The motion picture projectionist (Nov 1931-Jan 1933)

34 Motion Picture Projectionist November, 1931 and screen manufacturers have guided themselves accordingly. Screens of the perforated type in present use have a ratio of open to solid space of approximately 8 per cent; screens of the porous type have a rather larger ratio. Acoustic theory indicates a minimum of 5 per cent as desirable. Perforations generally are made after the screen is surfaced. Seams. — In assembling screens the seams should be placed vertically. Care must be taken not to stretch the screens too tightly. Butt joints are used with some metallic and beaded screens employing cellulose coated fabric as the base but are not generally used with others. Mechanics Size. — The distance between the front row of seats and the screen is one determining factor for the size. The larger the picture, the more plainly imperfections in the film, such as graininess, show up. This is very noticeable and objectionable in the nearer seats. Also, since the eye can satisfactorily accommodate itself to movement throughout a 60-degree angle, the distance between the front row and the screen should approximate 0.87 foot for each foot of screen width. For a 15-foot picture, a distance of at least 13 feet should therefore be provided. The size of picture should be determined by its distance from the rear seats. The width of the screen should be equal to approximately one-sixth the distance from the screen to the rear seats. For a distance of 120 feet, therefore, a 20-foot picture should be used, provided there are no seats nearer the screen than 17 feet and the projection angle is not very great. These rules are intended only as guides. The standardization of sizes is of primary importance to both manufacturer and exhibitor. Many errors are made in ordering screens because of confusion in description, resulting in considerable monetary loss. Sizes have already been standardized by several manufacturers and large users, but not always in the same way. The Committee is considering for recommendation a set of dimensions to be used as standards and sub-standards. Mounting. — Each manufacturer should determine the best method for mounting his own screens and advise purchasers accordingly. By taking proper care in mounting the screens, damage and cost of installation can be reduced considerably. A survey of instructions sent by manufacturers may lead to general rules. These may be drawn up into a revised instruction sheet for consideration by manufacturers. Masking. — The usual masking is black. This results in a very marked frame which reduces the effect of "jumping" of the picture caused by the film or projecting equipment. It has been felt that the resulting contrast is too great and various persons have advocated an intermediate gray. We are considering a suggestion that the mask be graded from black to lighter grays with the black edge next to the picture. Deterioration. — All screens deteriorate with age: "silver" screens tarnish, other types become yellow. Yellowing of the surface is accompanied by a reduction in reflection value and by an undesirable color tone which is imparted to the picture. Yellowing is usually caused by gums and binders and not by the pigments. We are informed that of late there has been marked improvement in this respect. In addition to discoloring screen, accumulated dirt also causes a deterioration of reflecting qualities. The amount of dirt collected depends on the condition of the air in the theatre, the precautions used to protect the screen, and the nature of its surface. It is essential that draperies surrounding the screen be cleaned regularly and that circulation of air through the openings of the screen be prevented. If possible, it should be enclosed when not in service, even though with the cheapest kind of material. The average useful life of a sound screen varies from one to two years, depending on the conditions of use. Cleaning. — Even with these safeguards the screen will gather dirt. An examination will indicate whether the dirt is dry or oily and, therefore, whether the screen may be brushed or not. If brushing is permissible, a long handled special brush should be obtained and the screen brushed once a week. It is also helpful to use a vacuum cleaner on the rear surface. More thorough cleaning should be done by experts who have sufficient scientific knowledge of screen materials to devise safe and suitable methods. Furthermore, certain types of screen cannot be cleaned satisfactorily at all. Each manufacturer should advise the users of his screens as to the possibilities. Reprocessing. — Renewing of the surface of diffusing screens by spraying is receiving considerable attention. When carefully done and when the proper materials are used, completely satisfactory results seem to be attainable. The spraying pigment should be highly reflecting, should not fill up perforations, and should not become yellow with age. The screen and surroundings should be thoroughly cleaned before the processing is undertaken. Such a renewal of the surface is not feasible on all types of screens. Light Reflection Total Reflection Factor. — There are several ways of defining the total reflection factor, based on the methods of test used in different laboratories. The laboratory which makes most of the commercial measurements of screen reflection characteristics employs a method in which the light is incident on the test sample from all directions within a cone of 180 degrees. The angle of observation is 12 degrees from the normal, and the light returned in this direction is taken to indicate the total reflection factor. The Committee advocates the adoption of this definition as standard. Angular Distribution. — One of the most important attributes of a screen is its ability to reflect the incident light to the observers. Angular distribution curves in the past have been obtained with light at normal in AN6LE OF OBSERVATION Distribution of Brightness in Horizontal Plane tor Beaded Screen Distribution of Brightness in Vertical Plane for Beaded Screen