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Multijunction subcell short-circuit currents are typically calculated by integrating the product of the external quantum efficiency and the input solar irradiance. The actual subcell current can be much greater than the result of this calculation when luminescence from a high bandgap subcell couples into a lower bandgap subcell. A model is developed, based on detailed balance, which quantifies luminescent coupling current under different spectral conditions. Steady-state laser and pulsed flash measurements on GaAs/GaInNAsSb dual-junction solar cells demonstrate a luminescent coupling factor of ~35%, compared with the theoretical maximum value of ~48%. The deviation from maximum is due to nonradiative current in the GaAs subcell.