The question about calibrating a sphere, I have answered several times since 2011. Perhaps it needs to become sticky’d.
There is a very simple, scientific way to do the calibration, without guesswork about some stock lights output.
All you need is a parallel beam of light (a thrower) and a luxmeter and the formula
lux x hole area (in square meters) = lumens
Thanks for the links, I never read those posts 
A few concerns with that method (in no way implying that these concerns are any worse than the concerns with other calibration methods):
-the sun’s brightness changes per second (at least here in the Netherlands), so using the same luxmeter for the sphere and the lux-measurement in front of the hole can lead to inaccuracies
-the hotspot of a thrower flashlight is never uniform in brightness so you have to work with an average brightness of the light entering the sphere.
-the method is vitally dependent on a luxmeter that is correctly calibrated for the colour temperature of the light source (sun or flashlight), I would not trust any cheap meter for that.
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
I use a flat multiplier for my device… and it’s .26. I don’t have any of those lights you’ve listed, so I can’t tell you how close your measurements are. I know some lights are quite off, per their supplied specs. I calibrated my device at first with a Fenix HL50 and later a Zebralight SC600MkII. I have since lost my Zebralight, but I’d like to get one of their current generation lights to see how close the light stacks up to my old calibration light.
Thanks for your response. I was just finishing reading this thread where you were posting about figuring out a multiplier. Are you using a round “polystyrene” sphere or a PVC light tube? Are you finding your calculated “lumens” seem consistent from low values to high values and also from throws to flooders? Glad to hear you are also seeing some lights being off stated specs.
-Garry
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.
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