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Instead of trying to emit 670nm, why not just use a wide spectrum source and a 670nm bandpass filter?

Like the sun and your closed eyelids. Light transmission goes up with wavelength https://www.spiedigitallibrary.org/journals/journal-of-biome... (see figure 5.) To speculate, perhaps we have evolved to benefit from closing our eyes in the sun.

That's a great connection. If it's that simple (and explained by human evolution), that would be awesome.