Remember the relationship between irradiance and incident radiance is E=∫xL(x′′→x′)cosθ′dwE = \int_x L(x'' \rightarrow x')cos{\theta'} dwE=∫xL(x′′→x′)cosθ′dw. Based on the graph, dwdwdw is cosθ′′dA′′(x′′−x1)2\frac{cos{\theta''}dA''}{(x'' - x1)^2}(x′′−x1)2cosθ′′dA′′. Replacing dwdwdw with dA′′dA''dA′′, We have the equation in the slide.
Remember the relationship between irradiance and incident radiance is E=∫xL(x′′→x′)cosθ′dw. Based on the graph, dw is (x′′−x1)2cosθ′′dA′′. Replacing dw with dA′′, We have the equation in the slide.