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Application Notes
Modern scanning electron microscopes (SEMs) all utilize digital scan controls, digital signal acquisition and processing of gray-scale digital images. In a working digital SEM, high energy electrons are focused into a fine beam which rasters across the surface of a specimen pixel by pixel in incremental steps. A complex beam/specimen interaction at each pixel generates a variety of signals which are detected and exhibited on a display unit scanning in synchronism with the beam scan on the specimen. When the focused beam is addressed to a certain point on the specimen, the signal intensity is measured by a detector integrating for the dwelling time, and represented as the brightness of the pixel in a digital image. The intensity is typically digitized into a range of 8 bits which gives 256 discrete gray levels. Apparently, such gray-level images can be interpreted to reveal some characteristics of the specimen. Since image contrast is always related to some properties of the specimen, understanding the concept of contrast and its numerical meaning is of great importance in scanning electron microscopy.
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