On 2022-03-21 10:58 (-4), Chris R wrote:
> I am working on a scene that is lit by moonlight coming through a window with
> partially open blinds.  I use LightSys, and have set up the light source as an
> area light just outside of the window itself.
> 
> I was wondering if anyone had suggestions for good color/lumens combinations to
> simulate moonlight.  I am currently using a made-up color of <0.45, 0.5, 0.75>
> and an intensity of 1.0 with no fading, (so I guess I'm not really using
> LightSys to great effect here).  Has anyone done any spectrum analysis for
> moonlight to suggest better values?

Lumens is not what you want for moonlight; this is a measure of the
total amount of light emitted (or reflected, in this case).  Lux is the
proper unit for a light source that is effectively at infinity.

Wikipedia says 0.05 to 0.3 lux.  That's quite a range!

To get a narrower precision, a preliminary calculation is:
  solar lux * lunar albedo * (solar radius / astronomical unit) ^ 2

Given 98,000 lux for overhead sunlight and an albedo of 7% for the Moon,
this yields 0.148 lux.

This should be multiplied by the ratio of the apparent areas of the Moon
and the Sun, but since they are approximately the same size in our sky,
that figure cancels.  The number should be further reduced because the
full Moon is not a flat reflector.  If I'm thinking correctly (not a
safe bet), a Lambertian reflector would halve the light; since the Moon
is not a Lambertian reflector, the true lux is somewhere between 0.074
and 0.148.

Finally, the closer the Moon to the horizon, the more the atmospheric
attenuation.  My previous calculations for sunlight (and moonlight
should be similar), assuming low turbidity, were:
  96% at 60 degrees
  86% at 45 degrees
  77% at 30 degrees
  47% at 15 degrees
  0% at moonrise
Obviously, those figures close to the horizon are too small, and that is
because I didn't consider the curvature of the Earth's atmosphere. My
guess, without consulting my musty calculus textbooks, is that the
attenuation is about 7-8% at moonrise.

Of course, with the Moon so far away, you do /not/ want fading; and you
should use a parallel light.

I have questions about the spectral data given for the moon rocks in
Lightsys IV, as the Moon does not appear blue to me; and this, even with
the blue bias that my vision appears to have in low light.  I have not
searched for better spectral data, but even with such data, you would
need to factor in solar spectral data (because the Moon only reflects
light) and atmospheric attenuation (ouch!).  Best I have done is to
compare the Moon to streetlights and indoor lighting, and I have
concluded that moonlight has a color temperature of around 4000 K when
the Moon is moderately high in the sky.  Lightsys IV macro Blackbody()
can calculate that color for you; remember to decide what white balance
you want.

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