An apparatus and devices for measuring fluorescence lifetimes of fluorescence sensors for one or more analytes, the apparatus comprising (c) one or more reference systems (3,6,7), said reference systems each comprising one or more reference light sources (3) and being adapted to receive one or more excitation signals (1a), to produce reference optical signals (6b) in response thereto, and to produce one or more electrical reference output signals (7b) in response to one or more excitation signals (1a); and (d) or more phase detectors (10), said phase detections being adapted to detect one or more delays of said one or more electrical output signals of said one or more fluorescence sensor systems and said one or more reference systems, and to produce one or more phase output signals; and a method of measuring concentration of one or more analytes using such apparatus and/or devices.