Help with my D.I.Y. Sphere Calibration

Hi, Djozz
I see what you mean about the sun’s brightness. I did not use the sun because the sun overloaded the sphere’s meter. The sun was just to illustrate the method of a parallel light beam. I used a thrower with a nice, even hot spot and was aware of the voltage sag so I took more than one sample pair of “lux in front of hole” and “sphere meter value”.
As you can see the sphere meter was occupied (build into the sphere) so the “lux in front of hole” has to be measured with a separate luxmeter.

It is impossible to say which is the best method, the imperical with an expensive flashlight or the scientific, by using the formula. That depends on the equipment used.
In my case there was no doubt about the method because I don’t own an expensive light other than an Zebralight H50. Fortunately it turned out to measure 60 lumens after my calibration which I think is the “correct” value. The sphere also confirms the binning of LED’s on stars.
I made that one calibration on the sphere in 2011 and have had no reason to distrust it since.
Happy measuring!

I really like the scientific method, going to give it a try sometime and see how it differs from my current calibration. :slight_smile:

And I still have in the planning to someday spend some serious money ($400) on first building a dead-constant led light source and then have it officially measured at a facility here in the Netherlands that does certified light measurements with a professional sphere. At least I already own a very good luxmeter that was once officially calibrated (but the certificate is expired now).

I’m using a PVC pipe. I am happy with my multiplier. In all my videos, since summer on of last year I compare my outputs with the manufacturers ratings. Sometimes they’re close and sometimes they’re not. Like my recent slew of Nitecore lights, I’ve found on the top end my lights measure slightly more than advertised on most. But in other lights, say the Acebeam T20, my top mode was less (only by a little bit) then theirs. Here are a few observations that I can say about my measurements.

1. Sometimes my highs and turbo modes read a bit higher than others who measure the same lights I do. BUT those measurements are still lower sometimes than the manufacturer.

2. High modes and turbo modes drop the fastest, so I calibrated with one of the medium modes on the Zebralight.

3. I tried several modes on the Zebralight, and created a multiplier for each. Then I tested each one of those multiplier against higher and lower modes on the Zebralight. The medium and the .26 multiplier seemed to produce the most accurate results compared to other modes.

4. Fenix and Zebralight both seem close when cross calibrated and compared to one another.

5. Sometimes when I test a brightness and it reads higher than the advertised spec, the runtime can be shorter than advertised as well.

6. When I do beamshot comparisons on “high modes” my tested specs and the relative brightness to one another is often reflected in the beamshot section. Although it can be hard to tell visually between a 900 lumens and 1000 lumens.

7.I think generally overall my specs are slightly higher than other reviewers. Not by much, but with random sampling I’ve done. As far as I know, no one has ever compared my testings to other peoples. I’ve only made 1 or 2 transcription errors that people ave brought up in my reviews. Those were actually because I forgot to double check my notes or math… not because of a bad reading on my lumen device. I’ve talked privately to one or two reviewers who got slightly lower readings on one light than I did. But no real “Those are wrong” call outs. That might be because people don’t take my findings as seriously as other reviewers, or that people are generally satisfied with my findings. I can’t say for sure, but I prefer the later explanation.

Thanks for the response. Your method on the Zebralight sounds like what I was trying to do on the Fenix HL55, except that I didn’t run my “calibration multiplier” back against the other modes.

-Garry

I like that and are seeing forward to the outcome!
The last 10 years of my working life I worked in a facility here in Denmark that does certified light measurements. We had a 2m professional sphere that got on sale. At first no one wanted to buy it but eventually it was sold for appr. 33000 EU, if I remember it right.
No wonder they charge really big money for trackable measurements.

I wouldn’t bother too much with the highest modes… in every light I’ve tested those always drop the fastest. Startup… and 30 seconds in are always significantly different. And they all continue to drop after 30 seconds anyway.

BTW - found a link to this thread someone was given the opportunity to measure 68 lights in a professional sphere (though measurements taken at 5secs not 30secs) and the results are pretty disappointing for someone like me trying to calibrate! Offers more assurance though that I’m not way off since I see many other lights missing their stated specs. Searching Ebay for used Fenix or Zebralight lights isn’t yielding much (at least not in my “affordable” range).

-Garry

OK, The sphere was calibrated first (using the method described), and afterwards exposed to the Zebralight that turned out to be 60 “MyLumens”.

Sorry, I was responding to mhanlen.

-Garry

I read that story last year and it was a really fun read, and indeed a pity that he did not measure at 30 seconds, not only because 30 seconds is according the ANSI method, but also because measuring after 5 seconds does not give an as repeatable indication of light output than measuring after 30 seconds.

Btw, last year I spent a really enjoyable weekend with folks from TLF and had the opportunity to re-measure 4 of those 68 actual flashlights in one of my integrating spheres that I brought with me. It appeared that my readings across those 4 lights (measured at 5 seconds to repeat the method) were almost consistently 9% higher. A TLF member present with his own lumen measuring device who had done his own calibration as well, also measured those 4 flashlights and found his values also 9% higher than what was measured earlier at the Zweibrüder sphere. So our calibrations agreed, but were higher than Zweibrüder.

djozz, did you see my post #13 above?

-Garry

Oh, sorry, yes I did, and then something else happened en then forgot to respond. (and now I need to sleep again, so a short answer for now):

I went along your lines with using a mildly 7135 driven power led for a constant light source, then for my sphere#3 (link via my sigline) I found that a ordinary white 5mm led driven at a constant 6mA by a simple circuit with a LM317 regulator is about the ideal constant light source for building into a sphere, but the low light level requires a luxmeter that can measure quite low levels.

Did the testing suggested and it appears my sphere has pretty good integration. However, I do see quite an affect of different size lights placed at the opening (and the Fenix headlamp I calibrated with fit the full width of my hole). I shot video of the tests and will try to upload them tomorrow.

-Garry

Videos of my tests posted to YouTube. During the 3rd video (testing my 3 inch PVC Light Tube) I ran out of battery in the camera. Basically my Thrunite Ti’s readings would vary greatly depending on where I placed the light, HOWEVER doing the 2nd test (testing for increased reflective surface area due to size of light placed at the opening) I saw extremely little affect even when placing my largest light (Courui) at the opening - which made sense to me since it seams the PVC Tube system is less likely to be reflecting light back toward the entry hole. I am however concerned how accurate measurements can be in the PVC since they vary so much as you move the light around. Perhaps me using only 3 inch PVC amplified that issue. Other’s see this issue in their PVC Light Tubes?

By the way, I had issues trying to do my test with dropping the light too far down into the entry hole and skewing the readings. (Was trying to watch the meter and not paying enough attention to where the light was positioned.) I tried to redo readings if I saw myself doing that, so be sure to watch the videos through to the end.

Videos:

Testing My Sphere for Proper Light Integration:

Testing Affect of Various Sized Lights Placed at the Opening:

Testing my PVC Light Tube:

Thoughts? Sphere seems to integrate light very well, wouldn’t you agree? I didn’t actually compute up how much change the size of the lighthead is causing. Seems to me if I calibrate with a medium sized light I should be within reason as I test smaller and larger lights. Problem is if I calibrate to a small light and then try to test a very large light. As I said in the previous post, the Fenix headlamp I calibrated to took up the full width of my opening, but it’s somewhat narrow leaving open space in the hole. Therefore I would consider it a medium to medium-large sized calibration light.

-Garry

So I ran a few lumen calcs on the Thrunite Ti by itself and then with another light in the opening (sphere, not PVC) and get the following increases:

With Home Depot 3AAA XB-D light: 2.57%

With 2D Mag LED: 8.2%

With Courui: 12.57%

I don’t know what affect the fact that I’m holding the Ti way off-center plays in though.

-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 :sunglasses: ), 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).

I think it is great that you made these video’s, I will watch them tonight.

The PVC-pipe device does not suffer much from entrance hole effects indeed because not much light is not coming back, it does not integrate very well. In fact: how better the integrating properties of the device the bigger the entrance hole effects, so improving integration creates an extra problem to be solved :-/

Thanks djozz. I’m liking the idea to “keep it simple” and have a calibration factor calculated for a medium sized light which should be within reason on smaller or larger lights. I’m okay with readings staying within about 10% range. I’ll have to read up on your Sphere #3 soon.

I’m using the LX1330B which reads down to 0.1 lux (not saying it’s accurate that low though).

mhanlen has graciously agreed to swap some lights (I send some of mine to be tested and he sends me some of his that have been tested) with me so that I have measurements in common with another user to compare my calibration against. I know what will happen though, I’ll get these measurements and then will change my sphere thereby negating all the measurements!

BTW djozz, where did your signature go?

Thanks,
-Garry

No idea where the signature has gone, it is there in my account information. :frowning:

edit: I deleted my signature line and then created it again, and it seems to be back now :slight_smile:

Disappointing is a good word for it. It looks like they only measured the high modes, which are usually the most variable and the least accurate. Would be nice if they had measured low and medium modes too, or perhaps every mode on each light. It looks like a big missed opportunity, using the most-precise equipment only on the least-precise modes.