Motion Picture News (Apr - Jun 1927)

1286 Motion Picture N e zv s W .Oi Project! or) fesJkctrleity.Prddical Meas Inquiries and Comments Effect of Aperture Size on Screen Illumination PART IV X this section of the discussion between Arthur Gray of Boston and the Editor, concerning /the possible effect Q2<^ which an increase in the size of the aperture in a motion picture projector would have upon screen intensity, we will give the results of some simple photometric tests which were conducted to check the statements made in the first part of our answer to Gray's arguments. (See preceding issue.) As will be recalled from a reading of the projection department in last week's issue of the News we claimed that the reason Gray observed an appreciable increase in screen intensity Avhen he opened the iris shutter (in front of the condenser lenses on his Brenkert Effect Projector, on which he conducted the test), and then increased the focal length of the projector lens to keep the size of the projected "spot" the same as it was in the other, shorter, focal length projection lens system, with which the comparison was made, was because the effective, or working, diameter of the projection lens was increased, and hence, the screen intensity rose in direct proportion to the increase in effective area of the lens. Used "Over-sized" Lens Gray, in effect, used an "over-sized" projection lens, which, in both cases, was far larger than that required to pick up the entire light beam and consequently, while no change in the physical size of the projection lens was made, nevertheless a real change in effective size resulted, which was responsible for the increased screen intensity he observed. The size of the aperture (iris opening) had nothing whatever to do with this increased screen intensity, and if he had used two lenses of, say, one-inch diameter in each case he would not have observed any change in screen intensity when switching from one system to the other. 'The fact that Gray conducted his test with a stereopticon optic system was responsible for the natural error he made because it is seldom that the projection lens of such a system, when used with an arc source of light, is completely filled — or reaches the saturation point — with light. The lens is always larger than required, except when such diffusing objects as design slides, etc., are being projected. Tf he had used a motion picture projec tion system, he would not have made this error, because it is seldom — except on very short focal length lenses — that the projection lens is not completely filled — or saturated— with light, so that with such a system, any widening of the angle of the light beam passing through the aperture will not result in increased screen intensity unless the physical diameter of the projection lens is also increased to pick up this extra light. Results of Photometric Tests In the photometric tests conducted, the results of which will now be given, a Macbeth illuminometer, which is generally considered to be the standard portable instrument for accurate results, was used. No effort was made to achieve extreme accuracy, as it was felt that the nature of the test did not warrant such a procedure since the differences in screen intensity, if they existed at all, would be of a large order. The results of these tests, therefore, are accurate to within 10 per cent, which is satisfactory for our purposes. Actually the percentage error is less than this, but the limit is 10 per cent. The first test consisted of duplicating Gray's original test wherein a stereopticon system, as found in the Brenkert effect projector, with an iris shutter in front of the converging condenser lens, was used in combination with a short focal length projection lens of 3 inches diameter (the standard with this particular machine). For the purpose of obtaining a steady source of light, a 1,000-watt incandescent lamp was used in each of the two systems under comparison. This iris shutter, of the short focal length system, was opened to the point where a "spot," 3 feet 6 inches in diameter was projected onto the screen at a distance of 27 feet. The intensity of illumination on 'the screen was then measured and recorded. The second "stereo" system was exactly the same as the first with the exception that a lonff focal length projection lens was used and the iris shutter opened to the point where a 3 "spot," 3 feet 6 inches in diameter was again formed on the screen at the same distance of 27 feet. The intensity of illumination with this system was also measured and recorded in the same manner as the first. We then had two separate stereopticon systems, the same in all respects, except that one used a short focal length projection lens and a small iris opening, whereas the other used a long focal length projection lens with a correspondingly larger iris opening in order to give the same size of projected "spot" at the same throw. Here are the results of the above test. Illumination Test No. 1 System No. 1 Light source 1,000 Watt, 120 Volt, incandescent lamp Projection lens f ...11 inches Projection distance.. .27 feet Iris shutter opening. .1 11/16 inches Size of projected spot. 3 feet 6 inches Intensity of screen illumination 12.8 foot-candles System No. 2 Light source 1,000 Watt, 120 Volt, incandescent lamp Projection lens f ...17 inches Projection distance. . . 27 feet Iris shutter opening. . 2^/2 inches Size of projected spot. 3 feet 6 inches Intensity of screen illumination 26.5 foot-candles Immediately after making this test and recording the results, the editor started to look around for the reason for the difference in screen intensity. He had not far to look. One glance at the projection lenses told the story. The size of the light source image as projected on the face of the 3-inch diameter projection lens in system No. 1 measured 1 X 1% inches. The effective, or working, area of this lens was, therefore : Area=l x 1^/4 ^1/25 sq. inches The rest of the lens area of this 3-inch lens could be tossed out of the window for all the good it did in providing light to the screen. The size of the light source image, as projected on the face of the 3-inch diameter projection lens in system No. 2, measured 1% ^ 1% inches. The effective area of this lens was, therefore, Area=iy2 x 13/i =2/625 sq. inches Illuminated Increase Due to Larger Effective Lens Area Now let us compare the two screen intensities, as measured with the photometer with each other. We find that system No. 2 gives a little more than twice the intensity as does system No. 1, as follows : Ratio= 26.5 12.8' =2.07 Now let us compare the effective lens area.s of both systems in order to find out whether this difference in screen intensity can be accounted for in this manner. We Iwive, as follows : System No. 1, lens area=1.25 sq. inches. System No. 2, lens area=2.625 sq. inches. Ratio= 2.62.') 1.25 =2.10 (Coiitinifcd on folloiclnrj page)