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Following along on Omarank's logic, if the C6 read diameter is actually 24mm in contact mode, that puts the virtual point ~131mm behind the face of the meter, and so putting the meter 5mm from the display would result in a read diameter of 25mm. If I extend that, then the read diameter at 63.5mm from meter face (where my frustum opening is) would be ~36mm (hence larger than my 30mm opening), and the read diameter at 136mm from the meter face would be ~49mm (safely inside my 60mm cylinder). So perhaps my next step will be to try various size openings inside the tube, to see if I can experimentally determine approximately what minimum size opening maintains the luminance reading, to see how it aligns with that theory.
 

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From the measurements I've made so far, it appears that the C6 HDR2000 read diameter is somewhere 36-42mm at 52mm from the meter face, and the i1Pro 3 read diameter is ~15mm at 48mm from the tip of the aperture. I don't know how to come up with FWHM/TLT math that squares with these measurements, however.
 

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Using a tube that's 136mm long and 60mm inner diameter, I measure that the read diameter of the C6 HDR2000 must be slightly larger than 60mm at that distance. Combined with my previous measurement at 52mm, both measurements are consistent with using an angle that is close to the TLT that was published for the C6, rather than using the FWHM, at these short distances. Using the published TLT of 15.9, and a virtual point that is 85.93mm behind the face of the meter, then the read diameter is: 24mm at the meter face, 38.5mm at 52mm in front of the meter, and 62mm at 136mm in front of the meter.
 

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I made further measurements using the i1Pro 3, and I measure a read diameter of ~29-30mm at 130mm from the aperture tip . If I combine that with my previous i1Pro 3 measurement at 48mm, the conclusion I draw is that the sensor characteristics must have changed between the i1Pro and the i1Pro 3 (I can't speak to the i1Pro 2), resulting in a different angle. If I use an angle of 11.1 degrees, with a virtual point that is 23.2mm behind the aperture tip, then the read diameter is: 4.5mm at the tip, 13.84mm at 48mm in front of the tip, and 29.77mm at 130mm in front of the tip. Close enough to believe it's in the right ballpark, given the relative imprecision of my measurements.
 
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