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From: beyer@houxs.UUCP (J.BEYER)
Newsgroups: rec.photo,sci.electronics
Subject: Re: Homebuilt transmission densitometer?
Message-ID: <671@houxs.UUCP>
Date: 15 Feb 88 13:51:21 GMT
References: <663@houxs.UUCP> <10862@sgi.SGI.COM>
Distribution: na
Organization: AT&T Bell Laboratories, Holmdel
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Summary: Specular, diffuse, and doubly diffuse density

The trouble with makeing a transmissioni densitiometer at home involves
the Callier effect. If collimated light is passed through a diffusing
medium, such as exposed and developed film, it is scattered.
If a densitometer is made by passing collimated light through the
film and the sensor is 'far' from the film, only the unscattered
light is measured. This is measuring specular density.
If collimated light is passed through the film and all the transmitted
light is gathered by the detector (by using an integrating sphere or
suitable opal glass arrangement) the setup measures diffuse density.
[it is equivalent to pass diffuse light through the film and use a detector
'far' from the  film.]
If diffuse light is incident on the film and a detector measures all the
light transmitted through the film, doubly-diffuse density is being measured.

These readings will all be different, especially in the higher density
areas. Now in most books and articles, diffuse density is what is referred
to. If your densitometer measures something else, you can get confused. If
you report something else, you may confuse others.
What density should you use? If making contact prints, you probably want
diffuse density. If enlarging with a diffusion enlarger, you probably want
diffuse density. If enlarging with a condenser enlarger, you could probably
use a specular density measurement, but that raises the issue of how specular.

The ANSI standard does define 2 kinds of specular density but these seem
to be related to movie and slide projection. It varies depending on the
aperture of the optical system.