Vacuum fluorescent display

A vacuum fluorescent display (VFD) is a display device used commonly on consumer-electronics equipment such as video cassette recorders, car radios, and microwave ovens. Unlike liquid crystal displays, a VFD emits a very bright light with clear contrast and can easily support display elements of various colours. The technology is related to both the cathode ray tube and the nixie tube.

The device consists of a hot cathode (filaments), anodes (phosphor) and grids 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, which emit electrons when heated by an electric current. These electrons are controlled and diffused by the grids, which are made up of thin metal. If electrons impinge on the phosphor-coated plates, they fluoresce, emitting light. Unlike the orange-glowing cathodes of traditional vacuum tubes, VFD cathodes are efficient emitters at much cooler temperatures, and are therefore essentially invisible.

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.

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 produces a different colour of light, for example, orange.

The light produced by most VFDs contains many colours and can often be filtered to produce a more-pure (saturated) colour such as a deep green or deep blue, depending on the whims of the product's designers.

Usage

Besides brightness, VFDs have the advantages of being rugged, inexpensive, and easily configured to display a wide variety of customized messages. Early on, the main disadvantage of this type of display was the consumption of significantly more power (0.2 watts) than a simple LCD. This was considered a significant drawback for battery-operated equipment like calculators, so VFDs ended up being used primarily in equipment powered by an AC supply or heavy-duty rechargeable batteries. Another advantage is that unlike LCDs, VFD's are not limited by the response of rearranging liquid crystals and are therefore able to function normally in sub-zero temperatures, making them ideal for outdoor devices in cold climates.

During the 1980s, this display began to be used for automotive applications, especially where car makers were dabbling with digital displays for vehicle instruments like the speedometer and odometer. A good example of these were the high-end Subaru cars made in the early 1980s (referred to by Subaru enthusiasts as a "digi-dash", or digital dashboard). The brightness of VFDs makes them well suited for use in the automotive industry. Current models of Renault MPV, Scenic and Espace both use VFD panels to show all functions on the dashboard including the radio and multi message panel.They are bright enough to read in full sunlight as well as dimmable for use at night. This panel uses four colours; the usual blue/green as well as deep blue, red and yellow/orange.

This technology was also used from 1979 to the mid-1980s in portable electronic game units. These games featured bright, clear displays but the size of the largest vacuum tubes that could be manufactured inexpensively kept the size of the displays quite small, often requiring the use of magnifying Fresnel lenses. While later games had sophisticated multi-colour displays, early games achieved colour effects using transparent filters to change the colour of the (usually green) light emitted by the phosphors. High power consumption and high manufacturing cost contributed to the demise of the VFD as a videogame display. LCD games could be manufactured for a fraction of the price, did not require frequent changes of expensive batteries (or AC adapters) and were much more portable. Since the late 1990s, backlit colour active-matrix LCD displays have been able to cheaply reproduce arbitrary images in any colour, a marked advantage over fixed-colour, fixed-character VFDs. This is one of the main reasons for the decline in popularity of VFDs, although they continue to be produced.

From the mid-1980s onwards, VFDs were used for applications requiring smaller displays with high brightness specifications, though now the adoption of high-brightness OLEDs is pushing VFDs out of these markets.

Fade

Fading is sometimes a problem with VFD displays. Light output drops over time due to falling emission and reduction of phosphor efficiency. How quickly and how far this falls depends on the construction and operation of the VFD. In some equipment, loss of VFD output can render the equipment inoperable.

Emission may usually be restored by raising filament power. Thirty-three percent voltage boost can rectify moderate fade, and 66% boost severe fade. This can make the filaments visible in use, though the usual green-blue VFD filter helps reduce any such red or orange light from the filament.

External links

* [http://hem.passagen.se/communication/vfd.html Vacuum Fluorescent Display (VFD) (including How to drive the filament)]
* [http://www.futaba1.de/index.php?option=com_content&task=view&id=32&Itemid=71 VFD-Page of Futaba (widely known manufacturer of VFDs)]
* [http://www.noritake-elec.com Noritake-Itron USA, Inventor and manufacturer of VFD, USA Link]
* [http://www.noritake-itron.jp Noritake-Itron Japan, Inventor and manufacturer of VFD, Japan Link]
* [http://www.noritake-itron.com Noritake-Itron UK, Inventor and manufacturer of VFD, UK Link]
* [http://www.phosphortech.com/VFD_Phosphor_Datasheets.php Datasheets and Technical Specs on VFD Phosphors]