My $1.00 densitometer is working now and Liam Lawless shootout

[Thodoris Tzalavras]

You could also sandwich 2 step wedges together (if you have a spare one) as a calibration tool, which would give you the ability to plot out your meter's response to densities all the way up to 6.00…

[ic-racer]

Irrespective of one's sensor or detector for the densitometer, the light source must follow certain standards.

I was just reviewing some history of Hurter and Driffield and recalled that they used collimated light in their densitometer. This lead to error in the readings. Later, densitometer light sources were defined by ISO (ISO 5-2 Geometric conditions for transmission density), to avoid that error.

Hurter and Driffield's photometer employed direct beams of light between two lamps and a grease-spot photometer, the negative being inserted in one of the beams. Some of the light transmitted by the negative was scattered out of the direct path and never reached the grease-spot. Thus the instrument recorded density values which were too high. Sensitometry since Hurter and Driffield, S.O Rawling, Nature, 1943

[Alan Townsend]

CdS photoresistors are not linear, i.e. resistance does not vary as the inverse of illumination. They are linear as resistors, i.e. current versus voltage --within limits-- but that is a different story. Look at the table in this datasheet:
https://www.farnell.com/datasheets/12656.pdf
specifically the two columns giving the resistance at, resp., 1 and 100 foot-candles, and see how the ratio of resistances is generally less than 100. Even that non-linearity is not universal: some CdS cells (when they were more common) had even less resistance ratio per decade of illumination.
Attachment 241381
So, if you interpret the ratio of resistances looking through two regions of a film as a ratio of transmittance, i.e., a difference of optical densities after taking the log, you are under-estimating the change of density.

The suggestion of ic-racer to calibrate out the non-linearity using a calibrated step wedge is correct in principle, but as you pointed out: "I only care about densities between 3.00 and 4.5, which is pretty much beyond the range of the calibrated step wedges ".

I would side with the suggestion of Eric Woodbury: photodiode. You have convenient circuits like the MAX4206 that will take as input the current from the photodiode, and provide the properly scaled logarithm. Of course, there is still some homework to do to choose the photodiode: spectral sensitivity, dark current, collecting area, availability... A long time ago I made a densitometer for the enlarger baseboard, using a Se photocell and a discrete log conversion circuit; output to a large galvanometer, 0-2.5D range, switch for +1 offset.

Bernard,

Thanks for the response. If you read my original post, I stated that the conductance, not the resistance, is linear with respect to the illumination. CdS cells are very linear over a wide range of illumination. They are also incredibly easy to read out using a good quality VOM and then converting resistance to conductance.

I built a densitometer about 50 years ago using a photodetector system that I believe was from either Science and Mechanics, or maybe it was Popular Science. It consisted of three CdS sensors with cabling, an analog meter with custom scales, and an analog circular calculator used to convert the meter scales into useful values. It was very interesting to me at the time. It had a sensor for use as a normal reflected light meter, a sensor designed for use under the enlarger in the darkroom, and a much smaller diameter sensor intended for a densitometer. I think it cost around $50, but seemed to me it could do everything, so I ordered one.

As an exposure meter, it was very clunky, so I rarely used that. In the darkroom, however, it worked rather well. It included plans for building a densitometer, and the calculator included a scale for reading density directly. It was in my memory how well this worked, so I simply duplicated the idea using a few off the shelf parts, which was extremely easy to do. I have have a math minor along with my electrical engineering degree so the calculations and theory are very easy for me to deal with.

A PIN photodiode would give much better response in low light levels, especially low red-light levels, but since I'm an old school black and white guy, I don't care about spectral response. CdS cells are close to orthochromatic film response. At the light levels I'm working with in my darkroom, the CdS cells response is ideal. What I have now is very simple and works very well.

The circuitry to support a PIN photodiode would be quite complex in detail, requiring several variable gain stages to cover a wide range of illumination. The readout is also more complicated. Using calibrated step wedges is another layer of complexity that's not needed. I use a lux meter to verify the CdS cell response and linearity. Very cheap and simple.

I'm amazed at how complicated some people want to make things when they are fundamentally fairly simple. In film photography a 1/4 f-stop accuracy is almost overkill and easily met by an uncalibrated CdS cell used differentially. I am not the National Bureau of Standards, or the International Standards Organization.

[Bernard_L]

Alan,

I understand your desire for simplicity, and as end user the design decisions are yours. Just for the record:

If you read my original post, I stated that the conductance, not the resistance, is linear with respect to the illumination.

That I understand. And resistance being inversely proportional to conductance, my statement:

i.e. resistance does not vary as the inverse of illumination.

is equivalent to stating that conductance is not proportional to illumination. In the datasheet I quoted, it was more like Conductance=K*(Illumination)^0.8

You can do a simple experiment that does not involve a step wedge. In projection under your enlarger, or in a contact printing configuration, whatever suits your usual work, measure with your apparatus the density of one piece of film, say Da, somewhere of order 1.0...1.5. Then another, of similar density Db. Then measure both sandwiched together. You should obtain Da+Db. Any deviation is an indication of systematic or typical random errors in the process. That is not NBS science.

[ic-racer]

Perhaps my mistake, but the title indicates "densitometer" and I presumed it was such. Though when I re-read the first post, I now get the impression it is a baseboard exposure meter which would be another kettle of fish.

[jnantz]

wow, I haven't read Liam Lawless' name except on the unblinking eye and the post factory industrial newsletter. what a treat
thanks for the flashback!

[Alan Townsend]

Perhaps my mistake, but the title indicates "densitometer" and I presumed it was such. Though when I re-read the first post, I now get the impression it is a baseboard exposure meter which would be another kettle of fish.

ic, I use this both as a densitometer for large format using an additional light source very close to the sensor, and as an enlarging meter. I can read densities from 35mm negatives under the enlarger with no problems using an old kettle.