American cinematographer (Jan-Dec 1941)

high illumination, because they absorb more light than the glass-beaded or aluminum type. Approximately 16 footcandles of illumination are required for these screens. The edges of the projection aperture are seldom sharp, even when clean, and for this reason, a black border round the screen-edges to act as a mask is desirable. Most screens sold by photographic houses are made on the roller-blind principle and housed in a case or box which is attached to the screen. This method affords protection to the surface, but if the screen is home-made of the flat rigid type some means of protection must be found which will keep it covered when not in use, as light has the effect of causing a loss of brilliance and discoloration. Screen-Size The average screen-size of a motion picture should be of such dimensions as will create an illusion of reality for the audience. It is difficult to compile tables for amateurs which give the correct picture-size needed for most rooms, because the first consideration for the home projectionist is the shape of the room available and the arrangement of heavy furniture which cannot be moved every time a show is given. A rule used by architects when planning the screen-size for a picture theatre is that the screen height should be one-sixth of the distance from the screen to the projection-booth. Thus a room 18 feet long needs a screen about three feet high (and four feet long). This rule cannot always be followed, but is one which applies to most rooms where home movies are shown. The ultimate in home projection is a specially designed projection-booth with a separate room adjoining that can be furnished with a proscenium, comfortable armchairs and light-dimmers, but of course this is beyond the realization of most cine-smiths. Although a special screen-illumination meter (Weston No. 703) is available for measuring screen-brightness, any exposure-meter which is calibrated in units of foot-candles between 0 and 25, such as the "Master" Weston and The General Electric Meter, is suitable for the accurate evaluation of light being emitted from a cine projector. Incident light can be measured from directly in front of the projection-lens or from the screen. Reflected light can be measured from the screen. It has been suggested* that the illumination should be measured from the centre of the screen, each of its four corners and the centre of the top, bottom and both sides. A ratio of 80 per cent is considered very good, and is obtainable with some projectors. Cleanliness Essential Generally the quality of substandard projection by both amateurs and professionals is deplorable. 35mm. theatre projectionists use great precautions to keep their machines and film free from foreign matter and oil-splashes, but although the magnification in 16mm. is 4% times more than in 35mm. projectors, sub-standard operators pay hardly any attention to cleanliness. Leaving film uncovered for days and neglecting to wipe it before projection is a common fault with 16mm. workers, and the main reason why most scratches and "rain" marks appear on the film. A spot of dust is magnified 40,000 times on the screen, therefore cleanliness is essential. Aperture-plates in substandard projectors should be easily removable to permit cleaning, but unfortunately they are usually fixed and hard to clean. This is one reason why they are often neglected. They should be frequently brushed or wiped with a piece of soft fluffless linen or at times, when a spot is obstinate, a wooden match might be used — nothing harder, as the highly polished projection gate is easily scratched. If it does become permanently marked, it should be taken out and repolished with jeweler's rouge, or replated. If one spot of film-emulsion adheres to the front or back pressure-plates of the projector it will cause the film to become scratched. But this is not the only cause for the scratches on cine film which are magnified so greatly on projection. Other causes are dirty gate, rewinding the film too quickly, or trying to tighten the film on the reel after it has been rewound. If a high-class projector is kept scrupulously clean it is possible to project one film thousands of times, but because the atmosphere is filled with small particles of dust it is sometimes necessary to clean the film with preparations sold by trade houses for this purpose. If these are not available, chemically pure carbon tetrachloride should be applied with a piece of silk plush or pure linen. Alcohol or cleaners containing alcohol should be avoided, especially for color-films, as alcohol dissolves the dyes used to form the picture. Sound Projectors Although at the present time there are three film-sizes of substandard sound projectors available, viz. 17.5mm., 16mm. and 9.5mm., only the 16mm. sound (Continued on Page 149) *Journal S.M.P.E., Volume 31, Number 5, p. 483. * Left, phantom view of mechanism of typical 16mm. sound projector. Right, sound on 16mm. is 25 frames ahead of its accompanying picture. m * A American Cinematographer March, 1911 123