Thread by @enn_nafnlaus

@spmanipulator @andjrison @GidMK Colour vision is weird, and a lot more complicated than one might at first think.

For a greenhouse project, we had the idea of using red-blue grow LEDs (mostly red, since it's super-efficient) for primary grow lighting but mixing in various small amounts of (inefficient...

@spmanipulator @andjrison @GidMK yellow) HPS with the idea of shifting the appearance of the light over the course of the day for our guests, from reddish sunrises to yellow dawn to white daylight. You know, we'd have red, yellow, and blue, and just blend through them in the right ratios - easy, right? Except...

@spmanipulator @andjrison @GidMK it totally didn't work in testing. If there was almost any blue at all (we couldn't fully shut off the channel), it looked purple/pink, unless the (inefficient yellow) HPS was cranked *way* up, wherein you'd get yellow light. And only with *tons* of blue and little of the...

@spmanipulator @andjrison @GidMK efficient red, you'd get white.

What's going is that we have different "axes". So-called "red" and "green" are actually heavily overlapped and are antagonistic; they both activate from the same light, but if one activates more than the other, then that indicates whether you're..

@spmanipulator @andjrison @GidMK more to the greener end or the redder end of the spectrum. You also have a "red+green" intensity value, the combined activation of these cones. On the other hand you have blue off doing its own thing, giving an intensity value. So your brain works a red-green ratio; a...

@spmanipulator @andjrison @GidMK red+green intensity; a blue intensity; and a blue vs. red+green intensity.

In a normal blackbody spectrum (like the sun or any hot glowing object), there's very little blue at all in your "warm light" spectrums, like sunsets, fire, etc. But in your "cool white / daylight"...

@spmanipulator @andjrison @GidMK light, there's tons of it. So the ratio between the blue components and the red+green components determines how warm or cool the light looks.

In a blackbody spectrum, when you do have blue present, longer wavelengths are much more shifted to green than red. So it's natural to..

@spmanipulator @andjrison @GidMK have blue when you have a red-green ratio on the green end. But when it's on the red end, that's an "unnatural" situation, often the result of pigments or other phenomena, and we perceive that as purple/pink.

So in short, if we wanted to use HPS to make a natural white light,..

@spmanipulator @andjrison @GidMK we had to really over dominate the red from LEDs, and really heavily pump up the blue, because that's what's needed to get more blue activation than red+green while having the red-green ratio be green-shifted. And while trying to achieve sunrise colours, we could hardly have ...

@spmanipulator @andjrison @GidMK any blue whatsoever, because again blue is hardly found in reddish blackbodies, so it gives you an unnatural purple look, even in small amounts.

(The real answer to doing what we wanted to do: red plus tunable-white LEDs. Tunable whites can shift from a warm light to a...

@spmanipulator @andjrison @GidMK daylight on their own, and so add in efficient deep red for sunrise / sunset - the longer the wavelength in the PAR range, the better, because you can deliver more moles of PAR relative to how bright it seems)