Phosphor persistence in oscilloscopic displays

Phosphor persistence in oscilloscopic displays

Vision Res. Vol. 33, No. 16, pp. 2337-2338, 1993 Printed in Great Britain. All rights reserved Copyright 0 0042-6989/93 $6.00 + 0.00 1993 Pergamon ...

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Vision Res. Vol. 33, No. 16, pp. 2337-2338, 1993 Printed in Great Britain. All rights reserved

Copyright

0

0042-6989/93 $6.00 + 0.00 1993 Pergamon Press Ltd

Research Note Phosphor Persistence in Oscilloscopic Displays GERALD

WESTHEIMER*

Received 27 April 1993

Display monitors with P31 phosphor controlled by a fast computer can emit light patterns that effectively decay in 2 msec when superimposed on even quite a dim background.

Phosphor persistence Visible persistence Phosphor decay

Groner, Groner, Muller, Bischof and Di Lo110 (1993) in these pages, based on very indirect experiments, have claimed that visual display monitors with P31 phosphor “produced persistence that lasted several hundred milliseconds even when a veiling light was projected on the screen”. Because monitors equipped with P31 phosphor are in wide use in vision laboratories, including mine, and because this claim runs counter to a general understanding of the properties of the P31 phosphor, I have measured the time-course of light emission of a HP1435A monitor whose specifications include this kind of phosphor. Light was measured by an RCA 931A photomultiplier tube whose output was captured by a digital storage scope (Tektronix 7854). Readout was digital and the record was subsequently graphed on a computer. The stimulus was a single dot, produced by z-axis pulses in a 2 x 2 matrix 8 pixels apart horizontally and 12 pixels apart vertically. Because the spot size on this scope is at least 0.25 mm, whereas the pixel spacing had been set at about 0.05 mm horizontally and about 0.04 mm vertically, the four individual phosphor excitations were not spatially resolved. For the purposes of the demonstration, this display program was repeated twice more as fast as the 20 mHz AT computer, controlled by a program partially in assembly language and partially in the C language, would allow. The total duration for the whole input to the HP1345A was < 100 ,usec. Figure 1 shows the time-course of the light emission of the phosphor. The time line represent 1 msec and also shows the photomultiplier output in complete darkness. The output level before the onset of phosphor emission reflects the fact that the screen had luminance of 0.14 cd/m2 due to dim general room illumination-it is at the minimum level common in our experiments, which are almost never conducted in complete darkness.

From the figure it appears that peak light output is reached within about 100 psec from the start. Decay to 10% of peak takes a further 600 ,~sec, and to 2% about 2 msec. The light level used here is not far from the peak emission of the display monitor. The intensity of a single light spot is expressed in candelas and, the resultant retinal flux depends on the observation distance and pupil diameter (Westheimer, 1985). The light intensity of a single brief light pulse would, therefore, have to be specified in units such as candela. sec. Because the integration time of the photopic visual system is at least 15-20 msec, the value of interest would thus be the integral under the curve in Fig. 1. For the record, these

FIGURE 1. Photomultiplier output in response to a spot of light stimulus on a HP1345A display monitor specified to be equipped with P3 1 phosphor. Length of time line is 1 msec. Stimulus consists of four closely-spaced single dots, displayed three times in succession as fast as computer program will allow. Rise time is about 100 psec and decay to 10% of peak takes a further 600~. Return to 2% (i.e. within the *Divisionof Neurobiology, 211 Life Science Addition, University of noise level when there is a 0.14 cd/m2 background) takes a total of California, Berkeley, CA 94720, U.S.A. about 2msec. Intensity control of scope was near maximum setting. 2337

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RESEARCH NOTE

light pulses were not visible from a distance of 2 m through a neutral filter of density 1.5. It is concluded that a standard HP1354A visual display with P31 phosphor controlled by a fast minicomPutcr can emit light Patterns with an effective decay time of 2 msec.

REFERENCES Groner, R., Groner, M. T., Muller, P., Bischof, W. F. & Di Lollo, V. (1993). On the confounding effects of phosphor persistence in oscilloscopic displays. Vision Research, 33, 913-918. Westheimer, G. (1985). The oscilloscopic view: Retinal illuminance and contrast of point and line targets. Vision Research, 25, 1097-l 103.