Decades before there were highly sensitive CMOS-based light sensors and charge-coupled devices (CCDs), light detection for image capturing was performed by vacuum tubes called photomultiplier tubes (PMTs). They amplify light by releasing electrons in response to a detector surface that answers to photon impingement. PMTs are still more sensitive and of lower noise level than the silicon devices. In fact, super-sensitive elements for many atom smashers and subterranean neutrino detectors still use photomultiplier tubes for that reason. My first encounter with a PMT was as part of a video map rendering system used on the airport surveillance radar (ASR) display that I worked on in the USAF. Air traffic controllers etched an overlay map of the airport area on a plate of coated glass. It was placed in a box that swept a light beam in synchronization with the ATC operator's PPI (plan position indicator) display while the video stream was added to the actual radar display. The method was of course crude by today's standards, but it actually worked quite well.
World's First Photon Counter
Counting the photons, or light particles, arriving from a star is the unusual occupation in which I. M. Levitt (left) and William Blitzstein were engaged when this photograph was made at the Flower Observatory in Highland Park, near Philadelphia. A photometer of their own design, which is the first astronomical device to count such photons, is used by the two scientists in their experiments. Levitt is shown adjusting the photomultiplier tube attached to the eyepiece of the telescope while Blitzstein is seen tabulating results obtained with two RCA Electronic Time Interval Counters, used to count electron impulses generated by the phototube.
Posted August 18, 2015