Technical Field

The present disclosure relates to a retro-reflective license plate and a retro- reflective sheet used therein.

Background Art

License plates (also known as number plates) attached to vehicles may be read by ALPR (Automated License Plate Reader) systems in order check for vehicle speeding violations, to investigate vehicles involved in crimes, and the like. Because ALPR systems use infrared cameras to read conventional coating-type license plates, when a retro-reflective license plate is used, the infrared light returned to the infrared camera is excessive, which generates halation. When halation is generated, the readability of the character information on the license plate is inhibited in the ALPR. Thus, Japanese Unexamined Patent Application Publication No. 2013-508749A (US Application Publication US 2012/0200710) discloses a number plate using a retro- reflective sheet including an infrared-opaque material in order to reduce the generation of halation.

Summary

Providing a layer containing an infrared-opaque material in the retro-reflective sheet in order to reduce the generation of halation as in the Patent Literature described above results in the whiteness of the license plate being reduced or the like, in the driver of the following vehicle not being able to easily read the characters on the license plate, and in a tendency for the visibility of the license plate to be reduced. That is, further improvements are needed to achieve both of a reduction in the generation of halation and an enhancement in the visibility to the driver.

One aspect on the present disclosure is a retro-reflective license plate with identification markings comprising: a plate base; and a retro-reflective sheet provided on the plate base and having a cube corner element, wherein the retro-reflective sheet has a retro-reflection coefficient which, upon an upper section and a lower section of the retro-reflective sheet being defined according to an orientation of the

identification markings and light being incident from a position shifted toward the upper section side from a normal direction of the retro-reflective sheet, decreases as an incident angle of the light is increased, and the retro-reflection coefficient when the incident angle of the light is 20 degrees toward the upper section side is 20 cd/lx/m ² or less. ALPR systems are often installed at points higher than the license plates of cars traveling normally. Therefore, according to the present aspect, when light from the ALPR system or the like is incident to the retro-reflective sheet from a position shifted to be above the retro-reflective license plate, the retro-reflection coefficient of the retro-reflective sheet decreases along with the incident angle of the light. As a result, the generation of halation is reduced at the infrared detector receiving reflected light from the retro-reflective sheet. In addition, since the retro-reflection coefficient increases when the incident angle of the light is 0 degrees, there is an advantage in terms of enhancing the visibility of the license plate for the driver of the following vehicle. That is, according to the present aspect, it is easy to achieve a reduction in the generation of the halation at an infrared detector such as an ALPR system and an enhancement in the visibility of the plate for the driver of the following vehicle.

In the retro-reflective license plate according to another aspect, the retro- reflection coefficient upon the incident angle of the light being 0 degrees may be 10 times or more the retro-reflection coefficient upon the incident angle of the light being 20 degrees toward the upper section side.

In the retro-reflective license plate according to another aspect, the retro- reflection coefficient upon the incident angle of the light being 0 degrees may be 30 times or more the retro-reflection coefficient upon the incident angle of the light being 20 degrees toward the upper section side.

In the retro-reflective license plate according to another aspect, the retro- reflective sheet may have a cube corner element in which, upon a vector, which divides three cube corner surfaces forming one cube into three equal parts and forms an equal angle with each of the three surfaces, being defined as an axis of symmetry, the axis of symmetry is inclined at 10 degrees or more with respect to a normal direction of the retro-reflective sheet.

The retro-reflective license plate according to another aspect may have a cube corner element where the axis of symmetry is inclined at 13 degrees or more to 15 degrees or less with respect to the normal direction of the retro-reflective sheet.

In one embodiment, the retro-reflective sheet is used in the retro-reflective license plate of any aspect described above.

According to one aspect of the present disclosure, it is easy to achieve both of a reduction in the generation of halation at an infrared detector and an enhancement in the visibility for the driver in the following vehicle.

Brief Description of Drawings

FIGS. lA and IB are diagrams illustrating one embodiment of a retro- reflective license plate. FIG. 2 is a diagram illustrating one embodiment of a cube corner element.

FIG. 3 is a diagram illustrating one embodiment of a work tool.

FIG. 4 is a diagram illustrating a cross-section taken along a line IV-IV in FIG.

2.

FIG. 5 is a diagram illustrating a cross-section taken along a line V-V in FIG. 2.

FIG. 6 is a diagram illustrating one embodiment of an effective aperture of a cube corner element.

FIG. 7 is a diagram in which an one embodiment of retro-reflective

characteristics of the retro-reflective sheet obtained by simulation.

FIG. 8 is a diagram illustrating an optical system for an identification marking reading test according to a reference experiment.

FIG. 9 is a table summarizing the results of evaluation according to the OC-i system and evaluation of the visibility for the driver of the following vehicle.

Description of Embodiments

In one embodiment, a retro-reflective license plate with identification markings comprises: a plate base; and a retro-reflective sheet provided on the plate base and having a cube corner element, wherein the retro-reflective sheet has a retro- reflection coefficient which, upon an upper section and a lower section of the retro- reflective sheet being defined according to an orientation of the identification markings and light being incident from a position shifted toward the upper section side from a normal direction of the retro-reflective sheet, decreases as an incident angle of the light is increased, and the retro-reflection coefficient when the incident angle of the light is 20 degrees toward the upper section side is 20 cd/lx/m ² or less.

In the present specification, the upper section and the lower section of a retro- reflective license plate are defined by the orientation of the Identification markings. In addition, the retro-reflection coefficient is defined as a value obtained by dividing the luminous intensity of the retro-reflective sheet to which the light is incident by the product of the brightness, which is received by a retro-reflective sheet placed orthogonally to the direction of the incident light, and the area of the retro-reflective sheet.

According to the present aspect, when light from the ALPR system or the like is incident to the retro-reflective sheet from a position shifted to the upper section side of the retro-reflective license plate, the retro-reflection coefficient of the retro- reflective sheet decreases as the incident angle of the light increases and the retro- reflection coefficient when the incident angle of the light is 20 degrees toward the upper section side is 20 cd/lx/m ² or less, thus the generation of halation is reduced in the infrared detector receiving reflected light from the retro-reflective sheet. In addition, since the retro-reflection coefficient increases when the incident angle of the light is 0 degrees, there is an advantage in terms of enhancing the visibility of the license plate for the driver of the following vehicle. That is, according to the present aspect, it is easy to achieve a reduction in the generation of halation in infrared detectors such as ALPR systems and an enhancement in the license plate visibility for the driver of the following vehicle. Embodiments of the retro-reflective license plate will be described below in detail while referring to the drawings. In the explanation of the drawings, the same elements are given the same reference numerals, and duplicate explanations are omitted.

FIGS. lA and IB are diagrams illustrating an example of a retro-reflective license plate according one embodiment. FIG. 1 A illustrates a front view of a retro- reflective license plate and FIG IB illustrates a cross-sectional view of the retro- reflective license plate taken along the line Ib-Ib in FIG. 1 A. As illustrated in FIGS. 1 A and IB, a retro-reflective license plate 1 is provided with a plate base 10, a retro- reflective sheet 20, and identification markings 30.

In the retro-reflective license plate 1, an upper section 2a and a lower section 2b are defined according to the orientation of the identification markings 30. In the case of Japan, identification information for identifying the vehicle, such as a place name and a classification number, is displayed as the identification markings 30. The contents of the identification information are determined according to the regulations, operations, and the like of the country in which the retro-reflective license plate 1 is to be used and the contents of the identification information are formed not only with simple patterns, but using distinguishing characters, symbols, or the like. That is, the up and down orientation of the identification information has the meaning of an up and down orientation suitable for a person to recognize the characters, symbols, or the like. The up and down orientation of the identification information corresponds to the up and down orientation of the identification markings 30 and also corresponds to the up and down orientation of the vehicle since the up and down orientation of the identification markings 30 is normally arranged to match the top and bottom of the vehicle. In addition, when the up and down orientation of the identification markings 30 is determined, the upper section 2a and the lower section 2b of the retro-reflective license plate 1 are also determined. For example, in a case of the rectangular retro- reflective license plate 1 which has a pair of long sides and a pair of short sides, the long side of the sides close to the lower section of the identification markings 30 is the lower section 2b and the long side of the sides close to the upper section of the identification markings 30 is the upper section 2a. The identification markings 30 are formed, for example, by printing with a roll coating method or the like. The retro-reflective sheet 20 is provided on the plate base 10 and the identification markings 30 are formed on a part of the surface of the retro-reflective sheet 20. The plate base 10 can be, for example, an aluminum plate with a white coating; however, the color of the plate base 10 is not limited to white and may be colors other than white. The upper section and lower section of the retro-reflective sheet 20 which are defined by the orientation of the identification markings 30 respectively correspond to the upper section 2a and lower section 2b of the retro- reflective license plate 1.

The retro-reflective sheet 20 is provided with a retro-reflective structural body 40 and an overlay layer 50. In addition, the retro-reflective sheet 20 is provided with a surface protective layer on the overlay layer 50 as necessary. The retro-reflective structural body 40, the overlay layer 50, and the surface protective layer are provided in this order. Here, a printed layer may be provided and a pattern may be formed between the retro-reflective structural body 40 and the overlay layer 50. The printed layer can be a coating film, a sheet, or the like. For example, the printed layer can be formed on a transparent film by an ink jet printer or the like using colored ink and/or clear ink according to the purpose. Examples of such inks include colored inks ("Mimaki ink", available from Mimaki Engineering Co., Ltd.) used in a JV5 ink jet printer (manufactured by Mimaki Engineering Co., Ltd.), and the like.

The retro-reflective structural body 40 has a retro-reflective layer 60 and an adhesive layer 70. The retro-reflective layer 60 is provided on the adhesive layer 70. The retro-reflective layer 60 has a main surface 60A and a structured surface 60B positioned on the opposite side to the main surface 60A and the structured surface 60B has a plurality of cube corner elements 80. The overlay layer 50 is provided on the main surface 60A.

Examples of preferable resins to be used for the retro-reflective layer 60 include polycarbonate, polymethyl methacrylate, polyethylene terephthalate, aliphatic polyurethane, ethylene copolymers, ionomers thereof, and the like. The cube corner elements 80 included in the retro-reflective layer 60 can be formed by casting directly onto a resin film as described in US5691846 A (Benson, Jr.). In a case where the retro-reflective layer 60 is formed by curing with radiation, examples of preferable resins include the cured products of radiation-curable compositions such as multifunctional acrylate, epoxy acrylate, and acrylic urethane. The resins described above are advantageous in terms of any one or a plurality of, for example, thermal stability, environmental stability, transparency, excellent release properties from a tool or mold, adhesion to other layers, and the like, and cured products of polycarbonate and epoxy acrylate are particularly advantageous in terms of the transparency, the thermal stability, and the like. The thickness of the retro-reflective layer 60 can be, for example, 40 μιη to 300 μπι. The thickness of the retro-reflective layer 60 is defined as the distance between the main surface 60A of the retro-reflective layer 60 and the highest point of the structured surface 60B. The height of the cube corner element 80 can be, for example, 50 μπι to 150 μπι. The height of the cube corner elements 80 is defined as the length to which the effective retro-reflective region of the side surfaces configuring the cube corner elements 80 extends in a direction orthogonal to the surface of the retro-reflective sheet 20.

The adhesive layer 70 includes one or more barrier layers 71 and can also include an adhesive 72. A barrier layer 71 provides a physical barrier between the adhesive layer 70 and the cube corner elements 80, and this barrier forms a low- refractive index layer 73 between the adhesive layer 70 and the cube corner elements 80. The barrier layer 71 has sufficient structural integrity to prevent the adhesive 72 from flowing into the low-refractive index layer 73 formed between the structured surface 60B and the barrier layer 71. The barrier layer 71 may come into direct contact with the leading ends of the cube corner elements 80, may be separated from the leading ends of the cube corner elements 80, or may be slightly pushed into the leading ends of the cube corner elements 80.

The barrier layer 71 optionally includes a material which prevents the adhesive 72 from coming into contact with the cube corner element 80, or the adhesive 72 from entering the low-refractive index layer 73. The material for the barrier layer 71 can be, for example, a resin, an ultraviolet-curable polymer, a film, an ink, a dye, a pigment, an inorganic material, particles, or beads. The dimensions and spacing of the barrier layer 71 may be varied and, in certain embodiments, the barrier layer 71 may form a pattern such as a grid, stripes, or dots on the retro-reflective sheet 20. The thickness of the barrier layer 71 can be, for example, 2 μπι to 10 μπι.

The adhesive 72 can be, for example, a cross-linked and plasticized acrylic pressure-sensitive adhesive or a hot melt pressure-sensitive adhesive. In addition, on a base of, for example, natural rubber, synthetic rubber, silicone, or other polymers, the adhesive 72 can include additives such as tackifiers, antioxidants, ultraviolet absorbers, plasticizers, or the like, as necessary. The adhesive 72 may be cross-linked by thermal cross-linking or radiation (for example, electron beams or ultraviolet light) using a cross-linking agent. The thickness of the adhesive layer 70 can be, for example, 30 μπι to 150 μπι.

The adhesive layer 70 also includes an adhesive region 74 that does not include the barrier layer 71. In the adhesive region 74, the adhesive layer 70 is adhered to the cube corner elements 80, and the low-refractive index layer 73 is efficiently sealed by the adhesive. The adhesive 72 holds the retro-reflective structural body 40 configured by the retro-reflective layer 60 and the adhesive layer 70 together as a whole and this removes the need for a separate sealing film and sealing process.

The low-refractive index layer 73 can include a material which has a refractive index lower than the refractive index of the cube corner elements 80, for example, air, or a low-refractive index material as described in U.S. Patent Application No.

61/324,249 (the contents of which are all incorporated herein). The low-refractive index layer 73 includes a material which has a lower refractive index than the cube corner elements 80, specifically, for example, a refractive index in the range of 1.0 to 1.5.

As illustrated in FIG. IB, light Lin incident to the retro-reflective layer 60 is retro-reflected via the overlay layer 50. This retro-reflection is the work of the cube corner elements 80 of the retro-reflective layer 60, and the cube corner elements 80 can have any suitable structure, as long as the cube corner elements 80 exhibit the retro-reflective function. For example, the cube corner elements 80 can be complete cubes such as full cubes, truncated cubes, triangular pyramids with cube corners, or the like. The cube corner elements include a trihedral structure having three sides substantially perpendicular to each other, and the retro-reflective sheet 20 is normally arranged with the identification markings 30 toward the observer and light source to be assumed. In the present embodiment, because the refractive index of the low- refractive index layer 73 is lower than the refractive index of the retro-reflective layer 60, the light Lin can be totally reflected. Since the cube corner elements 80 retro- reflect the light Lin by total reflection, it is possible to omit a manufacturing step, for example, for carrying out deposition onto the surface reflecting the light Lin.

The retro-reflective license plate 1 can have a liner 11 between the adhesive layer 70 and the plate base 10. The liner 11 includes a PET film, a polyethylene film, laminate paper, or the like which is subjected to a peeling process using silicone resin or the like.

The overlay layer 50 is provided as a protective layer of the retro-reflective layer 60 positioned under the overlay layer 50. The overlay layer 50 can include various types of resin films. When there is a demand that the retro-reflective sheet 20 has processability, deformability, or the like, it is desirable that the overlay layer 50 have at least one of flexibility or extensibility. For this reason, the overlay layer 50 can include, for example, at least one of vinyl chloride, vinyl chloride-vinyl acetate, urethane, acrylic, acrylate, olefins, or combinations thereof. When the overlay layer 50 includes these materials, cracks are not easily generated in the retro-reflective sheet 20 when used in license plate manufacturing and the retro-reflective sheet 20 can suitably follow the deformed shape of embossed or debossed portions. The overlay layer 50 is preferably transparent at least to visible light in order to ensure the visibility for the driver of the following vehicle. The thickness of the overlay layer can be, for example, 40 μιη to 200 μιη.

The surface protective layer can be transparent in the visible region and near- infrared region (0.4 μιη to 1 μιη) and can include, for example, polyurethane. A surface protective layer including polyurethane includes a reaction product of polyol having at least any one of a polyester skeleton or a polycarbonate skeleton and aliphatic isocyanate being trifunctional or higher. The glass transition temperature Tg of the surface protective layer including the reaction product is approximately 50°C or more, and the loss tangent tan5 at 120°C is approximately 0.1 or less. The surface protective layer having these properties has a high cross-link density and is able to impart weather resistance and chemical resistance to the retro-reflective sheet 20. The thickness of the surface protective layer including polyurethane can be, for example, 10 μπι to 60 μιη.

In the retro-reflective license plate 1, a bonding layer may be further provided between each of the layers as necessary. The bonding layer can be formed from a conventionally known adhesive and is appropriately selected according to the material of the bonded article. This adhesive can include materials such as, for example, acrylic resin, epoxy resin, and polyester resin as a base. The bonding layer provided between each of the layers is preferably transparent at least to visible light so as not to influence the license plate visibility for the driver of the following vehicle. The thickness of the bonding layer is not particularly limited, but can be, for example, 10 μπι to 100 μπι.

FIG. 2 is a diagram illustrating an example of a cube corner element according to one embodiment. In FIG. 2, the XY coordinates are depicted and the Y direction corresponds to the upper section 2a side of the retro-reflective sheet 20. The cube corner element 80 is a direct tool-type master and is provided with primary grooves 81, secondary grooves 82, and tertiary grooves 83. In the cube corner element 80, for example, a substrate 84 including polycarbonate is cut in the order of the primary grooves 81, the secondary grooves 82, and the tertiary grooves 83 using a work tool 35 (refer to FIG. 3). The work tool 35 is mounted on a post 36 and is provided with cutting surfaces 37 on either side of a tool center axis Cx. The cutting surfaces 37 are composed of a first cutting surface 37A which has a tool side angle P and a second cutting surface 37B which has a clearance angle Q. The tool side angle P is not zero and is preferably specified in order to make perpendicular or substantially

perpendicular cube optical surfaces. The clearance angle Q may be any angle, but is preferably in a range of 0 degrees to 30 degrees. In the manufacturing of the cube corner element 80, the clearance angle Q for the primary grooves 81 is, for example, 4 degrees and the clearance angle Q for the secondary grooves 82 and the tertiary grooves 83 is, for example, 20 degrees.

As illustrated in FIG. 2, a plurality of the primary grooves 81 are provided on the substrate 84 and the plurality of the primary grooves 81 are substantially parallel to each other along the X axis direction. A plurality of the secondary grooves 82 are provided on the substrate 84 and the plurality of secondary grooves 82 are divided into two groups. The first group of secondary grooves 82 are substantially parallel to each other along a first direction Axl, and the second group of secondary grooves 82 are substantially parallel to each other along a second direction Ax2. The first direction Axl is, for example, inclined by 50 degrees in a counterclockwise direction with respect to a reference of the X axis direction and the second direction Ax2 is, for example, inclined by 130 degrees in a counterclockwise direction with respect to the reference of the X axis direction. A plurality of the tertiary grooves 83 are provided on the substrate 84 and the plurality of tertiary grooves 83 are divided into two groups. The first group of tertiary grooves 83 are substantially parallel to each other along the first direction Axl and the second group of tertiary grooves 83 are substantially parallel to each other along the second direction Ax2. The secondary grooves 82 and the tertiary grooves 83 can be alternately formed in the first direction Axl, and the secondary grooves 82 and the tertiary grooves 83 can be alternately formed in the second direction Ax2.

A plurality of intersection points PI between the secondary grooves 82 and the tertiary grooves 83 are, for example, formed at equal intervals to each other so as to be separated at a distance Dl . The distance Dl is, for example, 278 μπι. The primary grooves 81 are formed at equal intervals separated by the distance Dl and, for example, have a distance 43 μπι of 0.155 Dl from intersection points PI between the secondary grooves 82 and the tertiary grooves 83.

The cube corner element 80 is provided with cubes with three different shapes, that is, first cubes 86, second cubes 87, and third cubes 88. The light Lin incident to the retro-reflective license plate 1 is retro-reflected by the first cubes 86, the second cubes 87, and the third cubes 88. In the present embodiment, the first cubes 86, for example, can have a triangular two-dimensional shape and the second cubes 87, for example, can have a quadrangular two-dimensional shape. The third cubes 88 can, for example, have a pentagonal two-dimensional shape. In FIG. 2, the first cubes 86, the second cubes 87, and the third cubes 88 are represented with shading.

FIG. 4 is a diagram illustrating a cross-section taken along a line IV-IV in FIG.

2. FIG. 4 illustrates a cross-sectional view of the first cube 86 and the second cube 87 and illustrates an example where an axis of symmetry Sx is inclined at an angle a with respect to the normal direction Rx of the substrate 84. The axis of symmetry Sx is defined as a vector which, when three cube corner surfaces forming one cube are divided into three equal parts, forms an equal angle with each of these three cube corner surfaces. The angle a, for example, can be an optional value preferably in an angle range of 10 degrees or more to 20 degrees or less. The normal direction Rx of the substrate 84 has the meaning of the normal direction of the retro-reflective sheet 20.

As illustrated in FIG. 4, the light Lin incident to the cube corner element 80 is retro-reflected by the first cube 86 and the second cube 87. In the first cube 86, for example, the incident light Lin is reflected by two or three surfaces out of a total of three surfaces which are one surface of the border 86a and two surfaces on both sides of a ridge line 86b to become reflected light Lout reflected substantially in parallel to the light Lin. In the second cube 87, for example, the incident light Lin is reflected by two or three surfaces out of a total of three surfaces, which are one surface of the border 87a and two surfaces on both sides of a ridge line 87b, to be reflected substantially in parallel to the light Lin. The light can be totally reflected at these surfaces. When the substrate 84 is, for example, a polycarbonate having a refractive index of 1.59, the critical angle 0c for total reflection is 39 degrees. When the substrate 84 is polycarbonate, if light Lin is incident to the structured surface 60B of the cube corner element 80 at an incident angle of 39 degrees or more, the light Lin can be retro-reflected.

FIG 5 is a diagram illustrating a cross-section taken along a line V-V in FIG. 2. FIG. 5 illustrates a cross-sectional view of the third cube 88 in which the axis of symmetry Sx forms an angle a with respect to the normal direction (the normal direction of the retro-reflective sheet 20) Rx of the substrate 84. In the third cube 88, for example, the incident light Lin is reflected by two or three surfaces out of a total of three surfaces, which are one surface of the border 88a and two surfaces on both sides of a ridge line 88b, to be reflected substantially in parallel to the light Lin.

FIG. 6 is a diagram illustrating an effective aperture of a cube corner element according to one embodiment. In FIG. 6, the effective aperture represents, for example, a region in which retro-reflection is possible by totally reflecting the light Lin incident to the cube corner element 80 having an incident angle of 60 degrees when the refractive index of the substrate 84 including the polycarbonate is 1.59. The first cube 86, the second cube 87, and the third cube 88 respectively have a first effective aperture 86e, a second effective aperture 87e, and a third effective aperture 88e corresponding thereto. In FIG. 6, the first effective aperture 86e, the second effective aperture 87e, and the third effective aperture 88e are represented with shading. The effective aperture degree of the whole cube corner element 80 is about 59 percent. FIG. 7 is a diagram in which an example of retro-reflective characteristics of the retro-reflective sheet according to one embodiment is obtained by simulation. The retro-reflective sheet has a cube corner element in which, with respect to the cube corner shape illustrated in FIG. 2, the axis of symmetry is inclined at 14.85 degrees with respect to the normal direction Rx of the substrate 84. This simulation was based on the premise of performing measurement in accordance with JIS Z 9117 as the optical system. In other words, the emitter emits light to be incident to the retro- reflective sheet 20 to form an incident angle with the normal direction of the retro- reflective sheet 20. The receiver, for example, observes the light reflected from the retro-reflective sheet 20 at an observation angle of 0.2 degrees. The observation angle is the angle between the illumination axis of the emitter and the observation axis of the receiver.

The horizontal axis in FIG. 7 indicates that the incident angle Θ of the light is positive when the light is incident to the retro-reflective sheet 20 from a position shifted to the upper section 2a side of the retro-reflective sheet 20 from the normal direction Rx of the retro-reflective license plate 1. The horizontal axis in FIG. 7 indicates that the incident angle Θ of the light is negative when the light is incident from a position shifted to the lower section 2b side. FIG. 7 shows the plotting of the retro-reflection coefficient of the retro-reflective license plate obtained for each of the incident angles Θ. The results shown in FIG. 7 show that the retro-reflective sheet 20 has a retro-reflection coefficient depending on the incident angle Θ of the light incident to the retro-reflective sheet 20 and that the retro-reflection coefficient is reduced as the incident angle Θ of the light increases when the light is incident to the retro-reflective sheet 20 from a position that is shifted from the normal direction of the retro-reflective license plate 1 to the upper section 2a side of the retro-reflective license plate 1. The changes in the retro-reflection coefficient of retro-reflective sheet 20 according to the incident angle Θ of the light incident to the retro-reflective sheet 20 are based on the changes in the effective aperture degree of the first effective aperture 86e, the second effective aperture 87e, and the third effective aperture 88e respectively corresponding to the first cube 86, the second cube 87, and the third cube 88 utilizing total reflection, according to the incident angle Θ of the light.

When the axis of symmetry is inclined by 14.85 degrees with respect to the normal direction Rx of the substrate 84, the reflected amount of the light incident to the retro-reflective sheet 20 from the upper section side of the retro-reflective sheet 20 is reduced as the incident angle Θ of the light increases. The retro-reflection coefficient when the incident angle Θ is 0 degrees is 763 cd/lx/m ² , and the retro- reflection coefficient when the incident angle Θ is 6 degrees is 101 cd/lx/m ² .

Furthermore, when the incident angle Θ is 20 degrees, the retro-reflection coefficient is reduced to 19 cd/lx/m ² . When the retro-reflection coefficient is reduced to 20 cd/lx/m ² or less, preferably 10 cd/lx/m ² or less, and more preferably 1 cd/lx/m ² or less, it is possible to effectively reduce and suppress the light reflected to the infrared detector in the automatic number plate recognition (N System) or the like among the ALPR systems. As a result, the retro-reflective license plate 1 reduces the generation of halation at the infrared detector.

The retro-reflective sheet 20 is not limited to the axis of symmetry being inclined at 14.85 degrees with respect to the normal direction Rx of the substrate 84, and the angle a which the axis of symmetry forms with the normal direction Rx of the substrate 84 is 10 degrees or more, and more preferably 13 degrees or more. Setting the angle a in this range makes it possible to sharply reduce the retro-reflection coefficient as the incident angle increases in a range where the incident angle Θ exceeds 0 degrees. In addition, setting the angle a to 20 degrees or less, preferably 17 degrees or less, or 15 degrees or less makes it possible to maintain the retro-reflection coefficient at a high value at an incident angle of 0 degrees. That is, when the angle a is in an angle range of 10 degrees = a = 20 degrees, more preferably 13 degrees = a = 15 degrees, the retro-reflection coefficient with respect to the light incident to the retro-reflective sheet 20 from the upper section side of the retro-reflective sheet 20 produces a high value when the incident angle Θ is 0 degrees, which can be sharply reduced as the incident angle Θ of the light increases. Using the retro-reflective license plate 1, it is possible to reduce the retro-reflection coefficient of the retro- reflective sheet 20, for example, to 1 cd/lx/m ² or less by changing the material of the cube corner elements 80 and the shape and size of the three surfaces configuring the cube corner elements 80 and changing the direction of the axis of symmetry.

Adjusting the retro-reflection coefficient makes it easy to achieve a reduction in the generation of halation at an infrared detector and an enhancement in the license plate visibility for the driver of the following vehicle.

As can be seen from the results shown in FIG. 7, by shifting the angle of the axis of symmetry in retro-reflective license plate 1, the retro-reflection coefficient when the incident angle Θ of light is 0 degrees can be 10 times or more or 20 times or more, and preferably 40 times or more the retro-reflection coefficient when the incident angle Θ of light is 20 degrees to the upper section side of the retro-reflective sheet. At this time, the license plate visibility is further enhanced for the driver of the following vehicle reading the identification markings of the retro-reflective license plate from a position at which the incident angle Θ of the light is approximately 0 degrees and, at the same time, it is possible to further lower and suppress the light reflected to the infrared detector in the automatic number plate recognition device (N System) or the like among the ALPR systems typically located above the car body. [Experimental Example]

Description will be given below of the results of experiments performed.

(Reference Experiment)

FIG. 8 is a diagram illustrating an optical system for a marking reading test according to a reference experiment. An optical system 90 for this test was provided with an emitter 91 and a receiver 92 and was able to examine the retro-reflective characteristics of the retro-reflective license plate 1 using the emitter 91 and the receiver 92. In the reference experiment, an irradiation apparatus was used as the emitter 91 and a CCD camera was used as the receiver 92. In the reference experiment, a license plate having the structure illustrated in FIG. 1 was used. In addition, the license plate had a cube corner element where the inclination of the axis of symmetry illustrated in FIG. 23 of Japanese Patent No. 3590061 (W095/11463) was 21.78 degrees with respect to the plate surface. During the measurement, the normal direction Rx of the license plate was inclined by 10 degrees with respect to the horizontal axis (X axis). The emitter 91 was installed in order to be able to input light from a position inclined by only 20 degrees with respect to the horizontal axis (X axis). The incident angle Θ was set to 10 degrees and the observation angle was set to 0.2 degrees. By installing the license plate described above inclined at 10 degrees, it was possible to measure approximately the same retro-reflective characteristics as in a state (a general use state) where the surface of the license plate corresponding to the reference experiment, that is, a license plate surface having a cube corner element for which the axis of symmetry is inclined by approximately 15 degrees with respect to the normal direction Rx of the substrate of the plate, is arranged to be approximately orthogonal with respect to the horizontal axis.

In the reference experiment, an image of the identification information read by an OC-i system (manufactured by A- TEC Co. Ltd.) during daytime and an image of the identification information read by the OC-i system at nighttime were confirmed. In the reference experiment, when the reading was performed by the OC-i system, the retro-reflection coefficient when the incident angle Θ was 10 degrees was suppressed to 20 cd/lx/m ² or less as illustrated in FIG. 7. For this reason, the amount of the near- infrared light received by the receiver 92 was reduced, the generation of halation at the receiver 92 was reduced, and thus and a good read image was obtained.

Furthermore, in the reference experiment, as illustrated in FIG. 7, the license plate visibility from the position of the driver of the following vehicle was also excellent because the retro-reflection coefficient at 0 degrees of the incident angle Θ exceeded 700 cd/lx/m ² . Comparative Example 1

The license plate of Comparative Example 1 was an existing 3M™ High Definition License Plate Sheeting Series 6700 manufactured by 3M. Here, the inclination of the axis of symmetry of the cube corner element of the license plate was approximately 3 degrees. The license plate of Comparative Example 1 had the plate structure illustrated in FIG. 1 in the same manner as in the reference experiment. However, in Comparative Example 1, unlike the reference experiment, the reading test of the identification information was performed using the optical system of FIG. 8 in a state where the license plate surface having the cube corner elements was arranged to be approximately orthogonal with respect to the horizontal axis without inclining the license plate. Here, the retro-reflection coefficient when the incident angle Θ was 5 degrees was 204 cd/lx/m ² , and the retro-reflection coefficient when the incident angle Θ was 20 degrees was 145 cd/lx/m ² .

In Comparative Example 1, when the reading was performed by the OC-i system, the amount of the near-infrared light received by the receiver 92 was not reduced and the generation of halation was noted at the receiver 92. As a result, a good read image was not obtained. In addition, in Comparative Example 1, the visibility from the position of the driver of the following vehicle was reduced in the experiment at nighttime.

Comparative Example 2

The license plate of the Comparative Example 2 was a coating-type aluminum plate which did not have retro-reflective characteristics. Similar to Comparative Example 1, an identification information reading experiment was performed using the optical system of FIG. 8 in a state where the license plate surface was arranged to be approximately orthogonal with respect to the horizontal axis.

In Comparative Example 2, significant generation of halation was not observed when the reading was performed by the OC-i system. However, in

Comparative Example 2, in the experiment at nighttime, the visibility from the position of the driver of the following vehicle was reduced.

FIG. 9 is a table summarizing the results of evaluation according to the OC-i system and evaluation of the visibility for the driver of the following vehicle. In both the results of the evaluation according to the OC-i system and the visibility evaluation for the driver of the following vehicle, the experiment results in which good reading was performed were evaluated as "A" and the experiment results in which good reading was not performed were evaluated as "B".

In comparison with Comparative Example 1 and Comparative Example 2, the results of the reference experiment show that, for the retro-reflective license plate 1, good reading can be performed in both the evaluation according to the OC-i system and the evaluation of the license plate visibility for the driver of the following vehicle. Using the retro-reflective license plate 1, it is easy to achieve a reduction in the generation of the halation at an infrared detector and to achieve an enhancement in the license plate visibility for the driver of the following vehicle.