TECHNICAL FIELD

The present invention relates to the field of optical devices for light emitting diodes, LEDs, and more particularly to a collimator structure for a LED light source.

BACKGROUND OF THE INVENTION

In general lighting mainly white light emitting diodes, LEDs, are used. A common way to produce such white LEDs is to combine blue emitting LED ^' s with a phosphor on top of the die, which results in so called phosphor converted LEDs. The spectrum of the emission from the LED can be tuned with the phosphor choice and the concentration and thickness of the phosphor. Furthermore, an inert scattering particle can be added to the phosphor to increase the light path length in the phosphor layer and to homogenize the light output distribution. With these parameters the average color point and color rendering index, CRI, can be engineered to the requirements of the light source.

The light emission spectrum of a phosphor converted LED is dependent on the emission angle. This is the angle between the surface normal of the LED and the ray direction. From small angles to large angles the amount of blue light decreases and the amount of converted yellow light increases. The phenomenon results in a yellow halo around a spot when these LED's are used in a spot application. In general the light in spot applications is collimated with a collimator such as a parabolic or elliptical reflector, a lens, or a combination thereof. The reflection/refracting surfaces are curved in such a way that a spot is generated with the required light beam width. Sometimes the optical surfaces are facetted to increase production tolerances.

A light source arrangement, which is known from US Patent No. US 7,731,388 B2, comprises LEDs arranged within a collimator which has a plurality of facetted surfaces arranged such that the collimator consists of a plurality of engaged concentric surface segments in a plane perpendicular to the optical axis of the collimator, each having a polygonal cross sectional shape. Further, a respective surface segment radius defined as the distance from the optical axis to a middle of a facetted surface of each surface segment increases in a direction from an entry of the collimator, where the LEDs are arranged, to an exit of the collimator, where the light emitted by the LEDs exits the collimator.

The collimator above, and other known examples of collimating devices, reflectors and lenses, have in common that a part of the reflector/lens builds a part of the exiting light beam, i.e. there is a direct continuously decreasing or increasing function between the light source emission angle and the light beam angle. Therefore, the color of the light in the collimated light beam is angle dependent which results in the known halo.

SUMMARY OF THE INVENTION

In view of the above, an object of the invention is to at least alleviate the problems discussed above. In particular, an object is to provide a color correcting collimator structure which is arranged to collimate light from light sources having a color distribution over angle in a manner such that the colors are mixed in the resulting light beam.

This object is achieved by a collimator according to the present invention as defined in claim 1.

The invention is based on the insight that by dividing a collimator surface into surface segments, which each is arranged to provide a light beam portion which has a light beam distribution corresponding to the light beam distribution of the whole resulting light beam, light emitted at small angles is mixed with light emitted at large angles such that a substantially increased mixing of colors over angle is achieved in the resulting light beam.

Thus, in accordance with an aspect of the present invention, a collimator structure for a light source comprising a collimator surface arranged to collimate light emitted by the light source when the light source is in a mounted position, and a light output exit for outputting a collimated resulting light beam. The collimator surface comprises a plurality of surface segments which each provides a light beam portion. The resulting beam is the sum of all light beam portions. Each surface segment is arranged such that the respective light beam portion has a predetermined light beam distribution which corresponds to a light beam distribution of the whole resulting light beam.

Thereby, there is provided a collimator structure in which each surface segment is arranged in such a way that it, when in use, will produce an light beam portion which at least with respect to its angular intensity distribution has a similar or equivalent width as the resulting collimated light beam. At a given distance, typically selected to be in the so called far field, all light beam portions of the surface segments overlap, and as a result light emitted by the light source at small angles is mixed with light emitted at large angles. That is, in a case of having a light source such as a phosphor converted LED, the color distribution over angle which is present at the light source is corrected for, or at least improved, and the halo disappears, or is at least reduced, in the resulting light beam. In which amount color correction is achieved depends on the details in the color distribution over angle of the light source, and the intensity profile provided from each surface segment.

In general it is not necessary that the surface segments will produce exactly the same intensity profile over angle as the whole beam, although a similar or equivalent width of the produced light beam is an important factor. The collimator structure is also applicable to other light sources having a color distribution over angle, i.e. which have a light emission spectrum which is dependent on the emission angle. When the size of the surface segments is selected small enough the angle dependency of the resulting light beam is reduced to such a level that it is unnoticeable.

According to an embodiment of the collimator structure, each surface segment is arranged to produce a divergent or a convergent light beam portion. These surfaces may be arranged differently in different applications. Further, a collimator surface comprising only surface segments producing divergent light beam portions, or only surface segments producing convergent light beam portions, is applicable.

According to an embodiment of the collimator structure, each surface segment has a predetermined shape to provide a light beam portion with a predetermined light beam width for a given distance. Generally in spot applications the far field beam angle is considered. However, the design of the surface segments of a collimator structure of the present inventive concept to mix very well close to the collimator, i.e. in the near field, is possible.

According to an embodiment of the collimator structure, the predetermined light beam widths for the surface segments are matched, which is advantageous for providing a good color mixing of a symmetrically emitting light source.

According to an embodiment of the collimator structure, the collimator surface is concave or convex.

According to an embodiment of the collimator structure, a plurality of the surface segments are arranged forming adjacently arranged circular surface segments, each having a circular cross sectional shape in a plane being perpendicular to an optical axis of the collimator surface.

According to an embodiment of the collimator structure, each surface segment is constituted by a plurality of sub segments arranged forming adjacently arranged surface segments, each surface segment having a polygonal cross sectional shape in a plane perpendicular to an optical axis of the collimator surface. This is advantageous to improve the position tolerance of a light source with respect to the collimator.

According to an embodiment of the collimator structure, the collimator structure further comprises at least one additional collimator surface.

According to an embodiment of the collimator structure, arranged for providing an alternating distribution of surface segments arranged to produce divergent light beam portions and surface segments arranged to produce convergent light beam portions. Alternating divergent and convergent producing surface segments have the advantage that the resulting collimator surface of the collimator structure has no kinks. (Expressed in mathematical form the derivative of the surface sag to the radius is continuous.)

According to an embodiment of the collimator structure, the number of surface segments is selected within a range of 2 - 20.

According to an embodiment of the collimator structure, the surface segments are reflecting and/or refracting.

According to an embodiment of the collimator structure, the collimator is one of a reflector collimator, a lens, a combination of a reflector and lens, and a total internal reflection collimator.

According to an embodiment of the collimator structure, the surface segments are adapted for translational symmetry of the collimator surface and/or the light source.

According to another aspect of the inventive concept, there is provided a lighting device comprising a collimator structure according to the present inventive concept, and a light source.

Other objectives, features and advantages will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non- limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein: Fig. la is a schematic perspective side view of a collimator structure and light source, and Fig. lb illustrates a resulting collimated light beam distribution of light emitted by the light source in the far field;

Fig. 2a is a schematic cross sectional side view of a light source and a collimator structure according to the present inventive concept, and Fig. 2b is a diagram illustrating a light beam distribution of the resulting collimated light beam, and a light beam distribution of a surface segment of the collimator structure of Fig. 2a;

Fig. 3a is a schematic perspective partly cut side view of a light source and a collimator structure according to the present inventive concept, Fig. 3b illustrates a resulting collimated light beam distribution of light emitted by the light source in the far field, and in Fig. 3c a collimator surface shape along the radius of the collimator structure of Fig. 3a is illustrated;

Figs. 4a - 4f are schematic illustrations of different embodiments of lighting devices and a collimator structure according to the present inventive concept,

Figs. 5a - 5c, illustrate symmetry and geometry relations for calculating the shapes of each surface segment, and

Fig. 6 illustrates symmetry and geometry relations for calculating the shapes of each surface segment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following a collimator structure generally comprises a collimator body, comprising a collimator surface which is arranged for collimating light of a light source arranged at or inside the collimator structure, and a light output exit for outputting the collimated resulting light beam.

Fig. la shows an illustrative example of a collimator structure 1 comprising a single surface segment. Further, a Active light source 11 with an extreme exaggerated color over angle distribution is used to make the problem of a Halo clearly visible. To continue, Fig. la is a schematic perspective side view of the collimator structure 1 and the light source 11. The collimator 1 comprises a collimator surface 2 which is a single reflective convex surface segment arranged to collimate light into a Gaussian light beam with a width of 20 degrees. The light source 11 is here a light emitting diode, LED, which emits blue light (400 nm) between 0 and 45 degrees and red light (600 nm) between 45 and 90 degrees with respect to a LED light emitting surface (not shown) of the light source 11. The light source 11 is arranged at a centre of the collimator 1 with the LED light emitting surface arranged facing the collimator surface 2. Light emitted by the light source 11 towards the collimator 1 (wherein light emitted in a direction 0 degrees coinciding with the optical axis of the collimator) is collimated by the collimator surface 2 and exits the collimator 1 along the optical axis of the collimator as a Gaussian light beam ½ with a width of 20 degrees. Fig. lb illustrates a true color chart of a simulated light beam distribution as a function of angle of the collimated light beam ½ in the far field. In Fig. lb, it is shown that the angle of the rays emitted by the LED 11 have a direct relation to the angle in the far field. Outside the collimated central portion CP which mainly comprises of blue light, a halo H is visible which has a broader light beam distribution then the central portion CP, and which consists mainly of the red light emitted by the light source 11.

An embodiment of a collimator structure according to the present inventive concept is now described with reference to Fig. 2a. For simplicity, collimator structure is herein after referred to as a collimator. In Fig. 2a, there is illustrated a reflector collimator 10 having an optical axis o.a. which coincides with the symmetry axis of the collimator 10. Here a light source 11, being a Lambertian emitter, is arranged at the centre of the collimator 10 and facing a collimator surface 12 of the collimator 10, such that light emitted by the light source 11 is collimated producing a resulting light beam ½ in the direction along the optical axis o.a. having a Gaussian light beam distribution, which is illustrated in Fig. 2b.

The collimator surface 12 is here a reflecting layer arranged on the inner surface of the collimator 10. Further, the collimator surface 12 is in a direction along the diameter of the collimator surface divided into three surface segments, a - c, one surface segment a in the middle and two encompassing circular surface segments b, c, which each is curved so as to produce a light portion I _s having a light beam distribution which corresponds to the light beam distribution of the resulting light beam ½. In Fig. 4e, the principle shape of the collimator surface 12 is schematically illustrated in more detail. To continue with reference to Fig. 2a, in the far field the light beam portions from the three surface segments, a - c, overlap, and the light emitted by the light source 11 at small angles, near the optical axis o.a., is mixed with light emitted at large angles with respect to the optical axis o.a. As a result of this the problem with the halo disappears. In Fig. 2b the light beam distribution for the resulting light beam ½ and the light beam distribution of a surface segment light beam portion Is are plotted in the same graph. Here it can be seen how the light beam portion of each surface segment, a - c, is designed to provide the whole light beam with respect to the width of the resulting beam, and in the far field, each surface segment contributes equally, with respect to angular distribution, to the resulting light beam distribution of the resulting light beam ½. In a preferred embodiment, the luminous flux between the surface segments is balanced. However, in alternative embodiments, in absolute luminous flux the contribution of each surface segment is selected to be different. This may be utilized to provide a required color mixing of the resulting light beam.

In all of the embodiments of the collimator herein, each surface segment of the collimator surface is curved to provide a light beam portion having a light beam distribution which provides light at all angles within the resulting light beam. This means that for light rays emitted by the light source, which are reflected at an edge of a surface segment, these rays will be distributed so as to, at a given distance for a near field application, or in the far field for a far field application, form an outer edge of the resulting light beam. Each surface segment may be reflecting and/or refracting, and may produce a divergent or convergent beam. Preferably, surface segments arranged to provide divergent beams and surface segments arranged to provide convergent beams alternate, but all permutations are applicable. The required number of surface segments on the collimator surface depends on the degree of color over angle distribution and complexity of the light source. Further, according to embodiments of the collimator the surface segments are symmetrically arranged but in alternative embodiments of the collimator non-symmetric arrangements are applicable.

The shape of each surface segment can be computed with a theory which is summarized below. Referring now to Figs. 5a - 5c, the shape of the optical surfaces, e.g. surface segments, of a collimator C can be exactly computed for a points source P. When the optical system has a rotational or translational symmetry with respect to the light source and the collimator, relative concise mathematical equations can be obtained to compute the desired surface profile.

In a rotationally symmetric system the luminous flux between two angles θι and Θ2 of a light source P placed at the origin of the coordinate system is computed according to equation Eq.1 :

source iS-L, 0 ₂ ) = 2π J^ ² I _source (s)sin(s)ds Eq. 1

, where l _S0U rce(s) is the intensity distribution of the source P as a function of angle s, see Fig. 5a. Angle s is the angle between the light direction and the optical axis o.a as depicted in Fig. 5a. The luminous flux at a target T, as illustrated in Fig. 5b, is computed according to equation Eq. 2:

®target (.<Pl. <P2 = ^2π $ζ[ harget tf)sin(t)dt Eq. 2 , where I _tar get(t) is the intensity distribution of the target T, and where t is the angle between the light direction and the optical axis o.a. In case of a near field problem, as illustrated in Fig. 5c, the luminous flux at a target a distance z is calculated according to equation Eq. 3:

^target iyi- yz) = ^2π ζ E _target (u)udu Eq. 3

, where E _target (u) is the illuminance at the target T and u the height coordinate in a plane perpendicular to the optical axis o.a. The geometries of the far field and near field target are shown in Fig. 5b and Fig. 5c, respectively.

For a collimator C with an optical efficiency of η the source and target luminous flux are related by

, which together with the previous three equations, Eq. 1 - Eq. 3, gives a mathematical relation between the source angle s and the target angle t or target position u. Here, 0 < η < 1 and contains the losses due to absorption and scattering of the optical media and surfaces.

A similar derivation can be performed in case of a translational symmetry that results in equations that describe again the relation between the source angle s and the target angle t or target position u.

A relation for the light ray direction before and after an optical surface is found from Snell's law for refraction. At a surface boundary between two media with a refractive index n and n' the following equation can be obtained

β{3) = ατααη { ™→ ^~ ^ } _Eq . ₅

^ ln'cos[s-t(s)]-n)

, where β is the angle between the surface normal and the light ray direction, see Fig. 6. Here it is assumed that the light source P is a point source or in other words the light source P is small compared to the optics. In case the optical surface OS is a mirror the relation n=-n ' is used. Furthermore, the geometry in Fig. 6 gives the differential equation for the optical surface profile:

- dr = ΐαη[β (s)] Eq. 6

, which has the solution:

r(s) = r ₀ exp {j _S ^S _o tantf (s')ds']} Eq. 7

, in which ro is a chosen start radius and SQ is the smallest angle between the optical axis o.a and the surface segment to be calculated.

Equations Eq. 1 to Eq. 7 describe the design problem for a collimator completely. These relations can be implemented in a programming language such as Fortran, C++ or MatLab and solved numerically for a given light source intensity profile and a target intensity or illumination profile. Fig. 3c gives an example of such an optical profile for a mirror collimator.

In case the light source is not small compared to the optics a further optimization of the obtained curves for the surface segments is needed. To this end the computed surface sag is fitted to for example a polynomial and optimized within a standard ray-trace program such as LightTools or ZEMAX. Further details can be found in

"Relationships between the generalized functional method and other methods of non-imaging optical design", John Bortz and Narkis Shatz, Applied Optics, Vol. 50, Issue 10, pp. 1488- 1500 (2011), and "Freeform reflector design with extended sources", Florian Fournier, PhD Thesis, CREOL, the college of Optics and Photonics at the University of Central Florida Orlando, Florida, USA. (2010).

According to an embodiment of the collimator, referring now to Fig. 3a, the collimator is a mirror reflector 20 having a collimator surface 22, The collimator surface is divided into one centre surface segment a, and ten circumferential ring surface segments, b- k, which form adjacently arranged circular surface segments (with increasing diameters), each having a circular cross sectional shape in a plane perpendicular to the optical axis of the collimator surface 22.

Each of the surface segments is arranged to provide a light beam portion of light emitted by a light source 11 arranged at the centre of the collimator, which light beam portion has a light beam distribution corresponding to the light beam distribution of the resulting light beam. The surface segments, a- k, are alternately convergent and divergent mirrors which results in a continuous smooth collimator surface 22, which is further illustrated in Fig. 3c which is a graph illustrating the profile of the inner surface, i.e.

collimator surface 22, when plotted from the symmetry axis x (which coincides with the optical axis o.a in this case) to the outer edge of the collimator 20. Fig. 3b illustrates a true color chart of the intensity distribution of a resulting light beam ½ when employing a Active light source with an extreme exaggerated color over angle distribution, similar to the collimator described with reference to Fig. 1. The light source 11 is a LED light source, which emits blue light (400nm) between 0 and 45 degrees and red light (600 nm) between 45 and 90 degrees. The collimator 20 collimates the light from the light source 11 into a Gaussian light beam with a width of 20 degrees. In the plotted color mesh in Fig. 3b blue and red rays are shown as a function of angle. When comparing the resulting beam ½ of Fig. 3b with the resulting beam ½ of Fig. lb, which both illustrates collimating a light source with the same color distribution over angle, Fig. 3 clearly show that when employing the collimator of Fig. 3a, the light emitted from the light source is mixed over all angles and the halo H which is visible in Fig. lb, when employing a one segment collimator, has practically disappeared.

According to an embodiment of the collimator (not shown) a surface segment is constituted by a plurality of sub segments which are circumferentially arranged on the collimator surface, and forming in a direction along the optical axis o.a adjacently arranged surface segments (with increasing diameters), similar to the circularly arranged surface segments of the collimator described with reference to Fig. 3a. However, instead of a circular cross sectional shape of an individual surface segment in a plane perpendicular to an optical axis of the collimator surface, each surface segment has a polygonal cross sectional shape in a plane perpendicular to an optical axis of the collimator surface. Each surface segment is arranged to produce a light beam portion having a light beam distribution corresponding to a light beam distribution of the whole resulting beam. According to embodiments of the inventive concept, the collimator may be a reflector, a lens with an appropriate bending surface or a combination of a reflector and lens.

In Figs. 4a-f , different embodiments of a lighting device comprising a collimator according to the present inventive concept, and some close ups of embodiments of a collimator according to the present inventive concept, are schematically illustrated. For sake of simplicity means for mechanical mounting means for e.g. the light source, heat sinks, means for providing electrical power and control signals needed to drive the light sources, housings etc. of the exemplifying embodiments have been left out in the illustrations. The skilled person will realize this. Further, the light source 11 is here a LED light source of any applicable kind. However, it should be mentioned that the present inventive concept is applicable to any other suitable type of color over angle distributing light sources.

Fig. 4a is a schematic illustration of a lighting device 30 comprising an embodiment of a collimator 31 according to the present inventive concept and a light source 11. The collimator 31 is here a concave plate, which may be a PMMA plate provided with a reflecting layer on a collimator surface 32 which is divided into five surface segments, a - e, each designed for providing a whole light beam distribution of the provided light beam portions. The light source 11 and the collimator 31 are arranged at an angle, which here is approximately 45 degrees. In this case each surface segment, a - e are further arranged such that, in addition to providing the same light beam distribution, each respective provided light beam portion is directed in a common direction which is separated from the optical axis of the concave collimator surface 32.

Fig. 4b is a schematic illustration of a lighting device 40 comprising an embodiment of a collimator 41 according to the present inventive concept and a light source 11. The collimator 41 comprises a reflecting collimator surface 42 which is divided into three ring surface segments, a - c, each designed for providing a light beam distribution of the provided light beam portion which corresponds to the whole light beam distribution of the resulting light beam I _B . Each surface segment, a- c, is arranged to produce a convergent light beam portion.

Fig. 4c is a schematic illustration of a lighting device 50 comprising an embodiment of a collimator 51 according to the present inventive concept and a light source 11. The collimator 51 is here a glass lens provided with two collimator surfaces 52a, 52b which each is divided into surface segments, a - d. The surface segments, a - d, of the collimator surface 52a, 52b, are further arranged in pairs of surface segments (a,a'), (b,b'),(c,c'), (d,d'). Each pair provides a light beam portion having a light beam distribution corresponding to the resulting light beam distribution of the resulting light beam ½.

Fig. 4d is a schematic close up of a convex collimator surface 62 which is divided into surface segments, a - d, which all are arranged for providing divergent light beam portions.

Fig. 4e is a schematic picture illustrating a lighting device as previously described with reference to Fig. 2a.

A lighting device 60 described with reference to Fig. 4f, comprises a collimator 61 according to the present inventive concept and a light source 11. The collimator 61 is here a total internal reflection (TIR) collimator. The collimator 62 is made in plastics, or glass, and comprises a collimator surface 62, comprising a centre surface segment a, and a ring surface segment b which collimate the light by refraction, and ring surface segments c - g which collimate light by total internal reflection. Each surface segment, a - g, is arranged for providing a light beam portion having a light beam distribution corresponding to the resulting light beam distribution of the resulting light beam ½, and they all add to the resulting beam ½. Suitable materials for TIR collimators are ceramics, a polymer like e.g. Polymehtyl Methacrylate, PMMA, or polycarbonate, a silicon resin, or combinations thereof.

According to the present inventive concept, the shape of the intensity or illumination profile of each surface segment of the collimator surface does not have to be the same, which advantageously adds design freedom. As was previously discussed, the present inventive concept is applicable for providing collimation of light in rotational symmetry systems, which are exemplified above with reference e.g. to Figs. 2a, 3a, 4b, etc., as well as in translational symmetry systems. As an example, referring now to the embodiment as illustrated in Fig. 4a, which show a lighting device 30 comprising an embodiment of a collimator 31 according to the present inventive concept being a concave plate provided with surface segment. In an alternative embodiment of the lighting device 30, the light source 11 is exchanged for a linear array of LEDs, i.e. a light source of a predetermined width. Further, the collimator surface 32 is arranged to collimate the light into a resulting elongated beam ½ in one direction. This embodiment is applicable for providing elongated LED based lighting devices for replacement of e.g.

fluorescent tube lighting. Further, all shown embodiments herein can be used in translational symmetry by adapting the curvature of each surface segment for a light source and/or collimator surface having translational symmetry in accordance with the method as described above.

In many of the embodiments above, the light source has been shown arranged at a centre position of the collimator. As illustrated, e.g. in Fig. 4a, it may however be placed off centre. The individual calculation or optimization of the shape of each surface segment to produce a light beam portion having a light beam distribution of the whole resulting light beam is then adjusted with respect to the positioning of the light source. The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended claims.