The name could be argued, IMHO sphere is anything that integrates the light to the point of beam profile/shape/angle not drastically effecting the readings. A light pipe classically does not do this so I feel it is closer to a sphere then a light pipe. Although the largest reason for calling it a sphere is that is what everyone else calls these devices and thus what they would search for.
Yeah, the first several prototypes of this setup were very overthought, didn’t work that well and were hard to use. When I finally said to heck with it and decided I didn’t care I went simple and to my surprised worked way better that way.
The real eye opening moment for me was when I realized the difference between a mathematically calibrated sphere and a standard calibrated sphere. DIY spheres are always standard calibrated. In this case there are only 2 things that matter, consistency and linearity. This checks both boxes so it is then just a matter of calibrating it to a good standard.
On a mathematically calibrated sphere (such as an ANSI certified sphere) things get way harder and that why they cost $10,000.
The electrical tape for tuning was another eye opening moment. I tested extensively to see if it would effect the constancy but it turned out that if the light was truly “integrated” it had no effect other then reducing the meter reading. As far as it peeling or currling, I have not had an issue yet but I also keep that fenix light nearby and check the readings from time to time. It always comes out within 2-3 lumens.
The largest issue I have is actually dust. I left the centering rings off of it at first when not is use and after awhile the dust started to effect the readings slightly. I blew it out and it was back to normal. So now I leave the smallest hole centering ring on it at all times to keep the dust out.
As far as the lumen measurement range, the min lumens it will detect is around ~5 or so, generally doesn’t detect moon modes and under 5 lumens I am not convinced of the accuracy.
The highest it has seen so far is a bit over 10k lumens from an 8X SRK.
For head sizes, it depends on the size of the PVC you use. 3” that I used Is cheap but the price goes up quickly as you go bigger so I decided to pass on that. You can get PVC upward of 6” though so max head size is not really an issue, just a matter of cost and how much space you want to take up.
I personally have measured from a BLF-348 up to a D01 myself, those are the smallest and largest lights I own at the moment, no issues with either at all.
I totally agree on being “good enough”, consistency is way more important then accuracy, as long as it is consistent then it is comparable.