Field of the Invention

The invention relates to multi-layered displays and especially multi layered displays in situations where power consumption is an issue.

Background

Historically mobile phones used monochrome screens. These were segmented displays similar to those used in digital watches and calculators. The information that could be displayed on these screens was limited to numeric data such as phone numbers.

The next generation included passive matrix displays. These were still monochrome. However they had the advantage they could be used to display pixilated data. These displays had super twisted liquid crystal sandwiched between two pieces of glass. The display had electrodes arranged in columns on one side and in rows on the other side. A voltage was applied across selected rows and columns. Where regions with applied voltage intersected, the LC tilted, causing the screen to go black in the intersecting regions. This system was relatively flexible. It allowed the display of numbers, letters and even simple images. However, the display was not sophisticated enough to allow the display of grey scale or colour images.

The next milestone was the development of a passive matrix system where the voltages applied could be controlled to give differing degrees of tilt at the liquid crystal. The different tilts allowed different amounts of light through the display. Accordingly this could be used to provide grey scale images. This passive matrix system could also be used in combination with colour filters to allow the display of colour images.

These screens though have a tendency to flicker. In addition, they have slow response times. In response to these issues active matrix screens were developed. These active matrix screens have a transistor and a capacitor associated with each pixel. This

allows for much quicker response times and much higher resolutions.

However, the inclusion of the transistors, capacitor and colour filters has a cost in terms of transmissivity. That is, the active matrix screens do not allow much light through them. As a result, the backlight needs to produce more light to compensate. This results in the backlight consuming a lot more power than its predecessors.

In recent times development has moved to alternative backlighting technologies such as LEDs and to mechanisms such as light collimation and polarization recycling, to increase the light output of the backlighting units. Multi layer displays are a relatively modern phenomenon. They are described in PCT/NZ98/00098, which is hereby incorporated into this specification. Multi layer displays generally comprise two overlaid layers of active matrix LCD screens.

The advantages of the multi layer display rest with the ability to display two layers of data at the same time. Complex data can be displayed on both the front and rear displays. Usually this data would need to be separated by displaying it on separate displays or in a minimized size so that both can be displayed on the same screen. The multi layer display however splits information across layers. As the information is presented at different distances the human brain is able to distinguish the information into its relative information sets. In this way more information can be presented to and cognised by a user.

The overlay of active matrix LCD screens is not without technical drawbacks. By way of example the overlay leads to the formation of a moire interference pattern. To overcome this an interstitial material is sandwiched between the active matrix LCD screens. The interstitial material slightly blurs the rear LCD, minimizing the moire interference. However, it also leads to degradation of the rear LCD image quality. This is discussed in detail in PCT/NZ03/00046.

In addition, the presence of two active matrix LCDs in the optical stack of the multi layer display leads to the loss of a significant proportion of the light exiting the backlight arrangement. This loss is between about 95 and 98%. As a result multi layer displays tend to have a luminance of 150 cdnrf ² as compared to a regular display with a luminance of 300-400 cdm ^"2 .

The transmissivity of the typical components of an active matrix display are

recorded in the table below. The transmissivity of specific components from different manufacturers will vary. However, the table provides an indication of the order of transmission you can expect from a typical active matrix display.

Note: the total transmissivity is the product of the transmissivities of the individual components.

To try an overcome this, the backlight of a multi layer display is significantly brighter than that in an ordinary display. This of course has a negative effect on the efficiency of the unit making the multi layer display technology unsuitable in situations where power consumption is limited by use of a battery, fuel cell or other storage device. .

Accordingly there is a need for an improved multi-layer display that overcomes some of the foregoing options or at least provides the public with a useful choice.

Object

It is an object of the invention to provide an improved multi-layer display, which goes some way to ameliorating the aforementioned problems, or at least provides the public with a useful choice.

Statement of Invention

In a first aspect the invention relates to a multi layer display device including a passive matrix display layer and an active matrix display layer.

Preferably the active matrix display layer includes colour filters.

Preferably the passive matrix display layer is located in front of the active matrix display layer.

More preferably the most forward display layer includes an overlay making it a touch screen.

Preferably the multi layer display device is a mobile phones, PDAs, MP3 players, laptops and tablet computers.

More preferably there is an interstitial glass layer or a space between the passive matrix display layer and the active matrix display layer. In a further related aspect the invention relates to a mobile electronic device incorporating a multi layer display.

Preferably the multi layer display includes a passive matrix display layer and an active matrix display layer.

More preferably the passive matrix display layer is located in front of the active matrix display layer.

More preferably there is an interstitial glass layer or a space between the passive matrix display layer and the active matrix display layer.

More preferably the front display layer includes a touch screen overlay.

The display medium can be any display matrix that is transmissive. Suitable mediums include liquid crystal, OLED, TOLED and electrophoretic ink.

The polariser may be standard stretched iodine or wire grid polariser.

The interstitial glass may be of substantially uniform refractive index. Different effects may be obtained by shaping or patterning or changing the refractive index of the glass. By way of example the interstitial glass may be adapted as a lens providing the illusion of greater depth.

Where a space is provided between the display layers, this is preferably between about 1 mm and 15mm.

The present invention may be useful for inclusion in mobile devices such as mobile phones, PDAs, MP3 players, laptops and tablet computers. In addition, the invention may be integrated into traditional display media such as desktop

monitors.

The front-most display layer may include a pressure-sensitive (resistive), electrically-sensitive (capacitive), acoustically-sensitive (SAW - surface acoustic wave) or photo-sensitive (infra-red) overlay so that it is becomes a touch or pen based display.

The advantage of the above arrangements is the avoidance of overlapping black matrix and colour filters. This is advantageous in a number of aspects. Firstly, the overlapping structures interact to cause Moire interference. Having two layers of colour filters substantially reduces the brightness of the display. By eliminating one of these structures we can increase the brightness of the display by a factor of between around 2 or 3.

Brief Description of the Drawings

One aspect of the invention will now be described with reference to the attached figure, which displays a schematic representation of one embodiment of the device.

Detailed Description of the Invention

The multi layer display of the present invention avoids the use of multiple layers of black matrix and colour filters by utilizing a mixture or passive and active matrix displays.

The invention centres on the realization that in mobile displays it is unnecessary to allow for the display of complex images on both the front and rear display layers of a multi layer display. In fact, the complex images need only be allowed for on one of the displays. It is preferable to have the complex image data on the rear display and relatively sparse symbolic or textual data on the front display.

This realization represents a significant departure from the current thinking where the drive is towards improving the backlight and other arrangements to ensure sufficient luminance despite the low transmissivity of the active matrix displays. In this invention then, a marked departure is made, and the passive displays are integrated into the display with the active display.

Referring to attached figure, in a preferred arrangement the multiplayer display of the present invention includes:

1. A linear polariser to analyse the light coming through the front most liquid crystal layer 2. Glass patterned with passive matrix rows to provide voltage for controlling liquid crystal

3. An alignment layer for liquid crystal to provide the correct pre-tilt.

4. Twisted nematic liquid crystal for optical activity to change light transmission state 5. Alignment layer to align liquid crystal

6. Glass patterned with passive matrix columns to provide voltage for controlling liquid crystal

7. Wire grid polariser to analyse light coming from rear most layer and provide polarised light for the next layer. 8. Active matrix ground ITO layer to provide a voltage reference to address ^• against

9. Colour filters to provide the ability to make a particular pixel appear a particular colour

10. Alignment layer for liquid crystal 11. Liquid crystal to provide optical activity required to change the light transmission state.

12. Active matrix display grid to provide voltage for controlling liquid crystal

13. Rear display glass to provide a substrate for subsequent layers.

14. Rear polariser to polarise light coming from a backlight device. Parts 1 to 6 form the passive matrix display layer. Parts 9 to 14 form the active matrix display layer. The distance between the displays provide the depth effect. This can be varied by including a space between the layers or including an interstitial glass or lens layer.

Those skilled in the art will appreciate that this order may be reversed depending

on the application. Additionally, it may be desirable to put the wire grid polariser on the other side of the interstitial glass, or to use two separate displays. The advantages provided by this stack over prior art include:

(a) Increased light transmission. This is because the front most liquid crystal component is now without colour filters, capacitors and transistors, the black matrix mask and colour filters. This in turn significantly reduces the burden on a portable power supply, which in turn means the power supply will last longer and be less of a burden in overall size, which is important in portable applications.

(b) No moire interference between colour filters on the front display layer and rear display layer. This generally results in the need to blur the image on the rear display layer, which results in a degradation of image quality. Also this blurring needs to be done in a very controlled manner between said polarisers, which results in a relatively expensive component because of the process constraints.

(c) Decreased display thickness. Mobile applications at present have a display stack that is about 2mm thick. This stack needs to exist inside the device package in conjunction with other components to make the device function. Increases in thickness of as small as 2 or 3 millimeters will make the device to large for users to carry comfortably.

(d) Reduced mass and manufacturing cost since the two middle pieces of glass has been combined into one.