A reciprocity theorem is presented that relates the short‐circuit current of a device, induced by a carrier generation source, to the minority‐carrier Fermi level in the dark. The basic relation is general under low injection. It holds for three‐dimensional devices with position dependent parameters (energy gap, electron affinity, mobility, etc.), and for transient or steady‐state conditions. This theorem allows calculation of the internal quantum efficiency of a solar cell by using the analysis of the device in the dark. Other applications could involve measurements of various device parameters, interfacial surface recombination velocity at a polycrystalline silicon emitter contact, for example, by using steady‐state or transient photon or mass‐particle radiation.