May be I put it wrong. I noticed for example a much bigger difference in mW/m2 output relative to distance (below 1 meter) in UVB as in longer wavelengths. Also I saw this graph on page 7 which is very interesting.
Very useful article, thanks.

But that graph refers to how angle of viewing affect the spectrum. And its not related to how light transmit, but to the LED source itself. This difference is caused in white LEDs by how the phosphor layer is added. LED manufacturers had improved it strongly on the last years, but is imperfect yet.

Standard LEDs dont emit almost any UV, and less specifically UVB.

I like the idea of intra/under canopy lighting a lot, it was next on my list.
Since no more than 15% (?) of the light penetrates through the canopy this should boost yield per m2 (I obviously like m3 better) and only little light is needed to get significant results.
Purdue University is working hard on this, and in general all researchers involved on the development of grow areas for NASA missions.

Planophills plant's species strongly benefit for intracanopy lighting. Average improvement in yield is being 35% for the same amount of light used, but some species has increased it by 100%, doubling yield with same amount of light.

Percentages of IC lighting generally varies from 25 to 50% of the total light.

Due undercanopy areas are mostly underlit, and many times severely underlit, very little light (compared to the amounts of light used overcanopy) transform lower leaves from sinks to sources of energy and resources. Bottom leaves arnt discarted, as happen when there is only top lighting, due the arey are a ballast for the plant. This mean undercanopy lighting achieve a way better use of plants resources and allows to use optimal irradiances for all the leaves of the plant, strongly improving photosynthetic efficacy.

We are getting excelent bottom buds using very low irradiances undercanopy, below half of used overcanopy (on uE/cubic feet basis).

I believe Ive linked it before, but check here the Excel spreadsheet