Using a scanner as a densitometer : LUSENET : B&W Photo - Film & Processing : One Thread

Does anyone have any experience using a film scanner and Photoshop to accurately read negative densities?

It seems to me that you might be able to use a Kodak reference grayscale strip as a reference...any thoughts??

Thanks...Craig Sterling

-- Craig Sterling (, October 04, 2001


I tested my filmscanner density range with a set of ND filter gels. Not with the idea of using it as a densitometer, but to get an idea of the output gamma and Dmax of the scanner itself.
It seems to me that you have to watch that you don't fall foul of the scanner's software.
The negative scanning mode on mine, for instance, automatically sets black and white points, according to the negative that's being scanned, and that makes a nonsense of any calibration.
The normal slide scanning mode is better, in that it's more consistent, but you still have to find the 'white point' of the scanner. This usually isn't 0.0D as you might expect, but somewhere in the region of 0.1 to 0.15D. This small density can be difficult to ascertain properly, but it's vital to know what baseline offset you've got.
You then have to take a few measurements of known density steps to find the gamma of the scanner, in order to work back from a pixel value to a real density.
I think most scanners default to the sRGB standard of gamma 2.2, but you'll have to check the scanner in question.
Then there's the question of long-term repeatability. My scanner shows a small, but noticeable temperature effect, both with lamp warm-up time, and with variations in ambient temperature. You'll probably need to check the calibration very frequently if you want accurate results.

Anyway, in brief: Yes, it's perfectly possible, as long as you know exactly what the scanner software is doing to the image before it lets you see it.

-- Pete Andrews (, October 05, 2001.

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