Light emitting module, a lamp, a luminaire and a display device
View Patent ↗A light emitting module includes at least one LED mounted on a reflective base and surrounded by reflective walls to form a cavity. A partially diffusive layer containing a phosphor is mounted above the LED with a gap therebetween. Some of the light emitted by the phosphor is backscattered toward the LED, base, and walls. The LED absorbs a significant amount of the light, while the reflective base and walls efficiently reflect over 90% of the light back toward the phosphor layer for exiting the module. Various equations are provided that allow the designer to select an optimal gap between the LED and the partially diffusive layer to maximize light extraction efficiency out of the diffusive layer. The optimal gap range for maximizing light extraction efficiency is a product of LED largest linear length, LED surface area, base area, wall area, and reflective coefficients of the various elements.
1. A light emitting module comprising:
a light exit window;
a base comprising a light reflective surface facing the light exit window, the light reflective surface having a base reflection coefficient (Rbase) being defined by a ratio between the amount of light that is reflected by the light reflective surface of the base and the amount of light that impinges on the light reflective surface of the base;
at least one solid state light emitter disposed on the base arranged for emitting light of a first color, the at least one solid state light emitter having a top surface and having a solid state light emitter reflection coefficient (R_SSL) being defined by a ratio between the amount of light that is reflected by the top surface and the amount of light that impinges on the top surface, wherein the top surface of the at least one solid state light emitter faces towards the light exit window,
a wall interposed between the base and the light exit window, the base, the wall and the light exit window enclosing a cavity, the wall comprising a reflective material forming a light reflective wall surface facing towards the cavity, the light reflective wall surface having a wall reflection coefficient (Rwall) being defined by a ratio between the amount of light that is reflected by the light reflective wall surface and the amount of light that impinges on the light reflective wall surface,
wherein an effective reflective coefficient (Reff) is defined as a weighted average of the base reflection coefficient (Rbase) and the wall reflection coefficient (Rwall),
wherein a solid state light emitter area ratio (ρ SSL ) is defined as the ratio between the area of the top of the at least one solid state light emitter and the sum of the area of reflective surface of the base and the area of the reflective wall surface, and wherein a largest linear size (d SSL ) of the top surface of the at least one solid state light emitter is defined as the longest distance from a point on the top surface of the at least one solid state light emitter to another point on the top surface of the at least one solid state light emitter along a straight line, and
a partially diffusive reflective layer, comprising a luminescent material for converting at least a part of the light of the first color into light of a second color, the light exit window comprising at least a part of the partially diffusive reflective layer,
wherein a gap with a distance h is present between the top surface of the at least one solid state light emitter and the partially diffusive reflective layer for which 0.3·d SSL ≦h≦0.75·d SSL for 0<ρ SSL <0.1, 0.15·d SSL ≦h≦0.3·d SSL for 0.1≦ρ SSL ≦0.25, and 0.1·d SSL ≦h≦0.2·d SSL for ρ SSL >0.25, and
wherein the value of the effective reflection coefficient (Reff) is larger than 70% and larger than the solid state light emitter reflection coefficient (R_SSL) and wherein the wall reflection coefficient (Rwall) is smaller than 95%.
2. A light emitting module according to claim 1 , comprising a plurality of solid state light emitters, wherein each one of the solid state light emitters is configured for emitting light in a specific color and each one of the solid state light emitters having a top surface, and wherein the solid state light emitter reflection coefficient (R_SSL) is defined as the average value of the reflection coefficients of the plurality of solid state light emitters.
3. A light emitting module according to claim 1 wherein the value of the effective reflection coefficient (Reff) is larger than the solid state light emitter reflection coefficient (R_SSL) plus a factor c times the difference between 1 and the solid state light emitter reflection coefficient (R_SSL), wherein 0.2≦c ≦1 for 0<ρ SSL <0.1, 0.3≦c≦1 for 0.1≦ρ SSL ≦0.25, and 0.4≦c≦1 for ρ SSL >0.25.
4. A light emitting module according to claim 1 , wherein at least a part of the reflective surface of the base is closer to the partially diffusive reflective layer than the top surface of the at least one solid state light emitter,
and wherein the distance h between the top surface and the partially diffusive reflective layer is 0.4* d SSL +Δh/2≦h≦5* d SSL +Δh/2for 0<ρ SSL <0.1, 0.15* d SSL +Δh/2≦h≦3* d SSL +Δh/2for 0.1≦ρ SSL ≦0.25, and 0.1* d SSL +Δh/2≦h≦2* d SSL +Δh/2 for ρ SSL >0.25, and wherein Δh is the absolute value of the difference between the distance (h) between the top surface of the at least one solid state light emitter and the partially diffusive reflective layer and the shortest distance between the reflective base surface and the partially diffusive reflective layer.
5. A light emitting module according to claim 1 , wherein the light reflective wall surface is tilted with respect to a normal axis of the base wherein the tilt increases the reflection of light towards the light exit window.
6. A light emitting module according to claim 1 , wherein the partially diffusive reflective layer forms the light exit window, the partially diffusive reflective layer having an edge, and the edge of the partially diffusive reflective layer being in contact with the base.
7. A light emitting module according to claim 1 , comprising a substantially transparent material arranged between the at least one solid state light emitter and the partially diffusive reflective layer, the transparent material being optically coupled to the at least one solid state light emitter.
8. A light emitting module according to claim 7 wherein the substantially transparent material is further optically and thermally coupled to the partially diffusive reflective layer.
9. A light emitting module according to claim 7 wherein the substantially transparent material is sintered translucent polycrystalline alumina with a grain size that is larger than 44 μm or smaller than 1 μm.
10. A light emitting module according to claim 1 , wherein the light exit window further comprises a diffuser layer for obtaining a diffuse light emission, for obtaining a spatially, color and color over-angle uniform light emission, and for obtaining a color mixed light emission.
11. A light emitting module according to claim 1 , wherein a diffuser layer for obtaining a diffuse light emission, for obtaining a spatially, color and color over-angle uniform light emission, and for obtaining a color mixed light emission is provided at a distance from a side of the partially diffusive reflective layer facing away from the at least one solid state light emitter.
12. A lamp comprising a light emitting module according to claim 1 .
13. A luminaire comprising a light emitting module according to claim 1 or comprising a lamp.
14. A display device comprising a light emitting module according to claim 1 .
15. A light emitting module according to claim 1 wherein the light reflective wall surface is curved for increasing the reflection of light towards the light exit window.
16. A light emitting module according to claim 1 , wherein the light exit window further comprises a dichroic layer for correcting color over angle variations or light uniformity.
17. A light emitting module according to claim 1 , wherein the light exit window further comprises an optical element for providing a desired light beam shape.