Figure 2 represents what an observer sees. The observer directly above the area element will be seeing the scene through an aperture of area dA 0 and the area element dA will subtend a (solid) angle of d Ω 0, which is a portion of the observer's total angular field-of-view of the scene. Since the wedge size d Ω was chosen arbitrarily, for convenience we may assume without loss of generality that it coincides with the solid angle subtended by the aperture when "viewed" from the locus of the emitting area element dA. Thus the normal observer will then be recording the same I d Ω dA photons per second emission derived above and will measure a radiance of