11/5/2014
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`Survey of Electronic Displays
`
`INTERNATIONAL...
`
`I Learn I Publications I Technical Papers
`
`Survey of Electronic Displays
`
`Paper #: 750364
`
`DOI:
`
`10.4271/750364
`
`Published: 1975-02-01
`
`Citation:
`
`Nolan, J., “Survey of Electronic Displays," SAE Technical Paper 750364,
`1975, doi:10.4271/750364.
`
`Author(s): J. F. Nolan
`Affiliated: Owens Illinois Technical Center
`
`Abstract: This paper presents a survey of the various types of electronic displays
`that are available now or are being investigated in research laboratories.
`The survey is limited to small or medium sized displays which might be
`suitable for automotive dashboard displays. Included in the survey are
`light emitting diodes, planar gas discharge,
`incandescent, vacuum
`fluorescent, electroluminescent, and liquid crystal displays, as well as
`some other types of displays which, although not commercially available
`at present
`in quantity, are being actively investigated in
`research
`laboratories. The discussion includes topics such as the distinction
`between active and passive displays and the advantages and limitations
`of each type, the principles of operation of the various types of displays,
`and their electrical and optical characteristics.
`Automotive
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`Topic:
`Displays
`Instrument panels
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`750364
`
`ABSTRACT
`This paper presents a survey of the various types of elec­
`tronic displays that are available now or are being investi­
`gated in research laboratories. The survey is limited to
`small or medium sized displays which might be suitable for
`automotive dashboard displays. Included in the survey are
`light emitting diodes, planar gas discharge, incandescent,
`vacuum fluorescent, electroluminescent, and liquid crystal
`displays, as well as some other types of displays which, al­
`though not commercially available at present in quantity,
`are being actively investigated in research laboratories. The
`discussion includes topics such as the distinction between
`active and passive displays and the advantages and limita­
`tions of each type, the principles of operation of the various
`types of displays, and their electrical and optical character-
`istics.
`
`Survey of Electronic Displays
`
`J. F. Nolan
`Owens Illinois Technical Center
`
`T HE FIELD OF E L E C T R O N IC DISPLAYS has been
`expanding rapidly in recent years both in the number of
`electronic displays being manufactured and in the variety
`of types of display devices available. The annual market
`for electronic numeric readout devices has increased from
`less than $10 million ten years ago to an estimated $160
`million in 1974 (1)*. In addition to NIXIE®** tubes,
`which have been available for almost 20 years, it is now
`possible to buy numeric readouts which use light emitting
`diode, planar gas discharge, vacuum fluorescent, incan­
`descent, electroluminescent, and liquid crystal displays.
`Also, some kinds of displays (a-c and d-c plasma panels)
`are now available as medium and large sized matrix dis­
`plays. In addition, a number of other display technol­
`ogies are being investigated in the laboratory with hope
`of commercial introduction in the near future. This latter
`group includes electrochromic, electrophoretic, and fluid
`dipole displays.
`There is an increasing interest in displays on the part of
`suppliers who are exploring new display technologies and
`improved fabrication techniques for existing displays as
`they are produced in larger volumes. There is also an in­
`creasing interest in the newer display technologies on the
`part of display users, since as increased volumes allow
`price reductions, some of the newer electronic display de­
`vices are becoming less expensive than the tranditional
`electromechanical displays. Five years ago the numeric
`readouts for desk calculators were either NIXIE tubes or
`electromechanical displays. Today all newly introduced
`calculators use recently developed multidigit electronic
`displays such as light emitting diodes, planar gas dis­
`charge, vacuum fluorescent, or liquid crystals. The recent
`development of pocket calculators was made possible by
`the newer electronic displays technologies as well as by
`
`♦Numbers in parentheses designate References at end of paper.
`***Registered trademark of Burroughs Corporation.
`
`the advances in integrated circuitry. It would not have
`been possible to miniaturize calculators to the degree that
`has occurred if designers were restricted to NIXIE tubes
`or electromechanical displays to provide the readout. O ne
`would expect a similarly interactive situation to occur in
`automotive dashboard displays; that is, it is likely that
`some of the presently used electromechanical displays will
`be replaced by electronic displays as the costs become
`comparable. Later it is likely that new functions will be
`performed by the new displays that are not being per­
`formed at present and new possibilities will arise in dash­
`board design as the characteristics of the electronic dis­
`play technologies become exploited. Also, it appears
`likely that the potential automotive market will have an
`influence on the development of electronic display tech­
`nology since the automotive environment is somewhat
`more demanding in terms of temperature, humidity,
`shock, vibration, and cost than most of the environments
`the electronic displays have been subjected to in the past.
`Since no single type of electronic display device has
`proven to be clearly superior for every application, it
`seems clear that for the near future at least the automo­
`tive design engineer will have to stay abreast of the char­
`acteristics of a number of different electronic display
`technologies.
`This paper presents an introductory survey of the field
`of electronic displays. It is addressed to those w ho have
`no familiarity with the field and is intended as a broad-
`brush overview of displays. A description is given of the
`different types of display devices that are available now or
`are being investigated in research laboratories. The dis­
`cussion is limited to small or medium sized displays which
`might be considered for automotive dashboard applica­
`tions. The discussion includes the principles of operation
`of the various devices and a description of their main
`characteristics. Although this survey does not go into
`great detail concerning the displays, references are given
`which will provide the interested reader with more de-
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`tailed information. The appendix gives a list of suppliers
`of the various kinds of display devices. This paper does
`not include a discussion of the electronic drive circuitry
`for operating displays; this topic is covered elsewhere
`(2,3).
`
`CLASSIFICATION OF DISPLAYS
`One way of classifying displays is according to whether
`they are active or passive. The distinction between active
`and passive displays is that active displays emit light
`themselves whereas passive displays simply modulate
`light from some other source; that is, they act as switch-
`able light valves. Table 1 gives a comparison of some of
`the main characteristics of active and passive displays.
`Examples of active displays are light emitting diodes and
`gas discharge displays. Examples of passive displays are
`liquid crystal and electrochromic displays. The brightness
`of an active display depends on the characteristics of the
`display itself and not on the brightness of the ambient
`light level. A passive display may include an auxiliary
`light source such as an incandescent bulb either behind
`the display if the display is transmissive or in an edge-
`lighting configuration if the display is reflective. For this
`kind of display the brightness of the display will depend
`on the brightness of the auxiliary light source. Alter(cid:173)
`natively, a passive display may impart information to the
`viewer by modulating ambient light, either in reflection
`or transmission. In this case, the brightness of the display
`depends on the brightness of the ambient light level, in(cid:173)
`creasing as the ambient increases.
`The legibility of a display is affected by the contrast.
`The contrast ratio is defined as the brightness of the dis(cid:173)
`play element divided by the brightness of the adjoining
`background. A contrast ratio of about 10:1 is normally
`judged to be adequate, but a larger contrast ratio gives a
`better appearing display. For an active display, the con(cid:173)
`trast ratio decreases as the ambient light level increases
`because of increased reflected light from the background.
`For example, the spot brightness of a light emitting diode
`segmented numeric display might be 100 foot-lamberts
`under normal operating conditions. This is quite ade(cid:173)
`quate for viewing under normal indoor lighting condi(cid:173)
`tions but in direct sunlight the ambient light level m ay be
`so high that the display is illegible. It is of course possible
`to use filters or anti-reflective coatings to diminish the ef(cid:173)
`fect of reflected ambient light and therefore improve the
`contrast ratio.
`For a passive display that uses ambient light the con(cid:173)
`trast ratio is approximately independent of the ambient
`light level. This can be an advantage for viewing in ex(cid:173)
`ceptionally bright ambients such as direct sunlight and in
`environments where the ambient light level goes through
`large changes (for example, direct sunlight to shadow)
`since the brighter the ambient is, the brighter will be the
`display. However, a passive display which uses ambient
`light can not be seen at all when the ambient is totally
`
`dark. It is of course possible to supply a passive display
`with an auxiliary light source which is turned on only
`when the ambient level is low.
`Another way of classifying displays is by physical char(cid:173)
`acteristics such as display size and format. The simplest
`kind of display is the single dot indicator which can tell
`the viewer which of two possible states is appropriate for
`the selected variable (for example, headlights in high or
`low beams). More versatile than this is the seven-segment
`numeric readout illustrated in Fig. 1. This format can be
`used to display information which consists solely of num(cid:173)
`bers such as would be used in a digital speedometer, digi(cid:173)
`tal gas gage, or a dashboard clock. Somewhat more ver(cid:173)
`satile than this is the 16-segment alphanumeric readout
`which is used in some displays. Still more versatile is the
`dot matrix format which can be used to display numbers,
`letters, and simple pictures.
`Another way of classifying displays is by the display
`technology that is used to impart the information to the
`viewer; that is, light emitting diodes, liquid crystals, etc.
`Table 2 lists a number of display technologies of interest
`at present for small and medium sized displays. These
`display types are grouped according to whether they are
`active or passive and according to whether the displays
`are commercially available in volume at present. The re-
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`938
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`J. F. N O L AN
`
`mainder of this paper consists of a discussion of the main
`characteristics of the nine types of displays listed in
`Table 2.
`LIGHT EMITTING DIODES
`A light emitting diode (LED) is a solid state device
`which emits light when electrons and holes recombine
`radiatively in the region around a p-n semiconductor
`junction. Light emitting diodes have been studied in
`many laboratories for more than ten years and compre(cid:173)
`hensive reviews are available (4-6).
`Usually LEDs are made from III-V compounds al(cid:173)
`though other materials may be used (7). The materials
`normally used are gallium, arsenic, and phosphorous in
`single crystal compounds grown on substrates using either
`vapor phase or liquid phase epitaxy. The operation of a
`light emitting diode display may be understood by refer(cid:173)
`ring to Fig. 2, which is a schematic representation of a
`typical gallium arsenide-gallium phosphide L E D. Using
`single crystal gallium arsenide of n-type conductivity as a
`substrate, two layers containing gallium phosphide are
`deposited as shown in Fig. 2. In the first layer the phos(cid:173)
`phorous concentration is tapered from 0% at the sub(cid:173)
`strate to 4 0% at the top. Then a second layer is deposited
`with a constant composition of 4 0% phosphorous. These
`layers are of the order of 20 n thick and typically are
`deposited by vapor phase epitaxy. It is possible to make a
`light emitting diode from gallium arsenide only, without
`gallium phosphide, but this would emit in the infrared.
`The gallium arsenide-gallium phosphide L ED shown in
`Fig. 2 is the familiar red-emitting L E D. A 2 jt deep zinc
`diffusion gives the p-type conductivity required to form
`the p-n junction. A voltage applied between the two
`metal electrodes causes current to flow. As electrons
`enter the p-type region they can recombine radiatively
`with holes in the p region. This radiative recombination
`of electrons and holes in the vicinity of the junction pro(cid:173)
`vides the photons which constitute the light emitted by a
`light emitting diode. Light is emitted in all directions
`from the junction, and some light escapes from the top,
`and this is the light seen by the viewer.
`Fig. 3 illustrates how the wavelength, or color, of the
`light emitted may be controlled to some degree by varying
`
`the composition of the diode material. Here the band
`structure is plotted in momentum space of GaAs and G aP
`and intermediate compounds. Consider the lowest curve
`which represents pure gallium arsenide. Electrons in(cid:173)
`jected into the conduction band of p-type GaAs can re(cid:173)
`combine radiatively with free holes in the valence band
`and generate a quantum of light with an energy approxi(cid:173)
`mately equal to the width of the energy gap between the
`valence band and conduction band. For GaAs the band
`gap energy is 1.43 eV. This corresponds to a photon
`wavelength of 910 nm, and this is the light emitted in the
`infrared GaAs light emitting diode.
`Combining GaAs and G aP to a ternary compound,
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`SURVEY OF ELECTRONIC DISPLAYS
`GaAsP, results in a conduction band structure between
`the two extreme cases of GaAs and GaP. With about 4 0%
`G aP in GaAs the band gap energy is increased to 1.92 eV
`and the wavelength is decreased to 650 nm. This is the
`familiar red-emitting L ED which has seen wide applica(cid:173)
`tion in the last several years. As the percentage of G aP is
`increased still further, one encounters the difficulty that
`the indirect minimum is lower than the direct minimum,
`and so, electrons will tend to accumulate in the indirect
`minimum. Since the indirect minimum does not occur di(cid:173)
`rectly above the valence band maximum, radiative re(cid:173)
`combination is very improbable because of the momen(cid:173)
`tum difference between the electrons and holes. If it were
`not for this difficulty it would be possible to fabricate
`yellow and green LEDs by simply increasing the percent(cid:173)
`age of GaP. It is still possible to fabricate yellow and
`green L E Ds using GaAsP alloys by introducing iso-elec-
`tronic traps via nitrogen dopings (8,9). Devices have
`been made in the laboratory in which the color of the
`emitted light may be varied by varying the current through
`the device (10).
`Light emitting diodes are available in a variety of for(cid:173)
`mats and physical sizes. A single L ED chip is typically
`about 0.020 in square. For the single L ED lamp a single
`chip is encased in a plastic cap which also acts as a lens.
`For alphanumeric displays, either segmented or dot ma(cid:173)
`trix, one needs a number of discrete LEDs. These dis(cid:173)
`plays may be made in hybrid form (11) from discrete di(cid:173)
`odes or in monolithic form with all diodes produced in a
`single chip (12, 13). The hybrid approach minimizes the
`use of the expensive single crystal L ED material and is
`the approach preferred at present for larger sized dis(cid:173)
`plays.
`One of the desirable characteristics of light emitting
`diodes is that they have a low operating voltage. Typi(cid:173)
`cally the diode's forward voltage drop is less than 2 V.
`They can easily be driven from the low voltage supplies
`normally found in digital equipment without any voltage
`step-up, and they are easy to multiplex. Light emitting
`diodes have a spot brightness which is typically in the
`range of 30-300 foot-lamberts. Response time is around
`10 ns. They have a very long operating life (> 10s h) and
`are rugged. They have been available in red for some time
`and have recently become available in yellow and green.
`One other consideration with light emitting diodes is that
`the display material is relatively expensive single crystal
`semiconducting material, and this becomes a considera(cid:173)
`tion as one considers displays of larger sizes. This does
`not necessarily mean that LEDs will never be able to com(cid:173)
`pete effectively in larger sized displays. Historically, semi(cid:173)
`conductor devices have become less expensive as volumes
`increase, and L ED numeric segment displays are becom(cid:173)
`ing available in larger sizes up to inch-high characters.
`However, the materials cost does mean that at present
`L E Ds compete most effectively in the smaller sized dis(cid:173)
`plays, and this is where they are finding large applications
`at the present time.
`
`PLANAR GAS DISCHARGE
`Gas discharge displays use the light emitted by excited
`atoms in a gas discharge to display information. Both d-c
`and a-c gas discharge displays are available. The distinc(cid:173)
`tion between d-c and a-c lies more in the construction of
`the devices than in the electrical signals used to drive
`them; d-c devices are typically driven by pulsed electrical
`signals rather than by a strictly d-c voltage; a-c gas dis(cid:173)
`charge devices have an insulating layer of dielectric ma(cid:173)
`terial over the electrodes which separates the gas dis(cid:173)
`charge from the conducting electrodes. This insulating
`layer acts as a capacitor in series with the gas discharge
`and imposes the requirement that the device must be op(cid:173)
`erated in an a-c mode. In a-c gas discharge segmented
`numeric displays it is possible to coat the entire back elec(cid:173)
`trode pattern with an absorbing black dielectric which
`improves the contrast of the display. Some a-c gas dis(cid:173)
`charge devices use the charge collected on the insulating
`dielectric to give the device memory (14).
`Recent multidigit numeric readouts are referred to as
`planar gas discharge displays to distinguish them from
`the older NIXIE tubes which use a number of shaped
`metal cathodes stacked one behind the other, with one
`cathode shaped to correspond to each of the ten numbers
`to be displayed. The planar gas discharge displays obtain
`different numbers by energizing different segments of a
`seven-segment display. Typical construction for a d-c gas
`discharge segmented numeric display is shown in Fig. 4.
`A metallic conductor is printed on a glass or ceramic sub(cid:173)
`strate in the seven-segment pattern using thick film tech(cid:173)
`niques. Corresponding segments from the different digits
`are normally connected together although this is not nec(cid:173)
`essary. If this is done it means that only seven leads (nine
`counting decimal points and commas) must be brought
`out from the segmented cathodes for connection to the
`external circuitry, and it also means that the display must
`be operated in the multiplex mode. The front plate has a
`number of transparent conductors, one per digit, which
`act as anodes. The transparent conductor pattern is typi(cid:173)
`cally made of etched tin oxide on a glass substrate. The
`two substrates are aligned with separation maintained by
`a spacer and sealed around the edges. The assembly is
`subsequently evacuated, filled with a neon-based gas mix(cid:173)
`ture, and hermetically sealed. The completed device also
`contains a small amount of mercury which greatly in(cid:173)
`hibits the harmful effects of cathode sputtering.
`The gas volume is enclosed between the glass substrates
`—typical spacing is around 0.020 in. When a potential
`difference is applied between electrodes that exceeds the
`breakdown voltage of the gas (typically somewhat greater
`than 150 V), electron avalanches occur in the gas volume
`leading to ionization and excitation of the gas atoms and
`subsequent emission of radiation as the excited states de(cid:173)
`cay to the ground state by emitting photons. The design
`shown in Fig. 4 has an opaque cathode and a transparent
`anode. Other designs are also possible; descriptions may
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`940
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`be found in the literature of both d-c (15) and a-c (16)
`numeric segment displays.
`The typical neon red-orange glow consists of a large
`number of spectral lines between 540-700 nm. One of the
`strongest is in the yellow at 585 nm. The lower level for
`these transitions is not the ground state of the neon atom
`but is one of four levels more than 16 electron V above
`the ground state. This energy is subsequently dissipated
`by radiation in the far ultraviolet or by destruction of
`metastable atoms. This means that a large amount of
`energy is being wasted from the point of view of produc(cid:173)
`tion of visible radiation. This is in spite of the fact that
`the neon gas discharge display is one of the more efficient
`of the active displays. There has been a fair amount of
`effort recently to investigate the possibility of converting
`some of this energy to the visible using phosphors with
`neon or other gases, thus obtaining higher efficiency and
`colors other than the familiar neon orange. A luminous
`efficiency of 1.8 lumens/W has been reported recently for
`a laboratory device using a green phosphor with a xenon-
`argon gas mixture (17). This is almost an order of magni(cid:173)
`tude larger than the efficiency of that standard neon de(cid:173)
`vice. Various colors (green, yellow, blue) have been
`obtained in the laboratory, and some of these should be
`available commercially in the future.
`Gas discharge displays are also available as matrix dis(cid:173)
`plays. Gas discharge dot matrix displays are often re(cid:173)
`ferred to as plasma panels and are available in a-c (18)
`and d-c (19,20) versions. Gas discharge displays have a
`
`J. F. N O L AN
`very sharp threshold characteristic, and this makes them
`especially suitable for matrix type displays. In a matrix
`display an element at the intersection of horizontal line
`X and verticle line Y is normally addressed by applying
`partial signals to line X and line Y. For example, if a
`voltage V is required to give the proper light output at the
`addressed intersection, this can be obtained by applying
`V/2 to line X and V/2 to line Y with polarities such that
`the voltages add at the intersection. This means that
`every element on line X and line Y other than the ad(cid:173)
`dressed element is subjected to a partial voltage. If this
`partial voltage were to result in a partial light output, an
`objectionable background light would be produced at un-
`addressed points. To avoid this it is desirable to have full
`light emitted from an element with full voltage applied
`and zero light emitted from an element with a partial
`voltage applied. Gas discharge matrix displays fulfill this
`requirement quite well because of the sharp threshold in
`the light output versus voltage curve.
`A-c plasma panels have been built with a display area
`of 17 x 17 in (1024 x 1024 elements) and are commer(cid:173)
`cially available in sizes up to 8 1/2 x 8 1/2 in (512 x 512
`elements) (21). This is the largest flat panel matrix dis(cid:173)
`play of any kind that is currently available. Gas dis(cid:173)
`charge dot matrix displays, both d-c and a-c, are available
`in a number of smaller sizes down to one-line displays of
`16 alphanumeric characters.
`Gas discharge displays operate at a relatively high volt(cid:173)
`age, typically around 170 V. This is somewhat of a dis(cid:173)
`advantage but not a severe one since, because of the sharp
`threshold, the difference between the on and off voltages
`is much less than 170 V, and electronic drive schemes
`have been devised which are cost competitive with the
`driving circuitry for LEDs. Gas discharge displays are
`bright (50-500 foot-lamberts), have a fast response time
`(1-10 jus), and a long operating life (> 104 h). At present
`the color is restricted to neon orange, but other colors
`should be available in the near future. The sharp thresh(cid:173)
`old makes them relatively easy to multiplex. There is no
`expensive materials cost consideration here, and a natural
`application would be the larger sized digital readout dis(cid:173)
`plays and dot matrix displays.
`VACUUM FLUORESCENT
`Vacuum fluorescent displays produce light by cathodo-
`luminescence in a manner similar to that used in a cath(cid:173)
`ode ray tube but with greatly reduced size and power and
`using electrons with much lower energy (22). A filament
`is operated at low power (below incandescence) to supply
`electrons which are then accelerated through a moderate
`voltage (about 25 V) to impinge on a phosphor and pro(cid:173)
`duce visible light, typically of a blue-green color. Vac(cid:173)
`uum fluorescent numeric readouts have been available for
`some time as individual tubes (one digit per tube) and
`have recently become available in multidigit form. Fig. 5
`shows a schematic diagram of the interior of a four-digit
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`S U R V EY OF ELECTRONIC DISPLAYS
`
`941
`
`vacuum fluorescent display. The seven-segment conduct(cid:173)
`ing anodes are printed on a ceramic substrate along with
`the segment interconnections. The substrate is then over-
`coated with a black insulating material in all areas ex(cid:173)
`cept over the display segments. The phosphor is depos(cid:173)
`ited on these segments. Several wires acting as filaments
`are stretched the length of the display, and each character
`has a separate grid between the filaments and the seg(cid:173)
`mented anodes. Electrons are emitted from the filaments
`and accelerated through a relatively low voltage (25-50 V)
`to strike the phosphor on the selected anode segments
`which then emit light. The filaments are between the light
`emitting anode segments and the viewer, but this does not
`detract from the appearance of the display since the fila(cid:173)
`ments are quite thin and are operated at a low enough
`temperature such that they do not glow visibly. The en(cid:173)
`tire structure shown in Fig. 5 is enclosed in an evacuated
`glass envelope. If the grids are biased slightly negative
`(around - 5 V) the current to the anode is cut off even if
`a positive voltage is applied to the anode segments. In
`this way the multidigit display may be operated in a mul(cid:173)
`tiplex mode. Designs other than that shown in Fig. 5
`have been used. Some designs use one filament wire per
`digit running vertically in front of each digit. Although
`the grid facilitates multiplexing, it is not essential for the
`operation of the device and some designs omit it. The es(cid:173)
`sential features are the electron source and the selectable
`anode segments with a phosphor coating, all in an evacu(cid:173)
`ated enclosure.
`Vacuum fluorescent displays have found application in
`calculator readouts and in digital clocks. The phosphor
`used in these devices (ZnO) is rather unusual in that it is
`a fairly good conductor and is capable of being excited by
`low energy electrons. One can not obtain other colors
`simply by replacing the phosphor with a different phos(cid:173)
`
`phor used in cathode ray tubes since this would require
`going to much higher voltages. However, the Z nO phos(cid:173)
`phor does emit over a rather broad band of wavelengths,
`and some other colors may be obtained by putting ap(cid:173)
`propriate filters in front of the display device.
`Vacuum fluorescent displays have a brightness of
`around 100 foot-lamberts, a response time in the micro(cid:173)
`second range, and an operating life of about 20,000 h.
`They are relatively easy to multiplex. They require a sep(cid:173)
`arate power supply for the filament, and for some designs
`damage to the filament due to vibrations is a concern.
`INCANDESCENT
`In an incandescent display a current is passed through
`a wire filament sufficient to heat the filament to incandes(cid:173)
`cence, and the incandescent filament provides the light
`for the display. Segmented numeric displays are avail(cid:173)
`able in which the segments consist of glowing filaments
`which are viewed directly. These displays are available in
`a variety of sizes. Incandescent displays are the brightest
`of the active displays; a brightness greater than 5000 foot-
`lamberts is not unusual. Since they emit light over a
`broad continuous spectrum, many different colors are
`available with the use of appropriate filters. They oper(cid:173)
`ate at a low voltage (5 V) and can be multiplexed al(cid:173)
`though this requires additional components in the driving
`circuitry. They have a relatively slow response time—
`in the millisecond range. Incandescent displays have been
`criticized for not being sufficiently resistant to vibration
`and shock, although the effects of shock and vibration
`can be minimized by operating at a reduced filament tem(cid:173)
`perature and by maintaining a reduced current through
`the filament when it is in the off state. There is a trade(cid:173)
`off between operating life and brightness, but long life
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`(> 105 h) can be obtained at a brightness (> 1000 foot-
`lamberts) which is large compared to other displays. Al(cid:173)
`though incandescent displays have been around for a long
`time, they are still the subject of some research activity.
`Work has recently been reported on an investigation of
`an incandescent matrix display fabricated using thin film
`techniques (23, 24).
`ELECTROLUMINESCENT
`Electroluminescence is the generation of light on the
`application of an electric field to certain solid materials.
`Materials which exhibit this property are called electro(cid:173)
`luminescent phosphors; perhaps the best known example
`is zinc sulfide activated with manganese. Electrolumines(cid:173)
`cent displays use the phenomenon of field effect electro(cid:173)
`luminescence, as distinguished from injection electrolumi(cid:173)
`nescence which is used in light emitting diodes. The use
`of electroluminescence to display information is a subject
`that has received attention in research laboratories for
`many years. A substantial research effort was undertaken
`at many laboratories in the 1950s in the hope of develop(cid:173)
`ing practical electronic displays for many applications in(cid:173)
`cluding television. The early promise was not realized,
`and the amount of effort dwindled. One of the main
`problems with these early devices was the great difficulty
`that was encountered in obtaining adequate brightness
`and operating life at the same time. Some recent work
`has been reported (25, 26) which shows significant prog(cid:173)
`ress in brightness and life, and interest in electrolumines(cid:173)
`cent displays is reviving.
`A number of different materials have been found which
`exhibit electroluminescence; typically they are II-VI com(cid:173)
`pounds. Most electroluminescent materials require acti(cid:173)
`vators; that is, impurities in the host material which give
`rise to luminescent centers that are responsible for the
`characteristics of the emitted radiation. The details of the
`physical processes which lead to electroluminescence are
`quite complex; a number of explanations have been pro(cid:173)
`posed to account for observed effects in different mate(cid:173)
`rials. In one model which has been applied to zinc sulfide
`systems, electrons and hole