Projection engineering (Jan 1932-Mar 1933)

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Page 8 PROJECTION ENGINEERING up as standard by one equipment manufacturer for theatres using their sound system. In some quarters this is thought lo be rather severe, but fortunately a well projected picture is possible with a screen meeting these requirements. Another company has allowed a slightly greater tolerance. Sound screen surfaces vary considerably ; some are hard, some sticky, some rough and some smooth. The perforations increase dirt collecting characteristics, not only from their physical presence, but due to the fact that air circulates through the openings. Metallic screens not only collect dirt and dust, but become tarnished, which tarnish lowers their light reflecting qualities considerably. The actual amount of dirt which collects upon any given screen depends upon characteristics of the theatre, and the care which is taken to protect the screen. Certain common sense precautions should be taken which will decrease the importance of this item of theatre maintenance. Curtains, draperies and travelers accumulate dirt which gets on to the screen. These articles should be kept scrupulously clean. Drafts should be prevented which would carry dirt and dust to the screen surface, and when not in use the screen should be covered in order that the dirt stirred up by the house cleaning will not settle upon the screen surface. The maintenance of screens resolves itself into four definite divisions — prevention of dirt upon the surface, freeing the surface of dirt which does collect there, thorough and complete periodic cleaning, and a periodic renewal of the screen surface. Fig. 3. Projector type television screen. Even with the best of precaution, it will be impossible to maintain an absolutely clean screen surface. Inspection will show whether what dirt has collected upon the screen is dry or greasy. Dry dirt obviously can be brushed off with a long-handle brush, and it is good practice to clean the rear screen surface weekly or semi-monthly with a vacuum cleaner. There is no entirely satisfactory way to clean greasy accumulations from screen surfaces. If such cleaning becomes necessary, clean soapy water applied by sponge probably does the least harm to the surface. Renewal of screen surface by spraying has come in for considerable attention of late, and when carefully done with good material a fairly satisfactory job may be obtained. The material used should be highly reflective. Installation problems connected with the motion picture screen are magnified somewhat by the lack of authentic data available on the subject. To date there is little information available upon the subject of keystoning, side distortion, optimum projection angle for various size theatres, and the relations between these various factors as concerned with theatres of any given characteristics. The screen itself, as a particular subject for engineering investigation, has to date not received consideration in keeping with its importance to the finished motion picture. There have been investigations made by various agencies, but in so far as a general investigation of the whole subject is concerned, taking advantage of the various items of knowledge available, I know of none. Installation problems connected with the screen increase in proportion to the size of the theatre. Certain fundamental considerations enter into the final solution. In the first place, the human eye is unable to accommodate itself to an angle over 45 or SO degrees, hence the distance of the front row of seats from the screen should be at least fifteen inches for each foot of screen width. The size of the picture is also determined by the distance from the rear seats, and for this consideration a screen in width approximately one-sixth of the distance from the screen to the last row of seats should be provided. As screen size increases, imperfections in the projected picture become more apparent. Graininess in the film shows up on a large screen, and small faults which might otherwise be unnoticeable in the print become obvious. On account of these considerations, a screen greater than 18 by 24 feet should never be used. Larger Picture Area In this connection, the recent craze for large pictures might receive some small attention here. A large picture was practically obtained by merely Fig. 2. Showing method of screen mounting. changing the physical set-up of projector and screen assembly, resulting in a projected image which was very unsatisfactory from a technical standpoint. The tendency towards large pictures has been somewhat discarded, and exhibitors are returning to a picture size which allows them to obtain some measure of technical perfection upon the screen. In a consideration of the actual problem of installation of the screen, it might be well to point out that illusion is best maintained if the screen be placed as near to the stage floor as possible. The floor of the stage in front of the screen should be painted with a non-reflecting, dull dark paint. A black frame around the screen decreases the jumping effect due to poor film or poor projection equipment, and should always be installed. In certain cases, improved projection will result from a tilting of the screen, due to a decrease in the keystone effect or to a re-distribution of the light throughout the auditorium. Tilting in general introduces complications of projection which are not present when the screen is upright. A tilted screen collects dirt much more readily than an upright. In conclusion, a word upon the relative importance of the screen to the finished motion picture might not be amiss. Large amounts of money are spent upon the production of a motion picture, as well as considerable effort on the part of a great number of people. Much effort in the theatre may be bent towards the perfection of projection. All of this will be wasted if the screen does not measure up to the standards which have been set in the other branches. There may be possible important advances in screen design which would improve the reproduction of projected pictures. The subject is one worthy of the attention of cinema engineers who are ever on the alert for points of attack in forwarding the art of the talking picture. In short, the screen is the final link in the motion picture chain from studio to patron, and as such deserves more attention than has been given it in the past.