Automated procedure speeds diffraction-pattern analysis
Electron diffraction is widely used in the characterizing of specimens for transmission electron microscopy. With the growing trend toward automating the measurement and analysis of electron diffraction patterns (EDPs), a number of companies have developed software for measuring, indexing, and simulating these patterns. Many software programs are stand-alone versions that perform either a specific aspect of EDP analysis or require significant user interaction, particularly in the measurement phase.
In some cases, the lack of suitable algorithms for measuring electron diffraction patterns limits these programs so that users still have to perform the standard procedures of measuring EDPs from negatives or prints. To automate this procedure, D. Carl Dufner of the Electron Microscopy Center at Texas A&M University (College Station, TX) has developed an automated method for on-line acquisition and measurement. Consisting of a Targa+16 frame-grabber board from Truevision (Indianapolis, IN), ImagePro Plus image-processing and analysis software from Media Cybernetics (Silver Spring, MD), and Excel spreadsheet software from Micro soft, the system can capture images from either a television camera on a transmission electron microscope or a videocassette recorder.
The Image-Pro Plus software is used to obtain the electron diffraction patterns. After an image is saved, a macro sequence written as a script file in Image-Pro Plus initiates the measurement process. During the measurement process, Image-Pro Plus counts the spots as bright objects within a specified area of interest, measures the areas, and determines the centroid positions.
Measurements are then exported to the Excel worksheet, where the distances between the spots are determined and converted into d-spacing values. Because the transmitted beam spot usually has a larger area than the diffracted beam spots, the area measurements are sorted by Excel in descending order to highlight that spot as a point of reference for the distance measurements.
After the measurements are completed, a hard copy of the electron diffraction pattern, along with the numbered spots, is made. The only interactive step is in selecting an area of interest in the form of a circle, square, or another user-defined shape. The measurement process is therefore streamlined with fewer user-interactive steps, which translates into time savings, particularly in the analysis of a large number of EDPs obtained from well-populated sample grids. ©