WO2006035379A1

WO2006035379A1 - System and method for polarisation conversion

Info

Publication number: WO2006035379A1
Application number: PCT/IB2005/053128
Authority: WO
Inventors: Bart Salters
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-09-28
Filing date: 2005-09-22
Publication date: 2006-04-06
Also published as: EP1797473A1

Abstract

A projection system for projecting an image onto a screen comprising a light source (106) for generating a light beam (105), a polarisation means (110, 112) for transmission of a first part of the light beam (105) polarized in a first direction and reflection of a second part of the light beam (105) polarized in a second direction perpendicular to the first direction, and polarisation conversion means (126) for converting polarisation direction of one of the first or second part of the light beam (105) into the polarisation direction of the other part. The polarisation means (110, 112) are positioned coaxially around a longitudinal centre (104) of the light beam (105) so that the second part of the light beam (105) substantially is reflected towards the centre (104) and that reflection means (122, 124) are positioned at the centre (104) for directing the second part of the light beam (105) in the same direction as the first part of the light beam (105).

Description

System and method for polarisation conversion

FIELD OF INVENTION

This invention relates to a system and method for converting polarisation of light in an optical system, such as a liquid crystal projection system.

BACKGROUND OF INVENTION

In general, a projection conversion system comprises a polarisation beam splitter having an entrance surface half the size of an exit surface of a first integrator plate following an ultra high powered lamp. The law of preservation of etendue dictates that the angular spread of the light will roughly double when the cross-sectional area of the light bundle is reduced by a factor two. The exiting area through which the light exits the polarisation beam splitter is double the size of the entrance area through which the light enters the polarisation beam splitter so as to achieve a light bundle having one polarisation direction. The combination of integrator plates and polarisation beam splitter have now doubled the angular spread to obtain a single polarisation direction. This doubling of angular spread is an undesired side effect of system design, since the larger angles of the exiting light in turn require larger lenses and larger liquid crystal display (LCD) panels and larger projection lenses further down the line in the system. Consequently the price of a projection system enlarges.

A projection converter, such as described in the article "Ultra high-efficiency LC projector using a polarized light illumination system" by Itoh, Y., Nakamura, J. I. et al in SID Digest of technical papers, XXVII, Society for information Display, Santa Ana, CA, pp. 993-996, 1997, comprises a shielding element to block approximately half of the non¬ polarized light from a lamp before a polarisation beam splitter so as to provide space for polarized light exiting the polarisation beam splitter. However, the shielding element thus reduces the brightness of the light driving the LCD panels, and therefore further solutions are required, which maintain good etendue and brightness. SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a polarisation conversion system for converting light without enlarging angular spread and for maintaining a good etendue. A particular advantage of the present invention is utilisation of a space which has a low distribution of light thereby optimizing the polarisation conversion, since a smaller amount of light is blocked.

A further advantage of the present invention is the provision of a light bundle having less angular spread, which reduces costs of a projector, e.g. a cheaper projection less. The above object and advantages together with numerous other objects, advantages and features, which will become evident from below detailed description, is obtained according to a first aspect of the present invention by a projection system for projecting an image onto a screen comprising a light source for generating a light beam, a polarisation means for transmission of a first part of the light beam polarized in a first direction and reflection of a second part of the light beam polarized in a second direction perpendicular to the first direction, and polarisation conversion means for converting polarisation direction of one of the first or second part of the light beam into the polarisation direction of the other part, and wherein the polarisation means are positioned coaxially around a longitudinal centre of the light beam so that the second part of the light beam substantially is reflected towards the centre and that reflection means are positioned at the centre for directing the second part of the light beam in the same direction as the first part of the light beam.

The projection system according to the first aspect of the present invention may thus improve etendue compared to prior art techniques by utilising an area of the light beam, in which the light distribution has been found to be very low.

The polarisation conversion means may according to the first aspect be adapted to convert the first part of the light beam and is positioned coaxially around the centre behind the polarisation means. Alternatively, the polarisation conversion means is adapted to convert the second part of the light beam and is positioned at the centre behind the reflection means. Hence the polarisation conversion may be configured to transmit light having a desired polarisation or to reflect light having a desired polarisation.

The reflection means according to the first aspect of the present invention may further be adapted to direct a part of the light beam received directly from the light source back towards the light source. Since the light distributed in this area is low the reflection means positioned in this space will block some light. However, having the reflection means adapted to reflect this light back toward the light source the light may be reused.

The reflection means according to the first aspect of the present invention may comprise a first flat piece- wise surface adapted to reflect part of the light beam in the centre back towards the light source. Further the reflection means may comprise a plurality of piece- wise flat mirrors angularly displaced relative to the longitudinal centre by substantially 45° and the plurality of piece-wise flat mirrors may be adapted to direct the second part of the light beam in the same direction as the first part. That is, the light of the second polarisation is directed towards the centre to refill the space at the longitudinal centre with light having the desired polarisation.

The polarisation means according to the first aspect of the present invention may comprise a plurality of flat piece-wise surfaces for reflecting the second part of the light beam toward the centre. The flat piece-wise surface is angularly displaced relative to the longitudinal centre by substantially 45°. Thus the light having the second polarisation is directed towards the plurality of piece-wise flat mirrors of the reflection means, which as described above reflects the second part to fill the space at the centre.

The flat first piece-wise surface and the plurality of flat piece-wise surfaces according to the first aspect of the present invention may have any polygonal shape such as rectangular shape. The above objects, advantages and features together with numerous other objects, advantages and features, which will become evident from below detailed description, is obtained according to a second aspect of the present invention by a polarisation conversion system for converting polarisation of a light beam comprising a polarisation means for transmission of a first part of said light beam polarized in a first direction and reflection of a second part of said light beam polarized in a second direction perpendicular to said first direction, and polarisation conversion means for converting polarisation direction of one of said first or second part of said light beam into the polarisation direction of the other part, and wherein said polarisation means are positioned coaxially around a longitudinal centre of said light beam so that said second part of said light beam substantially is reflected towards said centre and that reflection means are positioned at said centre for directing said second part of said light beam in the same direction as said first part of said light beam.

The polarisation conversion system according to the second aspect of the present invention may incorporated any features of the projection system according to the first aspect of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non- limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawing, wherein: figure 1, shows a cross-sectional view of light distribution density directly following a lamp; figure 2, shows a block diagram of a polarisation conversion system according to a first embodiment of the present invention; and figures 3a and 3b show views of a polarisation conversion system according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In the following description of the various embodiments, reference is made to the accompanying figures, which show by way of illustration various embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the present invention. Examination of the light distribution density of a lamp has revealed a non¬ uniform spreading of the light, which is shown in figure 1 in a cross-sectional view. State of the art polarisation conversion systems do not profit from this fact, since distribution is generally considered constant through out the full lighting area.

As shown in figure 1, the centre area 10 of the light distribution according to the examination contains a limited amount of light. The present invention suggests utilisation of this centre area 10 for positioning of a polarisation conversion system. The positioning of the polarisation conversion system in this centre area 10 ensures that only a small amount of light is discarded.

Figure 2 shows a block diagram of the polarisation conversion system 100 according to a first embodiment of the present invention. The polarisation conversion system 100 comprises a blocking grid 102 positioned along the longitudinal centre 104 (reference numeral 10 in figure 1) of the distributed light 105 from a lamp 106. The blocking grid 102 comprises a reflective surface 108 facing the lamp 106, which surface 108 reflects all incident light. The polarisation conversion system 100 further comprises a first and second polarizing beam splitter (PBS) 110 and 112, respectively, which PBSs 110, 112 are shifted laterally relative to the longitudinal centre 104 next to the blocking grid 102 so that surfaces 114 and 116 of the PBSs receive the light 105 from the lamp 106. The first and second PBS 110, 112 transmit first light 118 having a first polarisation, and reflect second light 120 having a second polarisation. The second light 120 is directed by the PBSs 110 and 112 towards a first 122 and a second 124 mirror positioned behind the blocking grid 102. The first and second mirrors 122 and 124 reflect the light 120 towards a half- wave plate 126 altering the polarisation of the light 120 into light 128 having the first polarisation. Hence a bundle of light having the same polarisation is achieved, which is what is required for LCD panels further down in a projector device.

Angular spread of the light 105 is not shown in figure 2, however, since only 45° mirrors are applied the angular spread of the entering light 105 is preserved. That is all angles of light present in at the entrance will also be present in the exiting light and no further new and/or larger angles will be created.

Figure 2 shows a two-dimensional case, however, it should obviously be understood, that the polarisation conversion system 100 may be implemented in three dimensions having elements positioned symmetrically about the symmetry-axis of figure 2.

Figures 3a and 3b show views of a second embodiment according to the present invention. In particular these figures show mirrors and PBSs of a polarisation conversion system.

As a ultra high power (UHP) lamp is rotational symmetrical a similar mirror/PBS construction seems appropriate, however, a rotational symmetrical mirror/PBS construction in fact increases angular spread due to the curved mirrors. Besides curved PBSs are hard to manufacture. Therefore a piece-wise flat solution is proposed.

Figure 3a shows in a three dimensional view a blocking grid 302 mounted with a plurality of piece-wise flat mirror elements 304, 306, 308, 310, 312, 314, 316, 318, 320, 322, 324 and 326. As described with reference to figure 2, the lower surface 303 of the blocking grid 302 reflects incident light back towards a lamp 106(not shown in figure 3). The mirrors 304, 306, 308, 310, 312, 314, 316, 318, 320, 322, 324 and 326 are tilted relative to the lower surface 303 of the blocking grid 302 so as to reflect incoming light toward a half- wave plate 126 (not shown in figure 3).

Figure 3b shows in top view the blocking grid 302 and mirrors 304, 306, 308, 310, 312, 314, 316, 318, 320, 322, 324 and 326 together with PBSs 328, 330, 332, 334 reflecting light having a first polarisation to the mirrors 304, 306, 308, 310, 312, 314, 316, 318, 320, 322, 324 and 326, and transmitting light having a second polarisation.

Hence the polarisation conversion system according to the present invention provides light distributed over a smaller angular spread of the light beam compared with prior art, while having an desired polarisation. Thereby the polarisation conversion system effectively improves etendue characteristics compared with prior art.

Obviously, since the lamp is circular and the PBSs 312, 314, 316, 318 rectangular some light having an undesired polarisation will pass the polarisation conversion system without being converted. Although this is undesirable it is not uncommon in prior art systems. In the prior art systems this problem is generally solved by positioning a blocking grid in front of the polarisation conversion system, however, this induces a loss of 20% of the incoming light.

In projection system according to the present invention, the polarisation conversion system converts the non-polarized light so that a substantial amount of the light exiting the polarisation conversion system has a first polarisation, such as 80 - 100% of the exiting light, while the remaining light has an undesired polarisation. The light having the undesired polarisation may be blocked by a clean-up polarizing means, which is placed before the LCD panels.

Claims

CLAIMS:

1. A projection system for projecting an image onto a screen comprising a light source (106) for generating a light beam 105, a polarisation means (1 10, 112) for transmission of a first part of said light beam polarized in a first direction and reflection of a second part of said light beam polarized in a second direction perpendicular to said first direction, and polarisation conversion means (126) for converting polarisation direction of one of said first or second part of said light beam into the polarisation direction of the other part, and wherein said polarisation means (110, 112) are positioned coaxially around a longitudinal centre (104) of said light beam (105) so that said second part of said light beam (105) substantially is reflected towards said centre (104) and that reflection means (122, 124) are positioned at said centre (104) for directing said second part of said light beam (105) in the same direction as said first part of said light beam (105).

2. A projection system according to claim 1, wherein said polarisation conversion means (126) is adapted to convert said first part of said light beam (105) and is positioned coaxially around said centre (104) after said polarisation means (110, 1 12).

3. A projection system according to claim 1, wherein said polarisation conversion means (126) is adapted to convert said second part of said light beam (105) and is positioned at said centre after said reflection means (122, 124).

4. A projection system according to any of claims 1 to 3, wherein said reflection means (122, 124) further is adapted to direct a part of said light beam (105) received directly from said light source (106) back towards said light source (106).

5. A projection system according to any of claims 1 to 4, wherein said reflection means (122, 124) comprises a first flat piece-wise surface (108) adapted to reflect part of said light beam (105) in said centre (104) back towards the light source (106).

6. A projection system according to any of claims 1 to 5, wherein said reflection means (122, 124) comprises a plurality of piece-wise flat mirrors (122, 124) angularly displaced relative to said longitudinal centre by substantially 45° and wherein said plurality of piece- wise flat mirrors (122, 124)are adapted to direct said second part of said light beam (105) in the same direction as said first part.

7. A projection system according to any of claims 1 to 6, wherein said polarisation means (110, 112) comprise a plurality of flat piece-wise surfaces (114, 116) for reflecting said second part of said light beam (105) toward said centre (104).

8. A projection system according to claim 7, wherein said flat piece-wise surface (114, 116) is angularly displaced relative to said longitudinal centre (104) by substantially 45°.

9. A projection system according to any of claims 5 to 8, wherein said flat first piece-wise surface (108) and said plurality of flat piece-wise surfaces (114, 115) are rectangularly shaped.

10. A polarisation conversion system for converting polarisation of a light beam (105) comprising a polarisation means (110, 112) for transmission of a first part of said light beam polarized in a first direction and reflection of a second part of said light beam polarized in a second direction perpendicular to said first direction, and polarisation conversion means (126) for converting polarisation direction of one of said first or second part of said light beam into the polarisation direction of the other part, and wherein said polarisation means (110, 112) are positioned coaxially around a longitudinal centre (104) of said light beam

(105) so that said second part of said light beam (105) substantially is reflected towards said centre (104) and that reflection means (122, 124) arc positioned at said centre (104) for directing said second part of said light beam (105) in the same direction as said first part of said light beam (105).