IR Filter ? (Not a "what should I buy" thread)...UV too

Josh66 · May 4, 2009

OK, I have two B+W IR filters. The 092 (89B) and the 093 (87C).

The 093 filter is supposed to block the entire visible spectrum.

So - why can I see through it?

Sometimes it's hard to tell these filters apart with a quick glance, so I hold them up to some light. The 092 filter does not block all visible light, so you can see through it OK in bright light. The 093 filter should be blocking all visible light, so it still looks black; but - I can see the sun through it.

If it's blocking all visible light, how is it that the sun is visible to the eye through it?

If it really did block all visible light I should not be able to see anything through it, including the sun, right?

And I know about the dangers of looking at the sun, I'm just talking about a brief glance through a very dark filter here.

One more question, just so I don't have to start a new thread...

The UV filters (not that kind) are looking pretty interesting too. I'm talking about the filters that block everything except for UV light. These.

Does anybody know what the sensitivity of the 350D (or any other body) is to UV light?

The glass in my lenses will absorb some of the light too (I think I remember reading that quartz lenses are best for this...$$$$), right?

I guess what I'm asking is, is it even worth trying?

Garbz · May 5, 2009

No filters are perfect. The 093 filter has an 830nm cutoff which is well into the infrared but the sun is such a bright source that would be visible since a couple of percent of the light is still visible at around 780nm and below. That said it also won't have a 100% perfect stop band.

All your assumptions are correct, except the one about looking at the sun. The filter is not dark, it is outright dangerous. All the bright light that causes heat (IR) passes through the filter. You put the filter to your eye and your pupils dilate because you think it is dark letting more light in. This lovely warming light then gets focused onto your retina. All it takes is a few seconds of this kind of exposure to raise the temperature enough to permanently blind you! IR is a very different beast than UV light, I suggest you don't do this again.

Also digital camera sensors are next to not reactive to UV light at all. Same as IR, except the IR is a result of a filter that can be removed. Also lenses block a lot of UV light naturally, often enough to make this kind of photography impossible with a digital camera. Not sure of Canons equivalent but for UV photography you need something like a Medical-Nikkor or UV-Nikkor lens to get going, whereas nearly any lens will do for IR photography.