If the lightsource is not sticked completely down the hole, its light does not illuminate the part of the inner surface near the entrance hole, so the direct path to the detector area is prevented. I found btw, because the aluminium makes the hole lining highly reflective, that the luxreading is fairly constant with the lightsource sticking at various depths in the entrance hole.
I did not try plain matt white paint, but I tested a latex/bariumsulphate mixture that I found an article on. It reduced the reflectivity of the surface and did not greatly change the wavelength 'response' of the sphere's inner surface. So my newest insight is to just leave the coating out, just carefully sand the styrofoam with fine sandpaper to a matt finish. If you do decide on a matt white paint finish, let me know how it turned out, and if you care to check it, take a luxreading on the same (stable) lightsource before and after the painting to see if the reflectivity has improved.
I put it together and did some test. The reading was very high. With my 8”/20cm diameter sphere (7”/17.5cm inner) and WF501 with XM-L2-U2 bulb, the meter shows 40000 Lux. I got the same value if I point the meter directly towards the Lux meter with a distance of 11” (28 cm) between them. When I use the same meter on my 4” pipe 3 elbow tester, the reading was 1600 with the same meter. Does that make sense? If not, what can go wrong?
The actual luxreading is not a number that tells you much, so to compare it with different other ways of using the luxmeter does not make a lot of sense. Once it is calibrated with a lightsource of known output you're good to go.
But that the number is so high does get you into trouble with the range, if the maximum output that you can measure before the luxmeter gets out of range is under 1000 lumen, that would make it less useful. I think what is to blame here is the limited wall thickness of the ball, just 1.25cm, compared to 2cm in the ball that I used. That transmits more light and gives higher readings than my version. It also affects the integration somewhat because more light is leaving 'the system'.
A solution can be a longer (black coloured) tube for mounting the luxmeter sensor so it sits farther away and sees less light. A bigger sphere or one with a thicker wall will do the job as well.
I found the problem is the direct path. After put a baffle between the flashlight and the lux meter sensor, the reading reduced a lot. Right now, my maximum reading (XM-L2 U2 at battery current of 1.5A) is about 10000.
By definition, if total light is 1 lumen over an area of 1 square meter, the lux reading should be 1 lux. I think the relationship of lumen and lux reading can be derived in theory. Of course due to various factors, the result will vary greatly.
When I use a big baffle made by carton board, there reading is about 10000 for 1.6A battery current and XM-L2 U2). However, when I use a thin carton photo paper as the baffle, the reading increase to 35000 again. Maybe some light leaks through. I need to test some more before post photos.
I cut the baffle to be smaller. Now the reading is about 15000lux. The key is I need to find the correction, or dividing, factor.
I took a few photos of the sphere but don’t knw how to attach them.
There's a thread about how to post pictures. You can not upload pictures directly to BLF,you have to upload them to an image hosting site and paste the URL of the picture into the insert picture menu:
Thanks!
The photos are attached. To have correct lumen values, it looks like I need to have a large dividing factor (>20). I don’t know if it is normal.
Ah, now I understand how you have build it. I'm not sure if it is done intentially, but your build is essentially different from the sphere in the OP in that you made a hole for the luxmeter. The sphere of the OP does not have that, the luxmeter sees the light through the sphere's wall, and that was for reason of simplicity:1) no baffle was needed this way, 2) some extra integration takes place, and 3) and the light reaching the luxmeter is decreased, getting the range useful without extra grey filters and such.
Now that you made it the way you did I think the best is to make the baffle as small as possible (and white coloured) , just big enough to stop the two holes 'seeing each other', and lower the luxreading in another way: a grey filter before the sensor, or making a longer black coloured tube with the luxmeter at the end of it. Using a dark coloured baffle to reduce the reading, like you do, also reduces the integration of the sphere, making it more sensitive to beam profile of the light source (the whole point of using an integrating sphere is to measure the amount of light of a source independant of what shape the beam is).
Thanks. What you said make sense. Now I am not sure my baffle is really needed. It does reduce the luxmeter’s reading but may or may not because the directly light was blocked. In any case, the reading was very high previously and I like it to be a little lower so what I measured will not exceed my luxmeter’s reading. I just want to compare different flashlights anyway so accuracy is less a concern.
Now, what I need to do is find a way to calibrate it.
BTW, just curious, what is your correction factor, i.e., the ratio of lux readings divided by estimated lumen?
You still need a baffle between entrance hole and detection hole so that no direct light from the source can get into the exit hole. It helps integration to great extend. A small roundish blockage in between can do the job.
The conversion factor (strictly spoken it is a conversion instead of a correction) differs a bit between different spheres that I made. As you can read there, the sphere of the OP has a conversion factor (I use lumen/klux for ease of use) of about 83, so 83lumen/klux measured. The max of the used luxmeter is 200klux, so the maximum output the sphere can measure is 16600 lumen.
Thanks for your clarification.
I read from one posting somewhere about a calibration method. I think it would be a good one. He suggested to measure the lux value of direct sunlight. Then pointing the IS’s inlet port directly face the same sunlight and measure the lux value at the sensing port. The lux value (directly from sunlight) times the port opening area gives value of lumen. The conversion factor can be computed from the computed lumen and the lux value measured form the IS’s sensing port. One may need to adjust the lux meter and the inlet port such that they are both at the highest values so they are measure the same thing.
I will need to paint the outside of the IS to be black so no light will come from the ball directly before I try it.
What do you think about this approach?
Calibration with the sun like this will work, but your calibration will not be independant from your luxmeter's calibration, if your lux is off, your lumen will be off. And luxmeters cheaper than several hundred dollars have big tolerances, even if they are correctly calibrated (which the cheap ones will not be).
Perhaps the best 'budget' calibration is using a household reflector led bulb of a trusted brand (Philips, Osram come to mind, they usually come with a (multiple) TIR), switch it on and let it settle for half a hour before using it for calibration. Your entrance hole must be big enough for it (50mm)
Do not paint your sphere black, with its thin walls it is even advised to paint it white for improved reflectivity. For a sun calibration event, put it in a cardboard box with a hole or something like that.
Thanks for your comments and suggestions. I don’t know how large is the error of my luxmeter. I will keep this in mind.
As for using household lamps for calibration, the problem is that my inlet light port is not large enough. In any case, I just want to have an idea about the flashlights that I have anyway.
I will let you know once I have more test results.
Glad I stumbled on this thread. Great work Djozz! Looks like I can locally pickup (2) 12 inch half balls @ $8each. Question, with a 12 inch ball (30.5cm), what's the largest size hole I could use and be within reason for decent results? Can I go up to 75mm? (i.e. for a Courui?) My use would be for bike lights which could also involve shapes that are not round (more oval).
Hi Gary, a 30.5cm outer diameter is about 26cm inner diameter. You're fine with a 75mm hole in that, it will affect the amount of integration quite a lot, but even up to 116mm is within 5% of the inner surface area (5% of the inner surface taken up by holes is a rule-of-thumb-maximum for proper integration, according to the Labsphere theory, but that is with a very high reflective inner surface, and sanded polystyrene is pretty white but not that white). But you could also make an insert in the 75mm hole for a smaller hole, say 42mm (a C8 would fit in that) by re-using the cut-out of the 75mm hole, if the cut is neatly made with a scalpel or exacto-knife. You need to use a separate multiplier for the two holes (so calibrate the sphere for each hole separately).
(for real accuracy the multiplier should be adjusted for each measured flashlight, but then you need to make use of an multiplier measurement light and that was not the focus of the 'cheap and easy one' :-) ).
I repeat this because someone above made a hole for the luxmeter also: do not make a hole for the luxmeter, the light is measured through the styrofoam in this design. The design of the OP assumes a wall thickness of around 2cm, this reduces the amount of light reaching the luxmeter enough to get the lumen-measurements into a workable range.
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