where I ( x , t ; n ^ , ν ) {\displaystyle I(\mathbf {x} ,t;\mathbf {\hat {n}} ,\nu )} denotes the spectral radiance (or specific intensity) at the point x {\displaystyle \mathbf {x} } at time t {\displaystyle t\!} and frequency ν {\displaystyle \nu \!} , n ^ {\displaystyle \mathbf {\hat {n}} } denotes a variable unit vector with origin at the point x {\displaystyle \mathbf {x} } , d ω ( n ^ ) {\displaystyle d\omega (\mathbf {\hat {n}} )} denotes an element of solid angle around n ^ {\displaystyle \mathbf {\hat {n}} } , and Ω {\displaystyle \Omega \!} indicates that the integration extends over the full range of solid angles of a sphere.