Sunlight is yellow parallel rays plus blue isotropic light

post by Thomas Kehrenberg (thomas-kehrenberg) · 2023-03-01T17:58:02.706Z · LW · GW · 5 comments

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When you look up the color temperature of daylight, most sources will say 6500K, but if you buy an LED with that color temperature, it will not look like the sun in the sky. It will seem bluer (or, less yellow-y). Yet, 6500K is arguably the correct number. What is going on?

The answer is Rayleigh scattering. What we perceive as sun rays on the Earth’s surface has traveled through a lot of atmosphere, whereby a lot of the blue light has been scattered away, such that the sun rays look a bit yellowish. But the blue light isn’t all lost! A lot of it does arrive on the surface eventually, but it does so as diffuse light – light that seems to come from the whole sky. Which is of course why the sky looks blue (if it isn’t blocked by clouds).

The result is that the parallel rays that come from the bright spot in the sky (aka, the sun) are (slightly) yellow, but as they mix with the diffuse blue light from the sky, they form white light with a color temperature of about 6500K. More specifically, the daylight standard D65 is defined as

the average midday light in Western Europe, comprising both direct sunlight and the light diffused by a clear sky.

And that’s where we get the 6500K color temperature.

(The sun as seen from space actually has a color temperature of 5800K to 5900K. I haven’t looked into why these numbers are different but I assume it’s because of absorption in the atmosphere.)

But most of the time, not everything is illuminated evenly. The parallel rays from the sun produce shadows and they are blue:

(Source. This picture was not taken at midday, so the sun is extra yellow, but this allows us to clearly see the blue shadows.)

So, in order to imitate daylight more faithfully indoors, I think you should separate the components of the light as our atmosphere does. So, use spotlights of yellowish light together with diffuse color-of-the-sky blue light that seems to come from everywhere, to produce blueish shadows.

Here is a video of someone doing this the hard way with actual Rayleigh scattering – to very impressive results – but I think you could just buy yellowish LED spotlights (with color temperature of 3500–4500K or so? I tried to find the color temperature of just the sun rays without the skylight, but couldn’t find anything) and blue LED strips mounted on all walls close to the ceiling. And of course, the lights should be very very bright in order to imitate the sun.

I suspect that the blueish shadows will make any white light feel much more pleasant.

I intend to do this myself for my office, but it will take some re-configuration of the lights, so it might take a while and I wanted to get this post out early. If someone manages to try this out before me, I would be very interested to hear about their experiences.

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