Here is a post I made on rec.photo.darkroom years ago on calibrating a CDS cell for a denisitometer. Though I am not sure of the absolute accuracy, it is repeatable, and was very useful in providing a measure of contrast and film speed. You may need to try out different CDS cells (with different resistance / light characteristics). With mine, the resistance was about 5 k-ohms when the enlarger is wide open and set to a height for approximately an 8X10 inch print.
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Using a CdS cell as a densitometer.
Required materials:
- CdS cell (about $2.00, even at Radio Shack inflated prices)
- Enlarger
- Digital multimeter, whose resistance measurement is accurate in the
megohm range.
Schematic:
- There really isn't one. Put the multimeter in resistance measurement mode
and connect the leads to the CdS cell.
Measurement considerations:
- Turn all room lights off;
- Be careful of enlarger reflections off clothes/walls etc.
- For low light levels, the CdS cell response is slow. Allow sufficient
time for the cell to stabilize.
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Calibration:
The intensity of light hitting a CdS cell is proportional to the log of the
resistance. In photography terms, doubling the intensity of light equates to
an increase in intensity of about 0.301 (or the log of 2). This is the
density scale on the Y-axis of densitometer curves.
How can the CdS be calibrated so that:
density = log( resistance ) * K
where K is a constant such that doubling the light will result in an
increase of 0.301.
In doing this, the CdS cell may be used as a densitometer.
Fortunately, if you need a densitometer, you likely have an enlarger which
can be used as an calibration instrument. Every f/stop provides an "intensity"
increase of 0.301, as it to doubles the light for opening one complete stop.
Likewise, every half-stop will increase the intensity by 0.5 * 0.301.
By placing the CdS cell under the enlarger, measuring the resitance at all
the f/stop openings and doing some math, one will be able to calibrate the
CdS cell.
(1) (2) (3) (4)
Log of
Enlarger Rel Log CdS Cell CdS Cell
Aperture Exposure Resistance Resistance
k-ohms
2.8 1.51 13.74 1.14
2.8/4 1.35 17.50 1.24
4 1.20 22.30 1.35
4/5.6 1.05 28.30 1.45
5.6 0.90 37.00 1.57
5.6/8 0.75 46.80 1.67
8 0.60 60.80 1.78
8/11 0.45 80.10 1.90
11 0.30 106.90 2.03
11/16 0.15 146.50 2.17
16 0.00 193.60 2.29
The data in column (2) is constant and should be treated as the Y-axix.
For the apertures shown, measurements should be made as in column (3).
These measurements will depend on the cell, enlarger light/housing and
magnification etc. The data in column (4) is simply the log of column (3).
Column (4) is the X axis.
With column (2) data and column (4) data, derive the slope using:
1. Linear regression;
2. -or- plot the data points on graph paper, draw the "best" line and
measure the slope (delta Y divided by delta X)
In both cases, change the sign to positive. Ignore the constant (y intercept)
as this will be eliminated when measuring film base plus fog.
From the data above, the following equation is generated.
Rel. Log Exposure = log(CdS Cell resistance) * 1.31
nb. the correllation coefficient should be very close to 1
(This data results in 1.00).
Thats is for calibration!
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To make a test film:
I meter on a white sheet of paper and photograph it with a paper
indicating the exposure (eg -4 stops)
Then, I take a photo of the sheets at
-6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8 stops
Though recently, I'm using:
-6, -5.7, -5.3, -5, -4.7, -4.3....-2, 0, 2, 2.3, 2.7, 3....6 stops,
as this gives a better toe representation.
Once the film is developed, I take a sample of a blank negative, for the film
base plus fog, set the enlarger to get about a 5 to 10 k-ohm resistance reading
and record the value.
Then, for each negative, record the resitance and exposure.
Apply the equation to all these readings and subtract the film base plus fog
reading. Now, you have a densitometer curve for you film!
Plot this.
From here, I follow Ansel Adams (The Negative) recommendations for
determining the true film speed (0.1 density = zone 1) and the contrast (eg. N
).
Thats All!
If you try this, post the results. I wonder if all will have good results
like mine!
