RGB vs LAB Luminosity Masks 

So does creating Luminosity Masks in LAB or RGB make a difference to the result given the way each colour space handles tonal information? Here’s a comparison of one Lights, Dark and Midtone luminosity mask created using the Calulations workflow In LAB (left) and then RGB (right).

The difference in the Midtone mask is quite noticeable between the colour spaces. The LAB version is slightly darker, with a bit more tonal separation than the RGB version.

The Darks 3 mask is almost indistinguishable between LAB and RGB.

LAB again had greater contrast. than RGB, but the highlights are punchier especially around the patterns in the central ceiling.