Equidensitometry


Equidensitometry is the technique of measuring equidensities in a photographic deposit or photographic layer, such as photographic films and photographic plates.
  • Equidensities are points, lines and areas having equal densities also called isodensities.
  • First order equidensities are points, lines and areas having isodensities obtained by applying one of the listed below techniques once, thus they exhibit one particular Density in the original
  • Second order equidentities are points and lines having isodensities obtained by applying one of the listed below techniques twice, thus they exhibit two particular Densities in the original. Second order equidensities do not normally show areas.Equidensity series is a sandwich of a number of different equidensities. Usually each equidensity is coded by color or raster.

Photographic and computerized image processing techniques

Four techniques of obtaining equidensities are:Tone separation process Sabattier effect, also called pseudo-solarizationAgfacontour Professional film Computerized LUTs Manipulating custom curves in photo-editing programs such as PhotoShop provide tools to obtain equidensities in digital image processing.

Applications

Interpretation or measurement of an image is sometimes aided by simplification of the density pattern. This results in data reduction and enables keying. This keying to singular grey- or colour values is known in modern image processing as using L.U.T.s.
The fact that a certain density of a photographic layer has been caused by a specific amount of actinic radiation can be used to qualify and perform spatial measurements of a physical phenomenon. A calibrated photographic process is used to obtain quantitative measurements as well.
Equidensitometry with Agfacontour made it possible to obtain reproducible equidensities similar to pseudosolarized photographic films and prints. The Agfacontour film contained two special emulsions with each different spectral sensitivity. Using color filters during the exposure on Agfacontour one could control the width of the equidensity. By varying the exposure one could change the density in the image that showed on Agfacontour as equidensity. Although the material exhibited very low sensitivity the results were very reproducible and second order equidensities were sharp and clear, something that with pseudosolarization was almost impossible to achieve without special procedures.
In the following fields equidensitometry is used extensively: