Electrical Engineering ⇒ Topic : Beam Focus

Beam Focus

This region has three anodes, the two end anodes at the same potential and the middle or focussing anode at a lower potential (Refer Figure (a). The end anodes are called astigmatism electrodes. The focus control reduces the beam to a tiny dot on the screen while astigmatism control makes the dot as round as possible without blur.

                                                                    figure (a)

The cylindrical anode stretches across its equipotential lines and they blur at the centre forming a convex surface. The p.d. between the ends of anodes and the middle anode creates a force. Electrons which strike the convex lens normally pass through while those striking at an angle get refracted towards normal and become parallel to the CRT axis. At the end anode, the electrons get further refracted and converge at the screen into a spot. The focal point on the screen can be varied by varying the voltage on the focussing anode.