Light distribution using a light emitting diode assembly

A fluorescent light tube retrofit with light emitting diodes (LEDs) that evenly distribute light to avoid bright spots is disclosed. One tube in the form of a conventional fluorescent tube includes two LEDs mounted to the tube on opposite sides of a single circumference of the tube. The LEDs can face the center of the tube, or the LEDs can be offset facing relative to the center of the tube. A reflecting surface can be disposed inside the tube to reflect light evenly toward an arc of the tube. Alternatively, at least one LED can be oriented to direct light into a light pipe that curves around the interior of the tube.

TECHNICAL FIELD

The present invention relates to a light emitting diode (LED) assembly, and more specifically, to a LED assembly that can replace a conventional fluorescent light in a conventional fluorescent light fixture.

BACKGROUND

Light emitting diodes (LEDs) have many advantages over fluorescent lights. LEDs are more efficient, last longer, and are less sensitive to vibrations and low temperatures. To take advantage of the benefits of LEDs, conventional fluorescent light tubes have been retrofit to include LEDs. For example, U.S. Pat. No. 7,049,761 discloses a tube having the shape of a conventional fluorescent light tube encasing a group of LEDs. Known fluorescent light tubes retrofit with LEDs are constrained by the directional light output of the LEDs, in contrast to the uniform non-directional light output of fluorescent tubes.

BRIEF SUMMARY

The present invention teaches LEDs in various orientations to evenly distribute light around the circumference and along the length of a tube, resulting in even lighting without obvious point sources of light. One such configuration includes a first LED assembly and a second LED assembly, each having a plurality of LEDs. A tube includes at least one tube portion, and the first and second LED assemblies are attached to longitudinal lengths of the tube portion and are oriented to face the interior of the tube. The areas of the tube that receive the least amount of light from each LED assembly receive light from multiple LED assemblies, while the sections of the tube that receive the greatest amount of light from each LED assembly only receive light from one LED assembly. Thus, in the aggregate, a similar amount of light strikes the tube around its entire circumference and along its entire length.

Another such configuration includes, for example, a tubular housing including at least one tube portion and at least one LED assembly including a plurality of LEDs. Each LED assembly is mounted to a longitudinal length of the tubular housing and is oriented to emit light parallel to a tangent of the tubular housing. This configuration also includes a light pipe associated with each LED assembly and curving inside at least a portion of the tubular housing.

Details of these embodiments, and others, are described in further detail hereinafter.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Known fluorescent light tubes retrofit with LEDs distribute light directly toward objects to be illuminated. However, distributing light directly toward objects to be illuminated can result in harsh, uneven light and the appearance of bright spots due to the narrow viewing angle of LEDs. In contrast, embodiments of a linear distribution light emitting diode assembly that provide even light are disclosed herein. By placing LEDs in certain orientations, the appearance of bright spots is overcome, and even light is provided.

Embodiments of a linear distribution light emitting diode assembly are illustrated inFIGS. 1-7. The light rays illustrated in the figures are for illustrative purposes only and are not intended to accurately portray the actual dispersion of light from the LEDs. As illustrated inFIG. 1, an LED lighting unit10includes a tube12and a LED assembly14. The tube12is shaped to enable the LED lighting unit10to be compatible with a conventional fluorescent light fixture and includes end caps for inserting the unit10into such a light fixture. The LED assemblies14extend along longitudinal lengths of the tube12, i.e., lengths of the tube12parallel to the axis of the tube12, in order to provide light to the entire length of the tube12. The tube12is formed by attaching two semi-circular tube portions13to the LED assemblies14. The attachment between the tube portions13and the LED assemblies14can be by way of glue, screws, snap-fit mechanisms, or other suitable attachment mechanisms known to those of skill in the art.

If, however, the LED lighting unit10includes only one LED assembly14on a circumference of the tube12as illustrated inFIG. 7, one tube portion13can extend nearly a full circle from one side of the LED assembly14to the other. Alternatively, the tube12can be a conventional fluorescent light tube with LED assemblies14attached to its interior as illustrated by example inFIG. 5. The attachment between the tube12and the LED assemblies14can be by way of glue, screws, snap-fit mechanisms, or other suitable attachment mechanisms known to those of skill in the art. Also, the specific shape of the tube12depends on the desired use of the LED lighting unit10. For example, the tube12need not be an elongated shape; it can be U-shaped, toroidal, or any other shaped required by the specific application. In such a case, the one or more LED assemblies14would still extend parallel to the axis of the tube (that is, would still extend in a longitudinal direction), but would be shaped to be compatible with the tube12. For example, if the tube12is intended to replace a conventional ring-shaped fluorescent light, the LED assemblies14extend longitudinally around the inner and outer circumferences of the ring-shaped tube to follow the curve of the tube12. The tube12can be formed of polycarbonate, glass, acrylic, and other materials known to those of skill in the art.

In the illustratedFIG. 2, the tube12includes a diffusing surface22. The diffusing surface22as illustrated is a diffusing film applied to the interior surface of the tube12. Alternatively, the diffusing surface can include light diffusing particles in a light transmitting resin applied to the interior of a transparent tube12. Or, instead of fixing a separate diffusing layer to a transparent tube, the tube10can be made of a translucent material. The tube12can also undergo a treatment to create a diffusing surface22on its interior, such as roughening the interior surface of the tube12. Alternatively, as illustrated inFIG. 7, light extraction structures, such as ridges32, dots, bumps, dimples, and other uneven surfaces, can be included on the interior surface of the tube12, in which case a separate diffusing layer can be included on the exterior of the tube12.

Each LED light assembly14includes a plurality of LEDs16and an electric circuit. The LEDs16included in the LED light assembly14emit white light. However, if desired, LEDs16that emit blue light, ultra-violet light or other wavelengths of light can be included. Printed circuit boards (PCB)18make up the electric circuitry in the illustrated embodiments. However, other types of circuit boards, for example metal core circuit boards, can be used in place of PCBs18. Alternatively, the circuitry can be formed directly on the interior surface of the tube12, such as by depositing copper onto the interior of the tube portions13before assembly. Likewise, wires can be used in place of a printed circuit board18, so long as the LEDs16are electrically connected and adequately supported. When wires are used, LEDs16can be glued directly to a heat sink20or, if no heat sink is necessary in the application, to the tube12. Because the danger of LED failure is low, the LEDs16can be connected in series or parallel. Heat sinks20are illustrated attached to each PCB18. However, the tube portions13can be formed of heat-conducting plastic materials that do not require heat sinks20. In an application where the tube12is arranged in a ring-shape, for example, it is desirable that the electric circuit include a flexible circuit board.

To facilitate a physical and electrical connection with a conventional fluorescent lighting fixture, end caps (not shown) are attached to each end of the tube12. The end caps include a transformer, if necessary, and any other required electrical components. Alternatively, the electrical components can reside in a portion of the tube12. The end caps include a necessary physical and electrical connection, such as the two-pin configuration commonly used in conventional fluorescent light fixtures. Such a structure is shown in, for example, U.S. Pat. No. 7,049,761.

In the embodiment illustrated inFIG. 2, two LED assemblies14are attached to longitudinal lengths of tube portions13to form tube12. The LED assemblies14are spaced apart 180° relative to the center of the tube12, and the LED assemblies14are oriented to face the center of the tube12. While LEDs16emit light in multiple directions, the direction a LED16is said to be “facing” is determined by reference to the direction in which emitted light travels. That is, if a line were to run in the direction an LED assembly14is oriented to “face”, an equal amount of light emitted by the LED16would pass on both sides of any plane including the line.

The light emitted by an LED16is the most concentrated in the region surrounding the direction the LED16faces. By placing two LED assemblies14on opposite sides of the tube12and orienting them to face the center of the tube12, an even distribution of light around the circumference of the tube12is achieved because the parts of the tube12that receive the least amount of light from each LED assembly14, such as the top and bottom portions of the tube12as shown inFIG. 2, receive light from both LED assemblies14. The parts of the tube12that receive the most amount of light from each LED assembly14, such as the area of the tube12in the region around where the LED16faces, only receive light from one LED assembly14. Thus, in the aggregate, a similar amount of light strikes the tube12around its entire circumference. Further, the diffusing surface22provides additional bright-spot eliminating capability by diffusing the light before it exits the tube12. While only two LED assemblies14are contemplated on a single circumferential path of the tube12in the embodiment illustrated inFIG. 2, additional LED assemblies14could be placed about the tube12for additional brightness. It is desirable but not necessary that such LED assemblies14be evenly-spaced about the tube12.

A second embodiment is illustrated inFIG. 3. Here, the LED assemblies14are in an offset orientation; i.e., instead of facing the center of the tube12, the LED assemblies14inFIG. 3are angled slightly above and below the center of the tube12, respectively. The LED assembly14orientation in the first embodiment results in some light being blocked from exiting the tube12by the opposing LED assembly14. Compared to the center-facing orientation of the first embodiment, the offset orientation of the second embodiment permits an increased amount of light to exit the tube12, resulting in an increased overall brightness of the LED lighting unit10. The number of LED assemblies14around one circumference of the tube12and the spacing of the LED assemblies14can be varied from the configuration shown inFIG. 3, but it is desirable that such assemblies14be distributed evenly around the circumference of the tube12as mentioned above. Additionally, the offset angle, i.e., the angle between the direction a LED16faces and the center-facing direction, can be varied. The greater the offset angle, the less light is blocked by the opposing LED assembly14. However, the trade-off of increasing the offset angle is that the light distribution becomes less even as the angle increases.

In another embodiment, as shown inFIG. 4, a reflecting surface24is placed inside the tube12. The reflecting surface24is made of a reflective material, such as a mirror made of glass or plastic with a metallic coating on its backside, and can include a diffusing surface if desired. The reflecting surface24spans a diameter of the tube12. Alternatively, the reflecting surface24can have a major length less than the diameter of the tube12and can be buttressed by brackets in the tube12or attached to end caps at each end of the tube12. The reflecting surface24has a convex shape designed to evenly distribute light throughout an arc of the tube12. The specific curvature of the reflecting surface24is dependent on the viewing angle of the LEDs16, the distance from each LED16to the reflecting surface24, and the number of LEDs16around the circumference of the tube12. For example, a LED16with a narrow viewing angle requires a greater angle of deflection than a LED16with a wide viewing angle in order to achieve the same distribution of light across an arc of the tube12. Additionally, a lip26projects from the reflecting surface24near the point where each LED16faces the reflecting surface24. The lip26is a projection from the reflecting surface24that directs light around the LED assembly14that would otherwise be reflected off the reflecting surface24right back toward the LED assembly14. Thus, the lip26increases the amount of light that is able to exit the tube12, thereby increasing the brightness of the LED lighting unit10.

Another embodiment includes a bend28in the reflecting surface24as illustratedFIG. 5. The reflecting surface24in this embodiment is similar to the reflecting surface24in the previous embodiment, except bends28are disposed near the junction of the reflecting surface24and the tube12. Each bend28is angled to direct light through the area of the tube12just outside the perimeter of an LED assembly14. By directing light through the tube12in the vicinity of the LED assembly14, the occurrence of dark spots created by the LED assemblies14is reduced. This embodiment also features a diffusing surface22on the reflecting surface24.

An embodiment illustrated inFIG. 6features at least one LED assembly14mounted radially to the tube12. In this orientation, the radially-mounted LED assembly14faces parallel to a tangent of the tube12at the location the LED assembly14is mounted. A first end of a light pipe30is adjacent to each LED16to receive the emitted light. The pipe30then curves around the inside of the tube12until the second end of the pipe30is adjacent to the backside of the next LED assembly14. The light pipe30tapers as it arcs around the circumference of the tube12. The large cross section of the light pipe30in the vicinity of LED16allows a high proportion of light to arc around the tube12instead of exiting. As the light arcs around the tube12and the quantity of light in the light pipe30decreases due to a portion of the light exiting the tube12, the smaller cross section of the light pipe30forces a higher proportion of light out of the tube12. Thus, an even amount of light exits the tube12through the entire arc of the tube12. The light pipe30is constructed of plastic with a metallic coating to reflect light. The light pipe30can also be constructed of mirrored glass. Regardless of the material selected, the light pipe30should have as close to total internal reflection as possible in order to maximize the brightness of the LED lighting unit10.

The surface of the light pipe30in this embodiment includes light extraction structures, specifically ridges32as illustrated. Light extraction structures can take other shapes, such as dots, bumps, dimples, and other uneven surfaces. The size and shape of such light extracting structures can vary over a circumference and a length of the tube12to create a uniform distribution of light over the circumference and length of the tube12. For example, the structures can be small and sparse near the near the LED16where the flux of light is high, and larger and more dense away from the LED16where the flux of light is low. If multiple LEDs16are placed around a circumference of the tube12, there can be multiple areas around the circumference of the tube12that have densely spaced light extracting structures. The placement of light extracting structures is determined by software, such as the software disclosed in Michael Zollers, “Integrated Optimization Capabilities Provide a Robust Tool for LED Backlight Design,”LEDs Magazine(October 2006), pp. 27-29, which is hereby incorporated by reference. The light extracting structure placement can also be determined in other ways, such as through experimentation or hand calculation. Alternatively, the surface of the light pipe30can be smooth; the light pipe30need not include light extraction structures.

If there is only one LED assembly14on a circumference of the tube12, the light pipe30completes almost an entire rotation inside the tube12before ending on the opposite side of the LED assembly14from which it started, thereby distributing light over nearly the entire circumference of the tube12. In operation, a portion of the light emitted by an LED16hits the tube12having an angle of incidence less the critical angle of the tube12and exits the tube12, a portion hits the tube12having an angle of incidence equal to or greater than the critical angle of the tube12and is deflected back into the tube12, and a portion initially contacts the light pipe30. The light pipe30deflects the light that hits it back toward the tube12. Thus, light rays can ricochet through an arc before exiting the tube12, resulting in an even distribution of light through the are.

An embodiment illustrated inFIG. 7features a side-emitting LED16and a light pipe30similar to the light pipe30in the fifth embodiment. The side-emitting LED16emits a disc of light at approximately a right angle to the direction the LED16faces. The LEDs16abut the tube12such that the LEDs16emit light parallel to a local tangent of the tube12. In this embodiment, the circuit board18and heat sink20are mounted below the LED16on the interior of the tube12. Alternatively, the circuit board18and heat sink20call be mounted on the outside of the tube12. The light pipe30curves around the inside of the tube12, extending from one side of the LED16to the other and forming a channel between the tube12and the light pipe30. The light pipe30is tapered such that the portion of the light pipe30furthest from the LED16is closest to the tube12. The tapered shape of the light pipe30causes a high proportion of light to exit the tube12when the quantity of light is low and results in an even distribution of light around the circumference of the tube12. Thus, light exits the side of the LED16and curves around a circumference of the tube12, reflecting between the tube12and the light pipe30until the light strikes the tube12at an angle less than the critical angle and exits the tube12. Alternatively, multiple LED assemblies14can be disposed about the circumference of the tube12, in which case a light pipe30extends between each of the LED assemblies14. Also, light extracting structures can be placed on the light pipe30as discussed in the previous embodiment.