I agree… let’s see if I manage to explain things correctly, maybe @Elle can step in if I’ll say something wring:
- luminance is the second component of an XYZ triplet. Relative luminance is Y divided by the luminance of the reference white point Yn (Yr = Y/Yn). However, the RGB -> XYZ matrix given by the RGB primaries of an ICC profile are such that Yn = 1, therefore the Y value computed from the ICC primaries is actually Yr.
- lightness (L*) is Yr encoded with the L* TRC (https://en.wikipedia.org/wiki/Lab_color_space#Forward_transformation)
- lightness (HSL) is a different thing, which unfortunately is called in the same way as L*…
The reason why I have used the term luminance in the B&W conversion tool is that the resulting image is encoded with the same TRC as the initial RGB data, and not necessarily with the L* TRC. However, the resulting image is equivalent to L* lightness.
Finally, the luminance blend mode in PhF does exactly what Elle describes in the article you are referring to. Moreover, since few days the LCH Lightness blend mode is also available…
Hope this is shading a bit of light on this lexical mess!