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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?  It should
> be pretty straightforward, unless there's something I missed.
> Since it's for monochrome, color sensitivity of the sensing element
> (photodiode?) shouldn't matter.

	Here are a few suggestions:

1.	You MUST use an incandescent white light source; for general purpose
	use, a single monochromatic light source such as an LED is unsuitable.
	The transmission spectrum of B&W film stock is not flat, and varies
	from manufacturer to manufacturer; consequently, the use of a single
	monochromatic light source is an invitation to trouble.

2.	In order to achieve sufficient signal-to-noise ratio in measuring
	higher densities (< 10% T, > 1.0 absorbance), LED's will not provide
	sufficient luminous intensity with a photodiode or photovoltaic
	detector, anyhow.  So, you really need an incandescent lamp.

3.	The incandescent lamp MUST be powered from DC (use a 6 or 12 volt
	lamp), and MUST be regulated using a solid-state constant-current
	regulator.  I cannot overemphasize the importance of having good
	regulation for the lamp supply.

4.	You may wish to consider using fiber optics to direct the light
	source to the sensing head.

5.	The calibration control for setting 100% T (0 absorbance) can
	either vary photodetector sensitivity or vary lamp current; some
	commercial instruments use the latter approach (I personally
	don't care for it, though).  If you use a microprocessor, no
	potentiometer is necessary - just a switch to tell the uP to
	take a reference reading.

6.	Calibration of a meter for % transmission is simple, because it is
	a linear relationship.  If you want an absorbance readout, you will
	require a logarithmic scale; there are clever analog circuits for
	this, or you can do the conversion in a microprocessor.  In this
	day and age, I recommend the latter.

7.	I recommend a photovoltaic detector, but a photodiode will work -
	provided that it has decent visible light response.  Forget about
	using a CdS photoresistive detector.  A photomultiplier tube is
	used on most commercial densitometers, but this is probably too
	complex and frought with potential problems for a DIY project.

8.	Use a Kodak or equivalent density wedge to verify operation of your
	completed densitometer.

9.	Beware of ambient light interference resulting from poor optics
	design.
	
> How about connecting the output to
> your PC (everybody has one, don't they)?

	Not a bad idea - as long as you have some type of analog input
board.  You could then do the absorbance calculation (if you want to deal
in absorbance units) on the PC.

> How about hearing from
> those of you with experience, or even ideas?

	This is the kind of stuff I have been designing for the past 18 years,
so you now have the benefit of some of what I've learned The Hard Way. :-)

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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