Integrating sphere #4 (the fast and cheap one)

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djozz
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The baffle position is not critical but should be somewhere in the middle between entrance hole and sensor, and as Jason pointed out it just prevents direct light from light source to sensor so that detected photons have undergone at least one reflection (not 2). Ideally, the baffle should be as small as possible while still blocking all direct light. This is all discussed in my integrating sphere threads before, and in the sphere theory article from Labsphere.

JasonWW
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garrybunk wrote:
JasonWW wrote:
The baffle is just to prevent direct light and should be opaque.

Ok, any ideas on a material to use that’s stiff enough to stay in place? And I guess styrofoam is not a good choice.

-Garry


If styrofoam is convenient, use it. Just glue a solid material over it, like posterboard, thick plastic or metal painted white. Just something to keep light from going through.

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Thanks guys. It’s a lot easier to block direct light only. Poster board painted matte white sounds simple enough. I’ll definitely err on the side of making it too big (but not way too big).

-Garry

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EasyB
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I just made one of these using a 11.9” OD hollow half balls from Amazon. Made a 70mm flashlight entry hole. I want to reiterate the importance of rigging it so the luxmeter is in the exact same position each time you use it, for repeatability. This was the most difficult thing to get right for me. Thanks djozz!

Enderman
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With the right paint coating, maybe you could make a sphere out of two of these 14” bowls?

Pretty cheap from ikea Smile
https://www.ikea.com/ca/en/p/blanda-blank-serving-bowl-stainless-steel-0...

djozz
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Should work nicely and will not melt from high power flashlights, but start with a good undercoat that sticks to SS.

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djozz wrote:
Should work nicely and will not melt from high power flashlights, but start with a good undercoat that sticks to SS.

Right, good point.
Wasn’t there some CPF post about a special powder that was good for integrating spheres?
Maybe a white/clear epoxy coated with the powder would be enough to stick to the SS if you sanded it first?
djozz
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Enderman wrote:
djozz wrote:
Should work nicely and will not melt from high power flashlights, but start with a good undercoat that sticks to SS.

Right, good point.
Wasn’t there some CPF post about a special powder that was good for integrating spheres?
Maybe a white/clear epoxy coated with the powder would be enough to stick to the SS if you sanded it first?

There is a whole science to that, and the knowledge level on cpf was not so high. The powder is bariumsulphate, which is widely used in professional integrating spheres. The suggestion on cpf was to “sprinkle” a layer on top of a still wet latex coat. But the literature says that BaSO4 is actually quite transparent for light, and that you need a layer of the stuff that is several mm thick to obtain enough reflectivity. So you need a very clear substrate to mix it in and I found that PVA (polyvinylalcohol) is sometimes used for that. I tried several layers of this mixture in a test sphere and does produce a fairly solid coating, but upon drying some fine cracking does occur (which I reckon does not significantly affect the reflectivity properties).

But no coating that I checked sofar leaves the tint of the light intact, the CCT drops at least 500K, which is mainly caused by reduction of the blue peak. This is a cause for inaccuracy: cool tints have a high blue peak and will be more affected (read lower) than warm tints. This is also dependent on how well your sphere integrates: the better integration of the sphere, the more reflections, the more tint shift. Integration works against preservation of tint.

But that is also true for professional spheres, they cause tint shift just as well. The way to compensate that is to make your detector a spectrometer instead of a luxmeter (which is maukka’s setup), then you can produce a spectral difference characteristic for your sphere and calculate the tint shift out of the result.

thefreeman
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djozz wrote:
So you need a very clear substrate to mix it in and I found that PVA (polyvinylalcohol) is sometimes used for that. I tried several layers of this mixture in a test sphere and does produce a fairly solid coating, but upon drying some fine cracking does occur (which I reckon does not significantly affect the reflectivity properties).

From what I’ve read, PVA doesn’t work well with thick/several layers as it will crack, if acrylic is suitable optically then it is preferable for thick layers.

djozz
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I think that the main thing to look for is something that does not yellow over time. All acrylic white paints that I used in the house do show yellowing. But that could be other ingredients too, no idea. Latex paints do not show visible yellowing but that is not a clear substrate, you can mix it with BaSO4 but the white latex itsef also affects the reflectivity and does not reflect as uniform over the spectrum as BaSO4 (there have been measurements of that).

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I’m not sure what modern latex (non natural) is, I’ve seen it mentionned as acrylic latex and styrene or acrylate latex, it doesn’t seem to be a specific material when not talking about natural latex.

I just received calibrated lights from Maukka but I still haven’t made my sphere Facepalm so I need to get on to it, I have a spectrophotometer so ultimately I can correct the spectrum change caused by the sphere. Still I wanted to try to make a sphere as good as possible by using a baryum sulfate coating, I found one source of it locally that is not too expensive, 98.5% purity, don’t know if that’s good enough (I don’t like that the seller describes it as a ”cream” color).

What quantity of powder did you use in the one of your baryum coated sphere ?

djozz
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This is the experiment from 2016, and the mix that I later used for the test sphere (I had no spectrometer back in 2016)

https://budgetlightforum.com/comment/978621#comment-978621

The “cream” colour of BaSO4 is perhaps in comparison with other white substances. Either they use optical whiteners that convert UV into blue (like copier paper) or reflect blue better than other parts of the spectrum (i.e. titanium oxide, often used in white paint). Your brain interprets white colours with lots of blue as “whiter than white”. While white substances that evenly reflect all wavelengths look less white than that.

djozz
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Just came across this article:

https://www.theguardian.com/environment/2021/apr/15/whitest-ever-paint-c...

Sounds all too familiar to the specs of an integrating sphere coating. If that makes it into a commercial product that is good news for us budget integrating sphere builders.

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Wow!

Quote:
The barium sulphate paint enables surfaces to be below the ambient air temperature, even in direct sunlight,
Love

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