See http://en.wikipedia.org/wiki/Pulse_oximetry http://en.wikipedia.org/wiki/Pulse_oximetry for more information, but basically you measure the difference in ab

See http://en.wikipedia.org/wiki/Pulse_oximetry http://en.wikipedia.org/wiki/Pulse_oximetry for more information, but basically you measure the difference in absorbance between two specific wavelengths of light (one is near-IR and might not be captured by a cellphone's camera) over time. The difference tells you how saturated your hemoglobin is (most of which has oxygen bound to it). Pulse oximetry is one of the more reliable/least invasive/cheapest methods of determining pulse rate. Additionally, there's a massive market opportunity for extremely cheap PulseOx sensors that keep a data history: busy hospital emergency rooms would love to slap such a sensor onto the finger of every patient that comes through the door (and subsequently has to sit and wait). When a patient crashes while sitting in the waiting room the pulse ox sensor would be able to give the nurses and doctors a little bit of information about what happened that lead to the crash.