WO1991010224A1

WO1991010224A1 - Display devices

Info

Publication number: WO1991010224A1
Application number: PCT/GB1991/000011
Authority: WO
Inventors: Alan Mosley; Michael George Clark; Piero Migliorato; Colin Teck Hooi Yeoh
Original assignee: The General Electric Company, Plc
Priority date: 1990-01-05
Filing date: 1991-01-04
Publication date: 1991-07-11
Also published as: EP0462259A1; JPH04506421A; GB9000252D0; GB2239729A; GB9100137D0; GB2239729B

Abstract

A display device for providing colour images, for use, for example, in television receivers and videophones, comprises a display panel (3) having a matrix of rows and columns of ferroelectric liquid crystal (FLC) elements. A sequential colour filter system (7-17) is located between a light source (5) and the display panel or between the display panel and the viewer. The rows of FLC elements are addressed in sequence, and each row in succession is set to a non-transmissive state at a constant predetermined time after it has been addressed, and for a constant predetermined period. The colour filter system may be divided into at least two independently switchable areas which are aligned with respective groups of the rows of FLC elements. The filter areas may be switched while the respective groups of rows of FLC elements are set to the non-transmissive state.

Description

Di splay Devices

This invention relates to display devices, and particularly to ferroelectric liquid crystal displays for providing coloured images.

There is currently a large amount of world-wide interest in the development of flat panel displays for use in videophones, office terminals and television receivers. It will be apparent that videophones and television receivers will require coloured displays, and it is probable that such displays will also be required in office terminals in due course. The foremost technology for forming flat panel displays for all of these applications is based on liquid crystal displays (LCDs), more especially active matrix addressed LCDs and ferroelectric LCDs. Indeed, pocket-size television receivers employing active matrix addressed LCDs are already available. In these displays the colour is introduced by the use of red, green, and blue filters. This use of filters leads to a reduction in resolution, because each picture element then consists of three (red, green, blue) sub-pixels. Furthermore, the fabrication of sub-pixel colour filters has proved to be an expensive and complicated process, often requiring four photolithographic steps, which are known to be a cause of loss of yield.

In the area of colour cathode ray tube (CRT) displays, a mesh or grill is used to obtain colour images by use of red, green and blue sub-pixels. It is known that a sequential colour shutter, coincidental ly also based on liquid crystal technology, such as two-frequency addressed devices or pi-cell devices, can be used in conjunction with a monochrome CRT to produce a high-resolution colour image. This shutter arrangement relies on the optical characteristics of the CRT; specifically that a given pixel is illuminated for only part of the frame period. For example the phosphor decay time may be aproxi ately 3ms, whereas the frame time is 20ms.

It will be apparent to those skilled in the art that a sequential colour filter of that type cannot be used with currently- known active matrix addressed LCDs and ferroelectric LCDs, because the optical response of these LCDs is significantly different from that of a CRT.

It is an object of the present invention to enable the use of a combination of a ferroelectric LCD with a sequential colour shutter.

According to the invention there is provided a display device comprising a matrix of rows and columns of ferroelectric liquid crystal elements; a light source; and a sequential colour filter system disposed between the light source and the matrix or between the matrix and the viewer; wherein rows of the elements are addressed in sequence; and wherein each row in succession is set to a non-transmissive state at a constant predetermined time after it has been addressed, and for a constant predetermined period.

Preferably the colour filter system is divided into at least two independently-switchable areas which are aligned with respective groups of said rows of elements.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which

Figure 1 is an exploded schematic pictorial view of a display device incorporating the invention,

Figures 2-5 illustrate, schematically, sections of waveforms in alternative schemes for driving the elements of a waveform-multiplexed ferroelectric liquid crystal display, and

Figures 6 and 7 are schematic timing diagrams of alternative modes of operation of an image display combined with a single optical shutter.

A ferroelectric liquid crystal display is a bistable optical device having a high switching speed (10-lOOkHz). In the conventional mode of addressing an image-forming dot matrix ferroelectric display, each line of the display is addressed once (or in some cases twice) per frame, and is, therefore, active for the full frame period. This mode of addressing cannot be used with a sequential colour filter, for the following reasons:

The sequential colour filter system operates by rapidly changing the colour (red, green or blue) transmitted by the filter. Each frame of the image display therefore consists of three sub-frames, each comprising red, blue or green components of the final image. Considering a sequence of these three sub-frames, the following problem becomes apparent. For the sake of example, it will be assumed that (a) the red sub-frame image is being defined, so that the sequential colour filter is set at red, (b) the display has been blanked to a dark state, (c) the first line of the display is ready to be addressed, and (d) the frame period is, for the purposes of this example, 25ms and the row address line is 50 JJS, so that there are 500 rows. The first line which is addressed will be on for the whole of the frame period (25ms), whereas the last line will be on for only 50 s, before the colour of the filter needs to be changed to the colour required for the next sub-frame. This will clearly lead to a highly distorted image, which can be obviated in the known system only by using a sequential colour filter which is sub-divided into a large number of rows (500 in this example) equal to the number of rows of the image-forming display. Obviously this solution greatly increases the complexity, and hence the cost, of the system.

The present invention alleviates this problem by, in effect, causing the ferroelectric display to mimic the optical characteristics of a CRT. This technique can be applied equally well to both a waveform-multiplexed, ie passive matrix, ferroelectric LCD and an active matrix addressed ferroelectric LCD. The rows of the image-forming ferroelectric liquid crystal, display are successively blanked to a dark state at a fixed time after they have been addressed, in order to ensure that all of the pixels of the image-forming display are activated for the same length of time. The fixed time at which blanking is effected can be set to an optimum value. The sequential colour filter system is divided into two or more independently-switchable areas which are aligned with p re-determined sets of rows of the image-forming display. In this way a high brightness efficiency is obtained by switching each of the areas while the part of the image display to which it is aligned is blanked.

Referring to Figure 1 of the drawings, a display device 1 incorporating the invention comprises a liquid crystal display 3 which receives light from a source 5 via a polariser 7, a shutter cell 9, a polariser 11, a shutter cell 13 and a polariser 15. A further polariser 17 is disposed between the display 3 and the viewer.

The polariser 7 transmits green (G) and blue (B ) vertically-polarised light and red (R) horizontally-polarised light. The polariser 11 transmits blue vertically-polarised light and red and green horizontally-polarised light. The polarisers 15 and 17 are neutral polarisers which transmit white (W) light polarised in the vertical and horizontal directions, respectively. The absorption axes of the polarisers 15 and 17 are indicated by arrows marked Bl. Suitable devices for forming the polarisers 7 and 11 may be obtained from Optical Devices Inc., of California, USA. The neutral polarisers 15 and 17 may be obtained from Nitto or Sanritzu, both of Japan.

The shutter cells 9 and 13 will rotate plane polarised light through an angle of 90° in their "selected" states, as indicated by a logic '1' in Table 1 below. In their "unselected" state they allow plane polarised light to be transmitted substantially unaltered. That state is indicated by a logic '0* in Table 1.

The image display 3 is a waveform - multiplexed ferroelectric liquid crystal display, in which a "selected" pixel, denoted by a '1' in Table 1, will rotate plane polarised light by 90°. A "non-selected" pixel, denoted by a '0' in the table, will allow plane polarised light to be transmitted substantially unaltered.

Table 1 indicates the eight possible combinations of states of the shutters 9,13 and the display 3, and the resulting colour of the transmitted light for each of the combinations. It will be seen that pixel s of the three primary colours (R,B,G) can be obtained and that, for each colour, changing the state of a display pixel from ' 1' to '0' turns the pixel black.

Table 1

In a waveform-multiplexed ferroelectric LCD it i s preferabl e that the blanking pulse be applied to one row at the same time as a row further down the image di splay is bei ng written. It is therefore essential that the blanking pulse wil l be effecti ve i rrespective of the data voltage being appli ed to the column electrodes of the waveform-multiplexed ferroelectric LCD. The reason for the above-mentioned preference is that separate bl anki ng pulses for the indi vidual rows would need to be of approximately the same pulse width as the writing pulses, and therefore the frame period would otherwise be doubled, which is disadvantageous. In the case of an active matrix addressed ferroelectri c LCD, the row blanki ng pul se can be appli ed either at the same time as a writing pulse to another row or in a different time slot. The latter is feasi bl e in an acti ve matrix addressed ferroelectric LCD because the addressing time (approximately 5μs) of the acti ve matrix is shorter than the addressi ng time of a row in the waveform-multiplexed ferroelectri c LCD. This is because, in the case of active matrix addressing, after the charge required to switch the ferroelectric liquid crystal has been deposited (in 5us) the reorientation (ie blanking or writing) of the ferroelectric liquid crystal can occur within its relaxation time since, after addressing, the pixel is then isolated from the data voltage.

An example of waveforms suitable for driving the rows and columns of the display in the present invention is shown in Figure 2. The waveforms are similar to waveforms disclosed in co-pending British Patent Application No.9021346.3. A writing row waveform (Figure2(a)) comprises a write pulse 21 of amplitude 2V_S and of duration t_s. A "select" data waveform (Figure2(b)) comprises a pulse 23 of amplitude + V immediately followed by a pulse 25 of amplitude - V. A "non-select" data waveform (Figure2(c)) comprises pulses 27 and 29 which are the inverse of the pulses 23 and 25. Each of the pulses 23, 25, 27 and 29 is of duration t_s. A blanking row pulse 31 (Figure 2(d)) is of amplitude - V_s and of duration 2t_s.

Examples of other waveforms which could be use for driving the display are shown in Figures 3-5 of the drawings. Similar waveforms are disclosed in British Patent Application No. 8719078. In Figure 3(a) a row writing waveform comprises a pulse 33 of amplitude V followed by a pulse 35 of amplitude - V_w. A dc offset voltage 37 of magnitude V₀ is used to avoid the presence of an overall residual dc component on the liquid crystal cells. The offset voltage is given by

V₀ = V-V_w where N is the number of rows N-l in the display.

A "non-select" data waveform (Figure3(b)) comprises a pulse 39 of amplitude + V_D followed by a pulse 41 of amplitude -V_D. A "select" data waveform (Figure 3(c)) comprises pulses 43 and 45 which are the inverse of the pulses 39 and 41. A blanking row waveform comprises a pulse 47 of amplitude -V_B followed by a pulse 49 of amplitude + V_β.

The pulses 33, 35 and 39-49 are all of duration t_s.

In Figure 4(a) the writing waveform is the same as in Figure 3(a). In this case, however, the "select" data waveform (Figure 4(b)) is a zero voltage 51. The "non-select" data waveform comprises a pulse 53 of amplitude + V_D followed by a pulse 55 of amplitude - V_D. The blanking waveform (Figure 4(c)) is similar to that of Figure 3(c). Again, the pulses are all of duration t_s.

In Figure 5(a) the row writing waveform is similar to that of Figures 3(a) and 4(a). The "select" column waveform (Figure 5(b)) comprises a zero voltage level 57. The "non-select" column waveform (Figure 5(c)) comprises alternate positive and negative pulses 59, 61 of magnitude V_D. The row blanking waveform comprises pulses 63, 65 (Figure 5(d)) of amplitude - Vg and + V_B, respectively, temporally displaced from the row writing and column pulses by plus half a pulse width. Alternatively, the row blanking waveform may comprise pulses 67, 69 (Figure 5(e)) similar to the pulses 63 and 65, but temporally displaced from the row writing and column pulses by minus half a pulse width.

In order to use the display device of Figure 1, it is essential that the liquid crystal display 3 be always blanked to a dark state (black), since blanking to a "bright" state, as required by some non-dc-compensated drive schemes used to drive waveform-multiplexed ferroelectric liquid crystal displays will result in the transmission of a coloured light and a loss of the integrity of the displayed image. The addressing schemes described above with reference to Figures 2-5 are all dc-compensated.

In the case of an active matrix addressed image display, if necessary, any residual dc voltage remaining on the pixel after the writing pulse may be removed by inserting into the addressing scheme a voltage-elimination pulse during which the potential difference across the pixel is reduced to zero.

As mentioned above, it is possible to address each row of an active matrix addressed ferroelectric LCD twice; once with a blanking pulse and once with a writing pulse, the two pulses being separated by a fraction of the frame period, depending on the number of independently-switchable areas of the shutters.

In order to implement the present invention it is necessary to synchronise the switching of the waveform-multiplexed ferroelectric LCD or the active matrix addressed ferroelectric LCD with the addressing of the sequential colour shutter. Furthermore, the electrode area of the sequential colour shutter should preferably be divided into at least two equal areas, the interelectrode gaps between those areas being aligned with interelectrode gaps in the liquid crystal display 3.

By way of example, a timing diagram for an image display operating in conjunction with a single optical shutter which is switched sequentially between transmitting red light and transmitting green light and the electrode area of which is divided into two equal areas separated by a small gap of, for example. 25pι, is shown in Figure 6. In this case the frame time, which must be short enough to produce a flicker free image, is given by 2(row address time x number of rows + 2S_t), where S_t is the time required to switch between the two states of the sequential colour filter. The image display matrix may be split into two electrically-independent halves by leaving a gap in the column electrodes. Each half can then be addressed independently, which may be advantageous for either waveform multiplexing or active matrix addressing of the image display, especially when the number n of equal areas which make up the shutter electrodes is even.

An alternative timing diagram is shown in Figure 7. In this case, the frame time is equal to 2(row address time x number of rows). However, the blanking time is greater than the writing times by an amount 2S_t. This will lead to a slightly darker display than that achieved by the scheme used in Figure 6. In general, the brightness efficiency y of the described combination of a ferroelectric LCD image display and a sequential colour shutter will depend on:-

u = (Frame time - Blank time) Frame time

Therefore, in order to increase the brightness of the display, the blanking time B_t (in Figures 6 & 7) should be made as small as possible. This parameter can be reduced by increasing the number n of equal areas which form the shutter electrode, since the total blank time Tg for a frame for the scheme illustrated in Figure 6 is given by:

T_B = Frame time n

As n increases so does the complexity of the device. Values of n between 2 and 10 are considered to be the most useful.

In Figures 6 and 7 the frame times increase as S^ increases, and it is therefore advantageous to minimise S_j.. The value of S_t will depend on the technology chosen for the sequential colour switch. At the present time three types of device might be used, a device relying on the two-frequency effect, a pi -cell, or a ferroelectric optical shutter, all of which are liquid crystal devices. Of these three devices the ferroelectric optical shutter has the shortest switching time S..

One of the advantages of the present invention is the greater overall brightness efficiency of the display device compared with the known devices. In the known display devices the information for each colour sub-frame is written on the display with no illumination, and the completed sub-frame is then illuminated briefly for a time t_η-. The brightness efficiency tι of such an arrangement is given by t._j /(sub-frame time), which is much smaller than that of the present invention.

Hence, the present invention enables relatively bright high-resolution images to be achieved and fabrication of complex arrays of colour filters is avoided.

In the embodiments described above, three sub-frames of, say, 6.6ms each must be written in a period of, for example, 20ms. While this can be readily achieved, some cost penalty may be involved, particularly in respect of display applications where high resolution is not required. Under such circumstances it can be advantageous to modify the display addressing sequence in two ways, namely 1) the rows can be interlaced, thereby reducing the number of rows to be addressed during a sub-frame period, and 2) the columns can be interlaced, thereby reducing the required data rate of the column drivers. Examples of these interlaced schemes are shown in Tables 2,3 and 4 below. Table 2

i.e. the colours are in horizontal stripes.

This scheme increases the row address time by a factor of 3.

Table 3

This scheme increases the row address time by a factor of 1.5.

Table 4

Interlacing of. Red and Blue Fields and Col umns

Field Field Field 1 Field 1 Field 2 Field 2 Field Field

In thi s scheme the data rate is reduced by a factor of 2, and the row address l ine is increased by a factor of 1.5. The use of a display device comprising a ferroelectric LC dot matrix image display and a sequential colour shutter (preferably also based on a ferroelectric LC device) as described has the advantages of producing, at relatively low cost, a compact, high -resolution, high-brightness colour display, such as required in, for example high-definition television. Furthermore, it could significantly reduce the cost of low-resolution colour ferroelectric LCDs. More especially, the display device of the present invention is more compact than an equivalent CRT-based system and could have higher resolution than display systems based on the above-mentioned RGB sub-pixel arrangement. Interlacing of the columns and/or the rows in the present invention can provide displays with resolutions equivalent to or greater than those achieved in displays with RGB sub-pixels, but without the need for relatively short row address times and high column data rates.

Although in the embodiment described above with reference to Figure 1 the colour filter system is disposed between the light source 5 and the display 3, alternatively the colour filter system could be disposed between the display and the viewer.

Claims

1. A display device characterised by a matrix (3) of rows and columns of ferroelectric liquid crystal elements; a light source (5); and a sequential colour filter system (7-17) disposed between the light source and the matrix or between the matrix and the viewer; wherein rows of the elements are addressed in sequence; and wherein each row in succession is set to a non-transmissive state at a constant, predetermined time after it has been addressed, and for a constant predetermined period.

2. A device as claimed in Claim 1, characterised in that the colour filter system (7-17) is divided into at least two independently-switchable areas which are aligned with respective groups of said rows of elements (3).

3. A device as claimed in Claim 2, characterised in that the independently-switchable areas of the colour filter system (7-17) are switched while the respective groups of rows of elements (3) are set to the non-transmissive state.

4. A device as claimed in any preceding claim, characterised in that the sequential colour filter system comprises a plurality of polarisers (7,11,15,17) which selectively transmit red, green and/or blue light of predetermined polarisation directions; and a plurality of shutter devices (9,13) each of which is switchable to either of two states in one of which the shutter device rotates the polarisation of light transmitted therethrough and in the other of which it transmits light therethrough substantially without altering the polarisation of .the light.

5. A device as claimed in Claim 4, characterised in that one of said polarisers (7) transmits vertically-polarised light of two of said colours and horizontally-polarised light of the other of said colours.

6. A device as claimed in Claim 4 or Claim 5, characterised in that one of said polarisers (11) transmits horizontally-polarised light of two of said colours and vertically-polarised light of the other of said colours.

7. A device as claimed in any one of Claim 4, 5 or 6, characterised in that at least one of said polarisers transmits white light which is polarised vertically or horizontally.

8. A device as claimed in any one of Claims 4-7, characterised by a further polariser (17) disposed on the side of the ferroelectric liquid crystal cell matrix (3) remote from the light source (5).

9. A device as claimed in any one of Claims 4-8, characterised in that the shutter devices (9,13) are ferroelectric optical shutters.

10. A device as claimed in any preceding claim, characterised in that the matrix (3) of rows and columns of elements is a waveform-multiplexed ferroelectric liquid crystal display.

11. A device as claimed in any one of Claims 1-9, characterised in that the matrix (3) of rows and columns of elements is an active matrix addressed ferroelectric liquid crystal display.