A large number of GaAs substrates GaAlAs double‐heterostructure (DH) wafers, and high‐radiance GaAlAs DH light‐emitting diodes (LEDS) were evaluated using transmission cathodoluminescence (TCL). We show that only epitaxial wafers with a high defect density as revealed by TCL readily develop dark line defects (DLDs) with current injection, optical excitation, or electron beam excitation. Furthermore, in agreement with the previous work, the electron‐beam‐induced DLDs originate at dislocations and their growth requires minority‐carrier injection. Based on these results, it is inferred that TCL can serve as a nondestructive screening technique for the selection of materials that produces a high yield of reliable LEDs.