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${\displaystyle L={\frac {d^{2}\Phi }{dAd\Omega cos\theta }}}$
Where ${\displaystyle \Phi }$ is radiant flux, ${\displaystyle A}$ is an area, and ${\displaystyle \Omega }$ is a solid angle.
Typical units are ${\displaystyle [{\frac {W}{m^{2}sr}}]}$ where "sr" means steradians.