So people have been requesting to see this sphere even though I warned them it was really nothing that amazing when you broke it down but here it is anyways.
To start out with it needs to be noted that I got the idea for the centering rings from JoshK’s spheres, they are truly a genuine idea and vastly improves the spheres performance.
Cliff notes version:
- Tired of screwing with a “normal PVC sphere”, so Dale sent me his JoshK sphere
- I learned a ton from messing with the Josh sphere and then decided to apply that to the PVC sphere in order to measure larger lights
- I added an “integrating disc” that I made out of styrofoam that diffuses the beam and ensures stable lux readings, other options exist as well.
- I 3D printed some flashlight “centering rings” and coated them in reflective tap to make the opening always look like a giant reflector and thus vastly lower any effects of head size on the readings.
- I tweaked the thickness of the integrating disc and added a small amount of electrical tape in order to get the lux meter to read lumens directly on the meter with no multiplier needed.
- I enjoy being able to take a quick reading without having to do math and know that it is consistent.
- Pictures at the end show the stability of the readings even with a silly angle on the light, in real world usage you have to try to get over a ~2% variation.
So you have all seen the many threads on building a PVC pipe based sphere, many of you have done this already. I had done it as well but quickly abandoned it as it was just too unreliable for me, slight changes in the way the flashlight was put in the sphere would drastically change the readings. Plus having to use a calculator to get the lumen readings was a step I didn’t feel was needed.
I gave up on it for awhile and then Dale was nice enough to send me his JoshK sphere which I then set about improving to allow it to handle the kind of output my lights put out. During this I figured out some simple things that I could apply to the PVC sphere and now I use the PVC almost exclusively, simply because it fits on my desk and allows for much larger flashlights to be tested.
So, now to the sphere itself, Some of these steps are not totally in order as there were a lot of trial and error steps not mentioned here in between.
The biggest issue with normal PVC spheres is that it doesn’t integrate the light, it simply reflects it until it reaches the lux meter and because of this the readings vary wildly due to the beam pattern and position.
I also wanted the lumens to read out on my lux meter directly instead of having to use a calculator, why make things harder then they need to be?
I started out with a basic 3” “P-Trap” setup, I used a 3” to 1.5” reducer on the meter end and with a little dremmel work the HS1010A sensor pops right into the hole and mounts snugly. The HS1010A works great for me BTW, I have 2 of them and they both read within a few lux of each other.
So after the tests with the Josh sphere I knew that an NR lens for a camera works well at reducing the lux reading without effecting the accuracy. Although finding one large enough and with the right transparency proved hard. So I reverted back to an earlier idea I had which was to use a styrofoam disc.
It reduced the lux reading nicely AND functioned as a great integrating layer. The light passing through it was perfectly smooth and “soft” thus there was no beam that would reflect around the pipe and cause odd readings.
I cut it to size and then started sanding it down slowly checking as I went. My goal was to sand it down until the lux meter read the lumens out and thus didn’t need a multiplier. I got it just right and then fully pressed it into the pipe and blew out all the dust, turns out the dust had been effecting the readings and it was now a bit too high.
Not to worry, I simply added a small amount of black electrical tape to reduce the reading back down and bam, the lux meter now reads lumens!
So the sphere now read within ~3% no matter where I pointed the light around the entrance point but I wanted it better and I also didn’t want to be blinded while testing lights.
So I printed up some “centering rings” for the end of the PVC. I put reflective tap on the inside so that to the sphere the entrance hole always looks like a giant reflector no matter what size the flashlight itself is. It also centers the light and gives me a consistent depth to insert it.
And that is basically it. I made a normal PVC sphere, added an “integrating disc” and a “reflective centering ring” and now instead of 40%+ change in readings based on flashlight position it is around 3%. How far I insert the light past the ring easily has a larger effect then that.
If I was to do it again I think that a 3D printer integrating disc could be used if it was printed thin enough. Or one of a bunch of other options as well. Besides that I would not change anything at this point, I love it!
Now for the pictures!
Example of the lack of effect that angle has, a lot of those losses are actually from having to stick the light further into the sphere to keep the beam “inside” the opening. In pratical terms the most variation you will get without an extreme like this is sub ~2% vs the 4% seen here. Just showing the worst case as an example.
TA's words of advice for this week:
Gun control is not about guns; it's about control.
Democracy must be something more than two wolves and a sheep voting on what to have for dinner. -- James Bovard , Civil Libertarian (1994)