This is a fascinating sensor. I have not been able to find out the details of how it works. It appears to use infrared light.
There is an 80 page datasheet plus some other info. Here is a 2 page blurb on theory.
There is an evaluation kit.
Sparkfun.com has a kit. There are some comments at Sparkfun.
Prices are ~ $6 for chip and ~ $20 for eval board