In reading up on the XYZ color space you regularly come across something to the effect that "Y is taken as the luminance" or "relative luminance" whether it be for the XYZ color space or in xyY. I'm struggling to gain any intuition for how this Y value is related to luminance from a photometric perspective. There, the luminence is closely related to the radiance, but weighted as a function of our eye's response. It "contains" all the colors we can see and has actual units. There's seemingly very little connection between the two.
Bottom line: I go out and measure the spectral radiant power reflected off a surface, and calculate the luminance. Then, I take an image that contains that surface. I convert the image to the XYZ color space. How is that Y value related to the calculated luminance? Do the physics/units go completely out the window?