This might help how to use these: In both cases, you're working with PS Info Palette and you hover the mouse over the grey patch.
In #1, click on "Color Space" and select "L*a*b" then in the "Channel" box select "Lightness". Then "L" should appear at the upper right.
In #2, insert the three RGB numbers (all will be the same because we're dealing with grey), click on "start" and in the next screen you'll see several results, among them L*ab. Ignore illuminant and observer.