1. A land-based vehicle including an arrangement for monitoring objects in or about a vehicle, comprising: a source from which modulated illumination is emitted into an area in or about the vehicle; a receiver arranged to receive illumination reflected from an object in the path of the modulated illumination; and circuitry coupled to said receiver and said source and arranged to compare a phase of the modulated illumination with a phase of the reflected radiation at a common frequency to determine whether there is a phase difference between the modulated illumination and the reflected illumination, whereby said phase difference is a measure of a property of the object.