Patent Description:
It is known to optically couple LED light into the side edges of a transparent light guide. A surface of the light guide has optical features, such as prisms or roughening, to allow the light to uniformly leak out. Since the LEDs have a Lambertian emission, it is difficult to couple into the edge of the light guide. Further, the light guide must be relatively thick to enable the edges to receive the light from the LEDs.

It is also known to form holes through the light guide around its perimeter and position an LED at the bottom of each hole. An opaque reflector layer is then positioned over the top opening of each hole to reflect any top emission from the LED back into the light guide. The reflector layer is typically a metallized plastic film. US patent application publication <CIT> describes a light guide with an LED positioned at the bottom of a hole with an opaque cusp-shaped reflector overlying each hole to reflect the light sideways into the light guide. The light is further mixed in the light guide. Such reflectors create dark spots over the LEDs and are difficult to precisely align with the LEDs. An opaque bezel must be positioned over the LEDs so the dark spots are not visible. Further, the reflector may only have a reflectivity of about <NUM>-<NUM>%, so there is significant attenuation of light rays reflected off the reflective layer. Other disadvantages exist.

Additionally, since the LEDs are only positioned along the perimeter of the light guide, it is difficult to uniformly leak light out across the top surface area of the light guide not covered by the bezel.

<CIT>, <CIT> and <CIT> each disclose a backlight assembly with light sources positioned in cavities of a light guide. The light guide comprises TIR structures above the cavities. Due to total internal reflection, the light generated by the light sources is reflected into the light guide, creating dark spots over the light sources.

What is needed is a lighting structure that uses a light guide and LEDs, where the structure does not suffer from the drawbacks of the prior art.

Elements that are the same or similar are labeled with the same numeral.

<FIG> illustrates a light guide <NUM>. <FIG> shows a cross-section through the light guide in accordance with one embodiment of the invention. The light guide <NUM> may be PMMA, PET, or other plastic having a thickness on the order of for instance <NUM>-<NUM>. The light guide <NUM> may be flexible.

The light guide <NUM> is molded, such as by pressing between two opposing heated rollers or plates having a negative of the molded features, to form top TIR structures <NUM> and bottom cavities <NUM>. An LED <NUM> is positioned in each cavity <NUM>. The TIR structures <NUM> and cavities <NUM> are precisely aligned due to the alignment of the molding rollers/plates. The TIR structures <NUM> and cavities are formed along one or more edges of the light guide <NUM> and are accordingly in this embodiment off centered in the light guide. Also asymmetrical and off-centered configurations of TIR structures and cavities with respect to the light guide are possible. For a small light guide <NUM>, only one TIR structure/cavity may be required along only one edge. For a much larger light guide <NUM>, the TIR structures <NUM> and cavities may be located along all four edges or distributed over the entire surface. The pitch of the TIR structures <NUM> and cavities depends on the number of LEDs needed for a desired brightness and the size of the light guide. Typical for this off centered configuration of <FIG>, the leakage through the TIR may be zero or small compared to the amount of light reflected by the TIR structure.

Instead of molding the light guide and/ or the cavities and/or the TIR structures, one or more of techniques of injection molding, hot pressing, machining, 3D printing, rolling, laser treatment and other manufacturing options may be used to create the light guide.

The LEDs may be grouped, for instance in one or more arrays, and may have same or different colors, for instance for color tuning, more in particular for generating white light. In particular also neighboring LEDs may have different colors. (Multiple LEDs of same or different colors can also be used under one cavity as another type of solution).

In one embodiment, an opaque bezel <NUM>, shown transparent with an opening defined by the dashed-line rectangle, overlies the TIR structures <NUM> if there are dark spots or bright spots to be hidden. The bezel <NUM> may be part of a frame that supports the light guide <NUM> or may be part of a housing.

<FIG> is a bisected view of only a single TIR structure <NUM> and its underlying cavity <NUM> in a light guide known from the prior art, whose basic principle of operation is explained in the following. An LED <NUM> is mounted on a thin printed circuit board <NUM> (PCB), which may be a flexible circuit, and the LED <NUM> is positioned in the middle of the cavity <NUM>. All the LEDs used may be on the same PCB <NUM> and be aligned with all the cavities <NUM> for a simple fabrication process. The PCB <NUM> is fixed in position relative to the bottom surface of the light guide <NUM>, such as via a frame.

In <FIG>, the TIR structure <NUM> has a symmetrical circular cusp shape so that light is reflected by TIR into the light guide <NUM>. TIR structures may also have an asymmetrical and freeform shape. <FIG> shows simulated light rays <NUM> directed by the shaped walls of the cavity <NUM> and the TIR structure <NUM> and also reflected by TIR off the smooth top and bottom surfaces of the light guide <NUM>. In a further variant of this light guide, the top or bottom surface of the light guide <NUM> has optical features for causing the leaking out of a percentage of light across the top surface of the light guide <NUM>. Surface optical properties of top or bottom cavity surfaces can also be tuned to control light leakage. If a diffuser sheet is used over the light guide, the emitted light will be uniform.

The LED <NUM> may be any type of LED, such as a GaN-based LED that emits blue light, with one or more phosphor layers that are energized by the blue light to add red and green components to create white light. Some of the blue light leaks through the phosphor. The phosphor may be a YAG phosphor (emits green-yellow light) along with a red phosphor to achieve the desired color temperature. The LED emission is generally Lambertian.

In the light guide of <FIG>, the light guide <NUM> is <NUM> thick, and the bezel <NUM> is <NUM> wide.

Light reflected by TIR is essentially not attenuated, while light reflected by an opaque reflective film (such as a metalized layer) may be attenuated by about <NUM>-<NUM>%. Therefore, using the TIR structure <NUM> is an improvement over using an opaque reflective layer.

The cavity <NUM> shape is tuned to the particular TIR structure <NUM> used. In <FIG>, the cavity <NUM> shape refracts the LED <NUM> light to optimally shape the LED emission for impinging on the cusp-shaped TIR structure <NUM> so that more LED light is uniformly intercepted by the TIR structure at above the critical angle for TIR. Therefore, the area over the TIR structure <NUM> may be a dark spot. The LED side light is also redirected for improved mixing in the light guide <NUM>.

In the light guide, there is only a <NUM> gap between the top of the cavity <NUM> and the point of the cusp. In other variants of the light guide shown in <FIG>, there may even be no gap and the top of the cavity <NUM> and the point of the cusp may intersect.

Also shown in <FIG> is a thin reflective film <NUM>, such as available from Toray Industries, Inc. , that is laminated over the PCB <NUM> and vertical edges <NUM> of the light guide <NUM> to reflect light back into the light guide <NUM>. A small air gap <NUM> is ideally created between the reflective film <NUM> and the light guide <NUM> to maximize TIR off the light guide <NUM> surfaces, which is more efficient than reflection off the reflective film <NUM>.

In one variant of the light guide shown in <FIG>, the TIR structures <NUM> and cavities <NUM> are circular and symmetrical. In another variant of the light guide shown in <FIG>, the TIR structures <NUM> and cavities <NUM> are asymmetrical so as to reflect more of the LED light toward the middle of the light guide <NUM> and less light toward the edges. This technique can also help to shape the beam, for example asymmetric beam patterns.

The TIR structures <NUM> and cavities <NUM> are shaped to leak out a controlled amount of light to blend in with the surrounding areas of the light guide <NUM>. In such a case, no bezel <NUM> is needed to block dark or bright spots. However, in the example of <FIG>, only a miniscule amount of light leaks through the point of the cusp.

<FIG> illustrates a different shape of the TIR structure <NUM> and cavity <NUM>. The TIR structure <NUM> is essentially cone shaped, allowing more leakage due to more of the incident light being at less than the critical angle. The shape of the cavity <NUM> is also tuned to the TIR structure <NUM> to cause more of the LED light to impinge upon the TIR structure <NUM> at less than the critical angle. The center portion of the cavity ceiling is generally flat so more of the LED light is directed toward the center of the TIR structure <NUM> to escape through the TIR structure <NUM>. A <NUM> gap is shown between the top of the cavity <NUM> and the TIR structure <NUM>. The amount of light leaking through the TIR structure <NUM> can be made uniform across the TIR structure <NUM> to match the brightness of light leaking through surrounding areas of the light guide <NUM>. In this way, no bezel is needed to hide any dark spots. Also, the LEDs may be arranged uniformly over the bottom surface of the light guide <NUM> rather than only along the edges. This improves uniformity and enables the fabrication of a light guide of any size. Simulated light rays <NUM> are shown. The remainder of <FIG> may be similar to <FIG>.

<FIG> shows a cross-section through the light guide in accordance with an embodiment of the invention and illustrates another cusp-shaped TIR structure <NUM> and an alternative shape of the cavity <NUM>. The cavity <NUM> has a convex ceiling shaped such that it refracts more of the LED light at less than the critical angle for the TIR structure <NUM> so that more LED light leaks out the TIR structure <NUM>.

<FIG> also illustrates the PCB <NUM>, which may be a flexible circuit, an insulated metal substrate, FR4, or other material. The PCB <NUM> has metal traces that connect the anode and cathode terminals of the LEDs to a power source. A reflective layer <NUM> is provided over the PCB <NUM>. The reflective layer <NUM> may be a reflective solder mask. Ideally, there is an air gap between the reflective layer <NUM> and the bottom of the light guide <NUM> so more light is reflected by TIR for improved efficiency (there can be optical contact (no airgap) also for some applications). The reflector layer <NUM> may be white to diffuse the light, or specular, or diffusing specular.

Over the top surface of the light guide <NUM> is laminated a diffusing sheet <NUM> or other type of sheet, such as an further optical structure, for instance using a transparent adhesion layer <NUM>. The adhesion layer <NUM>, covering all or part of the top surface of the light guide, may for instance be glue, tape or an index matching liquid. The diffusing sheet <NUM> helps mix the light exiting the light guide <NUM> for increased uniformity. The diffusing sheet <NUM> may contain light scattering particles and/or have optical features or a roughened surface. Suitable diffusing sheets are available from Toray Industries, Inc. Further, by not having an air gap above the light guide surface, there is less TIR off the surface (due to the higher index of refraction of the adhesion layer <NUM> and diffusing sheet <NUM>, and more light is extracted with fewer internal reflections, improving efficiency. Ideally, the adhesion layer <NUM> and diffusing sheet <NUM> have the same index as the light guide <NUM>.

<FIG> illustrates how a light guide <NUM> (only a small portion is shown) may have the TIR structures <NUM> distributed across the light emitting surface of the light guide <NUM>. The TIR structures <NUM> and underlying cavities are formed to leak out sufficient light to blend the leaked light with the light leaking out the surrounding areas of the light guide <NUM>. Thus, a very large light guide <NUM> may be formed. Further, the light guide <NUM> may be made thinner since there is less concern about light attenuation by the light guide <NUM>. The separation between LEDs <NUM> may for instance be on the order of <NUM>-<NUM>, depending on the type of LED used, the desired brightness, and the size of the light guide. The relative sizes of the LEDs <NUM> and TIR structures <NUM> are greatly exaggerated for clarity.

<FIG> illustrates a portion of the light guide <NUM> showing optical features <NUM> formed into the bottom of the light guide <NUM> to direct the light (using TIR) toward the top surface of the light guide <NUM> at less than the critical angle (relative to the top surface) for escaping the top surface. Multiple internal reflections are desirable to mix the light from the LEDs. A reflective film <NUM> is used to reflect light back into the light guide <NUM>. A light ray <NUM> is shown.

The optical features <NUM> may vary along the surface to most uniformly cause light to leak out across the light guide.

Accordingly, there may be five optical surfaces in the structure that direct light: the cavities, the TIR structures, the TIR of the smooth light guide surfaces, the light extraction features, and the diffusing sheet.

In one embodiment, the LED light coupling efficiency into the light guide is about <NUM>%. The index of refraction of the PMMA light guide used in the simulations is about <NUM>.

In another embodiment, the cavities and TIR structures may be other than circular, such as square shaped. Also the light guide may have different shapes such as rectangular, square, round or any irregular/asymmetric shape.

In one embodiment, the resulting light guide provides illumination as a down light, for instance for general illumination or for horticulture application. In another embodiment, the light guide serves as a backlight for an LCD display, such as in a monitor, smart phone display, or television. Since the LEDs used in the embodiments do not require a lens, the LEDs are very thin, enabling the formation of a very thin (less than <NUM>) backlight for use in a smart phone. Other embodiments of the invention may be used for automotive applications or in photo camera flashes, for instance in smart phones. Depending on the specific applications one or more LEDs may generate light in the UV or infrared spectrum. Although the LEDs in the shown embodiments are top-emitters, within the scope of the invention also other types of LEDs may be used, such as side-emitters and n-sided emitters.

Claim 1:
A light emitting structure comprising:
a light guide (<NUM>) having several surfaces, cavities (<NUM>) in a first surface and total internal reflection, TIR, structures (<NUM>) in an opposite second surface aligned with the cavities (<NUM>), wherein the TIR structures (<NUM>) reflect at least some light into the light guide (<NUM>) by TIR; and
a plurality of LEDs (<NUM>) positioned within the cavities (<NUM>),
wherein the light guide (<NUM>) leaks light from at least one of its several surfaces,
wherein the cavities (<NUM>) each have a convex ceiling formed to refract light from the LEDs (<NUM>) to impinge upon the TIR structures (<NUM>) in a desired manner, and
wherein the TIR structures (<NUM>) and cavities (<NUM>) are distributed over the first and second surfaces of the light guide (<NUM>)
characterized in that the TIR structures (<NUM>, <NUM>) and cavities (<NUM>, <NUM>, <NUM>) are designed to leak a controlled amount of light from the TIR structures (<NUM>, <NUM>).