I use a homemade densitometer that cost only a dollar. Here's how;
This densitometer is shown in my darkroom on the baseboard of my 35mm enlarger. It consists of a 3mm CdS cell, a good quality digital multimeter set to ohms, and two wires soldered to the CdS cell on one end, and jacks that plug into the DMM on the other. I bought a bag 20 small 10 Mohm CdS cells on Amazon for $5. I built a mounting board using some thin mdf that includes a strain relief, and used jacks I had already. I did not include the DMM in the cost, since everybody already has one. For highest accuracy, a good quality DMM is required.
It's not well known, but the lowly CdS cell is a very linear converter of light to current, It's completely passive, robust, and accurate provided a good quality DMM is used to measure the resistance of the cell. Then, we need to convert the resistance to conductance and make a few calculations to get us to density, but this is all very simple if you don't mind some number crunching. The enlarger lamp is a convenient way to measure density of materials placed directly on the CdS unit, or of negatives projected there from a negative. I have the enlarger all the way up to where it would make a 3 x 4 1/2 print, and measure my negative densities there. The sensor is about the right size for sheet film directly or 35mm at this size.
To measure a density, we turn on the enlarger, then measure the resistance without the material , and write that down A. Then we place our material to measure on the sensor and measure the resistance, and write that down B. This sensor is a 10M ohm one, so to calculate a conductance in arbitrary units, we calculate 10,000,000/resistance in ohms AR and write that down. Then we calculate the same for B in the same way and write that down BR. The amount of light measured is proportional to these conductances, so we are making a ratio AR/BR. This is the ratio of the amount of light measured. With ratios, the units cancel, so don't. With film, the ratio of two light transmissions is always the same their densities. The common LOG of this ratio is the density. It took a while to explain, but is easy to do. Put 10000000 into the calculator memory, then divide this by the two resistances, find the ratio, and take the LOG. This also works for finding density ratios, only we measure two points on a negative to compare them. If we pick the brightest and then the dimmest point, we have the density ratio of that negative.
The reason I used 10,000,000 is because that makes the calculation small whole numbers, and much easier to do. My meter is an old Radio Shack model that has an RS232 interface for reading on the PC directly. I used to have a simple program for doing these calculations on DOS, but it doesn't work on Windows without changes. When I was in high school, I bought a Science and Mechanics light meter that came with several probes, a small meter with a needle, and a computation circular calculator. They included directions on making a densitometer/sensitomer so I did that. It turns out the probes were CdS cells, the meter was a conductance meter, and the calculator converted everything to convenient units, like density for exampe, EV's and camera setting.
My Minolta SRT101 and my Gossen Luna Pro meter both use the same sensors in the same way, although they also have bridge circuits that adjust logarithmic current meters to work properly. These passive devices have no electronics to fail. The current meters are moving coil type, so can fail mechanically. DMM's also have bridge circuits for each range that were adjusted at the factory. My dollar densitometer is a little better than 1% accurate over a range of 10,000 to 1, which is a density of 4.00. Needs no calibration since we are using ratios. I really enjoy using it even it is a pain sometimes when in a hurry.
Comments and suggestions are welcome.
This densitometer is shown in my darkroom on the baseboard of my 35mm enlarger. It consists of a 3mm CdS cell, a good quality digital multimeter set to ohms, and two wires soldered to the CdS cell on one end, and jacks that plug into the DMM on the other. I bought a bag 20 small 10 Mohm CdS cells on Amazon for $5. I built a mounting board using some thin mdf that includes a strain relief, and used jacks I had already. I did not include the DMM in the cost, since everybody already has one. For highest accuracy, a good quality DMM is required.
It's not well known, but the lowly CdS cell is a very linear converter of light to current, It's completely passive, robust, and accurate provided a good quality DMM is used to measure the resistance of the cell. Then, we need to convert the resistance to conductance and make a few calculations to get us to density, but this is all very simple if you don't mind some number crunching. The enlarger lamp is a convenient way to measure density of materials placed directly on the CdS unit, or of negatives projected there from a negative. I have the enlarger all the way up to where it would make a 3 x 4 1/2 print, and measure my negative densities there. The sensor is about the right size for sheet film directly or 35mm at this size.
To measure a density, we turn on the enlarger, then measure the resistance without the material , and write that down A. Then we place our material to measure on the sensor and measure the resistance, and write that down B. This sensor is a 10M ohm one, so to calculate a conductance in arbitrary units, we calculate 10,000,000/resistance in ohms AR and write that down. Then we calculate the same for B in the same way and write that down BR. The amount of light measured is proportional to these conductances, so we are making a ratio AR/BR. This is the ratio of the amount of light measured. With ratios, the units cancel, so don't. With film, the ratio of two light transmissions is always the same their densities. The common LOG of this ratio is the density. It took a while to explain, but is easy to do. Put 10000000 into the calculator memory, then divide this by the two resistances, find the ratio, and take the LOG. This also works for finding density ratios, only we measure two points on a negative to compare them. If we pick the brightest and then the dimmest point, we have the density ratio of that negative.
The reason I used 10,000,000 is because that makes the calculation small whole numbers, and much easier to do. My meter is an old Radio Shack model that has an RS232 interface for reading on the PC directly. I used to have a simple program for doing these calculations on DOS, but it doesn't work on Windows without changes. When I was in high school, I bought a Science and Mechanics light meter that came with several probes, a small meter with a needle, and a computation circular calculator. They included directions on making a densitometer/sensitomer so I did that. It turns out the probes were CdS cells, the meter was a conductance meter, and the calculator converted everything to convenient units, like density for exampe, EV's and camera setting.
My Minolta SRT101 and my Gossen Luna Pro meter both use the same sensors in the same way, although they also have bridge circuits that adjust logarithmic current meters to work properly. These passive devices have no electronics to fail. The current meters are moving coil type, so can fail mechanically. DMM's also have bridge circuits for each range that were adjusted at the factory. My dollar densitometer is a little better than 1% accurate over a range of 10,000 to 1, which is a density of 4.00. Needs no calibration since we are using ratios. I really enjoy using it even it is a pain sometimes when in a hurry.
Comments and suggestions are welcome.
