WO2013088295A1

WO2013088295A1 - Optical element and method for homogenizing light

Info

Publication number: WO2013088295A1
Application number: PCT/IB2012/056930
Authority: WO
Inventors: Teunis Willem Tukker; Willem Lubertus Ijzerman; Sebastianus Adrianus GOORDEN
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-12-14
Filing date: 2012-12-04
Publication date: 2013-06-20

Abstract

An elongated optical element (100) arranged for homogenizing a bundle of light rays (110) is provided. A perimeter (120) of a transversal cross-section (121) of the optical element is defined by at least two edges (122) of zero curvature and vertices (123) between any two adjacent ends of the at least two edges, wherein at least one of the vertices is a segment with positive curvature, its length constituting at least 1% and at most 90% of the length of the perimeter.

Description

Optical element and method for homogenizing

FIELD OF THE INVENTION

The present invention relates to an optical element and a method for homogenizing a bundle of light rays. BACKGROUND OF THE INVENTION

Numerous systems in various fields of industry require a provision of a beam of light which is homogeneous (uniform) across the span of the light beam, in terms of properties such as a constant illuminance and/or colour. For example, in many medical applications such as laser therapy, laser bio-stimulation, and photodynamic therapy, it is highly desirable that the bundle of light rays has a homogeneous illuminance. As most light sources, however, emit a light which is non-homogeneous, light filtering and/or devices for light correction have been proposed to obtain the sought-for homogeneity of the light.

In a case where the light is generated by one or more light sources (e.g. LEDs with different colours), a mixing of the light may be performed with the aim of rendering a homogeneous light. The mixing of the light may be carried out by guiding the light from the light source(s) through an optical guide. Embodiments of optical guides are solid mixing rods (e.g. a glass/plastic fibre, rod, tube, or the like), utilizing the total internal reflection (TIR) at the interfaces towards the surrounding medium (reflection back and forth), such that the light reflected within the mixing rod has been mixed when exiting the mixing rod.

The structure of the mixing rod is of importance to obtain a preferred mixing of the light, and various geometric structures of the mixing rods have been proposed for this purpose. From practice, it is known that mixing rods having square cross-sections are superior to circular ones, but that rods with hexagonal cross-sections are even better for the purpose of obtaining a uniform light. Although hexagonal mixing rods are widely used nowadays, this geometric shape does not provide an adequate homogeneity of the

illuminance at the exit face of the mixing rod. More specifically, the light rays are not adequately mixed in the far field of the mixing rod since the mixing does not sufficiently alter the angles of the light rays. In view of this, there is a wish to provide an optical element which provides an improved mixing of the light.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical element and a method for homogenizing a bundle of light rays. This and other objects are achieved by an optical element and a method having the features set forth in the independent claims.

Preferred embodiments are defined in the dependent claims.

Hence, according to a first aspect of the present invention, there is provided an elongated optical element arranged for homogenizing a bundle of light rays. The perimeter of a transversal cross-section of the optical element is defined by at least two edges of zero curvature and vertices between any two adjacent ends of the edges. At least one of the vertices is a segment with positive curvature, wherein its length constitutes at least 1% and at most 90% of the length of the perimeter. Thus, the present invention is based on the idea of providing an optical element having a transversal cross-section wherein the vertices between the edges have positive curvature, i.e. wherein the vertices are outwardly curved/rounded. By the term "zero curvature", it is here meant that the edges of the transversal cross-section are even/straight, i.e. not curved in the plane of the cross-section. Furthermore, by the term "vertices", it is here meant corners/angles between the edges of the transversal cross-section of the optical element. By the term "positive curvature", it is here meant that the at least one vertex is rounded outwards from the perimeter (i.e. bulging convexly outwards). When light rays are directed into the optical element and are reflected therein, the curved vertices of the optical element lead to an improved mix of the light rays in the optical element, compared to a mixing rod without rounded corners/vertices. The purposefully shaped/designed perimeter of the optical element enhances the irregularity/chaos of the mixing of the light, and leads to an increased homogeneity of the bundle of light rays when exiting the optical element.

According to a second aspect of the present invention, there is provided a method for homogenizing a bundle of light rays. The method comprises the step of directing a bundle of light rays towards an entry face of an elongated optical element. Furthermore, the method comprises the step of mixing the bundle of light rays by means of the optical element, wherein a perimeter of a transversal cross-section of the optical element is defined by at least two edges of zero curvature and vertices between any two adjacent ends of the edges. At least one of the vertices is a segment with positive curvature, wherein its length constitutes at least 1% and at most 90% of the length of the perimeter. Furthermore, the method comprises the step of extracting the bundle of light rays from the exit face of the optical element.

The present invention is advantageous in that the elongated optical element provides an improved mixing of a bundle of light rays, such that a more homogeneous light is obtained after the bundle of light rays has passed through the optical element, compared to existing mixing rods. More specifically, the edges and the vertices of the optical element, wherein the vertices have positive curvature, provide an enhanced mixing of the light due to an improved scattering/reflection of the light within the optical element. This is realized as the perimeter of the optical element, comprising rounded/curved vertices, increases the number of directions of the light ray reflection/scattering as the normal angle of the curved vertices varies continuously. It will be appreciated that in an optical element with a transversal cross-section comprising only straight edges, a ray direction can generally only change by multiples of 2π/η, where n is the number of edges. In contrast, the optical element of the present invention decreases the number of stable trajectories of the reflected light, i.e. trajectories of the light ray reflections having a periodic propagation within the optical element, and provides a more homogeneous light (i.e. even distribution of the light components) at the exit face of the optical element with respect to one or more of e.g.

luminous intensity, colour point, wavelength spectrum, etc., compared to mixing rods in the prior art.

The present invention is further advantageous in that the improved mixing of the light is provided solely by the geometrical shape of the optical element. In other words, the enhanced homogeneity of the light is obtained merely by the geometrical features of the optical element, such that additional measures for the purpose of improving the mixing of the light (e.g. a coating or other treatments of the inside of the optical element and/or a provision of auxiliary elements to the optical element for the purpose of improving the reflectivity) may be rendered superfluous. Consequently, the optical element of the present invention is easy to manufacture, as the optical element may be produced merely from the material which has the purpose of guiding and mixing the light (e.g. a transparent material comprising glass, plastic, or the like), thereby omitting the need of other (auxiliary) materials. Moreover, as additional treatments (e.g. inside coatings) may be refrained from, rendering the manufacture of the optical element of the present invention relatively inexpensive. Since the element may be manufactured from a single material, the optical element is easily recyclable.

Another advantage associated with the present invention is that the geometry of the transversal cross-section of the optical element further provides an earlier homogeneity of the bundle of light rays in a direction from the entry face to the exit face of the optical element compared to known mixing rods. In other words, a bundle of light rays led into the entry face of the optical element is quickly mixed along the elongated optical element due to the optimized perimeter of the transversal cross-section according to the present invention. The optical element of the present invention is able to achieve the task of mixing the initially non-homogeneous bundle of light rays into a homogenized light earlier along its elongation compared to mixing rods in the prior art. Hence, the optical element of the present invention may have a relatively shorter length than other mixing rods in the prior art to fulfil this given task. This is highly advantageous, as the present invention thereby provides an even lower manufacture cost of the optical element, a lower weight, a more convenient handling and/or transportation, and/or a simplified procedure if the optical element is to be mounted into an optical system.

The elongated optical element is arranged for homogenizing a bundle of light rays. In other words, the optical element is arranged to mix/homogenize a bundle of light rays of one or more light sources, which light rays are directed into an entry face of the optical element. After passing through the optical element, the bundle of light rays are mixed into a homogeneous light at an exit face of the optical element.

The perimeter of a transversal cross-section of the optical element is partly defined by at least two edges of zero curvature, i.e. even/straight edges (not curved). As the perimeter (i.e. the boundary) of the transversal cross-section comprises at least two edges of zero curvature, the cross-section is not circular.

Furthermore, the perimeter of the transversal cross-section of the optical element is partly defined by vertices between any two adjacent ends of the at least two edges.

In other words, the perimeter is defined by the at least two edges and vertices between any two adjacent ends of these edges. At least one of the vertices is a segment with positive curvature, i.e. a segment outwardly rounded from the perimeter. Furthermore, it will be appreciated that the positively curved vertex thereby differs from an angle with a sharp corner having an undefined derivative.

At least one of the vertices, being a segment with positive curvature, has a length constituting at least 1% and at most 90% of the length of the perimeter. In other words, of the entire length of the perimeter, defined by the edges and the vertices between any two adjacent ends of the edges, the length of the curved vertices represents 1—90% of the length of the perimeter. Intervals defining suitable total vertex lengths may have one of the numbers 1 , 2, 5, 10, 15, 20, 30, 40, 50% as lower endpoint and may independently have any one of the numbers 90, 85, 80, 70, 60, 50, 40% as upper endpoint.

According to an embodiment of the present invention, the optical element may be shaped as a cylinder. In other words, the optical element of the present embodiment has an elongated shape, and has a transversal cross-section which is constant along the longitudinal axis of the optical element. The cylinder- shaped optical element is advantageous in that it is easily manufactured, e.g. by extrusion. Furthermore, the present embodiment is advantageous in a case the length of the optical element needs to be changed. For example, if the optical element is shortened, the cross-section of the exit face, as well as the cross-section between the entry face and the exit face will still be the same. Hence, the length of the optical element may be adapted more easily according to the required mixing of the light. Alternatively, the optical element may have a frustoconical shape, wherein the entry face and the exit face have different sizes. For example, the cross-section may decrease along the longitudinal axis of the optical element, such that the exit face has a smaller area than the entry face.

According to an embodiment of the present invention, at least one of the vertices may be a segment of a circular arc. In other words, the at least one vertex may be a portion of the circumference of a circle. The present embodiment is advantageous in that the vertex has a continuous and symmetric rounding which even further contributes to the mixing of the light.

According to an embodiment of the present invention, the radius of the circular arc may be equal to the length of at least one of the at least two edges. The present embodiment is advantageous in that it decreases the number of stable trajectories of light ray reflections within the transversal cross-section of the optical element, as can be verified by numerical simulations. For example, if the cross-section of the optical element has a polygon shape with an odd number of edges, only one stable trajectory exists between the segment of the circular arc and the opposing edge having the same length as the radius of the circular arc. Hence, the present embodiment will achieve to an even more improved mixing of the light.

According to an embodiment of the present invention, the radius of the circular arc may be greater than the length of at least one of the at least two edges. An advantage of the present embodiment is that it even further decreases the number of stable trajectories of light ray reflections within the optical element. For example, if the transversal cross-section of the optical element has a polygon shape with an odd number of edges, there exists a unique trajectory between the segment of the circular arc and the opposing edge, wherein the radius of the circular arc is greater than the opposing edge. Hence, the present embodiment will achieve an even more improved mixing of the light.

According to an embodiment of the present invention, all of the at least two edges may be of equal length and all of the vertices are of equal length. In other words, the edges of the transversal cross-section of the optical element is equilateral, and the vertices between any two adjacent ends of the edges are of equal length. The present embodiment is advantageous in that the optical element provides an n-fold rotational symmetry (wherein n is the number of edges). Consequently, an alignment of the optical element is facilitated, e.g. when mounting the optical element in an optical system.

According to an embodiment of the present invention, the tangent of at least one of the vertices and the tangent of any two adjacent ends of the edges may be equal at at least one point of intersection between the at least one of the vertices and any two adjacent ends. In other words, the vertex between two adjacent ends provides a smooth

rounding/connection/patching between the edges, wherein the tangent of any point on the vertex is within the interval bounded by the tangents of the two adjacent ends and equal at the points of intersection.

According to an embodiment of the present invention, the perimeter may comprise six edges. In other words, the six edges constitute an hexagonal cross-section of the optical element, further comprising rounded vertices between the edges. The present embodiment is advantageous in that the mixing of the light by the optical element of the present embodiment is superior to the mixing which is achieved by mixing rods in the prior art having merely a hexagonal cross-section without rounded corners. This is realized as the rounded vertices in the hexagonal cross-section of the present embodiment increases the number of possible/distinct light ray reflections within the optical element. Furthermore, as mixing rods with hexagonal cross-sections are known in the prior art, the present

embodiment is further advantageous in that the equipment for the manufacture of the mixing rods from the prior art is easily modified for the manufacture of the optical element according to the present embodiment, wherein the hexagonal cross-section further comprises rounded vertices to provide an improved mixing of the light.

According to an embodiment of the present invention, the perimeter may be defined by three edges wherein two edges are perpendicular and of equal length, the vertices being three circular arcs of equal radius, and wherein the radius is equal to the length of one of the two edges. In other words, the transversal cross-section of the optical element is shaped as a right-angled triangle, having two perpendicular edges as catheti and one edge as hypotenuse, but wherein the vertices are rounded such that no sharp corners exist. Further, the vertices are circular arcs having the same radius, wherein the radius is equal to the length of one of the two edges (catheti). As stable trajectories of light ray reflections are known to exist in any transversal cross-section that has n-fold rotational symmetry (wherein n is an integer), only one stable trajectory of light ray reflections is present in an optical element having the perimeter as defined in the present embodiment. Hence, an advantage with the present embodiment is that the homogeneity of the light at the exit face of the optical element is even further increased.

According to an embodiment of the present invention, there is provided an optical system for homogenizing a bundle of light rays, comprising an elongated optical element as defined in any one of the preceding embodiments and at least one light source arranged at a transversal entry face of the optical element, arranged for directing a bundle of light rays towards the entry face. The optical system may comprise two or more light sources, e.g., light sources emitting light with different properties which it is desired to mix into one homogeneous bundle.

According to an embodiment of the present invention, the optical system comprises a collimating means arranged at a transversal exit face of the optical element for collimating the bundle of light rays exiting the optical element. By "collimating means" it is here meant substantially any means for the purpose of collimating the light at the exit face of the optical element into a beam of a required/desired shape, wherein examples of a collimating means may be a lens, a Soller collimator or a TIR collimator.

Further objectives of, features of, and advantages with, the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art will realize that different features of the present invention can be combined to create embodiments other than those described in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention.

Fig. 1 is a schematic illustration of an elongated optical element according to an embodiment of the present invention;

Figs. 2a-b are schematic illustrations of transversal cross-sections of optical elements; Figs. 3a-b are schematic illustrations of trajectories of light ray reflections; Fig. 4 is a schematic illustration of a transversal cross-section of an elongated optical element according to an embodiment of the present invention; and

Fig. 5 is a schematic illustration of an optical system according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, the present invention is described with reference to an elongated optical element arranged for homogenizing a bundle of light rays.

Fig. 1 is a schematic illustration of an elongated optical element 100 according to an embodiment of the present invention. The optical element 100, which may be made of a transparent material like glass or plastic, is shaped as a cylinder and comprises an entry face 101 and an exit face 102. At operation, a bundle of light rays 110 is directed towards the entry face 101, wherein the bundle of light rays 110 undergoes total internal reflection (TIR) at the interfaces towards the surrounding medium. After being reflected within the optical element 100, the bundle of light rays 110 exits the optical element 100 through the exit face 102.

The contour of the optical element 100 in Fig. 1 is designed such that the perimeter 120 of a transversal cross-section 121 of the optical element 100 is defined by six edges 122 of zero curvature and six vertices 123 between any two adjacent ends of the edges 122. In the present embodiment, the edges 122 are of equal length and the vertices 123 are of equal length. The vertices 123 are segments with positive curvature (rounded segments), wherein the lengths of the vertices 123 in Fig. 1 constitute 30-50% of the length of the perimeter 120. When the bundle of light rays 110 is directed into the optical element 100, the curved/rounded vertices 123 of the optical element 100 lead to an improved mix of the angles and positions of the light rays in the optical element 100, compared to a mixing rod without rounded corners/vertices. In three dimensions, the corners/vertices 123 may alternatively be referred to as rounded ridges 123 extending longitudinally in the optical element 100. By virtue of its geometry, the optical element 100 provides an increased homogeneity of the bundle of light rays 110 when exiting the optical element 100 by the exit face 102.

Fig. 2a is a schematic illustration of a transversal cross-section 200 of a mixing rod according to the prior art, wherein the cross-section 200 is hexagonal, comprising six edges 201 and six corners 202. However, the geometric shape of the cross-section 200 as disclosed does not provide the desired homogeneity of the illuminance at the exit face of the mixing rod. More specifically, the light rays are not adequately mixed in the far field of the mixing rod since the mixing does not sufficiently alter the angles of the light rays.

Fig. 2b is a schematic illustration of the transversal cross-section 121 of the optical element 100 according to Fig. 1. Compared to the sharp corners 202 of the cross- section 200 as shown in Fig. 2a, the vertices 123 of the optical element 100 are segments with positive curvature. As a result, the cross section 121, comprising the vertices 123, provides an improved mixing of a bundle of light rays directed into the optical element 100 compared to the use of an optical element with the cross-section 200.

Fig. 3a is a schematic illustration of trajectories of light ray reflections within an exemplifying optical element 300. Here, the transversal cross-section 321 of the optical element 300 comprises four straight edges 322 and four vertices 323 with positive curvature. The four edges 322 of the cross-section 321 define an essentially quadratic shape, whereas the curved/rounded vertices 323 smoothen the sharp corners of the square. Although the cross-section 321 comprises rounded vertices 323, stable trajectories are still present in the contour of the optical element 300. Three exemplifying trajectories are indicated in Fig. 3a as 330, 331 and 332. The first trajectory 330 is located between the base edge and the top edge of the cross-section 321, as a light ray may be reflected back and forth between the parallel base edge and top edge of the cross-section 321. Analogously, the second trajectory 331 is located between the left edge and the right edge of the cross-section 321, as a light ray may be reflected back and forth between the parallel left edge and right edge of the cross-section 321. Furthermore, the third trajectory 332 is located diagonally in the cross-section 321, wherein the light ray is reflected from one edge to an adjacent edge by 90°, such that the trajectory 332 encloses a rectangular shape by reflection from all four edges 322.

Fig. 3b is a schematic illustration of trajectories of light ray reflections within an exemplifying optical element 350. Here, the transversal cross-section 371 of the optical element 350 comprises three edges 372 and three vertices 373 with positive curvature. The three edges 372 of the cross-section 371 define a triangular shape, whereas the

curved/rounded vertices 373 smoothen the sharp corners of the triangle. Analogously to Fig. 3a, stable trajectories are still present in the contour of the optical element 350. An exemplifying stable trajectory is indicated in Fig. 3b as 380, wherein the a light ray is reflected from the three edges 372 such that the trajectory 380 achieves a pattern of triangles.

Fig. 4 shows a schematic illustrations of a transversal cross-section 421 of an elongated optical element 400 according to an embodiment of the present invention. In this embodiment, the transversal cross-section 421 of the optical element 400 comprises three straight edges 422 and three vertices 423 with positive curvature. The three edges 422 of the cross-section 421 provides the shape of a right-angled triangle, having two perpendicular edges as catheti and one edge as hypotenuse, but wherein the vertices 423 are rounded such that no sharp corners exist. Further, the vertices 423 are circular arcs having the same radius R wherein the radius R is equal to the length L of one of the two edges (catheti). Compared to transversal cross-sections having n-fold rotational symmetry (wherein n is an integer), e.g. the cross-sections 321 and 371 as shown in Fig. 3a and Fig. 3b, respectively, the perimeter of the optical element as defined in the present embodiment has only one stable trajectory 390 of light ray reflection. This trajectory 390 is located in Fig. 4 as a reflection between the edge, which represents the hypotenuse of the cross-section 421, and its opposite vertex.

Hence, the present embodiment is advantageous in that the contour of the cross-section 421 provides a homogeneity of the light at the exit face of the optical element which is even further increased.

Fig. 5 is a schematic illustration of an optical system 500 according to an embodiment of the present invention. The optical system comprises an elongated optical element 100 as described in any one of the preceding embodiments. Three light sources 510 are arranged at a transversal entry face 501 of the optical element 100, arranged for directing a bundle of light rays 110 towards the entry face 501. The optical system 500 further comprises a collimating means 511 arranged at a transversal exit face 502 of the optical element 100 for collimating the bundle of light rays 110 exiting the optical element 100.

Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent to those skilled in the art after studying this description. The described embodiments are therefore not intended to limit the scope of the invention, which is only defined by the appended claims. For example, in Fig. 1, the relationship between the diameter of the entry face 101 and the length of the elongated optical element 100 may be different from that shown. For example, the optical element 100 may be thinner (longer) or thicker (shorter) in relation to the entry face 101, such that the ratio between the length of the optical element 100 and the entry face 101 becomes larger or smaller, respectively. Furthermore, in Fig. 2b, the vertices 123 may constitute a greater or a smaller portion of the length of the perimeter 120 than that shown. The light sources 510 as shown in Fig. 5 are shown as light bulbs for an enhance understanding of the figure, and it will be appreciated that the light sources 510 may alternatively be LEDs, as described earlier. It will also be appreciated that the number of elements shown/described may vary. For example, the three light sources 510 in Fig. 5 may alternatively be any number of light sources.

Claims

CLAIMS:

1. An elongated optical element (100) arranged for homogenizing a bundle of light rays (110), wherein a perimeter (120) of a transversal cross-section (121) of said optical element is defined by:

at least two edges (122) of zero curvature, and

- vertices (123) between any two adjacent ends of said at least two edges, wherein

at least one of the vertices is a segment with positive curvature, its length constituting at least 1% and at most 90% of the length of said perimeter.

2. The elongated optical element (100) as claimed in claim 1, wherein said optical element is shaped as a cylinder.

3. The elongated optical element (100) as claimed in claim 1, wherein at least one of said vertices (123) is a segment of a circular arc.

4. The elongated optical element (100) as claimed in claim 3, wherein the radius of said circular arc is equal to the length of at least one of said at least two edges (122).

5. The elongated optical element (100) as claimed in claim 3, wherein the radius of said circular arc is greater than the length of at least one of said at least two edges (122).

6. The elongated optical element (100) as claimed in claim 1, wherein all of said at least two edges (122) are of equal length and all of the vertices (123) are of equal length.

7. The elongated optical element (100) as claimed in claim 1, wherein the tangent of at least one of said vertices (123) and the tangent of said any two adjacent ends of said at least two edges (122) are equal at at least one point of intersection between said at least one of said vertices and said any two adjacent ends.

8. The elongated optical element (100) as claimed in claim 1, wherein said perimeter (120) comprises six edges.

9. The elongated optical element (400) as claimed in claim 1, wherein said perimeter (421) is defined by three edges (422) wherein two edges are perpendicular and of equal length, said vertices (423) being three circular arcs of equal radius (R), and wherein said radius is equal to the length of one of said two edges.

10. An optical system (500) for homogenizing a bundle of light rays (110), comprising:

at least one elongated optical element (100) as claimed in claim 1, and at least one light source (501) arranged at a transversal entry face of said optical element, arranged for directing said bundle of light rays towards said entry face.

11. The optical system (500) as claimed in claim 10, further comprising at least one collimating means (502) arranged at a transversal exit face of said optical element (100) for collimating said bundle of light rays (110) exiting said optical element.

12. A method for homogenizing a bundle of light rays (110), said method comprising the steps of:

directing said bundle of light rays towards a transversal entry face of an elongated optical element (100),

mixing said bundle of light rays by means of said optical element, wherein a perimeter (120) of a transversal cross-section (121) of said optical element is defined by at least two edges (122) of zero curvature and vertices (123) between any two adjacent ends of said at least two edges, and wherein at least one of the vertices is a segment with positive curvature, its length constituting at least 1% and at most 90% of the length of said perimeter, and

extracting said bundle of light rays from the exit face of said elongated optical element.