Light source and method of providing a bundle of light

A light source, includes a component (6) for emitting light, arranged to emit a major part of light generated within the component through a face (8) of the component (6), and at least one structure (7;18) for coupling out light, having a base (11;19) in contact with the face (8) of the component (6) and arranged to emit light through an aperture (12;20) opposite the base (11;19). An interface between the component (6) and an environment surrounding the base (7;18) is arranged to keep at least part of the light generated in the component confined to at least a layer of the component (6) adjoining the base (11;19). The base (11; 19) is in contact with the face (8) over an area (A2) smaller than the area (A0) of the face (8).

The invention relates to a light source, includinga component for emitting light, arranged to emit a major part of light generated within the component through a face of the component, andat least one structure for coupling out light, having a base in contact with the face of the component and arranged to emit light through an aperture opposite the base, wherein an interface between the component and an environment surrounding the base is arranged to keep at least part of the light generated in the component confined to at least a layer of the component adjoining the base.

The invention also relates to a method of providing a bundle of light, includingactivating a component for emitting light, arranged to emit a major part of light generated within the component through a face of the component, such that at least part of the light generated in the component is reflected at at least part of the face of the component.

The invention also relates to an optical apparatus comprising a light source and an optical system for accepting light from the light source.

It is known to cover the die of a Light-Emitting Diode (LED) with a silicone encapsulant. On top of that, a plastic cover is located. Apart from protecting the die and encapsulant, it also serves the purpose of a beam-shaping element. Depending on the application, the beam can be shaped such that the LED emits mainly in a forward direction, i.e. away from the face of the component for emitting of light.

A problem of the known configuration is that, when it is incorporated in a lighting apparatus with a further optical system for gathering the light from the light source, the amount of useful light remains limited. This problem is due to the limited étendue of the system gathering the light, and comes to the fore when the light source is incorporated in relatively compact lighting apparatus.

It is an object of the invention to provide a Light source, method of providing a bundle of light and optical apparatus for providing a relatively large throughput of light through an optical system capturing light from the light source.

This object is achieved by the light source according to the invention, which is characterized in that the base is in contact with the face over an area smaller than the area of the face.

The light source forms a source of light with relatively large brightness, defined as the luminous intensity per unit of surface area (A) of the light-emitting surface emitted into a solid angle (Ω). Because the base of the structure for coupling out light is in contact with the face of the component for emitting light over an area substantially smaller than the area of the face, the light-emitting area is reduced, tending to increase the brightness. Because the interface between the component and an environment surrounding the base is arranged to keep at least part of the light generated in the component confined to at least a layer of the component adjoining the base, light that is not directly coupled out will be reflected at the interface and then coupled out. For this reason, the intensity of light does not decrease proportionally to the decrease in surface area due to the fact that the structure for coupling out light is only in contact with the component over a fraction of the latter's surface area.

It is observed that WO 2005/001331 discloses a luminous body comprising a housing. The light is issued at a light emission surface which closes off the housing at its upper side. At the lower side of the housing, a plurality of light sources has been inserted. The light sources are composed of LED elements with electrical contacts and respective lens bodies provided thereon, from which the light is issued in lateral directions. The surfaces of the lens bodies facing (opposed to) the light emission surface of the housing are each coated with a reflecting layer which reflects back those light component which are inevitably also radiated in axial direction of the LED elements. The spatial radiation characteristics of the luminous body are essentially determined by the shape and gradient of the light emission surface and usually exhibit a Lambert-type character. WO 2005/001331 does not disclose a light source including a structure for coupling out light having a base in contact with the face of a component for emitting light and arranged to emit light from a surface opposite the base. The reflective layer of the lens bodies precludes the formation of a bundle of light. The known luminous body serves to distribute the light originating from more or less point-shaped light sources as evenly as possible over a light-emitting surface.

In a preferred embodiment, at least a layer of the component adjoining the base has an index of refraction larger than one.

Thus, for a planar component for emitting light, total internal reflection at the interface to the environment of the component for emitting light will tend to keep the light confirmed to the component. Application of a reflective coating around the base is not essential to keeping at least part of the light generated in the component confined to at least a layer of the component adjoining the base in this embodiment, making manufacturing easier.

An embodiment of the light source according to the invention includes a plurality of the structures for coupling out light, each having a base in contact with the face over an area substantially smaller than the area of the face.

This embodiment has the advantage of being able to provide a relatively large amount of light. The plurality of light structures each provide light at a relatively large brightness. Each has a sufficiently low étendue, the product of the solid angle in which light is emitted and the light-emitting area, that a relatively large part of the emitted light is captured by optical components in series with the light source.

An embodiment of the light source includes at least one arrangement for collimating at least the light passing through one of the structures for coupling out light.

The effect is to decrease the solid angle into which light is emitted further, whilst essentially preserving the étendue, which ensures that a large part of the emitted light is captured by any of a large number of different optical components that may be placed in series with the light source.

In a variant, at least one of the structures for coupling out light includes a collimating system integral thereto.

For small-scale light sources, this makes assembly less complicated, in particular alignment.

In a variant, at least one lenslet is provided over the respective apertures opposite the bases of the structure for coupling out light.

This allows the use of relatively simple to manufacture structures for coupling out light.

In an embodiment, the aperture opposite the base has a larger area than the base.

This ensures that relatively little light is lost in a structure for coupling out light. Light emitted through the area of contact over a relatively large solid angle is coupled out. This will be apparent by considering the fact that the structure for coupling out light is an étendue-preserving structure.

In an embodiment, the component for emitting light includes a layered device includingone or more active layers for converting electrical current into light,at least one further layer, transparent in a range of frequencies of light generated in the component and having a smaller absorption coefficient in that range than the active layers.

This has the effect that light is easily “spread” out in lateral directions, allowing a relatively large fraction of the generated light to reach the or an area of contact with the or a structure for coupling out light, instead of being absorbed in the device for emitting light. This is especially advantageous where the component is a Light-Emitting Diode (LED), because the active materials used in solid-state LEDs generally have a relatively high coefficient of absorption.

In a variant, the component for emitting light includes a layered device includingone or more active layers for converting electrical current into light andat least one layer between the face of the component and the active layers, comprising a luminescent material.

The effect is to increase the range of colors of light which the light source can be configured to produce. In particular where the component for emitting light comprises an LED, the active layers can be configured to emit blue light, for example, which the luminescent material converts into green or red light. The converted light is emitted with a relatively high brightness, making the light source suitable for use in displaying images.

In an embodiment, the component for emitting light includes a layered device, wherein at least one of the structures for coupling out light is integral with a layer of the component adjoining its base.

This obviates the need for intricate assembly of the structures and the component, making the light source suitable for miniaturization.

In an embodiment, a reflecting surface is provided opposite the face of the component for emitting light.

This increases the yield of the light source.

In an embodiment, the component for emitting light is formed by a light-emitting diode die.

This embodiment combines the favorable aspects of light-emitting diodes without the limited brightness of the light-emitting diode on its own.

According to another aspect, the method of providing a bundle of light according to the invention is characterized by coupling out light through at least one structure having a base in contact with the face over an area smaller than the area of the face.

An embodiment includes activating the component in an environment having an index of refraction substantially lower than the component for emitting light.

Thus, it is ensured that total internal reflection will occur at the interface of the component for emitting light to its environment. Due to this, a relatively large part of the light generated in the component will be emitted through the structure for coupling out light.

According to another aspect, the light source in the optical apparatus according to the invention is formed by a light source according to the invention.

Preferably, the optical system has an acceptance larger than the étendue of the light source.

In this context, acceptance means the product of the solid angle over which incoming light is captured and the light-capturing area. Because it is larger than the étendue of the light source, the maximum amount of light is captured.

An optical assembly1is shown inFIG. 1, and comprises a light source2and reflector arrangement3. The light source2is not shown in exact detail, as several possible embodiments will be discussed below. The reflector arrangement3captures the light emitted when the light source is energized. The reflector arrangement has a solid opening angle Ω1and light-capturing surface A1. Thus, the acceptance of the reflector arrangement3equals Ω1·A1. Instead of the reflector arrangement3, a lens or combination of lenses could have been used. The light source2has an étendue smaller than the acceptance Ω1·A1, to ensure that substantially all of the light emitted by it passes through the optical assembly1. The optical assembly1is located in an environment consisting essentially of air. In alternative embodiments, the environment may be formed by another gas or gas mixture. The optical assembly1, or at least the light source2, may also be placed in a liquid or solid medium, provided the medium has a lower index of refraction than the material of the light source2.

Applications of the optical assembly include projectors, also known as beamers. The light source2, as will be explained, emits light of a relatively high brightness, meaning it is well-suited to use in compact devices, since the opening angle and/or light-capturing area of the optical system in series with the light source2can be relatively small. Another application of the light source2is matrix illumination. In essence, this is a flat pixelated lamp that projects pixels onto a surface to be illuminated. Such a lamp can be used to convey a mood or display non-alphanumerical information. It can also be used to enhance the viewing experience when watching television by using it as ambient lighting that extends the image outside the boundaries of the television screen. Other applications include LED-based backlights for liquid crystal displays and LED-based headlights for vehicles. These advantageously make use of the relatively long lifetime of LEDs and low operating voltage, as well as their relatively low costs. The drawback of limited brightness is removed by incorporating the LED as a component for emitting light in the light source2that is described herein.

The general build-up of a first light source4is shown inFIG. 2. The first light source4comprises a heat sink5and an LED die6, as an example of a component for emitting light. A first structure7for coupling out light generated within the component for emitting light is attached to the component for emitting light. In the illustrated embodiment, only one first structure7for coupling out light is attached to an upper surface8of the LED die6. In alternative embodiments, several of the first structures7may be arranged in an array on the upper surface8. The first structure7increases the brightness of light emitted by the component for emitting light.

The component for emitting light can be based on one of a number of different mechanisms for generating light when powered by electrical current. In the example ofFIG. 2, as in all examples illustrated herein, it comprises the LED die6, which comprises a number of active layers, indicated generally by reference numeral9. In one embodiment, the layers9comprise InxGa1-xN, with x varying from layer to layer. The index of refraction of the active layers has a value of around 2.5 in that case. The active layers9have a total thickness in the order of several μm, for instance 5 μm. The active layers9are covered by a substrate10made of sapphire, which has an index of refraction of about 1.8. The substrate has a thickness one or two orders of magnitude higher than that of the stack of active layers, for instance 100 μm. An LED has a number of attractive properties, which include the long lifetime, low operating voltage, small form factor, almost pure spectral colors, fast modulation of lumen output, and the fact that it turns on instantaneously when energized. The first structure7for coupling out light increases the inherently low brightness of the LED die6.

The first structure7for coupling out light comprises a transparent medium having a relatively high index of refraction. It approximates that of the substrate10and/or active layers9more closely than the value of the index of refraction of the environment of the light source, i.e. air. Suitable materials include glass, quartz, sapphire and transparent ceramics. InFIG. 2, the first structure7for coupling out light has the shape of a collimator, namely a concentric-parabolic-concentrator. Thus, it comprises an integral system for collimating light. A base11of the first structure7for coupling out light is in contact with the upper surface8, and has an area A2. Light enters the first structure7for coupling out light through the base11and exits through an opposite light-emitting surface12with an area A3. The area A3is substantially larger than the area A2of the base. This area A2is a fraction of the total area A0of the upper surface8, in order to increase the brightness.

The brightness of emitted light is increased because the light flux does not decrease proportionally to the area A2of the upper surface8through which light is coupled out. The light flux decreases less than might be expected, due to at least partial reflection at side surfaces13and a lower surface14. In the case of the side surfaces13, this is due to the fact that the environment of the LED die6has a lower index of refraction than the substrate10and active layers9, so that total internal reflection acts to keep a substantial part of the light confined to the substrate10. In the case of the lower surface14, this is enhanced by the presence of a surface layer of reflective material (not shown in detail). The part of the upper surface8surrounding the base11may also be coated with a surface layer of reflective material. Alternatively, only part of the light generated in the active layers9may be reflected at the interface to the air surrounding the base11, namely that part incident on the surface at angles in a sub-range of the total geometrically possible range. This is the range of angles to the normal exceeding the critical angle. There is little emission through the side surfaces13, as the active layers9tend to absorb most of the generated light propagating in that direction. Moreover, the LED die5is planar, so that the area of the side surfaces13is small compared to that of the upper surface8.

The brightness B of the light source is given by the following equation:

B=Φπ·A,(1)
wherein Φ is the luminous flux (in lumen). This equation is based on a Lambertian pattern of emission.

To estimate the effect of coupling out light through an area A2smaller than a total area A0of the upper surface8, the brightness was modeled analytically using the following equation:

In a practical example, the area A0of the upper surface8is 1 mm2and the area A2of the base11is 0.2 mm2. Using equation (2), one can estimate that the brightness B=2.3 B0for Rb=0.7. The luminous flux Φ equals 0.45·Φ0.

This result has been confirmed using ray-tracing calculations in an optical simulation package (ASAP from Breault Research). The LED die6was modeled as two boxes with a reflective layer over the lower surface14. The active layers9were modeled as a box of 1×1×0.01 mm3, having a refractive index n=2.5. The substrate was modeled as a box of 1×1×0.1 mm3, having a refractive index n=1.8. The reflective layer is assumed to be an aluminum electrode having a reflectivity ρ=0.8. Light generation in the active layers9was modeled by adding two Lambertian emitting planes in back-to-back configuration.FIG. 3shows the results of the simulation. A first curve15shows the angular distribution of the normalized intensity. A second curve16shows the integrated intensity. From the figure, it follows that 50% of the light resides within an angular cone of 2×13°=26°. For this simulation, the entrance diameter of the first structure7for coupling out light was set at 0.5 mm. The diameter of the light-emitting surface12was set at 3 mm. The simulation confirmed the results obtained by using equation (2), in that they gave Φ=0.46·Φ0and B=2.2·B0.

To confirm the simulations, measurements were made on some actual light sources. The results of these measurements are illustrated inFIG. 4. For each of four quantities A-D, the left-hand bar represents the measured flux (in lumen), whereas the adjacent bar represents the flux obtained from the simulation. A is the flux output from a LED die with a conventional collimator that completely encapsulates the LED die6, i.e. both the upper surface8and side surfaces13. B represents the flux output of the LED die6without any encapsulation. C represents the flux output through the light emitting surface of the first structure7for coupling out light. D is the flux output lost through the side surfaces13. This loss is due to the fact that total internal reflection does not occur over the full range of angles. In fact, the light impinging on the side surfaces13tends to be incident at an acute angle, since the component for emitting light is planar.FIG. 4confirms the results obtained through simulation and shows that these results are not inherently biased.

A second light source17, operating in the same way as the first light source ofFIG. 2, is shown inFIG. 5. The component for emitting light is the same. Corresponding parts have been numbered in the same way as inFIG. 2. The second light source17includes a plurality of second structures18a-18efor coupling out light through the upper surface8of the LED die6. The second structures18a-18ehave the shape of an inverted cone. Respective bases19a-19eof the second structures18a-18eare in contact with the upper surface8over areas A2substantially smaller than the total area A0of the upper surface8. Each has an exit aperture20a-20eopposite its base19a-19ewith a larger area than the base19a-19eopposite it. The index of refraction of the second structures is matched to that of the substrate10, whereas the upper surface8has an interface to air over the area surrounding the bases19a-19e. A medium other than air may fill the interstices between the second structures18a-18efor coupling out light.

An array21of lenslets22a-22fis arranged over the exit apertures20a-20e. The lenslets22a-22fand second structures18a-18efor coupling out light form an integral component in the illustrated embodiment. In other variants, the array21may be bonded to the second structures18a-18e. In another variant, the array of second structures18a-18efor coupling out light is integral with the substrate10.

The first structure7for coupling out light may, for instance, be comprised in an array of similar structures in a configuration similar to that of the second structures18a-18e. The active layers9may include an organic light-emitting material. In a variant, transparent, ceramic layer of luminescent material is provided in the place of the substrate10. Such a component for emitting light is known as a P-converted LED. Thus, light-generation in the topmost layer of the component for emitting light is not precluded.