Friday, September 25, 2009

C. V. Raman’s Explorations in Color Science

by Michael H. Brill, Datacolor

This year (and hopefully this month) will mark the end of my ten-year chairmanship of CIE TC1-56, during which I found that the laws of color matching are not so simple as Hermann Grassmann thought. Is color science possible without Grassmann’s underpinnings? To find out, I look at a little-known corner of history….

Chandrasekhar Venkata (C. V.) Raman (1888-1970) was always interested in the science of color. Indeed, that interest seemed to motivate his Nobel-Prize-winning work in spectroscopy. A 1921 trip returning to India from England made him marvel at the blue of the ocean, and to posit that blue as arising from molecular scattering of light by water molecules, not just reflection of the blue of the sky (as Lord Rayleigh supposed). By 1928, Raman found that blue light through glycerin had a shift to the green, and that the shift was due to quantum transitions in molecular electron states. Raman spectroscopy was soon born, quantum mechanics and the photon theory of light were vindicated, and the Nobel Prize followed two years later.

In 1959, after a fruitful career in optics, acoustics, and the interaction of light with sound waves, Raman turned his attention exclusively to color, including the colors of plants and minerals and color blindness. He used only a pocket spectroscope, some black-and-white photographic film, and a few functional human visual systems. The color-science period of his life was to last more then ten years, and gave rise to many publications in the annals of the Indian Academy of Sciences in Bangalore. I found one book of this work [1] on a shelf in Datacolor earlier this year, and a Google search revealed this work and a lot more, on individual pdf pages from the Indian Academy of Sciences. Even partial retrieval of the work was painful, but it was worthwhile and timely for me.

Here is a quote from a 1966 work [2]:
“It is a remarkable fact that a person endowed with normal vision is capable of recognizing quite small differences in colour if these are presented to him in an appropriate fashion. For example, the two yellow lines in the spectrum of a mercury lamp, whose wavelengths are respectively 5770 Å and 5790 Å and which are of equal intensity when seen simultaneously through the eye-piece of a spectrometer exhibit an observable difference in colour, the former line appearing of a greenish hue while the latter is a pure yellow. This fact suggested to the author that an arrangement by which the entire continuous spectrum is presented as a series of discrete lines would be a useful device for the study of the spectrum colours and especially for exhibiting the differences in the rate of progression of colour in different parts of the spectrum.
“The idea indicated above can be realized in practice by setting two half-silvered plates of glass in parallel positions before the slit of a wave-length spectrometer and viewing the spectrum of a brilliant source of white light of restricted area normally through the combination. The entire spectrum is then seen as an array of discrete lines or bands in a dark field, their number and spacing being determined by the separation between the plates. By making one of the plates movable with respect to the other, the number of lines or bands seen in the spectrum can be varied within wide limits. […] A channeled spectrum of 100 bands […] was presented.” (p. 269)
“ A remarkable and convincing demonstration that Daltonian vision arises by reason of an abnormal enhancement of the sensation of yellow in relation to other colours in the spectrum…” (p. 270)

This work is experimentally ingenious. I’m especially impressed with Raman’s recognition of (and use of) the subtle property of human vision that enhances the discrimination of colors when they are separated by a dark boundary. But something is missing in the description. There’s no mention of trichromacy, none of the heritage of Newton, Young, Helmholtz, or Hering. That seems to be true of all of Raman’s work. And of course, Grassmann’s laws are also absent from the discussion

How could a 20th century physics Nobelist devote ten years to color research and write copious articles without once referring to the trichromacy of color vision? Perhaps the answer can be found in Raman’s famous 1968 lecture, “Why the sky is blue” [3]. Here, Raman recounts the familiar Rayleigh-scattering story, but adds much more. Why don’t we see the stars during the day? Because the atmosphere of the earth is a veil that hides them by scattering the Sun’s light. Why isn’t the sky blue on a moonlit night (for which the same spectrum acts in an attenuated form)? Well, here Raman isn’t so sure. He says it’s a difficult question why we don’t see colors at night. He never mentions rods and cones, or of the body of literature that culminated in the same year with Yves LeGrand’s second edition of Light, Color, and Vision. And yet Raman espouses a holistic philosophy of science: “Ultimately you find that you have to travel the whole field of science before you get the answer to the question: Why the sky is blue?” Another quote reveals how exploratory he is willing to be: “To get any colour, red, green, or blue, you have to take out the yellow. Yellow is the deadly enemy of colour.” Later, he seems to be getting closer to opponent-color theory: “It is the reduction of the yellow of the spectrum that is to say the predominance of the blue which is responsible for the blue light of the sky.” But here’s the final take-home lesson: “I want you to realize that the spirit of science is not finding short and quick answers. The spirit of science is to delve deeper.” I will guess that Raman was well aware of rods, cones, and trichromatic theory, but felt he had not been able to delve deeper, to add as much to the vision explanation as he had to the simple one-sentence “Rayleigh scattering” answer to “Why is the sky blue?”

Well, Dr. Raman, I am ready to delve deeper now that TC1-56 is at its end. Simple sound bites such as “linearity” and “trichromacy” aren’t going to cut it anymore.

1. “Memoirs of the Raman Research Institute No. 137: Floral Colours and the Physiology of Vision”, by Sir C. V. Raman (Bangalore, 1963); pp. 57-108.

2. C. V. Raman, The New Physiology of Vision, Chapter XXXIX. Daltonian Colour Vision, J. Indian Academy of Sciences, pp. 267-274 (1966). http://www.ias.ac.in/jarch/proca/63/00000280.pdf and subsequent pages (last 3 digits in filename).

3. C. V. Raman, “Why the Sky is Blue,” Lecture Dec. 22, 1968 at Ahmedabad. http://dspace.rri.res.in/bitstream/2289/1509/1/1968%20(Dec.%2022)%20Raman's%20Lecture%20-%20Ahmedabad.pdf

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