Sensitometry

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Page 10 of Raymond Davis and F. M. Walters, Jr., Scientific Papers of the Bureau of Standards, No. 439 (Part of Vol. 18) "Sensitometry of Photographic Emulsions and a Survey of the Characteristics of Plates and Films of American Manufacture," 1922. The next page starts with the H & D quote: "In a theoretically perfect negative, the amounts of silver deposited in the various parts are proportional to the logarithms of the intensities of light proceeding from the corresponding parts of the object." The assumption here, based on empirical observations, is that the "amount of silver" is proportional to the optical density.
Page 10 of Raymond Davis and F. M. Walters, Jr., Scientific Papers of the Bureau of Standards, No. 439 (Part of Vol. 18) "Sensitometry of Photographic Emulsions and a Survey of the Characteristics of Plates and Films of American Manufacture," 1922. The next page starts with the H & D quote: "In a theoretically perfect negative, the amounts of silver deposited in the various parts are proportional to the logarithms of the intensities of light proceeding from the corresponding parts of the object." The assumption here, based on empirical observations, is that the "amount of silver" is proportional to the optical density.

Sensitometry is the scientific study of light-sensitive materials, especially photographic film. The study has its origins in the work by Ferdinand Hurter and Vero Charles Driffield (circa 1876) with early black-and-white emulsions. They determined how the density of silver produced varied with the amount of light received, and the method and time of development.

Plots of film density (log of opacity) versus the log of exposure are called characteristic curves, Hurter–Driffield curves, HD curves, H & D curves, D–logE curves, or D–logH curves. The overall shape is a bit like an "S" slanted so that its base and top are horizontal. There is usually a central region of the HD curve which approximates to a straight line, called the "linear" or "straight-line" portion; the slope of this region is called the gamma. The low end is called the "toe", and at the top, the curve rounds over to form the "shoulder".

Useable values of gamma are typically between 0.8 and 1.2. (But values of up to 1.5 may be useful for slides). A full set of HD curves for a film shows how these vary with developer type and time. A curve is shown in the Kodak Publication Selecting Films > Sensitometric & Image-Structure Data.

[edit] References

  • Hurter, Ferdinand & Driffield, Vero Charles (1890) Photochemical Investigations and a New Method of Determination of the Sensitiveness of Photographic Plates, J. Soc. Chem. Ind. May 31, 1890
  • Langford, M. J. (1977, 4th edition), Basic Photography. A Primer for Professionals. Focal Press, London & Boston, ISBN 0-240-50955-2
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