All Things Color for Film and Digital Cinema
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Optical Density Explained. Density Pt 2.

As mentioned in Part 1, optical density or ‘log density’ is the scale used for measuring photographic optical ‘lightness’. 

In describing densitometry, three main terms are used. Transmission(T), Opacity(O) and Density(D).

Transmission (T) is a measure of the light passing ability of a material expressed as a percentage. It is calculated by taking the measured transmitted light and dividing it by the incidental light(the light source).                T= transmitted light/incident light

Opacity (O) is the ability of a medium to absorb light. Opacity is measured as     O= 1/transmitted light 

Optical Density (D) is the l                                                                                                  D=Log(O)

                                                               or                  D= (LOG10(1/T) 

Using the basic principles above we can best show the relationship between Transmission, Opacity and Density using a graph mapping out a black to white scale over a number of steps.



To the Left you can see how as the ‘linear’ transmittance of light increases the measured “optical density” of the image decreases. Obviously the more light being allowed through a medium the less ‘dense’ it becomes. 





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