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548 P. H. ARNOLD [j. s. M. p. E.
quality of the lamp, a neutral-density filter was employed. The results obtained with an 0.6 neutral-density filter are shown in Fig. 4 which affords a study of the toe or threshold sensitivity of the new film that was not possible under the sensitometric conditions shown in Fig. 3. The gamma was not changed appreciably by the use of the filter.
The Type lib sensitometer is highly regarded as a precision instrument, and, because of its general availability, is familiar to commercial motion picture laboratories. However, it provides only one type of sensitometric result : that produced by a time-scale apparatus operating with a certain level of illumination and exposure ratio. Access to other types of sensitometers made possible a study of the relative sensitometric characteristics of Ultra-Speed Panchromatic and other negative films under different sensitometric conditions and with somewhat different results.
A comparison of the results obtained with several types of sensitometers is shown in Fig. 5. The Ultra-Speed Panchromatic film has been given the same exposure and development as Superpan Negative film. A new panchromatic negative film, Agfa Supreme, was included in the comparison as representative of an intermediate negative-film type having a general sensitivity higher than that of the supersensitive panchromatic films but not as high as that of Ultra-Speed Panchromatic film.
Film-Speed Determination by Sensii,ometry. — Although some sensitometric methods of film-speed determination specify that the comparative speeds be determined from sensitometer strips developed to the same gamma, the data shown in Fig. 5 afford valid film-speed comparisons between Ultra-Speed Panchromatic and Superpan Negative, though the films have been developed for the same time and not to the same gamma. Since Ultra-Speed Panchromatic film has a lower gamma (slower rate of development) than Superpan Negative within the range of development employed for the test, its speed advantage would necessarily be increased by developing the films to the same contrast. Relative film-speed data derived from Fig. 5, therefore, must represent the minimum speed advantage of UltraSpeed Panchromatic over Superpan Negative, since in practice the "flatter" film would usually be given somewhat longer relative development. Longer development would naturally increase, rather than decrease, the speed of the "flatter" film relative to the more brilliant one. The minimum relative film-speed determinations can