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By observing the screen with the eyes so shielded that nothing but the picture could be seen, the ceiling lights were alternately turned off and on, and the effect upon the quality of the picture observed. While it is possible to detect a slight veiling of the deeper shadows when the room illumination is being used, the effect is so slight as to be inappreciable in causing a degradation of the contrast of the picture.
Undoubtedly the veiling illumination falling upon the screen from the general illumination in the room is sufficient to cause an easily measurable decrease in actual contrast of the picture. Actual observation, however, shows that this decrease in contrast is not apparent to the observer. The explanation of why this decrease in contrast produces such a small apparent effect is of considerable interest, and is, in fact, of fundamental importance. The explanation lies in the fact that the contrast sensibility of the retina increases as the adaptation level rises. The presence of the general room illumination is responsible for a rise in the adaptation level of the observer, and the corresponding increase in contrast sensibility permits the perception of smaller brightness differences. If in the projected picture a constant contrast has been maintained, this increase in contrast sensibility would tend to make the picture look more contrasty, but since the presence of veiling glare tends to decrease the actual contrast, the two effects work in opposite directions and tend to compensate each other, the result being no perceptible change in apparent contrast.
On the whole this arrangement was considered very satisfactory by all those observing. One point of interest noted was that much less visual discomfort resulted when the screen brightness was suddenly changed by the appearance of the title region of the film, and further that a slight residual flicker due to lack of precise shutter adjustment was less noticeable. In fact much less general eye fatigue resulted when the room lights were on than when they were turned off.
In order then that a permanent record of the quantity and distribution of illumination used might be obtained, a series of brightness and illumination measurements were made. For this purpose the Macbeth illuminometer properly calibrated was used. The points at which measurements were made are indicated in Fig. 2. The stations A, B, C, D, and E, represent points on the table plane, that is, 30 inches from the floor, while the stations, 1, 2, 3, 4, 5, 6, and 7 are situated at various points on the ceiling as indicated by the arrows and represent the points of maximum brightness as seen by an observer situated at a point near station D. All values of brightness are expressed in milli-lamberts (m. 1.) and those of illumination in foot candles (f. c). In order that a complete analysis of the distribution of the light might be made, sets of measurements were made under four conditions of illuminations as follows :
I. Ceiling lights on. Arc not operating. II. Ceiling lights on and arc operating. No picture in the machine.
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