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408 RICHARDSON'S BLUEBOOK OF PROJECTION
Here again the accurate alignment of the light source and condenser with the aperture and projection lens is imperative. (1K4) The figures following show the extent to which screen illumination is reduced when the alignment of the light source is inaccurate. One hundred per cent projection values are to be obtained only when the filament is centered on the optical axis. Here is an indication of what happens when the filament is off center.
Filament out of center up or down 1/16 inch, light 96.0% ; 1/8 inch, light 85.3% ; 3/16 inch, light 66.6%
Filament out of center sidewise 1/16 inch, light 92.8% ; 1/8 inch, light 76.2% ; 3/16 inch, light 57.7%
In the above tabulation we see that if all other optical train elements are in correct alignment, but the lamp filament is, for example, 1/8 inch too high or too low, almost IS per cent of the light is lost; if it is 3/16 inch too far to one side, almost 43 per cent of the light is wasted, which emphasizes the importance of centering the lamp filament exactly on the axis of the optical train.
(185) The majority of the high wattage projection lamps show a certain amount of coil distortion (sagging) towards the end of life. If allowed to continue it may result in a short circuit in the coils. Filament sag cuts down the illumination on the screen or it may cause the lamp to burn out immediately. It is best to replace such lamps to avoid trouble during a performance.
(186) Throwing the full operating load directly on these high wattage, heavy current projection lamp filaments often causes some distortion of the filament. (187) To offset this controls are available that permit the lamps to be burned at reduced current until the filament warms up. In 10 to 15 seconds the various parts of the filament warm up sufficiently so that it is safe to give it the full current load.
Properties of the Component Elements
Since the standard professional projector aperture and projection lens present openings of considerable size, a