Wikipedia
Monoscope
A monoscope was a special form of video camera tube which displayed a single still video image. The image was built into the tube, hence the name. The tube resembled a small cathode ray tube (CRT). Monoscopes were used beginning in the 1950s to generate TV test patterns and station logos. This type of test card generation system was technologically obsolete by the 1980s.
Usage examples of "monoscope".
From a jacket pocket he brought forth a small monoscope, twisted it open, and took off both lens shields.