How does the Y component in CIE XYZ relate to physical luminance?

Question

I understand that in CIE XYZ and xyY, the Y component is often described as luminance or relative luminance. What I’m trying to understand is how that relates to luminance in the photometric sense.

If I measure the reflected spectral radiant power from a surface and compute its luminance, then photograph that surface and convert the image data to XYZ, how should I interpret the resulting Y value? Is Y directly proportional to physical luminance, or only a relative value unless the camera/image pipeline is calibrated? In other words, when do the XYZ values retain real physical units, and when are they just normalized color coordinates?

user20809 · Answer

Luminance is basically Radiance weighted by the Human Visual System (HVS), i.e. Power per Unit Area per Unit Solid Angle. Whether it is given in relative or absolute units depends on how the values were measured, absolute values require a calibrated reference light source and/or measuring device.

If you have some absolute spectral radiometric measurements and need the corresponding absolute photometric quantity, you need to integrate them to CIE XYZ tristimulus values while using a normalization factor k equal to Km = 683lm.w-1, the maximum spectral luminous efficacy of the HVS. 1 watt of light at 555nm equals to 683 lumens. Y in that context would be an absolute measure of Luminance.

Colorimetry 4th Edition has most of the relevant details, the 3rd edition can be found online and has very similar content.

How does the Y component in CIE XYZ relate to physical luminance?

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