4 ms·

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 ~

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 13y 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 13y ago
  Is that noise inherent to film? Do webcams have the same noise?
  - jerich 13y 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 13y ago
      Why do they make them like that? Why not make the sensors 33/33/33?
      - jerich 13y 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 13y ago
  [deleted]

acuozzo 13y 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 13y ago
Is that noise inherent to film? Do webcams have the same noise?
- jerich 13y 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 13y ago
    Why do they make them like that? Why not make the sensors 33/33/33?
    - jerich 13y 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 13y ago
[deleted]