Utocolor or Uto paper was a light-sensitive paper for
color photography
Color photography is photography that uses media capable of capturing and reproducing colors. By contrast, black-and-white or gray- monochrome photography records only a single channel of luminance (brightness) and uses media capable only of ...
manufactured by Dr. J.H. Smith of
Zurich, Switzerland and available by early 1907.
Overview
Improved versions appeared in later years. It used the direct bleach-out method of color reproduction first proposed by
Louis Ducos du Hauron and
Charles Cros
Charles Cros or Émile-Hortensius-Charles Cros (October 1, 1842 – August 9, 1888) was a French poet and inventor. He was born in Fabrezan, Aude.
Cros was a well-regarded poet and humorous writer. As an inventor, he was interested in the field ...
in the late 1860s and based on the fact that a substance can only be affected by exposure to light if it absorbs some of that light.
[The Grotthuss-Draper law.] Unstable coloring matter is therefore bleached by light of its
complementary color
Complementary colors are pairs of colors which, when combined or mixed, cancel each other out (lose hue) by producing a grayscale color like white or black. When placed next to each other, they create the strongest contrast for those t ...
, which it strongly absorbs, but little affected by light of its own color, which it mostly transmits or reflects.
The Uto paper came coated with a mixture of
cyan, magenta and yellow dyes which appeared nearly black. It was sensitized with a catalyst that greatly increased the susceptibility of the dyes to fading. Where the coating was exposed to red light, that light was absorbed by the cyan dye and therefore bleached it, leaving the magenta and yellow dyes, which together appeared red; yellow light was absorbed by both the cyan and magenta dyes, bleaching them to leave only the yellow; green light bleached only the magenta, leaving cyan and yellow, which appeared green; blue light bleached only the yellow dye, leaving the cyan and magenta dyes, which in combination absorbed all colors of light except blue. Less strongly colored light bleached all three dyes to differing degrees, and white light bleached all three equally, leaving only a grayish dye mixture or the bare white paper. After exposure, the catalyst was rinsed out of the coating.
In practice, an hours-long exposure to direct sunlight was required to completely bleach the dyes, making Utocolor paper useless in a camera. It was suitable only for reproducing transparent colored originals, such as stained or "art" glass, paintings on glass, and early photographic color images on glass such as
Autochromes. Although rinsing out the catalyst stabilized the prints and made them much less sensitive to light, it did not actually "fix" them by removing all sensitivity, so they were unfit for prolonged open display and were best kept in the protective darkness of a closed album when not being examined.
References
Photographic processes
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