Abstract:
A side-emitting collimator employs a combination of refraction and internal reflection to organize light from a light source into oppositely directed collimated beams. A light source chamber over the light source is defined by substantially cylindrical and aspheric refracting surfaces positioned to gather light into the collimating lens. The aspheric refracting surfaces redirect a portion of the light from the light source into a direction perpendicular to the optical axis of the light source. The substantially cylindrical surfaces refract light from the light source onto an aspheric upper reflecting surface. Light incident upon the aspheric upper reflecting surface is collimated into a direction perpendicular to the optical axis of the light source. The side-emitting collimator includes mirror image collimator halves, each producing a collimated beam. The collimator halves are rotationally symmetric about a common axis of symmetry above a plane including the axis of symmetry.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to lenses for distributing light from a light source and more particularly to a lens for changing the effective direction of light emission for a light source. 
   2. Description of the Related Art 
   The use of LED&#39;s in warning and signaling lights is well known. Older models of LED&#39;s produced limited quantities of light over a relatively narrow viewing angle centered around an optical axis of the LED. These LED&#39;s were typically massed in compact arrays to fill the given illuminated area and provide the necessary light output. Modern LED&#39;s produce significantly greater luminous flux per component, permitting fewer LED&#39;s to produce the luminous flux required for many warning and signaling applications. It is known to arrange a small number of high-output LED&#39;s in a light fixture and provide each high-output LED with an internally reflecting collimating lens. The collimating lens gathers light from the LED into a collimated beam centered on the LED optical axis. Such an arrangement typically does not fill the light fixture, resulting in an undesirable appearance consisting of bright spots arranged against an unlit background. Light-spreading optical features on the outside lens/cover are sometimes employed to improve the appearance of the light fixture. 
     FIG. 1  illustrates a prior art collimator of a configuration frequently employed in conjunction with LED light sources. Light from an LED positioned in a cavity defined by the collimator is organized into a collimated beam aligned with the optical axis of the LED. The known internally reflecting collimator for an LED is a molded solid of light transmissive plastic such as acrylic. The radial periphery of the collimator is defined by an aspheric internal reflecting surface flaring upwardly and outwardly to a substantially planar light emission surface. The bottom of the collimator includes a cavity centered over the LED optical axis. The cavity is defined by a substantially cylindrical side-wall and an aspheric upper surface. The aspheric upper surface is configured to refract light emitted at small angles relative to the LED optical axis to a direction parallel with the LED optical axis. The shape of the aspheric upper surface is calculated from the refractive properties of the air/acrylic interface, the position of the LED point of light emission relative to the surface and the desired direction of light emission, e.g., parallel to the LED optical axis. The mathematical relationship between the angle of incidence of a light ray to a surface and the angle of the refracted ray to the surface is governed by Snell&#39;s Law: “The refracted ray lies in the plane of incidence, and the sine of the angle of refraction bears a constant ratio to the sine of the angle of incidence.” (sin θ/sin θ′=contant, where θ is the angle of incidence and θ′ is the angle of refraction) 
   To allow the collimator of  FIG. 1  to be easily extracted from a mold, the substantially cylindrical side-wall of the cavity is typically canted at an angle of between 1° and 3° relative to the central axis of the collimator. The cavity is narrower at the top where the side-wall joins the aspheric upper surface than at the bottom of the collimator. For any particular point on the substantially cylindrical side-wall, the path of light refracted into the collimator can be calculated using Snell&#39;s law. The shape of the peripheral aspheric internal reflecting surface is calculated from the path of light refracted by the substantially cylindrical side-wall surface and the desired direction of light emission, e.g., parallel to the LED optical axis. The resulting aspheric internal reflecting surface redirects light incident upon it in a direction parallel to the optical axis of the LED. 
   The result is that substantially all of the light emitted from the LED is redirected parallel to the optical axis of the LED to form a collimated beam. This arrangement efficiently gathers light from the LED and redirects that light into a direction of intended light emission. Unless the light is somehow spread, the light from each LED appears to the viewer as a bright spot the size and shape of the collimator. 
   SUMMARY OF THE INVENTION 
   Briefly stated, a side-emitting collimator according to the present invention comprises a pair of collimator halves configured to meet over an LED to form a cavity defined by refracting surfaces. The upper boundary of each collimator half is defined by a surface configured to internally reflect light refracted into the collimator lens by one of the refracting surfaces. Each half of the side-emitting collimator redirects light from the light source in a direction substantially perpendicular to the optical axis of the light source. The side-emitting collimator redirects the light from an LED into diametrically opposed collimated beams perpendicular to the optical axis of the light source. 
   Each half of the side-emitting collimator includes an aspheric upper internal reflecting surface whose shape is calculated from the path of light refracted by the corresponding internal refracting surface. As used in this application, the term “aspheric” means “not spherical”. The illustrated aspheric surfaces are created by rotating a non-circular curve about an axis of symmetry. These surfaces can be described as “rotationally symmetric” about the axis of symmetry. Each half of the side-emitting collimator defines one half of a cavity that receives the light source. The upper surface of the cavity includes two adjoining substantially cylindrical surfaces that meet over the optical axis of the LED light source. Each end of the cavity is defined by one half of an aspheric surface. The partial-cylindrical surfaces are positioned to refract light from the light source onto the upper internal reflecting surface. The partial aspheric refracting surface redirects light from the light source into a direction of light emission parallel to light reflecting from the partial aspheric upper internal reflecting surface. 
   These surface shapes and relationships are an example of surfaces that efficiently gather light from an LED and re-direct that light into a pair of opposed collimated beams perpendicular to the optical axis of the LED. The invention should not be construed as being limited to the particular disclosed surface shapes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side sectional view of a prior art collimator; 
       FIG. 2  is a sectional view through a side-emitting collimator in functional conjunction with an LED according to aspects of the present invention; 
       FIG. 3  is a sectional view of the side-emitting collimator and LED of  FIG. 2  in conjunction with a reflector according to aspects of the present invention; 
       FIG. 4  is a sectional view through a side-emitting collimator incorporated into an internally reflecting lens in functional conjunction with an LED according to aspects of the present invention; 
       FIG. 5  is a side plan view of the side-emitting collimator and LED of  FIG. 2 ; 
       FIG. 6  is a perspective view of a row of side-emitting collimator halves according to aspects of the present invention; 
       FIG. 7  is an exploded perspective view of a light assembly incorporating four rows of side-emitting collimator halves as shown in  FIG. 6 ; 
       FIG. 8  is an exploded perspective view of a light assembly incorporating two rows of side-emitting collimator halves as shown in  FIG. 6 ; 
       FIG. 9  is a perspective partially assembled view of the light assembly of  FIG. 7 ; and 
       FIG. 10  is a side view assembled view of the light assembly of  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Exemplary embodiments of the side-emitting collimator will now be described with reference to  FIGS. 2–6 .  FIG. 2  shows an exemplary side emitting collimator  10  in conjunction with an LED light source  20 . The side-emitting collimator  10  organizes light from the LED  20  into a pair of opposed collimated beams  11  perpendicular to the optical axis A O  of the LED  20 . The LED  20  includes a base  26  above a heat-transmissive slug  24 . A lens  28  extends upwardly from the base  26 . The LED lens  28  surrounds a point of light emission  22 . The point of light emission  22  in an LED  20  is defined by a semi-conductor chip (not shown) which emits light when energized by an electrical current. The illustrated LED lens  28  is of the high-dome or “lambertian” shape. An LED with this lens shape emits light in a half dome surrounding the optical axis A O  of the LED. The side-emitting collimator  10  is constructed about a focal point F that preferably coincides with the point of light emission  22  of the LED  20 . The left and right portions  10   a ,  10   b  of the side-emitting collimator  10  are identical. Each half  10   a ,  10   b  of the side-emitting collimator is a solid molded from optical-grade plastic, such as acrylic. Other forming methods and materials may also be compatible with the present invention. 
     FIGS. 3 and 4  are sectional views of light assemblies employing side-emitting collimators  10  according to aspects of the present invention. In the assembly of  FIG. 3 , angled reflecting surfaces  72  redirect the collimated beams after they emerge from the substantially planar light emission surfaces  18  of the side-emitting collimator  10 . The angled reflecting surfaces  72  are laterally spaced apart and configured to each intercept a portion of the collimated beam from each side-emitting collimator  10 . This arrangement distributes light from each LED  20  over a large area in a direction of light emission B for the light assembly. The reflecting surfaces  72  are also shown in  FIGS. 8 and 9 . 
     FIG. 4  is an alternative embodiment where the side-emitting collimator  10  is incorporated into a larger internally reflecting lens. The reflecting surfaces  72  shown in  FIG. 4  are internal reflecting surfaces. 
   In the illustrated embodiments, the reflecting surfaces  72  are substantially planar, extend the length of the light assembly and are laterally equidistantly spaced. The reflecting surfaces  72  are oriented at an angle of approximately 45° relative to the optical axes A O  of the LEDs  20 . The shape, height, length, lateral spacing, and angular orientation of the reflecting surfaces may be selected to produce a particular desired pattern of light distribution. For example, reflecting surfaces that are convex when viewed in section (not shown) would increase the spread of the light pattern for the assembly. 
     FIG. 5  illustrates the geometrical relationships between an LED  20  and the refracting and reflecting surfaces of one half  10   a  of the side-emitting collimator  10 . Each half  10   a ,  10   b  of the side-emitting collimator  10  includes an upper surface  16  produced by rotating a non-circular curve  90  about an axis of symmetry A C  that extends beneath both halves  10   a ,  10   b  of the collimator  10 . The shape of the curve  90  rotated about axis A C  is calculated to reflect light refracted by the corresponding refracting surface  12  into the desired path of the collimated beam. The path of light gathered by the refracting surface  12  in the collimator can be calculated from the known values of: the shape of the surface  12 ; the angle of light incident upon the surface  12 ; and the refractive properties of the collimator lens material (acrylic plastic)/air interface using Snell&#39;s Law. 
   These calculations produce a non-spherical, or aspheric reflecting surface  16 . As used in this application, the term “aspheric” means “not spherical”. The illustrated aspheric surfaces  14 ,  16  are created by rotating a non-circular curve  90 ,  92  about an axis of symmetry A C . The resulting aspheric surfaces can be described as “rotationally symmetric” about the axis of symmetry A C . The non-circular curve  90  extends upwardly and outwardly from a first end  15  to a second end  17 . When rotated at least approximately 180° about the axis of symmetry A C , the curve  90  defines the upper aspheric reflecting surface  16  of each collimator half  10   a ,  10   b.    
   The aspheric refracting surface  14  of each collimator half  10   a ,  10   b  is configured to refract a portion of the light from the LED  20  into a direction perpendicular to the optical axis A O  of the LED. The shape of the non-circular curve  92  used to define the aspheric refracting surface  14  is calculated from the known values of: the desired angle of refraction in the collimator; the angle of light incident upon the surface  14 ; and the refractive properties of the collimator lens material (acrylic plastic)/air interface using Snell&#39;s Law. The non-circular curve  92  extends between an origin  94  on the axis of symmetry A C  to a terminus  96  where the curve  92  intersects with the substantially cylindrical refracting surface  12 . When rotated at least approximately 180° about the axis of symmetry A C , the curve defines the aspheric refracting surface  14 . 
   The illustrated collimator halves  10   a ,  10   b  are semi-circular in lateral section (perpendicular to the view of  FIG. 5 ). A plane P, includes the axis of symmetry A C  and the focus F of the collimator  10 . Each collimator half  10   a ,  10   b , is rotationally symmetrical about the axis of symmetry A C  above plane P. Each half of the side-emitting collimator defines one half  30   a ,  30   b  of a cavity  30  that receives the light source (LED  20 ). The upper surface of the cavity includes two adjoining substantially cylindrical refracting surfaces  12  that meet over the optical axis A O  of the LED light source. Each end of the cavity  30  is defined by the aspheric refracting surface  14 . The substantially cylindrical surfaces  12  are positioned to refract light from the light source onto the upper internal reflecting surface. The aspheric refracting surface  14  redirects a portion of the light from the light source into a direction parallel to light reflecting from the aspheric upper reflecting surface. 
   Each half  10   a ,  10   b  of the side-emitting collimator  10  defines one-half  30   a ,  30   b  of a cavity  30  surrounding the LED lens  28 . The cavity portion  30   a ,  30   b  defined by each half  10   a ,  10   b  of the side-emitting collimator  10  is defined by two refracting surfaces  12 ,  14 . The substantially cylindrical surface  12  is centered on focal point F and extends perpendicularly to the optical axis A O  of the LED  20 . Surface  12  is described as a “substantially cylindrical” surface because it is not perfectly cylindrical with respect to the axis of symmetry A C . In the illustrated embodiments, the substantially cylindrical surface  12  has a larger diameter at its open end (to the right in  FIG. 5 ) than at the junction with the aspheric refracting surface  14 . This particular surface configuration is not necessary to the optical performance of the collimator  10 . Alternative surface configurations for the refracting surfaces may occur to one skilled in the art. 
   There are many ways to form the lens shapes employed in the side-emitting collimator  10 . One preferred method is to mold the lenses from clear plastic.  FIG. 6  illustrates an exemplary configuration where a row  40  of side-emitting collimator halves  10   a  are molded from optically clear acrylic plastic having an index of refraction of approximately 1.491 at a wavelength of 550 nm. A mirror image row  40  of side-emitting collimator halves arranged as shown in  FIG. 9  produces eight side-emitting collimators  10  for organizing the light from a row of eight LEDs  20 . Alternatively, the side-emitting collimators may be molded as a single unit (not shown). 
     FIGS. 7–10  illustrate exemplary light assemblies incorporating rows of side-emitting collimators  10  over corresponding rows of LEDs  20  according to aspects of the present invention. A molded plastic reflector  70  provides structural support for the assemblies. PC boards  50  include a linear array of eight LED&#39;s  20 . Two rows  40  of side-emitting collimator halves  10   a ,  10   b  are arranged with their respective cavity ends aligned over the row of LEDs to form side-emitting collimators  10  over each LED. The PC boards and rows of collimator-halves are clamped against the back of the reflector  70  by heat sinks  60  corresponding to the configuration of the PC boards  50 . The illustrated heat sinks  60  are molded from heat-transmissive plastic to disperse heat generated by the LED&#39;s  20 . A lens  80  protects the assembly and also may be provided with refracting features for spreading light coming off the parallel reflecting surfaces  72 . 
     FIG. 8  illustrates a light assembly employing a single PC board  50 , heat sink  60  and row of side-emitting collimators  10  constructed from two rows  40  of collimator halves  10   a ,  10   b . The heat sink  60 , reflector  70  and lens  80  of  FIG. 8  function in the same manner as the corresponding components of  FIG. 7 .  FIG. 3  illustrates the relative positions of the LED  20 , side-emitting collimator  10  and reflecting surfaces  72  of the light assembly shown in  FIG. 8 . These relationships organize light from the row of LED&#39;s  20  so that it is emitted substantially evenly over the area of the light assembly. The inventive side-emitting collimator  10  and reflector efficiently distributes the light from the LEDs to improve the aesthetic appearance of the light radiation pattern without impairing compliance with relevant standards for warning and signal light pattern and intensity. 
     FIG. 9  is a top perspective view of the reflector of  FIG. 7  in functional conjunction with the rows  40  of collimator halves  10   a ,  10   b . The opposed collimated beams from each side-emitting collimator  10  are incident upon five parallel reflecting surfaces  72  extending along either side of each row of LEDs. This configuration effectively spreads the light over the surface area of the reflector  70 .  FIG. 10  is a side view of the assembled components shown in  FIG. 7 . An advantage of the illustrated light assembly configuration is an extremely low profile as shown in  FIG. 10 . This low profile permits a light assembly according to the present invention to be mounted on the exterior of a vehicle without requiring a large cutout to accommodate a deep reflector. The rear surface of the heat sinks  60  will have a large surface contact with what will typically be the metal skin of the emergency vehicle. This large surface contact will enhance heat transfer away from the LEDs of the light assembly. 
   While exemplary embodiments of the foregoing invention have been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations and alternatives may occur to one of skill in the art without departing from the spirit and the scope of the present invention.