OPTICAL FIBER AND ILLUMINATION DEVICE

Abstract: An optical fiber (10) comprising (i) a core (1) capable of transmitting light entered at one end to another end and (ii) a covering tube (2) having a predetermined length and optically connected to the core (1), the covering tube (2) comprising a light-transmitting resin having dispersed therein light scattering particles, wherein the light scattering particles are contained in an amount of from 0.01 to 0.9 part by weight per 100 parts by weight of the light-transmitting resin.

Description: OPTICAL FIBER AND ILLUMINATION DEVICE DETAILED DESCRIPTION OF THE INVENTION Technical Field to which the Invention Belongs The present invention relates to an optical fiber and an illumination device, more specifically, the present invention relates to a so-called lateral illumination-type optical fiber which leaks light entered at least at one end in the longitudinal direction of the core, through the covering tube contacted to the peripheral surface (namely, lateral surface) of the core, and an illumination device using this optical fiber. Background of the Invention Representative lateral illumination-type optical fibers include the following two types: (1) an optical fiber in which a covering tube such as a clad, contacted to the peripheral surface of a core contains light scattering particles to scatter the light exited from the core into the covering tube, thereby causing light leakage, and (2) an optical fiber which has a light-diffusing reflection film bonded in the stripe form to a part of the peripheral surface of the core along the longitudinal direction of the core. The optical fiber of type (1) is disclosed, for example, in U.S. Patent 4,422,719. In the optical fiber disclosed here, a resin clad made of Teflon™ produced by Du Pont or the like and covering the core contains light-scattering particles each comprising a metal oxide such as titanium dioxide, in an amount of from 2 to 10 wt% based on the entire clad. In the case where the clad does not contain light-scattering particles, the light traveling through the core and reaching the interface between the core and the clad is reflected in a large proportion. However, in the case where the clad contains light-scattering particles, the light reaching the interface between the core and the clad is scattered and a part of the light is reflected towards the inside of the core and the remaining passes through the clad to leak outside. Through such an operation, light entering at one end of the core can afford illumination throughout the peripheral surface of the fiber with high brightness. However, since the amount of light-scattering particles is too large, the light is leaked in a large amount near the light entering end, and combined with the low transmittance of the clad, the brightness lowers at the position distant from the light entering end (e.g., center portion of the fiber in the longitudinal direction). Accordingly, a relatively long (for example, 2 m or more) fiber which can be used as a linear illuminant substitutable for a neon tubing cannot be illuminated with uniform brightness throughout the entire length. The optical fiber of type (2) has a light-diffusing reflection film which is a coating comprising a light-transmitting resin having dispersed therein light-scattering particles such as titanium dioxide, as disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 60118806. The light-diffusing reflection film diffuses and reflects light into the core when the light travels through the core and reaches the interface between the reflection film and the core. By this action of the diffusibing reflection film, in cooperation with the lens effect of the core, the light can be leaked with a directivity toward the direction crossing the longitudinal direction of the core to thereby exhibit illumination of high brightness over the longitudinal direction. However, since the above-described diffusibing reflection film is usually extremely low in the transmittance, illumination in a wide angle of visibility (in other words, over the entire peripheral surface) as that of a neon tubing cannot be obtained. It is also difficult to illuminate a relatively long fiber with uniform brightness. As described above, conventional lateral illumination-type optical fibers can easily enhance the brightness in the case of a relatively short fiber, however, they cannot illuminate a relatively long fiber throughout the entire length with uniform brightness. Summary of the Invention As described in the foregoing, according to the present invention, a lateral illumination-type optical fiber capable of illuminating a relatively long fiber with uniform brightness throughout the entire length can be obtained. This optical fiber of the present invention can be advantageously used as a linear illuminant substitutable for the neon tubing and at the same time, a linear illumination device substitutable for the neon illumination device can be provided. Accordingly, in some embodiments, the present invention provides a lateral illumination-type optical fiber which can uniformly illuminate a relatively long fiber - 2 throughout the entire length and can be used as a linear illuminant substitutable for a neon tubing. In addition, in some embodiments, the present invention provides an illumination device comprising the above-described optical fiber and capable of being used as a linear illumination device substitutable for a neon illumination device. In an aspect thereof, the present invention provides an optical fiber comprising (i) a core capable of transmitting light entered at one end to another end and (ii) a covering tube having a predetermined length and optically connected to the core, the covering tube comprising a light-transmitting resin having dispersed therein light scattering particles, wherein the light scattering particles are contained in an amount of from 0.01 to 0.9 part by weight per 100 parts by weight of the light-transmitting resin. In another aspect thereof, the present invention provides an illumination device comprising the above-described optical fiber of the present invention and a light source disposed such that the light enters at least at one end of the core, the core covered with the covering tube having a length of from 2 to 50 m and the covering tube having a coefficient of variation in the illumination brightness measured throughout from one end to another end in the longitudinal direction at an interval of 10 cm, of 70% or less. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view showing one preferred embodiment of the optical fiber according to the present invention, by closing up particularly to one end portion of the optical fiber. Fig. 2 is a perspective view showing another preferred embodiment of the optical fiber according to the present invention, by closing up particularly to one end portion of the optical fiber. Fig. 3 is a side view showing one embodiment of the illumination device using the optical fiber of the present invention. Fig. 4 is a graph showing the measurement results of the brightness of the illumination device shown in Fig. 3. Detailed Description of a Preferred Embodiment ■ 3 - The present invention is described below with respect to the mode of operation and embodiments thereof. The present invention is first described with respect to the mode of operation. The optical fiber of the present invention comprises a covering tube having the above-described characteristics and thereby can achieve uniform illumination throughout from one end to another end in the longitudinal direction. If the amount of light-scattering particles contained is too small, satisfactory brightness necessary for the linear illuminant substitutable for a neon tubing (for example, in the case of white illumination, 1,000 cd/m^ or more) cannot be obtained even if the strength of the light source (consumption power) is increased. On the other hand, if the amount is too large, because of the same reasons in the case of conventional optical fibers described above, a relatively long (for example, 2 m or more) fiber cannot be illuminated with uniform brightness throughout the entire length. Accordingly, the amount of light-scattering particles contained is preferably from 0J0 to 0.70 part by weight, more preferably from 0J5 to 0.60 part by weight per 100 parts by weight of the light-transmitting resin. The uniformity of illumination referred to in the foregoing is defined as the condition such that the coefficient of variation of the illumination brightness measured throughout from one end to another end of a covering tube covering the core in the longitudinal direction at an interval of 10 cm is 70% or less. The term "coefficient of variation" as used herein means a percentage (lOOxSD/Av) of an absolute value (SD) of the standard deviation of all measured values obtained by measuring the brightness throughout from one end to another end at an interval of 10 cm along the longitudinal direction of the covering tube (namely, the portion covered with the covering tube of the optical fiber), to the average value (Av) of all measured values. If the coefficient of variation as defined above exceeds 70%, the illumination is perceived to be non-uniform by the observer and the optical fiber may not be used as a linear illuminator substitutable for a neon tubing. The coefficient of variation is preferably 50% or less, more preferably 30% or less. The illumination uniformity of the optical fiber is usually evaluated using a metal halide lamp of 30 W as a light source by entering light at both ends of the core and illuminating the fiber. The covering tube usually has a thickness of from 0.1 μm to 3 mm, preferably from 0.5 μm to 2 mm, more preferably from 1 μm to 1 mm. If the thickness is less than 0J μm, - 4 - sufficiently high brightness may not be obtained, whereas if it exceeds 3 mm, the brightness lowers at the portion distant from the light insertion end (for example, in the vicinity of the center portion of the core in the longitudinal direction) and the uniformity of illumination brightness may be lost. The state that the covering tube is "optically connected to the core" is defined as the following state (a) or (b): (a) the peripheral surface of the core is directly contacted with the inner surface of the covering tube; or (b) a light-transmitting polymer layer is further provided to come into contact with the peripheral surface of the core and the polymer layer is in contact with the inner surface of the covering tube. In the case of state (b), the difference (A = P - C) between the refractive index (P) of the light-transmitting polymer layer and the refractive index (C) of the core is from -0.2 to 1.0. The optical connection is further described in detail later. The illumination device of the present invention has characteristics as described above and therefore, can be suitably used as a linear illumination device substitutable for a neon illumination device. It may be sufficient if the light is inserted at least at one end, however, a light source is preferably disposed so that the light can enter at both ends of the core. For example, a first light source for inserting light at one end of the core and a second light source for inserting light at another end of the core may be used as the light source. By inserting light at both ends of the core, the uniformity of brightness can be further increased. The same effect can also be obtained by using one light source and inserting light at both ends of the core using a light transmitting means such as a separate optical fiber. The core covered with a covering tube usually has a length of from 2 to 50 m, preferably from 2.5 to 30 m, more preferably from 3 to 15 m. If the length is less than 2 m, the illumination device may not be used as a substitution for the neon illumination device, whereas if it exceeds 50 m, the uniformity of brightness throughout the entire length of the fiber may be reduced. - 5 The light source which can be used may be a common light source such as a metal halide lamp, a xenon lamp, a halogen lamp, a light emitting diode and a fluorescent lamp. The consumption power of the light source is usually from 0.05 to 300 W. The optical fiber of the present invention and the constituent elements thereof are described in detail below. Optical Fiber One preferred embodiment of the optical fiber of the present invention is described by referring to Fig. 1. In an optical fiber 10, a covering tube 2 having a predetermined length is disposed to come into contact directly with the outer peripheral surface (also called circumeferentially lateral surface) of a light-transmitting core 1. The length of the covering tube 2 corresponds to the length of the portion of the core 1 intended to be illuminated and the length is usually the same as the length from one end to the other end of the core. In the example shown in Fig. 1, the peripheral surface of the core is directly in contact with the inner surface of the covering tube and therefore, the core and the covering tube are optically connected to each other. The core 1 usually has a refractive index of from 1.4 to 2.0. The material for forming the core is a light-transmitting material such as quartz glass, optical glass or polymer. The core may be a solid core formed from the above-described material or a liquid sealed-type core comprising a flexible plastic tube having sealed therein a liquid having a relatively high refractive index such as silicone gel. In the case of a solid core, the core is usually covered with a light-transmitting clad so as to prevent pollution and the above-described covering tube may be used concurrently as the clad. Also, a clad may be used as a protective tube further covering the outer peripheral surface of the covering tube. In another preferred embodiment of the optical fiber of the present invention, the optical fiber is constructed as shown in Fig. 2. More specifically, an optical fiber 10 is a three-layer structure comprising a core 1, a light-transmitting polymer layer 3 in direct contact with the peripheral surface of the core 1, and a covering tube 2 disposed so that its inner peripheral surface can be in contact with the polymer layer. In this case, the light- transmitting polymer layer 3 acts as a light-transmitting adhesive for optically connecting the core 1 and the covering tube 2. This embodiment is advantageous in that a covering ■ 6 - tube having a relatively small thickness (usually from 1 to 100 μm) can be bonded to the core without generating wrinkles or breakage. A covering tube having a relatively small thickness is advantageously used for increasing the brightness throughout the longitudinal direction of the optical fiber without impairing the uniformity. The polymer used for the light-transmitting polymer layer can be formed from a light-transmitting polymer such as acryl-base polymer, polymethylpentene, ethylene-vinyl acetate copolymer, polyvinyl chloride and vinyl acetate-vinyl chloride copolymer. The light-transmitting polymer is suitably an adhesive such as a pressure-sensitive adhesive, a hot-melt adhesive and a cure-type adhesive, because an optical fiber using a covering tube having a relatively small thickness can be easily produced. This polymer usually has a refractive index of from 1.4 to 1.7 and a total light transmittance of 80% or more. A polymer having a refractive index higher than the refractive index of the core is suitably used, because the brightness can be effectively prevented from reduction and the uniformity of brightness can be effectively elevated. The illumination brightness of the optical fiber is not particularly limited as long as the effect of the present invention is not impaired. ' For example, the value measured using a white light-emitting metal halide of 130 W by entering light at both ends of the core is usually 1,000 cd/m^ or more, preferably 2,000 cάlvP- or more, throughout the entire length of the optical fiber. With the brightness in this range, the optical fiber can be widely used as a linear illuminant substitutable for a neon tubing. The absolute value of the brightness can be easily elevated by increasing the output power of the light source. In the case where the optical fiber of the present invention is used as a linear illuminator substitutable for a neon tubing, a light source is disposed so that the light can enter the core at one end or both ends of the core. When the light is inserted only at one end, the following means is effectively used for more elevating the uniformity of brightness: disposing a specular reflection material at another end of the core so that the light reached another end can be reflected into the core, or forming the optical fiber with a taper such that a thickness (diameter) of the core t g is reduced from one end (light inserting end) toward the other end. Core In the case of forming the core from a polymer, a light-transmitting polymer such as acryl-base polymer, polymethylpentene, ethylene-vinyl acetate copolymer, polyvinyl chloride or vinyl acetate-vinyl chloride copolymer may be used. The polymer usually has a refractive index of from 1.4 to 1.7 and a total light transmittance of 80% or more. In order to impart mechanical strength sufficient to endure the deflection of the core itself, the polymer may be cross-linked. The production process of a solid core is described below by taking an acryl-base core as an example. A tubular reaction vessel extending in the longitudinal direction and having an opening at least at one end is filled with an acryl monomer (a mixture or the monomer alone) as a raw material of the core. The acryl monomer is progressively heated to a temperature higher than its reaction temperature so that the reaction of the acryl monomer proceeds from the other end of the container tube toward the opening. In other words, the heating position is moved from the other end toward the opening. The reaction is performed while pressurizing the acryl monomer by a pressure gas put into contact with the acryl monomer. After completion of the heating as far as the opening, the container tube as a whole is preferably further heated for several hours so as to completely finish the reaction. Examples of the acryl monomer as a raw material of the core include (i) a (meth)acrylate of which homopolymer has a glass transition temperature (Tg) higher than 0°C (e.g., n-butyl methacrylate, methyl methacrylate, methyl acrylate, 2-hydroxyethyl methacrylate, n-propyl methacrylate, phenyl methacrylate), (ii) a (meth)acrylate of which homopolymer has a Tg less than 0°C (e.g., 2-ethylhexyl methacrylate, ethyl acrylate, tridecyl methacrylate, dodecyl methacrylate), and a mixture of (i) and (ii). In the case of a mixture of (i) and (ii), the mixing weight ratio (H:L) of the (meth)acrylate (H) in (i) to the (meth)acrylate (L) in (ii) is usually from 15:85 to 60:40. A polyfunctional monomer as a cross-linking agent, such as diallyl phthalate, triethylene glycol di(meth)acrylate and diethylene glycol bisallylcarbonate, may be added to the mixture. The thus-formed acryl-base core can be a polymer homogeneous from one end to the other end of the core in the longitudinal direction and exhibiting good light transmitting capability and mechanical strength sufficient to endure the distortion of the core itself. Accordingly, this is suitable for forming an optical fiber having a length of 2 m or more. The container tube used in the above-described production process is usually a fluoropolymer such as tetrafluoroethylene-hexafluoropropylene copolymer (FEP). The container tube may also be used as a covering agent concurrently as a clad by previously incorporating light-scattering particles therein and holding the core as it is after completion of the reaction without taking it out. The production process of such a flexible optical fiber (core) is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 63-19604. The cross-sectional shape of the core in the cross direction (direction orthogonal to the longitudinal direction) is not particularly limited as long as the effect of the present invention is not impaired. For example, the shape is a geometrical shape capable of maintaining the flexibility of the core, such as circular, elliptical or semicircular or arched form having an area larger than a semicircle. If the core has a circular cross section it usually has a diameter of from 3 to 40 mm, preferably from 5 to 30 mm. Covering tube The covering tube is produced, for example, by melt-molding resin pellets comprising a light-transmitting resin having dispersed therein light-scattering particles. In order to control the amount of light-scattering particles contained in the covering tube, a resin free of light-scattering particles may be mixed with the above-described resin pellets. Examples of the molding apparatus which can be used include an extruder. The light-transmitting resin may be a polymer which can be used for the core or a polymer which can be used for the clad. The refractive index of the light-transmitting resin is usually selected so that the refractive index of the finished covering tube can be larger than the refractive index of the core. For example, in the case where the refractive index of the light-scattering particle is larger than the refractive index of the core, the refractive index of the light-transmitting resin may be set to be smaller than the refractive index of the core. The light-scattering particle is, for example, a white inorganic powder or color pigment having a refractive index of from 1.5 to 3.0. Preferred examples of the white inorganic powder include barium sulfate (refractive index: 1.51), magnesia (refractive - 9 index: 1.8) and titania (refractive index: 2.6). A coloring material such as a fluorescent dye may also be incorporated together with the light-scattering particle so as to leak the white light inserted into the core as colored light. In covering a core with a covering tube to optically connect to each other, for example, a heat-shrinkable resin is incorporated into a covering tube to form a tube having an inner diameter larger than the outer diameter of the core by approximately from tens of μm to 1 mm, a core is inserted into the tube, and the tube is heated to tightly bond the core with the covering tube. Alternatively, a light-transmitting polymer layer described above may be disposed between the heat-shrinkable covering tube and the core so as to optically connect the core to the covering tube through the light-transmitting polymer layer. Further, as described above, a tube used as a reaction vessel for forming a core may be made to serve as a covering tube. Furthermore, a precursor sheet of the covering tube may be cylindrically rolled up to form a covering tube. For example, a coating solution containing a light-transmitting resin and light-scattering particles is coated on the release surface of a liner to form a precursor sheet comprising the coating. Then, on the surface of this precursor sheet, a light-transmitting adhesive layer is laminated to form a precursor sheet with an adhesive layer. Finally, this precursor sheet with an adhesive layer is bonded along the peripheral surface of the core to optically connect the core to the covering tube. The above-described coating solution can be prepared using a usual dispersion apparatus such as a sand mill, and the coating solution can be coated using a commonly used coater such as a knife coater or a bar coater. The precursor sheet preferably has a thickness (namely, thickness of the covering tube) of from 0J to 100 μm, more preferably from 0.5 to 50 μm. The covering tube having such a small thickness is advantageous for elevating the brightness throughout the longitudinal direction of the fiber without impairing the uniformity (in other words, while maintaining the narrow range of the coefficient of variation). The adhesive layer usually has a thickness of from 1 to 100 μm. The covering tube may also be formed of a coating adhered directly to the core. For example, a coating solution containing a light-transmitting resin and light-scattering particles is coated by a coating method such as dipping to form a covering tube comprising 10 . the coating adhered to the peripheral surface of the core. At this time, out of two ends of the core, at least the end which becomes the light inserting end is treated to prevent it from being covered with the coating. In any case, the light transmittance of the covering tube is not particularly limited as long as the effect of the present invention is not impaired, however, it is usually 5% or more, preferably 10% or more. Further, the covering tube may contain various additives other than the above-described materials as long as the effect of the present invention is not impaired. Examples of suitable additives include a cross-linking agent, an ultraviolet absorbent, a heat stabilizer, a surface active agent, a plasticizer, an antioxidant, an antifungal, a light accumulating material, a pressure-sensitive adhesive and a tackifier. Examples The present invention is described below by referring to the Examples, however, the present invention should not be construed as being limited to the following Examples. Example 1 10 kg of Teflon (trademark) pellets ("Teflon™, product No.: FEP100-J", produced by Du Pont) and 1 kg of titanium dioxide-containing flororesin pellets ("Neoflon™, product No.: FEP NP20WH", produced by Daikin Kogyo KK, titanium dioxide content: 2.3 wt%) were mixed and formed into a covering tube having an outer diameter of about 12 mm and a thickness of 0.8 mm using an extruder. The amount of titanium dioxide contained was 0.21 part by weight per 100 parts by weight of the light-transmitting resin. Into the covering tube obtained a mixture comprising a monomer solution containing 2-ethylhexyl methacrylate, n-butyl methacrylate and triethylene glycol dimethacrylate at a weight ratio of 50:50: 1 and bis (4-t-butylcyclohexyl)peroxy dicarbonate as an initiator was injected. The mixture was thermally polymerized according to the above-described production process of a solid core to prepare an optical fiber of this Example. In the optical fiber of this Example, the covering tube and the core were directly adhering and optically connected to each other. The core had a refractive index of 1.48. 11 - Example 2 .An optical fiber of this Example was prepared in the same manner as in Example 1 except for changing the amount of titanium dioxide contained in the covering tube to 0.38 part by weight per 100 parts by weight of the light-transmitting resin. Example 3 0.07 g of titanium dioxide ("Tipaque™ product No.: CR-90", produced by Ishihara Techno) was added to a solution obtained by dissolving 65 g of a fluorine-base polymer (product No.: THV200P, produced by 3M, refractive index: 1.36) in 250 g of ethyl acetate, and the titanium dioxide was dispersed in the solution using a sand mill to obtain a dispersion solution. The resulting dispersion solution was coated on a liner ("Purex™, product No.: G1W", produced by Teijin Ltd.) by means of a knife coater to obtain a covering tube precursor sheet having a thickness of 20 μm. The resulting covering tube precursor sheet had a titanium dioxide content of 0J 1 part by weight per 100 parts by weight of the fluorine-base polymer. Separately, an ethyl acetate solution (solid contents concentration: 30 wt%) of a phenoxyethyl acrylate-acrylic acid copolymer (weight ratio of monomer units: 99:1, refractive index: 1.56) was coated on the liner to form an adhesive layer as a light- transmitting polymer layer. The adhesive layer had a thickness of 10 μm. The precursor sheet and the adhesive layer were laminated to form a precursor sheet with an adhesive layer and the precursor sheet with an adhesive layer was bonded to a core along the peripheral surface thereof to obtain a core with a covering tube. In the resulting core with a covering tube, the core and the covering tube were optically connected through a light-transmitting polymer adhering to the peripheral surface of the core. The core used was obtained by removing the clad from an optical fiber (product No. : LF120, produced by 3M) and had a refractive index of 1.48 and a diameter of 12 mm. The thus-prepared core with a covering tube was covered with a clad comprising a heat-shrinkable FEP tube (product No.: NF-120, produced by Junko Sha) to obtain an optical fiber of this Example. 12 Comparative Example 1 .An optical fiber of this Comparative Example was prepared in the same manner as in Example 3 except for changing the amount of titanium dioxide contained in the covering tube to 1.01 part by weight per 100 parts by weight of the fluorine-base polymer. Examples 4 and 5 and Comparative Example 2 As shown in Fig. 3, two metal halide lamps (product No.: LBM130H, manufactured by 3M, consumption power: 130 W) were connected to both ends of the optical fiber of Example 1 to manufacture an illumination device of Example 4. This illumination device had, as seen in Fig. 3, Light Source A for entering light at one end of the core and Light Source B for entering light at another end of the core. The illuminable optical fiber 10 had a length of 3 m. Both light sources of the illumination device were illuminated and the illumination state was observed. As a result, it was verified that the optical fiber portion exhibited uniform illumination throughout the entire length like a neon tubing. The coefficient of variation of the illumination brightness of the optical fiber was determined as follows. The brightness was measured by a luminance meter (product No.: CS-100, manufactured by Minolta) along the longitudinal direction of the portion covered with the covering tube (in this Example, the entire length of the illuminable optical fiber) of the optical fiber throughout from one end to another end at an interval of 10 cm. The coefficient of variation of the brightness was obtained using the values measured according to the definition described above. At each measuring point, the distance between the luminance meter and the peripheral surface of the optical fiber was 60 cm. In this Example, the coefficient of variation of the brightness was 6%. Illumination devices of Example 5 and Comparative Example 2 were manufactured in the same manner as in Example 4 except for using an optical fiber of Example 3 or Comparative Example 1. These illumination devices were determined on the coefficient of variation of brightness in the same manner as in Example 4. In Example 5 and Comparative Example 2, the coefficients of variation of brightness were 20% and 84%, respectively. With respect to the illumination state, in Example 5, the optical fiber portion exhibited uniform illumination throughout the entire length like a neon tubing. However, in 13 Comparative Example 2, the brightness in the vicinity of the center portion in the longitudinal direction of the optical fiber was apparently lower than the brightness near the light source, and the illumination was perceived to be non-uniform. This phenomenon occurred because the illumination amount in the vicinity of the light entering end was too large and accordingly, the brightness in the vicinity of the center portion of the optical fiber was by far lower than the brightness near the light entering end. on the other hand, in the optical fibers of Examples I and 3 (illumination devices of Examples 4 and 5), variation of brightness was very small throughout the entire length of the optical fiber. The measurement results of the brightness in Examples 4 and 5 and Comparative Example 2 are shown in Fig. 4. Examples 6 and 7 An illumination device of Example 6 was manufactured in the same manner as in Example 4 except for using the optical fiber of Example 2. Further, an illumination device of Example 7 was prepared in the same manner as in Example 6 except for changing the length of the illuminable optical fiber to 10 m. The coefficient of variation of illumination brightness was determined in the same manner as above and it was 11% in Example 6 and 43% in Example 7. Also, the illumination state was observed, as a result, it was perceived that in both the illumination devices of Examples 6 and 7, the optical fiber exhibited uniform illumination throughout the entire length like a neon tubing. The brightness in the vicinity of the center portion in the longitudinal direction of the optical fiber was 2,900 cd/m^ in Example 6 and 800 cd/m^ in Example 7. 14

Claims: What is Claimed is: 1. An optical fiber comprising (i) a core capable of transmitting light entered at one end to another end and (ii) a covering tube having a predetermined length and optically connected to said core, said covering tube comprising a light-transmitting resin having dispersed therein light scattering particles, wherein said light scattering particles are contained in an amount of from 0.01 to 0.9 part by weight per 100 parts by weight of said light-transmitting resin. 2. .An illumination device comprising an optical fiber according to claim I and a light source disposed such that the light enters at least at one end of said core, said core covered with said covering tube having a length of from 2 to 50 m and said covering tube having a coefficient of variation of the illumination brightness measured throughout from one end to another end in the longitudinal direction at an interval of 10 cm, of 70% or less. 3. The illumination device as claimed in claim 2, wherein said light source is disposed such that the light enters at both ends of said core. - 15

Inventors: IMAMURA KENGO, IRIE SHINICHI, MATSUMOTO KENJI, UDAGAWA ATSUSHI, IMAMURA, KENGO, IRIE, SHINICHI, MATSUMOTO, KENJI, UDAGAWA, ATSUSHI
Applicants: IMAMURA KENGO, IRIE SHINICHI, MATSUMOTO KENJI, MINNESOTA MINING & MFG, UDAGAWA ATSUSHI, IMAMURA, KENGO, IRIE, SHINICHI, MATSUMOTO, KENJI, MINNESOTA MINING AND MANUFACTURING COMPANY, UDAGAWA, ATSUSHI
Application year: 1999
Main classification: F21V8
IPC codes: F21V8, G02B6