I'd start by using AVFoundation to turn the light on. The answers in the linked post below include examples of how to do this:
How to turn the iPhone camera flash on/off?
Then as far as detecting the light change goes, you can probably use Brad Larson's GPUImage Framework. This framework includes a couple of helpful functions that you may be able to use to achieve this, including:
- GPUImageAverageLuminanceThresholdFilter
- GPUImageAverageColor
- GPUImageLuminosity
Using the filters listed above you should be able to measure color variations in your finger, and monitor the time intervals between the occurrences of these changes. Using this framework you may even be able to specify an arbitrary variance requirement for the color/luminosity change.
From there all you have to do is convert the time interval in between the color changes to a pulse. Here's an example of how to calculate pulse.
http://www.wikihow.com/Calculate-Your-Target-Heart-Rate
