Hello everyone! We promised that we would talk about how colored photographs are produced. Though the process is somewhat complicated, we will try to introduce an easy explanation.
Color photography is based upon the principle that the colors of nature can be represented to human eyes and brains by mixtures of blue, green, and red light. Mixtures of these kinds have been produced by projecting colored beams of light in the ‘register’. These beams of light emanate from properly prepared transparent positive images. They can also be produced by modulating or by silver images, microscopic blue, green, and red filters which are juxtaposed on support. In the method, the eye receives the required amounts of green blue and red light to reproduce the intended color. An ideal electronic analog of this system is a color television picture tube. The use of green, blue and red beams or filters is difficult in practice and requires a lot of light energy. Alternatively, most methods of color photography are based on the use of the complements of blue, green, and red, which are yellow, magenta, and cyan respectively. Yellow results when blue is absent from white light. Similarly, magenta is absent when the green light is not present, and cyan, when the red light is absent. Thus à yellow filter can prevent the blue component of white light and permit only green and red; magenta prevents the green light from passing and allows only blue and red light to pass, and cyan controls red while permitting blue and green light to go through. Combinations of different proportions and densities of yellow and magenta produce a variety of colors, generally including orange and red. Similarly, yellow and cyan produce a variety of colors including green and magenta and cyan produce a group of colors including blue. There is a way these combinations can be affected.
The way is to superimpose layers of dyes on a single support.
Originally pigment layers were applied on one upon another and that is how color prints were prepared.
But, as there are much better dye-transfer systems available now, this process has almost become obsolete.
However, there is still some highlight on the pigment process as prints thus produced possess more permanent colors than those done with the help of dyes.
Dyes for color photography are produced according to the following set of reactions:
Exposed silver salt + developer → oxidized developer + silver
Oxidized developer + coupler → dye
This process requires that only one layer develop at a time and also that all reactants
be washed out of the photographic material before the next dye is produced. Preparation of each layer for development requires careful control. The coupler is introduced to the emulsion by the manufacturer. Then the exposed silver salts can comfortably be developed in all layers.
The oxidized developer reacts with the coupler immediately and thus a proper dye is formed.
Two types of color photographic processes are available.
They are- negative and reversal. Negative processing is the one where an incorporated coupler is used.
It results in color development in the very region of the exposed silver salts. In the regions that are exposed to blue, a yellow dye is formed. Similarly, a magenta dye is formed in regions exposed to green and a cyan dye is formed in regions exposed to red. The negative can be used to produce a positive. In making the print, the dyes in the negative are responsible for controlling the amounts of blue, green, and red light reaching the layers of emulsion on the print material.
The emulsions also contain incorporated couplers.
The reversal process involves the development of exposed silver salts to a silver negative.
The remaining silver salts are not exposed universally but selectively and color is developed to yield dye layer by layer.
New processes of color photography are being developed. We hope that what is done in a somewhat difficult way nowadays can be done with much ease tomorrow.
We are awaiting that tomorrow eagerly.
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