WO2014068497A1

WO2014068497A1 - Optical element with total internal reflection surface portion for improved spatial light distribution

Info

Publication number: WO2014068497A1
Application number: PCT/IB2013/059787
Authority: WO
Inventors: Haifeng Zhang; Yuanjing PENG
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-11-05
Filing date: 2013-10-30
Publication date: 2014-05-08
Also published as: DE202013012202U1; DE112013005281T5; CN204964862U

Abstract

This invention relates to an optical element (7; 21) for modifying a spatial distribution of light emitted by a light source (4).The optical element comprises a light input surface (17; 27) for receiving light emitted by the light source (4), the light input surface (17; 27) defining a cavity (16) in the optical element (7; 21) for accommodating the light source (4).The optical element further comprises a light output surface (9; 22) for emitting an elongated light beam (10).The light output surface (9; 22) comprises at least a first total internal reflection surface portion (12; 23) arranged to receive light traveling from the light input surface (17; 27). The first total internal reflection surface portion (12; 23) is shaped to reflect at least a portion of the light towards the plane parallel to the supporting surface (8; 25) through total internal reflection.

Description

Optical element with total internal reflection surface portion for improved spatial light distribution

FIELD OF THE INVENTION

The present invention relates to an optical element for modifying a spatial distribution of light emitted by a light-source mounted on a carrier to provide an elongated light beam. The present invention also relates to a lighting system and to a light-output device.

BACKGROUND OF THE INVENTION

For the safety of motorists and pedestrians etc., roads are often illuminated at night time using street lights.

In street lighting, one of the challenges is to achieve selective illumination, i.e. to illuminate the street and not the surrounding areas. This may be accomplished by using optical elements such as reflectors and/or lenses. This allows for more efficient use of energy since the distance between adjacent street lights can be increased without losing desired lighting. Furthermore, by increasing the distance fewer street lights are needed and therefore it also reduces the cost of installation and maintenance.

However, conventional lighting technology such as e.g. discharge lamps, need a large amount of energy for each light in order to illuminate the road between the lights, in particular, if the distance between adjacent street lights is increased. Furthermore, they require frequent maintenance.

Recently, solid state light-sources, such as light-emitting diodes (LEDs) have been implemented in street lighting applications. For example, WO2012/080889A1 discloses a street light comprising an array of LEDs and an array of optical elements, one optical element arranged in front of each LED.

Each of the optical elements according to WO2012/080889A1 modifies the spatial distribution of the light emitted by its corresponding LED to provide an elongated illumination pattern on the road. This allows for increased spacing between street lights.

However, it would be desirable to further improve the spatial distribution of light and to provide an improved illumination pattern in LED based street lighting. SUMMARY OF THE INVENTION

In view of the above, it is a general object of the present invention to provide an improved optical element providing for an improved spatial distribution of light.

According to the present invention, it is therefore provided an optical element for modifying a spatial distribution of light emitted by a light source to provide an elongated light beam that is elongated in a first direction, wherein the optical element comprises: a supporting surface for supporting the optical element on a carrier; a light input surface for receiving light emitted by the light source, the light input surface defining a cavity in the optical element for accommodating the light source; a light output surface for emitting the elongated light beam following a first refraction at the light input surface and a second refraction at the light output surface, wherein the light output surface comprises at least a first total internal reflection surface portion arranged to receive light traveling from the light input surface in a second direction perpendicular to the first direction and deviating from a plane parallel to the supporting surface by an angle smaller than 45°, the first total internal reflection surface portion being shaped to reflect at least a portion of the light towards the plane parallel to the supporting surface through total internal reflection.

The light-source may advantageously be a so-called solid state light-source, which is a light-source in which light is generated through recombination of electrons and holes. Examples of solid state light-sources include light-emitting diodes (LEDs) and semiconductor lasers.

The present invention is based on the realization that the ratio of 'useful' illumination can be increased by providing an optical element that directs light rays that would otherwise by emitted in an unwanted direction back towards the supporting surface so that this light may be recycled. For example, the light thus redirected by the optical element may be reflected by a scattering reflector and thereafter enter the optical element in more favorable directions. For instance, the light-source may be mounted on a carrier, which may have a diffusely reflecting upper surface. After reflection at such a scattering reflector, 'recycled' light rays can contribute to the 'useful' illumination.

The present inventors have further realized that the above-mentioned direction of light rays towards the carrier can conveniently and efficiently be achieved through total internal reflection by suitably shaping a portion of the light output surface of the optical element.

By increasing the ratio of 'useful' illumination, that is, light that would otherwise not reach the intended area of illumination, the energy consumption of a light- output device comprising the optical element may be reduced while still providing the same amount of 'useful' illumination.

Since the first TIR surface is arranged to receive light in a second direction deviating from a plane parallel to the supporting surface (such as a surface of the carrier when the optical element is arranged to accommodate a light source arranged on a carrier) by an angle smaller than 45°, the first TIR surface portion is arranged to receive light rays emitted from the light source at small angles with respect to the supporting surface. This means that 'diverging' light that would otherwise escape from the optical element in an unwanted direction can be 'trapped' at the TIR surface portion and redirected towards the carrier.

The optical element may be manufactured from an optical material having a refractive index greater than 1. Examples of suitable optical materials are poly(metyhyl methacrylate) (PMMA) or polycarbonate (PC), but many other materials could be used, for example glass.

The first TIR surface portion may advantageously comprise a surface portion with a center of curvature different from the center of curvature of adjacent surface portions of the light output surface outside the first total internal reflection surface.

In other words, in every cross-section of the optical element comprising a connection between the first TIR-surface portion and another portion of the light output surface, a curve defining the outline of the cross-section has an interrupt point at the connection such that the outline curve is discontinuous at the connection.

The optical element may advantageously have a plane of symmetry that intersects the first TIR surface portion.

This may be advantageous for example the case of a divergent section being part of the plane of symmetry, in which case the diverging light rays may be collected and reflected back to the carrier by the TIR surface portion.

A cross-section of the TIR surface portion with respect to the plane of symmetry may advantageously be defined by a Bezier curve.

A Bezier curve is a curve that may be defined by three points and is an efficient way of controlling a curvature. By defining the cross-section of the first total internal reflection surface portion using a Bezier curve, design of the optical element can be facilitated.

The light output surface may advantageously further comprise a second TIR surface portion, the second TIR surface portion being arranged opposite from the first TIR surface portion with respect to the cavity of the optical element, such that the second TIR surface portion is intersected by the plane of symmetry of the optical element.

To further reduce the occurrence of stray light emitted "to the sides", the optical element may additionally comprise a second TIR surface portion. For example, with the additional TIR surface portion arranged opposite to the first TIR surface portion, the emitted light beam may be made narrower along the dimension between the two TIR surface portions.

In one embodiment, having the second TIR surface similar to the first TIR surface, the optical element of this embodiment could have two planes of symmetry perpendicular to each other.

The light input surface may advantageously comprise a refraction surface portion configured in such a way that an angle between a tangent to the surface portion and a plane parallel to the supporting surface is at least 40°.

The refraction surface portion is arranged such that light rays may be refracted by the refraction surface portion before reaching the TIR surface portion. The refraction surface refracts light rays in such a way that the incident angle of the light rays when they reach the TIR surface is large. When the angle between a tangent to the refraction surface portion and a plane parallel to the supporting surface is at least 40° the TIR surface portion of the light output surface can efficiently reflect the light back towards the carrier.

Furthermore, the refraction surface portion of the light input surface may advantageously be planar. However, the refraction surface portion may further be a curved surface portion as long as a tangent of the surface portion fulfills the condition of an angle of at least 40° with a plane parallel to the supporting surface.

The refraction surface portion is arranged adjacent to the TIR surface portion of the light output surface such that light rays may be refracted by the refraction surface portion before reaching the TIR surface portion. In the plane of symmetry, the refraction surface portion and the TIR surface portion are arranged "on the same side" with respect to the cavity.

The light output surface may advantageously comprise a surface portion defined by a Zernike polynomial.

In other words, a surface portion adjacent to the TIR surface portion may be defined by a Zernike polynomial. Zernike polynomials are commonly used to describe optical aberrations and are therefore useful when designing optics with high precision. A surface defined by Zernike polynomials forms a curved surface. In another embodiment, the light output surface may advantageously comprise two convergent surface portions bridged by a divergent surface portion. This configuration gives rise to a "peanut" shaped surface and is efficient in collecting emitted light rays into an elongated shape, where the elongation is in the first direction along the longitudinal axis of the "peanut" shaped surface.

Advantageously, the TIR surface may be arranged adjacent to the divergent surface portion.

In other words, the TIR surface may be arranged on the narrow portion in the middle of the "peanut". By arranging the TIR surface portion adjacent to the divergent section, the light rays diverging in the direction of the TIR surface portion may efficiently be reflected back to the carrier.

A plurality of optical elements according to various embodiments of the present invention may, furthermore, advantageously be arranged in an array to form an optical assembly. Such an optical assembly may comprise different optical elements, or all optical elements in the optical assembly may be substantially identical.

The optical elements of such an assembly may be arranged such that the emitted light by all the optical elements form a composite uniform output light beam with high intensity.

Advantageously, the plurality of optical elements in the optical assembly may be formed integrally in one piece

Making the assembly in a single piece facilitates the maintenance of a final product significantly. Furthermore, mass production of a device comprising the optical elements may be performed more efficiently with higher throughput if formed integrally in one piece.

According to one embodiment of the present invention there is further provided a lighting device comprising: a plurality of light-sources mounted on a carrier; and the above-mentioned optical assembly arranged on the carrier in such a way that each of the solid state light-sources is accommodated in the cavity of a corresponding one of the optical elements comprised in the optical assembly; wherein the carrier is a scattering reflecting carrier.

The carrier of the lighting device on which the light sources are mounted comprises a scattering reflecting surface in order to (diffusely) reflect and therefore recycle the light returning from the TIR surface. According to one embodiment of the present invention there is provided a luminaire comprising: a fixture; and the lighting device attached to the fixture.

For example, the fixture may be a post making the luminaire suitable for road or street lightning. It may further be a smaller post making the luminaire suitable for lighting of sidewalks, walking trails, garden lighting, or illumination of parks. The fixture may also be a ceiling of a tunnel such that the luminaire may be used for lighting the tunnel, or some other ceiling. Furthermore, a wall may be a fixture such that the luminaire can be attached to the wall for lighting of tunnels, or even indoor lighting.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing example embodiments of the invention, wherein:

Fig. 1 illustrates a luminaire comprising a lighting device and a post;

Fig. 2 illustrates an exemplary embodiment of a lighting device comprising optical elements according to the present invention;

Fig. 3 is an exploded view of an optical element according to a first embodiment of the present invention and a light emitting diode, mounted on a carrier;

Fig. 4 is a cross-sectional view of the optical element according to the first embodiment of the present invention and a light emitting diode, mounted on a carrier;

Fig. 5 illustrates an optical element according a second embodiment of the present invention;

Fig. 6 is a cross-sectional view of the second embodiment of the present invention mounted on a carrier; and

Fig 7 schematically shows a further exemplary embodiment of the optical element according to the present invention, having two TIR surface portions.

DESCRIPTION OF AN EXAMPLE EMBODIMENT OF THE PRESENT INVENTION

In the following description, the present invention is mainly described with reference to a road lighting application with optical elements arranged on a plate mounted on a post for illumination of a road.

It should, however, be noted that this by no means limits the scope of the invention, which is equally applicable to other applications, such as illumination of sidewalks, parks, gardens, etc. Fig. 1 illustrates an exemplary application for embodiments of the optical element according to the present invention in the form of a luminaire 1 comprising a fixture 2 and a lighting device 3 attached to the fixture 2. In this embodiment the application may be for illumination of for example a road, or a sidewalk.

Fig. 2 illustrates an embodiment according to the present invention in terms of a lighting device 3 comprising a plurality of solid state light-sources, here provided in the form of light-emitting diodes (LEDs) 4, mounted on a carrier 5. An optical assembly 6 is arranged on the carrier 5 such that each of the LEDs 4 is accommodated in the cavity of a corresponding one of the optical elements 7 of the optical assembly 6. In this way, the optical elements may modify the light emitted by the LEDs. The carrier 5 of this embodiment is a scattering reflecting carrier in order to serve for recycling light beams reflected by the TIR surface (see Fig 4) of the optical element. The lighting device may be mounted on a fixture 2 such as a post for illumination of for example a road.

In one embodiment of the lighting device illustrated in Fig. 2, the optical assembly comprises 16 optical elements with corresponding LEDs. The optical elements are arranged such that the emitted light efficiently produces a uniform composite light beam with high intensity and low energy consumption. If such a lighting device is mounted on a post for road lighting, one may achieve a spacing/height of 8/1, where spacing is the distance between adjacent lampposts and height is the height of the post.

Fig. 3 is an exploded view of an exemplary embodiment of an optical element

7 mounted on a carrier 5 for supporting the optical element 7 on the supporting surface 8. An LED 4 is arranged in a cavity formed by the optical element 7 and the carrier 5 when the optical element 7 is arranged on the carrier 5 (the cavity will be explained in more detail with reference to Fig. 4). There is a light output surface 9 for emitting an elongated light beam 10 elongated along a first direction 11. The light output surface 9 comprising at least a first TIR surface portion 12 arranged to receive light traveling from the light input surface (shown in Fig. 4) in a second direction 13 perpendicular to the first direction 11.

In the embodiment shown in Fig. 3, the light output surface 9 comprises two convergent surface portions 14 bridged by a divergent surface portion 15, and a first TIR surface portion 12 arranged adjacent to the divergent surface portion 15. This configuration yields a "peanut" shaped light output surface. The two converging surface portions 14 are distributed along a first direction 11, which make the emitted light beam 10 elongated in the first direction 11 as is indicated in Fig 3. The bridging divergent surface portion spreads the emitted light from the divergent surface towards the convergent surface portions. The TIR surface portion 12 is arranged adjacent to the divergent surface portion 15, and such that light emitted in a second direction 13, perpendicular to the first direction 11, and that falls within an angle smaller than 45° of a plane parallel to the supporting surface 8, may be total internal reflected towards the carrier 5. This way, with the TIR surface portion 12, the otherwise "useless" light emitted by the LED, can be reflected back towards the carrier and be recycled. In some embodiments, the angle may advantageously be smaller than 35° or 25°, so as to have the elongated light beam 10 a little wider.

Fig. 4 is a cross-sectional view of the optical element 7 mounted on a carrier 5 for supporting the optical element 7 on the supporting surface 8 and for supporting a LED 4, as shown in Fig 3. The cross-section is in the plane of symmetry that intersects the first TIR surface portion 12 along the second direction 13. As can be seen in Fig. 4, the cavity 16 that accommodates the LED 4 is defined by a light input surface 17 that comprises a planar refraction surface portion 18. The planar refraction surface portion is arranged such that an angle a of at least 40° is formed between the planar refraction surface portion and a plane parallel to the supporting surface 8 of the optical element 7. The upper surface 19 of the carrier 5 is such a plane parallel to the supporting surface 8 of the optical element.

In Fig. 4 an exemplary travel path 20 for a light beam is indicated. The exemplary light beam is emitted by the LED 4 and travels towards the light input surface 17 in the direction of the planar refraction surface portion 18. The light input surface 17 receives the light beam 20 at the planar refraction surface 18 and the light beam is refracted at the planar refraction surface 18. Following the refraction at the refraction surface 18, there is a light output surface 9 for emitting the light beam after a second refraction at the light output surface 9. The travel path 20 for the light beam in Fig. 4 reaches, after the first refraction at the planar refraction surface 18, the TIR surface portion 12 arranged to receive light traveling from the light input surface 17 in the second direction 13 perpendicular to the first direction 11 and deviating from a plane parallel to the supporting surface 8 by an angle smaller than 45°. The TIR surface portion is shaped to reflect at least a portion of the light towards the plane parallel to the supporting surface 8 through total internal reflection. Therefore, as is shown by the exemplary travel path 20, the light beam continues, after the total internal reflection at the TIR surface portion 12, towards the carrier 5. The carrier being such that light is reflected at the surface as indicated in Fig. 4, and may therefore be recycled.

The embodiment shown in Fig. 4 comprises a planar refraction surface portion 18. The refraction surface may in various embodiments according to the present invention be a curved surface portion provided that an angle between a tangent of the refraction surface portion and the plane of the carrier is at least 40°. For example, the refraction surface portion may be a convergent surface portion which equally well functions according to the present invention.

In various embodiments according to the present invention, the cross-section of the TIR surface portion 12 in the plane of symmetry may be defined by a Bezier curve. Using a Bezier curve is an efficient way to define curvatures. A Bezier curve may be defined by three points, with three points the curve is given by:

P(t) = (1 - 1) ² * P₀ + 2t(\ - 1) * P_l + 1² * P₂ (0 < t < 1), where Po, Pi, and P₂ are the three points that control the shape of the curve. For example, when t=0, then P(t)=Po, and when t=l , then P(t)=P₂, that means Po and P₂ are the start and end points of the Bezier curve. By changing Pi the curvature can be controlled.

In various embodiments according to the present invention the cross-section of at least one surface portion of the light input surface 17 in the plane of symmetry may be defined by a Bezier curve.

Fig. 5 illustrates an alternative embodiment of an optical element 21 according to the present invention, comprising a light output surface 22, a first TIR surface portion 23, a light output surface portion defined by Zernike polynomials 24, the TIR surface portion 23 being arranged adjacent to the Zernike light output surface portion 24.

Zernike polynomials are typically used to describe optical aberrations where different orders of the Zernike polynomials describe different orders of aberration. The function describing the surface is given by

.„ 66

1 -f- 1— (1 kjc^v ^

where, k is a conic constant, c is the curvature, c_n+i is the coefficient of Z_n (n=l to 66), Z_n is the nth Zernike polynomial, and r = ^x¹ + y² where x and y are perpendicular dimensions in the plane of the carrier 5.

Fig. 6 illustrates a cross-section in the plane of symmetry of the embodiment of the present invention shown in Fig, 5, the cross-section intersects the light output surface 22 comprising a surface portion defined by Zernike polynomials 24 and a first TIR surface portion 23 adjacent to the Zernike light output surface portion 24, a carrier 5 supporting the optical element 21 on a supporting surface 25 and that may support an LED, a refraction surface portion 26 arranged adjacent to the first TIR surface portion 23, and a light input surface 27.

In Fig. 6, the cross-section of at least one surface portion of the light input surface 27 in the plane of symmetry may be defined by a Bezier curve. Furthermore, the cross-section of the TIR surface portion 23 in the plane of symmetry may be defined by a Bezier curve.

Finally, an optical element 30 according to a further embodiment will be described with reference to Fig 7. The optical element in Fig 7 is similar to the optical element 7 described above with reference to Figs 3 and 4. While the optical element 7 in Figs 3 and 4 has a single TIR surface portion 12 and thus is non- symmetrical with respect to the cross-section shown in Fig 4, the optical element 30 in Fig 7 has a first 12 and a second 31 TIR surface portion to make the light beam shaped by the optical element 30 symmetrically narrower. Although not explicitly shown in Fig 7, it should be understood that the light input surface of the optical element 30 shown in Fig 7 may advantageously have an additional refraction surface arranged opposite the refraction surface portion 18 shown in Fig 4, so that the light input surface is also symmetrical in the cross-section view of Fig 4.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. For example the embodiments are by no means limited to having a single light-source in each optical element but can equally well comprise more than one light source. Furthermore, the present invention is described in embodiments having the "peanut" shaped light output surface and, in another embodiment a Zernike polynomial surface. However, the invention may work equally well with for example a spherical, elliptical, paraboloid or other polynomial light output surfaces.

The light input surface and the TIR surface portion is in various embodiments described in terms of Bezier curves. Neither is the mentioned order "three" of the Bezier curve limiting nor does it limit the function of the present invention. Furthermore, the TIR surface portion may equally well be a planar surface portion.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

Claims

CLAIMS:

1. An optical element (7; 21) for modifying a spatial distribution of light emitted by a light source (4) to provide an elongated light beam (10) that is elongated in a first direction (11), wherein said optical element (7; 21) comprises:

a supporting surface (8; 25) for supporting said optical element (7; 21) on a carrier (5);

a light input surface (17; 27) for receiving light emitted by said light source (4), said light input surface (17; 27) defining a cavity (16) in said optical element (7; 21) for accommodating said light source (4);

a light output surface (9; 22) for emitting said elongated light beam (10) following a first refraction at said light input surface (17; 27) and a second refraction at said light output surface (9; 22),

wherein said light output surface (9; 22) comprises at least a first total internal reflection surface portion (12; 23) arranged to receive light traveling from said light input surface (17; 27) in a second direction (13) perpendicular to said first direction (11) and deviating from a plane parallel to said supporting surface (8; 25) by an angle smaller than 45°,

said first total internal reflection surface portion (12; 23) being shaped to reflect at least a portion of said light towards said plane parallel to said supporting surface (8; 25) through total internal reflection.

2. The optical element (7; 21) according to claim 1, wherein said first total internal reflection surface portion (12; 23) comprises a surface portion with a center of curvature different from a center of curvature of adjacent surface portions of said light output surface (9; 22) outside said first total internal reflection surface (12; 23).

3. The optical element (7; 21) according to claim 1 or 2, wherein said optical element (7; 21) has a plane of symmetry that intersects said first total internal reflection surface portion (12; 23).

4. The optical element (7; 21) according to claim 3, wherein a cross-section of said total internal reflection surface portion (12; 23) with respect to said plane of symmetry is defined by a Bezier curve.

5. The optical element (7; 21) according to claim 3, wherein said light output surface (9; 22) further comprises a second total internal reflection surface portion, said second total internal reflection surface portion being arranged opposite from said first total internal reflection surface (12; 23) with respect to said cavity (16) of the optical element (7; 21), such that said second total internal reflection surface portion is intersected by said plane of symmetry of the optical element (7; 21).

6. The optical element (7; 21) according to claim 1 or 2, wherein said light input surface (17; 27) comprises a refraction surface portion (18; 26) configured in such a way that an angle (a) between a tangent to said surface portion (18; 26) and said plane parallel to said supporting surface (8; 25) is at least 40°.

7. The optical element (7; 21) according to claim 6, wherein said refraction surface portion (18; 26) of the light input surface (17; 27) is planar.

8. The optical element (7; 21) according to claim 1 or 2, wherein said light output surface (9; 22) comprises a surface portion defined by a Zernike polynomial (24).

9. The optical element (7; 21) according to claim 1 or 2, wherein said light output surface (9; 22) further comprises two convergent surface portions (14) bridged by a divergent surface portion (15).

10. The optical element (7; 21) according to claim 9, wherein said first total internal reflection surface portion (12; 23) is arranged adjacent to said divergent surface portion (15).

11. An optical assembly (6) comprising a plurality of optical elements (7; 21) according to claim 1 or 2 arranged in an array.

12. The optical assembly (6) according to claim 11, wherein said plurality of optical elements (7; 21) is formed integrally in one piece.

13. A lighting device (3) comprising:

a plurality of light sources (4) mounted on a carrier (5); and

the optical assembly (6) according to claim 11 or 12 arranged on said carrier (5) in such a way that each of said light sources (4) is accommodated in said cavity (16) of a corresponding one of said optical elements (7; 21) comprised in said optical assembly (6);

wherein said carrier (5) is a scattering reflecting carrier.

14. A luminaire (1) comprising:

a fixture (2); and

the lighting device (3) according to claim 13 attached to said fixture (2).