Image Contrast

Contrast C involves the signals produced by the detector for two points A & B on the sample, which depend on:

  • No. of signal electrons emitted from the sample
  • how readily these electrons reach the detector
  • the efficiency of the detector in recording the arriving e-
Subsequent amplification and processing of the signal produced by the detector can change the appearance of the image, but cannot alter its information content.

For two small objects to be detected against a background of random noise, studies show that their signal difference must be at least five times the noise level.

In order to detect objects of small size and low contrast in an SEM it is necessary to use a high beam current and a slow scan speed (i.e., improve signal to noise ratio).

The fact that certain expected features do not show up in an SEM image does not necessarily mean the features aren't there (particularly when working at high magnifications where beam currents must be very low to give the small beam diameter needed for good resolution) . It may simply be that under the operating contiditions being used they cannot be detected - it may be somewhat like trying to photograph a snowball against a white sheet in the moonlight.

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