British Kinematography (1950)

86 BRITISH KINEMATOGRAPHY Vol. 16, No. 3 Instead of absorbing only in the primary colour bands red, green or blue, the best available dyes formed from colour couplers also absorb light of one or both of the colours they should freely transmit. Generally speaking, magentas and cyans are the least perfect, while yellows are usually fairly satisfactory. Most magentas absorb green light as they should, but also absorb some of the blue light they should transmit ; while cyan dyes usually absorb some blue and green as well as the red they are intended to absorb. In Eastman Color Negative film, the undesirable blue absorption of the magenta dye is compensated by the provision of a correction mask which is coloured yellow, while the unwanted blue and green absorptions of the cyan dye are compensated by means of a mask which is coloured orange-red. These yellow and orange-red masking images are formed within the appropriate image layers of the film in an entirely automatic manner during the normal process of colour development. Automatic Colour-Masking Like any other multi-layer negative material, Eastman Color Negative comprises emulsion layers sensitive to red, green and blue, and incorporated in these layers are cyan, magenta and yellow couplers, the first two of which are themselves coloured at the time of incorporation in the emulsions. The cyan coupler is orange-red in colour while the magenta coupler is yellow. After exposure and colour development, three silver images are formed in the film, as well as three corresponding dye images. After removal of the reduced silver and remaining halides, the negative dye images will remain, together with the unused couplers in the three layers. In all other colour film using couplers, care has been taken to ensure that the residual couplers which remain after processing are transparent and colourless, but in Eastman Color Negative the original colours of the cyan and magenta couplers are not changed or destroyed when the developed colour images are formed, and the remaining, unused couplers comprise positive images which have been utilised to form the required colour correction masks. Whenever there is a magenta image density, there is also an unwanted yellow density accompanying it by reason of the unwanted absorption of the magenta dye itself. Where there is a maximum density of magenta there we shall also have a maximum density of unwanted yellow, and no additional yellow due to coloured coupler. The density of the yellow-coloured coupler layer is so calculated that the unwanted yellow density is just matched by the yellow due to the coloured coupler in areas where no silver is reduced. Where there is less than a maximum density of magenta, then there the whole of the yellow-coloured coupler will not have been used and the residue will add just sufficient yellow density to make the same overall total. It will now be evident that this uniform yellow density, which is continuous throughout the magenta image layer, will cancel the effect of the unwanted yellow component of the magenta image, and only density due to the true magenta component of the image will modulate light transmitted by the layer. The same principle is employed in masking the cyan image, but an orange-red coloured coupler is used because the cyan dye is responsible for unwanted absorption of both blue and green light. In case I have not succeeded in making the matter clear let me present the idea in the words of one of the patents which protect it : — ' "A method of producing a colour corrected colour component record for subtractive colour photography, in which an image constituting the record is developed to the required subtractive colour in a silver salt emulsion containing a colour coupler which gives such subtractive colour on coupling development, but is itself so coloured, that it absorbs light undesirably absorbed by said subtractive colour, whereby the uncoupled colour coupler forms the complementary image required for masking."