I consider the image orthicon and its 'bigger' brother, the image isocon, video camera tubes to be just about the most remarkable electron tubes to have ever been devised. Although first designed by RCA around 80 years ago, even by today's engineering standards, these ingenious quantum mechanical devices represent the pinnacle of vacuum tube technology.

For electron tubes, not only is their complexity unrivalled but they work extremely well at what they were designed to do, that is they are remarkably sensitive image-capturing devices that can work down to the threshold of the photoelectric effect. The image orthicon can record images by candlelight and the isocon by starlight (as it has such a good noise figure—so good they were used for imaging in astronomical telescopes). To achieve this, the photocathode uses multiple alkali metals in combination to achieve an optimal photon/electron work function over the usable visible spectrum.

These amazing electron tubes combine a diverse range of vacuum electronic phenomena and vacuum tube techniques into an integrated system that orchestrates electrons into do their bidding (I use orchestrate here intentionally because electrons are in effect orchestrated into performing quite complex operations to generate an electronic image). What essentially differentiates the orthicon from modern solid-state imaging devices is that it has a much more complicated architecture that involves many different processes whereas the latter repeats essentially one process on mass.

Briefly, these technologies include:

* Electron/thermionic emission, (Richardson's law), generated from a heated cathode enhanced with low work function alkaline earth metal oxides (oxides of Ba, Ca, and Sr, etc.).

* A Wehnelt cylinder electron gun that produces a fine beam of electrons.

* Photo-emissive semitransparent cathode combined with an imaging subsection (as mentioned).

* A image transfer system which involves the transfer of the image across a glass plate (in later EEV tubes, an electronically conducting glass (Elcon glass) was used).

* A scanning section consisting of forward and return electron beams, electrostatic focusing electrodes and electromagnetic focusing and scanning coils.

* 5-stage photomultiplier to amplify the signal in the return electron beam by between 500 and 1000 times.

* A metallic getter deposited onto the glass envelope to maintain a high vacuum (the highly reactive metal would absorb (react with) any stray gas molecules): https://en.wikipedia.org/wiki/Getter

The 4 1/2" image orthicon in the hands of a competent operator produces quite stunning images. One of their endearing characteristics is that the gamma could be altered dynamically on the fly by adjusting the electronic parameters (this way the visual 'mood' of a scene could be set by the CCU (camera control operator) as required). That said, they were prone to producing halo effects and negative halos when, say, photographing candles (e.g.: a lit candelabra resting on a piano). I can recognize an image orthicon image from an old recording in a split second, as they produce unmistakable images (albeit excellent ones).

Why then are they no longer used? First, they were horrendously expensive as they were all handmade; second, they were very large; third, they required high voltages and complicated scanning circuits to operate; forth, for colour TV the cameras were physically enormous (so big in fact that the orthicon† was only used for the luminance channel and the much smaller vidicons for the chroma channels where resolution was less important); and last, the physical nature of their construction meant that there were always slight variations in their physical geometry and that made convergence alignment between channels difficult.

† There were a rare breed of colour cameras that used three 3" orthicons but I've never heard of a camera that used three 4 1/2" ones.)

Images of 3" orthicon showing photo cathode, image section, and photomultiplier dynodes:

http://hakimweb.com/AntiqueXRayTubes/web10/Radiology_Related...

Brief outline of the theory of operation:

https://electronicsdesk.com/image-orthicon.html

A YouTube video describing the operation of an RCA type 5820 3" image orthicon (very good):

https://m.youtube.com/watch?v=-_L4f8_FgFM

Here are some excellent photos of the 4 1/2" EEV type 7259 image orthicon (many considered this the best image orthicon ever made). I actually own two of these and they still worked when I first came into possession of them (they were removed from service because of slight image blemishes):

http://www.r-type.org/exhib/abk0016.htm

Incidentally, you'll note the box the 7259 was transported in (vertical image with English Electric Camera Tubes in yellow on the side) and marked 'keep upright'. Well, in the side of this box was embedded a little circular plastic container that looked a bit like an hourglass. It was an anti-tip mechanism where if tipped upside down a ball bearing would drop out and it was nigh on impossible to put back. The reason why it was important that the tube not be inverted so its photocathode was face down was to stop any tiny loose fragments left over from the manufacturing process from falling onto the photocathode surface and possibly causing blemishes in the image. If the ball had popped out it was an insurance claim! (In the days before video games, used boxes used to be claimed for these anti-tilt devices, they then became get-the-ball-back puzzles that amused people for weeks.)