A method for determining at least one behavioural, postural or geometric-morphological characteristic of a person wearing spectacles, the method includes the steps of: a) illuminating the wearer's head using an infrared light source (220, 230, 240), b) acquiring an image of the wearer's head using an infrared image sensor (210); c) in the image, locating reflections from the infrared light surface, reflected by the two lenses of the spectacles; and d) using a computation unit (250) to deduce the required characteristic as a function of the positions of the reflections in the image.