Cathodeon monicon tube with test card "C" (BBC version). Serial No. 387.

The monicon was the special image generation cathode-ray tube which fit inside a "monoscope" camera. It would display the image etched on to the target plate and was used to produce test cards and logos - thus avoiding pointing a TV camera at a chart for a prolonged period of time (which would permanently burn the image into the camera's pick-up tube). Cathodeon was one of the Pye group of companies.