WO2015044060A1

WO2015044060A1 - Collimator, lighting device and luminaire

Info

Publication number: WO2015044060A1
Application number: PCT/EP2014/070071
Authority: WO
Inventors: Lin Wang; Oliver Dross; Yun Li
Original assignee: Koninklijke Philips N.V.
Priority date: 2013-09-29
Filing date: 2014-09-22
Publication date: 2015-04-02

Abstract

Disclosed is a collimator (10) for a lighting device (100) comprising at least one solid state lighting element (112a, 112b), the collimator comprising a reflective central portion (30) and a reflective annular portion (40) surrounding the reflective central portion, wherein the reflective central portion comprises a plurality of concentric prisms (32) each comprising a first surface (32a) oriented under a first angle (α-₁) with a central axis (50) of the collimator; and a second surface (32b) oriented under a second angle (α-₂) with said central axis, said first and second surfaces of each prism extending from a main surface of the collimator and terminating into a common end point; wherein incident light refracted by the first surface is reflected by the second surface and incident light refracted by the second surface is reflected by the first surface. A lighting device including such a collimator and a luminaire including such a lighting device are also disclosed.

Description

COLLIMATOR, LIGHTING DEVICE AND LUMINAIRE

FIELD OF THE INVENTION

The present invention relates to a collimator for a lighting device comprising at least one solid state lighting element, the collimator comprising a central portion and a reflective annular portion surrounding the central portion.

The present invention further relates to a lighting device including such a collimator.

The present invention yet further relates to a luminaire including such a lighting device.

BACKGROUND OF THE INVENTION

Lighting devices, e.g. light bulbs, based on solid state lighting (SSL) elements such as light emitting diodes (LEDs) are rapidly gaining popularity because of the green credentials of such devices. For instance, SSL element- based light bulbs are significantly more energy-efficient than their incandescent or halogen equivalents and have superior lifetime. However, the market penetration of SSL element-based lighting devices is hampered by a number of factors.

Firstly, SSL element-based lighting devices are more costly to purchase than their traditional counterparts, although increasing consumer awareness about the superior lifetime and energy consumption of SSL element-based lighting devices is at least partially overcoming this problem as consumers realize that over the lifetime of the SSL element-based lighting devices significant cost savings can be expected when comparing the SSL element-based lighting device with a traditional equivalent such as an incandescent lighting device.

Secondly, SSL elements are a fundamentally different light source than e.g. filaments in an incandescent light bulb. Whereas the filament produces omnidirectional light, the SSL element typically produces light over a range of 180° or less, which can give SSL element-based lighting devices a different appearance in terms of luminous distribution. This can lead to consumer dissatisfaction, as the luminous distribution patterns of traditional light sources such as incandescent light bulbs has very much become the accepted norm, and deviations from this norm are perceived as unnatural and/or unpleasant.

For this reason, SSL element-based lighting devices typically comprise optical elements to shape the luminous output of the SSL elements such that the luminous output distribution of the SSL element-based lighting device more closely resembles that of a traditional light source. The design of such optical elements quite often poses a real technical challenge because of size constraints; the size of the SSL element-based lighting device must match or closely resemble of its traditional equivalent, which puts significant size constraints on the desired optical elements.

One type of optical element that is particularly suitable for use in SSL element-based lighting devices because of its compact nature is a Fresnel lens, which often is used to collimate the luminous output of the SSL elements of a SSL element-based lighting device. A Fresnel lens typically comprises a central refractive (lens) element surrounded by annular reflective prisms or facets that are designed to totally internally reflect the incident light.

A drawback of such lenses is that the central refractive element essentially is an imaging device, which generates images of the SSL elements at far field. This can cause colour separation in case of a SSL element-based lighting device comprising SSL elements generating different colours that are supposed to combine to produce white light, as well as cause unwanted beam widening. Moreover, such lenses tend to replicate or image the shape of the LED carrier, which can produce a more or less rectangular beam pattern at far field in case of a rectangular carrier, which is aesthetically displeasing. In order to counter this effect, additional optical elements such as micro-lenses or diffusers at the exit surface of the lighting device can be used. This however increases the cost of the lighting device. SUMMARY OF THE INVENTION

The present invention seeks to provide a non-imaging collimator according to the opening paragraph.

The present invention further seeks to provide a lighting device including such a non-imaging collimator.

The present invention yet further seeks to provide a luminaire including such a lighting device.

According to an aspect, there is provided a collimator for a lighting device comprising at least one solid state lighting element, the collimator comprising a reflective central portion and an annular portion surrounding the reflective central portion, wherein the reflective central portion comprises a plurality of concentric prisms each comprising a first surface oriented under a first angle with a central axis of the collimator; and a second surface oriented under a second angle with said central axis, said first and second surfaces of each prism extending from a main surface of the collimator and terminating into a common end point; wherein incident light refracted by the first surface is reflected by the second surface and incident light refracted by the second surface is reflected by the first surface.

By replacing the refractive central portion of such a collimator with a reflective central portion, a non-imaging central portion is obtained capable of producing largely circular beam profiles at far field such that the need for additional optical elements to reshape the luminous output passing through the central portion of the collimator is obviated.

The annular portion may be reflective or refractive, or may be an annular body exhibiting total internal reflection.

The concentric prisms preferably are totally internally reflecting to optimize the efficiency of the collimator.

In an embodiment, the absolute value of the first angle equals the absolute value of the second angle for at least some of the concentric prisms and the absolute value of the first angle is different to the absolute value of the second angle for at least some other of the concentric prisms. For instance, for the prisms at or near the central axis of the collimator the absolute value of the first angle may equal the absolute value of the second angle as the first and second surfaces of these prisms will typically be exposed to incident light under very similar angles whereas the prisms at or near the periphery of the central region the absolute value of the first angle may be different to the absolute value of the second angle as the first and second surfaces of these prisms will typically be exposed to incident light under different angles. The first and second angles of each prism may be individually optimized as a function of the position of the prism relative to the position of the at least one SSL element in the lighting device in which the collimator is to be used.

In the context of the present application, the term 'prism' is used to describe an angled or faceted surface that refract incident light and comprises at least two adjoining surface portions under an angle with each other.

Each first surface and second surface may be a curved surface such as a truncated conical surface. The first surface and the second surface of each prism may be individually optimized, i.e. each first and second surface may have a unique shape.

In an embodiment, the annular portion comprises a plurality of further concentric prisms each comprising a first further surface and a second further surface extending from a main surface and terminating into a common further end point, wherein each first further surface is oriented under a first further angle with said central axis and the second further surface is oriented under a second further angle with said central axis. This yields a Fresnel-type collimator with a reflective central portion, thereby providing a very compact collimator.

In the Fresnel-type collimator, the first further angle may be the same for all the further concentric prisms. Each first further surface and second further surface may be a curved surface such as a truncated conical surface.

In an embodiment, the respective common end points are oriented on one of a virtual plane, a virtual convex surface and a virtual concave surface. The shaping of the virtual surface defined by the end points of the central reflective portion is a further degree of design freedom that can be utilized to tailor the shape of the luminous output produced by the collimator such that the collimator can be optimized for different application domains.

The collimator preferably is a total internal reflection collimator to optimize its efficiency.

According to another aspect, there is provided a lighting device comprising at least one solid state lighting element and the collimator according to an embodiment of the present invention, wherein the concentric prisms face the at least one solid state lighting element. Such a lighting device benefits from a desirable beam shape at reduced cost. The lighting device may be a light bulb such as a spot light bulb. The light bulb may be any suitable size and type, such as MR1 1 , MR16, GU10, AR1 1 1 , PAR38, PAR30, BR30, BR40, R20, and R50 light bulbs or any other suitable size.

In an embodiment, the reflective central portion of the collimator is aligned with a central axis of the luminous output distribution of the at least one solid state lighting element to ensure a symmetrical light distribution being produced by the lighting device.

The lighting device may comprise a plurality of solid state lighting elements arranged on a rectangular carrier.

The lighting device may comprise a plurality of solid state lighting elements including solid state lighting elements of different colour, e.g. for producing white light by mixing the different colours. The inclusion of the collimator according to an embodiment of the present invention ensures that a high degree of colour mixing with minor colour separation in the luminous output of the lighting device is obtained.

According to yet another aspect there is provided a luminaire comprising the lighting device according to an embodiment of the present invention. Such a luminaire may for instance be a holder of the lighting device or an apparatus into which the lighting device is integrated. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention are described in more detail and by way of non- limiting examples with reference to the accompanying drawings, wherein:

FIG. 1 schematically depicts a collimator according to an embodiment of the present invention;

FIG. 2-6 schematically depict embodiments of different aspects of the collimator of FIG. 1 ;

FIG. 7 schematically depicts a collimator according to an embodiment of the present invention in perspective view;

FIG. 8 shows an image of a far field beam profile of a prior art lighting device;

FIG. 9 shows an image of a far field beam profile of a lighting device according to an embodiment of the present invention;

FIG. 10 shows an image of a far field beam profile of a prior art lighting device comprising multiple colour LEDs;

FIG. 1 1 shows an image of a far field beam profile of a prior art lighting device comprising multiple colour SSL elements according to an embodiment of the present invention; and

FIG. 12 schematically depicts a lighting device according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

FIG. 1 schematically depicts a cross-section of a collimator 10 according to an embodiment of the present invention. The collimator 10 is characterized by a central portion 30, which may be a circular central portion 30, having a plurality of ring-shaped or concentric reflective prisms 32. The prisms 32 typically have individually optimized shapes, as will be explained in more detail below. The collimator 10 typically has a central symmetry axis 50 as shown in FIG. 1. The collimator 10 further comprises an annular portion 40 surrounding the central portion 30. The nature of the annular portion 40 is not particularly relevant. In FIG. 1 , the annular portion 40 comprises a plurality of reflective elements, e.g. further prisms 42. The reflective elements 42 preferably are total internal reflection elements. However, it is equally feasible to provide the annular portion 40 is a single body such as a total internally reflecting body or as a refractive portion, e.g. comprising annular refractive elements. Other embodiments of the annular portion 40 will be apparent to the skilled person.

The central portion 30 is designed to exhibit non-imaging behavior. This is achieved by designing the central portion 30 to have reflective prisms 32, which preferably exhibit total internal reflection behavior, wherein the shape of each prism 32 is a function of its position relative to the central axis 50 of the collimator, or more precisely, of the position of the prism 32 relative to the light sources to be collimated by the collimator 10. As the collimator 10 is usually positioned symmetrically relative to such light sources, e.g. in a lighting device such as a (spot) light bulb, in the remainder of this description reference will be made to the position of such prisms 32 relative to the central axis 50 of the collimator 10.

In an embodiment, the shape of each individual prism 32 may be optimized as follows. As shown in FIG. 2, the prism 32 has side surfaces 32a and 32b, which may have any suitable shape, e.g. may be straight or curved, e.g. have a truncated conical shape. This will dependent on the nature of the ray fans produced by the light sources to be collimated, e.g. solid state lighting elements such as light emitting diodes (LEDs). In FIG. 2 only an infinitesimally small region of the prism 32 around its end point is shown, such that the side surfaces 32a and 32b can be approximated by straight lines, as shown in FIG. 2.

The respective angles of the side surfaces 32a and 32b with the central axis 50 are labeled CH and 0:2. The respective angles of the incident rays from each side of prism 32 have tilt angle θι and Θ2, which tilt angle has been defined relative to a surface of origin of these rays, which surface is typically perpendicular to the central axis 50. Each light ray is refracted upon entry of the prism 32 by a first one of its side surfaces (e.g. side surface 32a for the ray emitted under angle θι) and reflected by the opposite side surface (e.g. side surface 32b for the ray emitted under angle θ-ι). This will cause the incident rays to exit the prism 32 under angles βι and β2 respectively. In other words, the angles βι and β2 are a function of the beam angles θι and Θ2, and the angles of the side surfaces 32a and 32b as follows:

In a preferred embodiment, the values of βι and β2 are chosen to be zero, as this ensures that the light produced by the prism 32 is fully collimated such that imaging effects are avoided. In other words, at these values of βι and β2, the central portion of the collimator 30 is perfectly non-imaging. So for a given θι and Θ2, Equation (1 ) and Equation (2) can be used to find the optimal values of the angles CH and 02 of the side surfaces 32a and 32b of the prism 32. It will be apparent that these optimal values will depend from the position of the prism 32 relative to the light sources to be collimated, such that each prism 32 of the central portion 30 typically comprises unique values of αι and a₂, i.e. each prism 32 has a unique shape, or in other words no two prisms 32 of the central region 30 have the same shape.

The above equations have been derived for the end portion of the prism 32 as explained above. In order to derive the overall shape of the side surfaces 32a and 32b, each side surface can be parameterized from the end point of the prism 32 (i.e. the point where the side surfaces 32a and 32b meet) using the following Taylor expansion: yi = a-ix + a₂x² + a₃x³ + (3)

y₂ = bix + b₂x² + b₃x³ + (4) The coefficients ai and bi can be obtained directly from angles cti and ct2 around the end point of the prism 32, after which the other unknown coefficients (a₂, a₃, b₂, b₃, ...) can be determined to collimate two selected ray fans. However, the propagation of each light ray through these side surfaces 32a and 32b gives a highly non-linear equation system, such that obtaining the exact solution of these Taylor expansions can be computationally challenging. In an alternative embodiment, these coefficients may be approximated numerically instead.

FIG. 3 and 4 show different aspects of an embodiment of a collimator 10 when used in a lighting device 100 comprising a carrier 1 10 carrying a plurality of solid state lighting elements, e.g. inorganic or organic semiconductor LEDs, including a first solid state lighting (SSL) element 1 12a and a second solid state lighting element 1 12b.

In FIG. 3, a prism 32 of the central region 30 of the collimator 10 is shown that is placed on the central axis 150 of the lighting device 100. As the collimator 10 is centered over the carrier 1 10, the central axis 150 of the lighting device 100 coincides with the central axis 50 of the collimator 10. Consequently, the side surfaces 32a and 32b of the prism 32 in FIG. 3 are equidistant to the SSL elements 1 12a and 1 12b, such that the side surfaces 32a and 32b are exposed to (near-)identical light patterns, i.e. θι = -02 in this embodiment. As will be understood from equations (1 )-(4) this results in a prism 32 having a symmetrical shape in which the first side surface 32a and the second side surface 32b are oriented under the same absolute angle with the central axis 150 of the lighting device, i.e. CH = -0:2.

In contrast, FIG. 4 schematically depicts an off-center prism 32 of the central reflective portion 30 of the collimator 10, wherein the first side surface 32a of the prism 32 is located nearer to a SSL element (here first SSL element 1 12a) than the second side surface 32b, such that the side surfaces 32a and 32b are exposed to different light patterns, i.e. θι ≠ -02. As will be understood from equations (1 )-(4) this results in a prism 32 having an asymmetrical shape. Each prism 32 of the central reflective portion 30 of the collimator 10 may have a shape that is optimized as a function of its position relative to the SSL elements of the lighting device 100, as explained above.

In FIG. 1 , the respective end points of the prisms 32 of the central reflective or non-imaging region 30 of the collimator 10 lie in a virtual single plane. However, it should be understood that the respective orientation of these end points relative to each other may be varied depending on the application of the collimator 10. For instance, as shown in FIG. 5, the respective end points of the prisms 32 of the central reflective portion 30 lie on a virtual convex surface 34, whereas in FIG. 6 the respective end points of the prisms 32 of the central reflective portion 30 lie on a virtual concave surface 36. This provides additional design freedom that can be utilized to optimize the collimator design for specific application domains. For instance, with the end points in a planar alignment, the thickness of the collimator 10 may be minimized, whereas with the end points in a convex or concave alignment, the optical design of the collimator 10 may be optimized, e.g. to achieve improved light mixing, more uniform light distribution, or improved light collection. A further design freedom can be provided by the shaping of the prisms 32, which for instance may be constant or variable width prisms and/or prisms having a constant or variable angular extension.

In an embodiment, the annular reflective portion 40 surrounding the central reflective portion 30 may also comprise a plurality of further prisms 42 as previously explained. The shape of the each further prism 42 may be individually optimized as previously explained or alternatively each further prism 42 may have the same shape.

FIG. 7 schematically depicts a perspective view of an embodiment of the collimator 10 in which the central reflective portion 30 with concentric prisms 32 protrudes from the annular portion including the further concentric prisms 42. At this point it is noted that the collimator 10 preferably is a total internal reflection collimator, i.e. a collimator in which both the prisms 32 and the further prisms 42 are total internal reflection prisms. For instance, the collimator 10 may be a Fresnel-type collimator although it should be understood that other collimator designs such as UFO, RXI and XRR type collimators are equally suitable, as such collimators all comprise a central lens portion that may be replaced by the reflective central portion 30 having individually optimized prisms 32 as explained above.

The collimator 10 according to embodiments of the present invention may be made in any suitable manner, e.g. using injection moulding. Suitable materials include optical grade polymers such as polycarbonate, poly ethylene terephthalate and poly (methyl methacrylate) although it should be understood that other transparent materials, e.g. glass, may also be contemplated.

The optical performance of the collimator 10 according to an embodiment of the present invention has been compared with the optical performance of a prior art collimator having a refractive central lens portion by simulation. In the simulation, a Fresnel-type collimator having a central reflective portion 30 according to an embodiment of the present invention is compared with a Fresnel- type collimator having a refractive central lens portion. In the simulation, the reflective annular portion 40 is set as an absorber such that only light through the central portion of the collimator is traced. In other words, the following simulation results compare the optical performance of a central reflective portion 30 having individually optimized prisms 32 with the optical performance of a known central lens portion. In the simulations, the SSL elements are simulated to be mounted on a square printed circuit board (PCB).

FIG. 8 shows the optical output of the prior art collimator and FIG. 9 shows the optical output of the collimator 10 according to an embodiment of the present invention at far field (200 mm from the collimator). As will be immediately apparent, the imaging properties of the central lens of the prior art collimator replicate the square shaped PCB whereas the non-imaging central reflective portion 30 having individually optimized prisms 32 of the collimator 10 provides a more aesthetically pleasing circular light spot. It has been found that in an embodiment, the side surfaces 32a and 32b of the prisms 32 were able to reflect over 50% of the incident light, with the remaining light being refracted under large angles such that the refracted light did not interfere with the formation of the image at far field. In another simulation, the PCB was simulated to carry three colour LEDs. FIG. 10 shows the optical output of the prior art collimator and FIG. 1 1 shows the optical output of the collimator 10 according to an embodiment of the present invention at far field (200 mm from the collimator). The arrows in FIG. 10 depict areas of distinct colour separation in the far-field image of the prior art collimator. In contrast, the spot produced by the collimator 10 comprising the non-imaging central reflective portion 30 having individually optimized prisms 32 exhibits excellent colour mixing and negligible colour separation.

FIG. 12 schematically depicts a cross-section of a non-limiting embodiment of a lighting device 100. The lighting device 100 typically comprises a carrier 1 10 carrying one or more SSL elements. In FIG. 12, a first SSL element 1 12a and a second SSL element 1 12b are shown by way of non-limiting example only; it should be understood that the lighting device 100 may comprise any suitable number of SSL elements. In an embodiment, the SSL elements include SSL elements of different colour, e.g. two or three different colours to be mixed in order to produce white light as is well-known per se. The carrier 1 10 may be any suitable carrier, e.g. a PCB, a heat sink, a combination of a PCB and a heat sink and so on. The carrier 1 10 may have any suitable shape, e.g. a rectangular or circular shape.

The lighting device 100 further comprises an embodiment of the collimator

10. The collimator 10 is typically aligned central to the lighting device 100, e.g. aligned with the central axis of the lighting device 100. In an embodiment, the collimator 10 is placed such that the prisms of the collimator face the SSL elements of the lighting device 100 to ensure a highly collimated luminous output.

In an embodiment, the lighting device 100 is a light bulb such as a spot light bulb. The light bulb may be any suitable size or shape. Non-limiting examples of suitable light bulb sizes include MR1 1 , MR16, GU10, AR1 1 1 , PAR38, PAR30, BR30, BR40, R20, and R50 light bulbs although it should be understood that many more suitable sizes will be apparent to the skilled person.

The lighting device 100 according to embodiments of the present invention may be advantageously included in a luminaire such as a holder of the lighting device, e.g. a ceiling light fitting, or an apparatus into which the lighting device is integrated, e.g. a cooker hood or the like to produce a luminaire capable of generating a high-quality luminous profile at far field.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A collimator (10) for a lighting device (100) comprising at least one solid state lighting element (1 12a, 1 12b), the collimator comprising:

a reflective central portion (30) and an annular portion (40) surrounding the reflective central portion, wherein the reflective central portion comprises a plurality of concentric prisms (32) each comprising:

a first surface (32a) oriented under a first angle (CH) with a central axis (50) of the collimator; and

a second surface (32b) oriented under a second angle (02) with said central axis, said first and second surfaces of each prism extending from a main surface of the collimator and terminating into a common end point;

wherein incident light refracted by the first surface is reflected by the second surface and incident light refracted by the second surface is reflected by the first surface.

2. The collimator of claim 1 , wherein the annular portion is selected from a reflective annular portion, a refractive annular portion and a total internal reflection body portion.

3. The collimator of claim 1 or 2, wherein the concentric prisms are totally internally reflecting.

4. The collimator (10) of any of claims 1 -3, wherein the absolute value of the first angle (α-ι) equals the absolute value of the second angle (02) for at least some of the concentric prisms (32).

5. The collimator (10) of any of claims 1 -4, wherein the absolute value of the first angle (CH) is different to the absolute value of the second angle (02) for at least some of the concentric prisms.

6. The collimator (10) of any of claims 1 -5, wherein each first surface (32a) and second surface (32b) is a curved surface such as a truncated conical surface.

7. The collimator (10) of any of claims 1 -6, wherein the annular portion (40) comprises a plurality of further concentric prisms (42) each comprising a first further surface and a second further surface extending from a main surface (20) and terminating into a common further end point, wherein each first further surface is oriented under a first further angle with said central axis and the second further surface is oriented under a second further angle with said central axis.

8. The collimator (10) of claim 7, wherein each first further surface and second further surface is a curved surface such as a truncated conical surface.

9. The collimator (10) of any of claims 1 -8, wherein the respective common end points are oriented on one of a virtual plane, a virtual convex surface (34) and a virtual concave surface (36).

10. The collimator (10) of any of claims 1 -9, wherein the collimator is a total internal reflection collimator.

1 1 . A lighting device (100) comprising at least one solid state lighting element (1 12a, 1 12b) and the collimator (10) of any of claims 1 -10, wherein the concentric prisms face the at least one solid state lighting element.

12. The lighting device (100) of claim 1 1 , wherein the reflective central portion (30) of the collimator (10) is aligned with a central axis (150) of the luminous output distribution of the at least one solid state lighting element (1 12a, 1 12b).

13. The lighting device (100) of claim 11 or 12, wherein the lighting device comprises a plurality of solid state lighting elements (112a, 112b) arranged on a rectangular carrier (110).

14. The lighting device (100) of any of claims 11-13, wherein the lighting device comprises a plurality of solid state lighting elements (112a, 112b) including solid state lighting elements of different colour.

15. A luminaire comprising the lighting device (100) of any of claims 11-14.