We consider ray bundles emanating from a source such as a camera or light source. We derive the full spatial and temporal structure to ¿rst order of the intersection of ray bundles with scene geometry, where scene geometry given as any implicit function. Further, we present the full details of 2 often used geometrical representations. The ¿rst order structure may be used as the linear approximation of the change of photons as the camera, ob jects, and light source change as function of space and time. Our work generalises previous work on ray differentials [Igehy, 1999] and photon differentials [Schjøth et al., 2007].