In relation to Kodak absorption filter graphs; What does “Transmittance” & “Diffuse Density” mean? and the difference between them?
Our collective wisdom is that the Wratten filters were measured using a GE Hardy spectrophotometer with the filters up against the sphere port. Thus the geometry was 8° incidence and total hemispherical collection. Assuming Helmholtz reversibility of optics, this is equivalent to diffuse illumination and nearly straight on viewing, a common photographic geometry, e.g., transparency viewer. Transmittance is the amount of light transmitted compared to the amount incident. Density is the “common logarithm of the reciprocal of transmittance” according to the Kodak Filter guide. So, 10% transmittance is equal to a density of 1.
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