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mean distribution of light, and this is the reading used when considering standards of efficiency for the projectionist to maintain.
You must expect, however, to find the mean distribution-figure lower than the centre reading, especially if there happens to be a bright spot in the centre of the screen or considerable fallingoff in brightness at the edges.
A standard of 8 foot-candles has been decided on as the average mat-screen reading for 16-mm. professional exhibiting.
So far we have considered only the light falling upon a surface and the subsequent measurement of that surface in foot-candles. Such measurements are not much help to the amateur, of course, for usually he tries to get as bright a picture as possible with lamps of low power.
Under the exacting conditions of professional exhibiting, however, the photometer is an invaluable aid in indicating, besides screen brightness, the possible falling-off in efficiency of a screen's surface, which occurs over such a lengthy period of time that the deterioration may develop unnoticed by the ever-present operator.
Every screen reflects and distributes light over an area the size of which depends on the type and texture of its surface. This light, which is light reflected from the screen into the eyes of the audience, can also be measured by means of a photometer — this time marked in foot-lamberts.
The meter is held facing the screen when standing in the middle of the seating area for a centre-reading, and afterwards held (always with it pointing at the screen) in various parts of the seating-area for readings necessary to arrive at an average figure. And now we come to an important difference between measuring screen-brightness and measuring screen reflectivity.
When we hold a meter against the screen we are measuring the light received from the lamp and we use such measurements to ensure that sufficient even light is reaching the screen. But when we direct the meter at the screen, we are measuring the light received by the audience, and an average reading in this case is of little practival value. Instead, a standard reading should be decided upon for that section of the seating-area which has the (normally) lowest reading, and subsequent readings for that section should never be allowed to fall far below that
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