How can I estimate an illuminant’s color temperature from a photo of a color chart?

Question

I’m developing a digital camera and want to estimate the illuminant’s correlated color temperature programmatically for white balance/color correction. The light source is not necessarily a standard illuminant with known Kelvin value.

If I photograph a standard color chart (for example, a 24-patch chart) under that light, how can I estimate the illuminant color temperature from the captured RGB or raw Bayer data? I’m especially interested in practical approaches or algorithms for deriving CCT from chart data captured by the camera’s own sensor.

user1730 · Answer

To keep things simple, use a neutral gray card instead of a color checker. Or use only a neutral patch of the color checker. You have to identify in the image the area that corresponds to the relevant patch/gray card, then average separately the R, G and B raw pixels on this area. Make sure none of those pixels is saturated, otherwise start over again with a lower exposure.

You end up with an (R, G, B) triplet which is the color of your illuminant in your camera's native color space. Convert then to the CIE standard color space using a formula like

⎛X⎞   ⎛            ⎞ ⎛R⎞
⎜Y⎟ = ⎜ 3×3 matrix ⎟ ⎜G⎟
⎝Z⎠   ⎝            ⎠ ⎝B⎠

where the matrix is the color matrix of your Bayer sensor, which you hopefully already know. From here, you can compute the correlated color temperature using standard formulas.

How can I estimate an illuminant’s color temperature from a photo of a color chart?

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