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As I understood, the full well capacity is the maximum number of charges(photoelectrons) that can be stored in a pixel before a leakage to adjacent pixels and causing problems like blooming.

In some websites, the full well capacity is given as the maximum color value stored in a pixel before saturation. But in my opinion it should be the maximum intensity value of a raw image, instead of color value (4 time larger). Am I correct?

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Both approaches are correct. At least as I understand what I think you are trying to say. I'm not sure, though, what you mean by, "(4 time bigger)".

Raw luminance values are monochromatic in the sense that there is only one intensity value for each sensel (what we call a pixel well). But every one of those monochromatic luminance values is the result of filtering for one of three colors of light. If a pixel is filtered for green light, then a full well value indicates the presence of intense light that contains green. If a pixel well is filtered for red light, then a full well value indicates the presence of intense light that includes a lot of red wavelengths. If a pixel well is filtered for blue light, then a full well value indicates the presence of a lot of blue light.

If the light striking a particular area of the sensor is white, then all the adjacent pixel wells should (at least after the data has been calibrated) return roughly the same value. But when the light striking a particular area of a sensor is predominately one color, such as blue, then the pixel wells filtered for blue light will have luminance values much higher than the surrounding red and green filtered pixel wells.

For more on various demosaicing approaches, please see What are the pros and cons of different Bayer demosaicing algorithms? and Would a demosaic algorithm for black and white be useful?

For what happens when demosaicing goes wrong, usually due to a hot or stuck pixel, please see: What is this bright squarish pattern that I'm getting in long exposure images?

  • I meant the maximum color value of a saturated raw image is 4 times larger than the maximum intensity value of the same image. As you already mentioned the filters, of course the color value for blue wavelength of a blue pixel should be larger, but I don't know how a pixel can give you 3 values for each color? How can the sensor differentiate how much of that white light was blue or red or green as there are only some photoelectrons produced. – faf Apr 29 '16 at 12:44
  • The sensor does not. The demosaicing algorithm used by the application to convert the raw data to a color image does. – Michael C Apr 29 '16 at 16:06
  • I agree, but how come I can extract RGB values from raw image?(demosaicing is done by interpolating RGB value of each pixel, i.e. red color from red pixel,etc.) – faf Apr 29 '16 at 16:14
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    @faf Demosaicing is a little more complex than that. It interpolates all three color values for each pixel based on not only the luminance value for that pixel but on the luminance values for surrounding pixels of the other two colors and values of other nearby pixels with the same color filter. The vast majority of demosaicing algorithms will not use a straight value from any pixel to assign any of the three colors to that pixel location. Even a pixel that is filtered for red will not just use the straight value of that pixel to assign the red value to that pixel. It will be interpolated. – Michael C Apr 29 '16 at 21:11

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