d. Once you have connected all the plotted densitometer readout on the plotting sheet, you are
ready to perform the GAMMA readout. This is done with the help of a gamma meter. Before we go
on with the gamma meter and how it is to be used, we will first tell you something about GAMMA and
how it helps you in quality control.
4-12.
HOW TO DETERMINE GAMMA.
a. GAMMA (Υ). In technical terms, gamma (signified by the Greek letter Υ) is a
sensitometric quantity which indicates the slope or gradient of the straight line portion of the
characteristic curve of photographic emulsions. It is commonly interpreted as a measure of the contrast
reproduced in a negative image; that is, the ratio of negative contrast to original subject contrast for a
given range of tonal values. Therefore, one of the primary uses of gamma is its employment as a
yardstick. It measures the degree of development of photographic materials, since changes in
development vary contrast or the slope of the curve. (Exposure changes shift the position of the points,
right or left, on the log E axis without altering the slope of the curve. Thus, the tendency is to expose
to control the density and development to control the contrast of the image reproduced. Remember the
expression, "Expose for density - develop for contrast.")
Gamma may be likened to the slope of a road. Mathematically, it is the ratio of the height
gained to the distance traveled in a horizontal direction. In determining gamma, the height is density
(D), and the horizontal base is the log of exposure (log E).
Materials capable of producing widely different gamma values are available.
Normal
photographic subjects call for films with a gamma value of around 1.0, varying from 0.6 to 1.5. Such an
emulsion will record the wide range of tones which is present in outdoor scenes. In practice, each of the
main groups of negative materials has its own individual characteristics. Gamma is useful to the
photographer because it tells him how his photographic material will respond to changes in exposure and
processing.
b. From the foregoing discussion, it may be seen that gamma is definable in different terms.
Some other useful definitions include the following:
(1) Gamma is the numerical measure of the contrast reproduced in a negative image.
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