Is the daytime sun yellow?

http://solar-center.stanford.edu/SID/activities/GreenSun.html

This page, which you will notice is from Stanford, goes to some lengths to argue that the sun is white. However, it seems to be pretty obvious that in the middle of the day, the Sun is pretty yellow when viewed from Earth. When light bulbs are labeled, daylight White is used on the box when clearly the LEDs are glowing somewhat in the yellow spectrum. Am I missing a point in Stanford’s argument here?

This is your argument for why they are wrong?

Light towards the blue/violet end of the spectrum gets scattered because its wavelength is shorter and is more likely to be absorbed by the atmosphere. So the more atmosphere the sun has to shine through the redder it gets as the blue end gets subtracted from its true white(ish) colour. Hence red sunsets.

So the daytime sun IS yellow. But not if you’re on the International Space Station.

“This is your argument for why they are wrong?”

A bit of it. I thought it was common knowledge that if you make a truly white bulb, people think of it as a little odd. But they often want is something with a bit of a yellow tinge to it, so that it looks like sunlight.

They are? The daylight white bulbs look white or even perhaps bluish to me (probably in comparison with typical tungsten lights.) I can’t stand daylight white bulbs in my house except where I want something close to a neutral white balance, because they are far too blue for my eyes.

This. The daylight white bulbs are much whiter to look at then the “soft light” bulbs. Because I have seasonal affective disorder, I much prefer the daylight bulbs in my non-sun-facing condo.

I think people often like yellower (warmer) light when they buy light bulbs. I don’t think those are typically labeled as daylight bulbs. Even if the bulbs labeled as daylight bulbs did produce a yellow light, that is more about how people think about the color of the sun and not what the actual color of the sun is.

In my own experience, glancing at the center of the sun’s disc during daylight hours, I would have said it’s white. I’m surprised anyone thinks otherwise, but color perceptions can be weird, I guess. Still, measuring the spectrum seems pretty straightforward.

The colour temperature of the Sun is about 5,900 K outside of the atmosphere. Movie lights that match sunlight on earth are 5,500 K. Colour temperatures above 5,000K are considered ‘cool’ (bluish).

@Johnny_L.A for the win. This is what I was taught.

And @eschrodinger, I believe you are correct about most people preferring the yellower /softer light as several neighbors have commented negatively about the blue-white shining out my windows at night. Maybe they shouldn’t look? :grinning_face_with_smiling_eyes:

It’s not because it looks more like sunlight, it is because it looks more like conventional tungsten-filament bulbs. People are just used to those. Photographers know that when taking photos under tungsten light you have to adjust the color otherwise it looks orange in the photo. A truly white bulb can look a little bit blue but this is a cognitive effect.

Sunlight colour is weird, as is most colour. In principle the sun is a black body radiator, and outside the atmosphere sunlight is close to that. The effective temperature is 5772 K.
On the Earth the light is scattered, so the visible sun disk itself has some blue removed, and it appears more yellow. The spectrum no longer fits a perfect black body curve. However there is significant radiation from the blue sky, and the overall illumination of objects in sunlight is more blue. Hence the quite high equivalent black body temperature of the illuminant.
As the day progresses the balance of sky blue and scattering changes, and the effective illuminant black body temperature drops.
Standard sunlight is defined as the illumination from just the solar disk at midday. So some yellowing relative to the actual sun, but without the extra blue of the sky.
Tungsten lamps are much cooler, and tungsten halogen lamps a bit hotter, but still cooler than the sun. They are usually naked black body radiators, but a some use dichroic filters to modify the light a bit. Temperatures vary from 2700 to 3200 K. Much cooler. Much yellower.
LED lamps are just terrible as they are not black bodies, and simply balance three broad line emissions to achieve a perceived colour that balances to something close to the defined black body emission for the temperature on the box. But that only works if they are illuminating white objects. Illuminating coloured objects and it all falls apart with effects that depend upon the manner in which the colour of the object is achieved.
The eye tries to balance the perceived light towards a perceived white, so small changes to the apparent temperature of the illumination are not as obvious as if you compare the light sources side by side.

Yep. If you’re shooting tungsten-balanced film in daylight, you need a #85 filter (orange); otherwise your footage will look blue. If you shoot daylight-balanced film indoors, you need to use a blue filter or else the footage will come out reddish. I don’t remember what the f-stop adjustment is for the blue filter, but it’s a lot greater than the orange one; so people I know tend to use tungsten film/Wratten 85 filter outdoors if they’re not using daylight film for whatever reason. That way they can use the same film indoors or outdoors without much of a compensation penalty.

ETA: Tungsten movie lights are around 3,200 or 3,400 Kelvin.

I haven’t bought tungsten-balanced film in many years. I haven’t bought any film in many years. :grinning:

Almost all white LEDs are actually a blue LED plus yellow phosphor:

The blue is a narrow spike but the phosphor has a fairly broad emission spectrum.

It’s a little more complicated than that. White objects tend to have a broad reflection spectrum, but that’s not necessarily the case–one could have a white spectrum with notches that line up badly with the blue spike here.

True. However I would say they are not white :slight_smile: . Colourists use a slab of pure chalk as a reference white material. White isn’t a colour, which is perhaps the bottom line problem with the question. We can define white as equal energy at each wavelength - but that doesn’t match other uses of the word. No black body radiator is white in that case. Colourists do however define a white colour. But it is only a perceptual thing, and relative to a defined spectral curve that is neither flat nor a black body, but derived from the sun as seen on Earth. So by that definition the sun is white, simply because it defined white.

“White”, then :slight_smile: . Agreed that there is a definitional problem. We can create an objective definition (and such standards exist, as you note), but it won’t necessarily appear white in all cases. And on the other hand, what looks white may not be white in any meaningful sense. Not to mention that there is a difference between white of emission vs. that of reflection.

I shot super-16 in 2015.

It’s not obvious at all, because you’re never looking at the midday Sun directly (or if you do, you’ll only do it once because you’ll go permanently blind very soon). There are techniques to view the Sun securely, such as projecting its image onto a surface, and when you do that it appears white, as it is. The question is not whether the Sun is white or yellow; it’s why we mentally think of it as yellow even though it is, in fact, white. Here is an article discussing that phenomenon; it’s behind a paywall, but perhaps you find an opportunity to get it from a library or a free version on the internet (the version that I read a while ago seems to be gone).

Yeah, if there’s anything at all that is white, the midday Sun is it. The question is just why we think of the Sun as yellow. I haven’t read @Schnitte 's paywalled article, but I’m guessing that the gist is that it’s by contrast with the blue sky.

Unless you’re an astronomer. In that case, Vega is by definition white, and the Sun is, by comparison, slightly yellowish-reddish.

But why are we only talking about the midday sun?

The reason people think of the sun as yellow is that during the times of the day and seasons when you can stand to look at the thing, it’s yellowish.

Or: to the question of whether the sun as viewed from the surface of earth is yellow or white, the answer is “yes”. Theres no reason, other than “i know a thing”, to correct people that the sun is white.