Optical interactions in multijunction solar cells result from radiative charge carrier recombination in a wide-bandgap subcell, which generates additional photocurrent in a narrower-bandgap subcell via photovoltaic conversion of the emitted photons. This paper proposes a method for the experimental determination of the optical interactions efficiency, including the measurement of its limiting (saturation) value, γS. The method was tested on triple-junction GaInP/GaAs/Ge solar cells. The parameter γS was recorded at ultra-high current densities through the p–n junction, achieved using small-area samples and specialized equipment. The relationship between the obtained values and the properties of the semiconductor structure is discussed, including the temperature dependence of the radiative recombination efficiency (luminescence intensity).