A VFD (which is an acronym for Vacuum Fluorescent Display) is a display device based on vacuum tube technology.
The IV-3 VFD tube is a very tiny tube with a digit size of 8 mm.
It consists of a vacuum sealed glass tube containing a thin tungsten wire filament coated by alkaline earth metal oxides, a control grid and seven phosphor coated segments + a decimal separator on a ceramic substrate which form the anodes.
When the filament wire is heated by electric current, the alkaline earth metal oxides on the wire start emitting electrons.
Unlike the orange-glowing cathodes of traditional vacuum tubes, VFD cathodes are efficient emitters at much lower temperatures, and are therefore essentially invisible.
When a positive voltage is applied to an anode (in regard to the filament), the free electrons which are emitted by the filament are accelerated towards the phosphor coating on the anode. This works because in the real world, electrons move actually from the negative terminal to the positive terminal. On impact, the energy of these electrons causes the phosphor to emit visible light, in this particular case a greenish glow. This is the same priciple as was used in cathode ray tubes (CRT) tubes in older television sets. However, phosphors used in VFDs are different from those in cathode-ray displays since they must emit acceptable brightness with much less electron energy. That's why these IV-3 tubes have an acceptable brightness at voltage levels around 30V, this in contrast to several thousand volts in a CRT.
The filament should be driven by AC current. When DC is used there will be a constant potential difference across the filament. When a positive voltage is then applied to the anodes, the actual potential between the filament and the anodes may vary according to the position of the anode in respect to the filament wire. This will be apparent as a luminance slant across the display.
The control grid between the filament and the anodes which looks like a fine metallic mesh allows turning on and off the segments of the entire tube regardless of the anode segment voltages. This allows driving multiple tubes using multiplexing. The grid functions somewhat like the control grid of a classic triode. Normally when an anode is at around 30V, electrons from the filament are accelerated towards that anode. By connecting a slightly negative voltage (-1V) to the control grid, electrons are repelled by the control grid and cannot reach the anode anymore. When applying 30V to the grid, electrons can move freely to the anode and the anode segment will light. It is important to apply a slightly negative voltage to grids and anodes that shouldn't be lit, otherwise ghosting can occur.
On top of the VFD tube you may notice a dark grey silvery substance, the getter. A getter is a deposit of reactive material that is placed inside a vacuum system, for the purpose of completing and maintaining the vacuum.
Getters are especially important in sealed systems, such as vacuum tubes, including cathode ray tubes (CRTs), which must maintain a vacuum for a long time. This is because the inner surfaces of the tube and its contents release adsorbed gases for a long time after the vacuum is established. The getter continually removes this residual gas as it is produced. Just below the getter you may notice a small metallic ring. When the VFD tube was manufactured, these did hold the getter material.
Once the system was evacuated and sealed, the material was heated, usually by RF induction heating, and evaporated, depositing itself on the walls of the tube to leave the silvery coating. This is called a flashed getter.
The picture below is a macro photo of an IV-3 tube. In front of the anode segments covered with a whitish phosphor, you can see the filament wire and the control grid clearly.