A piece of "infrablue" or red filter which you can use to turn your webcam or cheap point-and-shoot into an infrared/visible compositing "multispectral" camera. The filter allows you to take an infrared photo in the "red" channel of your camera, and a visible image in the "blue" channel. These can be used to measure photosynthetic activity; read more about the technique at http://publiclab.org/wiki/near-infrared-camera and http://publiclab.org/wiki/infragram.
Post-process your photos at Infragram.org , the free and open source image processing website which composites your images and helps you to assess plant health. See some example images at left.
Installing your filter:
Be aware that this is a relatively permanent change to your camera -- you may not want to convert your expensive DSLR, for example! We've also found that not every webcam will produce good infrablue images -- while all the legit point & shoot cameras are fine, we're keeping track of which webcams do and don't work well on this page: http://publiclab.org/wiki/infragram-convertible-cameras (if you're doing your own experiments, as many already are, please add your findings there!) Additionally, we have discovered that a Rosco Red Fire filter works better for a larger variety of cameras and we have begun moving to produce red with blue filter packs.
(The lens assembly removed and the infrared-blocking filter being extracted. Typically the IR filter that needs to be removed is glued to the backside of the lens assembly.)
Get involved in the process today by signing up on our "plots-infrared" discussion list: http://publiclab.org/wiki/mailing-lists