function [rad] = planck_wl(wl, T)
% rad = planck_wl(wl,T)
% returns planck monochromatic radiance in {W / (m^2 ster micron)}

c1 = 3.7247e8/pi;       % 2*h*c^2
c2 = 1.4388e4;          % h*c/k

rad = c1./wl.^5 ./(exp(c2./wl./T)-1);


