I'd read online about using exposed 35mm film or exposed slides as an IR band pass filter - for viewing infra-red only. I didn't have any of these on hand, but the inner part of a floppy disk works too.
|Unmodified camera||Without red glass filter||Without red glass, through floppy|
Having checked I could see the signal from my TV remote through the modified webcam with and without the DIY filter (in fact you could even pick this up with the unmodified camera, but not nearly as bright), I wanted to try something a little more interesting. I'd seen examples online of anti-fraud designs in USA Dollars and English bank notes visible under IR, so that seemed a good test case. Sadly I didn't have any handy, but I found it works on Japanese Yen too.
Here are the front sides of three Japanese bank notes. The row below shows the reverse side of the same bank notes. In order to get reasonable exposure, I had to adjust the lighting conditions for each image type.
The first column is taken with an unmodified Xbox Live Vision and is a reasonable representation of how the notes look to the naked eye. Sadly the focus isn't perfect - this is done manually by twisting the lens and I was holding the camera free hand.
The second column is from a modified Xbox Live Vision where the red glass filter has been removed making it also sensitive to infra-red light. This messes up the default colour balance, but a number of of security design features are apparent.
The third column is also from the modified Xbox Live Vision lacking its red glass filter, but shot through the floppy part of a 3.5" disk as a crude DIY infra-red pass filter. This cuts out most of the visible spectrum, leaving mostly infrared.
|Yen front, visible light||Yen front, visible plus infrared||Yen front, mostly infrared|
|Yen back, visible light||Yen back, visible plus infrared||Yen back, mostly infrared|
First, the ¥10,000 note (man en), with Fukuzawa Yukichi (福澤 諭吉) on the front, and on the back the phoenix from the Byōdō-in Buddhist temple in Kyoto (also on the 10 yen coin). Notice how on (on both sides) the number 10000 appears twice, once dark and once light under IR, while they both look dark under normal light.
Second, the ¥5,000 yen note (go sen en), with novelist Higuchi Ichiyō (樋口 一葉) on the front, and iris flowers on the back. Notice on the front under IR the colour of 五 (or rather, an old fashioned form of that kanji more like an 二 and an X, for ご or go, meaning 5) and 千円 (せんえん or sen en, meaning 1000 yen) are pale and dark respectively - but the same under normal light.
Finally, the ¥1000 yen note (sen en), with biologist Noguchi Hideyo (野口 英世) on the front, and Mount Fuji and cherry blossom on the back. Similarly on the front side 千 (せん or sen, meaning 1000) and 円 (えん or en, or as we normally write it, yen) are pale and dark respectively under IR, whereas under normal light they look the same.
If you're interested this page talks about some of the security features in this series of Japanese notes.
I also tried Polish 10 and 20 Zloty notes, and Royal Bank of Scotland £5 and £10 notes, but these didn't show any obvious pattern under IR.
Update - English Five Pounds Note
I tried this on a reasonably crisp English £5 note, and as expected from other examples online, only half the Queen's face is visible under infrared - but that seems to be the only interesting difference:
|English £5 under visible light||English £5 under visible and IR||English £5 under infrared|
Update - Other applications:
Found this page on combining balloon mapping and near-infrared camera photography (like this), apparently it can be used to measure plant health.