Abstract:
A method and system for condensing and collecting electromagnetic radiation onto a target surface comprised generally of a radiation source, a primary reflector and a retro-reflector having a shape complementary to the shape of the primary reflector is disclosed. The primary reflector has a reflecting surface for reflecting the radiation from the source which is substantially concave in shape. The radiation source emits substantially uniform radiation flux in substantially all directions which is collected by the primary reflector and redirected toward the target surface. The retro-reflector, having a complementary shape which depends upon the shape of the primary reflector, is positioned so as to intercept a portion of the radiation redirected toward the target surface. The retro-reflector reflects the intercepted portion of the radiation back toward said primary reflector along the same path such that the redirected radiation is channeled back through the source. In such a manner, flux density at the target surface is improved.

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods and systems for increasing the flux density of light exiting a source of electromagnetic radiation by retro-reflection. 
     BACKGROUND OF THE INVENTION 
     One of the major goals when collecting and condensing radiation, particularly visible light, from a source onto a target surface is the maximization of the flux density, or brightness, of the light at the target surface. Various configurations using on-axis elliptical and parabolic reflectors, and off-axis reflectors of various shapes have been used. Since the brightness of the image created at the target theoretically only can be conserved in an ideal optical system (and is reduced in a non-ideal system) it is impossible to increase the total flux at the target above the amount which is emitted by the source. 
     Specifically in the area of optical condensing and collecting systems which use reflectors, the fundamental system, exemplified by FIG. 1 a , is comprised of a primary reflector  2  having a generally concave shape. Concave reflectors having a variety of shapes are known in the art, including spherical, paraboloidal, ellipsoidal, and torroidal reflectors. FIG. 1 a  specifically depicts a common ellipsoidal shaped concave reflector  2  which has two focal points  4  and  5 . In such an ellipsoidal system, typically the source  1  of radiation will be placed near one focus  4 , and the target surface  3 , typically the input end of an optical fiber, homogenizer, or lens, is located near the other focus  5 . One of the natural reflecting properties of an ellipsoidal shaped reflector is that light emitted at one of its foci will be collected and focused onto its other focus. 
     A technique commonly used by the prior art to combat the fundamental limitation that the total flux at the target surface must be at most equal to the flux emitted by the source is the use of an arc lamp as the source in combination with a retro-reflector. This combination takes the light emitted from one side of the arc lamp and redirects it with the retro-reflector back through the arc of the lamp. Since the absorption of the reflected light by the arc is very small, light emitted from the opposite side of the arc lamp when a retro-reflector is used is comprised of both the light radiating from the arc itself as well as the retro-reflected light. Thus, the total light flux emitted from the side of the lamp opposite the retro-reflector is effectively doubled. Other prior art methods have extended this concept by reflecting light from the arc back into itself multiple times, thus increasing the flux further as in U.S. Pat. No. 4,957,759 to Goldenberg et al. 
     As depicted by FIG. 1 b , retro-reflectors have been commonly used in projection systems having an optical axis  17 . A spherical retro-reflector  16  is placed behind the source  11 , typically an arc lamp, with the arc  11   a  placed at the center of curvature  19  of the spherical retro-reflector  16 . This orientation causes the light collected at the back of the source  11  to be imaged back through the arc  11   a  itself and be collected by condensing optics  18 , such as lenses, at the front of the system. Such a retro-reflector  16  would effectively double the brightness being delivered to the condensing optics under the ideal circumstances, and in practice typically leads to around a 60% to 80% increase in flux density at the target surface  13 . 
     To improve the flux density of the light delivered by the a reflector-based condensing system such as in FIG. 1 a , a compound reflector system as shown by FIG. 2 has been developed by the prior art. Referring to FIG. 2, such a compound reflector system has on the opposite side of the source  21  from the target surface  23  an ellipsoidal primary reflector  22  which collects light from the source  21  located at a first focus  24  an reflects it toward a second focus  25 . A concave spherical retro-reflector  26 , situated with its center of curvature  29  being coincident with the first focus  24 , collects a portion of the radiation emitted by the source  21  and reflects it back through the source  21  such that its effective flux density is nearly doubled. This retro-reflected light is then collected by the ellipsoidal primary reflector  22  same as the original light and delivered to the second focus  25 , thus increasing the overall flux density at the target surface  23 . 
     FIG. 3 shows another configuration of such a compound reflector system where the concave spherical retro-reflector  36  is placed behind the source  31  and the ellipsoidal primary reflector is placed between the source and the target surface  33 . As with the compound reflector system depicted by FIG. 2, the source  31  is located near the first focus  34  of the primary reflector  32  and the center of curvature of the retro-reflector  36 , and the target surface is placed near the second focus  35 . Flux density at the target surface  33  in this case is also nearly doubled when compared to the case with no retro-reflection. 
     Although both the systems depicted by FIGS. 2 and 3 employ concave spherical retro-reflectors to increase the flux density at the target surface, the compound reflector system used in both is intricate and costly to manufacture. Furthermore, proper alignment between the lamp and the reflector is difficult. Thus, there remains a need in the art for an optimized system and method for optical condensing and collecting which increases the flux density of radiation emitted by a source toward a target surface which is simple and inexpensive to manufacture. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and system for condensing and collecting electromagnetic radiation to increase flux density at a target surface. Systems according to the present invention comprise a source of radiation emitting substantially uniform radiation flux, such as an arc lamp, a primary reflector having a substantially concave shaped reflective surface, a focal point, and an optical axis, and a retro-reflector having a non-concave shaped reflective surface. According to the present invention, the substantially concave shape of the primary reflector and the non-concave shape of the retro-reflector are chosen such that the two shapes are complementary. That is, the shapes of the primary reflector and retro-reflector are such that light directed from the primary reflector will intersect the non-concave reflective surface of the retro-reflector at a right angle such that the light hitting the retro-reflector will be returned to the source substantially along its original path. 
     The method of the present invention comprises the steps of emitting radiation from a source, collecting the radiation with a substantially concave shaped primary reflector and redirecting the emitted radiation in at least two portions toward a target surface, such as the input end of a fiber optic, a field homogenizer, or a lens. The method further comprises reflecting at least one of the portions of radiation redirected by the primary reflector substantially back along its original path and through the source using a non-concave retro-reflector which is shaped complementary to the primary reflector. 
     The present invention overcomes the disadvantages and drawbacks present in the prior art in that it efficiently condenses a light onto a target surface with high flux density without the need for costly and complicated compound reflectors or condensing lenses. The above and other advantages, features and aspects of the invention will be more readily perceived from the following description of the preferred embodiments thereof taken together with the accompanying drawings and claims. The present invention is illustrated by way of example and not limitation in the drawings, in which like reference numerals indicate like parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a schematic diagram of a traditional on-axis ellipsoidal reflector condensing and collecting system as is known in the art. 
     FIG. 1 b  is a schematic diagram of a condensing and collecting system employing lenses and a spherical retro-reflector as is known in the art. 
     FIG. 2 is a schematic diagram of a first embodiment of an on-axis compound reflector condensing and collecting system as is known in the art. 
     FIG. 3 is a schematic diagram of a second embodiment of an on-axis compound reflector condensing and collecting system as is known in the art. 
     FIG. 4 is a schematic diagram of a first embodiment of the present invention wherein a substantially concave ellipsoidal primary reflector is paired with a complementary convex spherical retro-reflector. 
     FIG. 4 a  is a schematic diagram depicting the retro-reflector of FIG. 4 as seen along the optical axis of the primary reflector to further illustrate its ring shape in that embodiment of the present invention. 
     FIG. 5 is a schematic diagram of a second embodiment of the present invention wherein a substantially concave ellipsoidal primary reflector is paired with a complementary convex spherical retro-reflector. 
     FIG. 5 a  is a schematic diagram depicting the retro-reflector of FIG. 5 as seen along the optical axis of the primary reflector to further illustrate its disk shape in that embodiment of the present invention. 
     FIG. 6 is a schematic diagram of a third embodiment of the present invention wherein a substantially concave ellipsoidal primary reflector is paired with a complementary convex spherical retro-reflector. 
     FIG. 6 a  is a schematic diagram depicting the retro-reflector of FIG. 6 as seen along the optical axis of the primary reflector to further illustrate its partial disk shape in that embodiment of the present invention. 
     FIG. 7 is a schematic diagram of an fourth embodiment of the present invention wherein a substantially concave paraboloidal primary reflector is paired with a complementary planar retro-reflector. 
     FIG. 7 a  is a schematic diagram depicting the retro-reflector of FIG. 7 as seen along the optical axis of the primary reflector to further illustrate its ring shape in that embodiment of the present invention. 
     FIG. 8 is a schematic diagram of a fifth embodiment of the present invention wherein a substantially concave paraboloidal primary reflector is paired with a complementary planar retro-reflector. 
     FIG. 8 a  is a schematic diagram depicting the retro-reflector of FIG. 8 as seen along the optical axis of the primary reflector to further illustrate its disk shape in that embodiment of the present invention. 
     FIG. 9 is a schematic diagram of a sixth embodiment of the present invention wherein a substantially concave paraboloidal primary reflector is paired with a complementary planar retro-reflector. 
     FIG. 9 a  is a schematic diagram depicting the retro-reflector of FIG. 9 as seen along the optical axis of the primary reflector to further illustrate its partial disk shape in that embodiment of the present invention. 
     FIGS. 10 a - 10   f  are schematic views of a plurality of polygonal lightguide (waveguide) targets in cross-sections which may be employed in embodiments of the present invention. 
     FIG. 11 is a schematic view of a circular cross-section lightguide target which may be utilized in the present invention. 
     FIG. 12 a  is a schematic side view illustrating an increasing taper lightguide target according to one embodiment of the invention. 
     FIG. 12 b  is a schematic side view illustrating a decreasing taper lightguide target in accordance with another embodiment. 
     FIG. 13 is a schematic cross-section of a hollow tube lightguide homogenizer which may be utilized in the present invention. 
     FIG. 14 is a schematic side view of an embodiment which may be utilized with the invention in which the target surface is an input end of a lens. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 4 and 4 a  depict a first preferred embodiment of the present invention comprised of a substantially concave primary reflector  42  having an ellipsoidal shape with symmetry along an optical axis  47 . A source  41  comprising an arc lamp is placed with its arc gap at a first focus  44  of the reflector  42  such that the axis of the lamp is collinear with the optical axis  47  of the ellipsoidal primary reflector  42 . 
     As depicted in the figure, the primary reflector  42  preferably is a section of an ellipsoid such that it collects a majority of the light emitted from the source  41  arc lamp. As will be appreciated by one skilled in the art, since the source  41  is at the first focus  44  of the ellipsoid, all the emitted light will be collected and directed to the second focus  45  of the ellipsoid. To increase the flux density of light at the target surface  43 , a convex spherical retro-reflector is placed in the path of the output from the primary reflector  42  between the first and second focus  44  and  45  such that its center of curvature  49  is located at the proximity of the second focus  45 . 
     This particular orientation of a spherical convex retro-reflector  46  and a substantially concave ellipsoidal primary reflector  42  is complementary in that all the light incident upon the reflective surface of the spherical retro-reflector  46  will be intersecting it at right angles. As a result, the light incident on the convex spherical retro-reflector  46  will be reflected 180° substantially retracing its own path back to the ellipsoidal primary reflector  42  and back into the arc itself, thus performing the retro-reflection function. In the preferred embodiment as depicted in FIGS. 4 and 4 a , an aperture  46   b  at the center of the convex spherical retro-reflector  46  is made to provide a high flux density and low numerical aperture light output to the target surface  43 . As shown in FIG. 4 a , this aperture  46   b  is made symmetric about the optical axis  47  of the primary reflector  42  to ensure that all retro-reflected light exits the aperture  46   b  such that it can be collected at the target surface  43 . As will be appreciated by one skilled in the art, this aperture  46   b  can be of any shape which is symmetrical about the optical axis  47 , but preferably the aperture  46   b  is circular as shown in FIG. 4 a  such that the retro-reflector  46  when viewed along the optical axis  47  has a ring shape. 
     FIGS. 5 and 5 a  schematically depict an alternative embodiment of the present invention which is similar to the previous embodiment in that it employs an ellipsoidal primary reflector  52  reflecting light from an arc lamp source  51 , located at a first focus  54 , onto a target surface  53 , located at a second focus  55 . Instead of using a ring shaped retro-reflector, the spherical substantially convex retro-reflector  56  having its center of curvature  59  substantially at the second focus  55  in this alternative embodiment is a solid disk shape centered at the optical axis  57  when viewed along the axis  57 , as shown in FIG. 5 a . The output is taken outside this convex spherical retro-reflector  56  with increased flux density at the target surface due to the retro-reflection. 
     FIGS. 6 and 6 a  schematically depict another alternative embodiment of the invention in which a convex spherical retro-reflector  66  is paired with a substantially concave ellipsoidal primary reflector  62 . As seen in the figures, the retro-reflector  66  is appears to have a half disk shape when viewed along the optical axis  67 . This configuration essentially divides the initial light output from the primary reflector  62  into two halves such that a first half of the light output (depicted as the bottom half in the figures) is reflected back into the primary reflector  62 . All the light delivered to the target surface  63  exits from the non-restricted (depicted as the upper half) portion of the primary reflector  62 . 
     FIGS. 7 and 7 a  schematically depict a second preferred embodiment of the present invention comprised of a substantially concave primary reflector  72  having a paraboloidal shape with symmetry along an optical axis  77 . Similar to embodiments employing ellipsoidal primary reflectors, a source  71  comprising an arc lamp is placed with its arc gap at the focus  74  of the primary reflector  72  such that the axis of the lamp is collinear with the optical axis  77  of the paraboloidal primary reflector  72 . 
     As shown in FIG. 7, a retro-reflector  76  comprising a substantially planar reflective surface is placed perpendicular to the optical axis  77  of the paraboloidal primary reflector  72 . As will be appreciated by one skilled in the art, it is a natural property of a substantially concave paraboloidal reflector that light emitted at its focus will be collimated into a plurality of rays all traveling parallel to the reflector&#39;s optical axis. Therefore, a planar retro-reflector  76  oriented perpendicular to the optical axis  77  is complementary to the paraboloidal primary reflector  72  because it will reflect all incident light from the primary reflector  72  180° thus substantially retracing its path back to the source  71 . 
     As shown by FIG. 7 a , in this preferred embodiment an aperture  76   b  is present in the center of the reflector. As with the embodiment of FIGS. 4 and 4 a , it should be appreciated that this aperture  76   b  need not be circular, but preferably is symmetric about the optical axis  77  of the primary reflector  72 . 
     FIGS. 8 and 8 a  schematically depict an alternative embodiment of the second preferred embodiment of the invention. In this alternative embodiment, the planar retro-reflector  86  is again fixed perpendicular to the optical axis  87  of the primary reflector  82  between the focus  84  and the target surface  83 . However, when viewed along the optical axis  87 , it can be seen that the retro-reflector  86  has a solid disk shape centered at the optical axis  87  as depicted in FIG. 8 a . Thus, as shown by FIG. 8, all light rays reaching the target surface  83  travel around the outside of the retro-reflector  86 . 
     FIGS. 9 and 9 a  schematically depict another alternative embodiment of the present invention using a parabolic primary reflector  92  and a planar retro-reflector  96 . Similar to the embodiment depicted by FIGS. 6 and 6 a , the planar retro-reflector  96  has a half disk shape when viewed along the optical axis  97 . In an embodiment of the invention employing this type of retro-reflector  96 , all the light delivered to the target surface  93  exits from the non-restricted portion of the primary reflector  92 . The resulting output will therefore be essentially semi-circular in cross-section and will have higher brightness than the original output without retro-reflection. 
     In all of the above embodiments, both the primary reflectors and the retro-reflectors can be made from suitable materials having a high reflectivity coating, preferably of metal. The reflectors can also be made with glass and coated with metallic coating, such as silver or aluminum, for broadband applications, or with multi-layer dielectric coatings for applications where only certain wavelengths of radiation are desired to be reflected. For example, a broadband visible coating for white light output, a blue or UV coating for curing of epoxies, or other wavelength specific coatings suitable for other particular applications. 
     Suitable sources according to embodiments of the present invention are any radiation source which emits substantially uniform high intensity radiation from a small area, such as an arc lamp. A particularly suitable arc lamp for use in embodiments of the present invention will have an arc gap which is small relative to the focal length of the primary reflector. Preferably, for fiber optic applications the, source comprises a high intensity arc lamp having an arc gap length on the order of about 1 mm to about 6 mm. Such lamps can be mercury lamps, mercury xenon lamps, xenon lamps, metal-halide lamps, HID lamps, tungsten halogen, or halogen lamps, or arc lamps having various dopants such as sodium. Those of ordinary skill in the art will readily appreciate that the lamp type and power rating should be chosen based upon particular application of the present invention. 
     The target surface according to embodiments of the present invention can be any surface upon which it is desirable to provide with concentrated radiation flux. Such surfaces, for example, can be, but are not limited to, the surface of a lens, the input surface of light guides, which can be a single fiber or a fiber bundle, homogenizers, hollow internally reflective tubes and other fiber optics, light guides and combinations thereof. 
     For example, in situations where it is desired to deliver light into a fiber optic made of low temperature plastic based materials, redistributing the profile of the radiation flux at the target surface with a homogenizer before introducing it into the fiber optic will help to prevent scorching of the fiber optic material. This is especially useful in embodiments of the present invention which use ellipsoidal primary reflectors as a highly concentrated spot of light is produced at the target surface. 
     Suitable homogenizers for use in the present invention can any intensity profile or numerical aperture transforming devices which also serve as lightguides. Suitable homogenizers for use in embodiments of the present invention include tapered or untapered polygonal waveguides, tapered single core optical fibers (such as cladded rods), a fused bundle of optical fibers, or a randomized fiber bundle. 
     Suitable lightguides I can be polygonal in cross-section as shown in FIGS. 10 a - 10   f  or circular in cross-section as shown in FIG.  11 . Further, lightguide I can be an increasing taper lightguide as shown in FIG. 12 a  or a decreasing taper lightguide as shown in FIG. 12 b . Additionally, lightguide I can be a hollow tube homogenizer as shown in FIG. 13 having reflective inner walls R, or the target I can be an input end S of a lens L as shown in FIG.  14 . 
     For embodiments of the present invention wherein paraboloidal primary reflectors are employed, the output can be redirected at the target surface into such fiber optic lightguides by using focusing lenses or reflectors as is known. in the art. 
     Since reflectors having an exact ellipsoidal shape as in the embodiments depicted by FIGS. 4-6 can be expensive to make, the ellipsoidal shaped substantially concave surface can be approximated by the use of spherical mirrors, or toroidal mirrors as taught by U.S. Pat. Nos. 5,414,600, 5,430,634, and 5,757,431, the disclosures of which are herein incorporated by reference. As taught be these references, the amount of degradation in coupling of light to the target surface depends critically on the dimensions of the system (e.g., the target surface to source distance) and the relative numerical apertures of the primary reflector and target surface. 
     As will be appreciated by one skilled in the art, the distance between the source and the target surface (thus, the dimensions of the ellipsoidal primary reflector) can be chosen such that a desired amount of magnification and desired numerical aperture is obtained at the target surface. Preferably, the magnification factor and numerical aperture of the system is chosen such that the output of the light at the target matches the input characteristics of the target (such as the critical numerical aperture of a fiber optic which is placed near the target surface). 
     While the present invention has been shown and described with reference to preferred embodiments for carrying out the invention, it should be understood that various changes may be made in adapting the invention to different embodiments without departing from the broader inventive concepts disclosed herein and encompassed by the claims which follow.