Improving the DIY integrating sphere

James0604

New member
Joined
Aug 18, 2021
Messages
1
I am truly amazed at the time and effort that many here have put into building integrating spheres. The cooperation seen in the past amongst the community in helping one another with calibration and builds has been one of CPFs high points. There has been a great deal of effort expended on deciding how best to build these spheres but I have found it intriguing that one of the most important aspects of an IS has completely been ignored. Sure there have been a few here and there that have made mention of the need for a proper coating but I know of no one yet that has actually done it. I know people here care about having as accurate a device as is possible so the only thing I can figure is that people just don't see the importance. Well hopefully we can change that.

If you do any amount of research into IS you will find how much they stress the importance of a good coating. Without the coating your IS is more of a S. than an IS. There are two main qualities you want in the coating. Diffuse reflectivity and it needs to reflect as much of the spectrum as possible. A standard flat white paint from the hardware store may fill the need for diffuse but it is doubtful it will faithfully reflect the spectrum of the light source. That would lead to incorrect readings.

Spectralon is the industry standard for coatings as it is an exceeding good diffuse reflector over a wide spectrum range. It is also exceedingly expensive and out of reach to the DIYer. So what to do?

In my research I found this article that mentions that barium sulfate powder alone on a surface actually performed better than Spectralon in the 425nm-600nm range which just so happens to be the sweet-spot for what we deal with here. Of course they rightly saw a need for a binder agent as just applying powder to a surface without having something to hold it all together and to the surface is not a good idea. So they mixed it in with a white latex paint. The problem is that the performance decreases as paint is added. This got me to thinking about the need to have the bare powder face out to get the best performance so I started to do a little experimenting. So far the best binder I have found is a white latex paint but I don't mix the BS into the paint but rather paint the surface and then while it is still wet dust it so that the powder adheres. This leaves a very flat(as in sheen) and diffuse reflecting surface. Perfect for an IS.

To show the difference in performance between the bare foam as is used in most of the DIY spheres I made a little demo and posted it over at my youtube account. I invite you to take a look at just what a difference it can make. Sorry that the sound seems out of step with the video. I have no idea how to fix that.:eek:

And here's the best part. It's incredibly cheap! Here is a link to a pound of the powder for only $6 plus shipping. This is an absolute must have in my opinion if you want your integrating sphere to be a true tool. So get cracking on those spheres.👍
It is really cool, what’s the integrating sphere coating? Is it BaSO4? I see an article to learn the BaSO4 reflective value can be 0.96 and this link for your reference: integrating sphere BaSO4 coating
I am truly amazed at the time and effort that many here have put into building integrating spheres. The cooperation seen in the past amongst the community in helping one another with calibration and builds has been one of CPFs high points. There has been a great deal of effort expended on deciding how best to build these spheres but I have found it intriguing that one of the most important aspects of an IS has completely been ignored. Sure there have been a few here and there that have made mention of the need for a proper coating but I know of no one yet that has actually done it. I know people here care about having as accurate a device as is possible so the only thing I can figure is that people just don't see the importance. Well hopefully we can change that.

If you do any amount of research into IS you will find how much they stress the importance of a good coating. Without the coating your IS is more of a S. than an IS. There are two main qualities you want in the coating. Diffuse reflectivity and it needs to reflect as much of the spectrum as possible. A standard flat white paint from the hardware store may fill the need for diffuse but it is doubtful it will faithfully reflect the spectrum of the light source. That would lead to incorrect readings.

Spectralon is the industry standard for coatings as it is an exceeding good diffuse reflector over a wide spectrum range. It is also exceedingly expensive and out of reach to the DIYer. So what to do?

In my research I found this article that mentions that barium sulfate powder alone on a surface actually performed better than Spectralon in the 425nm-600nm range which just so happens to be the sweet-spot for what we deal with here. Of course they rightly saw a need for a binder agent as just applying powder to a surface without having something to hold it all together and to the surface is not a good idea. So they mixed it in with a white latex paint. The problem is that the performance decreases as paint is added. This got me to thinking about the need to have the bare powder face out to get the best performance so I started to do a little experimenting. So far the best binder I have found is a white latex paint but I don't mix the BS into the paint but rather paint the surface and then while it is still wet dust it so that the powder adheres. This leaves a very flat(as in sheen) and diffuse reflecting surface. Perfect for an IS.

To show the difference in performance between the bare foam as is used in most of the DIY spheres I made a little demo and posted it over at my youtube account. I invite you to take a look at just what a difference it can make. Sorry that the sound seems out of step with the video. I have no idea how to fix that.:eek:

And here's the best part. It's incredibly cheap! Here is a link to a pound of the powder for only $6 plus shipping. This is an absolute must have in my opinion if you want your integrating sphere to be a true tool. So get cracking on those spheres.👍
It is really cool, what’s the integrating sphere coating? Is it BaSO4? I see an article to learn the BaSO4 reflective value can be 0.96 and this link for your reference: integrating sphere BaSO4 coating
 

VintageTech79

New member
Joined
Aug 23, 2021
Messages
1
Decades ago I worked for a color spectrophotometer company. One of the services we offered on one particular model was IS refurbishment. The spheres were cast two piece aluminum and bolted together The casting exterior was a roughly cubical shape. We removed them from instruments and separated the halves. All the old barium was rinsed off and the primer layer inspected for excessive discoloration from aging/contaminates(some were in paint stores)/mechanical damage. After drying acceptable castings for refurb were coated with -wait for it-Kilz White Primer I forget how many layers. This made a surprisingly good substrate for the Kodak Barium Suspension we sprayed on with a Graco gravity fed spray gun. Both operations required holding the sphere halve at arms length and applying a fairly rapid curling(swiveling?) motion to it as one sprayed. Checks were done between primer layer application to check for runs which nixed the halve for subsequent barium application. After both halves dried the sphere was reassembled and checked on a reference machine for quick ballpark reflectivity test then sent on to the customer machine to begin calibration with the new electronics or existing electronics depending on the revision/job requirement. Dont know if any of this will help the diy endeavor but thought it might be of interest.
 
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