A light therapy device (1) comprises: at least one light source (2); a control device (10) for controlling the operation of the light source; a boundary memory (11) containing boundary information defining boundaries for the distance (D) and relative angle (f) of a user's head with respect to the light source; monitoring means (30) for monitoring the actual distance and actual relative angle of the user's head and generating an output signal indicating the measured distance and angle. The control device has an input coupled to the memory for receiving the boundary information, and has an input coupled to the monitoring means for receiving the output signal from the monitoring means. The control device compares the output signal from the monitoring means with the boundary information, and issues a warning signal when the actual position and/or orientation of the user's head and/or eyes is/are outside said boundaries.
