Help with my D.I.Y. Sphere Calibration

[djozz]

@garrybunk

@djozz

For improving accuracy for this type of sphere, you can either . . . or you can measure the specific sphere reflectivity for each to be measured light source before doing the actual light measurement (using a constant output light source elsewhere in the sphere) and then adjust the multiplier accordingly.

In my sphere #4 with build-in reflectivity measuring light source, I do not adjust the multiplier, but I have build in a simple device that for each flashlight optically adjusts the reflectivity of the sphere to a constant value.

Is there a link showing how you’ve done this? And/or can you go into some detail on it? Could I do something like create a hole where I tightly fit a Convoy S2/S2+ being certain to always insert to where the light’s bezel is flush with the inside surface, and run the light at a low to medium level on a AMC7135 regulated driver? I guess that requires getting a very good lumen measurement on that S2/S2+ light to begin with. Then my next question is, is that “hassle” worth it? Are we talking narrowing down our overall accuracy from say 10% to 5? Or is it more like 30 to 5%?

Is there a source where BLF users are posting lumen measurements for guys like me to reference and compare to? Or are they just scattered through posts everywhere?

-Garry

The exact output of the reflectivity measuring light does not need to be measured, it just has to be very constant, and you can use your sphere to test that. Narrowing accuracy from 15% to 8% (I totally made up these numbers, a calculated wild guess you may call it, it higly depends on which other known and unknown variations are build in the measuring method) sounds reasonable for this measure, if a wide variety of flashlight sizes and textures are measured.

It looks like you use a standard chinese luxmeter, if I’m correct it measures down to 1 lux. Then the 6mA 5mm led lightsource for measuring reflectivity that I used in sphere#3 has not enough output to measure reflectivity in a significant number of digits. So for your sphere you better use a brighter light source. Yes, for a fairly constant output you could go for an S2+, on a 1x7135 chip driver. I think a simple AK-47 driver with two chips removed should do the job. Best is to use a high voltage led, like the newer batches XP-G2, so that the 7135 does not have burn off too much voltage and avoid temperature effects (these cheap 7135 clones suffer from temperature variation, but I can not find a source for original ADDtek 7135 chips). To avoid having to make a baffle inside the sphere between measuring light and detector, you can mount the flashlight not flush with the inner surface of the sphere but a bit recessed, with the sides of the hole made black (just like your measuring entrance hole). When inserting this light from the outside it needs a ‘hard stop’ (same for the detector, but you managed that by mounting it flat against the polystyrene so it can not go anywhere) because the measured light will be extremely sensitive to how deep in the hole the flashlight is inserted. Perhaps you can make a collar around the flashlight head for that that and hope that it does not deform the polystyrene when it is pushed against it. The position of this reflectivity measuring light I think would best be right next to the detector near the entrance hole.

I do not know a single BLF source for measured outputs. I know selfbuilt over at CPF incudes output numbers in his reviews (made with his corrected milk carton device ), and so does UPz in his reviews, who btw made a very thorough write-down of how his sphere is made and tested (although in spanish).