5 ms·

What in the world? Is this magic?

• deleted 14y ago
  [deleted]
• AdamTReineke 14y ago
  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.
  • huhtenberg 14y ago
    > ... unperceivably redder But why is he analyzing the green channel then? :)
    • mkopinsky 14y ago
      Look at the absorption spectra at http://omlc.ogi.edu/spectra/hemoglobin/index.html 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.
      • acuozzo 14y ago
        > 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.
        • mkopinsky 14y ago
          Is that noise inherent to film? Do webcams have the same noise?
          • jerich 14y ago
            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.
            • mkopinsky 14y ago
              Why do they make them like that? Why not make the sensors 33/33/33?
              • jerich 14y ago
                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.
        • deleted 14y ago
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