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Light Filters — Jones 149
to define sensitivity as directly proportional to the density which is produced for a fixed development time by the action of a constant amount of energy (ergs per cm. sq.) of the various wave-lengths as indicated by the scale estabhshed on the X-axis. Curve D in Fig. 7 represents the spectral distribution of sensitivity as defined in this manner.
The spectral distribution of energy in daylight is shown by curve B which may be represented formally by,
/=/(X) (15)
The ordinate of this curve at any wave-length, X, will be represented by the symbol J\. The curve as shown is computed from the data given in the previous communication^ (Fig. 7, p. 144). Measurements have shown that of the radiation incident on a horizontal plane so placed as to receive radiation from the entire sky hemisphere and from the sun, 80 per cent is sunlight and 20 per cent skylight. Using the curves representing the distribution of energy in radiation from sun and sky and combining these in the proportion 80-20 the curve B in Fig. 7 is obtained. On a vertical plane exposed to sunhght the percentage of skyhght is probably only about 10 per cent, but in the shadows a much greater proportion of the radiation is due to skylight so that the above ratio (80-20) is thought to represent a very probable composition of the average quality of natural illumination effective in photography. The curve as plotted shows only relative energy values, the maximum ordinate being arbitrarily adjusted to unity (1.0). It is not necessary in this case to use absolute values since we are interested only in determining the ratio of the filter exposure to the no-filter exposure.
In practical work the only other absorbing material of importance between the photographic plate and the object is the lens. This is usually made of three or more pieces of optical glass which may or may not be cemented together with a thin layer of Canada balsam. The absorption of energy by this lens in the visible region is relatively small and constant but in the ultraviolet, wave-length less than 400, the absorption is variable and becomes large as wavelength decreases. The lens, therefore, has an appreciable influence upon the spectral composition of radiation which reaches the photographic material. The spectrophotometric transmission curve of a typical motion picture objective is shown in curve C. This curve may be represented formally by