It turns out that by pointing a simple smartphone camera at a person's face, you can get a good idea of their heart-rate, so good that it can often work about as well as a smartwatch.
I've spent ~11000 words talking about it if you want to read about how I got skin detection running in real-time or the nitty-gritty of when it works and doesn't. It's implemented in Python, with heavy reliance on OpenCV, NumPy and SciPy amongst a few other libraries.
