The spatial distribution of the long-wavelength luminescence in thick In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) has been investigated using cathodoluminescence (CL) imaging and spectroscopy. The CL spectra show defect-induced broad bands between 1000≲λ≲1600 nm. These bands exhibit spatial variations which correlate with the dark line defects (DLDs) observed in the λ=950 nm exciton luminescence imaging. Transmission electron microscopy showed that -oriented misfit dislocations occur primarily at the substrate-to-MQW and GaAs capping layer-to-MQW interfaces. The large spatial variation of the luminescence intensities indicates that the DLDs observed in CL images are caused by the presence of nonradiative recombination centers occurring in the MQW region located between the interface misfit dislocations. This study provides new information describing the origin and nature of DLDs and differs from previous models which have regarded the electronic nature of dislocation cores as the primary mechanism for inducing DLD radiative contrast in luminescence imaging of strained InGaAs/GaAs.