Cousin Ricky <ric### [at] yahoocom> wrote:
> 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.


Wow ! Thanks a lot for giving it so much thought, all this will allow me to not
only add a moon prest to the other 24 existing light types of the Blender addon,
but also improve the existing sun when I get the time!

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