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A-C HighIntensity Arc Slide Projector
By ARTHUR J. HATCH
This paper describes a high-intensity arc slide projector which is powered from a 110-v, 60-cycle convenience outlet and requires only 10-amp supply. The resulting intensity of illumination is sufficient for screens of 35 ft in width.
A
modern slide projector, shown in Fig. 1, using a high-intensity a-c carbon arc as a light source, has been developed to cover both the large-screen areas of drive-in theaters and smaller screens where an exceptionally high level of illumination is desired.
With this high-intensity a-c arc adapted for 3| in. by 4 in. slides, 7500 Im are projected, with no slide in the carrier. Expressed a different way, this 7500 1m projected to a 35-ft wide screen will produce a screen brightness of approximately 9 ft-L, which incidentally is the lower limit of the SMPTE screen brightness range for 35mm projection. For a 50-ft wide picture the screen brightness will be nearly equal to that usually obtained on the average 50-ft drive-in screen with 35mm projection.
With small-size screens of 10 to 12 ft in width, the brightness may approximate 70 ft-L, which is sufficient to obtain a reasonably good contrast even with the normal room lighting remaining on.
The complete projector comprises the arc lamphouse, optical system, slide carriers, and fan and transformer, all assembled as a table unit 78 in. long and weighing 175 Ib. The table is adjustable in height by means of its four
Presented on April 22, 1952, at the Society's Convention at Chicago, 111., by Arthur J. Hatch, The Strong Electric Corp., 87 City Park Ave., Toledo 2, Ohio.
legs, from 36 in. to 56 in., and tillable from 10° upward to 30° downward.
The reflector-type arc lamphouse and power supply elements are essentially the same units used in the "Trouper" arc spotlight.* The lamphouse is complete with carbon holders, motor-driven carbon feed, reflector tilt adjustments, arc focus knob and arc imager screen.
The trim of 6 mm by 7 in. coppercoated high-intensity a-c carbons is burned in coaxial alignment at 45 amp and 21 v a-c. The burning time for a single trim of carbons is 1 hr 20 min.
Although the first development with this new a-c projector has been for 3j by 4 in. slides, simple adaptations can be used to project both larger and smaller material. However, in the case of 2 in. by 2 in. material or smaller, heat-removing means in the form of heat filters or heat deflectors will have to be used in the light beam to prevent damage to the slide.
The optical system is arranged so that the light from the arc is gathered by a lOj-in. diameter elliptical reflector which has a focus of 3 J in. and a working distance of 24 in. This reflector converges the beam of light through a plano-convex lens and thence through the slide aperture to the objective lens.
* R. Ayling "New portable high-intensity arc spotlight," Jour. SMPE, 53: 408-416, Oct. 1949.
October 1952 Journal of the SMPTE Vol.59
335