Abstract:
A variety of light-emitting devices for general illumination utilizing solid state light sources (e.g., light-emitting diodes) are disclosed. A light-emitting device can include a first light-emitting element (LEE) for emitting light having a first spectral composition, a second LEE for emitting light having a second spectral composition, and a scattering element surrounding at least in part the first and second LEEs to scatter light emitted from the first and second LEEs. The light-emitting device can also include electrical connections for connecting the first and second LEEs to a power source, where the electrical connections are arranged such that power to the first LEE is separately adjustable relative to power to the second LEE.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e)(1) of U.S. Provisional Application No. 61/788,737, filed on Mar. 15, 2013, which is incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The described technology relates to color tuning of light-emitting devices including a symmetric or asymmetric light valve. 
       BACKGROUND 
       [0003]    The described technology relates to color tuning of light-emitting devices including a symmetric or asymmetric light valve. 
         [0004]    Light-emitting elements are ubiquitous in the modern world, being used in applications ranging from general illumination (e.g., light bulbs) to lighting electronic information displays (e.g., backlights and front-lights for LCDs) to medical devices and therapeutics. Solid state light emitting elements, which include light emitting diodes (LEDs), are increasingly being adopted in a variety of fields, promising low power consumption, high luminous efficacy and longevity, particularly in comparison to incandescent and other conventional light sources. 
         [0005]    One example of a solid state light-emitting element increasingly being used for in luminaires is a so-called “white LED.” Conventional white LEDs typically include an LED that emits blue or ultraviolet light and a phosphor or other luminescent material. The device generates white light via down-conversion of blue or UV light from the LED (referred to as “pump light”) by the phosphor. Such devices are also referred to as phosphor-based LEDs (PLEDs). Although subject to losses due to light-conversion, various aspects of PLEDs promise reduced complexity, better cost efficiency and durability of PLED-based luminaires in comparison to other types of luminaires. 
         [0006]    While new types of phosphors are being actively investigated and developed, configuration of PLED-based light-emitting devices, however, provides further challenges due to the properties of available luminescent materials. Challenges include light-energy losses from photon conversion, phosphor self-heating from Stokes loss, dependence of photon conversion properties on operating temperature, degradation due to permanent changes of the chemical and physical composition of phosphors in effect of overheating or other damage, dependence of the conversion properties on intensity of light, propagation of light in undesired directions in effect of the random emission of converted light that is emitted from the phosphor, undesired chemical properties of phosphors, and controlled deposition of phosphors in light-emitting devices, for example. 
       SUMMARY 
       [0007]    The described technology relates to light-emitting devices that convert light and mix the converted light. The converted light can be mixed using a symmetric or asymmetric light valve, and/or a light guide, for example. 
         [0008]    Accordingly, various aspects of the invention are summarized as follows. 
         [0009]    In general, in a first aspect, the invention features a light-emitting device including a substrate having a first surface; at least one first light-emitting element (LEE) disposed on the first surface for emitting light having a first spectral composition; at least one second LEE disposed on the first surface for emitting light having a second spectral composition, the first and second spectral compositions being different; and a scattering element that includes an inelastic scattering material, where the scattering element surrounds, at least in part, the at least one first and second LEEs to scatter light emitted from the at least one first and second LEEs; and electrical connections for connecting the at least one first and second LEEs to a power source, where the electrical connections are arranged such that power to one or more of the at least one first LEE is separately adjustable relative to power to one or more of the at least one second LEE. 
         [0010]    The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. In some embodiments the at least one first LEE includes two or more first LEEs and the electrical connections can be arranged such that power to one or more LEE of the two or more first LEEs is separately adjustable relative to power to one or more other LEE of the two or more first LEEs. In some embodiments, the at least one second LEE includes two or more second LEEs and the electrical connections can be arranged such that power to one or more LEE of the two or more second LEEs is separately adjustable relative to power to one or more other LEE of the two or more second LEEs. 
         [0011]    In some embodiments, the light-emitting device can further include a sensor that is arranged to receive a fraction of light output by the scattering element, where the sensor can be configured to provide a sensor signal based on the fraction of light, and wherein the sensor can be in communication with a control circuit configured to control power provided to the at least one second LEE in response to the sensor signal. In some embodiments, the light-emitting device includes the control circuit. In some embodiments, the sensor signal can be configured to provide an indication of an intensity of the light output by the scattering element. In some embodiments, the sensor signal can be configured to provide an indication of a spectral density distribution of the light output by the scattering element. In some embodiments, the light-emitting device can further include a non-photonic sensor. In some embodiments, the non-photonic sensor includes at least one of a temperature detector and a voltage detector. 
         [0012]    In some embodiments, the light-emitting device can further include at least one third LEE disposed on the first surface for emitting light having a third spectral composition, where the third spectral composition is different from the first spectral composition. In some embodiments, the light-emitting device can further include at least one fourth LEE disposed on the first surface for emitting light having a fourth spectral composition, where the fourth spectral composition has a correlated color temperature different from the second spectral composition. In some embodiments, the scattering element includes a phosphor material. In some embodiments, the scattering element has a dome shape. In some embodiments, at least a portion of the first surface is a diffusely reflective surface. In some embodiments, at least a portion of the first surface is a specularly reflective surface. In some embodiments, the first spectral composition includes light with a narrow emission spectrum. In some embodiments, the second spectral composition includes light with a broad emission spectrum. In some embodiments, the at least one second LEE has a phosphor coating. In some embodiments, the second spectral composition has a low correlated color temperature. In some embodiments, the second spectral composition corresponds to white light. 
         [0013]    In some embodiments, the scattering element can be spaced apart from the at least one first and second LEEs and coupled to the substrate to form an enclosure, where the scattering element can have an input surface facing the at least one first and second LEEs and an output surface opposing the input surface. In some embodiments, the substrate forms a cup. In some embodiments, the substrate is flat. In some embodiments, an index of refraction of the scattering element is larger than an index of refraction of a medium in the enclosure and larger than an index of refraction of an ambient environment. 
         [0014]    In some embodiments, the light-emitting device can further include a light guide and light output by the scattering element can be coupled into the light guide. In some embodiments, the light-emitting device can further include an extractor element coupled to an output surface of the scattering element. In some embodiments, the light-emitting device can further include a light guide and light output by the extractor can be coupled into the light guide. In some embodiments, the light-emitting device can further include a light guide and the extractor can be coupled to the light guide. In some embodiments, an index of refraction of the scattering element can be larger than an index of refraction of the extractor element such that an acceptance angle for rays of light within the scattering element at the output surface is larger than an acceptance angle at an input surface of the scattering element. In some embodiments, an index of refraction of the scattering element can be equal or smaller than an index of refraction of the extractor element. 
         [0015]    In some embodiments, the scattering element can be spaced apart from the at least one first and second LEEs and coupled to the substrate to form an enclosure, and an index of refraction of the scattering element can be larger than an index of refraction of a medium in the enclosure. In some embodiments, the extractor element can have a transparent exit surface opposing the output surface of the scattering element that is shaped such that an angle of incidence on the exit surface of the light provided by the scattering element that directly impinges on the exit surface is less than a critical angle for total internal reflection at the exit surface. In some embodiments, the scattering element can be a coating applied to a surface of the extractor element. In some embodiments, the scattering element can be a roughened surface of the extractor element facing the at least one first and second LEEs. In some embodiments, the at least one first and second LEEs and the scattering element can be arranged and the substrate can be configured such that light having substantially isotropic chromaticity is output by the scattering element. 
         [0016]    In general, in a further aspect, the invention features a light-emitting device, including a substrate having a first surface; two or more light-emitting elements (LEEs) disposed on the first surface; and a scattering element that surrounds, at least in part, the two or more LEEs, where the scattering element includes inelastic scattering centers arranged to scatter light from the two or more LEEs, and where the scattering element includes multiple segments, each of the segments configured to provide light having one of two or more spectral compositions; and electrical connections for connecting the two or more LEEs to a power source, where the electrical connections are arranged such that power to at least one first LEE of the two or more LEEs is separately adjustable relative to power to at least one second LEE of the two or more LEEs. 
         [0017]    The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. In some embodiments, the electrical connections can be arranged such that power to at least one LEE of the two or more LEEs that correspond to at least one of the segments is separately adjustable relative to power to at least one other LEE of the two or more LEEs that correspond to another segment. In some embodiments, each segment of the multiple segments includes a phosphor layer that provides light with a corresponding correlated color temperature. In some embodiments, each segment of the multiple segments includes a phosphor layer that provides light with a corresponding chromaticity. 
         [0018]    In some embodiments, the light-emitting device can further include a sensor arranged to receive a fraction of light output by the scattering element, where the sensor can be configured to provide a sensor signal based on the fraction of light, and where the sensor can be in communication with a control circuit configured to control power provided to at least some of the two or more LEEs. In some embodiments, the light-emitting device includes the control circuit. In some embodiments, the sensor signal can be configured to provide an indication of an intensity of the light output by the scattering element. In some embodiments, the sensor signal can be configured to provide an indication of a spectral density distribution of the light output by the scattering element. In some embodiments, the light-emitting device can further include a non-photonic sensor. In some embodiments, the non-photonic sensor can include at least one of a temperature detector and a voltage detector. 
         [0019]    In some embodiments, the scattering element has a dome shape. In some embodiments, at least a portion of the first surface is a diffusely reflective surface. In some embodiments, at least a portion of the first surface is a specularly reflective surface. In some embodiments, the scattering element can be spaced apart from the two or more LEEs and coupled to the substrate to form an enclosure, where the scattering element has an input surface facing the two or more LEEs and an output surface opposing the input surface. In some embodiments, at least a portion of the substrate forms a cup. In some embodiments, the substrate is flat. In some embodiments, an index of refraction of the scattering element can be larger than an index of refraction of a medium in the enclosure and larger than an index of refraction of an ambient environment. 
         [0020]    In some embodiments, the light-emitting device can further include a light guide and light output from the scattering element can be coupled into the light guide. In some embodiments, the light-emitting device can further include an extractor element coupled to an output surface of the scattering element. In some embodiments, the light-emitting device can further include a light guide and light output from the extractor element is coupled into the light guide. In some embodiments, the light-emitting device can further include a light guide and wherein the extractor element is coupled to the light guide. In some embodiments, an index of refraction of the scattering element can be larger than an index of refraction of the extractor element such that an acceptance angle for rays of light within the scattering element at the output surface is larger than an acceptance angle at an input surface of the scattering element. In some embodiments, an index of refraction of the scattering element can be equal or smaller than an index of refraction of the extractor element. 
         [0021]    In some embodiments, the scattering element can be spaced apart from the at least one first and second LEEs and coupled to the substrate to form an enclosure, and an index of refraction of the scattering element can be larger than an index of refraction of a medium in the enclosure. In some embodiments, the extractor element can have a transparent exit surface opposing the output surface of the scattering element that is shaped such that an angle of incidence on the exit surface of the light provided by the scattering element that directly impinges on the exit surface is less than a critical angle for total internal reflection at the exit surface. In some embodiments, the scattering element can be a coating applied to a surface of the extractor element. In some embodiments, the scattering element can be a roughened surface of the extractor element. 
         [0022]    In general, in a further aspect, the invention features a method for generating output light, including providing at least one first light-emitting element (LEE) on a first surface of a substrate for emitting light having a first spectral composition; providing at least one second LEE on the first surface for emitting light having a second spectral composition, where the first and second spectral compositions are different; and scattering light emitted from the at least one first and second LEEs by a scattering element surrounding, at least in part, the at least one first and second LEEs to provide output light; and providing electrical connections for connecting the at least one first and second LEEs to a power source, where the electrical connections are arranged such that power to one or more of the at least one first LEE is separately adjustable relative to power to one or more of the at least one second LEE. 
         [0023]    The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. In some embodiments, the at least one first LEE includes two or more first LEEs and the electrical connections can be arranged such that power to one or more LEE of the two or more first LEEs is separately adjustable relative to power to one or more other LEE of the two or more first LEEs. In some embodiments, the at least one second LEE includes two or more second LEEs and the electrical connections can be arranged such that power to one or more LEE of the two or more second LEEs is separately adjustable relative to power to one or more other LEE of the two or more second LEEs. 
         [0024]    In some embodiments, the method can further include providing at least one third LEE disposed on the first surface for emitting light having a third spectral composition, where the third spectral composition can be different from the first spectral composition. In some embodiments, the method can further include providing at least one fourth LEE disposed on the first surface for emitting light having a fourth spectral composition, where the fourth spectral composition can have a correlated color temperature different from the second spectral composition. In some embodiments, the scattering element includes a phosphor material. In some embodiments, the scattering element has a dome shape. In some embodiments, at least a portion of the first surface is a diffusely reflective surface. In some embodiments, at least a portion of the first surface is a specularly reflective surface. In some embodiments, the first spectral composition includes light with a narrow emission spectrum. In some embodiments, the second spectral composition includes light with a broad emission spectrum. In some embodiments, the at least one second LEE has a phosphor coating. In some embodiments, the second spectral composition has a low correlated color temperature. In some embodiments, the second spectral composition corresponds to white light. 
         [0025]    In some embodiments, the scattering element can be spaced apart from the at least one first and second LEEs and coupled to the substrate to form an enclosure, where the scattering element has an input surface facing the at least one first and second LEEs and an output surface opposing the input surface. In some embodiments, at least a portion of the substrate forms a cup. In some embodiments, the substrate is flat. In some embodiments, an index of refraction of the scattering element can be larger than an index of refraction of a medium in the enclosure and larger than an index of refraction of an ambient environment. 
         [0026]    In some embodiments, the method can further include providing a light guide and light output by the scattering element is coupled into the light guide. In some embodiments, the method can further include providing an extractor element coupled to an output surface of the scattering element. In some embodiments, the method can further include providing a light guide and light output by the extractor element is coupled into the light guide. In some embodiments, the method can further include providing a light guide and the extractor element can be coupled to the light guide. In some embodiments, an index of refraction of the scattering element can be larger than an index of refraction of the extractor element such that an acceptance angle for rays of light within the scattering element at the output surface is larger than an acceptance angle at an input surface of the scattering element. In some embodiments, an index of refraction of the scattering element can be equal or smaller than an index of refraction of the extractor element. 
         [0027]    In some embodiments, the scattering element can be spaced apart from the at least one first and second LEEs and coupled to the substrate to form an enclosure, and where an index of refraction of the scattering element can be larger than an index of refraction of a medium in the enclosure. In some embodiments, the extractor element can have a transparent exit surface opposing the output surface of the scattering element that is shaped such that an angle of incidence on the exit surface of the light provided by the scattering element that directly impinges on the exit surface is less than a critical angle for total internal reflection at the exit surface. In some embodiments, the scattering element can be a coating applied to a surface of the extractor element. In some embodiments, the scattering element can be a roughened surface of the extractor element facing the at least one first and second LEEs. In some embodiments, the at least one first and second LEEs and the scattering element can be arranged and the substrate can be configured such that light having substantially isotropic chromaticity is output by the scattering element. 
         [0028]    In general, in a further aspect, the invention features a light-emitting device including: a substrate having a first surface; two or more light-emitting elements (LEEs) disposed on the first surface, where at least some of the two or more LEEs emit pump light and at least one LEE emits white light, and where a light output of the at least one LEE emitting white light can be adjusted; a first optical element having a first surface spaced apart from the two or more LEEs and positioned to receive light from the two or more LEEs, the first optical element including scattering centers arranged to scatter light from the two or more LEEs; and a second optical element having an exit surface, where the second optical element is transparent and in contact with the first optical element, there being an optical interface between the first and second optical elements at the place of contact, the optical interface being opposite the first surface of the first optical element, the second optical element being arranged to receive at least a portion of the light through the optical interface; where: a medium adjacent to the first surface of the first optical element has a refractive index n 0 ; the first optical element includes a material having a first refractive index n 1 , where n 0 &lt;n 1 ; the second optical element includes a material having a refractive index n 2 , where n 0 &lt;n 2 ; and the exit surface is a transparent surface that is shaped such that an angle of incidence on the exit surface of the light provided by the first optical element that directly impinges on the exit surface is less than a critical angle for total internal reflection. 
         [0029]    The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. In some embodiments, n 0 , n 1  and n 2  can be selected such that an amount of light transmitting from the first optical element into the second optical element is larger than an amount of light transmitting from the first optical element into the medium adjacent the first surface. In some embodiments, the light-emitting device can further include a light guide and light output by the second optical element is coupled into the light guide. In some embodiments, the light-emitting device can further include a light guide and the second optical element is coupled to the light guide. In some embodiments, the at least some of the LEEs that emit pump light are controlled independently from the at least one LEE that emits white light. 
         [0030]    In general, in a further aspect, the invention features a light-emitting device including: a substrate having a first surface; two or more light-emitting elements (LEEs) disposed on the first surface; a first optical element having a first surface spaced apart from the two or more LEEs and positioned to receive light from at least one of the two or more LEEs, where the first optical element includes scattering centers arranged to scatter light from the two or more LEEs, the first optical element includes multiple segments, where each segment is configured to provide light having one of two or more spectral compositions, and where a light output of LEEs corresponding to the multiple segments of the first optical element can be adjusted; and a second optical element having an exit surface, the second optical element being transparent and in contact with the first optical element, there being an optical interface between the first and second optical elements at the place of contact, the optical interface being opposite the first surface of the first optical element, the second optical element being arranged to receive at least a portion of the light through the optical interface; where: a medium adjacent to the first surface of the first optical element has a refractive index n 0 ; the first optical element includes a material having a first refractive index n 1 , where n 0 &lt;n 1 ; the second optical element includes a material having a refractive index n 2 , where n 0 &lt;n 2 ; and the exit surface is a transparent surface that is shaped such that an angle of incidence on the exit surface of the light provided by the first optical element that directly impinges on the exit surface is less than a critical angle for total internal reflection. 
         [0031]    The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. In some embodiments, n 0 , n 1  and n 2  can be selected such that an amount of light transmitting from the first optical element into the second optical element is larger than an amount of light transmitting from the first optical element into the medium adjacent the first surface. In some embodiments, the light-emitting device can further include a light guide and light output by the second optical element is coupled into the light guide. In some embodiments, the light-emitting device can further include a light guide and the second optical element is coupled to the light guide. In some embodiments, each segment of the multiple segments can include a corresponding phosphor layer. In some embodiments, LEEs corresponding to a set of the multiple segments can be controlled independently from LEEs corresponding to another set of the plurality of segments. 
         [0032]    Amongst other advantages, embodiments of the light-emitting devices can be configured to provide converted light with an emission spectrum that can be substantially independent of certain variations in the spectra of the pump light sources (e.g., long-term degradation of phosphor or drifting conversion properties of phosphor.) The light-emitting devices can include a scattering element that mixes light which is output by the light sources. The described technology can help stabilize the emission spectrum and thereby chromaticity and/or color temperature of the light provided by the illumination device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIGS. 1A and 1B  are cross-sectional views of an example of a light-emitting device with multiple light-emitting elements. 
           [0034]      FIG. 1C  is an example of a control circuitry for a light-emitting device. 
           [0035]      FIGS. 2A and 2B  are views of an example of a light-emitting device with a segmented scattering element. 
           [0036]      FIGS. 2C and 2D  are views of another example of a light-emitting device with a segmented scattering element. 
           [0037]      FIG. 2E  is another example of a control circuitry for a light-emitting device. 
           [0038]      FIG. 3  is a schematic diagram showing an example feedback circuit used to provide intra-system source feedback for a light-emitting device. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    Over the lifetime of phosphor-based light-emitting devices color variation can occur, for example, due to long-term degradation of phosphor. The level of phosphor degradation can be dependent on the operating temperature of the light-emitting device. Moreover, conversion properties of a phosphor may drift and depend on operating temperature, light exposure or other parameters, for example. Furthermore, color control by a user during operation by the light-emitting device may be desirable. 
         [0040]      FIGS. 1A and 1B  are cross-sectional views of an example of a light-emitting device  100  with multiple light-emitting elements  110  and  112 .  FIG. 1A  shows a side sectional view of the light-emitting device  100 .  FIG. 1B  shows a top view of the light-emitting device  100 . The example light-emitting device  100  includes a substrate  150 , multiple light-emitting elements  110  with a narrow emission spectrum (e.g., blue pump LEDs), one or more light-emitting elements that have a broad emission spectrum  112 , referred to as broadband LEE (e.g., a white LED), a scattering element  120 , and an extractor element  130 . Depending on the implementation, the scattering element  120  is configured to elastically, inelastically or both elastically and inelastically scatter light. In some implementations, the extractor element  130  is omitted from the light-emitting device  100 . Different broad-band LEEs can have different emission spectra, for example one or more white LEDs with about 2700 K correlated color temperature and one or more white LEDs with about 5000 K. Multiple LEEs with different broad-band emission spectra can help better approximate a desired chromaticity and/or correlated color temperature of the light emitted by the light-emitting device during control and/or compensation of ageing effects, for example. 
         [0041]    In some implementations, the light-emitting device  100  includes an optional light guide (not illustrated). Depending on the implementation, the scattering element  120  and/or the extractor element  130  are configured to couple light into the light guide. An optical coupling of the scattering element and/or the extractor with the light guide can be achieved via an additional suitably shaped hollow or solid coupler (not illustrated). The coupling also can be provided via a suitably configured extractor. Such an extractor can have various shapes. For example, the extractor can be shaped between radii R 0  and R 1 , to guide light via TIR or otherwise in a forward direction and then couple with the light guide. 
         [0042]    The light guide guides the light and may provide additional mixing and/or other functions, for example. Depending on the implementation, the light guide can guide light via specular mirror reflection and/or TIR. The length of the light guide parallel to the z-axis and the shape of the cross section of the light guide perpendicular to the z-axis and/or its variation along the z-axis can determine the degree of mixing provided by the light guide. The light guide can have straight and/or curved portions in an elongate extension parallel to the y-axis, for example. In some implementations, the light guide has a tubular, square, triangular, hexagonal or other regular or irregular cross section within planes perpendicular to the optical axis of the light-emitting device that is perpendicular to the z-axis, for example. 
         [0043]    The cross section of a light guide can change in orientation and/or size along the z-axis. For example, the section can have a hexagonal shape that rotates by Pi/3 every centimeter and/or widens towards half of its length before it tapers again towards the far end opposite of the scattering element. 
         [0044]    In some implementations, multiple hemi-spherical example light-emitting devices, such as light-emitting devices  100  as illustrated in  FIGS. 1A and 1B , may be arranged along a length of a light guide that is elongate along the y-axis, for example. In some implementations, multiple light-emitting devices with a tubular scattering element extending along the y-axis, and with multiple light-emitting elements  110 / 112 , may be arranged along a light guide that is elongate along the y-axis, for example. 
         [0045]    The substrate  150  can have a surface  155 . In some implementations, the surface  155  can be reflective (e.g., a mirror). The scattering element  120  can have a first surface  115  spaced apart from the light-emitting elements  110  and  112  and positioned to receive the light from the light-emitting elements  110  and  112 . The light-emitting elements  110  and  112  can be disposed on the surface  155  of the substrate  150 , in an opening that is, at least in part, defined by the first surface  115  (e.g., having a radius R O ). 
         [0046]    The substrate  150  can extend within the x-y plane up to (not illustrated) or beyond point R 1 . The surface  155  can be provided by a reflective layer (not illustrated) that can be wider or narrower in the x-y plane than the substrate  150 . Furthermore, the substrate  150  can be disposed on a reflective layer. The scattering element  120  can be located on the inside of the extractor element  130  adjacent an enclosure  140  (e.g., a semispherical enclosure of radius R O ) of the extractor element  130  and form an optical interface  125 . In some implementations, the scattering element can be a layer (e.g., coating) with scattering centers. In some implementations, the scattering element can be a roughening of the inner surface of the extractor. The enclosure  140  can be filled with a medium (e.g., gas or air) and encloses the light-emitting elements  110  and  112 , and at least a portion of the surface  155 . The scattering element  120  can have a spherical, hemi-spherical or other shape. Such shapes can range from dome-like via flat to bowl-like shapes. The scattering element can have one or more indentations. An example light-emitting device with a flat scattering element may have a substrate configured similarly to the one of the example light-emitting device of  FIGS. 2C and 2D . 
         [0047]    Depending on the implementation, the medium in the enclosure  140  can be a gaseous or other medium having a refractive index n 0  that is greater or equal to 1 and smaller or equal to a refractive index n 120  of the scattering element  120  (1≦n 0 ≦n 120 ), or the medium can have a refractive index n 0  that is greater or equal to a refractive index n 120  of the scattering element  120  (n 0 ≧n 120 ). The refractive index of the medium in the enclosure  140  may have a refractive index comparable to the refractive indices of the light-emitting elements, for example. The medium in the enclosure  140  surrounds the light-emitting elements  110 ,  112  and separates the light-emitting elements from the scattering element  120 . In some implementations, the medium can be of a high refractive index material (for example, a solid or a liquid). 
         [0048]    In some implementations, the refractive indices of the components on either side of the scattering element  120  relative to the refractive index of the scattering element  120  are chosen such that the acceptance angle at the optical interface  125  of the scattering element  120  is larger than the acceptance angle at the first surface  115  of the scattering element  120 . The acceptance angle in these cases refers to the incidence angles of light rays at the corresponding surfaces that do not undergo total internal reflection. As such and considering a scattering element with an isotropic photon density this situation may be referred to as an asymmetric light valve. It is noted that an asymmetric light valve is not necessarily limited to implementations in which TIR may occur at an optical interface  125 . For example, the scattering element  120  may have substantially the same or a large refractive index as the extractor element  130  in which case there is no TIR and hence the acceptance angle concept does not apply. As such and depending on the implementation an optical interface  125  may not exist except between the scattering centers and the surrounding host material in the scattering element, for example. 
         [0049]    In some implementations that include an extractor, the exit surface  135  of the extractor element  130  is shaped as a spherical or a cylindrical dome or shell with a radius R 1  in which the optical interface is disposed within an area defined by a respective notional sphere or cylinder that is concentric with the exit surface and has a radius R OW =R 1 /n 130 , wherein n 130  is the refractive index of the extractor element  130 . Such a configuration is referred to as Weierstrass geometry or Weierstrass configuration. It is noted that a spherical Weierstrass geometry can avoid total internal reflection (TIR) for rays passing through the area circumscribed by a corresponding notional R 1 /n 130  sphere irrespective of the plane of propagation. A cylindrical Weierstrass geometry can exhibit TIR for light that propagates in planes that intersect the respective cylinder axis at shallow angles even if the light passes through an area circumscribed by a corresponding notional R OW =R 1 /n 130  cylinder. 
         [0050]    It is noted that other light-emitting devices can have exit surfaces with other shapes and/or other geometrical relations with respect to the optical interface. For instance, a non-spherical or non-cylindrical exit surface of the extractor element  130  can be employed to refract light and aid in shaping an output intensity distribution in ways different from those provided by a spherical or cylindrical exit surface. The definition of the Weierstrass geometry can be extended to include exit surfaces with non-circular sections by requiring that the optical interface falls within cones, also referred to as acceptance cones, subtended from points p of the exit surface whose axes correspond to respective surface normals at the points p and which have an apex of 2*Arcsin(k/n 130 ), wherein k is a positive number smaller than n 130 . It is noted that the exit surface needs to be configured such that the plurality of all noted cones circumscribe a space with a non-zero volume. It is further noted that k is assumed to refer to a parameter that determines the amount of TIR at an uncoated exit surface that separates an optically dense medium, having n 130 &gt;1, on one side of the exit surface making up the extractor element  130  from a typical gas such as air with n˜1.00 at standard temperature and pressure conditions, on the opposite side of the exit surface. Depending on the embodiment, k can be slightly larger than 1 but is preferably less than 1. If k&gt;1, some TIR may occur at the exit surface inside the extractor element  130 . In some embodiments, this results in the optical interface being at least R(p)*(1−k/n 130 ) away from the exit surface in a direction normal to the exit surface at a point p thereof. Here, R(p) is the local radius of curvature of the exit surface at the point p, and n 130  is the refractive index of the extractor element  130 . For a spherical or cylindrical exit surface with k=1, the boundaries circumscribed by the noted cones correspond with a spherical or cylindrical Weierstrass geometry, respectively. Some embodiments are configured to allow for some TIR by choosing k&gt;1. In such cases, k/n 130  is limited to k/n 130 &lt;0.8, for example. 
         [0051]    In summary, an illumination device is said to satisfy the Weierstrass configuration if a radius R O  of the optical interface is less than or equal to R O ≦R OW =R 1 /n 130 , where R 1  and n 130  respectively are the radius and index of refraction of the extractor element  130 . Equivalently, the extractor element  130  of a light-emitting device is said to satisfy the Weierstrass configuration if a radius R 1  of an extractor element  130 , which has an index of refraction n 130 , is equal to or larger than R 1 ≧R 1W =n 130 R O , where R O  is the radius of the optical interface of the illumination device. 
         [0052]    In some implementations, the exit surface  135  of the extractor element  130  can have a radius R 1  that is concentric with the optical interface  125 , such that the extractor element  130  satisfies the Brewster configuration R 1 ≧R 1B . When the optical interface is between the extractor and air, the Brewster radius is given by R 1B =R O (1+n 130   2 ) +1/2 , where R O  is the radius of the optical interface  125  of the light-emitting device  100 , and n 130  denotes the index of refraction of the material of the extractor element  130 . As the extractor element  130  satisfies the Brewster configuration, an angle of incidence on the exit surface  135  of the scattered light that directly impinges on the exit surface  135  is less than the Brewster angle, and as such, the scattered light that directly impinges on the exit surface  135  experiences substantially no total internal reflection and limited Fresnel reflections thereon. 
         [0053]    In the example device illustrated in  FIGS. 1A and 1B , light propagation asymmetry arises, for example, from the materials on the inside (index n 0 ) and outside (index n 130 ) of the scattering element  120  with index n 120  being unequal. For instance, if n 120 =1.5 and n 0 =1.0, that is n 0 &lt;n 120 , a large fraction (˜75%) of the isotropically distributed photons impinging on the first surface  115  will be reflected by total internal reflection (TIR) back into the scattering element  120  and only a smaller fraction (˜25%) will be transmitted backwards into the enclosure  140  from where some may reach the light-emitting elements  110  or  112 . At the optical interface  125 , the condition n 120 ≦n 130  will guarantee photons reaching the optical interface  125  will transition into the extractor element  130  without TIR, and the Brewster condition will further guarantee that a substantial portion of these photons can transmit into the ambient environment without TIR through the exit surface  135 . Only a small fraction (down to about ˜4% depending on incidence angle) will be returned by Fresnel reflection at the exit surface  135 . 
         [0054]    In a general aspect, a light-emitting device (e.g.,  100 ) includes a substrate (e.g.,  150 ) having a surface (e.g.,  155 ); two or more LEEs (e.g.,  110 ,  112 ) configured to emit light, where at least one of the two or more LEEs (e.g.,  112 ) is coated with a phosphor layer; a first optical element (e.g., a scattering element  120 ) that has a first surface (e.g.,  115 ) spaced apart from the LEEs and positioned to receive light from at least one of the LEEs, where the first optical element includes scattering centers arranged to scatter light from the LEEs; and a second optical element (e.g., an extractor element  130 ) that has an exit surface (e.g.,  135 ), where the second optical element is transparent and in contact with the first optical element, there being an optical interface (e.g.,  125 ) between the first and second optical elements at the place of contact, where the optical interface is opposite the first surface of the first optical element, and where the second optical element is arranged to receive at least a portion of the light through the optical interface. A medium adjacent to the first surface of the first optical element has a refractive index n 0 ; the first optical element includes a material that has a first refractive index n 1 , where n 0 &lt;n 1 ; the second optical element includes a material that has a refractive index n 2 , where n 0 &lt;n 2 ; and the exit surface is a transparent surface that is shaped such that an angle of incidence on the exit surface of the light provided by the first optical element that directly impinges on the exit surface is less than a critical angle for total internal reflection. 
         [0055]    While the light-emitting device  100  shown in  FIGS. 1A and 1B  includes an extractor element, light-emitting devices without extractor element are also possible. Generally, light mixed in the scattering element can be output directly from the scattering element into the ambient environment without first passing through an extractor element. 
         [0056]    In implementations of light-emitting devices with an extractor element, the refractive index of the extractor element can be larger, equal, or smaller than the refractive index of the scattering element. In implementations of light-emitting devices, where the extractor element is omitted, the refractive index of the scattering element is generally larger than the refractive index of the ambient environment. 
         [0057]    Returning to  FIGS. 1A and 1B , the light-emitting device  100  can include multiple light-emitting elements  110 , such as blue LEDs  110  and one or more light-emitting elements  112 , such as white LEDs (a white LED can include a blue LED coated with a phosphor layer). The light-emitting element  112  can have a low color temperature. For example, the light-emitting device  100  can have a target correlated color temperature of 3000 Kelvin and the light-emitting element  112  can have a correlated color temperature of 2700 Kelvin. Initially, the light-emitting element  112  can be dimmed to a low light output. When, over time, the scattering element  120  of the light-emitting device  100  degrades, the emission spectrum of the light-emitting device  100  may shift to a bluer color. This color shift can be compensated by increasing the light output from the light-emitting element  112  such that the light-emitting device maintains a consistent emission spectrum over its lifetime. In other embodiments, for example when due to ageing the scattering element would shift to a redder color, a cooler white LED can be used to offset the ageing effects if adequately controlled. 
         [0058]      FIG. 1C  is an example of control circuitry  160  for a light-emitting device, such as light-emitting device  100 . Any number of light-emitting elements can be electrically connected in series (e.g., dependent on the desired light intensity and voltage drop). Generally, the light-emitting elements of the light-emitting device  100  can be split into groups that are controlled independently of each other to allow independent control of the brightness of each group of light-emitting elements. Generally, the split can be by correlated color temperature (CCT), chromaticity or otherwise. In case of  FIG. 1C , the split is by chromaticity and CCT. Light-emitting elements of each group can be electrically connected in series, in parallel or in a mixed series and parallel manner, which may be determined based on desired voltage drops and/or drive currents, for example. As shown in  FIG. 1C , light-emitting element  112  can be controlled by a current source  162   a  and the group of light-emitting elements  110  can be controlled by current source  162   b . The light-emitting elements can be split into n groups (e.g.,  112 ,  110 , . . .  110   n ) that are controlled independently by n current sources (e.g.,  162   a ,  162   b , . . .  162   n ). The groups of light-emitting elements are electrically connected in parallel to a voltage source, such as voltage source  164 , to provide, in conjunction with the corresponding current sources, power to the groups of light-emitting elements. 
         [0059]      FIG. 2A  is a cross-sectional side view of a light-emitting device  200  with a segmented scattering element  220 . The light-emitting device  200  includes a substrate  250 , multiple light-emitting elements  210   a / 210   b  (e.g., blue pump LEDs), a segmented scattering element  220 , and an extractor element  230 . In some implementations, the extractor element  230  is omitted from the light-emitting device  200 .  FIG. 2B  shows a top view of the segmented scattering element  220 . Different segments of the segmented scattering element  220  can have different shapes. Segments can have circular or spherical shapes or other regular or irregular shapes and can form a mosaic of segments. Different segments can have different light conversion properties including different absorption spectra, different white points and/or different color points of the converted light or other properties, for example. The substrate  250  can have a surface  255 . In some implementations, the surface  255  can be reflective (e.g., a mirror). The segmented scattering element  220  has a first surface  215  spaced apart from the light-emitting elements  210   a / 210   b  and positioned to receive the light from the light-emitting elements  210   a / 210   b.    
         [0060]    The light-emitting elements  210   a / 210   b  are disposed on the surface  255  of the substrate  250 , in an opening that is defined by the first surface  215  of the segmented scattering element  220  (e.g., having a radius R O ). The light-emitting elements can be placed such that one or more light-emitting elements correspond to a particular segment of the segmented scattering element  220 . In some implementations, sizes of the respective areas of the scattering element segments can be substantially the same or different relative to each other. 
         [0061]    The substrate  250  can extend within the x-y plane up to (not illustrated) or beyond point R 1 . The surface  255  can be provided by a reflective layer (not illustrated) that can be wider or narrower in the x-y plane than the substrate  250 . Furthermore, the substrate  250  can be disposed on a reflective layer. The segmented scattering element  220  can be located on the inside of the extractor element  230  adjacent an enclosure  240  (e.g., a semispherical enclosure of radius R O ) of the extractor element  230  to form an optical interface  225 . The enclosure  240  can be filled with a medium (e.g., gas or air) and encloses the light-emitting elements  210   a / 210   b , and a portion of the surface  255 . In this example, the extractor element  230  has a hemi-spherical shape. Other implementations can have extractors of other portions of a sphere or other shapes in general. 
         [0062]    In this example, the light-emitting device  200  includes multiple light-emitting elements  210   a  and  210   b , such as blue pump LEDs, disposed on the surface  255  of the substrate  250 . In some implementations, the segments of the segmented scattering element  220  can include or be formed of multiple phosphor layers (e.g., coatings,) such as phosphor layer  222  and  224 . The phosphor layers  222  and  224  can be configured such that the correlated color temperature of phosphor layer  222  is different from the correlated color temperature of phosphor layer  224 . 
         [0063]    In some implementations, the segmented scattering element  220  can include a uniform phosphor layer and additional phosphor layer applied to portions of the segmented scattering element  220 . For example, the uniform phosphor layer can produce a correlated color temperature of about 3000 Kelvin and the additional phosphor layer, which is included with portions of the segmented scattering element  220 , can alter the correlated color temperature of the corresponding portions of the segmented scattering element  220 . The altered correlated color temperature can be higher or lower than the correlated color temperature of the uniform phosphor layer. 
         [0064]    In this example, the light-emitting elements  210   a  are located below the phosphor layer  224  and arranged to pump the area covered by the phosphor layer  224  and the light-emitting elements  210   b  are located below the phosphor layer  222  and arranged to pump the area covered by the phosphor layer  222 . For example, the light-emitting elements  210   a  can be configured to pump the area covered by the phosphor layer  224  stronger than the light-emitting elements  210   b  pump the area covered by the phosphor layer  222 . Therefore, during initial operation, the light-emitting elements  210   a  located below the phosphor layer  224  can be dimmed. When, over time, the phosphor layers degrade, the emission spectrum of the light-emitting device  200  may shift to a blue color. This color shift can be compensated by increasing the light output of the light-emitting elements  210   a  located below the phosphor layer  224  or vice versa, such that the light-emitting device maintains a consistent emission spectrum over its lifetime. 
         [0065]    In some implementations, the segmented scattering element (e.g.,  220 ) can include segmented phosphor layers to produce multiple correlated color temperatures. In some implementations, the light-emitting elements (e.g.,  210   a  and  210   b ) can be of different colors. In some implementations, the described embodiments can be implemented to shift the correlated color temperature from one color to another color. 
         [0066]    While the light-emitting device  200  shown in  FIG. 2A  includes an extractor element, light-emitting devices without an extractor element are also possible. Generally, light mixed in the scattering element can be output directly from the scattering element into the ambient environment without first passing through an extractor element. 
         [0067]      FIG. 2C  is a cross-sectional side view of another example light-emitting device  270  with a segmented flat scattering element  280 . The example light-emitting device  270  includes a substrate  250 , multiple light-emitting elements  210   a / 210   b  (e.g., blue pump LEDs), a segmented scattering element  280 , and an extractor element  290 . In some implementations, the light emitting device  270  can also include a light guide as described herein.  FIG. 2D  shows a top view including the segmented scattering element  280 . Different segments of the segmented scattering element  280  can have different shapes. The segments can have polygonal, circular or other regular or irregular shapes and can form a mosaic of segments. Different segments can have different light conversion properties including different absorption spectra, different white points and/or different color points of the converted light or other properties, for example. The substrate  250  has a surface  255 . The surface  255  is reflective (e.g., a mirror). The segmented scattering element  280  has a first surface  286  spaced apart from the light-emitting elements  210   a / 210   b  and positioned to receive the light from the light-emitting elements  210   a / 210   b , and a light output surface  288 . 
         [0068]    The light-emitting elements  210   a / 210   b  are disposed in a recessed portion with an aperture having a radius of about R 0  of the substrate  250 . The scattering element  280  covers the aperture of the recessed portion of the substrate  250  defining an enclosure  285 . The light-emitting elements  210   a / 210   b  can be placed such that one or more light-emitting elements correspond to a particular segment of the segmented scattering element  280 . In some implementations, sizes of the respective areas of the scattering element segments can be substantially the same or different relative to each other. 
         [0069]    The substrate  250  extends within the x-y plane beyond point R 1 . The surface  255  can be provided by a reflective layer (not illustrated) that can be wider or narrower in the x-y plane than the substrate  250 . Furthermore, the substrate  250  can be disposed on a reflective layer. The enclosure  285  can be filled with a medium (e.g., gas or air or higher refractive index material) and encloses the light-emitting elements  210   a / 210   b , and a portion of the surface  255 . In this example, the extractor element  290  has a hemi-spherical shape. Other implementations can have extractor shapes that correspond with other portions of a sphere or have other shapes in general. 
         [0070]    In this example, the light-emitting device  270  includes multiple light-emitting elements  210   a  and  210   b , such as blue pump LEDs, disposed on the surface  255  of the substrate  250 . In some implementations, the segments of the segmented scattering element  280  can include or be formed of multiple phosphor layers (e.g., coatings,) such as phosphor layer  282  and  284 . The phosphor layers  282  and  284  can be configured such that the correlated color temperature of phosphor layer  282  is different from the correlated color temperature of phosphor layer  284 . 
         [0071]    In some implementations, the segmented scattering element  280  can include a uniform phosphor layer and additional phosphor layer applied to portions of the segmented scattering element  280 . For example, the uniform phosphor layer can produce a correlated color temperature of about 3000 Kelvin and the additional phosphor layer, which is included with portions of the segmented scattering element  280 , can alter the correlated color temperature of the corresponding portions of the segmented scattering element  280 . The altered correlated color temperature can be higher or lower than the correlated color temperature of the uniform phosphor layer. In some implementations, different segments of the scattering element  280  include different types of phosphors. This can help control amounts of converted light emitted by one phosphor reaching another phosphor and thereby help mitigate parasitic absorption of converted light. 
         [0072]    In this example, the light-emitting elements  210   a  are located below the phosphor layer  284  and are arranged to pump the area covered by the phosphor layer  284 . The light-emitting elements  210   b  are located below the phosphor layer  282  and are arranged to pump the area covered by the phosphor layer  282 . For example, the light-emitting elements  210   a  can be configured to pump the area covered by the phosphor layer  284  stronger than the light-emitting elements  210   b  pump the area covered by the phosphor layer  282 . Therefore, during initial operation, the light-emitting elements  210   a  located below the phosphor layer  284  can be dimmed. When, for example over time or during operation, the phosphor layers alter, the emission spectrum of the light-emitting device  270  may shift to a blue color. Such a color shift can be compensated by for example increasing the light output of the light-emitting elements  210   a  located below the phosphor layer  284  or vice versa, such that the light-emitting device maintains a consistent emission spectrum over its lifetime. 
         [0073]    In some implementations, a segmented scattering element (e.g.,  220  or  280 ) can include segmented phosphor layers to produce multiple correlated color temperatures. In some implementations, light-emitting elements (e.g.,  210   a  and  210   b ) can be of different colors. In some implementations, the described embodiments can be implemented to shift the correlated color temperature during operation from one color to another color by controlling the drive currents provided to different light-emitting elements. 
         [0074]    While the light-emitting device  270  shown in  FIG. 2C  includes an extractor element, light-emitting devices without an extractor element are also possible. Generally, light mixed in the scattering element can be output directly from the scattering element into the ambient environment without first passing through an extractor element. 
         [0075]    In implementations of light-emitting devices with an extractor element, the refractive index of the extractor element can be larger, equal, or smaller than the refractive index of the scattering element. In implementations of light-emitting devices, where the extractor element is omitted, the refractive index of the scattering element is generally larger than the refractive index of the ambient environment. 
         [0076]      FIG. 2E  is an example of control circuitry  260  for a light-emitting device, such as light-emitting device  200 , to control light-emitting elements of the light-emitting device. Generally, the light-emitting elements of the light-emitting device can be split into groups that are controlled independently of each other to allow independent control of the brightness of each group of light-emitting elements. Generally, the split can be by correlated color temperature CCT, chromaticity or otherwise. The split can be by association between phosphor layers and their corresponding light-emitting elements, such as phosphor layers  222  or  224  and light-emitting elements  210   a  and  210   b . As shown in  FIG. 2C , the group of light-emitting elements  210   a  can be controlled by a current source  262   a  and the group of light-emitting elements  210   b  can be controlled by current source  262   b . The light-emitting elements can be split into n groups (e.g.,  210   a ,  210   b , . . .  210   n ) that are controlled independently by n current sources (e.g.,  262   a ,  262   b , . . .  262   n ). The groups of light-emitting elements are electrically connected in parallel to a voltage source, such as voltage source  264 , to provide, in conjunction with the corresponding current sources, power to the groups of light-emitting elements. 
         [0077]    In some implementations, the light-emitting device can include a sensor that measures color coordinates of the light emitted by the light-emitting elements. In some implementations, the sensor can be configured to indicate estimates of light intensity, spectral density, or both. The sensor can be coupled with a control loop that can be configured to dim the color or brightness of individual light sources, for example, when a portion of a phosphor layer degrades less over time than other portions of the phosphor layer, or some phosphor layers degrade less over time than other phosphor layers. Such control mechanisms can maintain constant illumination pattern or color distribution of the light-emitting device over its lifetime. For example, when portions of a phosphor layer degrade over time, blue light may become more visible. The shift to blue light can be compensated by adjusting the output (e.g., dimming) of the light-emitting elements respective to their position in the light-emitting device, whether or not a light-emitting element is located below a degraded portion of the phosphor layer. 
         [0078]    Light-emitting devices, such as light-emitting devices  100 ,  200  and/or  270 , can be controlled in a feed forward, a feedback or a mixed feed forward and feedback manner. In a feed forward control scheme, drive currents and/or drive voltages of different groups of light-emitting elements may be determined based on one or more of these drive currents and/or drive voltages alone or in other ways, for example. 
         [0079]    In some implementations, a light-emitting device includes an optional light guide (not illustrated). Depending on the implementation, a scattering element, such as scattering element  220  or  280 , and/or an extractor element, such as extractor element  230  or  290  (if present) can be configured to couple light into the light guide. An optical coupling of the scattering element and/or the extractor with the light guide can be achieved via an additional suitably shaped hollow or solid coupler (not illustrated). The coupling also can be provided via a suitably configured extractor. Such an extractor can have various shapes. For example, the extractor can be shaped between radii R 0  and R 1  in such a way (not illustrated), to guide light via TIR or otherwise in a forward direction and then couple with the light guide. 
         [0080]    The light guide guides the light and may provide additional mixing and/or other functions, for example. Depending on the implementation, the light guide can guide light via specular mirror reflection and/or TIR. The length of the light guide parallel to the z-axis and the shape of the cross section of the light guide perpendicular to the z-axis and/or its variation along the z-axis can determine the degree of mixing provided by the light guide. The light guide can have straight and/or curved portions in an elongate extension parallel to the y-axis, for example. In some implementations, the light guide has a tubular, square, triangular, hexagonal or other regular or irregular cross section within planes perpendicular to the optical axis of the light-emitting device that is perpendicular to the z-axis (see e.g. FIGS.  2 A and  2 C,) for example. 
         [0081]    The cross section of a light guide can change in orientation and/or size along the z-axis. For example, the section can have a hexagonal shape that rotates by Pi/3 every centimeter and/or widens towards half of its length before it tapers again towards the far end opposite of the scattering element. 
         [0082]    In some implementations, multiple light-emitting devices, such as light-emitting devices according to  FIGS. 2A and 2B  or  2 C and  2 D, for example, may be arranged along a length of a light guide that is elongate along the y-axis, for example. In some implementations, such light-emitting devices may share one or more tubular scattering elements extending along the y-axis. Multiple light-emitting elements may be arranged along a light guide that is elongate along the y-axis, for example. Instead of spherical shaped segments (e.g., as illustrated in FIG.  2 B,) the tubular scattering element can have ring-shaped segments or other regular or irregular shaped segments, for example. 
         [0083]      FIG. 3  is a schematic diagram showing an example feedback circuit  300  used to provide intra-system source feedback in a light-emitting device, such as the light-emitting devices  100 ,  200 , and/or  270 . The feedback circuit  300  can be used to control one or more groups of light-emitting elements (e.g., adjust the brightness) of the light-emitting device. In this example, the feedback circuit  300  includes a photonic sensing unit  320  and a controller  330 . 
         [0084]    The photonic sensing unit  320  can be placed to sample scattered mixed light, for example downstream from a scattering element of the light-emitting device, to sense scattered light propagating within an extractor element of the light-emitting device. In some implementations, the photonic sensing unit  320  can include a color detector, an intensity detector, or a combination of both. In some implementations, one or more of the detectors can be arranged such that mostly scattered light that is Fresnel-reflected at an exit interface of the extractor element is being sensed. Moreover, the one or more detectors can be arranged such that the scattered light reflected by the exit surface of the extractor element and received by the sensor originates from a large portion of an optical interface between the scattering element and the extractor element. 
         [0085]    The controller unit  330  can be implemented as hardware, software or a combination of both. For example, the controller unit  330  can be implemented as a software driver executed by a specialized or general purpose chip. The controller unit  330  parses sensing signals received from the photonic sensing unit  320 . Parsed signal values are compared by the controller unit  330  to reference color values or reference intensity values, referred to as reference values. The controller unit  330  accesses such reference values in one or more lookup tables, for instance. For example, the controller unit  330  selectively transmits adjustment signals to a power driver to adjust relative power values for a combination of different color light-emitting elements  310 , in response to sensing that chromaticity of the scattered light propagating in the extractor element has changed. As another example, the controller unit  330  selectively transmits adjustment signals to the power driver to adjust power values for one or more light-emitting elements  310 , in response to sensing that the intensity of the scattered light propagating in the extractor element has changed. 
         [0086]    In some implementations, the feedback circuit  300  can include a non-photonic properties sensing unit  340 . Examples of non-photonic properties sensed by this unit are temperature, voltage drop, etc. In such implementations, the controller unit  330  parses the non-photonic sensing signals received from the non-photonic properties sensing unit  340  in combination with the photonic sensing signals received from the photonic sensing unit  320 . Values of the parsed combination of photonic and non-photonic sensing signals are used by the controller unit  330  to transmit adjustment signals to the driver that drives the LEEs  310 .