LIQUID CRYSTAL DISPLAYS

Abstract: In order to control the brightness of a fluorescent liquid crystal display (1), a photodiode (7) is located adjacent an edge (9) of a display cell (2). The photodiode receives light (8) which has been totally internally reflected within the cell and which is thereby emitted from the cell edge. The emitted fluorescence remains substantially constant during switching of the display and is a measure of the brightness of the display. The output of the photodiode is fed to a circuit (10) which controls the brightness of the light source (5) illuminating the display, so that a substantially constant display brightness is obtained.

Description: Liquid Crystal Displays This invention relates to liquid crystal displays (LCDs), and particularly to the automatic control of the brightness of fluorescent LCDs. Fluorescent liquid crystal displays combine the visual impact of emissive (i.e. non-liquid crystal) displays with- the virtues of liquid crystal display (LCD) technology. Fluorescent LCDs are uniformly bright and have a hemispherical angle of view, yet complex display patterns can be manufactured cheaply. They are readily visible in low ambient light levels at which conventional LCDs are hard to read, yet do not wash out in high ambient light levels as do conventional emissive displays. Such a fluorescent LCD is described in our British Patent Specification No: 2,169,092A and in an article by R. Van Ewyk et al in Displays, October 1986, 155-160. A pleochroic fluorescent dye, such as that described in our British Patent Specification No: 2,189,502A, is dissolved in a liquid crystal, and the mixture is used to fill a conventional LCD cell. Both the fluorescence and the light absorption by such a display are anisotropic. In a fluorescent LCD such as is described in the above references, when no electric field is applied to a segment of the display, there is strong absorption of some stimulating radiation and consequently strong fluorescence from the display. When a field is applied, however, the liquid crystal and dye molecules real i gn so that there are only weak absorption and weak fluorescence. Hence, the display general ly consists of dark characters on a bri ght background. It is also possible to construct a fluorescent l iquid crystal displ ay so that the unswitched area produces only weak fluorescence and the switched areas produce strong fluorescence. This type of displ ay then consists of bright characters on a dark background and is the reverse-contrast form of the displ ay. The stimulating radiation may interact either di rectly or indi rectly with the dye. In the former case, l ight is absorbed by the dye molecules whi ch subsequently fluoresce at a l onger wavelength. In the case of the perylene diesters which we have described in the above-mentioned publ ications, blue light is absorbed and green l i ght emitted. Alternatively, the fluorescence may be stimulated by indi rect excitation. In this case, the incident radiation is absorbed by some other species in the mi xture, such as the liquid crystal or some other deliberately-introduced molecule. This then t ransfers the absorbed energy, either radiatively or non-radiatively, to the dye which subsequently fluoresces. In the above-mentioned perylene system, indi rect stimulation can be effected by ultraviolet l i ght, whi ch i s invisible, whi le the fluorescence remains in the green. The stimulating radiation may be provided by a number of sources simultaneously. For example, a uv fluorescent tube acting as a backli ght may be the primary stimul ant, while additonal stimulation may be provided by ambient uv and/or by blue l i ght. Al l such sources will contribute to the bri ghtness of the displ ay. For a given l i ght output from the fluorescent tube, the bri ghtness of the displ ay will therefore vary in dependence upon the ambient light. As the fluorescent tube ages, its brightness will diminish and, for a given ambient l i ght level , the brightness of the displ ay wil l therefore decrease. In addition, the efficiency of the LCD itself wil l diminish with age. It is an object of the present invention to provide automatic control of the bri ghtness of a fluorescent LCD, in order to al leviate the above-menti oned disadvantages of known displ ays of that kind. According to the invention there is provided a fluorescent liquid crystal display (LCD) system, comprising a fluorescent liquid crystal cell having a first major surface at a viewing side thereof, a second major surface on the reverse side thereto and edges interconnecting said major surfaces; light-sensitive means positioned to receive fluorescence emitted from a said edge and to produce an output dependent upon the brightness of that fluorescence; and means responsive to the output of the light-sensitive means to control the brightness of a light source illuminating the cell. Preferably the light source is a fluorescent lamp and preferably the light source is positioned to illuminate the cell via said second major surface. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which Figure 1 is a schematic cross section of a liquid crystal display incorporating a fluorescent liquid crystal cell, and Figure 2 is a schematic block diagram of a circuit for controlling the brightness of the cell of Figure 1. Referring to Figure 1, a liquid crystal display 1 comprises a liquid crystal cell 2 which is constructed in a conventional manner and comprises two glass plates 3 which are assembled with a spacing of typically 8 jim. On its inner surface each plate 3 carries a transparent electrode pattern and a suitable aligning layer (not shown) for a liquid crystal layer 4. The liquid crystal layer comprises a nematic liquid crystal, for example a material designated E7 supplied by BDH Ltd., into which has been dissolved approximately 1% by weight of a fluorescent dye such as 3,9 or 10-bis (4-n-pentyl-bicyclo (2.2.2) oct-l-yl ) perylene dicarboxylate. The cell is illuminated by a fluorescent lamp 5 which is located behind the cell 2 and which emits light in the region of 350 nm, suitable for stimulating fluorescence of the dye indirectly. The display can be switched on and off by applying an electric field from an external voltage source (not shown) between the transparent electrodes. Much of the light emitted by the fluorescent dye in the liquid crystal material emerges from the viewed face 6 of the display. However, some fraction of this light is total ly internal ly reflected, general ly at the interfaces between the glass pl ates 3 and the outside atmosphere, and emerges from the edges of the cel l . The intensity of the light emitted at the edges is proportional to the brightness or intensity of the displ ay itself. Since the area of the display whi ch is being switched at any instant is generally small compared with that which is not being switched, and since the order parameters of the dye and the liquid crystal are not unity (i .e. they are imperfectly al igned), the light emitted from the edges of the cell remains substantial ly constant during switching. Hence, variati ons in the l i ght emitted from the edges of the cel l are largely caused by variations in the overall bri ghtness of the cell itself due to changes i n the level of stimulating radiati on or in the efficiency of fluorescence of the dye. Hence, when the cell is strongly stimulated by ambient li ght, the fluorescent lamp 5 can be switched off, and when the external stimul ation is reduced, the lamp is progressively switched on to maintain the cell bri ghtness at a constant level . Control of the bri ghtness of the cell is effected by a photodiode 7 whi ch is l ocated so as to detect the fluorescence 8 emerging from an edge 9 of the cel l . The photodi ode is connected to a control ci rcuit 10, which controls the intensity of the fluorescent l amp 5. A schematic bl ock di agram of the control circuit 10 is shown in Fi gure 2. The photodiode 7 is connected in series with a variabl e resistor 11 between a fixed voltage V Q and ground. A voltage V p at the junction 12 between the photodi ode 7 and the resistor 11 then depends on the intensity of l ight fal l ing on the photodiode. The voltage V p is fed into one input of a differential amplifier 13. A reference voltage the value of which is selectable by the user of the displ ay, is connected to the other input of the ampl ifier. The output of the amplifier 13 is fed to an integrating ampl ifier 14, the output of which is connected to a buffer amplifier 15. The voltage output of the ampl ifier 15 controls the fluorescent lamp 5 via a l amp drive circuit 16. By operation of the circuit, the intensity of the fluorescent lamp can be controlled, in dependence upon the light intensity detected by the photodiode 7, to maintain a constant cell brightness. The variable resistor 11 controls the sensitivity of the control circuit, and the required brightness of the display is set by selecting an appropriate value of V pe ^. An additional photodiode (not shown) could be incorporated in the reference voltage circuit to make automatic adjustments for the ambient brightness, as a less bright display is required in dim lighting than in strong lighting. The circuit of the present invention therefore operates to control the brightness of the backlight, i.e. the lamp 5 so that it is dim when there is sufficient excitation from external sources and bright when such sources are dim, thereby providing a constant fluorescence output from the cell. It also compensates automatically for changes in brightness of both the backlight and the cell as a function of time, for the output of both of these elements . decreases with increasing usage or age. Furthermore, it can serve to optimise the lifetime of these components, as their lives depend on the brightness which they are required to produce. Thus, early in their lives, both the backlight and the fluorescent LCD cell can be excited at less than their maximum output, thereby prolonging their working life (for example, if half the maximum brightness is acceptable, the half-life of the display will be approximately doubled). It is also possible to provide for the individual LCDs in a multi-cell display to be adjusted to give equal brightness, and to be maintained equally bright independently of their different fading rates. Moreover, if a single cell has to be replaced, its brightness can be set, and automatically held, equal to that of the other elements in the display. Although in the above embodiment a fluorescent tube acting as a backlight is controlled by the circuit 10, it would be possible for the circuit 10 to control, alternatively or additionally, another light source, either behind or in front of the cell, to maintain the fluorescence at the required level. The fluorescent liquid crystal cell in the above-described embodiment is of the conventional type, in whi ch the unswitched areas produce strong fluorescence and the switched areas produce weak fluorescence. However, the invention is also applicable to cel ls which operate in the reverse manner, i.e. in a reverse- contrast type of display.

Claims: CLAIMS 1. A fluorescent liquid crystal display (LCD) system, coπprising a fluorescent liquid crystal cell (2) having a first major surface (6) at a viewing side thereof, a second major surface on the reverse side thereto and edges interconnecting said major surfaces; light-sensitive means (7) positioned to receive fluorescence (8) emitted from a said edge (9) and to produce an output dependent upon the brightness of that fluorescence; and means (10) responsive to the output of the light-sensitive means to control the brightness of a light source (5) illuminating the cell. 2. A system as claimed in Claim 1, wherein the light source (5) is so located as to illuminate the cell (2) via said second major surface. 3. A system as claimed in Claim 1 or Claim 2, wherein the light source (5) comprises a fluorescent lamp. 4. A system as claimed in any preceding claim, wherein the means (10) to control the brightness of the light source (5) coπprises a differential amplifier (13) having a first input to which the output of the light-sensitive means (7) is coupled and a second input to which is fed a reference voltage. 5. A system as claimed in Claim 4, wherein the reference voltage is controllable (11) to adjust the brightness of the output of the cell (2). 6. A system as claimed in Claim 4 or Claim 5, wherein a lamp drive circuit (16) is coupled to the output of the differential amplifier (13). 7. A system as claimed in Claim 6, including integrating amplifier means (14) coupled between the differential amplifier (13) and the lamp drive circuit (16). 8. A system as claimed in any preceding claim, including second light-sensitive means operative to provide additional control of the brightness of the light source (5) in dependence upon the brightness of ambient light in the operating environment of the cell (2).

Inventors: WILTSHIRE MICHAEL CHARLES KEOG, WILTSHIRE, MICHAEL, CHARLES, KEOGH
Applicants: GEN ELECTRIC CO PLC, THE GENERAL ELECTRIC COMPANY, PLC
Application year: 1988
Main classification: G02F1
IPC codes: G02F1, G04G9, G09G3