WO2004107018A1

WO2004107018A1 - Led illumination system

Info

Publication number: WO2004107018A1
Application number: PCT/IB2004/050743
Authority: WO
Inventors: Adrianus J. S. M. De Vaan
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-06-02
Filing date: 2004-05-19
Publication date: 2004-12-09
Also published as: TW200508690A

Abstract

An LED illumination system is provided that couples light from red, green blue LEDs into a light integrator via cube prisms containing dichroic mirrors. Light efficiency is improved over prior art systems through the use of non-imaging collectors to reduce the angle of incidence of light that, due to otherwise high angles of incidence, would be totally internally reflected in the cube prisms, and would not be reflected into the light integrator.

Description

LED illumination system

TECHNICAL FIELD OF THE INVENTION

This invention relates to illumination systems, and in particular to an arrangement for maximising the efficiency of illumination systems using light emitting diodes.

BACKGROUND TO THE INVENTION

Light emitting diode (LED) sources for projection systems are rapidly becoming available in higher power packages. It is expected that LED's will soon be suitable for low lumen (5 to 100) and medium lumen (100 to 400) projection systems and these LED's will be favourable in respect to cost and lifetime compared to projection lamps, such as ultra high performance (UHP) lamps. LED's will then be suitable for use in consumer applications such as rear projection television systems, ultra portable projection systems, mobile telephones, personal digital assistants and automobiles.

One of the main problems in obtaining high system efficiency is that the LED material consists of high refractive index material (N > 3.5). A substantial amount of the light does not leave the LED material when it hits the LED material boundary since the high refractive index results in a poor critical angle of reflection, and total internal reflection of some of the light occurs. Further, the light that does leave the LED is emitted over a hemisphere and is therefore difficult to collect into a coherent light beam. It is therefore an object of the present invention to provide a compact, simple, low cost and highly efficient illumination system to couple light from LEDs into a light integrator for use in further optical systems.

SUMMARY OF THE INVENTION In accordance with an aspect of the invention, there is provided an , illumination system comprising: a plurality of light sources, each light source generating light of a respective colour; a plurality of non-imaging collectors, coupled to respective light sources, for collecting light from the light source and emitting the light from a respective exit aperture thereof; a plurality of reflective elements, coupled to respective collectors, for reflecting the light emitted from the exit aperture of the respective collector; and an integrator having an input surface for receiving the light reflected by the plurality of reflective elements.

According to another aspect of the invention, there is provided a projection system comprising an illumination system as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is an illumination system in accordance with a first embodiment of the present invention;

Figure 2 is a diagram of the light rays propagating from LED material in accordance with the invention; and

Figure 3 is a diagram of a projection system including an illumination system in accordance with the first embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 is an illumination system in accordance with a first embodiment of the present invention. The illumination system 1 comprises three light sources 2, 4 and 6 that each emit light of a particular colour, such as red, green and blue. The system also includes a light integrator 8 for channelling light to a further optical system (not shown), such as a projection system. The light integrator 8 is conventional and homogenises light input at an input surface 81 and outputs the homogenised light at output surface 82.

In this preferred embodiment, the light sources are panels made from LED material, and the panels 2, 4 and 6 emit blue, green and red light respectively.

The LED panel 2 emits blue light over a large angular range, and, to maximise the light efficiency of the illumination system 1, as much light as possible must be provided to the further optical system.

Therefore in accordance with the invention, LED panel 2 is coupled to a non- imaging collector 10 which collects blue light from the panel 2 and illuminates an exit aperture 101 of the collector 10 with the light. The collector 10 is a non-imaging collector so that light emitted from the LED panel 2 with a high angle of incidence is reflected towards the exit aperture 101 of the collector 10 and may be used by the further optical system. Although the collector 10 is shown as having straight sides in Figures 1, it will be appreciated that this is for ease of illustration only, and that in practice the most efficient collectors will have curved sides.

In particular, the non-imaging collector 10 may be a parabolic or elliptical collector.

The exit aperture 101 of the collector 10 is coupled to a surface 121 of a cube prism 12 which contains a dichroic mirror 14. The dichroic mirror 14 reflects blue light and transmits light having all other wavelengths. Surface 122 of the cube prism 12, which is perpendicular to the surface 121, is coupled to the input surface 81 of the light integrator 8. LED panel 4 is coupled to a second non-imaging collector 16 which collects green light from the panel 4 and illuminates an exit aperture 161 of the collector 16 with the light.

The exit aperture 161 of the collector 16 is coupled to a surface 181 of a cube prism 18 which contains a dichroic mirror 20. The dichroic mirror 20 reflects green light and transmits light having all other wavelengths. Surface 182 of the cube prism 18, which is perpendicular to the surface 181, is coupled to surface 123 of cube prism 12, which is opposite the surface 121.

LED panel 6 is coupled to a third non-imaging collector 22 which collects red light from the panel 6 and illuminates an exit aperture 221 of the collector 22 with the light. The exit aperture of the collector 22 is coupled to a surface 241 of a prism 24.

The prism 24 has a surface 243 which reflects the light from the panel 6 towards surface 242 of the prism 24, which is perpendicular to the surface 241. Surface 242 of the prism 24 is coupled to surface 183 of cube prism 18, which is opposite the surface 182.

In alternative embodiments, the prism 24 may be a cube prism with a dichroic mirror which reflects red light and transmits light with all other wavelengths.

Red light from LED panel 6 is collected by non-imaging collector 22 and is reflected off surface 243 of prism 24. The light passes straight through cube prism 18 and dichroic mirror 20 and cube prism 12 and dichroic mirror 14 without reflection and enters the light integrator 8 via input surface 81. Green light from LED panel 4 is collected by non-imaging collector 16 and is reflected off dichroic mirror 20 which reflects green light. The reflected light then passes through cube prism 12 and dichroic mirror 14 and enters the light integrator 8. Blue light from LED panel 2 is collected by non-imaging collector 10 and is reflected off dichroic mirror 14 which reflects blue light. The reflected light then enters the light integrator 8.

The combined red, green and blue light (i.e. white light) is output from output surface 82 of the light integrator 8 and is provided to a further optical system.

As the collectors 10, 16 and 22 collect light from the LED panels 2, 4 and 6 respectively which would otherwise be unusable by the further optical system, the efficiency of the illumination system is improved. The collectors collect and distribute the output light over a larger surface area (the area of the exit aperture) with a more uniform angular distribution. Consequently, the increased efficiency allows the same brightness to be obtained with smaller LED panels. This can reduce the power consumed by an illumination system.

It will be appreciated that the order in which the LED panels 2, 4 and 6 are placed with respect to the light integrator may be varied. In addition, some of the LED panels may be positioned either side of the light integrator 8 instead of along one side as shown in Figure 1.

Further, one of the LED panels may be positioned so that light is emitted from its collector in a direction that is parallel to the direction that light propagates through the light integrator 8. In this case, that particular LED panel does not require a reflective element to reflect the light into the light integrator 8.

It will be appreciated that the invention is applicable to many other configurations of LED panels and reflective elements, and it should be noted in particular that the invention is not limited to illumination systems in which there are only three LED panels. Figure 2 is a diagram showing how light rays may propagate from the LED material in accordance with the invention.

It will again be appreciated that although the collector is shown as having straight sides, this is for ease of illustration only, and that in practice the most efficient collectors will have curved sides. In Figure 2, light source 302 is coupled to non-imaging collector 304. Light source 302 is emitting light, represented here by light beams 306, 308, 310, 312 and 314.

In a system where the light source 302 is coupled directly to a cube prism, or other optical element, much of the light from the light source 302 would be unusable by further optical devices, such as a projection lens, as the angular acceptance of the further optical device may be limited, which would mean that much of the light would not propagate further through the further optical device and would be lost. For example, it is likely that the light represented by light beams 306, 308 and 310 would not be usable by a projection lens as the angle of incidence to the projection lens would be too high, resulting in the light being blocked at the stop in the projection lens.

However, in accordance with the invention, the collector 304 'collects' the light that would otherwise be unused by the system, and reduces its angle of incidence to the cube prism, so that it may be used by the further optical system.

For example, the collector 304 is constructed so that light beam 306 is reflected off the walls of the collector and the angle of incidence to the collector-prism boundary is reduced, thereby reducing the likelihood that the light beam will be blocked in the projection lens.

An ideal collector would reflect all incident light beams so that they are travelling perpendicular to the plane of the light source upon exiting the collector, thereby maximising the amount of light transmitted to the further optical system.

Figure 3 shows a projection system including an illumination system in accordance with the first embodiment of the present invention.

Here, the illumination system 1 is coupled to an exemplary optical system 400. This optical system 400 comprises a projection lens 402, polarising beam splitter 404, a relay system comprising lenses 406a, 406b and 406c, mirror 408 and light valve 410.

The illumination system 1 acts as a source of light, which passes through relay system lenses 406a and 406b and is reflected towards a polarising beam splitter 404.

The polarising beam splitter 404 reflects light having a first polarisation, and transmits light having a second polarisation, the first and second polarisation directions being orthogonal.

The light having the first polarisation is reflected by the polarising beam splitter 404 onto light valve 410, which modulates the incident light with image information.

The light valve 410 may, for example, be a liquid crystal on silicon panel which will alter the polarisation state of the incident light, in accordance with the image to be displayed.

Once the light has been reflected and modulated by the light valve 410, the light re-enters the polarising beam splitter 404. The polarising beam splitter 404 transmits the parts of the modulated light having the second polarisation to the projection lens 402 for display on a projection surface (not shown). It will be appreciated that the further optical system described above is purely for illustrative purposes only, and that the illumination system according to the present invention may be used in conjunction with optical systems having different components, such as rotating prisms or colour wheels. It will also be appreciated that the illumination system may be used in any type of projection system, such as front and rear projection televisions or projectors, in which light must be provided to a further optical system within the projection 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. An illumination system comprising: a plurality of light sources, each light source generating light of a respective colour; a plurality of non-imaging collectors, coupled to respective light sources, for collecting light from the light source and emitting the light from a respective exit aperture thereof; a plurality of reflective elements, coupled to respective collectors, for reflecting the light emitted from the exit aperture of the respective collector; and an integrator having an input surface for receiving the light reflected by the plurality of reflective elements.

2. An illumination system as claimed in claim 1, wherein at least one of the plurality of reflective elements is a cube prism, the or each cube prism containing a dichroic mirror for reflecting light of a particular wavelength and transmitting other wavelengths without reflection.

3. An illumination system as claimed in claim 2, wherein the dichroic mirror in the or each cube prism is selected so that it reflects the wavelength of light emitted from the respective collector coupled thereto.

4. An illumination system as claimed in one of claims 1 to 3, wherein the each light source is made of light emitting diode material.

5. An illumination system as claimed in any preceding claim, wherein there are three light sources.

6. An illumination system as claimed in claim 5, wherein the plurality of primary colours are red, green and blue.

7. An illumination system as claimed in any preceding claim, wherein each of the collectors are parabolic collectors.

8. An illumination system as claimed in one of claims 1 to 6, wherein each of the collectors are elliptical collectors.

9. An illumination system as claimed in any preceding claim wherein the surface area of a light source is less than the area of the exit aperture of a collector.

10. An illumination system as claimed in claim 1, comprising three light sources, three non-imaging collectors, and three reflective elements, wherein: each of the collectors extends in a respective direction generally parallel to the input surface of the integrator; a first reflective element, coupled to a first collector and associated with a first light source, is in the form of a first dichroic mirror, for reflecting light from the first light source towards the input surface of the integrator, and for transmitting incident light from the second and third light sources; a second reflective element, coupled to a second collector and associated with a second light source, is in the form of a second dichroic mirror, for reflecting light from the second light source through the first dichroic mirror towards the input surface of the integrator, and for transmitting incident light from the third light source; and a third reflective element, coupled to a third collector and associated with a third light source, is in the form of a mirror, for reflecting light from the third light source through the second dichroic mirror and the first dichroic mirror towards the input surface of the integrator.

11. A projection system comprising a light source, and an optical system for forming an image using light from the light source, wherein the light source comprises an illumination system as claimed in any one of claims 1 to 10.