There is disclosed a housing including a plurality of compartments for housing a plurality of LEDs or photo detectors. Each compartment has a number of control pads projecting inwardly and a number of protrusions. The control pads are configured to provide a contact surface for the LEDs or photo detectors to control the alignment and position of each of the plurality of LEDs or photodiodes within each of the plurality of compartments. Each of the protrusions urges each of the plurality of LEDs or photodiodes against the respective control pads to control the alignment of the LEDs or photodiodes in the housing. A detector array including a casing, an LED housing and a photodiode housing is also disclosed.