(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`(19) World Intellectual Property
`Organization
`International Bureau
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`(10) International Publication Number
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`\fl
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`(43) International Publication Date
`WO 2013/114259 A1
`8 August 2013 (08.08.2013) WIPOI PCT
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`(51)
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`International Patent Classification:
`G023 6/42 (2006.01)
`A613 18/22 (2006.01)
`G02B 27/09 (2006.01)
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`(21)
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`International Application Number:
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`PCT/IB2013/050665
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`(22)
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`International Filing Date:
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`25 January 2013 (25.01.2013)
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`(25)
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`(26)
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`(30)
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`(71)
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`(72)
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`(74)
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`(81)
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`Filing Language:
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`Publication Language:
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`English
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`English
`
`Priority Data:
`61/593,396
`
`1 February 2012 (01.02.2012)
`
`US
`
`[NL/NL];
`Applicant: KONINKLIJKE PHILIPS N.V.
`High Tech Campus 5, NL-5656 AE Eindhoven (NL).
`
`Inventors: TUKKER, Teunis, Willem; c/o High Tech
`Campus, Building 44, NL-5656 AE Eindhoven (NL).
`IJZERMAN, Willem, Lubertus; c/o High Tech Campus,
`Building 44, NL-5656 AE Eindhoyen (NL). GOORDEN,
`Sebastianus, Adrianus; c/o High Tech Campus, Building
`44, NL-5656 AE Eindhoven (NL).
`
`Agents: VAN EEUWIJK, Alexander, Henricus, Walter-
`us et al.; High Tech Campus Building 44, NL-5656 AE
`Eindhoyen (NL).
`
`Designated States (unless otherwise indicated, for every
`kind of national protection available): AE, AG, AL, AM,
`
`A0, AT, AU, Az, BA, BB, BG, BH, BN, BR, Bw, BY,
`BZ, CA, CH, CL, CN, CO, CR, CU, CZ, DE, DK, DM,
`DO, DZ, EC, EE, EG, ES, FI, GB, GD, GE, GH, GM, GT,
`HN, HR, HU, ID, IL, IN, IS, JP, KE, KG, KM, KN, KP,
`KR, KZ, LA, LC, LK, LR, LS, LT, LU, LY, MA, MD,
`ME, MG, MK, MN, MW, MX, MY, MZ, NA, NG, NI,
`NO, NZ, OM, PA, PE, PG, PH, PL, PT, QA, R0, RS, RU,
`RW, SC, SD, SE, SG, SK, SL, SM, ST, SV, SY, TII, TJ,
`TM, TN, TR, TT, TZ, UA, UG, US, UZ, VC, VN, ZA,
`ZM, ZW.
`
`(84)
`
`Designated States (unless otherwise indicated, for every
`kind of regional protection available): ARIPO (BW, GH,
`GM, KE, LR, LS, MW, MZ, NA, RW, SD, SL, SZ, TZ,
`UG, ZM, ZW), Eurasian (AM, AZ, BY, KG, KZ, RU, TJ,
`TM), European (AL, AT, BE, BG, CH, CY, CZ, DE, DK,
`EE, ES, FI, FR, GB, GR, HR, HU, IE, IS, IT, LT, LU, LV,
`MC, MK, MT, NL, NO, PL, PT, RO, RS, SE, SI, SK, SM,
`TR), OAPI (BF, BJ, CF, CG, CI, CM, GA, GN, GQ, GW,
`ML, MR, NE, SN, TD, TG).
`Published:
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`with international search report (Art. 21(3))
`
`before the expiration of the time limit for amending the
`claims and to be republished in the event of receipt of
`amendments (Rule 48.2(h))
`
`(54) Title: METHOD, OPTICAL SYSTEM AND LIGHTING ARRANGEMENT FOR HOMOGENIZING LIGHT
`
`
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`(57) Abstract: A method, an optical system and a lighting arrangement for homogenizing a bundle of light rays (110) by means of
`an elongated optical element (100) arranged for homogenizing light. The bundle of light rays is directed into a transversal entry face
`(101) of the optical element, and into at least one of the following geometrical regions: (a) a neighbourhood of the perimeter of the
`entry face; (b) a neighbourhood of at least a portion of a first line segment (R1) extending from the centre of the entry face to a mid-
`point of a vertex (123); (c) a neighbourhood of at least a portion of a second line segment (R2a) extending from the centre of the
`entry face to a midpoint of an edge (122).
`
`
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`Method, optical system and lighting arrangement for homogenizing light
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`FIELD OF THE INVENTION
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`The present invention generally relates to the field of optical components.
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`More precisely, it relates to a method, an optical system and a lighting arrangement for
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`homogenizing a bundle of light rays.
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`BACKGROUND OF THE INVENTION
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`Numerous systems in various fields of industry require a provision of a beam
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`of light which is homogeneous (uniform) across the span of the light beam, in terms of
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`properties such as a illuminance and/or colour. For example, in many medical applications
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`such as laser therapy, laser bio—stimulation, and photo—dynamic therapy, it is highly desirable
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`that the light beam has a homogeneous illuminance in the output profile of the light beam. As
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`most light sources, however, emit a light which is non-homogeneous, light filtering and/or
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`devices of light correction have been proposed to obtain the sought homogeneity of the light.
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`In a case where the light is generated from a plurality of light sources (e.g.
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`LEDs with different colours), a mixing of the light may be performed with the aim of
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`rendering a homogeneous light. The mixing of the light may be carried out by guiding the
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`light from the plurality of light sources through an optical guide. Embodiments of optical
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`guides are solid mixing rods (e. g. a glass/plastic fiber, rod, tube, or the like), utilizing the
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`total internal reflection (TIR) at the interfaces towards the surrounding medium (reflection
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`back and forth), such that the light reflected within the mixing rod has been mixed whcn
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`exiting the mixing rod.
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`The structure of the mixing rod is of importance to obtain a preferred mixing
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`of the light, and various geometrical structures of the mixing rods have been proposed for this
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`purpose. From practice, it is known that mixing rods having square cross—sections are
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`superior to circular ones, but that rods with hexagonal cross-sections are even better for the
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`purpose of obtaining a uniform light. Although hexagonal mixing rods are widely used
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`nowadays, this geometrical shape does not provide an adequate homogeneity of the
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`illuminance at the exit face of the mixing rod. More specifically, the light rays are not
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`adequately mixed in the far field of the mixing rod since the mixing does not sufficiently alter
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`the angles of the light rays.
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`Furthermore, the ability of the mixing rod to mix the light is dependent on how
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`the light from the plurality of light sources is directed into the mixing rod. More specifically,
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`the directing of the light into the mixing rod is dependent on the geometrical shape of the
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`mixing rod, although the precise relationship between mixing efficiency and light incidence
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`points have not been fully explored and reduced to practice.
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`In view of this, there is a wish to provide an improved mixing of the light.
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`SUMMARY OF THE INVENTION
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`It is an object of the present invention to provide a method, an optical system
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`and a lighting arrangement for homogenizing a bundle of light rays. This and other objects
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`are achieved by a method, an optical system and a lighting arrangement having the features
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`set forth in the independent claims. Preferred embodiments are defined in the dependent
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`claims.
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`Hence, according to a first aspect of the present invention, there is provided a
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`method for homogenizing a bundle of light rays by means of an elongated optical element
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`arranged for homogenizing light. The method comprises the step of directing the bundle of
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`light rays into a transversal entry face of the optical element, having a cylinder shape. The
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`entry face comprises at least two edges of zero curvature, and vertices between any two
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`adjacent ends of the at least two edges, wherein at least one of the vertices is a segment with
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`positive curvature. Furthermore, the method comprises the step of directing the bundle of
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`light rays into at least one of the following geometrical regions: (a) a neighbourhood of the
`
`perimeter of the entry face; (b) a neighbourhood of at least a portion of a first line segment
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`extending from the centre of the entry face to a midpoint of a vertex; and (c) a neighbourhood
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`of at least a portion of a second line segment extending from the centre of the entry face to a
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`midpoint of an edge. The method further comprises the step of extracting the bundle of light
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`rays from an exit face of the optical element.
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`According to a second aspect of the present invention, there is provided an
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`optical system for homogenizing a bundle of light rays. The optical system comprises an
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`elongated optical element having a cylinder shape, arranged for mixing light. The optical
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`element comprises a transversal entry face and a transversal exit face. The entry face has a
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`perimeter comprising at least two edges of zero curvature, and vertices between any two
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`adjacent ends of the at least two edges, wherein at least one of the vertices is a segment with
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`positive curvature. The optical system further comprises at least one light source arranged at
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`the entry face, wherein the light source is arranged for directing the bundle of light rays into
`
`the entry face. The optical system may comprise two or more light sources, e.g., light sources
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`emitting light with different properties which it is desired to mix into one homogeneous
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`bundle. The bundle of light rays is directed into at least one of the above geometrical regions
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`(a), (b) and (0).
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`According to a third aspect of the present invention, there is provided a
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`lighting arrangement comprising at least one light source adapted to emit a bundle of light
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`rays. The light source or sources are arranged along at least one of the following geometries:
`
`a perimeter of a polygon; at least a portion of a first line segment extending from a centre to a
`
`vertex of a polygon; and at least a portion of a second line segment extending from a centre
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`to a midpoint of an edge of a polygon. The polygon substantially coincides with the shape of
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`an entry face of an optical element, arranged to homogenize a bundle of light rays emitted by
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`the at least one light source. The optical element has a cylinder shape, and comprises a
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`transversal entry face and a transversal exit face. The entry face has a perimeter comprising at
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`least two edges of zero curvature, and vertices between any two adjacent ends of the at least
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`two edges, and wherein at least one of the vertices is a segment with positive curvature. The
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`at least one light source directs the bundle of light rays into at least one of the geometrical
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`regions (a), (b) and (c) of the optical element.
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`Thus, the present invention is based on the idea of homogenizing a bundle of
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`light rays by directing the bundle of light rays into the entry face of an elongated optical
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`element having a cylinder shape. The vertices between the edges have positive curvature, i.e.
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`the vertices are outwardly curved/rounded. By the term “zero curvature”, it is here meant that
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`the edges of the entry face are even/straight, i.e. not curved in the plane of the entry face.
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`Furthermore, by the term “vertices”, it is here meant comers/angles between the edges of the
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`entry face of the optical element. By the term “positive curvature”, it is here meant that the at
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`least one vertex is rounded outwards from the perimeter (i.e. bulging convexly outwards). By
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`the term “polygon”, it is here meant that the perimeter is defined by even/straight edges, but
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`wherein the comers/angles between the edges of the perimeter are rounded. When light rays
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`are directed into the optical element and are reflected therein, the curved vertices of the
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`optical element lead to an improved mix of the light rays in the optical element, compared to
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`a mixing rod without rounded comers/vertices. The purposefully shaped perimeter of the
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`optical element enhances/increases the irregularity/chaos of the mixing of the light within the
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`optical element.
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`Based on the described optical element of the present invention, it will be
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`appreciated that the inventors have realized a way of directing of the light into the entry face
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`of the optical element, which provides an increased homogeneity of the light at the exit face
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`of the optical element than if the light entered the entry face at random locations. This
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`increased homogeneity of the light is achieved by directing the bundle of light rays into one
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`or more of the geometrical regions (a), (b) and (c).
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`The present invention is advantageous in that an increased homogeneity of the
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`light is provided by selectively directing the bundle of light rays into specific geometrical
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`regions of the entry face of the optical element, dependent on the geometric shape of the
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`entry face. The inventors have realized that the optical element may provide a highly
`
`mixed/homogenized light after the bundle of light rays has passed through the optical
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`element, dependent on where the light incides on the entry face with respect to its
`
`geometrical shape. Furthermore, the inventors have realized that other regions of the entry
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`face, into which the bundle of light rays may be directed, will instead provide a poor
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`mixing/homogenization of the light. Hence, the present invention provides the advantage of
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`an improved homogenization of the light, which is obtained by a purposefiil directing of the
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`light into the entry face of the optical element. Furthermore, it will be appreciated that the
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`positions of the geometrical regions of the entry face, into which the light may be directed for
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`an increased homogeneity of the light, are not obvious to a person skilled in the art of optics.
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`On the contrary, the present invention suggests to direct light into specific regions of the
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`entry face of the optical element which the skilled person would not contemplate when given
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`the task of providing an improved mixing of the light. Hence, the directing of the light into
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`the optical element, as described in the present invention, provides a surprising effect related
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`to the homogenization of the light.
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`The method for homogenizing a bundle of light rays by means of an elongated
`
`optical element arranged for homogenizing light comprises the step of directing the bundle of
`
`light rays into the transversal entry face of the optical element. By the term “directing”, it is
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`here meant that the bundle of light rays is oriented towards the entry face. For example, one
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`or more light sources may be positioned immediately adjacent to the entry face, such that the
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`light from the light sources is directed directly into the entry face. Alternatively, the light
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`may be guided (e. g. by optical guiding means) towards the entry face of the optical element.
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`The bundle of light rays is directed into at least one of the geometrical regions
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`(a), (b) and (c). By the term “neighbourhood of the perimeter”, it is here meant a region in a
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`peripheral portion (the periphery) of the entry face. Furthermore, “a neighbourhood of at least
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`a portion of a line segmen ” refers to a region in the proximity of the portion of a line
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`segment.
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`The method filrther comprises the step of extracting the bundle of light rays
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`from an exit face of the optical element. In other words, after the bundle of light rays has
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`passed through the optical element, the homogenized light is extracted from the exit face of
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`the optical element, opposite the entry face.
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`With reference to the geometric regions discussed above, it is preferable that
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`region (b) is located in an entry face substantially shaped as an odd-numbered polygon.
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`Further, region (0) is preferably located in an entry face substantially shaped as an even-
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`numbered polygon. Similarly, with reference to lighting arrangements according to
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`embodiments of the invention, it is preferable that said one or more light sources are arranged
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`along at least a portion of a first line segment extending from a centre to a vertex of an Odd-
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`numbered polygon. Alternatively, such lighting arrangements may comprise one or more
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`light sources arranged along at least a portion of a second line segment extending from a
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`centre to a midpoint of an edge of an even-numbered polygon.
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`According to an embodiment of the present invention, and most preferably in
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`the case where the entry face is an even-numbered polygon, the bundle of light rays may be
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`directed into a neighbourhood of at least one portion of a second line segment, wherein the at
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`least one portion extends between the midpoint of the second line segment to a midpoint of
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`an edge. Hence, the bundle of light rays may be directed into a neighbourhood of the portion
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`of the second line segment from the midpoint of the line segment (i.e. a midpoint on the
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`radius) to the midpoint of the edge. This is particularly efficient if the optical element has an
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`entry face shaped as an even—numbered polygon. The present embodiment is advantageous in
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`that an increased homogeneity of the light is provided if light is directed into this specific
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`geometrical region, in the case where the entry face has the shape of a even—numbered
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`polygon.
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`According to an embodiment of the present invention, the bundle of light rays
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`may be directed outside a central region of an even-numbered polygon. Hence, in a case
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`where the optical element has an entry face in the shape of a rectangle, and in particular a
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`square, the bundle of light rays may be directed outside (i.e. not into) a central region of the
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`entry face. The inventors have come to the astonishing conclusion that light directed into the
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`central region of an entry face having the shape of a rectangle, will only be homogenized to a
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`limited extent (or not at all). Hence, the present embodiment is advantageous in that the light,
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`as extracted from the exit face of the optical element, may be even further homogenized if
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`one refrains from directing the bundle of light rays into the centre of the entry face but
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`instead directs the light into other regions of the entry face according to the described
`
`embodiments.
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`According to an embodiment of the present invention, the number of edges of
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`the even-numbered polygon may be four, six or eight. Analogously, according to another
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`embodiment of the present invention, the number of edges of the odd-numbered polygon may
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`be three, five or seven.
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`According to an embodiment of the present invention, the bundle of light rays
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`may be directed outside a central region of an odd-numbered polygon, wherein the number of
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`edges of the odd-numbered polygon is seven. Analogously with the case of entry faces of
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`even-numbered polygons, the inventors have realized that light, directed into the central
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`region of an entry face of a polygon with five or seven edges, Will only be homogenized to a
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`limited extent or, possibly, not at all. Hence, the present embodiment is advantageous in that
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`a more homogenized light may be obtained if one refrains from directing the bundle of light
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`rays into this region of the entry face, but instead directs the light into other regions of the
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`entry face according to the described embodiments.
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`According to an embodiment of the present invention, the length of the
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`vertices may constitute at least 1% and at most 90% of the length of the perimeter. In other
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`words, the length of the curved vertices represents 1—90% of the entire length of the
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`perimeter defined by the edges and the vertices between any two adjacent ends of the edges.
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`The present embodiment is advantageous in that the elongated optical element provides an
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`improved mixing of a bundle of light rays compared to both an optical element with perfectly
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`sharp comers and a round (or nearly round) entry face. Hence, a more homogeneous light is
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`obtained after the bundle of light rays has passed through the optical element of the present
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`invention, compared to existing mixing rods.
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`The features of the optical element of the present invention are advantageous
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`in that the element provides an improved mixing of a bundle of light rays, such that a more
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`homogeneous light is obtained after the bundle of light rays has passed through the optical
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`element, compared to existing mixing rods. More specifically, the edges and the vertices of
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`the optical element, wherein the vertices have positive curvature, provide an enhanced
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`mixing of the light due to an improved scattering/reflection of the light Within the optical
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`element. This is realized as the perimeter of the optical element, comprising rounded/curved
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`vertices, increases the number of directions of the light ray reflection/scattering as the normal
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`angle of the curved vertices varies continuously. It will be appreciated that in an optical
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`element with a transversal cross-section comprising only straight edges, a ray direction can
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`generally only change by multiples of 27t/n, where n. is the number of edges. In contrast, the
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`optical element of the present invention decreases the number of stable trajectories of the
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`reflected light, i.e. trajectories of the light ray reflections having a periodic propagation
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`within the optical element, and provides a more homogeneous light (i.e. even distribution of
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`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
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`the prior art.
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`The features of the optical element of the present invention are further
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`advantageous in that the improved mixing of the light is provided solely by the geometrical
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`shape of the optical element. In other words, the enhanced homogeneity of the light is
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`obtained merely by the geometrical features of the optical element, such that additional
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`measures for the purpose of improving the mixing of the light (e. g. a coating or other
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`treatments of the inside of the optical element and/or a provision of auxiliary elements to the
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`optical element for the purpose of improving the reflectivity) may be rendered superfluous.
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`Consequently, the optical element of the present invention is easy to manufacture, as the
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`optical element may be produced merely from the material which has the purpose of guiding
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`and mixing the light (e. g. a transparent material comprising glass, plastic, or the like), thereby
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`omitting the need of other (auxiliary) materials. Moreover, as additional treatments (e. g.
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`inside coatings) may be refrained from, rendering the manufacture of the optical element of
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`the present invention relatively inexpensive. Since the element may be manufactured from a
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`single material, the optical element is easily recyclable.
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`Another advantage associated with the features of the optical element of the
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`present invention is that the geometry of the transversal cross—section of the optical element
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`filrther provides an earlier homogeneity of the bundle of light rays in a direction from the
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`entry face to the exit face of the optical element compared to known mixing rods. In other
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`words, a bundle of light rays led into the entry face of the optical element is quickly mixed
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`along the elongated optical element due to the optimized perimeter of the transversal cross-
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`section according to the optical element of the present invention. The optical element is able
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`to achieve the task of mixing the initially non—homogeneous bundle of light rays into a
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`homogenized light earlier along its elongation compared to mixing rods in the prior art.
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`Hence, the optical element of the present invention may have a relatively shorter length than
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`other mixing rods in the prior art to fulfil this given task. This is highly advantageous, as the
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`optical element thereby implies an even lower manufacture costs of the optical element, a
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`lower weight, a more convenient handling and/or transportation, and/or a simplified
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`procedure if the optical element is to be mounted into an optical system.
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`A cylinder-shaped optical element with the features according to the present
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`invention is advantageous in that it is easily manufactured, e.g. by using extrusion.
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`Furthermore, the cylinder-shape of the optical element is advantageous in a case the length of
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`the optical element needs to be changed. For example, if the optical element is shortened, the
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`cross-section of the exit face, as well as the cross-section between the entry face and the exit
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`face will still be the same. Hence, the length of the optical element may be adapted more
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`easily according to the required/sought mixing of the light.
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`At least one of the vertices of the optical element may be a segment of a
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`circular arc. In other words, the at least one vertex is a portion of the circumference of a
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`circle. The present embodiment is advantageous in that the vertex has a continuous and
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`symmetric rounding which even further contributes to the mixing of the light.
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`The radius of the circular arc of the optical element may be equal to the length
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`of at least one of the at least two edges. The present embodiment is advantageous in that it
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`decreases the number of stable trajectories of light ray reflections within the transversal
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`cross-section of the optical element, as can be verified by numerical simulations. For
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`example, if the cross-section of the optical element has a polygon shape with an odd number
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`of edges, only one stable trajectory exists between the segment of the circular arc and the
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`opposing edge having the same length as the radius of the circular arc. Hence, the present
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`embodiment contributes to an even more improved mixing of the light.
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`The radius of the circular arc of the optical element may be greater than the
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`length of at least one of the at least two edges. An advantage of the present embodiment is
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`that it even further decreases the number of stable trajectories of light ray reflections within
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`the optical element. For example, if the transversal cross—section of the optical element has a
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`polygon shape with an odd number of edges, no stable trajectory exists between the segment
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`of the circular arc and the opposing edge, wherein the radius of the circular arc is greater than
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`the opposing edge. Hence, the present embodiment even further contributes to an improved
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`mixing of the light.
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`All of the at least two edges of the optical element may be of equal length and
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`all of the vertices are of equal length. In other words, the transversal cross—section of the
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`optical element is equilateral with respect to its edges, and the vertices between any two
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`adjacent ends of the edges are of equal length. The present embodiment is advantageous in
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`that the optical element provides an n—fold rotational symmetry (wherein n is the number of
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`edges). Consequently, an alignment of the optical element is facilitated, e.g. when mounting
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`the optical element in an optical system.
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`The gradient of at least one of the vertices and the gradient of any two adjacent
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`ends of the edges of the optical element may be equal at at least one point of intersection
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`between the at least one of the vertices and any two adjacent ends. In other words, the vertex
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`between two adjacent ends provides a smooth rounding/connection/patching between the
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`edges, wherein the gradient of any point on the vertex is within the interval bounded by the
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`gradients of the two adjacent ends and equal at the points of intersection.
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`The perimeter of the optical element may comprise six edges. In other words,
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`the six edges constitute an hexagonal cross-section of the optical element, further comprising
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`rounded vertices between the edges. The present embodiment is advantageous in that the
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`mixing of the light by the optical element of the present embodiment is superior to the mixing
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`which is achieved by mixing rods in the prior art having merely a hexagonal cross-section
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`without rounded comers. This is realized as the rounded vertices in the hexagonal cross-
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`section of the present embodiment increases the number of possible/distinct light ray
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`reflections within the optical element. Furthermore, as mixing rods with hexagonal cross-
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`sections are known in the prior art, the present embodiment is filrther advantageous in that
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`the equipment for the manufacture of the mixing rods from the prior art is easily modified for
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`the manufacture of the optical element according to the present embodiment, wherein the
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`hexagonal cross-section filrther comprises rounded vertices to provide an improved mixing of
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`the light.
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`The perimeter of the optical element may be defined by three edges wherein
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`two edges are perpendicular and of equal length, the vertices being three circular arcs of
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`equal radius, and wherein the radius is equal to the length of one of the two edges. In other
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`words, the transversal cross—section of the optical element is shaped as a right—angled
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`triangle, having two perpendicular edges as catheti and one edge as hypotenuse, but wherein
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`the vertices are rounded such that no sharp comers exist. Further, the vertices are circular
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`arcs having the same radius, wherein the radius is equal to the length of one of the two edges
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`(catheti). As stable trajectories of light ray reflections are known to exist in any transversal
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`cross—section that has n—fold rotational symmetry (wherein n is an integer), only one stable
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`trajectory of light ray reflections is present in an optical element having the perimeter as
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`defined in the present embodiment. Hence, an advantage with the present embodiment is that
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`the homogeneity of the light at the exit face of the optical element is even further increased.It
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`will be appreciated that the specific embodiments and any additional features described
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`10
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`above with reference to the method are likewise applicable and combinable with the optical
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`system according to the second aspect of the present invention and the lighting arrangement
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`according to the third aspect of the present invention.
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`Further objectives of, features of, and advantages with, the present invention
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`will become apparent when studying the following detailed disclosure, the drawings and the
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`appended claims. Those skilled in the art will realize that different features of the present
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`invention can be combined to create embodiments other than those described in the
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`following.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`These and other aspects of the present invention will now be described in more
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`detail, with reference to the appended drawings showing embodiment(s) of the invention.
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`Fig. 1 is a schematic illustration of an elongated optical element according to
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`an embodiment of the present invention;
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`Figs. 2a-b are schematic illustrations of transversal cross-sections of optical
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`elements; and
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`elements.
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`Figs. 3a-f are schematic illustrations of different entry faces of optical
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`DETAILED DESCRIPTION OF THE EMBODIMENTS
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`Fig. 1 is a schematic illustration of an elongated optical element 100. The
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`optical element 100, which may be made of a transparent material like glass or plastic, is
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`shaped as a cylinder and comprises a entry face 101 and an exit face 102. At operation, a
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`bundle of light rays 110 is directed towards the entry face 101, wherein the bundle of light
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`rays 110 undergoes total internal reflection (TIR) at the interfaces towards the surrounding
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`medium. After being reflected within the optical element 100, the bundle of light rays 110
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`exits the optical element 100 through the exit face 102.
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`The contour of the optical element 100 in Fig. 1 is designed such that the
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`perimeter 120 of a transversal cross—section 121 of the optical element 100 is defined by six
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`edges 122 of zero curvature and six vertices 123 between any two adjacent ends of the edges
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`122. In the present embodiment, the edges 122 are of equal length and the vertices 123 are of
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`equal length. The vertices 123 are segments with positive curvature (rounded segments),
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`wherein the lengths of the vertices 123 in Fig. 1 constitute 30—50% of the length of the
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`perimeter 120. When the bundle of light rays 110 from the light source(s) 130 is directed into
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`l l
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`the optical element 100, the curved/rounded vertices 123 of the optical element 100 lead to an
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`improved mix of the angles and positions of the light rays in the optical element 100,
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`compared to a mixing rod without rounded corners/vertices. The corners/vertices 123 may
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`alternatively be referred to as rounded ridges 123 extending longitudinally in the optical
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`element 100. Due to the rounded edges, the optical element 100 provides an increased
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`homogeneity of the bundle of light rays 110 when this leaves the optical element 100 by the
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`exit face 102.
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`The light sources 130 may e. g. comprise several LEDs with different colour
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`(e. g. one or more white LEDs wherein red LEDs may further be provided for improving the
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`colour rendering index (CRI)) wherein the LEDs may further be comprised in a tuneable
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`LED spotlight. Furthermore, a collimating means may be arranged at the exit face 102 of the
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`optical element for collimating the bundle of light rays exiting the optical e