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UltravioletPhotography

Rain-X changes a filter's transmission


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Some time ago there were a discussion about how to best maintain a healthy surface on some types of sensitive filter glass.

One of the suggested potential cures was to coat the filter-surface with some kind of rain repellant product.

On the market there are several such products.

I bought a bottle of Rain-X for my car.

https://www.rainx.co...t/#.XxmmwPj7QxM

These products change the surfaces contact properties making it more difficult for water to stick to glass like windshields, making them more hydrophobic.

 

Today I decided to test if such a treatment changed the transmission and set up an experiment.

When harvesting filter rings from cheap "UV-filters" I got some rather thin glasdisk leftovers.

They transmit UV reasonably well to below 300nm and are suitable for this test.

 

I cleaned one of them and divided the surface, in half, with a visual tape, on both sides.

Then I treated one half of the disk on both front- and back-sides according to the application instructions for the product.

Both sides of that half got two applications of the product.

 

When measuring I used the untreated surface as a 100% reference to detect any change.

I alternated between treated and untreated surface as documented in the top left spectra table.

As can be seen there is a slight improvement of the transmission at the 300nm -end.

post-150-0-48133100-1595515257.png

 

The improvement is measurable, but very small and totally insignificant for normal photography.

 

The change to more hydrophobic behaviour is very obvious. only working for big water droplets.

post-150-0-57661000-1595516151.jpg

When fogging the surface with my breath I could not see much difference.

 

My conclusion is that it is likely safe to treat filters with this product, but it will might not protect them at all from humid air.

The treatment will not harm the UV-transmission.

 

The treatment might make it easier to avoid or remove droplets and dirt when the filter is exposed to rain or harsh environments.

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Andy Perrin
Do you think the material is acting as a simple anti-reflective coating to boost transmission? I am having trouble understanding how that would work otherwise.
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Do you think the material is acting as a simple anti-reflective coating to boost transmission? I am having trouble understanding how that would work otherwise.

If that's the case, it should change changing the angle of the glass filter.
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Andy -

Yes that is what I think too. As the layer must be very thin here, the optimal wavelength for this "AR"-coating will be far deeper into UV.

 

Stefano -

You are correct about that but changing the angle also changes the traveling distance of the collimated beam in the glass-material and adds some interesting polarisation effects and literally complex math.

Here is some things about surface reflection due to the difference of refractive index:

https://en.wikipedia...efractive_index

 

It will not be easy to differentiate between those effects with a simple setup as the "AR"-effect is so small here.

I'll pass on that

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Thanks very much for doing this test Ulf.

You seem to have concluded some favorable results.

I think that the 'secret' ingredient could be silicone as that transmits well through to UVC.

My coated sample of S8612 is holding up well.

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My conclusion is that it is likely safe to treat filters with this product, but it will not protect them at all from humidity.

 

1) Why won't it protect from humidity?

2) Assuming it will not, then at some point the Rain-X will need to be removed, to clean the glass surface.

I would assume the same cleaners may remove the coating that will remove the oxidation, but I am wondering what would be best to clean the coating from the glass if desired?

Any ideas? What is the Rain-X made from and what removes it?

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Andy Perrin

1) Why won't it protect from humidity?

 

I thought he meant it wouldn't protect them from humidity because it still fogged up when he breathed on it. I'm not sure the conclusion is correct, though, because the water is sitting on the Rain-X, not the filter itself, so it may still protect the filter. (And I also don't see why he would draw the conclusion that it's not hydrophobic for small droplets. The small droplets shown in the image certainly look like they have a big contact angle. Some are nearly balled up.)

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I don't think that is what he meant. Anything will fog up when you breath on it, a Baader U will... any coated filter will. Right?
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Andy Perrin

Well, no doubt he will tell us, but that's all I could make from what he wrote that sounded relevant.

 

And no, not anything will fog up if you breathe on it -- if the surface is too hot then there will be no reason for vapor to condense on it. That's how defrosters work.

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Well, I have never heated up my Baader U, or frankly any other filters...

I have never breathed on any filter coated, or not that didn't fog up.

Yes, he will tell us.

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I think it will not protect against humidity as the function of the coating is with the water's surface-tension in the droplets, changing the contact angle.

 

I am just a bit pessimistic about how well it protects the glass surface from the environment or gasses like water vapour, but might be wrong here.

My guess is that free water molecules in the air will not be affected by this.

It might, if the barrier is sealing off the glass's surface but as it is very thin it might not have a full coverage.

Further tests is needed to tell.

 

At least we now know that there is no transmission loss in for us important wavelengths.

As I think a treatment might help keeping the filters more clean too it can be a good idea to try it on a few filters.

 

Cleaning the surface can very likely still be done with Cerium-oxide as that is a mild mechanical processing of the surface.

It is very difficult to tell if the other chemical cleaning processes will be changed, and if they get more difficult or not.

 

I do not think the ingredient is silicone as the product's liquid appears to be mainly some organic volatile solvent, carrying the active substance.

This is not my field and I can be completely wrong.

 

I have edited my conclusions a bit in the first post due to this discussion.

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Andy Perrin
The way to test it would be to take a half-coated piece (like Ulf did above) of S8612 or some other Schott glass and put it in a closed container with a small amount of water for a while. The humidity will be fixed in that case, and should reach 100% relative humidity. Then wait and examine for changes at a later date.
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The way to test it would be to take a half-coated piece (like Ulf did above) of S8612 or some other Schott glass and put it in a closed container with a small amount of water for a while. The humidity will be fixed in that case, and should reach 100% relative humidity. Then wait and examine for changes at a later date.

If the temperature is also slightly elevated the test speed will increase.

Place the container over some room heater or by the window allowing the sun to warm it.

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Andy Perrin
Ulf, the relative humidity would stay 100%, but when the temperature changes, the actual amount of water in the air will go up and down with the temperature, so I would think keeping it away from windows would be better. Maybe you could warm it some other way, like with a lamp?
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I have put my coated 'Invisible Glass' sample of S8612 with another BG glass that has heavily corroded in the past, but now cleaned, in a sealed container with a small amount of water.
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Ulf, the relative humidity would stay 100%, but when the temperature changes, the actual amount of water in the air will go up and down with the temperature, so I would think keeping it away from windows would be better. Maybe you could warm it some other way, like with a lamp?

A constant temperature is better, but not critical, as long as it is elevated. A higher temperature will speed things up.

A place where the temperature is reasonable high all the time would be best.

 

I was just thinking about a method without any fire hazard or added energy.

If the heat source together with the sample container were placed together in a thermally insulated bigger container it would not need much power for heat source.

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I would like to know if acetone or hydrogen peroxide will remove the coating.

I assume acetone would. I would like to clean the glass free of any coating before using cerium, I don't know why, superstition perhaps?

I like to start with HP when cleaning, before cerium, often that does the job very well and fast with no cerium needed.

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A constant temperature is better, but not critical, as long as it is elevated. A higher temperature will speed things up.

A place where the temperature is reasonable high all the time would be best.

 

I was just thinking about a method without any fire hazard or added energy.

If the heat source together with the sample container were placed together in a thermally insulated bigger container it would not need much power for heat source.

 

I have placed the S8612 in a sealed container with a little water in the hot water heaters cupboard, that is comfy warm.

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I would like to know if acetone or hydrogen peroxide will remove the coating.

I assume acetone would. I would like to clean the glass free of any coating before using cerium, I don't know why, superstition perhaps?

I like to start with HP when cleaning, before cerium, often that does the job very well and fast with no cerium needed.

I'll check that later on the test disk I used.
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I would warn everyone not to put anything on your filters unless you first know what is best appropriate to remove the coating.

A filter is more important than a coating.

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That is a very good advice and the reason to that I used a cheap discarded glass disc for the test.

 

I tested to remove the coating with acetone without any success. Acetone is surprisingly also beading on the surface.

Rain-X is designed to survive alcohols and other organics, normally added to the wiper fluid.

 

However it was really easy to remove the coating with Cerium oxide.

At first that liquid also beaded, but after rubbing it spread nicely and worked well, removing the coating.

post-150-0-20266100-1595674962.jpg

 

Remember that it is important to use quality cerium oxide that is pure.

I bought mine from Steve (Cadmium).

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my goodness !! You can find almost everything these days in Wikipedia.

Rain-X is some mix of polysilicates polysiloxanes, acetone and water.

https://en.wikipedia.org/wiki/Rain-X

 

It's also flammable and toxic.

 

If acetone is in Rain-X, then that might explain why acetone would not clean it off the glass?

 

 

Article in Wired says Rain-X also contains isopropanol (ethyl alcohol) and sulfuric acid.

https://www.wired.com/2010/06/st-whatsinside-rainx/

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my goodness !! You can find almost everything these days in Wikipedia.

Rain-X is some mix of polysilicates polysiloxanes, acetone and water.

https://en.wikipedia.org/wiki/Rain-X

 

It's also flammable and toxic.

 

Maybe someone with better understanding of chemistry can say if there is an improved chance of the protection we seek too:

From the article referenced above:

"The polysiloxanes have functional groups that bind to the hydroxyl group of the glass surface"

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