British Kinematography (1951)

February, 1951 stableford: back projection 6] effect) . If these particles are impregnated with a brown staining agent, such as tobacco tar, their very capacity to reflect has been destroyed to the extent of t:ie colour to which they have been stained. If a translucent type of screen is stained to the same degree, its inherent capacity for transmission of light has not been destroyed at all. All that has happened is that a transparent filter, from the deposition of tobacco tar, forms on its surface. Effect of Increased Brightness If by increased brightness, visual acuity is stimulated, then the magnification factor can be reduced. This is aptly illustrated when one views an object under a reading lamp for closer examination; the acuity increases and as the viewing distance or magnification is left unchanged, the sight stimulus is increased. The inference is that the increased brightness and increased acuity which the back projection screen can provide could lead to a substantial reduction in picture size without loss of visual sensation. A smaller picture will improve the viewing conditions and sight line problems in almost all theatres, although it is not suggested that any wholesale reduction in size be contemplated. As an illustration of what must be the extreme in this, a special high reflection type of screen using a 20 ft. picture was being tested. When showing a richlv coloured scene under starlight conditions, a group of experienced theatre controllers and technicians were able to see an acceptable picture at 1,800 feet viewing distance. This is the unbelievable ratio of 90:1. Sound Reproduction There is one final aspect of back projection that must be mentioned, but on which no opinion will be passed, and that is sound reproduction. Obviously one cannot have a perforated back projection screen, and sound must therefore be disseminated from outside the screen periphery. The standard practice nowadays is to put the high frequency speakers above the centre of the screen and the low frequency speakers on either side, all three radiating through the wool masking. Theatre engineers who service such theatres have reported that they receive no complaints on sound and that, in their opinion, this arrangement is a workable one. The advantages of back-projection were the subject of a number of demonstrations. Two identical 16mm. projectors ran matched prints side by side, one with front and the other back projection, the latter showing a marked superiority in brightness and contrast ratio. On a 2ft. screen, a picture was shown to be of adequate entertainment value at a distance of 30ft. — a ratio of 15 : 1. Another demonstration showed that the use of a mirror in the beam caused no perceptible loss of definition. The small effect of ambient lighting was also shown. Brit. Kine. Soc. 2. No. REFERENCES 1, Jan., 1939, 1 / p. 24. 2. /. Brit. Kine. Soc, 10, No. 1, Jan. /Feb., 1947, p. 37 ; 10, No. 2, Mar. /Apr., 1947, p. 58. 3. /. Soc. Mot. Pic. & Tel. Eng., 55, No. 2, Aug., 1950, p. 131. 4. B.S. 1404:1947; Brit. Kine., 14, No. 6, June, 1949, p. 196 ; /. Soc. Mot. Pic. Eng., 50, No. 3, Mar., 1948, p. 260. 5. /. Soc. Mot. Pic. Eng., 51, No. 1, July, 1948, p. 1. 6. /. Brit. Inst. Radio Eng., 10, No. 6, June, 1950, p. 219. 7. Ideal Kinema, Dec., 1949, p. 19 ; /. Soc. Mot. Pic. Eng., 50, No. 4, Apr., 1948, p. 367. 8. Bull. Comm. Sup. Tech., I, No. 4, July, 1947, p. 158 ; /. Soc. Mot. Pic. Eng., 54, No. 3, Mar., 1950, p. 280. 9. Brit. Kine., 15, No. 5, Nov., 1949, p. 141. 10. Brit. Kine., 16, No. 6, June, 1950, p. 189. DISCUSSION Mr. R. Pulman : I believe there are considerable mechanical difficulties to be overcome in sound head construction for rear projection. The question of speaker placement is very important; in view of the possibility of stereophonic sound, mavbe the last place we want the sound is in the centre of the picture. The Author: I think a mirror is essential, when the film can be laced up the normal way. Mr. Willis-Culpitt : Rear projection screens at the moment have sometimes a centre-to-side ratio of 4 or 5 to 1 . The Author : The curve relating to the new screen is appreciably flatter. In the studio, we