So, I’ve ordered the 30cm one at Opitec, thank you for mentioning this other source. The project seems easy enough to find the time for it and to make use of the Luxmeter I’ve bought some time ago from Banggood (Sunche HS1010A). The real problem will be (for me) to find reliable reference sources to somehow calibrate the setup for lumen readings…
What do you think: could I use LED light bulbs as a reference? Some of them have rather precise specs on their datasheets, e.g. this Osram: Deutsche Homepage | Licht ist OSRAM
According to the package it should have about 806 lm (10W/60W type). Perhaps E14 lamps would suit better (to make the opening not too large)…
Sorry for the late reply, missed this post. How are you getting on with the sphere?
An Osram led light bulb should be a very ok reference if you let it warm up for 30 minutes before measuring (it takes that long to get the light output constant, as I learned from texaspyro), but this bulb you linked is not suitable because you have stick it entirely into the sphere to let all the light in, and it will then also directly shine on the area on the inner surface where the luxmeter is behind. A led spotlight is a more suitable reference, like this one: http://www.osram.de/osram_de/produkte/led-technologie/lampen/consumer-led-reflektorlampen/led-superstar-par16/index.jsp, or any other led reflector lamp with reliable specs.
Very cool djozz, thanks.
Antenne, you could look up the measurements from some of our members of a flashlight that you own, and calibrate that way. That’s what I’d do if I built one of these.
Thank you for the suggestions. I have to admit, my sphere is still to be realized. I’ve got the styrofoam ball but had no time yet for the finish…
Anywhere to get the ball in the US?
Edit: I found smoothfoam balls at Jo-Ann Fabric and Craft. They have both 8” and 12” ones.
Hi, Djozz:
I just bought an 8” smoothfoam ball. Can you tell me what is the best way to make the holes on the ball?
I used a narrow blade scalpel but an exacto knife should work as well. I hope the sphere works for you, there has not been feedback yet of someone who build one of these.
Thanks for getting back to me… I found Exacto knife work pretty well. I have a couple more questions and home you can help.
- You used aluminum-tape to line the flashlight port. How this can prevent the direct path? Is it to prevent the light goes through the foam?
- Do I need to paint inside of the sphere? People suggested using matte white paint. I am wondering if flat paint will work well also.
Will keep you posted about my progress.
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.
Thanks! Will get back to you.
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.
Now I'm getting curious how it looks like :-)
I'm glad that with some troubleshooting you are getting it to work. What is the lumenrange you are getting now?
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:
https://budgetlightforum.com/t/-/15452
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).