Use this specially modified webcam to analyze plant health, in combination with the Infragram.org image processing tool to measure photosynthesis. Based on the same multi-spectral satellite imaging techniques used by NASA, this USB camera lets you take simultaneous infrared and visible photographs at 1600x1200 pixel resolution.
The pre-installed filter (also sold separately if you want to convert your own camera) 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; you can read more about the technique here and here.
This technique was developed by contributors to the Public Lab, an open network of collaborators who develop affordable environmental science tools. Weighing less than an ounce, it's perfect for connecting to your laptop, a Raspberry Pi, or a mobile sensing platform, and with unscrewable lenses and no infrared-block filter, it can be adapted for other uses as well.
For more information, check out their Kickstarter page!
