Motion Picture News (Sept-Oct 1918)

2230 Motion Picture News Projection^ Questions Answered/ The Characteristics and Efficiency of Projection Screens ANY effort to increase the efficiency of the motion picture projector from the viewpoint of light, or energy, conservation must also take into consideration the character of the screen upon which the pictures are projected, and likewise its efficiency as a reflector for returning the projected light to the eyes of the spectator. It seems logical, therefore, to follow up our recent articles on the efficiency of the projector optical system with some particulars as to the efficiency of projection surfaces, and we are fortunate in being able to present some intimate and accurate data on this subject, as the result of investigations made at the Research Laboratory of the Eastman Kodak Company.1 The information contained in the article cited deserves to be widely known amongst operators, exhibitors and manufacturers of projection appliances, and is freely used in the preparation of the following paragraphs, which are not, however, in all particulars a literal reprint of the original communication. Projection screens vary from mat white walls and cloth screens to special surfaces and coatings intended to concentrate the light within a narrow angular field relative to the normal to the screen surface. When there is an abundance of light there is no question but that a highly diffusing surface, such as a white plaster wall, gives a minimum of eye discomfort to the spectator. However, the question of obtaining sufficient screen illumination must be considered, on account of the expense and difficulty of operating intense light sources, such as high-power arcs, and on account of the liability of injury to the projected material when excessively high illumination is used. AO 1 A A LU & ST e f £ it c is ho L 1 — i % ■8 ft H FIG. 1 Accordingly, numerous attempts have been made to increase the efficiency of projection screens by increasing their reflecting power, and by so designing their surfaces that the light which they reflect is nearlj all concentrated within an angle of 30 degrees from the normal to the screen surface. Some of these specially prepared screens, however, produce considerable eye discomfort, hence the problem of selecting the best possible screen for different sizes and shapes of auditorium, or theatre, is very complex. To insure a comprehensive grasp of the problem the. reflection analyses of various types of screens and surfaces now in use will be discussed, then the properties of the ideal screen will be con sidered, and lastly the maximum efficiency which can be realized in a projection surface. In the subsequent data it is assumed, unless otherwise specified, that about 20 per cent, of the light incident upon the screen is absorbed and therefore lost. Table I gives the percentage of the incident light reflected by various surfaces, which have been used or experimented with as projection screens, at various angular distances from the normal to the screen surface. In obtaining the data contained in Table I the various surfaces were illuminated at perpendicular incidence by a disc source subtending a solid angle of .01 steradian, and the reflection values obtained are reduced to percentages of that for a perfectly diffusing surface having 100 per cent reflecting power under the same illumination. (Note — The steradian is the unit of solid angular measurement. It is derived from the radian, a unit of ayigular measurement. Angular measurement in radians is represented by the length of the arc for a given angle divided by the radius of the corresponding circle, and the radian, or unit in this method of measurement, is an arc equal in length to the radius of the circle of which it is a part. Now it is seen that the steradian, mentioned above, is the solid angle subtended at the center of a sphere by an area on the surface of the sphere equal to the square of the radius, or, in other words, a steradian is the solid angle subtended by unit surface on a sphere of unit radius.) TABLE 1 Screen 1. Magnesium carbonate block 2. Magnesium oxide 3. Mat photographic paper 4. White blotter 5. Ground opal glass 6 7 s 9. Mat varnish on foil 10. Mirroroid A (.Aluminum). 11. Mirroroid B (Aluminum) . 1 2 . Becker compound 13. Mirror with ground face. . . Angles of Observation 0° 0.875 0.80 0.78 0 76 0.69 0.83 1.01 1.18 1.93 4.9 0.875 0.80 0.78 0.76 0.69 0.715 0.85 1 04 1.80 4.55 0.875 0.80 0.78 0.76 0.69 0.615 0.66 0.87 1.70 3.86 15° 0.875 0.80 0.78 0.76 0.69 0.49 0.52 0.71 1.53 3.03 30° 0 832 0.77 0.78 0.73 0.68 0.28 0.25 0.28 0.83 0.78 1 Projection and Focusing Screens — by P. G. Nutting Communication No. 26 from the Research Laboratory of E. K. Co. Trans. I. E. S., Vol. 11 (1916), p. 92. The numerical data of Table I is shown plotted in the form of curves in Figs. 1 and 2, where the ordinates represent percentage reflection and the abscissae the angular distances from the normal to the screen surface. Fig 1 includes the flatter curves of the surfaces which give practically even reflection over a wide angular field, while Fig 2 shows the curves for those surfaces which reflect most highly within an angle of about 30 degrees. Fig. 2 is plotted to half the scale of Fig. 1, and.it will further be noted that the curve for No. 13 (mirror with ground face) is plotted in two sections. It is obvious that section b must be added to section a in order to graphically represent the percentage reflection at, and in the vicinity of, the normal. A large percentage of the projection screens in use are far below maximum efficiency and a slight accumulation of dust will lower the efficiency 50 per cent. Many roller screens of painted cloth of the non-metallic type reflect less than 40 per cent, of the incident, light when new, whereas a reflection of at least 70 per cent is easily attainable in this type of screen. Efficiency of Projection Screen* If we take as the ideal screen a screen which reflects all the light projected upon it, uniformly within a given angle from the normal (say 30 degrees), then a rational measure of diffusion efficiency in any screen is the ratio of the light reflected within this angle to the total light reflected. The light reflected within a given angle is obtained by multiplying the numerical values of