AM 67-16 

CROSS-MODALITY MATCHING OF LOUDNESS TO 

BRIGHTNESS FOR FLASHES OF VARYING LUMINANCE 

AND DURATION 

Approved by 

~~ J. RoBERT DILLE, M.D. 

CHIEF' CIVIL AEROMEDICAL 

INSTITUTE 

Mark F. Lewis, Ph.D. 

August 1967 

Released by 

~AM> P. v. SIEGEL, M.D. 

FEDERAL Am SURGEON 

FEDERAL AVIATION ADMINISTRATION 

Office of Aviation Medicine 
ACKNOWLEDGMENTS 

The author wishes to thank Glenda Wade and Marie Cape, who served as 

patient subjects. Faye Ashby and Henry Mertens assisted in the collection 

of the data. 

Qualified requesters may obtain Aviation Medical 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 
CROSS-MODALITY MATCHING OF LOUDNESS TO BRIGHTNESS 

FOR FLASHES OF VARYING LUMINANCE AND DURATION 

I. The Problem. 

The current study is directed to evaluating the 

effective brightness of signal lights for observers 

in aviation. Since signal lights are usually presented to the observer as flashes with finite duration, the effect of flash duration on apparent 

brightness assumes importance. At threshold the 

effect of flash duration is well known : threshold 

for detection of light flashes is a function of flash 

energy [the product of flash luminance (L) and 

duration ( t)] for all flash durations below some 

critical duration. This relation is known as 

Bloch's law. At durations exceeding the critical 

duration, the threshold for detection is a function 

of luminance. The aim of this study is to establish the effective brightness of flashes above the 

threshold for detection. 

A large number of experiments have been devoted to the Broca-Sulzer effect, a phenomenon 

in which the apparent brightness of a light flash 

of suprathreshold luminance does not increase 

monotonically with duration, but rather passes 

through a maximum and then decreases to a 

steady-state value. The effect has been described 

by Stevens1 •

2 as a 2-dB hump that occurs between 

30 and 300 msec., the duration at which the maximum brightness is reached tending to decrease 

systematically as flash luminance is increased. 

Typically, the effect is measured by having subjects adjust the luminance of a fixed-duration 

flash to match the brightness of flashes of varying 

duration and fixed luminance, although Katz 3 

assessed the phenomenon by having subjects adjust the luminance of flashes of varying duration 

to match the brightness of a flash of fixed duration and fixed luminance. Aiba and Stevens4 

utilized both procedures in their determination of 

the effect. In either case the measurement of apparent brightness was accomplished by use of a 

visual comparison stimulus. 

Attempts to obtain brightness functions without use of a visual comparison stimulus have 

1 

demonstrated that Bloch's law (Lt=O) holds 

(for constant brightness criteria) for all stimulus 

durations less than some critical duration, tc; 

when the ·duration of the stimulus exceeds the 

critical duration, brightness is determined solely 

by luminance. •~• 7 In a study by Raab, Fehrer, 

and Hershenson8 three subjects made brightness 

estimates to flashes of constant luminance and 

varying duration; no Broca-Sulzer effect was obtained. The authors hypothesized that BrocaSulzer maxima may occur only when a visual 

comparison stimulus is presented along with the 

test flash instead of being a sole function of flash 

duration. Of subsequent studies, the results of 

one6 were interpreted as supporting this hypothesis, while the results of two otherS5

•

7 were interpreted as providing evidence for Broca-Sulzer 

maxima in the absence of a comparison stimulus. 

These studies, discussed elsewhere,9 used variations of brightness estimation techniques and 

their results regarding the Broca-Sulzer phenomenon are not unequivocal. 

The current study attempts to determine the 

brightness function using cross-modality matching in which the intensity of an acoustic signal 

is adjusted until the subject reports that the tone 

is as "loud" as the flash is "bright." In other 

words, the subjective brightness of light flashes 

for observers in night flying conditions is evalu-

-ated without the contaminating effects (usually 

present in laboratory investigation~?) introduced 

by having observers make judgments against 

comparison lights. 

II. Method. 

A. Subjects. Subjects were female studentwives between 21-26 years of age. Both subjects 

were examined and found to be emmetropes with 

no color vision defects. Both were paid an hourly 

wage. 

B. Apparatus. An optical system was fiet up 

to deliver a 0.5° stimulus to the fovea by 
Maxwellian view. Light from a Sylvania glow 

modulator tube (R1131C) was collimated by one 

lens; an image of the 2.36-mm. crater was focused 

in the subject's pupil by a second lens after the 

light had passed through neutral density filters 

(Optics Technology & Bausch and Lomb) that 

were used to control stimulus intensity, a~ field 

stop, and a beam-splitter. The subject fixated 

four red fixation lines through a 2-mm. artificial 

pupil. Head position was controlled by a chin 

and forehead rest. Intensity of the fixation lines 

was controlled by the subject with a rheostat. 

The glow modulator tube was driven by an Iconix 

Light Driver; flash durations were controlled by 

an Iconix 6257 timebase with preset controllers 

(Iconix 6010) and associated logic. Flashes were 

monitored by an RCA 1P39 phototube. Temporal 

characteristics of the waveform of the light 

flashes showed that, at the current level used 

( 40mA.) , rise-time was less than 15 p.sec. and 

decay time less than 25 p.sec. Luminance calibrations were made with a S.E.I. exposure photometer by a method described earlier.6 

Acoustic stimuli were generated by a KrohnHite Oscillator (Model 440) and delivered monaurally through a vVestern Electric headset 

( # 1002F). Voltage through the headset was 

continuously monitored on a Tektronix type 535A 

oscilloscope. 

C. Procedure. Each session was preceded by 

ten minutes dark-adaptation. The subject was 

then required to adjust the intensity of the fixation lines, using the rheostat, until the fixation 

lines were just visible. The experimenter then 

read the following instructions to the subject: 

"When you hear the ready signal, sight through 

the tube and fix your gaze on the center of the 

cross. vVhen ready, press and hold down the 

switch. Every twenty seconds you will hear a 

tone and see a flash in the center of the cross. 

Using the large knob, adjust the intensity of the 

tone until it is as loud as the flash is bright. Not 

all the flashes are the same brightness or duration. 

Try not to allow the flash duration to influence 

your judgment. When you have achieved a 

satisfactory match release the switch." 

Seventy-eight stimuli were presented, ranging 

in duration from 2 msec. to 1024 msec., and in 

luminance from 7.9 mL. to 15,850 mL. On a 

single trial, the flash. presentation occurred every 

twenty seconds until the subject was satisfied 

2 

with his match. Flash onset occurred 2 msec. 

after onset of a two-second presentation of the 

1000-Hz comparison tone. For subject GW each 

stimulus was presented twice in each two-hour 

session in random order; for subject MC, who 

took longer to complete matches, each stimulus 

was presented once in each session. Thus a total 

of 18 matches was obtained for each stimulus 

from each subject. Each subject was given three 

practice sessions before the data reported were 

collected. 

III. Results and Discussion. 

Figure 1 shows for each subject the Sound 

Pressure Level of the 1000-Hz tone that matched 

each flash as a function of flash duration with 

luminance as. the parameter. The ordinate is the 

geometric mean (dB average) of 18 matches for 

each flash. At each luminance level, the intensity 

of the matching tone is an increasing, negatively 

accelerated function of flash duration. In some 

of the functions (from subject GW) there are 

inversions, but these appear to be unsystematic. 

No evidence of Broca-Sulzer maxima is seen. 

Figure 2 shows the same data plotted with 

points connecting equal energy stimuli. The 

horizontal functions indicate agreement with 

Bloch's law. No deviations from Bloch's law 

that would be consistent with the Broca-Sulzer 

effect are seen. The descending branches of the 

functions represent the loss of duration effects 

beyond the critical duration. For subject MC 

there appear to be duration effects well beyond 

the region where the critical duration should be. 

Similar effects have been observed with one subject using category judgments.6 

Figure 3 shows the same data replotted with 

luminance on the abscissa and duration as the 

parameter (for subject GW). Many of these 

functions (for subject GW) appear to be linear 

to a first approximation; but (for subject MC) 

there is a tendency :for some o:f them to appear 

negatively accelerated. 

The functions of Figure 3 were used to 

generate a family of constant-response functions 

that relate flash energy to flash duration. Several 

horizontal cuts were made through the functions 

of Figure 3. The log Lt (in msec.-mL.) marked 

by the intersections were plotted in Figure 4 as 

functions of log t, and straight-line segments 

were fitted to the points by the method of 

averages. 
The functions in Figure 4 indicate agreement 

with Bloch's law: Brightness is determined by 

flash energy up to the critical duration; for 

flashes longer than the critical duration, brightness is determined by flash luminance. This result is in agreement with previous .studies.4 •

5 •6 •7 

In addition, the functions of Figure 4 indicate a 

change in the critical duration for Bloch's law: 

As stimulus luminance increases, tc decreases. 

The extent of the change in tc is small when compared with the results of J. C. Stevens & Hall/ 

but the current result is not in disagreement with 

3 

the results of other studies where a change is reported. 4 ' 5 In one previous study 6 no reliable 

change in tc was detected. 

Finally, the contours of Figure 4 show no deviations consistent with the Broca-Sulzer effect. 

This result is in conflict with the interpretive 

findings of Raab5 and of J. C. Stevens & Hall,7 

but agrees with the previous findings of Lewis6 

and of Raab, Fehrer and Hershenson.8 The result 

is consistent with the hypothesis that the BrocaSulzer effect occurs only when a visual comparison stimulus is present. 
.;:.. 

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FIGURE 1. Geometric mean intensity (dB average) of matching 1000-Hz tone as a function of flash duration at twelve luminance levels. Data on the left 

are from subject G. W.; those on the right are from subject M. C. 
Ct 

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Log t (msec) 

FIGURE 2. Geometric mean intensity (dB average) of matching 1000-Hz tone as a function of flash duration at 11 energy levels. Data on the left are from 

subject G. W.; those on the right are from subject M. C. 
0:. 

110 r t (msec) 110 I t (msec) 

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Log L (mL) Log L (mL) 

FIGURE 3. Geometric mean intensity (dB average) of matching 1000-Hz tone as a function of flash luminance at ten durations. Data on the left are from 

subject G. W.; those on the right are from subject M. C. For greater clarity, functions for durations 64-1024 msec. have been shifted along the ordinate· 

by amounts indicated to the left of the functions. 
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FIGuRE 4. Equal-brightness functions relating flash energy (log mL.-msec.) to flash duration (log msec.). Data on the left are from subject G. W.; those 

on the right are from subject M. C. The points were obtained by making horizontal cuts through the functions of Figure 3 at the intensity levels of 

the matching tone indicated above by the parameter. 
REFERENCES 

1. STEVENS, S. S.: Duration, luminance and the brightness exponent. Percept. & Psychophys., 1966, 1, 96-100. 

2. STEVENs, S. S.: Concerning the measurement of brightness. J. Opt. Soc. Amer., 1966, 56, 1135--1138. 

3. KATZ, M. S. : Brief flash brightness. Vision Res., 1964, 

4. 361-373. 

4. AIBA, T. S. & STEVENs, S. S. : Relation of brightness 

to duration under light- and dark-adaptation. Vision 

Res., 1934, 4, 391-401. 

5. RAAB, D.: Magnitude estimation of the brightness of 

brief foveal stimuli. Science, 1962, 135, 42-43. 

8 

6. LEwrs, M. F. : Category judgments as functions of 

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

1965, 55, 1655-1660. 

7. STEVENs, J. C. & HALL, J. W. : Brightness and loudness 

as functions of stimulus duration. Percept & Psychophys., 1966, 1, 319-327. 

8. RAAB, D., FERRER, E., & HERSHENSON, M.: Visual 

reaction time and the Broca-Sulzer phenomenon. 

J. ercp. Psychol., 1961, 61, 193-199. 

9. LEWIS, M. F. : Category judgments and the luminanceduration function. J. Opt. Soc. Amer., 1966, 56, 1399-

1400.