Problems in Color Reproduction
- Ideal dyes are required for high color fidelity
- Real dyes are not ideal – cross-talk
- Color correction is required
- Masking using colored couplers
- Interimage effects using image-modifying couplers
In order to achieve high color fidelity we would like to have ideal dyes. Ideal dyes absorb only one color of light.
In the real world, the dyes are not ideal, but suffer from some overlap of their spectral absorptions, causing cross-talk and color contamination (less saturated or “muddy” colors).
We need to provide color correction to counteract this contamination of the colors.
Two methods are used. One is called “masking” and uses colored couplers. The other employs “interimage effects” and uses special image-modifying couplers.
In photography, colors are formed by combinations of cyan, magenta, and yellow dyes.
This is what we mean by cross-talk. Ideal dyes would absorb light only within their required band of wavelengths: yellow between 400 and 500 nm, magenta between 500 and 600 nm, and cyan between 600 and 700 nm.
Real dyes are broader than this and overlap each other. The magenta dye absorbs some blue light and the cyan dye absorbs some green light. We call these “unwanted absorptions”.
Here is the effect of the unwanted absorption of a dye on the characteristic curve. At the same time as the magenta forms a normal characteristic curve of green density, a smaller, unwanted secondary curve of blue density, of lower contrast, is also formed.
The principle of masking uses colored couplers. Masking allows us to form a negative amount of yellow dye that cancels the unwanted absorption.
Until now, we have assumed that the image-forming coupler was colorless and formed a color dye. Because the magenta dye absorbs some blue light, we can consider it to contain some yellow dye. By making the masking coupler yellow to start with and magenta after coupling, the initial yellow color is lost.
With the yellow-colored magenta coupler we also obtain a negative characteristic curve of blue density, equal and opposite to that of the unwanted absorption. If we add these blue curves together, the net effect is a constant amount of blue density. It is not image-wise and can be cancelled out by increasing the total amount of blue light when printing the negative.
The net effect of the yellow-colored magenta coupler is to superimpose a constant higher density of blue on the green curve. As this blue density is not image-wise, it can be cancelled by simply increasing the overall blue light level when printing or scanning.
Similarly, the cyan dye absorbs some green light.
We use a magenta colored cyan coupler to cancel the unwanted green light absorption of the cyan dye.
The net effect of the magenta-colored cyan coupler is to superimpose a constant higher density of green on the red curve. As this green density is not image-wise, it can be cancelled by simply increasing the overall green light level in printing or scanning.
The combination of these two colored couplers, the yellow-colored magenta coupler and a smaller amount of the magenta-colored cyan coupler, results in the familiar orange-colored mask that we see in all color negatives.
There are two other types of couplers. These cause so-called “interimage effects”. DIRs, or Developer Inhibitor Releasing couplers, react with oxidized developer in the normal way, but release a chemical called a developer inhibitor. As its name suggests, this reduces the amount of development locally and causes the dye clouds formed to be smaller. The result is a finer-grained and sharper image.
DIARs,or Developer Inhibitor Anchimeric Releasing couplers, have a time-delayed release of inhibitors. This allows them to migrate to adjacent imaging layers before inhibiting development there. They are used to correct the unwanted absorption of dyes in a different way from colored masking couplers.
The next topic I am going to cover will be graininess and sharpness.
To be continued……