WO2016020323A1

WO2016020323A1 - Lighting arrangement with end color suppression

Info

Publication number: WO2016020323A1
Application number: PCT/EP2015/067816
Authority: WO
Inventors: Tewe Hiepke Heemstra
Original assignee: Koninklijke Philips N.V.
Priority date: 2014-08-05
Filing date: 2015-08-03
Publication date: 2016-02-11

Abstract

A lighting arrangement (1) is disclosed. The lighting arrangement comprises an elongated carrier (6) defining a longitudinal direction (L) and having a center portion (8) arranged between two longitudinally separated end portions (7), two or more center light sources (10) arranged in a pattern having an extension in the longitudinal direction (L) on the center portion (8); and at least one end light source (11) arranged on at least one of the two end portions (7). The center light sources (10) comprises at least one light source configured to emit light having a first color and at least one light source configured to emit light having a second color which is different from the first color. The at least one end light source (11) is configured to emit light having a color which is different from the first and second colors.

Description

LIGHTING ARRANGEMENT WITH END COLOR SUPPRESSION

FIELD OF THE INVENTION

The present disclosure relates to an elongated lighting arrangement having light sources emitting light of different colors. BACKGROUND OF THE INVENTION

Many lighting arrangements that emit light of a certain color do so through the mixing of light of different colors. Light that appears white to the eye may be produced by mixing light from red, green and blue light sources, for instance. A light-emitting diode (LED) lamp exemplifying this technique is disclosed in US 8,376,602 B2. The LED lamp may include several differently colored LEDs, the light from which is combined to form white light.

This technique of mixing light of various colors provides for a good compromise between energy efficiency and high color rendering, but also presents a challenge when it comes to preventing the mixed light from showing color variations reflecting the light source positions. Further technical development in this area is warranted.

SUMMARY OF THE INVENTION

The general object of the present disclosure is to provide an improved or alternative lighting arrangement, the illumination from which is a mixture of the light from several light sources of different colors.

In a lighting arrangement wherein light of various colors is mixed, nonuniform color may be a particularly pertinent problem by the ends of an elongated lighting arrangement due to insufficient mixing of the light emitted by the end light sources. An aspect of particular interest is therefore to reduce discolorations by the ends of elongated lighting arrangements of this type, such as a tubular LED lamp providing white illumination light.

The invention is defined by the independent claim. Embodiments are set forth in the dependent claims, the description and the drawings. According to a first aspect, a lighting arrangement is disclosed. The lighting arrangement comprises: an elongated carrier defining a longitudinal direction and having a center portion arranged between two longitudinally separated end portions, two or more center light sources arranged in a pattern having an extension in the longitudinal direction on the center portion; and at least one end light source arranged on at least one of the two end portions. The center light sources comprise at least one light source configured to emit light having a first color and at least one light source configured to emit light having a second color which is different from the first color. The at least one end light source is configured to emit light having a color which is different from the first and second colors.

Providing an end of the arrangement of light sources with a light source of a different color than the center light sources may help suppress color variations in the light emitted by that end.

An efficient way to suppress color variations is to have the color of the light emitted by the at least one end light source being equal to the average color of the light emitted by the center light sources. Color variations may also be efficiently suppressed by adapting the color of the light emitted by the at least one end light source to the color of the light emitted by the closest center light source, i.e. the center light source that is closest to the at least one end light source. For example, the color of the light emitted by the at least one end light source may be slightly shifted away from the average color in order to compensate for the color of the light emitted by the closest center light source. In some embodiments, it may be advantageous to adapt the light emitted by the at least one end light source to the color of the light emitted by the closest center light sources, for example the two, three or four closest center light sources.

The light emitted by any two neighboring center light sources may have different colors. Each center light source may be configured to emit light having either the first color or the second color. The first color and the second color are typically two colors of light that combine to form white light. The first color may for example be red and the second color may be greenish-yellow so that the lighting arrangement provides white illumination light. Other combinations of two colors are of course possible. For example, the first color may be red, and the second color and associated flux may be chosen such that the average resulting color is near the black body curve.

The center light sources and the end light sources may be solid-state lighting devices, such as semiconductor light-emitting diodes, organic light emitting diodes or polymer light emitting diodes. Many further variants of the lighting arrangement are conceivable. For example, the center light sources and the at least one end light sources may form a single straight line. At least one end light source can be arranged on each of the two end portions. The lighting arrangement may comprise a housing which encloses the center light sources and the end light sources. The housing may be tubular. The housing may for example be a straight tube or a curved tube. The housing usually has a light-transmissive portion arranged to receive light emitted by the center light sources and the end light sources. The light- transmissive portion may be configured to diffuse the light emitted by the center light sources and the end light sources.

It is noted that the invention relates to all possible combinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 is a schematic cutaway view in perspective of a lighting arrangement. Fig. 2 is a schematic illustration of a CIE 1931 color space chromaticity diagram.

As illustrated in the figures, the sizes of layers and regions may be exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of embodiments of the present invention. Like reference numerals refer to like elements throughout.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

The lighting arrangement 1 shown in Figure 1 includes a tubular housing 2. The shape of the housing 2 in figure 1 is that of a straight tube having a circular transverse cross section, but other shapes and cross sections are conceivable. The housing 2 may have the shape of a curved tube, for instance. The transverse cross section of the housing 2 may be U-shaped. The length 1 of the housing 2 may be between a few centimeters and several meters, for example between 50 centimeters and 1.5 meters. The diameter d of the housing 2 may be in the range from 1 centimeter to 10 centimeters, for example between 2 centimeters and 4 centimeters. The housing 2 is adapted to transmit light and thus may comprise a light- transmissive portion 3. The housing 2 can for example be made of plastics or glass that transmits light. The light-transmissive portion 3 is typically provided with means for diffusing light, for example a light-diffusing layer 12. The light-transmissive portion 3 may be provided with a collimator, such as a lens. One or more connectors 4 for connecting to a mains electricity supply, or some other electrical power source, may be attached to the housing 2. A connector 4 is attached to each of the two ends of the housing 2 in figure 1. Each connector 4 may have one or more pins 5 for inserting into a power socket.

The housing 2 encloses, either partly or completely, an elongated carrier 6 defining a longitudinal direction L. The carrier 6 has two end portions 7 which are separated in the longitudinal direction L. A center portion 8 of the carrier 6 is arranged between the two end portions 7. The carrier 6 may be a circuit board, for example a printed circuit board or a wired circuit board. The carrier 6 may be in thermal contact with a heat sink 9 for transferring heat away from the carrier 6. The carrier 6 may for example be arranged on the heat sink 9. The heat sink 9 may be formed by the housing 2.

The carrier 6 provides electrical circuitry for several light sources which are mounted on the carrier 6. The light sources include two or more center light sources 10, which are arranged on the center portion 8, and at least one end light source 1 1 arranged on one of the end portions 7. In figure 1, each of the two end portions 7 is provided with one end light source 1 1. An example of an alternative embodiment is that of two or more end light sources being arranged on each of the two end portions 7. Another example is that of only one of the end portions 7 being provided with one or more end light sources 1 1. The center light sources 10 are arranged on the center portion 8 in a pattern having an extension in the longitudinal direction L. The center light sources 10 may for example be arranged along a slightly curved line or in a zigzag pattern. In figure 1, the center light sources 10 form a single straight line. Other possible embodiments include those in which the center light sources 10 form two or more straight lines. The center 10 and end 1 1 light sources are enclosed by the housing 2 so that light emitted by the center 10 and end 1 1 light sources is received by the light-transmissive portion 3. The center 10 and end 1 1 light sources are typically solid-state lighting devices, such as semiconductor light-emitting diodes, organic light emitting diodes or polymer light emitting diodes. The center light sources 10 emit light of more than one color. To be more precise, at least one of the center light sources 10 is configured to emit light of a first color and at least one center light sources 10 is configured to emit light of a different, second color. According to one embodiment, each of the center light sources 10 is configured to emit either the first color or the second color. The center light sources 10 may be arranged on the carrier 6 so that any two neighboring light sources are of different colors. If the center light sources 10 are arranged along a straight line and each center light source 10 is configured to emit light having either the first or the second color, the center light sources 10 may be alternately arranged. That is to say, every other center light source 10 may emit light of the first color while the remaining center light sources 10 emit light of the second color.

The end light sources 1 1 are configured to emit light having a different color than the first and second colors. The color of the light emitted by each of the end light sources 1 1 may for example be the average color of the light emitted by the center light sources 10. However, all of the end light sources 1 1 do not have to emit light of the same color. The color of an end light source 1 1 may be adapted to, or selected based on, the color of the closest center light source 10, alternatively two or more of the closest center light sources 10. For example, an end light source 1 1 whose closest center light source 10 emits light of the first color may emit light having a color that is slightly shifted away from the average color and the first color. Similarly, an end light source 1 1 whose closest center light source 10 emits light of the second color may emit light having a color that is slightly shifted away from the average color and the second color. Factors influencing which end light source color minimizes the color variation over the length of the lighting arrangement 1 include the shapes of the light spots on the light-transmissive portion 3 and the reflective properties of the inside ends of the housing 2.

The first and second colors may be any two colors. Usually, the first and second colors are such that the lighting arrangement 1 provides white illumination light. The first color and the second color can for example be greenish-yellow and red, respectively. By greenish-yellow light is meant light having a dominant wavelength in the range from about 515 nm to about 558 nm. By red light is meant light having a dominant wavelength larger than about 620 nm. The dominant wavelength represents the color perceived by the eye and may be derived from a CIE color space chromaticity diagram. Of course, in practice, the light emitted by any of the center 10 and end 1 1 light sources comprises a spectrum of

wavelengths. When the lighting arrangement 1 is in use, electricity is transmitted via the connector 4 and the carrier 6 to the center 10 and end 1 1 light sources which thereby emit light. The emitted light mixes to form, for example, white light. The light transmitted through the light-transmissive portion 3 of the housing 2 provides illumination. The light emitted by the end light sources 1 1 helps reduce color variations that might have appeared in the light emitted by the two ends of the lighting arrangement 1 had the lighting arrangement 1 only had the center light sources 10.

How the average color of the center light sources 10 may be determined is explained below with reference to figure 2. A common way of defining color is with respect to a CIE color space, such as the CIE 1931 XYZ color space. In this color space, the color and flux of the light emitted by a light source is represented by the so-called tristimulus values X, Y and Z, from which two chromaticity coordinates (here denoted by x and y) may be derived. A color may be specified by its chromaticity coordinates and graphically represented by a point in a CIE 1931 color space chromaticity diagram. The average color of two colors is, in such a diagram, specified by a point lying on the straight line connecting the two points representing the two colors. (If the coordinates are suitably weighted by the luminance values Y, the coordinate of the average color will be at the "center of mass.") An example is given in figure 2 in which the point P₃ represents the average color of the colors represented by the points Pi and P₂. In another example (not shown), red is in the lower right corner, greenish-yellow is e.g. CIEx=0.33; CIEy=0.4; and the mixed color can e.g. be CIEx=0.46; CIEy=0.41. These results are readily generalized to more than two colors.

Furthermore, the color point of the mixed light does not have to equidistant from both contributing color points.

It should be noted that in those embodiments in which the light emitted by the end light sources 1 1 equals the average color of the center light sources 10, the color of the light emitted by the end light sources 1 1 may not be exactly equal to said average color. More precisely, the color emitted by the end light sources 1 1 may deviate from the average color within certain bounds related to MacAdam numbers, while keeping the luminance of the lighting arrangement 1 near the average luminance. Knowing the light spot shape of all the center 10 and end 1 1 light sources, and keeping the flux of each light source about equal to the average light source flux, it is possible to determine the chromaticity coordinate of the end light sources 1 1 that minimizes the perceived severity of the variation in color of the light emitted by the lighting arrangement 1. Since energy losses in the lighting arrangement are typically only slightly influenced by the wavelength of the light exiting the housing 2, the chromaticity values x and y of the light emitted by all of the center light sources 10 may be derived from the sum of the tristimulus values X, Y and Z of each center light source 10. The tristimulus values of a light source may be derived from its spatially integrated emission spectrum. Alternatively, the average color of the light emitted by the center light sources 10 may be determined by measuring their combined spatially integrated emission spectrum, e.g. in an integrating sphere measuring system. It is also possible to measure the color of the surface of the light- transmissive portion 3 using a spectroradiometer or a color measuring video photometer.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the center 10 and end 1 1 light sources may be laser diodes the light from which is properly diffused. 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. 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. A lighting arrangement (1) comprising

an elongated carrier (6) defining a longitudinal direction (L) and having a center portion (8) arranged between two longitudinally separated end portions (7),

two or more center light sources (10) arranged in a pattern having an extension in the longitudinal direction (L) on the center portion (8); and

at least one end light source (11) arranged on at least one of the two end portions (7),

the center light sources (10) comprising at least one light source configured to emit light having a first color and at least one light source configured to emit light having a second color which is different from the first color,

characterized by the at least one end light source (1 1) being configured to emit light having a color which is different from the first and second colors.

2. The lighting arrangement (1) according to claim 1 , wherein the color of the light emitted by the at least one end light source (1 1) is equal to the average color of the light emitted by the center light sources (10).

3. The lighting arrangement (1) according to claim 1, wherein the color of the light emitted by the at least one end light source (1 1) is adapted to the color of the light emitted by the center light source (10) that is closest to the at least one end light source (1 1).

4. The lighting arrangement (1) according to claim 1, wherein the color of the light emitted by the at least one end light source (1 1) is adapted to the color of the light emitted by two or more of the center light sources (10) that are closest to the at least one end light source (1 1).

5. The lighting arrangement (1) according to any of the preceding claims, wherein any two neighboring center light sources (10) emit light having different colors.

6. The lighting arrangement (1) according to any of the preceding claims, wherein each center light source (10) is configured to emit light having either the first color or the second color.

7. The lighting arrangement (1) according to any of the preceding claims, wherein the first color is red and the second color is greenish-yellow.

8. The lighting arrangement (1) according to any of the preceding claims, wherein the center light sources (10) and the end light sources (1 1) are solid-state lighting devices.

9. The lighting arrangement (1) according to any of the preceding claims, wherein the center light sources (10) and the at least one end light source (1 1) form a single straight line.

10. The lighting arrangement (1) according to any of the preceding claims, wherein at least one end light source (1 1) is arranged on each of the two end portions (7).

1 1. The lighting arrangement (1) according to any of the preceding claims, further comprising a housing (2) which encloses the center light sources (10) and the end light sources (1 1), wherein the housing (2) has a light-transmissive portion (3) arranged to receive light emitted by the center light sources (10) and the end light sources (1 1).

12. The lighting arrangement (1) according to claim 1 1, wherein the light- transmissive portion is configured to diffuse light.