1. Field of the Invention
This invention relates to improvements in density check plates for checking the variation of the spectral characteristics of a densitometer.
2. Description of the Prior Art
Photographic color densitometers are generally provided with spectral filters for the three colors, red, green, and blue (hereinafter referred to as R, G, and B) of a color photosensitive material, or filters matched in visual density to measure the density of the three colors, R, G, and B, and the transmission density. The density value of te color photosensitive material to be measured greatly depends upon the characteristics of the spectral filters used in the photographic color densitometer. Accordingly, in the routine use of photographic densitometers, it is necessary to always check the density value so as to avoid a variation in the spectral characteristics of all optical systems in a device. This includes measuring the spectral filters.
For this reason, a density check plate employing six kinds (for high density and for low density of each color, R, G, and B; i.e., two kinds for each color), with each color, R, G, and B, of color plates which greatly vary in the value of the spectral density in the vicinity of the dominant wavelength of the three-colors, R, G, B spectral filter is generally used. The "Check Plaque" (product name) of the Eastman Kodak Co. is one example of such a density check plate.
The principle of the check plate as described above will be described with reference to FIG. 1. First, in the event a dye, for example, cyan, magenta, yellow recording on the photosensitive material, having a spectral density distribution D.sub..lambda. is measured with the use of a photographic color densitometer using a spectral filter having the spectral characteristic A.sub..lambda., a porton (hatched portion) in which both curves overlap is measured as a density value. However, if the characteristic A.sub..lambda. of the spectral filter should be changed into a characteristic as indicated at B.sub..lambda. due to a time variation (the time variation of the spectral filter is generally considered to be where the dominant wave-form is displaced and where more break-through occurs at the high density portion), the measured value greatly varies as may be seen clearly from the difference in the area of the hatched portion. The difference in the measured value becomes greater with the variation in the spectral characteristic of the dye in the vicinity of the dominant wavelength (435.8, 546.1, and 643.8 nm according to ASA Standards) of the three-color spectral filter increases. This makes it easy to check the variation in the spectral charcteristics of the spectral filter. Therefore, it is desirable to select a color plate having great variation in spectral characteristics to serve as a density check plate for the photographic color densitometer.
However, in a density check plate manufactured in a manner as described above, when the spectral characteristic of the spectral filter used in the photographic color densitometer varies, the density of the density check plate varies greatly due to the characteristic of the spectral filter, and as a result the same density check plate cannot be used.