Explanation of Density

[Alan Smithee]

Can someone go over how the photographic density scale relates to the RGB
values for density. Reflective vs. Transmissive? Thx.

 

[Bart van der Wolf]

Can someone go over how the photographic density scale relates to
the RGB
values for density. Reflective vs. Transmissive? Thx.

Optical Density, or OD=Log10(Opacity).
Opacity=1/Transmission.
Transmission=Transmitted/Incident.

You can replace transmission with reflection when using reflection
originals instead of trans-illuminated ones. To be even more exact,
the transmitted or reflected light should be diffuse, because
collimated light will be scattered differently depending on graininess
and density level (=Callier effect).

This transmission is measured at a linear gamma level, and if you have
a e.g. 50% transmission, it will be represented by 127.5 (or rounded
to 128) on a scale from 0 to 255 (8-bits/channel), or 32767.5 (or
rounded to 32768) on a scale from 0 to 65535 (16-bits/channel).

If you want to relate to gamma adjusted data, you'd need to specify
whether slope adjusted gamma or regular. And then we also have to
consider if color-management is involved.
We also have to assume the scanner's sensor is linear over it's entire
range, which it almost certainly isn't, so it must be calibrated for
exact calculations.

Bart

 

[CSM1]

Can someone go over how the photographic density scale relates to the RGB
values for density. Reflective vs. Transmissive? Thx.

The short answer, they don't.

The RGB values are just what you scanner produces at the time. They are not
directly related to any set value. Just a range of values produced by
whatever setting you used to scan the image with.

However in a gray scale step chart with known steps, the value of the scan
should be relative to the next step in the chart.

As an example, if there are 5 steps in the step chart that are white to
black with equal density between each step. The scanner should produce 5
values from 0 (black) to 255 (white). 0, 42.5, 85, 127.5, 170, 212.5, 255.
When set to 8 bits, not 16 bits per channel.

Since these are black and white gray scale, each R, G, and B channel should
have the same values.

No scanner in the real world will actually do that.

 

[Alan Smithee]

The short answer, they don't.

The RGB values are just what you scanner produces at the time. They
are not directly related to any set value. Just a range of values
produced by whatever setting you used to scan the image with.

However in a gray scale step chart with known steps, the value of the
scan should be relative to the next step in the chart.

As an example, if there are 5 steps in the step chart that are white
to black with equal density between each step. The scanner should
produce 5 values from 0 (black) to 255 (white). 0, 42.5, 85, 127.5,
170, 212.5, 255. When set to 8 bits, not 16 bits per channel.

Since these are black and white gray scale, each R, G, and B channel
should have the same values.

No scanner in the real world will actually do that.

I thank both you and Bart for the info. It's helping me work this out. Thx.