1. A sensor unit for sensing characteristics of a display system when used for imaging, the display system being configured to generate three dimensional images by combining at least first sub-images for a first eye and second sub-images for a second eye, the sensor unit comprising: a filter configured to filter radiation used for displaying of said first sub-images from radiation used for the displaying of said second sub-images; a detector configured to detect a first radiation signal comprising information related to separate properties of the radiation used by the display system for the displaying of said first sub-images and/or to detect a second radiation signal comprising information related to separate properties of the radiation used by the display system for the displaying of said second sub-images individually, and a processor configured to process detected signals from the detector and to generate control signals based thereon for adjusting the characteristics of the display system which characteristics are used when imaging the first sub-images and/or control signals adjusting the characteristics of the display system which characteristics are used when imaging the second sub-images.