Solid Angles

["AHOLA, CHRIS" <aholact@songs.sce.com>]

     Alex,
     
     A solid angle is the projection of an area on the surface of a sphere 
     divided by the square of the radius of the sphere.  The solid angle is 
     measured in units of steradians.  
     
     The solid angle (usually represented by the upper case Greek letter 
     Omega) is calculated as follows:
     
     Omega = A/r^2
     
     where:
     
     A = Surface area of sphere subtended by the solid angle and,
     
     r = radius of sphere
     
     
     For example, the maximum possible solid angle is that of a complete 
     sphere and is equal to 4 pi steradians.  It follows that the solid 
     angle of a hemisphere would be equal to 2 pi steradians (and so 
     forth...).
     
     
     Proof that the solid angle of a sphere is equal to 4 pi steradians is 
     as follows:
     
     Surface area of sphere = 4 pi r^2
     
     Omega = A/r^2 = 4 pi r^2 / r^2 = 4 pi steradians
     
     
     I hope this helps.
     
     Chris Ahola
     AHOLACT@SONGS.SCE.COM