An infusion pump (10) has an optical cassette detection system (50) for determining whether or not a cassette (14) of an administration tubing set is properly loaded in the pump. Operation of the pump may be enabled or disabled based on a determination of the cassette detection system. The cassette detection system includes a light emitter (52) and a corresponding photosensitive detector (54) aligned along an optical axis, and window (55) carried by the cassette. When the cassette is properly loaded in the pump, a light beam from the emitter enters the window, where a portion of the beam is absorbed and another portion of the beam is transmitted for receipt by the detector. The detector signal is evaluated by signal evaluation electronics to determine if the cassette is loaded. The determination may be based on an expected attenuation of the light beam corresponding to a predetermined light absorptance property of the window.