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426 FLICKER WITH MOVING PICTURES [Cn. XI
THEORY AND EXPERIMENTS ON FLICKER
§ 592. Experiments have been made to determine the speed at which flicker disappears, that is, the speed at which the eye is unable to distinguish between a continuous and an intermittent light.
These experiments show that at a certain speed the appearance of flicker disappears. This speed is practically the same for different people.
As the speed is increased the flicker disappears for the center of the field of vision before it does for the edge. Thus, the light on a screen may not appear to flicker when looked at directly but it may appear to flicker when looked at out of the "tail of the eye."
As the brightness of illumination is increased the appearance of flicker is increased and a higher speed is required for flicker to disappear. Thus, when showing a very dark film, the light may not appear to flicker at all, while with a very transparent film or no film at all the light may appear to flicker violently although the speed is the same.
If, instead of having the dark period and the light period equal, either the dark period or the light period is made less in proportion the flicker appears less violent, and it disappears entirely at a lower speed. This effect is, however, not very great.
Thus, the flicker with a shutter in which l/6 is light and % is dark, is the same as one in which % is light and Yt> is dark (§ SQ2a).
§ 592a. A formula to express these factors numerically was worked out by T. C. Porter of Eton College and published in the Proceedings of the Royal Society,, Vol. 63, p. 347; Vol. 70, p. 313-329 (1902).
The constants have been recalculated.
Let f = number of light flashes per second at which flicker disappears when light and dark flashes are equal.
Let n = number of flashes per second; light and dark flashes are unequal.
w = angle of white space in disc.
(360° — w) = angle of dark space in disc.
I = intensity of illumination in meter candles.
b = constant depending on illumination.
From experimental data the formula comes out
f = 26 -f 12.2 log I
b = 12.04 + 2-3?8 log I
n = f + b [logw — log (360° — w) — 4.5106].