AM 70-15 

TWO-FLASH THRESHOLDS AS A FUNCTION OF 

COMPARISON STIMULUS DURATION 

Approved by 

~~ CHIEF, CIVIL AEROMEDICAL 

INSTITUTE 

Mark F. Lewis, Ph.D. 

Henry W. Mertens, M.A. 

September 1970 

Department of Transportation 

Released by 

~AM) P. V: SIEGEL, M.D. 

FEDERAL Am SuRGEON 

FEDERAL AVIATION ADMINISTRATION 

Office of Aviation Medicine 
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TWO-FLASH THRESHOLDS AS A FUNCTION OF COMPARISON 

STIMULUS DURATION 

I. Problem. 

The recent proposal to approve use of strobe 

lights for collision a voidance instead of the rotating beacon suggests that the luminance requirement for collision avoidance lights is 

inappropriate for light flashes shorter than the 

critical duration of Bloch's law. According to 

Bloch's law, flashes of constant luminance (L) 

but different duration ( t) are of equal brightness 

only when the durations of the flashes exceed the 

critical duration (tc), generally taken to be 100 

msec. (L=C when t>tc). When the flash durations are shorter than the critical duration, equal 

flash brightness is achieved with equal flash energies, the product of luminance and duration 

(L x t=C' when t:-s;tc).2 Thus, for strobe lights 

producing flashes shorter than the critical duration, the luminance requirement should be increased to produce flash energy equivalent to that 

obtained when the luminance is presented in a 

flash that exceeds the critical duration. Consequently, it is essential that the value of the critical duration for human vision be accurately 

known. However, recent studies of two-flash 

thresholds have indicated that the critical duration may be much shorter than the 100 msec. 

value generally cited. 7 10 The current study was 

undertaken to determine whether the suggested 

shorter critical durations are valid. 

Based on Da vy's5 finding that in peripheral 

vision temporal discrimination ceases when 

flashes are wholly presented within the critical 

duration (tc) of Bloch's law, it was proposed9 

that two-flash thresholds may be used as direct 

measures of the critical duration. If responsesare equal to equal energy stimuli that are shorter 

than t"' then two-flash thresholds should be invariate with changes in the duration of comparison stimuli as long as these comparison stimuli 

are shorter than tc. Increases in the duration of 

comparison stimuli beyond tc should produce increases in the two-flash thresholds. 

1 

Nilsson11 examined the discrimination of twoflash stimuli as a function of the duration of the 

comparison stimulus. In a three-stimulus forcedchoice situation, he measured, for each comparison stimulus duration, the probability of correct 

discrimination as a function of the difference in 

duration between comparison and test stimuli at 

three luminances. Nilsson's results indicate that 

two-flash thresholds are an increasing function 

of the comparison stimulus duration. Nothing 

about critical durations may be inferred from 

these data; since test stimuli were varied in 15 

msec. steps, the possibility of detecting critical 

durations less than 15 msec. was excluded. 

Past work on the effect of luminance on twoflash threshold has indicated that there is a small 

effect of varying stimulus luminance above about 

1.0 log mL. 7 9 The current experiment also reexamines effect on two-flash threshold of varying 

luminance in the range between 1.0 and 3.0 log 

mL. 

II. Method. 

Subjects. The two male subjects (Ss) had 

normal acuity; one (BR) with correction. Both 

Ss had previous experience in a similar experiment, but neither was familiar with the purpose 

or design of the current experiment. Both were 

screened 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 anomaloscope examination. No evidence of color deficiency was found. 

Apparatus. The apparatus, previously described9 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 earlier.8 
Procedure. The procedure adopted was a 

variation of the Block Up and Down Two Interval Forced-Choice (BUDTIF) method described 

by Campbell,3 the method modified for a random 

double staircase.4 After dark adapting for 10 

min., the S adjusted the intensity of four fixation 

lines until they were just visible and, on an 

auditory ready signal, pressed a button to start 

a trial. A low intensity one-half sec. duration 

white noise defined three observation intervals 

which were separated by one-sec. intervals. A 

pair of one-msec. flashes was presented at the 

end of each observation interval: In two intervals a coinparison pair was presented with an 

interflash interval that was constant on all trials 

of a session and of 1, 2, 5, 7, 10, 15, or 20 msec. 

in duration, thus producing comparison stimulus 

durations of 3, 4, 7, 9, 12, 17, and 22 msec., respectively; in the other interval a test pair with 

a variable and always longer interflash interval 

was presented. A comparison stimulus was presented in the first observation interval, while test 

and second comparison stimulus alternated randomly from trial to trial in the second and third 

observation intervals. On each trial, S was instructed to report in which 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 S used two push buttons to indicate his choice and was informed of the accuracy 

of his responses by a noise that came on momentarily following correct responses. 

Within each staircase, the stimuli were presented in blocks of four trials with a minimum 

intertrial interval of 20 sec. The interflash interval for the test stimulus was changed according to the following rules: If the subject was 

right more than three times in a block of four 

trials, the interflash interval was decrei,tsed by 

one msec. If S was right exactly three times, the 

test stimulus was unchanged. If the S was right 

less than three times in a block of four trials, 

the interflash interval was increased by one msec. 

Two independent staircases were run, one starting 

at a short interflash interval, the other at a relatively long interval. On any trial, the particular 

staircase which was presented was randomly determined. Thirty blocks of four trials in each of 

the two staircases comprised a single session. 

Each session lasted approximately 120 min. Un2 

der each condition (of constant luminance and 

comparison stimulus duration), the staircases 

were maintained across sufficient sessions for 

stable estimates of threshold to be achieved; the 

staircases were miantained across at least three 

sessions at each condition. The mean interflash 

interval for the last two sessions at each luminance and comparison stimulus duration was 

taken to be the threshold. 

The luminances used were 1.0, 2.0, and 3.0 log 

mL. The random order in which each combination of luminance and comparison stimulus duration was selected for testing was different for 

each S. 

III. Results and Discussion. 

The results are given in Figure 1 with twoflash threshold plotted as a function of comparison stimulus duration. Luminance is the parameter. The data indicate that two-flash threshold, 

over the range of durations· tested, is an increasing function of comparison stimulus duration at 

all luminance levels and the amount of increas~ 

in two-flash threshold is an increasing function 

of stimulus luminance. Stimulus luminance also 

has a significant effect on two-flash threshold with 

thresholds doubling between the 1.0 and 3.0 log 

mL. levels in contrast to the smaller effects observed previously. 7 9 

In general, the data indicate that it may be 

inappropriate to apply the critical duration of 

Bloch's law to two-flash thresholds. That is, the 

discriminations made among equal energy stimuli 

need not reflect processes which determine threshold responses5 6 or responses to brightness criteria1 8 at different durations. 

The current study indicates that the previous 

suggestions of critical durations shorter than 100 

msec. are not valid. Consequently, in specifying 

brightness requirements for anticollision lights, 

an energy requirement is appropriate when flash 

duration is shorter than 100 msec. Although the 

current study indicates that well trained Ss 

may discriminate among equal energy flashes, 

the departures from Bloch's law are not great 

enough to justify variation from a Bloch's law 

requirement for flash specification. 

Previous research8 10 suggests that Bloch's law 

is applicable for the specification of anticollision 

light luminance. The results of the current study 

imply that deviations .from Bloch's law are rela-
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FIGURE 1. Two-flash thresholds as a function of comparison stimulus duration at three luminance levels. Luminance levels of 1.0, 2.0, and 3.0 log mL. are designated by open circles, closed circles, and squares, respectively. 

3 
tively insignificant. We conclude that use of a 

Bloch's law specification reflects a more general 

retinal process than does the current use of the 

Blonde! and Rey12 empirical equation. 

Conclusions : 

1. Two-flash thresholds are an increasing 

function of comparison stimulus duration over 

the range investigated. 

2. The extent to which two-flash thresholds 

are affected by comparison stimulus duration is 

reduced by decreases in luminance. 

3. The previous finding that two-flash thresholds are a decreasing function of luminance has 

been replicated at all values of comparison 

stimulus duration. This effect was larger than 

previously observed. 

4. When strobe lights are used as collision 

avoidance lights, the luminance requirement 

should be increased to produce flash energy 

equal to that produced by equal luminance 

flashes of 100 msec. duration. When the flash 

duration exceeds 100 msec., the luminance requirements is appropriate. 

REFERENCES 

1. Aiba, T. S. and Stevens, S. S. Relation of brightness 

to duration under light- and dark-adaptation. Vision Research, 4 :391-401, 1964. 

2. Bartlett, N. R. Thresholds as dependent on some 

energy relations and characteristics of the subject. 

In Graham, C. H. (Ed.), Vision and Visual Perception. New York: Wiley, 1965. 

3. Campbell, R. A. Detection of a noise signal of 

varying duration. Journal of the Acoustical Society 

of America, 35:1732-1737, 196S. 

4. Cornsweet, T. N. The staircase-method in psychophysics. American Journal of Psychology, 75 :485-

491, 1965. 

5. Davy, E. The intensity-time relation for multiple 

flashes of light in the peripheral retina. Journal of 

the Optical Society of America, 42 :937-941, 1952. 

6. Graham, C. H. and Kemp, E. H. Brightness discrimination as a function of the duration of the 

increment in intensity. Journal of General Physiology, 21:635-650, 1938. 

4 

7. Kietzman, M. L. Two-pulse measures of temporal 

resolution as a function of stimulus energy. Journal 

of the Optical Society of America, 57:809-813, 1967. 

8. Lewis, M. F. Category judgments as functions of 

flash luminance and duration. Journal of the Optical 

Society of America, 55:1655-1660, 1965. 

9. 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. 

10. Lewis, M. F. Two-flash thresholds as a function of 

flash luminance and area. Perception and Psychophysics, 4:241-244, 1968. 

11. Nilsson, T. H. Two-pulse-interval thresholds. Journal of the Optical Society of America, 59 :753-756, 

1969. 

12. Blondel, A. and Rey, J. The perception of lights 

of short duration at their range limits. Transactions 

of Illuminating Engineering Society, 7:625-662, 1912.