International projectionist (Jan-Dec 1947)

tion were based on the following general procedure. First of all, the arc lamp, film aperture, and projection lens were positioned as specified by the equipment manufacturers' instructions. By this means, the mirror-to-aperture (or condenser-to-aperture) distance was adjusted and the carbon crater-to-mirror (or craterto-condenser) distance was set approximately. Optical alignment was achieved by placing a straight rod on the optical axis of the lamp, and by moving the aperture and projection lens so that the optical axes of the lamp, aperture, and lens all coincided. For convenience, most measurements were obtained using a short projection throw of about 12 ft. and an illuminated screen area 2 to 3 ft. in width. However, a number of confirming tests were made with projection throws as long as 65 ft. and screen sizes as wide as 12 ft. Light intensities on the screen were obtained using accurately calibrated Weston Photronx cells equipped with Viscor filters. The cells were placed at five locations on the screen: one at the center, two on a horizontal line through the center and near the sides, and the other two near diagonally opposite corners of the illuminated screen area. The side and corner cells were spaced in from the boundaries of the lighted screen area by distances equal to five per cent of the horizontal and vertical dimensions of the lighted area. In order to obtain total lumens falling on the screen, it is necessary to have some method of averaging and weighing the values of intensity obtained at the five selected points. A satisfactorily close approximation to the average Intensity has been found to be given by giving the center intensity a weight of two, the average of the sides a weight of two, and the average of the corners a weight of one. The average intensity so obtained is multiplied by the illuminated screen area to give total lumens. Wire screens of calibrated transmission placed over the front of the lamp were found convenient to reduce the amount of light passing through the projection lens and to the screen. Some tests were carried out with no absorbing screens in order to check further on the accuracy of the transmission factors of the wire screens employed. Before any light measurements were made, the intensities at the sides and corners of the screen were balanced by tilting the mirror or by moving the condenser lenses vertically or horizontally. The distribution of light on the screen, i. e., the ratio of intensity, sides-to-center, was adjusted by axial movement of the carbons in the mirror lamps, or by axial adjustment of the condenser lenses the condenser lamps. In all cases, the positive carbon crater position, and the arc current and arc gap for the carbon trim were accurately maintained. The importance of close control of crater position and arc current to provide constant light on the screen has been described in a pevious paper.2 Discussion of Data Results of measurements of screen light are shown in Table 1. The description of the carbon trim used, its operating conditions and the lamp optics are specified in the first five columns of Table 1. Screen lumen values are next listed, both for a screen light distribution ratio of 80 per cent side-to-center, and for the ratio resulting when the system is adjusted to give maximum intensity at the center of the screen. With each such adjustment, an F: 2.5 and an F: 2.0 projection lens was employed. The F: 2.5 lens is untreated, and of 5.5 in. focal length, while the F: 2.0 lens, an example of a high-quality modern lens, is surfacetreated and of 5 in. focal length. Similar measurements with other 4.5 and 5 in. e.f. F: 2.0 lenses of various make were also made, but gave no significant difference in amount of screen light. The screen lumen values quoted are TABLE 1: Screen Illumination with Carbon Arc Motion Picture Projection Systems 35-mm Film Projection Systems — 0.600-in. x 0.825-in. Aperture Carbon Arc Lamp Optical System 5.5-in. E.F. //2.5 Untreated Lens 5-in. E.F Treated //2.0 Lens 80 Per Cent Dist. Max. Light3 Per Screen Cent Lumens Dist. 80 Per Cent Dist. Max. ] Screen Lumens -ight» Screen Lumens' Per Cent Dist.2 Screen Lumens Per Cent Dist. Per Cent Dist. Item Positive Negative Amp Volts Low Intensity J 12 mm X 8 in. Low Int. "One Kilowatt," D-C 8 mm X 8 in. Low Int. 32 55 10 in. Dia. //2.3 Mirror 2,500 70 3,400 70 2 7 mm X 12 or 14 in. "Suprex" Nonrotating High Intensity 6 mm X 9 in. "Orotip" C 40 27.5 11V« in. Dla. //2.5 Mirror 4,600 80 5,000 65 5,900 80 6,500 65 3 7 mm X 12 or 14 in. "Suprex" 6 mm X 9 in. "Orotip" C 42 33 14 in. Dia. //2.3 Mirror 4,900 80 5,500 60 6,600 80 7,500 60 4 7 mm X 12 or 14 in. "Suprex" 6 mm X 9 in. "Orotip" C 50 37 14 in. Dia. //2.3 Mirror 6,400 80 7,200 60 8,600 80 10,000 60 5 8 mm X 12 or 14 in. "Suprex" 7 mm X 9 in. "Orotip" C 60 36 14 in. Dia. //2.3 Mirror 7,600 80 8,200 65 10,300 80 11,000 65 6 8 mm X 12 or 14 in. "Suprex" Rotating High Intensity 7 mm X 9 in. "Orotip" C 70 40 14 in. Dia. f/2.3 Mirror 9,600 80 10,600 65 13,000 80 14,000 65 7 13.6 mm X 22 in. High Int. Vm X 9 in. "Orotip" 125 68 Condenser Lenses at //2.0 7,500 80 9,200 65 11,500 80 14,500 60 S 13.6 mm X 22 in. High Int.* >A X 9 in. "Orotip" 150 78 Condenser Lenses at//2.0 11,000 80 13,000 60 16,000 80 19,500 60 9 13.6 mm X 22 in. Super-High Int.* V. X 9 in. Heavy Duty "Orotip" 170 75 Condenser Lenses at//2.0 11,000 80 13,000 65 18,500 80 21,500 60 Notes: 1. Screen lumen figure is for systems with no shutter, film, or filters of any kind. 2. Per cent distribution refers to ratio of light intensity at side of screen to that at the center. 3. Maximum light is value with system adjusted to produce maximum light intensity at the center of the screen. 4. Heat filter may be necessary; reduce light approximately 20 per cent if Aklo or Phosphate glass is used. INTERNATIONAL PROJECTIONIST • April 1947