant, U.S. Coast Guard, 1980) (see

Figure 1). Design guidelines require that ranges have K factors between 1.5 and 4.5. A

range with a K factor less than 1.5 will not

change its alignment perceptibly with small

changes in lateral position. A range with a K

factor greater than 4.5 will change alignment

too rapidly with changes in lateral position. In

the following discussion, we will assume a

range 152 m (500 ft) wide by 1219 m (4000

ft) long, with the near end of the range 610 m

(2000 ft) from the range beacon. This gives

us a K factor of 4.5 at the near end of the

range and 1.5 at the far end, and provides a

basis for direct comparison of the various

range displays. For a given K factor,

thresholds are directly proportional to channel

width, so the results are applicable to any

range configuration.

Static Thresholds

Figure 16 compares performance of four

types of range displays, at both the near and

far ends of the channel. Relative performance

is similar for both ends of the channel, with

thresholds at the far end three times greater

than those at the near end, since the distance

21
from the beacon(s) is three times greater. The

two-point fixed and the extended source parallax displays are nearly identical, with

thresholds of approximately 4.5 m around the

range axis at the near end and 13.5 m at the

far end of the channel.

The rotating beam display appears to afford much poorer sensitivity than the parallax

displays, with thresholds that are nearly twice

the size. The threshold was 7.6 m at the near

end of the channel and 22.7 m at the far end,

with standard deviations equal to half the

measured threshold. Results showed, however, that the thresholds would be halved

when the lights were flashed at half the rate illustrated by these data, bringing the levels

similar to those of the parallax displays.

The frequency encoded display, on the

other hand, shows much better sensitivity than

any of the other range types. This result, however, depends on the design of the beacon. In

this type of display the centerline would be

indicated by a fixed-on beam, and with a

departure from centerline, the beam would

start to blink. Centerline sensitivity would

therefore depend on the angle covered by the

steady on-center beam. The data shown here,

1.1 m at the near end and 3.2 m at the far end,

are based on the 24 jnds within the range of

flash frequencies tested, as discussed in the

section on dynamic thresholds, which follows.

Figure 16 does not show data for the

chromatic range display. The study of that display shows wide and consistent differences

within and among observers, confirming previous studies on the variability in judgment of

hues (Laxar, Miller, & Wooten, 1988; Neitz

& Jacobs, 1986, 1990; Schefrin & Werner,

1990) and in judgment of white points

(Jameson & Hurvich, 1951; Richards, 1967;

Wright, 1969). These results make it difficult

to choose a "white" light to indicate center of

the channel that would be immediately recognized as white by a large proportion of observers with normal color vision. This

ON/OFF CHANNEL CENTERLINE

A

U,

CA

I.-

25

20

15

10

5

0

NEAR END FAR END

Fig. 16. Thresholds for perceiving position on or off range centerline, for two parallax and two single-station range displays,

at the near and far ends of the channel.

22
suggests that a chromatic range beacon may

not be an appropriate device for displaying

center of channel position.

Given the data from the present simulations, we consider it not meaningful to try to

specify the sensitivity afforded by this type of

beacon for identifying center of channel position. It cannot be predicted whether, from a

brief glance, a vessel operator could tell if

he/she were on or slightly off the channel

centerline. Performance could be estimated,

however, from the dynamic threshold data, as

discussed in the Application section, following. A slightly different procedure may have

yielded more useful results, that is, asking the

observer to judge whether on or off centerline

rather than make a color judgment.

Dynamic Thresholds

Figure 17 shows motion thresholds for the

four types of parallax and two types of singlestation range displays, on-axis and at the edge

of the channel. The results are given for the

far end of the channel (K = 1.5). Thresholds

for the near end of the channel (K = 4.5) are

one-third the size shown in Figure 17. Over

all, the extended source is the best of the parallax range displays, followed by the path indicator. At the far end of the channel, the

extended source, at 30.9 m sensitivity on axis,

was 4.5 m better than the currently used twopoint fixed-on display. At the channel edge,

the extended source was 4.9 m better. As

noted earlier, however, the extended source

was significantly different only from the twopoint flashing display.

W

~1ý

0

=

(A

50

40-

30-

20-

10-

0

DIRECTION OF MOTION

FAR END OF CHANNEL -9

ON-Axis CHANNEL EDGE

Fig 17. Thresholds for perceiving motion across the channel width for four parallax and three single-station range displays,

on the range axis and at the edge of the channel. Data describe performance at the far end of the channel. Thresholds for

the near end of the channel are one-third those shown.

23

z 0~ Z 0 o z0 0 Z0

* iI N hi

0 = 1 0 0= X hi 0

~ 0 W I b U WiL 0 iL 0 

X nU~I U X

L U .L UL L 6 Li ' 0 > . U

Z Z O U Z ZOO 0 0 UN

H W z. z I- W W z z z I

3 4C 0 19 3 X 0 W = I UJ 91. 1W IL W6 W- LU 9. 09 U_. W

M..Mý -- - -
Performance was significantly better with

motion toward the range centerline than away

for parallax range displays. The mean difference of 0.31 arc min shown for the twopoint fixed range display in Figure 5 is

equivalent to having sensitivity 2.6 m better at

the near end of the channel and 7.9 m better at

the far end. Results for the other types of

parallax range displays are comparable. This

means that with such displays, mariners are

less sensitive to motion when approaching the

edge of the channel than when moving toward

the centerline, perhaps contrary to what a

range indicator should be capable of displaying.

The greater accuracy found with the group

of highly experienced observers, 0.5 arc min,

is equivalent to 4.3 m at the near end of the

channel and 12.7 m at the far end. This suggests that with training or experience, performance can be improved for a variety of range

light configurations.

The single-station rotating beam display

shows higher thresholds than the others, at

47.9 m on-axis and 42.1 m at the channel

edge, at the far end of the channel. When the

lights were flashed at half the rate, sensitivity

was improved twofold for static thresholds.

Although we did not measure motion

thresholds, it is reasonable to assume that sensitivity to motion also would be greatly improved at slower flash rates. This could make

the rotating beam display as good as, or better

than, the parallax displays for static

thresholds. The dynamic conditions would

probably still remain difficult for the observers.

The frequency encoded display appears to

afford superior sensitivity. Based on the 24

jnds found between 0 Hz and 6.7 Hz, if this

range of flash frequency were displayed

across each side of the 152 m (500 ft) channel

width (one side red, one side green) at the far

end of the range (Figure 18), each half would

contain a 0 Hz segment around the range axis

plus 23 jnds, 1 + 2*(23) = 47 jnds across the

channel width. If equally spaced, these would

provide a sensitivity of 3.2 m (10.6 ft) perpendicular to the range axis. At the near end of

the range, the same angular display would subtend 50.8 m (167 ft), with a sensitivity of 1.1

m (3.6 ft). Extrapolating to a flash rate of 20

Hz would provide an additional 16 jnds on

each side if the display were to cover the full

channel width at its near end, as shown in Figure 18. This would afford a sensitivity of 3.2

m (10 ft) at the outside segments of the channel.

Care must be taken in interpreting the

results of the frequency encoded display, however. There are many factors that affect the

perception of flicker, which could influence

the frequency difference thresholds and alter

our conclusions. These include the luminance,

size, and color of the light, the duty cycle,

waveform, and amplitude of flicker, and the

background luminance. Operationally, factors

such as atmospheric conditions and sea state

could decrease an observer's sensitivity, thereby degrading performance.

Not evident here is the uncertainty the observers expressed in judging changes in flash

rate. Figure 10 showed large standard deviations in frequency difference thresholds. This

would further tend to worsen sensitivity and

increase uncertainty as the observer approached the edge of the channel. The sensitivity afforded by an operational frequency

encoded range indicator may be considerably

poorer than that shown here.

With the chromatic range display, as expected, discrimination of slowly changing

24
color stimuli proved much poorer than

measures of static discrimination found in the

literature. As shown in Figure 15, under the

optimal conditions of higher luminance level

and larger aperture size, there exist seven just

noticeable differences in the range of colors

tested. Let us assume the nominal range discussed above and shown in Figure 18. If a

single-station range beacon were to display

the extent of its colors over the channel width

at the far end of the range, the observer's sensitivity would be one-seventh of the width, or

21.8 m (71.4 ft). This means that the vessel

would have to move that distance across the

channel before the operator could determine

that the ship had moved and correctly identify lateral direction of motion. Under optimal conditions, therefore, the chromatic

beacon may provide better sensitivity to motion across a channel than the parallax or the

rotating beams displays. At the end of the

range near the beacon, however, the colors,

being displayed at the same angles, would

span only the central 51 m (167 ft) of channel

width. In some instances, this might be impracticably narrow, and careful consideration

would have to be given to the dimensions of

the range for which an angular display

beacon could be used.

Aperture size of the chromatic range display may be more important in determining

performance than the luminance levels tested

here. It has long been known that color discrimination becomes poorer with lights of

smaller subtense (Bedford & Wyszecki,

1958). With extremely small lights, discrimination becomes especially poor in the

blue-green region of the spectrum (Willmer

and Wright, 1945). The smaller aperture, 1 arc

min, would be 0.54 m (1.77 ft) in diameter

and the larger aperture, 3.5 arc min, would be

1.89 m (6.19 ft) in diameter at 1 nautical mile

distance. At greater distances, the lights

-0- Ks 4.5

-w- K=2.5

K- 1.5

NNN

I..0

23 Inds

500 ft 152 m

Fig 18. Single-station frequency coded range display on an

assumed range, showing flash frequencies and jnds. K is

the range sensitivity factor for a parallax range.

would have to be proportionately larger, and

might prove impractical for a single-station

beacon.

With the exception of the extreme green

end of the scale, errors on the color/saturation

dynamic thresholds averaged lower than those

25
found with current two-station range indicators, which were on the order of 11% (see

Table 1). The 22.4% error rate found at the

green extreme may be related to the relative

lack of saturation of that stimulus light, excitation purity of 48%, making it difficult to tell

when it was changing toward the yellowish

white. In the middle of the color range, errors

were virtually nil.

SAPPLICATION

The previous section discussed the relative

performance afforded by the various types of

simulated range displays within an assumed

channel configuration. In this section we will

present threshold data for each range display,

computed from the experimental results, as

they relate to a range of channel widths and

distances from the beacons. Since the types of

display used different methods of presenting

the lateral information, a comment will be

made on the computation of thresholds for

each type of range display.

Static Thresholds

Table 5 and Figure 19 show at what distances from the channel centerline, in meters, an

observer can just discern that he/she is off the

centerline and to which side. Smaller values

indicate better sensitivity to lateral position.

Thresholds are given for one parallax display

and the three single-station displays, for five

channel widths from 100 m to 500 m. In the

case of the parallax range, values were calculated for three different range sensitivities (K

factors). For the rotating beams display,

thresholds were calculated for distances of 0.5

or 5.0 nautical miles (NM) from the beacons.

Thresholds for the frequency encoded and the

chromatic beacons were calculated as being independent of these factors.

For parallax displays, data for the twopoint fixed range are presented; the results for

the extended source display were virtually

identical. For a given range sensitivity (K factor) and a given channel width, a deviation

from channel centerline can be computed in

terms of the horizontal separation of the two

able 5

Static Threshold Summary Data (meters)

Range Display

Channel Width (m)

100 200 300 400 500

Two-point fixed, K= 1.5

Two-point fixed, K=3.0

Two-point fixed, K=4.5

Rotating Beams at 0.5 NM

Rotating Beams at 5.0 NM

8.5 17.1

4.3

2.9

11.5

115.0

Frequency Encoded

Chromatic

2.1

14.3

25.6

8.5 12.8

5.7

11.5

115.0

4.3

28.6

8.6

11.5

115.0

6.4

42.9

26

34.2

17.1

11.4

11.5

115.0

8.5

57.1

42.7

21.3

14.3

11.5

115.0

10.6

71.4 Chromatic
300 460 500

E

0

I-M

0

Channel Width ( m )

Fig 19. Static thresholds for the simulated range displays by channel width.

range lights. This value is independent of the

distance of the observer from the nearer range

beacon and directly proportional to channel

width. Using the threshold for horizontal

separation of the two lights determined in the

experiment, 0.51 arc min, the deviation from

channel centerline was computed for values of

K = 1.5, 3.0, and 4.5 at the five channel

widths.

The rotating beams display provides a

simple angular static threshold of 42.7 arc min

from the centerline. This angle subtends 11.5

m at a distance of 0.5 NM, and 115 m at 5.0

nm. These values are independent of channel

width, since the angle subtended is a constant.

As pointed out in the Results section, the

standard deviation of this measure was onehalf the threshold, or ± 5.8 m at 0.5 NM, and

± 57.6 m at 5.0 NM.

For the frequency encoded display, we assumed 47 jnds, based on the dynamic

thresholds, equally spaced across the channel

width. For a 100-m channel width, for example, the threshold would be 100/47 or 2.1

m. These values, then, depend only on channel width and are independent of distance

from the beacon.

Similarly, we based the static thresholds

for the chromatic display on the number of

jnds obtained from dynamic thresholds, simply dividing channel width by 7 jnds. This may

be a conservative estimate of performance,

since static thresholds are typically smaller

than dynamic. Here, too, the values are independent of distance from the beacon.

Dynamic Thresholds

Table 6 and Figure 20 give the dynamic

thresholds about the channel centerline.

These figures represent the lateral distance the

27

150

Range Type

0 Chromatic

< FroQuelcy S.................

cp Rotating Boam 5.0 NM

O Rotating BSem 0.S NM

* Two-point K - 4.5

* Two-point K 3.0

a Two-point K t.5

125-

100-

75

50-

252

01

-a

-- 4

-. 4-

100 200
250o

225{

150

1251

100.

75-

50

16o 260 360 460 56o

E

n0

(_

I.C

I0

Channel Width ( n )

Fig 20. Dynamic thresholds at channel centerline for the simulated range displays by channel width.

Table 6

Dynamic Threshold Summary Data -- Channel Centerline (meters)

Range Display

Channel Width (m)

100 200 300 400 500

Two-point fixed, K=1.5

Two-point fixed, K=3.0

Two-point fixed, K=4.5

Two-point flashing, K=1.5

Two-point flashing, K=3.0

Two-point flashing, K=4.5

Rotating Beams at 0.5 NM

Rotating Beams at 5.0 NM

Frequency Encoded

23.2 46.5

11.6 23.2

7.98 15.5

25.9 51.8

12.9 25.9

8.7 17.3

24.2 24.2

242.4 242.4

2.1 4.3

28

A

-. - Av

254 4 -z

Range Type

0 ClWOMIIC

< Faquancy

no Rotating Saan a.o NU

C oblatino BEaka 0.5 NM

0 Two-Iont K a 45

7 Iwo-point K - M0

n Two-point K.1.5

69.7

34.8

23.3

77.7

38.9

25.9

24.2

242.4

6.4

92.9

46.4

31.1

103.6

51.8

34.7

24.2

242.4

8.5

116.1

58.0

38.9

129.5

64.8

43.4

24.2

242.4

10.6

cot .......... 4ý .......... qzý ----- --CO .. ......... =

0.
observer has to move before he/she discerns

motion and direction away from the centerline. Table 7 and Figure 21 show motion

thresholds at the channel's edge for each

given width.

For parallax displays, data are presented

for the two-point fixed and two point flashing

ranges; thresholds for the extended source and

path indicator were marginally better than

these. A linear regression function was fitted

to the two-point fixed ranges' threshold versus start position data shown in Figure 4,

giving an on-axis dynamic threshold of 1.39

arc min horizontal separation for the twopoint fixed range and 1.55 arc min for the twopoint flashing, with larger values as start

position increased toward channel edge. Onaxis thresholds were calculated in the same

manner as the static thresholds for the three K

factors and five channel widths. Edge-of-channel thresholds were similarly calculated, except that thresholds increased slightly with the

value of K due to the inherent configuration

of a two-station range display. For the sake of

clarity, the date for the two-point flashing

ranage are not illustrated in Figures 20 and

21.

Dynamic thresholds for the rotating beams

display were calculated in the same manner as

the static thresholds. The on-axis values were

based on the 90 arc min found in the experiment; the edge-of-channel values were obtained by means of linear regression on means

of the experimental data shown in Figure 8.

For the frequency encoded and chromatic

range displays, the values given are the same

dynamic thresholds as shown under static

thresholds.

The following conclusions can be drawn

from the summary data presented here:

Static thresholds are generally better

than dynamic thresholds.

For the two-point fixed range display,

thresholds become poorer as channel

width or K factor increases.

Thresholds for the rotating beams display are basically independent of channel width, except for edge of channel

at the 5.0 NM distance. In that case,

thresholds improve slightly with channel width, due to the decrease in

threshold at starting positions near the

channel centerline, as shown in Figure

8. In any case, sensitivity afforded by

the rotating beams display decreases

rapidly with distance from observer to

the range beacon, due to the strictly angular nature of the display.

For the frequency encoded and

chromatic displays, thresholds become

poorer with channel width, but are independent of distance from the beacon

if one assumes that all jnds are displayed across the channel width at that

one distance. For a given beacon, however, thresholds will vary with distance along particular range, as

pointed out in the Discussion section

on dynamic thresholds of the frequency encoded display and shown in Figure 18. Caution is once again advised

in reliance on these values, due to their

variability.

Whether the sensitivity afforded by the

various range displays, as shown by the

thresholds given here, is suitable for use on a

particular range is a question that involves

many parameters. Among them are the size,

speed, and maneuverability of the vessels

using the range, expected atmospheric

visibility, sea state, weather, wind velocity

29
Table 7

Dynamic Threshold Summary Data -- Channel Edge (meters)

Channel Width (m)

Range Display 100 200 300 400 500

Two-point fixed, K= 1.5

Two-point fixed, K=3.0

Two-point fixed, K=4.5

Two-point flashing, K=1.5

Two-point flashing, K=3.0

Two-point flashing, K=4.5

Rotating Beams at 0.5 NM

Rotating Beams at 5.0 NM

Frequency Encoded

Chromatic

-3

-

-:: - -

.4.

100 200 300 400 500

Range Type

- Nnatiig amm as &.0 NMMM

w Two-pint K 4.S

* Two-pi=nt K *3.0

a Two-pon K.,.5

Channel Width ( n )

Fig 21. Dynamic thresholds at channel edge for the simulated range displays by channel width.

30

26.8

15.2

11.4

29.0

16.0

11.8

23.2

240.0

2.1

14.3

53.7

30.4

22.8

58.0

32.1

23.6

31.5

237.5

4.3

28.6

80.5

45.7

34.2

87.0

48.2

35.2

39.7

235.0

6.4

42.9

107.3

60.8

45.6

116.0

64.2

47.1

47.8

229.2

8.5

47.1

134.2

76.1

57.0

145.0

80.3

58.9

76.1

222.5

10.6

71.4

E

.5

U2

0)

.._

IE

'U

€I

a)

224

150.

125,

100.

75

50

25

0

n.................. .........
and direction, and vessel traffic patterns, including density and direction relative to the

navigational aid.

CONCLUSIONS

From this research we conclude that it

would be difficult to improve on parallax

range displays as effective means for indicating position within a channel or direction of

motion across a channel. They afford good

sensitivity for proximity to range centerline,

and observers are confident in the judgments

made using them. They are effective for use

over the entire width and length of the navigational range, although thresholds become

poorer at lower K factors (Figures 19-21).

The two-point flashing display, however, was

associated with a higher error rate than the

other types of parallax displays. The extended

source, a proposed display configuration,

provided slightly better sensitivity to motion

across the range than the other types of parallax displays. Further research could be conducted to determine if this superiority were

greater with different vertical offsets, as the

present experiments were conducted with a

vertical offset of 4 arc min, the distance

specified as optimal for two-point ranges

(Commandant, U.S. Coast Guard, 1980).

The rotating beams system, being an angular display, affords relatively poor

thresholds at long ranges (Figures 19-21).

However, it shares the advantage of the parallax displays in its effectiveness for use over

the entire width and length of the navigational

range as long as the distance from observer to

beacon is not too great. As the observer

moves to a greater angle off the range axis,

the interval between the flashing lights simply

increases. This advantage does not apply to

the other two types of single-station range displays, the frequency encoded and the

chromatic displays. With the latter two, the

angle of displayed information is limited. If

the angle is set for full coverage of the channel width at the far end of the range, any

deviation beyond those angular limits would

provide no additional information as to distance or direction of motion from the centerline. As the observer approaches the beacon,

the channel width covered by the beacon's display angle proportionally decreases, so that

the use of such types of displays might best be

limited to relatively short or sector shaped ranges.

Figures 16 and 17 indicate somewhat superior sensitivity afforded by the parallax ranges

over the rotating beams display within the assumed channel. Additional results showed,

however, that the critical variable is the observer's ability to discriminate temporal intervals between flashes. Sensitivity for the

rotating beams display, therefore, could be improved by decreasing the average flash rate of

the three beams. Additional studies could be

conducted to determine the flash rate for maximizing sensitivity while still providing frequent enough information updates for optimal

navigation. Sensitivity for the rotating

beacons, however, is significantly affected by

many variables, including flash rate,

luminance, and refractive error, making it difficult to assign a sensitivity value. Moreover,

judgments with this display proved very difficult under the dynamic simulation condition.

The large standard deviations of the frequency encoded display shown in Figure 10

are evidence of the observers' variability in

judging change in frequency. When only the

averages are considered, as in Figures 16, 17,

19-21, sensitivity appears to be superior to

other types of beacons tested. The notable uncertainty on the part of the observers, however, makes the concept of frequency

31
encoding extremely dubious for use in a

single-station range display. Additional

studies with observers who are highly practiced in distinguishing flash rates might show

that such a display is acceptable for experienced operators, however.

The color/saturation concept shows

promise for a single-station range. For observers with normal color vision, sensitivity

was better than with current two-station and

some alternative single-station range indicators, although errors were very high at the

extreme green end of the color range. With

the colors of lights tested, however, it was unclear that such a beacon would perform adequately for signaling when the vessel was on

or near the centerline of the channel. Further

study with a more saturated green, a more

neutral white, and a longer wavelength red

would provide further information on the adequacy of a color/saturation single-station

range indicator. The colors would also have to

be carefully chosen and tested to ensure minimal confusion by color defective observers,

estimated to comprise over 8% of the male

population (Wyszecki & Stiles, 1982).

SUMMARY

This study has compared four different current and proposed parallax range indicators

and three types of proposed single-station

range indicators. Parallax ranges have been

used successfully for many decades, but

results found here show that equally good performance might be obtained under some conditions with single-station range indicators,

which may be associated with reduced costs.

All range indicators, however, showed poorer

sensitivity and increased uncertainty of judgment as the observer approached the channel

edge. These results, then, provide a basis for

conducting additional laboratory or field tests

for further determination of the adequacy of

single-station range indicators.

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America, 48, 129-135.

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presentation time. Journal of the Optical

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Brown, D. M. (1982). Rotating beam single

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Ciccolella, J. A. (1958). Single station range

lights: Past-- present--future. The Coast

Guard Engineer's Digest (CG 133) No.

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C. L. (1955). Difference limens for photic

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Jameson, D., & Hurvich, L. M. (1951). A

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32
Jameson, D., & Hurvich, L. M. (1955).

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(1988). Long-term variability in the

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Luria, S. M. (1990a). Perception of temporal

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(Report No. 1155). Groton, CT: Naval

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33
Priest, I. G., & Brickwedde, F. G. (1938).

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Perception of temporal order in adjacent

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Westheimer, G., & McKee, S. P. (1977b).

Spatial configurations for visual hyperacuity. Vision Research, 17, 941-947.

Willmer, E. N., & Wright, W. D. (1945).

Colour sensitivity of the fovea centralis.

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colour. New York: Van Nostrand Reinhold.

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science (2nd ed.). New York: Wiley.

34
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A comparison of Simulated Parallax and Single-Station Range Aids to Navigation: Final Report

12. PERSONAL AUTIHOR(S)

Kevin Laxar, S. M. Luria, and Marc B. Mandler

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Final I FROMI W TO 12I 1991 April 19 34

16. SUPPLEMENTARY NOTA ION

17. COSAI I ODES 18. SUBJECT TERMS (Contnue on reverse it necessary and idenbty by block number) Aids to

FIELD GROUP SUB-GROUP navigation; leading lights; range lights; lateral sensitivity; saturation

discnmination; hue discrimination; color perception; visual acuity; vernier

S ............. acuity: temporal discrimination, flicker frequency.

19. ABkS IHRACI (Contnue on reverse it necessary and identty by block number)

An appropriately designed parallax (two-station) range allows a mariner to accurately determine a range line--the correct path to steer his vessel--at great distances. Less expensive alternatives to parallax ranges are

desirable, and many ideas, principally single-station ranges, have been proposed. However, the mariners'

abilities to establish range lines with them have not been measured. The present work has quantified the

navigational sensitivity afforded by parallax ranges and three different types of single-station range display

systems. The single-station ranges used (a) temporal characteristics, (b) spatial representation, or (c) color

changes of the signal to represent changes in lateral position. Range systems were simulated either

opto-mechanically or on a high resolution computer display system. The mariners' ability to determine both lateral position in a channel and direction of motion across a

channel was assessed psychophysically for each range system. Performance with single-station range

systems was compared with that obtained with parallax ranges. This allowed us to evaluate the implications

of replacing parallax ranges with the single-station ranges.

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19. cont'd.

All three single-station range types showed the potential for providing navigational sensitivity as good as,

or better than, the parallax displays under certain range conditions. However, each of the single-station ranges has a set of variables associated with it that could markedly affect the sensitivity. In general, when compared with parallax ranges, single-station ranges were characterized by greater uncertainty on the part of the

observer. Under optimal conditions, the frequency encoded range provided the best sensitivity of the three

single-station displays, followed by the color-coded beacon, and the sequentially flashing beacon. Under

operational conditions, however, the sequentially flashing beacon would likely prove superior.

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