An architect recently emailed me asking if it was possible to use TM-30 metrics with daylight.  My short answer was something like, “I suppose you could, but why would you?”

The long answer is that all of the TM-30 measurements (Rf, Rg, the 16 chroma shifts, the 16 hue shifts) are relative – comparing the light source in question with the reference light source.  With daylight, you’d be comparing the daylight SPD you captured at a moment in time with the CIE definition of daylight at the same CCT.  On an average day I doubt that any of the measurements would deviate from 100 by more than a couple of points.  So, using TM-30 (or CRI Ra) is like measuring a ruler with another ruler.  You’re essentially comparing one thing to a definition of itself.