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THE COLOR PEOCESS 511
green series (CC33, CC34, and CC35), and Figure 119 shows the characteristics for the minus-red series (CC43, CC44, and CC45).
If these various sets of color compensating filters are compared, significant differences are apparent. Although each set represents a "family," only the minus-blue series seems to fit the "ideal" criterion that attenuation shall take place only in the band for which the filter is rated. Even this series does not have a "square wave" transmission characteristic. It would be unreasonable to expect such a characteristic from the coloring materials available for filter making. Since the differences, though small, are significant, it is well to summarize them.
Minus-Blue Series (CC23, CC24, CC25)
(1) One principal absorption wave length-430 nm,.
(2) Negligible attenuation in either the minus-green or the minus-red bands. ]
Minus-Green Series (CC33, CC34, CC35)
(1) Two principal absorption wave lengths-530 and 565 mu..
(2) Slightly greater attenuation at 565 mjn than at 530 mu..
(3) Slight attenuation in the minus-blue range (approximately 10% at 430 mu for CC35).
(4) Negligible attenuation in the minus-red range.
Minus-Bed Series (CC43, CC44, CC45)
(1) Broad band nonsymmetrical transmission centered about 500 m(x.
(2) Slight attenuation in the minus-blue band (approximately 15% at 430 mjj, for CC45).
(3) Greater attenuation at 565 mu, in the minus-green band than at 530 mpi (almost 15%).
(4) Appreciably greater attenuation at the longer wave length end of the minusred band than at the shorter wave length end (approximately one-third greater for CC45 at 700 than at 600 mpi).
Should it be necessary to use other filters for correction purposes it will be found that it is not convenient to compare quickly the published characteristics of filters made by different manufacturers owing to the differences in the manner in which such characteristics are presented. It is hoped that in the future all manufacturers will use standard scales for published data and standard methods of measurement so that optical filters for photographic purposes may be readily compared regardless of their origin. The color compensating filters (CC types) for printing are subtractive ; the accuracy of control is very poor in printing compared with additive color printing. Additive color filters may be used in such trichromatic printers as the DeBrie.