Abstract:
Apparatus for concentrating light rays arriving from at least one external source onto a receiver, individual beams of the light rays each arriving at the apparatus substantially collimated, the apparatus including a respective Fresnel lens assembly for each of a plurality of openings, the Fresnel lens assembly including a first Fresnel lens, and a second Fresnel lens, the first Fresnel lens being located between a respective one of the openings and the receiver, the second Fresnel lens being located between the first Fresnel lens and the receiver, the first Fresnel lens for making the light rays arriving from the respective one of the openings parallel with an optical axis of the first Fresnel lens, the second Fresnel lens converging the collimated light rays onto the receiver, each opening being located in front of the Fresnel lens assembly, on the focal plane of the first Fresnel lens, centered on the focal point of the first Fresnel lens, and the receiver being located behind the Fresnel lens assembly, on the focal point of the second Fresnel lens.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of copending application U.S. Ser. No. 11/667,639, filed on May 11, 2007, which is a National Phase of International Application No. PCT/IL2005/001182 with an International Filing Date of Nov. 10, 2005, which claims priority to Israel Application No. 165168, filed on Nov. 11, 2004, where the above applications are incorporated by reference herein in their entireties. 
    
    
     FIELD OF THE DISCLOSED TECHNIQUE 
     The disclosed technique relates to optics in general, and to methods and systems for concentrating light, in particular. 
     BACKGROUND OF THE DISCLOSED TECHNIQUE 
     The need to concentrate light is relevant in many technological applications. One such application is concentrating incoming light rays from a light source onto a photodetector. Such a photodetector may be detecting the wavelength of incoming light rays, its intensity, or various other properties or signals carried by incoming light rays. Photodetectors come in various sizes with regards to the size of their active detecting surface. Their price increases significantly with an increase in their active detecting surface. A photodetector, with an active detector surface in the square centimeter range, can cost many times the amount of a similar photodetector, with an active detector surface in the square millimeter range. It is therefore cost effective to be able to concentrate incoming light onto ever smaller photodetectors. Currently, optical systems exist which can achieve the goal of concentrating incoming light rays onto small photodetectors. The optical systems currently used to concentrate incoming light rays are usually either large, bulky or not cost effective. As such, they are not well suited to be used in cases where light concentration onto small detectors is required and the physical size of such optical systems needs to be small. 
     U.S. Pat. No. 5,604,607 issued to Mirzaoff, and entitled “Light concentrator system” is directed to a system for collecting and concentrating electromagnetic radiation. The system includes a photosensitive medium for capturing light and a planar array located proximate to the photosensitive medium for guiding the light into the photosensitive medium. The planar array includes a plurality of concentrator elements. The concentrator elements each have a circular input and a circular output opening, and a hyperbolic cross section. Between each input and output opening is a reflective inner wall. The reflective inner wall functions to guide and concentrate radiant energy or light impinging upon an input opening through the concentrator element to an output opening towards the photosensitive medium. 
     U.S. Pat. No. 6,541,694 issued to Winston et al., and entitled “Nonimaging light concentrator with uniform irradiance” is directed to a nonimaging light concentrator system and nonimaging optical mixer designs that produce uniform flux for use with photovoltaic dish concentrators. The system includes a primary collector of light, such as a reflector dish, for producing highly concentrated light flux. The system further includes an optical mixer located near the focal zone of the primary collector of light. The optical mixer includes a transparent entrance aperture and a transparent exit aperture. The optical mixer further includes an internally reflective housing for substantially total internal reflection of light. An array of photovoltaic cells is located near the transparent exit aperture. 
     The system works as follows. Light entering the system is collected by the primary collector of light and directed towards its focal zone. At the focal zone, the light enters an optical mixer. Light inside the optical mixer is provided to the array of photovoltaic cells by substantial total internal reflection inside the optical mixer. Substantial total internal reflection provides for uniform light flux on the array of photovoltaic cells. 
     U.S. Pat. No. 6,302,100 issued to Vandenberg, and entitled “System for collimating and concentrating direct and diffused radiation” is directed to a system and a method for collimating light energy falling randomly from a plurality of directions onto a fixed positioned thin flat surface. The system includes a collimator for collimating incident light, a lens for concentrating light collimated by the collimator, a light funnel for further concentrating light concentrated by the lens, and a receiver. The collimated light is concentrated by an assembly of converging and diverging prismatic slabs and optical means towards the light funnel. Each prism slab&#39;s longitudinal axis is parallel to the shortest side of the collector. Each diverging prism has a first side in contact with a converging prism, and a second side in contact with another converging prism. The index of refraction of the converging prisms differs from the index of refraction of the diverging prisms. 
     SUMMARY OF THE DISCLOSED TECHNIQUE 
     It is an object of the disclosed technique to provide a novel method and system for optically concentrating light rays using two Fresnel lenses. 
     In accordance with the disclosed technique, there is thus provided an apparatus for concentrating light rays arriving from at least one opening onto a receiver, wherein the individual beams of the light rays each arrive at the apparatus substantially collimated. The apparatus includes a respective Fresnel lens assembly for each opening. Each Fresnel lens assembly includes a first Fresnel lens and a second Fresnel lens. The first Fresnel lens is located between the opening and the receiver. The second Fresnel lens is located between the first Fresnel lens and the receiver. The first Fresnel lens collimates the light rays arriving from the opening, and the second Fresnel lens converges the collimated light rays onto the receiver. The opening is located in front of the Fresnel lens assembly, on the focal plane of the first Fresnel lens, centered on the focal point of the first Fresnel lens. The receiver is located behind the Fresnel lens assembly, on the focal point of the second Fresnel lens. 
     According to another aspect of the disclosed technique, there is thus provided an apparatus for concentrating light rays arriving from at least one opening onto a receiver, wherein the individual beams of the light rays each arrive at the apparatus substantially collimated. The apparatus includes a respective lens assembly for each opening. Each lens assembly includes a first lens and a second lens. The first lens is located between the opening and the receiver. The second lens is located between the first lens and the receiver. The first lens collimates the light rays arriving from the opening. The second lens converges the collimated light rays onto the receiver. The opening is located in front of the lens assembly, on the focal plane of the first lens, centered on the focal point of the first lens. The receiver is located behind the lens assembly, on the focal point of the second lens. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosed technique will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: 
         FIG. 1A  is a schematic diagram of the prior art illustrating the collimating properties of convex lenses and Fresnel lenses. 
         FIG. 1B  is a schematic diagram illustrating the converging properties of convex and Fresnel lenses; 
         FIG. 2  is a schematic diagram of a system, constructed and operative in accordance with an embodiment of the disclosed technique; and 
         FIG. 3  is a schematic diagram of another system, constructed and operative in accordance with an embodiment of the disclosed technique. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The disclosed technique overcomes the disadvantages of the prior art by providing an optical concentrator comprising two Fresnel lenses. One Fresnel lens is used to collimate incoming light rays, whereas the other Fresnel lens is used to concentrate the incoming light rays onto a small photodetector. The disclosed technique provides a product which is lightweight, compact, and cost effective. 
     Fresnel lenses produce the same optical effects of conventional lenses, be it concentrating light, collimating light, or dispersing light, for a fraction of the weight, volume and width of conventional lenses. This is due to their unique design. Reference is now made to  FIG. 1A , which is a schematic diagram illustrating the collimating properties of a convex lens and a Fresnel lens. Convex lens  20  has a property whereby light rays  21  passing through its surface are collimated provided such light rays pass through point  24 , an imaginary point known as the focal point. In fact, any light ray passing through line  26 , an imaginary line known as the focal plane, will be collimated by convex lens  20 . This property is due to the refractive nature of light rays passing through different media. The distance from focal plane  26  to convex lens  20  is known as the focal length and is specific for the particular curvature of convex lens  20 . Fresnel lens  22 , just like convex lens  20 , has the same property whereby light rays  21  passing through its surface are collimated, provided such light rays pass through its focal point  28  or its focal plane  30 . 
     Reference is now made to  FIG. 1B , which is a schematic diagram illustrating the converging properties of a convex lens and a Fresnel lens. Convex lens  20  has a property whereby collimated light  32  passing through its surface can be converged onto its focal point  24 . This property is due to the refractive nature of light rays passing through different media. Fresnel lens  22 , just like convex lens  20 , has the same property whereby collimated light  32  passing through its surface is converged onto its focal point  28 . 
     Reference is now made to  FIG. 2 , which is a schematic diagram of a system, generally referenced  100 , constructed and operative in accordance with an embodiment of the disclosed technique. System  100  includes opening  102 , first Fresnel lens  104 , second Fresnel lens  106 , and photodetector  108 . System  100  may be encapsulated in closed structure  114  to prevent ambient light or other external light sources from entering it. Opening  102  is located on the focal plane of first Fresnel lens  104 , centered along optical axis  112  of first Fresnel lens  104  and second Fresnel lens  106 . Opening  102  is located in front of first Fresnel lens  104 . Optical axis  112  passes through the focal points of both Fresnel lenses. Opening  102  can vary in size. If opening  102  is larger than the diameter of Fresnel lenses  104  and  106 , some of the light entering system  100 , via opening  102 , may not be concentrated on photodetector  108 . Side  114  of first Fresnel lens  104  is ridged, whereas side  116  of first Fresnel lens  104  is flat. Fresnel lenses can be referred to as ridged lenses as their unique design gives them the appearance of having ridges. First Fresnel lens  104  has its ridged side  114  facing opening  102 . First Fresnel lens  104  and second Fresnel lens  106  are substantially similar in size. Second Fresnel lens  106  is located behind first Fresnel lens  104 , centered on optical axis  112  such that optical axis  112  passes through its focal point. Side  120  of second Fresnel lens  106  is ridged, whereas side  118  of second Fresnel lens  106  is flat. Second Fresnel lens  106  has its flat side  118  facing opening  102 . In an embodiment of the disclosed technique, second Fresnel lens  106  and first Fresnel lens  104  are fit together by joining side  116  of first Fresnel lens  104  to side  118  of second Fresnel lens  106 . In another embodiment of the disclosed technique, first Fresnel lens  104  and second Fresnel lens  106  are located a distance apart from one another. In a further embodiment of the disclosed technique, first Fresnel lens  104  can have its flat side  116  facing opening  102 . In another embodiment of the disclosed technique second Fresnel lens  106  can have its ridged side  120  facing opening  102 . Photodetector  108  is located on the focal plane of second Fresnel lens  106 , centered on optical axis  112 . Photodetector  108  is located behind second Fresnel lens  106 . This means that photodetector  108  is located on the focal point of second Fresnel lens  106 . Photodetector  108  is significantly smaller than the size of opening  102 . 
     System  100  works in the following generalized manner. Light rays  110 A arrive at system  100  from multiple directions. Incoming, light rays  110 A, to be concentrated on photodetector  108 , pass through opening  102 . Since the size of opening  102  is, in general, much smaller than the distance between system  100  and the source of light rays  110 A, each individual beam of light passing through opening  102  will pass there through as a substantially collimated beam of light. Opening  102  is the only way for incoming light rays to fall incident on Fresnel lenses  104  and  106  because system  100  may be completely encapsulated within closed structure  114 . Since opening  102  is smaller than or equal to the diameter of Fresnel lenses  104  and  106 , and since opening  102  lies on the focal plane of first Fresnel lens  104 , any light rays passing through opening  102  and falling incident on first Fresnel lens  104  will become substantially collimated, or substantially parallel with one another, after passing through first Fresnel lens  104 . Once incoming light rays  110 A pass through first Fresnel lens  104 , they emerges as substantially collimated light rays  110 B due to the specific location of opening  102  vis-à-vis first Fresnel lens  104 . Substantially collimated light rays  110 B then fall incident on second Fresnel lens  106 . Since the light rays falling incident on second Fresnel lens  106  are substantially collimated, they will pass through second Fresnel lens  106  and then converge onto the focal point of second Fresnel lens  106 . Since photodetector  108  is located on the focal point of second Fresnel lens  106 , convergent light rays  110 C exiting second Fresnel lens  106  will be concentrated on photodetector  108 . 
     In general, there is no restriction on the distance between first Fresnel lens  104  and second Fresnel lens  106 , although due to the ridged nature of first Fresnel lens  104 , some of substantially collimated light rays  1108  may disperse slightly once they pass through first Fresnel lens  104 . The amount of dispersion, in terms of distance from the ends of second Fresnel lens  106 , depends on the distance between the two Fresnel lenses. As the distance between the two Fresnel lenses increases, the amount of dispersion, in terms of distance from the ends of second Fresnel lens  106 , also increases. If the amount of dispersion is significant, then the diameter of second Fresnel lens  106  needs to be increased accordingly to converge the dispersed rays that pass through first Fresnel lens  104  onto photodetector  108 . 
     It is noted that first Fresnel lens  104  and second Fresnel lens  106  can each be replaced by a cylindrical Fresnel lens. It is also noted that first Fresnel lens  104  and second Fresnel lens  106  can be replaced by a double-sided Fresnel lens. It is further noted that first Fresnel lens  104  and second Fresnel lens  106  can each be replaced by a spherical lens, a cylindrical lens, or a regular lens. 
     Reference is now made to  FIG. 3 , which is a schematic diagram of another system, generally referenced  140 , constructed and operative in accordance with an embodiment of the disclosed technique. System  140  includes three Fresnel lens sets which are each substantially similar to system  100  ( FIG. 2 ). System  140  includes openings  142   A ,  142   B  and  142   C , first Fresnel lenses  144   A ,  144   B  and  144   C , second Fresnel lenses  146   A ,  146   B  and  146   C , and photodetector  148 . One side of each of first Fresnel lenses  144   A ,  144   B  and  144   C  is ridged, whereas the other side of each of first Fresnel lenses  144   A ,  144   B  and  144   C  is flat. One side of each of second Fresnel lenses  146   A ,  146   B  and  146   C  is ridged, whereas the other side of each of second Fresnel lenses  146   A ,  146   B  and  146   C  is flat. First Fresnel lenses  144   A ,  144   B  and  144   C , and second Fresnel lenses  146   A ,  146   B  and  146   C , are substantially similar in size. 
     Opening  142   A  is located on the focal plane of first Fresnel lens  144   A , centered along optical axis  152   A  of first Fresnel lens  144   A  and second Fresnel lens  146   A . Opening  142   A  is located in front of first Fresnel lens  144   A . Optical axis  152   A  passes through the focal points of both Fresnel lenses. Second Fresnel lens  146   A  is located behind first Fresnel lens  144   A , centered on optical axis  152   A  such that optical axis  152   A  passes through its focal point. Opening  142   B  is located on the focal plane of first Fresnel lens  144   B , centered along optical axis  152   B  of first Fresnel lens  144   B  and second Fresnel lens  146   B . Opening  142   B  is located in front of first Fresnel lens  144   B . Optical axis  152   B  passes through the focal points of both Fresnel lenses. Second Fresnel lens  146   B  is located behind first Fresnel lens  144   B , centered on optical axis  152   B  such that optical axis to  152   B  passes through its focal point. Opening  142   C  is located on the focal plane of first Fresnel lens  144   C , centered along optical axis  152   C  of first Fresnel lens  144   C  and second Fresnel lens  146   C . Opening  142   C  is located in front of first Fresnel lens  144   C . Optical axis  152   C  passes through the focal points of both Fresnel lenses. Second Fresnel lens  146   C  is located behind first Fresnel lens  144   C , centered on optical axis  152   C  such that optical axis  152   C  passes through its focal point. 
     Photodetector  148  is located on the intersection of the focal planes of second Fresnel lenses  146   A ,  146   B  and  146   C . Photodetector  148  is located behind second Fresnel lenses  146   A ,  146   B  and  146   C . This means that photodetector  148  is located on the focal points of second Fresnel lenses  146   A ,  146   B  and  146   C . Second Fresnel lenses  146   A ,  146   B  and  146   C  are configured such that their respective focal points all coincide. Photodetector  148  is significantly smaller than the size of openings  142   A ,  142   B  and  142   C . Openings  142   A ,  142   B  and  142   C  can vary in size. If the openings are larger than the diameter of the Fresnel lenses, some of the light entering system  140 , via opening  142   A ,  142   B  and  142   C  may not be concentrated on photodetector  148 . In an embodiment of the disclosed technique, second Fresnel lenses  146   A ,  146   B  and  146   C  are each respectively fit together to first Fresnel lenses  144   A ,  144   B  and  144   C  by respectively joining the flat side of each of first Fresnel lenses  144   A ,  144   B  and  144   C  to the respective flat side of each of second Fresnel lenses  146   A ,  146   B  and  146   C . 
     Each Fresnel lens set and opening (for example opening  142   A , first Fresnel lens  144   A  and second Fresnel lens  146   A ) is configured to receive light rays, for example light rays  150   A ,  150   B  and  150   C , coming from different directions. It is noted that each individual beam of light, in each of light rays  150   A ,  150   B  and  150   C , arrives at system  140  substantially collimated. Each individual beam of light arrives at system  140  substantially collimated since the size of openings  142   A ,  142   B  and  142   C  are, in general, much smaller than the distance between system  140  and the source of light rays  150   A ,  150   B  and  150   C . The location and orientation of each Fresnel lens set and opening, with respect to one another, is such that photodetector  148  is simultaneously located at the focal point of each second Fresnel lens of each Fresnel lens set. In another embodiment of the disclosed, a plurality of Fresnel lens sets and openings are configured to receive light rays coming from different directions. The location and orientation of each Fresnel lens set and opening, with respect to one another, is such that photodetector  148  is simultaneously located at the focal point of each second Fresnel lens of each Fresnel lens set. System  140  allows light rays coming from a plurality of directions to be concentrated onto a single photodetector. 
     It will be appreciated by persons skilled in the art that the disclosed technique is not limited to what has been particularly shown and described hereinabove. Rather the scope of the disclosed technique is defined only by the claims, which follow.