Image Formation and Image Contrast in HREM

Abstract

Since the reports about directly observing atoms in crystals by means of high-resolution electron microscope images in the early seventies [1,2], the high-resolution electron microscopy (HREM) has become a powerful technique in determining structures and defects for minute crystals. The image formation and image contrast theories are given in detail in the book “Experimental High-Resolution Electron Microscopy” written by Spence [3]. The image formation process contains two steps — the interaction of electrons with the object and the formation of image by the lens. There are different ways to describe the electron diffraction theory [4]. The one by means of the physical optics approach [5] is widely accepted in HREM. In the present paper the interaction of substances with electrons and the image formation process by the lens are briefly introduced. Two approximations of image contrast theory — weak-phase object approximation (WPOA) and projected charge density approximation (PCDA) are given for interpreting the similarity between the image and the structure for very thin and slightly thicker crystals, respectively, and the pseudo-weak-phase object approximation (PWPOA) is introduced for discussing the image contrast change with the crystal thickness.