There is provided a method for determining presence of a person comprising a) receiving IR sensor data (50) during a first time period from a thermopile and using the IR sensor data to determine an IR background signal baseline (51) for the time period, and determining a variability of the IR sensor data (50), b) using the IR background signal baseline (51) and the variability of the IR background signal level to determine a threshold (52) with a value higher than the background signal baseline (51), and in such a way so that greater variability in the IR background signal (50) results in a higher threshold (52), then c) receiving further IR sensor data (50) during a second time period, which is after the first time period, and using the further IR sensor data (50), and the threshold (52) determined in step b) to determine that a person is present when the further IR sensor data (50) comprises a value that is higher than the threshold.