Method of manufacturing a plastic graded-index optical transmission element

Abstract: This invention relates to a refractive index distribution type plastic optical transfer member which has a round section having a radius r₀ of 0.4 ∼ 0.6 mm, has a refractive index distribution substantially approximate to a predetermined ideal refractive index distribution curve in a region of at least from 0,25r₀ ∼ 0.70r₀ from its center axis to the outer peripheral surface, and has a refractive index n₀ of 1.4 ∼ 1.6 at its center, a refractive index distribution constant of at least 0.15 mm⁻¹ but less than 0.3 mm⁻¹ and a modulation transfer function of at least 55 %. This optical transfer member is produced by arranging N (N ≧ 2) uncured liquid materials whose viscosity is 10³ ∼ 10⁸ poises and whose refractive indices n are n₁ > n₂ > n₃ ... n N , when they are cured, in such a way that n becomes progressively lower from the center to the outer peripheral surface, shaping them in an uncured strand fiber consisting of a plurality of concentric laminated layers, and curing the uncured strand fiber while, or after applying mutual diffusion treatment of the materials between the adjacent layers.

Description: The present invention relates to a method of manufacturing an optical transmission element, which is useful for optical transmission lines such as near parabolic optical fibers, rod-like convergent lenses, and photosensors, as well as for an image transmitting array employed in a copying machine using a white light source. BACKGROUND ART Optical transmission elements each with refractive indexes gradually distributed from the center toward the periphery on a cross-section thereof are disclosed in Japanese Examined Patent Publication No. 47-816, Japanese Examined Patent Publication No. 47-28059, and European Patent Application Publication No. 0208159. The graded-index optical transmission element disclosed in Japanese Examined Patent Publication No. 47-816 is made of glass and fabricated by an ion exchange method. This method, however, has a poor productivity and cannot produce articles having an identical shape (an identical length, in particular) and an identical performance. Even with an identical performance, the fabricated distributed refractive index type optical transmission articles have different lengths, and thus problems arise with the handling thereof. The plastic graded-index optical transmission element disclosed in Japanese Examined Patent Publication No. 47-28059 is made by mixing two or more transparent polymers having different refractive indexes and different solubilities with respect to a particular solvent. The mixed polymers are shaped into a rod or a fiber, and immersed in the solvent to extract a part of the polymers from the surface thereof, thereby changing a mixing ratio of the polymers from the surface toward the center thereof. Since the optical transmission article of this method is made of a mixture of two or more polymers having different refractive indexes, fluctuations in the refractive indexes of the article occur, deteriorating the transparency and causing light scattering, and therefore, the article cannot serve as the graded-index optical transmission element. Accordingly, little improvement can be expected in the application and development of this method. European Patent Application Publication No. 0208159 describes a method in which (A) at least one kind of thermoplastic polymer and (B) a monomer which is compatible with the polymer (A) when polymerized and forms a polymer having a refractive index different from that of the polymer (A), are mixed and formed into a rod-like shape. From the surface of the shaped body, the monomer (B) is volatilized to continuously distribute the monomer (B) from the surface toward the interior of the formed material, and thereafter, the non-polymerized monomer in the shaped body is polymerized to form a graded-index optical transmission plastic element. An ideal refractive index distribution curve of the distributed refractive index type optical transmission article is expressed as follows: N = N₀(1 - ar²) This curve is considered to be the same as curve " a " of Fig. 2. According to studies and measurements made by the inventor, using an Interfaco interference microscope under conditions to be explained later, however, the graded-index optical transmission element fabricated according to the above-mentioned method provides a refractive index distribution curve " b " of Fig. 2. In the range of 0.5r₀ to 0.75r₀ of radial distances from the center (i.e., in a range of c to d in the figure, with e being an outermost part), the curve b is relatively close to the ideal curve expressed by the equation (1). At the outer and inner sides of the above range, however, the refractive index distribution is greatly deviated from the ideal curve. When a grid pattern is observed through these optical transmission elements, if the optical transmission element has a refractive index pattern which almost correctly follows the quadratic curve defined by the equation (1), the article will provide a normal grid image as shown in Fig. 3(a). If, however, the refractive index distribution of the optical transmission element deviates from the ideal refractive index distribution, as indicated by (b) of Fig. 2, the article will provide a distorted grid image as shown in Figs. 3(b) and 3(c), since the article cannot transmit a correct image. In this case, a modulation transfer factor (MTF) indicating the resolution of the article is very low, i.e., less than 30%, which is not acceptable for use as an optical transmission article of a copying machine. Accordingly, the conventional graded-index optical transmission element with the refractive index distribution as shown by (b) of Fig. 2 must be cut or eluted by a solvent process to remove a portion outer than the position (d) of Fig. 2, thereby providing the optical transmission element with an optical path having a relatively ideal refractive index distribution. It is, however, difficult to provide an optical transmission element with a high resolution, and thus the productivity thereof is very low, and it is very difficult to constantly produce products having a uniform quality. DISCLOSURE OF THE INVENTION An object of the invention is to provide a method of manufacturing a plastic graded-index optical transmission element which can be applied to a copying machine employing a white light source. Compared with the conventional optical transmission elements, the optical transmission element manufactured according to the method of the invention has a higher resolution, is brighter, and obtains a remarkably improved productivity. The invention provides a method of manufacturing a plastic graded-index optical transmission element having a circular cross section of a radius r₀ and a refractive index distribution which substantially approximates a refractive index distribution curve defined by the following equation (1), at least in a limited range extending from the centre axis toward the peripheral surface of the optical transmission element: n(r) = n₀[1 - (g²/2)r²] where n₀ is the refractive index at the centre axis of the optical transmission element; n(r) is the refractive index at a position of a radius r away from the centre axis of the optical transmission element; g is the refractive index distribution constant (mm⁻¹) of the optical transmission element; and r is the distance (mm) from the centre axis toward the peripheral surface of the optical transmission element wherein, 1.4 ≦ n₀ ≦ 1.6 0.4 ≦ r₀ (mm) ≦ 0.6 0.15 ≦ g (mm⁻¹) < 0.3 characterised in that the method comprises the steps of preparing N (N > 2) uncured liquid substances each having a viscosity of between 10³ and 10⁸ poises and refractive indexes n of n₁ > n₂ > n₃ .... n N when cured, concentrically laminating the substances in such a way that the refractive indexes are successively reduced from the centre toward the periphery of the laminated substances to form an uncured strand fiber, mutually diffusing the substances between adjacent layers in such a way that the refractive indexes are continuously distributed between the layers, and at the same time or thereafter, curing the uncured strand fiber to form the optical transmission element, in that said limited range approximating equation (1) extends from 0.25r₀ to 0.70r₀, and in that said plastic graded-index optical transmission element has a modulation transfer function (hereinafter referred to as the MTF) of not less than 55%, calculated according to the following equation (2): MTF = [(i max - i min )/(i max + i min )] x 100 when a grid image of 4 line-pairs/mm is formed through the optical transmission element on a CCD line sensor and the maximum value i max and the minimum value i min of light quantities of the image are measured. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a refractive index distribution curve of an example of a plastic graded-index optical transmission element of the invention; Figure 2 shows a refractive index distribution curve of a plastic graded-index optical transmission element formed according to a prior art method; Figure 3 is a view of examples of grid images obtained by these optical transmission elements; Figure 4 is a schematic view of an apparatus for measuring the resolution of an optical transmission element; Figure 5 is a graph showing the light quantity levels of a grid image measured with a CCD sensor; Figure 6 is a schematic view of an apparatus preferably used for forming the plastic graded-index type optical transmission element of the invention; Figure 7 is a schematic view of a lens property measuring apparatus; and Fig. 8 shows a refractive index distribution curve of an example of plastic graded-index optical transmission optical element of the invention. BEST MODE OF CARRYING OUT THE INVENTION As indicated by (b) of Fig. 1, the refractive index distribution curve of the optical transmission element formed according to the invention must substantially approximate the ideal refractive index distribution curve ((a) of Fig. 1) expressed with the equation (1), at least in the range of 0.25r₀ to 0.70r₀ , preferably 0.20r₀ to 0.75r₀ , from the center axis. If a plastic graded-index optical transmission element has a refractive index distribution curve which does not approximate the refractive index distribution curve (a) of Fig. 1 expressed with the equation (1), in the range of 0.25r₀ to 0.70r₀ from the center axis, the optical transmission element cannot correctly transfer images or satisfy the requirements for an optical transmission article applied to a copying machine, and therefore, can not be applied for such a purpose. The plastic graded-index optical transmission element manufactured according to the invention must have a value n₀ of 1.4 ≦ n₀ ≦ 1.6. If the value exceeds 1.6, the optical transmission plastic element is difficult to produce, and if the value n₀ is smaller than 1.4, the difference of refractive indexes at the center axis and at the periphery of the element cannot be enlarged, to thus realize an optical transmission element having an excellent resolution and image transmitting characteristics. A value g is defined by the following equation 3: g = 2(N₀ - N r )/(N x r²) which defines a lens length and an image forming length. When the value g is greater than 0.3 mm⁻¹, the optical transmission element may have a chromatic aberration, and therefore, may not be appropriate for an optical transmission element used with a white light source. When the value g is smaller than 0.15 mm⁻¹, the optical transmission element may have a long image forming length and be difficult to handle. When the plastic graded-index optical transmission element manufactured according to the invention is used for a copying machine, etc., a plurality of such elements, instead of a single article, are usually arranged in a single row or a plurality of rows in a zigzag manner, to form an array of the optical transmission materials in which images provided by the elements partly overlap each other to form an image. The visibility of the overlapped images depends on the degree of overlapping, which is influenced by the diameter of each optical transmission element. To obtain a clear image, the radius r₀ must be in the range of 0.4 to 0.6 mm. If the element is thinner than this, the element may have an insufficient brightness and be difficult to produce with a uniform refractive index distribution. If the element is thicker than the above range, the images provided by the array of the optical transmission elements may unevenly overlap each other, and thus will not transmit clear images. The MTF, which indicates resolution of the distributed refractive index optical transmission plastic article of the invention, can be determined as follows. Namely, as shown in Fig. 4, a light source 42, a filter 43, a diffusion plate 44, a grid 45, and an array 47 composed of a plurality of plastic graded-index optical transmission elements 41 form a resolution measuring apparatus. A grid image having a spatial frequency of 4 (line-pairs/mm) is passed through the optical transmission elements 41 to form an image on a CCD line sensor 46. The formed grid image is read, and the maximum value (i max ) and the minimum value (i min ) of light quantities of the image are measured as shown in Fig. 5. According to the measured i max and i min , the MTF is calculated from the following equation (2): MTF(%) = {(i max - i min )/(i max + i min )} x 100 where the grid constant is the number of lines per millimeter (line-pairs/mm), in which each line comprises a pair of white and black lines as shown on the grid 45 of Fig. 4. The MTF of the plastic graded-index optical transmission element formed according to the invention must be not less than 55%. If the MTF is smaller than 55%, the optical transmission element provides a low resolution and cannot form a clear image when applied to a copying machine such as a facsimile. During the preparation of a plastic graded-index optical transmission element of the invention: if N ≧ 2 is established in the graded-index optical transmission element having the value g of 0.3 > g ≧ 0.15, a required difference n₁ - n N may be set between the center layer and the outermost layer of the optical transmission material, and accordingly, it will become easy to fabricate the optical transmission element of the invention having a refractive index distribution which approximates the curve of the equation (1) in the range of 0.25r₀ to 0.75r₀ from the center of the article. Accordingly, N must be 2 or greater than 2, preferably 2 to 7, and more preferably 3 to 5. When carrying out the invention, each of the uncured liquid substances must have a viscosity of between 10³ and 10⁸ poises and be curable. If the viscosity is smaller than 10³ poises, the strand will be easily broken and it will be difficult to form a strand-like shape. If the viscosity is greater than 10⁸ poises, the substance will have a poor shaping processability which will lead to a deterioration of the concentricity or the forming of irregular diameters. The liquid substances to be employed when carrying out the invention may be radical polymerizable vinyl monomers, or compositions comprising the monomers and polymers that are soluble with the monomers. The radical polymerizable vinyl monomers are, for example, methyl methacrylate (n = 1.49), styrene (n = 1.59), chlorostyrene (n = 1.61), vinyl acetate (n = 1.47), fluorized alkyl(meta)acrylate (n = 1.37 to 1.44) such as 2,2,3,3-tetrafluoropropyl(meta)acrylate, 2,2,3,3,4,4,5,5-octafluoropropyl(meta)acrylate, 2,2,3,4,4,4-hexafluoropropyl(meta)acrylate, and 2,2,2-trifluoroethyl(meta)acrylate, and (meta)acrylates having a refractive index between 1.43 and 1.62 such as ethyl(meta)acrylate, phenyl(meta)acrylate, benzyl(meta)acrylate, hydroxylalkyl(meta)acrylate, alkyleneglycoldi(meta)acrylate, trimethylolpropane-di or -tri(meta)acrylate, pentaerythritol-di, -tri, or -tetra(meta)acrylate, diglycerintetra(meta)acrylate, dipentaerythritolhexa(meta)acrylate, as well as diethyleneglycolbisallylcarbonate, fluorized alkyleneglycolpoly(meta)acrylate, etc. In order to adjust the viscosities of these cured liquid substances and make the distribution of the refractive index of an obtained strand fiber from the center toward the outside of the strand fiber, the uncured liquid substances are preferably composed of vinyl-based monomers and soluble polymers. The polymers able to be used must have a good compatibility with polymers produced from the radical polymerizable vinyl monomers. These polymers are, for example, poly(methyl methacrylate) (n = 1.49), poly(methyl methacrylate)-based copolymers (n = 1.47 to 1.50), poly-4-methylpenten-l (n = 1.46), ethylene/vinyl acetate copolymers (n = 1.46 to 1.50), polycarbonate (n = 1.50 to 1.57), poly(vinylidene fluoride) (n = 1.42), vinylidene fluoride/tetrafluoroethylene copolymers (n = 1.42 to 1.46), vinylidene fluoride/tetrafluoroethylene/ hexafluoropropene copolymers (n = 1.40 to 1.46), and poly(alkyl fluoride)(meta)acrylate polymers. To adjust the viscosities, it is preferable to use polymers having an identical refractive index for respective layers, because an optical transmission plastic article having refractive indexes continuously distributed from the center toward the surface of the material can thus be formed. In particular, poly(methyl methacrylate) has an excellent transparency and a high refractive index, and therefore, is preferably used for producing the plastic graded-index optical transmission element of the invention. To cure the strand fiber formed from the uncured substances, it is preferable to add a thermosetting catalyst and/or photocatalyst to the uncured substances. The fiber strand containing the thermosetting catalyst and/or photocatalyst is heated or irradiated with light, preferably ultraviolet light. The thermosetting catalyst may be a peroxide-based catalyst, and the photopolymerization catalyst may be benzophenone, benzoinalkylether, 4′-isopropyl-2-hydroxy-2-methyl-propiophenone, 1-hydroxycyclohexylphenylketone, benzylmethylketal, 2,2-diethoxyacetophenone, chlorothioxanthone, thioxanthone-based compounds, benzophenone-based compounds, 4-dimethylaminobenzoic ethyl, 4-dimethylaminobenzoic isoamyl, N-methyldiethanolamine, triethylamine, etc. The light source used for the photopolymerization may be a carbon arc lamp, a high-pressure mercury lamp, an ultra-high pressure mercury lamp, a low-pressure mercury lamp, a chemical lamp, a xenon lamp, or a laser beam, etc., emitting light having a wavelength of 150 to 600 nm. To produce an optical transmission element in accordance with the method of the invention, a strand fiber forming apparatus shown in Fig. 6, for example, may be used. A concentric composite nozzle 61 extrudes an uncured strand fiber 62, which is passed through a mutual diffusion portion 63 for mutually diffusing monomers of respective layers of the strand fiber to impart a refractive index distribution, as well as through a curing portion 64 for curing the uncured substances. The strand fiber is then passed between pulling rollers 65 and wound, as a plastic graded-index optical transmission element 66, around a winding portion 67. To remove volatilizing substances released from the strand fiber 62, from the mutual diffusion portion 63 and from the curing portion 64, an inert gas such as nitrogen gas is introduced from an inert gas introducing port 68, and discharged from a discharging port 69. The plastic graded-index optical transmission element obtained according to the above method may have a coating layer with a low refractive index. The coating layer can be formed by mixing trifluoroalkylacrylate, pentafluoroalkylacrylate, hexafluoroalkylacrylate, fluoroalkylenediacrylate, 1,1,2,2-tetrahydroheptadecafluorodecylacrylate, hexanedioldiacrylate, neopentylglycoldiacrylate, dipentaerythritolhexaacrylate, etc. As and when required, polymers of fluorized acrylate or methacrylate may be added to adjust the applicability and refractive indexes. In addition, it is preferable to add photopolymerization initiators. The invention will be explained in more detail with reference to Examples. The lens properties and refractive index distributions of the examples were measured as follows: I. Measurement of lens properties Evaluation apparatus The lens properties were measured with an evaluation apparatus shown in Fig. 7. Preparation of samples Each optical transmission element prepared according to the Examples was cut to a length of about one fourth (λ/4) of a period (λ) of a light beam. The period (λ) was determined from the wave form of a He-Ne laser beam passing through the element. The element was then polished with a polisher, to make both end faces of the sample parallel to each other and orthogonal to a longitudinal axis, and thus a sample to be evaluated was obtained. Measuring method As shown in Fig. 7, a sample table (76) was placed on an optical bench (71), and a sample (78) to be evaluated was placed on the sample table (76). A diaphragm (74) was adjusted so that light from a light source (72) passes through a condenser lens (73), the diaphragm (74), and a glass plate (75) and entirely irradiates an end face of the sample. Thereafter, the sample (78) and a Polaroid camera (77) were adjusted so that the light was focused on a film in a Polaroid (a trademark of the Polaroid company) camera. An image of a square grid was photographed, and the distortion of the grid was observed. The glass plate (75) was a chrome-plated photomasking glass, the chrome film on which was precisely processed to form a square grid pattern of 0.1 mm. II. Measurement of refractive index distribution An Interfaco interference microscope made by the Carl Zeiss company was used for this measurement. Example 1 Poly(methyl methacrylate) ([η] = 0.34 measured in methylethylketone (MEK) at 25°C) of 52 parts by weight, benzylmethacrylate of 35 parts by weight, methylmethacrylate of 13 parts by weight, 1-hydroxycyclohexylphenylketone of 0.2 parts by weight, and hydroquinone of 0.1 parts by weight were heated and mixed at 60°C to form an uncured substance as an original liquid for forming a first layer (a central portion). Poly(methyl methacrylate) ([η] = 0.34 measured in MEK at 25°C) of 50 parts by weight, benzylmethacrylate of 15 parts by weight, methylmethacrylate of 35 parts by weight, 1-hydroxycyclohexylphenylketone of 0.2 parts by weight, and hydroquinone of 0.1 parts by weight were heated and mixed at 60°C to form an uncured substance as an original liquid for forming a second layer. Poly(methyl methacrylate) ([η] = 0.34 measured in MEK at 25°C) of 50 parts by weight, methylmethacrylate of 50 parts by weight, 1-hydroxycyclohexylphenylketone of 0.2 parts by weight, and hydroquinone of 0.1 parts by weight were heated and mixed to form an uncured substance as an original liquid for forming a third layer (outer layer portion). The three kinds of the original liquids were simultaneously extruded from a composite nozzle to form a concentric strand fiber. During this extrusion, the viscosity of the first layer components was 4.7 x 10⁴ poises, that of the second layer components was 3.7 x 10⁴ poises, and that of the third layer components was 2.9 x 10⁴ poises. The temperature of the composite nozzle was 60°C. As shown in Fig. 6, the strand fiber 62 extruded from the nozzle was passed through the mutual diffusion portion (63), 45 cm in length, to mutually diffuse the monomers between layers of the strand fiber. Thereafter, the strand fiber was passed through the center of a light irradiating portion composed of 12 fluorescent lamps (120 cm long, output 40 W) equidistantly arranged in a ring shape, at a speed of 40 cm/min, and as a result, the monomers in the strand fiber were polymerized to form a plastic graded-index optical transmission element which was then drawn by nip rollers. When forming the strand fiber, a ratio f discharged quantities of the first, second, and third layers was 7:4:1. The plastic graded-index optical transmission element thus produced had a radius (r₀) of 0.59 mm, distributed refractive indexes measured by the Interfaco interference microscope of 1.508 at a central portion and 1.498 at a peripheral portion, and a refractive index distribution constant (g) of 0.20 mm⁻¹. As shown in Fig. 8, the refractive index distribution of the element substantially approximated the equation (1) in the range of 0.15r₀ to 0.75r₀ extending from the center toward the external face of the element. Both end surfaces of the optical transmission element were polished to a lens length of 18.4 mm, and an MTF thereof was measured with a grid of 4 line-pairs/mm; the MTF was 60% at a conjugate length of 42.4 mm. The obtained grid image was clear with only minor distortion. A plurality of the optical transmission elements were employed to form an optical transmission element array having a lens length of 18.4 mm as indicated by the numeral 47 in Fig. 4, the MTF thereof was measured with a grid of 4 line-pairs/mm, and it was found that the MTF was 52%. At this time, the conjugate length of the rod-like lens forming the optical transmission element array was 42.4 mm. This optical transmission element array, an LED light source, and a light receiving CCD element were assembled into an image scanner, which provided a high resolution and was able to transmit clear images. Example 2 The three kinds of original liquids used in the Example 1 and a fourth layer forming original liquid made of poly(methyl methacrylate) ([η] = 0.34 measured in MEK at 25°C) of 47 parts by weight, methylmethacrylate of 40 parts by weight, 2,2,3,3,4,4,5,5-octafluoropentylmethacrylate of 13 parts by weight, 1-hydroxycyclohexylphenylketone of 0.2 parts by weight, and hydroquinone of 0.1 part by weight were heated and mixed at 60°C to form uncured substances, and thereafter, the four kinds of original liquids were simultaneously extruded from a concentric four-layer composite spinning nozzle to form a concentric strand fiber. During this extrusion, the viscosities of the first, second, and third layers were substantially the same as those of the Example 1, and the viscosity of the fourth layer-forming components was 2.5 x 10⁴ poises. The temperature of the composite nozzle was 60°C. Thereafter, the same processes as in Example 1 were carried out to provide a plastic graded-index optical transmission element. When forming the strand fiber, the ratio of discharged quantities of the first, second, third, and fourth layers was 7:4:1:0.5. The optical transmission element thus produced had a radius (r₀) of 0.60 mm, distributed refractive indexes measured by the Interfaco interference microscope of 1.507 at a central portion and 1.496 at a peripheral portion, and a refractive index distribution constant (g) of 0.20 mm⁻¹. The refractive index distribution of the article substantially approximated the equation (1) in the range of 0.15r₀ to 0.80r₀ extending from the center toward the external face of the element. Both end surfaces of the optical transmission element were polished to a lens length of 18.4 mm, the MTF thereof was measured with a grid of 4 line-pairs/mm, and it was found that the MTF was 65% at a conjugate length of 42.4 mm. A plurality of the optical transmission elements were employed to form an optical transmission article array having a lens length of 18.4 mm, in a manner similar to the Example 1. The MTF of the array, measured with a grid of 4 line-pairs/mm, was 58% at a conjugate length of 42.4 mm. This optical transmission element array, an LED light source, and a light receiving CCD element were assembled into an image scanner, which provided a high resolution and was able to transmit clear images. Example 3 The four kinds of original liquids used in the Example 2 were used as original liquids for forming first to fourth layers. Poly(methyl methacrylate) ([η] = 0.34 measured in MEK at 25°C) of 40 parts by weight, methylmethacrylate of 18 parts by weight, 2,2,3,3,4,4,6,6-octafluoropentylmethacrylate of 42 parts by weight, 1-hydroxycyclohexylphenylketone of 0.2 parts by weight, and hydroquinone of 0.1 part by weight were heated and mixed at 60°C to form a fifth layer, and thereafter, the five kinds of original liquids were simultaneously extruded from a composite nozzle to form a concentric strand fiber. During this extrusion, the viscosities of the first, second, third, and fourth layers were substantially the same as those of Example 2, and the viscosity of the fifth layer-forming components was 2.2 x 10⁴ poises. The temperature of the composite nozzle was 60°C. Thereafter, the same processes as in Example 1 were carried out to provide a plastic graded-index optical transmission element. When forming the strand fiber, the ratio of discharged quantities of the first, second, third, fourth, and fifth layers was 7:4:1.1:0.6:0.4. The optical transmission element thus produced had a radius (r₀) of 0.60 mm, distributed refractive indexes measured by the Interfaco interference microscope of 1.507 at a central portion and 1.494 at a peripheral portion, and a refractive index distribution constant (g) of 0.20 mm⁻¹. The refractive index distribution of the element substantially approximated the equation (1) in the range of 0.15r₀ to 0.85r₀ extending from the center toward the external surface of the element. Both end surfaces of the optical transmission element were polished to a lens length of 17.8 mm, the MTF thereof was measured with a grid of 4 line-pairs/mm, and it was found that the MTF was 72% at a conjugate length of 32.6 mm. A plurality of the optical transmission elements were formed into an optical transmission element array having a lens length of 17.8 mm, in a manner similar to Example 1. The MTF of the array measured with the grid of 4 line-pairs/mm was 65% at a conjugate length of 32.6 mm. This optical transmission element array, an LED light source, and a light receiving CCD element were assembled into an image scanner, which provided a high resolution and was able to transmit clear images. Comparative Example 1 The same four kinds of original liquids as those used in the Example 2 were employed at a ratio of discharged quantities of the first, second, third, and fourth layers of 1:1:1:1. The other conditions were the same as those of Example 2 in forming a strand fiber. The monomers were diffused, and the curing process was carried out to prepare an optical transmission element having a radius (r₀) of 0.55 mm, distributed refractive indexes measured by the Interfaco interference microscope of 1.506 at a central portion and 1.486 at a peripheral portion, and a refractive index distribution constant (g) of 0.29 mm⁻¹. The refractive index distribution of the element agreed with the equation (1) for only about 10% of the radial range. Both end surfaces of the optical transmission element were polished to a lens length of 13.5 mm, and the MTF of the element measured with a grid of 4 line-pairs/mm, was 22% at a conjugate length of 24.7 mm. A plurality of the optical transmission elements were assembled into an optical transmission article array having a lens length of 13.5 mm in a similar manner to Example 1. The MTF of the array measured with the grid of 4 line-pairs/mm was 19% at a conjugate length of 24.7 mm. This optical transmission element array, an LED light source, and a light receiving CCD element were assembled into an image scanner, which provided a very poor resolution. Example 4 Poly(methyl methacrylate) ([η] = 0.34 measured in MEK at 25°C) of 51 parts by weight, benzylmethacrylate of 20 parts by weight, methylmethacrylate of 29 parts by weight, 1-hydroxycyclohexylphenylketone of 0.2 parts by weight, and hydroquinone of 0.1 part by weight were heated and mixed at 60°C to form an uncured substance as an original liquid for forming a first layer. The original liquid for forming the third layer used in the Example 1 was used as an original liquid for forming a second layer. The original liquid for forming the fourth layer used in the Example 2 was used as an original liquid for forming a third layer. These three kinds of original liquids were used to prepare a plastic graded-index optical transmission material in a manner similar to the Example 1. At this time, the viscosity of the first layer-forming components was 4.5 x 10⁴ poises. When forming a strand fiber, the ratio of discharged quantities of the first, second, and third layers was 7:3:1. The prepared optical transmission element had a radius (r₀) of 0.46 mm, distributed refractive indexes measured by the Interfaco interference microscope of 1.500 at a central portion and 1.490 at a peripheral portion, and a refractive index distribution constant (g) of 0.25 mm⁻¹. The refractive index distribution of the element substantially approximated the equation (1) in the range of 0.15r₀ to 0.81r₀ extending from the center toward the external face of the article. Both end surfaces of the optical transmission element were polished to a lens length of 15.6 mm, the MTF thereof was measured with a grid of 4 line-pairs/mm, and it was found that the MTF was 62% at a conjugate length of 29.0 mm. A plurality of the optical transmission elements were employed to form an optical transmission element array having a lens length of 15.6 mm in a manner similar to Example 1. The MTF of the array measured with the grid of 4 line-pairs/mm was 55% at a conjugate length of 29.0 mm. This optical transmission element array, an LED light source, and a light receiving CCD element were assembled into an image scanner, which provided a high resolution and was able to transmit clear images. Example 5 A polymer [A] (n₀ = 1.456, [η] = 1.00 measured in MEK at 25°C) of 50 parts by weight composed of methylmethacrylate of 50% by weight and 2,2,3,3-tetrafluoropropylmethacrylate of 50% by weight, methylmethacrylate of 50 parts by weight, 1-hydroxycyclohexylphenylketone of 0.2 part by weight, and hydroquinone of 0.1 part by weight were heated and mixed at 60°C to form an uncured substance as an original liquid for forming a first layer. The above polymer [A] of 48 parts by weight, 2,2,3,3-tetrafluoropropylmethacrylate of 22 parts by weight, methylmethacrylate of 30 parts by weight, 1-hydroxycyclohexylphenylketone of 0.2 part by weight, and hydroquinone of 0.1 part by weight were heated and mixed at 60°C to form an uncured substance as an original liquid for forming a second layer. The polymer [A] of 46 parts by weight, 2,2,3,3-tetrafluoropropylmethacrylate of 44 parts by weight, methylmethacrylate of 10 parts by weight, 1-hydroxycyclohexylphenylketone of 0.2 part by weight, and hydroquinone of 0.1 part by weight were heated and mixed at 60°C to form an uncured substance as an original liquid for forming a third layer. The three kinds of original liquids were spun to form a composite in a manner similar to Example 1 at the viscosities of 4.0 x 10⁴ poises for the first layer-forming components, 3.3 x 10⁴ poises for the second layer-forming components, and 3.1 x 10⁴ poises for the third layer-forming components. When forming the strand fiber, the ratio of discharged quantities of the first, second, and third layers was 7:4:1. The prepared optical transmission element had a radius (r₀) of 0.50 mm, distributed refractive indexes measured by the Interfaco interference microscope of 1.472 at a central portion and 1.459 at a peripheral portion, and a refractive index distribution constant (g) of 0.27 mm⁻¹. The refractive index distribution of the element substantially approximated the equation (1) in the range of 0.15r₀ to 0.78r₀ extending from the center toward the external face of the element. Both end surfaces of the optical transmission element were polished to a lens length of 14.0 mm, the MTF thereof was measured with a grid of 4 line-pairs/mm, and it was found that the MTF was 64% at a conjugate length of 29.0 mm. A plurality of the optical transmission elements were employed to form an optical transmission element array having a lens length of 14.0 mm in a manner similar to Example 1. The MTF of the array measured with the grid of 4 line-pairs/mm was 57% at a conjugate length of 29 mm. This optical transmission element array, an LED light source, and a light receiving CCD element were assembled into an image scanner, which provided a high resolution and was able to transmit clear images. INDUSTRIAL APPLICABILITY Compared with an optical transmission element of the prior art of the same kind, the plastic graded-index optical transmission element manufactured according to the invention substantially approximates the ideal distribution curve of the equation (1), at least in the range of 0.25r₀ to 0.75r₀ from the center of the element, and therefore, without a cutting of the peripheral portion thereof, the element of the invention provides excellent lens characteristics. The plastic graded-index optical transmission element of the invention, therefore, can be applied to an image transmitting array of a copying machine employing a white light source, and to an optical transmission line such as a near parabolic optical fiber, a rod-like convergent lens, and an optical sensor. The optical transmission element prepared according to the invention can be efficiently manufactured by a multilayer extrusion method of concentrically extruding two or more layers from uncured substances. LIST OF REFERENCE MARKS 41: optical transmission element 42: light source 43: filter 44: diffusion plate 45: grid 46: CCD line sensor 47: array 61: composite nozzle 62: uncured strand fiber 63: mutual diffusion portion 64: curing portion 65: take-up roller 66: optical transmission element 67: winding portion 71: optical bench 72: light source 73: condenser lens 74: diaphragm 75: glass plate 76: sample table 77: Polaroid camera 78: sample

Claims: A method of manufacturing a plastic graded-index optical transmission element having a circular cross section of radius r₀ and a refractive index distribution which substantially approximates a refractive index distribution curve defined by the following equation (1) at least in a limited range extending from the centre axis toward the peripheral surface of the optical transmission element: n(r) = n₀[1 - (g²/2)r²] where n₀ is the refractive index at the centre axis of the optical transmission element; n(r) is the refractive index at a position of radius r away from the centre axis of the optical transmission element; g is the refractive index distribution constant (mm⁻¹) of the optical transmission element; r is the distance (mm) from the centre axis toward the peripheral surface of the optical transmission element; and 1.4 ≦ n₀ ≦ 1.6 0.4 ≦ r₀ (mm) ≦ 0.6 0.15 ≦ g (mm⁻¹) < 0.3 characterised in that the method comprises the steps of preparing N (N ≧ 2) uncured liquid substances each having a viscosity between 10³ and 10⁸ poises and refractive indexes n of n₁ > n₂ > n₃ ..... n N when cured, concentrically laminating the substances in such a way that the refractive indexes are successively reduced from the centre toward the periphery of the laminated substances to form an uncured strand fibre, mutually diffusing the substances between adjacent layers in such a way that the refractive indexes are substantially continuously distributed between the layers, and at the same time or thereafter, curing the uncured strand fibre, to form the optical transmission element, in that said limited range approximating equation (1) extends from 0.25r₀ to 0.70r₀, and in that when a grid image of 4 line-pairs/mm is formed through the optical transmission element on a CCD line sensor and the maximum value i max and the minimum value i min of light quantities of the image are measured, the modulation transfer function (MTF) calculated according to the following equation (2): MTF = [(i max - i min )/(i max + i min )] x 100 is at least 55%. A method of manufacturing a plastic graded-index optical transmission element according to claim 1 wherein the uncured liquid substances comprise a mixture of poly(methyl methacrylate) and radical polymerisable vinyl monomers. A method of manufacturing an optical transmission element array comprising manufacturing a plurality of graded-index optical transmission elements according to the method of claim 1 or claim 2 and assembling said elements in a single line or a plurality of lines. A method according to any preceding claim in which the refractive index distribution of the plastic optical transmission element substantially approximates the refractive index distribution curve defined by the equation (1) at least in the range of 0.20r₀ to 0.75r₀ extending from the centre axis toward the peripheral surface of the optical transmission element.

Inventors: ISHIMARU TERUTA MITSUBISHI RAY, MISHINA YOSHIHIKO MITSUBISHI R, MURATA RYUJI MITSUBISHI RAYON, ODA MASAAKI MITSUBISHI RAYON C, TOYODA N MITSUBISHI RAYON CO L, UOZU YOSHIHIRO MITSUBISHI RAYO, ISHIMARU, TERUTA MITSUBISHI RAYON CO., LTD., MISHINA, YOSHIHIKO MITSUBISHI RAYON CO., LTD., MURATA, RYUJI MITSUBISHI RAYON CO., LTD., ODA, MASAAKI MITSUBISHI RAYON CO., LTD., TOYODA, N. MITSUBISHI RAYON CO. LTD., UOZU, YOSHIHIRO MITSUBISHI RAYON CO., LTD.
Applicants: MITSUBISHI RAYON CO, MITSUBISHI RAYON CO., LTD.
Application year: 1989
Main classification: G02B6
IPC codes: G02B3, G02B6