A head mountable display, HMD, system to display images to a user via an HMD includes an illumination detector to detect an illumination level of an ambient environment around the HMD; an operation detector to detect one or more operations indicative of one or both of: (i) initiation of the HMD being put on by the user; and (ii) initiation of the HMD being taken off by the user; and a controller to control the illumination level of the display images, the controller being configured to generate and apply a temporary illumination level variation to the display images in response to a detection of the one or more operations by the operation detector.