Abstract: A lens system comprises a first lens group and a second lens group, and is configured to form an image at a first magnification and at a second magnification. The lens system has a common optical axis in both magnifications. The lens system is further configured to form an intermediate image between the first lens group and the second lens group at the first magnification. The intermediate image formed in the first magnification is further imaged onto an optical detector. In the first magnification, the second lens group acts as a relay lens imaging the intermediate image onto the optical detector. In the second magnification, the first and second lens groups together form an image on the optical detector without forming an intermediate image.

Abstract: A cathode ray tube comprising an electron source and an electron beam guidance cavity having an input aperture and an output aperture, wherein at least a part of the wall of the electron beam guidance cavity near the output aperture comprises an isolating material having a secondary emission coefficient &dgr;1 for cooperation with the cathode and for forming an electron source. Furthermore, the cathode ray tube comprises a first electrode which is connectable to a first voltage source for applying, in operation, an electric field with a first field strength E1 between the cathode and the output aperture. &dgr;1 and E1 have values, which allow electron transport through the electron beam guidance cavity. According to the invention, the cathode structure comprises means for reducing the spread of the energy distribution of the electrons leaving the exit aperture between the input of the cavity and the accelerating grid.