Xref: utzoo rec.photo:1980 sci.electronics:2188
Path: utzoo!utgpu!water!watmath!clyde!bellcore!decvax!decwrl!labrea!agate!pasteur!ames!ll-xn!husc6!m2c!ulowell!cg-atla!mikkel
From: mikkel@cg-atla.UUCP (Carl Mikkelsen)
Newsgroups: rec.photo,sci.electronics
Subject: Re: Homebuilt transmission densitometer?
Message-ID: <1851@cg-atla.UUCP>
Date: 17 Feb 88 15:28:54 GMT
References: <2659@dcatla.UUCP> <191@ritcv.UUCP> <10997@sgi.SGI.COM>
Reply-To: mikkel@cg-atla.UUCP (Carl Mikkelsen)
Distribution: na
Organization: Compugraphic Corp. Wilmington, MA
Lines: 48

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 an analogue enlarging meter/densitometer that I
use routinely for both B&W and colour enlarging, although
theoretically it should not work as well for colour.  I have a CDS
cell on a probe that measures the amount of light transmitted through
the film and delivered to the paper.  There is a "pot" to null out the
intensity of any particular portion of the negative (such as base,
point of full blackness, etc), so that by subsequently probing the
highlight areas, can read "stops of negative contrast" directly.

	Based on negative contrast, I pick a polycontrast filter (yes,
I am not a purist).  With the proper filter, I set the pot to a
calibrated position, and adjust the f-stop to null the meter reading.
At this point, I have a pretty good (often "near perfect") selection
of both contrast grade and exposure.  From this point on, I adjust for
special effects desired, dodge and burn, etc.

	Internally, the meter contains two (2) crude log-amps, a
differential amplifier, and a metering circuit.  The whole thing is
powered by two 9v alkaline cells (that have so far lasted for over two
years), that also drive two small lights to illuminate the meter.

	BTW, although probably obvious to you all, I did not
anticipate a need to turn off the safelights when making meter
readings.  I modified my enlarger timer to kill the safelights by
adding a DPST NO relay energized by the enlarger circuit switching the
safelight circuit.

	All parts came from Radio %$#@ or my junk box.  I have also a
Beseler Analite 400, and find it to be total trash compared to this
meter.

	May your good shooting come out fine in the soup!


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      | Carl Mikkelsen           |   ..!ism780c\                  |
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