Blue light inhibits cells expansion, red light tends to increase cell expansion. One of the reasons to use a veg light is that it makes plants short and compact. I use a veg LED and a flower LED. The plant below was vegged for about 40 days under a Growcraft X3 light. It was topped at day 21 and LST'd and was then grown under their flower light.
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The only % "requirement" that I've been able to find is a comment by Bugbee that flower lights should have at least 4% blue to avoid misshapen plants.
Bugbee's latest (as of this Spring) advice was to use a white LED with some far red. He referred to the far red as being like a "flamethrower" for growth. This spectrum is from one of his recent videos:
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Bugbee's advice is that while spectrum is key to shaping plants, the most important issue is to maximize light levels. Mitch Westmoreland, a PhD student under Bugbee, validated that in the videos he released this spring in which he shared portions of the research he did for this thesis. In the videos, Westmoreland states that they were not able to find benefit from using UV light but acknowledges that there are a huge number of combinations of exposure time, different wavelengths, and different intensities so it's quite possible that UV light has a benefit. His issue is that they are unable to demonstrate any benefit.
If a grower has found an approach to using UV that is reproducible, I would highly recommend that the grower contact Mitch and share that information. He's just one person working in a lab so he's limited in what he can test. On the other hand, if someone finds a winning combination, everyone would benefit if it could be reproduced.
PAR, short for "photosynthetically active radiation, covers the 400 to 700nm range. That's been the historical definition but Bugbee has adovocated that ANSI introduce ePAR or "extended PAR" which would extend the upper limit from 700 to 750. The lower limit is not changed because light with a wavelength shorter than 400nm is not photosynthetically active, per above.
That's not to say UV doesn't impact how plants grow. The "not photosynthetically active" means that it doesn't play a role in photosynthesis.
The screenshot below is from this page:
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The graphic below shows the percentage make up of various grow lights and it confirms that HPS, historically used as a flower light, does have 4% blue in the spectrum.
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