Patent Application
20090032827
This invention relates to light emitting diodes (LEDs) and, in particular, to certain lens designs useful for backlighting.
LED dies typically emit light in a lambertian pattern. It is common to use a lens over the LED die to narrow the beam or to make a side-emission pattern. A common type of lens for a surface mounted LED is preformed molded plastic, which is bonded to a package in which the LED die is mounted. One such lens is shown in U.S. Pat. No. 6,274,924, assigned to Philips Lumileds Lighting Company and incorporated herein by reference.
When LEDs are the light source in a backlight, various techniques have been used to prevent the small LED die from appearing as a bright dot on the backlight's output surface. For example, for small backlights, LEDs may illuminate a solid, transparent light guide from the side edges. The LED light then mixes in the light guide and leaks out the top with a uniform emission profile. In larger backlights, designed by the present assignee, the LEDs are distributed on the base surface of a reflective backlight box, and each LED has a side emitting lens that greatly limits the light emission normal to the LED. The side emissions are mixed in the box and eventually leak out through the top opening of the box to create a uniform emission profile. In such a backlight, the reflected and mixed sidelight constitutes a vast majority of the light ultimately emitted by the backlight. It is inherent in such a design that light is attenuated by each reflection.
The present assignee has developed an overmolding technique that molds a lens directly over the LED in virtually any shape. Various backlights using side-emitting lenses formed by overmolding are described in U.S. application publication number US 2006/0102914, assigned to Philips Lumileds Lighting Company and incorporated herein by reference.
A new lens surface shape is disclosed for an LED, where the lens is concave over the LED die, and the rim of the concave portion smoothly transitions into the sidewalls. The rim is at a particular radius from the center line to achieve the desired emission pattern. The shape of the lens results in a maximum intensity between 35-65 degrees with respect to the normal of the LED die surface. Instead of minimizing the emission at the normal of the LED, as is typically done with side-emitting lenses, the intensity along the normal is 40-90% of the maximum intensity.
The lens is preferably silicone and formed by molding directly over the LED die.
One or more of the LEDs incorporating the lens are used in a reflective backlight box, where the light emission from the lens directly illuminates a light emitting surface of the backlight (e.g., a diffuser sheet or a brightness enhancement film) and exits the backlight. Although there will be some reflected light inside the backlight box (reflected off the backlight box walls), such reflected light does not form the majority of the light that ultimately exits the backlight. In one embodiment, at least 50% of the light exiting the backlight box is from direct illumination by the LEDs.
The LEDs in the backlight box may be blue, red, and green LEDs, or use phosphor conversion to create red, green, and blue light components. The optimum lens shape for each type of LED may be different to achieve the desired brightness profile.
The thickness of the lens, the width of the lens, the shape of the lens, and the distance between the top of the lens and the top surface of the backlight are optimized to maximize the efficiency and brightness uniformity of the backlight.
Elements labeled with the same numeral in the various figures may be the same or equivalent.
As a preliminary matter, a conventional LED is formed on a growth substrate. In the example used, the LED is a GaN-based LED, such as an AlInGaN LED, for producing blue or UV light. Typically, a relatively thick n-type GaN layer is grown on a sapphire growth substrate using conventional techniques. The relatively thick GaN layer typically includes a low temperature nucleation layer and one or more additional layers so as to provide a low-defect lattice structure for the n-type cladding layer and active layer. One or more n-type cladding layers are then formed over the thick n-type layer, followed by an active layer, one or more p-type cladding layers, and a p-type contact layer (for metallization).
Various techniques are used to gain electrical access to the n-layers. In a flip-chip example, portions of the p-layers and active layer are etched away to expose an n-layer for metallization. In this way the p contact and n contact are on the same side of the chip and can be directly electrically attached to the package (or submount) contact pads. Current from the n-metal contact initially spreads laterally through the n-layer. In contrast, in a vertical injection (non-flip-chip) LED, an n-contact is formed on one side of the chip, and a p-contact is formed on the other side of the chip. Electrical contact to one of the p or n-contacts is typically made with a wire or a metal bridge, and the other contact is directly bonded to a package (or submount) contact pad. A flip-chip LED is used in the various examples for simplicity, although a non-flip-chip LED may be used instead.
Examples of forming LEDs are described in U.S. Pat. Nos. 6,649,440 and 6,274,399, both assigned to Philips Lumileds Lighting Company and incorporated herein by reference.
Optionally, the metal pads on the LED dice are bonded to pads on a submount wafer, and the sapphire substrate is removed. The submount wafer is then singulated by sawing to separate out the LEDs. Electrodes of one or more submounts may then be bonded to a printed circuit board, which contains metal leads for connection to other LEDs and to a power supply. The circuit board may interconnect various LEDs in series and/or parallel.
The particular LEDs formed and whether or not they are mounted on a submount is not important for purposes of understanding the invention.
In the preferred embodiment for forming a lens over each LED die, an array of LEDs is mounted on a submount wafer. The submount may be a ceramic substrate, a silicon substrate, or other type of support structure with the LED dice electrically connected to metal pads on the submount. A lens is then overmolded onto each LED die simultaneously using the overmolding process described in U.S. application publication number US 2006/0102914, assigned to Philips Lumileds Lighting Company.
In this overmolding process, a mold has indentations in it corresponding to the positions of the LED dice on the submount wafer. The indentations are filled with a liquid, optically transparent material, such as silicone, which when cured forms a hardened lens material. The shape of the indentations will be the shape of the lens. The mold and the LED dice/support structure are brought together so that each LED die resides within the liquid lens material in an associated indentation.
The mold is then heated to cure (harden) the lens material. The mold and the substrate wafer are then separated, leaving a complete lens over each LED die, completely encapsulating the die. This general process is referred to as overmolding. The submount wafer is then singulated to separate out the LEDs.
In one embodiment, the inventive lens is the only overmolded lens encapsulating the LED. In another embodiment, a hemispherical lens is first overmolded on the LED to encapsulate the LED, followed by molding the inventive lens over the hemispherical lens.
The lens 26 has a concave shape above the die 22 and has a rounded rim at a certain radius, where the lens 26 is the thickest, then falls off. The sidewalls of the lens 26 are substantially vertical, such as at an angle of 10-15% with respect to vertical.
In one example, the surface of the lens 26 is described by the following equation, where Z is the vertical distance of the lens surface to the top of the LED die and R (radius) is the distance from the centerline. The dimensions are given relative to a center height of 1.0.
Z(R)=1.0+0.4*R4−0.0497*R14 eq. 1
Z(R)=1.0+0.2*R6−0.0921*R12 eq. 2
Depending on the specific requirements for a radiation pattern, the values of the polynomial coefficients can be optimized. Therefore, the general polynomial function is:
Z(R)=C0+C2*R2+C4*R4+C6*R6+C8*R8+C10*R10+C12*R12+C14*R14 eq. 3
The lens curvature is not restricted to polynomial functions.
Since the light is broadly concentrated within a certain angle, the light pattern is about 1.5-2 times wider than a Lambertian pattern for the same brightness level contour.
In the preferred embodiment, the peak intensity is between 35-65 degrees off the normal. The intensity along the normal is 10-60 percent less than the peak intensity (i.e., 90-40% of the peak intensity). With typical wide emitting lenses, the brightness along the normal is made as small as possible. The present lens is designed to produce a substantially uniform intensity upon a flat output surface of a backlight box (
The dimensions of the concave lens are selected to optimally illuminate a flat surface at a particular distance from the LED die. Any change to the lens thickness, width, or curvature will typically change the angle of peak intensity.
Backlight 40 is formed of reflective inner surfaces 42, a top diffuser sheet 44 (e.g., a roughened plastic sheet), and one or more brightness enhancement films (BEFs) 46. The diffuser sheet 44 and each BEF 46 may be very thin (less than 1 mm). The diffuser sheet 44 improves the brightness uniformity across the surface of the backlight. The BEFs 46 may be formed by a micro-prism pattern in a plastic sheet that redirects light within a narrow angle toward the viewer. A liquid crystal display 48 overlies the backlight 40 and, essentially, has a controllable shutter at each pixel location for the RGB pixels for displaying a color image. If the backlight 40 emits white light (containing RGB components), a red, green, or blue filter at the corresponding RGB pixel locations only passes the intensity-modulated red, green, or blue component.
The lens shape, the spacing between LEDs, and the distance to the top of the backlight are selected so that the emissions from adjacent LEDs merge to form a substantially uniform illumination over the backlight top surface. Since the light emitted through the center of the lens 26 normal to the LED surface travels the least distance to the backlight top surface, that light has the least spreading, while the peak intensity light emitted at a 35-65 degree angle travels further to impinge upon the top surface of the backlight and thus spreads out more before impinging on the backlight top surface. The combination of the intensity profile (
To further improve the brightness uniformity, each lens may have a generally rectangular shape, as shown in the top down view of a single LED 56 with lens 58 in
In one embodiment, the thickness of a lens 26 is 0.5 to 1 mm, and the width of the lens is about 2-3 mm, given that an LED die is about 1 mm×1 mm. The lens dimensions may be different depending on the LED type, the backlight configuration, the pitch of the LEDs, and other factors. In one embodiment, the distance between the top of the lens 26 and the backlight diffuser sheet 44 is 1-3 cm. In one embodiment, the total thickness of the backlight box is 3.5 cm. The optimum pitch of the LEDs on the backlight base, the number of LEDs, and the size of the lens may be determined empirically depending on the required size of the backlight and the required brightness of the backlight emission.
To create white light from each LED, the LED die may emit blue light, and phosphor particles energized by the blue light generate red, green, and/or yellow components that combine with the blue light to create white light, as illustrated in
In
In
In another embodiment, the LED die may be a non-flip-chip die, with a wire connecting the top n-layers to a metal pad on the submount. The lens then also encapsulates the wire.
While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.
