The Electron Microscope and the Computer: Present Trends and Future Prospects

Abstract

In the present generation of electron microscopes the roles of computers or microprocessors can be divided into control, acquisition and analysis of both spectral and image data. Not much, however, has been done to realise the full power of computer based systems and integrate all these functions with the electron optics. The control of practically all microscope columns is performed by an 8-bit or 16-bit processor usually running a program that repeatedly scans for user inputs and changes lens currents or alignment settings if necessary. External control for special experiments can either be implemented by scanning an additional user input from a serial port or by allowing external analog signals to replace those generated by the microscope control scheme. As the program loop typically takes 0.1 sec to complete it is preferable to implement some functions such as external beam scanning by providing analog ramps (even if generated by another computer).Computer acquistion of data was introduced to electron microscopy with analytical techniques, such as EDX, in which the computer was the basis of a multi channel analyser. In the case of energy loss and Auger spectroscopy, computer scanning of the spectrometer and acquisition of single electron pulse-counted data quickly displaced chart recorders as a means of collecting data. A computer based system not only could perform acquisition more efficiently, it could also provide a convenient means for processing the results and doing quantitative analysis. Furthermore digitally stored data could easily be transferred to other systems on disks or by direct link (such as ethernet) and analysed elsewhere. However for image acquisition there has been very little use of computers in acquiring data. Although microscopists are happy to consider a spectrum as an array of numbers they still prefer to deal with images as pictures rather than digital data sets. The problems are not entirely psychological since a major barrier to the widespread use of image processing in microscopy is the lack of a suitable detection system. TV cameras compare unfavourably with photographic plates in terms of both dynamic range and "resolution" as defined loosely in terms of pixel size or lines/mm. This argument does not apply to scanning microscopes but, even in scanning systems, frame buffers and powerful computer systems have only been integrated as part of the microscope electronics in the last few years. Microscopists still prefer to measure quantities from exposed photographs rather than work with digitized data in a workstation environment using high level image processing software. Until recently cost may have been a consideration but now computing platforms of sufficient capability are less than 1/5 of the cost of an average SEM.