The radiance equation is $$L = \frac{d}{dA} \frac{2(\phi)}{dW cos(\theta)} (watt/srm^2)$$ where $\phi$ is the flux.
I am thinking, should not be the cosine term on the numerator instead of the denominator? Having the cosine in the denominator will make L goes to infinity if $\theta = 90$ , which does not make sense to me. My understanding is that if $\theta = 90$ (i.e. flux direction is perpendicular to the surface normal), then $L$ should equal to zero (and not infinity).