One example given to motivate the study of bipartite heat engines in the previous chapter was light-harvesting molecular machines like photosystem II [1] and bacteriorhodopsin [2]. These machines are driven out of equilibrium by high-energy solar photons, and the light-induced reactions can be treated as coupling to a heat bath at the temperature of the photon source [3–5]. In this chapter (Secs. 8.2 and 8.3) I will provide theoretical background for this counterintuitive claim, first giving a general argument based on the statistical mechanics of blackbody radiation, then examining in detail a simple example system.