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most used in the motion picture laboratories of England and the U.S.A. is the Kastman Type 2B. Sensitometer, shown
in Fig. 1. This is a time scale instrument, that is, the exposures are increased from step to step by an
increase in time of exposure with a constant intensity light source. The exposure scale consists of twenty-one steps, each step receiving an exposure 1.414 times as long as the next shorter. There are two lamps used, one for positive film and one for negative. In the case of the former the lamp is standardised for candle-power at 2,560 deg. K. and it is used in conjunction with a filter in order to raise the radiation at the exposure plane to approximately 5,400 deg. K, which matches fairly well the colours of mean noon sunlight. The
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Lamp for positive film is used with a filter to raise the colour temperature at the exposure plane to 3,000 deg. K. This instrument was specially designed for the motion picture laboratory, is of rugged construction and is simple to operate.
Densitometers
There are a variety of densitometers in use in motion picture laboratories, mostly reading in diffuse densities, ranging from comparatively inexpensive instruments, such as the Kodak densitometer, to photocell densitometers complete with automatic curve plotting apparatus. The Kodak densitometer, shown in Fig. 2, is very simple to operate, reads directly in density values over a range from zero to 3.0, and having
a field of 0.5 square millimeter is particularly suitable for sound track density measurement.
General practice in the case of new developers iat all large laboratories is to make a series of sensitometric exposures on the appropriate film, and to develop these strips for a series of times within the working limits of the particular developing machine used. In this connection it is necessary for all sensitometric strips to be put through the continuous developing machine 'with the light exposure end leading the heavy exposure end in order to obviate variations due to ‘‘ directional effects.’’ It is customary, therefore, to punch-mark the light exposure end of each sensitometric strip, so as to give the required information to the developing machine operator.
The densities resulting from the processed sensitometric strips are then measured on the densitometer, and since the time of exposure for each density step of the sensitometric strip is known, it is possible to construct a series of density-log exposure curves From the information contained in these curves a time of development-gamma value curve is then drawn, from which in turn the correct time of development necessary to produce the required gamma value on the particular film stock used is readily obtained. A typical time-gamma curve for positive film is shown in Fig. 3.
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Although it is undesirable that a discussion on some of the methods and equip
ment used in a modern film laboratory
should give more than a shadowy outline of photographic sensitometry, it will perhaps be advisable to point out that gamma,