AM 68-20 

TWO-FLASH THRESHOLDS AS A FUNCTION OF FLASH 

LUMINANCE AND AREA 

23828 

Approved by 

~~ J. RoBERT DILLE, M.D. 

CHIEF, CIVIL AEROMEDICAL 

INSTITUTE 

Mark F. Lewis, Ph.D. 

September 1968 

Department of Transportation 

Released by 

~MD P. v. SIEGEL, M.D. 

FEDERAL Am SURGEON 

FEDERAL AVIATION ADMINISTRATION 

Office of Aviation Medicine 
Qualified requestors may obtain Aviation J\Iedical Reports from Defense Documentation 

Center. The general public may purchase from Clearinghouse for Federal Scientific 

and Technical Information, U.S. Dept. of Commerce, Springfield, Va. 22151. 
TWO-FLASH THRESHOLDS AS A FUNCTION OF FLASH LUMINANCE 

AND AREA 

I. Problem. 

The recent use of strobe lights in aviation 

suggests the possibility of using temporal patterns of short duration flashes as informationcarrying signals. The current study was directed 

at determining the human visual capacity for 

detecting information so-structured, by studying 

the limiting factors, under night flight conditions, for detecting the dark interval between two 

brief flashes of light. This procedure has been 

named the two-flash or two-pulse threshold. 

Recently there have been several investigations 

into the effects of flash luminance and duration 

upon two-flash thresholds. These studies indicate that at low energies (i.e., with short duration, low luminance flashes) two-flash thresholds 

are a decreasing function of flash luminance; 8 at 

higher energies (i.e., with flashes of longer duration and higher luminances) two-flash thresholds 

are independent of luminance.6 The current 

study replicates the low energy luminance effects 

and extends the problem to include the effects of 

stimulus area. 

II. Method. 

Subjects. Two subjects were an undergraduate 

man and woman of the University of Oklahoma. 

An additional subject (F.A.) was a woman research technician at the Civil Aeromedical Institute. Only the latter was familiar with the 

purpose and design of the experiment. All subjects were emmetropes. All subjects were screened 

for color vision deficiency on a battery of tests 

that included the A.O.-H.R.R. and the Dvorine 

plates, the Farnsworth-Munsell 100-hue test, the 

Farnsworth Dichotomous (Panel D-i5) test, and 

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FIGURE 1. Optical system used in the present study. Achromatic lens (B) collimates light from a Sylvania glow 

modulator tube (R11310), the collimated beam passing through neutral density filters (E), neutral density 

wedge ( 0), field stop (F), and beam splitters (D) and (G) before being focused in the plane of the observer's 

pupil (J), by achromatic lens (H). (I) is a two mm. artificial pupil. Light from an incandescent lamp (K) 

is collimated by an achromatic lens (L), this collimated beam passing through a red filter (Wratten No. 24) 

(l\1), and fixation field stop (N), before being added to the stimulus beam through beam splitter (G). Beam 

splitter (D) samples a portion of the light from the glow modulator tube passing it through Kodak Wratten 

filter No. 106 before being focused by lens ( Q) on ·-an RCA 1P39 photo multiplier tube used for stimulus 

monitoring. 

1 
an anomaloscope examination. No evidence of 

color deficiency was found. 

Apparatus. The apparatus, previously described8 included a Maxwellian view optical 

system (Figure 1) with a Sylvania Glow Modulator tube used as light source, and associated 

logic for control of stimulus durations. Luminance was controlled with neutral density filters 

(Bausch & Lomb). Luminance was calibrated 

with an SEI Exposure Photometer using a 

method described earlier.7 

Procedure. The procedure adopted was a 

variation of the Block "Gp and Down Two Interval Forced-choice (BUDTIF) method described by Campbell,2 the method modified for a 

random double staircase.3 After dark adapting 

for ten minutes, the subject 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 white noise 

defined two one-second observation intervals 

separated by one second. A pair of one-msec. 

flashes was presented in each interval: in one 

interval a comparison pair with an interflash 

interval of 1.0 msec. was presented so that the 

total duration of the comparison stimulus was 

3.0 msec. ; in the other intern} a test pair with a 

variable interflash interval was presented. Onset 

of test and comparison stimuli followed onset of 

noise by 40 msec. The observation interval in 

which the test stimulus appeared was randomly 

varied from trial to trial. The subject was instructed to report the temporal position of either 

the longer or the dimmer stimulus. The subject 

was informed about the accuracy of his responses 

by a tone that came on momentarily following 

correct responses. 

''Vithin each staircase the stimuli were presented in blocks of eight trials with a minimum 

intertrial interval of twenty seconds. The interflash interval for the test stimulus was changed 

according to the following rules: If the subject 

was right more than six times in a block of eight 

trials, the interflash interval was decreased by 

one msec. If the subject was right less than six 

times, the interflash interval was increased by 

one msec. If the subject was right exactly six 

times, the test stimulus was unchanged. Two 

independent staircases "-ere run, one starting at 

a short interflash inten·al, the other at a relatively long inten·al. On any trial the particular 

staircase which \vas presented was randomly de2 

termined. Twenty blocks of eight trials in each 

of the two staircases comprised a single session. 

Each session lasted approximately 120 minutes. 

At any fixed luminance and area the staircases 

were maintained across sufficient sessions for 

stable estimates of threshold to be achieved; the 

staircases \Yere maintained across at least three 

sessions at each condition. The mean interflash 

interval for the last two sessions at each luminance and area was the threshold. Initially several sessions were required for each subject to 

stabilize at a consistent threshold value. The 

required number of practice sessions varied from 

3 for subject R.M. to 18 for subject B.J. 

The luminances used were 2.0, 6.0, 20, 63, and 

200 mL. Diameters of the circular targets were 

5, 10, 20, and 30 minutes of arc. For each subject the order in which luminance and areas were 

presented was random. 

III. Results and Discussion. 

Figure 2 shows the two-flash thresholds for 

each subject as a function of flash luminance at 

each area. Figure 3 shows the same data plotted 

as a function of area with luminance as the 

parameter. In both figures the ordinate indicates 

the total duration of the test stimulus; the vertical bars represent one standard deviation. 

Figure 4 sho\vs the same data averaged across 

subjects. The dashed lines connect with the point 

for which (due to insufficient time) data from 

subject B.J. are not available. Inspection of 

Figures 2 and 3 indicates that the data from 

subject B.J. are elevated relative to those of subjects R.M. and F.A.; consequently, the point is 

probably depressed from the value to be expected 

were the datum from subject B.J. available. 

Figures 2 and 4 indicate that two-flash 

thresholds decrease in a negatively accelerated 

fashion as luminance increases. At the two 

larger areas there appears to be a reversal at the 

highest luminance. This effect was not clearly 

indicated in the data for all subjects (Figure 2), 

nor was a reversal of this kind found in previous 

studies.6 •

8 Examination of the data for two of 

the subjects suggests that the slope of the function decreases as area increases. 

Figures 3 and 4 indicate that two-flash 

thresholds decrease as area increases. This effect 

is most clearly seen at the lowest luminance. 

Examination of the data from individual subjects suggests that the effects of area decrease as 

luminance increases. 
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LOG L (ml) 

FIGuRE 2. Two-ftash threshold ( msec.) as a function of log luminance ( mL) for three subjects. Areas are indicated as follows : 

open circle 30, filled circle 20, open square 10, filled square 5 minutes of arc diameters. Curves have been shifted toward 

along the ordinate by an amount indicated next to each function. 
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FIGUR~: 3. Two-flash threshold ( msec.) as a function of the log diameter (minutes of arc) at each luminance level. Luminances 

are indicated as follows: open circle 2.3, filled circle 1.8, open square 1.3, filled square .8 log mL. Curves have been shifted 

upward along the ordinate by amounts indicated next to each function. 
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FIGURE 4. •.rwo-fiash threshold (msec.) as a function of log luminance (mL) and log diameter (minutes of arc). Average 

of three subjects. On the left stimulus diameters are indicated as follows : open circle 30, filled circle 20, open square 10, filled 

square 5 minutes of arc. On the right luminances are indicated as follows : open circle 2.3, filled circle 1.8, open square 1.3, 

and filled square .8 log mL. 
In a previous study8 it was suggested that the 

two-flash threshold may be interpreted as estimating the critical duration, tc, o£ Bloch's law. 

It was also suggested that two-flash thresholds 

yield direct estimates o£ the critical duration, 

unlike threshold studies \vhich estimate the critical duration £rom the intersection o£ the fitted 

line representing Bloch's law (Lt= C) with the 

fitted line representing the relation, L =constant. 

Indeed, it may be that calculating critical duration £rom such fitted lines overestimates the 

critical duration, here taken to be the duration 

at which complete reciprocity ends, by confounding it with what Pieron10 has called "utilization 

time," the duration within which there is only 

partial integration o£ energy. With this interpretation the results of the previous study indicate that the critical duration £or Bloch's law 

decreases as a negatively accelerated £unction o£ 

luminance in the dark-adapted eye. That result 

has been replicated in the current study. 

The relation between the critical duration and 

stimulus area has been the subject o£ a number 

o£ previous im-estigations. Graham and Margaria4 found that as stimulus area increased, 

there was less abrupt change £rom total integration (the relation Lt=C) to the relation L=constant; at a large area the transition was gradual; 

at the smallest area (two minutes) the transition 

was abrupt. Moreover the point o£ departure 

£rom strict reciprocity, i.e., the critical duration, 

decreased as area increased. The same result is 

obtained by fitting straight lines to their data 

and taking the intersection as the critical duration. Baumgardt and Hillman1 found abrupt 

changes to occur at approximately 100 msec., 

independent of area, but beyond the critical duration their data sho"- evidence £or partial integration occurring up to the longest duration 

employed (one second). The investigation by 

Sperling and Joliffe11 with three stimulus diameters ( 4.5', 45', & 3°) indicates departures £rom 

complete reciprocity occurred at shorter durations £or the larger areas; their data also indicate 

partial integration occurring at durations well 

beyond the critical duration. If the critical 

duration is derived £rom the Sperling and J oliffe 

data by following the convention o£ fitting 

6 

straight lines representing the relations Lt=C 

and L=constant, the critical durations obtained 

£rom the intersections o£ these straight lines do 

not vary with area, a situation that is in accord 

with the suggestion made above that such a procedure confounds critical duration with utilization time. If two-flash thresholds may be 

interpreted as directly estimating the critical 

duration rather than utilization time, then the 

data o£ the current study indicate that critical 

duration decreases as area increases; this effect 

is seen most clearly at the lowest luminance used. 

As the luminance level was increased relative to 

the level o£ adaptation, the effect o£ area became 

increasingly smaller. 

Mahneke9 has suggested that the two-flash 

threshold may be discussed as a limiting case £or 

critical flicker-fusion frequency. To facilitate 

comparisons with flicker data, the data o£ the 

current study have been replotted in Figure 5 

to show the reciprocal o£ the two-flash thresholds 

as a £unction o£ stimulus parameters. Figure 5 

indicates that with the two smaller areas critical 

flicker frequency (as measured with the two-flash 

threshold) increases with luminance; at the two 

larger areas there are significant reversals at the 

highest luminance investigated. 

These data appear to be consistent in form 

with the results o£ conventional investigations 

o£ C.F.F. as a £unction o£ luminance,5 although 

the slopes o£ the current £unctions are greatly 

reduced. However, in the case o£ the current 

data stimulus luminance was varied independently 

o£ adaptation level. 

Figure 5 indicates that at the lowest luminance 

level employed, C.F.F. is a linear £unction o£ log 

area, i.e., these data are proportional to the 

Granit-Harper law. At luminances well above 

the level o£ adaptation there are significant departures £rom the Granit-Harper law such that 

C.F.F. does not appear to be systematically related to stimulus area. It must be stressed that 

these data are not strictly comparable with conventional C.F.F. data in that pulse-to-cycle fraction is totally confounded with the dependent 

variable in the present study, whereas in conventional studies pulse-to-cycle fraction is usually 

held constant. 
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FIGURE 5. Reciprocal of two-flash threshold as a function of log luminance and log area. On the left stii)'lulus diameters are 

indicated as open circle 5, filled circle 10, open square 20, filled square 30 minutes of arc. On the right luminances are indicated as follows: open circle .8, filled circle 1.3, open square 1.8, filled square 2.3 log mL. '.rhe reciprocals have been multiplied by an arbitrary constant (103 ) to facilitate comparison with conventional C.F.F. data. 
REFERENCES 

1. BAGMGARDT, E. & HILL::\IAN, B. Duration and size 

as determinants of peripheral retinal response. J. 

opt. Soc. A mer., 51 :340-3H, 1961. 

2. CAMPBELL, R. A. Detection of a noise signal of 

varying duration. J. acoust. Soc. Amer., 35:1732-

1737, 1963. 

3. CoRNSWEET, T. X. The staircase-method in psychoph;rsics. Amer. J. Psychol., 75 :-!85--491, 1965. 

4. GRAHAM, C. H. & ~1ARGARIA, R. Area and the 

intensity-time relation in the peripheral retina. 

Amer. J. Physiol., 113 :299-305, 1935. 

5. HECHT, S. & VERRIJP, C. D. Intermittent stimulation 

by light. III. and IV. J. gen. Physiol., 17 :251-286, 

1933. 

6. KIETZMAN, ~I. L. Two-pulse measures of temporal 

resolution as a function of stimulus energy, J. opt. 

Soc. Amm·., 57 :809-813, 1967. 

8 

7. LEWIS, ::u. F. Category judgments as functions of 

flash luminance and duration. J. opt. Soc. Amer., 

55:1655-1660, 1965. 

8. LEWIS, :M. F. Two-flash thresholds as a function 

of luminance in the dark-adapted eye. J. opt. Soc. 

A mer., 57 :814-815, 1967. 

!>. :\IAHNEKE, A. Fusion thresholds of the human eye 

as measured with two or several light flashes. Acta 

Ophthalmol., 36:12-18, 1958. 

10. PI.ERON, H. Vision in intermittent light. In W. D. 

Neff (ed.), Contributions to sensory physiology, Vol. 

I. Xew York: Academic Press, 1965, pp. 179-26-!. 

11. SPERLING, H. G. & JoLIFFE, C. L. Intensity-time 

relationship at threshold for spectral stimuli in human vision. J. opt. Soc. Amer., 55:191-199, 1965.