The spontaneous radiation generated in the active region of (Al,Ga)As double‐heterostructure (DH) proton‐delineated stripe‐geometry lasers has been studied by measuring the luminescence (Ls) emitted from a window fabricated in the metallization on the substrate. Observations of the intensity of this luminescence with an image converter provide an effective means for detecting growth‐, processing‐, and aging‐induced defects. In addition, these measurements provide a simple procedure for separating changes in optical loss from changes in radiative‐recombination efficiency. Thus,Lsobservations are useful for quality control and laser‐aging analysis. For lasers with 8% aluminum in the active region,Lsis shown to be principally radiation reemitted from a very thin layer of GaAs substrate at the interface with then‐type ternary layer. Reasonably high spatial resolution (∼3 &mgr;m) of the radiation from the active stripe region is retained in this absorption‐reemission process. For devices with no aluminum added to the active layer, the luminescence is mostly single pass radiation from the active region. Quantitative current and spatial dependences ofLshave been obtained which probe the carrier concentration in the active volume. It has also been shown that the threshold current is easily measured by an abrupt change indLs/di(saturation) and that this slope also characterized the slope of the intensity of the stimulated emission (L) above threshold. In addition, abrupt changes indLs/diat threshold current have been measured near the edges of the stripe showing that saturation‐induced effects are observable over the entire stripe width. In the current increment in which near‐field examination ofLshowed spatial motion of a filament significant spatial changes inLswere observed (although only for lasers without aluminum in the active layer). Thus, spatial motion of lasing filaments can be correlated with changes in the steady‐state spatial carrier‐concentration profiles. Many lasers exhibited a decrease inLsabove threshold, most pronounced about the central axis of the lasing stripe. This decrease in the steady‐state carrier concentration does not seem to be consistent with a model based on saturable optically absorbing traps.