White balance

[KeithB]

Good article.
It got me musing what the "natural" white balance of the sensor is. What would a real raw image look like? They always seem to have *some* white point used before display.

Also, you might want to point out that the only reason we have to go through all this is that our eyes also do a white balance adjustment so that white paper looks white under most lighting types. If our visual system didn't do this, we could just use a fixed white point.

Also, the white piece of paper is not a good reference because it is dependent on exposure. if it comes out as 255, 255, 255, how do you know whether it is simply over exposed and it would be 255, 128, 128 when exposed "just to the right"?

[Bob Atkins]

White is good as long as it's not 255, 255, 255. However if you fill the frame with a white card and shoot with autoexposure you automatically get something close to a mid-grey because that's what exposure meters do. A white card included in a shot used as a reference for white balance when doing a RAW conversion shouldn't be 255, 255, 255 though. You have to make sure you're not blowing out one of the channels. You need a signal in all three color channels that's well above zero and below saturation, preferably something between maybe 128 and 250.

You also can't white balance if one or more colors is totally missing from the incident light. If all you have is green light with NO red or blue component, you can't generate a white balance. You have to have SOME response in all three color channels to generate a white balance that will work. So you can white balance a light with a pale green filter over it, but you can't white balance a light with a deep green filter over it that blocks out all red and blue.

There really is no white balance for RAW. You can't display a RAW file - it wouldn't look like an image. It depends on the relative response of the red, green and blue pixels and the fact that there are twice as many green pixels as there are red or blue. That may differ from sensor to sensor depending on the characteristics of the red, green and blue Bayer matrix filters over the pixels.

Raw files are displayed with whatever WB they were shot with. There's always some WB setting active in the camera. If you did a RAW conversion without a WB I'm pretty sure the result would look pretty horrible. I'm guessing that all RAW files contain default white balance information written in there by the camera. According to Wikipedia, the RAW file format includes sensor metadata which in turn includes the attributes of the CFA (color filter assembly aka Bayer filter) and its color profile, plus image metadata, which presumably includes a white balance correction.