Patent Publication Number: US-11662077-B2

Title: Color mixing optics for LED illumination device

Description:
PRIORITY CLAIM 
     This application is a continuation of U.S. application Ser. No. 17/013,214, filed on Sep. 4, 2020, which is a continuation of U.S. application Ser. No. 16/422,927, filed May 24, 2019, now U.S. Pat. No. 10,767,835, issued on Sep. 8, 2020, which is a continuation of U.S. application Ser. No. 15/653,608, filed Jul. 19, 2017, now U.S. Pat. No. 10,302,276, issued May 28, 2019, which claims priority to and is a divisional of U.S. application Ser. No. 14/505,671, filed Oct. 3, 2014, now U.S. Pat. No. 9,736,895, issued Aug. 15, 2017, which claims priority to U.S. Provisional Application No. 61/886,471, filed Oct. 3, 2013. Each of these applications are incorporated by reference herein in their entirety. 
     RELATED APPLICATIONS 
     This application is related to the following applications: U.S. application Ser. No. 12/803,805, which was issued as U.S. Pat. No. 9,509,525; Ser. No. 12/806,118, which was issued as U.S. Pat. No. 8,773,336; Ser. No. 13/970,944, which was issued as U.S. Pat. No. 9,237,620; Ser. No. 13/970,964, which was issued as U.S. Pat. No. 9,651,632; Ser. No. 13/970,990, which was issued as U.S. Pat. No. 9,578,724; Ser. No. 14/314,530, which was issued as U.S. Pat. No. 9,769,899; Ser. No. 14/314,580, which was issued as U.S. Pat. No. 9,392,663; and Ser. No. 14/471,081, which was issued as U.S. Pat. No. 9,510,416—each of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention relates to the addition of color mixing optics and optical feedback to produce uniform color throughout the light beam produced by a multi-color LED illumination device. 
     2. Description of Related Art 
     Multi-color LED illumination devices (also referred to herein as light sources, luminaires or lamps) have been commercially available for many years. For example, Cree has marketed a variety of primarily indoor downlights, troffers, and other form factor luminaires that combine white and red LEDs to provide higher color rendering index (CRI) and efficacy than conventional white LEDs alone can provide. 
     Philips Color Kinetics has marketed many multi-color LED products; however, most are restricted to indoor and outdoor saturated wall-washing color and color changing effects. Recently, Philips introduced the “Hue” product, which has an A19 form factor that provides colored, as well as white light. This product combines blue, red, and phosphor converted LEDs to produce saturated blue and red light, pastel green, and white light that can be controlled by a computer or smartphone. The phosphor converted LEDs produce a greenish light, but cannot produce a saturated green, like that of a red/green/blue/white (RGBW) LED combination. Since the Hue product has an A19 form factor, color mixing is achieved with simple diffusers arranged in the output light path above the LED package. Color accuracy in the Hue product is susceptible to LED aging, since it does not use optical feedback to compensate for the change in luminance over time for each of the differently colored LEDs. 
     Conventional color mixing optics typically use light guides, which tend to be large and inefficient. The rule of thumb for a light guide is that it should be about 10 times longer than the dimensions of the multi-color light source. A typical 90 Watt halogen bulb produces about 1200 lumens. An array of many large LEDs is necessary to produce such output light. For instance, 1200 lumen output LED arrays from Cree are about 5-6 mm in diameter. If such a light source comprised multi-colored LEDs, a 50-60 mm light guide would be needed to properly mix the colors. Considering that the light beam needs to be shaped after color mixing, the dimensions needed for a light guide become prohibitive. 
     No products currently exist on the market that provide both accurate white light along the black body curve and saturated colors. Further, no such products exist in a PAR form factor that provide uniform color throughout the standard 10, 25, and 40 degree beam angles. As such, a need exists for improved techniques to produce full color gamut LED light sources that do not change over time and that have uniform color throughout the entire light beam. 
     SUMMARY OF THE INVENTION 
     Illumination devices with improved color mixing optics and methods are disclosed herein for mixing the colors produced by a multi-colored LED emitter module to produce uniform color throughout the entire beam angle of the output light beam. Embodiments disclosed herein include a unique arrangement of multi-color LEDs in an emitter module, a unique exit lens with different patterns of lenslets formed on opposing sides of the lens, and other associated optical features that thoroughly mix the different color components, and as such, provide uniform color across the output beam exiting the illumination device. Additional embodiments disclosed herein include an arrangement of photodetectors within the primary optics structure of the LED emitter module that ensure the optical feedback system properly measures the light produced by all emission LEDs. As described herein, various embodiments may be utilized, and a variety of features and variations can be implemented as desired, and related systems and methods can be utilized as well. Although the various embodiments disclosed herein are described as being implemented in a PAR38 lamp, certain features of the disclosed embodiments may be utilized in illumination devices having other form factors to improve the color mixing in those devices. 
     According to one embodiment, an emitter module of an illumination device may include a plurality of emission LEDs that are mounted onto a substrate and encapsulated within a primary optics structure. In a preferred embodiment, the plurality of emission LEDs are electrically coupled as N chains of serially connected LEDs with N LEDs in each chain, and each chain may be configured to produce a different color of light. In some embodiments, the colors of LEDs included within the multi-color emitter module may be selected to provide a wide output color gamut and a range of precise white color temperatures along the black body curve. For example, chains of red, green, and blue (RGB) LEDs can be used to provide saturated colors, and the light from such RGB chains can be combined with a chain of phosphor converted white LEDs to provide a wide range of white and pastel colors. In one embodiment, each of the four RGBW LED chains may comprise four LEDs to provide sufficient lumen output, efficacy, and color mixing; however, the invention can be applied to various numbers of LED chains, combinations of LED colors, and numbers of LEDs per chain without departing from the scope of the invention. As described in more detail below, the illumination device improves color mixing, at least in part, by arranging the multi-color emission LEDs in a unique pattern. 
     According to one embodiment, the plurality of emission LEDs may be arranged in an array of N×N LEDs, where N is the number of LED chains and the number of LEDs included within each chain. In order to improve color mixing, the serially connected LEDs within each chain may be spatially scattered throughout the array, such that no two LEDs of the same color are arranged in the same row, column or diagonal. In the above example of four chains of four LEDs per chain (e.g., four red LEDs, four green LEDs, four blue LEDs and four white LEDs), the different colored LEDs are arranged in a four by four square, such that no two LEDs of the same color exist in the same row, column, or diagonal. It is generally desired that the LEDs be placed together as tightly as possible, and that the LED colors with the biggest difference in spectrum (e.g., red and blue) be grouped closer together. 
     It is worth noting that the inventive features described herein are not limited to a multi-colored LED emitter module having four chains of four LEDs per chain, and may be applied to a multi-colored LED emitter module including substantially any number of chains with substantially any number of LEDs per chain. For example, one alternative configuration may include four red, four blue, and eight phosphor converted LEDs for an application with higher lumen output, but smaller color gamut. In such a configuration, the additional four phosphor converted LEDs may replace the four green LEDs. Another alternative configuration may include chains of four red, four blue, four green and four yellow LEDs. Yet another alternative configuration may include chains of three red, three blue and three green LEDs. The number of LED chains, the number of LEDs per chain, and the combination of LED colors may be chosen to provide a desired lumen output and color gamut. 
     According to another embodiment, the plurality of emission LEDs within the emitter module may be spatially divided into N blocks, wherein N is an integer value greater than or equal to three (3). Each of the N blocks may consist of N LEDs, wherein each LED is configured for producing a different color of light. The N differently colored LEDs within each block are preferably arranged to form a polygon having N sides. For example, if N=3, the three differently colored LEDs (e.g., RGB) within each block are arranged to form a triangle. If N=4, the four differently colored LEDs (e.g., RGBW or RGBY) within each block are arranged to form a square. 
     The N blocks of LEDs may be arranged in a pattern on the substrate of the emitter module to form an outer polygon having N sides and an inner polygon having N sides. If N=3, the inner and outer polygons form triangles, and if N=4, the inner and outer polygons form squares. Within the outer polygon, the N blocks of LEDs are arranged on the substrate, such that: one LED within each block is located on a different vertex of the inner polygon, and the remaining LEDs within each block are located along the N sides of the outer polygon. To improve color mixing within the emitter module, the N blocks of LEDs are arranged, such that the LEDs located on the vertices of the inner polygon are each configured to produce a different color of light, and the LEDs located along each side of the outer polygon are also each configured to produce a different color of light. Such a configuration spatially scatters the differently colored LEDs across the substrate to improving color mixing within the illumination device. 
     According to another embodiment, the plurality of emission LEDs are mounted onto a ceramic substrate, such as aluminum nitride or aluminum oxide (or some other reflective surface), and encapsulated within a primary optics structure. As noted above, the plurality of emission LEDs may be arranged in a pattern on the substrate so as to form an outer polygon having N sides, where N is an integer value greater than or equal to 3. In one embodiment, the primary optics structure encapsulating the emission LEDs may be a silicone hemispherical dome, wherein the diameter of the dome is substantially larger (e.g., about 1.5 to 4 times larger) than the diameter of the LED array to prevent occurrences of total internal reflection. The dome may be generally configured to transmit a majority of the illumination emitted by the emission LEDs. In some embodiments, the dome may be textured with a slightly diffused surface to increase light scattering and promote color mixing, as well as to provide a slight increase (e.g., about 5%) in reflected light back toward photodetectors, which are also mounted on the substrate of the emitter module and encapsulated within the dome. 
     According to another embodiment, a plurality of photodetectors may be mounted on the substrate (e.g., a ceramic substrate) and encapsulated within the primary optics structure (e.g., within the hemispherical dome). The photodetectors may be silicon diodes, although LEDs configured in a reverse bias may be preferred. According to one embodiment, a total of N photodetectors may be mounted on the substrate and arranged around a periphery of the outer polygon having N sides, such that the N photodetectors are placed near a center of the N sides of the outer polygon. In one example, four photodetectors (detector LEDs or silicon diodes) may be mounted on the substrate, one per side, in the middle of the side, and as close as possible to the square N×N array of emission LEDs. In another example, three photodetectors (detector LEDs or silicon photodiodes) may be mounted on the substrate, one per side, near the middle and as close as possible to each side of the triangular pattern of 3 blocks of 3 differently colored LEDs. 
     In addition to having a desired arrangement on the substrate, the plurality of photodetectors are preferably connected in parallel to receiver circuitry of the illumination device for detecting a portion of the illumination that is emitted by the emission LEDs and/or reflected by the dome. In general, the receiver circuitry typically may comprise a trans-impedance amplifier that detects the amount of light produced by each emission LED chain individually. Various other patents and patent applications assigned to the assignee, including U.S. Publication No. 2010/0327764, describe means to periodically turn all but one emission LED chain off so that the light produced by each chain can be individually measured. This invention describes the placement and connection between the photodetectors to ensure that the light for all similarly colored emission LEDs, which are scattered across the substrate, is properly detected. 
     Any photodetector in a multi-color illumination device with optical feedback should be placed to minimize interference from external light sources. This invention places the photodetectors within the primary optics structure (e.g., the silicone dome) for this purpose. The four photodetectors are connected in parallel to sum the photocurrent produced by each photodetector, which minimizes any spatial variation in photocurrents caused by scattering the similarly colored emission LEDs across the substrate. According to one embodiment, the photodetectors are preferably red or yellow LEDs, but could comprise silicon diodes or any other type of light detector. The red or yellow detector LEDs are preferable since silicon diodes are sensitive to infrared as well as visible light, while the LEDs are sensitive to only visible light. 
     LED or silicon photodetectors produce current that is proportional to incident light. Such current sources easily sum when the photodetectors are connected in parallel. When connected in parallel, the N photodetectors function as one larger detector, but with much better spatial uniformity. For instance, with only one photodetector, light from one LED in a given chain may produce much more photocurrent than light from another LED in the same chain. As the emission LEDs age and the light output decreases, the optical feedback algorithm compensates for changes in the emission LED that induces the largest photocurrent simply due to LED and detector placement. N photodetectors connected in parallel resolves this issue. 
     In addition to the unique pattern in which the multi-colored LED chains are scattered about the emitter array, the advantageous placement of parallel coupled LED photodetectors within the primary optics structure, and the optionally diffused dome, additional embodiments disclosed herein provide unique secondary optics to provide further color mixing and beam shaping for the illumination device. According to one embodiment, such secondary optics may include an exit lens with substantially different arrays of lenslets formed on opposing sides of the lens, and a parabolic reflector having a plurality of planar facets (or lunes) that produce uniform color in the light beam exiting the illumination device and partially shape the light beam. 
     According to one example, a unique exit lens structure may comprise a double-sided pillow lens having an array of lenslets formed on each side of the lens, wherein the array of lenslets formed on an interior side of the exit lens is configured with an identical aperture shape, but different dimensions (e.g., size, curvature, etc.) than the array of lenslets formed on an exterior side of the exit lens. Such an exit lens breaks up the light rays from each individual emission LED and effectively randomizes the light rays to promote color mixing. The lunes in the parabolic reflector provide further randomization and color mixing, as well as beam shaping. 
     In some embodiments, the identical aperture shape of the lenslets formed on the interior and exterior sides of the exit lens may be a polygon having N sides, wherein N is an even number greater than or equal to four (4) (e.g., a square, hexagon, octagon, etc.). A polygon with an even number of straight sides is desirable, in some embodiments, since it provides a repeatable pattern of lenslets. However, the aperture shape is not limited to a polygon, and may be substantially circular in other embodiments. 
     The exit lens is preferably designed such that the lenslets formed on the interior side are substantially larger than the lenslets formed on the exterior side of the exit lens. As light rays from the emitter module enter the exit lens, the larger lenslets on the interior side of the lens function to slightly redirect the light rays through the interior of the exit lens, while the smaller lenslets on the exterior side of the exit lens focus the light rays differently, depending on the location of the individual smaller lenslets relative to the larger lenslets. The resulting output light beam has uniform color across the entire beam angle and softer edges than can be provided by a conventional exit lens, such as a single-sided pillow lens, wherein lenslets are provided on only one side of the lens, while a planar surface or Fresnel lens is provided on the other side. 
     In one example, the internal side of the exit lens may include a pattern of hexagonal lenslets that are, for example, three times larger than the diameter of the hexagonal lenslets included on the exterior side of the lens. In this example, an aperture ratio of the hexagonal lenslets formed on the interior side to the hexagonal lenslets formed on the exterior side may be 3:1. In another example, square or circular lenslets may be used on the interior and exterior sides of the exit lens. When square lenslets are used, the aperture ratio of the lenslets formed on the interior side to those on the exterior side may be 4:1. When circular lenslets are used, the aperture ratio of the lenslets formed on the interior side to those on the exterior side may be 3:1 or 4:1. Other aperture ratios may be used as desired. 
     In addition to aperture shape and size, the curvature of the lenslets, the alignment of the lenslet arrays and the material of the exit lens may be configured to provide a desired beam shaping effect. In some embodiments, the arrays of lenslets formed on the interior and exterior sides of the exit lens may be aligned, such that a center of each larger lenslet formed on the exterior side is aligned with a center of one of the smaller lenslets formed on the interior side of the exit lens. Aligning the lenslet arrays in such a manner significantly improves center beam intensity, which is important for focused light applications. In some embodiments, the curvature of the lenslets (defined by the radius of the arcs that create the lenslets) may also be chosen to shape the beam and improve center beam intensity. In one example, a curvature ratio of the lenslets formed on the interior side to those formed on the exterior side may be within a range of about 1:10 to about 1:9. It is noted, however, that the curvature ratio and the aperture ratios mentioned are exemplary and generally valid when the exit lens is formed from a material having a refractive index within a range of about 1.45 to about 1.65. Other curvature ratios and aperture ratios may be appropriate when using materials with a substantially different refractive index. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
         FIG.  1    is a picture of an exemplary illumination device. 
         FIG.  2    is a picture of various components included within the exemplary illumination device. 
         FIG.  3    is an exemplary block diagram of circuitry included within the driver board and LED emitter module of the exemplary illumination device. 
         FIG.  4    is an exemplary illustration of the color gamut provided by the exemplary illumination device on a CIE1931 color chart. 
         FIG.  5    is a picture of the exemplary heat sink and emitter module for the exemplary illumination device. 
         FIG.  6    is a close up view of the exemplary emitter module. 
         FIG.  7    is a computer drawing of the exemplary emitter module illustrating a unique arrangement of emission LEDs and photodetectors, according to one embodiment. 
         FIG.  8    is a diagram illustrating another unique arrangement of emission LEDs and photodetectors, according to another embodiment. 
         FIG.  9    is a diagram illustrating further details of the arrangement of emission LEDs and photodetectors shown in  FIG.  7   . 
         FIG.  10    is a picture of an exemplary reflector. 
         FIG.  11    is a picture of an exemplary exit lens. 
         FIG.  12    is an exemplary drawing of a portion of an exit lens illustrating the structure of the lens as a double-sided pillow lens comprising an array of lenslets formed on each side of the lens, according to one embodiment. 
         FIG.  13    is an exemplary drawing of a portion of an exit lens illustrating the structure of the lens as a double-sided pillow lens comprising an array of lenslets formed on each side of the lens, according to another embodiment. 
         FIG.  14    is an exemplary ray diagram illustrating the color mixing effect of the exit lens. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings,  FIG.  1    is a picture of an example illumination device  10 , which according to one embodiment, is an LED lamp with a PAR38 form factor. As described in more detail below, LED lamp  10  produces light over a wide color gamut, thoroughly mixes the color components within the beam, and uses an optical feedback system to maintain precise color over LED lifetime. LED lamp  10  is preferably powered by the AC mains and screws into any standard PAR38 fixture. The light beam produced by LED lamp  10  is substantially the same as the light beam produced by halogen PAR38 lamps with any beam angle, but typically between 10 and 40 degrees. 
     LED lamp  10  is just one example of a wide color gamut illumination device that is configured to provide uniform color within the beam and precise color control over LED lifetime. In addition to a PAR38 form factor, the inventive concepts described herein could be implemented in other standard downlight form factors, such as PAR20 or PAR30, or MR 8 or 16. Additionally, the inventive concepts could be implemented in luminaires with non-standard form factors, such as outdoor spot lights using light engines. As such,  FIG.  1    is just one example implementation of an illumination device according to the invention. 
       FIG.  2    is a picture of possible components included within example LED lamp  10  comprising Edison base  21 , driver housing  22 , driver board  23 , heat sink  24 , emitter module  25 , reflector  26 , and exit lens  27 . In the illustrated embodiment, Edison base  21  connects to the AC mains through a standard connection and provides power to driver board  23 , which resides inside driver housing  22  when assembled. Driver board  23  converts AC power to well controlled DC currents for controlling the emission LEDs (shown in  FIGS.  3  and  6 - 9   ) included within emitter module  25 . Driver board  23  and emitter module  25  are thermally connected to heat sink  24 . Driver board  23  also connects to the photodetectors (shown in  FIGS.  3  and  6 - 9   ) on emitter module  25 . 
     Light produced by the emission LEDs within emitter module  25  is shaped into an output beam by parabolic reflector  26 . The planar facets or lunes included within reflector  26  (shown in  FIG.  10   ) provide some randomization of light rays from emitter module  25  prior to exiting LED lamp  10  through exit lens  27 . Exit lens  27  comprises an array of lenslets formed on both sides of the exit lens. As described in more detail below, the lenslets formed on the interior side of the exit lens are preferably configured with an identical aperture shape, but different dimensions, than the lenslets formed on the exterior side of the exit lens. In some embodiments, each side of the exit lens  27  may include an array of hexagonally, square or circular shaped lenslets. However, the lenslets included on one side of the exit lens may be substantially larger than the lenslets included on the other side of the exit lens. Providing an exit lens  27  with different sized, yet identically shaped lenslets randomizes the light rays from emitter module  25 , while the reflector  26  further randomizes the light rays and also shapes the beam exiting LED lamp  10 . 
       FIG.  2    illustrates just one possible set of components for LED lamp  10 . If LED lamp  10  conformed to standard form factors, other than PAR38, the mechanics and optics could be significantly different than shown in  FIG.  2   . Likewise, the components would also be different for luminaires using light engines or other light sources. As such,  FIG.  2    is just one example. 
       FIG.  3    is an exemplary block diagram for the circuitry, which may be included on driver board  23  and emitter module  25 , according to one embodiment. In the illustrated embodiment, driver board  23  comprises AC/DC converter  30 , control circuit  31 , LED drivers  32 , and receiver  33 . AC/DC converter  30  functions to converter the AC mains voltage (e.g., 120V or 240V) to a DC voltage (e.g., typically 15-20V), which is used in some embodiments to power control circuit  31 , LED drivers  32 , and receiver  33 . In some embodiments, a DC/DC converter (not shown in  FIG.  3   ) may be included on the driver board  23  to further regulate the DC voltage from AC/DC converter  30  to lower voltages (e.g., 3.3V), which may be used to power low voltage circuitry included within the illumination device, such as a PLL (not shown), a wireless interface (not shown) and/or the control circuit  31 . LED drivers  32  are connected to emission LEDs  34  and receiver  33  is connected to photodetectors  35 . In some embodiments, LED drivers  32  may comprise step down DC to DC converters that provide substantially constant current to the emission LEDs  34 . 
     Emission LEDs  34 , in this example, comprise four differently colored chains of LEDs, each having four LEDs per chain. In one example, emission LEDs  34  may include a chain of four red LEDs, a chain of four green LEDs, a chain of four blue LEDs, a chain of four white LEDs. In another example, a chain of four yellow LEDs may be used in place of the chain of four white LEDs. In yet another example, an additional chain of white LEDs may be used in place of the chain of green LEDs. Although four chains of four LEDs per chain are shown in  FIG.  3   , the emission LEDs  34  are not restricted to the illustrated embodiment and may comprise substantially any number of chains with substantially any number of LEDs per chain. In addition, the emission LEDs  34  are not restricted to only the color combinations mentioned herein and may comprise substantially any combination of differently colored LED chains. In fact, the only restriction placed on the emission LEDs  34  is that the identically colored LEDs within each chain are serially connected, yet spatially scattered across the emitter module  25 . Unique arrangements of the emission LEDs  34  are described below with respect to  FIGS.  7 - 9   . 
     In general, LED drivers  32  may include a number of driver blocks equal to the number of LED chains  34  included within the illumination device. In the exemplary embodiment shown in  FIG.  3   , LED drivers  32  comprise four driver blocks, each configured to produce illumination from a different one of the LED chains  34 . Each driver block receives data indicating a desired drive current from the control circuit  31 , along with a latching signal indicating when the driver block should change the drive current supplied to a respective one of the emission LED chains  34 . Each driver block within LED drivers  32  typically produces and supplies a different current (level or duty cycle) to each chain to produce the desired overall color output from LED lamp  10 . 
     In some embodiments, LED drivers  32  may comprise circuitry to measure ambient temperature, emitter and/or detector forward voltage, and/or photocurrent induced in the photodetectors by ambient light or light emitted by the emission LEDs  34 . In one example, LED drivers  32  may include circuitry to measure the operating temperature of the emission LEDs  34  through mechanisms described, e.g., in U.S. application Ser. Nos. 13/970,944; 13/970,964; and 13/970,990. Such circuitry may be configured to periodically turn off all LED chains but one to perform forward voltage measurements on each LED chain, one chain at a time, during periodic intervals. The forward voltage measurements detected for each LED chain may then be used to adjust the drive currents supplied to each LED chain to account for changes in LED intensity caused by changes in temperature. In another example, LED drivers  32  may include circuitry for obtaining forward voltage and induced photocurrent measurements during the periodic intervals, so that the respective drive currents supplied to the LED chains can be adjusted to account for changes in LED intensity and/or chromaticity caused by changes in drive current, temperature or LED aging. Exemplary driver circuitry is described, e.g., in U.S. application Ser. Nos. 14/314,530; 14/314,580; and 14/471,081. 
     As shown in  FIG.  3   , a plurality of photodetectors  35  are connected in parallel to the receiver circuitry  33  of the illumination device for detecting at least a portion of the illumination emitted by the emission LEDs  34 . In one example, the plurality of photodetectors  35  may comprise four small red LEDs, which are connected in parallel to receiver  33 . However, the photodetectors  35  are not limited to red LEDs, and may alternatively comprise yellow or orange LEDs, silicon diodes or any other type of light detector. In some embodiments, red or yellow detector LEDs are preferable since silicon diodes are sensitive to infrared as well as visible light, while the LEDs are sensitive only to visible light. 
     LED or silicon photodetectors produce photocurrent that is proportional to incident light. This photocurrent easily sums when the photodetectors are connected in parallel, as shown in  FIG.  3   . When connected in parallel, the plurality of photodetectors  35  function as one larger detector, but with much better spatial uniformity. For example, preferred embodiments of the invention scatter or distribute the same colored LEDs within each chain across the emitter module  25  to improve color mixing. If only one photodetector were included within the emitter module  25 , light from one LED in a given chain would produce much more photocurrent than light from another LED in the same chain. By distributing the photodetectors  35  around a periphery of the emission LEDs  34  and connecting the photodetectors  35  in parallel, the photocurrents produced by each of the photodetector  35  is summed to minimize any spatial variation in photocurrents caused by scattering the same colored emission LEDs across the emitter module. 
     Receiver  33  may comprise a trans-impedance amplifier that converts the summed photocurrent to a voltage that may be digitized by an analog-to-digital converter (ADC) and used by control circuit  31  to adjust the drive currents produced by LED drivers  32 . In some embodiments, receiver  33  may further measure the temperature (or forward voltage) of photodetectors  35  through mechanisms described, e.g., in pending U.S. patent application Ser. Nos. 13/970,944, 13/970,964, 13/970,990. In some embodiments, receiver  33  may also measure the forward voltage developed across the photodetectors  35  and the photocurrent induced within the photodetectors  35  as described, e.g., in pending U.S. patent application Ser. Nos. 14/314,530, 14/314,580 and 14/471,081. The forward voltage and/or induced photocurrent measurements may be used by the control circuit  31  to adjust the drive currents produced by the LED drivers  32  to account for changes in LED intensity and/or chromaticity caused by changes in drive current, temperature or LED aging. 
     Control circuit  31  may comprise means to control the color and/or brightness of LED lamp  10 . Control circuit  31  may also manage the interaction between AC/DC converter  30 , LED drivers  32 , and receiver  33  to provide the features and functions necessary for LED lamp  10 . For example, control circuit  31  may be configured for determining the respective drive currents, which should be supplied to the emission LEDs  34  to achieve a desired intensity and/or a desired chromaticity for the illumination device. The control circuit  31  may also be configured for providing data to the driver blocks indicating the desired drive currents, along with a latching signal indicating when the driver blocks should change the drive currents supplied to the LED chains  34 . Control circuit  31  may further comprise memory for storing calibration information, which may be used to adjust the drive currents supplied to the emission LEDs  34  to account for changes in drive current, temperature and LED aging effects. Examples of calibration information and methods, which use such calibration information to adjust LED drive currents, are disclosed in the pending U.S. patent applications mentioned herein. 
       FIG.  3    is just one example of many possible block diagrams for driver board  23  and emitter module  25 . Driver board  23  could, for instance, be configured to drive more or less LED chains, or have multiple receiver channels. In other embodiments, driver board  23  could be powered by a DC voltage instead of an AC voltage, and as such, would not need AC/DC converter  30 . Emitter module  25  could have more or less emission LEDs  34  configured in more or less chains or more or less LEDs per chain. As such,  FIG.  3    is just an example. 
       FIG.  4    is an illustration of an exemplary color gamut that may be possible to produce with LED lamp  10 . Points  40 ,  41 ,  42 , and  43  represent the color respectively produced by exemplary red, green, blue, and white LED chains  34 . The lines  44 ,  45 , and  46  represent the boundaries of the colors that such a combination of emission LEDs could produce. All colors within the color gamut or triangle formed by lines  44 ,  45 , and  46  can be produced. 
       FIG.  4    is just one example color gamut. For instance, the green LED chain within LEDs  34  could be replaced with four more phosphor converted white LEDs to produce higher lumen output over a small color gamut. Such phosphor converted white LEDs could have chromaticity in the range of (0.4, 0.5) which is commonly used in white plus red LED lamps. Alternatively, cyan or yellow LED chains could be added to expand the color gamut or used in place of the chain of white LEDs. As such  FIG.  4    is just one example color gamut. 
       FIG.  5    illustrates an example placement of emitter module  25  within heat sink  24 .  FIG.  6    is a close-up picture of an exemplary embodiment of an emitter module  25  with a 4×4 array of emission LEDs  34  and four photodetector LEDs  35 , each arranged as close as possible to a different side of the LED emitter array. 
     As shown in  FIG.  6   , emission LEDs  34  and photodetectors  35  are mounted on a substrate  60  and are encapsulated by a primary optics structure  61 . In one embodiment, substrate  60  may comprise a laminate material such as a printed circuit board (PCB) FR4 material, or a metal clad PCB material. However, substrate  60  is preferably formed from a ceramic material (or some other optically reflective material), in at least one embodiment of the invention, so that the substrate may generally function to improve output efficiency by reflecting light back out of the emitter module  25 . In some embodiments, substrate  60  may comprise an aluminum nitride or an aluminum oxide material, although different materials may be used. In some embodiments, substrate  60  may be further configured as described, e.g., in U.S. application Ser. Nos. 14/314,530 and 14/314,580. 
     The primary optics structure  61  may be formed from a variety of different materials and may have substantially any shape and/or dimensions necessary to shape the light emitted by the emission LEDs  34  in a desirable manner. According to one embodiment, the primary optics structure  61  is a hemispherical dome. However, one skilled in the art would understand how the primary optics structure  61  may have substantially any other shape or configuration, which encapsulates the emission LEDs  34  and the photodetectors  35  within the primary optics structure  61 . In general, the shape, size and material of the dome  61  are configured to improve optical efficiency and color mixing within the emitter module  25 . 
     In the PAR  38  form factor, the diameter of the dome  61  is preferably larger than the diameter of the array of emission LEDs  34 , and may be on the order of 1.5 to 4 times larger, in some embodiments. Smaller or larger dome diameters may be used in other form factors. The dome  61  may comprise substantially any light transmissive material, such as silicone, and may be formed through an overmolding process, for example. In some embodiments, the surface of the dome  61  may be lightly textured to increase light scattering and promote color mixing, as well as to slightly increase (e.g., about 5%) the amount of light reflected back toward the detectors  35  mounted on the ceramic substrate  60 . 
       FIG.  7    is a computer drawing showing one embodiment of emitter module  25  comprising a 4×4 array of emission LEDs  34  and four LED photodetectors  35 . In this example, the 4×4 array of emission LEDs  34  comprises a chain of four red LEDs, a chain of four green LEDs, a chain of four blue LEDs, and a chain of four white LEDs. The emission LEDs  34  in each chain are electrically coupled in series, yet spatially scattered about the array, so that no color appears twice in any row, column or diagonal. Such a color pattern is unique for a 4×4 array and improves color mixing over other arrangements of emission LEDs that do not follow such rule. Although a particular pattern of LEDs  34  is shown in  FIG.  7   , the distribution of the same colored LEDs in each chain across the 4×4 array can change and the pattern can be rotated or mirrored. In some embodiments, the above rule can be expanded to N×N arrays of N LED chains with N LEDs per chain, where N is any number greater than three. In some cases, more than one LED chain may be provided with the same color of LEDs, provided the number of LEDs per chain is a multiple of N. Multiple patterns exist for arrays larger than 4×4. 
       FIG.  7    also illustrates an example placement of photodetectors  35  relative to the 4×4 array of emission LEDs  34 . In this example, the array of emission LEDs  34  forms a square, and the photodetectors  35  are placed close to, and in the middle of, each edge of the square. Photodetectors  35  may be any devices that produce current indicative of incident light. However, photodetectors  35  are preferably LEDs with peak emission wavelengths in the range of 550 nm to 700 nm, since such photodetectors will not produce photocurrent in response to infrared light, which reduces interference from ambient light. In one exemplary embodiment, photodetectors  35  may include red, orange, yellow and/or green LEDs. The LEDs used to implement photodetectors  35  are generally smaller than the emission LEDs  34 , and are generally arranged to capture a maximum amount of light that is emitted from the emission LEDs  34  and/or reflected from the dome  61 . 
     As shown in  FIG.  3    and described above, the photodetectors  35  are coupled in parallel to receiver  33 . By connecting the photodetectors  35  in parallel with the receiver  33 , the photocurrents induced on each of the four photodetectors are summed to minimize spatial variation between the similarly colored LEDs, which are scattered about the array. In other words, the photocurrent induced on each photodetector  35  by each similarly colored emission LED  34  will vary depending on positioning of that LED. By summing the photocurrents induced on the photodetectors  35  by all four similarly colored LEDs, the spatial variation is reduced substantially. The photocurrents are then forwarded to receiver  33  and on to control circuit  31 . 
     The above arrangement of photodetector LEDs  35  and the electrical connection in parallel allow the light output from many different arrangements of emission LEDs  34  to be accurately measured. The key to accurate measurement is that the multiple photodetectors  35  are arranged within the emitter module  25 , such that the sum of the photocurrents is representative of the total light output from each LED chain. In the embodiment of  FIG.  7   , one photodetector is placed on each edge of the emission LED  34  array and all photodetectors  35  are connected in parallel to receiver  33 . However,  FIG.  7    is just one example placement of photodetectors  35  within a multicolor LED emitter module  25 . 
     It is important to note that the arrangement of emission LEDs  34  and photodetectors  35  is not limited to only the embodiment shown in  FIGS.  6 - 7    and described above. In some embodiments, the emission LEDs  34  and photodetectors  35  may be arranged somewhat differently on the substrate  60 , depending on the number of LED chains and the number of LEDs included within each chain. 
     According to one embodiment, emitter module  25  may comprise a plurality of emission LEDs  34  that are electrically coupled as N chains of serially connected LEDs with N LEDs in each chain, wherein each chain is configured to produce a different color of light. Unlike the previous embodiment, in which emission LEDs  34  are arranged in an N×N array and similarly colored LEDs are distributed across the array, the emission LEDs  34  in this embodiment are spatially divided into N blocks, wherein N is an integer value greater than or equal to 3. 
     In some embodiments, each of the N blocks may consist of N LEDs, each configured for producing a different color or wavelength of light. The N differently colored LEDs within each block are arranged to form a polygon having N sides. For example, if N=3, the 3 differently colored LEDs (e.g., RGB) within each block would be arranged to form a triangle. If N=4, the 4 differently colored LEDs (e.g., RGBW or RGBY) within each block would be arranged to form a square, and so on. The N blocks of N LEDs are further arranged in a pattern on the substrate  60  of the emitter module  25 , so as to form an outer polygon having N sides and an inner polygon also having N sides. If N=3, the inner and outer polygons form triangles, and if N=4, the inner and outer polygons form squares. One skilled in the art would understand how different polygons may be formed when N&gt;4.  FIGS.  8 - 9    illustrate this concept. 
     In  FIG.  8   , three blocks  70  of three differently colored LEDs (e.g., RGB)  34  are arranged in a triangular pattern. The three blocks of three LEDs are arranged on the substrate, such that: one LED within each block is located on a different vertex of the inner triangle  72 , and the remaining LEDs within each block are located along the three sides of the outer triangle  74 . To improve color mixing within the emitter module, the three blocks  70  of LEDs are arranged, such that the LEDs located on the vertices of the inner triangle  72  are each configured to produce a different color of light (e.g., RGB), and the LEDs located along each side of the outer triangle  74  are also each configured to produce a different color of light (e.g., RGB). 
     In  FIG.  9   , four blocks  80  of four differently colored LEDs (e.g., RGBW)  34  are arranged in a square pattern. The four blocks of four LEDs are arranged on the substrate, such that: one LED within each block is located on a different vertex of the inner square  82 , and the remaining LEDs within each block are located along the four sides of the outer square  84 . As in the previous embodiment, the four blocks  80  of LEDs are arranged, such that the LEDs located on the vertices of the inner square  82  are each configured to produce a different color of light (e.g., RGBW), and the LEDs located along each side of the outer square  84  are also each configured to produce a different color of light (e.g., RGBW). 
     The configurations shown in  FIGS.  8 - 9    spatially scatter the differently colored chains of LEDs across the substrate  60  to improving color mixing in the illumination device. In order to provide an accurate measurement of the total light output by each LED chain, each of the embodiments shown in  FIGS.  8 - 9    includes N photodetectors  35 , which are mounted on the substrate  60 , encapsulated within the dome  61  and arranged around the outer polygons  74 / 84 , such that each photodetector  35  is placed substantially at the center of each side of the outer polygons  74 / 84 . As noted above, the N photodetectors  35  are electrically connected in parallel to receiver  33  for detecting a portion of the illumination emitted by each individual LED chain. By connecting the N photodetectors  35  in parallel with the receiver  33 , the photocurrents induced on each of the N photodetectors are summed to minimize spatial variation between the similarly colored LEDs, which are scattered across the substrate. 
     The photocurrents induced in the N photodetectors  35  by the emission LEDs  34  are measured for each LED chain, one chain at a time, to obtain a sum of photocurrents that is representative of the total light output from each LED chain. Exemplary methods for measuring such photocurrents are described, e.g., in U.S. patent application Ser. Nos. 14/314,580 and 14/471,081. 
     In one example, drive circuitry (e.g., LED drivers  32 ,  FIG.  3   ) within the illumination device may be coupled for driving the N chains of serially connected LEDs with respective drive currents substantially continuously to produce illumination, and for periodically turning the N chains of serially connected LEDs off for short durations of time to produce periodic intervals. During the periodic intervals, the drive circuitry may be configured for supplying a respective drive current to each LED chain, one chain at a time, to produce illumination from only one LED chain at a time. The receiver circuitry (e.g., receiver  33 ,  FIG.  3   ) within the illumination device is coupled to the N photodetectors  35  for detecting a sum of the photocurrents, which are induced in the N photodetectors  35  upon receiving a portion of the illumination produced by each LED chain, one chain at a time, during the periodic intervals. As noted above, the sum of photocurrents is representative of the total amount of the illumination produced by each LED chain, and also provides good spatial uniformity due to the spatial arrangement and parallel connection of the photodetectors  35 . The photocurrents detected by the receiver circuitry are then forwarded to control circuitry (e.g., control circuit  31 ,  FIG.  3   ), which utilizes the detected photocurrents (possibly along with other measurement values obtained during the periodic intervals) to adjust the drive currents supplied to one or more of the LED chains. The drive currents may be adjusted, in some embodiments, to achieve a desired intensity and/or a desired chromaticity for the illumination device, and/or to account for changes in drive current, temperature or LED aging effects. 
       FIG.  10    is a picture of an exemplary reflector  26  with planar facets or lunes  90  that focus the light beam from emitter module  25  and contribute to mixing the color produced by emitter module  25 . Reflector  26  is preferably an injection modeled polymeric but could comprise substantially any type of reflective material (such as aluminum or other types of metals) and may comprise substantially any shape. Lunes  90  are flattened segments in the otherwise round reflector  26  that slightly randomize the direction of the light rays from emitter module  25  and improve color mixing. 
       FIG.  11    is a picture of an exemplary exit lens  27  having an array of lenslets formed on each side of the lens, wherein the array of lenslets formed on an interior side of the exit lens (i.e., the side adjacent to the emitter module  25 ) is configured with an identical aperture shape, but different dimensions, than the array of lenslets formed on the exterior side of the exit lens. Such an exit lens  27  may be otherwise referred to herein as double-sided pillow lens. 
     In some embodiments, the identical aperture shape of the lenslets formed on the interior side and the lenslets formed on the exterior side may be a polygon having N sides, wherein N is an even number greater than or equal to 4 (e.g., a square, hexagon, octagon, etc.). A polygon with an even number of straight sides is often desirable, since it provides a repeatable pattern of lenslets. However, the aperture shape is not limited to such a polygon and may be substantially circular in other embodiments. 
     The exit lens  27  is preferably designed such that the lenslets formed on the interior side are substantially larger (i.e., have an aperture with a larger diameter) than the lenslets formed on the exterior side. In some embodiments, the difference in size between the lenslets formed on the interior and exterior sides of the exit lens  27  may be described as an aperture ratio, which is defined as the diameter of the larger lenslets to that of the smaller lenslets. 
     In addition to aperture shape and size, the curvature of the individual lenslets, the alignment of the interior and exterior lenslet arrays and the material of the exit lens  27  may be configured to provide a desired beam shaping effect. For example, the curvature of the lenslets (defined by the radius of the arcs that create the lenslets) should be chosen to shape the beam and improve center beam intensity. In addition, the lenslet arrays on the interior and exterior sides of the exit lens  27  should be carefully aligned, such that a center of each of the larger lenslets formed on the interior side is aligned with a center of one of the smaller lenslets formed on the exterior side. Aligning the lenslet arrays in such a manner significantly improves center beam intensity, which is important for focused light applications. Since refractive index affects the angle at which light entering and exiting the lens is refracted, the refractive index of the material used to implement the exit lens  27  should also be considered when selecting the desired aperture shape, size and curvature of the lenslet arrays. According to one embodiment, exit lens  27  preferably comprises injection molded acrylic (e.g., PMMA) having a refractive index between about 1.45 and about 1.65, but could comprise substantially any material that is transparent to visible light. 
       FIG.  12    illustrates one embodiment of an exit lens  27  comprising an array of larger hexagonal lenslets  100  formed on an interior side, and an array of smaller hexagonal lenslets  101  formed on an exterior side of exit lens  27 . It is noted that  FIG.  12    illustrates only a portion of the exit lens  27  and is magnified significantly to illustrate the difference in aperture size and the alignment between the lenslet arrays on the interior and exterior sides of the exit lens. The solid lines in  FIG.  12    illustrate the outline of the larger hexagonal lenslets  100  formed on the interior side, and the dotted lines illustrate the outline of the smaller hexagonal lenslets  101  formed on the exterior side of exit lens  27 . In the exemplary embodiment of  FIG.  12   , an aperture ratio of the larger hexagonal lenslets  100  to the smaller hexagonal lenslets  101  is 3:1. In one example, the interior side of the exit lens  27  includes an array of approximately 3 mm diameter hexagonal lenslets  100 , while the exterior side comprises an array of approximately 1 mm diameter hexagonal lenslets  101 . Alternative diameters for the hexagonal lenslets formed on the interior and exterior sides may be appropriate, as long as the aperture ratio remains 3:1. As shown in  FIG.  12   , the lenslet arrays are preferably aligned, such that the center of each 3 mm diameter lenslet  100  on the interior side of the exit lens is aligned with the center of one of the 1 mm diameter lenslets  101  on the exterior side of the exit lens. Although such an alignment provides the advantage of improving the center beam intensity, it is not required in all embodiments. 
       FIG.  13    illustrates an alternative embodiment of an exit lens  27  comprising arrays of substantially square lenslets  100 / 101  formed on the interior and exterior sides of the exit lens  27 . As with  FIG.  12   ,  FIG.  13    illustrates only a portion of the exit lens  27 , which is magnified significantly to illustrate the difference in aperture size and the alignment between the lenslet arrays on the interior and exterior sides of the exit lens  27 . The solid lines in  FIG.  13    illustrate the outline of the substantially larger square lenslets  100  formed on the interior side, and the dotted lines illustrate the outline of the substantially smaller square lenslets  101  formed on the exterior side of exit lens  27 . In one embodiment, an aperture ratio of the larger square lenslets  100  to the smaller square lenslets  101  is 4:1. In one example, the diameter of larger lenslets  100  may be 4 mm, and the diameter of the smaller lenslets  101  may be 1 mm. Alternative diameters for the square lenslets formed on the interior and exterior sides may be appropriate, as long as the aperture ratio remains 4:1. Like the previous embodiment, the arrays of square lenslets are aligned, such that the center of each larger lenslet  100  formed on the interior side is aligned with the center of one of the smaller lenslets  101  formed on the exterior side of the exit lens  27 . However, such alignment is not required in all embodiments. 
     The lenslet arrays formed on each side of the double-sided exit lens  27  are not limited to the aperture shapes and sizes shown in the embodiments of  FIGS.  12 - 13   . In general, the aperture shape of the lenslet arrays may be substantially any polygon having N sides, wherein N is an even number greater than or equal to 4 (e.g., a square, hexagon, octagon, etc.), or may be substantially circular. When circular lenslets are used, the aperture ratio of the lenslets formed on the interior side to those on the exterior side may be 3:1 or 4:1. Other aperture ratios may be used to provide a desired result. 
     Regardless of aperture shape, the curvature of the lenslets may be chosen to shape the beam and improve center beam intensity. As noted above, the curvature of lenslets  100  and  101  is defined by the radius of the arcs that create lenslets  100  and  101 . The curvature of the lenslets  100  and  101  may be described, in some cases, as a curvature ratio of the larger lenslets  100  formed on the interior side to the smaller lenslets  101  formed on the exterior side. In some embodiments, an appropriate curvature ratio may be within a range of about 1:10 to about 1:9. In one example, the radius of lenslets  100  is about 10 mm and the radius of lenslets  101  is about 1.2 mm. Alternative radii may be appropriate, as long as the curvature ratio remains within the desired range. 
     Although any combination of lenslets  100  and  101  size, shape and curvature are possible, the various shapes and dimensions described above have been shown to provide optimum color mixing and beam shaping performance. However, the exemplary dimensions mentioned above may only be valid when the exit lens  27  is formed from a material having a refractive index within a range of about 1.45 to about 1.65. Other curvature ratios and aperture ratios may be appropriate when using a material with a refractive index that falls outside of this range. 
       FIG.  14    is a light ray diagram illustrating the color mixing and beam shaping effects of exit lens  27 . As light rays  110  from emitter module  25  enter exit lens  27  from the left side of the figure, the larger lenslets  100  formed on the interior side of the exit lens  27  function to slightly redirect the light rays through the interior of the exit lens  27 . The smaller lenslets  101  formed on the exterior side of the exit lens  27  focus the incident light rays differently, depending on the location of the individual smaller lenslets  101  relative to each larger lenslet  100 . The effect of the dual sided exit lens  27  is improved color mixing, softer edges and improved center beam intensity for the resulting light beam  111 . 
       FIGS.  11 - 14    illustrate just a few examples of possible dual-sided exit lens  27  with different lenslet  100  and  101  patterns on each side. In other embodiments, different aperture shapes and aperture ratios could be used. Likewise, the curvature of the lenslets  100  and  101  could change significantly and still achieve the desired results. The exit lens  27  described herein provides improved color mixing and smoother edges with any shape, any ratio of diameters, and any lenslet curvature by generally providing an array of lenslets on each side of the double-sided exit lens, wherein each array comprises an identical aperture shape, but different dimensions. The exit lens  27  described herein further improves center beam intensity by aligning the lenslet arrays, such that the center of each larger lenslet  100  formed on the interior side is aligned with the center of one of the smaller lenslets  101  formed on the exterior side of the exit lens  27 . 
     It is further noted that other variations could also be implemented with respect to the above embodiments, as desired, and numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated.