GB2320359A

GB2320359A - Display device with overlapping sub-pixels

Info

Publication number: GB2320359A
Application number: GB9726571A
Authority: GB
Inventors: Jonathan Rennie Hughes; John Clifford Jones
Original assignee: UK Secretary of State for Defence; Sharp Corp
Current assignee: UK Secretary of State for Defence; Sharp Corp
Priority date: 1996-12-16
Filing date: 1997-12-16
Publication date: 1998-06-17
Anticipated expiration: 2017-12-16
Also published as: JP4484972B2; GB9726571D0; JPH10187085A; GB9626103D0; GB2320359B

Abstract

A liquid crystal display device comprises a layer of ferroelectric liquid crystal material in the smectic phase contained between two glass substrates arranged parallel to one another. Oppositely facing electrode structures applied to the inwardly directed faces of the substrates are in the form of electrode tracks arranged in rows and columns and crossing one another to form an addressable array of pixels at the intersections of the electrode tracks addressable by the application of suitable strobe and data pulses. Each pixel comprises two or more sub-elements arranged in overlapping layers and having different analogue grey levels dependent on the voltage waveform applied to switch the sub-element so that, in addition to the black state B and the white state W, the sub-element has one or more intermediate grey states G. Each pixel may comprise two sub-elements arranged in overlapping layers such that the total grey level of the pixel is obtained by multiplying together the grey levels of the two sub-elements. This arrangement can be used to provide a non-linear grey-scale progression, for example a logarithmic progression of grey-scale. Each of the sub-elements may have different analogue grey levels. For example, considering the white state as corresponding to a unit grey level of 1, one of the sub-elements may have analogue grey levels of 1 and 10 -2 and the other sub-element may have analogue grey levels of 1 and 10 -1 , giving three orders of magnitude of total grey level 1, 10 -1 , 10 -2 and 10 -3 .

Description

"Displav/Shutter Devices" This invention relates to display/shutter devices, and is concerned more particularly, but not exclusively, with optical shutter devices including spatial light modulators.

Liquid crystal devices are commonly used for displaying alphanumeric information and/or graphic images. Furthermore liquid crystal devices are also used as optical shutters, for example in printers and as spatial light modulators. Such liquid crystal devices comprise an array of individually addressable display/shutter elements which can be designed to produce not only black and white, but also intermediate tones, or colour variations in devices in which colour filters are used. The so-called grey-scale response of such a device may be produced in a number of ways.

For example, the grey-scale response may be produced by modulating the transmission of each element between "on" and "off' states in dependence on the applied drive signal so as to provide different levels of analogue grey scale. In a twisted nematic device, for example, the transmission of each element may be determined by an applied RMS voltage and different shades of grey may be produced by suitable control of the voltage. In active matrix devices the voltage stored at the picture element similarly controls the grey shade. On the other hand, it is more difficult to control the transmission in an analogue fashion in a ferroelectric liquid crystal device, although various methods have been reported by which the transmission may be controlled by modulating the voltage signal in such a device. In devices having no analogue grey-scale, a grey-scale response may be produced by so-called spatial or temporal dither techniques, or such techniques may be used to augment the analogue grey-scale.

In a spatial dither (SD) technique each element is divided into two or more separately addressable sub-elements which are addressable by different combinations of switching signals in order to produce different overall levels of grey. For example, in the simple case of an element comprising two equal sized sub-elements each of which is switchable between a white state and a black state, three grey levels (including white and black) will be obtainable corresponding to both sub-elements being switched to the white state, both sub-elements being switched to the black state, and one sub-element being in the white state while the other sub-element is in the black state. If both subelements are of the same size, the same grey level will be obtained regardless of which of the sub-elements is in the white state and which is in the black state, so that the switching circuit must be designed to take account of this level of redundancy. It is also possible for the sub-elements to be of different sizes which will have the effect that different grey levels will be produced depending on which of the two sub-elements is in the white state and which is in the black state. However a limit to the number of subelements which can be provided in practice is imposed by the fact that separate conductive tracks are required for supplying the switching signals to the sub-elements and the number of such tracks which can be accommodated is limited by space constraints.

In a temporal dither (TD) technique at least part of each element is addressable by different time modulated signals in order to produce different overall levels of grey.

For example, in a simple case in which an element is addressable by two sub-frames of equal duration, the element may be arranged to be in the white state when it is addressed so as to be "on" in both sub-frames, and the element maybe arranged to be in the dark state when it is addressed so as to be "off' in both sub-frames. Furthermore the element may be in an intermediate grey state when it is addressed so as to be "on" in one subframe and "off' in the other sub-frame. The dither frequency should be much greater than the frequency at which the device is refreshed to provide normal switching of the element so that the resulting dither will not be observable as flickering. Furthermore it is possible to combine such a temporal dither technique with spatial dither by addressing one or more of the sub-elements in a spatial dither arrangement by different time modulated signals. This allows an increased range of grey levels to be produced at the cost of increased circuit complexity.

In many display and shutter applications, such as in an optical shutter of a printer, there is a requirement for a very high dynamic range of grey levels to be generated, with little or no redundancy of grey levels. It is known for the elements of an array to be binary weighted, for example by dividing each element into three sub-elements having surface areas in the ratio 4 : 2 : 1 in a spatial dither arrangement. In this case, assuming that each sub-element is separately switchable between a black state B corresponding to a unit grey level of0 and a white state W corresponding to a unit grey level of 1, and that the total grey level is given by adding together the grey levels of the three subelements with the appropriate binary weighting, 8 different grey levels without redundancy are obtainable by addressing of the three sub-elements concurrently as follows: 4 2 1 Total B B B 0 B B W B W B 2 B W W 3 W B B 4 W B W 5 W W B 6 W W W 7 For the case where one form of digital dither (either spatial or temporal) is used exclusively, the number of grey levels achieved for b bits of dither is 2b, where the optimum distribution dither weightings are 20:21:22 .2be' However it will be appreciated that the dynamic range of grey levels obtainable with such an arrangement is limited by the number of bits which can be provided.

EP 0453033A1 discloses a display device of this type, as well as a means of attempting to minimise the number of conductive tracks required for producing a maximum number of grey scales by providing an optimum relationship between the ratios of the surface areas of the column electrode tracks and the ratios of the surface areas of the row electrode tracks.

It is an object of the invention to provide a display/shutter device with means for enabling a wide range of grey levels to be produced, with the grey levels preferably being spaced apart or in a sequence of desired weightings.

According to the present invention there is provided a display/shutter device comprising a first addressable array of display/shutter sub-elements, a second addressable array of display/shutter sub-elements arranged in overlapping relationship with the first addressable array of display/shutter sub-elements, and addressing means for selectively addressing each sub-element of each array in order to vary the tone or colour level of the sub-element relative to the tone or colour levels of the other subelements of the array, wherein the addressing means includes analogue level selection means for switching the sub-elements of at least one of the arrays between two or more different tone or colour analogue levels in response to different analogue switching signals, whereby a plurality of different overall tone or colour levels corresponding to a combination of the tone or colour levels of the sub-elements of the arrays are obtainable by selection of different combinations of switching signals applied to the subelements.

It should be understood that the black and white states are to be considered as constituting tone or colour levels in the context of this specification.

Such an arrangement utilizing analogue grey levels (which term includes black and white) enables a large number of suitably weighted grey levels to be produced over a wide range of grey levels without giving rise to unacceptable complications.

Furthermore the weighting can be chosen so as to minimise any redundancy in the overall grey levels (tone or colour levels).

EXAMPLES In order that the invention may be more fully understood, a number of different arrangements for producing different grey levels in display/shutter devices in accordance with the invention will now be described by way of example only.

Each of the examples to be described uses a ferroelectric display or shutter device comprising a layer of ferroelectric material in the smectic phase contained between two glass substrates arranged parallel to one another, and oppositely facing electrode structures applied to the inwardly directed faces of the substrates in the form of electrode tracks arranged in rows and columns and crossing one another to form an addressable array of display/shutter elements. The electrode tracks may alternatively be arranged to form a polar coordinate (r, 6) array a seven bar numeric array or some other x-y array. As is well known, the display/shutter elements or pixels at the intersections of the row and column electrode tracks are addressable by the application of suitable strobe and data pulses to the row and column electrodes. One such addressing scheme is disclosed in "The Joers/Alvey Ferroelectric Multiplexing Scheme", Ferroelectrics 1991, Vol. 122, pages 63 to 79. Furthermore each pixel, or each sub-element of each pixel where the pixel comprises two or more sub-elements arranged in overlapping layers, has n different analogue grey levels dependent on the voltage waveform applied to switch the pixel or sub-element, so that, in addition to the black state B and the white state W referred to above, the pixel or sub-element has one or more intermediate grey states G.

Although the following description will be given with reference to an array of display/shutter elements in which each element comprises two or more sub-elements in overlapping relationship to one another so that the overlapping sub-elements are arranged in series in the optical path, it should be understood that the description is also applicable to an alternative arrangement in which a first array of display/shutter elements (which is either a liquid crystal array or some other form of spatial light modulating array) overlaps a second array in the form of a multiplexed light source, such as an array of light-emitting diodes or vertical cavity lasers, which is also adapted to provide a modulated light output.

In a first example, each pixel comprises two sub-elements arranged in overlapping layers in a multi-layer device such that the total grey level of the pixel is obtained by multiplying together the grey levels of the two sub-elements (as opposed to adding together the grey levels of the sub-elements as in the additive arrangement utilizing three sub-elements arranged side-by-side described above). This arrangement can be used to provide a non-linear grey-scale progression, for example a logarithmic progression of grey-scale. Each of the sub-elements may have different analogue grey levels, although the grey levels may differ for the two sub-elements. For example, considering the white state as corresponding to a unit grey level of 1, one of the subelements may have three analogue grey levels of 0, 104 and 10 '3 and the other subelement may have three analogue grey levels of 104, 10-2 and 1.

The following table shows the total grey levels which would be obtained in this case. The levels extend over 5 orders of magnitude, although it should be noted that this has redundancy (0 occurs three times) and that two sets of three analogue grey levels result in only 7 different total grey levels. Such an arrangement also suffers from the fact that it is difficult to provide the sub-elements with the required logarithmic analogue grey levels.

1 2 Total 0 10-4 0 10-4 10-4 10.8 03 1 0-4 l 0 10.2 0 10-4 10.2 106 0 3 10.2 lOs 0 1 0 10-4 1 104 1 1 10-3 However, in order to produce a full white state, that is maximum transmission, it is necessary to have a level of 1 in each sub-element. If a full black state, that is zero transmission, is not required and a realistic minimum analogue level of 1/100 of full transmission is assumed, the further example shown in the following table can be implemented using two grey levels per sub-element to provide three orders of magnitude of total grey level.

1 2 Total 10.2 10.1 10 1 10.1 10.1 10.2 1 10.2 1 1 1 Thus significant additional grey level range can be obtained by such multiplicative dither as compared with the additive dither previously described. If such an arrangement is extended to the case of three sub-elements in series with grey levels ofl and 10-1 for one sub-element, l and 10.2 for a second sub-element and 1 and 104 for a third subelement, 8 different total grey levels are obtainable extending over 7 orders of magnitude, as shown in the following table: 1 2 3 Total 1 1 1 1 10.1 1 1 10-' 1 10.2 1 10.2 10.1 10.2 1 1 1 04 1 10-4 10-5 1 10.2 10-4 10-6 10-1 10.2 10-4 This approach may be further extended to cover the case in which each of two sub-elements has three grey levels, for example grey levels of 1, 10.1 and 10.2 for one sub-element and grey levels of 1, 10-3 and 10-6 for another sub-element, as shown by the following table: 1 2 Total 1 1 1 10.1 1 10.1 10.2 1 10.2 1 3 10-3 10-3 'l 10*3 10 4 10.2 10-3 10-5 1 10-6 10-6 10.1 10*7 10.2 10-6 10-8 Such an example provides 9 total grey levels extending over 8 orders of magnitude. It should be noted that, in these examples of multiplicative dither, each subelement has a fully "on" state but that a fully "off' state is excluded as such a state would lead to redundancy.

The grey levels can be changed from base 10 to other values to give other progressions. For example the case in which the grey levels of one sub-element are 1, 10.09 (0.125) and 10.1.8 (0.016) is shown in the following table (note the ratios of the indices are the same as in the previous table): 1 2 Total 1 1 1 (1) 100.3 1 10-0.3 (0.5) 1 10.0.6 (0.25) 1 10.0.9 10.0.9 (0.125) 10-0.3 10' 9 10-1.2 (0.063) 10-0.6 10' '9 10 ' (0.031) 10-10-1,8 10-lS (0.015) 10-0.3 10-1.8 10.2.1 (0.008) 10.0.6 10.1.8 10-2.4 (0.004) Other interesting progressions are possible by combining different base grey levels for different sub-elements, for example grey levels of 1, 104-01(0. 1) and 10-02(0.01) for one sub-element and 1, 10-03(0.5) and 10-07 10-07(0.2) giving a 1: 2 : 5 sequence over 3 decades.

It should be particularly noted that the serial arrangement of sub-elements used in such multiplicative dither provides a wide range of grey levels and a variety of logarithmic progressions, although it is difficult to accurately generate the extremely small transmission levels required.

Many more examples can be given where the sub-elements have different numbers of analogue grey levels, and where, if necessary, each pixel comprises more than two sub-elements arranged in overlapping layers.

Furthermore further examples can be given where the sub-elements have different spatial extents or are staggered relative to one another so that a sub-element in one layer partially overlaps two or more sub-elements in an adjacent layer, or where the number of sub-elements in one layer is substantially greater than the number of subelements in an adjacent layer, for example because two or more sub-elements are provided in one layer in correspondence with each sub-element in the adjacent layer. In the latter case it is possible for each sub-element in one of the layers to be divided into two or more sub-pixels in a spatial dither arrangement having suitable bit weightings, or alternatively or additionally for the sub-element to be subjected to temporal dither with appropriate bit weightings. The addition of such temporal or spatial dither enables further additive grey levels to be obtained. In the case of spatial dither the sub-pixels of each sub-element may have size weightings in the ratio n0:nb . . .n1 where n denotes the number of analogue grey levels and b denotes the number of sub-pixels. In the case where spatial dither is applied to both layers of a two-layer arrangement, it is preferred that sub-pixels of like sizes are arranged to overlap. In the case of temporal dither, to which similar weightings may be applied, the various sub-frames are preferably totally synchronised in the two layers. It is not necessary that the same analogue levels are used in each sub-frame, sub-pixel or sub-element but the digital weightings between subelements should coincide.

For example it is possible for two sub-elements to be arranged in series with both having two bits of spatial dither (i.e. two sub-pixels) with weightings in the ratio 1:3 and with the following possible grey levels: digital weighting 1 2 Sub-element 1 0,0.1, 1 0, 0.2, 2 3 AG (0, 0.1,1) Sub-element 2 0, 0.1, 1 0, 0.2, 2 3 AG (0, 0.1, 1) totals (0, 0.01,0.1, 1) (0,0.04,0.4,4) This leads to the following 16 grey levels being achieved: 0, 0.01, 0.04, 0.05, 1, 1.04, 1.4, 4, 4.01, 4.1, 5 Better distributions are possible using different analogue levels in one or more sub-pixels and/or one or more sub-elements. Further matching of the digital dither weightings may also be used to control the distribution.

Claims

CLATMS

1. A display/shutter device comprising a first addressable array of display/shutter sub-elements, a second addressable array of display/shutter sub-elements arranged in overlapping relationship with the first addressable array of display/shutter sub-elements, and addressing means for selectively addressing each sub-element of each array in order to vary the tone or colour level of the sub-element relative to the tone or colour levels of the other sub-elements of the array, wherein the addressing means includes analogue level selection means for switching the sub-elements of at least one of the arrays between two or more different tone or colour analogue levels in response to different analogue switching signals, whereby a plurality of different overall tone or colour levels corresponding to a combination of the tone or colour levels of the sub-elements of the arrays are obtainable by selection of different combinations of switching signals applied to the sub-elements.

2. A display/shutter device according to claim 1, wherein each of the sub-elements of each array incorporates a tone or colour analogue level corresponding to a transmission value which is not equal to zero so that an overall tone or colour level is obtained having a transmission value which is not equal to zero when such tone or colour levels of the sub-elements are combined.

3. A display/shutter device according to claim 1 or 2, wherein the sub-elements are arranged such that a sub-element in one layer overlaps two or more sub-elements in an adjacent layer such that the tone or colour levels of the sub-elements in said adjacent layer are combined with the tone or colour level of the sub-element in said one layer to obtain the overall tone or colour level.

4. A display/shutter device according to claim 1, 2 or 3, wherein the first and second addressable arrays form a combined addressable array of elements each of which comprises a respective sub-element in each of the first and second arrays, and the addressing means is adapted to switch the sub-elements of each of the first and second arrays between two or more different tone or analogue levels.

5. A display/shutter device according to claim 4, wherein the sub-elements of each element are of substantially the same surface area and are arranged in overlapping relationship such that the tone or colour levels of the sub-elements are multiplied together to obtain the overall tone or colour level of the element.

6. A display shutter device according to claim 4 or 5, wherein each of the subelements of each element incorporates a tone or colour analogue level corresponding to substantially full transmission so that an overall tone or colour level corresponding to substantially full transmission ofthe element is obtained when such tone or colour levels of the sub-elements are combined.

7. A display/shutter device according to claim 4, 5 or 6, wherein each of b subelements of each element is switchable between n different tone or colour analogue levels, where b and n are integers greater than 1.

8. A display/shutter device according to claim 7, wherein each of the b subelements of each element is switchable between two tone or colour analogue levels.

9. A display/shutter device according to claim 7, wherein each of the sub-elements of each element is switchable between three tone or colour analogue levels.

10. A display/shutter device according to claim 4, 5 or 6, wherein at least one of the sub-elements of each element is switchable between n different tone or colour analogue levels, and at least one further sub-element of each element is switchable between m different tone or colour analogue levels where n is an integer greater than 1 and m is an integer greater than n.

11. A display/shutter device substantially as hereinbefore described with reference to any of the foregoing examples.