POLARIZED, LED-BASED ILLUMINATION SOURCE

• US 20090040465A1
• Filed: 10/16/2008
• Published: 02/12/2009
• Est. Priority Date: 05/19/2005
• Status: Active Grant

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Oehler, Peter R., Conner, Arlie R., Donnelly, Sean M.
• Granted Patent: US 7,854,514 B2
• Time in Patent Office: Days
• US Class Current: 353/20
• CPC Class Codes: G02B 27/283 used for beam splitting or combining
• CPC Class Codes: G03B 21/16 Cooling; Preventing overheating
• CPC Class Codes: G03B 21/2033 LED or laser light sources
• CPC Class Codes: G03B 21/204 using secondary light emission, e.g. luminescence or fluorescence using different colours G03B33/00; if related to video signals H04N9/3197
• CPC Class Codes: G03B 21/2073 Polarisers in the lamp house peculiar to electronic spatial light modulators H04N9/3197
• CPC Class Codes: H04N 9/3141 Constructional details thereof details not peculiar to the presence of an electronic spatial light modulator G03B21/14

Abstract

An illumination source includes a number of light emitting diodes (LEDs) operating at a first wavelength. Light from the LEDs illuminates a phosphor material that generates light at a second wavelength. A reflective polarizer transmits light at the second wavelength in a first polarization state and reflects light at the second wavelength in a second polarization state orthogonal to the first polarization state. The light at the second wavelength reflected by the reflective polarizer is directed back towards the phosphor material without an increase in angular range. In some embodiments the LEDs, having a conformal layer of phosphor material, are attached directly to the first surface of a liquid cooled plate. A liquid coolant contacts a second surface of the plate.

Claims

1. An illumination source, comprising:

an array of one or more light emitting diodes (LEDs) capable of emitting light at a first wavelength;

a first light collecting/focusing unit to form at least some of the light at the first wavelength into a telecentric beam;

a phosphor capable of generating light at a second wavelength when illuminated by light at the first wavelength, the telecentric beam being directed to the phosphor; and

a reflective polarizer disposed to transmit light at the second wavelength, received from the phosphor, in a first polarization state and to reflect light at the second wavelength in a second polarization state back to the phosphor.

2. A source as recited in claim 1, further comprising a reflective filter positioned on an optical path of the light at the first wavelength, between the one or more LEDs and the phosphor, the reflective filter being reflective for light at the second wavelength and transmissive for light at the first wavelength.

3. A source as recited in claim 1, wherein the reflective polarizer comprises a polarizing beamsplitter.

4. A source as recited in claim 3, wherein the polarizing beamsplitter is disposed on an optical path of light at the first wavelength propagating between the one or more LEDs and the phosphor, the polarizing beamsplitter further comprising a reflector that transmits light at the second wavelength and reflects light at the first wavelength.

5. A source as recited in claim 1, wherein the first wavelength is a blue wavelength and the second wavelength is a green wavelength.

6. A source as recited in claim 1, wherein the phosphor material comprises SrGa₂S₄.

7. A source as recited in claim 1, wherein the light collecting/focusing unit comprises a tapered light pipe.

8. A source as recited in claim 7, wherein the tapered light pipe comprises rectilinear reflecting sides.

9. A source as recited in claim 7, wherein the tapered light pipe comprises curved reflecting sides.

10. A source as recited in claim 7, wherein the light collecting/focusing unit further comprises a focusing element disposed proximate an output end of the tapered light pipe.

11. A source as recited in claim 7, wherein the tapered light pipe comprises a transparent body with internally reflecting sidewalls.

12. A source as recited in claim 1, further comprising a second light collecting/focusing unit disposed on an optical path for the light at the second wavelength between the phosphor and the reflective polarizer.

13. A source as recited in claim 12, wherein the light at the first wavelength passes through the second light collecting/focusing unit to the phosphor.

14. A source as recited in claim 12, further comprising a dichroic separator disposed on an optical path for light at the first wavelength between the first and second light collecting/focusing units, the dichroic separator being reflective at the first wavelength and transmissive at the second wavelength or transmissive at the first wavelength and reflective at the second wavelength.

15. A source as recited in claim 1, wherein the reflective polarizer comprises a multilayer optical film polarizing beamsplitter.

16. A source as recited in claim 1, wherein the reflective polarizer comprises a polarizing layer disposed for substantially normal incidence of the light at the second wavelength.

17. A source as recited in claim 1, wherein the one or more LEDS are spaced apart from an input to the first light collecting/focusing unit, and further comprising a reflector at least partially surrounding the input so as to reflect light from the one or more LEDS towards the input that would otherwise not pass into the input.

18. A source as recited in claim 1, further comprising a polarization control element disposed between the phosphor and the reflective polarizer.

19. A source as recited in claim 1, further comprising a power supply coupled to provide power to the one or more LEDs.

20. A source as recited in claim 1, further comprising an image-forming device, light at the second wavelength being directed to the image-forming device.

21. A source as recited in claim 20, further comprising a projection lens unit disposed to project an image from the image-forming device and a screen on which the image is projected.

22. A source as recited in claim 20, further comprising a controller coupled to control an image formed by the image-forming device.

23. An illumination source, comprising:

an array of one or more light emitting diodes (LEDs);

a liquid cooled plate having a first surface and a second surface, the LEDS being attached directly to the first surface of the liquid cooled plate, a liquid coolant contacting the second surface of the liquid cooled plate; and

a phosphor material disposed on the one or more LEDs.

24. A source as recited in claim 23, wherein the LEDs are eutectically bonded to the first surface.

25. A source as recited in claim 23, wherein the liquid cooled plate comprises a metal plate having a thickness of less than one millimeter.

26. A method of manufacturing an illumination source, comprising:

providing one or more (LED) dies having a metallic layer on respective LED lower surfaces,

placing the LED dies in thermal contact with a first surface of a plate whose temperature is controllable by flowing a fluid past a second surface of the metal plate;

passing a heated fluid by the second surface of the plate so as to melt the metallic layer; and

cooling the metallic layer so that the metallic layer solidifies, thereby attaching the LED dies to the first surface of the liquid cooled plate.

27. A method as recited in claim 26, wherein the fluid is a gas.

28. A method as recited in claim 26, wherein the fluid is a liquid.

29. A method as recited in claim 26, further comprising placing flux on the plate before placing the LED dies in thermal contact with the plate.

30. A method as recited in claim 26, further comprising coating the LEDs with phosphor material.

31. An illumination system comprising:

a light source capable of emitting light at a first and a second wavelength and comprising;

an array of one or more light emitting diode (LED) dies, each LED die being capable of generating light at the first wavelength and being coated with a phosphor material, the phosphor material generating light at the second wavelength when illuminated by light at the first wavelength; and

a heat sink capable of being cooled with a coolant, the heat sink having a first surface and a second surface, the array of one or more LED dies being mounted on the first surface of the heat sink, a metal coating providing attachment between each LED die and the heat sink;

a light pipe disposed to receive light emitted by the light source;

a first reflective element for transmitting light at the second wavelength generated by the phosphor material and reflecting light at the first wavelength emitted by the array of one or more light emitting diode (LED) dies for recycling to the phosphor material; and

a reflective polarizer for transmitting light at the second wavelength in a first polarization state and reflecting light at the second wavelength in a second polarization state orthogonal to the first polarization state for conversion to the first polarization state.

32. The illumination system of claim 31, wherein the phosphor material comprises strontium thiogallate.

33. The illumination system of claim 31, wherein the light pipe is tapered.

34. The illumination system of claim 31, wherein the light pipe is hollow.

35. The illumination system of claim 31, wherein the light pipe is solid.

36. The illumination system of claim 31, wherein the light source is separated from an input side of the light pipe by an air gap.

37. The illumination system of claim 31 further comprising a liquid coolant contacting the second surface of the heat sink.

38. An optical system comprising the illumination system of claim 31 illuminating an imaging device, wherein the etendue of the illumination system is at least as large as that of the imaging device.

39. The illumination system of claim 31 further comprising a second reflective element surrounding the array of one or more light emitting diode (LED) dies.