My $1.00 densitometer is working now and Liam Lawless shootout

[Alan Townsend]

Here's a digital picture of my new densitometer that I just built cost about a buck.

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I'm using this for using the zone system in my darkroom. I just made a H&D plot of Regent Royal ortho-litho film developed in D-23 1:3 for six minutes. It goes all the way up to a density of 4.5. This requires a range of about 60,000 to one for the measurement range, not all that easy to do. But my meter works for this now.

My densitometer/ darkroom meter is a 5mm diameter cadmium sulfide photo resistive cell that I bought on Amazon in a 30 for $5.99 bag, or 20 cents each. Add four #10 machine screws, nuts, and washers and three pieces of 4x4 inch 1/8 in. thick mdf for a total of just under a buck. This is read out in ohms on the DMM I already owned, which has a hold button, almost a necessity in the darkroom.

For use, I need to calculate the conductivity of the cells by finding the inverse of the resistance, then multiplying by a large number to give arbitrary units for fining ratios. I do this by storing 10,000,000 in calculator memory, then dividing the memory read by the resistance in ohms. This gives reading ranging from 1 to a few hundred thousand since my meter reads a maximum of 10 megohms.

Liam Lawless wrote articles on making enlarged negatives using ortho litho film reversal processing in a peculiar way that includes an extremely healthy flash exposure about 13 years ago. His article does not give a theoretical description of the process, but rather a trial-and-error method that's pretty complicated. This has been discussed in this forum in the past. I assume digital negatives killed any interest in this.

I will be using this process to enlarge my 35mm and 4x5 negatives for alternate process printing. By understanding how this process works, and metering the original negatives, I will be able to produce enlarged negatives of almost any density range from negatives with variable ranges and get a good result on this first try. This is what I call zone system in the darkroom.

Ortho-litho films have short toes, followed by a short and steep linear area, which starts rounding down, followed by an enormous, rounded shoulder. At the top of the density range, the response is pretty linear and low contrast. This is why this process works. A density curve is needed to calculate both the negative exposure time and the flashing time. At the top of the scale, we find the exposure that gives maximum density and then the exposure that gives our desired density range subtracted from that maximum density. That will be close to the negative exposure time. We then push these up the graph scale with a flash exposure calculated to give the maximum density with the negative exposure time.

I will also use my Devtek processing drums, which is much simpler than the seven or eight trays you would need otherwise. I will need to build a light saber to stick down into the tube for re-exposure.

Comments and suggestions welcome.

Alan Townsend

[Graham Patterson]

Do you have to wait for the signal to stabilize? Historically, CdS cells have had slow responses to light change. Any delay would probably not be an issue in this application, though. Just curious.

[Alan Townsend]

Do you have to wait for the signal to stabilize? Historically, CdS cells have had slow responses to light change. Any delay would probably not be an issue in this application, though. Just curious.

Graham,

Yes, there is a delay. Since any light in the darkroom will affect the reading, all lights other than the enlarger need to be turned off. The hold button can be pushed in the cark, and then read with a weak light. This takes a few seconds anyway, so not an issue. I would say it takes about two seconds for the reading to stabilize.

I'm building some light meters for my view cameras as well. Simple boxes with input windows using same angles as lenses give. Some with black walls, some with white walls for averaging vs center weighted. Mounted to a camera, a light meter reading will be stable. Using a handheld sensor would be difficult due to this time delay.

A one-degree spot meter could be made using about an 8-inch-long piece of pvc pipe with a cover drilled 1/4-inch diameter. This would need a sight, like a pistol sight, for pointing and fastened to either a tripod or a camera with a small ball headed adapter so the aiming is constant for a few seconds. I plan on building one of these also later. The CdS cell color response is similar to orthochromatic film, so would work especially well for that. A handheld spot meter would not work well for that reason.

[Eric Woodbury]

Alan, this is very clever. congrats.

If you'd like a linear response, use a photo diode in series with a grounded, high value resistor. One meg-ohm would be a good starting point. Reverse bias the diode/resistor with 5V or 9V. Whatever is handy. Measure the voltage across the grounded resistor (thus the diode current). Photo diode current response is quite linear. The small change in voltage across the diode has only a small change in linearity that will be insignificant to your measurements.

[Alan Townsend]

Alan, this is very clever. congrats.

If you'd like a linear response, use a photo diode in series with a grounded, high value resistor. One meg-ohm would be a good starting point. Reverse bias the diode/resistor with 5V or 9V. Whatever is handy. Measure the voltage across the grounded resistor (thus the diode current). Photo diode current response is quite linear. The small change in voltage across the diode has only a small change in linearity that will be insignificant to your measurements.

Eric,

Thanks for the response. Yes, photodiodes are more accurate, particularly PIN photodiodes. To read out over a 30000 to one range requires at least an op amp or two, a circuit and power supply, and still the DMM to read a voltage. They are much more sensitive as well, particularly red sensitive. Noise in the amplifier circuit limits accuracy at low light levels.

On the other hand, the lowly CdS cell is also linear over this same range with around 5%-10% variance and requires no additional circuitry. This is good enough for film photography users, while labs and bureaus of standards likely would want something better. With a bag of 30 to choose from, I naturally chose the best one. :)

[Tin Can]

Very good!

I always do Sunny 16 before I meter

Good brain exercise!

I shot Pentax H1 never a meter over 30 years

Slides and Prints most well exposed and handheld

Magic KODAK had a printed Sunny 16 in every box of film

They should bring that back

I have the slides

[ic-racer]

Read each step of your calibrated step wedge and record the resistance. You can graph this to get the intermediate values.

[Alan Townsend]

Read each step of your calibrated step wedge and record the resistance. You can graph this to get the intermediate values.

Ice-racer,

To make accurate readings with a CdS cell, the measurements are differential. I read the sensor with no negative, and then with a negative, then take the log of the ratio. That takes care of the temperature coefficient and the various memory effects that cause errors in CdS cell world. I use arbitrary units but could convert to lumens by using my lumen meter, but this complicates and is not very valuable most of the time.

In this case, I only care about densities between 3.00 and 4.5, which is pretty much beyond the range of the calibrated step wedges I've used for photography. My arbitrary units correspond to apertures and times on my specific enlarger, so it becomes much simpler that way.

Thanks for the respons,

Alan Townsend

[ic-racer]

Ok, so there is no way to 'zero' the meter. So you mathematically zero it for each reading.
Yes that is pretty dense!

[Bernard_L]

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.
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.

[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.
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. :)