An appliance (1) for examining biological tissue comprises a light injection means (102) for injecting visible and/or close infrared light into the biological tissue, a detector (104) for converting light signals that exit the biological tissue into detection signals, an output device (130) allocated to the detector (104) for outputting information that depends on the detection signals and a shield (20; 106) allocated to the director (104) for shielding the detector (104) against extraneous light at least in the injected wave range. The appliance is also provided with a control device for controlling whether the arrangement of light injection means (102) and/or a detector and/or a shield (20; 106) is correct in relation to a substrate. The appliance can be used for examining biological tissue and supports the user for forcing the user to arrange the essential components of the examination device in the prescribed order before measuring.