A method of operating a wearable heads-up display that in use is worn on a head of a subject user includes projecting light to an eye of the subject user and detecting an ambient light condition in an environment of the wearable heads-up display by an ambient light sensor. The method includes measuring a pupil size of the eye of the subject user and determining a target luminous flux based on an output of the ambient light sensor and the pupil size. A current luminous flux of the light projected to the eye of the subject user is adjusted based on the target luminous flux.