ULTIMATE RESOLUTION AND INFORMATION IN ELECTRON-MICROSCOPY - GENERAL-PRINCIPLES

A new concept of resolution is introduced based on the idea that the electron microscope can be considered as a communication channel that transfers information from the object to the observer. The channel consists of three subchannels in series: (i) the transfer through the object, (ii) the transfer through the microscope and (iii) the recording of the image. For each of these subchannels the transfer function and resolution are discussed and from this the total information capacity of the whole channel can be estimated. Two different regimes of resolution exist. If the resolution is insufficient to discriminate the individual atoms of the object in projection, the necessary information exceeds the capacity of the channel so that a priori knowledge is needed from other techniques and only limited new information can be obtained with high-resolution electron microscopy (HREM). If the instrumental resolution is sufficient to discriminate the individual atoms, all atom positions can in principle be determined with relatively high precision and without a priori knowledge provided the information can be retrieved directly from the images. With the recent developments, the instrumental resolution approaches 0.1 nm which is close to the ultimate resolution which is limited by the object rather than by the instrument.