This can make pictures look funny, so manufacturers add a filter to block the infrared and give you a more natural image. Actually the filters don't do a perfect job and if you fire a TV remote control at your camera phone you should see it light up on the screen.
All this means you can have a bit of fun with your old web cam. You can remove the infrared filter then add a visible light filter so your webcam only sees infrared!
There are loads of different makes of webcam but most of them have a bit that looks like this:
It's the lens assembly. The end you can see is the end that sticks out of the webcam. At the other end is the IR filter.
I was able to slice off about 1mm from the end to release the filter using a stanley knife:
Then it just screws back into place.
So now you're letting in the infrared all you have to do is block out the visible. You can buy a special filter for the job, or (thanks to Alby Reid) you can cross two polarising filters because these generally only block out visible light. Try the lenses from a pair of 3D glasses (with ReadD branded glasses you'll need to flip them around in all different combination until you find the one that blocks light. That's because they filter circularly polarised light and are a little more complicated than straight polarisers) Here's my setup:
For a while I wasn't seeing anything, just a black screen. Then I realised all the light in room was from energy saving lightbulbs which give off no infrared. I switched to an old style incandescent bulb and the room lit up!
Here are some weird looking things (click to enlarge):
I think my Mona Lisa might be a fake!
Only the Queen is left when you take away visible light:
And here's my eyeball:
Plants aren't that into infrared: