Masking of colour materials

The spectral density distribution of a typical magenta image dye is shown in Figure 16.19(a). The characteristic curve of the green-sensitive emulsion which generates the dye is shown alongside and represents the green, blue and red densities of the dye image. It will be seen that the density to green light increases with exposure as would be expected, and that the unwanted blue absorption of the dye is shown by the increase in blue density over the same exposure range.

At the printing stage an unmasked negative record employing this dye will convey spurious information to the printing paper about the distribution of blue light in the camera image. This is undesirable and results, for instance, in bluish greens in the print reproduction, among other shortcomings.

In order to compensate for the variation of blue density with the magenta dye content of the negative, it is merely necessary to prepare a corresponding positive yellow dye record, as shown in Figure 16.19(c) and (d). The yellow positive is then superimposed in register with the negative, as shown in (e). Provided the blue-density characteristic curve of the positive is equal to the contrast of the negative, but in the opposite sense, the combined effect of the mask and the unwanted secondary density of the image dye is as shown in (f). The printing effect of the unwanted absorption of the magenta image dye has been entirely compensated at the expense of an overall increase in blue density.

Making separate coloured masks is usually inconvenient and modern colour masking systems rely on the formation of masks within the negative colour film. This is called integral masking, and is widely used in colour-negative films. The overall orange appearance of such negative films results from the yellow mask of the magenta dye together with the reddish mask required by the cyan image dye. The yellow image dye is usually sufficiently free from unwanted absorptions for masking to be unnecessary.

Integrally masked colour images cannot be used for projection purposes, as the colour cast is too great. If a colour negative (say of a poster) is required for direct projection it is necessary to use a slide film developed by a colour-negative processing method. It is worth noting that although integral colour masking cannot be used for materials intended for viewing (directly or via a projector), the use of inter-image

Figure 16.19 Elements of masking the blue absorption of a magenta image dye. (a) Spectral density distribution and (b) characteristic curves of the magenta dye image. (c) Spectral density distribution and (d) characteristic curve of the yellow mask. (e) Image and mask characteristic curves. (f) Integral characteristic curves of the masked green-sensitive layer

Figure 16.19 Elements of masking the blue absorption of a magenta image dye. (a) Spectral density distribution and (b) characteristic curves of the magenta dye image. (c) Spectral density distribution and (d) characteristic curve of the yellow mask. (e) Image and mask characteristic curves. (f) Integral characteristic curves of the masked green-sensitive layer effects is analogous to masking, and can be used to achieve a useful degree of colour correction.

Digital Cameras For Beginners

Digital Cameras For Beginners

Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.

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