The length of each wedge is the product of the diameter of the circle and cos(θ). The maximum rate of photon emission per unit solid angle is along the normal, and diminishes to zero for θ = 90°. In mathematical terms, the radiance along the normal is I photons/(s·m ·sr) and the number of photons per second emitted into the vertical wedge is I d Ω dA. The number of photons per second emitted into the wedge at angle θ is I cos(θ) d Ω dA.