Projection engineering (Jan-Dec 1931)

Page 22 PROJECTION ENGINEERING film. It is mainly for this reason that the ratio of the speeds of the two films is greater when exposed to the light of a tungsten source than when exposed to daylight. The difference in color sensitivity of the two types of film is shown in Fig. 2. This illustration shows prints of spectrograms of the two types of film when exposed to the light of a tungsten source. Comparisons of speed should not be made from these prints as the prints have been so made as to show the regions of the spectrum in which each emulsion is sensitive. The numbers in Fig. 2 represent wavelengths; beginning at 0.40, in the blue-violet region, the wavelength increases through the blue-violet, blue, green, yellow, and orange as far as red, at a wavelength of 0.68 microns. The supersensitive film shows an increase of sensitivity around 0.64 microns. Its sensitivity drops abruptly at the long-wave limit of visibility, whereas the present type of film is quite sensitive to the deep red, and even encroaches upon the near infra-red. This is an important advantage as it is the sensitivity at the longer wavelengths that contributes to the production of chalky highlights when tungsten sources are used. This is eliminated to a marked degree in the supersensitive film. A complete study of the color sensitivity of an emulsion requires the measurement of speed at the three major portions of the visible spectrum, namely, blue, green, and red. For the purpose of obtaining such information, speed curves similar to those shown in Fig. 1, later substantiated qualitatively by exposures made in a camera, were made, exposing the film to daylight through the No. 49 (blue), 58 (green), and 25 (red) filters. Such tests showed that the supersensitive film has 75 per cent 1.8 IA 1.2 10 OB 0.6 0<t 0.1 Exposure: Tungsten Development: i. sMin. in Borax 23»/<4 0.0 03 0.6 0.9 1.2 1.5 16 2.1 2.4 2.7 Fig. 3. Curve showing times of development required to produce the same gamma. more speed when exposed to the blue, 200 per cent more when exposed to the green, and 400 to 500 per cent more speed when exposed to the red than the present film. Another important thing to consider when comparing the two types of film relates to the contrast and to the rendering of details in the shadows and softer highlights. Fig. 3 shows, for exposures to tungsten sources, the difference between the times of development required to produce in the two types of film equal degrees of contrast (gamma). Both curves of Fig. 3 have the same gamma ; the data show that it was necessary to develop the supersensitive film three-quarters of a minute longer to produce this effect. Furthermore, greater density is obtained in the low-exposure region, due mostly to the greater speed of the supersensitive emulsion. It is this ability to respond to and differentiate between these low intensities which gives to the supersensitive film its great capability of rendering details in the shadows. On the other hand, it will be observed that in the high-exposure region the curve of the supersensitive emulsion tends to bend toward the horizontal, whereas the curve of the present type of film continues as a straight line. This is at least true of the series of exposures shown in the illustration. This characteristic leads to a softer rendering of high cJusryq-itisn pMA&nt 3-dm* Fig. 2. Spectrograms of the two types of film, obtained by exposing to the light of a tungsten source. lights and yet permits a very definite separation of highlights, providing good detail in this region. Fig. 4 shows in more detail the relationship existing between the contrast (gamma) and the time of development. These curves, commonly referred to as time-gamma curves, show the rate at which the gamma increases with the time of development. The rate of increase of the gamma of the supersensitive film is appreciably less than that of the present type of film, as inspection of the curves of Fig. 4 will show. This means that there is much less chance of over-developing or underdeveloping the film during processing. Errors of the order of 25 per cent in the time of development will show much less effect on the supersensitive film than on the present film. In other words, the new film provides for a much greater "development latitude." One important caution must be mentioned; due to the great sensitivity of the new emulsion, the film cannot be Tungsten lTresent Films 2. Super Sensitive Fium 6 S 12 15 19 21 Minutes in Bo*ax Fig. 4. Time-gamma curves of the two types of. film. successfully handled unless the light emanating from the present safelights is considerably reduced. It would be best to handle the film in total darkness ; no doubt this will be done, inasmuch as many camera loading rooms and laboratories where negative film is processed in machines are kept quite dark, if not totally so. It is felt, therefore, that this requirement will not cause any great hardship ; however, this word of caution is considered necessary on account of the greatly increased speed of the new film with regard to both white and colored light. Gray Base Film Since the introduction of the supersensitive negative another decided improvement has been made which will generally enhance photographic quality. The emulsion is now coated on a gray base, mainly for the purpose of preventing halation. The gray base was introduced to the motion picture industry in May, 1931. Halation has always been a source of trouble for photographers. The halation which appears as a glow encircling the highlights of the picture is due to the reflection by the rear surface of the film base of the light which has passed through the emulsion and the support,