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Avoidance of Stray Light
Light sources at the proscenium surround should not be directly visible if at all possible, for indirect illumination is much to be preferred. Some architects and engineers seem to be under the erroneous impression that as long as light sources are directly shielded from the audience, stray light can be allowed to reach the screen without any detrimental effect to picture presentation, and through this belief many presentations have been spoilt as far as picture contrast is concerned. If a kinema is lit as it is normally when picture presentation is in progress and observation is made from the centre of the screen, it will often be found that several relatively bright light sources are directly visible. Steps should be taken to ensure that directional baffles are provided to mask any such stray light from falling upon the screen.
6. THE SCREEN
Having obtained a certain quantity of light, covering a certain area at the position of the screen, the proper distribution of this light through reflection and refraction has to be considered. There are several types of screen material available, all having different distribution curves and reflection factors, which can be suited to certain shaped auditoria, and a very brief summary of these would not be out of place.
The glass-beaded screen, by refracting the reflected ray, can increase the
Fig. 8. Effect of Proscenium in dwarfing Picture.
apparent brightness by several hundred per cent. Unfortunately, this greatly increased brightness is only available over an arc of about 25 degrees, with a very high central node. Outside this arc, the apparent brightness of the picture falls off considerably. One further aggravation is that the glass screen is a high refractor. In other words, it throws the picture back into the projection room and starves the patrons in the stalls.
The silver screen has approximately the same angle of response as a glassbeaded screen, but in this case acts as a true reflector and throws the picture, with great brilliance, into the front of the stalls, and, governed by the projection rake, starves the patrons in the circle.
Under conditions found in a modern kinema, where at least 90 degrees horizontal arc of response from the screen is required, it becomes clear why the matt white diffusing screen, which spreads the picture it receives everywhere, is, up to now. the one mainly used.
Improving Screen Efficiency
The disadvantage of the flat white screen is that it throws the picture out over a complete hemisphere. Since the audience is disposed within an arc of about 00 degrees in the horizontal axis and about 30 degrees in the vertical