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From: kar@ritcv.UUCP (Kenneth A. Reek)
Newsgroups: rec.photo,sci.electronics
Subject: Re: Homebuilt transmission densitometer?
Message-ID: <191@ritcv.UUCP>
Date: 10 Feb 88 17:32:10 GMT
References: <2659@dcatla.UUCP>
Reply-To: kar@ritcv.UUCP (Ken Reek)
Distribution: na
Organization: Rochester Institute of Technology, Rochester, NY
Lines: 35

In article <2659@dcatla.UUCP> ittfb@dcatla.UUCP (Thomas F. Blakely) writes:
>A handy tool around the Black & White darkroom these days is a
>transmission densitometer.  While relatively inexpensive ones are
>available (ca. $200) it might be an interesting project to build
>one.  Has anyone had any experience along these lines?  

	I have built one of these using a CDS cell from Radio Shack.  As was
pointed out, the spectral sensitivity of the cell (and the lamp, too, for
that matter) is of little interest to B&W workers.  The problem is to get
the answer in the right kind of numbers.

	I connect an ohmmeter to the cell and read its resistance in ohms;
this value is determined by the density of the portion of the negative I'm
reading.  Unfortunately, it is proportional to opacity rather than density,
so to use these values you need a step wedge of known densities with which
to "calibrate" your ohmmeter (e.g. knowing that 42,500 ohms corresponds to
 .65 density on the step wedge and 49,350 ohms is .8 density on the wedge
allows you to estimate pretty well the density that gives a reading of 46,000
ohms).

	I worked out my calibration in this way and got surprisingly good
results, verified by readings on a Macbeth densitomiter here at RIT.  Be
prepared to recalibrate the thing each time you use it, however, unless you
have a regulated power supply for the lamp, and have a lamp whose output
changes little during its lifetime.  The cell might be sensitive to
temperature, too, and God knows what else.

	To get a readout directly in density units, you need an amplifier
with a logarithmic characteristic to convert the resistance of the cell
(proportional to opacity) into density (log of opacity).  I've not yet
done that.
-- 
	Kenneth A. Reek
	Rochester Institute of Technology
	{allegra,seismo}!rochester!ritcv!kar