I think I'm going to try this tomorrow. I'm pretty sure I bought a couple of photo cells a while ago, but wasn't sure how to use them. I've got a decent multimeter, so it's worth a try. There's certainly a lot of potential there, and it's definitely cheap. Thanks!
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My Dollar Densitometer
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bernard_L
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Yes. That is shown on the plot in my post #5 above
No. See below.
Thank you; at least I'm not alone having... strong reservations.
First off, what is a linear measurement M of light L? A device that delivers
M = a*L
where a is a constant. In native units of L versus M, the plot is a straight line through the origin.
The plot of conductance --of a CdS photoresistor-- is a straight line in log-log coordinates. This means
log(C) = p*log(L) + q
where p and q are constants; this is equivalent to:
C = L^p * 10^q where "^" has the usual meaning "raised to the power"
Unless p happens to be exactly 1, that relation is not linear. The plot of C versus L (just C and L, not logarithms) is not a straight line. For 0<p<1, it is curving with concavity down, and for p>1, it is curving up. No amount of dividing one value by anther one will change this.
This!
If conductance C (of the CdS photoresistor) were a linear measure of light L, it should double when light is doubled, as in opening the enlarger's diaphragm by one stop. If it does not, your basic assumption "The conductance is linear on linear-linear scale" is falsified. No pesky logarithms involved.
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Chan Tran
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Assume the plot on your post #5 is absolutely correct one can devise a formula to calculate the density with the resistance reading but I have played with CdS cell a lot and I found their repeatability is very poor. I had a decent densitometer to compare (X-Rite 810). It's not even good enough for +/-0.20 density tolerance. Something like this would do much better as it already has log output.
Light Sensor 70000 lux - 1143_0 - Phidgets
Measures light intensities of up to 70 kilolux on a logarithmic scale and connects to an Analog Input or VINT Hub port.
www.phidgets.com
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Chan Tran
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Yes true even companies like X-Rite don't make the kind of densitometer used in the darkroom any more. Their spectrophotometer can be used as reflection densitometer but not as transmission densitometer.
Clip leads will work OK if both wires are near each other or twisted together. The meter is very high impedence on MOhm scale, so can be noisy. The resistance of a contact connection can be high enough to cause trouble also, so it's best to have soldered connections on both ends of the wires, but especially at the CdS cell end. I used a twin lead cable, and soldered on the CdS end, but have set screw connection on the meter end. This works OK, but sometimes I need to tighten the set screws a bit. Best would be a coaxial cable with soldered ends, but that would be more than a buck. Also, when the meter changes range to MOhm for KOhms, you will need to wait some time before the reading is stable. My meter is not wonderful reading high resistance, so I need to wait about 15 seconds for stable reading in MOhms.
We are measuring densities, not light intensities. CdS cells are plus or minus 10% devices at measuring light intensities. Densities are ratios of these intensity measurements, and as long as we make our two readings at almost the same time, these errors cancel out for the most part. Another point is for example, if one resistance is 10% high, the other 10% low when measuring a 3.00 density, the reading will be between 2.97 and 3.03, which is close enough to have value to a darkroom worker. That 0.03 error would be a 1% error in reading a 3.00 density.
I think you are over complicating things a bit. Sharktooth pointed out I was doing the same. CdS cells are not the most accurate light sensors in the world, but within limits, and being used for film photography, they are accurate enough to give very useful information.
This is false. Sensors can have a variety of gains, including negative. I just measured my CdS cell, and with a doubling of light the resistance goes down approx. 1/2.1 per fstop over five fstops with no ND filters in place. This about a halving of resistance for a doubling of light, which shows the inverse relationship with resistance, and the almost doubling of conductance therefore. I have been claiming that measuring densities from 0.00 to 3.00 has about 1% error, meaning is "linear" within some range.
A really high quality CdS cell would work much better than mine, but mine is good enough for what I'm doing. Most of the time, I use my densitometer at 0 to 2.00 range, where it is slightly more accurate.
As a photographer, I like CdS cells in my camera TTL meter and my hand held meter because they are accurate enough and very robust due to their non-electronic nature. Same in my darkroom Techno-nerds always want the latest technology. CdS cells are very slow in response and mostly green sensitive with very little blue sensitivity but they use extremely low power. Compare my old SRT101 Minolta to my later X700 and two X570's that both failed due to electronics. The early ttl meter cameras with mechanical everything else were extremely reliable. The later electronified models were extremely unreliable due to electronic failures. Without electronics, reliability goes way up. But with it, all kinds of other gadgets and gizmos can be thrown into the mix to confuse everybody about what they really need.
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bernard_L
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??
Here is some reality. Not theory, not argument. Measurements.
De-soldered temporarily the CdS cells from two enlarger light meters: a small Ilford "wand" and a larger Philips one. Put them on the baseboard, connected to an ohm-meter. EL-Nikkor 50/2.8. Stepped through the f-stop clicks, recorded resistance values. Light intensity is normalized to the smallest value (smallest aperture), so values are 1, 2, 4, 8... Compute conductance as inverse of resistance. Plot.
Below: blue line and dots: measurements. Red line: linear extrapolation from the values at low light.
Would you measure your negatives with such a sensor?
Note: I did this for the sake of accuracy of information on the forum. Now anybody can have his/her own opinion. I'm done with this topic.
Plot for the CdS cell from the Philips device.
Plot for the CdS cell from the Ilford device. I forgot to measure the last point.
The main thing that distorts CdS cell readings is temperature. The current going through the cell will cause it to heat up on higher light levels. That will give an error. The old meters using these cells, including the Science and Mechanics meter I showed, used a mercury cell, which gives 1.3 volts, and gives less heating than the DMM will, which likely uses a 5 volt reference. This will give almost twenty times the heating as 1.3 volts. If I watch my meter when the enlarger is turned on and full aperture, which is where I make all my measurements, the resistance will go up constantly at a slow rate. If I then put a dense negative on the sensor, the reading will go up to the higher readin quickly, but then gradually go down as it cools off with the lower current. This happens more quickly when used under an enlarged negative, since the sensor is open to air. So my normal technique was to turn everything on and read reference value first quickly before heating occurs. This is the sensor looking at the lens with no density. Then I place the dense material on the sensor and the autorange function kicks in and changes the read to the MOhm scale. I then need to wait about 15 seconds for cool down. A better way now, with the help of Sharktooth, is to read the low density first and to not wait very to do that, then remove the dense material and wait only about second before taking the reading. So I can put the resistances in the calculator directly then press LOG. This method gives most accuracy. The way you measured, those conductances relate to much higher current levels than I see, so you would need to quickly make the measure, than turn meter off for cool down, or use a small fan to help cool the sensor.
De-soldered temporarily the CdS cells from two enlarger light meters: a small Ilford "wand" and a larger Philips one. Put them on the baseboard, connected to an ohm-meter. EL-Nikkor 50/2.8. Stepped through the f-stop clicks, recorded resistance values. Light intensity is normalized to the smallest value (smallest aperture), so values are 1, 2, 4, 8... Compute conductance as inverse of resistance. Plot.
Maybe you could have shared the meter readings on those meters over the same range.
From you charts, your enlarger puts out a lot more light than mine, and the CdS cell is much larger and lower impedance than mine? Also, regression lines are normally drawn to minimize errors, not maximize them. I could easy draw lines through those points that show less error from linearity. Oh, I forgot this is reality and not theory. Yes I would, I have, and will continue to use such a sensor. Also more correct to have increasing values on X-axis, not reducing values.
Looks to me like someone trying to prove a point using bad science. The second largest source of error for CdS cells is the so called memory effect. Both the very large heating effect and the large memory effect are the same or very similar at a given time, so tend to cancel out in a ratio. It is a bit of a kluge using the DMM more than the CdS cell, but with care it works well enough.
Thanks for the link. That appears to be a lux meter circuit for a phone. I just got my phone out, and used my lux meter app again to verify my densitometer. I have done this before, and it works to just above a 2.00 density. My enlarger baseboard is only 100 lux, so not enough.Assume the plot on your post #5 is absolutely correct one can devise a formula to calculate the density with the resistance reading but I have played with CdS cell a lot and I found their repeatability is very poor. I had a decent densitometer to compare (X-Rite 810). It's not even good enough for +/-0.20 density tolerance. Something like this would do much better as it already has log output.
Light Sensor 70000 lux - 1143_0 - Phidgets
Measures light intensities of up to 70 kilolux on a logarithmic scale and connects to an Analog Input or VINT Hub port.
A density I measured with my CdS densitometer was 2.20 using it, and it measures 2.19-2.32 with my phone app. The accuracy is proven again. We need to measure densities, not light intensities to determine accuracy for these. So the smartphone is even less than a buck, since I already had it and the app is free, but it can measure density ranges up to 2.2 with my basement clamp lamp I have. The readings do jump as I indicated, but would be better with a brighter desk lamp.
I just measured again with a much brighter desk lamp and got 2.08 with very stable readings. That's about a 5% difference.
Chan Tran
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What is your formula to convert your resistance reading into density?
I found a cds cell I bought at an electronics store a year ago for a few bucks. It was in a little plastic baggie, with a sticker that said 5K to 500K, and 100V. I clipped the two wire leads to my multimeter for some seat-o-the-pants testing. Here's what I found.
The resistance values changed with differing light levels, so you don't need any special electronic circuit to make this useful. I next covered the sensor with pieces of film of known density values, and measured the resistance. I took the log of the resistance ratio, and compared that to the density difference of the two pieces of film. The result was nowhere close, which implies that the response is non-linear. That was disappointing, but it just means that it's not going to be as easy as pie.
I could plot out a characteristic curve for the sensor so I could then use that to determine light values from resistance measurements. I'd need an accurate light meter to do that, which I have, but I could just use that to do the measurements and not need the cds cell.
It's still neat that you can get varying resistance values with varying light levels using a cheap sensor and a multimeter. There's lots of potential fun there, but I can't think of a good use for it at the moment.
The resistance values changed with differing light levels, so you don't need any special electronic circuit to make this useful. I next covered the sensor with pieces of film of known density values, and measured the resistance. I took the log of the resistance ratio, and compared that to the density difference of the two pieces of film. The result was nowhere close, which implies that the response is non-linear. That was disappointing, but it just means that it's not going to be as easy as pie.
I could plot out a characteristic curve for the sensor so I could then use that to determine light values from resistance measurements. I'd need an accurate light meter to do that, which I have, but I could just use that to do the measurements and not need the cds cell.
It's still neat that you can get varying resistance values with varying light levels using a cheap sensor and a multimeter. There's lots of potential fun there, but I can't think of a good use for it at the moment.
In fact, you do. Part of the trick (insofar it works) is in the fact that @Alan Townsend's "$1" meter relies on a >$150 DMM since anything more modestly priced will generally not perform sufficiently well in high-impedance applications. Hence, part of the non-linearity you're running into is likely caused by meter itself.
As I alluded to in my earlier post, there are also challenges in managing the actual light path. With low-density samples, these effects drop away sufficiently in some cases, but as you try to measure higher densities, things get tricky. This is in addition to more fundamental issues mentioned earlier.
Like I said before, the principle (sort of) works, but the question is how well it works, and what conditions need to be met to make it work sufficiently well. Once those are figured out, you've re-invented the densitometer...
It reminds me of some old light meters. CdS cells were a viable solution; they effectively pushed selenium cell designs out of the market, for which it is now nearly impossible to find new replacement parts. CdS cells are practical and easy to integrate with a simple analog galvanometer (which was far cheaper than a digital multimeter at the time), enabling the earliest TTL-metering SLRs. While CdS is slow and its metering range isn't particularly wide, film speeds back then were limited, and the pace of shooting wasn't fast enough for the meter's lag to be a major issue. There were many ingenious designs—for instance, galvanometers specifically engineered with a non-linear response to match the sensor's output. Just three key elements, battary, CdS cell, g.alvanometer, maybe some resisters, from light measurement to read-out. CdS meters from the 1960s, such as those found on the Pentax SP, can still remain reliable.
Later, SPDs (Silicon Photodiodes) offered superior performance in terms of speed and low-light sensitivity, but their output signal is too weak to drive a needle directly. They require amplification circuitry, and that’s where things start getting complicated. On one hand, microelectronics enabled more complex circuitry; on the other, the rise of auto-exposure cameras, which required faster metering and denser matrices, led to the universal adoption of SPDs. However, repairing these systems has become quite a headache. In some cases, building and calibrating an SPD-based light meter with modern microcontroller and AD/DA converters is actually easier than trying to repair the original circuitry.
Later, SPDs (Silicon Photodiodes) offered superior performance in terms of speed and low-light sensitivity, but their output signal is too weak to drive a needle directly. They require amplification circuitry, and that’s where things start getting complicated. On one hand, microelectronics enabled more complex circuitry; on the other, the rise of auto-exposure cameras, which required faster metering and denser matrices, led to the universal adoption of SPDs. However, repairing these systems has become quite a headache. In some cases, building and calibrating an SPD-based light meter with modern microcontroller and AD/DA converters is actually easier than trying to repair the original circuitry.
Density=log(r1/r2) where r1>r2. For phone lux meter Density=log(Lux1/Lux1) where Lux1 is reading the lamp without the density, Lux2 is reading through the density with same lamp.
Generally, add a microcontroller like the Arduino Pro Mini (ATmega328P)—using either its built-in 10-bit ADC or an external one for higher precision such as 24 bit—combined with a simple circuit can read the current through a CdS cell. Although the response is non-linear, you can implement arbitrary responces curves and sensor-specific calibration parameters in the code, resulting in quite accurate measurements.
Once you have this DIY light meter, the modern microcontroller is usually powerful enough to drive a display for shutter/aperture combinations or even integrate a laser rangefinder module——which is very useful to some old camera. For higher requirements, you may calibrate for temperature, wavelength, and battery voltage; however, some times film is forgiving enough that such calibration is not so necessary.
That's a problem if you want a meaningful density or illumination range.
24 bit would be more feasible on paper, but getting 24 usable bits of resolution is another matter. 16 bits is challenging enough already, and that's still a limited range unless you do the log stuff in analog circuitry. But that opens up a whole new can of worms...
About voltage, if you don't mind a slight increase in cost, you can easily provide a highly stable 1.25V supply at a few milliamperes using a precision LDO or voltage reference (such as the LT1004, ). And then test it with a multimeter’s current range—or, use an analog galvanometer, calibrate and draw a custom scale. The end result is at least as good as old meters. This is also an effective way to replace mercury batteries with lithium button cells or USB cable.
That's a pretty low impedance and range CdS cell. The heating by current issue will happen, so more care needed. With meter off, put density on cell, then turn meter on and read withing a second or two. Then remove the density and read again without delay. Turn meter off and leave it off unless making a reading. The meter current will cause the sensor to heat up slightly and increase the resistance.
You should do it the other way to reduce heating effects. All room light must be turned off. Use only the meter internal lamp to illuminate meter scale if it has one. Have the mater turned off for some period of time first so there is no meter current in the cell to allow cool down. Put density on cell, turn meter on and read that immediately, remove densiity and read meter again with no delay.
I took the log of the resistance ratio, and compared that to the density difference of the two pieces of film. The result was nowhere close, which implies that the response is non-linear.
Or that there was some experimental error. What were the densities, readings and calculations so that we know what "not even close" means. Why not read those densities with your phone ap and let us know what they are. A CdS cell with such a low range of impedances is a bit unusual. My cheapo reads from about 100ohms to 10,000,000 ohms, and I paid $5 for a bag of 20. I picked the one with the highest dark state resistance to reduce the heating effects of the meter current, which is a source of error in reading the cells with a DMM.
That was disappointing, but it just means that it's not going to be as easy as pie.
Ah, I get your point—but for a project with just a few dollars, can we really ask for more? I think the goal is simply to match the performance of those old analog light meters.
From a performance perspective, a logarithmic response can be easily achieved using Programmable Gain Amplifiers (PGAs) or PGA-ADCs controlled by a microcontroller. Dedicated logarithmic amplifiers also exist. But if goes this far, use an SPD would be better in performance.
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I think the idea behind this thread is not so much to mimic a light meter, but a densitometer. The challenge here is the large dynamic range required which typically needs to cover a logD 4.0 range (although arguably logD 3.0 would be fine for all intents and purposes).
That helps a little, but the gain range is in reality quite limited in the light of what you typically need/want in a densitometer. I think you pretty quickly end up realizing you need to fashion some kind of custom PGA that offers 3 or so major steps; instead of 1-8x as typically offered by the PGA inside a ADC, something like 1x-64x-1024x or so. That's not necessarily very complicated and ultra low noise isn't even required as long as linearity is OK. But it brings us very quickly very far away from "CdS cell plus multimeter makes $1 densitometer" idea.
Also, while the PGA-based concept works OK on paper, things tend to get a little tricky if you start implementing it and you want it to be consistent. How to deal with the signal ranges where the gain settings overlap? I.e. how to avoid nasty 'jumps' in response as you switch between gain settings?
Like so often, the concept is easy - the tricky bit is in getting it to work elegantly.
Yeah, that would be my thought as well.
Chan Tran
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I must say a DMM with autoranging is quite capable of measuring a wide range.
I see your point, but it might be more manageable in practice than it seems.
Old-day solutions, I’ve seen old cameras use simple 'analog optical' tricks—like using multiple CdS cells with different attenuators for variable gain, or splitting a Selenium cell into large and small zones to handle different light levels. They don't even use potentiometers or resistors.
For normal design, the build-in PGA works in tandem with the ADC resolution. With the ADS1112's 16-bit base plus 4-bit PGA, we get a theoretical 20-bit range (approx. D 6.0). Even accounting for noise and false bits, reaching D 4.0 is not impossible. This is based on a automated method, but manual range selection is also possible and more simple.
And, if we really need more headroom, there’s no need for custom circuit now. Off-the-shelf chips like the AD8310 log amp offer 95dB (approx.D 9.5) of range via internal six-stage gain. That’s likely far more than here requires. The issue you mentioned was resolved by ADI engineers, they tested it and reported the error range.
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