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Join Date: May 2013
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Optical Coating Removal

05/24/2013 6:30 PM

I'm trying to remove an infrared filter coating from an optical glass. The glass is in front of a CCD sensor. I tried 93% sulfuric acid soaking and hydrochloric acid without success.

Before trying any mechanical method (abrasion) that may alter optical quality is ther any solvent or acid/base able to remove such coating without attacking the glass?

Replacing optical glass is not possible since exact thickness is required in order to maintain focussing performance.

Thanks in advance

C.

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#1

Re: Optical coating removal

05/24/2013 6:48 PM

And the coating is what?

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#2

Re: Optical coating removal

05/24/2013 7:09 PM
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#3

Re: Optical coating removal

05/24/2013 8:12 PM

Surprisingly, the household cleaner, 409 is extremely good at removing optical coatings. I used it to clean a wristwatch and it dissolved the anti-glare coating on the crystal.

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#4

Re: Optical coating removal

05/24/2013 8:53 PM

The coating is probably a dielectric coating. It may look metallic, but not be. So acids might not work on it. You can try nitric acid or a combination of sulfuric and boric acids.

If those don't work, you can try polishing it off using an optical grade polishing compound like cerium oxide. (I used it once to polish a telescope mirror a long time ago.) It's a fast-cutting powdery material, usually just mixed with some water to make a slurry. By using cerium oxide you're polishing the coating off, and not grinding the glass surface. You should be left with a glossy, clear, smooth finish.

Here's a Wiki description of the material:

http://en.wikipedia.org/wiki/Cerium%28IV%29_oxide

Here's a source of a small sample that might be enough for your needs:

http://www.walmart.com/ip/19418840?wmlspartner=wlpa&adid=22222222227000000000&wl0=&wl1=g&wl2=&wl3=21486607510&wl4=&wl5=pla&veh=sem

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#5

Re: Optical Coating Removal

05/26/2013 6:48 AM

Try chromic acid, by mixing sulphuic acid and potassium dichromate, heat.

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#6

Re: Optical Coating Removal

05/26/2013 7:24 AM

Typical materials used for coatings include Magnesium Fluoride, Calcium Fluoride, Silicon Dioxide, Zinc Sulphide, metal oxides inc Titanium Dioxide, Metals such as Gold, Molybdenum.

Try Aqua Regia ie add Nitric acid to your Hydrochloric acid.

Caustic Soda (Sodium Hydroxide) unfortunately attacks glass, but it might be worth trying Washing Soda (Sodium Carbonate) or Baking Soda (Bicarb, preferred). These are both mildly caustic, carb more so than bicarb, which means they will attack glass slowly, but they may remove the coating, or at least lift it.

I suspect the mechanical methods will prove to be the best ones.

Check with camera people who dismantle and refurbish lenses. Lenses get scratched and get fungus on them. They would have to do what you are trying to do..

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#7

Re: Optical Coating Removal

05/26/2013 1:30 PM

Are you certain that the infrared filter is a coating? Many kinds of glass are inherently poor transmitters of infrared. If your glass is one of these, the only option would be replacement. If I'm not mistaken, quartz is better than glass...

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#9
In reply to #7

Re: Optical Coating Removal

05/26/2013 7:52 PM

I would try a solution of nitric acid, sulfuric acid, and glycerol alcohol. I'm certain it will remove just about any coating...

Crap! Before messing with all the acids and bases, you really might want to try a few of the organic solvents. You can always use the nasty stuff later. And you wont have to worry about a solvent taking out your glass as well.

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#8

Re: Optical Coating Removal

05/26/2013 3:10 PM

There are some good suggestions here to try to remove the coating. If that does not work, replacing the glass may not be as bad as you think.

The back focal distance from the lens mount to the CCD sensor surface will have been increased slightly to take account of the glass layer. The increase will have been [glass thickness -(glass thickness/refractive index)]. The glass is typically only 1mm or so thick & the R.I. will be around 1.4 so the adjustment is likely to have been about 0.35mm. As long as you are within about 0.1mm of the original focal length calculation you should be OK so this effectively gives you ±0.3mm on the glass thickness compared to the existing piece.

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