The image orthicon

Another type of camera tube, quite sensitive, but having a faster response to image changes, is image orthicon. It is constructed like the vidicon, apart from that there is a target electrode behind the photocathode. When an electron from the photocathode strikes this target electrode, many secondary electrons are emitted as a result. This is the form of current amplification. The image orthicon behaves as a photomultiplier, improving the sensitivity of it.

                                                                    Figure--Simplified cutaway view of an image orthicon.

A beam of electrons, emitted from electron gun, scans target electrode. The secondary electrons makes this beam to be reflected back toward the electron gun. Areas of target electrode with the most secondary-electron emission create the greatest return beam intensity; regions with least emission produce lowest return beam intensity. The greatest return beam intensity corresponds to brightest parts of video image. The return beam can be modulated as it scans the target electrode in the pattern like the one in TV receiver CRT. The return beam is picked up by the receptor electrode.The basic disadvantage of the image orthicon is that it generates significant noise in addition to signal output. But when a fast response is required and the light ranges from very dim to very bright, the image orthicon is camera tube of choice. It is very common in commercial broadcasting.