A patient monitoring system for monitoring the location of a patient at a distance comprises a projector 52 for projecting a pattern of light onto the surface of the patient and an imaging system 50 for obtaining images of the patient on to whom the pattern of light is projected. A heat sink 55 is further associated with the projector 52 and a heat source 74, such as an array of resistors, is configured to apply heat to the heat sink 55 when the projector 52 is not being operated, thus reducing the variation in the temperature of the heat sink 55. This in turn reduces variation in the thermal expansion and contraction of the monitoring system effects which can potentially introduce source of error in determining the position of a patient. The heat source 74 may further be responsive to the output of a thermometer measuring the temperature of the heat sink 55 applying heat to the heat sink 55 when the projector is not operated so as to maintain the heat sink 55 at a constant temperature.
