stylistic:style_transfer, quality:correction
Spectral sensitivity

Early photographic plates and films were usefully sensitive only to blue, violet and ultraviolet light. As a result, the relative tonal values in a scene registered roughly as they would appear if viewed through a piece of deep blue glass. Blue skies with interesting cloud formations photographed as a white blank. Any detail visible in masses of green foliage was due mainly to the colorless surface gloss. Bright yellows and reds appeared nearly black. Most skin tones came out unnaturally dark, and uneven or freckled complexions were exaggerated. Photographers sometimes compensated by adding in skies from separate negatives that had been exposed and processed to optimize the visibility of the clouds, by manually retouching their negatives to adjust problematic tonal values, and by heavily powdering the faces of their portrait sitters.

In 1873, Hermann Wilhelm Vogel discovered that the spectral sensitivity could be extended to green and yellow light by adding very small quantities of certain dyes to the emulsion. The instability of early sensitizing dyes and their tendency to rapidly cause fogging initially confined their use to the laboratory, but in 1883 the first commercially dye-sensitized plates appeared on the market. These early products, described as isochromatic or orthochromatic depending on the manufacturer, made possible a more accurate rendering of colored subject matter into a black-and-white image. Because they were still disproportionately sensitive to blue, the use of a yellow filter and a consequently longer exposure time were required to take full advantage of their extended sensitivity.

In 1894, the Lumière Brothers introduced their Lumière Panchromatic plate, which was made sensitive, although very unequally, to all colors including red. New and improved sensitizing dyes were developed, and in 1902 the much more evenly color-sensitive Perchromo panchromatic plate was being sold by the German manufacturer Perutz. The commercial availability of highly panchromatic black-and-white emulsions also accelerated the progress of practical color photography, which requires good sensitivity to all the colors of the spectrum for the red, green and blue channels of color information to all be captured with reasonable exposure times.

However, all of these were glass-based plate products. Panchromatic emulsions on a film base were not commercially available until the 1910s and did not come into general use until much later. Many photographers who did their own darkroom work preferred to go without the seeming luxury of sensitivity to red—a rare color in nature and uncommon even in human-made objects—rather than be forced to abandon the traditional red darkroom safelight and process their exposed film in complete darkness. Kodak's popular Verichrome black-and-white snapshot film, introduced in 1931, remained a red-insensitive orthochromatic product until 1956, when it was replaced by Verichrome Pan. Amateur darkroom enthusiasts then had to handle the undeveloped film by the sense of touch alone.