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EXHIBITORS DIGEST SECTION
Zhe ‘Drive-in . .
Picture Sizes and Light Sources For Wide-Screen and 3-D Projection
[ Drive-in screen images have always been huge compared to those of indoor projection, and screen brightness levels had to be accordingly much lower. Projection light sources have been developed for greater output; but widescreen presentation at drive-in calls for further increase in picture size. The following deals with the problem of lighting this yet larger picture on the basis of a brightness standard that accords with practical experience in the outdoor field.\
TO attack the problem as logically as possible, let us first examine the light source. It is possible to use certain carbon trims which will give tremendous light output and still operate within practical current rates. Data of the National Carbon Company provides the following example :
Positive 10mm x 20-in. “Hitex” with 7/16 x 9-in. “Orotip” negative, operated at 125 amperes, 64 volts; reflector //1.9 (16-inch); total light output 24,000 lumens; burning rates, positive 19*/2 inches per hour, negative 2.4 inches per hour.
It is thus judged possible, with this type of carbon trim and a 16-inch reflector, to obtain 24,000 lumens of light over entire screen area at 125 amperes. This assumes that a lens with a speed of at least // 1.9 is used.
Since we wish to utilize the greatest part of this light, the shutter transmission factor should be high. With that much arc output, means of cooling the film is necessary. Lamps of that calibre are or can be equipped with heat filter systems. An air blast system can cool the film without accompanying reduction of visible light at the aperture ; however, a filter system reduces the useful light by a small amount, according to authoritative rating of dichroic transmission, which would introduce a re
duction factor of around 12% in the calculations given below.
Ignoring that factor for the moment, and considering that the shutter transmission is 56%, then the light available at the screen carbon trim and optical system specified would be 24,000 times 56%, which equals 13,500 lumens.
SCREEN BRIGHTNESS
Although authoritative projection groups recommend a minimum screen brightness
of at least 10 foot-lamberts, drive-in theatres have never attempted to reach this level (the assumption being that they were getting all the light possible with the equipment available, and in many instances that was long the case — most drive-in theatres of 800-car capacity, for example, have had screens from 50 to 60 feet wide, and they apparently have tended to regard the brightness of the pictures as satisfactory).
Thus with the drive-in, we have established conditions of successful operation in which a maximum standard of screen
BETTER THEATRES Section, Guide Number of 1954
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