As outlined above, most early colour photography processes were ‘additive’—they relied on the principle of adding together red, green and blue light.
However, there is an alternative method of reproducing colour photographically: ‘subtractive’ colour synthesis.
What were the drawbacks of additive colour?
Additive colour processes had several disadvantages:
- They relied on the use of filters, which block out light
- This resulted in long exposure times and very dense transparencies
- The colour photographs made using these processes could only be viewed by transmitted light—i.e. by projection or by using special viewing devices
What is subtractive colour reproduction?
The original theory for subtractive colour reproduction can be traced back to the French physicist and inventor Louis Ducos du Hauron, who explained the method in his book Les couleurs en photographie, solution du problème (1869). Du Hauron proposed that colour separation negatives should be used to produce three positive images, which would then be dyed the complementary colours of cyan (bluegreen), magenta (blue-red) and yellow.
Each of these complementary colours absorbs—or subtracts (hence the name)—one of the primary colours. Cyan absorbs red light, reflecting a mixture of blue and green light. A cyan image, therefore, performs the same function as the red filter used in an additive process. Similarly, magenta absorbs green light and yellow absorbs blue light. By accurately superimposing these three complementary colours, all other colours can be reproduced. The colour in subtractive processes comes from dyes or pigments rather than coloured filters.
With subtractive colour, white, for example, is represented by clear glass or white paper rather than by light passing through three filters. This means that subtractive processes are much less wasteful of light.
More importantly, they work with reflected rather than transmitted light, meaning they can be used to produce colour photographs on paper.
How did subtractive colour processes work?
The development of subtractive colour processes followed two distinct paths. Firstly, the design of specialised cameras—for taking sets of colour separation negatives—and secondly, the search for practical methods of making and superimposing three positive images in the complementary colours.
When taking colour separation negatives of stationary subjects—e.g. a vase of flowers—a conventional camera could be used. The colour filter simply needed to be changed after each exposure. This procedure could be made simpler through the use of a ‘repeating back’, a moving part of the camera which allowed filters of different colours to drop into place.
A number of devices of this sort were marketed. The simplest type were long plateholders, fitted with three filters, which the photographer would manually slide along the camera back in three steps. The most complex were fitted with clockwork motors, enabling three negatives to be exposed in rapid succession in as little as two or three seconds.
Science Museum Group Collection When photographing subjects where movement was likely to occur—such as portraits—even automatic repeating backs were not fast enough. For these, a camera that could expose all three negatives simultaneously was needed.
Over the years, many designs for such ‘one-shot’ cameras were patented, and a number were produced commercially. These used various arrangements of mirrors and prisms to split the light entering the camera into three separate beams, each of which went to a plateholder fitted with a different coloured filter. Among the most successful designs were the Jos-Pe, Bermpohl, Klein and Mirkut cameras.