Complete Guide to Digital Cameras


What color are pixels captured by a digital cameras CCD?
Oct 18, 1999


Q: A lengthy discussion has ensued on one of the forums concerning the color of pixels in a raw digicam photo when enlarged on a computer so that individual pixels can be clearly seen. There seems to be two points of view:

1. The digicam attempts to show color as accurately as possible by inserting various colors into an area as close as possible to the color of the original scene. That is, an enlarged area of a blue sky may show green, gray, pink or other color pixels.

2. Since the digicam reads a single primary color for each pixel and then interpolates the other two primaries and combines all into a single color reading, each pixel in an enlarged photo should be a replica of the original scene. That is, an enlarged view of a blue sky should have only blue pixels of the appropriate color.

Which of the above two alternatives views is correct (or is there a more correct answer)?

Rodger C.


A: I guess the question is interesting from an academic point of view, but very few cameras will let you see the original data that comes from the CCD. I think some of Canon's cameras output a file that is supposed to be the original unaltered CCD data, but I don't know how unaltered that is.

Anyway, imagining that you could get that data, and the camera was a single chip CCD (which most digital cameras are) then this is what you should see:

A CCD is not sensitive to color; only to brightness. To make it sensitive to color, a color filter is placed in front of the CCD(s.) With a single CCD, each individual element of the CCD (called a photosite) has it's own colored filter placed in front of it. This is typically in a Red/Green/Blue pattern, though at least one manufacturer has used a CMY pattern.

Continuing with the RGB cameras, most of these use the Bayer pattern for the filter, which actually complicates the RGB pattern by adding twice as many green pixels than blue and red pixels. If you search on the web you can probably find a graphic that shows what this looks like. This pattern was developed by a guy at Kodak (Bayer) because the human eye is more sensitive to green than the other colors, so he wanted to increase the accuracy of the sampling for greens. So for every 100 pixels, 50 will be detecting green, 25 will measure Blue and 25 will measure Red.

So you point your camera at a blue sky and take a picture. The blue light travels through the lens, then through either a red, green or blue filter, and then hits the CCD photosite. If you could see the data coming from the CCD, then what you should see is that the photosites with blue filters would receive a lot of light, so the voltage they would return would be high. The Red and Green photosites would receive a lot less, so the voltage they return would be much lower.

This voltage is then converted into a digital value, which represents the level of light hitting the photosite. So the first photosite might have a high level (because it had a blue filter), while the second would have a very low level because it had a green filter.

If you had the file and could open it, the pixels would not appear to be blue, green or red, since the file would just contain a single value for each pixel representing the value coming from each photosite. In other words, the file wouldn't be in RGB format with the Red pixel values placed in the Red Channel, etc. Instead it would probably look like a grayscale file that was strangely dithered, and you'd have to know which pixel had which kind of filter associated with it. But you would see a widely variable brightness level from pixel to pixel.

It might be possible to translate that into an RGB format to "see" the pixels as the blue, red or green data. But no one would bother doing that. Instead, the camera attempts to average the colors by taking the values from surrounding pixels and averaging them, and then saves an RGB data file.


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