Abstract: A lamp device comprises an optical fibre which receives an incident light from a light source and emits an outgoing light, a light .conducting path lens connected to the optical fibre so as to receive the light emitted from the optical fibre and a light shielding member which has a window disposed between an outgoing light plane of the optical fibre and an incident light plane of the light conducting path lens. The shape of incident light on the incident light plane of the light conducing path lens is made rectangular.

Abstract: The present invention provides a pole light for directing or delivering light in a desired pattern to a surface proximate to the pole light. The present invention also provides a method for lighting a plurality of desired regions on a surface in a precise and predetermined pattern useful, for example, as a traffic management and guidance system.

Abstract: Fiber optic light emitting panel assemblies include one or more light emitting layers which are sealed along the side edges and/or an end edge to hold the fill threads in position and keep the light emitting portions from fraying at the edges. A thin film, sheet or coating may be applied to one or both sides of the light emitting portions of the panel assemblies. At one or both ends of the light emitting portions is a light cable which may be formed by randomly distributing the optical fibers which comprise the light cable into a plurality of discrete bundles each including fibers extending from different locations across substantially the entire width of the light emitting portions, bringing the discrete bundles together to form a single bundle of all of the fibers in each light cable, and applying a connector assembly to the cut ends of all of the fibers in each light cable.

Abstract: A fiber optic connector and method for connecting composite materials within which optical fibers are imbedded. The fiber optic connector includes a capillary tube for receiving optical fibers at opposing ends. The method involves inserting a first optical fiber into the capillary tube and imbedding the unit in the end of a softened composite material. The capillary tube is injected with a coupling medium which subsequently solidifies. The composite material is machined to a desired configuration. An external optical fiber is then inserted into the capillary tube after fluidizing the coupling medium, whereby the optical fibers are coupled.

Abstract: A light beam from an optical fiber held by waveguide support is guided to an illuminated region of a light-receiving section. A light beam from one light-deflecting device inside this region is reflected by a reflecting surface and passed as a light beam through a second light-deflecting device in another illuminated region to arrive at a second optical fiber. A light beam from the second optical fiber takes the reverse route to the foregoing route. In this way, various light-deflecting devices in one illuminated region are optically coupled to light-deflecting devices in other illuminated regions with substantially equal optical coupling intensity.

Abstract: A display system having a greyscale control arrangement for controlling light output from a plurality of optical fibers includes a light source effective for supplying a high brightness light output to the input ends of the plurality of optical fibers. A plurality of piezoelectric shutter members are disposed adjacent the light delivery ends of the optical fibers and are effective for controlling the delivery of light output through the optical fibers as a function of the on or off condition thereof. The delivery ends of the optical fibers are disposed relative to one another so as to form a plurality of display pixels. At least two of the optical fibers are combined to form each of the pixels. The diameters of the at least two optical fibers that make up each pixel are different thereby allowing that at least four different greyscale levels can be achieved for each display pixel.

Abstract: A device for data transmission by light has a waveguide doped with a fluorescent material. A light source is arranged to provide modulated light to enter the waveguide through an upper surface, the fluorescent material being responsive to the modulated light to emit light omnidirectionally. The emitted light is guided towards and emitted through end surfaces of the waveguide.

Abstract: An optical fiber manifold is provided for coupling light from an illumination source to a plurality of spaced, large diameter output fibers, or "light pipes", which are used for a variety of purposes, such as illuminating pools, spas, hazardous material zones, jail cells, and other applications where direct lighting is dangerous, difficult to maintain, or subject to vandalism. The manifold comprises a light converging element, which may be either a lens or a reflector, for converging light separately on each of the spaced optical fibers. The light converging element is segmented, with each segment corresponding to one of the optical fibers, and is precise enough that substantially all of the convergent light is received by the respective cores of each of the spaced output fibers, thereby minimizing light loss.

Abstract: The invention relates to a display matrix having light-conveying fibers that are maskable by disks each having a plurality of sectors and rotated by non-reversible motors each having two stable positions and under the control of a display control device. Each disk is provided with a detector device for detecting at least one angular position of said disk and connected to a device for monitoring matching, which device is itself connected to the display control device and acts on an activation device for the motor of said disk. The detection device advantageously includes a photoelectric sensor and a reflecting element disposed on a light-blocking sector of the disk.

Abstract: A lightguiding device, which can be an integral part of a lightguiding system, can increase the light intensity exiting from a lightguide by concentrating the transmitted light into a small portion of the lightguide's cross section. The device consists of a section of a lightguiding material which has been bent at an angle greater than 45 degrees relative to the axis of light transmission, preferably at an angle of 90 degrees with a small bend radius. In the bent section, a homogeneous refractive index is required at least for the lightguide's core. The lightguide must be terminated immediately beyond the bent section. Alternatively, a preformed bent section of appropriate lightguiding material can be attached to an end of a straight section of lightguide to increase the exiting light intensity. All types of lightguiding systems can be used to enhance output intensities, including glass or pure silica lightguides, hollow waveguides, liquid core lightguides and plastic lightguides.

Abstract: A modular fiber optic light line unit provides a uniform linear beam of illumination. A number of these modular units can be butted together end to end to create a long, seamless line of uniform strip lighting. The unit has a housing in which a distal portion of the fiber optic bundle fans out so that the termini of the fibers are arrayed in a single continuous fiber row at a distal face plate of the housing. The ends of the row are flush with end edges of the distal face plate. In one embodiment the housing has first and second halves that sandwich the fiber termini, and there are respective sawtooth or similar recesses that each hold a respective fiber end or terminus. The housing can be anodized aluminum, and the optical fibers can be polymer fibers, for example with a diameter of 0.030 inches.

Abstract: A light scattering light guide, which is capable of converting a light flux of a relatively small cross-sectional area into a light flux of a relatively large area with high efficiency and uniformity, manufacturing method thereof, and applied optical apparatuses such as backlight light source apparatuses and light branching/mixing devices which make effective use of the characteristics of said light scattering light guide. Such light scattering light guide is obtained by utilizing various structures with irregular refraction indexes generated in the process of polymerization of organic materials. The values of scattering power (effective scattering irradiation parameter E, etc.) are selected according to applications. The mechanism for forming the structures with irregular refraction indexes by the polymerization process of organic materials is based on, for example, the difference in compatibility, reactivity, density, etc. among the organic materials.

Abstract: An optical fiber light emitting apparatus (200) comprises at least one optical fiber (201) arranged in a repeating, recurrent pattern extending substantially throughout a predetermined area (206). The optical fiber (201) has a plurality of locations along the length thereof permitting light entering at least one end of the optical fiber (201) for transmission therein to be selectively emitted by the optical fiber (201) at the plurality of locations (210) for producing substantially uniform illumination substantially throughout the predetermined area (206). The repeating, recurrent pattern may form a spiral or serpentine pattern, for example. The repeating, recurrent pattern eliminates the space required for the fan-in arrangement (115) of prior art optical fiber light emitting panels (100). The optical fiber light emitting apparatus (200) is beneficial for illuminating keypads (613) and displays (611) in electronic devices (600), such as portable radio telephones, having limited space available.

Abstract: An optical arrangement that provides a telecentric helmet mounted display optical system. The optical arrangement comprises an image source including a faceplate wedge attached to a liquid crystal display, such as an active matrix liquid crystal display, for example, that is illuminated with collimated fight. The output image is used as a light source for an off-axis or on-axis reflector used to produce an output image. The faceplate wedge corrects the anamorphic distortion exhibited in the off-axis reflector and directs the light energy into the off-axis entrance pupil of the reflector to maintain high uniformity over the exit pupil without sacrificing brightness. More particularly, the present invention comprises the input light source, and the liquid crystal display that is illuminated with collimated light from the light source. The faceplate wedge is optically coupled to the liquid crystal display, and the reflector is optically coupled to the faceplate wedge for producing an output image.

Abstract: A polyolefin resin light diffusion tip is attached to an optical fiber end for conducting a pulse laser beam.

Abstract: A display device has an insulated basic substrate. A plurality of light waveguides are arranged on the basic substrate in parallel to each other. A plurality of signal wires is arranged in parallel and in a manner to be crossed with the light waveguides, respectively. A plurality of photoconductive layers three-dimensionally are laid between the light waveguides and the signal wires and directly connected with each light guiding portion of the light waveguides at respective crosspoints between the light waveguides and the signal electrodes. A plurality of pixel electrodes provided are to be connected with the photoconductive layers, respectively. An insulated opposed substrate located in opposition to the basic substrate with a display medium therebetween and having an opposed electrode on the surface opposed to the basic substrate. And the relation among an index of refraction n.sub.1 of the light guiding portion, an index of refraction n.sub.2 of the photoconductive layer, and an angle of incidence .theta.

Abstract: A corner supply type plane light source device having a simple structure and capable of forming a uniform luminescent part with a small power consumption is disclosed. The corner supply type plane light source device comprises a flat-plate light-scattering light guide 1, diffusion reflective sheets 2, 4, and LEDs 3 disposed at corners. The light-scattering light guide 1 is composed of a material provided with a uniform scattering power, and the effective scattering irradiation parameter E and the correlation distance a satisfy the conditions of 9.ltoreq.E[cm.sup.-1 ].ltoreq.100 and 0.06.ltoreq.a[.mu.m].ltoreq.35. Turning on the LEDs 3, light is supplied from each corner. The LED radiation light having directivity immediately undergoes the scattering in the light-scattering light guide 1 and the propagation direction is expanded in the lateral and vertical directions, and also undergoes the internal reflection on the bottom 12 and sides 13, and diffusion reflection by the diffusion reflective sheets 2, 4.

Abstract: A backlighting device for use with display panels has a plurality of light diffusing elements formed in dots or stripes on one of the two major faces of a light conducting plate that is made of a light-transmissive material. The device also has a linear light source provided in proximity to the end portion of at least one side of the light conducting plate. In the device, at least one sheet that is made of a light-transmissive material and which has a multiple of prisms or raised structures having straight ridgelines defined by the apices and minima of the raised structures at small intervals on the same side of the sheet.

Abstract: An optical illumination system comprising a waveguide that accepts light generated by a diffuse light source and transmits the light via total internal reflection. Attached on one surface of the waveguide is a low index of refraction layer and an array of microprisms, with each microprism having an light input surface and at least one sidewall tilted at an angle from the direction normal to the surface of the waveguide such that light escapes from the waveguide, reflects off the tilted sidewalls and emerges from the microprism as a spatially-directed light source. The low index of refraction layer prevents low angle light from entering the microprisms. An array of microlenses may be positioned to accept the output of the microprisms so that the light exiting from the microlenses is a substantially collimated light source.

Abstract: A reading magnifier formed by a bundle of juxtaposed longitudinally tapered optical fibers having a viewing end and a flat base end. The flat base end is cut at a bias across the bundle of optical fibers such that a line normal to the flat base end forms an acute angle with the direction of orientation of the optical fibers. The viewing end of the bundle may also be cut at an angle relative to the optical fibers. The reading magnifier provides a cone of light admittance that is skewed at an angle relative to a cylinder normal to the flat base end. The tilted admittance cone may also be achieved by bending the bottom portion of a bundle of tapered continuous optical fibers.

Abstract: Light processing apparatus creates visual effects with light from a light source. An input bundle of flexible fibre optic light guides receives light from the light source. A reconfiguration plate reconfigures the output ends of the light guides into a plurality of separated input zones of selected shape. A filter having colored areas shaped to correlate with the input zones filters the light. An output bundle of flexible fibre optic light guides distributes the light transmitted through the filter. A configuring plate configures the input ends of the output bundle of light guides into output zones which correlate with the input zones. The filter may be a rotatable color wheel or a computer-controlled liquid crystal display. When the filter is activated, the colored areas change colors, thereby creating various visual effects, such as painting with light.

Abstract: A light guiding plate has grooves or ridges with respective bottom or peak portions having surfaces with curvilinear sectional contours. The curvilinear portions replace angular intersections of slanted surfaces of the grooves or ridges and thereby provide uniform luminance over an entire face of the light guiding plate.

Abstract: A display apparatus of the present invention includes at least one display element having a display screen and an optical element for transiting an image from an input end face thereof to an output end face thereof. The optical element includes a first optical block and a second optical block each having a plurality of optical fibers combined together. The first optical block is connected to the display screen. One end face of the second optical block constitutes the output end face of the optical element. The numerical aperture of the optical fiber of the first optical block is smaller than the numerical aperture of the optical fiber of the second optical block. A diameter of the optical fiber of the first optical block is smaller than the diameter of the optical fiber of the second optical block.

Abstract: A lighting device for use in a vehicle which has a lighting unit for receiving light from a light source and emitting the received light. The lighting unit is provided with an optical fiber for receiving light from the light source and emitting the received light and an optical waveguide directly connected to the optical fiber for directly receiving the light emitted from the optical fiber and emitting light which has a desired luminous intensity distribution. Thereby, the lighting unit can have a simple structure and can be provided at low cost.

Abstract: Light-transmitting and light-directing shields, panels or lenses for supplying illumination to a locus remote from the heat and electricity of the light source. The present illuminating devices are assemblies of transparent waveguide panels or sheets and optical fibers associated with a remote light source. The waveguide is provided with a plurality of internal mirror surfaces which reflect light out of the plane of the waveguide and direct it in a predetermined direction for illumination purposes.

Abstract: A fiber optic light guide luminaire includes an elongated fiber optic light guide having a first end accepting light from a remote source of illumination and a second end emitting light on an optical axis disposed coaxially near the focus of a paraboloidal reflector. The paraboloidal reflector has a first end wherein the light guide is disposed and a second open end emitting light from the fiber optic light guide. The second end of the light guide extends into the first end of the reflector towards the second end of the reflector and past the focus of the paraboloid at a distance approximately equal to the diameter of the light guide, whereat the diameter of the second end of the light guide intersects the paraboloidal surface of the reflector at an angle of approximately 20.degree. to the optical axis. In a preferred embodiment the exterior of the luminaire is generally cylindrical and externally threaded.

Abstract: A lighting panel providing uniform illumination is described. The panel includes a transparent light guide which has a faceted grooved major surface and a parallel planar surface. The lighting panel introduces light into one end of the light guide. The facets facing the light source are inclined so that they reflect the incident light primarily along the light guide to be reflected from the opposite end. The facet facing away from the light source is inclined to reflect light out through the planar major surface.

Abstract: The near field optical microscopy probe has a conically tapered tip formed from the inner core of a fiber optic cable. The tapered tip protrudes longitudinally from the outer cladding and has a metallized optically opaque coating over all the tip except for the light-emitting aperture at the tip apex. The optical probe is manufactured by wet chemical etching. The protruding conical tip tapers at an acute angle on the order of about 15.degree. to 35.degree., such that the tip length is on the order of a few wavelengths. By this construction, illumination traverses only a very small nonpropagating mode or evanescent mode region with resulting high optical efficiency. The probe and specimen may be supercooled, causing the metallized coating to be highly conductive and therefore optically opaque. The result is a high efficiency, high resolution probe suitable for such demanding applications as DNA sequencing.

Abstract: An outer prism light guide surrounds a cover which in turn surrounds a light source. The outer guide is formed of prism light guide wall material having uniform shape in cross-sections perpendicular to a first octature symmetry direction, with its optical center line parallel to the first octature symmetry direction. Surfaces of the wall material are substantially in octature relative to the first octature symmetry direction. A surface normal vector at any point on any internal surface of the material is either substantially parallel or substantially perpendicular to a first notional line extending from the point through and perpendicular to the optical center line. The cover has cylindrical symmetry about the optical center line to intercept a substantial portion of light emitted by the source.

Abstract: An improved NVG compatible day/night backlighting assembly for a color liquid crystal display includes a daytime high intensity light source, an edge-mounted nighttime low intensity light source, and an integral collimating/reflecting/image splitting lens which collimates and image splits the light emitted from the high intensity daytime light source, and reflects upward the low intensity light rays emanating from the low intensity edge-mounted light source.

Abstract: An optical coupling assembly is useful for coupling a source of light, having high brightness and being non-coherent and focussed at a focal plane, to a plurality of optical light conductors. The coupling assembly includes an elongated light transmissive coupling member which has a central longitudinal axis and is disposed between the focal plane of the light source and leading ends of the optical light conductors. The coupling member includes oppositely flared front and rear end portions and a middle portion disposed along the longitudinal axis, with the middle portion extending between and connected to the front and rear end portions. The coupling member can have either a solid or tubular configuration. The middle portion of the coupling member has cylindrical shape and cross-sectional size of uniform dimensions and being less than the respective cross-sectional sizes of an inlet end of the front end portion and an outlet end of the rear end portion.

Abstract: A ring light collector having a light source with a plurality of light pipes is located in a fixed relation with the light source. Each light pipe has a face which collectively substantially form a ring having a predetermined width between an inner diameter and an outer diameter. A concave reflecting surface having an axis of symmetry is fixed relative to the light source and plurality of light pipes. The concave reflecting surface extends in a rotated ellipsoidal fashion and is defined by the outermost surface of an ellipsoid having a major axis forming a predetermined angle with the axis of symmetry. The major axis of the ellipsoid is rotated about the axis of symmetry. The first focal point of the ellipsoid is located a predetermined distance from said axis of symmetry and a second focal point is located substantially between the outer diameter and inner diameter of the ring. The light source is located substantially at the first focal point.

Abstract: An information indicator suitable for information processors such as personal computers, word processors and the like includes a light source, a scattering plate which scatters light from the light source, an indicator plate irradiated with scattered light from the scattering plate, and a prism plate operatively arranged with respect to the scattering plate and the indicator plate and having a prism-like protuberance structure formed at a side thereof. The prism plate is so disposed that the prism-like protuberance structure faces the scattering plate, and the angle 2.theta. constituting the prism-like protuberance structure is defined by the formula, 2.theta.>2/3.times.(.pi./2+2.times.arcsin (n'/n)), in this inequality, n is the refractive index of said prism plate medium, and n' is the refractive index of a material in which the prism plate is arranged.

Abstract: A liquid crystal display including a sheet for converging light from a back light source. The sheet may be an array of truncated prisms, an array of prisms separated by flat portions, asymmetrical prisms with dual vertices, or prisms with different vertices.

Abstract: A display apparatus of the invention includes: a plurality of display devices arranged in one direction, each of the plurality of display devices having a display area; and a plurality of image transmission means corresponding to the respective display devices, one end face of each of the image transmission means being coupled to a display area of the corresponding one of the display devices, other end faces of the image transmission means corresponding to the adjacent ones of the display devices being in contact with each other, the plurality of image transmission means being bent toward the one direction in which the display devices are arranged.

Abstract: A traceable transmission conduit assembly includes an optically transmissive jacket over a transmission conduit. The conduit, which may be an optical fiber, includes a light transmissive and translucent jacket thereover. Light is injected into the jacket at one location therealong. The light is emitted along the length of the jacket so that the jacket illuminates. One conduit bundled among a group of similar conduits may be traced by so illuminating the jacket of the one conduit.

Abstract: An illumination panel for use in large-scale stationary or mobile applications is disclosed. The illumination panel includes a cover and a reflecting back surface held together by a supporting element such that a cavity is defined therebetween; a light source; fiber optic cable extending from the light source and into the cavity, wherein the fiber optic cable consists of a plurality of unsheathed optical fiber cores bundled together within a hollow clear plastic flexible PVC tube.

Abstract: An interactive optical panel assembly 34 includes an optical panel 10 having a plurality of ribbon optical waveguides 12 stacked together with opposite ends thereof defining panel first and second faces 16, 18. A light source 20 provides an image beam 22 to the panel first face 16 for being channeled through the waveguides 12 and emitted from the panel second face 18 in the form of a viewable light image 24a. A remote device 38 produces a response beam 40 over a discrete selection area 36 of the panel second face 18 for being channeled through at least one of the waveguides 12 toward the panel first face 16. A light sensor 42,50 is disposed across a plurality of the waveguides 12 for detecting the response beam 40 therein for providing interactive capability.

Abstract: A luminous pull-cord for facilitating the operation of an electrical switch, comprising a length of fiber optic cord coupled to and illuminated by a light source as a means for actuating said electrical switch whereby said light conducting fiber is grasped and then pulled.

Abstract: A transmissive, backlit color twisted-nematic or super-twisted-nematic liquid crystal display employing a front fiber-optic faceplate as a front retaining element of a liquid crystal cell that increases viewing angle between the display and a viewer while minimizing or eliminating undesirable variations in luminance, contrast ratio and chromaticity is described. This novel configuration effectively decouples the liquid crystal cell and other optical elements behind the front fiber optic faceplate from the angular orientation of the viewer with respect to the display. The liquid crystal display described in this invention further provides a high quality color display, without viewing parallax, using a color selection filter mosaic outside of the liquid crystal cell.

Abstract: A surface lighting device receiving incident rays through two opposing faces of a light guide from light source to emit the incident rays from a face different from the two faces. The light guide is divided into plural divisions which are jointed together.

Abstract: An illuminator for vehicle exterior lamps using a remote light source. Light is transmitted to the illuminator by a light guide, such as a fiber optic light pipe. The illuminator utilizes a complex mirror and a reflector to collect and collimate over 80% of the light from remote light source into a desired parallel beam for use as an automotive headlight.

Abstract: A thermally insulated distributed light network employs an array of discrete, thermally insulative light guides that are spaced slightly away from a central light source and provide optical coupling to optical fibers that have a lower thermal capability. The light guides can either be solid such as quartz, focusing lenses, or a combination of the two. The light receiving ends of the light guides are preferably held by a specially designed coupler so that the guides do not diverge from each other by more than the fibers' optical acceptance angle. The light guides are shaped so that their adjacent edges touch and form an enclosure around the light source, and the fibers are provided in bundles with matching geometries. The network includes an edge-lit display panel that is illuminated by a fiber that has either internal light scattering centers in the form of reflective particles or gas bubbles, or a series of edge notches, to redirect light into the panel.

Abstract: An illumination device able to maintain a high level of light utilization and provide essential functions using a compact design. The illumination device includes a light source, at least one fiberoptic cable for transmitting light from the light source, and at least one lamp device for producing a desired light distribution. The lamp device includes a light bending and conducting path that directly receives the light emitted from the fiberoptic cable. The light bending and conducting path bends the light beam by a desired angle. The lamp device further includes light conducting path lenses that are directly connected to the light bending and conducting path. The lenses produce the desired lights distribution.

Abstract: An optical fibre has a light emitting region. In the light emitting region are reflecting surfaces, at least one of which has a cross sectional area less than that of the fibre. Light striking the reflecting surfaces will be reflected out of the fibre.

Abstract: An improved optical fiber for laterally directing a laser beam having a waveguide including a tip for communicating electromagnetic radiation in a propagation direction to the tip of the waveguide, a transmitting surface on the tip of the waveguide, a reflecting surface on the tip of the waveguide for internally reflecting electromagnetic radiation communicated by the waveguide in a direction lateral to the propagation direction toward a particular area on the transmitting surface, and wherein the particular area and the reflecting surface are disposed so that substantially all electromagnetic radiation reflected by the reflecting surface is incident on the particular area at below a critical angle for transmission through the transmitting surface in the lateral direction.

Abstract: An illumination device for a vehicle comprises at least one light generator with a light source, a plurality of light conductors receiving light from the light source, a plurality of light distributing units connected with the light conductors and operating for different illuminating functions, and a screening device associated with at least one of the light distributing units and switchable between a passive position in which the screening device permits supplying light in the light conductor associated with the one light distributing unit to the light distributing unit or permits exiting of light from the light conductor associated with the one light distributing unit, and an active position in which the screening device screens light of the light conductor associated with the one light distributing unit from entering the one light distributing unit or exiting the one light distributing unit.

Abstract: A distributed light delivery system for the illumination of printed circuit boards during fault detection operations is provided. The system has at least one light delivery bar having at least two optical light lines. Each optical light line is composed of an array of optical fibers. The entire array of optical fibers are oriented within the light delivery bar at one of at least two predetermined angles. When the light bar is properly mounted and directed on a staging apparatus, the light delivery bar is effective to illuminate all four sides and the top of three-dimensional components. The distributed light delivery system thus provides uniform and substantially shadow-free illumination of discrete through-hole technology components inserted through printed circuit boards, solder paste screened onto bare circuit boards, and/or SMD technology components mounted on printed circuit boards.

Abstract: A composite optical cable for lateral emission of light flux therefrom includes a plural number of sets of transparent optical fibers with each set of fibers assembled into a single cable and with a plurality of single cables assembled in the composite optical cable. The sets of optical fibers forming the single cables may be wound in one direction of rotation to form a spiral of fibers with length, and the single cables may be wound in the opposite direction of rotation to provide the composite optical cable. Selected masking schemes provide apertures at predetermined locations along the single cables through which light flux carried by the cables may be emitted for special lighting effects.

Abstract: An illuminator apparatus has a light source and a liquid-core light guide comprising a tubular flexible sheath of a fluorocarbon polymer material which is filled with a lightguiding fluid such as glycol, a salt solution containing a chloride, fluoride or phosophate, phenylmethyl silicone oil (n.apprxeq.1.57), PCTFE oil, polychlorotrifluoroethylene oil (n=approx. 1.40) or also pure water. The tubular sheath is internally coated with a thin film of Teflon AF.RTM., the refractive index of which is lower than that of the sheath material.