Hopefully this is a fairly straightforward question:

 

I am currently writing on Solar radiative output, and have run into a term several times that I cannot find a definition for,

 

"Centre-to-limb radiance variation" f (μ, λ)

 

 

If talking about total solar irradiance as measured by a radiometer, I am figuring the centre is the centre of the Sun, however, I cannot find an answer as to whether the limb is referring to the range of the radiometer from the centre, or whether it is referring to the centre to edge of disc radiance variation in the direction of the radiometer.

 

Any help is muchly appreciated as those 5 words are driving me crazy at the moment!

I think it's because sun is being modeled as a 2-D disk but there are edge (or limb) effects — the irradiance is not a constant over the disk for various reasons. The concept might be clearer if you read about limb darkening

http://en.wikipedia.org/wiki/Limb_darkening

Thanks! That was just what I was looking for, just couldnt get the search term correct.