The observer at angle θ to the normal will be seeing the scene through the same aperture of area dA 0 (still corresponding to a d Ω wedge) and from this oblique vantage the area element dA is foreshortened and will subtend a (solid) angle of d Ω 0 cos(θ). This observer will be recording I cos(θ) d Ω dA photons per second, and so will be measuring a radiance of