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A vacuum fluorescent display (VFD) is a display device once commonly used on consumer electronics equipment such as
video cassette recorder A videocassette recorder (VCR) or video recorder is an electromechanical device that records analog audio and analog video from broadcast television or other source on a removable, magnetic tape videocassette, and can play back the recordin ...
s,
car radio Vehicle audio is equipment installed in a car or other vehicle to provide in-car entertainment and information for the vehicle occupants. Until the 1950s it consisted of a simple AM radio. Additions since then have included FM radio (1952), 8 ...
s, and
microwave oven A microwave oven (commonly referred to as a microwave) is an electric oven that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces polar molecules in the food to rotate and produce t ...
s. A VFD operates on the principle of
cathodoluminescence Cathodoluminescence is an optical and electromagnetic phenomenon in which electrons impacting on a luminescent material such as a phosphor, cause the emission of photons which may have wavelengths in the visible spectrum. A familiar example is ...
, roughly similar to a cathode ray tube, but operating at much lower voltages. Each tube in a VFD has a phosphor-coated carbon
anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic ...
that is bombarded by electrons emitted from the cathode filament.Chen, J., Cranton, W., & Fihn, M. (Eds.). (2016). Handbook of Visual Display Technology. doi:10.1007/978-3-319-14346-0 page 1610 onwards In fact, each tube in a VFD is a
triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's ...
vacuum tube because it also has a mesh control grid. Unlike liquid crystal displays, a VFD emits 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 (which determines the nature of the phosphor) 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.
Cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...
was commonly used in the phosphors of VFDs in the past, but the current
RoHS The Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1), short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Unio ...
-compliant VFDs have eliminated this metal from their construction, using instead phosphors consisting of a matrix of alkaline earth and very small amounts of group III metals, doped with very small amounts of rare earth metals. VFDs can display seven-segment numerals, multi-segment alpha-numeric characters or can be made in a dot-matrix to display different alphanumeric characters and symbols. In practice, there is little limit to the shape of the image that can be displayed: it depends solely on the shape of phosphor on the anode(s). The first VFD was the single indication DM160 by Philips in 1959. The first multi-segment VFD was a 1967 Japanese single-digit, seven-segment device. The displays became common on calculators and other consumer electronics devices. In the late 1980s hundreds of millions of units were made yearly.


Design

The device consists of a
hot cathode In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating elemen ...
( filaments), grids and
anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic ...
s ( phosphor) encased in a
glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...
envelope under a high
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
condition. The cathode is made up of fine
tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...
wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is c ...
s, coated by
alkaline earth The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are al ...
metal oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
s (barium, strontium and calcium oxides), which emit
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
s when heated to 650 °C by an electric current. These electrons are controlled and
diffused Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size (mass) of ...
by the grids (made using
Photochemical machining Photochemical machining (PCM), also known as photochemical milling or photo etching, is a chemical milling process used to fabricate sheet metal components using a photoresist and etchants to corrosively machine away selected areas. This process ...
), which are made up of thin (50 micron thick) stainless steel. If electrons impinge on the phosphor-coated anode plates, they
fluoresce Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
, 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 Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...
, 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 copper-activated
Zinc oxide Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement ...
, which generates light at a peak wavelength of 505 nm. The cathode wire to which the oxides are applied is made of tungsten or ruthenium-tungsten alloy. The oxides in the cathodes are not stable in air, so they are applied to the cathode as carbonates, the cathodes are assembled into the VFD, and the cathodes are heated by passing a current through them while inside the vacuum of the VFD to convert the carbonates into oxides. The principle of operation is identical to that of a vacuum tube
triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's ...
. 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 display Multiplexed displays are electronic display devices where the entire display is not driven at one time. Instead, sub-units of the display (typically, rows or columns for a dot matrix display or individual characters for a character oriented dis ...
s 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 A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
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 Persistence of vision traditionally refers to the optical illusion that occurs when visual perception of an object does not cease for some time after the rays of light proceeding from it have ceased to enter the eye. The illusion has also been d ...
. 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 Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter m ...
to enhance the
color saturation Colorfulness, chroma and saturation are attributes of perceived color relating to chromatic intensity. As defined formally by the International Commission on Illumination (CIE) they respectively describe three different aspects of chromatic ...
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.


Use

Besides brightness, VFDs have the advantages of being rugged, inexpensive, and easily configured to display a wide variety of customized messages, and unlike LCDs, VFDs are not limited by the response time of rearranging liquid crystals and are thus able to function normally in cold, even sub-zero, temperatures, making them ideal for outdoor devices in cold climates. Early on, the main disadvantage of such displays was their use of significantly more power (0.2
watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...
s) than a simple LCD. This was considered a significant drawback for battery-operated equipment like calculators, so VFDs ended up being used mainly in equipment powered by an AC supply or heavy-duty rechargeable batteries. During the 1980s, this display began to be used in automobiles, especially where car makers were experimenting with digital displays for vehicle instruments such as speedometers and odometers. 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 For business applications, see Dashboard (business). A dashboard (also called dash, instrument panel (IP), or fascia) is a control panel set within the central console of a vehicle or small aircraft. Usually located directly ahead of the drive ...
). The brightness of VFDs makes them well suited for use in cars. The Renault Espace and older models of Scenic used 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 colors; 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 lens A Fresnel lens ( ; ; or ) is a type of composite compact lens developed by the French physicist Augustin-Jean Fresnel (1788–1827) for use in lighthouses. It has been called "the invention that saved a million ships." The design allows the c ...
es. While later games had sophisticated multi-color displays, early games achieved color effects using transparent filters to change the color of the (usually light blue) 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 Handheld electronic games are very small, portable devices for playing interactive electronic games, often miniaturized versions of video games. The controls, display and speakers are all part of a single unit. Rather than a general-purpose s ...
could be manufactured for a fraction of the price, did not require frequent changes of batteries (or AC adapters) and were much more portable. Since the late 1990s, backlit color active-matrix LCD displays have been able to cheaply reproduce arbitrary images in any color, a marked advantage over fixed-color, fixed-character VFDs. This is one of the main reasons for the decline in popularity of VFDs, although they continue to be made. Many low-cost DVD players still feature VFDs. From the mid-1980s onwards, VFDs were used for applications requiring smaller displays with high brightness specifications, though now the adoption of high-brightness
organic light-emitting diode An organic light-emitting diode (OLED or organic LED), also known as organic electroluminescent (organic EL) diode, is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light i ...
s (OLEDs) is pushing VFDs out of these markets. Vacuum fluorescent displays were once commonly used as floor indicators for
elevator An elevator or lift is a cable-assisted, hydraulic cylinder-assisted, or roller-track assisted machine that vertically transports people or freight between floors, levels, or decks of a building, vessel, or other structure. They a ...
s by
Otis Elevator Company Otis Worldwide Corporation ( branded as the Otis Elevator Company, its former legal name) is an American company that develops, manufactures and markets elevators, escalators, moving walkways, and related equipment. Based in Farmington, Connec ...
worldwide and Montgomery Elevator Company in North America (the former from the early 1980s to the mid-2000s in the form of (usually two) 16-segment displays, and the latter from the mid 1980s to the mid 1990s in the form of (usually 3) 10x14
dot-matrix display A dot-matrix display is a low cost electronic digital display device that displays information on machines such as clocks, watches, calculators, and many other devices requiring a simple alphanumeric (and/or graphic) display device of limited res ...
s). In addition to the widely used fixed character VFD, a graphic type made of an array of individually addressable pixels is also available. These more sophisticated displays offer the flexibility of displaying arbitrary images, and may still be a useful choice for some types of consumer equipment. Multiplexing may be used in VFDs to reduce the number of connections necessary to drive the display.


Use as amplifier

Several
radio amateurs An amateur radio operator is someone who uses equipment at an amateur radio station to engage in two-way communication, two-way personal communications with other amateur operators on Frequency, radio frequencies Amateur radio frequency allocatio ...
have experimented with the possibilities of using VFDs as triode amplifiers. In 2015,
Korg , founded as Keio Electronic Laboratories, is a Japanese multinational corporation that manufactures electronic musical instruments, audio processors and guitar pedals, recording equipment, and electronic tuners. Under the Vox brand name, th ...
released th
Nutube
an analogue audio amplifier component based on VFD technology. The Nutube is used in applications such as guitar amplifiers from Vox and the Apex Sangaku headphone amplifier. The Nutube is sold by Korg but made by Noritake Itron.


Fade

Fading is sometimes a problem with VFDs. 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. Fading can be slowed by using a display driver chip to lower the voltages necessary to drive a VFD. Fading can also occur due to evaporation and contamination of the cathode. Phosphors that contain sulfur are more susceptible to fading. Emission may usually be restored by raising filament voltage. 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.


History

Of the three prevalent display technologies – VFD, LCD, and LED – the VFD was the first to be developed. It was used in early handheld calculators. LED displays displaced VFDs in this use as the very small LEDs used required less power, thereby extending battery life, though early LED displays had problems achieving uniform brightness levels across all display segments. Later, LCDs displaced LEDs, offering even lower power requirements. The first VFD was the single indication DM160 by Philips in 1959. It could easily be driven by transistors, so was aimed at computer applications as it was easier to drive than a neon and had longer life than a light bulb. The 1967 Japanese single digit seven segment display in terms of anode was more like the Philips DM70 / DM71 Magic Eye as the DM160 has a spiral wire anode. The Japanese seven segment VFD meant that no patent royalties needed to be paid on desk calculator displays as would have been the case using Nixies or Panaplex neon digits. In the UK the Philips designs were made and marketed by Mullard (almost wholly owned by Philips even before WWII). The Russian IV-15 VFD tube is very similar to the DM160. The DM160, DM70/DM71 and Russian IV-15 can (like a VFD panel) be used as
triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's ...
s. The DM160 is thus the smallest VFD and smallest triode valve. Th
IV-15
is slightly different shape (se

for comparison).


See also

*
Nixie tube A Nixie tube ( ), or cold cathode display, is an electronic device used for displaying numerals or other information using glow discharge. The glass tube contains a wire-mesh anode and multiple cathodes, shaped like numerals or other symbo ...
*
Sixteen-segment display A sixteen-segment display (SISD) is a type of display based on sixteen segments that can be turned on or off to produce a graphic pattern. It is an extension of the more common seven-segment display, adding four diagonal and two vertical segments ...
* LCD *
LED Display A LED display is a flat panel display that uses an array of light-emitting diodes as pixels for a video display. Their brightness allows them to be used outdoors where they are visible in the sun for store signs and billboards. In recent ...


References


External links


Noritake's Guide to VFD Operation


* ttp://www.tubeclockdb.com/vfd-tubes.html Photos and specs for antique Russian VFD tubes
Simple VFD Test Circuit




* ttp://www.radiomuseum.org/tubes/tube_iv-15.html The Russian VFD indicator like a DM160 {{Authority control Vacuum tube displays