On each heartbeat, blood is pushed and makes the skin nearly unperceivably redder. By watching for and tracking this subtle change, you can extract a heartbeat.

> ... unperceivably redder

But why is he analyzing the green channel then? :)

Look at the absorption spectra at http://omlc.ogi.edu/spectra/hemoglobin/index.html. Green is ~510nm, red is ~650nm, blue is ~475nm. The difference between the absorption of Hb and HbO2 in the red range is the reason why pulse oximeters use red. In this case, we don't care about differentiating oxy- vs. deoxy-hemoglobin, we just care about the total absorption. Since more light is absorbed at green than at red, it will show more of an effect with pulse.

I am not confident in this explanation, and still don't know why green over blue.

> I am not confident in this explanation, and still don't know why green over blue.

I'm not sure either, but I can tell you that if you examine digitized (RGB) 35mm film -- across all 35mm film stocks -- the blue channel is the most noisy.

So my guess is: He might have chosen green over blue due to the blue channel being noisier.

Is that noise inherent to film? Do webcams have the same noise?

The green channel has the least noise for Bayer-sensor digital cameras (which is almost all of them).

50% of the sensor sites are green (25% each for red and blue), so there is more signal given the same amount of noise.

Why do they make them like that? Why not make the sensors 33/33/33?

From Wikipedia on the Bayer Filter: "Bryce Bayer's patent (U.S. Patent No. 3,971,065) in 1976 called the green photosensors luminance-sensitive elements and the red and blue ones chrominance-sensitive elements. He used twice as many green elements as red or blue to mimic the physiology of the human eye. The luminance perception of the human retina uses M and L cone cells combined, during daylight vision, which are most sensitive to green light."

The Foveon sensor does sample RGB at every pixel by using 3 layers, but it is only used in a few cameras.