Summary
The amount of light energy a particle absorbs does not depend upon correct focus. The change in the path of light rays brought about by defocusing causes absorbing areas to be registered as areas of higher transmittance than when in focus. Already wellknown in photometry, this effect is put to use by the “blind focus” method at the television texture analysis system (TAS, Leitz). Some chromophores within the object to be measured are compared to a preset value of transmittance, for exampleT=0.40.
Only the area representing the structures as dense or denser than the preset density are registered. If the structures are out of focus the size of the registered area is too low, since by defocusing, structures to be measured become pale and diffuse, the correct focus corresponds to the largest area to a preset value of transmittance.
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Decher, J., Bachmann, P. “Blind focusing”: An objective method for avoiding errors of focus at the television texture analysis system (TAS). Histochemistry 63, 221–226 (1979). https://doi.org/10.1007/BF00644544
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DOI: https://doi.org/10.1007/BF00644544