WO2005004497A1

WO2005004497A1 - Colour projection systems with improved lifetime

Info

Publication number: WO2005004497A1
Application number: PCT/IB2004/051077
Authority: WO
Inventors: Wouter Oepts
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-07-04
Filing date: 2004-07-01
Publication date: 2005-01-13

Abstract

Colour projection systems are provided in which the light valve has an increased lifetime compared to prior art systems. Increasing the area of the light valve illuminated by blue light with respect to the areas illuminated by red and green light, reduces the intensity of incident blue light an therefor increases the lifetime of the light valve. Image quality is maintained by ensuring that the total energy of the light beams of each colour incident on the light valve during an image frome is unchanged with respect to prior art systems.

Description

Colour projection systems with improved lifetime

TECHNICAL FIELD OF THE INVENTION This invention relates to colour projection systems, and in particular to scrolling colour projection systems using a liquid crystal on silicon panel.

BACKGROUND TO THE INVENTION In scrolling colour projection systems, light of different colours is sequentially scanned across a light valve to produce full colour images. Where the light valve is a liquid crystal on silicon (LCoS) panel, the polarisation of the incident light is changed as it is reflected from the panel- The reflected light is passed through a polarising beam splitter to produce the image to be displayed. In order to produce high quality bright images with high contrast, light having a high intensity is used in the system- It is also desirable to create a projection system that is reliable and has a long lifetime. It has been found in "Degradation of Liquid Crystal Alignment upon High Intensity Illumination in Microdisplays", Eurodisplay Digest '02 (2002), pages 201-204, by Wouter Oepts, Eri Ito, Mireille Reijme, Alwin Verschueren, Els Alexander and Cees van der Marel that the lifetime of liquid crystal microdisplays in projection applications is limited due to degradation upon high intensity illumination. The document "Liquid Crystal Alignment Degradation Induced by Photo- irradiation", SID '01 Digest, pages 980-983 (2001) by Hayley Bama, Minhua Lu, Mahesh Samant, Chen Cai and Alan Rosenbluth, discloses one partial solution to this problem, namely filtering UV light out of the incident light. There is a need to provide a colour projection system including a liquid crystal on silicon panel which has an increased lifetime with respect to known colour projection systems, in which the quality of the displayed image is not degraded- SUMMARY OF THE INVENTION It has been found that the lifetime of the LCoS panel is mainly limited by the intensity of the blue light component. Further, it has been found that, as the intensity of the blue light component decreases, the lifetime of the LCoS panel increases exponentially. Therefore, according to a first aspect of the present invention, there is provided a colour image display system for displaying a sequence of colour images, one colour image being generated each image frame, the system comprising: a light valve for modulating light illuminating a surface thereof with image information; an optical device comprising: splitting means for generating blue, red and green light beams from a light source, the splitting means generating at least one light beam of each colour, the blue, red and green light beams having first relative light powers; and stripe forming means for forming blue, red and green light stripes from the respective blue, red and green light beams, each light stripe illuminating a portion of the surface area of the light valve, the blue, red and green light stripes also having the first relative light powers; the optical device repeatedly sweeping the light stripes sequentially across the surface of the light valve, the light stripes sweeping across the whole of the surface of the light valve during an image frame; and a controller, for addressing the portions of the light valve with an image signal corresponding to the colour of the light stripe illuminating said portions so as to allow modulation of the light with the image information; wherein the total surface area of the light valve illuminated by the blue light is greater than the total surface area of the light valve illuminated by the red or green light- As the white light is separated into red, green and blue components in the same way as in a conventional scrolling colour system, increasing the area of the lightvalve illuminated by blue light - without reducing the amount of blue light incident upon the light valve during each image frame - results in a reduced blue light intensity, and hence an increase in the lifetime of the light valve. As the amount of blue light impinging upon the lightvalve during an image frame is unchanged with respect the amount of red or green light or the amount of blue light in conventional systems, the quality, colour balance or brightness of the generated image is not degraded- According to a second aspect of the present invention, there is provided a colour image display system for displaying a sequence of colour images, one colour image being generated each image frame, the system comprising: a light valve for modulating light illuminating a surface thereof with image information, the light valve having an array of rows of addressable pixels; an optical device comprising: splitting means for generating blue, red and green light beams from a light source, the splitting means generating at least one light beam of each colour, the blue, red and green light beams having first relative light powers; and stripe forming means for forming blue, red and green light stripes from the respective blue, red and green light beams, each light stripe illuminating a plurality of rows of addressable pixels, the blue, red and green light stripes also having the first relative light powers; the optical device repeatedly sweeping the light stripes sequentially across the rows of addressable pixels, the light stripes sweeping across each of the rows of addressable pixels during an image frame; and a controller, for addressing the rows of pixels with an image signal corresponding to the colour of the light stripe illuminating said rows so as to allow modulation of the light with the image information; wherein the length of time that each row of addressable pixels is illuminated by blue light is greater than the length of time that each row of addressable pixels is illuminated by the red or green light- According to a third aspect of the present invention, there is provided a colour image display system for displaying a sequence of colour images, one colour image being generated each image frame, the system comprising a light valve for modulating light illuminating a surface thereof with image information; an optical device for generating blue, red and green light beams sequentially from a light source, each light beam illuminating the surface area of the light valve for a portion of the image frame; the optical device illuminating the surface of the light valve with each of the coloured beams once per image frame; and a controller, for addressing the light valve with an image signal corresponding to the colour of the light beam illuminating the light valve so as to allow modulation of the light with the image information; wherein, during an image frame, the blue light illuminates the surface of the light valve for a greater length of time than either the red or green light.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 shows a scrolling colour projection system in accordance with a first embodiment of the present invention; Figure 2 shows an illumination pattern produced by the scrolling colour projection system according to the first embodiment of the present invention; Figures 3 and 4 show illumination patterns produced by a scrolling colour projection system according to a second embodiment of the present invention; Figure 5 is a graph illustrating the improvement in lifetime of a scrolling colour projection system in which the intensity of the blue light is halved; and Figure 6 shows a colour projection system with a rotating colour wheel according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a scrolling colour projection system in accordance with a first embodiment of the present invention. The scrolling colour projection system 2 comprises a light source 4, a light valve 6 with a controller 7 and an optical system 8- In this preferred embodiment, the light valve 6 is a liquid crystal on silicon (LCoS) panel, although it will be appreciated that the invention is also applicable to scrolling colour projection systems using other types of light valve whose elements are degraded by light having a particular colour. The optical system 8 has three main functions- The first function is to split the light from the light source 4 into separate coloured light beams, the second function is to form the separate light beams into light stripes of appropriate widths for illuminating parts of the surface of the light valve 6, and the third function is to cause the light stripes to be scrolled over the light valve 6. In this example, the light from the light source 4 is split into three component light beams and these three light beams will be referred to below for convenience as blue, red and green respectively- As each beam illuminates a part of the light valve 6, the light beams are modulated with image information. As a stripe falls off the edge of the light valve 6, it appears again at the opposite edge of the light valve 6. In fact, as a portion of a light stripe falls off of the edge of the light valve 6, it is discontinuously redirected to the opposite edge- The light valve 6 is addressed by controller 7 at three locations ahead of each of the coloured stripes. A particular row of pixels in the light valve 6 will be addressed with blue image information and then illuminated by the blue light stripe- When the blue light stripe has passed that row of pixels, the pixels will then be addressed with red image information. The row will then be illuminated by the red light stripe- Once the red light stripe has passed, the row of pixels will be addressed with green image information, and then illuminated with the green light stripe. Once the green light stripe has passed the row of pixels, the row is addressed with blue image information again and the cycle repeats. A single image frame may be produced by a single scanning cycle, or may be produced by two or more scanning cycles- In a conventional scrolling colour projection system, the stripes each cover an equal amount of surface area of the light valve 6 at a time- However, in accordance with the first aspect of the present invention, the incident blue light is distributed over a larger area of the light valve 6 than the red or green light. An exemplary optical system 8 in accordance with the invention is shown in Figure 1. It will be appreciated that this optical system is for illustrative purposes only, and that many other configurations of the optical system are possible that are capable of putting the invention into effect- Optical system 8 is based upon three rotating square prisms 10, 12 and 14, one for each of the blue, red and green light components respectively. Each of the prisms rotates to effect the scanning motion of the light stripes across the LCoS panel 6- As a corner of a prism crosses the light beam, discontinuous jumping of the light beam is caused. Therefore, a light stripe is scanned over the whole light valve by the rotation of the prism through 90°. The prisms all rotate at the same speed, however their phases relative to each other are different, so that their respective light stripes illuminate different parts of the light valve 6- The phases of the prisms are selected so that overlapping of the separate light stripes on the surface of the light valve 6 does not occur. The splitting function of the optical system 8 is performed, in this preferred embodiment, by a pair of dichroic mirrors 16a and 16b. The separate light components are created and directed towards their respective prisms by the dichroic mirrors 16a and 16b-

Dichroic mirrors 16c and 16d direct the separate light stripes to a polarising beam splitter 17 that reflects the light onto LCoS panel 6. Light is incident on the first dichroic mirror 16a which reflects the blue light component of the light and transmits the red and green components. The blue light component is then reflected from mirror 18 and passes through rotating prism 10. The red and green light components are then incident onto dichroic mirror 16b which transmits the green light component to the rotating prism 14. The red light component is reflected from dichroic mirror 16b through rotating prism 12- Dichroic mirror 16c transmits the blue light stripe from the rotating prism 10, and reflects the incident red light stripe from prism 12 to dichroic mirror 16d. The red and blue light stripes are reflected by dichroic mirror 16d towards the polarising beam splitter 17- The green light from the rotating prism 14 is transmitted by dichroic mirror 16d towards the polarising beam splitter 17. The blue, red and green light stripes are in a particular polarising state so the polarising beam splitter 17 reflects the light stripes onto the LCoS panel 6. As the stripes scan across the rows of pixels in the LCoS panel 6, the pixels, under the control of controller 7, alter the polarisation state of parts of the light stripes as they are reflected off the LCoS panel 6. The amount by which the polarisation is altered depends upon the specific colour to be displayed. The polarising beam splitter 17 then transmits the parts of the light stripes having a particular polarisation and reflects the other parts. The transmitted parts are displayed on a display surface by a projection lens (not shown). A polarising conversion system 22 may be provided that polarises the light from the light source 4. Lenses 19a, 19b, 20a, 20b and 20c are used to focus the light beams into appropriately sized stripes for scanning across the LCoS panel 6. In a conventional optical system, these lenses create stripes that are equal in size to each other. However, in accordance with the invention, lenses 19a and 19b are chosen so that they create a blue light stripe on the LCoS panel 6 that is wider (in the sense that a larger number of rows of pixels are illuminated at once) than the red and green light stripes created by lenses 20a, 20b and 20c. The red or green light stripes created by lenses 20a, 20b and 20c may have the same width as in a conventional system, or if there is insufficient space on the surface of the light valve 6, may be narrower than in a conventional system. It should be noted that the term 'light beams' used herein is used to refer to the separate light components of the light from the light source 4 before they have passed through the stripe- forming elements of the optical system 8 (i.e. lenses 19a, 19b, 20a, 20b and 20c in this illustrated embodiment), and the term 'light stripe' is used to refer to the separate light components after they have passed through the stripe-forming elements of the optical system 8. As in a conventional system, the light from the light source 4 is split by the dichroic mirrors 16a and 16b into the blue, red and green components, and therefore the power of the light source 4 is split between the coloured beams. The exact distribution of the light source power between the red, green and blue components depends upon the exact spectrum of the light source, and as light power is inversely proportional to the wavelength of the colour of the light, the powers of the three light beams are not equal to each other. Since light intensity is given by power/area, the blue stripe in a conventional system (i.e. a system having stripes of equal width) generally has a higher intensity than the red or green light stripes. As described above, in accordance with this embodiment of the invention, the width of the blue light stripe is increased relative to the widths of the red and green light stripes. The distribution of the power of the light source between the blue, red and green light components remains the same as in a conventional system, and therefore the blue light stripe has a lower intensity at the surface of the LCoS panel 6 than in a conventional system. It should be noted that the relative light power between the blue, red and green light beams is the same as the relative light power between the blue, red and green light stripes on the surface of the light valve. As a result of this, the colour balance of the displayed image is unaffected by widening the blue stripe in accordance with the invention. In other words, the same amount of blue light energy per image frame as used in a conventional system is distributed over a wider area of the LCoS panel 6 and therefore results in a reduced intensity. As the lifetime of the LCoS panel 6 has been found to be dependent upon the intensity of the incident blue light, reducing the intensity of the blue light increases the lifetime of the LCoS panel 6. In addition, as the amount of light energy of each colour illuminating the LCoS panel 6 during an image frame is the same as in a conventional system, the increase in system lifetime is achieved without degrading the quality, brightness and colour balance of a displayed image. The width of the blue light stripe may be increased by any amount, depending upon the desired increase in lifetime, provided the amount of light of each colour impinging upon the LCoS panel 6 during an image frame is the same as the amount of light of each colour impinging upon the LCoS panel 6 during an image frame in a conventional system. It should be noted that the width of the red and green light stripes may be reduced to allow for the increased width of the blue light stripe. If so, the intensity of the red and green light stripes will be increased relative to a conventional system. As the lifetime of the LCoS panel 6 is not dependent upon the intensity of the red and green light, this increase in intensity of the red or green light stripe does not significantly affect the lifetime of the LCoS panel 6. In a preferred embodiment, the red and green light stripes are of equal width. In alternative embodiments, the red and green stripes may have different widths- Such an arrangement would be implemented by varying the lenses 19b and 19c. As mentioned above, this optical system is purely exemplary, and many other optical systems capable of producing a blue light stripe having a larger width than the red or green light stripes will be readily apparent to a person skilled in the art. Figure 2 shows an illumination pattern produced by the scrolling colour projection system according to the first embodiment of the present invention. Three light stripes are shown on the surface of the light valve 6. The red and green light stripes have widths W_R and w_G respectively, and, in a preferred embodiment, w_R and W_G are equal. The blue light stripe has a width w₈ that is substantially greater than the widths of the red and green light stripes- The blue light stripe according to this embodiment of the invention may have a lower intensity than either the red or green stripes, provided the width of the blue light stripe is large enough relative to the widths of the red or green stripes- However, if the width of the blue light stripe is not sufficiently larger than the widths of the red and green light stripes, the intensity of the blue light stripe, whilst being lower than in a conventional system, may still be higher than the intensity of the red and green light stripes. Even in this case, an improvement in the lifetime of the LCoS panel 6 will be achieved- It should be noted that this illumination pattern is exemplary and that many different illumination patterns could be generated by the optical system described in Figure 1, depending upon the lenses 19a, 19b, 20a, 20b and 20c and the phases of the prisms 10, 12 and 14 relative to each other. For example, the order that the light beams scroll down the light valve may be different if the phases of the rotating prisms are adjusted, or the gaps (as indicated by the dark bands between the light beam stripes) may be different sizes to those shown, and may be different sizes to each other, if the lenses are varied. Figures 3 and 4 show illumination patterns produced by a scrolling colour projection system according to a second embodiment of the present invention. Four light stripes are shown on the surface of the light valve 6. The red and green light stripes have widths W_R and WG respectively, and, in this illustrated embodiment, W_R and w_G are equal- The two blue light stripes have a width WB that is equal to the widths W_R and W_G of the red and green light stripes. In this illustrated embodiment, as all of the light stripes have an equal width, the blue light is illuminating twice as much of the surface area of the light valve 6 as either the red or green light. This illumination pattern may be produced by an optical system similar to that described in Figure 1. However, to produce four light stripes, two of which being blue, the blue light component from the light source 4 can be split by a half-transmissive mirror. The two blue components can then be passed through separate rotating prisms to effect the scanning motion across the light valve 6. The lenses will be selected to focus the light beams into stripes of appropriate width. Thus, the system will have four rotating prisms, one for each of the red and green light components, and two for the separate blue light components. To provide the differing illumination patterns shown in Figures 3 and 4, the phases of the rotating prisms can be adjusted. Of course, it will be appreciated that illumination patterns having any number of blue stripes are possible in accordance with the invention, provided the combined surface area of the light valve that the blue light stripes illuminate is greater than the surface area of the light valve illuminated by either the red or green light stripes. Figure 5 is a graph illustrating the improvement in lifetime of a three-prism scrolling colour projection system illuminated with white light in which the intensity of the blue light is halved. The solid line on the graph shows how the lifetime of the LCoS panel varies with the intensity of the incident white light. The dotted line shows how the lifetime of the light valve varies with the intensity of the incident white light when the intensity of the blue light component is halved in accordance with the invention- As an example, consider the lifetime of the LCoS panel when the intensity of the incident white light is 10 Mlux- The solid curve gives an expected lifetime of 1338 hours. If the intensity of the white light were 5 Mlux, the expected lifetime would be 4595 hours- Therefore, as the lifetime is mainly affected by the intensity of the blue light component of the white light, halving the intensity of the blue light component from 10 Mlux to 5 Mlux by doubling the width of the blue light stripe increases the lifetime of the light valve to 4595/2=2297 hours (since the panel is being illuminated for twice as long by the lower intensity blue light). An improvement in the lifetime of the LCoS panel is therefore achieved, compared to the lifetime at 10 Mlux. It can be seen that the increase in the lifetime depends upon the initial intensity of the white light, and the lifetime can be approximately doubled when the intensity of the white light is 3 Mlux and the intensity of the blue light component is halved. As the amount of red, green and blue light illuminating the LCoS panel is unchanged with respect to prior art systems, the brightness and quality of displayed images are not impaired. It will be appreciated that the invention is also applicable to scrolling colour projection systems in which light beams having colours other than red, green and blue are used- For example, a system may use red, green, blue, cyan and yellow to generate the images. In this case, an optical system using the rotating prism concept will comprise five rotating prisms, one for each of the five coloured light beams- It will also be appreciated that one of the effects of increasing the width of the blue light stripe(s) relative to the widths of the red and green light stripes is to increase the amount of time that each row of addressable pixels in the light valve is illuminated by blue light during a single image frame- As the intensity of the blue light is lower (due to the increased area of the light valve being illuminated by the blue light), the increased illumination time means that the amount of blue light illuminating the light valve during an image frame is unchanged with respect to prior art systems- It will also be appreciated that the principle of the invention is applicable to other types of projection systems in which coloured stripes are scrolled sequentially across the LCoS panel. One such system is the rotating colour wheel system described in International application WO 02/096122 to Koninklijke Philips Electronics, where a 'spiralling' colour wheel is disclosed- Here, the colour wheel comprises a number of colour filters, with each colour filter occupying a spiral-shaped portion of the colour wheel. Light illuminates the different colour filters which produce the different coloured stripes, and these stripes are scrolled across the LCoS panel by the rotation of the colour wheel. In this type of system, the invention may be implemented by increasing the effective width of the blue spiral section- This may be achieved by, for example, creating the colour wheel with four spirals, two of which being blue, or may be achieved by increasing the width of the existing blue spiral, and reducing the widths of the red and green spirals. In order to ensure that the colour balance of the image is unchanged compared to conventional systems, the amount of blue light must be reduced, and the amount of red and green light correspondingly increased. This may be achieved by altering the dichroic filters of the colour wheel. In other types of colour projection systems having a LCoS panel, it may not be possible to adjust the width of a blue stripe to reduce the blue light intensity. In these systems, dichroic filters may be used to reduce the intensity of the incoming blue light. However, to ensure that the amount of blue light used to generate each image remains unchanged compared to a conventional system, the length of time that the blue light illuminates the LCoS panel must be increased. Such a system may be, for example, a system using a colour wheel that has red, green and blue segments, which rotates through 360° each image frame to sequentially illuminate a light valve with red, green and then blue light. A colour wheel projection system according to this aspect of the invention is illustrated in Figure 6. The colour wheel projection system comprises a light source 50, integrating rod 51, colour wheel 52, transmissive light valve 54, and projection lens arrangement 56. This system provides a colour image 58 on a projection screen 60- Light from the light source 50 passes through integrating rod 51 and illuminates part of the colour wheel 52. The colour wheel comprises several coloured segments, 52a, 52b and 52c, which, in this illustrated embodiment, are green, red and blue respectively. Light of the appropriate colour passes through the illuminated segment of the colour wheel 52 and onto the light valve 54, which modulates the coloured light with appropriate image information. The remaining light is reflected by the segment back towards integrating rod 51. This light is reflected in integrating rod 51 back towards the colour wheel 52. This means that the light reflected by the colour wheel 52 is recycled in the system- As the colour wheel 52 rotates, the colour of the light illuminating the light valve sequentially changes- Where the transmissive light valve 54 adjusts the polarisation of the incident coloured light, the system further comprises a polariser in order to produce the image 58 for display on the projection screen 60. In accordance with the invention, the intensity of the blue light component is reduced, while maintaining the amount of each colour illuminating the light valve with respect to conventional systems. Reducing the transmission of the blue segment 52c of the colour wheel 52 reduces the amount of blue light illuminating the light valve 54, and hence reduces the intensity of the blue light- In order to maintain the amount of blue light illuminating the light valve 54 during an image frame, the length of time that blue light illuminates the light valve 54 must be increased. This is achieved by increasing the size of the blue segment 52c. In this illustrated embodiment, the blue segment 52c is twice the size of each of the green and red segments, 52a and 52b- To maintain the amount of red and green light illuminating the light valve 54 during an image frame, the transmission of the green and red segments, 52a and 52b, are increased, allowing more green and red light through, and increasing the intensities of the red and green light components- Therefore, as the intensity of the blue light is reduced, the lifetime of the light valve 54 is increased with respect to conventional systems having a light source 50 of equivalent power, without degrading the colour balance of the displayed image. There are therefore provided colour projection systems having liquid crystal on silicon panels, which have an increased lifetime with respect to known colour projection systems- It should be noted that the term "comprises" or "comprising", as used herein, means that the stated features or elements are present, but does not exclude the possibility that additional features or elements may also be present- Similarly, the word "a" or "an" does not exclude the possibility that a plurality of the stated features may be present.

Claims

CLAIMS:

1. A colour image display system for displaying a sequence of colour images, one colour image being generated each image frame, the system comprising: a light valve for modulating light illuminating a surface thereof with image information; an optical device comprising: splitting means for generating blue, red and green light beams from a light source, the splitting means generating at least one light beam of each colour, the blue, red and green light beams having first relative light powers; and stripe forming means for forming blue, red and green light stripes from the respective blue, red and green light beams, each light stripe illuminating a portion of the surface area of the light valve, the blue, red and green light stripes also having the first relative light powers; the optical device repeatedly sweeping the light stripes sequentially across the surface of the light valve, the light stripes sweeping across the whole of the surface of the light valve during an image frame; and a controller, for addressing the portions of the light valve with an image signal corresponding to the colour of the light stripe illuminating said portions so as to allow modulation of the light with the image information; wherein the total surface area of the light valve illuminated by the blue light is greater than the total surface area of the light valve illuminated by the red or green light.

2. A colour image display system as claimed in claim 1, wherein, at the surface of the light valve, the intensity per unit area of the blue light is less than the intensity per unit area of the red or green light.

3. A colour image display system as claimed in claim 1 or 2, wherein the light valve comprises an array of rows of addressable pixels; the controller addresses the rows of pixels with the image signal coπesponding to the colour of the light stripe illuminating said rows; and wherein the length of time that each row of addressable pixels is illuminated by blue light is greater than the length of time that each row of addressable pixels is illuminated by the red or green light-

4. A colour image display system as claimed in claim 1, 2 or 3, wherein the splitting means is adapted to generate three light beams, and the stripe forming means is adapted to form three light stripes such that the blue light stripe illuminates a surface area of the light valve that is greater than the surface area of the light valve illuminated by the red or green light stripes.

5. A colour image display system as claimed in claim 4, wherein the surface area of the light valve illuminated by each of the red and green light stripes are equal.

6. A colour image display system as claimed in claim 1, 2 or 3, wherein the splitting means is adapted to generate four light beams, two of said light beams being blue and the other two light beams being red and green respectively.

7. A colour image display system as claimed in claim 6, wherein the stripe forming means is adapted to form four light stripes such that the four light stripes illuminate equal areas of the surface of the light valve-

8. A colour image display system as claimed in one of claims 1, 2 or 3, wherein the splitting means is further adapted to generate yellow and cyan light beams from the light source, the stripe forming means is further adapted to form yellow and cyan light stripes from the respective yellow and cyan light beams and wherein the total surface area of the light valve illuminated by the blue light is greater than the total surface area of the light valve illuminated by the red, green, yellow or cyan light.

9. A colour image display system as claimed in any preceding claim, wherein the light valve is a liquid crystal on silicon panel.

10. A colour image display system for displaying a sequence of colour images, one colour image being generated each image frame, the system comprising: a light valve for modulating light illuminating a surface thereof with image information, the light valve having an array of rows of addressable pixels; an optical device comprising: splitting means for generating blue, red and green light beams from a light source, the splitting means generating at least one light beam of each colour, the blue, red and green light beams having first relative light powers; and stripe forming means for forming blue, red and green light stripes from the respective blue, red and green light beams, each light stripe illuminating a plurality of rows of addressable pixels, the blue, red and green light stripes also having the first relative light powers; the optical device repeatedly sweeping the light stripes sequentially across the rows of addressable pixels, the light stripes sweeping across each of the rows of addressable pixels during an image frame; and a controller, for addressing the rows of pixels with an image signal corresponding to the colour of the light stripe illuminating said rows so as to allow modulation of the light with the image information; wherein the length of time that each row of addressable pixels is illuminated by blue light is greater than the length of time that each row of addressable pixels is illuminated by the red or green light.

11. A colour image display system as claimed in claim 10, wherein, at the surface of the light valve, the intensity per unit area of the blue light is less than the intensity per unit area of the red or green light.

12. A colour image display system as claimed in claim 10 or 11, wherein the total surface area of the light valve illuminated by the blue light is greater than the total surface area of the light valve illuminated by the red or green light.

13. A colour image display system as claimed in claim 10, 11 or 12, wherein the splitting means is adapted to generate three light beams, and the stripe forming means is adapted to form three light stripes such that the blue light stripe illuminates a surface area of the light valve that is greater than the surface area of the light valve illuminated by the red or green light stripes.

14. A colour image display system as claimed in claim 13, wherein the surface area of the light valve illuminated by each of the red and green light stripes are equal-

15. A colour image display system as claimed in claim 10, 11 or 12, wherein the splitting means is adapted to generate four light beams, two of said light beams being blue and the other two light beams being red and green respectively-

16- A colour image display system as claimed in claim 15, wherein the stripe forming means is adapted to form four light stripes such that the four light stripes illuminate equal areas of the surface of the light valve.

17. A colour image display system as claimed in one of claims 10, 11 or 12, wherein the splitting means is further adapted to generate yellow and cyan light beams from the light source, the stripe forming means is further adapted to form yellow and cyan light stripes from the respective yellow and cyan light beams and wherein the length of time that each row of addressable pixels is illuminated by the blue light is greater than the length of time that each row of addressable pixels is illuminated by the red, green, yellow or cyan light.

18. A colour image display system as claimed in one of claims 10 to 17, wherein the light valve is a liquid crystal on silicon panel.

19. A colour image display system for displaying a sequence of colour images, one colour image being generated each image frame, the system comprising: a light valve for modulating light illuminating a surface thereof with image information; an optical device for generating blue, red and green light beams sequentially from a light source, each light beam illuminating the surface area of the light valve for a portion of the image frame; the optical device illuminating the surface of the light valve with each of the coloured beams once per image frame; and a controller, for addressing the light valve with an image signal corresponding to the colour of the light beam illuminating said light valve so as to allow modulation of the light with the image information; wherein, during an image frame, the blue light illuminates the surface of the light valve for a greater length of time than either the red or green light.

20- A colour image display system as claimed in claim 19, wherein the optical device comprises a colour wheel having red, green and blue segments-

21- A colour image display system as claimed in claim 20, wherein the colour wheel comprises three segments and the blue segment is larger than either the red or green segments.

22. A colour image display system as claimed in claim 18, wherein the colour wheel comprises four segments, all segments having the same size, two segments being blue and the other two being red and green respectively.