Vacuum fluorescent display

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An entirely-typical vacuum fluorescent display used in a videocassette recorder

A close-up of the VFD highlighting the multiple filaments, tensioned by the sheet metal springs at the right of the image

A vacuum fluorescent display (VFD) is a type of display used commonly on consumer-electronics equipment such as video cassette recorders 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 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.

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 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 color of light, for example, orange.

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

[edit] Usage

Besides brightness, VFDs have the advantages of being rugged, inexpensive, and easily configured to display a wide variety of customised messages. Early on, the main disadvantage of this type of display was that it consumes "large" amounts of power (a few watts) in comparison to a simple LCD. This was considered a disadvantage 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.

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 (a portmanteau of "digital" and "dashboard"), or digital dashboard). The reason this technology is considered appropriate for electronic displays in the automotive context is that the displays are very bright.

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 reliably be manufactured kept the size of the displays quite small, often requiring the use of magnifying Fresnel lenses. 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 phospors. High power consumption and high fragility contributed to the demise of the VFD as a videogame display, along with high manufacturing cost. 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. As of the late 1990's, 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, however they continue to be produced today.

From the mid 1980s onwards, VFDs have been used for applications requiring smaller displays with high brightness specifications though, now they are even being pushed out of these markets with the adoption of high brightness OLEDs.

Unlike LCDs, most VFDs continue to function normally in sub zero temperatures, making them ideal for outdoor devices in cold climates.

[edit] External links

• Vacuum Fluorescent Display (VFD) (including How to drive the filament)
• VFD-Page of Futaba (widely known manufacturer of VFDs)
• Noritake-Itron, another well known manufacturer of VFDs in Japan