LANTERN SCREENS 800. General Notes. Types and Efficiency of Screens. Pictures may be projected on an opaque screen, the lantern being on the same side of the screen as the spectators (projection by reflection), or on a translucent screen, the lantern being on one side of the screen and the spectators on the other (projection by parence). The choice between these two methods usually turns on the nature of the premises. Where the slides are shown to assist a lecture, it is usually preferable for the lanternist to see the lecturer because he can then foresee his requirements, collaboration between the two being thereby better assured. Also, for an image projected by transparence to be satis factorily seen it is necessary that the spectators should view it from a position not far from the projection axis, so that their number is smaller than when an opaque diffusing screen is used.
For a long time the only screens used were opaque ones with a matt surface. Since 1910 screens with semi-diffusing and semi-reflecting surfaces (metallized screens : J. Anderton, 1891 A. and L. Lumiere, 1901, etc.) have also been used, a very much greater luminous efficiency being sometimes claimed for them. A sheet newly coated with magnesia throws back about 90 per cent of the light it receives, and a much higher efficiency is inconceivable. The essential difference between the matt screen and the metallized screen lies in the manner in which the light is distributed by the screen. A matt screen throws it back almost uniformly in all directions a spectator viewing the screen at an angle almost grazing its surface sees it nearly as brightly lit as a spectator occupying a position opposite the centre of the picture. With a metallized screen, on the other hand, the major part of the light is sent back according to the laws of specular reflection ; a spectator opposite the screen sees it very brightly, but as soon as a position away from the perpendicular to the screen-surface is taken, the brightness decreases, at first slowly and then with increasing rapidity, especially on that side of the screen which is farthest from the viewer. If a sheet of good
quality white blotting-paper be placed against the surface of a metallized screen by an assistant, the metallized surface will appear much brighter than the paper when one faces the screen, but the screen soon appears less bright than the latter as one moves to the side.
A metallized screen and a matt one are there fore not interchangeable. It cannot be said that one is brighter than the other. The former is better for use in a long, narrow hall of which the screen takes up nearly the whole width, whilst a matt-surfaced screen is better in a very wide hall in which the spectators in the front rows see the screen under considerable obliquity.
Between the above two extreme cases there are various intermediate types, usable within an angle of about 30 degrees on either side of the normal ; screens with a rough metallic surface, metallized screens covered with a layer of small glass beads in contact, etc.
The screens in common use are of flat surface. Under the pretext of suggesting a sensation of relief or of rendering the illumination more uniform, screens of curved surface have fre quently been advocated. The fact that the perception of relief in viewing an image projected on to a curved screen depends exclusively on auto-suggestion has been established by the very people who have devised such screens, for not only have they made claims for curves of the most diverse kinds, hut the number of those advocating a concave surface is about equal to that of the advocates of a convex one. As regards more uniform illumination, it is no doubt correct that a screen formed of the concave portion of a cylinder with a vertical axis would be advantageous if the projection were made before a single spectator placed, as the lens of the lantern, on the axis of curvature of the cylinder, but this advantage disappears in the case of a spectator removed to any extent from this ideal position.