Movie Makers (Jun-Dec 1928)

SCREEN Surface CHARACTERISTICS An Article to Aid Amateurs in Selecting or Making the Right Screen for Their Particular Purposes IN a recent article I told you in a general sort of way something about screens and screen surfaces. I shall now set forth certain characteristics of screen surfaces and other information having to do with screen illumination. As I said in the previous article, when light from the projector lens strikes the screen surface a certain percentage of it is reflected back and a certain percentage is absorbed by the surface and lost. The amount absorbed will depend upon the character of the surface and whether it be pure white or tinted. It must be remembered that the light-source of non-professional projectors is limited to a much lower value than those used in theatrical projection. Hence they provide only a part of the light available to theatre screens, particularly in view of the fact that the losses in the optical systems of the two types of projector are essentially the same. Of course, the amateur projectionist requires less light with 16mm. projectors, as the area of the film illuminated is only about one-sixth that of 35mm. film. Generally speaking, however, it is well to avoid anything which tends to a loss of light in non-professional projection. It must also be remembered that a certain, definitely limited, amount of light passes through the projector aperture to the surface of the screen. This amount is further reduced by approximately 50 per cent, by the projector rotating shutter. This light will be distributed by the lens, presumably evenly, over the whole surface of the screen. If you project a picture six feet wide, that picture, if undistorted, will have an area of By F. H. Richardson about twenty-seven square feet. If you project a picture four feet wide, it will have a total area of about twelve square feet. You thus see that by increasing the picture width from four feet to six feet you have more than doubled its area. Your available light, therefore, must be spread over more than double the surface, with consequent greatly diminished brightness of the picture. From this you learn that picture size has much to do with the brightness of screen illumination when the amount of available light is a fixed quantity. The projectors designed for home use have light-sources of fixed quantities within certain limitations. When used in the average room in the average home these limitations are not exceeded. It is when a larger picture for a special purpose is desired that the light-source provided is not sufficient; therefore the screen surface must be looked to for the additional brilliancy required. If you want a bright picture you must select a white surface of good reflection power and renew that surface with reasonable frequency. Every ninety days is not too frequently if it be constantly exposed to the atmosphere. White paint, white kalsomine, white cardboard and white plaster are all good surfaces; or you may choose one of the many patent screen surfaces. Now another important equation enters: the direction in which the surface reflects the major portion of the light. Some surfaces are very bright to those immediately in front of them, but at the same time very dim to those at one side. This is known to professional projectionists as "fade away." It is due to the fact that different surfaces reflect light at different angles, as shown by the accompanying drawings. A perfectly polished reflection surface reflects all incident light directly back. Hence in a mirror you see perfect images. A perfect diffusing surface reflects incident light in all directions, or angles. A surface which is not sufficiently polished to produce an image may still reflect a very great proportion of the light more or less directly back. In figure A we see the action of the mirror, in which all light from point X will be reflected straight back to point X. In figure B we see a surface which reflects most of the light from X more or less directly back, but not all of it directly back; hence no reflection is formed. To one seated directly in front of it the surface will be much brighter than to one seated far to one side. In figure C we see an approximately perfect diffusion surface, such as non-gloss paint, kalsomine, dull-surface paper, or white-finish plaster. Such a surface will appear much less bright to one seated directly in front of it than would surface B, but it will appear almost equally bright to persons seated directly in front, or far to one side; whereas surface B would have "fade away" and appear very dull to the one seated at a wide angle to its surface, but very bright to one seated directly in front of it. Taking the reflection power of a scraped block of magnesium carbonate (which is a perfectly smooth, textureless surface) as 100 (its reflec (Continued on page 420) FIG> A FlCr £ TIGC 400