Light Engine

Abstract

The invention describes a Light engine (1,2,3,4,5) comprising a chamber (6) with at least one aperture (7) and a number of LED elements (13) positioned inside this chamber, where effectively all inner surfaces of the chamber (6) are realized as high-reflective surfaces (20) which are essentially non-absorbing towards light within a desired wavelength region

Description

Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. In the drawings, wherein like reference characters denote the same elements throughout:

FIG. 1 shows a first embodiment of a light engine according to the invention;

FIG. 2 shows an enlarged image of a part of the walls of the chamber of the light engine according to FIG. 1;

FIG. 3 shows an enlarged portion of a wall of a chamber of a light engine according to a second embodiment of the invention;

FIG. 4 shows an enlarged portion of a wall of a chamber of a light engine according to a third embodiment of the invention;

FIG. 5 shows an enlarged portion of a wall of a chamber of a light engine according to a fourth embodiment of the invention;

FIG. 6 shows a fifth embodiment of a light engine according to the invention;

FIG. 7 shows a sixth embodiment of a light engine according to the invention;

FIG. 8 shows a seventh embodiment of a light engine according to the invention;

FIG. 9 shows an eight embodiment of a light engine according to the invention;

FIG. 10 shows a simplified schematic representation of the shape of a chamber for a light engine according to a ninth embodiment of the invention.

FIG. 11 is a diagram illustrating the influence of the aperture fraction f on:

• • the fraction T of the internally generated light that is emitted (transmitted) from the light engine;
  • the brightness ratio B denoting the brightness of the emitted light beam from the aperture of the light engine normalised with respect to the brightness of an individual LED element;
  • a quality parameter Q.

FIG. 12 is a diagram showing the dependence of the quality parameter Q on the aperture fraction f for various reflectivities of the inside reflecting wall surfaces and LED surfaces.

FIG. 13 is a diagram showing the dependence of the quality parameter Q on the aperture fraction f for several packing densities θ_LED of the LED elements on the inside walls of the light engine.

FIG. 14 is a diagram showing the dependence of the light concentration factor L on the collimation angle θ_C for various aperture fractions f.

FIG. 15 is a diagram showing the dependence of a light concentration factor L on the collimation angle θ_C for various LED packing densities θ_LED for a particular first aperture fraction f.

FIG. 16 is a diagram showing the dependence of a light concentration factor L on the collimation angle θ_C for various LED packing densities θ_LED for a particular second aperture fraction f.

The dimensions of the objects in the figures have been chosen for the sake of clarity and do not necessarily reflect the actual relative dimensions.

FIGS. 1 and 2 show a particularly preferred embodiment of a light engine according to the present invention, whereby FIG. 1 shows a cross-section through the entire light engine, and FIG. 2 shows an enlarged cross-section through the chamber wall.

The light engine 1 comprises a chamber 6, constructed, for example, in a rectangular or cylindrical manner. An opening or aperture 7 of surface area A_exit is located at the top of the chamber 6 and connects to a collimating element 8. LED elements 13 are positioned on the inside wall 10 of the chamber 6 at a certain distance from each other, i.e. in a particular grid, along the mantle and on the inside surface opposite the aperture 7. These LED elements 13 are connected via outcoupling elements 15 to a transparent covering plate 11.

This transparent covering plate 11 is positioned in the chamber 6 at a certain distance to the inside wall of the chamber 6. All walls 10 of the chamber 6, including the top side with the aperture 7, are covered by the transparent covering plate 11. The gap between the transparent covering plate 11 and the inside surfaces of the walls 10 of the chamber 6 is filled with a diffuse reflective white powder. Suitable candidates for the reflective white powder are Al₂0₃, TiO₂, YBO₃, BaSO₄, Ca-pyrophosphate, Ca-halophosphate, or MgO. Suitable materials for the transparent covering plate 11 include PMMA (polymethyl-methacrylate), PC (polycarbonate), resinous silicone compounds, and glass. This construction ensures that all inside surfaces 20 of the chamber 6, not occupied by a LED die, are highly reflective.

The construction of the walls can be seen in detail in FIG. 2. Here, the individual LED dies 13 are mounted on mounting slugs 14 which, preferably, also feature a reflective top surface around the LED dies. Transparent truncated inverted pyramids or cones serve as outcoupling elements 15 that are optically coupled to the transparent covering plate 11. Furthermore, these outcoupling elements 15 are optically coupled to the LED dies 13 by means of a resin or some other suitable glue-like material. Instead of optically coupling these outcoupling elements 15 with resin or a similar material to the transparent covering plate 11, they can preferably also be directly formed as part of the transparent covering plate 11. The outcoupling elements 15 guide the emitted light towards the interior 9 of the light engine 1. The cross-section of the conical outcoupling elements 15 widens in the direction facing away from the associated LED dies 13. Preferably, the outcoupling elements feature an angle of inclination between 5° and 65° with respect to the vertical, more preferably featuring an angle of inclination between 20° and 50° with respect to the vertical, and most preferably featuring an angle of inclination of about 45° with respect to the vertical.

The distance between the transparent covering plate 11 and the inside surface of the non-transparent outer wall 10 of the chamber 6, i.e. the thickness of the diffuse reflective powder layer 12, is preferably about 2-3 mm. The powder layer 12 provides the highly reflective surfaces 20 of the chamber 6, which enable internal light recycling. A collimating element 8 is arranged on the aperture 7, and is made from, for instance, transparent plastic material, and receives light that is emitted from the aperture 7 of the light engine 1. The shape of the collimating element 8 is chosen such that substantially no light is emitted from the exit surface of the collimating element 8 at an angle greater than the collimation half angle θ_C measured with respect to the propagation direction of the emitted light beam.

In order to improve the light outcoupling from the transparent covering plate 11 into the inside of the chamber 6, and to simplify the coupling of the light from the chamber 6 into the collimating element 8, the interior 9 of the entire chamber 6 is filled with a solid or liquid medium 22 which has a refractive index approaching or, more preferably, matching that of the transparent covering plate 11 and possibly also that of the collimating element 8. Unwanted light-loss inducing reflections at the boundary interfaces between the covering plate 11 and the medium 22, and at the interface between the collimating element 8 and the medium 22 are thereby avoided or at least diminished. In case the medium 22 is a liquid medium, the liquid can also be utilised for front-end LED cooling purposes, for instance by pumping the liquid medium 22 between the chamber 9 and an external cooling device.

FIG. 3 shows a somewhat modified construction of the inside surface of the wall 10 of the chamber 6. Here, the LED dies 13 are mounted directly on the inside surface of the chamber wall 10. An optical contact layer 16 is positioned on each LED die 13. This contact layer 16 may contain scattering particles to promote light outcoupling from the LED die 13. The transparent covering plate 11 features block-shaped outcoupling elements 15′, which protrude from the transparent covering plate 11 towards the LED die 13, acting as an extension or bridge, and providing optical contact with the contact layer 16. The space between the transparent covering plate 11 and the inside surface of the wall 10 is here also filled with a reflective dry white powder 12.

FIG. 4 shows a further possible construction. As in FIG. 3, the LED dies 13 are positioned on the inside wall 10. To facilitate outcoupling of the light emitted by the LED die 13 through the LED die surface, the LED dies 13 are preferably surrounded by a transparent scattering layer 17 that is in optical contact with the LED die surface, thereby promoting light outcoupling from the LED dies 13 into the chamber 6. A highly diffuse-reflective white particle/binder layer 18 covers the surfaces of the inside wall 10 that are located between the individual LEDs 13.

In FIG. 5, a further possible construction can be seen, where LED device bodies 23, each with a LED die element (not shown in the diagram), are mounted on the inside surface of the outer wall 10. The LED die elements themselves are enclosed in LED domes 19, which ensure good outcoupling of the light emitted from the LED dies. A covering plate 21, with suitable openings in a grid pattern through which the LED domes 19 protrude, covers the LED device bodies 23. The surface of the covering plate 21 between the LED domes 19 is covered with a white diffuse-reflective particle/binder coating 18 possessing a sufficient thickness to yield a highly reflective coating layer 18.

A further example construction of a light engine 2 is shown in FIG. 6. The basic difference between it and the example shown in FIG. 1 is that the chamber 6 is constructed differently than that of the light engine 1 in FIG. 1. Here, the chamber 6 features a floor wall 10, upon which the individual LEDs are mounted as in the example shown in FIG. 1. However, the side walls 10′ now extend conically from the floor wall 10 towards the aperture 7. No LEDs are positioned on these side walls 10′. To give the desired highly reflective inside surface 20, a transparent covering plate 11 is, as for the floor wall 10, arranged at a distance of about 2-3 mm from the inside of the side walls 10′, and the space between the covering plate 11 and the side walls 10′, as well as the space between the floor wall 10 and the covering plate 11 in between the LED mounting elements 14, dies 13, and outcoupling elements 15 are filled with a highly reflective white powder 12. Again, a collimating element 8 is arranged at the aperture 7. The advantage of this light engine 2 over the light engine 1 lies in its reduced volume and, in particular, in its reduced height. On the other hand, the number of LED elements relative to the total area of the chamber's inside walls is lower, since the side walls 10′ are not occupied by LED elements.

A further embodiment of a light engine 3 according to the present invention is shown in FIG. 7. The housing 6 of this light engine 3 features the same geometry as the housing of the light engine 2. However, the LED elements are mounted on the base 10 in the same manner shown in FIG. 5, i.e. LED device bodies 23, supporting LED domes 19 in which the LED dies (not shown in the diagram) are enclosed, occupy the base 10. Both the surface of the base wall 10 upon which the LEDs are mounted as well as the side walls and the tops of the LED device bodies 23 are covered with a white diffuse reflective coating 18 leaving only the protruding domes 19 to remain uncoated. A transparent covering plate 11′, with suitable openings in a grid pattern through which the LED domes 19 protrude, covers the LED device bodies 23. The space between this transparent covering plate 11′ and the inside surface of the outer wall 10 is filled with a reflective white dry powder 12. The conical side wall 10′ narrowing to the aperture 7 with the reflective material 12 disposed between the inside surface of the side wall 10′ and a transparent covering plate 11 is constructed in the same manner as for the light engine 2 of FIG. 6.

FIG. 8 shows a further embodiment of a light engine 4 according to the present invention, which, as regards outer housing 6, is constructed in a similar manner as the example described in FIG. 7. Other than in the example of FIG. 7, however, neither a transparent covering plate 11′ nor a reflective white dry powder 12 are used here. Instead, the conical chamber walls 10′ are now also covered on the inside with a white diffuse-reflective particle/binder layer 18 to give a highly reflective surface 20. In addition, a white diffuse-reflective particle/binder layer 18 is present on the inside surfaces of the chamber wall 10, and on the surfaces of the LED device bodies 23 located between the transparent domes 19.

FIG. 9 shows a further embodiment of a light engine 5 according to the present invention, which essentially only differs from the examples in FIG. 1 and FIG. 6 in the outer shape of the chamber 6. The lower part of the chamber 6 is cylindrically or rectangularly shaped, with a base wall 10 and a side wall 10, each occupied by LED elements 13 arranged in a certain grid pattern. The upper part of the chamber 6 narrows conically to the aperture 7, in the same way as the conically formed side wall 10′ of the light engine 2 in FIG. 6. This conical wall 10′ of the upper part of the chamber 6 is not occupied on the inside by LED elements 13, having only a highly reflective surface 20. This highly reflective surface 20 is formed again by a transparent covering plate 11 arranged at a distance from the walls 10, 10′ and a white reflective powder 12 filling the space between the inside surface of the walls 10, 10′ and the covering plate 11.

In all cases shown in the FIGS. 6 to 9, the interior 9 of the chamber 6 is preferably filled with a solid or liquid medium 22 possessing a suitable refractive index, as described in connection with the light engine 1 of FIG. 1.

The different examples show that the chamber 6 can basically have any kind of external geometry. Furthermore, it must be stressed that the aperture 7 does not necessarily have to be a circular opening in a side wall and that it is not necessarily provided with an optical element 8. Any side wall, preferably of relatively small dimensions, can be simply left out of the construction, giving an aperture 7. This is shown by the cylindrical chamber 6 of the simplified schematic in FIG. 10. Basically, such a chamber 6 can have any basic surface geometry, for example an aperture on opposite sides. For example, one can also imagine elongated light engine cubes with both small faces open to the outside world. This depends on the intended function of the light engine, and the spatial constraints under which the light engine will operate.

The exact construction parameters such as chamber geometry, number of LED elements in the chamber, size of aperture etc., depend on constraints such as the maximum size of the light engine, and the desired output parameters. The following therefore describes how the attainable output parameters depend on the construction parameters of the light engine:

Consider the light engine box depicted in FIG. 1 possessing a single aperture or exit port 7 of surface area A_exit, and a total interior surface area A_engine, which includes the exit port surface area A_exit. Suppose that a total number N_LED Of individual LED die elements 13, each possessing a projected) flat top area A_LED, are mounted on the inner surface of the wall 10 of the light engine 1. Each LED element 13 is assumed to possess a reflectivity R_LED and emit a lumen flux φ_LED from its die area A_LED. A white diffuse-reflective wall 20 of reflectivity R_wall is laterally present around the LED elements 13.

The transmitted fraction T of the internally produced light that escapes via the aperture exit port 7 into the outside world follows from the series:

$T=\frac{A_{\mathrm{exit}}}{A_{\mathrm{engine}}}+\left(1-\frac{A_{\mathrm{exit}}}{A_{\mathrm{engine}}}\right)R_{\mathrm{av}}\frac{A_{\mathrm{exit}}}{A_{\mathrm{engine}}}+{\left(1-\frac{A_{\mathrm{exit}}}{A_{\mathrm{engine}}}\right)}^2R_{\mathrm{av}}^2\frac{A_{\mathrm{exit}}}{A_{\mathrm{engine}}}+\dots$

which, with the ‘aperture fraction’

$f=\frac{A_{\mathrm{exit}}}{A_{\mathrm{engine}}}$

is equivalent to

$\begin{array}{cc}T=\frac{f}{1-R_{\mathrm{av}}\left(1-f\right)}& \left(1\right)\end{array}$

Thereby, R_av denotes the averaged internal reflectivity R_av of the non-exit part of the light engine's inner wall surface according to

$\begin{array}{cc}\begin{array}{c}{R}_{\mathrm{av}}=\frac{N_{\mathrm{LED}}A_{\mathrm{LED}}R_{\mathrm{LED}}+\left(A_{\mathrm{engine}}-A_{\mathrm{exit}}-N_{\mathrm{LED}}A_{\mathrm{LED}}\right)R_{\mathrm{wall}}}{A_{\mathrm{engine}}-A_{\mathrm{exit}}}\\ ={\theta }_{\mathrm{LED}}R_{\mathrm{LED}}+\left(1-{\theta }_{\mathrm{LED}}\right)R_{\mathrm{wall}}\end{array}\mathrm{wherein}& \left(2\right)\\ {\theta }_{\mathrm{LED}}=\frac{N_{\mathrm{LED}}A_{\mathrm{LED}}}{A_{\mathrm{engine}}-A_{\mathrm{exit}}}& \left(3\right)\end{array}$

denotes the fraction of the internally reflecting light engine surface area A_engine−A_exit that is covered with LED elements 13.

The above equations do not assume any specific shape of the internal light engine wall. On the other hand, the series expansion in Equation (1) only holds for small aperture fractions f. In the extreme case of a light engine comprising a single flat light-emitting surface, one has a maximum f=0.5 and, by definition, T=1 since no reflecting surfaces are in the way of the emitting light sources. In this case, Equation (1) erroneously predicts a T<1 but the error is still not substantial as long as R_av>0.90, which can readily be accomplished.

For realistic light engines, an upper limit f≈0.3-0.4 should preferably be maintained, but the concept of a light engine according to the invention is obviously more interesting for much smaller values of f. For example a light engine embodied as a square cube that is open on only one of its six sides possesses an aperture fraction f=0.17. Smaller values for the aperture fraction f can be easily obtained by making the cube more rectangular while keeping only one of its two small sides open. In a preferred embodiment of the invention, the aperture fraction f should be ≦0.15, more preferably ≦0.1. For example, a light engine 1 according to FIG. 1 with a diameter of 2 cm, a chamber length of 3 cm and an aperture diameter of 1 cm has an aperture fraction f=0.03.

In case R_LED=R_wall=R_av=1, no light losses are present and one obtains T=1 according to Equation (1) for any arbitrarily small aperture fraction f. This would theoretically allow the creation of extremely high brightness levels when f→0. In reality, however, this is impossible since optical light losses can never be fully avoided.

It is therefore also of interest to derive an equation for the obtainable brightness at the aperture exit port 7 of the light engine 1 as a function of the system parameters. The brightness ratio B, denoting the brightness B_exit at the aperture exit port 7 (assuming that no collimating element 8 is present) normalised with respect to the brightness level B_LED of an individual LED die element 13, follows from

$\begin{array}{cc}\begin{array}{c}B=\frac{B_{\mathrm{exit}}}{B_{\mathrm{LED}}}=\frac{\frac{{\mathrm{TN}}_{\mathrm{LED}}{\varphi }_{\mathrm{LED}}}{A_{\mathrm{exit}}}}{\frac{{\varphi }_{\mathrm{LED}}}{A_{\mathrm{LED}}}}\\ =\frac{{\mathrm{TN}}_{\mathrm{LED}}A_{\mathrm{LED}}}{A_{\mathrm{exit}}}=T\frac{N_{\mathrm{LED}}A_{\mathrm{LED}}}{A_{\mathrm{engine}}-A_{\mathrm{exit}}}\frac{A_{\mathrm{engine}}-A_{\mathrm{exit}}}{A_{\mathrm{exit}}}\\ =T{\theta }_{\mathrm{LED}}\left(\frac{1}{f}-1\right)=\frac{f}{1-R_{\mathrm{av}}\left(1-f\right)}{\theta }_{\mathrm{LED}}\frac{1-f}{f}\\ =\frac{1-f}{1-R_{\mathrm{av}}\left(1-f\right)}{\theta }_{\mathrm{LED}}\\ =\frac{\left(1-f\right){\theta }_{\mathrm{LED}}}{1-\left(1-f\right)\left[{\theta }_{\mathrm{LED}}R_{\mathrm{LED}}+\left(1-{\theta }_{\mathrm{LED}}\right)R_{\mathrm{wall}}\right]}\end{array}& \left(3\right)\end{array}$

which is valid when both the LED dies 13 and the aperture exit port 7 emit non-collimated light (i.e. Lambertian light with θ_c=90°).

In addition, it is instructive to derive a relation for a second brightness ratio L(θ_c), also called ‘brightness concentration factor’ in the following, with

$\begin{array}{cc}L\left({\theta }_c\right)=\frac{B_{\mathrm{exit}}\left({\theta }_c\right)}{B_{\mathrm{screen}}\left({\theta }_c\right)}& \left(4\right)\end{array}$

which denotes the ratio of the brightness B_exit(θ_c) of the light-emitting exit surface (which may be the projected light-emitting exit surface A_col of the collimating element 8) to the screen-averaged brightness B_screen(θ_c) of an imaginary flat screen of surface area A_screen=A_engine−A_exit whereupon LED elements 13 are mounted at a packing density θ_LED. Here, light is assumed to be emitted as a collimated beam that is angularly bounded within a collimation half-angle θ_c with respect to the propagation direction of the beam. For non-collimated light, one has θ_c=90°.

Knowledge of the brightness concentration factor L(θ_c) indicates whether or not a net light concentration has been achieved by packing N_LED dies together inside a light engine 1 at a surface packing density θ_LED in comparison with the simpler situation wherein the N_LED dies are simply mounted on a flat light emitting screen at the same surface packing density. A value L(θ_c)>1 indicates a relative light (brightness) concentration and a value L(θ_c)<1 indicates a relative light (brightness) dilution. Evidently, a value of L(θ_c) as large as is practically possible, and certainly higher than 1, is generally desirable.

In case the light engine 1 is made to emit 2D-collimated light, as shown in FIG. 1, the relevant exit port surface area becomes that of the projected output surface A_col of the collimating element 8 mounted on the exit port 7 of the light engine 1. Following the etendue law, for a given collimation half-angle θ_c, the minimum required output surface area A_col of the collimating element 8 relates to the output area A_exit of the aperture 7 of the light engine 1 in FIG. 1 according to

$\begin{array}{cc}{A}_{\mathrm{col}}=\frac{A_{\mathrm{exit}}}{{\sin }^2{\theta }_c}& \left(5\right)\end{array}$

and thus indicates an inevitable enlargement of the emitting surface A_col at decreasing θ_c.

The screen-averaged brightness level B_screen(θ_c) relates to B_LED(θ_c) according to

B _screen(θ_c)=θ_LEDB_LED(θ_c)   (6)

In the embodiment of the light engine 1 according to FIG. 1, the individual LED elements are provided with a collimating element in the form of pyramidal outcoupling elements 15. Therefore, the apparent light emitting surface area of an individual LED also increases but these can be directly accommodated on the mounting screen (the imaginary flat screen of surface area A_screen defined above for derivation of equation (4)) without enlarging the screen as long as the LED packing density constraint

θ_LED≦sin²(θ_c)   (7)

on the flat mounting screen is satisfied. The screen surface area A_screen can then be taken to be independent of θ_c.

From the above, and bearing in mind that B_exit in Equation (4) denotes the brightness B_col at the light output surface of the collimating element 8 mounted on the aperture opening 7 of the light engine 1 as soon as θ_c<90°, it follows that L(θ_c) can be obtained from:

$\begin{array}{cc}\begin{array}{c}L\left({\theta }_c\right)=\frac{B_{\mathrm{col}}}{B_{\mathrm{screen}}}=\frac{\frac{{\mathrm{TN}}_{\mathrm{LED}}{\varphi }_{\mathrm{LED}}}{A_{\mathrm{col}}}}{{\theta }_{\mathrm{LED}}B_{\mathrm{LED}}}\\ =\frac{{\sin }^2{\theta }_c}{{\theta }_{\mathrm{LED}}}\left(\frac{\frac{{\mathrm{TN}}_{\mathrm{LED}}{\varphi }_{\mathrm{LED}}}{A_{\mathrm{exit}}}}{\frac{{\varphi }_{\mathrm{LED}}}{A_{\mathrm{LED}}}}\right)\\ =\frac{{\sin }^2{\theta }_c}{{\theta }_{\mathrm{LED}}}B=\frac{\left(1-f\right){\sin }^2\left({\theta }_c\right)}{1-\left(1-f\right)\left[{\theta }_{\mathrm{LED}}R_{\mathrm{LED}}+\left(1-{\theta }_{\mathrm{LED}}\right)R_{\mathrm{wall}}\right]}\end{array}& \left(8\right)\end{array}$

For the special case θ_c=90° (Lambertian light) FIG. 11 shows calculated values for the transmission T and the brightness ratio B as a function of the aperture fraction f at a packing density θ_LED=0.05 and at reflectivities R_wall=0.98 and R_LED=0.50 which corresponds to realistic conditions.

It is clear from FIG. 11 that, at decreasing aperture fraction f, the attainable brightness ratio B at the exit port increases, albeit at a significantly decreasing lumen output, which is proportional to the transmitted fraction T of the internally produced light that leaves the light engine. Because lumen flux and brightness exhibit opposite trends in their relationship with the aperture fraction f, it makes sense to define a quality parameter Q according to

$\begin{array}{cc}Q=\mathrm{BT}=\frac{{\theta }_{\mathrm{LED}}\left(1-f\right)f}{{\left[1-\left[{\theta }_{\mathrm{LED}}R_{\mathrm{LED}}+\left(1-{\theta }_{\mathrm{LED}}\right)R_{\mathrm{wall}}\right]\left(1-f\right)\right]}^2}& \left(9\right)\end{array}$

In FIG. 11 the quality parameter Q is also plotted as a function of the aperture fraction f. From this plot it becomes apparent that Q goes through a maximum at an optimum aperture fraction f_opt. Values for f_opt follow from

$\begin{array}{cc}{f}_{\mathrm{opt}}=\frac{1-R_{\mathrm{av}}}{2-R_{\mathrm{av}}}& \left(10\right)\end{array}$

with R_av given by Equation (2).

However, Q does not strongly depend on the aperture fraction f near f=f_opt. In case a high T is more important than a high B (for example when a general lighting application is concerned), one is advised to choose a value f>f_opt. The reverse is true when a high B is more important than a high T.

FIG. 12 shows calculations of the quality parameter Q factor as a function of the aperture fraction f for various values of R_LED and R_wall at a packing density θ_LED=0.05 and for θ_c=90°. (I: R_wall=0.98 and R_LED=0.7; II: R_wall=0.98 and R_LED=0.5; III: R_wall=0.96 and R_LED=0.5). For aperture fractions around f_opt, the quality parameter Q drops noticeably, i.e. the curve flattens out, if the reflectivities R_LED and R_wall decrease.

Furthermore, FIG. 13 shows calculations of the quality parameter Q as a function of the aperture fraction f for various values θ_LED at constant R_wall=0.98 and R_LED=0.5 and for θ_c=90°. As can be seen, the quality parameter Q increases over the entire range of f with increasing packing density θ_LED.

FIGS. 14, 15 and 16 show calculations for the light concentration factor L(θ_c) at realistic reflectivities R_wall=0.98 and R_LED=0.5 for various values of the aperture fraction f (in FIG. 14 with a constant packing density θ_LED=0.05) and for various values θ_LED (in FIG. 15 with a constant aperture fraction f=0.05 and in FIG. 16 with a constant aperture fraction f=0.1). In all figures the lines are drawn subject to the constraint according to Equation (7).

It is evident that, at least for θ_c=60° (general lighting applications), use of a light engine according to the invention allows a significant brightness concentration to be accomplished with a numerical value up to a factor 5 at 80% lumen output (i.e. T=0.8). Also, higher light concentration factors L are achievable by reducing the aperture fraction f but at the cost of a reduced lumen efficiency.

Taking for granted that R_wall and R_LED are always chosen as high as practically possible, it is primarily the packing density θ_LED of the LED elements on the inner wall that affects the performance as a function of the aperture fraction f. A compromise will always have to be sought between brightness on the one hand and lumen efficiency on the other hand. Also the total required lumen output must be considered, whereby the size of the light engine is directly proportional to the total lumen output.

In case a high lumen efficiency is most important, it is advisable to choose the aperture fraction f≈0.10−0.12 at a low θ_LED≈0.01. This allows for T≈0.8 and B≈0.07 which, at θ_c=90°, is still seven times brighter than the screen-averaged brightness of the mounting wall. The brightness concentration factor L(θ_c) decreases at decreasing θ_c but remains substantial down θ_c=40°.

In case a high brightness is most important, it is advisable to choose a higher LED packing density θ_LED≈0.05, or even more if practicable. To raise the maximum attainable LED packing density, cooling of the LED elements should be provided, for example, by means of a matching refractive index cooling liquid as proposed above. At f≈0.1, one has a smaller T=0.65 but a higher brightness ratio B=0.3, which, at θ_c≈=90°, is still six times brighter than the screen-averaged brightness B=θ_LED of the mounting wall. The brightness can be further increased by decreasing the aperture fraction f down to, for example, f=0.05. At f≈0.1, the quality parameter Q significantly improves at increasing θ_LED. To obtain most benefit from the light engine according to the invention, it is therefore of great interest to increase θ_LED up to levels at and beyond θ_LED=0.10

Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. For the sake of clarity, it is also to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.

Claims

1. Light engine (1, 2, 3, 4, 5) comprising a chamber (6) with at least one aperture (7) and a number of LED elements (13) positioned inside this chamber, where effectively all inner surfaces of the chamber (6) are realized as high-reflective surfaces (20) which are essentially non-absorbing towards light within a desired wavelength region.

2. A light engine according to claim 1, comprising outcoupling means (15, 15′, 16, 17, 19) for enhancing outcoupling of the light emitted by the LED elements (13) into the chamber (6).

3. A light engine according to claim 1, where the high-reflective surfaces (20) are realized by a diffuse-reflective material (12, 18) distributed over the inside surface of the chamber walls (10, 10′).

4. A light engine according to claim 3, where the diffuse-reflective material (12) is enclosed between a transparent covering plate (11, 11′) and the inside surface of the chamber walls (10, 10′).

5. A light engine according to claim 3, where the diffuse-reflective material (12) comprises a reflective dry powder (12).

6. A light engine according to claim 4, where the transparent covering plate (11) covers the LED elements (13), and the outcoupling means (15, 15′) comprise a number of transparent outcoupling elements (15, 15′) each of which extends from a light emitting surface of an associated LED element (13) to the transparent covering plate (11).

7. A light engine according to claim 6, where the transparent outcoupling elements (15) have a cross section which is wider at an interface between the outcoupling element (15) and the transparent covering plate (11) than at an interface between the outcoupling element (15) and the associated LED element (13).

8. A light engine according to claim 1, where the outcoupling means (19) comprise transparent domes (19) each of which is optically connected to a light emitting surface of an associated LED element.

9. A light engine according to claim 8, where the domes (19) protrude through holes (22) in a covering plate (11′, 21) covering the chamber wall (10) on which the LED elements are mounted.

10. A light engine according to claim 3, where the reflective material (12) distributed over the inside surface of the chamber walls (10, 10′) comprises a light converting substance.

11. A light engine according to claim 1 where LED elements of different wavelength characteristics are positioned inside the chamber.

12. A light engine according to claim 1 comprising a light collimating element (8) positioned at the aperture (7) of the chamber (6).

13. A light engine according to claim 1, where the chamber (6) is filled with a material (22) which has a refractive index that approaches or, preferably, matches the refractive index of the transparent covering plate (11, 11′) and/or of the outcoupling means (15, 15′, 19) and/or of the collimating element (8) and/or of the LED elements (13).

14. A light engine according to claim 13, where the material (22) is a liquid material that is also used for the front-end cooling of the LED elements (13).

15. A light engine according to claim 1, where an aperture fraction (f) defined by the ratio of a surface area (Aexit) of the aperture (7) to a total interior surface area (Aengine) of the light engine ((1, 2, 3, 4, 5), which includes the surface area (Aexit) of the aperture (7), is preferably ≦0.15, more preferably ≦0.1.

16. Automotive light system enclosing a light engine (1, 2, 3, 4, 5) according to claim 1.