It is well known in photographic color printers to employ a scanning system to scan a photographic negative to determine the amount of exposure required in each of three primary colors for printing the photographic negative onto photographic color paper. U.S. Pat. No. 4,589,766 issued May 20, 1986 to Fursich et al discloses a color copier having such a scanning system and teaches that if the spectral sensitivities of the measuring system and the copying system are exactly matched, the type of photosensitive material or film being printed in the copier is of no consequence. A copying system as used herein means a given photographic paper exposed by a given printer lamp. Thus, theoretically, if the spectral sensitivities of the color channels in the scanner exactly match the spectral sensitivities of the color photographic paper in the photographic printer, the exposure in the printer can be controlled so as to obtain properly colored copies independently of the type of dye or density of the masks used in making the film.
Although in theory this may be correct, in practice it has not proven to be feasible to achieve a wavelength by wavelength match between the spectral response of the scanning system and the spectral response of the photographic paper in a color photographic printer. This has been due to the difficulty and expense of designing color filters and light source combinations for the scanner that exactly match the spectral sensitivity of the photographic paper at all wavelengths. For example, a line of photographic printers of the type described in U.S. Pat. No. 4,589,766 is manufactured and sold by the AGFA corporation as the MSP and MSC printers. The MSP and MSC printers are designed to be used with all kinds of film without the need to readjust the printer for film type. FIG. 3 shows a measured spectral response of the scanner in an AGFA MSP printer with filters designed to be used with Kodak color photographic paper versus a typical spectral response of the Kodak color photographic paper when exposed by the MSP printer lamp. The curves labeled 10, 12, and 14 are the measured spectral response curves of the blue, green and red channels of the scanner respectfully. The curves labeled 16, 18, and 20 are the measured spectral response curves of the blue, green and red sensitive layers of a color photographic paper, respectively. As can be seen from FIG. 3, the spectral responses of the scanner in the MSP photographic printer are an approximation of the spectral responses of the photographic paper in the printer.
Although the scanner filters and light source employed in the scanner of the AGFA MSP printer were chosen to provide a reasonably approximate match of the particular color photographic paper, it was found that all films could not be printed with consistent color as a function of negative exposure level. When the printer was balanced to print any one of the films throughout the exposure scale to the same color balance, other films experienced a color bias on the order of 0.10 reflection print density units. For example, if the printer were to be set up to yield standard color prints from AGFA 100 brand color negative film, Kodacolor VR 200 brand film would experience a green color bias for high density negatives on the order of 0.10 density units. Conversely, if the printer were to be set up to print Kodacolor VR 200 brand film, the AGFA 100 brand film would print with magenta bias on the order of 0.10 density units.
It is therefore the object of the present invention to provide an improved color scanner for a color photographic printer of the type having a spectral response like that shown in FIG. 3 that overcomes the problems noted above.