WO2009150580A1

WO2009150580A1 - Light emitting device

Info

Publication number: WO2009150580A1
Application number: PCT/IB2009/052361
Authority: WO
Inventors: Denis Fournier; Elvira J.M. Paulussen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-06-13
Filing date: 2009-06-04
Publication date: 2009-12-17

Abstract

The invention relates to a light emitting device comprising a light source assembly (1) emitting at least two distinct wavelengths or ranges of wavelengths, and a reflector assembly (2) comprising at least two reflective surfaces (S1, S2, S3) facing the light source assembly (1). Each reflective surface is capable to selectively reflect back some of said wavelengths or ranges of wavelengths and to transmit the non reflected wavelengths. The light source assembly (1) is located at one side of the light emitting device and the reflector assembly (2) at another side.

Description

LIGHT EMITTING DEVICE

TECHNICAL FIELD The present invention relates to the field of light emitting devices and, more particularly, of light emitting devices capable of producing a collimated and narrow beam of well-mixed colors. Said light emitting devices can be solid state lighting sources such as Light Emitting Diodes, hereinafter designated as LEDs.

BACKGROUND OF THE INVENTION

The development of LEDs has permitted huge improvements in lighting technologies. In comparison with older technologies, such as incandescent or halogen bulbs, light emitting devices based on LEDs are significantly:

- less energy-consuming - more shock-resistant;

- more efficient;

- endowed with a higher lifetime;

- capable of producing more saturated colors;

- not damaged by switching on/off. One well-known embodiment of the LEDs technology in light emitting devices comprises at least two preferably three kinds of LEDs having different colors, typically red, blue and green. Such polychromatic LEDs assembly can be both used to produce white light, with higher luminous power and precise control of the color temperature, or it can be used to produce any other color, fixed or changing with a real-time control. Moreover, some other LEDs with corresponding colors can be added in these assemblies in order to enlarge the colorimetric space of the resulting light.

These polychromatic LEDs devices can be employed in various applications, such as proximity lighting, grazing lighting, lighting with fiber devices, small light-up

(illumination), spot lighting, shop lighting and more generally accent lighting, i.e. decorative lighting, highlighting of elements of interior design such as pictures, plants, of buildings or of landscaping.

A relevant assessment criterion of the quality of the color mixing is the stability and the homogeneity of the global color of the beam.

In some applications where a narrow, sharp and collimated beam is required, (e.g. proximity lighting, grazing lighting and accent lighting), it is preferable that the color mixing remains good on long distances from the light sources.

In this context, the document EP-B-0734077 discloses a light emitting device having a stacked optical structures successively aligned along a light path, each optical structure comprising one LED and one dichroic parabolic mirror arranged for back- reflecting the color of the LED along the light path while transmitting the other colors to the next optical structure.

Such a light emitting device emits parallel light rays with a good color mixing on long distances.

SUMMARY OF THE INVENTION

Our invention improves the situation, by proposing a light emitting device capable of producing a beam of mixed wavelengths, comprising

- a light source assembly emitting at least two distinct wavelengths or ranges of wavelengths, and

- a reflector assembly comprising at least two reflective surfaces facing the light source assembly, each reflective surface being capable to selectively reflect back some of said wavelengths or ranges of wavelengths and to transmit the non reflected wavelengths, wherein the light source assembly is located at one side of the device and the reflector assembly at another side.

Preferably, the reflector assembly is arranged and positioned such that the wavelengths or ranges of wavelengths back-reflected by the corresponding reflective surfaces are mixed along a light path.

By proposing a light source assembly on one side and a reflecting assembly on an opposite side, the light emitting device proposes a solution which facilitates its use and maintenance by a user. For example, since the light sources are placed at one side, it can be easier to replace them or to connect them easily to a control unit.

Furthermore this light emitting device offers more flexibility in the design conception, since the light source assembly is physically separated from the reflector assembly.

A first example of a new design that might be provided: the light emitting sources are placed on a single cooling system (e.g. heat sink). This new arrangement decreases significantly the manufacturing costs and reduces the volume of the light emitting device.

A second example of a new design: two reflecting surfaces might be provided on each side of a single substrate (e.g. a lens). This device needs then one component for two mirrors instead of two components for two mirrors as it is known from prior art. This exemplary second new design might decrease the manufacturing costs and/or reduce the whole volume of the light emitting device, while keeping a very good mixing of the reflected light rays preferably collimated on a long distance, especially if the reflecting surfaces are arranged for back reflecting some parallel rays. Moreover this particular new design enhance the efficiency or the cost reduction of the light emitting device with regard to prior art: indeed, the known light emitting devices comprise one component provided with solely one mirror on a surface (typically the front surface) and with either no coating or coated with an anti-reflective material on its opposite surface. In the first case (no coating), there is a loss due to parasite reflection and absorption at the opposite surface. In the second case (anti-reflective coating), the coating costs money.

These first and second examples of designs might be applied solely or in combination in the light emitting device according to the invention.

Other kinds of new designs can be proposed in the following detailed description.

Other optional features of the invention, taken sole or in combination, are as follows:

- the beam produced by the light emitting device according to the invention, is collimated.

- the light source assembly of the light emitting device according to the invention comprises at least two LEDs (Light Emitting Devices); these LEDs are preferably located on a single cooling system;

- the reflective surfaces are disposed successively along the light path - the light emitting device according to the invention is such that: o the light source assembly comprises at least two light sources (designated as Lk where k = 1 :n with n > 2) emitting respective wavelengths λk or ranges of wavelengths Δλk; o the reflective surfaces (designated as Sk' where k' = l :m with m > 2) of the reflector assembly are disposed successively (from k'=l to k'=m) along the light path, each reflective surface Sk' reflecting at least one wavelength λk or one range of wavelength

Δλk and transmitting wavelengths λ(k+l :n) or range of wavelengths Δλ(k+l :n) corresponding to the wavelengths back-reflected by reflective surfaces S(k'+1 :m);

- the light assembly comprises a number of light sources equal to the number of reflective surfaces; - two of said reflective surfaces are provided on each side of a single substrate, such as a lens; this substrate may be made of a dielectric material;

- the light source assembly comprises Lambertian light sources emitting towards the half-space of the reflector assembly;

- the light source assembly comprises a plurality of light sources aligned on a line perpendicular to the light path;

- the light source assembly comprises light sources aligned on a line not perpendicular to the light path;

- the light source assembly comprises three light sources emitting respectively red, green and blue; - the light source assembly comprises at least two parallel lines of light sources and at least two parallel reflective surfaces extending in directions parallel to the lines of light sources; - the reflective surfaces Sk' are surfaces whose shapes are calculated by means of an algorithm consisting in: — > considering that the reflective surfaces Sk' are paraboloids and that the first reflective surface S(k'=l) is preferably a paraboloid of revolution; — > adapting the equation of the first reflective surface S(k'=l) so as to minimize the spot size obtained by the convergence, onto a focal plane of a paraxial convergent lens through which goes a back reflected beam of rays: o emitted by the first emitting source L(k=l), o reflected by the first reflective surface S(k'=l); — > adapting the equation of the reflective surface S(k'=2 to m-1) so as to minimize the spot size obtained by the convergence, onto the focal plane of the paraxial convergent lens 9 through which goes a back reflected beam of rays emitted by L(k=2 to n-1): o emitted by the emitting source L(k=2 to n-1), o refracted by the reflective surface(s) S(k'=l to [k'-l]), o reflected by the reflective surface S(k'=2 to m-1);

— > adapting the equation of the reflective surface S (k'= m) so as to minimize the spot size obtained by the convergence, onto the focal plane of the paraxial convergent lens 9 through which goes a back reflected beam of rays emitted by L(k= n): o emitted by the emitting source L (k= n), o refracted by the reflective surfaces S(k'= 2 to [m-1]), o reflected by the reflective surface S(k'= m); the integers n and m preferably corresponding to 3.

In a particular embodiment of the instant invention, wherein n = m = 3, the equations of the shapes of optical surfaces S(k' = 1), S(k' = 2) and S(k' = 3) are calculated by means of an algorithm consisting in: → considering that the surfaces corresponding to reflective surfaces S(k' = 1), S(k' = 2) and S(k' = 3) are paraboloids and that S(k'=l) is preferably a paraboloid of revolution; — > adapting the equation of the first reflective surface Sl so as to minimize the spot size obtained by the convergence, onto the focal plane of a paraxial convergent lens through which goes a back reflected beam of rays: o emitted by the first emitting source Ll , o reflected by the first reflective surface S 1 ;

— > adapting the equation of the second reflective surface so as to minimize the spot size obtained by the convergence, onto the focal plane of the paraxial convergent lens through which goes a back reflected beam of rays: o emitted by the second emitting source S2, o refracted by the first reflective surface S 1 , o reflected by the second reflective surface S2;

— > adapting the equation of the third reflective surface so as to minimize the spot size obtained by the convergence, onto the focal plane of a paraxial, convergent lens through which goes a back reflected beam of rays: o emitted by the third emitting source S3, o refracted by the first and the second reflective surfaces S2, o reflected by the third reflective surface S3.

- the starting points Pk of the light emitted from the light sources Lk, are substantially cop lanar.

- L(k = 1), L(k = 2) and L(k = 3) emit respective wavelengths λk = 1, λk = 2 and λk = 3 corresponding respectively to the colors red, green and blue.

In another aspect, the invention relates to an algorithm for calculating the equations of the shapes of the optical reflective surfaces S(k'), this algorithm consisting in:

— > considering that the reflective surfaces Sk' are paraboloids and that the first reflective surface S(k'=l) is preferably a paraboloid of revolution;

— > adapting the equation of the first reflective surface S(k'=l) so as to minimize the spot size obtained by the convergence, onto a focal plane of a paraxial convergent lens 9 through which goes a back reflected beam of rays: o emitted by the first emitting source L(k=l), o reflected by the first reflective surface S(k'=l);

— > adapting the equation of the reflective surface S(k'=2 to m-1) so as to minimize the spot size obtained by the convergence, onto the focal plane of the paraxial convergent lens 9 through which goes a back reflected beam of rays emitted by L(k=2 to n-1): o emitted by the emitting source L(k=2 to n-1), o refracted by the reflective surface(s) S(k'=l to [k'-l]), o reflected by the reflective surface S(k'=2 to m-1);

- said algorithm may be an algorithm for calculating the equations of the shapes of the surfaces of the optical surfaces S(k' = 1), S(k' = 2) and S(k' = 3), consists in: → considering that the surfaces corresponding to reflective surfaces S(k' = 1), S(k' = 2) and S(k' = 3) are paraboloids; — > adapting the equation of the first reflective surface so as to minimize the spot size obtained by the convergence, onto the focal plane of a paraxial lens, of rays: o emitted by the first emitting source , o reflected by the first reflective surface;

— > adapting the equation of the second reflective surface so as to minimize the spot size obtained by the convergence, onto the focal plane of a paraxial lens, of rays: o emitted by the second emitting source , o refracted by the first reflective surface, o reflected by the second reflective surface;

— > adapting the equation of the third reflective surface so as to minimize the spot size obtained by the convergence, onto the focal plane of a paraxial lens, of rays: o emitted by the third emitting source , o refracted by the first and the second reflective surfaces, o reflected by the third reflective surface.

In another of its aspects, the invention concerns a method for machining the optical surfaces S(k' = 1), S(k' = 2) and S(k' = 3) from the optical material, said surfaces being calculated with the algorithm defined above.

These and other aspects, features and advantages of the invention will become apparent to those skilled in the art upon reading the disclosure provided here in connection with the attached drawings. The detailed description, while indicating preferred embodiments of the invention, is only given by way of illustration. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more details by way of example with reference to the accompanying drawings, in which:

- Figure 1 is a schematic view of a light emitting device according to one embodiment of the invention. - Figure 2 is a detailed view of part II of Figure 1.

- Figure 3 is a schematic view of the light emitting device of Figure 1, showing the initial step of the algorithm used to determine the equations of the three reflective surfaces.

- Figure 4 is a schematic view of the light emitting device of Figure 1, showing the first step of the algorithm used to determine the equation of the first reflective surface. - Figure 5 is a schematic view of the light emitting device of Figure 1, showing the second step of the algorithm used to determine the equation of the second reflective surface.

- Figure 6 is a schematic view of the light emitting device of Figure 1, showing the third step of the algorithm used to determine the equation of the third reflective surface.

- Figure 7 is a schematic view of the light emitting device of Figure 1, showing the mixing of the rays emitted by the light emitting source along the light path.

DETAILED DESCRIPTION OF THE INVENTION It must be noted that as used in this specification and the claims, the singular forms

"a", "an", and "the" include plural referents unless the context clearly dictates otherwise.

Unless defined otherwise, all technical and scientific terms herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

The foregoing description of preferred embodiments of the invention is not intended to be exhaustive or to limit the invention to the disclosed embodiments. Various changes within the scope of the invention will become apparent to those skilled in the art and may be acquired from practice of the invention.

In the various drawings, the same reference numerals designate identical or similar elements. Figure 1 shows a light emitting device which emits narrow and sharp light beams with and a good color mixing over a long distance from the light source, which means no significant variation of chromaticity over the beam. Such light emitting device is used for instance in proximity, grazing or accent lighting.

This light emitting device of Figure 1 comprises a light source assembly 1 at one side of the light emitting device and a reflector assembly 2 facing the light source assembly

1, at another side, preferably the opposite side, of the light emitting device. The light source assembly 1 emits lights beams which are then reflected back by the reflector assembly 2 such that the light emitting device outputs a light around an optical axis designated as XX. Said light source assembly 1 and said reflector assembly 2 are mounted in a housing 4, e.g. a parallelepipedal case, containing light transmissible material 5, air for instance.

Said housing 4 comprises two opposite longitudinal walls 4L, 7S and two opposite lateral walls 4' & 4".

Said housing 4 can be made of metal or of any suitable polymer, for example a thermoplastic polymer or a thermosetting plastic polymer such as polycarbonate or ABS.

In this preferred embodiment, the number of light sources Ll, L2 & L3 (k = 1 to 3) of the light source assembly is equal to the number of reflective surfaces Si, S₂ and S3 (k' =

1 to 3) of the reflector assembly. The light source assembly 1 includes three light sources Ll, L2 & L3, three LEDs in this preferred embodiment, set up on a single cooling system or heatsink 3 which is assembled on the inner side of the longitudinal wall 4L of the housing 4. Each LED Ll, L2 or L3 emits at least one (in this case one) wavelength λl, λ2 or λ3 (or ranges of wavelengths Δλl, Δλ2 or Δλ3). For instance, Ll may emit green light, L2 may emit blue light and L3 emit red light.

LEDs Ll, L2 & L3 may emit light preferably towards the reflector assembly 2, e.g to the corresponding half-space, meaning they are Lambertian light sources.

In this preferred embodiment, the separate light sources Ll, L2 & L3 of the light source assembly 1, are advantageously disposed side by side and as close as possible from each other, for offering a compact structure which avoids any blocking of the light coming from the reflective surfaces Sl, S2 and S3. The inventors noticed that the obstacle formed by such a light source assembly 1 is not visible from a human eye and thus does not interfere in the output lighting.

In this preferred embodiment, each of the LEDs Ll, L2, & L3 exhibits e.g. a starting point Pl, P2 or P3 of the light (of the luminous rays), as shown in Figure 2. Said starting points Pl, P2 and P3 of the light are substantially coplanar and substantially as close as possible from each other, preferably on the same axis. This latter axis might be orthogonal to the optical axis XX. "Substantially" means for instance that the margin of error is comprised between +/- 10mm, preferably 5 mm.

In addition, Pl (then Ll also) is advantageously on the optical axis XX.

In an embodiment, the light source assembly 1 is coaxial with the optical axis XX.

In another embodiment, the light source assembly 1 could have an optical axis different from XX and the reflective surfaces Sl, S2 and S3 are arranged for back- reflecting the light around a XX axis tilted to the optical axis of the light source assembly 1.

In another embodiment, the light source assembly 1 could have an optical axis different from XX and an additional optical means can be used to deviate the incident light from the light assembly 1 to the XX-axis of the reflector assembly 2.

Advantageously, the light sources Lk (Ll, L2, L3), preferably coplanar, can be either in a linear or triangular arrangement. If k were equal to 4, the square arrangement might be chosen, in order to satisfy to the compactness requirement.

The reflector assembly 2 comprises three reflective surfaces, Si, S₂ and S3 facing and opposite to the LEDs Ll, L2, & L3 in the housing 4.

The reflective surfaces Si and S₂ are the two dioptres, respectively inner and outer, of a lens 6 which is placed or fixed in the housing 4 by its peripheral edge mounted on the lateral walls (4', 4") and/or on the transversal walls (not shown on the drawings) of the housing 4. This lens 6 is for instance made of a mineral glass (for example crown borosilicate glass -BK7-, SFl 1, etc.), or of an organic glass (e.g. polycarbonate, polyamide, PMMA, PE, Plexiglas etc.), or of any other dielectric material (for instance saphire, ZnO, ITO, Quartz, CaF₂, MgF₂, etc). Sl and/or S2 can be obtained by coating the lens 6 with an appropriate multilayer filter having alternated layers of low refractive indicia and high refractive indicia, made for example of alternating layers of TiO2 and SiO2.

The lens 6 is generally perpendicular to the optical axis XX and this latter corresponds roughly to the central geometrical axis of the lens 6.

The reflective surface S3 is defined by the inner face of the longitudinal wall 7S of the housing 4. As for Sl and S2, the optical axis S3 is XX. S3 can be obtained by means of an appropriate surface treatment, such as pure Aluminium.

The free spaces comprised, on the one hand, between the LEDs Ll, L2, & L3 and the lens 6 (Sl), and on the other hand, between the lens 6 (S2) and the inner face (S3) of the longitudinal wall 7S, is a light transmissible material 5, e.g. air. It could be emphasized that in this preferred embodiment, Sl is a parabolic of revolution focus is Pl such that it reflects back parallel rays around XX-axis.

The reflective surfaces Sl, S2 & S3 are arranged and positioned such that the wavelengths or ranges of wavelengths back-reflected by the corresponding reflective surfaces are mixed along a light path. Furthermore, each reflective surface Sl, S2 or S3 is designed so as to selectively reflects the different colors emitted by Ll, L2 or L3, depending on their wavelengths.

The selective properties of reflexion of Sl, S2 & S3 can be obtain by adapting shapes of the surfaces and/or covering said surface with a pass-band film having a maximum transmission around the wavelength or range of wavelengths emitted by the light source. In this preferred embodiment, it means that:

1. Sl reflects the wavelength or the range of wavelengths emitted by Ll (e.g. color blue) - see figure 4 - and transmits the wavelength or the ranges of wavelengths emitted by L2 and L3 (e.g. colors: red for L2 - see figure 5 - and green for L3 - see figure 6) emitted by L2 & L3; 2. S2 reflects the wavelength or the range of wavelengths emitted by L2 (e.g. color red) - see figure 5 - and transmits the wavelengths or the ranges of wavelengths emitted by L3 (e.g. color green for L3 - see figure 6); 3. S3 reflects the wavelengths or the ranges of wavelengths emitted by L3.

In the route from the light source assembly 1 to the reflector assembly 2, the wavelengths transmitted by Sl and S2, are refracted respectively by the lens 6 and by the air between S2 and S3, and vice versa in the route back of the reflected rays.

Figure 7 shows the path of all the rays emitted together by Ll, L2 and L3. It shows also how the mixing of the wavelengths of the back-reflected beam, i.e. how the rays from the light source assembly 1 are distributed over the whole reflecting surfaces Sl, S2, S3 and how the rays forming the resultant back-reflected beam are preferably parallel to each other, and thus well mixed and collimated.

The shapes of the three reflective surfaces Sl, S2 and S3 are calculated by the means of an algorithm, which is described as follows, so as to output rays around a single axis XX, and preferably with parallel rays.

The calculation can be made with a well-known optical calculation software, such as Speos® or Soltis Odyssey® commercialized by Optis, Zemax® commercialized by Zemax Development Corporation or Code V® or LightTools® commercialized by Optical Research Associates.

The modelling system preferably used in the invention, is represented in figure 3. It comprises the light sources Li, L₂ and L3, the reflective surfaces Si, S₂ and S3, a converging (paraxial) lens 8 and a screen 9 placed outside the light emitting device, beyond the outlet of the light emitting device. The lens 8 has a primary focal point, Fl, and a secondary focal point, F2, Fl being located at the light emitting device side. The screen 9 coincides with the secondary focal point F2.

A three-dimensional orthogonal system (X,Y,Z) is defined with X collinear to XX.

The three optical surfaces Si, S₂ and S3 are considered as diopters. Depending on the materials used for the lens, the diopters can be for example air/glass or air/polycarbonate.

The model used for the algorithm considers that the light emitted by the light sources is composed of multiple rays, with defined wavelength or range of wavelengths.

In general cases, the algorithm is the following: In the modelling, the assumptions were as follows:

• Light source assembly: 3 starting points Pl, P2, P3 of the light Ll (blue), L2 (red) and L3 (green).

• The distance between Pl and P3 is 2 mm, P1,P2,P3 being equidistant.

• The distance between Pl and the apex of the parabolic Sl is 10 mm. • The lens 6 is designed, according to a 1^st approximation, from a sphere having a diameter of 32 mm.

• Sl, S2 and S3 are ideal dioptres and mirrors, i.e.: no reflection/transmission loss per color.

• The distances between the centers of Sl & S2, and of S2 & S3 = 2 mm. • The substance between Sl and S2 (i.e. lens 6) is BK7 and the substance 5 between S2 and S3 and between the light source assembly 1 and Sl is air.

• The material of the housing 4 is of polycarbonate or ABS and the surface treatment used to manufacture the S3 mirror is of pure aluminium. • Three configurations are successively implemented in three steps: Config I = Ll (blue), Config 2 = L2 (red) and Config 3 = L3 (green).

• Each source Ll L2 L3 emits a conical beam of 120° from its respective starting point Pl P2 P3. • The focal point without convergent lens 8 of the three parallel and collimated in the three configurations is the infinity.

In this modelling, the approach is for example to output parallel back-reflected rays per color (using paraxial) and to get the three colors on top of each other by coincidence of spots in focal point F2 of lens 8 (Steps 1-3: see figures 4-6). In a 4^th step, the degree of collimation of the rays is assessed through the coincidence of the spots issued from Ll L2

L3, by means for example of a merit function.

A gaussian quadrature-algorithm which uses three rings and six arms is e.g. implemented. It determines the following number of rays traced in this model:

Config 1 (rotational symmetrical): amount of rays = amount of rings = 3 - Config 2: amount of rays = arms * rings = 3 * 6 = 18

Config 3: amount of rays = arms * rings = 3 * 6 = 18 - Altogether 3 + 18 + 18 = 39 rays are traced.

The first step of the algorithm is represented in figure 4 (Config 1). It determines the equation of the first reflective surface S 1.

The initial equation of the first reflective surface Sl is a polynomial: n n

X(Y,Z) = ∑∑A ' Y' ' Z^J , with n being the order of the polynomial, for example ι=0 j=0 n = 6.

The accuracy of the calculation increases with the order of the polynomial. In the first step, only the rays emitted by the first light source Li are considered.

After being emitted, these rays are reflected backward by the first reflective surface S₁. After leaving the light emitting device, the rays passes through the lens which focuses the rays onto the screen.

The first step considers several luminous rays emitted by the first light source L₁, typically 3.

By successive iterations, the coefficients \A \ can be optimized in order to obtain the global minimum of the merit function.

The second step of the calculation algorithm is represented in figure 5 (config 2). It determines the equation of the second reflective surface S2. The initial equation of the second reflective surface is a polynomial: n n

X(j,Z) ⁼ ∑ ^' ∑B_IJ ' Y' Z^J with n being the order of the polynomial, for example z=0 j=0 n = 6.

In the second step, only the rays emitted by the second light source L₂ are considered. After being emitted, these rays are refracted by the first reflective surface S₁, reflected backward by the second reflective surface S₂ and refracted again by the first reflective surface S₁. After leaving the light emitting device, the rays passes through the lens 8 which focuses the rays onto the screen 9.

The second step considers several luminous rays emitted by the second emitting source E₂, typically 18. By successive iterations, the coefficients μ?_;7 | _{^ 0 n} can be optimized in order to obtain the global minimum of the merit function.

The third step of the calculation algorithm is represented figure 6. It determines the equation of the third reflecting surface. The equations of the first and the second reflective surface are the one determined in the first and second step. The initial equation of the third reflecting surface is a polynomial: n n

X(Y,Z) = ∑∑C - Y' ^■ Z^J with n being the order of the polynomial for example z=0 j=0 n = 6.

The accuracy of the calculation increases with the order of the polynomial. In the third step, only the rays emitted by the third emitting source E₂ are considered. After being emitted, these rays are refracted by the first reflective surface S₁, refracted by the second reflective surface S₂, reflected backward by the third reflective surface S3, refracted by the second reflective surface S₂ and finally refracted by the first reflective surface S₁. After leaving the light emitting device, the rays passes through the lens which focuses the rays onto the screen. The third step considers several luminous rays emitted by the second emitting source E₂, typically 18.

By successive iterations, the coefficients can be optimized in order to obtain the global minimum of the merit function.

For example, the following coefficient of the above mentioned equations of the steps 1, 2 & 3 have been found:

The fourth step of assessment of the degree of collimation of the rays.

If the rays emitted by Li L2 & L3 were perfectly collimated, the intersection of the focused rays and the screen would be a single point. As a consequence, a merit function is calculated in order to evaluate the degree of collimation of the rays. For example, the merit function can take into consideration the diameter "d" of the spot from Ll L2 or L3 which has the smallest diameter and which enclose every point obtained by the intersection of the luminous rays and the screen 9.

The method of manufacturing the optical surfaces S(k' = 1), S(k' = 2) and S(k' = 3) from the optical material differs according to the material used and the required degree of precision.

In a first step, a mould or a set of moulds might be realized.

To this end, the output result of the calculation software, that is the equations of the optical surfaces S(k' = 1), S(k' = 2) and S(k' = 3), is entered into a CAD (Computer- Aided Design) software, for example Catia®.

The process for calculating the surfaces may include the following operations:

- Define a geometrical entity depending on the global geometry of the light emitting device.

- Assemble at least one of the above mentioned surfaces into this device. Then, the solid geometry is converted into mould geometry, depending on the material used, the manufacturing process and the solid geometry.

The output mould geometry is then entered into a CAM (Computer-Aided Manufacturing) independent software or module. For example, Catia® can be used with such a CAM module. The CAM software converts the mould geometry into a CNC (Computer Numerical Control) program, this program being able to command the CNC.

Then, the mould can be machined from raw metal with the above mentioned program. The manufacturing process of the surfaces differ according to the material used. As an example, considering a thermoplastic material such as Polycarbonate, the manufacturing process of the optical surfaces S(k' = 1), S(k' = 2) and S(k' = 3) from the optical material can be as described as hereinafter:

Polycarbonate enters the production process in the form of granules. Firstly, the granules are dried with warm air then loaded into position on the press. Then, the molds are positioned and the machine is programmed.

The plastic granules are fused and injected into the press. After that, the material is solidified by conduction through the mold. Finally, the press and the mold support-block are opened. Then, the optical surfaces Sl and S2 are coated with a band-pass reflective fϊlteraround the wavelength or range of wavelengths emitted by the corresponding light sources. For example, this filter can be made from a multilayer structure consisting in a succession of high refractive and low refractive layers (e.g. successive TiO2/SiO2 layers) adapted to filtering out most of the wavelengths outside the wavelength or the range of wavelengths emitted by the light source associate respectively to these surfaces. This kind of multilayer structure can be made from any know technique, such as for example one technique of Chemical Vapor Deposition (CVD).

Other manufacturing methods can be also employed, such as: machining, eventually high speed machining, of a preform made of dielectric material, preferably plastic or glass, and possibly resurfacing, depending on required precision; spin casting of a dielectric material, preferably plastic or glass, and possibly resurfacing, depending on required precision.

This invention can be applied in the realization of spots (Spot RGB indoor and LEDBeamer RGB projects at BU Luminaires) using multi-dye LED packages (OsiP and Maverick projects at BU SSL) or individual LEDs on a printed circuit board. These spots are used in outdoor and retail/shop lighting segments. It is to be understood that the invention is not limited to a light emitting device having a LED assembly 1 of three monochromatic LEDs but can also have two LEDs or more LEDs emitting different colors or the same colors. The corresponding reflecting surfaces might be calculated accordingly by a person skilled in the art, by using for example known modelling techniques, such as those aforementioned. Moreover, the calculation of the reflective surfaces Sl, S2, S3 may also be performed so as to output non parallel rays around XX-axis, and thus the light emitting device outputting a conical beam.

Moreover, the number of reflecting surfaces might be different from the number of light sources, one reflective surface can be calculated for back-reflecting the light emitted by two or more LEDs so as to have a final acceptable color mixing.

In a particular embodiment, Ll L2 and L3 can represent respective parallel lines of LEDs which extend perpendicularly to XX-axis, and the reflective surfaces Sl, S2, S3 are calculated not for being of revolution but for being extended along respective lines parallel to Ll, L2, L3 lines: the reflected light is thus spread on longer surface.

Claims

1. A light emitting device, comprising:

- a light source assembly (1) emitting at least two distinct wavelengths or ranges of wavelengths, and

- a reflector assembly (2) comprising at least two reflective surfaces (Sl, S2, S3) facing the light source assembly (1), each reflective surface being capable of selectively back-reflecting some of said wavelengths or ranges of wavelengths and to transmit the non reflected wavelengths, wherein the light source assembly (1) is located at one side of the light emitting device and the reflector assembly (2) at another side.

2. The light emitting device according to claim 1, wherein the reflector assembly (2) is arranged and positioned such that the wavelengths or ranges of wavelengths back-reflected by the corresponding reflective surfaces (Sl, S2, S3) are mixed along a light path (XX).

3. The light emitting device according to claim 1, wherein the beam is collimated.

4. The light emitting device according to claim 1, wherein the light source assembly (1) comprises at least two LEDs (Light Emitting Devices).

5. The light emitting device according to claim 4, wherein the LEDs are located on a single cooling system (3).

6. The light emitting device according to claim 2, wherein the reflective surfaces are disposed successively along the light path (XX).

7. The light emitting device according to claim 1, wherein: o the light source assembly (1) comprises at least two light sources (designated as Lk where k = l :n with n > 2) emitting respective wavelengths λk or ranges of wavelengths Δλk; o the reflective surfaces (designated as Sk' where k' = l :m with m > 2) of the reflector assembly (2) are disposed successively (from k'=l to k'=m) along the light path, each reflective surface Sk' reflecting at least one wavelength λk or one range of wavelength Δλk and transmitting wavelengths λ(k+l :n) or range of wavelengths Δλ(k+l :n) corresponding to the wavelengths back-reflected by reflective surfaces S(k'+1 :m).

8. The light emitting device according to claim 1, wherein the light assembly (1) comprises a number of light sources equal to the number of reflective surfaces.

9. The light emitting device according to claim 1, wherein two of said reflective surfaces are provided on each side of a single substrate, such as for example a lens (6).

10. The light emitting device according to claim 9, wherein the substrate is made of a dielectric material.

11. The light emitting device according to claim 1, wherein the light source assembly comprises Lambertian light sources emitting towards the half-space of the reflector assembly (2).

12. The light emitting device according to claim 2, wherein the light source assembly (1) comprises a plurality of light sources aligned on a line perpendicular to the light path

(XX).

13. The light emitting device according to claim 2, wherein the light source assembly (1) comprises light sources aligned on a line not perpendicular to the light path (XX).

14. The light emitting device according to claim 1, wherein the light source assembly (1) comprises three light sources emitting respectively red, green and blue.

15. The light emitting device according to claim 1, wherein the light source assembly (1) comprises at least two parallel lines of light sources and at least two parallel reflective surfaces extending in directions parallel to the lines of light sources.