So I was just reading up on how to create more Physically Based Lights and wandered into the realm of photometry. I read the paper "Moving FrostBite to PBR" and in it the author claims he decided to have all the light sources in units of Photometry (Lumens/Luminance etc).

The author also writes how to convert between radiometric and photometric units earlier on. But later he decides to ditch this conversion and have his lights in photometric from the get go. I don't understand how is this compatible?

If I want to use the same technique for my normal path tracer, is there a different form of the rendering equation for photometry? Should I just swap the radiometric parts for the photometric parts (radiance for luminance, radiant flux for lumens etc). Or maybe I should convert from photometric to radiometric like the paper mentioned. How does this all tie up, if anybody can clear it.

EDIT:- Just saw a similar question (sorry my net is slow). As nathan suggested in his answer rendering RGB images proceeds more naturally in radiometric units. If I opt the approach to convert between radiometric and photometric units using luminous efficacy for light sources will there still be a problem as Nathan suggested (wavelength dependance of photometric units) or that problem exists solely when rendering exclusively in Photometric units?

  • $\begingroup$ Whether you pick one or the other, both have an error compared to using spectral radiance and corresponding materials. I think it's really about how you prefer to have your pipeline. I would stick to trichromatic rendering approximating radiometry, simply because it's easier to derive and reason about. You can always integrate the radiance in the end. I'll emphasize though, that if you want to be accurate you should use neither, and rather opt for spectral radiance. $\endgroup$ – lightxbulb Feb 12 '19 at 21:42

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