B&W film sensitivity

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I read in a chemistry text that black & white film, especially non-panchromatic type films, are most sensitive to blue light. Can anyone either explain why this is (and what specifically differentiates panchromatic films from other B&W films), or refer me to a source where I might be able to locate the information?

Any help would be appreciated. Thanks!


-- Erica Mito (rikki@jps.net), December 01, 1998



Among other sources, Kodak's "Professional Black and White Films" (catalogue # 152 8298, publication F-5, $10.95 US) discusses the spectral sensitivity of blue-sensitive, orthochromatic, panchromatic, extended-red panchromatic, and infrared films at length. There are charts/graphs showing the spectral-sensitivity curves of these films in wavelengths (nanometres).

Some brief excerpts:

"Blue-sensitive films are sensitive only to ultraviolet radiation and blue light. Normal contrast, blue-sensitive films are no longer available; today, the only blue-sensitive films available have higher-than-normal contrast, and are used for copying or black-and-white reversal applications".

"Orthochromatic films are sensitive to ultraviolet radiation and blue and green light. Kodak produces only one normal-contrast orthochromatic film, Tri-X Ortho Film/4163. This film is frequently used for portraits of men because the darker gray-tone rendering of reds can provide a more masculine look. When you expose this film through a yellow filter (e.g. wratten #8) outdoors, this film produces results similar to those of panchromatic films exposed through a green or yellow-green filter".

"Panchromatic films are sensitive to all colors of light as well as ultraviolet radiation. They produce gray-tone renderings of subject colors that approximate their visual brightness".

"Extended-red panchromatic films (like Technical Pan) are more sensitive to red light than conventional panchromatic films".

"Infrared films are not sensitive only to infrared radiation; they are sensitive to ultraviolet radiation and to all wavelengths of visible light as well. They are not very sensitive to green light. When you expose these films through a deep yellow or red filter, the filter blocks most of the ultraviolet radiation and blue and green light so that the image is formed mainly by red light and infrared radiation".

The reasons for the above, and the precise chemical properties of film emulsions, are way beyond my level of comprehension. Kodak produces many other publications, one of which may answer your questions.

Hope this helps, and gets you started. Sergio.

-- Sergio Ortega (s.ortega@worldnet.att.net), December 01, 1998.

A purely intuitive explanation:

Light is composed of little packets of energy called photons. The shorter the wavelength the more energy in each photon. Blue and UV photons have shorter wavelengths, and thus more energy, than green or red photons. When photons strike a silver halide crystal the structure is upset. Electrons are knocked out of orbits. It takes a minimum amount of energy to knock these electrons out of orbit. Unless the silver halide crystal is modified in some way only blue and ultraviolet photons have enough energy to do this.

-- Tim Brown (brownt@ase.com), December 06, 1998.

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