LAB colour space part 2
So I want to quickly get into diagnosing a colour cast using the LAB colour space.
Every colour in the LAB space can be defined by three numbers, one for L, one for A and one for B. This is the same as RGB where for example a tone of red might be R=238, G= 13, B=13. In LAB, the L-value gives the brightness or Luminance of the pixel from 0 (black) to 100 (white). The A-value gives a colour from Magenta to Green, and the B-value gives a colour from Yellow to Blue. Neutral tones (i.e., those with no colour) are coded A=0, B=0. That goes for black, white and all the neutral gray tones in between. Numbers on the plus side of A code for increasing Magenta (a warm colour), and on the minus side code for increasing Green (a cool colour). Numbers on the plus side of B code for increasing Yellow (a warm colour), and on the minus side code for increasing Blue (a cool colour). So that same red I mentioned above in LAB works out to be L=59, A=86, B=73. This means it's made up of lots of Magenta and lots of Yellow. A Blue I looked at in LAB worked out to be 56,-19,-52 for L, A, and B, so in this case some Green and lots of blue.
So again neutral tones in LAB get values of A=0, B=0. This is the basis of diagnosing a colour cast in LAB. To look at LAB values in parts of an image, open up the Info panel and pull down the small menu, top right and choose Panel options, and select LAB color for the Second color readout. Then mouse around an image and see how the LAB values change. In the attachment I show Lance's original image. I've placed the cursor over a portion of the image that I judge should be neutral- in this case some white feathers in the shade. The Info panel shows that the pixels under the cursor have values of A=4 and B=-20, which means a little on the Magenta side of neutral in A and heavily Blue in B. Again, if those feathers were truly neutral they should be reading close to A=0, B=0.
So that's how you diagnose a colour cast in the LAB colour space. The process is somewhat subjective because you have to decide what parts of the image "should" be neutral and then proceed from there. In warm, evening or morning light the whites and greys are going to look warm and the A and B values in LAB will both be positive. You would not necessarily want to correct for this because you would remove the very effect you are trying to achieve by going out early or staying up late.
In the final installment, I'll run through the method of correcting a colour cast in LAB once diagnosed.