Enrique, I had thought about coating the inside of the sphere, but thought that the first one, in raw polystyrene, would be a good reference. My first sensor port was a simple hole bored through the sphere wall - the high reflectance of the cut foam + lack of baffle gave inflated readings. Then a tube (painted flat black inside) was installed, which gave undervalued readings. That was removed & replaced with a tube painted flat white inside, with a 45° slant cut to shield the sensor ... which worked out the best so far. My reflectance is still below optimum, as the radius squared correction factor is 25, but the actual factor is 18. Which doesn't bother me, considering the cost of the Munsell coating
teflon seems to be a very good material for light reflectance
Don, PTFE, and probably a number of other plastics (like HDPE) would make a nice, optically 'soft' surface. I don't know where to purchase these in sphere form, but they would be rigid enough to need no external support box. Split in half & hinged like a commercial sphere, the 'innards' could be configured differently ... like putting the source light inside the sphere, shining directly at the baffle, with the photodetector location behind the baffle.
Your idea for machining reducing ports is great, and I have some scrap HDPE solid round that will work. Just need to make one for each diameter light, or figure out sizes so that they can nest inside one another.
LuxLuthor, thanks for those links! Even with reading pages of information on spheres, the first Labsphere article never popped up, even though I'd read the second one.
I'm wondering about the quality and applicability of the sensor being used with specific narrow wavelength LED (still reading) and how well this sensor reads/converts various LED ranges to CIE Photopic Spectrum
That has concerned me from Day One. General purpose (inexpensive) light meters are heavily biased to look at all light & read out the 'sunlight equivalent'. The spectral response of these meters is much more attuned to the incan portion of the spectrum vs the LED portion. If both incans & LEDs were tested side-by-side, the results would probably be unacceptable using an identical correction factor. But a correction factor for incans could easily be developed that would allow dual use.
I believe that the Edmund design would not be terribly expensive to build in the home shop ... the challenge is to find a CCD array (Charge Coupled Device, like a digital camera sensor) that reads only the 360nm to 1000nm spectrum. Either there are limited spectrum CCDs, or optical filtering is used to allow passage of only the desired wavelengths. Linear CCD Array Cameras, like the LARRY 2048 (Linear ARRaY 2048 element) are available, but couldn't find a cost.
I hope I'm not taking this too much off topic ...
I appreciate your input. If it wasn't for half-watt & MrGman, this design would have stalled early, and been of limited use. Now, with William building his versions, there will be a basis for comparison. Which is really neat