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Discussion Starter · #1 ·
I've just been watching today's total lunar eclipse live on the internet (http://events.slooh.com/) since it isn't visible where I live and a topic came up that could be of interest to folks in this forum. I forget the actual question, but in the discussion that followed it was said that even though the full moon looks very bright to us the actual amount of sunlight being reflected is only about 10%! In astronomical terms this is called the moon's albedo.

"Albedo, or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it. Being a dimensionless fraction, it may also be expressed as a percentage, and is measured on a scale from zero for no reflecting power of a perfectly black surface, to 1 for perfect reflection of a white surface."
http://en.wikipedia.org/wiki/Albedo

The commentators also said that if the moon was totally covered in Black-Top (macadam, asphalt paving) that it would appear to have the same brightness it does now! The surface of the moon is really a very dark color.

That piqued my curiosity so I did a bit of checking and found that albedo is also used in reference to terrestrial materials. On a scale of 0 to 1 (0% to 100% reflective) green grass is about 0.25, desert sand 0.40, new concrete 0.55, fresh fallen snow 0.8 to 0.9. Clouds have a maximum reflectance of less than 0.8.

To put this into perspective for this Projection Screen forum the above would have the following equivalent Munsell N values (although they would not necessarily be neutral in color):

Moon - N3.8
Green grass - N5.7
Desert sand - N6.9
New concrete - N7.9
Clouds - maximum below N9.1 (lets just call this N9)
Fresh snow - maximum of N9.6
 

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Discussion Starter · #3 ·
That moon sure is getting hit with a lot of lumens! :bigsmile:
Yeah, glad I'm not the one paying THAT light bill! It was a real shocker to me though to find out the moon reflects so little light. :unbelievable: Just think how bright it would be it we painted it with a good screen mix! :rubeyes: :bigsmile:
 

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Discussion Starter · #6 ·
While on this subject we can also bring up a common situation in home theaters called "ambient light". Ambient light is either light from another light source than the projector hitting the screen, or it can also come from the light being reflected from the screen during use that is re-reflected off of objects or room surfaces back onto the screen. No matter the source of ambient light it will decrease image contrast if there is enough of it.

A similar situation happens when we see Earthshine at night. Earthshine can be seen when the moon is in it's early or late crescent phases when just a sliver of moon is illuminated by the sun. If one were to be on the moon during these phases one would see a "Full Earth". What is happening is that the dark portion of the moon is being illuminated by the sunlight that is being reflected from the earth back to the moon. That light is re-reflected back to the earth again and we call it Earthshine. The fascinating thing, at least to me, is that the average albedo of the earth is only 30% (3 times more than the moon) when means that earthshine is only 10% of 30% (I think that works out to 3%) of the regular intensity of moonshine (no alcohol jokes... I can already hear you snickering :foottap: ).

http://home.hiwaay.net/~krcool/Astro/moon/earthshine/
 
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