PUR vs. PAR? We barely know the actual requirements that various plants have for light, whether measured in PAR, lux, or PUR. For most plants we only know a range of light intensities that are adequate for those plants. For plants that are desired for their brilliant colors, we know roughly how much light it takes to grow them and we know that it takes even more light, in general, to get the good colors we want.
We have only a rough idea how much light a specific light fixture will give us, at some distance from the light. For many light fixtures we can only guess at how much light we can get from them.
We know for sure that any light we use will give a lot more intensity up near the waterline than down at the substrate, so almost all of the plants we use will be growing in a range of intensities, higher near the top of the plant and lower near the bottom of the plant.
Given all of those unknowns, and barely knowns, it doesn't make any sense to me to even consider PUR. If we had readily available PUR meters, that might be the better way to determine "exactly" how much light we are using, but we don't. Most of us don't even have access to a PAR meter.
I think we should set PUR aside and perhaps reconsider it in about 5-10 years.