densitometer (with tip for homebrewed densitometer)

If you have an enlarger, you only need a photo cell (about US$ 1) and a digital electronic tester (US$ 10-20).

1. Mount the cell at the bottom of a opaque black 35mm canister (not the cassette; the plastic that the cassete comes in); make two smallish holes at opposite sides of the canister walls for the wires to the cell, and a bit larger hole in the top of the canister to let light in.
2. Connect the wires leading out of the canister to the tester, and put it into resistance measuring mode.
3. Turn on the enlarger with a lens, but no negative in the carrier , note the value displayed in the tester. Let's call it 'nothing'.
4. Put a blank negative in the carrier, and note the value displayed now. It'll be bigger. Call it 'fb-f'
5. Put the negative which you want to measure its density in the carrier, and note the resistance value. Call it 'neg_density'.
6. Now, the fun part. Turn on a calculator (or a logarithm table, if you are so inclined - when I was in highschool I had to learn it the hard way :), and take the logarithm of the values collected.

   To know fb-f density, do log (fb-f) - log(nothing).

   To know your selected negative frame density, do log (neg_density) - log (nothing).

I've checked the method against a Stouffer calibrated transmission tablet. It works.

In fact, it'll probably be better than a densitometer, as the enlarging conditions where you'll make your print will be automatically considered. Commercial densitometers take a middle road between specular and diffused density.

What kind of photoresistor are you talking about? If CdS, usual ones' intensity-resistance relations are not very linear to use as you described...

"In fact, it'll probably be better than a densitometer, as the enlarging conditions where you'll make your print will be automatically considered."

Better for what? Without a well defined illumination and spectral response, highly linear circuits (which are not trivial) AND calibration against a known and well-defined standard whatever values one gets are not more than personal. Its a bit like a homebrew thermometer without calibrations and scales. Even if one hand calibrates the measures against transmission references you still don't know the response to other negatives and other conditions. For this one needs to have a well-defined illumination and filtering. For doing some basic film testing and B&W quality control it might be sufficient but there is more to sensiometry.

" Commercial densitometers take a middle road between specular and diffused density."

Its ISO (and national ANSI, DIN etc.) Standards.

Even among "commercial densitometers" there are many different types and responses (Status types). The most common transmission types for our uses is Status-A (for transparencies), Status-M (for colour masked negatives), Visual, Ortho and UV. For B&W one probably wants to use "Visual" (not Ortho!) but for contact printing using UV sensitive papers such as platinotypes you, by logical contrast, want "UV".

I use a CDs cell. Regarding linearity, I assume you are talking about the log(value) graph, and not the actual resistance values, which are exponential.

To check it, I took readings of the light coming out of the enlarger lens at f-stops 2.8, 4, 5.6, 8, 11,16 and 22. Each f-stop is, of course, a reduction in half of the light reaching the cell. I've attached a gif of the resulting graph. It's sufficiently linear for me.

With regards to the differences to commercial densitometers, I'm afraid I wasn't clear enough.

This in no way substitutes a commercial one, but I think that for this narrowly defined application domain, is more than enough.

I do use it for checking negative densities, which will be printed by projection in the same enlarger as the check is conducted. No contact printing, no AZO, no transparencies, no color negatives, just plain old enlarged 135 and 120 B&W.

As you are aware, projection densities are different from contact densities. A commercial densitometer does not take in account that, or the light source characteristics of the enlarger.

With regards to ISO standards, remember that a film speed test is all about differences between densities (eg. 0.10 above fb-f, 1.30 above fb-f, etc) and not absolute values. I'm not submitting film speeds to ISO, but defining a personal EI. I really don't care about the exact density value of a negative, but I care very much about its difference against a clear base negative.

And yes, I'm aware of the complexities of linear circuits. I come, (as if it's not evident) from an engineering background.

If you go to feeBay you can find lots of great color analyzers for around $25. Use there photocells and meters for the described precedure. Beats having to micky mouse something together.

Introduce yourself to a local printer. A good printer prepress department will always have a densitometer in service.

Santiago

A brilliant homebrew idea. Just wanted to check the formula for negative density. I thought it would involve subtracting the film base fog reading (not log nothing)from the negative density reasding.

But I was never any good at maths. Let me know.

Thanks Arthur