Integrating sphere #4 (the fast and cheap one)

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djozz
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Integrating sphere #4 (the fast and cheap one)

(Wait, I never mentioned sphere #3 ! Undecided. Well, I did make it, but although sphere #3 was supposed to be the simple one, during the build I got carried away and added all kinds of useful things. I have not done a build thread on it yet, but I will, one of these days.)

 

Another day, another integrating sphere. The aim of this one is to really be simple: see how much I can leave out without sacrificing too much on the performance. In this integrating sphere is much more thinking time involved than actual building time. In fact after figuring it out it was made and tested in 1.5 hours. I made it as a proof of concept so I starting out with a small 15cm diameter styrofoam ball just because I had one.

The inner diameter is 11cm, and because a rule of thumb is that less than 2% of the inner diameter must be occupied by holes (to maintain enough reflectivity for good integration), I made the entrance hole 30mm diameter. So that is the widest flashlight that it can measure. You can make this sphere with bigger size styrofoam balls and bigger holes, it will even be a better integrating sphere.

Now I cover the inside of the hole with aluminium-tape (the stuff you get in a plumbing shop), tin foil plus some (suitable for styrofoam) glue should work too. This is done because the detector will be next to the hole and it should not pick up light that enters the styrofoam layer directly from the light source.

I need a plastic ring to clamp the detector on a fixed spot on the outside of the sphere. I use a section of an old pill pot. you can use anything that is white, black or transparent (not coloured).

I taped it to the sphere with cellotape on the outside (cellotape sticks very wel to styrofoam). It can be anywhere near the top of the sphere, it just has to be in a fixed position so that the detector will be in the same position every time you attach it.

Stick the detector of your luxmeter onto the ring with a piece of tape and you are ready to go.

 

Why is this sphere made this way?

*With small spheres you run into the problem that your luxmeter is out of range with even weak flashlights. I have solved that before with a neutral density filter before the detector, now I did it by measuring through the styrofoam (idea stolen from texaspyro). The advantage is less light to measure and extra integration on top of the multiple reflections inside the sphere, the disadvantage is a possibly slight change of the spectrum of the measured light source.

*by covering the inside of the hole with aluminium, the detector just sees light that has already seen integration inside the sphere, direct light from the source going into the styrofoam is avoided.

*the detector is near the entrance hole so that light that goes into the styrofoam directly in front of it has undergone at least one reflection, this improves the integrating properties

*no baffle is needed, in fact the alu-foil works as the baffle

 

Does it work?

*With my cheap Tondaj luxmeter, the range of this particular sphere is 1 lumen to 17.000 lumen. Edit: if your sphere is bigger than this one, your range goes up which is not good for measuring low light levels. An adjustment to get the range down again is suggested in post #57.

*The integration (insensitivity to beam shape, just sensitive to the amount of light) of this sphere will be way better than the pipe measuring thing. Shining around a zoomed zoomie around the sphere in different angles gives a very constant reading. Moving around the luxmeter over the surface of the upper half of the sphere also gives a fairly constant reading.

*Because of the -relative to inner surface- large opening of this small sphere it is a bit sensitive to size, shape and reflectivity of the flashlight to be measured. For improved accuracy it would need a correction light source (like in my other spheres), but I chose not to add one. The error can be up to a few percent. A bigger sphere with -realtive to the inner surface- smaller hole will reduce the error (my 50cm sphere with 80mm hole hardly needs correcting).

*I measured three constant output flashlights to test the performance, a larger neutral white (XM-L2 4C) one and two identical flashlights, one with a 3000K XM-L2, one with a 6500K XM-L2. I used the neutral flashlight for calibration and used the found conversion factor to measure the two others. Then I compared it to the values of the same lights with my big integrating sphere (#2).

  Sphere #4, measured lux Sphere #4, calculated lumen Sphere #2, calculated lumen deviation of Sphere #4 from #2
3000K flashlight 1920 159 172 -7.5 %
4000K flashlight 4500 373 373 -
6500K flashlight 2720 225 228 -1.5 %

That is quite a deviation from the other sphere. What is the cause? First, the bigger flashlight with reflective bezel causes an improved reflectivity inside the sphere and measures relatively high compared to the two small black ones. Another error is the different luxmeters: the cheap Tondaj used here and the expensive Mobilux in Sphere #2 (I made a thread on the differences between these luxmeters: http://budgetlightforum.com/node/33929 ). The Tondaj overrates blue, that is why the 6500K flashlight measures 9% higher with it compared to the 3000K one. Another possible effect could be the light-path through the styrofoam possibly causing some spectrum shift.

 

Conclusion

I could do numerous more tests on this sphere (I may dive more into it one day) but I think the point is made: this sphere, with the cheap luxmeter, works within 10 percent from my best sphere with all sorts of corrections and state-of-the-art luxmeter. I think it is very usable. (for the record: 10% off is not that much in light measurements)

For improved accuracy, you can carefully sand the inner surface with 100o grit sandpaper (I'm going to do that, quick fix), use a larger innersurface-hole ratio, a better luxmeter or add a correction light source (may do that as well). I do not recommend experimenting with coatings: no fun, poor result IMO.

 

Thanks for enduring another post on integrating spheres Tongue Out

Edited by: djozz on 02/14/2016 - 08:25
MG
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And good write-up. If I can find the styrofoam ball this will go on the to-do list. Thanks!

mhanlen
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Nice… this makes it look so easy. I need to make one, one of these days. I even have a cheap lux meter.

MG
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It’s only $14 with free shipping from US. No wonder they sold 272 of them. Now 273! Thx again djozz….

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MG wrote:
It’s only $14 with free shipping from US. No wonder they sold 272 of them. Now 273! Thx again djozz….

You mind throwing up a link?

MG
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Here it is:

http://www.ebay.com/itm/High-Accuracy-50-000-Lux-Digital-LCD-Light-Meter-Photometer-3-Range-Luxmeter-/400806580914?pt=LH_DefaultDomain_0&hash=item5d51ef12b2

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Thanks djozz. Another spherelly good thread. You also have another new item I just cant recall what it is at the moment. Have you moved into bigger premise's?Smile

 

djozz quotes, "it came with chinese lettering that is chinese to me".

                      "My man mousehole needs one too"

old4570 said "I'm not an expert , so don't suffer from any such technical restrictions".

Old-Lumens. Highly admired and cherished member of Budget Light Forum. 11.5.2011 - 20.12.16. RIP.

 

djozz
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MRsDNF wrote:

Thanks djozz. Another spherelly good thread. You also have another new item I just cant recall what it is at the moment. Have you moved into bigger premise's?Smile

No, unfortunately not, instead I moved to smaller spheres. The response of my lovely and understanding co-occupant upon showing this sphere was: Well, that big one can now be discarded then!

Tired

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Do I sense someone has a big thumb print in the middle of their forehead. Cry Your not alone.Smile

 

djozz quotes, "it came with chinese lettering that is chinese to me".

                      "My man mousehole needs one too"

old4570 said "I'm not an expert , so don't suffer from any such technical restrictions".

Old-Lumens. Highly admired and cherished member of Budget Light Forum. 11.5.2011 - 20.12.16. RIP.

 

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Awesome work Djozz, I have been looking for a simple integrating sphere recipe, going to build one of these and try it out, thanks

blueb8llz
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Thanks for this write up. I may end up doing one some time soon

mhanlen
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A few questions.

Will this smaller ball handle higher output lights? A 100 lumen light and a 3000 lumen light?
Would it perform better if the outside was covered in a black paint since the walls are thin? Maybe that’s what you meant by not using a coating?
Where is a hollow ball available?

djozz
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mhanlen wrote:
A few questions. Will this smaller ball handle higher output lights? A 100 lumen light and a 3000 lumen light? Would it perform better if the outside was covered in a black paint since the walls are thin? Maybe that's what you meant by not using a coating? Where is a hollow ball available?

As said in above post, using the Tondaj luxmeter with range from 1lux to 200klux, the lumen range is 1-17,000 lumen. If the sphere is bigger the range will shift to higher numbers, i.e. a 30cm sphere (26cm inner diameter) has a inner surface that is 5.6 times that of the 15cm sphere, and because the light is spread out over this larger surface, the range with this same luxmeter will be 6-95,000 lumen. This is of course a less desirable range, but still workable. My Mobilux luxmeter goes down to 0.01lux, so that would also help with bigger spheres. Perhaps really big spheres still need a hole for the luxmeter to get the range right, and then a baffle must be used to block direct light from the light source.

About the walls of the sphere: styrofoam directly reflects only part of the light, a lot is getting into the foam and eventually may get back into the cavity of the sphere, escape on the outside, or it gets absorbed. Most of the 2cm wall thickness of this sphere is actually needed for added 'reflection' of the light. A simple test: you can see a slight 'greying' effect of a black background through half a cm of styrofoam. Through 2cm I can not see any greying.

Painting the outside black should not have any effect, the sphere does not see a difference if light coming out of the sphere is absorbed by black paint or if it escapes into the world, it is lost either way.

In europe, there appears only one manufacturer of hollow styrofoam balls, all hobby shops and display material shops sell the same balls, they come in a range of sizes the wall thickness is 2cm for all sizes.

I do not know about the US.

sandalian
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I’m wondering where can I get those kind of styrofoam here in my country..

I'm switching my BLF account into LumenZilla. Thanks!

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Just don’t step on it.

Old-Lumens wrote:
I love modding, but I don't have much use at all for flashlights in general.
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Djozz, do you need my address?

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After a quick search I’ve found this one (for German buyers):
http://www.filzenundspinnen.de/epages/62245016.sf/de_DE/?ObjectPath=/Sho...
I think this size (30cm) would do even better?

djozz
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Antenne wrote:
After a quick search I've found this one (for German buyers): http://www.filzenundspinnen.de/epages/62245016.sf/de_DE/?ObjectPath=/Sho... I think this size (30cm) would do even better?

It is the same series styrofoam balls as I can get here. Mine was from Opitec, I believe that is a german shop too.

With my 15cm ball I can not make the hole much wider than 30mm, with 30 cm you can go as wide as 60mm, making it suitable for more flashlights (but a smaller hole will give a bit better integration, but that was not bad anyway)

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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: http://www.osram.de/osram_de/produkte/led-technologie/lampen/consumer-le...
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)…

djozz
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Antenne wrote:
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: http://www.osram.de/osram_de/produkte/led-technologie/lampen/consumer-le... 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-le..., or any other led reflector lamp with reliable specs.

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Very cool djozz, thanks.

cabfrank
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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.

Antenne
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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…

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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.

Twinclouds

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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?

Twinclouds

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twinclouds wrote:
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.

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Thanks for getting back to me.. I found Exacto knife work pretty well. I have a couple more questions and home you can help.
1. 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?
2. 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.

Twinclouds

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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.

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Thanks! Will get back to you.

Twinclouds

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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?

Twinclouds

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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.

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