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From: Chris R
Subject: Moonlight
Date: 21 Mar 2022 11:00:00
Message: <web.623892ac2227bb25cc71334a5cc1b6e@news.povray.org>
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?

-- Chris R.


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From: Cousin Ricky
Subject: Re: Moonlight
Date: 21 Mar 2022 14:41:56
Message: <6238c6f4$1@news.povray.org>
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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From: Cousin Ricky
Subject: Re: Moonlight
Date: 21 Mar 2022 23:42:42
Message: <623945b2@news.povray.org>
On 2022-03-21 14:41 (-4), Cousin Ricky 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:
> [snip]

I just noticed that nowhere in your OP did you mention that the Moon is
full.  Please keep in mind that my reply, as well as the Wikipedia
figures, are for a full Moon.


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From: Chris R
Subject: Re: Moonlight
Date: 22 Mar 2022 08:20:00
Message: <web.6239bdd497e56063cc71334a5cc1b6e@news.povray.org>
Cousin Ricky <ric### [at] yahoocom> wrote:
> On 2022-03-21 14:41 (-4), Cousin Ricky 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:
> > [snip]
>
> I just noticed that nowhere in your OP did you mention that the Moon is
> full.  Please keep in mind that my reply, as well as the Wikipedia
> figures, are for a full Moon.

Yes, I am using a full moon for this scene.  I have added a visible element for
the moon, but none of the views I have been working on for the scene actually
make it visible, so the texture of the moon object itself is not as important.
I am mostly looking to get the right color/intensity for the light source.

I am pretty sure LightSys does not have anything that converts lux to
light_source color, so I'll have to look into that.



-- Chris R.


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From: Chris R
Subject: Re: Moonlight
Date: 22 Mar 2022 09:25:00
Message: <web.6239cd6397e56063cc71334a5cc1b6e@news.povray.org>
"Chris R" <car### [at] comcastnet> wrote:
> Cousin Ricky <ric### [at] yahoocom> wrote:
> > On 2022-03-21 14:41 (-4), Cousin Ricky 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:
> > > [snip]
> >
> > I just noticed that nowhere in your OP did you mention that the Moon is
> > full.  Please keep in mind that my reply, as well as the Wikipedia
> > figures, are for a full Moon.
>
> Yes, I am using a full moon for this scene.  I have added a visible element for
> the moon, but none of the views I have been working on for the scene actually
> make it visible, so the texture of the moon object itself is not as important.
> I am mostly looking to get the right color/intensity for the light source.
>
> I am pretty sure LightSys does not have anything that converts lux to
> light_source color, so I'll have to look into that.
>
>
>
> -- Chris R.
I spent a few minutes of internet research and found that lux to lumens
calculations are pretty simple, so I updated my moon light-source generating
macro to do that conversion, and am using your lux values to try things out from
there.  I am using the size of the area_light in converting from lux to lumens,
and am getting reasonable values, using a very cool whitepoint, so I'm happy
with it.

Thanks for the information!


-- Chris R.


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From: Chris R
Subject: Re: Moonlight
Date: 23 Mar 2022 16:10:00
Message: <web.623b7dcb97e56063cc71334a5cc1b6e@news.povray.org>
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.

Any thoughts?

-- Chris R.


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From: Kenneth
Subject: Re: Moonlight
Date: 23 Mar 2022 16:25:00
Message: <web.623b80dd97e560634cef624e6e066e29@news.povray.org>
Cousin Ricky <ric### [at] yahoocom> wrote:
> On 2022-03-21 10:58 (-4), Chris R wrote:
> >
> > ...I am currently using a made-up color of <0.45, 0.5, 0.75>
> > and an intensity of 1.0 with no fading...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...
> [clip]

That's a masterful analysis; thanks for taking the time to describe the details
so clearly.

I find it interesting that the full Moon as seen at zenith (with just our eyes,
not through a telescope) *appears* to be a Lambertian 'disc', not a sphere--
that is, it seems to have equal illuminance from all points on its surface...
even though the Sun's rays are hitting the sphere at progressively shallower
angles, out towards the rim. I'm guessing that this is a result of our automatic
 eye/brain adjustments having to do with 'local contrast' against a dark sky,
and because of the behavior of our eyes' receptors in *relatively* dim light.

The color temperature of Moonlight, as reflected from objects, seem to be
somewhat subjective from what I've read so far. We think of it as being slightly
blue-ish-- but the Moon itself, reflecting Sunlight, has a LOWER color
temperature than the Sun, something like 4100-deg Kelvin. Taken by itself, that
would mean that Moonlight should be slightly *yellowish*(!) in comparison. But
other things come into play-- the Earth's atmospheric scattering, our eyes'
reaction to dim light, and probably other mysteries that I don't know about...
all of which combine to create a slightly blue-ish result.  It's also possible
that we have been 'trained' to think of Moonlight as being blue-- from centuries
of artists' interpretations, how it is usually reproduced in movies, etc. No one
wants to see yellow-ish Moonlight, it just looks... wrong! ;-)

Take a look at this discussion (although I don't think it comes to any definite
conclusion)...

https://physics.stackexchange.com/questions/244922/why-does-moonlight-have-a-lower-color-temperature

Here's another little monkey-wrench to throw into the mix: the D65 'white point'
of computer monitors and some color standards. That's a 6500-deg Kelvin color
temperature, which is meant to match 'daylight'-- meaning, the Sun's light AND
the surrounding blue sky. That's more blue-ish than the Sun by itself (which is
around 5400 to 5700K?) But does this viewing 'environment' still hold true when
trying to create a 'low-light' Moonlit scene in POV-ray (or any other graphics
program)? By nature, such a scene should be reproduced very dim and dark to look
'natural'-- compared to a 'Sun-lit' scene. But that's not how we would usually
render it, because we want it to look...normal. Meaning, colors and brightnesses
rendered from 0 to 1.0 (0 to 255) or thereabouts-- just like a typical scene in
'daylight'. This...discrepancy(?) in how we render low-light scenes and their
associated 'color temperatures' seems like an interesting conundrum. Do I have a
solution to it? No! (ha). I just thought I would bring it up as a philosophical
discussion point. Maybe we should just follow the great artists of the past, and
color the Moonlight blue regardless!


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From: Chris R
Subject: Re: Moonlight
Date: 23 Mar 2022 17:30:00
Message: <web.623b90bf97e56063cc71334a5cc1b6e@news.povray.org>
"Kenneth" <kdw### [at] gmailcom> wrote:
> Cousin Ricky <ric### [at] yahoocom> wrote:
> > On 2022-03-21 10:58 (-4), Chris R wrote:
> > >
> > > ...I am currently using a made-up color of <0.45, 0.5, 0.75>
> > > and an intensity of 1.0 with no fading...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...
> > [clip]
>
> That's a masterful analysis; thanks for taking the time to describe the details
> so clearly.
>
> I find it interesting that the full Moon as seen at zenith (with just our eyes,
> not through a telescope) *appears* to be a Lambertian 'disc', not a sphere--
> that is, it seems to have equal illuminance from all points on its surface...
> even though the Sun's rays are hitting the sphere at progressively shallower
> angles, out towards the rim. I'm guessing that this is a result of our automatic
>  eye/brain adjustments having to do with 'local contrast' against a dark sky,
> and because of the behavior of our eyes' receptors in *relatively* dim light.
>
> The color temperature of Moonlight, as reflected from objects, seem to be
> somewhat subjective from what I've read so far. We think of it as being slightly
> blue-ish-- but the Moon itself, reflecting Sunlight, has a LOWER color
> temperature than the Sun, something like 4100-deg Kelvin. Taken by itself, that
> would mean that Moonlight should be slightly *yellowish*(!) in comparison. But
> other things come into play-- the Earth's atmospheric scattering, our eyes'
> reaction to dim light, and probably other mysteries that I don't know about...
> all of which combine to create a slightly blue-ish result.  It's also possible
> that we have been 'trained' to think of Moonlight as being blue-- from centuries
> of artists' interpretations, how it is usually reproduced in movies, etc. No one
> wants to see yellow-ish Moonlight, it just looks... wrong! ;-)
>
> Take a look at this discussion (although I don't think it comes to any definite
> conclusion)...
>
>
https://physics.stackexchange.com/questions/244922/why-does-moonlight-have-a-lower-color-temperature
>
> Here's another little monkey-wrench to throw into the mix: the D65 'white point'
> of computer monitors and some color standards. That's a 6500-deg Kelvin color
> temperature, which is meant to match 'daylight'-- meaning, the Sun's light AND
> the surrounding blue sky. That's more blue-ish than the Sun by itself (which is
> around 5400 to 5700K?) But does this viewing 'environment' still hold true when
> trying to create a 'low-light' Moonlit scene in POV-ray (or any other graphics
> program)? By nature, such a scene should be reproduced very dim and dark to look
> 'natural'-- compared to a 'Sun-lit' scene. But that's not how we would usually
> render it, because we want it to look...normal. Meaning, colors and brightnesses
> rendered from 0 to 1.0 (0 to 255) or thereabouts-- just like a typical scene in
> 'daylight'. This...discrepancy(?) in how we render low-light scenes and their
> associated 'color temperatures' seems like an interesting conundrum. Do I have a
> solution to it? No! (ha). I just thought I would bring it up as a philosophical
> discussion point. Maybe we should just follow the great artists of the past, and
> color the Moonlight blue regardless!

So, I'm feeling less bad about my arbitrary adjustments to the moon's light
color now.  I found a whole slew of information about the eye's dark adaptation
ability, and what it can do to our perception of color and brightness,
(specifically the Purkinje effect causing a shift towards blue sensitivity,
which provides another explanation for our perception of moonlight as blue.)

So, to make my macros look more technically astute, I have just added a "Dark
Adaptation" color value when computing the color of the moon's light before
applying the lux value and atmospheric attenuation.  It still means I'm mostly
just adjusting things until it looks right, but I can pretend there's some math
and science behind it.  :-)

-- Chris R.


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From: Cousin Ricky
Subject: Re: Moonlight
Date: 24 Mar 2022 00:27:26
Message: <623bf32e$1@news.povray.org>
On 2022-03-23 16:22 (-4), Kenneth wrote:
> 
> That's a masterful analysis; thanks for taking the time to describe the details
> so clearly.

You're welcome.

> I find it interesting that the full Moon as seen at zenith (with just our eyes,
> not through a telescope) *appears* to be a Lambertian 'disc', not a sphere--
> that is, it seems to have equal illuminance from all points on its surface...
> even though the Sun's rays are hitting the sphere at progressively shallower
> angles, out towards the rim. I'm guessing that this is a result of our automatic
>  eye/brain adjustments having to do with 'local contrast' against a dark sky,
> and because of the behavior of our eyes' receptors in *relatively* dim light.

It's an effect called opposition surge.  Because the Sun is illuminating
the Moon head on from our point of view, we are not seeing the shadows
cast by craters and mountains.

> The color temperature of Moonlight, as reflected from objects, seem to be
> somewhat subjective from what I've read so far. We think of it as being slightly
> blue-ish-- but the Moon itself, reflecting Sunlight, has a LOWER color
> temperature than the Sun, something like 4100-deg Kelvin. [snip]

So my estimate was very close!  :)

>
https://physics.stackexchange.com/questions/244922/why-does-moonlight-have-a-lower-color-temperature

The simplest way to summarize ProfRob's answer is that the Moon is not a
neutral gray; it is slightly brownish.

> Here's another little monkey-wrench to throw into the mix: the D65 'white point'
> of computer monitors and some color standards. That's a 6500-deg Kelvin color
> temperature, which is meant to match 'daylight'-- meaning, the Sun's light AND
> the surrounding blue sky. That's more blue-ish than the Sun by itself (which is
> around 5400 to 5700K?)

For your "viewing environment," think about where you want "white" to
be.  To give you some context, warm LEDs are 2700K; incandescent lights
are around 2800K; cool fluorescent lights are more or less around 4200K;
and daylight fluorescents and LEDs are 5000K.  Think about how bluish
your computer monitor looks, and you can see that D65 is way too high
for typical scenes; everything will look reddish.

I read somewhere that outdoor photographic film is calibrated to 5600K.

This post shows an outdoor sunny scene under various white points:
  https://news.povray.org/617229c5%40news.povray.org
This might give you an idea of what white point to choose.

Alas, I do not have a simple explanation for how this works; I don't
completely understand it myself.  I just examined test_lightsys.pov, and
used the technique on a solar spectrum to get my light_source colors.
Lightsys IV can do white point adjustments without requiring a spectal
curve, but I haven't figured out how to do that yet.  Have a look at
scenes im_test_day.pov and im_test_inc.pov to see if that helps.

> But does this viewing 'environment' still hold true when
> trying to create a 'low-light' Moonlit scene in POV-ray (or any other graphics
> program)? By nature, such a scene should be reproduced very dim and dark to look
> 'natural'-- compared to a 'Sun-lit' scene. But that's not how we would usually
> render it, because we want it to look...normal. Meaning, colors and brightnesses
> rendered from 0 to 1.0 (0 to 255) or thereabouts-- just like a typical scene in
> 'daylight'. This...discrepancy(?) in how we render low-light scenes and their
> associated 'color temperatures' seems like an interesting conundrum. Do I have a
> solution to it? No! (ha). I just thought I would bring it up as a philosophical
> discussion point. Maybe we should just follow the great artists of the past, and
> color the Moonlight blue regardless!

I know nothing about the psychology of low light, but I do know that we
lose color sensitivity in the dark.  So regardless the actual color of
moonlight, you might want to gray it down.  I'm thinking this would be
best done in post-processing, though.


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From: Mr
Subject: Re: Moonlight
Date: 24 Mar 2022 06:15:00
Message: <web.623c43c497e5606316086ed06830a892@news.povray.org>
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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