Pulse Oximetry with Arduino - Help Required

I’m helping a student of mine build a pulse oximetry system using arduino. Now, we’ve managed to build the basic oximeter with 1 IR led, 1 RED led and 1 phototransistor. there is difference in the analog values sensed for a finger and non-finger however we are interested in finding the oxygen saturation levels and are clueless about going forward. Any suggestions from anyone here? Most of the stuff i’ve looked up use DSP processors and complex algorithms. Are there any open source ideas? anybody done anything like this before here? :-/