Patent Publication Number: US-2006002108-A1

Title: Phosphor based illumination system having a short pass reflector and method of making same

Description:
RELATED PATENT APPLICATIONS  
      The following co-owned and copending U.S. patent applications are incorporated herein by reference: PHOSPHOR BASED ILLUMINATION SYSTEM HAVING A LONG PASS REFLECTOR AND METHOD OF MAKING SAME (Attorney Docket No. 59757US002); PHOSPHOR BASED ILLUMINATION SYSTEM HAVING A LONG PASS REFLECTOR AND METHOD OF MAKING SAME (Attorney Docket No. 59707US002); PHOSPHOR BASED ILLUMINATION SYSTEM HAVING A PLURALITY OF LIGHT GUIDES AND A DISPLAY USING SAME (Attorney Docket No. 59758US002); PHOSPHOR BASED ILLUMINATION SYSTEM HAVING A PLURALITY OF LIGHT GUIDES AND A DISPLAY USING SAME (Attorney Docket No. 59884US002); PHOSPHOR BASED ILLUMINATION SYSTEM HAVING A SHORT PASS REFLECTOR AND METHOD OF MAKING SAME (Attorney Docket No. 59885US002).  
     BACKGROUND  
      White light sources that utilize light emitting diodes (LEDs) in their construction can have two basic configurations. In one, referred to herein as direct emissive LEDs, white light is generated by direct emission of different colored LEDs. Examples include a combination of a red LED, a green LED, and a blue LED, and a combination of a blue LED and a yellow LED. In another configuration, referred to herein as phosphor-converted LEDs (PCLEDs), a single LED generates light in a narrow range of wavelengths, which light impinges upon and excites a phosphor or other type of emissive material to produce light having different wavelengths than those generated by the LED. The phosphor can include a mixture or combination of distinct emissive materials, and the light emitted by the phosphor can include broad or narrow emission lines distributed over the visible wavelength range such that the emitted light appears substantially white to the unaided human eye.  
      An example of a PCLED is a blue LED illuminating a phosphor that converts blue light to longer wavelengths. A portion of the blue excitation light is not absorbed by the phosphor, and the residual blue excitation light is combined with longer wavelengths emitted by the phosphor. Another example of a PCLED is an ultraviolet (UV) LED illuminating a phosphor that absorbs and converts UV light either to red, green, and blue light, or a combination of blue and yellow light.  
      Another application of PCLEDs is to convert UV or blue light to green light. In general, green LEDs have a relatively low efficiency and can change output wavelength during operation. In contrast to green LEDs, green PCLEDs, can have improved wavelength stability.  
      Advantages of white light PCLEDs over direct emission white LEDs include better color stability as a function of device aging and temperature, and better batch-to-batch and device-to-device color uniformity/repeatability. However, PCLEDs can be less efficient than direct emission LEDs, due in part to inefficiencies in the process of light absorption and re-emission by the phosphor.  
     SUMMARY  
      The present disclosure provides illumination systems that utilize emissive materials and interference reflectors for filtering components. In some embodiments, the interference reflectors of the present disclosure may include multilayer optical films including individual optical layers, at least some of which are birefringent, arranged into optical repeat units through the thickness of the film. Adjacent optical layers have refractive index relationships that maintain reflectivity and avoid leakage of p-polarized light at moderate to high incidence angles.  
      In one aspect, the present disclosure provides an illumination system, including a light source that emits light having a first optical characteristic and a light guide including an output surface. The system further includes a first interference reflector positioned between the light source and the output surface of the light guide, where the first interference reflector substantially transmits light having the first optical characteristic and substantially reflects light having a second optical characteristic. The system further includes emissive material positioned between the first interference reflector and the output surface of the light guide, where the emissive material emits light having the second optical characteristic when illuminated with light having the first optical characteristic.  
      In another aspect, the present disclosure provides a method of manufacturing an illumination system, including providing a light source that emits light having a first optical characteristic, and positioning emissive material to receive light emitted by the light source, where the emissive material emits light having a second optical characteristic when illuminated with light having the first optical characteristic. The method further includes positioning a first interference reflector between the light source and the emissive material, where the first interference reflector substantially transmits light having the first optical characteristic and substantially reflects light having the second optical characteristic; and positioning a light guide to receive the light emitted by the emissive material, where the light guide directs at least a portion of light emitted by the emissive material through an output surface of the light guide.  
      In another aspect, the present disclosure provides a display that includes an illumination system and a spatial light modulator. The illumination system includes a light source that emits light having a first optical characteristic, and a light guide having an output surface. The system further includes a first interference reflector positioned between the light source and the output surface of the light guide, where the first interference reflector substantially transmits light having the first optical characteristic and substantially reflects light having a second optical characteristic. The system further includes emissive material positioned between the first interference reflector and the output surface of the light guide, where the emissive material emits light including the second optical characteristic when illuminated with light including the first optical characteristic. The spatial light modulator is optically coupled to the illumination system and includes controllable elements operable to modulate at least a portion of light from the illumination system.  
      In another aspect, the present disclosure provides a method of providing illumination to a desired location, including illuminating a first interference reflector with light having a first optical characteristic, where the first interference reflector substantially transmits light having the first optical characteristic and substantially reflects light having a second optical characteristic. The method further includes illuminating emissive material with the light transmitted by the first interference reflector such that the emissive material emits light having the second optical characteristic; and directing at least a portion of the light emitted by the emissive material to the desired location.  
      The above summary of the present disclosure is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The Figures and Detailed Description that follow more particularly exemplify illustrative embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  schematically illustrates one embodiment of an illumination system having a short pass interference reflector.  
       FIG. 2  schematically illustrates one embodiment of an illumination system having a short pass interference reflector and a long pass interference reflector.  
       FIG. 3  schematically illustrates one embodiment of an illumination system having a short pass interference reflector and one or more optical elements.  
       FIG. 4  schematically illustrates one embodiment of an illumination system having a long pass interference reflector.  
       FIG. 5  schematically illustrates one embodiment of an illumination system having a short pass interference reflector, a long pass interference reflector, and an optical cavity.  
       FIG. 6 (A) is a schematic top plan view of one embodiment of an illumination system having an optical cavity that includes one or more facets.  
       FIG. 6 (B) is a schematic cross-section view of one portion of the optical cavity of the illumination system of  FIG. 6 (A).  
       FIG. 6 (C) is a schematic side view of the illumination system of  FIG. 6 (A).  
       FIG. 6 (D) is a schematic side view of another embodiment of an illumination system having an optical cavity that includes one or more facets.  
       FIG. 6 (E) is a schematic side view of another embodiment of an illumination system having an optical cavity that includes one or more facets.  
       FIG. 7  is a schematic top plan view of another embodiment of an illumination system having four optical cavities that each include one or more facets.  
       FIG. 8 (A) is a schematic top plan view of an embodiment of an illumination system having a short pass interference reflector located within one or more optical cavities located within a light guide.  
       FIG. 8 (B) is a schematic cross-section view of the illumination system of  FIG. 8A  taken along line  8 B- 8 B.  
       FIG. 9 (A) is a schematic side view of an embodiment of an illumination system having one or more optical cavities adjacent an input surface of a light guide.  
       FIG. 9 (B) is a schematic top plan view of the illumination system of  FIG. 9 ( a ).  
       FIG. 10 (A) is a schematic perspective view of one embodiment of an illumination system having one or more optical cavities adjacent an input surface of a light guide.  
       FIG. 10 (B) is a schematic cross-section view of the illumination system of  FIG. 10 ( a ) taken along line  10 B- 10 B.  
       FIG. 11  schematically illustrates an embodiment of an illumination system having a short pass interference reflector positioned between emissive material and an output surface of a light guide.  
       FIG. 12  schematically illustrates an embodiment of an illumination system having a short pass interference reflector positioned adjacent an output surface of a light guide and one or more phosphor dots positioned on the short pass interference reflector.  
       FIG. 13  is a schematic perspective view of one embodiment of an illumination system having a wedge-shaped light guide.  
       FIG. 14  is a schematic cross-section view of one embodiment of an illumination system having one or more light guides.  
       FIG. 15  schematically illustrates another embodiment of an illumination system having one or more light guides.  
       FIG. 16  schematically illustrates a display assembly including an illumination system and a display device.  
       FIG. 17  schematically illustrates an embodiment of an illumination system having a long pass interference reflector positioned such that emissive material is between an output surface of a light guide and the long pass interference reflector.  
       FIG. 18  is a schematic cross-section view of another embodiment of an illumination system having one or more light guides. 
    
    
     DETAILED DESCRIPTION  
      The present disclosure provides illumination systems that include a light source, one or more light guides, emissive material, and one or more interference reflectors. In some embodiments, the illumination systems provide white light for various applications. As used herein, the term “white light” refers to light that stimulates red, green, and blue sensors in the human eye to yield an appearance that an ordinary observer would consider “white.” Such light may be biased to the red (commonly referred to as warm white light) or to the blue (commonly referred to as cool white light). Further, such light can have a color rendering index of up to 100.  
      In general, these illumination systems include a light source that emits light including a first optical characteristic. The systems of the present disclosure also include emissive material that emits light having a second optical characteristic when illuminated with light having the first optical characteristic. The first optical characteristic and second optical characteristic may be any suitable optical characteristic, e.g., wavelength, polarization, modulation, intensity, etc. For example, the first optical characteristic may include a first wavelength region, and the second optical characteristic may include a second wavelength region that is different than the first wavelength region. In one exemplary embodiment, the light source may emit light having a first optical characteristic, where the first optical characteristic includes a first wavelength region including UV light. In this illustrative embodiment, the UV light emitted by the light source illuminates emissive material, which cause such material to emit light having a second optical characteristic, where the second optical characteristic includes a second wavelength region including visible light.  
      Some embodiments of the present disclosure include a short pass (SP) interference reflector. As used herein, the term “short pass interference reflector” refers to a reflector that substantially transmits light having a first optical characteristic and substantially reflects light having a second optical characteristic. In one exemplary embodiment, an illumination system includes a SP interference reflector that substantially transmits UV light from a light source and substantially reflects visible light emitted by emissive material that has been illuminated by the transmitted UV light.  
      Further, in some embodiments, the illumination systems include a long pass (LP) interference reflector. As used herein, the term “long pass interference reflector” refers to a reflector that substantially transmits light having a second optical characteristic and substantially reflects light having a first optical characteristic. For example, in one exemplary embodiment, an illumination system includes a LP interference reflector that substantially transmits visible light emitted by emissive material and substantially reflects UV light from a light source that had illuminated the emissive material.  
      In general, when the first optical characteristic and second optical characteristic are associated with wavelength, the emissive materials of the present disclosure may down-convert shorter wavelength light (e.g., UV light) to longer wavelength light (e.g., visible light). Alternatively, it is also possible to up-convert infrared radiation to visible light. For example, up-converting phosphors are well known in the art and typically use two or more infrared photons to generate 1 visible photon. Infrared LEDs needed to excite such phosphors have also been demonstrated and are very efficient. Visible light sources that use this process can be made more efficient with the addition of LP interference reflectors and/or SP interference reflectors, although the functions of each are reversed in this case compared to the down-converting phosphor systems. A SP interference reflector can be used to direct IR light towards the phosphor while transmitting the visible light, and an LP interference reflector can be placed such that the phosphor is between the LED and the LP interference reflector, where the LP interference reflector directs emitted visible light outward towards the intended system or user.  
      Although the exemplary embodiments of the present disclosure generally associate the first optical characteristic and second optical characteristic with wavelength, it is understood that such exemplary embodiments can also associate the first optical characteristic and second optical characteristic with other suitable characteristics of light, e.g., polarization, modulation, intensity, etc. For example, a SP interference reflector may be selected such that it substantially transmits light of a first polarization while the LP interference reflector substantially transmits light of a second polarization.  
      The illumination systems of the present disclosure may be used in any suitable application. For example, in some embodiments, an illumination system may be used as a light source for displays, light fixtures, headlamps, signs, etc.  
      In some embodiments, one or both of the SP interference reflector and LP interference reflector include polymeric multilayer optical films. Polymeric multilayer optical films are films that have tens, hundreds, or thousands of alternating layers of at least a first and second polymer material. Such layers have thicknesses and refractive indices that are selected to achieve a desired reflectivity in a desired portion of the spectrum, such as a reflection band limited to UV wavelengths or a reflection band limited to visible wavelengths. See, e.g., U.S. Pat. No. 5,882,774 (Jonza et al.). The polymeric multilayer optical films can be processed so that adjacent layer pairs have matching or near-matching, or deliberately mismatched refractive indices associated with a z-axis normal to the film such that the reflectivity of each interface between adjacent layers, for p-polarized light, decreases slowly with angle of incidence, is substantially independent of angle of incidence, or increases with angle of incidence away from the normal. Hence, such polymeric multilayer optical films can maintain high reflectivity levels for p-polarized light even at highly oblique incidence angles, thereby reducing the amount of p-polarized light transmitted by the reflective films compared to conventional inorganic isotropic stack reflectors. In some embodiments, the polymeric materials and processing conditions are selected so that, for each pair of adjacent optical layers, the difference in refractive index along the z-axis (parallel to the thickness of the film) is no more than a fraction of the refractive index difference along the x- or y- (in-plane) axes, the fraction being 0.5, 0.25, or even 0.1. The refractive index difference along the z-axis can be of the same or opposite sign as the in-plane refractive index differences.  
      Such polymeric multilayer optical films can be formed into any suitable shape as is further described herein. For example, polymeric multilayer optical film can be permanently deformed by embossing, thermoforming, or other known techniques to have a 3-dimensional shape such as a portion of a paraboloid, a sphere, or an ellipsoid. See, e.g., U.S. Patent Application Publication No. 2002/0154406 (Merrill et al.). See also U.S. Pat. No. 5,540,978 (Schrenk).  
      A wide variety of polymer materials are suitable for use in multilayer optical films for illumination systems. In certain applications according to various embodiments of the disclosure, it is desirable that the multilayer optical film includes alternating polymer layers composed of materials that resist degradation when exposed to UV light, e.g., a polymer pair of polyethylene terephthalate (PET)/co-polymethylmethacrylate (co-PMMA). The UV stability of polymeric reflectors can also be increased by the incorporation of non-UV absorbing light stabilizers such as hindered amine light stabilizers (HALS). In some cases, the polymeric multilayer optical film can also include transparent metal or metal oxide layers. See, e.g., PCT Publication WO 97/01778 (Ouderkirk et al.). In applications that use particularly high intensity UV light that would unacceptably degrade even robust polymer material combinations, it may be beneficial to use inorganic materials to form the multilayer optical films. The inorganic material layers can be isotropic or can be made to exhibit form birefringence as described, e.g., in PCT Publication WO 01/75490 (Weber) and thus have the beneficial refractive index relationships that yield enhanced p-polarization reflectivity as described herein.  
      In general, the interference reflectors described herein include reflectors that are formed of organic, inorganic, or a combination of organic and inorganic materials. The interference reflector can be a multilayer interference reflector. The interference reflector can be a flexible interference reflector. A flexible interference reflector can be formed from polymeric, non-polymeric materials, or polymeric and non-polymeric materials. Exemplary films including a polymeric and non-polymeric material are disclosed in U.S. Pat. Nos. 6,010,751 (Shaw et al.); 6,172,810 (Fleming et al.); and EP 733,919A2 (Shaw et al.).  
      The interference reflectors described herein can be formed from flexible, plastic, or deformable materials and can itself be flexible, plastic, or deformable. These flexible interference reflectors can be deflected or curved and still retain their pre-deflection optical properties.  
      Known self-assembled periodic structures, such as cholesteric reflecting polarizers and certain block copolymers, are considered to be multilayer interference reflectors for purposes of this disclosure. Cholesteric mirrors can be made using a combination of left and right handed chiral pitch elements.  
      In some embodiments of the present disclosure, the interference reflectors can be selected to substantially transmit or partially transmit light having a selected optical characteristic.  
      For example, a LP interference reflector that partially transmits blue light can be used in combination with a thin yellow phosphor layer in order to direct some blue light from a light source back onto the phosphor layer after the first pass through the phosphor.  
      In addition to providing reflection of blue light and UV light, a function of the multilayer optical film can be to block transmission of UV light so as to prevent degradation of subsequent elements inside or outside the illumination system, including prevention of human eye damage. In some embodiments, a UV absorber may be included on the side of the UV reflector furthest away from the light source. This UV absorber can be in, on, or adjacent to the multilayer optical film.  
      Although the interference reflectors of the present disclosure may include any suitable material or materials, an all polymer construction can offer several manufacturing and cost benefits. If high temperature polymers with high optical transmission and large index differentials are utilized in the interference reflectors, then an environmentally stable reflector that is both thin and very flexible can be manufactured to meet the optical needs of SP and LP interference reflectors. In some embodiments, coextruded multilayer interference reflectors as taught, e.g., in U.S. Pat. No. 6,531,230 (Weber et al.), can provide precise wavelength selection as well as large area, cost effective manufacturing. The use of polymer pairs having high index differentials allows the construction of very thin, highly reflective mirrors that are freestanding, i.e., have no substrate but are still easily processed. Alternatively, the interference reflectors of the present disclosure may be formed by casting as is described, e.g., in U.S. Pat. No. 3,711,176 (Alfrey, Jr. et al.).  
      An all polymeric interference reflector can be thermoformed into various three-dimensional shapes, e.g., hemispherical domes (as is further described herein). However, care must be taken to control the thinning to the correct amount over the entire surface of the dome to create the desired angular performance. Interference reflectors having a simple two-dimensional curvature are easier to create than three-dimensional, compound shaped interference reflectors. In particular, any thin and flexible interference reflector can be bent into a two-dimensional shape, e.g., a part of a cylinder, in this case an all polymeric interference reflector is not needed. Multilayer inorganic interference reflectors on thin polymeric substrates can be shaped in this manner, as well as inorganic multilayers on glass substrates that are less than 200 μm in thickness. The latter may have to be heated to temperatures near the glass transition point to obtain a permanent shape with low stress.  
      Optimum bandedges for SP and LP interference reflectors will depend on the emission spectra of both the light source and the emissive material in the system. In an illustrative embodiment, for a SP interference reflector, substantially all of the emission from the light source passes through the SP interference reflector to excite the emissive material, and substantially all of the emissions directed back toward the light source are reflected by the SP interference reflector so they do not enter the light source or its base structure where they could be absorbed. For this reason, the short pass defining bandedge of the SP interference reflector is placed in a region between the average emission wavelength of the light source and the average emission wavelength of the emissive material. In an illustrative embodiment, the SP interference reflector is placed between the light source and the emissive material. If, however, the SP interference reflector is planar, the emissions from a light source can strike the SP interference reflector at a variety of angles normal to a surface of the SP interference reflector, and at some angle of incidence be reflected by the SP interference reflector and fail to reach the emissive material. Unless the interference reflector is curved to maintain a nearly constant angle of incidence, one may desire to place the design bandedge at a wavelength larger than the midpoint of the emissive material and the light source emission curves to optimize the overall system performance. In particular, very little emissive material emission is directed to the interference reflector near zero degrees angle of incidence (i.e., normal to a surface of the interference reflector) because the included solid angle is very small.  
      In another illustrative embodiment, LP interference reflectors are placed opposite the emissive material from the light source to recycle the light source light back to the emissive material to improve system efficiency. In an illustrative embodiment, a LP interference reflector may be omitted if the light source emissions are in the visible spectrum and large amounts are needed to balance the color output of the emissive material. However, a LP interference reflector that partially transmits shorter wavelength light, e.g., blue light, can be used to optimize the angular performance of a blue-light source/yellow-phosphor system via the spectral angle shift that would pass more blue light at higher angles than at normal incidence.  
      In a further illustrative embodiment, the LP interference reflector is curved to maintain a nearly constant angle of incidence of the emitted light from the light source on the LP interference reflector. In this embodiment, the emissive material and the light source both face one side of the LP interference reflector. At high angles of incidence, a LP interference reflector having a substantially planar shape may not reflect shorter wavelength light. For this reason, the long wavelength bandedge of the LP interference reflector can be placed at as long a wavelength as possible while blocking as little of the emissive material emission as possible. Again, the bandedge placement can be changed to optimize the overall system efficiency.  
      In some embodiments, the multilayer interference reflectors described herein may have a lateral thickness gradient, i.e., a thickness that differs from one cross-section of the reflector to another cross-section of the reflector. These reflectors may have thicker interference layers as the emitted light angle of incidence increases toward an outer region of the multilayer reflector. Increasing the reflector thickness at the outer region of the reflector compensates for band shifting, since the reflected wavelength is proportional to the optical thickness of the high and low index interference layers and the incidence angle.  
       FIG. 1  schematically illustrates one embodiment of an illumination system  10 . The system  10  includes a light source  20  and a light guide  12  having an output surface  14 . In some embodiments, the light guide  12  can also include an input surface  16 . The system  10  also includes a first interference reflector  30  positioned between the light source  20  and the output surface  14  of the light guide  12 . Positioned between the first interference reflector  30  and the output surface  14  of the light guide  12  is emissive material  40 .  
      The light source  20  can include any suitable light source or light sources, e.g., electroluminescent devices, cold cathode fluorescent lights, electrodeless fluorescent lamps, LEDs, organic electroluminescent devices (OLEDs), polymer LEDs, laser diodes, arc lamps, etc. As used herein, the term “LED” refers to a diode that emits light, whether visible, ultraviolet, or infrared, whether coherent or incoherent. The term as used herein also includes incoherent epoxy-encased semiconductor devices marketed as “LEDs,” whether of the conventional or super-radiant variety. The term as used herein also includes semiconductor laser diodes.  
      In some embodiments, the light source  20  can be positioned adjacent one or more sides of the light guide  12 , and/or one or more major surfaces of the light guide  12 . As illustrated in  FIG. 1 , the light source  20  is positioned adjacent the input surface  16 . Although  FIG. 1  illustrates illumination system  10  as having one light source  20 , illumination system  10  may include two or more light sources positioned adjacent the same or other input surfaces of the light guide  12 .  
      The light source  20  emits light having a first optical characteristic. Any suitable optical characteristic may be selected. In some embodiments, the first optical characteristic can include a first wavelength region. For example, the light source  20  may emit UV light. As used herein, the term “UV light” refers to light having a wavelength in a range from about 150 nm to about 425 nm. In another example, the light source  20  may emit blue light.  
      In some embodiments, the light source  20  includes one or more LEDs. For example, the one or more LEDs can emit UV light and/or blue light. Blue light also includes violet and indigo light. LEDs include spontaneous emission devices as well as devices using stimulated or super radiant emission, including laser diodes and vertical cavity surface emitting laser diodes.  
      The light guide  12  of system  10  may include any suitable light guide, e.g., hollow or solid light guide. Although the light guide  12  is illustrated as being planar in shape, the light guide  12  may take any suitable shape, e.g., wedge, cylindrical, planar, conical, complex molded shapes, etc. Further, the input surface  16  and/or the output surface  14  of the light guide  12  may include any suitable shapes, e.g., those described above for the shape of the light guide  12 . It may be preferred that the light guide  12  is configured to direct light through its output surface  14 . Further, the light guide  12  may include any suitable material or materials. For example, the light guide  12  may include glass; acrylates, including polymethylmethacrylate, polystyrene, fluoropolymers; polyesters including polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and copolymers containing PET or PEN or both; polyolefins including polyethylene, polypropylene, polynorborene, polyolefins in isotactic, atactic, and syndiotactic sterioisomers, and polyolefins produced by metallocene polymerization. Other suitable polymers include polyetheretherketones and polyetherimides.  
      The illumination system  10  also includes a first interference reflector  30  positioned between the light source  20  and the output surface  14  of the light guide  12 . In the embodiment illustrated in  FIG. 1 , the first interference reflector  30  is a SP interference reflector, i.e., it substantially transmits light having the first optical characteristic from the light source  20  and substantially reflects light having a second optical characteristic. For example, as is further described herein, the emissive material  40  may emit visible light when illuminated with UV or blue light from the light source  20 . In such an embodiment, the first interference reflector  30  may be selected such that it substantially transmits UV light and substantially reflects visible light. In other embodiments, the emissive material  40  may emit infrared light when illuminated with light from the light source  20 . In such embodiments, the first interference reflector  30  may be selected such that it substantially transmits light from the light source  20  and substantially reflects infrared light.  
      The first interference reflector  30  may be positioned in any suitable location between the light source  20  and the output surface  14  of the light guide  12 . In some embodiments, the first interference reflector  30  may be positioned on the input surface  16  of the light guide  12 , within the light guide  12 , or on the light source  20 .  
      The first interference reflector  30  may include any suitable interference reflector or reflectors described herein. Further, the first interference reflector  30  may take any suitable shape, e.g., hemispherical, cylindrical, planar, etc.  
      The first interference reflector  30  can be formed of a material that resists degradation when exposed to UV, blue, or violet light, such as discussed herein. In general, the multilayer reflectors discussed herein can be stable under high intensity illumination for extended periods of time. High intensity illumination can be generally defined as a flux level from 1 to 100 Watt/cm 2 . Suitable illustrative polymeric materials can include UV resistant material formed from, for example, acrylic material, PET material, PMMA material, polystyrene material, polycarbonate material, THV material available from 3M Company (St. Paul, Minn.), and combinations thereof. These materials and PEN material can be used with light sources that emit blue light.  
      The illumination system  10  also includes emissive material  40  positioned between the first interference reflector  30  and the output surface  14  of the light guide  12 . The emissive material  40  emits light having a second optical characteristic when illuminated with light having the first optical characteristic from the light source  20 . The second optical characteristic may be any suitable optical characteristic, e.g., wavelength, polarization, modulation, intensity, etc. In some embodiments, the light emitted by the emissive material 40 may include a second wavelength region when the emissive material  40  is illuminated with light emitted by the light source  20  that includes a first wavelength region. For example, in some embodiments, the emissive material  40  may emit visible light when illuminated with UV or blue light form the light source  20 . As used herein, the term “visible light” refers to light that is perceptible to the unaided eye, e.g., generally in a wavelength range of about 400 nm to about 780 nm. In other embodiments, the emissive material  40  may emit visible light and/or infrared light. As used herein, the term “infrared light” refers to light in a wavelength range of 780 nm to 2500 nm.  
      In general, the embodiments disclosed herein are operative with a variety of emissive materials. In some embodiments, suitable phosphor materials may be used. Such phosphor materials are typically inorganic in composition, having excitation wavelengths in the 150-1100 nm range. A phosphor blend can comprise phosphor particles in the 1-25 μm size range dispersed in a binder such as silicone, fluoropolymer, epoxy, adhesive, or another polymeric matrix, which can then be applied to a substrate, such as an LED or a film. Phosphors include rare-earth doped garnets, silicates, and other ceramics. In other embodiments, the emissive materials can also include organic fluorescent materials, including fluorescent dyes and pigments, sulfides, aluminates, phosphates, nitrides. See, e.g., Shionoya et al.,  Phosphor Handbook , CRC Press, Boca Raton, Fla. (1998).  
      In embodiments that utilize emissive materials  40  having a narrow emission wavelength range, a mixture of emissive materials can be formulated to achieve the desired color balance, as perceived by the viewer, for example a mixture of red-, green- and blue-emitting materials. In other embodiments, emissive materials having broader emission bands can be useful for mixtures having higher color rendering indices. In some embodiments, the emissive materials can have fast radiative decay rates.  
      The emissive material  40  can be formed in a continuous or discontinuous layer. The emissive material  40  can be a uniform or non-uniform pattern. The emissive material  40  can include regions having a small area, e.g., “dots,” each having an area in plan view of less than 10000 μm 2 . In an illustrative embodiment, the dots can each be formed from a phosphor that emits longer wavelength light having one or more different peak wavelengths. For example, at least one dot can include a first emissive material that emits a peak wavelength in the red region, and at least another phosphor dot can include a second emissive material that emits a peak wavelength in the blue region. The dots emitting visible light having a plurality of peak wavelengths can be arranged and configured in any uniform or non-uniform manner as desired. For example, the emissive material  40  can include dots in a pattern having a non-uniform density gradient along a surface or an area. The dots can have any regular or irregular shape and need not be round in plan view. In addition, emissive material  40  can be in a co-extruded skin layer of a multilayer optical film.  
      Structured emissive materials can be configured in several ways to provide benefits in performance as described herein. When multiple phosphor types are used to provide broader or fuller spectral output, light from shorter wavelength phosphors can be re-absorbed by other phosphors. Patterns including isolated dots, lines, or isolated regions of each phosphor type can reduce the amount of re-absorption.  
      Multilayer emissive material structures can also reduce absorption. For example, layers of each emissive material may be formed in sequence, with the longest wavelength emitter nearest the excitation source. Light emitted nearer the emitter will, on average, undergo multiple scattering within the total emissive material to a greater extent than light emitted near the output surface of the emissive material. Since the shortest wavelength emitted is most prone to both scattering and re-absorption, it may be preferred to locate the shortest wavelength emissive material nearest the output surface of the emissive material. In addition, it may be preferred to use different thicknesses for each layer to compensate for the progressively lower intensity of the excitation light as it propagates through the multilayer structure. For emissive materials with similar absorption and emission efficiency, progressively thinner layers from excitation to output side would provide compensation for the decreasing excitation intensity in each layer. In some embodiments, one or more SP interference reflectors may be positioned between the different emissive material layers to reduce the emitted phosphor light that is scattered backward and re-absorbed by layers earlier in the sequence.  
      Non-scattering emissive materials can provide enhanced light output in combination with multilayer optical films. For example, non-scattering phosphor layers can include conventional phosphors in an index-matched binder (e.g., a binder with high index inert nanoparticles), nanosize particles of conventional phosphor compositions (e.g., where particle sizes are small and negligibly scatter light), or quantum dot emissive materials. Quantum dot emissive materials are light emitters based on semiconductors having low band gaps, e.g., cadmium sulfide, cadmium selenide or silicon, where the particles are sufficiently small so that the electronic structure is influenced and controlled by the particle size. Hence, the absorption and emission spectra are controlled via the particle size. See, e.g., U.S. Pat. No. 6,501,091 (Bawendi et al.).  
      The emissive material  40  may be positioned in any suitable location between the first interference reflector  30  and the output surface  14  of the light guide  12 . In some embodiments, the emissive material  40  may be positioned on the input surface  16  of the light guide  12 . Alternatively, the emissive material  40  may be placed within the light guide  12 . In other embodiments, the emissive material  40  may be dispersed within the light guide  12 . In other embodiments, the emissive material  40  may be positioned on an output surface  32  of the first interference reflector  30 . Any suitable technique may be used to position the emissive material  40  on the first interference reflector  30 , e.g., those techniques described in co-owned and co-pending U.S. patent application Ser. No. 10/727,023 (Ouderkirk et al.). For example, the emissive material  40  can be disposed or coated on the first interference reflector  30 . The emissive material  40  can be laminated, as a solid layer, adjacent the first interference reflector  30 . In addition, the emissive material  40  and the first interference reflector  30  can be thermoformed sequentially or simultaneously. The emissive material  40  can be compressible, elastomeric, and can even be contained in a foamed structure.  
      In some embodiments, the system  10  can also include a TIR promoting layer positioned on the emissive material  40  between the emissive material  40  and the first interference reflector  30 . The TIR promoting layer may include any suitable material or materials that provide a refractive index that is lower than the refractive index of the binder in the emissive material  40 . The TIR promoting layer may, in some embodiments, be an air gap. Such an air gap enables total internal reflection of light traversing at high incidence angles in the emissive material  40 . In other embodiments, the TIR promoting layer may be a microstructured layer having a microstructured surface. The microstructured surface can be characterized by a single set of linear v-shaped grooves or prisms, multiple intersecting sets of v-shaped grooves that define arrays of tiny pyramids, one or more sets of narrow ridges, and so forth. When the microstructured surface of such a film is placed against another flat film, air gaps are formed between the uppermost portions of the microstructured surface and the flat film.  
      Certain types of emissive materials can produce heat, for example, when converting light from a first wavelength region to a second wavelength. The presence of an air gap near the emissive material  40  may significantly reduce heat transmission from the emissive material  40  to surrounding materials. The reduced heat transfer can be compensated for in other ways, such as by providing a layer of glass or transparent ceramic near the emissive material  40  that can remove heat laterally.  
      In general, the light source  20  emits light having a first optical characteristic, at least a portion of which illuminates the first interference reflector  30 . In turn, the first interference reflector  30  substantially transmits the light from the light source  20 . At least a portion of the transmitted light illuminates the emissive material  40 . The emissive material  40  emits light having a second optical characteristic when illuminated with light having the first optical characteristic. Generally, the emissive material  40  may emit light in any direction. In other words, some light may be emitted back toward the light source  20 , and some light may be emitted toward the light guide  12 . Light emitted by the emissive material  40  that illuminates the first interference reflector  30  is substantially reflected such that the light does not reach the light source  20  where it can be absorbed. The light guide  12  directs at least a portion of the light emitted by the emissive material  40  through the output surface  14  where it can then be directed to a desired location using any suitable technique.  
      Some embodiments of illumination systems of the present disclosure may include more than one interference reflector. For example,  FIG. 2  schematically illustrates one embodiment of an illumination system  100  that includes a light guide  112  having an output surface  114  and an input surface  116 , and a light source  120 . The system  100  also includes a first interference reflector  130  positioned between the light source  120  and the output surface  114  of the light guide  112 , and emissive material  140  positioned between the first interference reflector  130  and the output surface  114 . All of the design considerations and possibilities described herein with respect to the light guide  12 , the light source  20 , the first interference reflector  30 , and the emissive material  40  of the embodiment illustrated in  FIG. 1  apply equally to the light guide  112 , the light source  120 , the first interference reflector  130 , and the emissive material  140  of the embodiment illustrated in  FIG. 2 .  
      One difference between the system  10  of  FIG. 1  and the system  100  of  FIG. 2  is that system  100  includes a second interference reflector  150  positioned such that the emissive material  140  is between the first interference reflector  130  and the second interference reflector  150 . In some embodiments, the second interference reflector  150  is a LP interference reflector, i.e., a reflector that substantially transmits light having a second optical characteristic and substantially reflects light having a first optical characteristic. For example, in some embodiments, the emissive material  140  may emit visible light (i.e., the second optical characteristic) when illuminated with UV or blue light (i.e., the first optical characteristic). In such embodiments, the second interference reflector  150  may be selected such that it substantially transmits visible light and substantially reflects UV or blue light. In other embodiments, the emissive material  140  may emit infrared light when illuminated with UV or blue light. In these embodiments, the second interference reflector  150  may be selected such that it substantially transmits infrared light and substantially reflects UV or blue light.  
      The second interference reflector  150  may include any suitable interference reflector or reflectors described herein. Further, the second interference reflector  150  may take any suitable shape, e.g., hemispherical, cylindrical, or planar.  
      The second interference reflector  150  may be positioned in any suitable location between the emissive material  140  and the output surface  114  of the light guide  112 . In some embodiments, the second interference reflector  150  may be positioned on the input surface  116  of the light guide  112 . In other embodiments, the second interference reflector  150  may be positioned within the light guide  112 . In some embodiments, the emissive material  140  may be positioned on the second interference reflector  150  as is further described, e.g., in co-owned and co-pending U.S. patent application Ser. No. 10/726,997 (Ouderkirk et al.). Alternatively, the first interference reflector  130 , emissive material  140 , and second interference reflector  150  may form an assembly where the emissive material  140  is in contact with both the first interference reflector  130  and the second interference reflector  150 . Any suitable technique may be used to form such an assembly, e.g., those techniques as described in co-owned and co-pending U.S. patent application Ser. No. 10/727,023 (Ouderkirk et al.).  
      The presence of the first interference reflector  130  and second interference reflector  150  can enhance the efficiency of the illumination system  100 . The second interference reflector  150  reflects at least a portion of the light that is not absorbed by the emissive material  140 , and that would otherwise be wasted, back into the emissive material  140 . This increases the effective path length of the light from the light source  120  through the emissive material  140 , thereby increasing the amount of light absorbed by the emissive material  140  for a given thickness of the emissive material layer or layers. The recycling of the light from the light source  120  also allows use of thinner layers of emissive material  140  for efficient light conversion.  
      In general, at least a portion of light having a first optical characteristic emitted by the light source  120  illuminates the first interference reflector  130 , which substantially transmits such light. At least a portion of light transmitted by the first interference reflector  130  illuminates the emissive material  140 . When illuminated with light having the first optical characteristic, the emissive material  140  emits light having a second optical characteristic. At least a portion of the light emitted by the emissive material  140  illuminates the second interference reflector  150 , which substantially transmits light having the second optical characteristic. At least a portion of the transmitted light enters the light guide  112  and is directed through the output surface  114  by the light guide  112 . Any light from the light source  120  that illuminates the second interference reflector  150  is substantially reflected towards the emissive material  140  where it may excite the emissive material  140  causing further light emission. In addition, light emitted by the emissive material  140  that illuminates the first interference reflector  130  is substantially reflected back toward the second interference reflector  150  and/or the light guide  112 .  
      The illumination systems of the present disclosure may include one or more optical elements. For example,  FIG. 3  schematically illustrates an illumination system  200  that includes one or more optical elements  260 . The system  200  further includes a light guide  212  having an output surface  214  and an input surface  216 , and a light source  220 . The system  200  also includes a first interference reflector  230  positioned between the light source  220  and the output surface  214  of the light guide  212 , and emissive material  240  positioned between the first interference reflector  230  and the output surface  214  of the light guide  212 . All of the design considerations and possibilities described herein with respect to the light guide  12 , the light source  20 , the first interference reflector  30 , and the emissive material  40  of the embodiment illustrated in  FIG. 1  apply equally to the light guide  212 , the light source  220 , the first interference reflector  230 , and the emissive material  240  of the embodiment illustrated in  FIG. 3 . The system  200  may also include one or more additional interference reflectors (e.g., a LP interference reflector) as is further described herein.  
      The one or more optical elements  260  may be positioned between the emissive material  240  and the output surface  214  of the light guide  212 , between the light source  220  and the first interference reflector  230 , between the first interference reflector  230  and the emissive material  240  and/or adjacent the output surface  214  of the light guide  212 . The one or more optical elements  260  can include any suitable optical element or elements, e.g., optical coupling agents such as adhesives or index matching fluids or gels, optical brightness enhancing films such as BEF (available from 3M Company), and short-wavelength absorbing materials such as ultraviolet light absorbing dyes and pigments, reflective polarizing films such as DBEF (also available from 3M Company), diffusers, and combinations thereof. In some embodiments, the one or more optical elements  260  are configured to control the angle of light emitted by the emissive material  240  that is directed into the light guide  212 .  
      In some embodiments, the one or more optical elements  260  may include one or more reflective polarizers. In general, a reflective polarizer can be disposed adjacent the emissive material  240 . The reflective polarizer allows light of a preferred polarization to be transmitted, while reflecting the other polarization. The emissive material  240  and other film components known in the art can depolarize the polarized light reflected by a reflective polarizer, and either by the reflection of the emissive material  240 , or emissive material  240  in combination with the first interference reflector  230 , light can be recycled and increase the polarized light brightness of the system  200 . Suitable reflective polarizers include, for example, cholesteric reflective polarizers, cholesteric reflective polarizers with a ¼ wave retarder, wire grid polarizers, or a variety of reflective polarizers available from 3M Company, including DBEF (i.e., a specularly reflective polarizer), and DRPF (i.e., a diffusely reflective polarizer). The reflective polarizer preferably polarizes light over a substantial range of wavelengths and angles emitted by the emissive material  240 , and in the case where the light source  220  emits blue light, may reflect the blue light as well.  
      Although the one or more optical elements  260  are illustrated in  FIG. 3  as being outside of the light guide  212 , the one or more optical elements  260  may be positioned on or inside the light guide  212 . In some embodiments, the one or more optical elements  260  may be positioned on the emissive material  240 . If a LP interference reflector is included in system  300  and positioned between the emissive material  240  and the output surface  214 , then the one or more optical elements  260  may be positioned on the LP interference reflector.  
      In some embodiments, an illumination system may include a LP reflector without a SP reflector. For example,  FIG. 4  schematically illustrates another embodiment of an illumination system  300 . The system  300  includes a light guide  312  having an output surface  314  and an input surface  316 , and a light source  320 . The system  300  further includes emissive material  340  positioned between the light source  320  and the output surface  314  of the light guide, and an interference reflector  350  positioned between the emissive material  340  and the output surface  314  of the light guide  312 . All of the design considerations and possibilities described herein with respect to the light guide  112 , the light source  120 , the emissive material  140 , and the second interference reflector  150  of the embodiment illustrated in  FIG. 2  apply equally to the light guide  312 , the light source  320 , the emissive material  340 , and the interference reflector  350  of the embodiment illustrated in  FIG. 4 .  
      Although depicted as being positioned outside the light guide  312 , the interference reflector  350  can be positioned in any suitable position between the emissive material  320  and the output surface  314  of the light guide  312 . For example, the interference reflector  350  may be positioned on the input surface 316 of the light guide  312 , or inside the light guide  312 . In some embodiments, the interference reflector  350  is positioned on the emissive material  340 .  
      Further, in some embodiments, system  300  may include one or more optical elements positioned between the light source  320  and the emissive material  340 , between the emissive material  340  and the interference reflector  350 , between the interference reflector  350  and the output surface  314  of the light guide  312 , and/or adjacent the output surface  314  of the light guide  312  (e.g., one or more optical elements  360  of  FIG. 3 ).  
      In general, the light source  320  emits light having a first optical characteristic, at least a portion of which illuminates the emissive material  340 . When illuminated with light having the first optical characteristic, the emissive material  340  emits light having a second optical characteristic. At least a portion of the light emitted by the emissive material  340  illuminates the interference reflector  350 . The interference reflector  350  substantially transmits light having the second optical characteristic and substantially reflects light having the first optical characteristic. At least a portion of the transmitted light is directed by the light guide  312  through the output surface  314  of the light guide  312 . Any light emitted by the light source 320 that is not converted by the emissive material  340  is substantially reflected by the interference reflector  350  and directed back toward the emissive material  340  where it can be converted. Light directed through the output surface  314  can be directed to a desired location using any suitable technique.  
      Some light sources that may be utilized in the illumination systems of the present disclosure emit light in a broad emission cone. For example, some LEDs emit light in a hemispherical pattern having a solid angle of 2π steradians or greater. Some embodiments of the present disclosure provide non-imaging optical devices to collect and/or direct excitation light from the light source into the light guide.  
      For example,  FIG. 5  is a schematic perspective view of one embodiment of an illumination system  400 . The system  400  is similar to the illumination system  100  of  FIG. 2 . System  400  includes a light guide  412  having an output surface  414  and an input surface  416 , and a light source  420 . The system  400  also includes a first interference reflector  430  positioned between the light source  420  and the output surface  414  of the light guide  412 , emissive material  440  positioned between the first interference reflector  430  and the output surface  414 , and a second interference reflector  450  positioned between the emissive material  440  and the output surface  414 . All of the design considerations and possibilities described herein with respect to the light guide  112 , the light source  120 , the first interference reflector  130 , the emissive material  140 , and the second interference reflector  150  of the embodiment illustrated in  FIG. 2  apply equally to the light guide  412 , the light source  420 , the first interference reflector  430 , the emissive material  440 , and the second interference reflector  450  of the embodiment illustrated in  FIG. 5 .  
      One difference between the system  100  of  FIG. 2  and the system  400  of  FIG. 5  is that system  400  also includes an optical cavity  470  optically coupled to the light source  420 , i.e., light from the light source  420  can be directed into the optical cavity  470 . When two or more devices are optically coupled, such devices are in the same optical path and can direct light to each other using any suitable technique, e.g., reflection, transmission, emission, etc. The optical cavity  470  is configured to direct light emitted by the light source  420  toward the first interference reflector  430 . The optical cavity  470  may be positioned in any suitable location. In some embodiments, the optical cavity  470  may be positioned in contact with the first interference reflector  430 . In some embodiments, a TIR promoting layer or layers may be positioned between the optical cavity  470  and the first interference reflector  430  as is further described herein.  
      The optical cavity  470  may take any suitable shape, e.g., elliptical, wedge, rectangular, trapezoidal, etc. It may be preferred that the optical cavity  470  take a parabolic shape.  
      The optical cavity  470  may be made using any suitable material or materials. In some embodiments, the optical cavity  470  may include a broadband interference reflector  472 . The broadband interference reflector  472  may be positioned on an optically clear body to form optical cavity  470 . Such an optically clear body may be made of any suitable material or materials, e.g., glass; acrylates, including polymethylmethacrylate, polystyrene, fluoropolymers; polyesters including polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and copolymers containing PET or PEN or both; polyolefins including polyethylene, polypropylene, polynorborene, polyolefins in isotactic, atactic, and syndiotactic sterioisomers, and polyolefins produced by metallocene polymerization. Other suitable polymers include polyetheretherketones and polyetherimides. In some embodiments, the broadband interference reflector  472  may be formed into the desired shape to form optical cavity  470 . The broadband interference reflector  472  may be made using any suitable material or materials and using any suitable techniques, such as those materials and techniques described, e.g., in U.S. Pat. No. 5,882,774 (Jonza et al.).  
      In some embodiments, the optical cavity  470  may be solid. Alternatively, the optical cavity  470  may be filled with any suitable medium, e.g., gas, or liquid.  
      The optical cavity  470  is formed such that the light source  420  emits light into the optical cavity  470 . Any suitable technique may be used such that light is directed into the optical cavity  470 . For example, the light source  420  may be placed within the optical cavity  470 . Alternatively, the light source  420  may be optically coupled to the optical cavity  470  via one or more openings or ports formed in the optical cavity  470 .  
      In some embodiments, the optical cavity  470  may include one or more apertures (not shown) that allow light from the light source  420  to illuminate the first interference reflector  430 . In one exemplary embodiment, the optical cavity  470  may include an elongated aperture that extends along at least a portion of the length of the optical cavity  470 . The elongated aperture may be positioned adjacent the first interference reflector  430 . In some embodiments, the optical cavity  470  may include diffusers or facets that may direct light substantially normal to a major surface of the first interference reflector  430 .  
      In general, the light source  420  emits light having a first optical characteristic, which is directed by the optical cavity  470  toward the first interference reflector  430 . The first interference reflector  430  substantially transmits light from the light source  420  such that it illuminates the emissive material  440 . At least a portion of the light that is not transmitted by the first interference reflector  430  is collected by the optical cavity  470  and redirected toward the first interference reflector  430 . When illuminated with light having the first optical characteristic, the emissive material  440  emits light having a second optical characteristic. At least a portion of light emitted by the emissive material  440  illuminates the second interference reflector  450 . Any light emitted by the emissive material  440  toward the optical cavity  470  is substantially reflected by the first interference reflector  430  back toward the emissive material  440 . Light emitted by the emissive material  440  that may be transmitted by the first interference reflector  430  is collected by the optical cavity  470  and directed back toward the first interference reflector  430 . The second interference reflector  450 , which substantially transmits light having the second optical characteristic and substantially reflects light having the first optical characteristic, substantially transmits light emitted by the emissive material  440  toward the input surface  416  of the light guide  412  where it is directed through the output surface  414  and subsequently to a desired location. Any light from the light source  420  that illuminates the second interference reflector  450  is substantially reflected back toward the emissive material  440  where it may be converted to light having the second optical characteristic.  
      Although  FIG. 5  illustrates illumination system  400  as including a first interference reflector  430 , in some embodiments, the system  400  may not include a first interference reflector. In such embodiments, the optical cavity  470  is positioned adjacent the emissive material  440  such that at least a portion of excitation light from the light source  420  illuminates the emissive material  440  without first illuminating an interference reflector.  
      Although not shown in  FIG. 5 , the illumination system  400  may also include one or more TIR promoting layers positioned adjacent one or both major surfaces of the emissive material  440  as is described, e.g., in reference to illumination system  10  of  FIG. 1 .  
      The optical cavities of the present disclosure may use any suitable technique to direct light from a light source onto an interference reflector, emissive material, or light guide. For example, FIGS.  6 (A)-(C) are schematic diagrams of another embodiment of an illumination system  500  that includes an optical cavity  570 . The system  500  also includes a light guide  512  having an output surface  514  and an input surface  516 , and a light source  520  optically coupled to the optical cavity  570 . The system  500  further includes a first interference reflector  530  positioned between the light source  520  and the output surface  514  of the light guide  512 , and emissive material  540  positioned between the first interference reflector  530  and the output surface  514  of the light guide  512 . All of the design considerations and possibilities described herein with respect to the light guide  12 , the light source  20 , the first interference reflector  30 , and the emissive material  40  of the embodiment illustrated in  FIG. 1  apply equally to the light guide  512 , the light source  520 , the first interference reflector  530 , and the emissive material  540  of the embodiment illustrated in FIGS.  6 (A)-(C). The system  500  can also include a LP interference reflector as is further described herein (e.g., second interference reflector  150  of  FIG. 2 ).  
      The optical cavity  570  includes an extended aperture (not shown) adjacent the first interference reflector  530 . The light source  520  may be optically coupled to the optical cavity  570  using any suitable technique. For example, in FIGS.  6 (A)-(C), the optical cavity  570  includes a collector  571  that collects light emitted by the light source  520  and directs it into optical cavity  570 . In some embodiments, the collector  571  also collimates the emitted light. As used herein, the term “collector” refers to a non-imaging optical device that collects light emitted by one or more light sources and directs the collected light toward emissive material or an interference reflector.  
      In some embodiments, it may be preferred that the z-dimension  502  of the optical cavity  570  has a minimum value such that etendue is preserved while also maintaining total internal reflection (TIR) at the surfaces of the optical cavity  570 . Such TIR at least in part depends on the refractive index of an interior space  574  of the optical cavity  570  and the etendue of the light source  520 . If the light source  520  includes an LED die, then, in some embodiments, the LED die is assumed to emit into 27π steradians, in which case the TIR angle for a refractive index of 1.5 is about 42°. In such embodiments, the z-dimension  502  for a 300 μm LED die is equal to (300 μm)/sin(48°)= 400  μm. If the z-dimension of the light guide  512  is 1000 μm, then the angle of incidence of the light on the first interference reflector  530  is equal to sin −1  ((300 μm)/(1000 μm))=17.5°.  
      The formula for band-edge shift with angle for a multilayer film is 
 
λ=λ(0)cos(Θ) 
 
 where Θ is the angle in the medium. The reflective band-edge shifts down by about 4%. Therefore, the blue band-edge selection for the first interference reflector  530  can be approximately 4% higher than one would choose for normal incidence. 
 
      The optical cavity  570  also includes an interior space  574 . The interior space  574  includes one or more facets  576 . Each facet  576  has a facet angle  578  that is selected such that the facets  576  direct excitation light toward the first interference reflector  530  at a substantially normal angle to a major surface of the first interference reflector  530 . Each facet  576  has a reflective face  577  that reflects light from light source  520 . Any suitable material or materials may be used to form facets  576 .  
      If the reflective surface  577  of facet  576  includes a multilayer optical film, then the minimum x-dimension  504  of the optical cavity  570 , which insures that there is little or no leakage through the facet  576  depends upon the facet angle  578 . For example, if the facet angle  578  is 45°, then some light may exceed the TIR angle at the surface  577  if the spread of light exceeds ±3°. A facet angle  578  of 45° out-couples rays at substantially normal incidence to the first interference reflector  530 . However, in some embodiments, it may not be necessary to illuminate the first interference reflector  530  at perfectly normal incidence. For example, 10° or 20° incidence can be sufficient to ensure that substantially all of the light from the light source  520  transmits through to the emissive material  540 . If the light spread ΔΘ is ±3° in the optical cavity  570 , then the x-dimension equals 5700 μm or 5.7 mm. Table 1 includes x-dimensions  504  for the optical cavity  570  given various light spread (ΔΘ) values.  
                       TABLE 1                       LED x-dimension   ΔΘ   Optical Cavity x-dimension                  300 μm    ±3°   5.7 mm       300 μm    ±5°   3.4 mm       300 μm   ±10°   1.7 mm       300 μm   ±15°   1.2 mm                  
 
      Although the optical cavity  570  is positioned adjacent the input edge  516  of the light guide  512 , the optical cavity  570  first interference reflector  530 , and emissive material  540  may positioned in any suitable location relative to the light guide  512 . For example, in some embodiments, the optical cavity  570 , first interference reflector  530 , and emissive material  540  may be positioned adjacent a major surface of the light guide  512  as is further described herein.  
      Some handheld light guides (e.g., light guides used in displays for handheld electronic devices) are about 1 mm in thickness. The slim 1 mm dimension can increase the complexity of converting and assembling the first interference reflector  530  and the emissive material  540 . If the thickness of the light guide  512  is less than 1 mm, then the embodiment schematically illustrated in  FIG. 6 (D) may be more useful. In  FIG. 6 (D), the optical cavity  570   d  is adjacent a sloped input surface  516   d , which may allow for a larger first interference reflector  530   d . The wedge formed by the light guide  512   d  provides a zone for light to expand and match the numerical aperture (NA) of the light guide  512   d . An optional second interference reflector  550   d  that substantially transmits light emitted by the emissive material  540   d  and substantially reflects light emitted by the light source  520   d  may be positioned on the output surface  514   d  and/or end of the light guide  512   d  opposite the input surface  516   d  to help prevent light that is not converted by the emissive material  540   d  from leaving the light guide  512   d.    
       FIG. 6 (E) schematically illustrates another embodiment of an illumination system  500   e  where the optical cavity  570   e  is positioned adjacent a bottom surface  518   e  of light guide  512   e . Such a design may allow for larger first interference reflectors  530   e  and emissive material  540   e  areas for smaller light guides  512   e . The system  500   e  also includes a second interference reflector  550   e  positioned on the output surface  514   e  and an end of the light guide  512   e  to prevent light that is not converted by the emissive material  540   e  from leaving the light guide  512   e.    
      Although FIGS.  6 (A)-(E) include systems having one light source, some embodiments can include two or more light sources. For example,  FIG. 7  schematically illustrates an illumination system  600  including four light sources  620 , each optically coupled to optical cavities  670 . Optical cavities  670  may include any suitable optical cavity described herein, e.g., optical cavity  570  of FIGS.  6 (A)-(C). Each optical cavity  670  is positioned adjacent an input surface  616   a  and  616   b  of the light guide  612 . The illumination system  600  may include any suitable system as described herein, e.g., illumination system  100  of  FIG. 2 . Although system  600  includes optical cavities  670  adjacent two input surfaces  616 ( a ) and  616 ( b ) of light guide  612 , the system  600  may include any suitable number of optical cavities positioned in any suitable location such that additional light sources may be provided.  
      As is also further described herein, any of the disclosed interference reflectors may be curved to aid in maintaining a substantially normal angle of incidence of light emitted by a point source onto the interference reflectors. For example, FIGS.  8 (A)-(B) schematically illustrate one embodiment of an illumination system  700  having a curved first interference reflector  730 . The illumination system  700  is similar to the illumination system  10  of  FIG. 1 . The system  700  includes a light guide  712  having an output surface  714  and an input surface  716 , and one or more light sources  720 . The system  700  further includes a first interference reflector  730  positioned between the one or more light sources  720  and the output surface  714 , and emissive material  740  positioned between the first interference reflector  730  and the output surface  714  of the light guide  712 . All of the design considerations and possibilities described herein with respect to the light guide  12 , the light source  20 , the first interference reflector  30 , and the emissive material  40  of the embodiment illustrated in  FIG. 1  apply equally to the light guide  712 , each of the one or more light sources  720 , the first interference reflector  730 , and the emissive material  740  of the embodiment illustrated in FIGS.  8 (A)-(B). The system  700  may also include optional second interference reflector  750  positioned between the emissive material  740  and the output surface  714  of the light guide  712  as is further described herein.  
      In the embodiment illustrated in FIGS.  8 (A)-(B), the one or more light sources  720  may be mounted on interconnect assembly  724 . Any suitable interconnect assembly may be used, e.g., those assemblies described in co-owned and copending U.S. patent application Ser. No. 10/727,220 (Schultz et al.).  
      The system  700  further includes one or more optical cavities  770  positioned within the light guide  712 . In the embodiment illustrated in FIGS.  8 (A)-(B), each light source  720  is associated with an optical cavity  770 . The one or more optical cavities  770  may take any suitable shape, e.g., cylindrical, hemispherical, etc. In the embodiment illustrated in FIGS.  8 (A)-(B), each optical cavity  770  is hemispheric in shape. All of the one or more optical cavities  770  may take the same shape. Alternatively, one or more optical cavities  770  may take different shapes. Further, each optical cavity  770  may be of any suitable size.  
      The optical cavities may be bounded by reflective surface  772 . Any suitable material or materials may be used to form reflective surface  772 . It may be preferred that the reflective surface  772  include a broadband interference reflector as described, e.g., in U.S. Pat. No. 5,882,774 (Jonza et al.).  
      In the embodiment illustrated in FIGS.  8 (A)-(B), the one or more optical cavities  770  are positioned in an interior space  717  of the light guide  712 . The one or more optical cavities  770  may be formed using any suitable technique. For example, the one or more optical cavities  770  may be formed as indentations in the input surface  716  of the light guide  712 . Any suitable number of optical cavities  770  may be included in the illumination system  700 . Further, although FIGS.  8 (A)-(B) illustrate optical cavities  770  on one edge of light guide  712 , the system  700  may include optical cavities  770  on two or more sides of the light guide  712  or on one or more major surfaces of the light guide  712 .  
      In some embodiments, each light source  720  may be positioned proximate a center of curvature of each optical cavity  770 . By placing the light source  720  proximate the center of curvature of each optical cavity  770 , light emitted by the light source  720  may illuminate the first interference reflector  730  substantially normal to a major surface of the first interference reflector  730 , thereby eliminating some bandedge shift. In other words, spacing the first interference reflector  730  away from the light source  720  and curving it in towards the light source  720  may help reduce the range of incident angles of light impinging on the first interference reflector  730 , thereby reducing the leakage of light through the first interference reflector  730  caused by the blue-shift effect as described herein.  
      In general, light having a first optical characteristic is emitted by the light source  720  and is substantially transmitted by the first interference reflector  730 . The transmitted light illuminates the emissive material  740 , causing the emissive material  740  to emit light having a second optical characteristic. Any light emitted by the emissive material  740  toward the light source  720  is substantially reflected by the first interference reflector  730 . Further, any light not transmitted by the first interference reflector  730  is substantially reflected by the reflective surface  772  and directed back toward the first interference reflector  730 . The light emitted by the emissive material  740  is then directed by the light guide  712  through the output surface  714  to a desired location. If an optional second interference reflector  750  is included between the emissive material  740  and the output surface  714 , it may be preferred that the second interference reflector  750  substantially transmits light having the second optical characteristic and substantially reflects light having the first optical characteristic. In such an exemplary embodiment, the light emitted by the emissive material  740  would be substantially transmitted by the second interference reflector  750  and directed by the light guide  712  through the output surface  714  to a desired location. Light emitted by the light source  720  that passes through the emissive material  740  without being absorbed is substantially reflected by the second interference reflector  750  back toward the emissive material  740 .  
      As previously described herein, some light sources of the present disclosure emit excitation light in a pattern having a solid angle of 2π steradians or greater. In some embodiments, a collector may be used to collect light emitted by a light source and collimate the collected light such that the light is directed toward an interference reflector or emissive material at substantially normal angles.  
      FIGS.  9 (A)-(B) schematically illustrate one embodiment of an illumination system  800  having one or more collectors  880 . The illumination system  800  includes a light guide  812  having an output surface  814  and an input surface  816 , and a light source  820 . In the embodiment illustrated in FIGS.  9 (A)-(B), the light source  820  includes one or more LEDs  822  optionally mounted on an interconnect assembly  824  as is further described herein. The system  800  also includes a first interference reflector  830  positioned between the light source  820  and the output surface  814 , and emissive material  840  positioned between the first interference reflector  830  and the output surface  814  of the light guide  812 . All of the design considerations and possibilities described herein with respect to the light guide  12 , the light source  20 , the first interference reflector  30 , and the emissive material  40  of the embodiment illustrated in  FIG. 1  apply equally to the light guide  812 , the light source  820 , the first interference reflector  830 , and the emissive material  840  of the embodiment illustrated in FIGS.  9 (A)-(B). Although not shown, the system  800  may also include an optional LP interference reflector between the emissive material  840  and the output surface  814  as previously described herein.  
      One difference between the illumination system  800  of FIGS.  9 (A)-(B) and the illumination system  10  of  FIG. 1  is that each LED  822  is associated with a collector  880 . Each collector  880  forms an optical cavity  882  that directs light emitted by the LED  822  toward the first interference reflector  830 . Each collector  880  may take any suitable shape, e.g., spherical, parabolic, or elliptical. It may be preferred that each collector  880  take a shape that allows for collimation of the light emitted by the light source  820 . Further, it may be preferred that each collector  880  be shaped such that it collects the light emitted by the LED  822  and directs the light toward the first interference reflector  830  such that the excitation light is incident upon the first interference reflector  830  at an angle that is substantially normal to a major surface of the first interference reflector  830 . The collectors  880  can reduce the angular spread of light impinging on the first interference reflector  830 , thus reducing the blue-shift of the reflection band as is further described herein. Each collector  880  may be in the form of simple conical sections with flat sidewalls, or the sidewalls can take on a more complex curved shape as is known to enhance collimation or focusing action depending on the direction of light travel. It may be preferred that the sidewalls of the collectors  880  are reflective and the two ends are not. It may also be preferred that the collector&#39;s sidewalls include a broadband interference reflector as is further described herein. Each collector  880  may be positioned in any suitable relationship to the first interference reflector  830 . For example, each collector  880  may be spaced apart from the first interference reflector  830 . Alternatively, one or more collectors  880  may be in contact with the first interference reflector  830 .  
      Although the system  800  is illustrated as having a light source  820  positioned adjacent one input surface  816  of light guide  812 , the system  800  can include two or more light sources positioned adjacent two or more input surfaces of the light guide  812 .  
      Any suitable device or technique may be used with the embodiments of the present disclosure to direct light from a light source toward an interference reflector such that the light is incident upon the interference reflector at substantially normal angles. For example, FIGS.  10 (A)-(B) schematically illustrate one embodiment of an illumination system  900  that includes an optical cavity  970  having collectors  980  formed in the optical cavity  970 . The system  900  includes light source  920 . The light source  920  includes one or more LEDs  922 . In this embodiment, the light source  920  is positioned adjacent an input surface  916  of light guide  912 . The system  900  further includes a first interference reflector  930  positioned between the light source  920  and the output surface  914 , and emissive material  940  positioned between the first interference reflector  930  and the output surface  914 . All of the design considerations and possibilities in regard to the light guide  12 , the light source  20 , the first interference reflector  30 , and the emissive material  40  of the embodiment illustrated in  FIG. 1  apply equally to the light guide  912 , the light source  920 , the first interference reflector  930 , and the emissive material  940  of the embodiment illustrated in FIGS.  10 (A)-(B). The system  900  may also include a LP interference reflector, e.g., second interference reflector  150  of  FIG. 2 .  
      The optical cavity  970  is positioned to direct light emitted by the light source  920  into the light guide  912 . The optical cavity  970  includes collectors  980  formed in the optical cavity  970 . Each LED  922  has a corresponding collector  980 . In some embodiments, two or more LEDs  922  may be positioned within a single collector  980 . The collectors  980  may each take any suitable shape, e.g., hemispherical, parabolical, or cylindrical. In FIGS.  10 (A)-(B), the collectors  980  are shaped as a two-dimensional conic sections. It may be preferred that each collector  980  is shaped such that light emitted by each LED  922  illuminates the first interference reflector  930  at substantially normal angles to a major surface of the first interference reflector  930 . The collectors  980  collect light emitted by the LEDs  922  and direct the collected light such that it illuminates the first interference reflector  930 . Further, it may be preferred that one or more LEDs  922  be positioned proximate a focus of one or more collectors  980 .  
      Any suitable technique may be used to form optical cavity  970  and collectors  980 . In some embodiments, the LEDs  922  may be potted with a flat slab encapsulant and index-matched to the optical cavity  970 . Further, the first interference reflector  930  and the emissive material  940  may be optically coupled to the optical cavity  970  using any suitable material or materials, e.g., optical adhesives, etc. It may be preferred that a TIR promoting layer may be positioned between the optical cavity  970  and the light guide  912  for better NA match of the light emitted by the emissive material  940  into the light guide  912 .  
      In some embodiments, the LEDs  922  may be mounted on an interconnect assembly  924 . Any suitable interconnect assembly may be used, e.g., those interconnect assemblies described in co-owned and copending U.S. patent application Ser. No. 10,727,220 (Schultz et al.). In an exemplary embodiment, the optical cavity  970  may be formed on interconnect assembly  924  using any suitable technique.  
      The collectors  980  may include a reflective inner surface such that light emitted by each LED  922  is reflected toward the first interference reflector  930 . It may be preferred that one or more collectors  980  include a broadband interference reflector positioned in the collector  980  to reflect light toward the first interference reflector  930 .  
      As previously described herein, the interference reflectors and emissive material of the present disclosure may be positioned in any suitable relationship to the light guide. For example, the first interference reflector  30  and emissive material  40  of illumination system  10  of  FIG. 1  are positioned adjacent the input surface  16  of the light guide  12 . In some embodiments, conversion of light can occur adjacent an output surface of a light guide. In other words, light from a light source may be directed by a light guide through an output surface of the light guide and subsequently converted by emissive material positioned on or adjacent the output surface of the light guide. Depending upon the types of light sources and interference reflectors selected, positioning the emissive material and interference reflectors a distance from the light source may prevent damage to the emissive material and/or the interference reflectors.  
      For example, polymeric interference reflectors can be degraded by overheating which can cause material creep thereby changing the layer thickness values and therefore the optical characteristics of the light (e.g., wavelength) that the reflector reflects. In the worst case, overheating can cause the polymer materials to melt, resulting in rapid flow of material and change in optical characteristic selection as well as inducing non-uniformities in the filter.  
      Degradation of polymer materials can also be induced by short wavelength (actinic) radiation such as blue, violet, or UV radiation, depending on the polymer material. The rate of degradation is dependent both on the actinic light flux and on the temperature of the polymer. Both the temperature and the flux will, in general, decrease with increasing distance from the light source. Thus it is advantageous in cases of high brightness light sources, particularly UV emitting light sources, to place polymeric interference reflectors as far from the light source as the design can allow.  
       FIG. 11  schematically illustrates one embodiment of an illumination system  1000  including a light guide  1012  having an output surface  1014  and an input surface  1016 , and a light source  1020 . The light source  1020  emits light having a first optical characteristic. The system  1000  also includes emissive material  1040  positioned to receive light from the output surface  1014  of the light guide  1012 , and a first interference reflector  1030  positioned between the emissive material  1040  and the output surface  1014  of the light guide  1012 . The emissive material  1040  emits light having a second optical characteristic when illuminated with light having the first optical characteristic. The first interference reflector  1030  substantially transmits light having the first optical characteristic and substantially reflects light having the second optical characteristic. All of the design considerations and possibilities described herein with respect to the light guide  12 , the light source  20 , the first interference reflector  30 , and the emissive material  40  of the embodiment illustrated in  FIG. 1  apply equally to the light guide  1012 , the light source  1020 , the first interference reflector  1030 , and the emissive material  1040  of the embodiment illustrated in  FIG. 11 . The system  1000  may also include a second interference reflector  1050  positioned such that the emissive material  1040  is between the second interference reflector  1050  and the first interference reflector  1030 . Any suitable interference reflector described herein may be utilized for the second interference reflector  1050  (e.g., second interference reflector  150  of  FIG. 2 ). The second interference reflector  1050  may help to prevent some or all of the light emitted by the light source  1020  from reaching a viewer facing the output surface  1014  of the light guide  1012 . The second interference reflector  1050  may be positioned in any suitable location. In some embodiments, the second interference reflector  1050  may be positioned on and in contact with the emissive material  1040 .  
      The first interference reflector  1030  may be positioned adjacent the output surface 1014, on the output surface  1014 , on the emissive material  1040 , or in any other suitable location. In one exemplary embodiment, the first interference reflector  1030  may be on and in contact with both the emissive material  1040  and the output surface  1014  of the light guide 1012. In some embodiments, system  1000  may also include one or more TIR promoting layers between the output surface  1014  and the first interference reflector  1030 , and/or an extraction device or devices on output surface  1014  to extract light from the light guide  1012 . Any suitable extraction device may be utilized.  
      In some embodiments, an extraction device or devices may be included adjacent a bottom surface  1018  of the light guide  1012  to direct at least a portion of light within the light guide  1012  though the output surface  1014 . Any suitable extraction device or devices may be utilized.  
      In some embodiments, the illumination system  1000  may include a TIR promoting layer in contact with the emissive material  1040  between the first interference reflector  1030  and the emissive material  1040 . It may be preferred that the TIR promoting layer include an index of refraction at the wavelength of light emitted by the light source  1020  that is less than the index of refraction of the emissive material  1040 . Any suitable material or materials may be used for the TIR promoting layer. The TIR promoting layer may include an air gap; alternatively, the TIR promoting layer may include a microstructured layer.  
      A second TIR promoting layer may be positioned in contact with the emissive material  1040  between the emissive material  1040  and the optional second interference reflector  1050 . It may be preferred that the second TIR promoting layer include an index of refraction at the wavelength of light emitted by the light source  1020  that is less than the index of refraction of the emissive material  1040 .  
      Although not shown, the system  1000  may include one or more optical elements positioned to receive light emitted by the emissive material  1040 . Alternatively, the one or more optical elements may be positioned between the output surface  1014  and the first interference reflector  1030 , and/or between the light source  1020  and the output surface  1014  of the light guide  1014 . If a second interference reflector  1050  is included, then one or more optical elements may be positioned between the emissive material  1040  and the second interference reflector 1050, and/or such that the second interference reflector is between the emissive material  1040  and the one or more optical elements. The one or more optical elements may include any suitable optical element as is further described herein.  
      In general, light having a first optical characteristic is emitted by the light source  1020 , at least a portion of which enters the light guide  1012  and is directed through the output surface  1014 . At least a portion of the light from the light guide  1012  illuminates the first interference reflector  1030  and is substantially transmitted. At least a portion of the transmitted light illuminates the emissive material  1040 , thereby causing the emissive material  1040  to emit light having a second optical characteristic. Light emitted by the emissive material  1040  can then be directed to a desired location using any suitable technique. Any light emitted by the emissive material  1040  toward the first interference reflector  1030  is substantially reflected back toward the emissive material. If a second interference reflector  1050  is included in system  1000 , then light emitted by the emissive material  1040  that illuminates the second interference reflector  1050  is substantially transmitted and directed to a desired location. Any light emitted by the light source  1020  that illuminates the second interference reflector  1050  is substantially reflected back toward the emissive material  1040  where it may be converted to light having the second optical characteristic.  
      Alternatively, some embodiments of illumination systems of the present disclosure may include a LP interference reflector and no SP interference reflector. For example,  FIG. 17  schematically illustrates an embodiment of an illumination system  1600  that includes a light source  1620  and a light guide  1612  having an output surface  1614  and an input surface  1616 . The light source  1620  emits light having a first optical characteristic. The system  1600  also includes emissive material  1640  positioned to receive light from the output surface  1614  and an interference reflector  1650  positioned such that the emissive material  1640  is between the output surface  1614  and the interference reflector  1650 . The emissive material  1640  emits light having a second optical characteristic when illuminated with light having the first optical characteristic. In this exemplary embodiment, the interference reflector  1650  substantially transmits light having the second optical characteristic and substantially reflects light having the first optical characteristic. All of the design considerations and possibilities described herein with respect to the light guide  1012 , the light source  1020 , the emissive material  1040 , and the second interference reflector  1050  of the embodiment illustrated in  FIG. 11  apply equally to the light guide  1612 , the light source  1620 , the emissive material  1640 , and the interference reflector  1650  of the embodiment illustrated in  FIG. 17 . The illumination system  1600  may also include other elements as described in reference to illumination system  1000  of  FIG. 11 , e.g., one or more optical elements, TIR promoting layers, etc.  
      In general, light having a first optical characteristic is emitted by the light source  1620 , at least a portion of which enters the light guide  1612  and is directed through the output surface  1614 . At least a portion of the light from the light guide  1612  illuminates the emissive material  1640 , thereby causing the emissive material  1640  to emit light having a second optical characteristic. At least a portion of the light emitted by the emissive material  1640  is substantially transmitted by the interference reflector  1650  and directed to a desired location using any suitable technique. Any light emitted by the light source  1020  that illuminates the interference reflector  1050  is substantially reflected back toward the emissive material  1040  where it may be converted to light having the second optical characteristic.  
       FIG. 12  schematically illustrates another embodiment of an illumination system  1100 . The system  1100  includes a light guide  1112  having an output surface  1114  and an input surface  1116 , and a light source  1120 . The light source  1120  emits light having a first optical characteristic. The system  1100  further includes a first interference reflector  1130  positioned adjacent the output surface  1114 . The first interference reflector  1130  substantially transmits light having the first optical characteristic and substantially reflects light having a second optical characteristic. The first interference reflector  1130  includes indentations  1134  formed in a first major surface  1132  of the first interference reflector  1130 . The system  1100  also includes emissive material  1140  positioned to receive excitation light from the output surface  1114  of the light guide  1112 . The system  1100  may also include an optional LP interference reflector (not shown) positioned such that the emissive material  1140  is between the LP interference reflector and the first interference reflector  1130 . All of the design consideration and possibilities in regard to the light guide  112 , the light source  120 , the first interference reflector  130 , the emissive material  140 , and the second interference reflector  150  of the embodiment illustrated in  FIG. 2  apply equally to the light guide  1112 , the light source  1120 , the first interference reflector  1130 , the emissive material  1140 , and the optional LP interference reflector of the embodiment of  FIG. 12 .  
      The emissive material  1140  includes dots  1142  that are positioned within indentations  1134  formed in the major surface  1132  of the interference reflector  1130 . Each phosphor dot can have any suitable size. For example, each dot can have an area in plan view of less than 10000 μm 2  or from  500  to 10000 μm 2 . In an illustrative embodiment, the dots can each be formed from emissive material that emits light having the second optical characteristic when illuminated with light having the first optical characteristic. In some embodiments, the emissive material  1140  includes one or more dots that emit one or more emitted wavelengths of visible light, e.g., a dot emitting red, a dot emitting blue, and a dot emitting green. For example, phosphor dot  1142 R may emit red light when illuminated with light from the light source  1120 , phosphor dot  1142 G may emit green light, and phosphor dot  1142 B may emit blue light.  
      The dots  1142  may be arranged and configured in any uniform or non-uniform manner as desired. For example, the emissive material  1140  can be a number of dots with a non-uniform density gradient along a surface or an area. The dots can have any regular or irregular shape and need not be round in plan view.  
      In general, structured phosphor layers, e.g., dots, can be configured in several ways to provide benefits in performance as described herein. When various types of emissive materials are used (e.g., red emitters, green emitters, etc.), light emitted from shorter wavelength emissive materials can be re-absorbed by other emissive materials. Patterns including isolated dots, lines, or isolated regions of each type can reduce the amount of reabsorption.  
      Any suitable technique may be used to provide indentations  1134  in the major surface  1132  of interference reflector  1030 , e.g., thermoforming, embossing, knurling, laser marking or ablating, abrading, cast and cure, etc. Alternatively, the first interference reflector  1030  may be thermoformed to provide reflective wells or pockets within which emissive material  1140  may be placed. The indentations  1134  may be formed in any pattern. Each indentation  1134  may have any suitable depth. It may be preferred that each indentation  1134  be relatively shallow such that the first interference reflector  1130  is not excessively thinned. Such thinning may cause a large wavelength shift due to thickness or angle effects.  
      Although  FIG. 12  illustrates the emissive material  1140  as including dots  1142 , the emissive material  1140  may be formed in any suitable shape and/or pattern, e.g., lines, discrete shapes, or half-tones patterned in graded density and/or size.  
      The light guides of the present disclosure can take any suitable shape. For example,  FIG. 13  is a schematic diagram of another embodiment of an illumination system  1200 . The system  1200  is similar in many respects to the illumination system  1000  of  FIG. 11 . The illumination system  1200  includes a light guide  1212  having an output surface  1214  and an input surface  1216 , and a light source  1220 . The system  1200  also includes emissive material  1240  positioned to receive light emitted by the light source  1220  from the output surface  1214  of the light guide  1212 , and a first interference reflector  1230  positioned between the emissive material  1240  and the output surface  1214 . All of the design considerations and possibilities described herein with respect to the light guide  1012 , the light source  1020 , the first interference reflector  1030 , and the emissive material  1040  of the embodiment illustrated in  FIG. 11  apply equally to the light guide  1212 , the light source  1220 , the first interference reflector  1230 , and the emissive material  1240  of the embodiment illustrated in  FIG. 13 . The system  1200  may also include a LP interference reflector (e.g., second interference reflector  1050  of  FIG. 11 ) positioned such that the emissive material  1240  is between the first interference reflector  1230  and the LP interference reflector.  
      The system  1200  also includes an optical cavity  1270  optically coupled to the light source  1220  that directs excitation light from the light source  1220  into the light guide  1212 . Any suitable optical cavity  1270  may be used, e.g., optical cavity  470  of the embodiment illustrated in  FIG. 5 .  
      The light guide  1212  also includes a reflective bottom surface  1218  that forms an angle with the output surface  1214  such that the light guide  1212  takes a wedge-like shape that tapers distal from the input surface  1216 . The reflective bottom surface  1218  may include any suitable reflective material or materials. It may be preferred that the reflective bottom surface  1218  include a broadband interference reflector  1290  as described, e.g., in U.S. Pat. No. 5,882,774 (Jonza et al.). The broadband interference reflector  1290  may be in contact with or spaced from bottom surface  1218 .  
      In some embodiments, a TIR promoting layer may be positioned between the output surface  1214  and the first interference reflector  1230 , and/or between the first interference reflector  1230  and the emissive material  1240  as is previously described herein.  
      The use of a wedged light guide  1212  may provide substantially normal incidence of the light emitted by the light source  1220  on the first interference reflector  1230 , thereby allowing transmission of substantially all of the light on the first pass towards the first interference reflector  1230 . In embodiments where there is a TIR promoting layer between the output surface  1214  and the first interference reflector  1230 , light directed toward the output surface  1214  from within the light guide  1212  at oblique angles may be directed back into the light guide  1212  by the TIR promoting layer. Such redirected light may then be reflected by the reflective bottom surface  1218  and directed through the output surface  1214  at substantially normal incidence to the output surface  1214 . Some light within the light guide  1212  may be directed through the input surface  1216  toward the light source  1220 . Such light may be collected by optical cavity  1270  and redirected into the light guide  1212  through the input surface  1216 .  
      In general, the light source  1220  emits light having a first optical characteristic that is directed into the light guide  1212  by the optical cavity  1270 . At least a portion of light is directed by the light guide  1212  and/or the reflective bottom surface  1218  of the light guide  1212  through the output surface  1214  such that it illuminates the first interference reflector  1230 . The first interference reflector  1230  substantially transmits light having the first optical characteristic onto the emissive material  1240 . When illuminated with light having the first optical characteristic, the emissive material  1240  emits light having a second optical characteristic. Some light may be emitted by the emissive material  1240  back toward the output surface  1214  of the light guide  1212 . The first interference reflector  1230  may substantially reflect such light back away from the output surface  1214 .  
      The illumination systems of the present disclosure may include any suitable type of light guide or guides. For example,  FIG. 14  schematically illustrates one embodiment of an illumination system  1300  that includes light guides  1312  each having an input surface  1316  and an output surface  1314 , and a light source  1320 . The light guides  1312  are optically coupled to the light source  1320 . The light source  1320  emits light having a first optical characteristic. The system further includes emissive material  1340  positioned to receive light from at least one light guide  1312 , and a first interference reflector  1330  positioned between the emissive material  1340  and the output surfaces  1314  of the light guides  1312 . The first interference reflector  1330  substantially transmits light having the first optical characteristic and substantially reflects light having a second optical characteristic. The emissive material  1340  emits light having the second optical characteristic when illuminated with light having the first optical characteristic. The system  1300  may also include an optional second interference reflector 1350 positioned such that the emissive material  1340  is between the second interference reflector  1350  and the first interference reflector  1330 . All of the design considerations and possibilities in regard to the light guide  112 , the light source  120 , the first interference reflector  130 , the emissive material  140 , and the second interference reflector  150  of the embodiment illustrated in  FIG. 2  apply equally to the light guides  1312 , the light source  1320 , the first interference reflector  1330 , the emissive material  1340 , and the optional second interference reflector  1350  of the embodiment illustrated in  FIG. 14 .  
      In some embodiments, the light guides  1312  may include one or more optical fibers  1313 . The optical fibers  1313  may include any suitable type of optical fibers, e.g., large-core polymer clad silica fibers (such as those marketed under the trade designation TECS™, available from 3M Company, St. Paul, Minn.), glass fibers, plastic core optical fibers, etc.  
      The optical fibers  1313  are optically coupled to the light source  1320 . As previously described herein, the light source  1320  may include any suitable type of light source or sources. In some embodiments, the light source  1320  may include discrete LED dies or chips disposed in an array pattern. Further, in some embodiments, the illumination system  1300  can include one optical fiber  1313  for each light source  1320 .  
      Any suitable technique may be used to couple light emitted by the light source  1320  into the light guides  1312 . For example, the illumination system  1300  may include one or more collectors that can convert isotropic emission from a corresponding LED die into a beam that will meet the acceptance angle criteria of a corresponding light-receiving light guide as described in the following co-owned and copending patent applications: U.S. patent application Ser. No. 10/726,222 (Henson et al.); U.S. patent application Ser. No. 10/726,244 (Simbal); U.S. patent application Ser. No. 10/726,248; U.S. patent application Ser. No. 10/727,220 (Schultz et al.); U.S. patent application Ser. No. 10/726,225 (Henson et al.); U.S. patent application Ser. No. 10/726,257 (Aguirre et al.); and U.S. patent application Ser. No. 10/739,792 (Ouderkirk et al.).  
      The emissive material  1340  as well as the first interference reflector  1330  and/or second interference reflector  1350  may take any suitable shape as is further described herein. In some embodiments, the emissive material  1340  and one or both interference reflectors  1330  and  1350  may be in the form of a continuous layer or layers. In other embodiments, the emissive material  1340  and one or both of the interference reflectors  1330  and  1350  may be curved. Further, in some embodiments, the emissive material  1340  and one or both interference reflectors  1330  and  1350  may be non-continuous segments that are formed on and in contact with one or more output surfaces  1314  of the light guides  1312 .  
      The emissive material  1340  may be positioned in any suitable relationship to the output surfaces  1314  of the light guides  1312 . In some embodiments, the emissive material  1340  may be spaced apart from the output surfaces  1314 . In other embodiments, the emissive material  1340  may be positioned on one or both of the first interference reflector  1330  and the optional second interference reflector  1350 . In other embodiments, a TIR promoting layer or layers may be positioned on the emissive material  1340  between the emissive material  1340  and the first interference reflector  1330 , between the emissive material  1340  and the optional second interference reflector 1350, or on both sides of the emissive material  1340  as is further described herein. See also U.S. patent application Ser. No. 10/762,724 (Ouderkirk et al.).  
      The first interference reflector  1330  may be positioned in any suitable location relative to the output surfaces  1314  and the emissive material  1340 , e.g., spaced apart from the output surfaces  1314 , spaced apart from the emissive material  1340 , on the output surfaces  1314 , on the emissive material  1340 , on both the output surfaces  1314  and the emissive material  1340 , etc. In some embodiments, a TIR promoting layer or layers may be included between the output ends  1314  and the first interference reflector  1330 . Further, in some embodiments, the output surfaces  1314  of the light guides  1312  and the first interference reflector  1330  may be index-matched using any suitable technique or materials, e.g., using index matching fluids, gels., adhesives, pressure sensitive adhesives, UV cured adhesives, or cements.  
      The illumination system  1300  may also include one or more optical elements  1360 . The one or more optical elements 1360 may be positioned to receive light from the emissive material  1340 , between the emissive material  1340  and the first interference reflector  1330 , and/or between the output surfaces  1314  of the light guides  1312  and the first interference reflector  1330 . The one or more optical elements  1360  may include collimating optics for directing light within a predetermined angle toward a display or other device. For example, the one or more optical elements  1360  may include brightness enhancement films, turning films, lenses, diffusers, gain diffusers, contrast-enhancing materials, reflective elements, etc. In some embodiments, the one or more optical elements  1360  may include a walk-off plate or crystal to provide a more uniform light distribution. Walk-off plates or crystals include layers that separate a ray of light into two rays that are displaced from each other, where such displacement results from the two polarization states of a light ray that each encounter different degree of refraction when impinging upon the walk-off crystal. Typical walk-off plates are made from a material that has different refractive indices for different polarizations of light (i.e., birefringence). Typically, the high refractive index direction is skewed from at least one of the in-plane axes of the plate.  
      In some embodiments, the one or more optical elements  1360  may include a reflective polarizer that allows light of a preferred polarization to be emitted by the system  1300 , while reflecting the other polarization. Any suitable reflective polarizer may be utilized, e.g., cholesteric reflective polarizers, cholesteric reflective polarizers with a ¼ wave retarder, wire grid polarizers, and a variety of reflective polarizers available from 3M Company, including DBEF (i.e., a specularly reflective polarizer), DRPF (i.e., a diffusely reflective polarizer). Light reflected by the reflective polarizer  1360  can be depolarized by the emissive material  1340 , and/or the interference reflectors  1330  and  1350  and recycled such that light of the selected polarization can be emitted with greater efficiency.  
      In general, light from the light source  1320  illuminates the input surfaces  1316  of the light guides  1312  and is directed by the light guides  1312  through the output surfaces  1314  where at least a portion of such light illuminates the first interference reflector  1330 . The first interference reflector  1330  substantially transmits the light from the light source  1320  such that at least a portion of the light illuminates the emissive material  1340 .  
      The emissive material  1340  emits light having the second optical characteristic when illuminated with light having the first optical characteristic. For example, the emissive material  1340  may be selected such that it emits visible light when illuminated with UV or blue light from the light source  1320 . At least a portion of the light emitted by the emissive material  1340  illuminates the optional second interference reflector  1350 , which substantially transmits such light. Any light from the light source  1320  that is not converted by the emissive material  1340  is substantially reflected by the optional second interference reflector  1350  back toward the emissive material  1340 . Further, any light emitted by the emissive material  1340  that illuminates the first interference reflector  1330  is substantially reflected.  
      As previously mentioned herein, any suitable technique may be used to couple light from the light source  1320  into the light guides  1312 . For example,  FIG. 15  schematically illustrates another embodiment of an illumination system  1400  that includes light guides  1412  including optical fibers  1413 . See, e.g., co-owned and copending U.S. patent application Ser. No. 10/726,222 (Henson et al.). The system  1400  includes a light source  1420 , emissive material  1440  positioned to receive light from the light source  1420 , and a first interference reflector  1430  positioned between the light guides  1412  and the emissive material  1440 . All of the design considerations and possibilities in regard to the light guide  1312 , the light source  1320 , the first interference reflector  1330 , and the emissive material  1340  of the embodiment illustrated in  FIG. 14  apply equally to the light guide  1412 , the light source  1420 , the first interference reflector  1430 , and the emissive material  1440  of the embodiment illustrated in  FIG. 15 . The system  1400  may also include a second interference reflector (not shown) as is further described herein.  
      Light source  1420  includes an array  1422  of LED dies  1424  that are positioned in optical alignment with an array of optical elements  1428 , which can include passive optical elements, such as focusing lenses  1429  or optical concentrating elements, such as reflectors. The array of optical elements  1428  are in turn optically aligned to an array of optical fibers  1413 . The array of optical fibers  1413  can be connectorized, where the connectorization can include a connector  1417  to support and/or house input surfaces  1416  of fibers  1413 . The connectorization can also include a connector  1415  to support and/or house output surfaces  1414  of fibers  1413 . Any suitable connector or connectors may be used at either the input surfaces  1416  or output surfaces  1414  of the optical fibers  1413 , e.g., those described in U.S. Pat. No. 10/726,222 (Henson et al.). As would be apparent to one of ordinary skill in the art given the present description, the output surfaces  1414  of the fibers  1413  may be bundled to form a point-like source or a shaped-array, such as a linear array, circular array, hexagonal array, or other shaped-array.  
      In an exemplary embodiment, the array  1422  of light source  1420  includes an array of discrete LEDs  1424 , such as an array of single LED dies or chips, which are mounted individually and have independent electrical connections for operational control (rather than an LED array where all the LEDs are connected to each other by their common semiconductor substrate). LED dies can produce a symmetrical radiation pattern, making them desirable light sources for the present disclosure. LED dies are efficient at converting electrical energy to light and are not as temperature sensitive as most laser diodes. Therefore, LED dies may operate adequately with only a modest heat sink compared to many types of laser diodes. In an exemplary embodiment, each LED die  1424  is spaced apart from its nearest neighbor(s) by at least a distance greater than an LED die width.  
      In addition, LED dies can be operated at a temperature from −40 to 125° C. and can have operating lifetimes in the range of 100,000 hours, as compared to most laser diode lifetimes around 10,000 hours or halogen automobile headlamp lifetimes of 500-1000 hours. In an exemplary embodiment, the LED dies  1424  can each have an output intensity of about 50 Lumens or more. Discrete high-power LED dies are commercially available from companies such as Cree and Osram. In one exemplary embodiment, an array of LED dies  1424  (manufactured by Cree), each having an emitting area of about 300 μm×300 μm, can be used to provide a concentrated (small area, high power) light source. Other light emitting surface shapes such as rectangular or other polygonal shapes can also be utilized. In addition, in alternative embodiments, the emission layer of the LED dies  1424  utilized can be located on the top or bottom surface.  
      In an alternative embodiment, the array  1422  may be replaced with a white vertical cavity surface emitting laser (VCSEL) array. The passive optical element array  1428  may be used to redirect that light emitted from each VCSEL into a corresponding fiber  1413 .  
      An aspect of the illustrated embodiment of  FIG. 15  is the one-to-one correspondence between each light source  1412 , a corresponding passive optical element of the array of optical elements  1428  (lens, focusing, concentrating, or reflective element), and a corresponding optical fiber  1413 . When powered, each LED die  1424  acts as an individual light source that launches light into a corresponding fiber  1413 . The present exemplary embodiment includes large-core (for example, 400 μm to 1000 μm) polymer clad silica fibers (such as those marketed under the trade designation TECS™, available from 3M Company, St. Paul, Minn.). Other types of optical fibers, such as conventional or specialized glass fibers may also be utilized in accordance with the embodiments of the present disclosure, depending on such parameters, e.g., as the output wavelength(s) of the LED dies  1424 .  
      In addition, as would be apparent to one of ordinary skill given the present description, other waveguide types, such as planar waveguides, polymer waveguides, or the like, may also be utilized in accordance with the present teachings.  
      Optical fibers  1413  may further include fiber lenses on each of the output surfaces  1414  of the optical fibers  1413 . Similarly, the input surfaces  1416  of the optical fibers  1413  may each further include a fiber lens. Fiber lens manufacture and implementation is described in co-owned and copending U.S. patent application Ser. Nos. 10/317,734 (Smithson et al.) and 10/670,630 (Jennings et al.).  
      The individual optical fibers  1413  are collected together to provide remote lighting at a distance from the original light sources. A further description of an LED-based lighting assembly that is implanted as a bulb replacement is described in co-owned and copending U.S. patent application Ser. No. 10/726,225 (Henson et al.).  
      In some embodiments, the LED dies  1424  may be independently controllable such that one or more LEDs  1424  can be selectively activated. For example, the system  1400  may include a controller (not shown) that is in electrical communication with each LED  1424 . The controller is operable to selectively activate one or more LEDs  1424 . Any suitable controller or controllers may be used, e.g., those described in co-owned and copending U.S. patent application Ser. No. 10/726,222 (Henson et al.). Such controllable output of the LEDs  1424  may be used in various types of applications, e.g., steerable headlamps for motor vehicles, pixilated displays, projection systems, signs, etc.  
      In general, light having a first optical characteristic is emitted by one or more LEDs  1424  of the light source  1420 , such light is directed into one or more optical fibers  1413  through their input surfaces  1416  by optical elements  1428 . The light is directed by the optical fibers  1413  through their output surfaces  1414  and illuminates the first interference reflector  1430 . The first interference reflector  1430  substantially transmits the light such that it illuminates emissive material  1440 . The emissive material  1440  converts at least a portion of the light from the light source  1420  into light having a second optical characteristic. Light emitted by the emissive material  1440  that is directed toward the first interference reflector  1430  is substantially reflected by the first interference reflector  1430 . If a LP interference reflector (e.g., second interference reflector  150  of  FIG. 2 ) is included in system  1400 , then the light emitted by the emissive material  1440  is substantially transmitted by the LP interference reflector. Any light from the light source  1420  that illuminates the LP interference reflector is substantially reflected back toward the emissive material  1440  where it may then be converted into light having the second optical characteristic. Light emitted by the emissive material  1440  and/or transmitted by the optional LP interference reflector can then be directed to a desired location using any suitable technique.  
      In some embodiments, the illumination systems  1300  of  FIG. 14  and  1400  of  FIG. 15  may include a LP interference reflector and no SP interference reflector. For example,  FIG. 18  schematically illustrates an illumination system  1700  that includes an interference reflector  1750  positioned to receive light from emissive material  1740 . The system  1700  also includes a light source  1720 , and light guides  1712  optically coupled to the light source  1720 . All of the design considerations and possibilities in regard to the light guides  1312 , the light source  1320 , the emissive material  1340 , and the optional second interference reflector  1350  of the embodiment illustrated in  FIG. 14  apply equally to the light guides  1712 , the light source  1720 , the emissive material  1740 , and the interference reflector  1750  of the embodiment illustrated in  FIG. 18 . The system  1700  may include other features similar to those described in respect to illumination system  1300  of  FIG. 14 , e.g., one or more optical elements, TIR promoting layers, etc.  
      In general, the light source  1720  emits light having a first optical characteristic. Such light illuminates input surfaces  1316  of light guides  1312  and is directed by the light guides  1712  through the output surfaces  1714  where at least a portion of such light illuminates the emissive material  1740 . The emissive material  1740  emits light having a second optical characteristic when illuminated with light having the first optical characteristic. At least a portion of the light emitted by the emissive material  1740  illuminates the interference reflector  1740 , which substantially transmits light having the second optical characteristic and substantially reflects light having the first optical characteristic. The substantially transmitted light is then directed to a desired location using any suitable technique.  
      The illumination systems of the present disclosure may be used in any suitable manner for providing illumination. For example, some or all of the illumination systems described herein may be used to provide illumination for displays.  FIG. 16  schematically illustrates a display assembly  1500  that includes an illumination system  1510  optically coupled to a display device  1512 . The illumination system  1510  may include any illumination system described herein, e.g., illumination system  10  of  FIG. 1 . The illumination system  1510  provides illumination light to the display device  1512 . The display device  1512  may be any suitable display device, e.g., LCD, electrochromatic or electrophoretic devices, spatial light modulator(s), transmissive signs, etc.  
      For example, the display device  1512  may include one or more spatial light modulators. In some embodiments, the one or more spatial light modulators may include an array of individually addressable controllable elements. Such spatial light modulators may include a suitable type of controllable element. For example, the spatial light modulator may include a variable-transmissivity type of display. In some embodiments, the spatial light modulator may include a liquid crystal display (LCD), which is an example of a transmission-type light modulator. In some embodiments, the spatial light modulator may include a deformable mirror device (DMD), which is an example of a reflection-type light modulator.  
      The display device  1512  may include any suitable optical and non-optical elements for producing a display image, e.g., lenses, diffusers, polarizers, filters, beam splitters, brightness enhancement films, etc. The illumination system  1510  may be optically coupled to the display device  1312  using any suitable technique known in the art.  
      All references and publications cited herein are expressly incorporated herein by reference in their entirety into this disclosure. Illustrative embodiments of this disclosure are discussed and reference has been made to possible variations within the scope of this disclosure. These and other variations and modifications in the disclosure will be apparent to those skilled in the art without departing from the scope of the disclosure, and it should be understood that this disclosure is not limited to the illustrative embodiments set forth herein. Accordingly, the disclosure is to be limited only by the claims provided below.