page_title: Vacuum fluorescent display
section_title: Design
hierarchical_section_title: Vacuum fluorescent display / Design
caption_attribution_description: English: Macro Shot of a Vacuum Flourescent Digit Deutsch: Makroaufnahme eines Zeichend einer Fluoreszenzanzeige
context_page_description: A vacuum fluorescent display is a display device once commonly used on consumer electronics equipment such as video cassette recorders, car radios, and microwave ovens. LCDs, OLED displays and LED segment displays have now largely replaced VFDs.
  A VFD operates on the principle of cathodoluminescence, roughly similar to a cathode ray tube, but operating at much lower voltages. Each tube in a VFD has a phosphor coated anode that is bombarded by electrons emitted from the cathode filament. In fact, each tube in a VFD is a triode vacuum tube because it also has a mesh control grid.
  Unlike liquid crystal displays, a VFD emits a very bright light with high contrast and can support display elements of various colors. Standard illumination figures for VFDs are around 640 cd/m² with high-brightness VFDs operating at 4,000 cd/m², and experimental units as high as 35,000 cd/m² depending on the drive voltage and its timing. The choice of color and display brightness significantly affect the lifetime of the tubes, which can range from as low as 1,500 hours for a vivid red VFD to 30,000 hours for the more common green ones.
context_section_description: The device consists of a hot cathode (filaments), grids and anodes (phosphor) encased in a glass envelope under a high vacuum condition.  The cathode is made up of fine tungsten wires, coated by alkaline earth metal oxides (barium, strontium and calcium oxides), which emit electrons when heated to 600°C by an electric current.  These electrons are controlled and diffused by the grids (made using Photochemical machining), which are made up of thin metal. If electrons impinge on the phosphor-coated anode plates, they fluoresce, emitting light. Unlike the orange-glowing cathodes of traditional vacuum tubes, VFD cathodes are efficient emitters at much lower temperatures, and are therefore essentially invisible. The anode consists of a glass plate with electrically conductive traces (each trace is connected to a single indicator segment), which is coated with an insulator, which is then partially etched to create holes which are then filled with a conductor like graphite, which in turn is coated with phosphor. This transfers energy from the trace to the segment. The shape of the phosphor will determine the shape of the VFD's segments. The most widely used phosphor is Zinc-doped Zinc oxide, which generates light at a peak wavelength of 505 nm. 
  The principle of operation is identical to that of a vacuum tube triode. Electrons can only reach (and "illuminate") a given plate element if both the grid and the plate are at a positive potential with respect to the cathode. This allows the displays to be organized as multiplexed displays where the multiple grids and plates form a matrix, minimizing the number of signal pins required. In the example of the VCR display shown to the right, the grids are arranged so that only one digit is illuminated at a time. All of the similar plates in all of the digits (for example, all of the lower-left plates in all of the digits) are connected in parallel. One by one, the microprocessor driving the display enables a digit by placing a positive voltage on that digit's grid and then placing a positive voltage on the appropriate plates. Electrons flow through that digit's grid and strike those plates that are at a positive potential. The microprocessor cycles through illuminating the digits in this way at a rate high enough to create the illusion of all digits glowing at once via persistence of vision.
  The extra indicators (in our example, "VCR", "Hi-Fi", "STEREO", "SAP", etc.) are arranged as if they were segments of an additional digit or two or extra segments of existing digits and are scanned using the same multiplexed strategy as the real digits. Some of these extra indicators may use a phosphor that emits a different color of light, for example, orange.
  The light emitted by most VFDs contains many colors and can often be filtered to enhance the color saturation providing a deep green or deep blue, depending on the whims of the product's designers. Phosphors used in VFDs are different from those in cathode-ray displays since they must emit acceptable brightness with only around 50 volts of electron energy, compared to several thousand volts in a CRT. The insulating layer in a VFD is normally black, however it can be removed to allow the display to be transparent. AMVFD displays that incorporate a driver IC are available for applications that require high image brightness and an increased number of pixels. Phosphors of different colors can be stacked on top of each other for achieving gradations and various color combinations. Hybrid VFDs include both fixed display segments and a graphic VFD in the same unit. VFDs may have display segments, grids and related circuitry on their front and rear plass panels, using a central cathode for both panels, allowing for increased segment density. The segments can also be placed exclusively on the front instead of on the back, improving viewing angles and brightness.
caption: Macro image of a VFD digit with 3 horizontal tungsten wires and control grid.
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