But even then it's not that simple. You need different wavelengths at different phenological stages, and what wavelengths you need depend on the crop (and variety, but you can at least control that for crops you can multiply vegetatively) you're growing, and then still you need to distribute that light to all leaves and not just the top ones. So now you need either LED's that can emit many different wave lengths, or you need many LED's that are only in use part of the time (killing your capital expenditure efficiency), or you need to move plants around to be under the right lights. And you need to move the light source to places where they can hit the leaves as much as possible, but you also need to keep the excess heat away from the plants (which is a problem even with today's highly efficient LED's - I mean, these are not like that IKEA nightlight you bought last week for $5.)

You’re right that they’re not like the ikea lamp I bought last week... they’re like the hydroponic lamps I bought two years ago for a shit ton of money.

Everything you said is right. It’s still more efficient to use the suns energy and then distribute the leds than to directly try to use the suns light. Modern hydroponic grow led controllers can adjust the wavelengths as the plants grow

The black leaf thing is interesting. Is that an oversimplification or do plants really only absorb visible light?

Plants only absorb very particular aspects of light. As far as I remember, it's only visible light, yes.

More information here: http://www.webexhibits.org/causesofcolor/7A.html

Hydroponic lighting is optimized for those particular wavelengths in the ~450 nm and ~650 nm peeks. Some plants absorb additional wavelengths, and could likely be targeted with other lights. The key though is that chlorophyll are like mitochondria -- highly preserved between species -- and they evolved to only absorb particular frequencies.

That’s really interesting, thanks for the explanation. Makes me think of tuning LED lighting the same way a mechanic might set spark timing. Obviously in reality the right lighting for a plant is known but it’s neat to be able to see the effect.

It may be well known but there are many practical considerations. It certainly requires a lot of tuning. For example, if you want to target all four peaks of absorption by cholorphyll a and b, that’s four wavelengths, but many people settle for just two due to cost. It’s all a giant optimization problem!

Research is inconclusive, but there are indications that UV light (although not required for photosynthesis) can have effects on yields and either on resistance against pests (insects/molds/fungi) or directly attack/kill/weaken those pests. So basically 'it depends', and people do their own tests and then determine if, in their specific circumstances, it is a net positive (economically) to add UV lights to their setups.

I think this is the main message. People in this thread are talking in absolutes about 'agriculture', but the reality is that there are many, many things that we don't understand well enough at the above-micro-level scale to be able to say 'you need to measure x, y and z and then we can calculate/model the optimal setup to grow something'. People are basically relying on (informed) trial and error, with feedback cycles measures in months or years, so it just takes a long long time for progress to be made - and even then, we usually don't know exactly why something works in a specific case. It's not like 'oh let's run an A/B test for the best color of our 'buy' button for a few days'.