Perfecting an image - signal processing
The SEM image is a constructed (virtual) intensity map (either digital or analogue) of numbers of electrons ejected from the sample material. The electron signal from each dwell point in the SEM is displayed in a sequence, as pixels on a line on a screen, line by line to build the image. The strength of the signal at each point is a reflection of the electrons generated from the topography or composition. Through signal processing each quantum of signal information (gained from each dwell point of the beam) can be changed to some new value that bears a rigorous relationship to the original one, before it is displayed. In this way we can adjust the signal to change contrast and brightness of our final image.
In most cases the unprocessed image contains enough "natural contrast" for the operator to extract useful information from the image. Natural contrast can be considered as the contrast contained in the signal that comes immediately from the specimen+ detector system. If the natural contrast is too low or too high, then signal changes corresponding to important detail may be lost. In this case we see the image as having a lot of black or white regions. A good quality image has a gradation of greys with very little of the image fully black or white. Signal processing techniques manipulate the natural contrast so that the eye can perceive information through contrast in the image. Although signal processing allows the user to manipulate the natural contrast, there is no addition of information, only enhancement of that already present.
This image of part of a small hive beetle shows too little contrast on the left and too much contrast on the right. The central image is correct.
The image on the left can be adjusted after collection, by modifying the spread of greyscale “Levels” in software like Photoshop, but the image on the right is not able to be corrected since the pure black and white areas are absolute (no further data can be retrieved from these regions).
It should be noted that signal processing can greatly change the appearance of an image relative to that which might usually be expected, and therefore the SEM operator is under an obligation to state whether processing has taken place. Normally however, it is considered routine to adjust the image quality using the contrast and brightness knobs ["contrast control" and "black level control"]. However, if some other differentiation had been applied to give a crisp appearance to a SE image, a written report should describe the exact nature of the processing.
Older models of SEMs generally have a graphical display of contrast and brightness that can be used to adjust the image. More modern machines rely on automatic adjustment (ACB buttons), supplemented by machine operator preferences corrected by eye, using the contrast and brightness controls.
Tilting to increase SE contrast
Another mechanism to increase SE contrast in an image is to tilt the sample so that it is at an angle to the probe (typically 30 to 60°). As a result of tilting, more SE are generated per unit of projected specimen area and this enhances contrast by making the distribution of light and dark areas more pronounced.
When tilting a sample use a small sample and tilt only as far as the machine instruction manual advises because it is possible to damage equipment around the stage by tilting too far or with too large a sample.