The theory of image formation by a high-resolution electron microscope is reviewed based on the wave-optics. The image formation in an electron microscope is characterized by strong interaction of electrons as well as a large spherical aberration of an objective lens. Thus the interference between scattered waves at recording an image becomes important. Ananalvtical expression of the transmission-cross-coefficient is derived, which describes the degree of coherence between each two scattered waves due to the aberrations. The transmission-cross-coefficients control the information transfer through the electron microscope, and thus limit the attainable resolution of the microscope. The characteristics of high-resolution electron microscopy will be made clear by comparison with optical microscopy.
