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Author Topic: What is the best definition for albedo?  (Read 920 times)

Offline thedoc

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What is the best definition for albedo?
« on: 07/12/2015 10:50:02 »
Ed Wilson  asked the Naked Scientists:
   Most definitions of albedo talk in terms of real-world planetary surfaces, so a value of 1 describes a "perfectly white" surface.  Some definitions say the perfectly reflective surface is a mirror.  How do the two definitions relate to each other?  Does a white surface, if it gets more and more perfectly white, become a mirror?

When considering albedo with reference to global warming, wavelengths other than the visible spectrum are relevant.  Does a surface have the same albedo value for all wavelengths?  I doubt it.




 
What do you think?
« Last Edit: 07/12/2015 10:50:02 by _system »


 

Offline evan_au

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Re: What is the best definition for albedo?
« Reply #1 on: 08/12/2015 11:19:27 »
Quote from: Ed Wilson
a value of 1 describes a "perfectly white" surface.  Some definitions say the perfectly reflective surface is a mirror.  How do the two definitions relate to each other?
The essence of both definitions is that they reflect 100% of the light that falls on them, without absorbing any of it.
One difference is that:
  • a white surface gives a diffuse reflection, ie incident light is reflected in all directions, like snow.
  • A mirror gives a specular reflection, obeying the "angle of incidence=angle of reflection" rule;
  • from certain angles, an illuminated mirror may appear black, while an illuminated sheet of paper will appear bright when viewed at all angles. 

Quote
Does a white surface, if it gets more and more perfectly white, become a mirror?
I would say that
  • a perfectly white surface, as it gets more and more smooth, provides a more specular reflection (like magazine paper vs newspaper)
  • A mirror surface (like aluminum foil or aluminized mylar), as it gets more and more crumpled, provides a more white reflection.
  • ...although you could probably still tell the difference by looking at the reflected polarization of a polarized light source.

Quote
Does a surface have the same albedo value for all wavelengths?
No. All chemicals (like paper) have an infra-red spectrum, corresponding to energy levels of the molecules.

Metallic mirror surfaces have fairly uniform reflectance over a wide range of frequencies (although the color of gold & copper shows that they are not entirely uniform over the visible spectrum).

But if you go too high in frequency (eg X-Rays and Gamma rays), they tend to go straight through most materials, rather than being reflected. 
 

Offline evan_au

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Re: What is the best definition for albedo?
« Reply #2 on: 09/12/2015 20:29:58 »
One anomalous case of albedo is with optical whiteners, as used in some clothes detergents.

These use the fact that sunlight and some indoors lighting have a component of invisible ultraviolet (UV) light.

Using fluorescence, the invisible UV produces visible light, so the object appears to emit more than 100% of the visible light falling on it. This results in an "impossible" albedo at visible wavelengths, although it is compensated by a very low albedo at UV wavelengths.

Clothes treated this way glow brightly in locations lit by UV lamps.

Some clothes detergents were advertised as being "whiter than white"; when challenged as being an unscientific advertising slogan, they were able to show that it is based on good science (and good chemistry).

 

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Re: What is the best definition for albedo?
« Reply #2 on: 09/12/2015 20:29:58 »

 

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