3.2.2.1. Magnification, Brightness and Contrast

Electron images are obtained by rastering the electron beam across the specimen surface (using the deflection coils inside the objective lens) and synchronously rastering the output signal of the detector on a cathode-ray tube (CRT). The ratio of the area rastered on the specimen to that of the CRT gives the magnification. For example, a rastered area of 200 μm² (200 x 10^-6 m²) displayed on CRT with an area of 20 cm (200,000 x 10^-6 m²) yields a magnification of 1000x. This is a very different process than the production of an image by an optical microscope. Electronic images are sequentially "constructed" during the rastering of the beam, whereas in optical systems all parts of the sample are imaged simultaneously.

As the electron beam is rastered over than sample, the beam pauses for a fixed time at each point. During this dwell time, the beam interacts with the sample and produces various effects (backscattered, secondary and Auger electrons; luminescence, etc.). The time required to generate these effects in the sample is very much smaller than the dwell time, so as soon as the electron beam leaves a spot, the effects stop. The magnitude and nature of the effects, which are detected by various electronic means, reveals information about the sample. Note that the rastering produces a digital image; each dwell point (pixel) can be represented by a number (signal strength). The resulting image can be saved and reprocessed in the same way that a grayscale image can be modified by PhotoShop or other image program.

The overall signal produced from the sample may be summarized by its brightness and contrast , both of which can be electronically modified. The brightness of an image simple reflects the size of the signal being produced from the sample and will vary across the rastered area (Figure 3.2.2.1a). Brightness can be increased by amplifying the signal from the sample, or decreased by decreasing the amount of amplification. Brightness can be affected by topography, composition, electrical conductivity, and other properties of the sample.

Figure 3.2.2.1a. Variation in brightness shown as a histogram. Pixels that are pure white plot on the extreme right; pure black pixels plot on the left. Most pixels have brightnesses in the middle. Brightening the image would result in shifting the entire histogram to the right.

Contrast reflects the variation in the signal from point to point. It may be expressed as C = ΔS / S_average, where ΔS = the change in signal strength between any two points, and S_average = the average signal strength. Contrast can also be enhanced by electronically increasing the difference between small and large signals (Figure 3.2.2.1b). In as much as variations in the signal reflect real compositional (or other) variations in the sample, the operator should try to optimize the contrast. However, increasing the contrast too much will mask subtle variations within the sample.

Figure 3.2.2.1b. Enhancing the contrast. Previous values that were in the middle have been pulled towards both ends. Notice how histogram has been stretched.