Patent Publication Number: US-2022228726-A1

Title: Adjustable white light illumination devices

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of U.S. Provisional Application Nos. 62/811,551 (filed on Feb. 28, 2019) and 62/677,405 (filed on May 29, 2018). The foregoing applications are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to adjustable white light illumination devices, and in various embodiments more specifically to white light illumination devices comprising light-emitting diode (LED) devices. 
     BACKGROUND 
     An increasing number of light fixtures utilize LEDs as light sources due to their lower energy consumption, smaller size, improved robustness, and longer operational lifetime relative to conventional filament-based light sources. Conventional LEDs emit light at a particular wavelength, ranging from, for example, red to blue or ultraviolet (UV) light. However, for purposes of general illumination, the monochromatic emitted light by LEDs must be converted to broad-spectrum white light. 
     Conventional white LEDs are typically constructed as phosphor-converted LEDs where a blue LED is covered with a phosphor coating that converts a portion of the emitted blue light to yellow light so as to create white light. The photometric characteristics of the emitted light, such as a color correlated temperature (CCT) value or chromaticity coordinates in the CIE 1931 color space, are generally fixed. While such conventional LED lighting devices may be suitable for some uses, commercial establishments may have different demands—e.g., preferring the CCT of the light to change throughout the day along with the circadian rhythm of individuals (e.g., employees, customers, etc.) exposed to the light. For example, the CCT of the white light may desirably be lower in the late evenings to facilitate a healthy sleep cycle and higher in the afternoon to combat afternoon fatigue. Consequently, there is a need for LED devices that can emit white light with an adjustable CCT value. 
     SUMMARY 
     Accordingly, various embodiments of the present disclosure relate to an LED illumination device capable of emitting white light with tunable characteristics (e.g., a CCT value). In one embodiment, the LED illumination device employs an LED array having multiple LEDs that can be controlled individually or in a group to generate white light having a tunable CCT value within a range. For example, the LED illumination device may include one or more groups of LEDs, each group including at least one red LED emitting red light having an emission wavelength between approximately 600 nanometers (nm) and approximately 670 nm, one “warm” white LED emitting white light having a CCT value between approximately 1800K and approximately 2700K, and one “cool” white LED emitting white light having a CCT value between approximately 3000K and approximately 6500K. Alternatively, each group may include one red LED and two blue LEDs having an emission wavelength between approximately 400 nm and approximately 530 nm; light emitted from the blue LEDs may be converted to the cool white light and warm white light using one or more photo-luminescent materials (e.g., phosphors, quantum dot materials, etc.). In some embodiments, each group of the LEDs includes three blue LEDs. Again, light emitted from two of the blue LEDs may be converted to the cool white light and warm white light. In addition, light emitted from the third blue LED may be converted to red light using, for example, deep-red quantum dots. Optionally, each of the LEDs may be disposed within a “cup-shaped” (e.g., parabolic) reflector for reducing “crosstalk” interactions between the light emitted from an LED and the photo-luminescent material(s) disposed above a neighboring LED In addition, the reflector may be made of a high-reflectivity material so as to redirect upward light from the respective LED, thereby achieving at least partial collimation of the beam. 
     In various embodiments, the LEDs and/or photo-luminescent material(s) are encapsulated within a waveguide material made of, e.g., silicon. Light emitted from the LEDs, including unconverted light and light converted by the photo-luminescent material(s), can be mixed in a mixing region inside the waveguide and then directed to an output region for outputting white light for illumination. Because the waveguide material may cause more blue light to be extracted from the LEDs, the outputted white light may have a higher CCT value (corresponding to cooler white light). To at least partially counteract such an effect, in some embodiments, one or more photo-luminescent materials may be utilized to shill the CCT value of the light emitted from the cool white LEI) toward a green CCT value and/or to shift the CM′ value of the light emitted from the warm white LED toward a yellow CCT value. The illumination device may also include control circuitry for varying a parameter (e.g., the amplitude and/or duty cycle of the applied current or voltage) associated with each LED (or, in some embodiments, each group of the LEDs), thereby adjusting the CCT value of the mixed light to a target value. 
     Accordingly, in one aspect, the invention pertains to a lighting device producing white light having a target color correlated temperature (CCT) value. In various embodiments, the lighting device includes multiple LEDs having (i) one or more red LEDs emitting red light having a wavelength between approximately 600 nm and approximately 670 nm, (ii) one or more warm white LEDs emitting white light having a CCT value between approximately 1800K. and approximately 2700K, and (iii) one or more cool white LEDs emitting white light having a CCT value between approximately 3000K and approximately 6500K; one or more photo-luminescent materials (e.g., a phosphor, a quantum dot material and/or a fluorescent dye) for shifting (i) the CCT value of the light emitted from the cool white LED(s) toward a green CCT value and/or (ii) the CCT value of the light emitted from the warm white LED(s) toward a yellow CCT value; and a waveguide material having (i) a mixing region for mixing the shifted and unshifted light so as to generate white light having the target CCT value and (ii) an output region for outputting the white light. 
     The lighting device may further include control circuitry for adjusting a parameter associated with the red LED(s), warm white LED(s) and/or cool white LED(s) so as to change the target CCT value of the generated white light. In one implementation, the parameter includes an amplitude and/or a duty cycle of a current and/or a voltage associated with the red LED(s), warm white LED(s) and/or cool white LED(s). In addition, the control circuitry may be further configured to adjust the parameter of each of the red LED(s), warm white LED(s) and cool white LED(s) individually. In some embodiments, the LEDs include the first group of the red LEDs electrically coupled to one another; the second group of the warm white LEDs electrically coupled to one another, and the third group of the cool white LEDs electrically coupled to one another; the control circuitry is further configured to adjust each group of the red LEDs, warm white LEDs and cool white LEDs individually. 
     In various embodiments, the lighting device further includes multiple cup-shaped reflectors for at least partial collimation of light emitted from the LEDs. Each reflector may have a top aperture and a bottom aperture and the bottom aperture has one of the LEDs disposed therein. One or more of the reflectors may be a parabolic reflector, and the respective LED(s) disposed therein may be located at or near the focus of the parabolic reflector(s). In one embodiment, the reflectors include or consist essentially of silicone. In addition, the lighting device may further include an encapsulant material filled in a cavity space above one of the LEDs and surrounded by the respective reflector. 
     In various embodiments, one or more of the warm white LEDs and/or one or more of the cool white LEDs include (i) a blue LED emitting blue light having a wavelength between approximately 400 nm and approximately 530 nm, and (ii) a secondary photo-luminescent material (e.g., a (Gd, Y) 3 (Al, Ga) 5 O 12  phosphor), different from the photo-luminescent material(s), for converting at least a portion of the blue light to light having a wavelength longer than the blue light. In addition, the lighting device may further include a, circuit board for mounting the LEDs thereon. In one embodiment, the lighting device further includes a heat-dissipation structure thermally coupled to the circuit board for dissipating heat generated by the LEDs. In addition, the lighting device may further includes one or more reflectors located in the mixing region of the waveguide fix promoting mixing of light. In one implementation, the waveguide material includes or consists essentially of silicone. 
     In another aspect, the invention relates to a lighting device producing white light having a target color correlated temperature (CCT) value. In various embodiments, the lighting device includes multiple LEDs including (i) one or more blue LEDs emitting blue light having a wavelength between approximately 400 nm and approximately 530 nm, (ii) one or more warm white LEDs emitting white light having a CM′ value between approximately 1800K. and approximately 2700K, and (iii) one or more cool white LEDs emitting white light having a CCT value between approximately 3000K and approximately 6500K; one or more photo-luminescent materials (e.g., a phosphor, a quantum dot material and/or a fluorescent dye) for shifting (i) the CCT value of the light emitted from the cool white LED(s) toward a green CCT value and/or (ii) the CCT value of the light emitted from the warm white LED(s) toward a yellow CCT value; a secondary photo-luminescent material (e.g., a deep-red quantum dot material), different from the photo-luminescent material(s), for converting the blue light to red light having a wavelength between approximately 600 nm and approximately 670 nm; and a waveguide material having (i) a mixing region for mixing the shifted and unshifted light so as to generate white light having the target CCT value and (ii) an output region for outputting the white light. 
     The term “color” is used herein to denote the monochromatic or peak wavelength (or wavelengths) of light emitted by one or more LEDs. In addition, the term “uniform,” as used herein, refers to a light intensity distribution whose lower and upper intensity limits are within a factor of four, preferably within a factor of two of each other. As used herein, the terms “approximately,” “roughly,” and “substantially” mean±10%, and in some embodiments, ±5%. Reference throughout this specification to “one example,” “an example,” “one embodiment,” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the present technology. Thus, the occurrences of the phrases “in one example,” “in an example,” “one embodiment,” or “an embodiment” in various places throughout this specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, routines, steps, or characteristics may be combined in any suitable manner in one or more examples of the technology. The headings provided herein are for convenience only and are not intended to limit or interpret the scope or meaning of the claimed technology. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, with an emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
         FIGS. 1A and 1B  depict a top view and a bottom view, respectively, of an example illumination system in accordance with various embodiments; 
         FIGS. 1C and 1D  depict exemplary configurations of the LEDs in an illumination system in accordance with various embodiments; 
         FIG. 2A  schematically depicts an exemplary three-dimensional configuration of an illumination system in accordance with various embodiments; 
         FIG. 2B  depicts a spatial arrangement of LEDs and associated conversion layers for converting the wavelength of at least a portion of the LED light in accordance with various embodiments; 
         FIGS. 2C and 21 ) depict LEI) arrays including various combinations of different types of LEDs in accordance with various embodiments; 
         FIG. 2E  depicts a conversion layer including multiple regions in accordance with various embodiments; 
         FIGS. 3A and 3B  depict an implementation of reflectors surrounding the LEDs in an illumination system in accordance with various embodiments; 
         FIGS. 4A and 4B  depict exemplary color coordinates of the light emitted from a warm white LED, a cool white LED, and a red LED in the CIE 1931 color space in accordance with various embodiments; 
         FIG. 4C  depicts shifts of color coordinates in the color space resulting from a waveguide/encapsulant material and one or more photo-luminescent materials in accordance with various embodiments; 
         FIG. 4D  depicts a green region and a yellow region in the CIE 1931 color space in accordance with various embodiments; and 
         FIG. 4E  depicts adjustments of the CO′ value along the Black Body Curve (BBC) in accordance with various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  conceptually illustrates an exemplary illumination system  100  including one or more strip lighting devices  102  in accordance herewith; each strip  102  has an array of multiple LEDs  104  mounted to a circuit board  106  (e.g., printed circuit board, PCB). Each of the LED groups  104  may include one or more LED dies for emitting light with the same or different characteristics (e.g., colors, powers and/or CCT values). The LEDs  104  may be electrically coupled, via the circuit board  106 , to connectors  108  mounted on each end of the circuit board  106 . The connectors  108  may then electrically couple the LEDs  104  to an external device  110  (e.g., another lighting device, a dimming device, a power supply, an “Internet of things” (IoT) device, or a combination thereof) such that the LEDs  104  may receive power from the external device  110  via the connectors  108  and emit light. 
     In some embodiments, the LEDs  104  are electrically coupled to control circuitry  112  in the strip lighting device(s)  102 . The control circuitry  112  may be configured to control operation of the LEDs  104  (e.g., by regulating the amplitude and/or duty cycle of the current and/or voltage applied to the LEDs  104 ), thereby regulating a characteristic (e.g., intensity or brightness) of the light emitted from the LEDs  104 . For example, the control circuitry  112  may adjust the brightness of individual LEDs using pulse width modulation (PWM). For example, the control circuitry  112  may rapidly turn individual LEDs on and off at a high frequency that is imperceptible to humans. In this example, the brightness of the individual LEDs  104  may be changed by adjusting the ratio of on-time to off-time within a particular cycle (sometimes referred to as a “duty cycle”). The higher the ratio of on-time to off-time, the brighter the LEI). Conversely, lowering the ratio of on-time to off-time dims the LED Thus, the duty cycle may positively correlate to the average flux of the LED being controlled. The control circuitry  112  may vary the ratio of on-time to off-time based on control signals received from the external device  110  via the connectors  108 . In one embodiment, the control circuitry  112  is implemented in circuitry that is external to the illumination system  100 . For example, circuitry in the external device  100  may be configured to regulate the current and/or voltage applied to the LEDs  104 , thereby directly controlling operations thereof. In this case, the control circuitry  112  may be omitted from the illumination system  100  altogether. 
     Generally, the LEDs  104 , control circuitry  112 , and/or the connectors  108  are all mounted to the circuit board  106 . For example, the circuit board  106  may include one or more conductors to electrically couple the components mounted thereto. In addition, the circuit board  106  may be flexible to enable the illumination system  100  to conform to uneven surfaces. Referring to  FIG. 1B , in some embodiments, the bottom surface of the circuit board  106  is connected to a heat dissipation structure  120  (e.g., a conventional heat sink) for dissipating heat generated by the LEDs  104 . 
     The strip lighting device(s)  102  in the illumination system  100  may have particular dimensions to enable a wide range of applications. For example, the lighting devices  102  may have a depth of no more than approximately 1 inch, a length of no more than approximately 25 inches, and a width of no more than approximately 4 inches. It should be appreciated that the strip lighting devices  102  may be constructed with other dimensions, and may be two-dimensional arrays of LED groups rather than one-dimensional strips. 
     In various embodiments, the LEDs  104  are separated by a distance (e.g., 25 millimeters (mm) or 3 mm). In addition, each of the LEDs  104  may be configured to emit light with the same or different characteristic (e.g., wavelength, CCT value, etc.). In one embodiment, the strip lighting devices  102  include one or more groups of LEDs, each group including at least one red LED  104 - 1  having a wavelength between approximately 600 nm and approximately 670 nm, one “warm” white LED  104 - 2  emitting white light having a CCT value between approximately 1800K and approximately 2700K, and one “cool” white LED  104 - 3  emitting white light having a CCT value between approximately 3000K and approximately 6500K. The group of LEDs  104 - 1 ,  104 - 2 ,  104 - 3  may be aligned consecutively on the same strip lighting device  102  (as depicted in  FIG. 1A ) or in any suitable configurations for generating white light with an adjustable characteristic (e.g., a CCT value) as further described below. For example, referring to  FIG. 1C , the group of LEDs  104 - 1 ,  104 - 2 ,  104 - 3  may be disposed on the same column on consecutive strips  102  abutting one another. Alternatively, referring to  FIG. 1D , the two white LEDs  104 - 2 ,  104 - 3  may be disposed next to each other on the same strip  102  while the red LED  104 - 1  may be disposed next to one of the white LEDs  104 - 2 ,  104 - 3  but on a different strip. 
     The LEDs  104  may be operated individually or in a grouped manner. For example, each LED may be independently coupled to the control circuitry  112  such that the control circuitry  112  can separately control individual LEDs. Alternatively, some of the LEDs  104  may be wired together to allow the control circuitry  112  to control them as a single unit; different groups may or may not share one or more LEDs  104 . For example, as described above, the illumination device  100  may include multiple groups of LEDs, each group including at least one red LED  104 - 1 , one warm white LED  104 - 2 , and one cool white LED  104 - 3 . In one embodiment, the LEDs  104 - 1 ,  104 - 2 ,  104 - 3  in each group are electrically coupled such that the control circuitry  112  can control the LEDs  104 - 1 ,  104 - 2 ,  104 - 3  equivalently. In another embodiment, the red LEDs  104 - 1  in at least some groups are electrically coupled together; this allows the control circuitry  112  to control equivalently all red LEDs  104 - 1  that are electrically coupled. Similarly, the warm white LEDs  104 - 2  in at least some groups may be electrically coupled together, and the cool white LEDs  104 - 3  in at least some groups are electrically coupled together. This way, the groups of warm white LEDs  104 - 2  and cool white LEDs  104 - 3  may be separately controlled by the control circuitry  112  in a group manner. 
     Referring to  FIG. 2A , in some embodiments, light emitted from the LEDs  104  travels through the space of a surrounding cavity  202  and is incident upon one or more conversion layers  204  that include one or more photo-luminescent materials (e.g., phosphors, quantum dot materials, etc.) for converting the LED light. The conversion layer(s)  204  absorbs at least some of the light emitted from the LEDs  104  and re-emits at least some of the absorbed light in a spectrum containing one or more wavelengths that are different from the absorbed light. In various embodiments, the photo-luminescent material(s) contained in the conversion layer(s)  204  is chosen based at least in part on the waveguide material. This is because the waveguide material may cause a larger portion of the blue light from the white LEDs  104 - 2 ,  104 - 3  to be extracted; this may result in a shift of the CCT value of light emitted from the white LEDs  104 - 2 ,  104 - 3  toward a higher CCT value (i.e., cooler white light). In one embodiment, the photo-luminescent material(s) on the conversion layer(s)  204  is chosen such that the wavelength shift thereby can at least partially counteract the shift resulting from the waveguide material as further described below. For example, the same or different photo-luminescent materials (e.g., phosphor QMK58/F-U2) may be applied to shift the CCT value of the light emitted from the cool white LED  104 - 3  toward a green CCT value and/or or to shift the CCT value of the light emitted from the warm white LED  104 - 2  toward a yellow CCT′ value. 
     In one embodiment, the conversion layer(s)  204  is constructed from a foil that includes a composition of photo-luminescent materials. For example, the foil may be premade using a conventional substrate material (e.g., one or more layers of polymer such as PET) and a binder material (such as silicone); the composition of photo-luminescent materials is then disposed on the substrate surface. Referring to  FIG. 2B , when multiple conversion layers  204  are used, the foils including different compositions of photo-luminescent materials may be placed on top of each other with or without a gap therebetween. In one embodiment, one or more layers made of polymer can be implemented to separate the conversion layers. In addition, a second layer made of the substrate material may be applied to the conversion layer(s)  204  so as to cover the photo-luminescent materials thereon. In some embodiments, the foil in the conversion layer(s)  204  includes one or more quantum dot materials. In addition, the foil may include a quantum dot enhancement film (QDEF) made by 3M Inc. or Nanoco Technology Ltd. to provide a geometry for deploying the quantum dot materials. 
     As described above, the conversion layer(s)  204  may absorb at least some of the light emitted from the LEDs  104  and re-emit (or converts) at least some of the absorbed light in a spectrum containing one or more wavelengths that are different from (typically longer than) the light emitted by the LEDs  104 . The wavelength of the converted light may depend on the composition ratio of the photo-luminescent materials, the characteristics associated with each photo-luminescent material, and the wavelength of the light emitted from LEDs  104 . The LEDs may include a monochrome LED with a narrow band spectrum (e.g., a red LED having a wavelength between approximately 600 Tim and approximately 670 nm, a blue LED having a wavelength between approximately 400 nm and approximately 530 nm, and/or an UV LED having a wavelength between approximately 100 nm and approximately 400 nm) and/or a phosphor-converted LED with a wider band spectrum (e.g., the warm white LED  104 - 2  and/or cool white LED  104 - 3 ). The converted and unconverted light may then be mixed in the waveguide material to generate light having a target characteristic (e.g., color and/or CCT value); the target characteristic may be tunable within a range as further described below. 
     In some embodiments, each group of the LEDs depicted in  FIGS. 1A, 1C and 1D  may include one red LED  104 - 1  and two blue LEDs  104 - 2 ,  104 - 3  (instead of one red LED and two white LEDs described above). In addition; referring to  FIG. 2C , the conversion layer(s)  204  may be disposed above the blue LEDs  104 - 2 ,  104 - 3  only, and not the red LED  104 - 1 . The photo-luminescent material(s) may convert at least some of the blue light emitted from the blue LEDs  104 - 2 ,  104 - 3  to light having a longer wavelength. For example, a (Gd, Y) 3 (Al, Ga) 5 O 12  phosphor may convert blue light to yellow light. The converted light (e.g., yellow light) and unconverted blue light may then be mixed to generate white light. Thus, by choosing the photo-luminescent material(s) and/or adjusting the composition thereof, the light emitted from the blue LEDs  104 - 2 ,  104 - 3  may be converted to thereby generate warm white light and cool white light, respectively. In one embodiment, one or more additional conversion layers  204  are utilized to shift the CCT value of the light emitted from the cool white LED  104 - 3  toward a green CO′ value and/or or to shift the CCT value of the light emitted from the warm white LED  104 - 2  toward a yellow CCT value as further described below. 
     In some embodiments, the LED array includes blue LEDs only. For example, referring to  FIG. 2D , light emitted from two of the blue LEDs  104 - 2 ,  104 - 3  may be converted to generate warm white light and cool white light as described above. In addition, the conversion layer(s)  204  having suitable photo-luminescent material(s) (e.g., deep-red quantum dots by Nanoco Technology Ltd.) may be disposed above the third blue LEDs  104 - 4  so as to convert the light emitted therefrom to red light having a peak wavelength at approximately 650 nm. Again, the light emitted from the LEDs, including both converted and unconverted light, may be mixed to generate light having a characteristic (e.g., color, CIE chromaticity coordinates and/or CCT value) that is tunable within a range. 
     Referring to  FIG. 2E , in various embodiments, the conversion layer(s)  204  is divided into multiple regions  232 - 242 ; each region is either uncoated or coated with the same or different photo-luminescent materials. For example, regions  232 ,  234  may be uncoated to allow unconverted blue light from the LED to travel through; regions  236 ;  238  may be coated with the first type of photo-luminescent material such that the converted light, after being mixed with the unconverted light, generates cool white light; and regions  240 ,  242  may be coated with the second type of photo-luminescent material such that the converted light; after being mixed with the unconverted light, generates cool white light. Thus, by utilizing different types of photo-luminescent materials having different characteristics at different locations over the LEDs  104 , a target spectral power distribution (SPD) of the light may be achieved. 
     Referring again to  FIG. 2B , the light emitted from an LED  104  may interact with the photo-luminescent material(s) disposed above a neighboring LED, cause a “crosstalk” interaction, and thereby result in additional colors. To reduce the crosstalk interaction, referring to  FIG. 3A , each LED  104  in the strip lighting device  102  may be surrounded by a “cup-shaped” reflector  302 . As shown, each cup-shaped reflector  302  typically has a top aperture  304  and a bottom aperture  306 ; the LED  104  is disposed inside of the bottom aperture  306 . The shapes of the apertures  304 ,  306  may be, for example, circular, elliptical, rectangular, square, etc., and may be the same or different from each other. In one embodiment, the reflectors  302  abut each other such that the bottom portions  308  thereof form a continuous surface. The reflectors  302  may be made of a high reflectivity material, such as MS-2002 silicone from DOWSIL. 
     In some embodiments, the geometry of the cup-shaped reflectors  302  is configured to provide a uniform distribution of the light intensity at a specific distance, D, above the LED  104  where the conversion layer(s)  204  is typically disposed. In one embodiment, the reflector  302  is a parabolic reflector (i.e., a reflecting optic whose reflective surface forms a truncated paraboloid), and the LED  104  is placed at or near the focus of the paraboloid. Thus, a light beam emitted from the LED  104  onto the reflector  302  may be redirected upward for at least partial collimation of the beam. 
     Referring to  FIG. 3B , in various embodiments, an encapsulant material is potted over the LED  104  within a cavity space  310  created by the reflector  302  to at least partially, encapsulate the LED  104 . In one embodiment, the height of the encapsulant material above the LED  104  approximately corresponds to the specific distance D described above, thus the light intensity on the top surface  312  of the cavity space  310  may be uniformly distributed without having any visible high intensity spots thereon. The encapsulant material may include, consist of, or consist essentially of a clear material such as silicon. Alternatively, the encapsulant material may form a cover having a convex or domed shape on top of the reflector aperture  304 ; the cavity space  310  can be filled with gas or instead can be under vacuum. In addition, one or more conversion layers  204  including one or more types of photo-luminescent materials (e.g., phosphors, quantum dot materials, etc.) may be coated inside and/or outside the top surface  312  of the encapsulant material to convert the light emitted from the LEDs  104  as described above. In some embodiments, the cavity space  310  is at least partly filled by the encapsulant material that includes a composition of the photo-luminescent material(s) and waveguide material so as to allow the photo-luminescent material(s) to be embedded in the waveguide material. 
     Referring again to  FIG. 2A , in one embodiment, the cavity  202  formed between the circuit board  106  and the conversion layer(s)  204  is filled with a waveguide material (e.g., silicone) such that the waveguide material is in direct contact with the top surface of the circuit board  106  and the conversion layer(s)  204 . In addition, one or more reflectors  210 - 214  may be disposed on the top, bottom and/or side surfaces of the waveguide, respectively, such that the light emitted from the LEDs  104 , including both unconverted and converted light by the conversion layer(s)  204 , can be mixed inside a mixing region  216  of the waveguide; the mixed light then propagates to an output region  218  of the waveguide for outputting the light. In addition, a reflector  219  may be disposed on the top surface of the circuit board  106 . In one embodiment, at least one of the reflectors  210 - 214 ,  219  is made of a high-reflectivity silicone (e.g., CI2001 from DOWSIL). Alternatively, a high-reflectivity foil may be used as one or more of the reflectors  210 - 214 ,  219 . It should be noted that although  FIG. 2A  depicts the LEDs  104  and conversion layer(s)  206  disposed on one side  220  of the waveguide only, they may be disposed on another side  222  with the similar spatial arrangement. In addition, the location of the output region  218  may be anywhere on the waveguide and is not limited to the top surface of the waveguide as depicted in  FIG. 2A . 
     In one implementation, the entire circuit board  106  is encapsulated inside the waveguide; the illumination system  100  may include a heat-conducting path connecting the bottom surface of the circuit board  106  to an outer surface of the waveguide for dissipating heat generating by the LEDs  104 . In one embodiment, the heat-conducting path is formed by using a heat conductive material as a part of the waveguide material and disposing the circuit board  106  to be in directly contact with the waveguide. 
     As discussed above, the LEDs  104  mounted on the circuit board  106  may be controlled individually or in a group manner to generate light having a tunable CCT value within a range. The particular range in which the CCT value can be varied may depend on the configurations of the LEDs, such as the particular combination of the LEDs.  FIG. 4A  depicts exemplary color coordinates  402 ,  404 ,  406  of the warm white LED  104 - 2 , cool white LED  104 - 3 , and red LED  104 - 1 , respectively, in the CIE 1931 color space in accordance with various embodiments. As shown, the color coordinates  402 ,  404 ,  406  form vertices of a triangular region  408 ; thus, the color coordinates of light produced by such a combination of LEDs  104 - 1 ,  104 - 2 ,  104 - 3  can be tuned within the triangular region  408 .  FIG. 4B  depicts an enlarged view of a region of the triangular region  408 . As shown, the CCT value of the light generated by the red LED, warm white LED and cool white LED can be tuned along the Black Body Curve  410  with a deviation of less than 3.0 SDCM (Mac Adam&#39;s ellipse). Further details about combining various LEDs to generate white light having a tunable CCT value are provided, for example, in International Application No. WO 2018/157166 (tiled on Feb. 27, 2018), the entire content of which is incorporated herein by reference. 
     As described above, the LEDs  104  may be encapsulated in a waveguide material ( FIG. 2A ) and/or an encapsulant material ( FIG. 3B ). As a result, a large portion of the blue light from the LEDs is extracted from the LEDs, which in turn causes shifts of the CCT values associated with the warm white light and cool white light. For example, referring to  FIG. 4C , the color coordinates of the light emitted from the warm white LED  104 - 2  may be shifted from a location  402  (approximately 2700K) to a location  412  (3000K); similarly, the color coordinates of the light emitted from the cool white LED  104 - 3  may be shifted from a location  404  (approximately 6500K) to a location  414  (8000K). The degree of shifting may, depend on the material characteristics of the waveguide and/or am encapsulant. It should be noted that because the waveguide material and/or encapsulant material has no (or at least limited) effect on the color coordinates  406  of the red light emitted from the red LED  104 - 1 , there may be no need for applying the photo-luminescent material(s) thereto. 
     In various embodiments, the color coordinate shifts resulting from the waveguide and/or encapsulant are at least partially counteracted by using, for example, one or more photo-luminescent materials (e.g., phosphor QMK58/F-U2) disposed on the conversion layer(s)  204 . In one embodiment, the photo-luminescent material(s) shifts the CCT value of the light emitted from the cool white LED  104 - 3  toward a green CCT value (e.g., from the location  414  to a location  424 ) and/or the (Cx, Cy) value of the light emitted from the warm white LED  104 - 2  toward a yellow (Cx, Cy) value (e.g., from the location  412  to a location  422 ). As a result, the color coordinates of the light generated by mixing the cool white light, warm white light and red light that have color coordinates at locations  424 ,  422 ,  406 , respectively, can be tuned within a new triangular region  428  formed by the new vertices  424 ,  422 ,  406 . In various embodiments, the CCT′ value of the mixed light can be tuned along the Black Body Curve  410  with a deviation of less than 1.5 SDCM. 
     It should be noted that the green CCT value and yellow CCT value toward which the CCT values of the cool white light and warm white light are shifted do not necessarily correspond to specific CCT′ values. Rather, referring to  FIG. 4D , the green CCT value and yellow CCT value can be any color coordinates located within the green region  432  and yellow region  434  in the color space. In addition, for purposes hereof, the green region  432  includes all color coordinates in the color space corresponding to wavelengths between approximately 480 nm and approximately 550 nm, and the yellow region  434  includes all color coordinates in the color space corresponding to wavelengths between approximately 550 nm and approximately 590 nm. 
     Referring to  FIG. 4E , in various embodiments, the CCT value of the mixed light is adjusted along the BBC with a deviation of less than 1.5 SDCM so as to match or complement the human circadian rhythm. The adjustment of the CCT value can be achieved by changing the intensity of the light emitted from one or more of the LEDs  104 - 1 ,  104 - 2 ,  104 - 3 . For example, when a target CCT value of the mixed light changes from a location  436  to a location  438  in the color space, the intensity of the cool white LED  104 - 3  may be reduced, while the intensity of the warm white LED  104 - 2  and/or red LED  104 - 1  may be increased. In various embodiments, the intensity contribution of the light from each LED negatively correlates to the distance between the color coordinates of the LED light and the target color coordinates in the color space. For example, assuming the target color coordinates being at the location  438 , because the distance, d 1 , between the color coordinates  422  of the warm white light and the target color coordinates  438  is smaller than the distance, d 2 , between the color coordinates  424  of the cool white light and the target color coordinates  438 , the intensity contribution from the warm white light may be larger than that from the cool white light. Similarly, because the distance d 2  is smaller than the distance, d 3 , between the color coordinates  406  of the red light and the target color coordinates  438 , the intensity contribution from the cool white light may be larger than that from the red light. 
     In some embodiments, the control circuitry  112  adjusts the intensity of the light emitted from one or more of the LEDs  104 - 1 ,  104 - 2 ,  104 - 3  by varying the amplitude and/or duty cycle of the current and/or voltage associated therewith. In addition, the control circuitry  112  may include a look-up table that maps particular target CCT values to a set of intensity ratios for the LEDs within the LED array. Thus, when the control circuitry  112  receives information indicative of a desired CCT value, it may access the look-up table to retrieve the corresponding intensity ratios, and, based thereon, adjust the intensities of the LEDs. 
     The control circuitry  112  may include or be connected to one or more modules implemented in hardware, software, or a combination of both. For embodiments in which the functions are provided as one or more software programs, the programs may be written in any of a number of high level languages such as PYTHON, FORTRAN, PASCAL, JAVA, C, C++, C#, BASIC, various scripting languages, and/or HTML. Additionally, the software can be implemented in an assembly language directed to the microprocessor resident on a target computer; for example, the software may be implemented in Intel 80×86 assembly language if it is configured to run on an IBM PC or PC clone. The software may be embodied on an article of manufacture including, but not limited to, a floppy disk, a jump drive, a hard disk, an optical disk, a magnetic tape, a PROM, an EPROM, EEPROM, field-programmable gate array, or CD-ROM. Embodiments using hardware circuitry may be implemented using, for example, one or more FPGA, CPLD or ASIC processors. 
     The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.