GB2199645A

GB2199645A - Hybrid incandescent/fluorescent light source for liquid crystal displays

Info

Publication number: GB2199645A
Application number: GB08728737A
Authority: GB
Inventors: Elias Savas Haim; Francis L Leard; Richard Alan Anderson
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1986-12-15
Filing date: 1987-12-09
Publication date: 1988-07-13
Anticipated expiration: 2007-12-09
Also published as: GB2199645B; FR2608301A1; DE3742245A1; JPH01170916A; SE8704830L; SE465848B; FR2608301B1; GB8728737D0; SE8704830D0

Description

.DTD:

HYBRID INCANDESCENT/FLUORESCENT LIGHT SOURCE FOR LIQUID CRYSTAL DISPLAYS 2199645 This invention relates to a light source for a liquid crystal display and, more particularly, to a hybrid fluorescent/incandescent light source.

.DTD:

Flat panel liquid crystal displays typically include a liquid crystal panel with a light source for illuminating the liquid crystal display positioned behind the panel. In many circumstances, the liquid crystal display must be viewed under very high ambient light conditions, for example, liquid crystal display used in the cockpit of an aircraft, must be viewable at ambient light levels which may be as high as i0,000 foot lamberts. Under these ambient conditions, the illumination level for the display has to be 2,000 foot lamberts or greater. The light source for the display must thus be capable of providing this level of illumination and must also be capable of doing so at extremely low temperatures, i.e. temperatures as low as -50 to -55 C.

.DTD:

It is operation at these extremely low temperatures that introduces problems with spectral light sources which otherwise are a prime candidates for the application. That is, fluorescent lamps which are capable of providing high intensity light outputs (in the excess of 2,000 foot lamberts at 40-50 C) are very poor performers at low temperatures. In fact, around -50 C, fluorescent light sources are essentially inoperative. Incandescent light sources on the other hand, are temperature insensitive, i.e., the light output is essentially constant over a range of temperatures from -60 C to +70-75=C. Although incandescent light sources are effective at low temperatures, their output, however, is quite low (around 600 to 700 foot lamberts) and their efficiency, in foot lamberts/watt, is quite low compared to fluorescent lights.

.DTD:

Applicant has found that high light output and high efficiency operation, may be achieved at low temperatures by means of a hybrid light assembly in which a plurality of incandescent and fluorescent lamps are positioned adjacent to each other in a parallel array. At low temperatures the incandescent lamp elements provide light for the display as well as acting as a radiant heat source for the adjacent fluorescent lamps. The heating effect of the incandescent lamps raises the temperature in the immediate vicinity of the fluorescent lamps sufficiently to allow the fluorescent lamps to operate and produce efficient light output even though the ambient temperature in which the light source and the fluorescent flat panel is operating is at or below the temperatures at which the fluorescent lamps are either inoperative or have very low outputs.

.DTD:

Accordingly there is disclosed herein an arrangement in which the light source for a liquid crystal display panel consists of a plurality of elongated fluorescent and incandescent lights mounted on a support panel in a parallel array, with fluorescent and incandescent lamps alternately mounted adjacent to each other. Radiant energy emitted by each incandescent lamp when operated at low temperatures, act as a heat source for the fluorescent lamps on either side of the incandescent lamp. As a result, the temperature level in the immediate vicinity of each fluorescent lamp is raised substantially above the ambient temperature, thereby allowing the fluorescent lamps to operate at ambient temperatures at which they are normally inoperative or at which they emit very little light.

.DTD:

The novel features which are characteristic of this an' ventzon" are set forth with particularity in the appended claims. The invention itself, however, together with the objectives and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

.DTD:

Figure I is a partially broken away perspective of a display panel arrangement including the hybrid fluorescent/incandescent light source; Figure 2 is a graphic illustration of the light emission characteristics of fluorescent and incandescent as a function of temperature; Figure 3 is a graphic illustration of the light emission characteristics of the hybrid fluorescent/incandescent light source.

.DTD:

Liquid crystal displays, as well known, typically consist of two istransparent glass substrates separated by a sealant to define a cavity in which a liquid crystal solution is retained. The interior surfaces of the transparent glass plates have the display elements which may be in the form of signs, symbols or depictions.

.DTD:

Thus, a metallic backplane is deposited on the interior surface of one substrata and individual transparent conductive electrodes. (arranged in any desirable form as for e xampie in the form of rows and columns) are deposited on the interior surface of the other substrata. The transparent electrodes are formed of a transparent metal, such as indium tin oxide (ITO), or the lke.

.DTD:

Figure 1 shows a liquid crystal display element 1 mounted in a housing 2. Housing 2 has an opening 3 in which a bezel 4 is supported to allow viewing of the display. Positioned at the lower end of the housing is a driver electronic panel 6 which supports the row and colum driver electronics for selectively energizing the individual pixels of a matrix liquid crystal display panel. Positioned immediately behind the liquid display panel is a diffuser element 7 which receives light from the hybrid light source 9 positioned innediately to its rear. Positioned behind the light source 9 is a printed circuit board 10 mounted on a bracket ii. Printed circuit board I0 supports the power supply and other energizing circuitry.

.DTD:

The hybrid fluorescent/incandescent light source 9 consists of a plurality of fluorescent lamps 12 mounted horizontally on board I0. Incandescent lamps 13 are positioned between the fluorescent lamps so that source 9 consists of an alternate array of fluorescent and incandescent lamps. The fluorescent and incandescent lamps are positioned so as to be in a heat exchange relationship with each other.

.DTD:

That is, radiant energy emitted by the incandescent lamps raises the temperature level in the immediate vicinity of the fluorescent lamps so that the fluorescent lamp temperature is substantially higher than the ambient temperature within the housing thereby allowing the fluorescent lamps to operate more efficiently than the ambient temperature would normally permit.

.DTD:

At extremely low temperatures, i.e. -50 C or less, the incandescent lamps are the principal light source for the liquid crystal panel displays. Simultaneously, the radiant energy emitted by the incandescents heats the fluorescents and increases their Output. In a relatively short time interval, I0 minutes or less, the hybrid system operating at an ambient temperature level of -50 C increases the temperature of the fluorescents lamp surfaces approximately 40 C above ambient, thereby allowing its light output to rise substantially even though the ambient temperature remains extremely low.

.DTD:

Figure 2 illustrates graphically the photopic output of fluorescent and incandescent lamps as a function of temperature. Thus, in Figure 2, the photopic output in foot lamberts is plotted on a logarithmic scale along the ordinate and temperature in =C is plotted linearally along the abcissa.

.DTD:

Curve 20 represents the photopic output as a function of temperature of a fluorescent lamp and curve 21 is the photopic output of an incandescent lamp as a function of temperature. The data plotted as curve 20 in Figure 2 represent the photopic outputs of a fluorescent lamp operated at 38 volts, 0.70 amps, i.e. a power input of 21.8 watts. Curve 21 represents the light output of an incandescent lamp operated at 12.1 volts and 1.55; i.e., a power input of 18.8 watts. It may be noted from curve 20 that below -20 C the output of the fluorescent lamp is extremely low around 40 foot lamberts,and at -50=C around I0 foot lamberts, (less than 0.5 ft lamberts/watt). Under such ambient conditions, which are not uncommon in aircraft applications where an aircraft may be parked overnight in extreme cold, a light source consisting of fluorescent lamps will produce very little output.

.DTD:

The output of the fluorescent lamp does not reach 3,000 to 4,000 foot lamberts until the ambient temperature is approximately 20 to 60 C. Thus, at its optimu. operating condition at 40 to 60 degrees C the output of a fluorescent lamp is around 4,000 foot lamberts or approximately 200 foot lamberts/watt.

.DTD:

The light output of an incandescent lamp as illustrated by curve 21, is essentially constant with temperature. However, the efficiency of an incandescent lamp is relatively low compared to the optimum efficiency of a fluorescent lamp. That is, light output of an incandescent lamp with a 19 watt input is approximately 600 foot lamberts thereby or approximately 32 lamberts watts; an efficiency which is almost an order of magnitude smaller than that of the optimal efficiency fluorescent lamp.

.DTD:

Figure 3 illustrates the photopic response of a hybrid incandescent/fluorescent light source embodyin9 the invention as function of time and shows the effect of the radiant energy from the incandescent lamp on a fluorescent lamp energized initially at an ambient temperature of -55 C. As will be pointed out in detail below, within I0 minutes, the actual lamp temperature rises from -55 C to approximately +200C thus raising the output of the fluorescent lamp from virtually zero to approximately 1,600 foot lamberts. In Figure 3, the photopic output in foot lamberts is plotted along the ordinate and the time in minutes is plotted along the abcissa. Curve 23 shows the total light output of the hybrid lighting system (fluorescent plus incandescent). Power inputs to the incandescent and fluorescent lamps, the same as those to the units described in connection with Figure 2; that is a 21.8 watt power input to the fluorescent and an 18.8 watt input to the incandescent.

.DTD:

The unit was initially soaked in a Tenny oven at -55 degrees C for one hcur before the test was begun. When the lamps were initially energized time equals 0 the ambient temperature measured in the housing was -53 C, and light output was approximately 690 fool lamberts. The light output initially is thus essentially the output of the incandescent lamp since, (as shown in Figure 2), the light output of the incandescent lamp is approximately 680 foot lamberts over the entire temperature range. Once energized, the light output of the hybrid system increases rapidly with time. Ten minutes later the ambient temperature has risen to -19 C. The light output, however, is in excess of 2,300 foot lamberts. Since approximately 680 lamberts represent constant light output from the incandescents the remaining 1,600 foot lamberts represents the output of the fluorescent lamps.

.DTD:

It will be noted from Curve 20 of Figure 2 that the output of the fluorescent lamp rises to approximately 1,600 foot lamberts at +20 C. It is thus apparent that the incandescent lamps raises the lamp surface temperature approximately 40 C higher than ambient and allows the fluorescent lamp to operate at much higher efficiencies and light outputs than would be possible at the given ambient temperatures.

.DTD:

It will thus be apparent that a hybrid fluorescent/incandescent light source operates very effectively at extremely low temperatures; i.e. temperatures at which the highly efficient fluorescent lights are normally either inoperative or operate with very low light outputs. It is therefore clearly apparent that such a light source is capable of operating at extremely low temperatures with high light outputs, and with the high efficiency of fluorescent light sources which are normally incapable of operating efficiently and effectively at these temperatures.

.DTD:

Thus there has been provided a hybrid incandescent/ fluorescent lioht source, operative at low temperatures, which takes advantage of the best operating characteristics of both the incandescent and fluorescent elements While the instant invention has been described in connection with a preferred embodiment thereof, the invention is by no means limited thereto since other modifications of the instrumentalities employed may be made and still fall within the scope of the invention and it is intention of the appended claims to cover all such modifications that fall within the true spirit and scope of the invention.

.DTD:

.CLME:

Claims

CLAIMS:

.CLME:

i. A liquid crystal display system compris&ng a liquid crystal display together with a light source therefor including at least one fluorescent and one incandescent light element, with said incandescent light element being positioned to act as heat source for said fluorescent element to raise the temperature of said fluorescent element above the ambient temperature whereby the light output from said fluorescent element is greater than it would be at ambient temperature.

.CLME:
2. A liquid crystal display system according to claim 1 wherein said light source includes a plurality of fluorescent and incandescent light elements.

.CLME:
3. A liquid crystal display system according to claim 2 wherein said fluorescent and incandescent light elements are mounted in a parallel array.

.CLME:
4. A liquid crystal display system according to claim 3 wherein said parallel array comprises alternate fluorescent and incandescent light elements.

.CLME:
5. A hybrid fluorescent and incandescent light source comprising:

.CLME:

a) a support means, b) at least one fluorescent and one incandescent light element mounted on said support, c) said incandescent light element being positioned to act as a heat source for said fluorescent elementto raise the temperature of fluorescent element above ambient.

.CLME:

I0
6. A hybrid light source according to claim 5 wherein said light elements are mounted in heat exchange relationship and said fluorescent element is exposed to radiant heat from said incandescent element.

.CLME:
7. A hybrid light source according to claim 5 or 6 wherein said light source includes a plurality of fluorescent and incandescent elements.

.CLME:
8. A hybrid light source according to claim 7 wherein said plurality of elements are mounted in parallel array.

.CLME:
9. A hybrid light source according to claim 8 wherein said parallel array comprises alternate fluorescent and incandescent elements.

.CLME:

i0. A hybrid light source, or liquid crystal display embodying such light source, substantially as hereinbefore described with reference to the accompanying drawings.

.CLME:

Published 1988 at The Patent Office, Suate House, 6671 High Ho]born, London WCIR 4P Further coples may be obtained from The Patent Ofce, 8a/es Branch, St Mar, Cray. Orpucgon Kent BR5 3RD Printed by Mu/tlplex techniques ltd. St Mary Cray, Kent. Con. 1/87.

.CLME: