1. An X-ray examination apparatus comprising: an X-ray tube for producing an X-ray beam; an X-ray image intensifier tube having an input screen disposed for receiving an X-ray image from the X-ray beam and an output screen for producing an optical image; image read out means, which includes an image signal processor, for producing a visual representation of the optical image; optical means for transmitting the optical image from the output screen to the image read out means; auxiliary light detecting means comprising a two-dimensional matrix of photodetector elements disposed in the form of an orthogonal matrix, for selecting and detecting a portion of the optical image being transmitted from the output screen to the image read out means which portion of the image contains image information from substantially the entire output screen, and for producing an output signal corresponding to said portion, the output signal being connected to control one or more signal transfer characteristics of the image signal processor; and means for individually controlling the photodetector elements by means of two mutually orthogonal line controls so that the output signal includes spatial image information from the output screen.