July 1967

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 57 NUMBER 7 JULY 1967

Subjective Brightness of a Very-Short-Persistence Television DisplayCompared to One With Standard Persistence

RICHARD A. EASTON, JOSEPH MARKIN, AND ALAN SOBEL

Zenith Radio Corporation, Chicago, Illinois 60639

(Received 26 January 1967)

In a zero-persistence television display, such as one employing a deflected laser beam or injection lumines-cent diodes, the "on" time per picture element is determined solely by the system's finite resolution and isabout 120 nsec under current standards, as compared to 60-psec persistence to 10% of initial luminance for aconventional cathode-ray-tube television display. Therefore, at a given luminance level, much higher peakluminances are encountered in the zero-persistence display. An experiment was performed to see if reasona-ble approximations to these two types of displays had equal averaged photometric luminances when adjustedfor equal subjective brightnesses-i.e., does the Bunson-Roscoe luminance-time reciprocity law hold underthese conditions?

One half of a television picture was displayed on a cathode-ray tube with 120-nsec decay to 37% of initialluminance; the other half was displayed on a television phosphor whose decay under these conditions was40 times as long, or about 5 psec. The viewer was asked to match the two halves for brightness and thismatch was checked with a photometer. The display covered a 12 X 14 degree rectangular field of view undersimulated home-viewing conditions. Tests were run at approximately 110 and 340 candelas per m2 (33 and100 ft-L) highlight luminances, with both a standard blank television raster and an Indian-head testpattern. No significant departure from reciprocity was noted under these conditions. A 95% confidence inter-val of 6% on the raster tests and 13% on the Indian-head-pattern tests was attained.INDEX HEADINGS: Vision; Television; Laser; Photometry.

C ONSIDER a television display as an array of injection luminescent diodes' the phosphor decay timeC picture elements. The usual TV display device is a is eliminated and the "on" time per element is now onlycathode-ray tube. The light from each picture element the 120 nsec determined by the finite resolution of thedecays to 10% of its initial luminance within about system. Since luminous energy is the integral of60,gsec (for black and white tubes) after the element luminance with respect to time, making the on timehas been struck by the electron beam. However, in a _

display device utilizing a deflected laser beam' or 2 A. Kawaji et al., in Proceedings of the 7th National Symposiumon Information Display (Soc. Information Display, Boston,

'A. Korpel et al., Proc. IEEE 54, 1429 (1966). 1966), p. 255.

957

EASTON, WIAARKIN, AND SOBEL V.

shorter requires that the luminance be made higher ifa fixed luminous energy output is to be maintained.

The reciprocity law (also called the Bunsen-Roscoelaw) states that brightness sensations depend upon theamount of luminous energy striking the retina butnot on the time-luminance distribution of this energy.This law has been verified in many experiments in whichthe luminance was well above rod threshold level andin which the integration time was less than about 100msec. However, the maximum peak-luminance levels forwhich this law holds are open to question, and we couldfind no references to previous reciprocity experimentsinvolving TV displays.

This paper describes an experiment designed to deter-mine if the reciprocity law holds for TV displays ofnear-zero persistence and average luminance of up to340 cd/m2.3

THE PROBLEM

A TV display contains 491 active horizontal scanninglines per frame.4 Each frame consists of 2 fields, inter-laced so that the lines in the second field fall exactlvhalfway between the lines of the first field. Since 30frames are transmitted every second, a given point ina frame will be scanned once in every 1/30 sec.

The picture-highlight luminance for home TV ispractically limited to about 340 cd/n 2 for black andwhite sets and to 140 cd/mi2 for color sets, by physio-logical, technical, and cost considerations. In a dimlylighted room, people generally choose average pictureluminances of only 70 cd/mt5.

Because of the limitations in the standards on thebandwidth of the transmitted signal, the minimumduration of a single picture element (measured in thehorizontal direction) is about 120 nsec. At 30 frames persecond this amounts to a duty factor of 3.6X10-6, ifwe make the simplifying assumption that pictureelements make instantaneous transitions from off toon and off again. This assumption corresponds towhat might be expected from a display device com-prising an array of injection-luminescent diodes, oneper picture element, operated without any storagedevices. It also approximates what might be expectedfrom a deflected-laser-beam display viewed from adistance at which individual picture elements can justbe resolved. With this duty factor, the zero-persistencedisplay will deliver luminance peaks 278 000 times theaverage luminance. For the usual P4 TV phosphor,integration of the area under the luminance-vs-timecurve yields a ratio of peak to average luminance of1600:1. The ratio of the peak luminances for theinstantaneous-transition and conventional displays ismore than 170:1.

3 Luminance in ft-L = (luminance in cd/M 2) XO.292.This and the following TV-display parameters are current

U. S. A. standards.I D. G. Fink, Television Engineering Handbook (McGraw-Hill

Book Company, New York, 1957).

We are not aware of any published results of reci-procity experiments which adequately cover these TV-display conditions. Gilmer' and Brindley7' 8 reportedthat there is no reciprocity failure for repetitive pulsessomewhat similar to those encountered in televisiondisplays, but they used small fields, confined to thefoveal region of the retina, and their peak luminanceswere about 1/100 of those we required. Since the periph-eral retina has temporal responses different from thatof the fovea,9 the small field sizes used also make theresults questionable for our purposes; the TV imagewill cover about 12X 14 deg of visual arc if the vieweris located four times the screen height away from thescreen. Miller10 used high-energy pulses in the luminanceand field-size ranges which concern us, but she usedsingle rather than repetitive pulses and was concernedonly with afterimage effects. Within these restrictions,she reported no reciprocity failure. However, we aredealing with repeated flashes and (presumably) primaryimages rather than afterimages, so her results are notdirectly applicable to the present problem.

Previous experimenters did not comment on anypossible effects due to dark or light adaptation orcontrast ratio nor did they attempt to use structuredimages. Since these factors do affect acuity and criticalflicker frequency, 9 they may also affect time-luminancereciprocity. For these reasons, we decided to make ourtests under actual TV-viewing conditions before con-cluding that the reciprocity law holds in this case.

EXPERIMENTAL DESIGN

The test consisted of having subjects look at theleft half of a TV display of near-zero persistence sideby side with the right half of a standard-persistencedisplay. They would then adjust the brightness of theleft half to match a preset brightness of the right half.

In this experiment the shorter-persistence displaywas a flying-spot-scanner tube with a blue-green P24phosphor screen. A laser display was not used because,at the time that this experiment was started, laserdisplays capable of the required brightness levels werenot available. Also, we considered that the results wouldbe more generally applicable if the experiment employedwavelengths close to the center of the visible range andcloser to white light than the monochromatic red ofthe available helium-neon laser. As was previouslymentioned, a zero-persistence display still has aneffective on time of 120 nsec, owing to the finite numberof resolvable picture elements in the display. The P24phosphor output decays to less than 40% of its initialluminance in this period, so it gives a fair approximation

6 T. Gilmer, J. Opt. Soc. Am. 27, 386 (1937).G. S. Brindley, J. Physiol. (London) 118, 135 (1952).G. S. Brindley, J. Physiol. (London) 1t17, 194 (1959).H. Pieron in W. Ness, Contributions to Sensory Physiology

(Academic Press Inc., New York, 1965), p. 180.'1 N. Miller, J. Opt. Soc. Am. 55, 1661 (1965).

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July 1967 BRIGHTNESS OF VERY-SHORT-PERSISTENCE DISPLAY

Rel.Output

100%r

60

20

I I I l100 300 n sec. t

FIG. 1. Decay characteristic of the short-persistence P24phosphor (from Phosphors, manual TPM-1508A, RCA ElectronTube Division, Harrison, N. J., 1961).

to zero persistence. Figure 1 shows the decay charac-teristics of the P24 phosphor.

The comparison display was a second flying-spotscanner tube with a P20 phosphor, which is the yellow-green longer-persistence component of the standard P4TV phosphor. (The P4 phosphor has two componentswhich differ both in color and persistence and togetheryield white light.) The P20 phosphor (see Fig. 2)has 40 times the persistence of the P24 phosphor(measured at 10% of initial luminance). It was usedin place of P4 for two reasons: It is a single-componentphosphor and thus has only one persistence at a givenluminance and it provides a closer spectral match tothe P24 phosphor. (The P24 is also a single-componentphosphor. For both phosphors, the shape of the decaycurve is independent of wavelength.) However, it was

Rel.Output

100%

60

0 40 80 P sec. tFIG. 2. Decay characteristic of the medium-short-persistence

P20 phosphor curve, measured experimentally for a raster at345 cd/m 2 average luminance.

still necessary to place a green filter (Wratten No. 13)in front of the P20 tube and a yellow filter (WrattenNo. 9) in front of the P24 tube in order to provide atolerable color match. The spectral peaks were thusmatched to within 5 nm, with the resulting peaks atabout 540 nm, which is green, near the peak of thespectral-response curve for photopic light-adaptedvision. Note that the literature value for the thresholdof wavelength discrimination at 540nmu is 4 nm.'1

Figure 3 illustrates the color match obtained. Thepurity of the two colors was not quite the same andthe subjective effect of this difference made brightnessmatching difficult, although not intolerably so.

The two images were placed side by side by meansof a system of mirrors. Each tube was driven by aseparate modified TV chassis. The common high-

Rel.Sensitivity %

420 PhosphorP

and f Ilter \ P24 Phosphor

20- g and filter

I I500 550 600 mat

FIG. 3. Comparison of the calculated spectral distributionsof the P20 phosphor after passing through a Kodak Wratten No. 13color-correcting filter and the P24 phosphor with a No. 9 Wrattenfilter. These curves are calculated from published (1965) KodakWratten filter data and RCA phosphor curves.

voltage anode power supply was regulated at 25 kV andprovision was made for monitoring the electron-beamcurrents in the two tubes. Figure 4 is a schematicdiagram of the complete apparatus.

The experiment was designed so that the observerswould be able to resolve at least one picture element.If resolution is better than this, no harm is done, sincedifferent areas within a picture element will have thesame time-luminance behavior. However, if the eyedoes not resolve individual picture elements theluminance-time distributions seen will be different fromthose that we assumed in setting up the experiment.For example, if two adjacent picture elements on thedisplay are fused by the observer into one, the energyper frame on the retina, of this one fused element, will

11 F. H. Adler, Physiology of the Eye (C. V. Mosby Company,St. Louis, Missouri, 1965).

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EASTON, MARI(IN, AND SOBEL

SIGNAL INPUT

FIG. 4. Schematic diagram of the experimental apparatus forthe luminance-time reciprocity study. See text for a more detailedexplanation of the various elements. 1-Wratten No. 9 yellowfilter. 2-P24 phosphor flying-spot-scanner tube. 3-P24 beam-current meter. 4&8-Modified TV sets for cathode-ray-tubescanning and low-voltage supply. 5-P24 luminance control.6-Selector for blank raster or Indian-head test pattern. 7-P20luminance control. 9-Regulated 25-kV power supply. 10-P20beam-current meter. 11-P20 phosphor flying-spot-scanner tube.12-Wratten No. 13 green filter. 13-First-surface mirrors.14-Tunnel with white mat diffusing surface providing evensurround illumination from light source 15. 16-Viewing port forsubject or photometer (30 cm from screen).

be twice that which would have been produced by eachof the two resolved elements.

Taking the resolving power of the eye as 1 min ofarc and assuming the picture to have 491 horizontallines (the remaining 34 lines of the nominal 525-linepicture are used for vertical retrace and so do not appearon the screen), the maximum viewing distance shouldbe about 7 times the picture height for resolution of allthe lines. Preferred viewing distances range from 3 to8 times the screen height, with a marked preferencefor 5 times.5 It would be desirable to have a display thesize of a typical TV picture so that the viewer couldsit several feet away. However, large-size direct-viewtubes with the fast P24 phosphors are incapable ofadequate luminance for this experiment, owing to thelow efficiency of the phosphor. The alternative wouldbe to use a magnifying lens and small CRT's but theefficiency would again be too low to yield the desiredluminances. Fresnel lenses were tried and discarded ashaving too-low resolution and undesirable fringeingeffects. A picture height of 5.7 cm and a viewingdistance of 30 cm were chosen as the best compromise.The picture was viewed with one eye to eliminate anyproblems with seeing double images. Resolution of the

display devices, including the effects of phosphor,electron gun, and video amplifiers was established asadequate by observing the resolution-chart portions ofthe Indian-head test pattern used as the picture insome of the experiments and by insuring that theindividual raster lines could be distinguished.

The area surrounding the image was illuminated withwhite light to simulate watching the picture in a roomwith subdued lighting. Preliminary tests indicated thatthis surround illumination had no significant effect onthe experimental results, but it did make the test some-what more comfortable for the observer. Further testsindicated that it did not matter which half was heldat a fixed brightness; we chose the right half.

In the final test, each subject matched the brightnessof the short-persistence display to the preset brightnessof the longer-persistence display. Matches were madewith both a blank raster and the highlight brightnessesin a standard Indian-head test pattern. The presetluminances were 110 and 340 cd/M2. Six trials weremade for each presentation at each luminance level;the results were averaged and their relative standarddeviations were computed.

A failure of reciprocity is presumably related to theilluminance of the retina, so that an ideal experimentwould measure this quantity and relate it to picturebrightness. However, to do this adequately wouldrequire measuring the size of the observer's pupil andcontrolling the distance between the pupil and thesource.' 2 In the interest of simplicity these steps werenot taken. It is possible to estimate retinal illuminancefrom the geometry of the experiment and the averagepupil diameter of the observer, but there is so muchvariation of both of these factors that we have notreported such calculations here.

The average luminances of the phosphors, JfI dt, forthe two displays were measured with a Spectra modelUB Spot Brightness Meter containing a photomulti-plier corrected for visual response. Thus, an objectivecomparison was obtained to test the observer's sub-jective one. The Spot Brightness Meter was calibratedagainst a standard Weston model 759 photometer.Absence of meter overload by the high instantaneousilluminance was established by placing a neutraldensity filter in front of the meter and noting thatthe drop of reading matched the reduction of illuminanceproduced by the filter. Curves of luminance as functionsof cathode-ray-tube beam current were plotted and thedata were taken in terms of this current, so that re-peated use of the Spot Brightness Meter was notrequired. For the pulse durations in this experiment,

12 The unit of retinal illuminance is the troland, which is usuallydefined in terms of the retinal illuminance produced by a uniform,extended source. [See, for example, S. S. Stevens, Ed., Ilandbookof Experimenttal Psychology (Jolin Wiley & Sons, New Vork,1951), pp. 814-817.] In this experiment, the instantaneous sourceis a point, so the retinal illuminance will be a function of thedistance between the source and the observer.

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July1967 BRIGHTNESS OF VERY-SHORT-PERSISTENCE DISPLAY

there is no reason to anticipate reciprocity failure inthe photomultiplier.

The test population consisted of 22 people, alladults. Sixteen men and six women were included, witha wide range of ages (14 aged 20 to 29, 5 aged 30 to 44,and 3 over 45). About half of the sample had some pre-vious experience involving brightness matching. Some25% reported some eye fatigue before starting the test.The subjects all had either normal vision or vision cor-rected to normal. Each person was read a previouslyprepared set of instructions and was given a chance tomake several trial matches before any data were taken.

RESULTS

The results are summarized in Table I. Table IIshows the individual distributions of results. Theresults given in Table I are based on the data betweenasterisks in Table II. (Inclusion of all the data inthese tables would not change the mean values appreci-

TABLE I. Summary of reciprocity-experiment results.'

Fixed display Variable displayMean Median Relative 95%O % Off

highlight highlight standard Confi- reci-Highlight lumi- lumi- devia- dence procityluminance nance nance tion of interval, of

cd/mi cd/mr cd/m 2 mean, % % mean

110Raster 110 110 13 6 3

110Indian head 140 140 28 13 20

340Raster 330 340 3 6 3

340Indian head 360 370 21 10 4

aThe results are based on the data between asterisks in Table II.

ably, but would broaden the distributions, typicallyincreasing the relative standard deviation about 50%.)

Clearly, reciprocity holds for the group as a whole

TABLE Ha. Experimental results.'

110 cd/m2 Blank raster 110 cd/M 2 Indian head 340 cd/M 2 Blank raster 340 cd/M 2 Indian headMean match Mean match Mean match Mean match

cd/mr2 ±R.S.D.%b cd/Mr2 ±R.S.D.% cd/m2 ±R.S.D.% cd/Mr2 ±R.S.D.%

62 22 86 25 205 22 165 1372 12 93 25 210 25 ...79be 20 93 17 225a 9 250 1279c 6 96 33 255 7

130 8 260 30 260 3096 5 134 36 270c 19 300 1996 25 134 15 285 18 315 7

100 17 137 14 300 11 320 40100 12 141 23 305 18 320 7100 11 141 5 310 22 330 14103 13 144 24 310a 13 365 21107 16 144 20 315 4 370 24110d 10 151 9 330 16 370 18110 10 154 20 330 10 375 6110 11 165 14 335 13 380 15110 22 165 12 335d 15 410 6110 17 165 13 345 10 440 14110 19 175 22 355 16 440 11113 7 182 18 360 7 445 12120d 10 182 14 365 12 465 9123 12 196 19 375 6130 6 385d 10 515 9137 14 390d 15 520 26137c 11 390c 19

*-- 400 15154 7 405 11185b 12

515b 16Mean for all data Mean for all data Mean for all data Mean for all data100 cd/mr2 i21% 140 cd/M2 ±28% 330 cd/mr2 ±16% 370 cd/mr2 432%

TABLE IIb. Number of subjects whose R.S.D.'s were less than the indicated percentages.

No. of subjects R.S.D., % No. of subjects R.S.D., % No. of subjects R.S.D., % No. of subjects R.S.D., %

8 10 3 10 6 10 7 1017 15 9 15 14 15 14 1522 20 14 20 22 20 16 20

20 25 19 26

a Each entry in Table Ila is the mean of 6 matches with the R.S.D. of these matches.b R,S.D. means Relative Standard Deviation.

V Where the symbols a, b. c, and d, appear they signify the results of two tests for each of these subjects on different days.

961

EASTON, MARKIN, AND SOBEL

at all of the levels tested although the matches variedconsiderably from person to person and from day today for the same person. Most of the people tested alsoshowed a 10%6-20%0 relative standard deviation for thematches reported at any given level. When statisticalconfidence-interval tests' 3 were applied to the meansreported in Table I, they indicated that the truemeans for all four levels were within 13% of the reportedmeans with at least a 95% certainty.

DISCUSSION

The larger 95% confidence intervals for the matchesinvolving the Indian-head test pattern (13% vs 6%7)were probably due to the fact that the contrast and greyscale of the variable display were affected somewhatby the brightness-control settings. Thus, although thesubjects were asked to disregard contrast, grey scale,and color, this was harder to do than in the matchesinvolving only a blank raster.

The only possible failure of reciprocity occurred withthe Indian-head pattern at the 110 cd/m 2 highlight-luminance level. In this test, the picture contrast washigher in the variable display than in the fixed display,owing to differences of phosphor sensitivity. In otherwords, the shapes of the CRT grid-voltage vs light-output curves were noticeably different at this lumi-nance. Thus, when the highlight luminances of the twodisplays were equal, the variable display had moredark area and therefore lower average luminance. This,in turn, may have been the reason why the subjectstended to set the highlight luminance of the variabledisplay above that of the fixed display, thus makingtheir average brightnesses more nearly equal. Thiseffect might have been avoided by operating bothCRT's at maximum luminance and attenuating thelight with neutral filters for the lower-luminancemeasurements. However, this would have shortenedthe life of the CRT's, so luminance was bias-controlledinstead. At the 340 cd/M2 level, the picture contrastswere more nearly equal and the effect was absent. For

13 William Volk, A pplied Statistics for Engineers (McGraw-HillBook Company, New York, 1958), Ch. 6.

this reason, it appears that the deviation in the 110cd/m2 case does not indicate a true failure ofreciprocity.

The accuracy of this experiment is limited by theextent to which the spectral distributions of the twophosphors were matched and by the accuracy withwhich the spectral response of the photomultiplierequals that of the human eye. Note that there is likelyto be considerable variation of visual response fromperson to person. Furthermore, the accuracy of thephotometric determinations depend on the accuracy andrise time of the photomultiplier Spot Brightness Meter,since this must respond to short, sharply-rising lightpulses. In our case this is fairly good; the manufacturerclaims 5%. Linearity and freedom from overload of themeter were checked as indicated earlier. The absoluteluminance calibration depends upon the accuracy of theWeston photometer standard. Again the manufacturerclaims 5%. While the accuracy of this experiment isnot as high as for some of those mentioned earlier, itshould be sufficiently accurate to detect any deviationsfrom reciprocity serious enough to affect the future ofsuch new display schemes as laser-beam deflection.

While it would not be advisable to extrapolate ourresults much beyond the limits that we chose (120nsec or longer pulse durations, at 340 cd/m2 averageluminance) the results here indicate that there are nosignificant reciprocity failures in going in the directionof a zero-persistence display. (The results of the experi-ments referred to earlier indicate that the law would holdif new display techniques were to evolve in the oppositedirection toward long persistence and low peak lumi-nances.) Therefore, given these limits, reciprocityfailure should not be a problem in the development ofnew TV displays.

ACKNOWLEDGMENTS

The authors wish to thank Norma Miller of OhioState University for comments, Paul Gleichauf of Rau-land Corporation for supplying the flying-spot scannertubes, Frank Wilczynski for help in constructing thetest apparatus, and, most especially, those people whoconsented to be test subjects for this experiment.

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