These subroutines calculate the power (in watts) radiated per unit surface area (m2) per unit solid angle (sr) by a black body at temperature='kelvin'. The power can be calculated either per unit frequency, at a given frequency (both in hertz), or per unit wavelength (in metres) at a given supplied wavelength (in angstroms).
The subroutines are designed to be portable and as fast yet as accurate as possible.
