The assumption that the speed of light is infinite underlies many established models in computer graphics and vision. Researchers exploring time-of-flight based sensors are moving into a domain that implicitly requires relaxation of this assumption. The classic rendering equation provides a rigorous foundation for understanding light transport, but fails to encompass the transient effects of light propagation at finite speeds. In this paper, we will introduce a physically-relevant generalization of the rendering equation and a method for approximating this equation, and define a summary measure of transient light patterns, which is used as a basis for general sensor model.