DISCRIMINATION OF SHORT .... DURATION (TWO PULSE) 

FLASHES AS A FUNCTION OF SIGNAL LUMINANCE 

AND METHOD OF MEASUREMENT 

Henry W. Mertens, M. A. 

Mark F. Lewis, Ph.D. 

FAA Civil Aeromedical Institute 

P.O. Box 25082 

Oklahoma City, Oklahoma 73125 

November 1971 

Availability is unlimited. Document may be released 

to the National Technical Information Service, 

Springfield, Virginia 22151, for sale to the public. 

Prepared for 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

Office of Aviation Medicine 

Washington, D.C. 205~0 
FAA-AM-71-42 

4. Title and Subtitle 

DISCRIMINATION OF SHORT-DURATION 

(TWO-PULSE) FLASHES AS A FUNCTION OF SIGNAL 

LUMINANCE AND METHOD OF MEASUREMENT 

7. Authorls} 

Henry W. Mertens, M.A. 

Mark F Lewis Ph D 

9. Performing Organization Name and Address 

FAA Civil Aeromedical Institute 

P. 0. Box 25082 

Oklahoma City, Oklahoma 73125 

5. Report Date 

November 1971 

6. Performing Organization Code 

8 . Performing Organization Report No . 

10 . Work Unit No. 

11. Contract or Grant No . 

13. Type of Report and Period Covered ~------------------~---------------------------4 12. Sponsoring Agency Nome and Address 

Office of Aviation Medicine 

Federal Aviation Administration 

800 Independence Avenue, S. W. 

Washin~t:on D r. ?O')Q() 

15. Supplementary Notes 

OAM Report 

14. Sponsoring Agency Code 

This research was conducted under Task No. AM-A-70-FSY-15 and AM-A-71-PSY-15. 

16. Abstract 

The recent introduction of strobe lights for anticollision purposes raises the 

possibility of using temporal patterns of short duration flashes as information 

carrying signals. The current experiments are concerned with the detection of 

the minimum duration dark interval between signal light pulses as a function o

signal luminance and the psychophysical method of measurement. Experiment I 

tested the theory of signal detectability (TSD) prediction that obse~ver 

sensitivity is independent of the psychophysic?l method used in measurement. 

Discrimination of a constant duration comparison stimulus (three msec) and a 

variable duration test stimulus (seven to 32 msec) was measured with a twoalternative Forced-Choice (FC) procedure and a Yes-No procedure. Sensitivity 

was comparable under the two psychophysical procedures, thus supporting the 

application of TSD to the sensory, processes involved in discrimination of twopulse stimuli. Experiment II measured discrimination with the FC procedure at 

three luminance levels: 31.8, 318, and 3183 candelas per square meter. 

Discriminability increased with luminance. Thus, pulses separated by a dark 

interval short enough so that only a single flash is seen over the entire 

scotopic intensity range may, however, be seen at photopic intensities as two 

pulses, or appear to flicker, or otherwise appear to be of different character. 

To maintain a homogeneous flash appearance over the entire range of signal 

intensities, the dark interval duration for multi-pulse flashes should be 

determined for the highest intensity condition at which operational observation 

is anticipated. 

17. Key Words 

Vision 

Visual Signals 

Perception 

19. Security Classif. (of this report} 

Unclassified 

Form DOT F 1100;7 la-s9l 

18. Distribution Statement 

Availability is unlimited. Document m

be released to the National Technical 

Information Service, Springfield, Virg

22151, for sale to the public. 

20. Security Classif. (of this page} 21. No. of Pages 22. Price 

Unclassified 5 $3.00 
DISCRIMINATION OF SHORT-DURATION (TWO-PULSE) FLASHES AS A 

FUNCTION OF SIGNAL LUMINANCE AND METHOD OF MEASUREMENT 

I. Introduction. 

The recent introduction of strobe lights for 

anticollision raises the possibility of using temporal patterns of short duration flashes as information carrying signals. An i:::1dividual flash 

can be produced by multiple brief light pulses, 

simulaneous or successive, as long as the component pulses are sufficiently close in time and 

space as to not be perceptually resolvable. The 

duration and intensity of a particular flash thus 

produced can be varied by ma:::1ipulation of the 

temporal spacing and the number of component 

pulses. Variation of signal intensity will also 

normally occur with varying visual range, windshield transmissivity, atmospheric conditions and 

any other factors which affect the amount of 

light reaching the observer's eye.1 In order to 

design multi-pulse signal flashes it is essential to 

know how the minimum detectable dark interval 

between brief pulses varies with signal intensity. 

The effect of luminance on the discrimination of 

duration differences between multi-pulse flashes is 

examined for the limiting case of two-pulsfl stimuli presented under night flight conditions. 

Recent studies of temporal discrimination of 

two-pulse stimuli differing only in duration have 

produced varying luminance effects. Lewis2 

measured thresholds for •temporal differences between two-pulse stimuli at luminances of 1.02 

to 3183 candelas per square meter ( cd/ m2 ). Twopulse thresholds were found to decrease in a 

negatively accelerated fashion as luminance increased, with only a small effect in the data of 

one observer for luminances above 31.8 cd/ m2 • 

In a subsequent study, Lewis3 measured twopulse thresholds as a function of pulse luminance and area. Data for the 30-min stimulus, 

which .was similar to the size used in other 

studies cited here, showed little, if any, effect of 

luminance in the range of 19.1 to 636 cd/ m2 • A 

progressively greater effect of luminance was obtained as stimulus diameter was decreased. 

1 

The above studies obtained thresholds with a 

variation of the method of limits ,the Block Up 

and Down Two Interval Forced Choice (BUDTIF) method developed by Campbell.4 Similar 

findings were obtained by Kietzman5 and Nilsson6 

using a variation of the method of constant stimuli involving a three-alternative forced choice response. Little effect of luminance on two-pulse 

threshold was observed when it was varied over a 

range of 7.96 to 1948 cd/ m2 by Kietzman, and 159 

to 6366 cd/ m2 by Nilsson. All studies cited here 

have used a one-msec duration for compo::1ent 

pulses of the two-pulse stimuli except for Kietzman's which used a five-msec duration. 

Lewis and Mertens7 measured two-pulse thresholds as a function of comparison stimulus duration at luminances between 31.8 and 3183 cd/m2 • 

Again, the BUDTIF technique was used. Twopulse thresholds were found to be an increasi::J.g 

function of comparison stimulus duration with 

the rate of increase rising with luminance. This 

large effect of luminance in a range above 31.8 

cd/ m 2 is in disagreement with the findings discussed above a::1d warrants further investigation. 

The measures of sensitivity derived from the 

models of the theory of signal detectability 

(TSD) have not yet been applied to the case of 

temporal discrimination of brief two-pulse stimuli. It has been demonstrated in several auditory 

and visual experiments,8 however, that TSD 

measures of sensitivity have the advantages of 

being independent of the observer's decision criterion and the particular psychophysical method 

used in taking the measurements, as predicted by 

TSD. Experiment I of the current study obtained psychometric functions for temporal discrimination of brief two-pulse stimuli derived 

from a Yes-No (YN) procedure and a ForcedChoice (FO) procedure. This experiment compares performance under two methods of 

controlling the observer's decision criterio::1 and 

provides a test of internal consistency for TSD. 
stimuli. 

II. Experiment I. 

Method for Experiment I 

Observers. Three men served as observers. 

They had normal acuity, with correction, and 

were scree~ed for color vision deficiency on a 

battery of tests that included the A.O.-H.R.R. 

and Dvorine plates, the Farnsworth-Munsell 100-

Hue test, the Farnsworth Dichotomous (Panel 

D-15) test, and an anornaloscope examination. 

No evidence of color defect was found with BR 

or DM:; HM: was a deuteranope. Of the three 

observers, only DM: had no experience in visio~ 

experiments. BR and HM were highly experienced observers, but only HM: (the senior author) 

was familiar with the purpose and design of the 

experiment. BR and DM: were paid an hourly 

wage. 

Apparatus. The apparatus, which has been 

described previously,2 included a Maxwellian 

view optical system with a Sylvania Glow Modulator tube used as a light source, and associated 

Iconix logic for control of stimulus duration. 

Luminance was calibrated with an SEI exposure 

photometer using a method described eariler. 9 

The stimulus image was a white disc subtending 

30' which was presented in the center of four red 

fixation lines forming an open cross. Viewing 

was monocular with the right eye. A 2 rnrn viewing aperature was used. Stimulus luminance was 

co~stant at 3183 cd/rn2 • 

FO Procedure. At the beginning of each experimental session, the observer dark adapted for 

at least five min and then adjusted the intensity 

of the fixation lines until they were just visible. 

On an auditory ready signal, the observer pressed 

a button to start a trial. Three successive observation intervals were the~ presented during 

each trial. Each observation interval was defined by an auditory signal consisting of a low 

intensity one-half-sec duration burst of white 

noise. One-sec intervals of silence separated the 

observation intervals. A pair of one-rnsec stimulus pulses was presented at the end of each observation interval. In two of these intervals, a 

comparison pair was presented with an interpulse interval that was always one-rnsec in dura2 

pair with a longer interpulse interval was presented. Test stimulus interpulse intervals of 5, 

10, 15, 20, 25, and 30 rnsec in duration were used, 

thus producing test stimulus durations of 7, 12, 

17, 22, 27, and 32 rnsec, respectively. A comparison stimulus was always presented in the first 

observation interval, while test and second 

comparison stimuli alternated randomly from 

trial-to-trial in the second and third observation 

intervals. The observer was instructed to report 

in w4ich interval, the second or third, the different stimulus most likely occurred. He was 

told to use any characteristic (apparent duration, 

brightness, color, etc.) of the flashes which he 

found useful to make the discrimination. The 

observer used two push buttons to indicate his 

choice and was informed of the accuracy of his responses by a noise that carne on momentarily following correct responses. Two hundred-fifty 

trials were presented in a single session. Intertrial intervals were at least 12 sec. A rest period 

was permitted whenever the observer felt he 

needed it. Each session lasted approximately 

120 min. Each condition of test stimulus duration was presented in two sessions. The proportion of correct responses P (C) was computed for 

each condition and, therefore, was based on 500 

responses. The random order in which test stimulus durations were presented was different for 

each observer. 

YN Procedttre. The YN procedure was similar to the FO procedure except that a test stimulus was not presented on every trial. \Vhen a 

test stimulus was not scheduled, a third comparison stimulus was presented in its place. The 

same six test stimulus durations were used with 

the YN procedure and the duration used was constant in a particular session. Comparison stimulus duration was always three rnsec. The 

observer's criterion was manipulated by varying 

the probability of occurrence of a test stimulus. 

The probabilities used were 0.1, 0.3, 0.5, 0.7, and 

0.9 and the probability was constant during a 

particular session. 

Each combination of test stimulus duration 

and probability of test stimulus occurrence was 

presented in two sessions; one in which the test 

stimulus appeared only in the second observation interval of each trial and one session in 

which it only appeared in the third observation 
gg 

the test stimulus would be presented in and what 

the probability of test stimulus occurrence would 

be for that session. His task was to indicate at 

the end of each trial whether or not he thought 

a test stimulus had been presented. All experimental conditions were presented with the 

test stimulus in the second position before replicating the experiment with the test stimulus in 

the third position. The experimental conditions 

of test stimulus duration were presented in a different random order in each replication. The 

relative area under the ROC curve P (A) was the 

dependent variable. As there were 250 trials in 

each session, each point in the ROC curve was 

based on 500 responses. 

Two weeks of practice preceded data collection 

under each procedure. Data collection for the 

FO procedure took one month and preceded data 

collection with the Y N procedure which took five 

months. 

Results and Discussion of Experiment I 

The distribution-free and theoretically similar8 

measures of sensitivity provided by P(C) for 

the FO method and P(A) for the YN method 

are plotted as a function of test stimulus duration for each observer in Figure 1. The measure 

D(L,m, s), suggested by Green and Swets8 is 

given in Table 1 and provides descriptive informatio~ about the ROC curves of the present 

experiment obtained from the YN procedure. 

The term L,m refers to the intercept of the ROC 

curve with P(S/ s) = 0.50 and sis the slope; both 

are obtained from plots on double probability 

paper. 

TABLE 1 

D (A!!~ .!,) Obtained from the Yes-No Procedure as a 

Test Stimulus 

Duration 

12 

17 

22 

27 

32 

Function of Test Stimulus Duration . 

A!!! .! 

-0.10 1.03 

0.43 1.07 

0 . 99 0.93 

1.45 0.92 

2.18 0.79 

2 . 89 0.61 

Sub ect 

A!!! 

-0.13 

BR 

.! 

0 . 99 

0 . 30 1.00 

0 . 77 0.79 

0 . 97 0.90 

1.78 0 . 92 

2.63 0.73 

Ll!!! 

0 . 17 

0.27 

0 . 70 

1.37 

2.11 

2.47 

HI! 

.! 

0.90 

0 . 83 

0.89 

0 . 62 

0.64 

0.97 

3 

.9 

.8 

.7 

.6 

.5 

1.0 

.9 

5 .8 

a. 

0 

z .7 <( 

Q 

a. .6 

.5 

0 

0 

1.0 

.9 

.8 

.7 

.6 

.5 

0 

5 10 15 20 25 30 35 

OM 

5 10 15 20 25 30 35 

HM 

5 10 15 20 25 30 35 

TEST STIMULUS DURATION 

FIGURE 1. Discrimination of two-pulse stimuli as a 

function of test stimulus duration under FC and YN 

procedures measured with three observers. Values of 

P (C) from the FC procedure are indicated by closed 

circles and values of P (A) from the YN procedure 

are indicated by open circles. 
procedures are in close agreement for two of the 

three observers. The decrease in performanc of 

BR under the YN procedure is discussed below. 

Although there is a tendency for discriminability 

to be higher under the FO procedure, there appears to be no significa!lt difference between 

indices of sensitivity obtained from the two procedures. The similarity of P(C) and P(A) 

measures in the data of two observers should encourage additional efforts to apply TSD in similar studies of temporal discrimination. 

III. Experiment II. 

Method for Experiment II 

Temporal discrimination of brief two-pulse 

stimuli was measured at three luminance levels, 

31.8, 318, and 3183 cd/ m2 • The procedure used 

in this experime!lt was identical to the FO procedure of Experiment I. Comparison and test 

stimuli were also identical to those used in Experiment I, with the addition of a 42 msec duration ( 40-msec interflash interval) test stimulus. 

Only BR and HM participated in Experiment 

II. Schedule changes prevented DM from serving as an observer. O!!e week of practice was 

given before the data collection was begun. Data 

collection took one month. 

Results and Discussion of Experiment II 

The data are presented in Figure 2. The proportion of correct responses P (C) is given as a 

function of test stimulus duration with lumi!lance 

as the parameter. The data of both observers 

show an increase in P(C) with increasing test 

stimulus duration; the rate of increase rises with 

luminance. 

Differences etween psychometric functions of 

the two\ obser' ers are apparent. Discrimination 

i!lcrease~ with test stimulus duration much more 

rapidly 'or HI than for BR at the 318 and 3183 

cd/ m2 l~vels. At the 31.8 and 318 levels, discrimination oes not differ for BR until test 

stimulus duration exceeds 17 msec, whereas for 

HM a clear difference is present at these luminances for all durations of test stimulus. Significant individual differences are also apparent 

in the two-pulse threshold data of previous experime!lts. 2 5 7 

4 

.9 

.8 

.7 

.6 

.5 

Q 0 10 15 20 25 30 35 40 4

a. 

1.0 

HM 

.9 

.8 

.7 

.6 

0 10 15 20 25 30 35 4 0 45 

TEST STI MULus· DURAT ION 

I!'IGURE 2. Discrimination of two-pulse stimuli as a 

function of test stimulus duration at three luminance 

levels as measured with two observers with the FC 

procedure. The luminances were 31.8, 318, and 3183 

cd/m' and are represented by closed circles, open 

circles, and open squares, respectively. 

The 3183 cd/ m2 luminance was used in both 

Experiments I and II with the FO procedure

Comparison of performance in these conditions 

provides information about the reliability of this 

particular temporal discrimination response. The 

FO curves for BR in Figures 1 and 2 for the 

3183 cd/ m2 luminance reveal a dow!lward shif

in discriminability in Experiment II. The curve 

from Experiment II is actually closer to the Y N 

curve of Experiment I than the FO curve of Experiment I. The corresponding curves for HM 

show a shift in the opposite direction. As these 

changes involve shifts of entire curves rather 

than the variability of individual data pobts, it 

is probable that they reflect shifts in the sensitivity of the observers. These shifts may be 

due to practice effects or change in general level 

of motivation and attention over the five month 

period which separated the two sets of measure-
effects observed. 

The current study supports the previous indication7 of a luminance effect at higher levels. 

Nilsson's6 suggestion that luminance effects on 

temporal discrimination of brief two-pulse stimuli exist only in the scotopic range is clearly contradicted. His criterion for scotopic stimuli was 

luminance less than 63.7 cd/ m2 for a total timulus 

"on" duration of 2 msec. The 318 and 3183 

cd/ m2 lumina:tces of the current study are above 

this range, yet discrimination clearly differs between the two conditions. 

As the stimuli used in all previous studies2 3 5 6 7 

were similar, other methodological factors are 

likely to have produced the variability among 

findings. The methodological differences between 

these experiments can:tot be related to the appearance or absence of luminance effects at pres

for varying luminance and test stimulus duration 

in experimental sessions, the ::lumber of observa

tions per data point, the use of feedback, and the 

preliminary practice of observers. 

This study indicates that the ability of the eye 

to detect the dark interpulse interval in two

pulse flashes increases with intensity in the 

photopic as well as the scotopic range. Thus

pulses separated by a dark interval short enough 

so that only a si:tgle flash is seen over the entire 

scotopic intensity range may, however, be seen a

photopic intensities as two pulses, or appear to 

flicker, or otherwise appear to be of differen

character. To maintain a homogeneous flash apperance over the entire range of signal intensities

the dark interval duration for multi-pulse flashes 

should be determi:ted for the highest intensity 

condition at which operational observation is 

anticipated. 

REFERENCES 

1. Gerathewohl, S. J., E. W. Morris, and J. A. Sirkis: 

Anti-collision Lights for the Supersonic Transport 

(SST). FAA Office of Aviation Medicine Report No. 

7Q-9, 1970. 

2. Lewis, M. F.: Two-Flash Thresholds as a Function of 

Luminance in the Dark-Adapted Eye, JOURNAL OF 

THE OPTICAL SOCIETY OF AMERICA, 57:814-815, 

1967. 

3. Lewis, M. F.: Two-Flash Thresholds as a Function 

of Flash Luminance and Area, PERCEPTION AND 

PSYCHOPHYSICS, 4:241-244, 1968. 

4. Campbell, R. A. : Detection of a Noise Signal of 

Varying Duration, JOURNAL OF THE ACOUSTICAL 

SOCIETY OF AMERICA, 35:1732-1737, 1963. 

5. Kietzman, M. L.: Two-Pulse Measures of Temporal 

Resolution as a Function of Stimulus Energy, JOUR5 

NAL OF THE OPTICAL SOCIETY OF AMERICA

57 :809-813, 1967. 

6. Nilsson, T. H. : Two-Pulse-Interval Thresholds, JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

59 :753-756, 1969. 

7. Lewis, M. F., and H. W. Mertens: Two-Flash Thresh

olds as a Function of Comparison Stimulus Duration

FAA Office of Aviation Medicine Report No. 7Q-15

1970. 

8. Green, D. M., and J. Swets: Signal Detection Theory 

and Psychophysics, New York, John Wiley and Sons

1966. 

9. Lewis, M. F.: Category Judgments as Functions o

Flash Luminance and Duration, JOURNAL OF THE 

OPTICAL SOCIETY OF AMERICA, 55:1655-1660

1965.