APPARATUS FOR COLLECTING SUNLIGHT

Abstract: An apparatus for collecting sunlight comprising a con­ vergent lens system for receiving and concentrating arriving sunlight and a light defocusing rod for collimating the light from the lens system. The light defocusing rod has a gradient refractive index which increases from the central axis toward the periphery thereof so that the incident convergent light rays are substantially collimated and introduced into an optical fiber cable.

Description: APPARATUS FOR COLLECTING SUNLIGHT FIELD OF THE INVENTION This invention relates to a solar energy collector, more particularly to an apparatus for collecting sunlight for concentration and introduction into an optical conductor for transmission therealong. BACKGROUND OF THE INVENTION Hitherto, various attempts have been made to utilize the solar energy reaching the earth's surface. Most of the prior art systems involve the transformation of the captured solar energy into electric or thermal energy. There have been very few that directly utilize the light of solar radiation as a source of optical energy. For lighting applications, however, the direct use of solar energy in the form of light is preferable as there is no transformation loss. Sunlight can be directly used by collecting it on the earth's surface where the sunlight arrives, concentrating the sunlight into a bundle of dense light by means of a convergent lens system, introducing it into an optical conductor or optical fiber cable for transmission therealong, then allowing it to issue out of the exit of the optical conductor or optical fiber cable to illuminate the region where illumination is desired. The problem with such sunlight collectors is that it is difficult to substantially collimate the collected and concentrated sunlight. The reason why it is necessary to substantially collimate the light is that the sunlight introduced into the optical fiber cable propagates therealong by repeated reflection on the boundary surface between the core and the cladding. The light that impinges on the boundary surface at an angle of incidence larger than the critical angle of incidence is not reflected back to the core, but is transmitted toward the cladding to leak out of the cable. Accordingly, it is essential for a sunlight collector that the outgoing light be substantially collimated. This is particularly true in the case where a tubular light guide with or without inner mirror surface is employed as a light conductor. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an apparatus for collecting sunlight which is capable of effectively collimating the collected and concentrated light. Another object of the present invention is to provide an apparatus for collecting sunlight which is simple in construction and is easy to manufacture. In one embodiment of the invention, there is provided an apparatus for collecting sunlight which comprises a convergent lens system, for receiving and concentrating sunlight arriving thereon, and a light defocusing rod aligned with said convergent lens system for collimating sunlight entered therein, said light defocusing rod having a gradient refractive index which increases from the central axis toward the periphery thereof, said convergent lens system being arranged with respect to said light defocusing rod such that the focus thereof is located within said light defocusing rod. With this arrangement, the sunlight collected and condensed by the convergent lens system is substantially collimated so that it is introduced without loss into an optical fiber cable or light guide. This enables the use of an optical fiber cable having a smaller numerical aperture. In another embodiment of the invention, there is provided an apparatus for collecting sunlight which comprises a convergent lens system for receiving and concentrating sunlight arriving thereon, a light defocusing rod aligned with said convergent lens system for collimating sunlight entered therein, said light defocusing rod having a gradient refractive index which increases from the central axis toward the periphery thereof, said convergent lens system being arranged with respect to said light defocusing rod such that the focus thereof is located within said light defocusing rod, and an optical conductor aligned with said light defocusing rod at the opposite side of said convergent lens system for receiving the concentrated and collimated sunlight from said light defocusing rod for transmission therealong. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of the apparatus according to the invention; and Fig. 2 is a graph showing the distribution of the refractive index of the light defocusing rod of the apparatus of Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, there is shown an embodiment of the apparatus according to the invention. The apparatus includes convergent lens 1 and light defocusing rod 2 mounted on a suitable frame (not shown), in alignment with one another. Optical fiber cable 3 is positioned at the opposite side of convergent lens 1 in such a manner that the inlet thereof is located adjacent to and in alignment with light defocusing rod 2. Light defocusing rod 2 has a gradient refractive index which increases from the central axis toward the periphery along a parabolic curve, as shown in Fig. 2. Such light defocusing rod with parabolic gradients of refractive index may be prepared by the two-step copolymerization of diethylene glycol bis (allyl carbonate) with phenyl methacrylate or vinyl benzoate (see Appl. Phys. Lett., 23, 247(1973); Appl. Opt., 16, 1305(1977); and, Appl. Opt., June 1, 1981). As illustrated in Fig. 1, the relative position of light defocusing rod 2 and convergent lens 1 is such that focus F of the lens is located within light defocusing rod 2. In use, sunlight passed through convergent lens 1 is converged toward focus F. On entering light defocusing rod 2, the convergent light undergoes refraction to swerve to become a pencil of collimated light which transmits toward optical fiber cable 3 along the central axis of light defocusing rod 2, as shown by the arrows. The light defocusing rod and optical fiber cable may be selected according to the following information: Assuming the diameter of light defocusing rod 2 to be d mm, the diameter of the bundle of parallel light rays to be d -0 the length of light defocusing rod 2 to be z mm, the refractive index at the central portion of light defocusing rod 2 to be n , and the refractive-index distribution 0 constant to be b, the refractive index nr of light defocusing rod 2 at the peripheral portion distant by r from the central axis is determined by the following equation: nr = no (1 + ¸ b r2) (1) Assuming b = 0.003, d = 10.2 mm, and no = 1.5: : n(r = 5.1) = 1,5(1 + ¸ x 0.003 x 5.12) = 1.559 (2) This means that the refractive index at the peripheral portion of the light defocusing rod is 1.559. Further, assuming the distance between focus F and the inlet surface of the rod to be Q1 t the reduction ratio y(do/d) may be expressed as follows: y = no #b x #1 sin h (#b x z) + cos h (#b x z) (3) Actual use of a light defocusing rod having refractive indices of no = 1.5 and nr = 1.559, a diameter d of 10.2 mm, and a length z of 29.9 mm showed it was possible to obtain a pencil of parallel light having a diameter do of 1.734 mm. Accordingly, by substituting the value 0.17 for the reduction ratio y(d0/d) in equation ( EMI4.1 To find the value of Q1 from equation (4), #1 is -8.658. That the resultant value is negative is due to the fact that the distance Ql 1 was measured in the direction of the path of the light. Thus, the actual position of the image is 8.658 mm from the inlet surface of the light defocusing rod. Accordingly, if optical fiber cable 3 having a diameter equal to or slightly greater than do mm is connected to the exit of light defocusing rod 2, it is possible to effectively introduce light rays into optical fiber cable 3.

Claims: CLAIMS 1. An apparatus for collecting sunlight, which comprises: a convergent lens system for receiving and concentrating sunlight arriving thereon; and a light defocusing rod aligned with said convergent lens system for collimating sunlight entered therein; said light defocusing rod having a gradient refractive index which increases from the central axis toward the periphery thereof; said convergent lens system being arranged with respect to said light defocusing rod such that the focus thereof is located within said light defocusing rod. 2. An apparatus for collecting sunlight, which comprises: a convergent lens system for receiving and concentrating sunlight arriving thereon; a light defocusing rod aligned with said lens system for collimating sunlight entered therein; said light defocusing rod having a gradient refractive index which increases from the central axis toward the periphery thereof; said convergent lens system being arranged with respect to said light defocusing rod such that the focus thereof is located within said light defocusing rod; and a light conductor aligned with said light defocusing rod at the opposite side of said convergent lens system for receiving concentrated and collimated sunlight from said light defocusing rod for transmission therealong.

Inventors: MORI, KEI
Applicants: MORI, KEI
Application year: 1982
Main classification: G02B6
IPC codes: F24J2, G02B6, G02B27