fig1 is a diagrammatic view of an apparatus that is an optical sight 10 , and that embodies aspects of the present invention . the sight 10 is designed to be mounted on a not - illustrated weapon , such as a rifle or pistol . a person uses the sight 10 to accurately aim the weapon . in particular , radiation from a remote scene 11 travels through the sight 10 along a path of travel 13 to the eye 12 of the person who is using the sight . the sight 10 has a housing that is represented diagrammatically in fig1 by a broken line 16 . an optical system is provided within the housing 16 , and includes an objective lens 17 , a prism assembly 18 , and an eyepiece lens 21 . when radiation from the scene 11 is traveling along the path of travel 13 , it passes successively through the objective lens 17 , prism assembly 18 , and eyepiece lens 21 . in the illustrated embodiment , the prism assembly 18 is a configuration of a known type , and includes three prisms 26 , 27 , and 28 . the prism assembly 18 includes several prism surfaces that reflect the radiation as it travels through the prism assembly 18 along the path of travel 13 . one of these surfaces is identified by reference numeral 31 in fig1 . an optical coating 32 of a known type is provided on the prism surface 31 . the coating 32 is reflective to visible radiation that is traveling along the path of travel 13 . in a known manner , the coating 32 has at least one not - illustrated opening etched through it , in the shape of a reticle . for example , the reticle may have the form of crosshairs of a known type . the sight 10 further includes a reticle illuminating portion 41 , which is represented diagrammatically in fig1 by a broken line . fig2 is a diagrammatic view showing a portion of fig1 in an enlarged scale , including a portion of the prism 28 , a portion of the coating 32 , and the internal structure of the reticle illuminating portion 41 . the reticle illuminating portion 41 includes an internal light source which , in the disclosed embodiment , is a tritium light source 51 . the tritium light source 51 is a radioluminescent device of a type known in the art . more specifically , tritium is a radioactive isotope of hydrogen , with atoms having three times the mass of ordinary light hydrogen atoms . the tritium material is provided within a capsule that is made from glass or some other suitable material , and that has a phosphor coating on its inner surface . as the tritium material decays , it emits soft beta rays that , when they strike the phosphor coating , are converted into visible light . the half life of tritium is approximately 12 . 5 years , and thus the tritium light source 51 has a usable life of more than 15 years . consequently , the tritium light source glows continuously for a long time , thereby providing a safe and reliable source of light , without any need for a power source such as a battery . the reticle illuminating portion 41 also includes two small lenses 52 and 53 that are supported at spaced locations . a beam splitter 54 of a known type is disposed optically between the lenses 52 and 53 . the beam splitter is transmissive to radiation having one wavelength or color , and is reflective to radiation at a different wavelength or color . light 56 emitted by the tritium light source 51 has a wavelength for which the beam splitter 54 is transmissive . thus , the light 56 passes through the lens 52 , and then some or all of this light then passes through the beam splitter 54 and the lens 53 in a direction toward the coating 32 , where some of this radiation then passes through the not - illustrated opening ( s ) in coating 32 that define the reticle . referring again to fig1 , a radiation collector 61 is fixedly supported on the exterior of the housing 16 . an optical fiber 62 of a known type extends from the radiation collector 61 to the reticle illuminating portion 41 . the fiber 62 has a core that is made from a material such as polystyrene , and the core is surrounded by a cladding made from a material such as a clear acrylic . due to differences in the refractive indexes of the cladding and core , most of the visible light within the core of the fiber is optically trapped there , and is successively reflected within the core in a manner that causes it to travel lengthwise within the core . ambient radiation 63 impinges on and enters the radiation collector 61 . in the disclosed embodiment , the ambient radiation 63 encompasses a relatively wide range of wavelengths , including both visible light and ultraviolet light . the radiation collector 61 internally converts non - visible light ( such as ultraviolet light ) into visible light , in a manner discussed later . some of the visible light from within the radiation collector 61 is then transmitted through the core of the optical fiber 62 to the reticle illuminating portion 41 . one end of the optical fiber 62 is visible in the lower portion of fig2 . a small lens 64 is provided between the beam splitter 54 and the illustrated end of the fiber 62 . visible light emitted from this end of the fiber 62 includes a wavelength as to which the beam splitter 54 is reflective . consequently , this light from the fiber passes through the lens 64 and travels at 66 to the beam splitter 54 , where at least part of it is reflected by the beam splitter 54 . the reflected light then passes through the lens 53 and propagates toward the coating 32 , where at least some of it serves to illuminate the reticle . fig3 is a diagrammatic perspective view of the radiation collector 61 and also an end portion of the optical fiber 62 . fig4 is a diagrammatic top view of the structure shown in fig3 . in the disclosed embodiment , the radiation collector 61 is a single integral part that is plate - like and generally rectangular . the radiation collector 61 has parallel top and bottom surfaces 81 and 82 that are polished and that are each rectangular and planar . along each of its four edges , the radiation collector 61 has two planar edge surfaces that are polished , that extend parallel to that edge , and that converge outwardly with respect to each other . two of these edge surfaces are designated by reference numerals 86 and 87 . fig5 is a diagrammatic fragmentary sectional view of the radiation collector 61 , taken along the section line 5 - 5 in fig3 . as shown in fig5 , the surfaces 86 and 87 form an angle 88 of 90 ° with respect to each other . the surface 86 forms an angle 91 of 135 ° with respect to the top surface 81 , and the edge surface 87 forms an angle 92 of 135 ° with respect to the bottom surface 82 . the edge surfaces 86 and 87 intersect each other along a line 96 that extends perpendicular to the plane of fig5 . similarly , the edge surface 86 intersects the top surface 81 along a line 97 , and the edge surface 87 intersects the bottom surface 82 along a line 98 . in the disclosed embodiment , the radiation collector 61 is a single integral part that is made of a material such as polystyrene , and that has an index of refraction different from the indexes of refraction of almost everything adjacent to the radiation collector 61 , including ambient air . due to the differing refractive indexes , if visible radiation is propagating within the radiation collector 61 and impinges on any external surface thereof at an angle greater than what is commonly called the “ critical ” angle , the visible radiation will experience total internal reflection . in this regard , as is known in the art , the critical angle is measured from an imaginary reference line that is perpendicular to the surface at the point where the radiation in question impinges on the surface . the illustrated shape of the radiation collector 61 , including the arrangement of external surfaces , is intended to ensure that visible radiation propagating within the radiation collector 61 will impinge on any external surface it may reach at an angle greater than the critical angle , and will therefore always experience total internal reflection . as a result , most of the visible radiation within the collector 61 will not be able to escape from the radiation collector 61 through any external surface thereof . in addition , the material of the radiation collector 61 is doped with a special fluorescent dye of a type known in the art . when certain wavelengths of non - visible light ( such as ultraviolet light ) enter the radiation collector 61 , the fluorescent dye absorbs that light and then emits visible light . in essence , the fluorescent dye converts the received optical energy from an initial wavelength outside the visible spectrum to a different wavelength within the visible spectrum . the material of the dye determines the wavelength and thus the color of the visible light that is emitted . the visible light produced by the fluorescent dye is then effectively trapped within the radiation collector 61 , in the manner discussed above . due to the fact that much of the visible radiation within the radiation collector 61 is not able to escape , the radiation collector 61 is relatively efficient at collecting visible radiation . in addition , to the extent that the fluorescent dye converts ultraviolet or other non - visible radiation into visible radiation , the radiation collector 61 is more efficient at collecting visible radiation than if the dye were not present . stated differently , when the dye is present , the amount of visible radiation within the radiation collector 61 will be greater than the amount of visible radiation that enters the collector from externally thereof . thus , the radiation collector 61 effectively provides a degree of gain in regard to the collection of visible radiation . as best seen in fig4 , a horizontal cylindrical opening 106 extends a short distance into the radiation collector 61 from one corner thereof . one end of the optical fiber 62 is disposed within the opening 106 , and is fixedly secured there by a commercially - available adhesive , or in any other suitable manner . visible light that is trapped within the radiation collector 61 will effectively be bouncing around therein in virtually all directions . some of this visible radiation will enter the core of the optical fiber 62 at the end surface 108 , and then will propagate through the optical fiber 62 to the reticle illuminating portion 41 ( fig1 ), in order to illuminate the reticle . with reference to fig1 - 2 , when the weapon sight 10 is in an environment where the ambient light includes sunlight or some other strong source of visible and / or non - visible radiation ( such as ultraviolet radiation ), the visible light emitted from the fiber 62 within the radiation illumination portion 61 will be significantly brighter than the light emitted by the tritium light source 51 . thus , in this type of situation , the illumination of the reticle is effected primarily by the light produced by the radiation collector 61 and transmitted through the optical fiber 62 . in contrast , when the weapon sight 10 is being used in darkness or in some other environment with little or no ambient light , the optical fiber 82 will be emitting little or no visible light within the reticle illumination portion 41 , but the tritium light source 51 will still be active , and will provide suitable illumination for the reticle . fig6 is a diagrammatic perspective view of the optical fiber 62 and a radiation collector 131 that is an alternative embodiment of the radiation collector in fig1 - 5 . the radiation collector 131 is cylindrical , and has polished exterior surfaces , including a polished conical surface 132 or 133 at each end . all portions of the conical surfaces 132 and 133 extend at an angle of 45 degrees with respect to a central axis of the radiation collector 131 . the cylindrical opening 106 extends a short distance coaxially into the radiation collector 131 from one end , such that the conical surface 133 is actually frustoconical rather than fully conical . an end portion of the optical fiber 62 extends into the opening 106 , and is fixedly secured there by a suitable adhesive . the radiation collector 131 is made of the same material as the radiation collector 61 , and is doped with a fluorescent dye . visible and non - visible radiation can enter the radiation collector 131 from externally thereof ( including ultraviolet radiation ), and the fluorescent dye will convert at least some of the non - visible radiation into visible radiation of a certain wavelength or color . as with the radiation collector 61 , the arrangement of external surfaces on the radiation collector 131 ( including the conical end surfaces 132 and 133 ) ensures that when visible radiation propagating within the collector 131 impinges on any external surface , it will do so at an angle greater than the critical angle . consequently , most of the visible radiation within the radiation collector 131 will be effectively trapped there . fig7 is a diagrammatic perspective view of a radiation collector 141 that is an alternative embodiment of the radiation collector 131 of fig6 . the embodiment of fig7 is effectively identical to the embodiment of fig6 , with one difference . the radiation collector 141 has the conical surface 132 at one end , but its other end extends all the way to the reticle illuminating portion 41 , and has a planar end surface 144 that is perpendicular to the central axis of the radiation collector 141 . when visible light that is propagating within the radiation collector 141 reaches the end surface 144 , it will impinge on the surface 144 at an angle less than the critical angle . consequently , this radiation will pass through the end surface 144 and into the reticle illuminating portion 41 , where it will illuminate the reticle in a manner similar to that described earlier for the radiation 66 in fig2 . although several selected embodiments have been illustrated and described in detail , it will be understood that they are exemplary , and that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention , as defined by the following claims .