WO2009040758A2

WO2009040758A2 - Spectrum-sequential display

Info

Publication number: WO2009040758A2
Application number: PCT/IB2008/053909
Authority: WO
Inventors: Erno H. A. Langendijk
Original assignee: Koninklijke Philips Electronics N.V.; Nxp Bv
Priority date: 2007-09-26
Filing date: 2008-09-25
Publication date: 2009-04-02
Also published as: WO2009040758A3

Abstract

A spectrum-sequential display comprises means (1, 2) for sequentially producing a number of backlight color fields (Y, B; W, B; R, C; M, G) and groups of addressable light transmission filter elements (RGW, MCW; RCW, MGW, WGB, RWB) for producing color outputs by multiplication of backlight color field and light transmission filter characteristics, in which at least one of the groups of addressable light transmission filter elements comprises a white transmission filter (W).

Description

SPECTRUM-SEQUENTIAL DISPLAY

The invention relates to a spectrum-sequential display comprising means for sequentially producing a number of backlight color fields and groups of addressable light transmission filter elements for producing color outputs by multiplication of backlight color field and light transmission filter characteristics.

FIELD OF THE INVENTION

Currently, most conventional LCD displays have a (continuous) white backlight and an LCD panel with addressable elements with red, green, and blue color transmission filters. Such a display uses 3 subpixels to make an image pixel and absorbs two- third of the light in the color filters.

Color sequential displays flash the backlight sequentially to red, green and blue and consequentially do not need color filters to make a color image, because they can flash the backlight sequentially to red, green and blue. Hence, in theory they can be three times as efficient as a conventional LCD display and can display the same resolution with only one-third of the number of (sub)-pixels. However, these displays also have some drawbacks. In order to avoid flicker they need to run at refresh rates as high as 180 Hz. Yet, at this frequency the observer will still see an annoying color breakup (color flash) when moving his eyes across the screen. Extremely high refresh rates of well above 600 Hz are needed to avoid this color breakup. With today's direct view LCDs, such refresh rates are not possible, due to the slow response of the LC molecules. In fact, even a refresh rate of 180 Hz is quite difficult to achieve with current LCDs; the pixel response, especially from black to white, is still too slow and the high update frequency generates high current to each subpixel, which requires a large TFT in each subpixel. These effects will limit the aperture of the subpixel and counteract the brightness advantage of a color sequential display.

So-called spectrum sequential displays, also referred to as a hybrid spatio- temporal color displays, combine the conventional LCDs and color sequential LCDs. Such a display conventionally has addressable elements with two (broadband) color filters (e.g. magenta and cyan) and two types of backlight color fields (e.g. cyan and yellow) although other combinations of color fields and color filters can be used such as for instance:

(1) magenta and cyan color filters with yellow and blue color fields, and

(2) magenta and green color filters with yellow and cyan color fields.

BACKGROUND OF THE INVENTION

A spectrum-sequential display is disclosed in the article Hybrid spatial- temporal color synthesis and its applications by Louis D. Silverstein et al, the Journal of SID, 14/1, 2006, pages 3 to 13. To provide the color fields usually a number of background spectral sources are provided, sequential color fields are conventionally produced by producing backlight of a particular color, i.e. having a particular spectrum, by using one or several of the background spectral sources. In such designs there are effectively three primary colors: red, green, and blue, provided by the combination of the emission spectra of the color fields and the transmission spectra of the filter elements.

SUMMARY OF THE INVENTION

Conventional Spectrum sequential displays have relatively broad color filters, removing some of the visible colors (corresponding to some of the backlight spectrum frequencies), and transmitting e.g. approximately 2/3 of the visible range. Magenta color filters transmit blue and red light, cyan color filters transmit blue and green.

Since it is desirable to have -especially for wide gamut displays- relatively narrow final primary outputs, the spectra of the backlight illuminations presented in sequence should ideally partially overlap with transmission ranges and partially overlap with blocking ranges of the broadband display (e.g. LCD) filters. There is an ever growing need to provide improved variants spectrum sequential displays, with respect to picture quality, brightness, and choice of filters. The presently used filters are often non-standard filters when compared to filters used in regular LCD devices, which increases costs.

To meet one or more of the above stated desires and/or needs in the spectrum- sequential display according to the invention at least one of the groups of addressable light transmission filter elements comprises a white transmission filter.

The inventors realized that near neutral/ hardly colored/ largely transmitting, or in other words white filters provide interesting properties. It provides an increase in brightness, or a decrease in energy requirements at the same brightness.

In preferred embodiments it enables the use of red color filters, thereby enabling a reduction in costs. It also enables an improvement in resolution.

The use of white filters seems to be very counter- intuitive, as the skilled person is always inclined to look for colored filters, since the very basis of the design of a conventional spectrum sequential display is to provide colors by a combination of colored backlights and colored filters. One can also use multiprimary mathematics to drive the display (backlights and addressable elements) optimally, to reproduce the colored pictures.

E.g., when using a first yellow color field and a second blue color field, one can tailor the blue of the second field to produce (part of) the color luminance as in conventional RGB displays, but also e.g. to produce a neutral white addition as a complementary to (compensator of) the yellow output.

Preferably the white transmission filter has a transmission curve having an average transmission over the visible range of at least 60%, more preferably of more than 80%.

A white transmission filter means, within the framework of the invention a filter which does not substantially influence the color point of white light shining through the white transmission filter.

In preferred embodiments the device comprises three groups of addressable light transmission filter elements having red, green and white filters.

This allows the use of standard color filters thereby reducing costs. In other preferred embodiments the device comprises three groups of addressable light transmission filter elements having magenta, cyan and white color filters.

The advantage is in increase in blue light output, since all filters transmit blue, which enables an overall increase in light output.

In embodiments of the invention one can most advantageously make use a naturally rather bright first backlight spectrum, e.g. yellow, and a rather dark second one, e.g. blue, and profit from an increase in addressable resolution and/or perceived sharpness, and also good temporal sharpness. One may design the spectra so that there is a second bright output primary, e.g. by selecting a second modus in the brighter of the spectra near the maximum of the psychovisual luminance curve, e.g. in the greenish region, other bright resulting primaries allowing further resolution increase.

BRIEF DESCRIPTION OF THE DRAWINGS The above object and desirable features of the present invention will be more apparent from the following description of the preferred embodiments with reference to the drawings, wherein:

Figs.l and 2 illustrate a conventional spectrum-sequential display; Figs. 3 and 4 illustrate embodiments of a spectrum-sequential display according to the invention;

Fig.5 illustrates the CIE color chart

Fig. 6 illustrates a number of alternative embodiments of color filter schemes for a spectrum-sequential display in accordance with the invention.

Figs. 7 and 8 illustrate arrangements of color filters for a spectrum-sequential display in accordance with the invention.

Fig. 9 illustrates a conversion and subpixel scaling of an RGB input. Fig. 10 schematically illustrates a display device with means for conversion of an incoming signal.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The figures are not drawn to scale. Generally, identical components are denoted by the same reference numerals in the figures.

Fig. 1 illustrates a conventional spectrum-sequential display. The display device comprises a means for generating color fields. In this example the display comprises for instance yellow (Y) and blue (B) phosphorescent elements 1 to produce sequentially a yellow (Y) and blue (B) backlight color field 3 a, 3b. In another embodiment the backlight sources are formed by Red, Green and Blue (R, G, B) LED-lights 2.

The color fields, in Figure 1 schematically indicated by 3 a and 3b are transmitted through an LCD panel 4 comprising switchable LCD elements provided with magenta (M) and Cyan (C) color filters 5. The magenta filter (M) transmits a blue and a red spectral part of a backlight color field, the cyan filter (C) transmits a blue and a green spectral part. Thus, during the yellow color field 3a, the output 6 is composed of subpixels in red (R) transmitted by the magenta color filter and green (G) transmitted by the cyan color filters. During the blue field 3b, the output 7 is composed of subpixels in bleu transmitted by both the magenta and cyan filters.

Fig. 2 illustrates the scheme for a conventional display in a more schematic manner, the top rows gives the spectral distribution of the two color fields. In this example the color fields are a yellow color field, made by mixing the light of for instance red and green LED light elements, and a blue color field, using blue light sources. The right most column illustrates the spectral transmission characteristic of the used filters. The horizontal axis denotes the wavelength in nm. The vertical axis gives the transmission. The magenta filter filters out a central green part while transmitting a red and blue part of the visible spectrum, a cyan color filter filters out a red part of the visible spectrum. The combination of the yellow color field and the magenta color filter provides for a red output, the combination of a cyan color filter and the yellow color field provides for a green output, the blue color field is transmitted by both the magenta and cyan color filter.

The lowest row schematically indicates at the left most part an arrangement of color filters, in this case a simple side-by-side positioning of magenta (M) and cyan (C) color filters, the middle and right most part illustrate the colors that are transmitted during the yellow color field 1 and the bleu color field 2, being red (R) and green (G) respectively blue (B) and blue (B).

The conventional two-color filter spectrum-sequential color displays provide a significant reduction in color breakup compared to the full color sequential displays. A number of problems arise however. A major problem for fast introduction of this technology is the fact that color filters other than red, greed, and blue are not yet very good for display applications. For example, current magenta color filters have a rather low transmission in blue (typically half of that of a blue color filter). These alternative color filters (yellow, magenta, cyan) also have more scattering of the light than the conventional red, green, and blue color filters, which decreases the panel contrast. Another problem is the light output.

Being able to use conventional red, green, and/or blue color filters, but still having a brightness and resolution would provide a remarkable cost advantage.

Preferably the perceived resolution is higher than a conventional RGB display having the same number of subpixels per unit area.

Thirdly it would be useful to design a display that allows matching better with a natural image e.g. the maximum luminance factor of reflective objects of particular colors. Preferably the display should have a yellow output that is quite close in luminance to display white (e.g. 90% of display white) and a (wide gamut) red, green, and blue primary that can have a much lower luminance, e.g. a luminance of 12%, 31%, and 7% of display white, respectively.

Figures 3 and 4 illustrate two embodiments of a spectrum-sequential display according to the invention. A scheme similar to the one illustrated in Figure 2 is used to illustrate the invention in Figure 3.

The color fields 1 and 2 in this example are yellow and blue, two colored filter elements are used, for instance red and green color filters (fig. 3) or magenta and cyan color filters (fig. 4). The essential feature of the invention is the additional use of a third filter, namely a white filter (W). Basically, a white filter is a filter that transmits substantially the same amount light in all parts of the visible spectrum or at least in the parts of the visible spectrum relevant for normal color perception; such a filter can be for instance a simple glass plate. White does not necessarily mean that the transmission is 100% over all the visible range merely that the color point of white light transmitted through the filter is not substantially changed. A high overall transmission is preferred.

Use of white filters seems to be very counter- intuitive, as the skilled person is always inclined to look for colored filters, since the very basis of the design of a conventional spectrum sequential display is to provide colors by a combination of colored backlights and colored filters. A simple glass plate, possibly with anti-reflection coating, constitutes an example of a white filter.

However, using a white color filter allows for instance to use (fig. 3) a color display with a red, green, and white color filter and a yellow and blue color field. The red and green color filters can be conventional color filters enabling a strong reduction in costs; the white color filter is greatly transparent (it absorbs light in the visible spectrum as little as possible) and is therefore also standard/easy to make. The overall transmission is greatly increased.

The yellow field can be made with standard red and green LEDs, with, for example, a peak intensity at 628 nm and 520 nm, respectively. The blue field can be made with a blue LED, with, for example, a peak intensity at 466 nm.

Using a white color filter has a number of advantages.

First, the transmission of the "white" subpixel (and thus the pixel) gets higher, because the white subpixel passes most of the light (whereas a colored transmission filter passes only part of the light, thus reducing light output). Second, the resolution of the display gets higher. This is because one can use subpixel sampling to take advantage of the fact that there are two bright subpixels per pixel; in this example the green one and the white one. These aspects form advantages common for this and all following examples. Furthermore, the color breakup is much less than with other spatio-temporal color displays because the eye is much less sensitive for temporal modulations between yellow and blue than for red-green ones.

In Figure 3 the bottom row schematically shows an arrangement of color filters, in this example a simple side-by-side arrangement of red, green and white color filters; the middle and right most part of the bottom row illustrate the colors that are transmitted during the yellow field 1 and the bleu color field 2, being red (R), green (G) and yellow (Y) respectively nothing, a little bit of blue (b), and blue (B).

One can also make further use of the possibilities the invention offers for instance as follows:

In a conventional display the luminance ratio between the R:G:B primaries is about 20:70:10. With today's LEDs (Luxeon III, Philips LumiLEDs) this typically means that you need a ratio of LED's of R:G:B is 2:2:1. Hence, twice as many red, and green LEDs compared to blue ones are used. In order to have a good display white in our invention, we need to double the blue light, because red and green pass through the red and green subpixel and through the white one, whereas the blue light only passes through the white subpixel (and only a little through the green one). For today's LEDs that is actually an advantage because now we have again a ratio of R:G:B is 1 :1 :1.

In a good display design with wide gamut primaries the ratio between red, green, blue, yellow, and white is about 12:31:7:91 :100 (See Ref.4.). In a conventional wide gamut display this is typically 26:68:6:94:100 (R+G=Y, R+G+B=W), hence far too much green, which is never used. In the design of Figure 3 the ratio is about 13:34:6:94:100 (R+G+"R+G"=94, R+G+"RG"+B=W). Hence, a very good match with the wide gamut display design rule is possible.

The yellow field preferably has a spectrum that contains energy in the pass band of the red and green color filter. Typically in the 510-560 nm range for green, and the 590-640 nm range for red. In that case the yellow light through the red and green color filter gives enough luminance for the red and green primary, respectively.

Using white filters thus allows a number of advantages, an increase of light output, the possibility of using conventional red and green color filters and in fact making more efficient use of the LED's. Figure 4 illustrates a further example of a spectrum-sequential display according to the invention.

This embodiment comprises, as the embodiment of Figure 3, a white (W) filter. The red and green color filters have been replaced by magenta (M) and cyan

(C) color filters. The disadvantage compared to the embodiment of Figure 3 is that more costly filters are used, since the conventional Red and Green filters are replaced by Magenta and Cyan. However, the overall light output increases, since more blue light is transmitted, since now all the filters transmit blue light. The increase in light output could also be used to change the proportion of different LED's used in the display. Thus a greater freedom in design is offered.

Figure 5 illustrates the CIE X-Y color diagram. A white color filter is a color filter that does not substantially change the X-Y color coordinates of white light transmitted through the filter. The color point of white light transmitted through the white filter falls within the central area around the centre of the CIE color triangle between the dotted lines. This area of the CIE X-Y color diagram is commonly denoted as "white". The dot indicates roughly the centre position of the "white" area. The white area W is located near and around the centre of the color triangle (indicated by the triangle in the figure) for conventional RGB display devices. Preferably the white transmission filter has a transmission curve having an average transmission over the visible range of at least 60%, more preferably of more than 80%. The other areas within the diagram schematically indicate other parts representing colors, for instance G for Green, R for Red Y for Yellow.

Figure 6 illustrates two more embodiments of a spectrum-sequential display according to the invention. In these examples yellow and blue color fields are used but different color filters are used namely red (R), cyan (C) and white (W) and magenta (M), green (G) and white (W). The primary colors produced being RGY for filed 1 and ObB, where 0 stands for no transmission, b for a little bit of blue and B for Blue, are also indicated.

Yet another possible embodiment is using WGB (White-Green-Blue) color filters and a cyan and red color field. Yet another possible embodiment is using RWB (Red- White-Blue) color filters and using a magenta and green field. The latter two embodiments, however, show a relatively considerable color breakup visibility.

In preferred embodiments a yellow and a blue color field are used. However, in other preferred embodiments a white and a blue field are used. Using a white and blue field has two advantages. First, a temporal white-blue modulation has even less color breakup that a yellow-blue one. Second, we do not need to install additional blue to make the same peak white as in a conventional display. A disadvantage is the fact that saturated yellow is now rather dark. Turning on the blue LED together with the red and green LED can make the white field. Another option is to use a phosphor converted white LED (blue LED + yellow phosphor or blue LED + red and green phosphors) to make the white field. This has a cost advantage.

Table 1 below provides a number of possibilities for using a white color filter. Various combinations are possible. R stands fro Red, Green for Green, B for Blue, b for a little bit of transmission for blue, 0 for no or hardly any transmission, Y for yellow, M for Magenta, C for Cyan, W for White. The first column gives the three filters, wherein one of the filters is white, the column 'field 1 ' provides the color for one of the temporal fields, the column 'field 2' the color for the other temporal field.

Table 1 : examples of combinations of light transmission filters and backlight fields

filters field 1 field 2 yellow (Y) blue (B)

RGW RGY ObB

MCW RGY BBB

RCW RGY ObB

MGW RGY BbB

white (W) blue (B)

RGW RGW ObB

MCW MCW BBB

RCW RCW ObB

MGW MGW BbB

red (R) cyan (C)

WGB ROO CGB magenta (M) green (G)

RWB RMB OGO

In all embodiments the advantages of a higher light output, because of the use of the white filter and/or of a higher resolution, because white can be made in two different manners, are provided. The invention allows for various structural arrangements of color filter elements.

In a first embodiment the pixels are square, and the three subpixels, i.e. the color filters have a horizontal/vertical ratio of substantially 1 :3, just like in a conventional display). With subpixel sampling we can double the horizontal resolution in that case.

Figures 7 and 8 illustrate various further structural arrangements of color filter elements.

The pixels may be square and positioned in columns (see Figure 7, part A, the so-called striped arrangement), or seen along the vertical direction may be shifted over one or more subpixels (B, a delta-nabla arrangement), or may be shifted over a fractal of a sub-pixel, for instance 0.5 or 1.5 subpixels (C, another example of a delta-nable arrangement). In another embodiment the pixels are rectangular with an hv-ratio of 2:1, and thus the subpixels, i.e. the color filters, have an hv-ratio of substantially 2:3 (D). The advantage of the latter arrangement is that the perceived resolution is equal in both horizontal and vertical direction when using subpixel sampling.

Fig. 8 shows a few more examples of possible color filter arrangements. In these embodiments the filter elements differ in horizontal/vertical ratio. Such difference is preferably more than 25%. This allows further freedom of design.

The invention can be applied in any LCD, from small mobile displays up to large LCD-TVs. It can also be applied in other light valve displays, which consist of a backlight and a fast switching light valve (e.g. Electro Wetting displays, LCOS and LCD projection displays).

The method to drive such a display can be realized with software, e.g. a plug- in for a photo-display or video application. Figure 9 illustrate a method for conversion and scaling of an RGB input signal into subpixel signals for a display in accordance with the invention.

The input in RGB is converted (Conv) into an intermediate four primary color data (RGBY). Sub-pixel scaling is used to drive the RGBY panel.

If we assume that the input video is aligned with the green subpixel than the red and yellow subpixel have a horizontal offset of approximately 1/3 of a pixel with respect to the position of the pixels in the input video. In order to compensate for that and to have increased resolution advantage, preferably sub-pixel scaling is applied. The input is therefore first converter to RGBY if for instance yellow and blue fields and RGW filters are used, or for instance RGBW if a white and a blue field are used and RGW filters are used. The third subpixel can be either yellow or white (one field) or blue (other field). Table 1 provides for the various further combinations of filters and color fields the outputs and thus the conversion to be used. For instance, using a red and cyan filed and WGB filters the input is to be converted into RCGB, wherein the first subpixel is either R (first field) or cyan (second field).

Effectively in one of the embodiments an RGBY display (RGY in one field + B in the other), or an RGBW display (RGW in one field and B in the other) is provided. Below as examples two simple algorithms for a number of the multi-primary conversions are described.

Simple algorithms for RGB-to-RGBY conversion (for each pixel in image):

First implementation:

Y' = (R+G)/2; R' = R; G' = G; B' = B;

Alternative implementation:

Y' = min(R,G) R' = R-Y'; G' = R-Y'; B' = B;

Simple algorithm for RGB-to-RGBW conversion (for each pixel in image):

First implementation:

W' = (R+G+B)/3;

R' = R;

G' = G;

B' = B; Alternative implementation:

W = min(R,G,B) R' = R-W; C = R-W; B' = B-W;

Once the conversion for the pixels is completed subpixel scaling is performed. An example of such sub-pixel scaling is as follows:

Simple algorithm subpixel scaling:

Input (first row of pixels):

(ROGOBO) RlGlBl R2G2B2 R3G3B3 R4G4B4 ...

Multi-primary conversion (see above for each pixel):

(ROGOBOYO) RlGlBlYl R2G2B2Y2 R3G3B3Y3 R4G4B4Y4...

Subpixel scaling:

First row:

Rl ' = 1/3 *R0+2/3 *R1 , i.e. the value for red for subpixel 1 is 1/3 the value for pixel RO plus

2/3 the value for pixel Rl, since the red sub-pixel is offset 1/3 to pixel RO seen from the aligned green sub-pixel

Gl ' = Gl Since the green pixel is aligned with the input video

Yl ' = 2/3*Rl+l/3*R2 the yellow and blue pixel are at the same spot so have Bl ' = 2/3*Bl+l/3*B2 the same combination of values, they are offest 1/3 to pixel R2 from the aligned green pixel. Second row:

R2' = l/3*Rl+2/3*R2 G2' = G2

Y2' = 2/3*R2+l/3*R3 B2' = 2/3*62+1/3*63

The series continues as we go along the row and repeats itself for the next rows. Note that if the index of the input pixel is outside the range of the input video, such as ROGOBOYO and R(n+l)G(n+l)B(n+l)Y(n+l) where n is the number of input pixels in the first row and RlGlBlYl is the first input pixel of the row, we take the values of the nearest input pixel (RlGlBlYl and RnGnBnYn, respectively).

For improved scaling filtering might be required to prevent scaling artifacts, such as aliasing. Note also that this embodiment of subpixel sampling does a simple linear interpolation. More advanced upscaling techniques can be used to make better use of the higher resolution. Figure 10 schematically illustrates a display device comprising a means (conv) for conversion of an input RGB data signal into an intermediate four primary color signal (RGBY, RGBW) and a means for application of sub-pixel scaling (SC) to the intermediate four primary color signal to provide a converted and sub-scaled signal, the device comprising means for addressing AD the addressable filter elements (4) and/or the light sources (1, 2) in accordance with the converted and sub-scaled signal. This leads to an image on a display of the display device, schematically indicated in Figure 10 by the face.

Preferably one of the backlight sources has a spectrum with several peaks i.e. produces a high relative output power in several ranges of the visible and preferably at least some of those peaks are positioned near the maximum of the human visual luminance curve. The yellow and white fields have such characteristics.

The invention is also embodied in any computer program product for a method of conversion from input RGB values into an intermediate four primary color data followed by sub-pixel scaling to be used in a display in accordance with the invention.

Under computer program product should be understood any physical realization of a collection of commands enabling a processor -generic or special purpose-, after a series of loading steps (which may include intermediate conversion steps, like translation to an intermediate language, and a final processor language) to get the commands into a processor, to execute any of the characteristic functions of an invention. In particular, the computer program product may be realized as data on a carrier such as e.g. a disk or tape, data present in a memory, data traveling over a network connection -wired or wireless- , or program code on paper. Apart from program code, characteristic data required for the program may also be embodied as a computer program product.

The invention is also for some embodiments embodied in a video data signal comprising video data for two sequential fields, wherein the video signal comprises data for red, green and yellow or white subpixels in one of the fields and blue pixels in the other field.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses or even if not placed between parentheses shall not be construed as limiting the claim.

It will be clear that within the framework of the invention many variations are possible. It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Reference numerals in the claims do not limit their protective scope.

For instance, different embodiments of the invention may be used for different parts of the display, for instance using one embodiment for the center of the image, while using another for the edges of the display. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements other than those stated in the claims. Use of the article "a" or "a" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. A spectrum-sequential display comprising means (1, 2) for sequentially producing a number of backlight color fields (Y, B; W, B; R, C; M, G) and groups of addressable light transmission filter elements (RGW, MCW, RCW, MGW, WGB, RWB) for producing color outputs by multiplication of backlight color field and light transmission filter characteristics, in which at least one of the groups of addressable light transmission filter elements comprises a white transmission filter (W).

2. A spectrum-sequential display as in Claim 1, in which the white transmission filter (W) has a transmission curve has an average transmission over the visible range of at least 60%.

3. A spectrum-sequential display as in Claim 2, in which the white transmission filter (W) has an average transmission of at least 80%.

4. A spectrum-sequential display as in Claim 1, wherein at least on the other groups of addressable light transmission filter elements has a red (R) transmission filter.

5. A spectrum-sequential display as claimed in Claim 4, wherein the display comprises three groups of addressable light transmission filter elements having red, green and white (RGW) light transmission filters.

6. A spectrum-sequential display as claimed in Claim 1 wherein the display comprises three groups of addressable light transmission filter elements having magenta, cyan and white (MCW) transmission filters.

7. A spectrum sequential display as claimed in Claim 4, wherein the display comprises three groups of addressable light transmission filter elements having red, cyan and white (RCW) transmission filters.

8. A spectrum sequential display as claimed in Claim 1 wherein the display comprises three groups of addressable light transmission filter elements having magenta, green and white (MGW) transmission filters.

9. A spectrum sequential display as claimed in any of the preceding Claims wherein one of the color fields is blue (B).

10. A spectrum-sequential display as claimed in Claim 9, in which one of the other color fields is yellow (Y).

11. A spectrum-sequential display as claimed in any of Claim 9 in which one of the other color fields is white (W).

12. A spectrum -sequential display as claimed in any of the preceding Claims wherein the color fields are red and cyan and the display comprises three groups of addressable light transmission filter elements having white, green and blue (WGB) transmission filters.

13. A spectrum -sequential display as claimed in any of the preceding Claims wherein the color fields are magenta and green and the display comprises three groups of addressable light transmission filter elements having red, white and blue (RWB) transmission filters.

14. A spectrum-sequential display as claimed in any of the preceding Claims, wherein the light transmission filter elements have a horizontal/vertical ratio of substantially 1 :3.

15. A spectrum-sequential display as claimed in any of the preceding Claims, wherein the light transmission filter elements have a horizontal/vertical ratio of substantially 2:3.

16. A spectrum-sequential display as claimed in any of the preceding Claims, wherein the light transmission filters differ in horizontal/vertical ratio.

17. A spectrum-sequential display as claimed in any of the preceding Claims wherein means (1, 2) for sequentially producing a number of backlight color fields comprise a backlight source having a spectrum (Y, W) with several peaks i.e. producing a high relative output power in several ranges of the visible range.

18. A spectrum-sequential display as claimed in Claim 17, in which at least some of those peaks are positioned near the maximum of the human visual luminance curve.

19. Device as claimed in any of the preceding Claims comprising a means (conv) for conversion of an input RGB data signal into an intermediate four primary color signal

(RGBY, RGBW) and a means for application of sub-pixel scaling (SC) to the intermediate four primary color signal to provide a converted and sub-scaled signal, the device comprising means (4) for addressing the addressable filter elements in accordance with the converted and sub- scaled signal.

20. Method of conversion of a RGB input signal wherein the input signal is converted (conv) into an intermediate four primary color signal (RGBY, RGBW) and sub- pixel scaling (SC) is applied to the intermediate four primary color signal (RGBY, RGBW).

21. Method as claimed in Claim 20 wherein the four primary color signal comprises data for red, green, blue and yellow subpixels.

22. Method as claimed in Claim 20 wherein the four primary color signal comprises data for red, green, blue and white subpixels.

23. Video data signal comprising video data for two sequential fields, wherein the video signal comprises data for red, green and yellow subpixels in one of the fields and blue pixels in the other field.

24. Video data signal comprising video data for two sequential fields, wherein the video signal comprises data for red, green and white subpixels in one of the fields and blue pixels in the other field.

25. Computer program product for a method of conversion from input RGB values into an intermediate four primary color data followed by sub-pixel scaling to be used in or for a display in accordance with the invention.

26. Computer program product comprising program code means stored on a computer readable medium for performing an encoding method as claimed in any one of claims 20 to 23.