Don't get all excited now - "annular" isn't the opposite of "oral".
There has been interest expressed in the transmissive chromatic neutrality of the central portion of the Color Parrot (at the overlay "spot") compared to that of the annular region around the spot.
Why might this matter? Well, because it is likely that the light from the central portion of the diffuser plays a prominent role at the center of the sensor, where (in Canon EOS cameras, at least) the measurement of chromaticity for white balance reference purposes is made.
(Some seem to think that such a situation might also mean that the chromatic response of the diffuser varies with angle off axis. This is less likely, in my opinion.)
We recently made some simple tests to look into this. A Canon EOS 20D was used. Basically, after setting the white balance of the camera to suit the ambient light at a certain location (using CWB with a reference frame taken from a WhiBal gray card at that location), we then took (with the camera at that same location, exposed to that same ambient light) three frames with the Color Parrot in place, as follows:
1. With the entire Color Parrot face uncovered.
2. With all of the face outside the overlay spot covered.
3. With the overlay spot covered.
We then measured the chromaticity averaged over a small area in the center of the sensor frame (essentially emulating what the camera would do if regarding that frame for custom white balance purposes).
We then plotted those the chromaticities on the CIE u'v' plane, arbitrarily putting the chromaticity of the "full face" shot at the origin. Thus, we only look at the relative chromaticities for the three situations. (This simplified the test procedure, since we did not have to ascertain chromaticity on an absolute basis, or by reference to a "credible" reference device.
We see that chart here:
These results are very interesting. They suggest the following:
1. The basic "stack" of the Color Parrot has a substantial departure from neutrality.
2. The overlay disk beings the central portion closer to neutrality.
3. Evidently (for the particular lens situation tested), the light on the center of the sensor comes preponderantly from the central region of the diffuser (not too surprising). And a good thing that is - it makes the actual chromatic neutrality performance of the Color Parrot close to the rather good performance of the central region.
Again, note that, owing to the reading of the chromaticities on an integer RGB basis, the process is susceptible to an uncertainty on the order of 0.001 u'v' unit along either axis.
Note that we are still bereft of any explanation of the supposed role of the central overlay, but it does seem to provide a nice neutrality correction where it does the most good.
For reference, the lens situation for these tests (with the diffuser in place) was as follows:
- EF 24-105 mm f/4.0 IS at 24 mm
- f/4
- Focus at infinity
We recognize that a different result might well be attained for different combinations of lens parameters.
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