Vacuum Fluorescent Display tubes contain a thin layer of phosphor mixed with inert gases in a high pressure glass enclosure. Their amazingly cool blue glow is based on the principle of cathodoluminescence, whereby electrons produced by a filament hit the phosphor and cause the emission of photons visible to the human eye.
The cathode, in this case, is made up of… Wait. Do you hear something?
‘HELLO? IS ANYBODY THERE?’
‘WHO IS THIS?’ you ask.
‘IT’S YOUR SECRET, INSIDE NERD, OF COURSE!’
‘AND WHAT DO YOU WANT?’
‘I WANTED TO LET YOU KNOW – I’VE BEEN READING ALONG AND I HAVE FALLEN IN LOVE. I HAVE FOUND THE ONE.’
‘THE ONE? BUT I ALREADY HAVE A GIRLFRIEND.’
‘DOES SHE EMIT EXCITED ELECTRONS TRAVELLING AT THE SPEED OF LIGHT WITH THE SOLE PURPOSE TO RELEASE THEIR ENERGY ON IMPACT?’
‘ERR, I GUESS NOT.’
‘DIDN’T THINK SO. YOU CAN KEEP HER, THEN. JUST GET ME THE CHRONIKER AND WE’LL CALL IT EVEN.’
‘BUT I CAN’T AFFORD THE CHRONIKER RIGHT NOW!’
‘DIDN’T YOU SAY YOU HAD A GIRLFRIEND…?’
If we may interrupt this touchy conversation, we’re here to get down to the science. To temporarily quench your noisy inner nerd, read on. If you’re actually hearing voices, though, you might want to see a shrink about that.
In any case, you already know that Vacuum Fluorescent Display tubes operate very similarly to vacuum tubes triode. The layer of phosphor within the VFD functions as an anode, while the cathode is made up of fine tungsten wires, coated in alkaline earth metal oxides. When heated by electric current, the cathode emits electrons, which are controlled and diffused with the help of the thin metal grids.
Interestingly enough, several phosphor plate elements can be incorporated in Vacuum Fluorescent Display tubes, but electrons can only reach a given plate element if both it and the grid are at a positive potential in relation to the cathode. This allows the creation of multiplexed displays, where the number of signal pins required is minimal. The microprocessor behind the display places positive voltage on a digit’s grid and on the appropriate plates, lighting up each digit at a time.
What is truly fascinating is that the microprocessor in Vacuum Fluorescent Display tubes can pass from illuminating one digit to another at such a high frequency that it creates the illusion of all digits glowing at once.
This wonderful phenomenon is called persistence of vision, and it is somewhat similar to what happens to you when your girlfriend starts talking about her day. Indeed, have you ever noticed how her entire speech turns into one long stream of sounds and you never really know what part of the day she’s talking about? Would you look at that – science is everywhere around us!