How to use CIELAB to obtain illumination invariance in image processing?

Question

I found out that taking the Euclidean distance in RGB space to compare two colors in applications like image segmentation is not recommended because of its dependence on illumination and lighting conditions. Furthermore, because of the numerical instability of the HSV hue value at low intensity, the CIELAB color space is said to be a better alternative.

My problem is that I don't understand how to actually use it: Since CIELAB is device independent, you cannot simply convert to it from some RGB values without knowing anything about the sensor that was used to obtain these RGB values. As far as I know, you have to convert to CIEXYZ in an intermediate step first, but there are several different matrices available depending on the exact RGB working space of the source.

Or is it irrelevant which matrix you choose if you only want to use CIELAB to compare two colors (as I said, for example to perform image segmentation)?

Answer 1

If you don't know the exact color space that you're converting from, you may use sRGB - it was designed to be a generic space that corresponded to the average monitor of the time. It won't be exact of course, but it's likely to be acceptable. As you observe, perfect accuracy shouldn't be necessary for image segmentation, as the relative distances between colors won't be materially affected.