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November, 1950
SANDVIK AND VEAL I FILMS FOR TELEVISION
145
which result from the various exposures. The results are expressed in graphical form by plotting curves of the density as a function of the logarithm of the exposure. Numerical constants are derived from these curves which are used in specifying the characteristics of the material.
When several similar series of exposures are made and are developed for different lengths of time, a family of curves results, as shown in Fig. 3. These are the sensitometric characteristic curves, more popularly known as the H and D curves. The slope of the straight-line portion of these curves is called " gamma." It will be noted that the slope of these curves increases with the length of time of development ; therefore, the difference in the densities corresponding to two successive exposure values increases with development time.
Gamma and Contrast.
The luminance relations between the different areas of the projected positive image should be the same as those between the corresponding areas in the object. This can be accomplished over a considerable range by developing the negative and the positive to the proper gammas. In general, the product of
Fig. 4. A tone reproduction
diagram of the photographic
process.
-0.0
-0.5 \
\ \ \
Lcc? exposure \ 1 , ,
c s x / o / / /
\ a/ .b/y
o> / / /
Log luminonce of scene
a\S\\c
-1 5 ^
-0.5 1 ^/
^^^"^ Log exposure -GO
— -1 1 1 1. , 1 .
20 2-5 10 1-5 00 05
the gammas of the negative and the positive should be equal to unity. However, owing to the difference in the viewing conditions of the original scene and the projected picture, and to some other factors which will not be discussed here, the gamma product should be somewhat greater than unity. When the gamma product is too low, the contrast of the picture is also too low and the picture appears " flat " ; conversely, when the gamma product is too high, the contrast is also too high.
It should be noted, however, that gamma and contrast are not synonymous. The contrast of the picture depends upon other factors, especially upon the lighting of the subject. The contrast in a photograph of a scene taken on a bright, sunny day will be greater than that in a photograph of the same scene taken on a cloudy day, when the two are developed to the same gamma.
Tone Reproduction.
The process of photographic tone reproduction can be illustrated graphically by means of the tone-reproduction diagram in Fig. 4. This figure is only illustrative ; for a more complete treatment of photographic tone reproduction the reader may consult references 18 and 19 in the bibliography.