Motion Picture Herald (Oct-Dec 1951)

Lomb, Western Electric, General Electric, and university optical institutes, etc., were requested to report on the various functions which had any bearing upon the necessary level of screen illumination in the motion picture theatre. The reports made in 1936 dealt with and set forth all the then existing knowledge concerning the human psychological and physiological functions of human vision. Experiments were conducted under laboratory conditions to determine the best level of screen illumination. As a result of that work, the committee was inclined to accept the conclusions that the brightness level should be something of the order of 30 footlamberts at the center of the screen. They reasoned that if such brightness were attainable, logical brightness limits would be 20 foot-lamberts for a minimum, and 45 foot-lamberts for a maximum. PRACTICAL STANDARDS Using a similar light source for a screen 30 feet wide, the center screen brightness could approach only 7 foot-lamberts. This was the best illumination attainable, at the time, for screens of the so-called deluxe houses, and it was felt that a minimum brightness level could not be set above this value so as not to penalize the majority of large theatres. TABLE OF VIEWING VALUES: Ability to see changes in picture contrast compared with changes in screen brightness. A B c D E • • • • • Screen Ability for Ability for Ability for Brightness in Highlight Shadow Ability tor Average Foot-Lamberts Brightness Brightness Focal Area Brightness 7 0.97 0.33 0.75 0.69 10 0.99 0.37 0.80 0.74 14 1.00 0.41 0.87 0.80 18 1.00 0.44 0.92 0.85 22 1.00 0.46 0.94 0.88 26 1.00 0.48 0.96 0.92 30 1.00 0.50 0.98 0.94 The committee then proceeded to set a minimum value of screen brightness, and a maximum value of screen brightness, so that motion picture prints could be processed at the studios for a definite mean value of screen brightness and would not suffer too greatly in picture contrast ratios when exhibited in different theatres. The top and bottom levels had to be set at values such that the quality of a release print adjusted for projection at the mean level would not suffer if projected at either of the extremes. The committee then turned to the best projection equipment available at that time, which they considered the 13.6mm high-intensity carbon operating at 125 amperes, with an optical system of f/2.4 speed. These systems were capable of delivering a maximum value of 5000 lumens to the screen. Assuming a screen reflectivity of 75%, this means that for a screen 25 feet wide, and a side-to-center light distribution of 80%, the center screen brightness could approach only 10 footlamberts. We must admit that this reasoning is quite logical if we consider that the apparent picture contrast can be materially altered by changes in picture brightness. Laboratory practice in making release prints, limits the quality variation {gamma) For STEADY PROFITS you need TODAY’S FINEST SCREEN IMAGE Replace NOW with B&L Super Cinephor Lenses . . . for Steady Patronage Keep your patrons coming back! Don’t let dim, eyestraining screen images cut profits. Show today’s sharpest, brightest pictures . . . edge-to-edge brilliance and contrast . . . with B&L Super Cinephor lenses. for complete information to Bausch & Lomb Optical Co., 679-11 St. Paul Street, Rochester 2, N. Y. 22 MOTION PICTURE HERALD. NOVEMBER 10. 1951