Abstract:
A signal is described that includes one or more LEDs that emit light and a lens that receives and collimates the light from the LED array. A distribution optic receives light from the collimating lens and distributes the light in a predetermined pattern according to a specification. A light absorbing/reflecting element is located in an area proximate the one or more LEDs to minimize light received from an external source from exiting the signal.

Description:
BACKGROUND 
     The present invention relates to LED-based lighting systems and, in particular, traffic signals. The exemplary embodiments find particular application in conjunction with minimizing reflection of light received from an outside source, such as the sun. One approach is to utilize one or more retroreflectors to reflect the externally originating light back toward the source. Another approach is to use a lens to direct externally originating light into an aperture within the signal housing. 
     Automotive, railway, vehicular, waterway, illumination, and/or pedestrian signals are employed to regulate motorists and pedestrians via various commands. These commands are provided by an illumination source with particular colors and/or shapes that are each associated with an instruction. For example, light emitting diodes can illuminate an appropriate signal that indicates a command to motorists and/or pedestrians. 
     In order to provide a signal that is clearly visible, signals can locate the light elements on a reflective substrate and further use reflectors to direct light emitted from the illumination source. A common problem with traffic signals occurs when external light (e.g., from the sun) enters the front of the signal, is reflected off internal specular surfaces and exits the signal at an angle that reaches a driver&#39;s and/or pedestrian&#39;s eyes. 
     The problem can be exacerbated by one or more optical element utilized to direct the light from the illumination source out of the signal. In general, light generated by the illumination source is directed out of the signal via optical elements, such as a lens, a collimator, a diffuser and the like. However, the same components can direct externally originating light into the signal following substantially the same path. In this manner, light that is received from an external source is directed toward the illumination element typically located at the back of the signal. The externally originating light can then be further reflected by the reflective substrate and out the signal on the same path as light generated by the illumination source light. In this manner, it can appear that the signal is on, even when the illumination source is unit. 
     Accordingly, it would be advantageous to have systems and methods which minimize reflection of light received by a signal from an outside source. 
     BRIEF DESCRIPTION 
     In one aspect, a light emitting device includes one or more LEDs that emit light and a lens that receives and collimates the light from the one or more LEDs. A distribution optic receives light from the collimating lens and distributes the light in a predetermined pattern. A light absorbing/reflecting element is located in an area proximate the one or more LEDs to minimize the amount of light received from an external source which exits the signal. 
     In another aspect, an LED traffic signal includes a rear housing wall and an LED array mounted to the rear housing wall. A lens receives and collimates the light from the LED array and a distribution optic receives light from the lens and distributes the light in a predetermined pattern. A converging element receives external light from the lens and directs it to a predetermined location on the rear housing. 
     In yet another aspect, an LED traffic signal comprises a housing that includes a rear housing wall. An array of LEDs is mounted to the rear housing wall and a lens receives and collimates the light from the LED array. A distribution optic receives light from the lens and distributes the light in a pattern according to a specification. A plurality of retroreflectors are mounted to the rear housing wall below the LED array to receive external light and minimize sun phantom effect associated with an external light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exploded isometric view of a prior art LED traffic signal. 
         FIG. 2  illustrates an exemplary lens utilized with the LED traffic signal, in accordance with an aspect of the subject invention. 
         FIG. 3  illustrates a side view of an LED traffic signal with an array of light absorbing/reflecting elements, in accordance with an aspect of the subject invention. 
         FIG. 4  illustrates a retroreflector, in accordance with an aspect of the subject invention. 
         FIG. 5  illustrates LEDs with an array of retroreflector elements, in accordance with an aspect of the subject invention. 
         FIG. 6  illustrates a side view of an LED traffic signal with rays to show the path of light travel within the signal, in accordance with an aspect of the subject invention. 
         FIG. 7  illustrates an exploded isometric view of an LED traffic signal with a light absorption element, in accordance with an aspect of the subject invention. 
         FIG. 8  illustrates an exploded isometric view of an LED traffic signal with a light absorption element, in accordance with an aspect of the subject invention. 
     
    
    
     DETAILED DESCRIPTION 
     In describing the various embodiments of the lighting system, like elements of each embodiment are described through the use of the same or similar reference numbers. 
       FIG. 1  illustrates an exploded view of a traditional LED traffic signal  100 . It is noted that while the description herein is of a traditional signal, many features thereof are equally relevant to the present invention. A housing including a rear wall  104  supports an LED array. Not shown is an intervening housing body that joins rear wall  104  and a lens  110 . In this manner, the signal can be mechanically coupled together utilizing tabs, snaps, or other joining elements. 
     An array of LEDs  106  is mounted to a PCB  118  that is coupled to a power supply (not shown) that delivers power to the LED array  106 . The PCB  118  is mounted to the wall  104 . In this example, the LEDs are configured in a linear array; however it is to be appreciated that substantially any configuration (e.g., circle, square, parallelogram, etc.) can be employed. Alternatively, the LED array  106  could be mounted directly to the wall  104  without an intervening PCB  118 . Traditionally, the surface upon which LED array  106  is mounted (PCB or rear housing wall) will provide at least limited light reflection properties. 
     The rear wall  104  can be made of a thermally conductive material to act as a heat sink for the LED array  106  mounted thereon. Alternatively or in addition the rear wall  104  can include a separate element (not shown) to draw heat away from the LED array  106 . 
     The LED array  106  is energized via a control system (not shown) to produce light to direct pedestrian and/or vehicular traffic. The LED array can include substantially any type of LED devices including, for example, batwing, side-emitter, and/or Lambertian. When active, the LED array  106  transmits light through a lens  110  and a distribution optic  108  and out the front of the LED signal  100 . Light emitted from the LED array  106  is received by the lens  110  directly from the LEDs and reflected from the intervening body housing and other surfaces and therefore at a plurality of angles. Lens  110  collimates the light so that it is emitted along substantially the same axis which is typically normal to the surface of the lens  110  from which it exits. Lens  110  can be a Fresnel lens. 
     Distribution optic  108  and lens  110  are oriented with respect to the LED array  106  to emit light from the signal  100  in a particular pattern. Such orientation can cause the signal  100  to capture and direct various amounts of light emitted from the LED array  106  into one or more areas across the face of the signal  100 . Disparate light distribution patterns can be defined by a variety of specifications for traffic signal light emission in conformance with one or more government standards such as those promulgated by the American Association of State Highway and Transportation Officials (AASHTO), the Institute of Transportation Engineers (ITE), the National Electrical Manufacturers Association (NEMA), the European Telecommunications Standards Institute (ETSI), the European Committee for Electrotechnical Standardization (CENELEC), and the European Committee for Standardization (CEN). 
     In one embodiment, the lens  110  includes a plurality of collimating zones to provide an output that is substantially uniform across the surface of the distribution optic  108 . Distribution optic  108  can have a pattern inner or outer surface to selectively distribute light from the lens  110 . Similarly, the path can be created by a masking element separate from the distribution optic. Alternatively, or in addition, the distribution optic  108  can be located between the lens  110  and the wall  104  to first pattern the light. In yet another embodiment, the collimation and/or distribution and/or patterning of light can be accomplished via a single optical element. 
     With regard to patterning, the outer surface of the distribution optic  108  can direct light out of the signal in a particular direction (e.g., upward, downward, etc.). In one example, light is preferentially transmitted laterally and downward via the distribution optic  108  for European applications. In another example, light is transmitted laterally, upward and downward for U.S. designs as illustrated in  FIG. 6 . However, the present invention is not limited to any particular light distribution pattern. 
     While lens  110  is shown having a circular configuration, any shape including square, rectangular (horizontally or vertically elongated), and elliptical are feasible. For example, a railroad application may use a rectangular vertical elongated lens as the required horizontal viewing aspect is very narrow (e.g., generally the width of the train track). A tall vertical aspect allows viewing of the signal from a wide vertical range to correspond to viewing locations near and far from the signal at either track or train cab level. Similarly, an automobile traffic signal may be designed with a rectangular horizontally elongated lens to have a wide spread horizontally, across several lanes of traffic. Ray tracing (e.g., as illustrated in  FIG. 6 ) is employed to calculate specific optical solutions for both the distribution optic  108  and the lens  110 . Suitable software for performing ray tracing, such as Optics Lab, OpTaliX, Zemax, etc., is well known in the art. The lens can be made of an acrylic, vinyl, polycarbonate and glass as examples. 
       FIG. 2  illustrates a detail view of lens  110  that includes a center portion  146  and an edge portion  148 . In this embodiment, lens  110  is a Fresnel lens that collimates light emitted from a source within a short distance. In order to receive and collimate light from disparate angles, lens  110  contains a plurality of concentric rings emanating from the center portion  146  to the edge portion  148 . Three of these concentric rings are illustrated as a ring  140 , a ring  142 , and a ring  144 . The surface angle of each section increases as its radial distance increases from the center  146 . Thus, the surface angle of the ring  144  is greater than the surface angle of the ring  142 . Similarly, the surface angle of the ring  142  is greater than the surface angle of the ring  140 . In this manner, light is collimated such that light from a source on one side exits the lens  110  in a parallel fashion on the other side. 
     Lens  110  includes a plurality of collimating zones that can be circular or linear. Each collimating zone collimates light emanating from its respective LED ring or linear row. The LED light patterns can slightly overlap within and between the rings and rows to prevent the appearance of shadows, lines and/or rings. Due to the overlap, individual LED  106  failure, or variation in LED  106  output between adjacent LEDs  106  will not be discernable by the viewer. 
     Referring again to  FIG. 1 , It is known that light received by the signal  100  from an external source (e.g., the sun) can create the appearance that the signal  100  is illuminated when in fact it is not. Light from an external source  102  can enter the LED signal  100  via the distribution optic  108  and is focused by the lens  110  onto the rear wall  104 . Light directed at the rear of the housing can reflect off substantially any surface contained thereon whether such surface is specular or diffuse. Such reflection can occur regardless of color of the surface upon which the light hits. 
     Once the light has reflected off the rear wall  104 , the lens  110  collimates the light and the diffuser  108  diffuses the light as it exits the LED signal  100  along an optical path  112 . It is to be appreciated that the optical path  112  and the optical path  102  are for illustrative purposes only and that a plurality of incoming and outgoing optical paths can exist. However, the illustration demonstrates that external light on optical path  102  can be reflected out of the signal on optical path  112  resulting in a potential phantom on light to an observer of the signal. 
       FIG. 3  illustrates the LED signal  100 , such as the type described in  FIG. 1 , but further including an array of light absorbing/reflecting elements  214  placed in an area  212  beneath the LED array  106 . The area  212  can be located anywhere within the signal  100  and is primarily dependent on the orientation and configuration of the distribution optic  108  and the lens  110 . Moreover, area  212  is preferably located where distribution optic  108  and lens  110  direct external light within the signal  100 . 
     In one embodiment, light received from an external source  102  is refracted/redirected by the diffuser  108  and the lens  110   102  in a downward direction. In this manner, external light  102  is directed to area  212  that is located just below the LED array  106 . By placing the light absorbing/reflecting elements  214  in one or more locations where the external light  102  is directed, external light reflected out of the signal  100  can be minimized. 
     It is to be appreciated that the light absorbing/reflecting elements  214  can have one of reflection and absorption properties. In the reflection function, each light absorbing/reflecting element  214  utilizes a retroreflector (e.g., corner cube) geometry to reflect received light along a path that is substantially parallel to the received light but in the opposite direction. This particular characteristic occurs since the three surfaces, upon which the received light is reflected, are configured normally to one another. In this fashion, the reflected light is directed back in the same direction as its source and is not directed to the eyes of one observing the signal. In the absorption function, the elements  214  can be made of a material that is a dark color (e.g., black) to absorb received light. The material can also have particular properties (e.g., structure, density, etc.) to promote light absorption. For example, a black felt material could be particularly effective. 
     The number, configuration, and location of the light absorbing/reflecting elements  214  can be selected based on a number of factors such as the path of the external light  102 , the number, configuration, and placement of the LED array  106 , the diameter of the signal  100 , the orientation of the lens  110  and the distribution optic  108 , etc. Such optical properties are known to the skilled artisan and based on the teachings herein will allow a suitable number and location of absorbing/reflecting elements to be included in the housing. 
     The light absorbing/reflecting elements  214  reduce a sun phantom effect of a signal. Sun phantom is generally defined as the amount of external light reflected out of a signal. Sun phantom class is measured as a ratio of light output when a signal is on divided by light output when sunlight is striking the lens at 10 degrees to normal. An advantage of the present invention is that the reduction of sun phantom enhances design options such as reducing cost by utilizing fewer LEDs to meet the same sun phantom class. Alternatively, the same number of LEDs can be employed and an improved sun phantom rating achieved. A third advantage is that with a lower sun phantom, less power is required to illuminate the signal  100  to provide a desired light output. 
     In a preferred embodiment, the light absorbing/reflecting elements  214  are retro-reflected made from a specular material. The elements  214  have a shape of cube corners that are trimmed, for example, to one of 3, 4, or 6 sided polygons. The elements  214  are arranged in an array such that each of the elements  214  is in contact with one or more disparate elements  214  to eliminate gaps therebetween. Hexagonal, square, triangular shapes may be employed to optimize packing efficiency. In this embodiment, the orientation of each element  214  is identical to one another. However, such orientation is not critical since it is only a goal to redirect light along the same axis in which it is received. The nature of corner reflectors, such as the light absorbing/reflecting elements  214  will accomplish such reflection regardless of the axis of received light. 
     Each element  214  is typically from 0.0625″ to 0.25″ in size. Preferably, the width of the array of elements  214  is slightly larger than the width of LED array  106 . However, for functional purposes, there is no size restriction as long as the elements  214  can fit within the signal  100  and do not block light emitted from the LED array  106 . 
     The elements  214  can be made of injection molded material in conformance with standard manufacturing methods. Injection molding is a common and cost effective way to manufacture cube corner retroreflectors such as the elements  214 . However, any material that is opaque and/or specular can be employed (e.g., metal, glass, granite, etc.). 
     In the light reflecting embodiment, the light absorbing/reflecting elements  214  preferably direct the external light  102  along a path  112  that is the same or parallel to the external light  102  and out of the signal  100 . In this manner, incoming light  102  is reflected directly back to the source (e.g., sun) and thus is not returned (or viewed) to one or more pedestrians or motorists. Since the reflected light cannot be viewed, it will not appear that the signal is illuminated when in fact it is not. 
     As shown in  FIG. 4 , the light absorbing/reflecting elements  214  are shown as a corner cube retroreflector  400 . The retroreflector reflects a wave front back along a vector that is parallel to, but opposite in direction from the angle of incidence. The retroreflector  400  includes a first surface  402 , a second surface  404 , and a third surface  406  which are mutually perpendicular to each other in three disparate axes. In this embodiment, each of the perimeter of the surfaces  402 ,  404 , and  406  are relatively square to one another and flat. It is to be appreciated, however, that the perimeter of surfaces  402 ,  404 , and  406  can be substantially any shape (e.g., elliptical, oval, parallelogram, etc.). The ray path of the external light intersecting one of the surfaces  402 ,  404 , or  406  is irrelevant since they are mutually perpendicular to one another. 
     To illustrate the principle, light is received by the retroreflector  400  along path  410  by the first surface  402 . The light is reflected off the first surface  402  to the second surface  404  along path  412  that is substantially normal to the path  410 . The light is reflected from the second surface  404  to the third surface  406  via a path  414  that is substantially normal to the path  412 . The light is reflected by the third surface  406  in a path  416  that is substantially parallel to the path  410  in the opposite direction. 
     The light absorbing/reflecting elements  214  can be oriented in a position that corresponds with the orientation and configuration of the distribution optic  108  and the lens  110  and/or the incoming path of external light. Such orientation is not critical as long as light is received on any one of the surfaces  402 ,  404 , and  406  since light received is returned along the same axis in the opposite direction. In one example, the signal  100  is mounted to a fixed structure, such as a post, wherein light redirection is desired above the center line of the signal  100 . Thus, the light absorbing/reflecting elements  214  would be angled slightly above horizontal in anticipation of the external light location. 
       FIG. 5  illustrates an array  450  of light absorbing/reflecting elements  214 . In this embodiment, each of the elements  214  is a retroreflector  400 , one of which is designated within the array  450 . The LED array  106  is coupled to the PCB  118 . The retroreflectors  400  are placed side-by-side to insure that light received substantially anywhere within the array  450  is reflected back along the same axis in the opposite direction. 
     In an alternative embodiment,  FIG. 7  illustrates a signal  500  that includes a plurality of elements  214 ′ designed to redirect incoming external light into a particular location within the signal  500 . In one example, the location is a hole  510  in the back wall of the housing that is employed to trap the external light so that it escape back out of the signal  500 . In one embodiment, the hole  510  can be surrounded by a light absorbing material (not shown) to further decrease the amount of external light reflected. 
     In the signal  500 , the elements  214 ′ are mirrors (or equivalent) that are capable of directing received light via reflectance. The elements  214 ′ can be positioned and/or oriented in substantially any location within the signal  500 . In one example, the elements  214 ′ are positioned along a circumference of a circle defined by a radius  516  to circumscribe the hole  510 . The radius  516  can be determined based on optical properties of the signal  500 . This includes the size, orientation, location and type of distribution optic  108  and the lens  110 . When light is received at or within the circumference defined by radius  516 , it is reflected by one or more of the elements  214 ′ toward the hole  510 . The distribution optic  108  and the lens  110  can direct external light into a particular area, as described above that correlates to the radius  516  regardless of the angle/direction of external light into the signal  500 . 
     In yet another embodiment,  FIG. 8  shows a signal  600  that includes a converging element  506  that is positioned between the lens  110  and the rear housing wall  104 . The converging element  506  is employed to direct light incident upon it to a particular location via convergence. In one example, the converging element  506  is a positive lens such as a biconvex, a plano-convex, or a positive meniscus type. 
     In one embodiment, the converging element  506  is employed with the light absorbing/reflecting elements  214  and/or the elements  214 ′. The size, location, and orientation of the converging element can be based at least in part upon one or more of the size of signal  100 , the lens  110  type, size, orientation and placement, the distribution optic  108  type, size, orientation and placement, and the distance from the lens  110  to the LED array  106 , as described above. 
     The invention has been described with reference to the exemplary embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.