Patent Publication Number: US-11378730-B2

Title: Illumination structure including cavity and TIR structure

Description:
FIELD OF THE INVENTION 
     This invention relates to a lighting structure comprising a light guide mixing and emitting the light from light emitting diodes (LEDs) and, in particular, to forming structures in the light guide to optically couple the LED light and spread the light. The structure may be used for backlighting, general lighting or for other applications. 
     BACKGROUND 
     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 US 20080049445 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 85-90%, 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. 
     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. 
     SUMMARY 
     In one embodiment, a thin plastic light guide is formed to have cavities on one surface and TIR (total internal reflection) structures directly (can be off-centered also) above the cavities. A through-hole is not formed. LEDs mounted on a printed circuit board (including a flexible circuit) are positioned in the cavities. The LEDs may be grouped, for instance in one or more arrays. The LEDs may have a Lambertian emission and no lens, so are very thin. At least the top ceiling of each cavity is shaped to direct the LED light to most efficiently make use of the TIR structures. For example, the TIR structures may have a cusp shape, and the top ceiling of the cavities may reflect the LED light to optimally impinge on the TIR structure to maximize the TIR and the mixing of light. Further, the TIR structures and cavities may be shaped to cause a controlled amount of light to leak out through the TIR structures so there are no dark spots and no need for a bezel. Depending on the configuration and function of the light guide the leakage through the TIR may be zero or small compared to the amount of light reflected by the TIR structure. 
     The TIR structures and cavities may be symmetrical or asymmetrical. An asymmetrical shape takes into account that the LEDs along a perimeter should have more of their light directed toward the middle of the light guide. This technique can also be used to shape the beam or making asymmetric light beam. The cavities may for instance be roughened, coated, filled with material and/or partly reflective to support directing the light to the TIR structure. 
     The bottom or top surface of the light guide has additional optical features, such as tiny prisms, dots, or roughening, to cause the uniform leaking of light through the top surface. The optical features may vary along the surface to leak out different percentages of the light to improve uniformity across the light guide. 
     As mentioned above, the TIR structures and cavities may be formed to leak out a controlled amount of light so there are no dark spot over the LEDs. The light leaking through the TIR structure may be the same brightness as the light leaking out other areas of the light guide surface. This allows the LEDs to be distributed over the entire bottom surface of the light guide, which improves uniformity and enables the light guide to be any size and shape. 
     By molding the TIR structures in the light guide along with the cavities, the TIR structures and cavities can be precisely aligned. The light guide can possibly be very thin, even such as less than 3 mm, and flexible. 
     A diffuser sheet or other type of sheet may be laminated over the light guide to further mix the light for increased uniformity or perform another function. A reflective film may be laminated with or without airgap on the bottom surface of the light guide and its edges to ensure all light exits the top surface of the light guide. 
     Other embodiments are disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top down view of a light guide, in accordance with one embodiment of the invention, showing areas where LED cavities and TIR structures are located around the perimeter. The relative size of each area is greatly enlarged for clarity. 
         FIG. 2  is a bisected view of a single cavity and TIR structure (a cusp shape) in the light guide of  FIG. 1 , showing a simulation of light rays being shaped by the walls of the cavity and reflected of the TIR structure, where only a miniscule amount of light is leaked through the TIR structure. 
         FIG. 3  is a bisected view of an alternative single cavity and TIR structure (a cone shape), showing a simulation of light rays being shaped by the walls of the cavity and reflected off the TIR structure, where a controlled percentage of light is leaked out through the TIR structure so there is no dark or bright spot above the LED. 
         FIG. 4  is a bisected view of another single cavity and TIR structure (a cusp shape), where a diffusion sheet or other type of sheet is laminated over the light guide, and a reflector sheet is positioned below the light guide. 
         FIG. 5  illustrates how the cavities and TIR structures can be distributed over the entire area of the light guide, where the TIR structures uniformly leak out light to blend with light leaked out from surrounding areas of the light guide, enabling the creation of a light guide of any size and shape. 
         FIG. 6  illustrates optical features formed in a top or bottom surface of the light guide for leaking out light. 
     
    
    
     Elements that are the same or similar are labeled with the same numeral. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a light guide  10  in accordance with one embodiment of the invention. The light guide  10  may be PMMA, PET, or other plastic having a thickness on the order of for instance 2-5 mm. The light guide  10  may be flexible. 
     The light guide  10  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  12  and bottom cavities  40 . An LED  18  is positioned in each cavity  40 . The TIR structures  12  and cavities  40  are precisely aligned due to the alignment of the molding rollers/plates. The TIR structures  12  and cavities are formed along one or more edges of the light guide  10  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  10 , only one TIR structure-cavity may be required along only one edge. For a much larger light guide  10 , the TIR structures  12  and cavities may be located along all four edges or distributed over the entire surface. The pitch of the TIR structures  12  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. 1 , 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  14 , shown transparent with an opening defined by the dashed-line rectangle, overlies the TIR structures  12  if there are dark spots or bright spots to be hidden. The bezel  14  may be part of a frame that supports the light guide  10  or may be part of a housing. 
       FIG. 2  is a bisected view of only a single TIR structure  12  and its underlying cavity  16 . An LED  18  is mounted on a thin printed circuit board  20  (PCB), which may be a flexible circuit, and the LED  18  is positioned in the middle of the cavity  16 . All the LEDs used may be on the same PCB  20  and be aligned with all the cavities  16  for a simple fabrication process. The PCB  20  is fixed in position relative to the bottom surface of the light guide  10 , such as via a frame. 
     In  FIG. 2 , the TIR structure  12  has a symmetrical circular cusp shape so that light is reflected by TIR into the light guide  10 . TIR structures may also have an asymmetrical and freeform shape.  FIG. 2  shows simulated light rays  22  directed by the shaped walls of the cavity  16  and the TIR structure  12  and also reflected by TIR off the smooth top and bottom surfaces of the light guide  10 . In an actual embodiment, the top or bottom surface of the light guide  10  has optical features for causing the leaking out of a percentage of light across the top surface of the light guide  10 . 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  18  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 example of  FIG. 2 , the light guide  10  is 3 mm thick, and the bezel  14  is 10 mm 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 10-15%. Therefore, using the TIR structure  12  is an improvement over using an opaque reflective layer. 
     The cavity  16  shape (e.g., a convex, freeform, spline or polynomial shape) is tuned to the particular TIR structure  12  used. In  FIG. 2 , the cavity  16  shape refracts the LED  18  light to optimally shape the LED emission for impinging on the cusp-shaped TIR structure  12  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  12  may be a dark spot. The LED side light is also redirected for improved mixing in the light guide  10 . 
     In the example, there is only a 0.1 mm gap between the top of the cavity  16  and the point of the cusp. In other embodiments there may even be no gap and the top of the cavity  16  and the point of the cusp may intersect. 
     Also shown in  FIG. 2  is a thin reflective film  24 , such as available from Toray Industries, Inc., that is laminated over the PCB  20  and vertical edges  24  of the light guide  10  to reflect light back into the light guide  10 . A small air gap  26  is ideally created between the reflective film  24  and the light guide  10  to maximize TIR off the light guide  10  surfaces, which is more efficient than reflection off the reflective film  24 . 
     In one embodiment, the TIR structures  12  and cavities  16  are circular and symmetrical. In another embodiment, the TIR structures  12  and cavities  16  are asymmetrical so as to reflect more of the LED light toward the middle of the light guide  10  and less light toward the edges. This technique can also help to shape the beam, for example asymmetric beam patterns. 
     In another embodiment, the TIR structures  12  and cavities  16  are shaped to leak out a controlled amount of light to blend in with the surrounding areas of the light guide  10 . In such a case, no bezel  14  is needed to block dark or bright spots. However, in the example of  FIG. 2 , only a miniscule amount of light leaks through the point of the cusp. 
       FIG. 3  illustrates a different shape of the TIR structure  30  and cavity  32 . The TIR structure  30  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  32  is also tuned to the TIR structure  30  to cause more of the LED light to impinge upon the TIR structure  30  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  30  to escape through the TIR structure  30 . A 1 mm gap is shown between the top of the cavity  32  and the TIR structure  30 . The amount of light leaking through the TIR structure  30  can be made uniform across the TIR structure  30  to match the brightness of light leaking through surrounding areas of the light guide  36 . 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  36  rather than only along the edges. This improves uniformity and enables the fabrication of a light guide of any size. Simulated light rays  38  are shown. The remainder of  FIG. 3  may be similar to  FIG. 2 . 
       FIG. 4  illustrates another cusp-shaped TIR structure  12  and an alternative shape of the cavity  40 . The cavity  40  has a top ceiling shaped such that it refracts more of the LED light at less than the critical angle for the TIR structure  12  so that more LED light leaks out the TIR structure  12 . 
       FIG. 4  also illustrates the PCB  20 , which may be a flexible circuit, an insulated metal substrate, FR4, or other material. The PCB  20  has metal traces that connect the anode and cathode terminals of the LEDs to a power source. A reflective layer  42  is provided over the PCB  20 . The reflective layer  42  may be a reflective solder mask. Ideally, there is an air gap between the reflective layer  42  and the bottom of the light guide  43  so more light is reflected by TIR for improved efficiency (there can be optical contact (no airgap) also for some applications). The reflector layer  42  may be white to diffuse the light, or specular, or diffusing specular. 
     Over the top surface of the light guide  43  is laminated a diffusing sheet  44  or other type of sheet, such as an further optical structure, for instance using a transparent adhesion layer  46 . The adhesion layer  46 , 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  44  helps mix the light exiting the light guide  43  for increased uniformity. The diffusing sheet  44  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  46  and diffusing sheet  44 , and more light is extracted with fewer internal reflections, improving efficiency. Ideally, the adhesion layer  46  and diffusing sheet  44  have the same index as the light guide  43 . 
       FIG. 5  illustrates how a light guide  50  (only a small portion is shown) may have the TIR structures  52  distributed across the light emitting surface of the light guide  50 . The TIR structures  52  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  50 . Thus, a very large light guide  50  may be formed. Further, the light guide  50  may be made thinner since there is less concern about light attenuation by the light guide  50 . The separation between LEDs  18  may for instance be on the order of 10 mm-100 mm, depending on the type of LED used, the desired brightness, and the size of the light guide. The relative sizes of the LEDs  18  and TIR structures  52  are greatly exaggerated for clarity. 
       FIG. 6  illustrates a portion of the light guide  10  showing optical features  56  formed into the bottom of the light guide  10  to direct the light (using TIR) toward the top surface of the light guide  10  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  42  is used to reflect light back into the light guide  10 . A light ray  58  is shown. 
     The optical features  56  may vary along the surface to most uniformly cause light to leak out across the light guide. 
     In another embodiment, roughening the light exit surface of the light guide  10  results in the controlled leakage of light. 
     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 96%. The index of refraction of the PMMA light guide used in the simulations is about 1.4936. 
     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 3 mm) 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. 
     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.