WO2008139355A1

WO2008139355A1 - Illumination system

Info

Publication number: WO2008139355A1
Application number: PCT/IB2008/051725
Authority: WO
Inventors: Elvira J. M. Paulussen; Joseph L. A. M. Sormani
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-05-11
Filing date: 2008-05-05
Publication date: 2008-11-20
Also published as: TW200914767A

Abstract

The present invention relates to a light output system (10),comprising: an array of adjacently arranged non-imaging optical elements (12), wherein each optical element is adapted to homogenize and collimate light from a light source unit (18) associated with the optical element; and a lens (20) adapted to receive and further shape the light homogenized and collimated by the optical elements. The present system has improved lumen output and compact size.

Description

ILLUMINATION SYSTEM

FIELD OF THE INVENTION

The present invention relates to a light output system.

BACKGROUND OF THE INVENTION In light emitting diode (LED) illumination applications, mixing (e.g. color mixing) and collimation optics are often discussed topics. To this end, compound parabolic concentrators (CPCs) are used in many applications where light mixing and beam collimation are desired. In such applications, the CPC has a small input end where light from an LED is introduced and a large output end where manipulated light exits the CPC. However, to achieve high collimation (low collimation output angle), the CPC has to be considerably long. To reduce the total length, the CPC can be replaced by a shorter CPC, which has a larger output angle, and an aspherical lens placed in front of the shorter CPC for providing additional collimation. The total combination (CPC+lens) will be much shorter compared to a single (longer) CPC with the same collimation angle. However, when such a CPC-lens combination is used together with a single

LED, the output flux is limited. Hereto, several LEDs can be provided for the CPC-lens combination. Though the lumen output is increased, the several LEDs require a significantly larger CPC and lens, resulting in a system having increased overall size, making it not suitable for use in certain applications where size matters, for instance in portable devices.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partly overcome this problem, and to provide an improved light output system. A particular object is to provide a light output system having improved lumen output and compact size. These and other objects that will be apparent in the following summary and description are achieved by a light output system according to claim 1, comprising: an array of adjacently arranged non- imaging optical elements, wherein each optical element is adapted to homogenize and collimate light from a light source unit associated with the optical element; and a lens adapted to receive and further shape the light homogenized and collimated by the optical elements.

The invention is based on the understanding that by placing several such optical elements or components close together in an array, a single lens can be used to further shape the light manipulated by the several optical elements, which provides for a compact system (one lens only, small optical elements), while the lumen output may be increased (several light source units compared to a single light source). The adjacently arranged optical elements may be placed with or without a small gap between flanking elements, as will be explained more below. Also, 'collimate' in the context of the present application generally means that the light beam is made less divergent, not necessarily with completely parallel light rays.

Preferably, an output end of each optical element is arranged immediately at the side of the output end of at least one other optical element. Each output end may for instance be in contact with the output end of at least one other optical element of the array. In this way, when the lens is adapted to image the output ends of the optical elements of the array in the far field, seams or gaps between the imaged output ends in the resulting illumination window may be reduced or eliminated, which provides for a uniform illumination window. A small gap (for instance up to about 10 per cent of the output end diameter) between the output ends of the optical elements of the array could be accepted, with reduced - but still decent - performance (with respect to for instance illumination uniformity) as a result (if there is a gap, seams may show in the illumination window in the far field). A too large gap however may result in an inconsistent intensity in the illumination windows. If there is a small gap, the lens can for instance be adapted to image a surface slightly behind the output ends, in order to blur the seams in the illumination window and make them less evident. Also, instead of imaging in the far field, a DLP or LCD in a projector can be illuminated, for instance.

The lens is preferably an aspherical lens, though other suitable lenses could be used, such as a spherical lens. An aspherical lens has however better performance than such other lenses. In one embodiment, the optical elements are m*n in number, m and n are integers with at least one of m and n larger than 1, and the m*n optical elements are arranged in m rows and n columns. For instance, m=n=2 results in four optical elements arranged in a 2x2 array. Other arrays include 3x3, 3x2, etc. In any case, the number of optical elements and the field of the lens (acceptance angle) should be matched. Preferably, the optical elements are square shaped. For instance, the output ends of the optical elements may be square shaped. Hereto, in a 2x2 array, each optical element may provide one quadrant, the quadrants together (seamlessly) forming a square or rectangular (i.e. four sided with essentially right angles) illumination window in the far field. A square or rectangular illumination window is advantageous in illumination applications like video/still cameras or camera equipped mobile phones, (video) projectors, illumination of objects (buildings, art decoration, windows, etc.), and so on. Alternatively, the optical elements may have other shapes: rectangular, circular, honeycomb, etc. By selecting suitable shapes and arrays, various aspect ratios of the illumination window may be provided, e.g. 1 :1, 4:3, 16:9, etc.

Also preferably, the above non-imaging optical elements are compound parabolic concentrators (CPCs), which have superior performance when it comes to collimation and homogenization (i.e. making the intention distribution uniform). Alternatively, other non-imaging optical elements could be used, such as rods, tapered structures, etc. Also, the optical element could be hollow or solid. A hollow optical element may have a reflective inner surface, while a solid optical element may be filled with a material having a suitable refractive index.

Further, in one embodiment, each light source unit comprises at least one LED. Advantages of LEDs include instant on, long life time, etc. Preferably, one LED per optical element is used, in order to minimize the size of the optical elements and the lens. For instance, using one LED per optical element requires an optical element having about half the size of an optical element required for four LEDs. Instead of LEDs, other suitable light sources could be used.

In another embodiment, the light output system further comprises a diffuser placed between the optical elements and the lens, in order to further homogenize the luminous intensity distribution to provide a uniform illumination window. Alternatively, or as a complement, a similar diffuser may be positioned behind the lens, i.e. on the opposite side of the lens compared to the optical elements, to make the illumination more uniform. Further, the light output system is preferably an illumination system, i.e. it has a lumen output high enough to at least partly illuminate an object. BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention. Fig. 1 is a schematic perspective view of a light output system according to an embodiment of the present invention.

Fig. 2 illustrates an exemplary illumination window of the system of fig. 1.

DETAILED DESCRIPTION Fig. 1 is a schematic perspective view of a light output system 10 according to an embodiment of the present invention. The light output system 10 is preferably an illumination system adapted to at least partly illuminate an object.

The system 10 comprises an array with four compound parabolic concentrators (CPCs) 12. Each CPC has a small input end (input aperture) 14 and a large output end (output aperture) 16. At the input end 14 of each CPC 12, there is provided a light source unit comprising an LED 18, resulting in a total of four LEDs 18. As in prior art, per se, each CPC 12 is a non- imaging optics serving to collimate the light emitted from the associated LED 18 and to make the illumination intensity uniform. The CPCs 12 are "short", meaning that they generally do not produce perfect collimation. Further, the CPCs 12 are square shaped, each having at least a square output end 16. The input ends 14 could be round or squared. Also, the CPCs 12 are similarly oriented, that is, the optical axes of the CPCs 12 are essentially parallel and the output ends

16 are facing essentially the same direction.

Further, the CPCs 12 are arranged in a 2x2 array (two rows and two columns) so that the output end 16 of each CPC 12 is arranged immediately next to the output end 16 of at least one other CPC 12. In fig. 1, each square output end 16 has two sides in contact with sides of two other output ends 16, respectively.

Upon operation, light emitted from each LEDs 18 will be shaped to a narrower beam by its associated CPC 12 acting as primary optics. Generally, the light is collimated to about ±20-40 degrees (i.e. it diverges up to about ±20-40 degrees from the optical axis). The light emitted from each CPC-LED combination 12, 18 will soon after exiting the output ends

16 be overlapping due to the nearness of the CPCs 12.

Next, the system 10 further comprises a single aspherical lens 20 (i.e. a lens having a non- spheric surface) placed in front of the CPCs 12. The lens is adapted to further shape, preferably collimate, the output of the four CPCs 12, functioning as secondary optics. Hereto, the lens 20 should be sized and placed so that it receives essentially all light emitted by the four CPC-LED combinations 12, 18. Thus, the single lens 20 serves all the CPC-LED combinations 12, 18. Thus, the system 10 may provide a collimated illumination with high brightness.

Further, the lens 20 is an imaging lens. In particular, the lens 20 is adapted to image the adjacent square output ends 16 of the CPCs 12 of the array in a far field (e.g. 2 m away), and a resulting exemplary illumination window 22 of the system 10 (the output from the lens 20) in the far field is shown in fig. 2. Hereto, each CPC-LED combination 12, 18 contribute with one quadrant 24, together forming a square, pixilated illumination window 22. To this end, the aspherical lens 18 may be designed so that the quadrants 24 originating from the adjacent output ends 16 are almost perfectly joint together without seams.

The uniformity of the illumination window 22 will be enough for some applications, but it may be increased by placing an optional diffuser 26, for instance a Lambertian diffuser with an angle spread of 10-20 degrees or a holographic diffuser, between the CPCs 12 and the lens 20 (see fig. 1). The diffuser 26 may for instance be placed at the surface of the lens 20 facing the CPCs 12, covering essentially the entire surface, as in fig. 1. A diffuser generally diffuses or spreads out light. Hereto, the diffuser 26 serves to level or smooth out any variations (inter and/or infra) in luminance, e.g., of the above quadrants 24. The exemplary illumination window 22 in fig. 2 is with the diffuser 26, where darker/more dense areas indicate somewhat different intensity.

The present light output system is more compact than the system with several LEDs per CPC (e.g. the CPC length can be halved). Further, the lumen output is increased compared to the prior art single CPC-lens combination since light from several (four) light sources is used.

Applications of the present light output system includes illumination in video/still cameras, camera phones and projectors, illumination of objects (buildings, art decoration, windows, etc.), etc.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiment described above. On the contrary, many modifications and variations are possible within the scope of the appended claims, as indicated in the above summary and description. For instance, the CPCs could be placed somewhat away from each other, still with decent performance. Also, instead of CPCs, other optical components could be used. Also, the diffuser could be arranged in a different position between the CPCs and the lens, for instance just in front of the output ends of the CPCs. Alternatively, or as a complement, a similar diffuser may be positioned on the other side of the lens compared to the diffuser shown in fig. 1.

Claims

CLAIMS:

1. A light output system (10), comprising: an array of adjacently arranged non-imaging optical elements (12), wherein each optical element is adapted to homogenize and collimate light from a light source unit (18) associated with the optical element; and a lens (20) adapted to receive and further shape the light homogenized and collimated by the optical elements.

2. A light output system according to claim 1, wherein an output end (16) of each optical element is arranged immediately at the side of the output end of at least one other optical element.

3. A light output system according to claim 1 or 2, wherein the lens is an imaging lens.

4. A light output system according to any one of the preceding claims, wherein the lens is an aspherical lens.

5. A light output system according to any one of the preceding claims, wherein the optical elements are m*n in number, m and n are integers with at least one of m and n larger than 1, and the m*n optical elements are arranged in m rows and n columns.

6. A light output system according to any one of the preceding claims, wherein the optical elements are square shaped.

7. A light output system according to any one of the preceding claims, wherein the optical elements are compound parabolic concentrators (CPCs).

8. A light output system according to any one of the preceding claims, wherein each light source unit comprises at least one light emitting diode.

9. A light output system according to any one of the preceding claims, further comprising a diffuser arranged between the optical elements and the lens.

10. A light output system according to any one of the preceding claims, further comprising a diffuser arranged behind the lens.

11. A light output system according to any one of the preceding claims, wherein the light output system is an illumination system.