Selecting photocells for detectors.

Choosing the right photocell can be an important part of the SMART calibration process. Cadmium sulfide photocells are recommended in that the maximum sensitivity to light is in the visible part of the spectra, near green, and thus they are the most appropriate for use in calibrating our projectors

I strongly recommend that you get a "good" photocell from the SMART Products Store as it takes much of the uncertainty out of the SMART process. If you want to buy your own, the following are some of my notes on my experience finding a "good" photocell.

The linearity of the photocell in response to varying light levels is an important criteria. When I first started using photocells, I bought a pack of Cadmium-Sulfide Photocells (5), 276-1657, from Radio Shack for $2.29. I quickly checked the sensitivity and linearity of two of them against my calibrated light meter and found that were both quite satisfactory for my purposes, although the "metal" one seemed slightly better in terms of linearity.

Before we go any further, let me state that perfect linearity is not that critical for use with SMART, since the main purpose is to achieve consistent grayscale tracking among the three colors while optimizing black levels and contrast ratios.

Linearity is important however when you want to start comparing contrast ratios between projectors or when verifying or significantly modifying the projectors gamma tracking. Fortunately, virtually all of the SONY10HTs I have seen have very good gamma tracking. This is no doubt due to SONY's use of the 3D gamma matrix in the factory calibration process.

With the 10HT one can change the various gain settings over a considerable range without adversely affecting the gamma tracking other than of course, the case when things are overdriven. Furthermore, if deviations from a smooth curve are encountered at some IRE values, they can be easily tweaked using the 3D gamma matrix.

Several things have recently led me to reinvestigate the photocell linearity issue.
First, I have seen several cases where the contrast ratio in a spreadsheet sent to me seemed really much too low, and in all cases selecting a different photocell cured this.

I have also found that some people would really like to start with a pre-calibrated detector as they believe that their factory low may not be a good starting point. For these reasons, I recently bought another 5 pack of photocells from Radio Shack and tested all five for linearity and suitability for SMART.

First of all, not all RS packages contain the same assortment, but hopefully the guidelines I am listing here will help in selecting a proper photocell. The package I bought contained 1 large, 1/2inch photocell in a metal case or can, 1 large 1/2inch photocell in a plastic case, and three smaller, photocells in a 1/4inch plastic case. The detector I have been using to date is in a 1/4inch metal case.

The measurements were made with the detector 91 inches from the projector, CC filter in place,, etc. and with some new colored filter material at the detector that I'll describe below. My reference photocell was measuring a green contrast ratio of 286 and showed very good gamma tracking on the White Levels chart.

The three small plastic photocells all gave contrast ratios well above 200:1 and showed good gamma tracking as well. The two larger photocells gave contrast ratios of 37 and 57 and poor gamma tracking. This was disappointing as the large metal one "looked better". Obviously, looks can be deceiving. I know of others who are using an RS photocell in a 1/2 inch metal can with good success so it would seem that looks are unreliable as well whe choosing photocells.

What I did notice, however, was that all of the good photocells gave resistance values at IRE 0 of several M Ohms or more, i.e. between 1.5 and 7 M Ohms. The two bad ones gave resistance values at IRE 0 between 125 to 155 K Ohms. This would therefore seem to be a fairly simple first test - good photocells have a resistance above 1 M Ohm at IRE 0.

Once you have done a run calibration run on low there are two other things to check. Gamma tracking should be good, and one should find the white and theoretical gamma=2.2 curve in good agreement in the White Levels chart. You should also find contrast ratios for low between say 90 to 150 or so. I you see contrast ratios of 10 or 30, chances are something is wrong, and most likely it is the photocell.

If you want to further test your detector's linearity there are a couple or other ways. The easiest way is to repeat the measurement series at a different distance. Ideally, you should get exactly the same traces and contrast ratio, but with the measured light level scaled by the reciprocal of the square of the distance of the detector from the meter

Another way of checking detector linearity is via use of ND filters. The point of this test is that an ND filter should attenuate the light by the same factor, e.g.2, at all IRE levels.