Patent Publication Number: US-9897284-B2

Title: LED-based MR16 replacement lamp

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims the benefit under 35 USC § 119(e) of U.S. Provisional Application No. 61/617,029 filed Mar. 28, 2012, the disclosure of which is incorporated herein by reference in its entirety for all purposes. 
    
    
     BACKGROUND 
     The present disclosure relates generally to lighting devices and in particular to an LED-based lamp having a form factor compatible with standard MR16 lamps. 
     One popular type of halogen lamp is the multifaceted reflector (“MR”) type. MR lamps are generally conical in shape, with a halogen bulb placed in front of a multifaceted reflector that directs the light toward a front face. The facets of the reflector provide a pleasingly soft edge to the emergent light beam. “MR16” refers to an MR-type lamp with a 2-inch diameter at the front face. Numerous lighting systems and fixtures have been designed to accommodate MR16 lamps. 
     It is known that the efficiency of light-emitting diodes (LEDs), measured, e.g., in lumens/watt, is generally higher than that of halogen bulbs. Therefore, it would be desirable to provide an LED-based lamp having a form factor compatible with fixtures designed for MR16 lamps. 
     BRIEF SUMMARY 
     Embodiments of the present invention provide LED-based lamps that can be made to have a form factor compatible with fixtures designed for MR16 lamps. Such a lamp can have a housing that provides an external electrical connection. Inside the housing is disposed a single emitter structure having a substrate with multiple light-emitting diodes (LEDs) arranged thereon. Different LEDs produce light of different colors (or color temperatures). For example, at least one LED can produce a warm white light, while at least one other LED produces a cool white light and at least one other LED produces a red light. A total-internal-reflection (TIR) lens is positioned to collect light emitted from the single emitter structure and adapted to mix the light from the LEDs to produce a uniform white light. A diffusive coating is applied to a front face of the TIR lens for further color mixing. 
     The following detailed description together with the accompanying drawings will provide a better understanding of the nature and advantages of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified cross-sectional side view of an LED-based lamp according to an embodiment of the present invention. 
         FIG. 2  is a simplified top view of a nine-die LED package that can be used in the lamp of  FIG. 1  according to an embodiment of the present invention. 
         FIG. 3  is a perspective view of a TIR lens that can be used in the lamp of  FIG. 1  according to an embodiment of the present invention. 
         FIG. 4  is a cross-section side view of the TIR lens of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide LED-based lamps that can be made to have a form factor compatible with fixtures designed for MR16 lamps. Such a lamp can have a housing that provides an external electrical connection. Inside the housing is disposed a single emitter structure having a substrate with multiple light-emitting diodes (LEDs) arranged thereon. Different LEDs produce light of different colors (or color temperatures). For example, at least one LED can produce a warm white light, while at least one other LED produces a cool white light and at least one other LED produces a red light. A total-internal-reflection (TIR) lens is positioned to collect light emitted from the single emitter structure and adapted to mix the light from the LEDs to produce a uniform white light. A diffusive coating is applied to a front face of the TIR lens for further color mixing. 
       FIG. 1  is a simplified cross-sectional side view of an LED-based lamp  100  according to an embodiment of the present invention. Lamp  100 , which is cylindrically symmetric about an axis  101 , has a housing  102 , which can be made of aluminum, other metals, plastic, and/or other suitable material. Housing  102  holds the various components of lamp  100  together and can provide a convenient structure for a user to grip lamp  100  during installation or removal from a light fixture. The exterior of housing  102  can include mechanical and/or electrical fittings  103  to secure lamp  100  into a light fixture and/or to provide electrical power for producing light. These fittings can be compatible with existing MR16 lighting systems. In some embodiments, housing  102  may include fins or other structures to facilitate dissipation of heat generated during operation of lamp  100 . The exterior shape of housing  102  can be made to conform to a standard lamp form factor, such as MR16. 
     Within housing  102  is an emitter package  104 . Package  104  includes a substrate  106  in which is formed a recess  107 . Substrate  106  can be a multilayer structure with ceramic and metal layers. Examples are described in U.S. Patent Application Pub. No. 2010/0259930, the disclosure of which is incorporated herein by reference. Other substrates can also be used. 
     LEDs  108  are mounted on substrate  106  within recess  107 . In some embodiments, the top surface of recess  107  is patterned with a number of metal pads, each accommodating a single LED  108 . Each LED  108  can be a separate semiconductor die structure fabricated to produce light of a particular color in response to electrical current. In some embodiments, LEDs  108  can be covered with a material containing a color-shifting phosphor so that LED  108  produces light of a desired color. For example, a blue-emitting LED die can be covered with a material containing a yellow phosphor; the emerging mixture of blue and yellow light is perceived as white light having a particular color temperature. As described below, in some embodiments different ones of LEDs  108  produce light of different colors; LEDs  108  need not be identical. 
     Lamp  100  also includes a primary lens  110 , which can be made of glass, plastic or other optically transparent material, that is positioned to direct light emitted from LEDs  108  into secondary optics  112 . Secondary optics  112  advantageously include a total-internal-reflection (TIR) lens that also provides mixing of the colors of light emitted from LEDs  108  such that the light beam exiting through front face  114  has a uniform color. Examples of suitable lenses are described in U.S. Patent Application Pub. No. 2010/0091491; other color-mixing lens designs may also be used. 
     Lamp  100  also includes a diffusive coating  120  on front face  114  of lens  112 . Coating  120  provides further color mixing of the light exiting secondary optics  112  without requiring additional space, a significant consideration when designing a lamp with a compact form factor such as MR16. Various coatings  120  can be used. In some embodiments, coating  120  can be a holographic diffuser film, such as a light-shaping diffuser film made by Luminit Co. of Torrance, Calif. (website at www.lumintco.com). In these films, the diffusive coating is provided as a diffusive material disposed in a desired pattern on an optically transparent substrate film (e.g., acrylic, polyester, polycarbonate, glass or fused silica). The film is easily applied to front face  114 . Other types of coatings can also be applied; for example, diffusive material can be applied directly to front face  114 . Coating can improve color mixing without requiring additional space, a significant consideration with a small form factor such as MR16. 
     In some embodiments, lamp  100  includes a control circuit  116  that controls the power provided from an external power source (not shown) to LEDs  108 . In some embodiments, control circuit  116  allows different amounts of power to be supplied to different LEDs  108 , allowing for tuning of the color as described below. 
       FIG. 2  is a simplified top view of a nine-die emitter  200  implementing emitter package  104  of  FIG. 1  according to an embodiment of the present invention. In this embodiment, substrate  206  includes a recess  207  in which nine LEDs  208   a - i  are disposed as shown. LEDs  208   a - d  are cool white (CW) LEDs; LEDs  208   e - h  are warm white LEDs, and LED  208   i  is a red (R) LED. “Cool” white and “warm” white, as used herein, refer to the color temperature of the light produced. Cool white, for example, can correspond to a color temperature above, e.g., about 4000 K, while warm white can correspond to a color temperature below, e.g., about 3000 K. It is desirable that cool white LEDs  208   a - d  have a color temperature cooler than a target color temperature for lamp  100  while warm white LEDs  208   e - h  have a color temperature warmer than the target color temperature. When light from cool white LEDs  208   a - d  and warm white LEDs  208   e - h  is mixed by mixing lens  112 , an intermediate color temperature can be achieved. Red LED  208   i  provides additional warming. Examples of techniques for selecting LEDs for an emitter to provide a desired output color are described, e.g., in U.S. patent application Ser. No. 13/240,796, the disclosure of which is incorporated herein by reference. 
     In some embodiments, LEDs  208  are advantageously provided with electrical connections such that different groups of the LEDs are independently addressable, i.e., different currents can be supplied to different groups of LEDs. For example, a first group can include cool white LEDs  208   a - d , a second group can include warm white LEDs  208   e - h , and a third group can include red LED  208   i . (A “group” of one LED is permitted.) These electrical connections can be implemented, e.g., using traces disposed on the surface of substrate  206  and/or between electrically insulating layers of substrate  206 . 
     Where the different LED groups are interpedently addressable, package  200  provides an emitter that can be tuned to produce light of a desired color (e.g., color temperature) by adjusting the relative current delivered to different groups of LEDs  208 , e.g., using control circuit  116 . Techniques for tuning an emitter have been described, e.g., in U.S. patent application Ser. No. 13/106,808 and U.S. patent application Ser. No. 13/106,810, the disclosures of which are incorporated herein by reference. 
     In other embodiments, the color temperature of the light produced by the lamp can be controlled by selecting cool white LEDs  208   a - d  and warm white LEDs  208   e - h  such that the desired color (e.g., color temperature) is achieved when equal currents are supplied to all LEDs  208  (including red LED  208   i ). Selection of LEDs for a given substrate can be done by testing individual LED dice prior to substrate assembly to determine the color temperature of light produced and binning the LED dice according to color temperature. By selecting the warm white and cool white LEDs for a substrate from appropriately paired warm-white and cool-white bins, a desired color temperature for the lamp can be achieved when all LEDs are supplied with the same current. Accordingly, color tuning by adjusting the relative current supplied to different groups of LEDs is not required. 
     In the embodiment of  FIG. 2 , the LEDs are arranged to provide a roughly uniform circular distribution of cool white and warm white LEDs. That is, the cool white and warm white LEDs are intermixed and arranged such that warm and cool light are produced in approximately equal intensities across different parts of the emitter substrate. This allows for optimal color mixing using secondary optics such as TIR lens  112  of  FIG. 1 , to produce a uniformly white light from LEDs that are not uniform in color. 
       FIG. 3  is a perspective view of a TIR lens  300  that can be used in secondary optics  112  of lamp  100  of  FIG. 1  according to an embodiment of the present invention, and  FIG. 4  is a cross-section side view of TIR lens  300  showing illustrative dimensions, all of which can be varied as desired. TIR lens  300  can be made of an optically transparent material such as glass or plastic (e.g., polymethylmethacrylate (PMMA)) and can be manufactured, e.g., using conventional processes such as molding processes in the case of a plastic lens. TIR lens  300  has a smooth side wall  302 , a front (or top) face  304  and a flange  306 . As shown in  FIG. 4 , a central cavity  402  is created inside lens  300 , extending partway to front face  304 . Cavity  402  is open at the rear (or bottom), and primary lens  110  of package  104  ( FIG. 1 ) can extend into cavity  402 . Bottom (or rear) edge  404  of lens  300  can be sized and shaped to contact the edges of package  104  surrounding primary lens  110 , as shown schematically in  FIG. 1 . This provides alignment of the package with respect to the TIR lens. 
     As shown in  FIG. 3 , front face  304  of lens  300  is patterned with hexagonal microlenses  308 . Microlenses  308  provide beam shaping, and the pattern can be chosen to create a desired beam width. In  FIG. 4 , front face  304  is shown as having a concave shape. Each microlens  308 , however, has a convex curvature, providing small local excursions from the generally concave contour of front face  304 . 
     As noted above, a diffusive coating, such as a holographic diffuser film, can be applied over front face  304 . This coating can follow the general shape of face  304 . The diffusive coating enhances color mixing while allowing lens  300  to remain small. This facilitates the use of color mixing lenses in lamps with small form factors. 
     Side wall  302  can be shaped to optimize total internal reflection for an emitter disposed at a position determined by bottom edge  404  and cavity  402 . In some embodiments, side wall  302  of lens  300  can be coated with a reflective material, or a reflective housing can be placed around sidewall  302  to reduce light loss through side wall  302 . 
     Flange  306  extends peripherally from top face  304  and can be used to secure lens  300  in a housing such as housing  102  of  FIG. 1 . In some embodiments, flange  306  does not affect the optical properties of lens  300 ; the size and shape of flange  306  can be modified based on mechanical design considerations (e.g., retention of the lens within the housing of an assembled lamp). 
     The beam angle produced by lens  300  can controlled by suitable selection of various design parameters for the lens, in particular the size and shape of microlenses  308 . Examples of the effects of changing a microlens pattern and other lens design parameters are described, e.g., in U.S. Pat. No. 8,075,165, the disclosure of which is incorporated herein by reference. The particular configuration shown in  FIGS. 3 and 4  results in light with a beam angle of about 35-40 degrees, but other configurations can provide different beam angles. 
     In some embodiments, nine-die emitter  200  of  FIG. 2  and lens  300  can be placed within an exterior lamp housing (shown schematically as housing  102  in  FIG. 1 ) whose outer shape conforms to a standard MR16 lamp form factor. This housing, which can be made primarily or entirely of metal, can be a solid structure, a finned structure, a webbed structure or the like. Housing  102  can incorporate various mechanical retention features (e.g., slots, flanges, through-holes for screws or other fasteners, or the like) to secure emitter  200  and lens  300  in the desired arrangement. In some embodiments, housing  102  is also designed to facilitate dissipation of heat produced by package  200  during lamp operation, and metals or other materials with good heat transfer properties can be used. 
     An LED-based MR16 replacement lamp as described herein can provide high performance and improved energy efficiency as compared to existing halogen lamps. For example, a 12-watt lamp constructed as described herein can generate approximately 600 lumens with a color temperature of about 2700-2800 K. In a floodlight configuration (beam angle of 35-40 degrees), center beam candle power (CBCP) of approximately 2000 candelas is obtained. These numbers compare favorably with existing halogen MR16 lamps operating at higher power (e.g., 35-50 watts). 
     While the invention has been described with respect to specific embodiments, one skilled in the art will recognize that numerous modifications are possible. For example, the emitter can include a different number or arrangement of LEDs. The LEDs can be arranged in various ways; in some embodiments, rotationally symmetric arrangements (e.g., as shown in  FIG. 2 ) are preferred for optimum color mixing. Use of a single emitter with multiple LEDs in combination with a color-mixing lens and a diffusive coating provides uniform color of a desired temperature with a compact form-factor. 
     The shape of the TIR color-mixing lens can also be varied, subject to constraints based on the overall form factor of the lamp and the need for electrical, mechanical, and heat-dissipation structures. In general, the optimum lens shape depends in part on the characteristics of the emitter, and if the emitter is changed, the lens design can be reoptimized taking into account the desired color mixing and light output efficiency. The lens can be constructed of any material with suitable optical properties. In some embodiments, the outer side surface of the lens can be coated with a reflective material to further increase light output. 
     The front face of the secondary lens can be coated with a diffusive material to further improve the color uniformity of the light. A variety of materials can be used, including film coatings, spray-on materials, curable materials, or other materials as desired. 
     The housing holds the various components together and provides electrical and mechanical fittings usable to install the lamp in a light fixture. These fittings can be adapted to particular standards. In some embodiments, the housing can include a reflective holder surrounding the sides of the TIR color-mixing lens. The housing can also incorporate heat-dissipation structures (e.g., fins or webs of metal or other material with high thermal conductivity). 
     While specific reference is made herein to MR16 lamps to define a form factor, it is to be understood that similar principles can be applied to design compact LED-based lamps with other form factors. 
     Thus, although the invention has been described with respect to specific embodiments, it will be appreciated that the invention is intended to cover all modifications and equivalents within the scope of the following claims.