International projectionist (July-Dec 1934)

December 1934 INTERNATIONAL PROJECTIONIST 11 Projection Practice Committee H. Rubin, Chairman J. 0. Baker T. C. Barrows G. C. Edwards J. K. Elderkin J. J. Finn E. R. Geib S. Glauber C. L. Greene H, J. J. Hopkins W. C. Kunzmann R. H. McCullough R. MlEHLING P. A. McGuire M. D. O'Brien F. H. Richardson V. A. Welman Griffin mirror. The Committee recommends the use of these guards. (10) The comparative advantages of the new d-c. light sources using coppercoated Suprex carbons may be judged on the basis of two factors: (a) quality and quantity of projected light, and (b) cost of operation, the latter being of primary importance to the smaller theatres. The following resume of operating costs per hour is based on the prevailing prices for carbons in standard shipping-case quantities, and an average current cost of 5 cents per kwb.: Carbon Cost per Hour (6 and 7-mm. trim, allowing for stubs) Amperes Cents 10.4 40 45 50 14.4 18.5 Arc Current Cost per Hour (5 cents per kwh.) Amperes Cents 40 6.0 45 72 50 8.75 Supply Line Current Cost (Allowing for losses) D-C. Line and 80-v. Average Amperes Generator Generator Rectifier (cents) (cents) (cents) 40 20 15 9 45 24 17.5 11 50 29 22 13.5 Low-Intensity Costs, Cents per Hour, at 30 Amperes Carbon 4.9 Current on line side of motor-generator, 80-volt tvpe at 5 cents per kwh. 19.7 Cost with rectifier 11.5 High-Lou Costs, Per Hour Carbons 17.2 Current from line side of motor generator, at five cents per kwh. 47 From the standpoint of quality and quantity of light, there is no reasonable basis for comparison between these new arcs using the Suprex carbons and lowintensity arcs, as there is a pronounced favorable contrast in those respects in favor of the former. The low-intensity carbon arc delivers an intense white light which is very pleasing to the eye. A comparison of the Suprex carbon arc with the high-low arc at a current of from 50 to 60 amperes, showed that the Suprex carbon arc provides a light of equal intensity but with a more even field — and, of course, at a much lower operating cost. Suprex Arc Endorsed The Committee regards the d-c. Suprex carbon arc as one of the most important developments in the projection field within recent years. It fulfills the demand for improved screen illumination — both as to quantity and quality of light — in a manner that leaves no room for question. It enables the smaller theatres to offer for the first time a quality of screen illumination comparable with that found heretofore only in the largest and finest theatres. In addition, the Committee's test proved the arc to be economical in operation. Not only will the screen illumination benefit through use of this new type arc but the general illumination of the theatre can be improved, certainly a very desirable advance. Colored motion pictures, the number of which is progressively increasing, demand a light-source of high-intensity and good quality, a requirement that is fulfilled by this new arc. The Projection Practice Committee recommends the use of the Suprex d-c. arc. The Committee extends its thanks to the manufacturers who cooperated by supplying the equipment necessary for conducting the tests. The Committee is particularly indebted to the International Projector Corporation, which not only provided quarters in which to conduct the tests, covering a period of two weeks, but also contributed generously of its personnel, equipment, supplies, and electric power. Discussion : Mr. Richardson : I believe we should have included in the report the new a-c. light source, which is very economical and furnishes a very excellent projection light. The statement was made that a glass guard causes negligible light loss. I understand that a loss of at least 4 per cent occurs for each polished surface of glass through which the light passes. You stated that the negative consumption curve rises gradually to a point after which, comparing it with the curve for the positive carbon, the negative burns rather steadily, indicating that the arc will remain at the focus of the mirror. But when the positive burns faster or slower than the negative, the light source will be out of focus constantly. In this particular case it would enlarge the spot. Mr. Rubin : As was stated in the report, separate feed screws or adjustments for both negative and positive can adjust that. Actually this is a test curve. In the theatre, the arc would be set for 50 amperes, and the adjustment would not be changed. Mr. Brenkert : As the current increases, the rate of consumption of the positive carbon increases faster than that of the negative. The procedure to follow as the current is increased is to step up the speed of the motor that feeds both carbons, then slow down the negative feed by a separate adjustment. On the arcs put out years ago that could not be done, but it can be done today on most arc lamps. Mirror Guards Efficient Mr. Sachtleben : The light reflected by the light-guard will be 4 per cent at each surface, but because the surfaces of the light-guard are concentric with the surface of the mirror, the light reflected from these surfaces, except for second-order reflections and a very slight absorption in the glass itself, will be added to the light from the mirror. This loss of light will be very small, as was found by the investigations of the Committee. Mr. Brenkert: The best optical glass has a reflection loss at each surface, as Mr. Richardson stated, of 4 per cent. The guard provides two surfaces through which the light must pass on its way to the mirror, and on the return the light must pass through those surfaces again. Four such surfaces must be considered. Mr. Rubin : In all tests that we made, with any instrument, the loss could not be detected. It can not be detected with the naked eye. That is why we could not report on it. The effect of using the guard (Continued on page 23) | 70 ±60 3 50 40 io 6,S ■S MM CAftt ^ONS "* 6 7nn Carl OHS oo Pow F/f /A 20 ' WA1 OO [rs C OMSO M£P 3rAfl c FIGURE 5. Variation of power with light intensity