RELATED APPLICATIONS

[0001] This patent application claims the priority of German patent application 10319274.3, the disclosure content of which is hereby incorporated by reference. Field of the Invention

[0002] The invention relates to a light source including a plurality of semiconductor chips and at least two different electromagnetic-radiation-emitting chip types having different emission spectra. The electromagnetic radiation of the chips is radiated in a common light cone and intermixed. The invention relates, in particular, to light sources of this type in which the semiconductor chips emit visible electromagnetic radiation.

BACKGROUND OF THE INVENTION

[0003] Most known light sources have incandescent lamps, arc lamps or high-pressure lamps. During the operation thereof, a relatively large proportion of electrical energy is converted into heat, which may be problematic for surrounding devices or components. In this case, in order to generate light of a specific colour locus on the international standard CIE (International Commission on Illumination) chromaticity diagram, use is generally made of optical filters in order to eliminate undesirable colour components to the greatest possible extent. However, this reduces the efficiency of the respective light source.

[0004] As an alternative, there are light sources which use light-emitting diodes (LEDs), which have for example the advantage of a long service life, a fast response and also a relatively high electrical efficiency. Moreover, polychromatic light of a specific colour locus can be generated by combining LEDs of different colours. Colour filter are not absolutely necessary in this case.

[0005] A light source for the backlighting of an LCD display, said light source having LEDs with different emission spectra, is described in U.S Pat. No. 6,540,377 for example, in which red-, green- and blue-emitting LEDs are arranged in a manner intermixed with one another on a common plane. During operation of the light source, the electromagnetic radiation of the LEDs is radiated in a common light cone and intermixed by means of diffusor material.

[0006] One disadvantage of such a light source is that the light intensity that it radiates per solid angle is limited by the maximum emission intensity of the LEDs, as a result of which its applicability is restricted. Moreover, it is necessary to provide for sufficient intermixing of the electromagnetic radiation emitted by different LEDs if homogeneous light is desired.

SUMMARY OF THE INVENTION

[0007] One object of the present invention is to provide a light source based on semiconductor chips emitting visible electromagnetic radiation which has a high light intensity radiated per solid angle, which light intensity is not limited by the maximum emission intensity of the semiconductor chips.

[0008] Another object of the invention is to enable electromagnetic radiation from semiconductor chips with different emission spectra to be intermixed simply and effectively with one another.

[0009] These and other objects are attained in accordance with one aspect of the present invention directed to a light source comprising a plurality of semiconductor chips and at least two different electromagnetic-radiation-emitting chip types having different emission spectra, each of the semiconductor chips having a chip coupling-out area through which the electromagnetic radiation is coupled out. A plurality of primary optical elements is provided, each of the semiconductor chips having a respective one of the primary optical elements corresponding thereto. Each primary optical element has a light input and a light output, and reduces the divergence of at least part of the electromagnetic radiation emitted by the semiconductor chip during operation thereof. The semiconductor chips with the respectively corresponding primary optical elements are arranged in at least two groups that are spatially separate from one another, such that the groups emit separate light cones during operation of the semiconductor chips, and a secondary optical arrangement is provided, wherein the separate light cones of the groups are superposed by means of the secondary optical arrangement to form a common light cone.

[0010] Here and below the term light cone in each case means an arbitrarily shaped volume which is transilluminated by light from the light source, excluding regions in which the brightness is more than one order of magnitude lower than the maximum brightness given the same distance from the light source.

[0011] The superposition of separate light cones of different groups of semiconductor chips results in additive intermixing of radiation from different semiconductor chips. Improved mixing of the radiation from semiconductor chips with different emission spectra can thereby be achieved in a technically simple manner. Furthermore, the light intensity radiated per solid angle can be significantly increased by the superposition of separate light cones independently of the maximum light intensity of individual semiconductor chips.

[0012] In one embodiment, the separate light cones of the groups are superposed by means of the secondary optical arrangement to form a common light cone with a colour locus that is uniform to the greatest possible extent. In this case, the colour locus of the common light cone lies in the white region of the CIE chromaticity diagram.

[0013] At least one of the groups has semiconductor chips of only one chip type.

[0014] In addition or as an alternative, the semiconductor chips of a chip type are in each case contained only in one of the groups. This is useful when it is desirable to radiate light that is homogeneous to the greatest possible extent with a uniform colour locus.

[0015] In one embodiment, the plurality of semiconductor chips has a first, a second and a third chip type, the emission spectra of which are such that white light can be generated by superposition of the radiation emitted by semiconductor chips of different chip types.

[0016] In this case, the first chip type emits green radiation, the second chip type emits red radiation and the third chip type emits blue radiation, i.e. the respective emission spectrum of semiconductor chips of a chip type has radiation having wavelengths lying primarily in the green, red and blue region, respectively.

[0017] The semiconductor chips of the first chip type can be arranged in a first group and the semiconductor chips of the second and third chip types are arranged in a second group, the semiconductor chips of different chip types of the second group being arranged in a manner uniformly intermixed with one another.

[0018] As an alternative, the semiconductor chips of a chip type are arranged in each case in a group of their own and the groups are arranged in a manner spatially separate from one another, that is to say such that they do not overlap.

[0019] The secondary optical arrangement, by means of which the separate light cones are superposed, has at least one selectively reflective unit. The latter is tranmissive to the radiation of at least one separate light cone and reflective to the radiation of a further separate light cone. At least two separate light cones are superposed by means of the selectively reflective unit.

[0020] The selectively reflective unit contains a dichroic layer system.

[0021] In a particular embodiment, the secondary optical arrangement has at least one combining cube based on at least one selectively reflective unit. The combining cube has at least two light inputs, into each of which a light cone is incident. The light cones radiated into the combining cube are superposed in the interior thereof by means of the selectively reflective unit and emitted jointly from a light output of the combining cube. The incident light cone may in each case be an original separate light cone originating directly from a group of semiconductor chips or a light cone in which a plurality of such separate light cones have already been superposed.

[0022] In one embodiment, an aperture angle of a separate light cone is between 0 and 60°, preferably between 0 and 40°, particularly preferably between 0 and 20°, the limits being included in each case. As explained above, the expression light cone in this context does not relate to the shape of a cone in the mathematical sense and may accordingly have more than one aperture angle. If this is the case, then the above statement relates to the maximum aperture angle.

[0023] The fact that the divergence of the separate light cones is limited to such an extent by the primary optical elements makes it possible to achieve a higher luminance or light intensity radiated per solid angle of the light source.

[0024] In particular in order to realize such a small aperture angle of a light cone emitted by the primary optical element, the light output of the primary optical element in one embodiment has a light input area or a light input opening, the size of which is less than or equal to twice the chip coupling-out area. Particularly preferably, the size of the light input area or of the light input opening is at most 1.5 times as large, in particular at most 1.1 times or 1.05 times as large as the chip coupling-out area.

[0025] The size of the light input area or of the light input opening is preferably greater or not significantly less than the chip coupling-out area.

[0026] The light input of the primary optical element that is thus provided is not only suitable for a great reduction of the divergence of a light cone, but also enables a significant miniaturization of the primary optical element and thus production of a compact light source with a high emitted luminance.

[0027] Preferably, the primary optical elements of at least one group are at least partly arranged in such a way that their light outputs are densely packed. They preferably adjoin one another without any gaps. As a result, it is possible to achieve a higher luminance and better homogeneity of a separate light cone and thus also of the common light cone.

[0028] In one embodiment, the semiconductor chips of all the groups are essentially arranged in a common plane. This may significantly simplify mounting of the semiconductor chips.

[0029] In one embodiment, the semiconductor chips with the primary optical elements are arranged in matrix-like fashion, i.e. regularly in rows and columns, at least partly or at least in partial groups.

[0030] In one embodiment of the light source, the primary optical element is in each case an optical concentrator which is arranged with respect to the light source in such a way that its actual concentrator output is now the light input. Compared with the customary application of a concentrator (to which the term “actual” concentrator output used above relates), in the present case light from the semiconductor chips passes through the concentrator in the opposite direction. Consequently, this light is not concentrated, but rather leaves the concentrator with reduced divergence through the actual light input, now used as a light output. Once again, “actual” relates to the actual utilization as a concentrator.

[0031] The concentrator is particularly preferably a CPC-, CEC- or CHC-like concentrator, which means, here and below, a concentrator whose reflective side walls at least partly and/or at least to the greatest possible extent have the form of a compound parabolic concentrator (CPC) a compound elliptic concentrator (CEC) and/or a compound hyperbolic concentrator (CHC). Primary optical elements configured in this way enable an efficient reduction of the divergence of light, as a result of which the light intensity radiated per solid angle can be increased.

[0032] As an alternative to the CPC-, CEC- or CHC-like concentrator, the concentrator has side walls which connect the light input to the light output and along which direct connecting lines run essentially straight between the light input and the light output. The side walls have, instead of side walls curved in paraboloid, ellipsoid or hyperboloid-like fashion, side walls that essentially run straight, so that the primary optical element has for example a basic form of a frustum of a pyramid or a frustum of a cone.

[0033] The concentrator can have a cross-sectional area in the form of a regular polygon, such as a square cross-sectional area, in a region on the side of the light input. It likewise can have a cross-sectional area in the form of a regular polygon, such as a triangular, quadrangular, hexagonal or octagonal cross-sectional area, in a region on the side of the light output. The cross-sectional area undergoes transition from one form to the other between these regions. The light input of the concentrator can thus be adapted to the customary form of semiconductor chips and the light output can be configured for example in such a way that the light outputs of a plurality of primary optical elements can be arranged regularly with respect to one another and without any gaps. The use of such primary optical elements makes it possible to arrange the semiconductor chips at a relatively large distance from one another without the light cone radiated by this arrangement appearing to be illuminated with gaps. The distance between the semiconductor chips means that the heat generated by them can be dissipated better.

[0034] In one embodiment, the concentrator has a basic body defining a cavity, the inner wall of which basic body is reflective to a light emitted by the semiconductor chip, and/or the inner wall of which basic body is essentially provided with a layer or layer sequence, preferably with a metallic layer, which is reflective to a light emitted by the semiconductor chip.

[0035] As an alternative, the concentrator is a dielectric concentrator, the basic body of which is a solid body composed of a dielectric material with a suitable refractive index, so that light coupled in via the light input is reflected in it by total reflection at the lateral interface—connecting the light input to the light output—of the solid body to the surrounding medium. This has the effect that there are to the greatest possible extent no light losses on account of reflections in the concentrator.

[0036] In one embodiment, the dielectric concentrator has, as the light output, a surface, which is curved in a lens-like fashion and may be curved e.g. spherically or aspherically. As a result, it is possible to obtain a further reduction of the divergence of a light cone. The light output is preferably curved like an aspherical lens, as a result of which it is possible to take account for instance of the size of the chip coupling-out area. Spherical lenses are optimal for point light sources and, in the case of non-point light sources, may have significantly poorer properties with regard to a reduction of the divergence of a light cone.

[0037] In a further embodiment, the dielectric concentrator is advantageously at least partly provided with a layer or layer sequence, preferably with a metallic layer, which is reflective to light emitted by the respective semiconductor chip. This may be useful e.g. if the semiconductor chip is incorporated in the material of the concentrator, in order to prevent the radiation which does not meet the condition of total reflection in a first part of the concentrator from being laterally coupled out of the concentrator.

[0038] The concentrator can be arranged downstream of the semiconductor chip in the main radiating direction thereof, and there is a gap between the chip coupling-out area and the light input of the concentrator. The said gap is free of solid or viscous material to the greatest possible extent.

[0039] What is thereby achieved is that, in particular, beams which are emitted at a particularly large angle with respect to the main radiating direction of a semiconductor chip and would expand a separate light cone to an excessively great extent do not impinge on the light input, but rather pass by the latter laterally and are not even coupled into the concentrator at all. In the case of a dielectric concentrator, the gap has the effect that the proportion of beams reflected at the interface of the light input is larger, the larger the angle of incidence of said beams on the light input. Consequently, the highly divergent proportion of the light that passes into the concentrator is attenuated in each case.

[0040] In this context, the primary optical element can be assigned one or more reflector elements, which are arranged and/or are of a form such that some of the light beams which do not pass directly from the semiconductor chip into the concentrator are multiply reflected at said reflector elements and directed onto the light input of the concentrator at a smaller angle with respect to the main radiating direction of the semiconductor chip. This leads to the increase in the intensity of the light passing into the concentrator.

[0041] The basic body of the concentrator expediently comprises a transparent glass, a transparent crystal or a transparent plastic. In the latter case, the concentrator is preferably produced in a transfer moulding method and/or an injection moulding method.

[0042] The material of the basic body is resistant to a radiation emitted by the semiconductor chip, in particular to an electromagnet radiation from the blue or UV spectral region. For this purpose, the material has e.g. silicone or comprises the latter.

[0043] In one embodiment, the semiconductor chip is an electromagnetic-radiation-emitting diode, preferably an electromagnetic-radiation-emitting diode with a radiating characteristic which complies at least approximately with Lambert's law, preferably a thin-film light-emitting diode chip.

[0044] A thin-film light-emitting diode chip is distinguished in particular by the following characteristic features:

[0045] a reflective layer is applied or formed at a first main area—facing toward a carrier element—of a radiation-generating epitaxial layer sequence, which reflective layer reflects at least part of the electromagnetic radiation generated in the epitaxial layer sequence back into the latter;

[0046] the epitaxial layer sequence has a thickness in the region of 20 μm or less, in particular in the region of 10 μm; and

[0047] the epitaxial layer sequence contains at least one semiconductor layer with at least one area which has an intermixing structure which ideally leads to an approximately ergodic distribution of the light in the epitaxial layer sequence, i.e. it has an as far as possible ergodically stochastic scattering behaviour.

[0048] A basic principle of a thin-film light-emitting diode chip is described for example in 1. Schnitzer et al., Appl. Phys. Lett. 63 (16), 18 Oct. 1993, 2174-2176, the disclosure content of which is in this respect hereby incorporated by reference.

[0049] A thin-film light-emitting diode chip is to a good approximation a Lambert surface radiator and is therefore particularly well suited to application in a directional light source.

[0050] In one embodiment of the light source, the semiconductor chips are arranged on a respective carrier on which they are surrounded in each case by a frame. The primary optical element is fitted at or in the frame. It is held by the said frame and/or adjusted by the said frame relative to the chip coupling-out area.

[0051] At least parts or a part of the carriers and/or in each case the carrier and the frame are formed in one part with one another.

[0052] In a further embodiment of the light source, the inner area of the frame and/or free areas of that surface of the carrier which faces toward the radiating direction of the semiconductor chip is or are reflective to radiation emitted by the respective semiconductor chip. In addition or as an alternative, the inner area and/or free areas is or are at least partly provided with a layer or layer sequence, preferably with a metallic layer, which is reflective to radiation emitted by the respective semiconductor chip.

[0053] In one embodiment, a plurality of primary optical elements are formed in one part with one another. This may significantly simplify both production of the primary optical elements and mounting thereof in a light source, which may lead for example to lower production costs.