what are the values of surround space constant

F(x,y) = k* exp(-r2 / c2 )

here the function f is a gaussian function.this function used in retinex algorithm.

R(x,y)=log[I(x,y)]-log[I(x,y)*F(x,y)]
R is the retinex output,I is the input image,r=x2+y2;
c is the surround space constant and k is such that ∫∫ F(x,y) dx dy =1.

what is mean by surround space constant

please help me about the values of c and k