LED-based lamp including shaped light guide

A light guide (302) for a lamp includes a proximal end (510) configured to receive light from at least one LED. The received light forms an internal beam inside the light guide. The light guide angularly redistributes the LED light to have a specified angular output distribution. The light guide includes a distal end (520) having a peripheral portion (522). The light guide has a longitudinal axis (A) extending from the proximal end to the distal end. The light guide includes a lateral side (530) extending from the proximal end (510) to the peripheral portion (522) of the distal end (520). In a cross-section that includes the longitudinal axis, the lateral side of the light guide includes both a convex region and a concave region.

TECHNICAL FIELD

The present disclosure relates to lamps using at least one light emitting diode (LED) and a light guide.

BACKGROUND

In recent years, light-emitting-diodes (LEDs) have emerged as a new technology for illumination and lighting applications. LEDs have potential advantages over fluorescent lamps in that they may be more efficient, may produce less heat, may have longer lifetimes, and may function more efficiently at cold temperatures. For these reasons and others, there has been a recent effort to incorporate LEDs into lighting applications, such as retrofit chandelier or candelabra lamps.

An example of a known LED-based lamp is discussed in U.S. Patent Application Publication No. 2010/0208488 (Luo).FIG. 1shows a known light bulb1, which is discussed in Luo. The light bulb1has an envelope5that encompasses a light pipe2, a base4, and a reflector3.FIG. 2shows the light pipe2ofFIG. 1. An LED (not shown) is disposed within a hemispherical recess6at a lower end of the light pipe2. The lateral side of the light pipe is a tapered cylinder7.

SUMMARY

An exemplary embodiment includes a lamp with a housing having at least a transparent portion. A light guide is disposed within the transparent portion. The light guide includes a proximal end configured to receive light from at least one LED. The received light forms an internal beam inside the light guide. The light guide angularly redistributes the LED light to have a specified angular output distribution. The light guide includes a distal end. The distal end has a peripheral portion thereon. The light guide has a longitudinal axis extending from the proximal end to the distal end. The light guide includes a lateral side extending from the peripheral portion of the proximal end to the distal end. In a cross-section that includes the longitudinal axis, the lateral side of the light guide includes both a convex region and a concave region.

This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the subject matter. The detailed description is included to provide further information about the present patent application.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS INCLUDING BEST MODE

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It may be evident, however, to one skilled in the art, that the subject matter of the present disclosure may be practiced without these specific details.

FIG. 3is a plan drawing of a lamp300, according to an example embodiment. The lamp300includes a housing304. The housing304includes circuitry for powering one or more light emitting diodes (LEDs) within the housing304. Light from the LEDs is directed through a light guide302within the housing304, which angularly directs the LED light into a specified angular output. The housing304includes a transparent portion306, through which light emerges at the specified angular output. The housing304also includes a threaded base308. An exterior of the housing304optionally includes one or more decorative features, such as ridges, or simulated candle wax drippings. In some examples, the housing304is sized to match a volume envelope of a particular standardized light bulb, such as a B10 or B12. For these examples, the letter “B” denotes a particular specified bulb shape and size, such as a candelabra. The numerals “10 or “12” indicate that the external thread base diameter is 10 mm (“Miniature Edison Screw”) or 12 mm (“Candelabra Edison Screw”), respectively. Other bulb shapes, bulb sizes, and thread sizes are also possible.

FIG. 4is an exploded-view drawing of the lamp300, showing an example configuration of the elements within the housing304. The threaded base308may be a base candelabra screw that provides electronic connections to a corresponding socket. A suitable material for the threaded base308is brass, although other materials may also be used. The housing304may be a heat sink, and may be formed from aluminum or another suitable material. An LED driver422includes circuitry that can convert AC current, from the socket, to a DC current suitable to power the LEDs. The LED driver422is housed in an LED housing412. There is a potting410between the LED driver housing412and the heat sink housing304, which is used to increase thermal conductivity between the LED driver housing412and the housing304, and decrease electrical conductivity between the LED driver housing412and the housing304. A typical potting410is made with a liquid high-viscosity material, such as a gel. The high-viscosity material is then dried out in air, and becomes hard and solid. A bezel418is a metal part above the heat sink, typically made from aluminum, which increases the thermal conductivity between the LED driver422and the housing304. Thermally conductive adhesive414is used to secure many of the components, including the bezel418, in place. A plastic disc416increases electrical isolation between the LED driver422and the bezel418. Thermal grease420is used to provide smooth thermal conductivity between adjacent parts. Screws424,426secure the LED driver422to the LED housing412. A plastic cover428covers the active area of the LED driver422. Additional screws430,432secure the plastic cover428in place. It will be understood that the elements ofFIG. 4are merely an example, and other suitable mechanical configurations may also be used.

FIG. 5is a cross-sectional drawing of the light guide302ofFIG. 3. In an example, the light guide302is rotationally symmetric about a longitudinal axis (A), so that features and elements of the light guide302shown inFIG. 5are cross-sections of respective surfaces of revolution around the longitudinal axis (A). The light guide302is formed from a material that is transparent in the visible portion of the spectrum, such as polymethyl methacrylate (PMMA). The light guide may be formed from molding, grinding and polishing, or another suitable manufacturing process.

Light is produced by one or more LEDs590, shown near the bottom ofFIG. 5. In some examples, there may be three, four, or five LEDs590. The LEDs590are not part of the light guide302. The LEDs590are distributed around the longitudinal axis (A) of the light guide302. Although the light guide302is shown in cross-section inFIG. 5, the single LED shown inFIG. 5, and in subsequentFIGS. 6-10, is illustrated for convenience as one among a plurality of LEDs, and is disposed in front of or behind the plane of the page of the corresponding figures. In most cases, none of the LEDs590are aligned with the longitudinal axis (A), and all are disposed away from the longitudinal axis (A). The LEDs590may include a common emission plane that is perpendicular to the longitudinal axis (A). The LEDs590emit light in a Lambertian distribution, which has a characteristic emission pattern that peaks along the direction of longitudinal axis (A) and decreases to zero at angles perpendicular to the longitudinal axis (A). Most of the light leaving the LEDs590travels upward inFIG. 5, with a smaller amount being directed angularly toward the lateral sides of the longitudinal axis (A).

In some examples, the LEDs590all emit light at the same wavelength. In some of these examples, the LEDs590may be dimmable, with a wavelength spectrum that remains invariant as the intensity is varied. In other examples, at least two of the LEDs590emit light at different wavelengths. In some examples, the LEDs590include individual LEDs that emit light in the red, green, and blue portions of the spectrum. For these examples, the combined light from the LEDs590may simulate a specified color target, such as white light, or the light produced by a compact fluorescent lamp. For some of these examples, the light output of each of the differently colored LEDs may be controlled independently, so that the combined light from the LEDs590may be tunable to a desired color target. The tuning may be performed automatically, or may be performed manually by a user. For some of the tunable examples, the LEDs590may be dimmable, with a combined wavelength spectrum that remains invariant as the combined intensity is varied.

The light from the LEDs590propagates upward inFIG. 5, and enters the light guide302through a proximal end510of the light guide302. Light propagates within the light guide302, with a variety of propagation directions, toward a distal end520of the light guide. For some propagation directions, light travels from the proximal end510directly to the distal end520. For some propagation directions, the light reflects from a lateral side530of the light guide302, before reaching the distal end520of the light guide302. For some propagation directions, the light exits through the lateral side530of the light guide302. The proximal end510, the distal end520, and the lateral side530all extend across a number of features and regions, which are described in detail below.

The proximal end510of the light guide302may include an optional flat portion512surrounding the longitudinal axis (A). Such a flat portion512may be laterally sized to accommodate a particular portion of the propagation angles from the LEDs590; see, for instance,FIG. 10. For examples in which the flat portion512is omitted, the region surrounding the longitudinal axis (A) is concave. The proximal end510of the light guide302includes a concave portion514. The concave portion514can include the flat portion512, or can surround the longitudinal axis (A) if the flat portion512is omitted. In some examples, the concave portion514is spherical in shape, with a center of curvature located at or near the intersection between the longitudinal axis (A) and the emission plane of the LEDs590. For examples in which the flat portion512is omitted, the concave portion514may be spherical in shape in the area surrounding the longitudinal axis (A). The proximal end510of the light guide302includes a convex peripheral portion516that surrounds the concave portion514. In some examples, the cross-section of the peripheral portion516is spherical in shape, with a center of curvature located external to the light guide302. In practice, the optional flat portion512, the concave portion514, and the convex peripheral portion516may fully surround the half-plane emergent from the LEDs590; these three regions512,514,516of the proximal end510may receive essentially all the light emitted from the LEDs590. Note that the prior art light pipe2, shown inFIG. 2, includes a hemispherical recess6that extends to a corner, and lacks such a convex peripheral portion516. The proximal end510of the light guide302may additionally include non-optical features beyond the convex peripheral portion516, such as a ridge588. The non-optical features may be used for mechanically locating the light guide against specified mechanical features in the housing304, mechanically mounting the light guide302, or other suitable purposes. These non-optical features do not receive a significant amount of light from the LEDs590, and do not significantly affect the light output of the lamp300. The proximal end510of the light guide302optionally includes an anti-reflection thin-film coating. The optional anti-reflection coating may extend over the optional flat portion512, the concave portion514, and the convex peripheral portion516. The optional anti-reflection coating may or may not extend over the non-optical features of the proximal end510. Alternatively, the proximal end510of the light guide302may be devoid of a thin-film coating.

The distal end520of the light guide302includes a central portion524that surrounds the longitudinal axis (A). The central portion524may be laterally sized to accommodate a particular portion of the propagation angles from the LEDs590; see, for instance,FIG. 10. In some examples, the central portion524may be sized to accommodate the same portion of the propagation angles from the LEDs590as the optional flat portion512of the proximal end510of the light guide302. The central portion524may be flat, convex, or concave. The distal end520of the light guide302includes a peripheral portion522that surrounds the central portion524. The peripheral portion522extends in the distal direction (e.g., away from the LEDs590) at increasing distances away from the longitudinal axis (A). The most distal location on the distal end520is farthest away from the longitudinal axis (A). The distal end520of the light guide302appears as a depression, with the most-depressed portion (e.g., the most proximal portion) being the central portion524. In a cross-section that includes the longitudinal axis (A), the peripheral portion522is convex in shape. In some examples, the cross-section of the peripheral portion522is spherical in shape. In other examples, the cross-section of the peripheral portion522is an optically optimized shape, which lacks a single center of curvature. The distal end of the light guide502may be devoid of a thin-film coating, or may optionally be anti-reflection coated.

In an example, the peripheral portion522of the distal end520of the light guide302is frosted. For a frosted optical surface, a particular fraction of incident light is diffusely scattered (e.g., scattered with a random angular component to a propagation direction) into a reflected and/or transmitted space. The remaining fraction of incident light is specularly reflected or transmitted; this fraction is referred to herein as being reflected in part, or transmitted in part, respectively. The amount of scattering varies between 0% and 100%, and depends on the amount of frosting. For instance, a mild frosting may have a very low percentage of incident light being scattered and a very high percentage of incident light being specularly reflected or transmitted. Similarly, a more severe frosting may have a very high percentage of incident light being scattered and a very low percentage of incident light being specularly reflected or transmitted. For the simulated performance curves presented below inFIG. 15, the frosting level is set so that 50% of the incident light is scattered, and 50% is specularly reflected or transmitted. The level of 50% is merely an example, and other suitable frosting values may also be used.

The lateral side530of the light guide302extends from a location adjacent the peripheral portion522of the distal end520to a location adjacent the proximal end510. The lateral side530has a first region550, a second region560, a third region570, and a fourth region580, extending successively from the proximal end510to the distal end520. The first region550is concave. The second region560is convex. The fourth region580is concave. The third region570is a smooth and continuous transition region or inflection region between the convex second region560and the concave fourth region580. In a cross-section that includes the longitudinal axis (A), as inFIG. 5, the lateral side530includes both a convex region and a concave region. In some examples, the lateral side530transitions between the convex and concave regions smoothly and continuously, such as without corners or discontinuities. The convex region of the lateral side530includes the second region560. The concave regions of the lateral side530include the first region550and the fourth region580. The lateral side530is shaped so that for relatively low angles of propagation away from the LEDs590(such as at regions560,570,580), light that strikes the lateral side530will be reflected via total internal reflection. This behavior is shown in more detail inFIGS. 7 and 8. The lateral side530is also shaped so that at relatively high angles of propagation away from the LEDs590(such as at region550), light that strikes the lateral side530will be transmitted through the lateral side530. This behavior is shown in more detail inFIG. 6. The concave first region550of the lateral side530overlaps the convex peripheral portion516of the proximal end510, so that at least a portion of the concave first region550is at the same longitudinal location along the longitudinal axis (A) as at least a portion of the convex peripheral portion516. The concave fourth region580of the lateral side530overlaps the distal end520, so that at least a portion of the concave fourth region580is at the same longitudinal location along the longitudinal axis (A) as at least a portion of the distal end520. The overall shape of the lateral side530is an optically optimized curve, which does not have a single center of curvature. The lateral side530is devoid of a thin-film coating. The lateral side530of the light guide302may additionally include non-optical features at or near its proximal end, which do not significant affect the light output of the light guide302.

During operation, light emerges from the LEDs590with a full angular bundle of rays that extend over a full half-plane. The full angular bundle may be referred to as an internal beam. Different portions of the internal beam exhibit different behaviors, and light travels differently from surface-to-surface within the light guide302for the different portions. It is beneficial to analyze separately these different portions of the internal beam, keeping in mind that during operation, the internal beam exhibits all of these behaviors simultaneously.FIGS. 6-10show traces of light rays inside the light guide302for different portions of the internal beam.

FIG. 6is a schematic drawing of one group of light rays propagating from the LEDs590, and through the light guide302ofFIG. 5. This particular group of rays is referred to as a first portion600of the internal beam. This first portion600extends directly from the proximal end510of the light guide302to the first region550of the lateral side530of the light guide302. The first portion600then exits the light guide302by transmitting through the first region550. Because the first region550is configured for transmission therethrough, and not reflection therefrom, it is desirable that the rays transmitting through the first region do so at relatively low angles of incidence. It is also desirable that the rays enter the light guide302at relatively low angles of incidence through the convex peripheral portion516of the proximal end510of the light guide302. Referring back toFIG. 5, the convex peripheral portion516of the proximal end510of the light guide includes a location592and has a surface tangent596at the location. Also referring toFIG. 5, the first region550of the lateral side530of the light guide302includes a location594and has a surface tangent598that is parallel to surface tangent596. In other words, at least a location592on the convex peripheral portion516of the proximal end510of the light guide302is parallel to at least a location594on the first region550of the lateral side530of the light guide302.

It is advantageous that the rays in the first portion600of the internal beam contribute to the output of the lamp300. In contrast, for many known lamps, rays exiting the LED(s) at high angles of exitance are blocked by the geometry of the light guide from contributing to the lamp output. For instance,FIG. 16is a schematic drawing of a newly-performed ray trace through the known light pipe2ofFIG. 2. In this raytrace, a group of rays1600exits an LED located at the center of hemispherical recess6. The group of rays1600is shown inFIG. 16to exit the light pipe2with a downward direction; in practice, these rays would strike a mounting surface and would not contribute to the output of the lamp. The present light guide302includes the rays at the high angles of exitance as part of the lamp output, as shown inFIG. 6, and therefore achieves a significant improvement in performance over the known light pipe2ofFIG. 2.

FIG. 7is a schematic drawing of another group of light rays, referred to as a second portion700of the internal beam, propagating from the LEDs590, and through the light guide302ofFIG. 5. The second portion700extends directly from the proximal end510of the light guide302to the second region560of the lateral side530of the light guide302. The second portion700reflects via total internal reflection from the second region560. After reflection, the second portion700crosses the longitudinal axis (A) of the light guide302, then strikes the peripheral portion522of the distal end520of the light guide302. The peripheral portion522is frosted, so that a specified fraction of the second portion700diffusely scatters at the peripheral portion522, and a specified fraction of the second portion700specularly reflects. In some examples, the specified fraction is 50% for both the scattered portion and the specularly reflected portion. The specularly reflected portion strikes the fourth region580of the lateral side530of the light guide302, transmits through the fourth region580, and exits the light guide302.

FIG. 8is a schematic drawing of another group of light rays, referred to as a third portion800of the internal beam, propagating from the LEDs590, and through the light guide302ofFIG. 5. The third portion800extends directly from the proximal end510of the light guide302to the third region570of the lateral side530of the light guide302. The third portion800reflects via total internal reflection from the third region570. After reflection, the third portion800strikes the peripheral portion522of the distal end520of the light guide302. The peripheral portion522is frosted, so that a specified fraction of the third portion800diffusely scatters at the peripheral portion522, and a specified fraction of the third portion800specularly transmits. In some examples, the specified fraction is 50% for both the scattered portion and the specularly transmitted portion. The specularly transmitted portion transmits through the peripheral portion522of the distal end520, and exits the light guide302.

FIG. 9is a schematic drawing of another group of light rays, referred to as a fourth portion900of the internal beam, propagating from the LEDs590, and through the light guide302ofFIG. 5. The fourth portion900extends directly from the proximal end510of the light guide302to the peripheral portion522of the distal end520of the light guide302. The peripheral portion522is frosted, so that a specified fraction of the fourth portion900diffusely scatters at the peripheral portion522, and a specified fraction of the fourth portion900specularly reflects. In some examples, the specified fraction is 50% for both the scattered portion and the specularly reflected portion. The specularly reflected portion strikes the fourth region580of the lateral side530of the light guide302, transmits through the fourth region580, and exits the light guide302.

FIG. 10is a schematic drawing of another group of light rays, referred to as a fifth portion1000of the internal beam, propagating from the LEDs590, and through the light guide302ofFIG. 5. The fifth portion1000extends directly from the proximal end510of the light guide302to the central portion524of the distal end520of the light guide302. The fifth portion1000transmits through the central portion524of the distal end520, and exits the light guide302.

In practice, light in the fifth portion1000is usually considered undesirable, since this light exits the top of the lamp300and propagates directly upward from the lamp300. Note that light in the fifth portion1000originates from a point on-axis or close to on-axis (e.g., at or near the intersection between the longitudinal axis (A) and the emission plane of the LEDs590). As a result, it is possible to reduce or eliminate light in the fifth portion1000by using an off-axis configuration of the LEDs590. Specifically, if all the LEDs590are located off-axis, then there is little or no light propagating along the longitudinal axis (A) or close to the longitudinal axis (A), as inFIG. 10. Light from the off-axis LEDs590, instead of striking the central portion524, strikes the peripheral portion522, as inFIG. 9, and forms a desirable portion of the output from the lamp300.

There are many possible configurations for the LEDs590. In three examples, shown inFIGS. 11-13, the LEDs are distributed in a circle, and are evenly spaced around the circle. In each of these examples, the LEDs are arranged circumferentially around the longitudinal axis (A) of the light guide, and are spaced away from the longitudinal axis (A).FIG. 11is a schematic drawing of an LED configuration1100having three LEDs1102,1104,1106, arranged circumferentially around the longitudinal axis (A) of the light guide.FIG. 12is a schematic drawing of an LED configuration1200having four LEDs1202,1204,1206,1208, arranged circumferentially around the longitudinal axis (A) of the light guide.FIG. 13is a schematic drawing of an LED configuration1300having five LEDs1302,1304,1306,1308,1310, arranged circumferentially around the longitudinal axis (A) of the light guide. Other configurations are possible, in which the LEDs are spaced unevenly around a circle, or are arranged in another shape around the longitudinal axis (A) of the light guide. In other examples, fewer than three or more than five LEDs are used.

FIGS. 14 and 15compare the present light guide ofFIG. 5to the known light guide ofFIG. 2.FIG. 14is a polar plot of simulated light output for the known lamp ofFIG. 1, which includes the known light guide ofFIG. 2.FIG. 15is a polar plot of simulated light output for the lamp ofFIG. 3, which includes the light guide ofFIG. 5. In addition to the angular distribution of the output light, both figures also show a value of calculated efficiency, expressed as a number between zero and one. The calculated efficiency is the total light output of the light guide, summed over all directions, divided by the total light output of the LEDs.

The calculated value of efficiency for the present light guide is 0.939 (i.e., 93.9%), compared with 0.892 (i.e., 89.2%) for the known light guide. One contributor to this increase in efficiency is that the present light guide more effectively handles rays at high angles of exitance from the LEDs, such as those shown inFIG. 6. In addition to the increase in efficiency, the calculated performance of the present light guide shows a reduction in the sharp peaks on the polar plot, which is desirable. This reduction in peaks may desirably give the lamp a more uniform appearance, when viewed from a variety of angles or viewing positions.

The description of the invention and its applications as set forth herein is illustrative and is not intended to limit the scope of the invention. Variations and modifications of the embodiments disclosed herein are possible, and practical alternatives to and equivalents of the various elements of the embodiments would be understood to those of ordinary skill in the art upon study of this patent document. These and other variations and modifications of the embodiments disclosed herein may be made without departing from the scope and spirit of the invention.

GLOSSARY

A Non-Limiting Summary of Above Reference Numerals

1known light bulb2light pipe of known light bulb3reflector of known light bulb4base of light pipe of known light bulb5envelope of known light bulb6hemispherical recess at distal end of light guide of known light bulb7tapered cylinder on lateral side of light guide of known light bulb300lamp302light guide304housing306transparent portion of housing308threaded base410potting412LED driver housing414thermally conductive adhesive416plastic disc418bezel420thermal grease422LED driver424screw426screw428plastic cover430screw432screw510proximal end of light guide512flat portion of proximal end of light guide514concave portion of proximal end of light guide516convex peripheral portion of proximal end of light guide520distal end of light guide522peripheral portion of distal end of light guide524central portion of distal end of light guide530lateral side of light guide550first region of lateral side of light guide560second region of lateral side of light guide570third region of lateral side of light guide580fourth region of lateral side of light guide588ridge590at least one light emitting diode592location on the convex peripheral portion of proximal end of light guide594location on the first region of the lateral side of the light guide596surface tangent at location592598surface tangent at location594600first portion of internal beam700second portion of internal beam800third portion of internal beam900fourth portion of internal beam1000fifth portion of internal beam1100LED configuration having three LEDs1102LED1104LED1106LED1200LED configuration having four LEDs1202LED1204LED1206LED1208LED1300LED configuration having five LEDs1302LED1304LED1306LED1308LED1310LED1600group of raysA longitudinal axis