Patent Publication Number: US-2017351101-A1

Title: Light Mixing Lenses And Systems

Description:
RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 15/200,843, filed Jul. 1, 2016, which is a continuation of U.S. patent application Ser. No. 13/729,459, filed Dec. 28, 2012, which claims priority to Provisional Application No. 61/582,083, filed Dec. 30, 2011, all applications are herein incorporated by reference in their entirety. 
    
    
     FIELD 
     The present patent application generally relates to lenses and lighting systems, and more particularly to lenses and lighting systems for light mixing and/or color mixing. 
     INTRODUCTION 
     Lenses and lighting systems for high-power light sources, such as light emitting diodes, can have a wide variety of configurations. In many cases, a particular configuration can be characterized by the illumination pattern it produces, by the coherence, intensity, efficiency and uniformity of the light projected by it, and so on. The application for which the lens and/or lighting system is designed may demand a high level of performance in many of these areas. 
     Many applications call for the ability to mix light from multiple sources, e.g., sources producing light of different colors. Further, light mixing is also useful for systems with large light sources. In both cases, it is difficult to produce uniformly mixed light and reduce source imaging. To date, light-mixing systems have typically provided textured surfaces to spread the light from a light source. The efficiency and capabilities of such systems are limited and their illumination characteristics are typically sub-par. 
     Accordingly, there is a need for improved light-mixing lenses and lighting systems. 
     SUMMARY 
     In accordance with one aspect, an optical device is provided that includes a light pipe having a proximal end adapted for optical coupling to a light source, e.g., a light emitting diode (LED), to receive a plurality of light rays therefrom and a distal end providing an output surface for the light rays. The light pipe can be configured to cause mixing of the light rays via reflection at one or more lateral surfaces thereof as the light rays pass through the light pipe. The optical device can include a lens optically coupled to the output surface to receive light therefrom. In various aspects, the output surface can comprise one or more surface features configured to modulate an output of the light passing therethrough. For example, the output surface can include a texture. In some aspects, the surface features can comprise a plurality of microlenses such that light rays exiting the light pipe exit through the microlenses. 
     In some embodiments, the light source can include a plurality of light emitting sources generating light at different colors. For example, the light source can be two LEDs providing light at different colors 
     In some embodiments, the lens can be a zoom lens that can move axially relative to an output surface of the light pipe to change, e.g., the angular spread of the beam. In some embodiments, rather than utilizing a single zoom lens, a zoom lens system comprising two or more lenses, at least one of which is axially movable relative to the output surface of the light pipe, is employed to change the divergence of the output beam, e.g., between a narrow-beam spread and a wide-beam spread. For example, the zoom lens system can include a lens providing a positive optical power and another providing a negative optical power. In some embodiments, a multi-lens zoom system can provide, in the wide-beam position, an output beam exhibiting a divergence (e.g., as characterized by full width at half maximum (FWHM) of the light intensity distribution in a plane perpendicular to the direction of propagation) in a range of about 20 degrees to about 80 degrees. 
     In some embodiments, the lateral surface(s) of the light pipe can be configured to cause total internal reflection of at least some of the light rays incident thereon. In other embodiments, the lateral surface(s) of the light pipe can be metalized to cause specular reflection of light rays incident thereon. 
     In various embodiments, the light pipe comprises an input surface at the proximal end for receiving said light rays from the light source. In some aspects, the output surface has a greater cross-sectional area than said input surface. In one aspect, the light pipe has a longitudinal dimension that is at least 10 times greater than any linear dimension of said input surface. In various aspects, the light pipe can have a parallelepiped shape characterized by said input surface, said output surface, and four lateral surfaces. In a related aspect, any two opposed lateral surfaces of the light pipe diverge in a direction from said proximal end to said distal end. A draft angle associated with the divergence of the opposed lateral surfaces can be in a range of about 0 degree to about 20 degrees. 
     In some embodiments, the light pipe includes at least two portions having different cross-sectional shapes. For example, the light pipe can include a proximal portion (a portion proximate to the light source) having one cross-sectional shape (e.g., circular), an intermediate portion having a different cross-sectional shape (e.g., hexagonal) and a distal portion having yet another cross-sectional shape (e.g., square). 
     In some embodiments, the shape of the output surface of the light pipe is selected to impart a desired cross-sectional shape to the light beam exiting the light pipe. For example, in some embodiments, the output surface of the light pipe can have a generally square shape with rounded corners. In other embodiments, the output surface of the light pipe can have a hexagonal or an octagonal shape. In some embodiments, the shape of the input surface of the light pipe is different that the shape of its output surface. For example, the light pipe can include a square input surface and a hexagonal output surface. 
     In some embodiments, the optical system can include a light interface unit coupled to the proximal end of the light pipe for facilitating the delivery of light from a light source to the light pipe. In some cases, the light interface unit can also provide some mixing of light. In some implementations, the light interface unit is integrally formed with the remainder of the light pipe, e.g., the light interface unit can form the proximal portion of the light pipe. In some embodiments, the light interface unit can include a cavity, which can be formed by a curved input surface, for receiving light from a light source. The input surface receives the light and couples the light, e.g., via refraction, into the light interface unit. In some embodiments, the input surface is configured such that a substantial portion of the received light is redirected toward a peripheral surface of the light interface unit. The peripheral surface can in turn reflect the light incident thereon, e.g., via total internal reflection or specular reflection, to the light pipe (or in cases in which the light interface unit is formed integrally with the light pipe to the remainder of the light pipe). 
     In some embodiments, the light interface unit and the light pipe can be formed as separate pieces that are optically coupled to one another, e.g., via an adhesive and/or mechanical couplings. 
     In some embodiments, the light pipe can include an input surface that is shaped to conform to the shape of an output surface of a light source that is optically coupled thereto. For example, the light pipe can include a concave input surface that conforms to a convex surface of a dome of an LED. 
     In some embodiments of the optical system, a baffle is coupled to the lens. For example, a baffle can be coupled to a zoom lens utilized in the optical system so as to capture certain light rays, e.g., those rays exiting the light pipe that fail to strike the zoom lens when the zoom lens is moved away from the output surface of the light pipe to be at its distal location. 
     In some embodiments, the light pipe can be formed of polymethyl methacrylate (PMMA), polymethacrylmethylimid (PMMI), glass, polycarbonate, cyclic olefin copolymer, cyclic olefin polymer, silicone, or other suitable materials. 
     In some aspects, the optical device can include a holder providing a housing for the light pipe. In some embodiments, the holder can include a plurality of legs for coupling to openings in a substrate, e.g., a printed circuit board (PCB), on which a light source is mounted. 
     In a related aspect, an optical device is disclosed, which comprises a light pipe having a proximal end adapted for coupling to a light source to receive light therefrom and a distal end through which light exits the light pipe, said light pipe being configured to cause mixing of the light via reflection at one or more lateral surfaces thereof as the light propagates from its proximal end to its distal end. A lens, e.g., a zoom lens, is optically coupled to the distal end of the light pipe to receive at least a portion of the light exiting the light pipe, said lens being configured to project the received light as an output beam. The lens comprises at least one surface exhibiting a plurality of surface modulations for modulating (adjusting) at least one characteristic of the output beam. For example, the surface modulations can adjust a maximum divergence angle of the beam and/or the beam&#39;s cross-sectional shape. In some embodiments, the surface modulations can include at least two different types of modulations disposed on at least two different portions of the lens surface. For example, the different surface modulations can include sinusoidal modulations at two different frequencies. In some implementations, the two types of surface modulations are employed to impart different maximum divergence angles to the light exiting through different portions of the lens surface. 
     In some embodiments, the profile of the lens surface having surface modulations can be characterized as a combination of a base profile and a modulation profile, each of which can in turn be characterized by a mathematical function exhibiting a continuous first derivative. 
     In a related aspect, an optical system is disclosed that comprises a plurality of light modules disposed adjacent to one another, each of said light modules comprising: a light pipe for receiving light from a light source at a distal end thereof and guiding the received light at least partially via reflections at its one or more peripheral surfaces to a distal end thereof through which light exits the light pipe, and a lens for receiving at least a portion of the light exiting the light pipe to form an output beam. The light modules are positioned and oriented relative to one another such that the output beam of each module at least partially overlaps with the output beam of at least another module on a target surface so as to provide collectively a desired illumination pattern on that surface. For example, the output beam of each module can at least partially overlap with an output beam of at least another module at one or more spatial locations. 
     In some embodiments of the above optical system, at least two of said light modules include light pipes having different cross-sectional shapes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The skilled person in the art will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the applicant&#39;s teachings in any way. Like numerals are utilized to designate similar features of various embodiments. 
         FIG. 1A  depicts a partial cross-sectional view of an exemplary lighting module in accord with various aspects of the applicant&#39;s teachings. 
         FIG. 1B  schematically depicts a zoom lens system in an embodiment according to the teachings of the invention in a narrow-beam position, where the zoom lens system includes a positive and a negative lens. 
         FIG. 1C  schematically depicts the zoom lens system of  FIG. 1B  in a wide-beam position. 
         FIG. 1D  schematically depicts a zoom lens comprising a single lens in an embodiment according to the teachings of the invention in a narrow-beam position. 
         FIG. 1E  schematically depicts the zoom lens of  FIG. 1D  is a wide-beam position. 
         FIG. 2  depicts an exploded view of the exemplary lighting module of  FIG. 1A . 
         FIG. 3  depicts in detail a proximal end of the exemplary lighting module of  FIG. 1A . 
         FIG. 4  depicts in detail a distal end of the exemplary lighting module of  FIG. 1A . 
         FIG. 5A  schematically depicts an exemplary embodiment of the output surface of the zoom lens of  FIG. 1A . 
         FIG. 5B  is a schematic top view of the output surface of a zoom lens according to an embodiment in which a central portion of the surface exhibits one type of surface modulation and a peripheral portion of the output surface exhibits a different type of surface modulation. 
         FIG. 6A  schematically depicts an exemplary shape of an output surface of a light pipe utilized in an embodiment of an optical system according to the teachings of the invention, 
         FIG. 6B  schematically depicts another exemplary shape of an output surface of a light pipe utilized in an embodiment of an optical system according to the teachings of the invention. 
         FIG. 7  schematically depicts a light pipe according to an embodiment of the invention having a light interface unit at a proximal end thereof. 
         FIG. 8  schematically depicts a light pipe according to an embodiment of the invention having a concave input surface for receiving light from a light source. 
         FIG. 9  schematically depicts a light pipe according to an embodiment of the invention in which different portions exhibit different cross-sectional shapes. 
         FIG. 10A  schematically depicts an optical system according to an embodiment of the invention having a holder with two legs for coupling to a printed circuit board (PCB) on which a light source, e.g., an LED, is mounted. 
         FIG. 10B  is a partial schematic view of the optical system of  FIG. 10A  depicting the two legs of the holder. 
         FIG. 10C  schematically depicts a PCB on which a light source is mounted, where the PCB includes two openings for receiving the tips of the legs of the holder of the optical system shown in  FIGS. 10A and 10B . 
         FIG. 11A  is a partial schematic view of an optical system according to an embodiment of the invention which includes a zoom lens coupled to a baffle with the zoom lens positioned at a distal location relative to an output surface of the light pipe. 
         FIG. 11B  is a schematic view of the optical system of  FIG. 11A  in which the zoom lens is positioned at a proximal location relative to the output surface of the light pipe. 
         FIG. 12  depicts in detail an exemplary lighting system incorporating multiple exemplary lighting modules of  FIG. 1A . 
     
    
    
     DETAILED DESCRIPTION 
     The present application discloses, among other things, lighting modules and lighting systems containing one or more lighting modules, and associated methods, that receive light from one or more light sources for projecting it onto a target surface, e.g., in a uniform, patterned, or other controlled manner. In various embodiments, the lighting modules and lighting systems can be used to mix the light generated by one or more sources. By way of example, the projected light can be homogenized so as to create an output distribution of substantially uniform luminosity. In many cases, two or more light sources can be used to generate light of different wavelengths. In various embodiments, the lighting modules and systems can be effective to mix the light of different colors. Further, in some embodiments, the lighting systems of the invention can be utilized for effective mixing of light generated by spatially separate individual sources of single color that collectively make up a single large source of light. 
     The devices and methods disclosed herein can be used with a wide variety of light sources, including light-emitting-diodes and incandescent bulbs, or other coherent or non-coherent sources. Such devices and methods can have a wide range of applications, including, for example, in spot lighting, customizable/adjustable lighting systems, household lighting, flashlights, wearable headlamps or other body-mounted lighting, among others. 
     Throughout this application, the term “e.g.” will be used as an abbreviation of the non-limiting term “for example.” It should be understood that regardless of whether explicitly stated or not, all characteristics of the lighting modules and lighting systems described herein are by way of example only, and not necessarily requirements. All Figures merely depict exemplary embodiments of lighting modules and/or systems that incorporate various aspects of the applicants&#39; teachings. Further, the features of one embodiment can be combined with those of any other embodiment. 
     With reference now to  FIG. 1A , one exemplary embodiment of an optical system (herein also referred to as a lighting module)  100  in accord with the applicants&#39; teachings is depicted. As shown in  FIG. 1A , the lighting module  100  can include one or more light source(s)  110 , a light pipe  130 , and a holder  120 . Additionally, the lighting module can be associated with a zoom lens  140  that can selectively focus the light exiting the light pipe  130 , as discussed in detail below. 
     Although any number of light sources can be employed in the lighting module  100 ,  FIG. 1A  depicts a single light source  110  being associated with the lighting module  100 . Though in the exemplary embodiment depicted in  FIG. 1A  the light source  110  is depicted as a single LED chip having a substantially flat output surface, a person skilled in the art will appreciate that any number of a wide variety of light sources, including flat and domed light-emitting diodes, incandescent bulbs, or other coherent or non-coherent sources can be used with the systems and methods described herein. Such systems and methods can have a wide range of applications, including, for example, in spot lighting, customizable/adjustable lighting systems, household lighting, flashlights, wearable headlamps or other body-mounted lighting, among others. Further, as will be discussed below, one or more light sources can be associated with each of the lighting modules. By way of example, the lighting module  100  can be configured to project the light from two or more light sources  110  emitting light at the same or different wavelengths. As will be otherwise discussed herein, the lighting module  100  can be effective to mix the light of the two or more light sources. 
     The light pipe  130  can have a variety of configurations but is generally configured to receive the light generated by the light source  110  and output the light towards the target surface. In the exemplary embodiment depicted in  FIG. 1A , for example, the light pipe  130  can include a proximal surface  132 , a distal surface  136 , and one or more lateral surfaces (e.g., sidewalls  134 ) extending therebetween. Generally speaking, in this embodiment, the proximal surface  132  can receive light from the light source  110  and act as an input surface for coupling the light from the light source  110  into the light pipe  130 . In various embodiments, as the light from the light source  110  enters the light pipe  130 , the light from the light source  110  can be refracted towards the sidewall(s)  134 . The term “refraction” is meant to indicate that the light rays change direction, as can occur, for example, when they travel from one medium (e.g., air outside the light pipe  130 ) to another (e.g., the material making up the light pipe  130 ). As one skilled in the art will appreciate, some light generated by the light source  110  can enter the light pipe  130  without refraction, for example, if the light strikes the input surface  132  in a direction normal to the input surface  132 . 
     After entering the light pipe  130 , the light can be transmitted therethrough toward the distal surface  136 . The light can be transmitted directly through the light pipe  130  from the input surface  132  to the output surface  136  or indirectly via one or more reflections from the sidewall(s)  134 . The sidewall(s)  134  can be adapted to reflect light incident thereon via a wide range of mechanisms, for example via total internal reflection or via specular reflection, such as can be achieved by forming a metallic coating thereon. The reflected light can travel to the distal surface  136 , through which light can be coupled out of the light pipe  130  (e.g., via refraction). 
     Though the input surface  132  is shown as a substantially flat surface disposed in a facing relationship with the exemplary LED chip, it should be appreciated that the proximal surface  132  of the light pipe  130  can have a variety of shapes and/or surface profiles for coupling light generated by a source into the light pipe  130 . For example, the surface can be shaped so as to avoid or reduce the passage of light through the light pipe  130  without at least one reflection from the sidewalls  134  and/or to avoid or reduce imaging the light source  110  in the projected light. In some embodiments, for example, the proximal input surface can be configured to receive a light source. By way of example, the input surface  132  can be substantially concave so as to define a cavity in which a light source can be disposed. 
     As will be appreciated by a person skilled in the art, the output surface  136  of the light pipe  130  can also have a variety of configurations. As shown in  FIG. 4 , for example, the distal surface  136  can comprise surface features (e.g., texturing) formed thereon configured to further spread and or mix the light incident thereon in order to alter the output beam characteristics. For example, in the depicted exemplary embodiment, the distal surface can include an array of microlenses  138 , each of which can be effective to focus, diverge, or diffract light incident thereon out of the light pipe  130 . In various embodiments, the array of microlenses  138  can be effective to alter the distribution of light relative to a substantially flat output surface, for example, without substantially increasing the maximum divergence of light out of the light pipe  130 . 
     The microlenses  138  can have a variety of shapes and/or sizes. For example, each microlens  138  can be dimensioned such that it is at least about 10 times smaller than the surface area of the output surface  136 . As will be appreciated by a person skilled in the art, other structures for spreading or mixing the light at the distal surface  136  can also be employed as part of or in place of the array of microlenses  138 . By way of example, surface texturing (e.g., texturing created in the die-mold or using chemical or mechanical etching or roughening) can be used on the distal surface  136  as dictated by the desired output pattern and/or characteristics. 
     The size and the shape of the light pipe can also vary and can be selected so as to optimize the mixing of light. As best shown in  FIG. 2 , an exemplary light pipe  130  can be a substantially elongate member, for example, having a length that is substantially greater than its width. By way of example, the length of the light pipe  130  can be at least 10 times the width of the proximal surface. 
     The light pipe  130  can additionally have any number of sidewalls and can have a variety of cross-sectional shapes. By way of non-limiting example, the cross-section of the light pipe  130  can be square, circular, elliptical, hexagonal, star-shaped, etc. In various embodiments, the shape and/or number of the sidewalls can be selected based at least in part, for example, on the input surface  132  of the light pipe  130 . By way of example, in a light pipe in which the input surface  132  forms a semi-hemispherical cavity in which the light is coupled from the source  130  (e.g., a domed LED), the light pipe  130  can be selected to have a circular cross-sectional shape (i.e., one sidewall). Alternatively, for example, a flat input surface  132  may lend itself to a square and or rectangular cross-section to improve the mixing of light within the light pipe  130 . 
     Moreover, each cross-section along the length of the light pipe can be of the same shape but have different dimensions. For example, in the depicted embodiment, the cross-sectional area of the light pipe  130  adjacent the proximal surface  132  can be smaller than the cross-sectional area of the light pipe  130  adjacent to the distal surface  136 . 
     With specific reference to the exemplary embodiment depicted in  FIGS. 1A-5B , for example, the light pipe includes four sidewalls of equal linear dimensions such that the light pipe  130  has a square cross-section along its entire length. Moreover, the dimensions of the proximal end  132  can be smaller than the dimensions of the distal end  136  such that the light pipe  130  is shaped as an inverted frustum. One of skilled in the art will appreciate based on the teachings herein, for example, that the relative divergence of one sidewall relative to the opposed sidewall, for example, can be varied so as to alter the output light distribution. By way of example, a light pipe  130  configured such that the sidewalls  134  diverge along the length of the light pipe from the proximal surface  132  to the distal surface  136  can have a smaller output beam angle (as characterized by full width at half maximum (FWHM) of the light intensity distribution in a plane perpendicular to the direction of propagation) than the output beam angle FWHM of a light pipe in which the sidewalls  134  converge towards the distal surface  136 . A draft angle associated with the divergence of the opposed sidewalls can be, for example, in a range of about 0 degree to about 20 degrees. 
     As noted above, the lighting module  100  can additionally include a holder  120 . The holder  120  can have a variety of configurations but generally defines a bore  122  in which the light pipe  130  can be disposed. In various embodiments, the bore  122  can have a cross-sectional area that is generally configured to match that of the light pipe  130 . Accordingly, the light pipe  130  can be inserted into the holder  120  and can be retained therein, at least partially, for example, by way of a frictional fit between the holder  120  and the light pipe  130 . 
     In various embodiments, the holder  120  can also be configured to seat the light source  110 . With reference now to  FIG. 3 , the proximal end  124  of the holder  120  can contain a cavity in which the light source can be disposed. By way of example, the holder can include a shoulder  126  which can abut at least a portion of the light source  110 . Moreover, the shoulder  126  can include one or more vents  128  that allows for the dissipation of heat from the light source  110 . 
     Accordingly, as shown in  FIG. 3 , for example, a light source can be secured within the proximal end  124  of the holder  120  with the light pipe  130  disposed within the bore  122  of the holder  120 . The holder  120  can thus be configured to align the input surface  132  of the light pipe  130  and the source  110  to efficiently couple light generated by the source  110  into the light pipe  130 . Moreover, a gap (e.g., an air gap) between the light source  110  and the input surface  132  of the light pipe  130  can be in fluid communication with the vents  128  so as to prevent overheating of the source  110 . 
     The holder  120  can additionally include one or more coupling mechanisms to enable the holder  120  to couple to adjacent lighting modules  100 , as will be discussed in detail below. With reference now to  FIG. 4 , the holder  120  can include, for example, a distal flange  123  having a bore  125  formed therethrough. As will be appreciated by a person skilled in the art, the bore  125  can enable coupling between adjacent lighting module(s)  100 . By way of non-limiting example, a pin can be inserted through the bores  125  of adjacent lighting modules to movably or fixedly couple the lighting modules  100 . Moreover, the distal end of the holder  120  can be configured to retain the light pipe  120  within the bore. By way of example, the distal end of the holder can include one or more tabs  127  that can prevent the light pipe  120  from moving distally relative to the holder  130 . 
     With reference again to  FIG. 1A , the lighting module  100  can also be associated with a zoom lens  140  disposed distal to the output surface  136  of the light pipe  130  to receive light that is coupled out of the light pipe  130  and direct it towards a target surface. In various embodiments, the zoom lens  140  can be moved axially relative to the output surface  136  to alter the focusing power of the light being directed therethrough. By way of example, the zoom lens  140  can be translated along the central axis of the light pipe  130  so as to vary its distance from the output surface  136  of the light pipe  130 , thereby varying the width (i.e. the convergence-divergence) of the output light distribution with respect to the central axis of the light pipe  130 . In some embodiments, for example, the zoom lens  140  can be moveable between a position near the output surface  136  to a distal position at the focal length of the output surface  136 . In such an embodiment, axial movement of the lens toward the holder  120  (e.g., to the wide zoom position  140   a ) can be effective to increase the width of the beam distribution on the target surface whereas movement of the lens away from the holder  120  (e.g., to the narrow zoom position  140   b ) can decrease the width of the beam distribution on the target surface. 
     As will be appreciated by a person skilled in the art, the zoom lens  140  can be movable relative to the holder  120  using a variety of mechanisms. By way of example, the zoom lens  140  can be movable relative to the holder  120  and/or light pipe  130  via a step motor that can be controlled to position the zoom lens  140  at a desired location relative to the output surface  136  of the light pipe  130 . As will be appreciated by a person skilled in the art, in lighting systems in which multiple lighting modules  100  are associated, a zoom lens  140  associated with each light pipe  130  can be moved individually relative to its respective holder  120  or in tandem with the other zoom lenses in the system. Alternatively, for example, the zoom lens  140  can be rotated within a housing in a first direction (e.g., clockwise) to move the zoom lens  140  away from the output surface  132  and in a second direction (e.g., counterclockwise) to move the zoom lens  140  toward the output surface  132 . 
     As will be appreciated by a person skilled in the art, the zoom lens  140  can be shaped so as to provide a desired final output distribution on the target surface. As shown in  FIG. 1A , for example, the zoom lens  140  can include an input surface  142 , a lens body  144 , and an output surface  146 . The input surface  142  can be disposed in a facing relationship with the output surface  136  of the light pipe  130  and can be configured to couple the light received therefrom into the lens body  144 , for example, via refraction as otherwise discussed herein. Light can traverse the lens body  144  and can exit through the output surface  146  of the zoom lens  140 . 
     In some embodiments, rather than utilizing a single lens, the zoom lens (herein also referred to as the zoom lens system) can include a plurality of lenses, at least one of which is axially movable relative to the output surface of the light pipe. For example,  FIGS. 1B and 1C  schematically depicts a zoom lens system  10  (herein also referred to as a doublet zoom) that includes a lens  12  providing a positive optical power and a lens  14  providing a negative optical power. As least one of the lenses, and in some cases both, is axially movable relative to the output surface of the light pipe to change the angular spread of the output beam between a narrow-beam spread (shown in  FIG. 1B ) and a wide-beam spread (shown in  FIG. 1C ). The use of a doublet lens system with one positive lens and one negative lens can be advantageous in applications where a wide beam range, e.g., a divergence (FWHM) in a range of about 20 to about 80 degrees, is required. In embodiments in which the zoom lens includes a single positive lens (herein also referred to as a singlet zoom), in the “intrafocal position” (i.e., when the lens is placed close to the output surface of the light pipe), the positive power of the lens can reduce the divergence of the beam exiting through the output surface of the light pipe (See  FIGS. 1D and 1E  for schematic representation of a narrow-beam and wide-beam spread of the output beam in a system in which a singlet zoom  16  is employed). In contrast, the positive and the negative optical powers of the lenses of a doublet zoom allow achieving much wider beam spread, e.g., in a range of about 20 degrees to about 80 degrees (FWHM), when zoom system is in the “intrafocal position.” Further, in many applications, a multiple-lens zoom lens system can provide other advantages, such as conventional advantages known in the art. 
     As will be appreciated by a person skilled in the art, the input surface  142  and output surface  146  of the zoom lens  140  can be configured to control the final output beam characteristics. By way of example, the output surface  146  can include surface features formed thereon that can provide additional control over the cross-sectional shape and/or the maximum divergence angle of the output beam and/or its “texture” on a target surface. For example, the output surface  146  can be configured to further mix the light such that the final output beam can have a cross-sectional shape that differs from the light received at the input surface  142 . Although a beam of light emitted from a light pipe having a square cross-section can be similarly shaped (i.e., substantially square), the output surface  146  of the zoom lens  140  can be configured to mix the light, for example, so as to reduce the sharpness of the edges of the projected light pattern. By way of example, the output surface  146  of the zoom lens  140  can comprise surface features (e.g., annular rings, perturbations, microlenses) that are configured to modulate the output light distribution in a controlled manner. 
     For example, with reference now to  FIG. 5A , a pattern of sinusoidal grooves can be formed in the output surface  146  of the zoom lens  140 . The sinusoidal grooves can have a variety of configurations. By way of example, the tangent of the sinusoidal grooves at one or more points along the surface can deviate less than about ±5 degrees relative to the tangent of a nominal surface  148  in which the grooves are formed at those points as depicted in  FIG. 5A . In various embodiments, the sinusoidal grooves can be effective to scatter and/or modulate the light output from the light pipe  130  to decrease the sharpness of the edges of the output light distribution (e.g., reduce the appearance of a square output pattern on a target surface) and/or modulate the maximum divergence of the output light beam. By way of example, the sinusoidal grooves can reduce, or eliminate, high frequency components in a Fourier transform of the cross sectional intensity of the output light distribution. It should be understood that the controlled modulation of the output surface  146  can be achieved by employing patterns other than sinusoidal. Further, such controlled modulation can be implemented on either surface, or both surfaces, of the zoom lens. Yet, in other embodiments, rather than, or in addition to, imparting controlled modulation to one or more surfaces of the zoom lens, one or more additional optical elements (e.g., lenses) having at least a surface exhibiting such controlled modulation can be employed. 
     In some embodiments, the output surface  146  can be represented by a mathematical function as a combination of a base profile and a modulation profile. By way of illustration, the surface profile along a cross-sectional cut passing through the center of the surface (i.e., the intersection of an optical axis of the lens with the surface) along one dimension of the surface (herein denoted as the x-dimension) can be defined as follows: 
         F ( x )= f ( x )+ P ( x )  Eq.(1),
 
     wherein, 
     F(x) denotes the surface profile along the cross-sectional cut, 
     f(x) denotes a base profile, and 
     P(x) denotes a modulation profile. 
     P(x) is selected to be a function having a continuous first derivative. In other words, the first derivative of P(x), herein denoted as dP(x)/dx or P′(x) exhibits no discontinuity. Further, f(x) can be selected to exhibit a continuous first derivative. 
     A rotation of the above cross-sectional profile about an optical axis of the surface would generate a two-dimensional rotationally symmetric surface profile. In other embodiments, the surface profile may not be rotationally symmetric. For example, the cross-sectional profiles along two orthogonal dimensions (e.g., x and y dimensions) can be different. By way of example, in the above example, the surface profile along a cross-sectional cut passing the center of the surface along an orthogonal dimension (the y-dimension) can be characterized by a different base profile and/or modulation profile. 
     A variety of functions can be employed for the base and the modulation profiles defined above in Equation (1). By way of example, the surface can be rotationally symmetric with a semi-circular base profile and a sinusoidal modulation profile along one dimension of the surface (which corresponds in two dimensions to a hemispherical base profile and an undulating surface having sinusoidal variations along each direction along the surface). For example, the surface profile along one dimension (herein the x-dimension) can be defined as follows: 
         F ( x )=√{square root over ( R   2   −x   2 )}+∝ sin(β· x )  Eq.(2)
 
     wherein R is a constant corresponding to the radius of the base profile), a is a constant corresponding to an amplitude of the sinusoidal modulations, and denotes the frequency of the sinusoidal modulations. The cross-sectional profile defined by the above Equation (2) can be rotated about an optical axis of the surface to generate a two-dimensional surface characterized by a hemispherical base profile on which a plurality of sinusoidal modulations are disposed. The amplitude and the frequency of the sinusoidal modulations can be selected based on a desired modulation of the output beam. For example, for a given amplitude of the modulations, increasing the modulation frequency can increase the divergence angle of the beam, e.g., as characterized by full width at half maximum (FWHM) of the light intensity distribution in a plane perpendicular to the direction of propagation. Further, changing the amplitude of the modulations while keeping the frequency of modulations constant can also result in a change in the divergence angle of the output beam. In some cases, the amplitude and the modulation frequency can be both changed so as to keep the divergence angle of the beam constant while changing another characteristic of the beam, e.g., its cross-sectional shape or its texture on a target surface. 
     In some embodiments, one type of surface modulation is employed over one portion of the output surface  146  of the zoom lens  140  and another type of surface modulation is employed over another portion of the surface of the lens  140 . In this manner, the spread of the light exiting through the system via one portion of the output surface of the zoom lens  140  can be different than the spread of the light exiting the system via another portion of that output surface. By way of example,  FIG. 5B  shows a top view of an embodiment of the output surface  146  of the zoom lens  140  in which sinusoidal surface modulations with one frequency are present in a central region (CR) of the lens output surface  146  and sinusoidal surface modulations with a different frequency (e.g., a lower frequency) are present in a peripheral region (PR) of the lens output surface. In this embodiment, the portion of the light leaving the lens through the central portion of the lens can show a maximum angular divergence that is different, e.g., greater, than a maximum angular divergence exhibited by the light rays exiting the lens through the peripheral region of the surface. 
     In some embodiments, the frequency of the surface modulations can vary continuously from the center of the output surface  146  of the zoom lens  140  to an outer boundary of that output surface. Further, while in many embodiments discussed herein, the surface modulations are disposed on the output surface of the zoom lens, in some other embodiments, the surface modulations can be disposed on the inner surface of the zoom lens or both the inner and outer surfaces. 
     As noted above, in some embodiments, the output surface of the light pipe, e.g., the output surface  136  depicted in  FIG. 1A , can be shaped so as to impart a desired cross-sectional shape to the light exiting the light pipe. By way of example,  FIG. 6A  schematically depicts an output surface of the light pipe, which is shaped generally as a square with rounded corners. The rounded corners ensure that the emitted light output, though generally square shaped, lacks sharp corners. As another example,  FIG. 6B  schematically depicts another embodiment of an output surface of the light pipe, which has a hexagonal shape. In some implementations of this embodiment, the light pipe can have a square cross-section while its output surface has a hexagonal shape. Yet, in another embodiment, the output surface of the light pipe can have an octagonal shape. In some implementations of such an embodiment, the light pipe can have a square or hexagonal cross-sectional shape while its output surface has an octagonal shape. The octagonal shape of the output surface of the light pipe can impart a similar shape to the cross-sectional profile of the light beam exiting the light pipe. The octagonal shape of the light beam can be further adjusted via modulations of the output surface of the zoom lens to provide an output beam that has an approximately round cross-sectional shape. 
     In some embodiments, a light interface unit is employed to facilitate coupling of the light emitted by one or more light sources, e.g., one or more LEDs, into the light pipe. By way of example, the light interface unit can include an input surface that conforms to an output surface of a lighting unit in which the light sources are disposed, e.g., a hemispherical dome cover of an LED unit, to facilitate the delivery of the light emitted by the light sources to the light pipe. In some embodiments, the light interface unit can be formed integrally with the remainder of the light pipe, e.g., it can form a proximal portion of the light pipe. In some embodiments, the light interface unit can not only facilitate the coupling of the light emitted by the source into the light pipe, but it can also provide some mixing of the light. 
     By way of example,  FIG. 7  schematically depicts a light pipe  700  according to such an embodiment, which includes a proximal portion  700   a  that functions as a light interface unit for receiving light from a light source  110  and delivering the light to the remainder of the light pipe  700   b . The light interface unit  700   a  further provides some mixing of the light rays (see, e.g., exemplary light rays A and B) prior to their delivery to the remainder of the light pipe. In this embodiment, the light interface unit includes a cavity  701  formed by a curved input surface  702  that terminates in an apex  703 . The input surface  702  receives the light from the light source  110  and couples the light into the light interface unit  700 . The input surface  702  is configured such that a substantial portion of the light incident thereon (e.g., at least about 80%, or at least about 90%, or 100%) is refracted to be redirected toward a peripheral surface  704 . In this embodiment, the peripheral surface is configured in a manner known in the art to cause total internal reflection of the light rays incident thereon, or at least the total internal reflection of a substantial portion of the incident light rays (e.g., at least about 80% or 90% of the incident light rays), so as to redirect those rays toward the remainder portion  700   b  of the light pipe  700 . In some other embodiments, the peripheral surface can be metalized to cause specular reflection of the light rays incident thereon. Although in this embodiment, the light interface unit is formed integrally with the remainder of the light pipe, in other embodiments, it can be a stand-alone unit that is optically coupled to the light pipe. In such cases, the light interface unit and the light pipe can be coupled to one another, e.g., by using an adhesive or seating the light interface unit and the light pipe in a holder in optical alignment relative to one another. By way of further illustration, the light interface unit  700   a  can be formed similar to the lenses disclosed in U.S. Published Application No. 2010/0226127 entitled “Light Mixing Optics and System,” which is herein incorporated by reference in its entirety. 
     As another example,  FIG. 8  schematically depicts a light pipe  800  that includes a concave light receiving surface  801 , which provides a cavity in which a light source  802 , herein an LED having a convex dome  802   a , can be seated. In this embodiment, the concave light receiving surface  802  conforms substantially to the convex surface of the LED dome The matching of these surfaces can advantageously allow efficient coupling of the light emitted by the LEDs into the light interface unit. 
     In some embodiments, different portions of the light pipe can exhibit different cross-sectional shapes. For example, a portion of the light pipe can have a circular cross-section while another portion of the light pipe can have a square or a hexagonal cross-section. By way of example, the different cross-sectional shapes can be utilized to modulate, e.g., enhance, coupling of the light into and out the light pipe while ensuring effective mixing the light as it propagates along the length of the light pipe. By way of illustration,  FIG. 9  schematically depicts a light pipe  900  having a proximal portion  900   a  that exhibits a circular cross-section, an intermediate portion  900   b  that exhibits a hexagonal cross-section, and a distal portion  900   c  that exhibits a square cross-section. The light is coupled into the light pipe via an input surface  901  of the proximal portion and is coupled out of the light pipe via an output surface  902  of the distal portion  900   c . In this embodiment, the portions  900   a ,  900   b , and  900   c  are formed as an integral unit. In other embodiments, these portions can be formed as separate units and then assembled in a manner known in the art. Further, the ordering of the cross-sectional shapes can be different than those depicts in  FIG. 9 . For example, in some embodiments, the distal portion  900   c  can have a circular cross-section to impart a circular shape to the cross-section of the light beam exiting the light pipe. 
     In some embodiments, a holder employed in a system according to the teachings of the invention in which the light pipe is seated can include a plurality of legs for coupling to a plurality of openings in a printed circuit board (PCB) on which a light source, e.g., an LED, is mounted. By way of example,  FIGS. 10A and 10B  show schematically an optical system  1000  according to an embodiment of the invention that includes a holder  1001  in which the light pipe  130  is seated. The light pipe  130  is optically coupled to a light source  110  for receiving light therefrom. Similar to the previous embodiments, the optical system  1000  further includes a zoom lens  1004  that receives light from an output surface of the light pipe and projects the received light onto a target surface. The holder includes two legs  1005  and  1006  at its proximal end. The legs  1005  and  1006  include, respectively, tips  1005   a  and  1006   a  that are configured for insertion into openings  1100   a  and  1100   b  of a PCB  1100 , shown schematically in  FIG. 10C . The legs advantageously allow optical alignment of the light pipe  130  seated in the holder with the light source  110 , e.g., an LED, disposed on the PCB  1100 . 
     In some embodiments, the zoom lens in a system according to the invention can include a baffle for preventing certain light rays exiting the output surface of the light pipe, e.g., those light rays that miss the zoom lens, from reaching the external environment. For example,  FIGS. 11A and 11B  are partial schematic views of an embodiment according to the teachings of the invention in which the zoom lens  140  includes a baffle  141 . As shown in  FIG. 11A , when the zoom lens is moved away from the output surface  136  of the light pipe  130  to be at the distal location, some of the light rays, e.g., the exemplary light ray A, that leave the output surface of the light pipe at relatively large divergence angles, are not incident on the lens, i.e., they miss the lens. The baffle captures these light rays and prevents them from reaching the external environment. In some embodiments, an inner surface  141   a  of the baffle is black and/or is covered with light absorbing material to absorb the light rays incident thereon, or at least a substantial portion of those light rays. As shown in  FIGS. 11A and 11B , as well as  FIG. 10A , distal flange  123  of the holder  1001  is offset relative to the output surface  136  of the light pipe  130  (it is set back from the output surface) so as not to interfere with the baffle  141  as the zoom lens is moved closer to the output surface, e.g., when the zoom lens is positioned at its proximal location. Such offset of the flange can also be useful in certain embodiments in which the zoom lens includes a concave surface facing the output surface of the light pipe, rather than a convex surface shown in the above embodiments, in that it can provide room for the peripheral portions of the lens as the lens is moved closer to the light pipe. 
     With reference now to  FIG. 12 , an exemplary lighting system  600  in accord with various aspects of applicants&#39; teachings is schematically depicted. As shown in  FIG. 12 , the lighting system  600  can comprise multiple lighting modules  100   a - c , as generally described above with reference to  FIGS. 1A-5B , and can be arranged such that their individual output light distributions substantially overlap on the target surface. In various embodiments, the lighting module  100   a - c  can contain a different light source generating light of a different wavelength such that the lighting system  600  can be effective to mix different colored light in the combined output light distribution. 
     In some exemplary embodiments, each of the lighting modules  100   a - c  can be rotated slightly on its axis relative to one or more of the other lighting modules  100   a - c . In embodiments in which the light pipes  130  have a non-rotationally symmetric cross-section, rotation of one or more of the lighting modules  100   a - c  can be effective to change the total output light distribution on the target surface, and reduce, for example, the final appearance of the output light distribution having a shape corresponding to that of the cross-section of the light pipe. 
     By way of example, in some implementations, the lighting system  600  can include 30 light modules with the light pipes and/or the light sources (e.g., the LEDs) of the modules rotated about their longitudinal axes to varying degrees relative to one another. For example, in some cases each of the LEDs can be progressively rotated by 30 degrees (i.e., thirty positions from 0 degree to 357 degrees). 
     Any of the lenses and or light pipes described above can be made of polymethyl methacrylate (PMMA), PMMI, glass, polycarbonate, cyclic olefin copolymer and cyclic olefin polymer, or any other suitable material. By way of example, the zoom lens can be formed by injection molding, by mechanically cutting a reflector or lens from a block of source material and/or polishing it, by forming a sheet of metal over a spinning mandrel, by pressing a sheet of metal between tooling die representing the final surface geometry including any local facet detail, and so on. Reflective surfaces can be created by a vacuum metallization process which deposits a reflective metallic (e.g., aluminum) coating, by using highly reflective metal substrates via spinning or forming processes. Faceting on reflective surfaces can be created by injection molding, by mechanically cutting a reflector or lens from a block of source material and/or polishing it, by pressing a sheet of metal between tooling die representing the final surface geometry including any local facet detail, and so on. 
     In some embodiments, the light pipe can be formed of silicone, which can provide thermal resistance to the heat generated by the light source. In some such embodiments, the light pipe made of silicone is seated within a holder such that the holder ensures that the light pipe would retain its shape even as its temperature rises due to heat generated by the light source. In some such embodiments, the peripheral surface of the light pipe is metalized and/or an inner surface of a holder in which the light pipe is seated is metalized to provide specular reflection of the light incident on the peripheral surface of the light pipe. 
     Any appended claims are incorporated by reference herein and are considered to represent part of the disclosure and detailed description of this patent application. Moreover, it should be understood that the features illustrated or described in connection with any exemplary embodiment may be combined with the features of any other embodiments. Such modifications and variations are intended to be within the scope of the present patent application.