# Multi-mode fiber radiance calculator

## About this calculator

This calculator computes the radiance of a laser beam propagating in a multi-mode or double-clad optical fiber. It can also be used for free space beams by replacing core diameter with focal spot diameter. Radiance is kind of a measure of beam quality with power included. At best radiance can only be conserved, not improved in optical components such pump combiners. Radiance is defined as

where is the optical power, is the cross-sectional area of the focal spot or the core area of the fiber, and is the beam's divergence solid angle defined by its NA. This calculator computes as

where is the divergence angle. For small NA values, a useful approximation formula is