Glow Modulator

A glow modulator is a source of intense light of small area. A typical use in fax machines of old uses a photocell to scan the original image, the current variations of which are fed or transmitted to a glow modulator. This then converts the current variations into brightness variations which are then projected onto photo-sensitive paper.

In a hollow cathode tube, a hole (the crater) is drilled in the cathode rod. The cathode is enclosed in a ceramic material to prevent ionisation occurring outside the crater. The anode is mounted above the crater and a hole in the anode permits the light beam from the hollow cathode to pass through.

The light output is dependent on the gas composition used, the gas pressure, and the hole diameter and depth. A neon-argon mixture is suitable, provided the argon content is greater than 10%.