Depending on the spectrum of the phone camera's flash LED, you might be able to get away with just putting a diffraction grating in front of the camera. I recall, from reading a TI app note about pulse oximeters a while back, that these work by comparing the relative intensity of IR and a visible-red light transmitted through the finger (and that there was also some way to do it with reflected light). I wonder if there is a clever way to calibrate the grating without any extra hardware?