WO2008029359A1

WO2008029359A1 - One element beam combiner

Info

Publication number: WO2008029359A1
Application number: PCT/IB2007/053575
Authority: WO
Inventors: Jorgen M. Van Der Veen; Pieter Stroobach
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-09-07
Filing date: 2007-09-05
Publication date: 2008-03-13
Also published as: TW200821626A

Abstract

The present invention provides a beam combiner (1) based on a cube prism (10) having faces (11, 12, 13 and 14) arranged to collimate and shape incoming light beams (101, 201 and 301) and an exiting beam (400). Such a beam combiner is compact, efficient and does require very little alignment procedure.

Description

One element beam combiner

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an optical component, in particular to a beam combiner used for combining light beams emitted from several light sources.

BACKGROUND OF THE INVENTION

Illumination systems, projection display systems and other optically-based systems typically comprise a plurality of optical elements. One key optical element of such systems is a beam combiner. The function of a beam combiner is to combine the beams of different light sources into one beam. For example, for the purpose of creating a white beam in a projection display system, the beams of light sources emitting blue, red and green light, i.e. the three primary colours, can be combined. Several methods can be used to form a beam combiner. Two of these methods are described below.

One method is to combine the beams using an arrangement of dichroic mirrors. Depending on the wavelength at which the light is emitted, the light that is incident on a dichroic mirror will either be reflected or transmitted. In such beam combiners, the arrangement of the mirrors creates light paths for the different beams so that they are combined into one single beam.

Another method is to use an x-cube prism, which is a prism structure composed of four triangular prisms arranged in the form of a cube having two partially reflecting diagonal surfaces. In such beam combiners, the light emitted from the light sources that will impinge on the partially reflecting diagonal surfaces can either be reflected or transmitted, depending on the coatings of these partially reflecting diagonal surfaces. When used in for instance projection systems, a beam combiner based on an x-cube prism provides three positions at which the light sources can be located and a fourth position at which light exits.

However, further optical devices in addition to a prism or an arrangement of mirrors are needed in order to accomplish an operative beam combiner or a final product such as a projection display system. US 2005/0219476 discloses such a system in which an x- cube prism is used to combine the light emitted from three light-recycling illumination systems. Before light beams emitted from the recycling illumination systems enter the x-cube prism, the light beams pass through light-collimating means. In another embodiment, the light also passes through a beam-splitting prism polarizer.

A problem of the projection display system set forth in US 2005/0219476 is that alignment of several optical elements is required, thus making difficult the assembly of such a system. Another problem is that each individual component introduces a power loss in the system, thus resulting in low efficiency.

SUMMARY OF THE INVENTION An objective of the invention is to reduce above mentioned problems of prior art and to provide a beam combiner which is more efficient, and/or more compact, and/or easy to assemble, and/or which enables control of the light beam exiting the beam combiner. The present invention is based on the understanding that a beam combiner can be arranged to comprise a single optical structure having further functionalities in addition to combining beams. A basic idea of the present invention is to use four prisms assembled together to an integrated prism structure having the form of a cube and having curved faces for e.g. shaping the beam.

According to a first aspect of the present invention, there is provided a beam combiner comprising four prisms arranged in a prism structure having the form of a cube with two partially reflecting diagonal surfaces wherein, of four faces of the prism structure which are not perpendicular to planes defined by the partially reflecting diagonal surfaces, one face is defined as a front face and at least two of the faces other than the front face are arranged to receive light beams, said beam combiner combining said received light beams into a beam exiting the prism structure at the front face, wherein at least one face of the prism structure is arranged to shape the exiting beam.

The faces of the prism structure can be arranged for shaping or collimating the exiting beam. In addition, lenses can be arranged at the faces of the cube for focusing the received light beams or for focusing the exiting beam. In other words, the present invention provides a single optical structure for combining a plurality of light beams into an exiting beam and for shaping, collimating and focusing this exiting beam.

The main advantage of this single-element beam combiner as compared to currently used beam combiners is that it does not require any alignment of different interacting optical components since principal functionalities (collimating, focusing and shaping) are integrated in a single optical structure. Another advantage is that the beam combiner becomes very compact and thereof enables production of, for instance, very compact projection display systems. Further, the integration of all these functionalities in one single structure improves system efficiency since overall power loss is reduced as compared to prior art beam combiners. In another embodiment of the invention, there is provided a beam combiner wherein the lens arranged at one or more faces of the prism structure is a liquid crystal lens for selectively altering the focus of the exiting beam. Thus, one and the same illumination system could at one instant of time be used for illuminating an entire room and at another instant of time be used as a reading lamp. In another embodiment of the invention, there is provided a beam combiner further comprising light sources arranged at the faces other than the front face for producing light beams. These light sources can be single point light sources such as light emitting diodes (LEDs) or laser diodes. The use of LEDs and laser diodes enable provision of optical systems that are smaller as compared to prior art systems which e.g. uses high pressure lamps, since these lamps are bulky in comparison to LEDs and laser diodes. Another advantage of LEDs and laser diodes is that their light distributions are relatively narrow in comparison to other light sources. Thus, requirements on collimation of the light are milder for LEDs and laser diodes than for many other light sources. As a consequence, beam combiners based on LEDs and laser diodes can be combined with high resolution imaging devices in e.g. projection display systems. Further, characteristics of the LEDs and the laser diodes, such as wavelength and intensity, are easily controllable.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the [element, device, component, means, step, etc]" are to be interpreted openly as referring to at least one instance of said element, device, component, means, step, etc., unless explicitly stated otherwise. Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawing.

BRIEF DESCRIPTION OF THE DRAWING

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, in which: Fig. 1 is a top view of a prism structure comprising four prisms arranged adjacent to each other to form a cube having two partially reflective diagonal surfaces and having curved faces in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

With reference to Fig. 1, a first embodiment of the invention will be described below.

Referring to Fig. 1, four prisms are arranged adjacent to each other to form a prism structure 10 having the form of a cube (referred to as a cube prism in the following). The four prisms are preferably triangular but may also have other shapes. In the present embodiment, the beam combiner 1 further comprises three light sources Ll, L2 and L3 having three different wavelengths. The light sources Ll, L2 and L3 are positioned at faces 11, 12 and 13 of the cube prism 10 leaving one face 14 free from light sources (referred to as front face in the following). These four faces of the cube 10 are the faces which are not perpendicular to the diagonal surfaces 15 and 16 of the cube prism 10. As can be seen in Fig. 1, the face 12 of the cube prism 10 is arranged to collimate the light beam 201 emerging from the light source L2. Similarly, the faces 11 and 13 could be arranged to collimate the light beams 101 and 301 emerging from the light sources Ll and L3, respectively. In addition, the faces 11, 12, 13 and 14 can be arranged to collimate and shape the light beam 400 exiting the cube prism 10. In a preferred embodiment, the faces 11, 12, 13 and 14 of the cube prism 10 are curved and may include several different curvatures along horizontal, vertical as well as diagonal directions of each face of the cube prism 10 (not shown).

In one embodiment, the diagonal surfaces 15 and 16 of the prism structure 10 shall be coated with standard "DC blue" and "DC red" coatings, as provided by Unaxis. Such a DC blue coating reflects light having a wavelength below 450 nm, i.e. blue light, and transmits light having a wavelength in the range 510-800 nm, i.e. green and red light. Similarly, the DC red coating reflects light having a wavelength above 650 nm, i.e. red light, and transmits light having a wavelength in the range 400-575 nm, i.e. green and blue light. In a glass/coating/air arrangement, reflection coefficients of these coatings are in the range of 90-95%. However, in a glass/coating/glass arrangement such as in the present invention, these values would be slightly lower. It will be appreciated by a person skilled in the art that other coatings than the two coatings presented here can be used and that the used coatings may cover other reflection and transmission ranges. In addition, the faces 11, 12 and 13 of the cube prism 10 can be coated with transmission layers suitable for the selected wavelengths of the light sources Ll, L2 and L3, respectively. The front face 14 would preferably be coated with a wide band pass-band filter for transmitting all wavelengths emitted from the light sources Ll, L2 and L3. It will be appreciated by a person skilled in the art that more than one layer could be deposited on the diagonal surfaces 15 and 16 and on the faces 11, 12, 13 and 14 of the cube prism 10 in order to improve the quality of the coatings.

As illustrated in Fig. 1, the light source Ll emits at e.g. 440 nm a blue beam 101 entering the cube prism 10 through the face 11. The beam 101 will then impinge on the diagonal surface 16 and be reflected, thus exiting the cube prism 10 via the front face 14. If the beam first impinges on the diagonal surface 15, it will be transmitted and subsequently impinge on the diagonal surface 16, against which it will be reflected. Similarly, the red beam 301 emitted from the light source L3 will enter the cube prism 10 via the face 13, and then be reflected against the diagonal surface 15. If the beam 301 first impinges on the diagonal surface 16, it will be transmitted and subsequently be reflected against the diagonal surface 15. In any case, the red beam 301 will eventually exit the cube prism 10 through the front face 14. In the present embodiment, as the partially reflective diagonal surfaces are arranged for reflecting blue and red beams, the green beam 201 emitted from the light source L2 will enter the cube prism 10 via the face 12 and be transmitted trough the diagonal surfaces 15 and 16 to finally exit the cube prism 10 via the front face 14. As a result, the blue, red and green beams 101, 201 and 301 are combined together to form an exiting beam 400, which in this case would result in a white beam.

The fact that a number of different functionalities, such as shaping and collimating, are integrated in a single optical structure reduces the loss of light power as compared to systems comprising a plurality of separate, non- integrated optical components. In such multi-component beam combiners based on e.g. mirrors, each mirror (or each optical element) introduces a power loss of at least 1% per side of the mirror. This means that when three mirrors are used, the system has a power loss of about 6%. Integrating components within the faces 11, 12, 13 and 14 of the cube prism 10 reduces the number of optical interfaces in the light path and thereby reduces the light power loss. Hence, throughout embodiments of the present invention, a beam combiner 1 is provided where each light path passes through two faces only, for instance faces 13 and 14 for the red beam 301, faces 11 and 14 for the blue beam 11 or faces 12 and 14 for the green beam 201, which advantageously results in a light power loss of about 2% only. The cube prism 10 combines the beams, but since the angles at which the light sources Ll, L2 and L3 emits light against the faces 11, 12 and 13, respectively, may be mutually different, only a small portion of a cross-section of the exiting beam 400 would comprise white light. Thus, lenses can be arranged at the faces 11, 12 and 13 at which the light sources Ll, L2 and L3 are positioned in order for the incoming beams 101, 201 and 301 to coincide. The lenses at the faces 11, 12 and 13 of the cube prism 10 considerably reduce the size of the beam combiner 1 in comparison to beam combiners where separate, non- integrated components are used for collimating the incoming beams before entering the cube prism. In addition, in such multi-component systems, a relatively large distance is required between the light sources and the collimating components, which further increases the size of the beam combiner.

In an embodiment of the invention, the lens incorporated at the front face 14 of the cube prism 10 is a liquid crystal lens. Such a lens makes selective altering of the focus of the exiting beam 400 possible, which enables implementation of illumination systems adapted to both room illumination conditions and reading illumination conditions.

In yet a further embodiment, alignment of the light sources Ll, L2 and L3 is performed to create a uniform exiting beam 400. The diameter of the cross-section of the exiting beam is controlled by adjusting the distance between the light sources Ll, L2 and L3 and the faces 11, 12 and 13, respectively, of the cube prism 10. By adjusting the position and the tilt of the light sources Ll, L2 and L3 with regard to the faces 11, 12 and 13, respectively, of the cube prism 10, the angle and the shape of the exiting beam 400 can be controlled.

In the following, shaping of an exiting beam 400 created from a combination of laser diode beams is described.

A laser diode usually has an elliptical beam profile. Using different curvatures along e.g. two directions of a face of the cube prism and rotating the laser diode so that its orientation matches its light-distributing angle enable alteration of elliptical profile of a beam. By adjusting the distance between a laser diode and a corresponding face of the cube prism, the profile of the beam can be narrowed, and an exiting beam having for instance a circular cross-section can be created. Other shapes can be created using different designs for the optics, i.e. using different types of curvatures for the faces of the cube. It is possible to obtain beam profiles of any shapes e.g. square, rectangular or triangular. The main purpose of the shaping is to adapt the exiting beam profile to the final application. For instance, a round beam profile is suitable for an illumination system while square, rectangular and even triangular beam profiles are more suitable for beamers and head up displays. Depending on the curvature of the faces, the Gaussian profile of a diode laser can also be changed to a flattop profile. This could be of interest for illuminating a well-defined area in which each part of the illuminated area needs to be illuminated evenly.

It will be appreciated by a person skilled in the art that any type of light sources Ll, L2 and L3 may be used in combination with the cube prism 10 of the present invention. In an embodiment, light emitting diodes or laser diodes are used. These types of light sources are small and therefore enable compact beam combiners. An advantage of using diodes is that they emit light with a narrow beam distribution, thus enabling creation of a narrow exiting beam which can be directed to individual pixel positions on a screen (not shown in the Figure). Therefore, the use of LEDs enable combination of the beam combiner with high resolution imaging devices such as LCD panels or LCOS panels. As compared to LEDs, the use of laser diodes is even more preferable since laser beams per se are collimated. Thus, the image stays sharp regardless of the distance between the beam combiner and the display. Further, the use of laser diodes in e.g. television using three colours increases the spectral coverage and almost reaches the spectral coverage to which the human eye is sensitive, since laser diodes emit light of separate colours as compared to conventionally used fluorescent materials emitting mixed colours.

In another embodiment of the present invention, the light sources Ll, L2 and L3 emit blue, green and red light, respectively. These are the three primary colours which, after passing through the cube prism 10, results in a white exiting beam 400 suitable for projection display systems or illumination applications. However, it will be appreciated by the person skilled in the art that other wavelengths can be used and that a white exiting beam 400 can be obtained by the combination of other colours than red, blue and green.

The cube prism 10 can be manufactured using conventional methods such as grinding and polishing, hot glass pressing and plastic injection moulding. In some cases, lenses located at the faces 11, 12, 13 and 14 of the cube prism 10 are directly incorporated during the production process, thus providing a cheap technique to produce the single- element beam combiner. In other cases, these lenses are added later by gluing them onto the faces 11, 12, 13 and 14 of the cube prism 10 with optical transparent adhesives. In one embodiment, a projection display system is realized using a beam combiner 1 of the present invention. Using a beam combiner 1 equipped with curved faces, lenses and appropriate coatings of the diagonal surfaces and the faces, only a scanner mirror is needed to fully realize the projection display system. Further, such a projection display system would preferably be provided with light sources such as LEDs or laser diodes since the spatial modulation required to compose the image on the display easily can be controlled by electrical modulation of the LEDs or laser diodes. This is advantageous as compared to systems where the modulation only occurs in the display device.

In another embodiment, an illumination system is provided using a beam combiner 1 of the present invention.

The present invention is applicable in various display technologies applied in e.g. television, computers, automotive industry and mobile phones and also for lighting applications in which LEDs or laser diodes are used.

The invention has mainly been described above with reference to a number of explicitly disclosed embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims

CLAIMS:

1. A beam combiner (1) comprising four prisms arranged in a prism structure (10) having the form of a cube with two partially reflecting diagonal surfaces (15, 16) wherein, of four faces (11, 12, 13, 14) of said prism structure (10) which are not perpendicular to planes defined by said partially reflecting diagonal surfaces (15, 16), one face (14) is defined as a front face and at least two of said faces other than said front face (14) are arranged to receive light beams (101, 201, 301), said beam combiner (1) combining said received light beams (101, 201, 301) into a beam (400) exiting said prism structure (10) at said front face (14), wherein at least one face of said prism structure (10) is arranged to shape said exiting beam (400).

2. The beam combiner as defined in claim 1, wherein said at least one face of said prism structure (10) is arranged to collimate said exiting beam (400).

3. The beam combiner as defined in any one of the preceding claims, wherein at least one face of said prism structure (10) is arranged with at least one lens for focusing at least one of said received light beams (101, 201, 301).

4. The beam combiner as defined in any one of the preceding claims, wherein at least one face of said prism structure (10) is arranged with at least one lens for focusing said exiting beam (400).

5. The beam combiner as defined in claim 4, wherein said at least one lens is a liquid crystal lens for selectively altering the focus of said exiting beam (400).

6. The beam combiner as defined in any one of the preceding claims, wherein said prism structure (10) is an x-cube prism.

7. The beam combiner as defined in any one of the preceding claims, wherein each of said two partially reflecting diagonal surfaces (15, 16) is coated with at least one light-reflecting layer adapted to reflect at least one of said received light beams (101, 201, 301).

8. The beam combiner as defined in any one of the preceding claims, wherein said four faces (11, 12, 13, 14) of said prism structure are coated with transmission layers.

9. The beam combiner as defined in any one of the preceding claims, wherein a wide band pass-band filter is provided at said front face (14).

10. The beam combiner as defined in any one of the preceding claims, further comprising light sources (Ll, L2, L3) arranged at said faces (11, 12, 13) other than said front face (14) for producing said received light beams (101, 201, 301).

11. The beam combiner as defined in any one of the preceding claims, wherein said light sources (Ll, L2, L3) are single point light sources or an array of single point light sources.

12. The beam combiner as defined in any one of the preceding claims, wherein said light sources (Ll, L2, L3) are light emitting diodes, laser diodes or organic light emitting diodes.

13. The beam combiner as defined in any one of the preceding claims, wherein at least two of said light sources (Ll, L2, L3) emit light of different wavelengths.

14. A projection system comprising at least one beam combiner according to any one of the preceding claims.

15. An illumination system comprising at least one beam combiner according to any one of the preceding claims.