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UltravioletPhotography

Before, after, and thoughts.


Cadmium

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While trying to find a couple other filters last week, I by accident found these two filters from the same year, 2016, hiding in the same drawer.

These are both 4 years old, they are both similar BG type filter glass, but not exactly the same filter glass.

One is Chinese glass, QB39 2mm, the other is Schott S8612 3mm.

I probably only used each of these once for testing after I first made them.

These have never been cleaned until now.

Here are pics before and after I cleaned them.

I didn't need to use Cerium Oxide on these, I simply soaked both in hydrogen peroxide for 2 days, then scrubbed them with HP, and thus the results, very clean and ready to use, no residual surface anomalies.

You may think I replaced the glass with newer glass, but no, these are the very same filters.

Also, the retaining rings were stubborn to remove after being tightly in contact with the oxidizing glass for 4 years, so I soaked the entire filter, glass, ring, and label, in the HP.

This may not be good for the labels, but even after 2 days, the labels seem OK to me, but the glass is most important,

and the retaining rings unscrewed much easier, the soaking loosened up the stuck retaining rings.

These are indeed the most oxidized filters I have ever refurbished.

I was so glad I found these, because they are both the same age, one is Chinese glass and the other Schott, and this dispels the idea that one or the other has better or worse oxidation longevity.

After removing the filter glass from the rings, I scrubbed them while still wet from the HP with my finger wrapped in a PEC pad, each side...

and then wiped them clean and dry.

Here are the before and after results.

 

Before cleaning

post-87-0-13927400-1597125802.jpg

 

After cleaning

post-87-0-00480400-1597125819.jpg

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Thoughts.

There are three ways to deal with oxidation.

1) Raw filter glass. Clean them, more often the better.

2) Cladding. This is to seal your filter so nothing can oxidize them.

3) AR/protective coating. The same idea as cladding.

 

There are problems with cladding glass. It can still oxidize and have other void problems over the years, under that cladding, under the glue.

You may have seen this in older glued lens elements before,

but you probably have not seen oxidation between glued filter glass layers, but I have, it can happen.

So which lasts longer, raw glass or clad glass? Probably clad glass,

but after a while the clad glass can develop problems that are then impossible to resolve or clean unless you are able and willing to chemically separate the cladding from the filter glass.

If you remove the cladding from the filter glass, then I will bet you will be wondering why you would reattach the cladding to the filter.

Why not just leave the filter raw so you can clean it in the future without removing the cladding again.

My preference, use raw filter glass and clean them.

 

Now, that brings us to coating the filter glass. This remains to be seen exactly how well this will work over time.

It may or may not have similar problems as the cladding. It may last longer, but it may still have issues that will be impossible to resolve.

The coated version is on its way, it will cost twice the price, and it will require many years to find out what the longevity will be and if there will be any problems.

My personal preference is raw, because they can be cleaned easily, you are in control, and they are less expensive.

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The main advantage of the coating to me would be reduced flare issues. I have periodically had trouble with taking UV photos when the sun is too close to the camera direction and I would hope it might at least help with that issue. Otherwise I would go with raw glass also. I have never had significant oxidation issues, perhaps due to my local indoor climate.
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Coating is a concept covering a very wide range of treatments to achieve different things.

What kind of coating is it you are waiting for Steve?

 

Some will definitely protect the surface from decomposition or ageing.

Others might not. (Rain-X??)

Some will improve optical performance in some ways.

AR-coating is an example of that.

It is difficult to create an AR-coating that is efficient over a wide range of wavelengths.

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Here is an example of 'voids' and oxidation in and under glued cladding.

I had this filter made by someone in about 2012, I didn't make it, I would rather not say who made it. It is S8612 1mm clad on both sides with 0.5mm thick clear glass, which I 'think' was BK7, not sure, but not important for this example.

This filter was perfect when I got it, but over the years it has developed these 'air pockets' or 'voids', I don't know what is contained in those pockets, it might be air, or something else. The filter had no voids originally.

The voids are easy to see with your eye, but much harder to see is the oxidation which is happening under the glue, directly on the S8612 glass.

The oxidation looks like spots, rings, and blotches.

I don't know the process or the glue used in this cladding, but this filter is an interesting example of how bad things can happen regardless of cladding.

I would need to un-glue this filter to clean it, not something anyone wants to have to do.

 

This shows only the voids, these are empty pockets forming between the S8612 filters glass and the clear cladding glass, in the glue layer.

post-87-0-04362600-1597153906.jpg

 

Here you can see both the void pockets and the spots of oxidation under the glue on the S8612 (shot in full spectrum, why the ring is red).

post-87-0-07599400-1597154180.jpg

 

post-87-0-37220100-1597154111.jpg

 

post-87-0-06841600-1597154192.jpg

 

post-87-0-18594100-1597154221.jpg

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I had to abandon BG39 and BG40 years ago because I didn't know they could be cleaned with HP. :sad:

 

Either coating and cladding should be effective. I remember reading that Leica uses special glass material that is oxidized easily, and they place the grounding process and the coating process as close together as possible in the factory, so that the ground lens elements can be coated as soon as possible. If I remember correctly, the Andrea-U filter by UVR optics is of the three-layered clad type. But then the filter should be well sealed with the filter ring and its retaining ring in order to protect the side of the filter glass against oxidization. Otherwise, the raw glass would be better for the DIY UV shooters to maintain the filter.

 

Having said that, I wonder if the BG filter is still necessary since the Baader U filter is readily available?

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Having said that, I wonder if the BG filter is still necessary since the Baader U filter is readily available?

The reason for preferring filter glass over a Baader U is that the latter is a dichroic filter and will have artifacts with any wide angle lens. Also, the Baader is more expensive than even a two filter stack.

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Coatings: Can peel or flake, usually due to heat. Can become scratched. May also chemically deteriorate depending, of course, on the composition. Transmission of the coating fluid must be taken into account also.

 

Even coated and clad filters must be properly stored and maintained.

 

I hope everyone realizes that there is no perfect answer here. :grin:

Oh well.

 

Thank you for the interesting report, Cadmium.

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We need an optical grade two dimensional uv band pass diffraction grating. Remove glass all together, just allow 350nm to 395nm through.

Should be fun to invent.

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Is the peroxide you used the common 5% solution, the less common 30% solution, or something else? In any case, the cerium oxide will remove the efflorescence more quickly, if you have it--it only takes minutes.
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Akira, If you want to shot a UV+Blue+Green photo then you will need S8612 for that, and UG5 or U-330 too, all of which oxidize and will need to be cleaned.

There are many important and unique uses for S8612 and other BG filters that go beyond the limited use of a Baader U and other UV-only filters.

S8612 and BG filters are used to limit the range of other filters to a UV+Visual range, without them many other filters would have limited usefulness.

S8612 is the best all-in-one UV transmission and IR suppression filter. No equivalent in any brand.

 

Clark, I used the common 3% variety, which is all I have ever had.

USPS has mailing restriction for hydrogen peroxide. I think up to 8% is OK, 8% to 20% is another class, and surface only, and above 20%, I am not sure?

It might be interesting to try using some higher strength, but I have not tired it.

Also, it might be interesting to try using an ultrasonic cleaner in conjunction with HP, that might clean much better and faster, something to try.

 

Cerium is great, and it cleans fast, however, I believe it is best to try using HP first.

Try to dissolve and remove as much of the oxidation before you use cerium, that will help to keep the surface flatter.

Cerium doesn't differentiate between spots of oxidation build up and flat areas of clean glass.

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The reason for preferring filter glass over a Baader U is that the latter is a dichroic filter and will have artifacts with any wide angle lens. Also, the Baader is more expensive than even a two filter stack.

 

Certainly! I forgot about the angle-of-incident issue of the dichroic filter. Thank you for the correction.

 

Akira, If you want to shot a UV+Blue+Green photo then you will need S8612 for that, and UG5 or U-330 too, all of which oxidize and will need to be cleaned.

There are many important and unique uses for S8612 and other BG filters that go beyond the limited use of a Baader U and other UV-only filters.

S8612 and BG filters are used to limit the range of other filters to a UV+Visual range, without them many other filters would have limited usefulness.

S8612 is the best all-in-one UV transmission and IR suppression filter. No equivalent in any brand.

 

Oops, again, I forgot about the multi-spectral images. Thank you for the reminder.

 

I do have U-330 filter whose surface is also oxidized. My U-360 has been intact for more than 10 years. According to the catalog, the chemical characteisticrs (Dw and Da) are (4 and 5) for U-330 and (3 and 2) for U-360.

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S8612 is an excellent cyan filter. However, there are some cyan filters that are nearly the same as S8612. One might require another 0.5mm to get the same transmission curve, but it would also provide a good cost savings. One glass company shows a curve identical to the S8612, but their price is even higher than Schott and they only provide a melt cut to 5mm thickness. Given all of the variables - price, thicknesses, bubbles, diameters, minimum quantity, delivery times, shipping, dependability, etc. - it takes a while to find the right supplier of glass. And dichroic coatings are even more expensive to get right. Usually, you need to buy an entire batch from the coating machine. With coating, if one piece is not to spec, the whole batch is probably trash. Ah, well. It is all still fun.
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  • 5 weeks later...

I had some s8612 filters which were showing some surface 'oxidation'. 24 hours in 3% hydrogen peroxide solution and they cleaned up fine, and the labels were not affected. However I couldn't undo the retaining rings even after a 24 hour soak. Might try it for longer next time.

 

Times like this I wish I still had access my my old PhD lab and the surface analysis kit in there - would be interested to know what this 'oxidation' is.

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We really need one of those X-ray fluorescence thingies that Cadmium linked. Instant readout of all the elements...

If they only read elements, you then have to figure out the possible molecules. Scraping off some oxide (or whatever it is) and then analysing it could tell you something. Doing that with the glass below will read both the "stuff" and the glass.
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You know that stuff "Oxi-Clean"? Comes in different forms, and in other brands too. It is a solid form on Hydrogen Peroxide, when it is mixed with water, it breaks down into hydrogen peroxide.

I think they put the same stuff in all sorts of things, laundry detergent, dish-washing detergent...

I know they put HP in all sorts of things, tooth pastes...

Oxi-Clean, AKA, Sodium percarbonate = sodium carbonate + hydrogen peroxide.

https://en.wikipedia...um_percarbonate

 

One might even try that stuff, given that the HP percentage by weight is 30+%.

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It has been a humid summer where I live. I recently was on a trip to the Wyoming/Montana area and reached for my wide-angle with the UG11/S8612 stack filter. I took one look at the UG11 side and--uh-oh. I had no cerium oxide with me, so decided to try the peroxide soak with a bottle from a local grocery. I also looked at my Kolari deconverter, which has given me chronic trouble before, and it was at it again.

 

A day and a half later, and I can say that the efflorescence is gone from both filters for now..BUT

 

from the BG side of the sandwiched filter, one can clearly see a ring where a film of liquid has penetrated a few mm from the rim, implying incipient delamination. What the optical effects of this will be remains to be seen--but perhaps caution is indicated when using soaking baths on cemented filters.

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I have cleaned some glued stacks after soaking them in hydrogen peroxide, and I have not seen that happen.

The glue used is a UV cured type, and is very tough and should not be touched by 3% hydrogen peroxide.

I will test it.

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