Reading last night in Adams book 5 on artificial light and his take on luminance was refreshing. He compared brightness and luminance in very simple terms.
His explanation of brightness deals with the inherent property of reflected light from an object, without necessarily dealing with the amount of light directed at it. A side in shadow reflects x amount of light and a side in full light reflects y amount of light, simple zone system parlance and metering.
His take on luminance deals not with the reflected light from surfaces, but with the light falling upon it. He then explains how light intensity can be manipulated (additive or subtractive) to allow for creative interpretation of a given subject in a finished print. It really is a shame he died so soon, as his understanding of light and the ability to explain it was wonderful.
I would have liked to have seen him in a discussion of techniques with Phil Davis. We owe both of them a huge debt.