A METHOD AND DEVICE FOR DISPERSING SUBSTANTIALLY COLLIMATED LIGHT ISSUING FROM A LIGHT GUIDE

Abstract: A method and device for dispersing or diffusing substan­ tially collimated light issuing from a light guide for illumina­ tion purposes. The method makes use of a light defocusing rod as a light ray dispersing means. Collimated light rays introduced into the light defocusing rod swerve by refraction toward the outer periphery of the light defocusing rod and are thus dispersed. The light rays dispersing device accord­ ing to the invention comprises a light defocusing rod provided at its lower end with a reflecting surface.

Description: A METHOD AND DEVICE FOR DISPERSING SUBSTANTIALLY COLLIMATED LIGHT ISSUING FROM A LIGHT GUIDE BACKGROUND OF THE INVENTION The present invention relates to the art of illuminating an aerial or aqueous space by using especially, but not exclusively, solar radiation as a source of light energy and by using a light guide to conduct that light energy. More particularly, this invention relates to a method and device for dispersing substantially collimated light issuing from a light guide. The use of visible light energy from solar radiation for interior lighting purposes has been proposed and various lighting systems utilizing solar radiation have been contemplated. Such lighting systems generally comprise a solar beam receiver for collecting and concentrating solar radiation and an optical conductor for conducting light energy from the receiver to the area to be illuminated. Lighting systems utilizing solar radiation as a source of light energy have acquired considerable industrial applicability because of the development of high quality optical fiber cables. The optical fiber bundle now available on the market is not only able to transmit light energy at an extremely low transmission loss, but has sufficient flexibility to enable light to be conducted along a curved complex optical path provided between the light source and the area to be illuminated. An optical fiber cable or bundle generally comprises a plurality of individual optical fibers, each consisting of a central core glass and a surrounding cladding. During transmission through the core glass, light having an angle of incidence smaller than the critical angle of incidence of the core-forming glass is not totally reflected by the interface or boundary defined between the core and cladding and, therefore, penetrates wholly or partially therethrough toward the cladding to leak out of the optical fiber. Thus, only that light having an angle of incidence larger than the critical angle of incidence of the core material is totally reflected by the boundary and, on repeated reflection, transmitted throughout the fiber. As a result, the cone angle defined by the light issuing from the exit of the optical fiber is limited. The light allowed to issue out of the end of the optical fiber cable is collimated to a considerable degree and illumination takes place only in a spotlight-like fashion. Normally, the cone angle of light issuing from an optical fiber cable does not exceed about 480 for an optical fiber cable having a numerical aperture (NA) value of about 0.4. It has also been proposed to use solar radiation for underwater lighting purposes. A solar receiver or collector is provided on the sea or water surface and an optical fiber cable is extended therefrom to the bottom of the water. The visible or nonvisible light energy conducted through the fiber cable can be used to illuminate underwater areas, for gathering fish or raising plant planktons or edible sea vegetables. When lighting an aerial space with optical fiber cable, there is a substantial refraction at the exit of the cable due to the difference between the indices of refraction of the two adjacent mediums, i.e., the core glass, which has a high index of refraction, and the air, which has a low index of refraction. However, when lighting an aqueous space with optical fiber cable, the core glass of the cable is obliged to contacts the water. The index of refraction of water is lower than that of the core glass but higher than that of air. There is thus even less of a difference between the index of refraction of the core glass and water, so the cone angle of issuing light becomes smaller even less than 480. SUMMARY OF THE INVENTION The object of the present invention is to provide a method and a device for dispersing substantially collimated light issuing from a light guide. Throughout the specification and the appended claims, the term "substantially collimated light" is intended to include not only parallel beams but also those light rays which intersect each other at an angle not larger than, for example, 450. In one aspect, the present invention provides a method for dispersing substantially collimated light issuing from a light guide so as to illuminate the surrounding medium with the dispersed light, which method comprises introducing light issuing from a light guide into a light defocusing rod through at least one end thereof, said light defocusing rod being made from a rigid transparent material and defined by a cylindrical peripheral surface and a pair of opposite end surfaces perpendicular thereto, said light defocusing rod having a parabolic gradient of refractive index which increases from the longitudinal axis toward the circumferential periphery thereof so that light introduced into said light defocusing rod is refracted at least in part toward said circumferential periphery to issue outwardly therefrom. Light defocusing rods per se are known in the field of geometrical optics. However, in the prior art there is no teaching that suggests their use for lighting. Use of light defocusing rods makes it possible to disperse collimated light in a simple manner even in an aqueous medium. In another aspect, the present invention provides an optical device for dispersing collimated light issuing from a light guide so as to illuminate the surrounding medium with the dispersed light, which device comprises a light defocusing rod made from a rigid transparent material and defined by a cylindrical peripheral surface and a pair of opposite end surfaces perpendicular thereto, said light defocusing rod having a parabolic gradient of refractive index which increases from the longitudinal axis toward the circumferential periphery thereof so that light introduced longitudinally into said light defocusing rod through one of said end surfaces is refracted at least in part toward said circumferential periphery to issue outwardly therefrom, the other of said end surfaces being provided with a reflecting surface for reflecting outwardly and backwardly light arriving at said other end surface. The reflecting surface may be convex toward the inside of the light defocusing rod. Alternatively, it may be convex toward the outside of the light defocusing rod. When the reflecting surface is convex toward the outside of the light defocusing rod, it is preferable to provide a second reflecting surface on the cylindrical peripheral surface of the rod adjacent to the other end surface so that light rays reflected by the first-mentioned reflecting surface are further reflected and dispersed by the second reflecting surface. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic side elevational view of the light defocusing rod showing the method according to the invention; Fig. 2 is a graph showing the distribution of refractive index of the light defocusing rod; and Figs. 3 through 5 are schematic cross-sectional views, partly cut away, of various embodiments of the light defocusing rod according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, optical fiber cable 12 is aligre with light defocusing rod 14 so that light emitting end iG directly faces the rod. The light receptive end (not shown) of optical fiber cable 12 is optically connected to a light source, such as a sunlight receiver or collector (not shown) so that visible and non-visible light is intro duced into the optical fiber cable and conducted therethough in a substantially collimated fashion. Light defocusing rod 14 is defined by a cylindrical peripheral surface 16 and a pair of opposite end surfaces 1 and 20 perpendicular thereto. As shown in Fig. 2, light defocusing rod 14 has a parabolic gradient of refractive index which increases from the longitudinal axis toward the cylindrical periphery thereof. As shown in Fig. 2, the refractive index at the center of light defocusing rod 14 is lower than refractive index nl of water but higher than refractive index n2 of air and gradually increases toward the periphery of the rod along a parabolic curve. Such light defocusing rods 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., 20, June 1, 1981). According to the method of the invention, the light issuing from optical fiber cable 12 through light emitting end 10 is directed to entrance end 18 of light defocusing rod 14. That light which enters light defocusing rod 14 at an angle with respect to the longitudinal axis of the rod swerves outwardly by refraction and, on doing so, is dispersed away from the longitudinal axis of light defocusing rod 14 to issue roughly uniformly from the outer periphery of the rod, as shown by the arrows in Fig. 1. That light which enters the rod roughly parallely to the longitudinal axis thereof do not undergo so much refraction and issue from lower end surface 20. Although in the foregoing embodiment, an optical fiber cable is arranged at only one end of light defocusing rod 14, it is also possible to provide another optical fiber cable in front of lower end surface 20 of the rod so that light is introduced into light defocusing rod 14 from two directions. Figure 3 illustrates a part of a light defocusing rod suitable for use in the method according to the present invention. The lower end surface of light defocusing rod 30 is made concave and is provided with reflective coating 32 so that light arriving thereon is reflected radially outwardly, as shown by the arrows. Figure 4 shows another embodiment of the light defocusing rod according to the invention. In this embodiment, the lower end surface of light defocusing rod 40 is made convex and is provided with reflective coating 42. Provision of the reflective coating results in that light which would otherwise issue through the lower end surface at small angles being reflected backwardly and outwardly. Figure 5 shows a modified form of the embodiment of Fig. 4. In this embodiment, light defocusing rod 50 is provided with first reflecting surface 52 on the convex lower end surface and second reflecting surface 54 arranged on the cylindrical periphery of the rod adjacent to first reflecting surface 52. Thus, the light reflected by first reflecting surface 52 is then reflected by second reflecting surface 54 so that it issues from the upper portion of light defocusing rod 50 as shown by the arrows.

Claims: CLAIMS 1. A method of dispersing substantially collimated light issuing from a light guide so as to illuminate a surrounding medium with the dispersed light, which comprises: introducing light issuing from a light guide into a light defocusing rod through at least one end thereof, said light defocusing rod being made from a rigid transparent material and defined by a cylindrical peripheral surface and a pair of opposite end surfaces perpendicular thereto, said light defocusing rod having a parabolic gradient of refractive index which increases from the longitudinal axis toward the circumferential periphery thereof so that light introduced into said light defocusing rod is refracted at least in part toward said circumferential periphery to issue outwardly therefrom. 2. An optical device for dispersing substantially collimated light issuing from a light guide so as to illuminate a surrounding medium with the dispersed light, which comprises: a light defocusing rod made from a rigid transparent material and defined by a cylindrical peripheral surface and a pair of opposite end surfaces perpendicular thereto, said light defocusing rod having a parabolic gradient of refractive index which increases from the longitudinal axis toward the circumferential periphery thereof so that light introduced longitudinally into said light defocusing rod through one of said end surfaces is refracted at least in part toward said circumferential periphery to issue outwardly therefrom, the other of said end surfaces being provided with a reflecting surface for reflecting outwardly and backwardly light arriving at said other end surface. 3. An optical device as claimed in claim 2, wherein said reflecting surface is convex toward the inside of the light defocusing rod. 4. An optical device as claimed in claim 2, wherein said reflecting surface is convex toward the outside of the light defocusing device. 5. An optical device as claimed in claim 4, further comprising a second reflecting surface provided on said cylindrical peripheral surface adjacent to said other end surface for reflecting light from said reflecting surface on said other end surface.

Inventors: MORI, KEI
Applicants: MORI, KEI
Application year: 1982
Main classification: G02B6
IPC codes: G02B5, G02B6, F24J2, F21V8