Possible to "disable" IR filter by chemicals?

[hap2001]

Hi.
I'm going to do "IR convert" on my cameras, but finding a suitable replacement glass is tricky and expensive.
Is the actual filtering happen on the surface by a coating, or inside the glass itself?
If it's only on the surface, can I use some kind of chemical to dissolve them? What chemical should I use?
Thanks.

[switchabl]

Yes, it is almost certainly a multi-layer dielectric coating. It might be possible to remove it chemically but the first problem is that you don't even know the materials (SiO2, TiO2, Al2O3, MgF2, ...?). And some of the stuff that could work is pretty nasty and will attack the glass substrate as well. If you really have to, polishing it off should work, it won't be more than a couple of microns.

Most optics/glass suppliers will do custom sized windows (with a suitable AR coating). If you can't afford that, you could try buying a larger sheet in a standard size and cut that yourself.

You will probably get more helpful replies if you ask on a photo/astronomy forum, like Cloudy Nights.

[Someone]

Alternate opinion: IR (long cut) filters in cameras are predominantly not dichroic/interference filters, although some are. Even those that aren’t thin film filters they usually have some anti reflective coating, and may include LPF.

Unpopular opinion: Your lenses won’t be focused far outside their visible wavelengths anyway. Replacing an IR filter with an AR coated plain glass is nice in theory, but makes very little difference to just leaving it open.

[switchabl]

I think I was extrapolating mainly from the discrete (lab-grade) long cut filters I've seen, so you may very well be right. There is a relatively easy way to check for your camera without taking it apart: shine an infrared source on the sensor and check the reflection. A dielectric filter should be highly reflective in the IR.

If you want to retain full VIS performance (autofocus on DSLRs and image quality with wide-angle lenses) I think you really need to keep the optical path length intact. Otherwise, leaving the window off might indeed be an option.

EDIT: I found a Canon service manual (http://cholla.mmto.org/photography/gear/repair/eos_40d_service_manual.pdf) that indeed mentions an "Infrared-absorption glass" in the sensor assembly.

[Stray Electron]

   I looked into converting my old Olympus E-2 to IR use and I found that most people simply removed the filter and left it out. AFIK it's only a filter and not a lens.

   YMMV.

[magic]

Flat glass still refracts light and fast and/or wide angle lenses may be picky about it. Goes both ways: some film lenses don't like digital, some digital lenses could lose sharpness on film or a filterless sensor.

Speaking of Olympus, the glass in MFT is said to be 4mm thick and FT may be similar. That's much thicker than other systems (I presume Canikony aimed for at least decent compatibility with their existing film lenses).

[hap2001]

Thank you everyone.

I tried only remove and not replace on a rubbish point-and-shoot years ago, and as expected, it did not focus to infinity.
So to guys who don't need a replacement glass: you are lucky.

If I don't change lens frequently, maybe I can just put a convex lens in front of my DSLR lens...

[magic]

Yeah, I can see how this can be a problem too.

BTW, what thickness do you need? For 0.17mm or 1mm you could try microscope cover or slide glass, respectively. Except for bottom of the barrel stuff at $2 on eBay, they are decently flat (hold one in front of your eye and see if the scene behind warps when the glass is moved). Made of fairly ordinary glass, probably no less transparent to NIR than any random photographic lens (if that's what you plan to use). I just tried shooting a remote control through a microscope slide, no problem at all.

[MarkT]

Some black perspex is completely transparent at near IR, so you could even filter out visible light if you made a filter from that (though you won't get optically flat samples I fear).