Re: Pure white

[Leigh Palmer <palmer@sfu.ca>]

> If I remember correctly from my astronomy days, the Luminance curve for our
> sun peaks in the yellow portion of the spectrum.  That doesn't mean it
> necessaarily looks yellow (a psychological phenomenon).

Even through the atmosphere (so long as there is not excess pollution)
the spectrum of the Sun peaks in the region of the spectrum we would
associate with a green spectral line, but it also mixes in other light
which makes it approximate very nearly a 5770 K color temperature white
illuminant. Superimosed against a blue sky, if it were one ten
thousandth as bright as it is, it might look yellow. Color, which is
not a physical property, depends upon context, and especially on the
nature of the contiguous ground against which it is seen.

Incidentally, the astrophysics textbook I used this year says that the
Sun looks yellow even though it peaks in the green. I suspect the
authors have succumbed to this common misconception, or perhaps they
have noticed how yellow the Moon looks, illuminated as it is by the
yellow sunlight!

> However, if you
> look at the spectrum for the sky, it is distinctly bluish (check out the
> shadows in photographs of snow scenes).

I don't have to; I can look directly at the sky sometimes here in
British Columbia. I know it is bluish, though in my native Los Angeles
it was often white (when viewed from within) or brown (when viewed at
the same time from the Angeles Crest adjacent to LA) at the same time.

> The sun is decidedly more yellow
> than this.  If I stared at something as bright as the sun, it probably
> wouldn't matter a whole lot what color it is!

Exactly right. Looking at the Sun itself is a bad idea. Check out the
color of a white sheet of paper in full sunlight. You'll see just how
yellow the illuminant is. The white paper does not impart any special
quality to the light, and when I look at it it sure doesn't look yellow!

Oh, yes; you can also do the experiment described above with snow. I
must caution those of you who see yellow snow that its cause is probably
unrelated to the nature of the illuminant. In particular, you probably
should not eat any.

Leigh