The micro-spectrophotometer gives the curve of the spectral density distribution, of the light as in Fig. 1, transmitted through the small image on the color film and the curve shows the apparent peak densities (
Dc, Dm, Dy in Fig. 1) which answer to the combination of the three component dyes (
c, m, y). Starting from the values of three apparent peak densities, the numerical values based on the amount of exposure of red, green and blue band (
Er, Eg, Eb) contained in the light affect to the photographic image are decided, through an analytical calculation to elliminate the effect of false densities (
Cm, Cy, Mc, My, Ym, Yc in Fig. 1).
Then, we tried to establish the Color Index system,
Crg and
Cgb, defined by the expressions shown below, to express numerically the spectral character of light from the object photographed.
Crg=Log
Er-Log
Eg,
Cgb=Log
Eg-Log
Eb.
The examples of the color index of some objects decided through the Ektachrome-X film are shown in Table 2 and 3.
The real reversal color film has not sufficient faithfullness to perform the strict photometry based on the three color bands, because it has yet pretty large sources of error, such as the inter image effect (interlayers development effect) and as the inperfectness of color separation of the sensitive layers. The color index decided through color film is not the strict photometric value, and it is merely the value depend upon the arbitrary standard procedure. However, at least, it is clear that the color index is the value concerning to the spectral intensity distribution of the beam from the object.
We believe that the color index system, even through usual color film, will be significant in the photographic observation to investigate the color character of objects in the field and also in the case of automatic processing by computer to extract the image with designated color index on the color picture.
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