According to a first aspect disclosed herein, there is provided a controller for controlling a first light source to emit light into a pupil of a human user in order to reduce at least one biological effect of light from a second light source on the human user, the controller comprising: a first input for receiving light property data indicating a property of the light from the second light source, said property being of a type which causes a biological effect on the human user； a second input for receiving pupil data indicating a size of the pupil of the human user； an output for sending control commands to the first light source； a processor configured to: determine, using the light property data received via the first input, that the light from the second light source has said property； in response to said determination that the light from the second light source has said property, determine, using said pupil data received via the second input, a current size of the pupil； and on condition that the current size of the pupil is determined to be larger than a minimum size, control the first light source to emit light not having said property into the pupil and thereby reduce the size of the pupil.
