Abstract:
An LED-powered replacement for the conventional incandescent screw-in light bulb comprises a phosphor coated sphere emitting white light into the same spherical pattern as a frosted incandescent bulb. In one embodiment inside the hollow sphere there is a dielectric cone emitting blue light, which causes the phosphor coating to glow. The blue light comes into the cone from a dielectric totally internally reflecting concentrator (DTIRC), which receives light from a conical reflector surrounding an LED array. The array has blue chips for energizing the phosphor and red chips for supplementing the phosphor light, enabling separate electronic control of the color temperature as well as the overall luminosity of the LED Lamp. Both blue and red chips are controlled by a quantum dimmer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional Patent Application No. 61/216,182, filed May 13, 2009 by Sun et al. for “Dimmable LED Lamp.” 
         [0002]    This application is related to commonly owned co-pending U.S. patent application Ser. No. 12/378,666 (publication no. 2009-0225529), filed Feb. 18, 2009 by Falicoff et al. for “Spherically emitting remote phosphor”; co-pending U.S. patent application Ser. No. 12/589,071 (publication no. 2010-0097002), filed Oct. 16, 2009 by Shatford et al. for “Quantum Dimming via Sequential Stepped Modulation of LED Arrays”; co-pending U.S. patent application Ser. No. 12/687,620, filed Jan. 14, 2010 by Falicoff et al. for “Heat sink with helical fins and Electrostatic Augmentation”; co-pending U.S. patent application Ser. No. 12/210,096 (publication no. 2009-0067179) filed Sep. 12, 2008 by Chaves et al. for “Optical Device for LED-Based Lamp”; U.S. Pat. No. 7,144,121 issued Dec. 5, 2006 to Miñano et al. for “Dichroic Beam Combiner Utilizing Blue LED with Green Phosphor”; and U.S. Pat. No. 7,286,296 issued Oct. 23, 2007 and No. 7,380,962 issued Jun. 3, 2008, both to Chaves, Falicoff et al. for “Optical Manifolds for Light Emitting Diodes.” All of the above patents and applications, together with any earlier provisional applications to which any of them relates, are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Conventional incandescent lamps have an efficacy of 10 to 12 lumens per watt, so that a 60 W incandescent lamp (a common wattage) has a flux output of 600 to 720 lumens. In addition, the color temperature of these lamps is under 3000° K and they achieve a color rendering index (CRI) near 100. It would be desirable to have a retro-fit high efficiency solid state lamp that can replace a 60 to 75 W incandescent lamp, and that has a similar color temperature and a high CRI (above 85 and preferably 90 or more). Also, it would be desirable if the solid state lamp was dimmable (a major problem with the vast majority of the compact fluorescent bulbs that are at present the commonest substitutes for incandescent bulbs.) Finally, it would be desirable if the output from this solid state lamp replicate the spherical output from incandescent sources. Such a solid state device is especially needed at this time, because the European Union (EU) and the USA have mandated that incandescent lamp will be phased out in the near future (2010 for the EU). Current solid state replacement lamps are typically limited to a flux of around 400 to 480 lumens (equivalent to the flux output of a 40 W incandescent bulb), primarily because of thermal management issues. Also, the light output from these lamps does not match the spherical output from filament-based sources. 
       SUMMARY OF THE INVENTION 
       [0004]    With embodiments of the lamps described herein, it is believed to be possible to meet all the aforementioned goals to at least some extent. 
         [0005]    Embodiments of the present lamps make it possible to overcome the above mentioned thermal limitation by employing the helical vane cooling system described in the above-mentioned U.S. patent application Ser. No. 12/687,620. The invention also employs optical features taught in the above-mentioned U.S. patent application Ser. Nos. 12/210,096 and 12/378,666. These novel optical approaches provide solutions that make it possible to produce a spherical light output similar to that from an incandescent filament in a relatively small package. Embodiments of the present lamps make use of the driver and dimming systems taught in the above-mentioned U.S. patent application Ser. No. 12/589,071. 
         [0006]    It is also desirable for the LED lamp to have a very high efficiency. This can be achieved using the approach known as “remote phosphor” taught in the above-mentioned U.S. Pat. Nos. 7,286,296 and 7,380,962. A further principle used in the present lamps to achieve high efficiency is to combine the output from blue LEDs, blue stimulated yellow or green phosphor, and red LEDs. Such a system can achieve high efficiencies without compromising the CRI of the source. This approach to achieving high efficiency was taught in the above-mentioned U.S. Pat. No. 7,144,121. In that patent the outputs from blue LEDs, green phosphor and red LEDs are combined via a dichroic mirror after first being collimated. In embodiments of the present lamps, the output from these three light sources is combined in a spherical phosphor mixing chamber. Embodiments of the present lamps have in common with the devices of U.S. Pat. No. 7,144,121 that the red light is first collimated but differ in that the blue light source is first collimated on its own and then concentrated and sent into a spherical mixing chamber. A portion of the blue light is used to excite the remote phosphor while the remainder escapes through the phosphor. (A small portion of this light is reflected back to the LEDs.) The phosphor layer, which acts as a homogenizer, may reside on the inside of a hollow glass or other transparent or translucent dielectric sphere or alternatively may reside on the outside of a solid or hollow dielectric sphere. In U.S. Pat. No. 7,144,121 the green phosphor is proximate to the blue LED light source, and the combined blue/green light is collimated. Embodiments of the present lamps have an advantage in that the phosphor mixing chamber both homogenizes the three wavelength sources and outputs the desired spherical beam output profile to emulate a filament source. 
         [0007]    In order to achieve a high CRI it is useful to employ the approach taught in U.S. Pat. No. 7,144,121 where the white light is created by a combination of blue, green/yellow and red light sources. Experiments carried out by the Inventors showed that a system that uses this approach to make white light can achieve a CRI above 90 and a system efficacy from 60 to 100 lumens per watt using currently available LED chips and phosphors. 
         [0008]    Because of the thermal challenges of being confined to the volumetric envelope of a conventional incandescent lamp, the success of an LED substitute is as dependent upon the effectiveness of its heat-removal system as it is dependent upon the effectiveness of its optics or electronics. The present application describes embodiments of a complete dimmable LED lamp system, including helical fins enabling the LED light source to operate at much higher power levels than conventional LED lamp substitutes. Embodiments of the system include an array of red and blue LEDs mounted on a common board, which in turn has thermal vias connected to the helical vane thermal management system. This passive thermal system is configured to create an air current that cools the vanes over a wide range of orientations. This makes it possible to construct a device suitable as a replacement for A19 and other incandescent lamps. The prior art approach for solid state lamps is to use vertical or horizontal fins, which operate well only for certain orientations (for example, vertical fins orientated vertically). 
         [0009]    In order to achieve the highest efficacy and CRI, embodiments of the lamp produce white light by combining the output of the blue and red LEDs with green or yellow light from a remote phosphor source. A portion of the blue light is used to excite the phosphor. In an embodiment, the phosphor is remote from the LEDs and resides on the inside surface of a glass sphere at the exit aperture of an optical transfer system. Alternatively, the phosphor can reside on the outside of a solid dielectric sphere. In this embodiment the phosphor is usually protected, either by its being immersed in a suitable material, such as epoxy, or by a covering. 
         [0010]    The optical transfer system in these embodiments can consist of an open reflector and a solid dielectric optic. In an embodiment, the solid dielectric optic has both active and inactive surfaces. In general, an active surface is a surface that is contacted by light rays and guides the light, for example, by reflecting or refracting light rays, while an inactive surface is a surface that is not contacted by a significant proportion of light that will contribute to the light output of the lamp. The inactive surface or surfaces can provide a site for a feature used to hold and align the optic with respect to the LED array. A first active surface of this optic is a convex lens. The angular divergence of the light from the open reflector is reduced upon entering the convex lens. Light is transferred through the solid dielectric optic by a combination of TIR and direct radiation. In one embodiment there is a solid dielectric conical ejection feature at the end of the solid dielectric optic. In another embodiment there is a partial solid dielectric sphere. In both the conical feature and the spherical feature the base of the conical or spherical feature is smaller in area than the area inside the perimeter of the LED array. This is possible because of the change in index of refraction from air to a solid. In an embodiment, the conical feature is surrounded by the hollow glass sphere, which has the remote phosphor on its inside surface. The conical feature then ejects the light toward the remote phosphor. In the embodiment with the spherical ball at the end, the phosphor layer resides on the outside of the spherical dielectric. The phosphor layer also homogenizes the different wavelength light and ejects it in a spherical pattern similar to a traditional filament. 
         [0011]    Part of the transfer optic may be exposed before it enters the sphere. The hollow glass sphere may then be mounted to an opaque shield which prevents light leakage from the sides of the transfer optic prior to its entry into the conical ejection feature. In the embodiment with the solid dielectric spherical emitter the sphere can be molded as one piece with the transfer optic. The opaque shield also is employed in this embodiment. 
         [0012]    An exemplary system with equivalent output to a 60 W incandescent lamp consists of 9 light emitting diode chips, each approximately 1 mm by 1 mm in area. The preferred ratio of the number of blue chips to red chips is an integer. For example, for the case with 9 chips, there can be 4 blue chips and 4 red chips (and one empty space) or there can be 6 blue chips and 3 red chips. This preferred integer ratio makes it easier to dim the lamp using the quantum dimming approach of above-mentioned U.S. patent application Ser. No. 12/589,071. In the case where there are 4 blue and 4 red LEDs, there are four levels of output (25%, 50%, 75% and 100%), while in the case with 6 blue and 3 red LEDs, three levels can be obtained. This is possible without using pulse width modulation for either the blue or red sources. In the case where there is a non-integer ratio between the number of blue and red LEDs then the system still can work but one or more of the LEDs may require pulse width modulation in order to achieve a reasonable number of dimming levels without a color shift. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The above and other aspects, features and advantages of the present invention will be apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
           [0014]      FIG. 1  is a side view of an embodiment of an LED light bulb system. 
           [0015]      FIG. 2  is a partly cross-sectional side view of a second embodiment of a light bulb system. 
           [0016]      FIG. 3  is a perspective cutaway view of the light bulb system shown in  FIG. 2 . 
           [0017]      FIG. 4  is a perspective view of the light bulb system shown in  FIG. 2  from a different viewpoint. 
           [0018]      FIG. 5  shows exemplary quantum-dimming waveforms for a multi-chip LED light engine. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    A better understanding of certain features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings, which set forth illustrative embodiments in which various principles of the invention are utilized. 
         [0020]      FIGS. 1 through 4  show embodiments of a dimmable LED lamp, indicated generally by the reference numbers  100  and  150 . Referring initially to  FIG. 1 , lamp  100  comprises a dielectric sphere  102 , an external shroud  104 , a support ring  106  with holes  108 , a body comprising an electronics compartment  110 , and a screw-in base  112 . The holes  108  enable air currents to flow within support ring  106  no matter at what orientation light bulb  100  is installed. On the outside of dielectric sphere  102  is a thin layer of phosphor  114 . The phosphor typically is embedded inside a layer of optical grade epoxy, silicone or other suitable material known to those skilled in this art. Alternatively, the phosphor can be deposited on dielectric sphere  102  by a slurry process or some other process. A protective layer (not shown) is then formed over the phosphor. 
         [0021]      FIG. 2  is a cutaway side view of a second embodiment of dimmable LED Lamp  150 , in which features identical to those of  FIG. 1  are identified with the same reference numbers, and the description of those features is not unnecessarily repeated. Lamp  150  has a conical ejector  152  and hollow dielectric sphere  154 , further showing dielectric total internally reflecting concentrator  116  within shroud  104 . Shroud  104  surrounds dielectric total internally reflecting concentrator  116 . The interior surface of shroud  104  is highly reflective, to catch stray light The elements within shroud  104  are present in lamp  100  as well as in lamp  150 , and the disclosure of lamp  100  includes elements of lamp  150  that are common to both lamps, that is to say, substantially all elements except features in and around balls  102 ,  154 . 
         [0022]    For clarity, a gap is shown between shroud  104  and sphere  102 ,  154 . However, shroud  104  may touch sphere  102 ,  154  unless it would obstruct a part of the sphere through which light is intended to emerge. In  FIG. 2 , the hollow sphere  154  may be mounted on shroud  104  to minimize contact with the active surfaces of concentrator  116  and ejector  152 . Concentrator  116  receives light at its convex wide end  117  and conveys the light to conical exit surface of conical ejector  152 , from which light radiates onto an interior phosphor coating  160  on the interior surface of a hollow dielectric sphere  154 . Although the lamp  100  could operate with a solid dielectric sphere  102  having an external phosphor coating, practical considerations favor the adoption of the hollow sphere  154  of the preferred embodiment of  FIG. 2 . 
         [0023]    Optional external reflector  118  is provided on part of the external surface of sphere  102 , to conserve its light and prevent light from going towards the lower body of the lamp, where the light would be wasted. External reflector  118  is shown as separate lines for clarity but would actually usually be a very thin coating on the external surface of sphere  102 . External reflector  118  may be omitted. Conical ejector  152  directs little light to the part of the sphere  154  immediately around the base of the ejector  152 , so the amount of wasted light is relatively small. Also, in some uses the support ring  106  and cooling fins  128  (see  FIGS. 3 and 4 ) may be considered a decorative feature that is desirably illuminated. 
         [0024]    Conical ejector  152  in  FIG. 2 , or sphere  102  in  FIG. 1 , is mounted on the smaller, exit end of total internal reflection (TIR) concentrator  116 , which is hidden within shroud  104 . Shroud  104  is spaced from concentrator  116 , to avoid interfering with the TIR at the surface of concentrator  116 . A flange  120  holds concentrator  116  over the wide end of a reflective cone  122 , from which concentrator  116  receives both directly radiated light from LED array  124  (best seen in  FIG. 3 ) and light from LED array  124  reflected off the interior of cone  122 . This array  124  is mounted on circuit board  126 , which thermally communicates with external cylindrical surface  130  of body  110 . 
         [0025]    As shown in  FIG. 2 , hollow ball  154  is provided with a projecting flange  158  that is mounted on shroud  104 , and an opening inside flange  158  for the ejector cone  152 . This simplifies manufacture, because the conical reflector  122 , TIR concentrator  116 , and ejector cone  152  can be mounted to the LED  124  as one sub-assembly, and shroud  104  and ball  154  can then be mounted as a separate sub-assembly. An air-gap can then be left between the base of ejector cone  152  and hollow ball  154  to prevent light from bridging across. However, any light that leaks into the dielectric wall of hollow ball  154  through points of contact with ejector cone  152  is typically small in amount and typically escapes fairly easily into the phosphor coating  160 , so it is not usually worth taking great measures to avoid all contact. 
         [0026]    The wider, inlet end of concentrator  116  is convex, and acts as a convex lens  117 , collecting and starting to concentrate the diverging light from the cone  122 . The convex end  117  and the sidewalls of concentrator  116  have shapes tailored to work with cone  122  so that nearly all the light from cone  122  reaches conical ejector  152 . There will, however, be some inevitable Fresnel reflectance from the convex surface  117  back into cone  122 . Some of the light reflected from the convex surface  117  is retro-reflected by reflector  122  at angles at which it will enter surface  117  but will not stay within the sidewalls of concentrator  116 . Instead this stray light escapes as leakage that could be highly noticeable. The interior surface of shroud  104  is mirrored, and has the functions of masking, intercepting, and returning this stray light. 
         [0027]      FIGS. 3 and 4  are further perspective views of dimmable LED lamp  150 , showing in more detail helical heat-sink fins  128 .  FIGS. 3 and 4  also apply to dimmable LED lamp  100  of  FIG. 1 , because the differences between the two lamps are not shown in detail in  FIGS. 3 and 4 . Heat sink fins  128  extend radially between external cylindrical surface  130  of the lamp body and support ring  106 . In conjunction with holes  108  on support ring  106 , these fins are much more thermally effective than conventional heat sinks. Holes  108 , together with the gaps (visible in  FIG. 4D ) between the halves (in an axial direction) of the helical fins  128 , allow radial and circumferential, as well as axial, air flow. Helical fins  128  and holes  108  are especially beneficial when lamp  100  or  150  is mounted horizontally, or obliquely to the vertical, as they enable an effective upward convection current almost regardless of the orientation of lamp  100 ,  150 . With presently available LED chips, it is desirable to prevent the red LED chips heating above a junction temperature of about 100° C., or an external temperature of about 70° to 80° C., because they tend to degrade if overheated, and their life is shortened. Blue LED chips are also affected by overheating, though they can typically tolerate slightly higher temperatures than red LEDs. Also, if the temperature of the dielectric TIR concentrator  116  is kept below about 70° C., then polymethyl methacrylate (PMMA) can be used. PMMA molds well, and has high transparency, but has a lower melting point than alternative materials. The size of cooling fins  108  and mounting ring  106  may be larger or smaller than shown, depending on the heat output and temperature tolerance of the components used in a specific lamp. 
         [0028]      FIG. 5  shows examples of the operation of a quantum dimming scheme of dimmable LED lamp  100  of  FIG. 1  or LED lamp  150  of  FIG. 2 . LED array  124  is shown with 9 LED chips, comprising six blue chips B 1  through B 6  and three red chips R 1  through R 3 . The red chips are more efficient than a phosphor for producing red light, and when red chips R 1  through R 3  are provided, phosphor  160  of  FIG. 1  or  FIG. 2  can have a greenish or yellow spectrum lacking much red light. Separate intensity control of the red chips R 1 -R 3  from the blue chips B 1 -B 6  enables the color temperature of the array  124  to be adjusted separately from its overall luminosity. However, in a preferred embodiment each red LED chip is paired with two blue LED chips to form a triad, and each of the three triads is separately switched. In this approach, quantum dimming down to one third power can be achieved by turning on at any one time one triad set of LEDs (2 blue and 1 red). By turning on two sets, two thirds power can be obtained. Alternatively, a more complex quantum dimming approach, or a combined quantum dimming and PWM approach, can be used to achieve finer control of dimming. For example, if it is required to dim the lamp to one sixth power, then the red LEDs can be pulse width modulated so that when one blue LED is on, the red chip is on for one half the time of the blue. As long as a 2 to 1 flux ratio of the blue to red is maintained when averaged over a short time period (any fluctuations must be too brief to be perceived by the eye) then the light source will appear to have a constant color temperature over all dimmed settings. Using 6 blue chips and 3 red chips, six distinct quantum dimming settings can be achieved where the red chip is sometimes pulse width modulated and other times is not. This is illustrated in  FIG. 5 . 
         [0029]    On the upper left of  FIG. 5  is shown group  201  of current waveforms, comprising the six current inputs, labeled B 1  through B 6  for the blue chips shown within array  124 . This is the lowest of six blue-luminosity levels of the quantum dimming method without pulse-width modulating the blue chips. Each blue chip takes its turn at running at full current for its share (one sixth) of a cycle. The observer perceives a steady light equal in intensity to a single blue LED chip shining continuously, with at most a very slight flicker at the switching points. 
         [0030]    On the upper right of  FIG. 5  is shown group  202  of current waveforms, comprising the three current inputs, labeled R 1  through R 3  for the red chips within array  124 . This is the lowest of six red-luminosity levels, corresponding to the blue-luminosity level of waveform group  201  of the quantum dimming method. Each red LED is on for half the time of one of the blue chips. Each red LED corresponds to two blue LEDs, so two of the cycles of the red LEDs in group  202  are shown coincident with the single cycle of group  201 . In group  202 , there is one red LED on for half the time, so there is effectively a 50% pulse-width modulated (PWM) red light, with a PWM cycle time of ⅓ of the overall red group  202  cycle time or ⅙ of the overall blue group  201  cycle time. In that configuration, the PWM cycle time must be too short for the user to notice a flicker, even though the overall group  202  cycle time is longer. 
         [0031]    On the lower left of  FIG. 5  is shown group  203  of current waveforms, comprising the three current inputs R 1  through R 3  for the red chips within array  124 . Each red chip takes its turn running at full current. This is the second of six dimming levels (one third dimming) using quantum dimming. Two of its cycles are shown coincident with the single cycle of group  204 . 
         [0032]    On the lower right of  FIG. 5  is shown group  204  of current waveforms, comprising the six current inputs B 1  through B 6  for the blue chips within array  124 . This coincides with the reds in group  203  to produce the second of six blue-luminosity levels of the quantum dimming method without PWM. One of its cycles is shown coincident with the two cycles of group  203 . At any one time two of the blue chips are on and four are off. As shown in group  204 , each blue LED is on for two successive sixths of the group cycle, but alternatively each blue LED may be on for two separate sixths of the group cycle. 
         [0033]    In the first example  201 ,  202  a combination of quantum dimming for the blue chips with pulse width modulation for the red chips is used, whereas in the second example  203 ,  204  only quantum dimming is needed. Further control of chip luminosity is obtained through modifying the level of current for activated LEDs either up or down, although overdriving can accelerate chip ageing. By adjusting the current of each LED and by incorporating pulse width modulation for at least the reds, a wide range of color temperatures is possible for every dimming level. 
         [0034]    Various blue and red chips may require a different ratio than the 6:3 shown in  FIG. 5 , for example 5:4 or 7:2. The 6:3 ratio is shown because the blue and red waveforms of individual LEDs synchronize explicitly. A more complex ratio usually results in patterns in which the red LEDs are switched at different times from the blue LEDs, and/or in which PWM of the LEDs of at least one color is required at most or all dimming levels. 
         [0035]    Various modifications of the described embodiments are possible. For example the ejector  152  has been described as a cone, and the emitter  102  or  154  has been described as a sphere. Those shapes are simple to design, simple to manufacture, and provide in operation a good approximation to the light output distribution of a conventional incandescent bulb (approximately spherical except for a shadow around the base  112 ). However, the conical ejector  152  need not be exactly conical, the sphere need not be exactly spherical, and other shapes entirely may be substituted. The skilled person will understand how to select a shape that will provide a desired light output distribution, and even shapes that provide a less than optimal light distribution may be selected for other reasons, including the appearance of the lamp  100 ,  150  itself. 
         [0036]    The cone  122  between the LED array  124  and the lens  117  may be replaced by a differently shaped collimator. However, the cone  122  is simple to fabricate, because it can be rolled from a piece of flat reflective material, and is adequate for the purpose, when assisted by the lens  117 , of ensuring that almost all the light from the LED chips  124  enters the concentrator  116  within the acceptance angle of the concentrator. 
         [0037]    Although the lamp  100 ,  150  is shown with an Edison screw fitting  112  that is standard for mounting and power supply in the USA, of course a different form of fitting may be substituted. That may be done for compatibility with other standards (at least three sizes of Edison screw fitting are standard in the USA alone) or with the standards of other places, or for incompatibility with standards, to compel or prevent fitting of the lamp  100 ,  150  in selected receptacles. 
         [0038]    Although  FIGS. 1 through 4  show various modifications of the described embodiment, the skilled reader will understand how to select and combine among those modifications. 
         [0039]    The preceding description of the presently contemplated best mode of practicing the invention is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The full scope of the invention should be determined with reference to the Claims.