Objective Lens Properties of Very High Excitation
Recently the image quality of the electron microscope has been highly improved, because of technical advances made for the exclusion of mechanical and electrical instabilities in the instrument itself. To further improve the resolution, it is important to minimize the chromatic aberration, as well as the spherical aberration. This is true, even though the accelerating voltage has excellent stability; because of the unavoidable velocity fluctuations of electrons, resulting from thermal emission from the cathode, and the energy-loss in the specimen.Therefore, the specimen should be immersed deeply into the lens field, to make these aberrations small. The result will be very high lens excitation. In 1962, Professor E. Ruska and Dr. W. Riecke had investigated the single field condenser-objective lens at the center of which the specimen is placed. Prior to that, in 1960, Dr. S. Suzuki, one of the authors, had pointed out that this new objective lens, in which the specimen is placed at the image side from the center of the lens field,(of the condenser-objective lens: but no special illuminating system is necessary.