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.
Going back to this again, I realized there were a number of errors I introduced
when I moved from my guess at moon light and calculations using Lux and
including a parallel, circular area light. So, I'd like to go back to your
calculations above, which give you a Lux value for the moon in various
configurations, and determine how to combine that with the Blackbody() macro for
the moon's light temperature? In LightSys, the light color gets multiplied by
the lumens, and then light fading takes care of reducing that back to normal
brightness levels. When using LightSys for the sun, as a non-fading light,
small brightness constants are used instead of lumens, and I pick them
arbitrarily, (1.0, 2.0, ...).
Should I just be multiplying the Blackbody() color by the calculated lux value?
I've tried that, and it seems to be too dim based on the light I have seen
coming through the skylights in my house on a full moon, so I am wondering if I
am missing something. For now, I just introduce a brightness fudge factor,
similar to what I do with the sun, but that doesn't make me happy. Of course, I
could also be misperceiving the brightness of the real moon coming through my
windows for other reasons, too, and the dimmer color is more accurate.
-- Chris R.
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