An apparatus includes: a diagnostic system configured to diagnostically interact with a current target (110c) traveling along a trajectory (TR) and before the current target enters a target space; a first detection apparatus (120) configured to detect first light; a second detection apparatus (130) configured to detect second light; and a control system (150) in communication with the first and second detection apparatuses. The first light includes: light (140) produced from an interaction between the current target and the diagnostic system, and light (142) emitted from a plasma produced by a previous target. The second light includes the light (142) emitted from the plasma produced by the previous target. The control system (150) is configured to: produce an analysis signal based on first and second signals produced from respective outputs of the first and second detection apparatuses; and estimate a property of the current target based on the produced analysis signal.