Patent GB-670504-A (GB)
Title: Process and apparatus for printing color positives from color negatives
IPC: G03B27/72
Filed: 1948-12-14
Abstract: 670,504. Photographic printing of colour positives from colour negatives. GENERAL ANILINE & FILM CORPORATION. Dec. 14, 1948 [Dec. 31, 1947], No. 32326/48. Class 98 (ii). In printing a multilayer positive having three separate differentially sensitized emulsion layers from a multilayer negative having yellow, magenta and cyan images in individual layers the gamma of which varies when illuminated with light of varying wavelengths, the variations in gamma are established, radiation from a polychromatic light source is dispersed into a continuous spectrum and projected at a printing area between which, and the negative and positive to be printed, there is relative movement, predetermined portions of the projected blue, green and red lights are selectively blocked off, and one or two of the blue, green and red lights are further confined within wavebands to one side or other of a point at which maximum spectral densities are obtained by use of three spaced parallel adjustable lightpassing slits and mechanisms for moving each of the slits into registry with portions of the spectrum of the predetermined wavelengths and for adjusting the width of each slit independently, whereby the printing gamma of the corresponding layer or layers in the colour negative is reduced, thereby substantially equalizing the effective gammas of the separate layers of the positive after colour development. The correction which has to be applied may be experimentally determined from the spectral density curve of the negative (wavelength plotted against density), and the characteristic curve of the positive (density plotted against log exposure); the data may, however, be published or supplied by the film manufacturer. The invention includes the printing apparatus, and in this, as shown in Fig. 2, light from source 11 is focused by lens 12 into entrance slit 13 collimated by lens 14 and projected on to diffraction grating 15, to be focused by lens 16 into the plane of slit mechanism 17 wherein three slits permit passage of selected bands of blue, green and red light on to the continuously moving copy material. In Fig. 8 a prism 98 replaces the diffraction grating 15. Fig. 3 shows the positive and negative films passing over rollers 22, 23 in a plane close to the slit mechanism comprising a mash having transverse members 24, 25, 26 and 27 and movable pairs of blades 28, 29, 30, 31 and 32, 33, of which the transverse members mash off intermediate portions of the spectrum, as shown, and the movable blades permit selection of particular wavebands in the blue, green and red areas. In one form of the actual apparatus, Figs. 4, 5 (not shown), the slit-providing mechanism consists of three pairs of somewhat overlapping plates, each pair of which may be both driven bodily along the exposure area embracing the spectrum and moved closer together or further apart by screw mechanism; and in another form, Figs. 6, 7 (not shown), the slits are provided by three pairs of tensioned apertured curtains made of fabric or metal and mounted on spring rollers.