International projectionist (Jan 1959-Dec 1960)

AUDIO VISUAL EDUCATIONAL • INDUSTRIAL • COMMERCIAL Ambient Illumination and Projection Light I^HE subject of projector lumen requirements is seemingly never exhausted. Occasionally we have discussed the light outputs of 16-mm projectors and gave a table showing the sizes of pictures to be obtained on the average for 10 foot-lamberts of screen brightness with bulbs and arcs of different powers. (10 foot-lamberts is the professional mini mum-brightness standard; 20 is common in theatres, and 5 is acceptable in 16-mm projection.) Because motion-picture theatres are reasonably dark during the projection of films, the optimum picture-brightness level can be established within narrow limits for professional projection. A different state of affairs prevails in classroom projection because the room may not be quite dark, and is sometimes deliberately illuminated dimly for purposes of discussion and notetaking. Projection-light requirements vary enormously when ambient light falls upon the screen. Minimum Allowable Brightness Projection in a totally darkened room is, of course, ideal from the point of view of the projectionist. The theatrical minimum brightness level of 10 foot-lamberts is regarded as extremely desirable in the dark-room projection of instructional slides and 16mm films, while 20 foot-lamberts may be accepted as a practicable maximum. As a concession to the relatively low illuminating power of most nontheatrical projectors, the minimum allowable picture brightness has been set at 5 foot-lamberts for 16-mm showings. Projection screens, even the flatwhite matte screens, vary in reflective power. A screen which reflected 90% of the light when new may reflect only 80%, or even less, when old or dusty. Unlike indoor-theatre screens, most classroom screens are unperforated, and accordingly have the advantage of an 8% higher reflectance for the same type of surface. Calculating Light Output On the basis of an unperforated matte-screen reflectivity of 87%, 5, 10, and 20 foot-lamberts of "blankfield" brightness correspond to 5.75, 11.5, and 23 foot-candles of incident projector light measured with a lightmeter. A lumen is a unit of light flux, so the number of lumens illuminating a screen is the same as the number of lumens issuing from the lens of a projector. To determine the lumens of light output required of a projector for a given brightness level (footlamberts), multiply the area of the screen in square feet by the desired number of foot-lamberts and divide the product by screen reflectance. L = Ab R As an example, for 10 foot-lamberts on a 4' x 3' screen of 0.87 reflectance we need 138 lumens of projection light. 12 x 10 0,87 138. This light-output requirement is satisfied by the average 500-watt 16mm projector having a 65% shutter transmission and an F:1.6 coated lens (approx. 150 lumens), or by a 750watt machine using an F:2.0 coated lens (also approx. 150 lumens). Manufacturers of both slide and mo tion-picture projectors arc usually will'ing to furnish lumen-output data for their machines with various bulb, lens, and shutter combinations. (The use of flickcrlcss 3-blade shutters reduces screen illumination about 25% from the level obtained with 2-blade shutters having the same angular width of blade.) When the level of ambient light falling upon the surface of the screen is high, projection light must be stronger to make the entire range of contrasts and colors of the film and slide pictures visible. As a general rule, the 5-foot-lambert level set forth as a minimum 16-mm picture-brightness level should be applied only to rooms which are almost totally dark. The preferred 10-foot-lambert level is suitable when the brightness of the ambient light on a matte screen does not exceed 0.1 foot-lambert. (Newspaper headlines set in 1-inch type can be made out only with difficulty under this level of ambient light.) Ambient Light Factor Now, to determine the best piujection screen brightness under other intensities of ambient illumination, multiply the foot-lamberts of spilllight brightness at the surface of the matte screen by 100. Thus for 0.5 footlambert of ambient light on the screen, projection-light brightness should be This 80-foot window-on-the-world is strictly an illusion, it is actually the world's largest color photo — 80 feet long and 20 feet high — calculated to bring the warm Pacific breezes from Hawaii's famed Diamond Head into the main ballroom of the Cincinnati Music Hall. Produced by Color Corp. of America (New York and Tampa, Florida) the giant photo-mural was mace from a color negative only 825/100,000 the size of the final print. The picture was so big it had to be mounted in an airplane hangar and required 2550 square feet of photographic paper. INTERNATIONAL PROJECTIONIST SEPTEMBER 1959 13