This calculator will help you to compute the radiance of a laser beam propagating in a multi-mode or double-clad optical fiber
$\theta$ = arcsin(NA)
$\Omega$ = $\pi$NA$^2$ $\Omega$ = 2$\pi$(1-cos$\theta$)
A = $\pi\frac{D^2}{4}$ R = $\frac{P}{A\Omega}$
Optical Power, P; Fiber Core Diameter, D; Fiber Numerical Aperture, NA; Beam's Divergence Solid Angle, $\Omega$; Fiber Core Area, A; Radiance, R;