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Physics & Physiology of Color

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Did Maxwell Know
He Couldn't Photograph
Red And Green?
 

Follow this chronology carefully:

1802: Thomas Young postulated that human eyes are sensitive to red, green & blue light.
1861: J. C. Maxwell demonstrated a color photographic process, using red, green & blue light.
1873: Hermann Vogel discovered how to make photographic materials sensitive to green light
(Red came even later).
Woops!

When James Clerk Maxwell exhibited a three-color photographic process before the Royal Institution of Great Britain on May 17, 1861, he seemed to have verified Thomas Young's theory.

Maxwell photographed a colored ribbon on photographic plates. He made three exposures: one through a red filter, one through a green filter, and one through a blue filter. He probably then re-exposed those images onto other plates, or somehow processed them into positive rather than negative images; the published paper is unclear on the process.

Then, he used magic lanterns to project his transparencies, superimposed the three images, and filtered the projectors as he had filtered the original images—with red, green & blue filters. He produced a colored image, ".a coloured image was seen, which, if the red and green images had been as fully photographed as the blue, would have been a truly-coloured image of the ribbon."

He went on to suggest that his results would be greatly improved with the development of photographic materials more sensitive to green and red light. From this comment, it's obvious that Maxwell knew his emulsions were not equally sensitive to all the colors of light. However, he seems not to have known that his process was completely insensitive to red and green light. All photographic emulsions were sensitive only to blue light until 1873, and none were reasonably sensitive to all the colors until 1882.

Then, how did Maxwell obtain images at all through red and green filters? Well, his filters consisted of glass cells, filled with colored liquids. Each of his filters passed a considerable amount of blue light, to which his emulsions were sensitive. So, it's reasonable to assume that Maxwell created not a trio of records of the red, green & blue light reflected by his subject, but rather, three slightly different records of the blue light reflected from the ribbon.

But how then, did he produce a color image? That question is not so easy to answer.

Today, with modern films, and high quality filters, it's relatively easy to accomplish what Maxwell thought he had done. The logic that led Maxwell to attempt his experiment was sound. We now know that the process says little about how our eyes see color; it's just a very practical demonstration of some elementary math and physics.

It's probably important to say that Maxwell's photographic experiments were only a small part of his investigations, and he apparently didn't think they were very significant. He seems not to have realized the questions he raised with the photographic experiments. But, those questions remain unanswered.

Maxwell did produce a color image. He did it without photographic sensitivity to red and green light. And he demonstrated his process several times. What was really happening?

© J. C. Adamson, 1997