GB2211010A

GB2211010A - LCD devices

Info

Publication number: GB2211010A
Application number: GB8723708A
Authority: GB
Inventors: Kevin Chung Ping Lo; Michael Dean Smith
Original assignee: GP ELECTRONICS Ltd
Current assignee: GP ELECTRONICS Ltd
Priority date: 1987-10-09
Filing date: 1987-10-09
Publication date: 1989-06-21
Anticipated expiration: 2007-10-09
Also published as: GB2211010B; GB8723708D0

Abstract

The display of an LCD device is selectively changeable to the forward or reverse type e.g. black on white or white on black, by modifying an LC device of a known type comprising transparent sheets 11, 12 having patterned electrodes 13, 14 of an oxide sandwiching a liquid crystal layer 10 and associated polarisers 15, 16 to further include an LC device, formed by an LC layer 23 and non-patterned electrodes 18, 19 of an oxide formed on sheet 12 and on another sheet 17, which is energised by a switching circuit 21, 22. Operation in the two types or modes is thus achievable in one instead of two separate display devices. The first LC device 10-14 introduces a 90 DEG shift in the light polarised by element 15 and the second LC device introduces a further 90 DEG or 0 DEG shift depending upon whether switch 22 is open or closed. Operation in the two modes corresponding to day or night conditions may be controlled manually or by a light sensor. Applications may be in radios, vehicles, clocks, computers, toys, medical appliances etc. <IMAGE>

Description

AN IMPROVED LIQUID CRYSTAL DISPLAY This invention relates to liquid crystal displays (LCD's) and more particularly to an improved display having forward or reverse characteristics.

Conventional LCD's use liquid crystals which are organic materials whose optical properties can be influenced by electrical fields. They are termed liquid crystals because their molecules are free to move. The class mostly used in LCD's are nematic liquid crystals which have rod-shaped molecules which are more or less aligned in one direction.

In one known construction of an LCD, twisted nematic liquid crystals are sandwiched in a space of a few microns between two parallel sheets of glass, the inner surfaces of which have transparent conductive coatings which form electrodes, defining symbols, characters or other indicia to be displayed. The surfaces in contact with the liquid crystals are treated to induce the molecules to align in one direction parallel to the plate. The induced. direction at one sheet is at right angles to the other.The liquid crystal structure is twisted through 900 between one glass sheet and the other which rotates the plane of polarisation of transmitted light through 90 . By applying a suitable voltage between two opposite electrodes this twisted structure is broken as the molecules align themselves parallel to the electric fields.

Depending on the polarity of the polarizers they will either pass light or stop light from passing through the LCD, these are termed a 'forward' or light passing LCD or 'reverse' for a non-light passing LCD respectively.

An aim of the present invention is to provide an improved LCD which has both 'forward' and 'reverse' characteristics.

According to the present invention there is provided a liquid crystal display comprising first and second sheets of transparent material between which are sandwiched a layer of liquid crystals, the inside surfaces of the sheets being coated with an electrode forming oxide to form a pattern, and first and sccun polarizers located adjacent the outside surface of the first and second sheets respectively, wherein at least a third sheet of transparent material is located between the second sheet and the second polarizer, the inside surface of the third sheet and the outside surface of the second sheet having an electrode forming coating of oxide without a pattern, a second layer of liquid crystals is inserted between the second and third sheets and the non-patterned electrode coatings on these sheets are connected to an electrical switching circuit to energise the second layer of liquid crystals to change the display to the 'forward' or 'reverse' type.

Preferably, the electrode forming coating is indium-tin oxide.

Conveniently, the first and second polarizers have the same polarity.

In another arrangement of the LCD the polarity of the first and second polarizers are at 900 to each other.

The transparent material used for the sheets will normally be glass although other suitable transparent materials may be used if desired.

Preferably, the liquid crystal display is of the twisted nematic type.

Advantageously, the LCD may be controlled by a remote connection to synchronize the 'forward' and 'reverse' effects.

The LCD can conveniently be automatically shifted from the 'forward' to the 'reverse' condition or back again under control of a light sensor which measures the ambient light conditions.

The shifting may alternatively be effected by a manually operated switch.

The present invention may also extend to an electrical apparatus including an LCD as set forth above.

An embodiment of a liquid crystal display according to the invention will now be described, by way of example only, with reference to the drawings, in which: Fig. 1 is a diagrammatic exploded view of a conventional liquid crystal display of the 'forward' type; Fig. 2 is a diagrammatic view, similar to Fig. 1, showing a display of the 'reverse' type; Fig. 3 is a diagrammatic exploded view of a liquid crystal display according to the invention, in the non-energized light passing condition; and Fig. 4 is a diagrammatic view, similar to Fig. 3, of the display in an energized non-light passing condition.

The liquid crystal display shown in Fig. 1 is a conventional 'forward' type display comprising a layer of twisted nematic liquid crystals 1 sandwiched between two sheets of glass 2 and 3, the inner surfaces of which are coated with transparent layers 4 and 5 of indium-tin oxide in the form of a pattern. These layers 4 and 5 act as electrodes to energise the liquid crystals 1.

On the outside of each sheet of glass 2 and 3 are light polarizers 6 and 7 which will only pass light of the same polarization as the polarizer. Non-polarized light 8 entering the first polarizer 6 is polarized and this polarized light 9 passes through the glass sheet 2, the oxide layer 4 and through the liquid crystal layer 1. At this point the light's polarity shifts by 900 due to the molecular structure of the liquid crystals. The shifted light then passes through the second oxide layer 5 and the glass sheet 3 into the second polarizer 7.

Depending upon the polarity of the second polarizer 7, light will either pass from or stop at the second polarizer.

This liquid crystal display is called a light passing LCD of the 'forward' type.

Fig. 2 illustrates an LCD of the non-light passing or 'reverse' type. The non-polarized light 8' passes through a polarizer 6' polarizing the light 9' which then passes through the sheet of glass 2', the oxide layer 4' which forms a transparent invisible pattern and contacts the liquid crystal layer 1'. Once again the polarized light is shifted through 900 by virtue of the molecular structure of the liquid crystal layer 1' and passes through transparent layer 5', glass sheet 3' and emerges in a 90" shift direction. The second polarizer 7t is at 0 with respect to the first p6larizer 6' so that the light stops at this polarizer and there is no light output.

As the transparent electrodes are energized, the liquid crystals in between these electrodes will no longer shift the polarized light by 900 as the helix structure of the liquid crystals are broken. This will make the electrode pattern appear as either transparent or opaque to light depending on the polarity of the second polarizer.

An improved LCD construction, according to the invention will now be described with reference to the liquid crystal display illustrated in Figs. 3 and 4.

The LCD comprises a conventional display having twisted nematic liquid crystals 10 sandwiched between two sheets of glass 11 and 12. The inside surfaces of the glass sheets are coated with transparent layers 13 and 14 of indium-tin oxide or any other suitable material which acts as an electrode to energize the liquid crystals. These electrode layers are formed in patterns to define symbols, characters or other indicia to be displayed. Two polarizers 15 and 16, which are of the same polarity, are arranged outside the glass sheets 11 and 12.

According to the improved display one or more sheets of glass 17 are added to the known construction. The inside surface of the glass sheet 17 and the outside surface of the glass sheet 12 are coated with transparent oxide layers 18 and 19, without any pattern, which act as further electrodes. These electrodes are connected to an electrical circuit 20 with a power supply 21 and switch 22. Sandwiched between the oxide coated glass sheets 12 and 17 is a second twisted nematic liquid crystal layer 23 which is controlled electrically by energizing the electrodes 18 and 19.

As with a conventional LCD, non-polarized light 24 passes through the first polarizer 15 to be polarized, the polarized light 25 then passes through the liquid crystal layer 10 to be shifted by 900. As the shifted light 26 leaves this layer it will pass through the second layer of liquid crystals 23 sandwiched by the transparent electrodes 18 and 19. The liquid crystal layer 23 can be switched to shift the light by 0 or 900 depending on the status of the electrode supply being on or off respectively.

The shifted light then passes into the second polarizer 16 which will either allow the light to pass or stop the light from the previous layers.

The embodiment shown ' in Fig. 3 shows the improved construction in the 'forward' condition where the circuit is switched off not energizing the liquid crystal layer 23 to provide a light output at the second polarizer 16 while the embodiment shown in Fig. 4 shows the display in the 'reverse' condition in which the circuit is switched on energizing the liquid crystal layer 23 providing no light output at the second polarizer.

The second polarizer 16 can be 0 or 900 with respect to the first polarizer 15 depending on whether a 'reverse' or 'forward' effect is required without the liquid crystal layer 23 being energized. Thus the LCD can be electrically programmed to be of the 'forward' or 'reverse' type.

By using the second liquid crystal layer 23 to shift the LCD into the 'forward' or 'reverse' mode, both conditions can be achieved providing the most desirable effects during night-time or day-time conditions.

Although the embodiments described use a manual switching device for shifting the LCD from 'forward' to 'reverse' or back again, it will be possible to use a light sensor to measure the ambient light level and to shift the LCD automatically from the 'forward' to the 'reverse' condition. In the case where displays are lit at night, such as car dashboards, a remote control connection can be made to the improved LCD to synchronise the 'forward' or 'reverse' conditions.

There are many applications of the improved LCD according to the invention the most advantageous being the dramatic improvement of night and day viewing of the LCD pattern.

At night the 'reverse' LCD can emulate the appearance of a highly visible and cosmetically appealing LED or a light emitting diode display. During the day or during high ambient light conditions, the 'reverse' type display is not easy to read due to the small amount of light that can enter the display from the front to be reflected back out, and also due to contrast ratios, the rear lighting of the display cannot practically be increased to a high enough level to still function in a high ambient light environment.

The improved LCD of the present invention can be used with a variety of products such as radios, vehicle dashboards, clocks and instruments, domestic appliances, computer LCD, flat panel displays, toys as well as many military, medical and industrial applications which require LCD's for use in both daylight and low light conditions. It will be appreciated, however, that the essence of the invention is to provide a switchable LCD which can be used either in the forward or reverse mode according to the wishes or desires of the user.

Claims

-1. A liquid crystal display device comprising first and second sheets of transparent material between which are sandwiched a layer of liquid crystals, the inside surfaces of the sheets being coated with an electrode forming oxide to form a pattern, and first and second polarizers located adjacent the outside surfaces of the first and second sheets respectively, wherein at least a third sheet of transparent material is located between the second sheet and the second polarizer, the inside surface of the third sheet and the outside surface of the second sheet having an electrode forming coating of oxide without a pattern, a second layer of liquid crystals is sandwiched between the second and third sheets and the nonpatterned electrode coatings on these sheets are connected to an electrical switching circuit to energise the second layer of liquid crystals to change the display to the "forward" or "reverse" type.
2. A liquid crystal display device according to claim 1, wherein the electrode forming coating consists of indium-tin oxide.
3. A liquid crystal display device according to claim 1 or claim 2, wherein the first and second polarizers have the same polarity.
4. A liquid crystal display device according to claim 1 or claim 2, wherein the polarity of the first and second polarizers are at 900 to each other.
5. A liquid crystal display device according to any preceding claim, wherein said first and second sheets of transparent material are made of glass.
6. A liquid crystal display device according to any preceding claim, wherein the liquid crystal display is of the twisted nematic type.
7. A liquid crystal display device according to any preceding claim, wherein said device is provided with a remote connection arranged to control said device to synchronize the "forward" and "reverse" effects.
8. A liquid crystal display device according to claim 7, wherein said remote connection includes a light sensor arranged to shift the device from the "forward" to the "reverse" condition or back again automatically according to the ambient light conditions sensed by the sensor.
9. A liquid crystal display device according to claim 7 or claim 8, wherein said remote connection includes a manually operated switch.
10. A liquid crystal display device substantially as described herein with reference to Figs. 3 and 4 of the drawings.