(12) United States Patent
`Beeson et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,960,872 B2
`Nov. 1, 2005
`
`US006960872B2
`
`(54) ILLUMINATION SYSTEMS UTILIZING
`LIGHT EMITTING DIODES AND LIGHT
`RECYCLING TO ENHANCE OUTPUT
`RADIANCE
`
`(75) Inventors: Karl W. Beeson, Princeton, NJ (US);
`Scott M. Zimmerman, Baskin Ridge,
`NJ (US)
`
`(73) Assignee: Goldeneye, Inc., Carlsbad, CA (US)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 64 days.
`
`(21) Appl. No.: 10/814,043
`(22) Filed:
`Mar. 30, 2004
`
`(65)
`
`Prior Publication Data
`
`US 2004/0232812 A1 Nov. 25, 2004
`
`Related US. Application Data
`
`63 C '
`'
`' p
`f ppl'
`' N 10/445 136 ?l d
`e on
`,
`,
`ication o.
`ontmuation-m- arto a
`May 23, 2003, now Pat. No. 6,869,206.
`
`(51) Int. c1.7 .......................... .. F21V 7/00; H01L 33/00
`(52) US. Cl. ..................... .. 313/113; 313/498; 362/247;
`362/545
`
`(58) Field of Search ....................... .. 313/110, 112—113,
`313/498—512; 362/247, 310, 240, 544—545,
`555, 800, 347, 31, 217, 26
`References Cited
`U.S. PATENT DOCUMENTS
`
`(56)
`
`4/1995 Murase et al.
`5,410,454 A
`6,144,536 A 11/2000 Zimmerman et al.
`6,185,357 B1
`2/2001 Zou et al.
`6,186,649 B1
`2/2001 Zou et al.
`6,280,054 B1 * 8/2001 Cassarly et al. .......... .. 362/231
`6,488,389 B2 * 12/2002 Cassarly et al. .......... .. 362/231
`6,550,942 B1
`4/2003 Zou et al.
`
`* cited by examiner
`Primary Examiner—Karabi Guharay
`(74) Attorney, Agent, or Firm—William Propp, Esq.
`(57)
`ABSTRACT
`
`An illumination system has a light source that is at least
`partially enclosed Within a light-recycling envelope. The
`light source is a light-emitting diode that emits light, and a
`fraction of that light Will exit the light-recycling envelope
`through an aperture. The light-recycling envelope recycles
`part of the light emitted by the light source back to the light
`source in order to enhance the output radiance of the light
`exiting the illumination system.
`
`38 Claims, 16 Drawing Sheets
`
`104
`
`110 108
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`100
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`114
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`W
`
`116
`
`118
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`U.S. Patent
`
`Nov. 1,2005
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`Sheet 1 0f 16
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`US 6,960,872 B2
`
`FIG. 1A
`(Prior Art)
`
`FIG. 1B
`(Prior Art)
`
`,4
`27 j
`
`29
`
`28
`
`21
`
`20
`
`/
`
`26
`/
`
`Qout
`
`22
`Area“,
`
`23
`
`Lens
`
`25
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 2 0f 16
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`US 6,960,872 B2
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`FIG. 2A
`(Prior Art)
`
`32
`
`36
`
`3O /
`
`34
`
`FIG. 2B
`(Prior Art)
`
`40
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`43
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`ll \
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`U.S. Patent
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`Nov. 1, 2005
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`Sheet 3 0f 16
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`US 6,960,872 B2
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`FIG. 3A
`
`WWII/WWW
`
`100
`
`108
`
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`
`102
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`100
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`
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 4 0f 16
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`US 6,960,872 B2
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`FIG. 3D
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`100
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`114 /
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`104
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`118
`
`FIG. 3E
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`100
`
`120
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`2
`
`106
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`112
`
`102
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 5 0f 16
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`US 6,960,872 B2
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`130
`
`132
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`FIG. 4
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`110
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`
`106
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`112
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`102
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 6 6f 16
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`US 6,960,872 B2
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`FIG. 5
`
`140
`
`108
`
`110
`
`5
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 7 0f 16
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`US 6,960,872 B2
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`FIG. 6A
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`160
`
`a“ 168
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`162
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`168
`
`108
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`110
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 8 of 16
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`US 6,960,872 B2
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`FIG. 6B
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`170
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`172
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`176
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`176
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`
`
`108
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`110
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 9 0f 16
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`US 6,960,872 B2
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`FIG. 6C
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`180
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`188
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`190
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`186
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`‘"182
`
`184
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`/--—104
`
`192\\
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`190
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`
`110
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`192
`
`106
`
`112
`
`102
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 10 0f 16
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`US 6,960,872 B2
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`FIG. 7A
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`200
`
`220
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`
`224
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`
`106
`
`112
`
`102
`
`222
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 11 0f 16
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`US 6,960,872 B2
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`FIG. 7B
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`256
`
`272
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`260
`
`108
`
`110
`
`104
`
`A
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`258
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`106
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`112
`
`102
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 12 0f 16
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`US 6,960,872 B2
`
`FIG. 8
`
`600
`
`638
`
`642
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`630
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`110
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 13 0f 16
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`US 6,960,872 B2
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`310
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`\\/////////¢
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`
`FIG. 9A
`
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`FIG. 9B
`
`300
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`320 330
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`U.S. Patent
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`Nov. 1,2005
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`Sheet 14 0f 16
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`US 6,960,872 B2
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`FIG. 10A
`
`FIG. 10B
`
`.400
`
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`
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`U.S. Patent
`
`Nov. 1,2005
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`Sheet 15 0f 16
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`US 6,960,872 B2
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`FIG. 11A
`
`FIG. 11B
`
`510a
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`U.S. Patent
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`V.
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`
`US 6,960,872 B2
`
`1
`ILLUMINATION SYSTEMS UTILIZING
`LIGHT EMITTING DIODES AND LIGHT
`RECYCLING TO ENHANCE OUTPUT
`RADIANCE
`
`CROSS REFERENCE TO RELATED
`APPLICATION
`
`10
`
`This application is a continuation-in-part of US. patent
`application Ser. No. 10/445,136 ?led on May 23,2003, now
`US. Pat. No. 6,869,206, entitled “ILLUMINATION SYS
`TEMS UTILIZING HIGHLY REFLECTIVE LIGHT
`EMITTING DIODES AND LIGHT RECYCLING TO
`ENHANCE BRIGHTNESS,” Which is herein incorporated
`by reference. This application is also related to US. patent
`application Ser. No. 10/814,044 entitled “ILLUMINATION
`SYSTEMS UTILIZING MULTIPLE WAVELENGTH
`LIGHT RECYCLING” and to US. patent application Ser.
`No. 10/815,005 entitled “PROJECTION DISPLAY SYS
`TEMS UTILIZING LIGHT EMITTING DIODES AND
`20
`LIGHT RECYCLING,” both of Which are ?led concurrently
`With this application and Which are herein incorporated by
`reference.
`
`15
`
`TECHNICAL FIELD
`
`This invention relates to illumination systems incorporat
`ing light-emitting diodes (LEDs). Light-emitting diodes
`include inorganic light-emitting diodes and organic light
`emitting diodes (OLEDs).
`
`BACKGROUND OF THE INVENTION
`
`Illumination systems are used as either stand-alone light
`sources or as internal light sources for more complex optical
`systems. Examples of optical systems that utiliZe or incor
`porate illumination systems include projection displays,
`?at-panel displays, avionics displays, automotive lighting,
`residential lighting, commercial lighting and industrial light
`ing applications.
`Many applications require illumination systems With high
`brightness and a small effective emitting area. An example
`of a conventional light source With high brightness and a
`small effective emitting area is an arc lamp source, such as
`a xenon arc lamp or a mercury arc lamp. Arc lamp sources
`may have emitting areas as small as a feW square millime
`ters. An example of a complex optical system that can utiliZe
`an illumination system With high brightness and a small
`effective source area is a projection television display.
`Current projection television displays typically project the
`combined images of three small red, green and blue cathode
`ray-tube (CRT) devices onto a vieWing screen using projec
`tion lenses. More recent designs sometimes use a small-area
`arc lamp source to project images from a liquid crystal
`display (LCD), a liquid-crystal-on-silicon (LCOS) device or
`a digital light processor (DLP) device onto a vieWing screen.
`Light sources such as LEDs are currently not used for
`projection television displays because LED sources do not
`have sufficient output brightness.
`The technical term brightness can be de?ned either in
`radiometric units or photometric units. In the radiometric
`system of units, the unit of light ?ux or radiant ?ux is
`expressed in Watts and the unit for brightness is called
`radiance, Which is de?ned as Watts per square meter per
`steradian (Where steradian is the unit of solid angle). The
`human eye, hoWever, is more sensitive to some Wavelengths
`of light (for example, green light) than it is to other Wave
`lengths (for example, blue or red light). The photometric
`
`2
`system is designed to take the human eye response into
`account and therefore brightness in the photometric system
`is brightness as observed by the human eye. In the photo
`metric system, the unit of light ?ux as perceived by the
`human eye is called luminous ?ux and is expressed in units
`of lumens. The unit for brightness is called luminance,
`Which is de?ned as lumens per square meter per steradian.
`The human eye is only sensitive to light in the Wavelength
`range from approximately 400 nanometers to approximately
`700 nanometers. Light having Wavelengths less than about
`400 nanometers or greater than about 700 nanometers has
`Zero luminance, irrespective of the radiance values.
`In US. patent application Ser. No. 10/445,136, brightness
`enhancement referred to luminance enhancement only.
`Since luminance is non-Zero only for the visible Wavelength
`range of 400 to 700 nanometers, US. patent application Ser.
`No. 10/445,136 is operative only in the 400- to 700
`nanometer Wavelength range. In the present application,
`hoWever, brightness enhancement refers to radiance
`enhancement and is valid for any Wavelength throughout the
`optical spectrum.
`In a conventional optical system that transports light from
`an input source at one location to an output image at a
`second location, one cannot produce an optical output image
`Whose radiance is higher than the radiance of the light
`source. A conventional optical system 10 of the prior art is
`illustrated in cross-section in FIG. 1A. In FIG. 1A, light rays
`18 from an input light source 12 are focused by a convex
`lens 14 to an output image 16. The conventional optical
`system 10 in FIG. 1A can also be illustrated in a different
`manner as optical system 20 shoWn in cross-section in FIG.
`1B. For simplicity, the input source 22, the lens 24 and the
`output image 26 are all assumed to be round. In FIG. 1B, the
`input source 22 has area, Areal-n. The light rays from input
`source 22 ?ll a truncated cone having edges 21 and 23. The
`cone, Which is shoWn in cross-section in FIG. 1B, extends
`over solid angle 27. The magnitude of solid angle 27 is QM.
`Lens 24 focuses the light rays to image 26 having area,
`Areaom. The light rays forming the image 26 ?ll a truncated
`cone having edges 25 and 29. The cone, Which is shoWn in
`cross-section, extends over solid angle 28. The magnitude of
`solid angle 28 is 90,”.
`If the optical system 20 has no losses, the light input ?ux
`at the input source 22,
`
`[Equation 1]
`<I>,-,,=(Radiance,-,,) (AreamXQm),
`equals the light output ?ux at the output image 26,
`
`25
`
`30
`
`35
`
`40
`
`45
`
`<I>out=(RadianceUM) (AreaUMXQUM).
`
`[Equation 2]
`
`In these equations, “Radiancein” is the radiance at the input
`source 22, “Radianceom” is the radiance at the output image
`26, “Areal-n” is the area of the input source 22 and “Areaom”
`is the area of the output image 26. The quantities QM and
`Q0,” are, respectively, the projected solid angles subtended
`by the input source and output image light cones. In such a
`lossless system, it can be shoWn that
`
`55
`
`Radiancem=RadianceoM
`
`[Equation 3]
`
`60
`
`and
`
`(Aream)(§2m)=(AreaW)(90M)
`
`[Equation 4]
`
`If the index of refraction of the optical transmission medium
`is different at the input source and output image positions,
`the equality in Equation 4 is modi?ed to become
`
`65
`
`("l-n2)(Afeam)(QMFWMZ)(AreaWXQWL
`
`[Equation 5]
`
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`US 6,960,872 B2
`
`3
`Where nm is the index of refraction at the input position and
`no,” is the index of refraction at the output position. The
`quantity (n2)(Area)(Q) is variously called the “etendue” or
`“optical extent” or “throughput” of the optical system. In a
`conventional lossless optical system, the quantity (n2)(Area)
`(Q) is conserved.
`In US. Pat. No. 6,144,536, herein incorporated by
`reference, Zimmerman et al demonstrated that for the special
`case of a source that has a re?ecting emitting surface, an
`optical system can be designed that recycles a portion of the
`light emitted by the source back to the source and transmits
`the remainder of the light to an output position. Under
`certain conditions utiliZing such light recycling, the effective
`luminance of the source as Well as the output luminance of
`the optical system can be higher than the intrinsic luminance
`of the source in the absence of recycling, a result that is not
`predicted by the standard etendue equations. In US. Pat. No.
`6,144,536, the term “luminance” is used for brightness. As
`previously stated, the term “luminance” is only useful for
`visible optical Wavelengths betWeen 400 and 700 nanom
`eters. Therefore US. Pat. No. 6,144,536 is operative only in
`that spectral region.
`An example of a light source With a re?ecting emitting
`surface is a conventional ?uorescent lamp. A cross-section
`of a conventional ?uorescent lamp 30 is shoWn in FIG. 2A.
`The ?uorescent lamp 30 has a glass envelope 32 enclosing
`a holloW interior 36. The holloW interior 36 is ?lled With a
`gas that can emit ultraviolet light When a high voltage is
`applied. The ultraviolet light excites a phosphor coating 34
`on the inside surface of the glass envelope, causing the
`phosphor to emit visible light through the phosphor coating
`34. The phosphor coating 34 is a partially re?ecting surface
`in addition to being a light emitter. Therefore, it is possible
`to design an optical system that recycles a portion of the
`visible light generated by the phosphor coating 34 back to
`the coating 34 and thereby cause an increase in the effective
`brightness of the conventional ?uorescent lamp.
`The disclosures on light recycling in US. Pat. No. 6,144,
`536 relate to linear light sources that have long emitting
`apertures With aperture areas greater than 100 square milli
`meters
`Such con?gurations, Which typically use
`?uorescent lamps as light sources, are not suitable for many
`applications such as illumination systems for large projec
`tion displays. At the application date for US. Pat. No.
`6,144,536, a typical LED had an output of only 1 lumen per
`square millimeter (mm2) and a light re?ectivity of less than
`20%. To make an illumination system that produces 1000
`lumens output for a projection display Would require at least
`1000 LEDs having a total LED surface area of 1000 mm2.
`If 1000 loW-re?ectivity, loW-output LEDs are placed on the
`inside surface of a light-recycling envelope that has a 10
`mm2 output aperture and that has a total inside area of 1010
`mm2 (including the area of the output aperture), the overall
`output e?iciency Will be less than 2%. Less than 20 lumens
`from the original 1000 lumens Will exit the light-recycling
`envelope. Such an illumination system is not very practical.
`Recently, highly re?ective green, blue and ultraviolet
`LEDs and diode lasers based on gallium nitride (GaN),
`indium gallium nitride (InGaN) and aluminum gallium
`nitride (AlGaN) semiconductor materials have been devel
`oped. Some of these LED devices have high light output,
`high radiance and have a light-re?ecting surface that can
`re?ect at least 50% of the light incident upon the device. The
`re?ective surface of the LED can be a specular re?ector or
`a diffuse re?ector. Typically, the re?ective surface of the
`LED is a specular re?ector. Radiance outputs close to 7000
`Watts per square meter per steradian and total outputs of
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`approximately 0.18 Watt from a single packaged device are
`possible. Light outputs per unit area can exceed 0.045
`Watt/mm2. As such, several neW applications relating to
`illumination systems have become possible. Advantages
`such as spectral purity, reduced heat, and fast sWitching
`speed all provide motivation to use LEDs and semiconduc
`tor lasers to replace ?uorescent, incandescent and arc lamp
`sources.
`FIG. 2B illustrates a cross-sectional vieW of a recently
`developed type of LED 40 that has an emitting layer 46
`located beloW both a transparent top electrode 43 and a
`second transparent layer 44. Emitting layer 46 emits light
`rays 45 When an electric current is passed through the device
`40. BeloW the emitting layer 46 is a re?ecting layer 47 that
`also serves as a portion of the bottom electrode. Electrical
`contacts 41 and 42 provide a pathWay for electrical current
`to ?oW through the device 40. It is a recent neW concept to
`have both electrical contacts 41 and 42 on the backside of
`the LED opposite the emitting surface. Typical prior LED
`designs placed one electrode on top of the device, Which
`interfered With the light output from the top surface and
`resulted in devices With loW re?ectivity. The re?ecting layer
`47 alloWs the LED to be both a light emitter and a light
`re?ector. Lumileds Lighting LLC, for example, produces
`highly re?ective green, blue and ultraviolet LED devices of
`this type. It is expected that highly re?ective red and infrared
`LEDs With high outputs and high radiance Will also even
`tually be developed. HoWever, even the neW green, blue and
`ultraviolet gallium nitride, indium gallium nitride and alu
`minum gallium nitride LEDs do not have sufficient radiance
`for many applications.
`LEDs, including inorganic light-emitting diodes and
`organic light-emitting diodes, emit incoherent light. On the
`other hand, semiconductor laser light sources, such as edge
`emitting laser diodes and vertical cavity surface emitting
`lasers, generally emit coherent light. Coherent semiconduc
`tor laser light sources typically have higher brightness than
`incoherent light sources, but semiconductor laser light
`sources are not suitable for many applications such as
`displays due to the formation of undesirable speckle light
`patterns that result from the coherent nature of the light.
`It Would be highly desirable to develop incoherent illu
`mination systems based on LEDs that utiliZe light recycling
`to increase the illumination system output radiance. Possible
`applications include projection displays, ?at-panel displays,
`avionics displays, automotive lighting, residential lighting,
`commercial lighting and industrial lighting.
`
`SUMMARY OF THE INVENTION
`This invention is an illumination system that is comprised
`of a light source, a light-recycling envelope and at least one
`light output aperture. The light source is at least one light
`emitting diode that emits light. The at least one light
`emitting diode is further comprised of an emitting layer that
`emits light and a re?ecting layer that has re?ectivity RS and
`that re?ects light. The total light-emitting area of the light
`source is area AS and the light emitted by the light source has
`a maximum intrinsic source radiance.
`The light-recycling envelope at least partially encloses the
`light source and has re?ectivity RE. The light-recycling
`envelope re?ects and recycles part of the light emitted by the
`emitting layer back to the re?ecting layer of the light
`emitting diode.
`The at least one light output aperture is located in a
`surface of the light-recycling envelope. The total light output
`aperture area is area A0 and areaAO is less than area AS. The
`light source and the light-recycling envelope direct at least
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`US 6,960,872 B2
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`5
`a fraction of the light out of the light-recycling envelope
`through the at least one light output aperture. The fraction of
`the light that exits the at least one light output aperture exits
`as incoherent light having a maximum exiting radiance.
`Under some conditions utiliZing light recycling, the maxi
`mum exiting radiance is greater than the maximum intrinsic
`source radiance of the light source.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`A more complete understanding of the present invention,
`as Well as other objects and advantages thereof not enumer
`ated herein, Will become apparent upon consideration of the
`folloWing detailed description and accompanying draWings,
`Wherein:
`FIGS. 1A and 1B are cross-sectional side vieWs of con
`ventional optical systems of the prior art.
`FIGS. 2A and 2B are cross-sectional vieWs of prior art
`light sources that have both emitting and re?ecting surfaces.
`FIGS. 3A, 3B, 3C, 3D and 3E illustrate an embodiment of
`this invention that has one light-emitting diode.
`FIG. 4 is an embodiment of this invention in Which the
`light-recycling envelope is partially ?lled With a light
`transmitting solid.
`FIG. 5 is an embodiment of this invention that further
`comprises a planar re?ecting polariZer.
`FIGS. 6A, 6B and 6C are embodiments of this invention
`that further comprise light-collimating elements.
`FIG. 7A is an embodiment of this invention that further
`comprises both a light-collimating element and a planar
`re?ective polariZer.
`FIG. 7B is an embodiment of this invention that further
`comprises both a light-collimating element and a beam
`splitting prism polariZer.
`FIG. 8 is an embodiment of this invention that comprises
`tWo light sources, tWo light-recycling envelopes, tWo light
`collimating elements and a beam-splitting prism polariZer.
`FIGS. 9A and 9B illustrate an embodiment of this inven
`tion that further comprises a light guide.
`FIGS. 10A, 10B and 10C illustrate an embodiment of this
`invention that has tWo light-emitting diodes.
`FIGS. 11A, 11B and 11C illustrate an embodiment of this
`invention that has four light-emitting diodes.
`FIGS. 12A and 12B illustrate an embodiment of this
`invention that has tWelve light-emitting diodes.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`The preferred embodiments of the present invention Will
`be better understood by those skilled in the art by reference
`to the above FIGURES. The preferred embodiments of this
`invention illustrated in the FIGURES are not intended to be
`exhaustive or to limit the invention to the precise form
`disclosed. The FIGURES are chosen to describe or to best
`explain the principles of the invention and its applicable and
`practical use to thereby enable others skilled in the art to best
`utiliZe the invention.
`The embodiments of this invention are comprised of at
`least a light source, a light-recycling envelope and a light
`output aperture located in the surface of the light-recycling
`envelope.
`The preferred light source of this invention comprises at
`least one light-emitting diode (LED). Preferred LEDs are
`inorganic light-emitting diodes and organic light-emitting
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`diodes (OLEDs) that both emit light and re?ect light. More
`preferred LEDs are inorganic light-emitting diodes due to
`their higher light output brightness. The light emitted by the
`light source is preferably greater that 200 nanometers in
`Wavelength and less than 3000 nanometers in Wavelength.
`Various illumination systems that utiliZe LEDs are illus
`trated in FIGS. 3—12. An LED depicted in FIGS. 3—12 may
`be any LED that both emits light and re?ects light. Examples
`of LEDs that both emit and re?ect light include inorganic
`light-emitting diodes and OLEDs. Inorganic light-emitting
`diodes can be fabricated from materials containing gallium
`nitride, aluminum gallium nitride, indium gallium nitride,
`aluminum nitride, aluminum indium gallium phosphide,
`gallium arsenide, indium gallium arsenide or indium gallium
`arsenide phosphide, for example, but are not limited to such
`materials. OLEDs may be constructed from a variety of
`light-emitting organic small molecules or polymers. Appro
`priate small molecules include, for example, tris
`(8-hydroxyquinoline) aluminum(III), Which can be abbrevi
`ated as Alq3, and certain types of chelates, oxadiaZoles,
`imidaZoles, benZidines and triarylamines, but are not limited
`to such materials. Appropriate polymers include, for
`example, poly(ethylene dioxythiophene) and poly(styrene
`sulfonate).
`For purposes of simplifying the FIGURES, each LED in
`FIGS. 3—12 is illustrated in an identical manner and each
`LED is shoWn as being comprised of tWo elements, an
`emitting layer that emits light and a re?ecting layer that
`re?ects light. Note that typical LEDs are normally con
`structed With more than tWo elements, but for the purposes
`of simplifying the FIGS. 3—12, the additional elements are
`not shoWn. Some of the embodiments of this invention may
`contain tWo or more LEDs. Although each LED in FIGS.
`3—12 is illustrated in an identical manner, it is Within the
`scope of this invention that multiple LEDs in an embodi
`ment may not all be identical. For example, if an embodi
`ment of this invention has a plurality of LEDs, it is Within
`the scope of this invention that some of the LEDs may be
`inorganic light-emitting diodes and some of the LEDs may
`be OLEDs. As a further example of an illumination system
`having multiple LEDs, if an embodiment of this invention
`has a plurality of LEDs, it is also Within the scope of this
`invention that some of the LEDs may emit different colors
`of light. Example LED colors include, but are not limited to,
`Wavelengths in the infrared, visible and ultraviolet regions
`of the optical spectrum. For example, one or more of the
`LEDs in a light-recycling envelope may be a red LED, one
`or more of the LEDs may be a green LED and one or more
`of the LEDs may be a blue LED. If an embodiment, for
`example, contains red, green and blue LEDS, then the red,
`green and blue LEDs may be poWered concurrently to
`produce a single composite output color such as White light.
`Alternatively, the red, green and blue LEDs in this example
`may each be poWered at different times to produce different
`colors in different time periods.
`Preferred LEDs have at least one re?ecting layer that
`re?ects light incident upon the LED. The re?ecting layer of
`the LED may be either a specular re?ector or a diffuse
`re?ector. Typically, the re?ecting layer is a specular re?ec
`tor. Preferably the re?ectivity R S of the re?ecting layer of the
`LED is at least 50%. More preferably, the re?ectivity R5 is
`at least 70%. Most preferably, the re?ectivity R5 is at least
`90%.
`Each LED in FIGS. 3—8 and 10—12 is illustrated With an
`emitting layer facing the interior of the light-recycling
`envelope and a re?ecting layer positioned behind the emit
`ting layer and adjacent to the inside surface of the light
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`7
`recycling envelope. In this configuration, light can be emit-
`ted from all surfaces of the emitting layer that are not in
`contact with the reflecting layer. It is also within the scope
`of this invention that a second reflecting layer can be placed
`on the surface of the emitting layer facing the interior of the
`light-recycling envelope. In the latter example, light can be
`emitted from the side surfaces of the emitting layer that do
`not contact either reflecting layer. A second reflecting layer
`is especially important for some types of LEDs that have an
`electrical connection on the top surface of the emitting layer
`since the second reflecting layer can improve the overall
`reflectivity of the LED.
`The total light-emitting area of the light source is area As.
`If there is more than one LED within a single light-recycling
`envelope, the total light-emitting area As of the light source
`is the total
`light-emitting area of all
`the LEDs in the
`light-recycling envelope.
`Alight source, whether comprising one LED or a plurality
`of LEDs, has a maximum intrinsic source radiance that
`depends on the light source design and the driving electrical
`power applied to the light source. The maximum intrinsic
`source radiance is determined in the following manner. First,
`the radiance is measured for each LED in the light source
`when the light-recycling envelope is not present and when
`no other LED is directing light to the LED under measure-
`ment. The measurements are done with each LED powered
`at the same level as in the illumination system and are done
`as a function of emitting angle. From these radiance
`measurements, a maximum radiance value can be deter-
`mined for all the LEDs. This maximum value is defined as
`the maximum intrinsic source radiance.
`
`The light-recycling envelope of this invention is a light-
`reflecting element that at least partially encloses the light
`source. The light-recycling envelope may be any three-
`dimensional surface that encloses an interior volume. For
`
`example, the surface of the light-recycling envelope may be
`in the shape of a cube, a rectangular three-dimensional
`surface, a sphere, a spheroid, an ellipsoid, an arbitrary
`three-dimensional facetted surface or an arbitrary three-
`dimensional curved surface. Preferably the three-
`dimensional shape of the light-recycling envelope is a
`facetted surface with flat sides in order to facilitate the
`
`attachment of LEDs to the inside surfaces of the envelope.
`Preferable three-dimensional shapes have a cross-section
`that is a square, a rectangle or a polygon.
`The light-recycling envelope reflects and recycles part of
`the light emitted by the light source back to the light source.
`Preferably the reflectivity RE of the inside surfaces of the
`light-recycling envelope is at least 50%. More preferably,
`the reflectivity RE is at least 70%. Most preferably,
`the
`reflectivity RE is at least 90%. Ideally, the reflectivity RE
`should be as close to 100% as possible in order to maximize
`the efficiency and the maximum exiting radiance of the
`illumination system.
`The light-recycling envelope may be fabricated from a
`bulk material that is intrinsically reflective. A bulk material
`that is intrinsically reflective may be a diffuse reflector or a
`specular reflector. Preferably a bulk material that is intrin-
`sically reflective is a diffuse reflector. Diffuse reflectors
`reflect light rays in random directions and prevent reflected
`light from being trapped in cyclically repeating pathways.
`Specular reflectors reflect light rays such that the angle of
`reflection is equal to the angle of incidence.
`Alternatively, if the light-recycling envelope is not fabri-
`cated from an intrinsically reflective material, the interior
`surfaces of the light-recycling envelope must be covered
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`with a reflective coating. The reflective coating may be a
`specular reflector, a diffuse reflector or a diffuse reflector that
`is backed with a specular reflecting layer. Diffuse reflectors
`typically need to be relatively thick (a few millimeters) in
`order to achieve high reflectivity. The thickness of a diffuse
`reflector needed to achieve high reflectivity can be reduced
`if a specular reflector is used as a backing to the diffuse
`reflector.
`
`Diffuse reflectors can be made that have very high reflec-
`tivity (for example, greater than 95% or greater than 98%).
`However, diffuse reflectors with high reflectivity are gener-
`ally quite thick. For example, diffuse reflectors with reflec-
`tivity greater than 98% are typically several millimeters
`thick. Examples of diffuse reflectors include, but