GB2058385A

GB2058385A - Liquid Crystal Displays

Info

Publication number: GB2058385A
Application number: GB8028477A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1979-09-07
Filing date: 1980-09-03
Publication date: 1981-04-08
Also published as: FR2465287A1; JPS5650369A; DE3032344A1

Abstract

Viewing of a liquid crystal display cell, e.g. in an aircraft cockpit, is facilitated by securing a partially transmissive, partially reflective diffuser 24, formed for example of opal glass, to the rear surface of the liquid crystal display cell 12 so that the diffuser is disposed between the liquid crystal display cell and a micro- corner cube retroreflector 26. Scattered light transmitted by the diffuser is reflected by the micro- corner cube retroreflector back to the diffuser along the same paths as the light emerged from the diffuser, causing the reflected light to be scattered through the liquid crystal display cell within a wide viewing cone centered about the direction of the light ray incident on the liquid crystal display cell. In this way, maximum use is made of the ambient light to achieve a display with increased brightness. An index matching layer 28 may be provided between diffuser 24 and reflector 26, and the latter may include a reflective layer 29 to increase the angle over which each corner cube can reflect light. <IMAGE>

Description

SPECIFICATION Liquid Crystal Displays This invention relates generally to liquid crystal displays and more particularly to a liquid crystal display having improved illumination.

To safely and efficiently operate various aircraft, particularly large, multi-engine commerciai and military jet liners, the pilot and other flight crew members must continuously be apprised of navigation data and aircraft operating data such as heading, altitude and direction, and engine speed, engine oil pressure, and the quantity of fuel remaining, respectively, to name a few. Typically, such navigation data and aircraft operating data are derived by electronic or electromechanical sensors, processed by associated sensor circuitry and displayed by a combination of electro-optical and electromechanical display means secured in the aircraft cockpit within viewing distance of the pilot and flight crew.

Typically, these electro-optical and electromechanical display means are secured within the aircraft cockpit so that they must be viewed at an angle, either to the right, the left, or downwards from the normal viewing position. With the only illumination within the aircraft cockpit being provided by light passing through the aircraft wind-shield, the only portion of these displays that are illuminated to any extent is a quarter sphere located above and to the side of the display. Since the level of ambient lighting within the aircraft cockpit is generally low, especially at night, it is crucial that such electro-optical and electro-mechanical display means provide the brightest display possible so that data displayed thereby are visible to the pilot and flight crew.

Presently, electro-optical devices utilized for the display of alphanumeric navigation and aircraft operating data have included both the reflective and transmissive type of liquid crystal displays (commonly referred to as LCD's) and light emitting diode displays (commonly referred to as LED's). While both the reflective and transmissive type liquid crystal displays are less expensive to construct and consume less power in comparison with light emitting diode displays, both reflective and transmissive type liquid crystal display require external illumination in order for information displayed thereby to be visible to user personnel. With transmissive type liquid crystal displays, an external light source is disposed behind the liquid crystal display cell to illuminate the cell by transmitting light therethrough.With the reflective type liquid crystal display cell, ambient light incident on the liquid crystal display cell provides liquid crystal display cell illumination. Since the reflective type liquid crystal display relies on ambient light for liquid crystal display cell illumination, it is desirable, particularly where low levels of ambient light exist, to maximize the ambient light reflected by the liquid crystal display cell to provide the brightest display possible. Thus, the present invention is concerned with maximizing the illumination of the reflective type liquid crystal display.

In accordance with the present invention there is provided a liquid crystal display cell having opposed front and rear surfaces, and further comprising: optical means disposed adjacent to the rear surface for partially reflecting and partially transmitting light emerging from the rear surface; and reflector means, spaced parallel to and apart from the optical means in registration therewith, the reflector means reflecting light transmitted by the optical means back to the optical means along substantially the same path as light incident on the reflector means such that the optical means scatters light through the cell in a viewing cone centered about the incoming light ray incident on the front surface.

In a preferred embodiment of the invention the optical means comprises a diffuser which may be formed, for example, from a layer of opal glass.

The reflector means preferably comprises a micro array of corner cube retroreflectors, and a layer of transparent material such as glass, having an index of refraction equal to that of the corner cube retroreflectors, may be disposed between the diffuser and micro array.

A display embodying the invention makes maximum use of the ambient light to achieve a display with increased brightness. Such a display is particularly suited for displaying navigation data and operating data in an aircraft cockpit.

By way of example only, an embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a side view of a reflective liquid crystal display according to the prior art; and Figure 2 is a side view of the liquid crystal display according to the present invention.

Figure 1 illustrates a liquid crystal display according to the prior art which comprises a reflective type liquid crystal display cell 12.

Cell 12 includes a pair of transparent plates 14 and 1 6 that are spaced parallel to, and apart from one another, each plate being formed from a layer of glass, plastic or the like. The front and rear opposing surfaces of plate 14 are designated 1 4a and 1 4b, respectively. Similarly, the front and rear opposing surfaces of plate 16 are designated 1 6a and 1 sub, respectively.

Secured to each of the rear and front surfaces 1 4b and 1 spa, respectively, of plates 14 and 16, respectively, is a respective one of transparent electrodes 1 boa and 1 sub. Electrodes 1 boa and 1 8b are each formed by depositing a thin layer of indium tin oxide or the like on plate surfaces 1 4b and 1 6a, respectively. Typically, one of electrodes 1 boa and 1 8b is configured in the shape of a character, symbol, or other indicia while the other electrode is configured of solid area. Disposed between electrodes 1 boa and 1 8b is a layer 20 of liquid crystal material, such as a cyanobiphenyl held by suitable gasket means (not shown) located between liquid crystal electrodes.

Each of a pair of electrical conductors, 22a and 22b is electrically cdnnected to a respective one of electrodes 1 boa and 1 8b. When a potential is applied to conductors 22a and 22b, an electric field develops between electrodes 1 boa and 1 bob, causing layer 20 of liquid crystal material to alter the molecular alignment, allowing ambient light to pass therethrough.Light that passes through layer 20 of liquid crystal material, electrode 1 8b and plate 16 is reflected by a slightly scattering rough reflector 23, typically formed of a thin layer of reflective material, such as aluminum or silver, deposited on slightly roughened plate surface 1 6b, back through the liquid crystal display cell with the central lobe of diffused light from rough reflector 23 being centered about the spectral reflective angle B with the normal N of reflector 23, which angle B is equal to the angle , the incident angle which the incoming ambient light ray makes with the normal N to reflector 23.

Reflection of light in this way creates a viewing cone of scattered light whose apex angle, ranging from 20 to 30 degrees, is centred about the spectral reflective angle B.

Use of the prior reflective type liquid crystal displays for displaying navigation and aircraft operating data incurs the disadvantage that little of the ambient light is reflected within the quarter spheres located above and to the sides of the display which is exactly where ambient illumination is greatest for such displays when secured in the aircraft cockpit.

Figure 2 illustrates a novel reflective-type liquid crystal display providing improved illumination which comprises a conventional reflective type liquid crystal display cell 12 configured identically to display cell 12 described with respect to Figure 1. A partially reflective, partially transmissive diffuser 24 is secured in face-to-face relationship with plate surface 1 6b of cell 12. Spaced parallel to, and apart from diffuser 24 is a retroreflector 26 which reflects light incident thereto back along the same paths.

Diffuser 24 can be configured of either a surface diffuser or a solid diffuser. In the presently preferred embodiment with diffuser 24 configured of a solid diffuser, typically formed from a layer of opal glass or the like, retroreflector 26 comprises a solid micro array of corner cube retroreflectors.

Each of the corner cube retroreflectors within micro array is typically of a prismatic shape, and reflects substantially all of the light incident thereto even though incident light may be substantially off the central axis of the corner cube retroreflector. For a further, more detailed discussion of corner cube retroreflectors, reference should be had to The RCA Electro Optics Handbook published by RCA Corporation, (1974) at pages 217-224.

With retroreflector 26 configured of a solid micro array of corner cube retroreflectors, it is desirable to fill the void between diffuser 24 and retroreflector 26 with a layer 28 of transparent material, such as glass or plastic, having an index of refraction approximately equal to that of retroreflector 26 to achieve surface matching and thereby avoid internal light trapping. It is also desirable to add a layer of reflective material 29 on the surface of the micro array of corner cube retroreflectors to increase the angle over which each individual corner cube retroreflector can reflect light.

In operation, when the molecular alignment of layer 20 of liquid crystal material is altered in response to an electrical potential applied to conductors 22a and 22b, ambient light is transmitted through layer 20, electrode 1 8b and plate 16 is scattered by diffuser 24. Scattered light incident on retroreflector 26 is reflected back to diffuser 24 along the same path as the light incident on retroreflector 26. Thus, reflected light previously scattered by diffuser 24 is now rescattered by the diffuser into a wider viewing cone centered about the direction of the incoming ambient light ray incident on liquid crystal display cell 12. In this manner, diffuser 24 and retroreflective reflector 26 advantageously maximize utilization of available ambient light to provide the brightest display possible.

Typically, the goniophotometric transmissive and reflective characteristics of diffuser 24, that is, the directional light distribution transmission and reflection characteristics, are chosen to match the desired viewing cone angle, usually between 20 to 30 degrees. The diffuser thickness typically ranges from 1 to 20 mils.

The foregoing describes apparatus, comprised of a diffuser and retroreflector, for improving the illumination of liquid crystal displays by scattering light exiting the liquid crystal display cell back through the cell in a wide viewing cone centered about the direction of the incoming ambient ray incident on the liquid crystal display cell to provide the brightest display possible. Since the diffuser and retroreflector may be added to a liquid crystal display cell without substantial cost, such apparatus provides an inexpensive means for improving liquid crystal display illumination.

Claims

1. A liquid crystal display cell having opposed front and rear surfaces, and further comprising: optical means disposed adjacent to the rear surface for partially reflecting and partially transmitting light emerging from the rear surface; and reflector means, spaced parallel to and apart from the optical means in registration therewith, the reflector means reflecting light transmitted by the optical means back to the optical means along substantially the same path as light incident on the reflector means such that the optical means scatters light through the cell in a viewing cone centered about the incoming light ray incident on the front surface.

2. A cell according to claim 1 wherein the optical means comprises a diffuser.

3. A cell according to claim 2 wherein the diffuser comprises a layer of opal glass.

4. A cell according to claim 1 wherein the reflector means comprises a micro array of corner cube retroflectors, the array having a first surface and a second opposite facing surface, the first surface facing the optical means.

5. A cell according to claim 4 further including: a layer of transparent material having an index of refraction equal to the index of refraction of the micro array of corner cube retroreflectors, the layer being disposed between the optical means and the micro array; and a layer of reflective material disposed on the second micro array surface for increasing the angle over which the micro array can reflect light.

6. A liquid crystal display comprising: (a) a liquid crystal cell having opposed front and rear surfaces; (b) optical means, disposed adjacent to the rear surface of the cell for partially transmitting and partially reflecting light transmitted by the cell; and, (c) reflector means spaced parallel to and apart from the optical means in registration therewith, the reflector means reflecting the light transmitted by the optical means back to the optical means along substantially the same path as light incident on the reflector means thereby causing the optical means to scatter light through the cell in a viewing cone centered about the incoming ambient ray incident on the front surface of the cell.

7. A display according to claim 6 wherein the optical means comprises a diffuser.

8. A display according to claim 7 wherein the diffuser comprises a layer of opal glass.

9. A display according to claim 6 wherein the reflector means comprises a micro array of corner cube retroreflectors having first and second opposing surfaces, the first micro array surface facing the diffuser.

10. A display according to claim 9 further including: a layer of transparent material having an index of refraction substantially equal to that of the micro array of corner cube retroreflectors, the layer of transparent material being disposed between the diffuser and the micro array; and a layer of reflective material disposed on the second micro array surface for increasing the angle over which the retroreflector can reflect light.

11. A cell according to claim 1 and substantially as herein described with reference to Figure 2 of the accompanying drawings.