noise canceller

In a cold-field emission gun, short-time intensity fluctuations of the emission current (called "emission noise (chip noise)" exist. As a result, light-and-dark horizontal line contrast appears in an image of SEM or STEM. A system to reduce this adverse contrast is called "noise canceller." The noise canceller consists of a detector and an operational circuit. The canceller detects a part of the emission current, feeds back the fluctuation signal to the image signal to remove the effect of the fluctuation, and reduces the horizontal line contrast on-line. In this process, the emission current is detected by a dedicated aperture for current measurement or a condenser aperture fitted with a current-detection mechanism.