S-shaped distortion

"S-shaped distortion" means the distortion characteristic of the electromagnetic lens for the electron beam. Unlike the light beam in the optical lens, the electron beam suffers rotation in the electromagnetic lens. The rotation magnitude increases as the electron beam deviates from the optical axis, and then an image that should be observed as a line on the image plane (fluorescent screen) is viewed as an s-shaped image. The distortion becomes a problem for the projector lens because there exist electron beams which pass largely apart from the optical axis. Combination of lenses with opposite polarity can cancel the distortion, but this technique has not been used yet.