Patent Publication Number: US-2019170909-A1

Title: Method of forming film of lens and fresnel lens

Description:
TECHNICAL FIELD 
     The technical field relates to a method of forming a film of a lens and a Fresnel lens. 
     BACKGROUND 
     As an optical member for displaying video and the like by extending a light source, a Fresnel lens is used. In the Fresnel lens, a plurality of grooves are formed in parallel straight lines on a lens surface  102  of a Fresnel lens  101  as shown in a perspective view of  FIG. 1A . A cross-sectional view obtained by enlarging a portion A of the lens surface  102  in  FIG. 1A  is shown in  FIG. 1B . As shown in the cross-sectional view, the grooves  103  on the lens surface  102  of the Fresnel lens  101  form side walls which are not vertical but slightly inclined. 
     There has been a case where optical characteristics are deteriorated when a projection light is radiated in the Fresnel lens  101 . That is because the projection light is reflected on side walls of the grooves  103  formed on the lens surface  102  of the Fresnel lens  101  and the reflected light becomes scattered light inside the Fresnel lens  101  thereby causing deterioration of optical characteristics when the projection light is incident on the lens surface  102  of the Fresnel lens  101 . 
     Accordingly, in order to suppress the scattered light on the side walls of the grooves  103 , there is disclosed in JP-A-H4-163539(Patent Literature 1) that a light absorber  104  is applied on the side walls of the grooves  103  formed on the lens surface  102  of the Fresnel lens  101  as in a cross-sectional view of the Fresnel lens  101  shown in  FIG. 1C . 
     As a method of forming the light absorber  104  on the side walls of the grooves  103  formed on the lens surface  102  of the Fresnel lens  101 , there is a method of forming the light absorber  104  by sticking a film in which the light absorber  104  is formed to each other as described in, for example, JP-A-S58-186732 (Patent Literature 2). 
     However, positioning and so on are necessary when the film in which the light absorber  104  is formed is stuck to each other, therefore, the method is limited to lenses in which the grooves  103  have a certain degree of size. 
     Moreover, the film in which the light absorber  104  is formed has to be fabricated for each design of the Fresnel lens  101 , which takes cost and time for obtaining the grooves  103  with an arbitrary shape. 
     It is necessary to form a large number of grooves  103  with an arbitrary minute shape for reducing the size and improving characteristics of the Fresnel lens  101 , and a method of forming the light absorber  104  that blocks light on side walls of the grooves  103  is required. 
     In recent years, as a method of forming a fine pattern at low cost in a simple manner, a coating method using ink jet is widely used. The coating method using ink jet is a method of performing coating by discharging a small amount of ink to prescribed positions from plural nozzles provided in coating heads of an ink-jet apparatus. Ink containing the light absorber  104  is discharged to groove inclined portions on the lens surface  102  of the Fresnel lens by the ink-jet method, thereby forming the light absorber  104  on side walls of minute grooves formed at arbitrary positions of the Fresnel lens. 
     However, the ink discharged by ink jet is generally directed to a vertical direction, therefore, an object to be coated by ink jet is a flat shape. Accordingly, it is difficult to coat the side walls of the grooves  103  in the Fresnel lens  101  with ink. 
     Accordingly, as a method of coating a side wall of an object by ink jet, a method shown in JP-A-2008-263094 (Patent Literature 3) is proposed. A side view of  FIG. 2A  shows a coating method by ink jet according to Patent Literature 3. 
     In  FIG. 2A , a strip-like insulator as an object  106  is set on a conveying belt  105 . Coating heads  107  of ink jet are installed obliquely upward. A paste  108  for a side-surface electrode is discharged from the coating heads  107  from oblique directions toward side surfaces of the object  106 . The side surfaces of the strip-like object  106  are coated with the paste  108  for the side-surface electrode. 
       FIG. 2B  is a perspective view showing a state after the strip-like insulator as the object  106  is coated with the paste  108  for the side-surface electrode. As the paste  108  for the side-surface electrode is discharged from oblique directions while conveying the strip-like insulator on the conveying belt, the entire side walls of the strip-like object  106  can be coated with the paste  108  for the side-surface electrode. 
     A coating method used when the side walls of the grooves  103  of the Fresnel lens  101  are coated by the method shown in Patent Literature 3 is shown in  FIG. 3A . A Fresnel lens  110  is installed in a jig  111  for conveyance. Then, an ink-jet coating head  112  is installed obliquely upward of the jig  111  for conveyance. 
     According to the structure, an ink  113  containing the light absorber  104  is discharged obliquely from the coating head  112  while moving the jig to which the Fresnel lens  101  is fixed. 
     A cross-sectional view of  FIG. 3B  is a view showing a case where the ink  113  containing the light absorber  104  is discharged to a side wall  114  of the groove  103  in the Fresnel lens  110 . The ink  113  containing the light absorber  104  is discharged toward the side wall  114  of the groove  103  in the Fresnel lens  110 . Accordingly, the light absorber  104  is hit on the side wall  113 . After the ink  113  containing the light absorber  104  is hit on the side wall  114  of the groove  103  in the Fresnel lens  110 , the light absorber  104  is formed on the side wall  114  of the groove through drying, curing and so on. 
     SUMMARY 
       FIG. 4A  shows a cross-sectional view showing a case where the light absorber  104  is formed on the side wall  114  of the groove  103  in the Fresnel lens  101  by the method shown in Patent Literature 3. As shown in the cross-sectional view, after the ink  113  containing the light absorber  104  is hit on the side wall  114  by ink jet, the ink  113  flows to a lower bottom surface of the side wall  114  along the side wall  114 . After that, the ink  113  is spread further from the bottom surface. Accordingly, the light absorber  104  is wetly spread not only to a width of the side wall  114  but also over a large distance from the bottom surface. 
       FIG. 4B  shows a top view showing a case where the light absorber  104  is formed on the side wall  114  of the groove  103  in the Fresnel lens  101 . As shown in the top view, the light absorber  104  is formed not only in a width  115  of the side wall  114  of the Fresnel lens  101  but in a wider width  116  on the lower bottom surface of the side wall  114 . When projection light is incident on the Fresnel lens  101  in this case, light is absorbed in the light absorber  104  formed widely on the lower bottom surface of the side wall  114  in the Fresnel lens  101 , which makes the shade and largely reduces optical characteristics of the lens. 
     The present disclosure has been made for solving the above problem, and an object thereof is to provide a method of forming a film of a lens and a Fresnel lens capable of suppressing spreading of the light absorber formed in plural grooves. 
     In order to solve the above problem, a Fresnel lens including a Fresnel lens body, grooves formed on the lens surface of the Fresnel lens body and having bottom surfaces and inclined surfaces and a light absorber formed over the inclined surfaces and the bottom surfaces in which a contact angle at an end portion of the light absorber with respect to the bottom surface is larger than 70 degrees is used. 
     Moreover, a Fresnel lens including a Fresnel lens body, grooves formed on a lens surface of the Fresnel lens body and having bottom surfaces and inclined surfaces and a light absorber formed over the inclined surfaces and the bottom surfaces in which a layer having a thinner thickness than thicknesses of other parts is provided at an end portion of the light absorber is used. 
     Moreover, a Fresnel lens including a Fresnel lens body, grooves formed on the lens surface of the Fresnel lens body and having bottom surfaces and inclined surfaces and a light absorber formed over the inclined surfaces and the bottom surfaces in which an end portion of the light absorber has a corrugated shape is used. 
     Furthermore, a method of forming a film of a lens including the steps of coating inclined portions of a plurality of grooves formed on a lens surface with ink, fixing the ink and removing unnecessary portions of the ink by laser is used. 
     As described above, an ink-jet printing method according to the present disclosure can execute reciprocating printing by using a photocurable ink without changing conditions such as the UV radiation dose between forward and backward paths as well as without giving the feeling of strangeness in color for human eyes to thereby realize both tact and quality in the ink-jet printing method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a related-art Fresnel lens,  FIG. 1B  is an enlarged cross-sectional view of groove portions of the Fresnel lens and  FIG. 1C  is a cross-sectional view obtained at the time of forming a light absorber in the groove portions of the Fresnel lens; 
         FIG. 2A  is a cross-sectional view showing a coating method to sections by a related-art ink-jet method and  FIG. 2B  is a perspective view obtained at the time of coating side walls of a rectangular work by the related-art ink-jet method; 
         FIG. 3A  is a cross-sectional view showing a method of forming a light absorber in groove portions by the related-art ink-jet method, and  FIG. 3B  is a cross-sectional view showing a method of forming the light absorber in groove portions by the related-art ink-jet method; 
         FIG. 4A  is a cross-sectional view obtained at the time of forming a light absorber in groove portions by the related-art ink-jet method, and  FIG. 4B  is a top view obtained at the time of forming the light absorber in groove portions by the related-art ink-jet method; 
         FIG. 5A  is a perspective view of a Fresnel lens according to an embodiment,  FIG. 5B  is a cross-sectional view obtained at the time of forming a light absorber in groove portions of the Fresnel lens according to the embodiment and  FIG. 5C  is a cross-sectional view obtained by removing the light absorber of the Fresnel lens according to the embodiment by laser; 
         FIG. 6A  is a cross-sectional view showing a state just after the related-art Fresnel lens is coated with ink,  FIG. 6B  is a cross-sectional view obtained at the time of removing the light absorber of the Fresnel lens according to the embodiment by laser and  FIG. 6C  is a cross-sectional view obtained by removing the light absorber of the Fresnel lens according to the embodiment by laser; 
         FIG. 7A  is a perspective view showing a state where the related-art Fresnel lens is coated with ink and  FIG. 7B  is a perspective view obtained at the time of removing the light absorber of the Fresnel lens according to the embodiment by laser; and 
         FIG. 8A  to  FIG. 8D  are plan views showing examples of ink curves according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Structure of a Fresnel Lens 
     A Fresnel lens according to an embodiment is shown in a perspective view of  FIG. 5A . A plurality of grooves  3  are formed in parallel straight lines on a lens surface  2  of a Fresnel lens  1 .  FIG. 5B  and  FIG. 5C  are cross-sectional views obtained by enlarging a portion A of the lens surface  2 . The grooves  3  have a V-shape in this case, however, the shape is not limited to the V-shape. One of both flat surfaces of the V-shape may be horizontal or vertical. 
     A base material and a thickness for forming the Fresnel lens  1  are not particularly limited but transparent materials with no coloring and turbidity are preferable in consideration of optical characteristics. An antireflection film may be formed on a surface of the lens according to need. 
     A method of forming the grooves  3  in the Fresnel lens  1  is not particularly limited, which includes a method of cutting a base material, an injection molding of a resin material, a transfer method by heating, etc. However, it is necessary to form the grooves  3  in accordance to a design value for securing optical characteristics. In this case, it is necessary that burrs or stains do not remain due to processing of the grooves  3 . 
     In the structure of the grooves  3  in the Fresnel lens  1 , the grooves  3  are not formed vertically but formed with a slightly inclined angle as shown in  FIG. 5B  and  FIG. 5C  to thereby allow the Fresnel lens  1  to have optical characteristics. 
     When the Fresnel lens  1  transmits given light, diffracted light is generated at inclined portions  4 , or bottom surfaces and corner portions at apexes of the grooves  3 . Accordingly, the inclined portions  4  are coated with ink  6  having light shielding property for suppressing diffracted light. The inclined portions  4  of the grooves  3  coated with the ink  6  are shown in  FIG. 5B . 
     Coating Method of Fresnel Lens 
     In the present embodiment, a process of forming a film by the ink  6  and a process of removing unnecessary portions by laser are included. 
     Ink 
     As the ink  6 , a black color having a high optical absorption rate is preferable. However, the color of the ink does not matter as far as diffracted light can be suppressed. 
     A material of the Fresnel lens  1  is a resin material. The ink  6  that hardly permeates and is hardly absorbed into the resin material is used. 
     Accordingly, an ultraviolet curable ink or a solvent-type ink containing a volatile solvent is preferably used as the ink  6 . This is for suppressing spreading of the ink  6 . 
     The ultraviolet curable ink is an ink containing a color pigment of black or the like such as carbon particles in an ultraviolet curable resin. After the Fresnel lens  1  is coated with the ink  6 , the ink  6  is cured and fixed by ultraviolet rays. The ultraviolet curable resin with a peak curing wavelength in a range from 350 nm to 450 nm may be preferably used. 
     The solvent-type ink is an ink in which a color dye is mixed into the volatile solvent, for example, alcohol and the like. When the volatile ink solvent is used, the ink  6  can be fixed by volatilizing a solvent component after the inclined portions  4  are coated with the ink  6 . 
     Laser Processing 
     The coating of the inclined portions  4  with the ink  6  can be performed by various methods as described above. However, it is difficult to control a width of the ink  6  used for coating as the ink  6  spreads due to the accuracy of the coating method, the difference in surface condition between the ink  6  and the material of the Fresnel lens  1  and so on. Accordingly, the width of the ink is widened. 
     It is necessary to process a line width of the ink  6  to be 50 μm or less according to optical characteristics. Accordingly, excessive portions of the ink  6  are removed by a laser  5  as shown in  FIG. 5C . The unnecessary portion of the ink  6  on the inclined portion  4  is irradiated with the laser  5  to remove the portion, thereby processing the ink to be a state of an ink  7 . 
     As the laser  5 , a green laser (wavelength 500 nm to 550 nm) is used for removing only the ink  6  without damaging the transparent material of the Fresnel lens  1 . Accordingly, even when a region where the ink  6  does not exist is irradiated with the laser  5 , the Fresnel lens  1  is hardly damaged by the laser  5 . 
     An irradiation diameter of the laser  5  is reduced by an aperture of a lens, for example, a FΘ lens to be approximately ϕ20 μm to 50 μm. The narrowed laser  5  is radiated so as to correspond to a removal amount of the ink  6  in the inclined portion  4 , thereby to remove the ink. 
     Cumulative irradiation time of the laser  5  for removing the ink  6  to be removed is adjusted in accordance with an amount of ink to be removed. However, when the amount of ink  6  to be removed is large and damage to the material of the Fresnel lens  1  around the ink  6  occurs, intervals between irradiation time and irradiation stop time are adjusted and pulse irradiation of the laser  5  is performed, thereby removing only the ink  6  by the laser  5  without damaging the Fresnel lens  1 . 
     As for an alignment method between the irradiation position of the laser  5  and the groove  3 , positional adjustment can be performed by providing a cross-shaped alignment mark on an outside of a lens effective region in the Fresnel lens  1 . The shape of the alignment mark is not limited to the cross-shaped mark but may be a circle shape and the like as far as relative positional relation between the irradiation position on the laser device&#39;s side and the Fresnel lens can be secured by a recognition method using a camera or the like. 
     Details of Laser Processing 
     Cross-sectional shapes of the ink  7  processed by the laser  5  will be shown in  FIG. 6A  to  FIG. 6C . 
     A case where inclined portions  21  of the grooves  3  are coated with an ink  23  is shown in  FIG. 6A . An end portion of the ink  23  applied to the inclined portion  21  slightly protrudes due to surface tension. A contact angle  24  of the end portion with respect to a bottom surface  22  is 70 degrees or less. 
     The unnecessary portion of the ink  23  existing on the bottom surface  22  is removed by the laser  5 . As a result, a contact angle  25  at the end portion of the ink becomes larger than 70 degrees as shown in  FIG. 6B . The end portion is preferably a fracture surface with the contact angle  25  of 80 degrees or more. 
     The end portion of the ink  23  is held by the surface tension in  FIG. 6A , and a film thickness is gradually reduced at the endmost portion of the ink  23 . Accordingly, a portion where the projection light can be blocked and a portion where the projection light is not blocked are generated stepwise, therefore, it is difficult to obtain good contrast. 
     On the other hand, the contact angle  25  at the end portion of the ink  23  is 80 degrees or more in  FIG. 6B . Accordingly, there is no portion where the film thickness of the ink  23  is gradually reduced, and there is an effect that the projection light can be blocked at the end portion of the ink  23  with good contrast. 
     In order to minimize heat damage at the time of removing the ink, it is preferable that an ink  26  with a thinner thickness than other parts remains as shown at a circle place of  FIG. 6C . When the ink remains as a thin film with a thickness of approximately 1 μm, damage to the bottom surface  22  by the laser  5  is prevented, and further, the ink becomes hardly recognized by human eyes. 
     The film thickness at the endmost portion of the ink  23  in  FIG. 6C  can be controlled not in the portion where the film thickness is gradually reduced but in the portion where the projection light can be blocked and the portion transmitting light (portion of the thin ink  26  of approximately 1 μm) in the same manner as described above. 
     The thickness of the thin ink  26  does not always have to be 1 μm as long as it is 5 μm or less. The thickness is preferably at least 0.1 μm or more to 5 μm or less. 
     Detailed Front Shape of Ink  7   
     The detailed front shape of the ink  7  formed by the laser  5  will be explained with reference to  FIG. 7A  and  FIG. 7B . In a case where an ink  33  is just applied to an inclined portion  31 , an ink  34  on a bottom surface  32  makes a random curve due to surface tension as shown in  FIG. 7A . It is difficult to control the ink  34  spreading on the bottom surface  32  and a width sometimes reaches 50 μm or more. 
     Accordingly, unnecessary ink protruding to the bottom surface  32  is removed by the laser  5  as shown in  FIG. 7B . Accordingly, an ink curve  35  having regular cycles can be formed. It is preferable to form a line including one or more flection points or a curve formed of an aggregation of points. The reason of the above is as follows. The diffracted light may be slightly generated in the portion shielded by the ink and the portion where there is no ink and transmitting light. Accordingly, when a boundary line between the light shielding portion and the transmitting portion is a straight boundary line with a constant distance or a simple boundary line, it may be recognized as diffracted light by human eyes. Therefore, the ink curve  35  that is not a simple boundary line and includes regular cycles is formed. The ink curve  35  is provided with, for example, flection points or non-continuous points. 
     Examples of the ink curve  35  having regular cycles are shown in plan views of  FIG. 8A  to  FIG. 8D . 
     In  FIG. 8A , a curve is a cyclic line. In  FIG. 8B , a curve is a cyclic polygonal line. In  FIG. 8C , a curve is a cyclic curve with a notch  36  or a projection  37  at part of a cyclic curve. In  FIG. 8D , a curve has a straight-line unevenness. All curves are not simple. 
     It is preferable to form a straight-line shape or free curves by moving the laser  5  on a prescribed track while performing irradiation of laser  5 . 
     Effect 
     As the applied ink can be removed without damaging the Fresnel lens in the method of producing the Fresnel lens according to the present disclosure, it is possible to coat only the side wall having an almost vertical shape and part of the bottom surface with the ink in a line width of 50 μm or less. 
     The method of producing the Fresnel lens according to the present disclosure can be widely used for forming a functional film or for performing a three-dimensional design decoration with respect to side walls of devices having a three-dimensional structure.