Xref: utzoo rec.photo:1919 sci.electronics:2078
Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!sgi!donl
From: donl@foist
Newsgroups: rec.photo,sci.electronics
Subject: Re: Homebuilt transmission densitometer?
Summary: Color analyzers as densitometers
Message-ID: <10720@sgi.SGI.COM>
Date: 10 Feb 88 04:30:43 GMT
References: <2659@dcatla.UUCP> <11237@shemp.UCLA.EDU>
Sender: daemon@sgi.SGI.COM
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Organization: Silicon Graphics Inc, Mountain View, CA
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In article <11237@shemp.UCLA.EDU>, korfhage@CS.UCLA.EDU writes:
> Recently I was wondering about using my color meter as a transmission
> densitomiter. I have the low end Beseler model, and it uses a photomultiplier,
> seems quite sensitive and has a very small metering aperature.

I recently bought a used Cosar Mornick color analyzer to reduce testing in
color printing.  I was quite excited that it had a density scale on it as
well as the time and filtration scales, though i am not yet sure how accurate
it is.  The density scale is basically just the log of the time scale, and
color filtration numbers are (supposed to be) density x 100.  If the density
scale were not present, the log of the exposure time scale would do fine.

> The general idea, it seems to me, is to compare the unknown density with
> some known references until you find a match, or until you get close enough
> that you can make a good estimate.  The question is how do you get your
> reference? A calibrated Kodak step wedge runs about $81 at the local store,
> and I didn't care to spend this much money on a whim. An uncalibrated wedge is
> less (about $45, as I recall), but then you need to find a densitomiter you
> can use to calibrate it.  [...]

There are different sizes of Kodak step tablets; the #2 is about 3/4
inch wide, and about 5 inches long.  Mine is uncalibrated, and i think
it was about $23.  I've mounted one of these in a sheet of 4x5 film so
that i can project it when i want, which makes for quick paper-testing
exposures.

> If anybody sees anything wrong with this scheme, I would like to know.
> It would not be as accurate as a real densitomiter, but it would seem like
> an inexpensive way to figure out 0.1 above film base + fog, or whatever,
> with accuracy good enough for a hobbyist.

I think there is a tendency to be too worried about how accurate the
readings are in this sort of thing.  I suspect that shutter and
aperture variations, processing variations, metering problems,
differing amounts of flare, light falloff in the enlarger, etc. all add
up to a much larger variation than the inaccuracy of using an
inexpensive meter as a densitometer.  Reasonable tolerances suggested
by the master are .09 to .11 for film speed, which is 10%, which should
be pretty easy to meet.  If you can show that the meter you are using
doubles and halves pretty accuratly, the density readings you obtain
with it should be more than adequate.  I have found the uncalibrated
step tablet to be accurate enough for my needs; i have found steps that
are a little high or low using a rather inexpensive ($75) black-and-white
enlarging meter as densitometer, but the overall slope is plenty close.

- donl

{sun,pyramid,adobe,decwrl,ucbvax,allegra,research}!sgi!donl
donl@sgi.com