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638 KOLB December
projection conditions. Likewise, air cooling was provided by Waller15 in the Waller Flexible Gunnery Trainer, where the radiant flux on the film is not high but the dimensional stability is extremely critical.
In the field of slide projection, Edouart4 described air cooling in the Paramount Process Stereopticons, which have an air duct designed for minimum resistance to flow and baffles inserted to direct air onto the slides. Popovici11 described a variation using refrigerated air. An even more critical installation was described by Tuttle14 who discussed the cooling of slides in a glass-enclosed gate, so that the air stream could be completely directed and controlled past the slide.
There were other similar uses of air cooling, but so far as the published accounts show, none realized the high air velocities at the film surface recommended in this report, none appreciated the dual cooling and positioning functions of air on cine film, and none were successful in extending the safe projection limits to the levels reached in these experiments.
MEASUREMENT OF RADIANT ENERGY
Early in the program of studying high-intensity projection, it became apparent that a reproducible method of measuring radiant energy incident upon the film was essential. Such a basis was made possible by the work of Zavesky, Null, and Lozier,17 who reported energy measurements on a number of typical carbon arcs, but unfortunately their methods of measurement can be duplicated only in the laboratory and require a special setup. Hatch6 has proposed a portable meter for indicating radiant energy at the center only of the aperture in a projector head. We have built such a meter (which Zavesky, Null, and Lozier have calibrated against the more accurate measurements made in the National Carbon Company's Research Laboratory8) and used it for determining the level of radiation intensity in all of our experiments and for correlating a limited amount of information from the trade.
One caution should be observed in comparing our results. Zavesky, Null, and Lozier17 report radiation output from the various arcs as that value obtained with no shutter, heat-absorbing glasses, or other absorbing elements in the beam. We have found it more convenient, on the other hand, to report what is actually incident upon the film — taking into account the reduction in energy produced by the rear shutter plus any reduction produced by draft glasses, heat-absorbing glasses, and so forth. To prevent confusion, we have chosen to call