A method of determining a distance from a first luminaire (22) to a second luminaire (22) comprises causing a light source co-located with the second luminaire to emit light directed towards a reflecting surface (A). The method further comprises detecting a level of reflected light from the reflecting surface using a sensor co-located with the first luminaire (22), and accessing a reflectance value determined for the reflecting surface (A). The method further comprises modifying the level of reflected light to take into account the accessed reflectance value, and using the modified measure to provide a measure of a distance between the first and second luminaires.