An optical inspection system is for visually inspecting the blades of a turbine at turning gear operation. The present invention provides a seal-leak detection arrangement that, when incorporated into air moving equipment, monitors the air passing through the air moving equipment for the presence of contaminants such as lubricant oil, which could damage the equipment downstream. The clipping circuit includes a PNP transistor which detects a change in the output voltage of the operation amplifier and a voltage source connected to a base of the PNP transistor.