The Blackening of a Photographic Plate as a Function of Intensity of Light and Time of Exposure.

Abstract

Blackening of Photographic Plates as a Function of Light Intensity, Wave-length and Exposure.—Three emulsions, a slow one (Seed 23), a rapid one (Seed $27X$) and a very rapid one (Seed Graflex) were coated on plate glass and illuminated for various lengths of time with various intensities of monochromatic light of wave-length 450, 550 and 650 μ μ. The results are given in curves and are found to be well represented, for a given emulsion and wave-length, by the empirical equation for blackening, $D=\left(\frac{\mathrm{I}}{a}\right)log[b-(b-\mathrm{I}\left)e^{-ct}\right]$, where $logc=A+BlogI+c{log}^{2}I$, and $a$, $b$, $A$, $B$ and $C$ are constants. A graphical method of determining these constants employing a monographic chart is described. Schwarzschild's exponent $p$ is shown to be equal to ${(B+ClogI)}^{-1\mathrm{}}$, and is found to vary from 0.7 to 1.95 according to the emulsion, wave-length and intensity. In using the photographic method of measuring the intensity of a source of light, it is obviously important to determine the constants of the equation for the blackening of the emulsion for the proper wavelength. The method of doing this is explained in detail.