Abstract:
A light funnel collimator has a central lens surface and a back reflecting surface, shaped to provide a wider back-ground beam and a narrower hotspot beam within but off-center of the wider beam. One of the beams is on-axis of the collimator, and the other beam is off-axis. The reflector is at least partly asymmetrical relative to the axis, and provides or contributes to the off-axis beam.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit of U.S. Provisional Patent Application No. 61/298,030, filed Jan. 25, 2010 by Dross et al. for “Off-axis collimation optics.” 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Light emitting diodes (LEDs) are widely available, inexpensive, and efficient light sources. For uses such as sport headlamps, one or two state of the art LEDs provide adequate light. While simple light distributions of rotational symmetry are sufficient for low quality products or less demanding uses, more complex light distributions are being employed for better vision when walking, running, or cycling with a headlamp. It is beneficial to produce a relatively narrow hotspot of typically 10° full width at half maximum (FHWM) so to illuminate objects far away from the user, while a lower level intensity background light is needed to provide lighting of the ground close to the user. Such a background light is not needed upwards from the hotspot so that a background beam that is tilted relative to the hotspot beam is beneficial. 
         [0003]    A collimator configuration as seen in  FIG. 1  is used in a current headlamp product made by Silva Sweden AB that produces a wide beam off-axis background light around a narrow intense on-axis hotspot. The lens  100  is of a type herein called a “photon funnel” that has a central “collimator cavity” containing the light source. The wall of the cavity has a front or center lens  103  and a side or peripheral cavity surface. In a center section of the photon funnel, light passes by refraction through the center lens  103  and an exit surface (which in  FIG. 1  is part of a front surface  104 ), while the majority of the light passes through the cavity side surface by refraction, is reflected by total internal reflection (TIR) at a back surface  102 , and exits through the front surface  104  by refraction. 
         [0004]    In the Silva product, the center lens  103  of lens  100  is a rotationally symmetric surface that has its rotational axis tilted with respect to the light source axis to provide an off-axis background light while surface  102  collimates the majority of the light from the LED to form a narrow hot spot. This architecture works well, if the amount of light that is needed for the background illumination is roughly one third of the full light emitted by a Lambertian LED, as this is the typical amount of light collected by the center lens of a conventional photon funnel. If more or less light is wanted in the off-axis beam, this configuration cannot be used. Moreover the center lens provides a relatively wide beam by nature of the lens  103 , so that if a narrow off-axis beam is wanted, the center lens cannot provide such beam. 
         [0005]    In all of the described embodiments, the cavity side surface is a surface of rotation about a center axis, and the light source is an LED chip centered on and coaxial with the center axis of the cavity. A typical LED chip is flat, and is a Lambertian emitter with its emission symmetrical about an axis perpendicular to the flat chip. The LED chip thus typically has a well-defined central axis. In the present specification, the terms “on-axis” and “off-axis” are used here with respect to the common center axis of the collimator cavity and the LED chip. In all of the embodiments, one of the hotspot beam and the background beam is directed along the center axis, and the other beam is directed along a second axis, referred to as a “tilted axis,” diverging from the center axis. In all of the embodiments, the exit surface of the optics is flat, and the surface normal of the exit surface coincides with the cavity center axis. However, exit surfaces of other shapes and orientations can be implemented. 
         [0006]    The head lamp itself often provides means to adjust the direction of light emission of the entire lamp, so that the narrow beam can be adjusted for far vision while the wide beam will provide near vision. Thus, as will be shown below with reference to  FIG. 4 , the same functionality as in the Silva lamp can be achieved by the “dual” case in which a tilted center lens provides a hotspot beam along the tilted axis, and an on-axis reflector provides an on-axis background beam. However, the simple dual configuration will then typically direct two-thirds of the light into the background beam and one-third into the hotspot beam, which may not be optimal. 
         [0007]    Other applications besides sport headlamps of partially or fully off-axis LED collimators would be in architectural lighting to create certain lighting effects, such as illuminating a wall from a lighting fixture that is oriented parallel to the wall, in street lighting, and many other applications. 
       SUMMARY OF THE INVENTION 
       [0008]    The optical approach explained in detail below does not rely solely on the center lens of a photon funnel to provide off-axis illumination. Using the TIR reflective back surface of a photon funnel for off-axis illumination has several advantages, among them: that much more flux impinges upon this surface; and that by the nature of reflection, modifications of the back surface make much larger off-axis beam tilt angle possible than with a single refraction at the center lens. In all of the embodiments described below, the optical designs are modified rotational designs. The rotational designs are obtained with common methods, either with point source approximation numerical or analytic methods or with extended source optimization using common iterative numerical methods. The starting point design can be a narrow-beam on-axis collimator, or part of the surfaces can be calculated to provide a wider on-axis beam. In a subsequent step some optical surfaces are modified to deviate from the rotational symmetry. All other surfaces may be left unchanged, including the so-called cavity surfaces (the circumferential wall of the central cavity, through which light enters the photon funnel dielectric on a path towards the back reflective surface) and the front (exit-) surface of the dielectric. In the following detailed description and drawings, examples of photon funnels with a fully or partially modified TIR back surface are described and shown. The center lens may or may not also be modified, to provide additional on and off axis illumination, and all combinations of modified center lenses and modified mirrors are possible. The back surface may be modified so that a modified section of the reflector surface provides off-axis light while an unmodified section provides on-axis light. Both beam spreads, the angle of tilt between the on and off-axis portions and their intensity patterns and levels can be controlled. When modifying both the center lens and back surface completely, all light can be sent off-axis, either in a single beam or in two (or more) differently tilted beams. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The above and other aspects, features and advantages of the present invention will be apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
           [0010]      FIG. 1  shows a photon funnel of the prior art. 
           [0011]      FIG. 2  shows the first preferred embodiment of a photon funnel with a back surface to provide off-axis narrow beam illumination and on-axis wide background light provided by the center lens. 
           [0012]      FIG. 3A  shows an intensity distribution in the horizontal and vertical direction, of a photon funnel with an off-axis hotspot and an on-axis low intensity background intensity. 
           [0013]      FIG. 3B  shows the same radiation pattern as a  2  dimensional distribution. 
           [0014]      FIG. 4  shows a photon funnel that provides a narrow off-axis hotspot from the center lens and a wide on-axis background illumination from the back surface. 
           [0015]      FIG. 5  shows a photon funnel that provides a narrow on-axis hotspot from the center lens and a wide off-axis background illumination from the back surface. 
           [0016]      FIG. 6  shows a photon funnel that provides a narrow off-axis hotspot from a top section of the back surface. 
           [0017]      FIG. 7  shows a photon funnel that provides wide beam on-axis background illumination from a bottom section of the back surface. 
           [0018]      FIG. 8  shows a three dimensional view of a photon funnel as constructed according to  FIG. 6  and/or  FIG. 7 . 
           [0019]      FIG. 9  shows the 2D wavefront method to calculate meridian cross-sections for an improved embodiment for off-axis illumination from the back surface. 
           [0020]      FIG. 10  shows a three dimensional view of the back surface constructed according to  FIG. 9 . 
           [0021]      FIG. 11  shows the 3D wavefront method to calculate a freeform back surface. 
           [0022]      FIG. 11A  shows prefixed photon funnel surfaces from a standard rotational design. 
           [0023]      FIG. 11B  shows the 3D wavefronts from the source and target used to derive the freeform back surface. 
           [0024]      FIG. 11C  shows the exit surface and the calculated freeform back surface. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings, which set forth illustrative embodiments in which the principles of the invention are utilized. 
         [0026]    In the following drawings all architectures are described in more detail: 
         [0027]    Referring to  FIG. 2 , a lens  200  represents a photon funnel that collimates the light from a source  201  into an off-axis hot spot and also creates a low-intensity background illumination. The photon funnel  200  comprises a dielectric lens with a central cavity. Lines  212  in  FIG. 2  are cross sections of a conical surface of rotation with respect to the source axis  203 , forming the side wall of the cavity. Surface  202  forms a central lens, closing off the front end of the cavity, and is a rotationally symmetric surface around axis  203 . Lens surface  202  is shaped so that rays such as  204  and  205  that are refracted at lens surface  202  and at flat exit surface  211  produce a uniform background illumination. 
         [0028]    In general, references to a “surface of rotation,” “rotationally symmetric surface,” or similar indicate that the surface can be generated by rotation of a generator line, which may be straight, curved, or of a more complicated shape, about an axis, but do not require that the physical surface forms a complete annulus about the axis, nor that the axial ends of the physical surface form circles coaxial with the axis. As will be explained below, many of the embodiments described can be formed with surfaces that may be either a single annulus or two or more distinct arcs, and in many of the embodiments an axial boundary may be a junction (with or without an optically inactive step) between surfaces that are rotationally symmetric about different axes. 
         [0029]    The back of photon funnel  200  is formed by a single continuous surface, represented by curves  206  and  207  in  FIG. 2 , that is rotationally symmetric with respect to an axis  208  that is tilted relative to the source axis  203 . Thus, the curves  206 ,  207  are not identical, because they represent different parts of the surface of rotation, with curve  207  starting closer to the (tilted) axis  208 . Reflective surface  206  is shaped so that rays such as  209  and  210  that are refracted at side surface  212  undergo total internal reflection at surface  206  and then exit through surface  211 , forming an off-axis narrow beam pattern. 
         [0030]      FIG. 3A  shows two-dimensional horizontal and vertical sections through a typical intensity pattern produced by a partially off-axis collimator such as that shown in  FIG. 2 . The background emitted by the center lens is located with its center on-axis. The light has a FWHM of about 45 deg. The hotspot, emitted by the back surface  206 , is located off-axis. In  FIG. 3B , a two-dimensional intensity pattern is shown to illustrate the same radiation pattern. The scales on the horizontal axis in  FIG. 3A  and the vertical and horizontal axes in  FIG. 3B  represent angles in degrees from the center axis  203 . The contour lines in  FIG. 3B  and the vertical axis in  FIG. 3A  are normalized intensity levels. 
         [0031]    Lens  400  in  FIG. 4  is an optical device that collimates the light from a source  401  into an off-axis narrow hot spot and also creates a low-intensity background of rotational symmetry. A central cavity is bounded by a conical surface of rotation with respect to the source axis  403 , seen in cross section as lines  412  and  413 . Lens surface  402  is a rotationally symmetric surface around tilted axis  408 . As a result, the line where lens surface  402  meets conical surface  412 ,  413  is tilted, shown by the greater length of section line  413  than section line  412 . Rays such as  404  and  405  that are refracted at lens surface  402  and planar exit surface  411  will form the narrow off-axis beam pattern. Surface  406  forming the back of the light funnel  400  is rotationally symmetric around (non-tilted) light-source axis  403 . The cross-section of surface  406  is calculated to provide a wide background pattern. Rays such as  409  and  410  that are refracted at surface  412 ,  413  undergo total internal reflection at the back surface, represented by curves  406  and  407 , and then exit through planar surface  411 . 
         [0032]    Lens  500  in  FIG. 5  is an optical device that collimates the light from source  501  into an on-axis hot spot and also creates a low-intensity off-axis background illumination. A central cavity is bounded by conical surface  512 , which is a surface of rotation with respect to the source axis  503 , and by lens surface  502 , which is a rotationally symmetric surface around source axis  503 . Rays such as  504  and  505  that are refracted at lens surface  502  and planar exit surface  511  will form the narrow on-axis beam pattern. The back surface, represented by curves  506  and  507 , is a rotationally symmetric surface around tilted axis  508 . The cross-section of surface  506 ,  507  is calculated to provide an off-axis wide background pattern. Rays such as  509  and  510  that are refracted at conical surface  512  undergo total internal reflection at surface  506 ,  507  and then exit through planar surface  511 . 
         [0033]    Lens  600  in  FIGS. 6 and 7  is an optical device that collimates the light from source  601  into an off-axis hot spot and also creates an on-axis low-intensity background. By splitting the back surface into sections, the amount of light directed into on-axis and off-axis beams can be adjusted to the application. Side surface  609  of the central cavity is a conical surface of rotation with respect to the source axis  605 . Surface  602 , which forms the front part of the TIR back surface of light funnel  600 , is rotationally symmetric around tilted axis  604 . Tilted reflector surface  602  may be seen as being obtained by tilting a surface  611  that is rotationally symmetric around source axis  605  around the center of source  601 . Rays such as  606  and  607  that are refracted at cavity side surface  609 , then undergo total internal reflection at tilted reflective surface  602 , and then exit passing through flat front surface  608  will form the off-axis hot spot. 
         [0034]    Lens surface  615  and reflective surface  613 , which forms the rear part of the TIR back surface of light funnel  600 , is rotationally symmetric around source axis  605 . Rays  705  (see  FIG. 7 ) that are refracted at cavity side surface  609 , then undergo total internal reflection at on-axis reflective surface  613 , and then exit passing through flat front surface  608  contribute to the on-axis background illumination. Rays  704  that are refracted at the front lens surface  615  of the cavity and then at the flat front surface  608  also contribute to the on-axis background illumination. 
         [0035]    As may be seen from  FIGS. 6 and 7 , the relative intensities of the hot-spot and background beams may easily be set by choosing the position of the transition between the front and rear sections  602  and  613  of the reflector surface. 
         [0036]      FIG. 8  provides a three dimensional view of a photon funnel lens  800 , with source axis  805 , which may be similar to the lens  600  shown in  FIGS. 6 and 7  described above. 
         [0037]    Various methods of construction can be used to obtain a back surface of a photon funnel to provide off-axis illumination or non rotationally symmetric illumination: 
         [0038]    1. The whole or a section of a rotationally symmetrical collimating back surface such as surface  611  is tilted ( FIG. 6 ) around the source center by an angle equal to ArcSin((Sinθ)/n), where θ is the desired off axis angle of the center of the illumination pattern and n is the index of refraction of the dielectric used. Because of the refraction of the light at the cavity wall  609 , the system does not behave like a light source in an air filled reflector, so that for large pitch angles around the source center no optically “correct” surface for the illumination task can be obtained. For small angles of pattern center shift (up to approximately 20 deg) this method works sufficiently well. This solution provides an asymmetric exit aperture that is off centered from the optical axis  605 . 
         [0039]    2. In  FIG. 9  a more precise procedure is illustrated. A meridian cross-section  902  is calculated as a generalized Cartesian oval derived from off-axis wavefront  909  (outgoing wavefront after refraction at the exit surface  908 ) and source wavefront  911  (source wavefront after refraction at cavity wall  910 ). The source is treated like a point source, so that wavefront  911  can be represented as a spherical wavefront. The meridian cross-section  903  is calculated similarly from wavefronts  912  and  913 . Both cross-sections are rotated around a tilted axis  904  and result in surfaces  1001  and  1002  in  FIG. 10 . In  FIG. 10 , only the back surfaces of the photon funnel are shown. The surfaces  1001  and  1002  do not necessarily intersect. The left half (as seen in  FIGS. 9 and 10 ) of  1001  and the right half of  1002  are cut and connected by optically inactive surfaces  1003  and  1004 . 
         [0040]    3. For large off-axis angles or for a more complex off-axis or non rotationally symmetrical radiation pattern, a new back surface can be derived: In  FIG. 11A , a source  1104  and a cavity, consisting of cylinder  1103  and cavity lens  1102 , are shown. Exit surface  1101  is a flat surface. The back surface is derived as follows. An outgoing wavefront  1105  ( FIG. 11B ) is chosen that contains the information of what off-axis radiation pattern is to be obtained. The outgoing wavefront is in this case a cylindrical surface, although any other suitably well-behaved wavefront with or without any symmetry can be used. However it must be possible to propagate the wavefront free of caustics throughout the space in which the back surface is being created. A centered cylindrical wavefront  1105  would provide an extended oval beam pattern, centered at the source axis. The outgoing wavefront must be traced back through the exit surface. The source wavefront, in the point source approximation, is a spherical wavefront that, when propagated and refracted at the cavity, becomes an aspheric rotationally symmetric surface  1106 . A generalized Cartesian surface that couples wavefronts  1105  and  1106  can be numerically calculated and is shown as freeform surface  1107  ( FIG. 11C ), which will be the back reflecting surface for the photon funnel (cavity and lens are not shown in  FIG. 11C  for simplicity). 
         [0041]    Although specific embodiments have been described, the person skilled in the art will understand how features of different embodiments may be combined, and how features may be substituted or modified, without departing from the scope of the claimed invention. 
         [0042]    For example, although the reflectors  206 ,  207 ,  406 ,  407 ,  506 ,  507 ,  602  have each been described as a single rotationally symmetrical surface, any of them may be designed by any of the methods described with reference to any of  FIGS. 9 and 11 , and may therefore be two (or more) surfaces separated by an axial cut line, as in  FIG. 10 , or an asymmetrical surface as shown in  FIG. 11 , or both. 
         [0043]    For example, although a light source consisting of one or more LEDs in a plane has been described, other forms of light source, including light sources hereafter to be invented or developed, may be used. However, if the light source is not Lambertian, the shape of the lens and the reflecting back surface for a given beam pattern may be different. In the interests of simplicity, the LED has been approximated to a point source. Those skilled in the art will understand how a light source of non-negligible size will affect the shapes of the lens and reflector, and may limit the attainable precision of the beam pattern. 
         [0044]    For example, although in all the embodiments the lens  202 , etc. is a single optical surface producing a single beam, the lens may, like the reflector  602 ,  613  or  1001 ,  1002 , be divided into two or more sections producing distinct beams. 
         [0045]    The preceding description of the presently contemplated best mode of practicing the invention is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The full scope of the invention should be determined with reference to the Claims.