@emarkd, correct, the luxmeter has to be matched with its sphere and then the calibration done per set.
Averaging a series of very well regulated (see selfbuilt’s reviews for which lights on which modes are best regulated) and ANSI-rated lights is the cheap way to calibrate the spheres. Sending one of them off for a certified calibration is the expensive way.
If one sphere was sent out for certified calibration, could that one sphere then be used to calibrate the rest of them?
I’m not aware of places where they certify spheres. And you can’t couple one unique number to an integrating sphere like this anyway because the number that the luxmeter gives is not independent on the physical appearance of the light source.
It is much more robust to have a constant light source calibrated, and a calibrated light source is more practical too, i.e. if you decide to alter the sphere design it is very easy to re-calibrate it.
Ah, my misunderstanding. I see what you mean now. Thank you.
I do Jos. All the time. 
Like a reference manual, you and Match.
When manxbuggy1 and rdrfronty built my PVC trap box (and their own and TomE’s and Richards) they used 20-30 factory ANSI rated lights and averaged the results. Both are/were collectors and had a variety of lights to work with, manxbuggy1 preferring smaller hot rod style lights and rdrfronty going for a host of name brand large lights by Nitecore, Olight, Fenix… all the big favorites. So there is a wide assortment of test subjects with a background of good ANSI ratings used to get our multiplier.
I still don’t know much about this sort of stuff, which is partly why I’m involved in it. I do have a “light box” built from an amazon shipping box. I cut a cardboard baffle, taped it into place and spray-bombed the whole inside white. Two holes in the sides and it was done. Its…okay, but I can actually make my results vary by as much as 10% just because the walls are too flimsy to hold the light and the meter still. Just a bit of tilt one way or another and the numbers fluctuate. So I want something better. I calibrated my box using some of my own factory lights, which I do have a lot of … about 100. Many are modded and useless for this sort of stuff, but many aren’t. So again I’m happy to loan anything I’ve got for calibration purposes.
But I do like the idea of sending a light off for true ansi calibration, which could then be used to calibrate our spheres. I’d be in for my part of the costs of that, assuming its not outrageous.
Don’t fret guys, we will use whatever uniqueness is required to get impressive results. That’s why I need lots of data, so I can know and compensate. DB Custom, yes, averaging many lights would be awesome. Who do I PM to ask about borrowing them? I didn’t quite follow who owned them.
Guys, how big of a hole do we want? Would a single 2” work? Please share some models and head sizes.
2” diameter works out to be 4.9% 1.0% of the total area within the 12” ball.
According to manufacturer, the walls are 7/8” thick, so the internal radius is 5-1/8”. Internal sphere surface area is 4 x pi x r^2 = 64.4 330 sq. in. Area of 1” radius (2” diameter) hole is approximate 3.14 sq. in. Doing the math, that comes out to be roughly 4.9% 1.0%.
Note: My wish is for a 3” diameter hole. That works out to be about 11% 2.2% of the internal sphere surface area. I know that this is bad. Just stating my personal preference.
The 3” diameter hole would accommodate my flashlights up to the head diameter size of a Thorfire S70.
A 3.5” diameter hole (2.9% of internal sphere surface area) would be even better. It would accommodate my Nitecore TM16GT.
Edited in bold typeface.
Although a 2”/50.8mm is in the best size range for accuracy I would be OK going very slightly larger if that’s the “vox populi”. I don’t think it will be a big problem going larger since we’re not talking ‘lab grade’ expectations. 2” gives just a little clearance on my largest C8 clone, so I’m good with it.
Phil
…think you left out a ^2 there. A 3” (diameter) hole is only about 2% of the surface area of a 5.125” (radius) sphere.
I thought the whole point of using the larger sphere was so we could have a larger hole and therefore test larger lights. I was definitely hoping for a 3” hole. That covers bigger hosts like the Maxtoch M24, Olight M3SX, even barely some stuff like the big Thrunite/Acebeam lights like the TN32, etc.
But I know nothing about all this. I already see where djozz says 3” is a bad idea (why? too much missing interior surface?) so if its a bad idea then do the good idea thing. Having a 3” opening is pointless if the results aren’t trustworthy. I’ll be happy with whatever comes, as long as it works.
Didn’t do the math, but I’d be OK at ~7% at the most with 5% being better, but 2” or 50mm as the minimum regardless of the foregoing.
Phil
Thanks! I was in a rush to pick up my son after school, didn’t get a chance to check my calcs.
And I’m simply drained from discussing his school problems. Everything is the teacher’s fault, she sent him to the principal’s office for “no reason”, he was just studying at the time like he was supposed to. Some/most of his fellow students are mean to him, but he’s never mean to them. Nothing is ever his fault.
Sometimes, being a parent isn’t much fun.
Ok. I will plan to taper the 2” hole, so as long as the BEAM is 2” or less you are golden. The head can be a bit larger than 2”.
I received the 1/2 sphere they had in stock just today. I have to say, shining a 1400 lumen flashlight from the outside in lights the interior evenly and perfectly. Shining it inside is too intense. I think we will stick with the 2” hole for low lumen work and use the through-wall method for high lumen work. There won’t be a hard size limit for through-wall. From what I have seen, we are on track for excellent results!
For the “thru-wall” readings is it going to be necessary to center the light on a specific spot? If so, then that spot should be accurately marked permanently somehow. Consistency between all users is what makes this project different and so valuable to us.
It would also do well to include usage instructions (could be posted on BLF somewhere) so that everyone knows how to position the light being tested correctly, to allow 30 seconds runtime before getting the reading, etc.
Phil
Yea I plan to mark it. There will be a video too.
According to this lumens-to-lux calculator, for a 5.125”R sphere (area = 2.29 sf), it would take over 40,000 lumens to exceed the LX1330B’s 200K lux range.
Granted, that’s a theoretical calculation.
In our model, we’d have an adjustment factor that could be higher or lower depending on how the light is collected within the sphere. However, I’d suggest that it is most likely that the sphere could accommodate a beam of 10K to 20K lumens without fear of overtaxing the light meter. The higher meter range might be less accurate than the lower range, but it would probably be more accurate than using the through-wall method.
With that in mind, I’d suggest that the 3” diameter opening still be considered for discussion.
The problem with a 3” head inside the sphere is the beam angle crawls up the wall so far. The meter starts seeing the outter hot spot. Through-wall is totally solving that I see.
When you cut out the hole would it make sense to save the plug that is removed? That way you can reinstall it and have a solid sphere for the shine-through-the-wall readings?