Abstract:
A light barrier and/or light reflector for PCB-mounted LED&#39;s has reflective walls defining interior chambers which redirect the light from the LED&#39;s in a substantially orthogonal direction and shield each LED from the light emitted by adjacent LED&#39;s. The device is particularly suited for redirecting light from LED&#39;s on a PCB to a status indicator panel thereby replacing conventional light pipes.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to light barriers or shields. More particularly, it relates to devices for directing and shielding the light from light emitting diodes (LED&#39;s) that are mounted on printed circuit boards (PCB&#39;s). 
     2. Description of the Related Art 
     LED&#39;s are particularly suited for use as status indicators for various electronic circuits and systems. They are small, relatively inexpensive, have modest power requirements, produce little heat and have a long working life. LED&#39;s are available from a variety of sources in packages which permit direct mounting to a printed wiring board (PWB). However, since a PWB or PCB is typically mounted within a chassis of some sort, LED&#39;s on the PCB are usually not directly viewable by the user of the device. 
     In the past, light pipes comprised of rods or bundles of fibers of clear plastic or glass have been used to direct the light from PCB-mounted LED&#39;s to lenses or windows on, for example, the front panel of an electronic device where they may be viewed by the user. However, there are a number of disadvantages associated with this system: the light pipes must be aligned with both the LED and the viewing lens or window; and, the light from adjacent LED&#39;s can “bleed over” into an adjoining light pipe producing indefinite indications to the user. Moreover, light pipes are relatively costly and labor-intensive to install. The present invention solves these problems in a particularly efficient and cost-effective way. 
     SUMMARY OF THE INVENTION 
     A combination light barrier and reflector comprising individual “compartments”, “chambers”, “cavities”, “wells” or “pockets” is designed to fit over PCB-mounted LED&#39;s and redirect the light from such LED&#39;s in an orthogonal direction to lenses or windows on a nearby panel. 
     In one embodiment, the invention comprises a one-piece, plastic unit with an angled top and partition walls defining compartments which accommodate LED&#39;s that may be mounted on a printed circuit board. The partition walls are preferably opaque and substantially prevent light from one LED from entering an adjacent compartment. The angled top portion of the unit reflects the light from the LED to a status panel in proximity to the PCB containing the LED&#39;s. The status panel may comprise lenses or windows for viewing the light from the LED&#39;s and the partition walls of the light barrier may be further configured to shield the lenses or windows on the status panel from extraneous light. 
     The light barrier may be fabricated by the injection molding of a thermoplastic resin and may optionally be equipped with snap-in type mounting studs for quick and easy attachment to a printed circuit board. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of one embodiment of the light barrier showing the underside of the device. 
         FIG. 2  is a bottom plan view of the embodiment shown in  FIG. 1 . 
         FIG. 3  is a front view of the embodiment illustrated in  FIG. 1 . 
         FIG. 4  is a top view of the embodiment illustrated in  FIG. 1 . 
         FIG. 5  is a cross sectional view of the embodiment illustrated in  FIG. 1  taken along line A—A 
         FIG. 6  is an exploded view of a device employing the light barrier illustrated in  FIG. 1 . 
         FIG. 7  is a partial cross sectional view showing the light barrier and a surface-mounted LED attached to a printed circuit board. 
         FIG. 8  is a cross sectional view of an alternate embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates in perspective view one particular embodiment of the invention which hereinafter may be referred to as a “light barrier” or a “light reflector” or a “barrier/reflector”. The light barrier  10  illustrated in  FIG. 1  is adapted for mounting to a printed circuit board. The perspective view of  FIG. 1  shows the surface which abuts the PCB and the front surface of the light barrier  10 —i.e., the surface from which light is emitted. If the light barrier is mounted on the upper surface of a horizontal PCB,  FIG. 1  may be considered a perspective view of the underside of light barrier  10  taken from the front side. 
     Light barrier  10  has segmented top  12  which comprise flat segment  24  and angled segment  26 . End walls  14  connect top  12  to back wall  22 . Interior walls  16  are attached to top  12  and back wall  22  to define compartments  15 . Each compartment  15  is open on its bottom and front sides. 
     Interior walls  16  each comprise wall bottom  36  and wall front  28 . Wall bottoms  36  are coplanar as are wall fronts  28 . The plane of wall bottoms  36  is orthogonal to the plane of wall fronts  28  and defines the bottom surface of light barrier  10 .  FIG. 2  is a bottom plane view of light barrier  10 . Similarly, the plane of wall fronts  28  defines the front surface of light barrier  10 .  FIG. 3  is a front plan view of light barrier  10 . A wall bottom  36  may be provided with a notch  34  to accommodate a component lead on the PCB to which the light barrier  10  is mounted. 
       FIG. 4  is a top plan view of light barrier  10  showing mounting feet  20  and “core outs”  40  which may be used in some embodiments for molding purposes to maintain a consistent wall thickness. 
     Light barrier  10  is equipped with barbed mounting studs  18  for engaging corresponding holes in a receiving PCB. Studs  18  are attached to lands  30  which are connected via step portions  32  to top  12 . The transverse cut or void in the mounting stud  18  allows the barbed portions to flex inward toward the center of the stud  18  during insertion into the mounting hole on the PCB and then to return to the extended position when the barb clears the hole on the opposite side of the PCB. The distance between the shoulder of the barb and the surface of land  30  may be chosen to accommodate the particular thickness of the PCB to which the light barrier is to be mounted. Mounting feet  20  may be provided along back wall  22  for engaging corresponding receivers on the PCB which, in some embodiments, may be 0.125″ diameter holes in the PCB. 
     The surfaces of flat top segment  24 , angled top segment  26 , interior walls  16  and back wall  22  which face the interior of each chamber  15  are preferably reflective to visible light. To that end, it is desirable to have a smooth finish on these surfaces. 
     The light barrier of the present invention may be fabricated of any dimensionally stable, opaque material. For ease of manufacture and low cost, the light barrier may be formed by injection molding of a thermoplastic resin. One particularly suitable plastic is LEXAN® polycarbonate resin manufactured by GE Plastics. 
     It will be appreciated by those skilled in the art, that a material of high Lambertian reflectance is preferred—i.e., incoming light is partially absorbed and partially transmitted equally in all directions. For this reason, a white material may provide the greatest brightness to the viewer. However, it has surprisingly been found that even black plastic (which typically is the least expensive plastic resin) may be used to fabricate the light barrier if those portions of the mold producing the interior surfaces of the “pockets” are polished to a #3 finish or better. 
     A Lambertian surface is any surface whose radiance is independent of direction. Such a surface obeys Lambert&#39;s cosine law that states that the reflected or transmitted luminous intensity in any direction from an element of a perfectly diffusing surface varies as the cosine of the angle between that direction and the normal vector of the surface. As a consequence, the luminance of that surface is the same regardless of the viewing angle. 
     When a photon hits a rough surface, it rebounds in a direction not much related to its incoming direction. We talk here about the case where photons reflect in a statistically independent direction. This is called Lambertian reflection and applies to ray tracing with diffuse surfaces, or MonteCarlo physics calculations. A lambertian surface is a surface of perfectly matte properties, which means that it adheres to Lambert&#39;s cosine law. Ideal diffuse reflectors are said to be Lambertian reflectors. 
     At the other extreme is mirror or specular reflection exhibited by shiny metal surfaces such as chrome, silver or pure aluminum. Specular reflectance obeys the law of reflection, where the angle of reflection equals the angle of incidence. It is most important to realize that although specular reflections produce a clear image in the surface of the material, the actual amount of light reflected may be deceptively low. A matt white painted surface, for instance, has a reflectance of 85% to 90% compared with only 60% specular reflectance from a polished stainless steel surface, while polished aluminum will be approximately 85%. 
       FIG. 5  is a cross-sectional view taken along line A—A in  FIG. 1 . In the particular embodiment illustrated, the angle θ between angled top segment  26  and flat top segment  24  is about 17.5°. The plane of flat segment  24  may be parallel to the plane of the printed circuit board to which the device is mounted. 
     In alternative embodiments, the top  12  of light barrier  10  may comprise any number of segments. It will be appreciated by those skilled in the art that as the number of segments increases, the profile of top  12  may approach that of a segment of a parabola. 
     In an alternative embodiment, as shown in  FIG. 8 , top  12  may be a portion of a parabola with the LED being positioned at approximately the focus of the parabola. A parabolic reflector or parabolic dish is a reflective device formed in the shape of a paraboloid of revolution. Parabolic reflectors can either collect or distribute energy such as light. The parabolic reflector functions due to the geometric properties of the paraboloid shape: if the angle of incidence to the inner surface of the collector equals the angle of reflection, then any incoming ray that is parallel to the axis of the dish will be reflected to a central point, or “focus”. Similarly, energy radiating from the “focus” to the dish can be transmitted outward in a beam that is parallel to the axis of the dish. Accordingly, light from the LED striking the reflective parabolic surface  12  will be redirected out of the front of light reflector  10 . 
     A Lambertian source is an optical source that obeys Lambert&#39;s cosine law, i.e., that has an intensity directly proportional to the cosine of the angle from which it is viewed. Conventional (surface-emitting) LEDs are approximately Lambertian. They have a large beam divergence. This results in a radiation pattern that resembles a sphere (or a hemi-sphere in the case of an SMT packaged LED). Thus, most of their total optical output is not coupled into optical fibers or light pipes. 
       FIG. 6  is an exploded view of a device—e.g., a Fibre Channel switch—which employs the light barrier of the present invention to illuminate status indicators on its front panel. The illustrated device comprises chassis  46  having front portion  50 . Printed circuit board  42  is housed within chassis  46 . PCB  42  comprises a plurality of surface mount LEDs  44  in a generally linear array near the front edge of PCB  42 . Light barrier  10  is positioned above the linear array of LEDs. 
     LEDs are available in packages designed for surface mount technology (SMT). This provides much more design freedom in allowing SMT LEDs to be economically placed directly onto circuit boards, with the viewed panel or indicator placed in a different location. There are a variety of different SMT styles, with a choice of J-wing, gull-wing, yoke-bend, and Z-bend leads. These surface mount LEDs are designed with flat top and sides for the ease of pick-and-place by automatic placement equipment. They are compatible with consecutive IR and vapor phase reflow soldering. Ceramic high-reliability LEDs are able to withstand the heat of wave soldering. Commercially available devices combine a red, blue, and green LED chip all into a single SMT package, making full-color displays possible. 
     As illustrated in  FIG. 7 , light from surface mount LED  44  is emitted in a generally upward direction from PCB  42  upon which LED  44  is mounted. The emitted light reflects off the undersurface of light barrier  10  and exits the light barrier  10  as light rays  60 . 
     Front panel  50  of chassis  46  may have receptacles  58  for receiving, for example, a network connector (not shown). Associated with each receptacle are one or more LEDs  44  for indicating the status of the connection. Front panel  50  has holes  48  which align with the individual compartments  15  in light barrier  10 . Similarly, bezel  52  has lenses  54  which align with holes  48  and which, in some embodiments, may be optically dispersive to the light from LEDs  44  so as to provide a readily-observed, lighted indicator. Openings  56  in bezel  52  permit the insertion of connectors into receptacles  58 . 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.