WO2004105401A1

WO2004105401A1 - Display device with improved brightness level homogeniety

Info

Publication number: WO2004105401A1
Application number: PCT/IB2004/050686
Authority: WO
Inventors: Adrianus J. S. M. De Vaan
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-05-21
Filing date: 2004-05-13
Publication date: 2004-12-02

Abstract

A display device, such as a Liquid Crystal on Silicon (LcoS) projection device, is provided that generates a first image having at least one non-uniform brightness level and a second image, so that when the first and second image are superimposed on a display surface to form a composite image, the composite image has improved brightness level uniformity relative to the first image.

Description

Display device with improved brightness level homogeniety

TECHNICAL FIELD OF THE INVENTION

This invention relates to projection systems, and in particular to a method and apparatus for improving the homogeneity of the black level in projection systems based on

Liquid Crystal on Silicon (LCoS) technology.

BACKGROUND TO THE INVENTION

In a projection display system, an image from one or more source panels is projected onto a screen. In Liquid Crystal on Silicon (LCoS) display systems, the source panel is a reflective Liquid crystal display (LCD) on a silicon backplane. An LCoS display panel contains a thin layer of liquid crystal material between two substrates, divided up into a large number of pixels. The LCD modulates the intensity of the transmitted or reflected light by changing the polarisation of the light passing through the liquid crystal layer. The amount that the polarisation of the light changes depends upon the voltage applied to the liquid crystal layer. In projectors using reflective LCoS panels, a polarising beam splitter is positioned in front of the LCoS panel. This polarising beam splitter (PBS) selectively reflects light of one linear polarisation and transmits light having an opposite linear polarisation.

During operation of the LCoS display system, polarised light is guided by the polarising beam splitter onto the LCoS display panel. The voltage applied to the liquid crystal layer in a particular pixel is varied depending upon the colour required from that pixel.

For example, if a particular pixel is required to display 'white' on the screen,

(i.e. all of the incident light is reflected from the LCoS panel onto the screen) the applied voltage orients the liquid crystal layer in that pixel such that the polarisation of the reflected light is rotated by 90° after passing the liquid crystal layer. As the polarisation of the light has been changed by 90°, it can pass through the polarising beam splitter to the projection lens, and a white pixel is projected on the screen. If a pixel is required to display 'black' on the screen, (i.e. none of the incident light reflected from the LCoS panel must reach the screen) the applied voltage orients the liquid crystal layer so that the polarisation of the incident light is unaffected. Therefore, the light reflected back by the LCoS panel is guided away from the screen by the polarising beam splitter.

If an intermediate 'grey' colour is required, the liquid crystal layer can be oriented so that the polarisation of the light is rotated up to 90° so that some of the light is guided through the polarising beam splitter and onto the screen.

To obtain full colour images from an LCoS system, two different techniques can be used. In a single-panel LCoS system, red, green and blue light is sequentially scanned over the LCoS panel. If the scanning occurs at a sufficiently high rate, the sequence of coloured images projected onto the screen will be perceived as a full colour image.

Alternatively, the system may have three LCoS panels, one for each of the primary colours. To provide high quality images, a projection system should have a high contrast ratio, and this ratio must be uniform across the whole of the displayed image.

The contrast ratio is defined as the ratio of the brightest part of the displayed image to the darkest part.

The level of brightness or light present in the darkest or blackest portion of an image is known as the 'black level' .

Similarly, the 'white level' is the level of brightness or light present in the lightest or whitest portion of an image.

'Grey levels' are levels of brightness that lie between the 'white' and 'black' levels. In LCoS display systems, the uniformity of each of these levels, and hence the uniformity of the contrast ratio, over the projected image is degraded by a number of factors.

These factors include the quality of the materials used to manufacture the polarising beam splitter, the thermal gradient across the polarising beam splitter caused by the incident light, the angle of incidence of the light on the polarising beam splitter, the amount of mechanical stress applied to the polarising beam splitter, thickness homogeneity of the LCoS panel, the temperature profile over the LCoS panel and the non-telecentricity of the illumination beam.

This brightness level uniformity degradation results in lighter or darker areas of an image which otherwise should have a uniform brightness. For example, if an entirely uniform 'black' image were intended to be displayed (i.e. an image with as little light as possible reaching the screen), the brightness level inhomogeneities would result in lighter or darker shades of black in certain parts of the image, depending upon the particular characteristics of the projection system.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an LCoS projection system in which the contrast ratio is uniform over the whole of the projected image.

Therefore according to a first aspect of the present invention, there is provided a projection display device for displaying an image, comprising a light source that generates a light beam; first light modulating means changing the polarisation of incident light in accordance with image information for a first image, and outputting a first modulated light beam to a polarising beam splitter; the polarising beam splitter providing parts of said first modulated light beam having a first polarisation to a projection lens, said projection lens displaying the first image on a projection surface, the displayed first image having at least one non-uniform brightness level; the device further comprising second light modulating means for changing the polarisation of a portion of said light beam in accordance with image information for a second image, and outputting the second modulated light beam to said polarising beam splitter; wherein the polarising beam splitter provides parts of the second modulated light beam having a second polarisation to the projection lens, said projection lens superimposing said second image over said first image on the projection surface to provide a composite image, wherein the image information for the second image corrects for the at least one non-uniform brightness level of the first image, such that the composite image has improved brightness level uniformity relative to the first image.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an LCoS projection system according to a first aspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a projector device 2 displaying an image on a screen 4 via a projecting lens arrangement 6. Although the invention is described below with reference to a projector device and a screen, it will be appreciated that the invention is applicable to any projection systems that use LCoS technology, including front and rear projection televisions.

As is conventional, the projector device 2 comprises a light source 8, a light collection system 9, containing integrator optics and a polarisation conversion system, and a lens 10 for generating a uniform and polarised beam of white light.

Integrator optics and polarisation conversion systems are well known in the art and neither their structure nor function will be described further herein.

The beam of light is reflected from a mirror 12, through lens 14 and into a polarising beam splitter (PBS) 16.

The beam of white light is pre-polarised so that it will pass through the polarising beam splitter 16 with a minimum of reflections.

Thus, the light beam is transmitted through the PBS 16 and enters the Philips prism architecture 18. The Philips prism 18 splits the white light beam into red, green and blue components and outputs one of them from each of the three faces of the prism.

A Philips prism architecture, also known as a trichroic prism architecture, is well known in the art and neither its structure nor function will be described further herein. As is conventional, reflective liquid crystal displays (R-LCDs) 20, 22 and 24 are positioned at the output faces of the Philips prism 18 such that the red, green and blue components of the light from the Philips prism 18 are reflected back into the prism after the polarisation of the incident light is modified by the respective R-LCD. A R-LCD panel is provided for each of the primary colours, blue, green and red, which correspond to R-LCDs 20, 22 and 24 respectively.

Philips prism 18 then acts to recombine the blue, green and red components of the light, and the recombined modulated light beam is output from the Philips prism 18 back into the polarising beam splitter 16. The polarising beam splitter 16 transmits and reflects parts of the modulated light beam depending upon the polarisation state of the various light components.

The reflected parts of the modulated light beam are projected onto the screen 4 via the projecting lens arrangement 6 and form the image to be displayed.

The transmitted parts of the modulated light beam traverse back to the illumination system, where they are normally lost. As described above, to provide high quality images, the projection system should have a high contrast ratio, and this ratio should be uniform across the whole of the displayed image.

To provide this uniform contrast ratio, the 'white', 'black' and 'grey' levels should each be uniform.

Hereinafter, the term 'brightness level' refers to either the 'white' level, 'black' level or one of the 'grey' levels, and the term 'brightness levels' refers to two or more of the 'white' level, 'black' level and 'grey' levels.

Therefore, in accordance with the invention, an additional lightvalve 26 is provided to compensate for the inhomogeneities introduced by the projection system, and in particular by the polarising beam splitter 16 and the R-LCD panels 20, 22 and 24.

The lightvalve 26, which, in a preferred embodiment is a transmissive LCD panel, is positioned on the otherwise empty surface of the polarising beam splitter 16.

A semi-reflective mirror 28 reflects a fraction of the pre-polarised light beam through a lens 30 and through the lightvalve 26 via a mirror 32.

The percentage of light that is reflected by the semi-reflective mirror 28 is very low compared with the amount of light transmitted, but the reflected light is sufficient to correct the contrast distortion. Typically, less than 1% of the light is reflected by the semi- reflective mirror 28. The lightvalve 26 comprises a number of pixels. In a preferred embodiment, each pixel is greater in area than the pixels in the R-LCDs 20, 22 and 24. That is, the resolution (number of pixels per unit area) of the lightvalve 26 is lower than the resolution of the R-LCDs 20, 22 and 24.

However, it will be appreciated that the performance of the present invention will improve as the resolution of the lightvalve 26 increases.

However, an advantage of the present invention is that a low-resolution lightvalve 26 will have an appreciable affect on the uniformity of one or more of the brightness levels in a displayed image, without adding significantly to the overall cost of the display system. Where the lightvalve 26 is a transmissive LCD panel, each of the pixels contains a thin layer of liquid crystal material, to which a voltage can be applied. As in the R-LCDs, this voltage determines the polarisation direction of the liquid crystals, and hence the polarisation direction of the transmitted light. Depending upon an applied voltage, each of the pixels in the lightvalve 26 alters the polarisation of the incident light beam so that some, none or all, of the incident light for that pixel is transmitted by the polarising beam splitter 16. This transmitted light is displayed on the screen 4 along with the image generated from the light from the Philips prism 18 and polarising beam splitter 16.

As the non-uniformity of one or more of the brightness levels results in lighter or darker areas of an image which otherwise should have a uniform brightness, the pixels in the lightvalve 26 are configured so that additional light is transmitted by the polarising beam splitter 16 to the parts of the displayed image that are darker than other parts of the image as a result of the inhomogeneities.

No additional light is transmitted to the part or parts of the image where one or more of the brightness levels are lightest.

Therefore, the level to which the particular brightness level is homogenised will be determined by the lightest part of that particular brightness level in the image. Thus, the one or more brightness levels will be homogenised to the level of the lightest part of the inhomogeneity in the uncorrected image.

The amount of additional light required to provide the uniform brightness levels will depend upon the amount of inhomogeneity in the various parts of the image introduced by the projection system. It will be appreciated that the projection system, and in particular the polarising beam splitter 16, will also introduce brightness level distortion into the additional light provided by the lightvalve 26. In accordance with an embodiment of the invention, when configuring the pixels in the lightvalve 26, the distortion caused by the projection system on the additional light is taken into account, thereby further improving the brightness level correction in the displayed image.

In further embodiments, the amount of correction introduced by the lightvalve 26 can be modified depending on the variations of the inhomogeneities over time, such as inhomogeneities caused by changes in temperature, degradation of materials, etc.

Although the invention is described and illustrated as a three-panel LCoS system with a type of Philips prism, it will be appreciated that the invention is applicable to other types of three-panel LCoS systems, and also to single-panel LCoS systems, such as a rotating prism system.

Therefore, there is disclosed an arrangement for homogenising the brightness levels, and hence the contrast in an image displayed by an LCoS display system. 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 projection display device for displaying an image, comprising: a light source that generates a light beam; first light modulating means changing the polarisation of incident light in accordance with image information for a first image, and outputting a first modulated light beam to a polarising beam splitter; the polarising beam splitter providing parts of said first modulated light beam having a first polarisation to a projection lens, said projection lens displaying the first image on a projection surface, the displayed first image having at least one non-uniform brightness level; the device further comprising: second light modulating means for changing the polarisation of a portion of said light beam in accordance with image information for a second image, and outputting the second modulated light beam to said polarising beam splitter; wherein the polarising beam splitter provides parts of the second modulated light beam having a second polarisation to the projection lens, said projection lens superimposing said second image over said first image on the projection surface to provide a composite image, wherein the image information for the second image corrects for the at least one non-uniform brightness level of the first image, such that the composite image has improved brightness level uniformity relative to the first image.

2. A projection display device as claimed in claim 1, wherein the second light modulating means comprises a lightvalve.

3. A projection display device as claimed in claim 2, wherein the lightvalve is a transmissive liquid crystal display panel.

4. A display device as claimed in any preceding claim, wherein the first light modulating means comprises three reflective liquid crystal display panels, the reflective liquid crystal panels modulating incident red, green and blue light beams respectively.

5. A display device as claimed in claim 4, wherein the first light modulating means further comprises a type of Philips prism for splitting the light beam into said red, green and blue light beams.

6. A display device as claimed in claim 4 when dependent on claim 3, wherein the transmissive liquid crystal display panel has a lower resolution than each of the reflective liquid crystal display panels.

7. A display device as claimed in any preceding claim, wherein the amount of light from said light source used to form said second image is less than the amount of light used to form said first image.

8. A display device as claimed in any preceding claim, wherein the second image remains substantially the same over time.

9. A display device as claimed in any preceding claim, wherein the at least one non-uniform brightness level comprises a black level.

10. A display device as claimed in any one of claims 1 to 9, wherein the at least one non-uniform brightness level comprises a grey level.

11. A display device as claimed in one of claims 1 to 9, wherein the at least one non-uniform brightness level comprises a white level.