Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/12—Reflex reflectors
  • G02B5/122—Reflex reflectors cube corner, trihedral or triple reflector type
  • G02B5/124—Reflex reflectors cube corner, trihedral or triple reflector type plural reflecting elements forming part of a unitary plate or sheet
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/60—Systems using moiré fringes

Definitions

• the invention relates to articles comprising a sheeting having a microstructured surface that reflects light in a moire-like pattern.
• the invention further relates to methods of making a master and replicas thereof including tooling that results in sheeting having such pattern.
• a preferred method comprises forming V-shaped grooves in a substrate (e.g. metal plate) wherein the intersections of three grooves form cube-corner elements.
• the cube-corner elements are formed in such a manner that adjacent parallel grooves have substantially the same groove spacing and groove depth throughout the array.
• Retroreflective cube-corner sheeting is commonly employed for various decorative and safety purposes. Cube-corner sheeting is often preferred for such uses as traffic signs, pavement markings, vehicle markings and personal safety articles, in view of its high retroreflected brightness. Since its inception, various improvements have been made, such as described by the patent literature that relates to cube-corner retroreflective sheeting.
• U.S. Patent No. 6,206,525 teaches retroreflective sheeting for forming orientation free cones of reflected light encompassing a 0.5 degree angle of observation formed of small metal backed cube-corner prisms in an array in which the size of the prisms are in a range between 0.0005 inch to 0.003 inches on center.
• the array is formed by casting transparent plastic prisms in a mold formed by ruling three sets of grooves, which intersect at an angle. The grooves are spaced apart in the range of 0.0005 inch to 0.003 inches on center.
• the prisms are coated with a reflective material such as a metal.
• very small prisms The main disadvantage of very small prisms is described as being that it is very difficult to rule an array of 0.002" on center prisms over a large area, as the ruling cutting diamond wears out.
• very small prisms are described as having advantages as well, including increased flexibility.
• the present invention relates to retroreflective sheeting comprising a polymeric sheet having an array of geometric elements (e.g. cube-corners) wherein the sheeting comprises a moire-like pattern.
• the elements are interconnected by a continuous land layer.
• the elements are preferably formed from three sets of mutually intersecting V- shaped grooves.
• the grooves preferably have an average groove spacing ranging from 0.0005 inches (0.0127 mm) to 0.007 inches (0.1778 mm) and more preferably less than 0.004 inches (0.1016 mm).
• the cube-corners elements are substantially the same size throughout the array or sub-array.
• Also disclosed is a method of making a tooling comprising providing a master that provides such moire-like pattern, electroplating the master forming a negative tooling, removing the tooling from the master, optionally electroplating the negative tooling at least once forming at least one positive tooling, and optionally electroplating the positive or negative tooling forming a multigenerational tooling.
• a method of making a retroreflective sheeting comprising providing a tooling that provides such moire-like pattern, casting a fluid resin composition on the microprismatic surface of the tooling, allowing the composition to harden forming a sheet, and removing the tooling.
• An alternate method comprises providing a tooling that provides such moire-like pattern, providing a moldable substrate, contacting a surface of the tooling to the substrate such that the pattern is imparted on the substrate, and removing the tooling.
• the methods may further comprise applying a reflective coating to the resin or substrate.
• the resin composition is preferably transparent.
• a preferred resin is polycarbonate.
• Fig. 1 depicts a magnified plan view of a portion of a master comprising three sets of V-shaped parallel grooves.
• An actual 6-inch-square (15.24-centimeter-square) master having a groove spacing of 0.004 inches (0.1016 mm) would have 1500 cube-corner elements along the width totaling over 2 million for the entire area.
• This figure also depicts an exploded view of a portion of retroreflective sheeting formed from a tool that was a negative replica of the master.
• Fig. 2 depicts a photograph of a tooling having a moire-like pattern in accordance with the present invention.
• the tooling was a negative replica of a master having a constant groove spacing of less than 0.003500 inches (0.088900 mm) throughout the array wherein the grooves were cut with a precision at least as precise as +/- 100 nm.
• the photograph was taken with an Olympus C700 digital camera using the flash at a distance of about 10 feet (3 meters).
• the present invention relates to masters, tooling and in particular sheeting comprising a moire-like pattern.
• the invention further relates to methods of making a master and replicas thereof comprising such pattern.
• "moire-like pattern” refers to the observed appearance being similar to that depicted in Fig. 2 when evaluated by shining the light from an ordinary flash light held near an observer's eyes at about the position of the nose at a distance from a retroreflective master or replica of up to about twenty feet, illuminating the retroreflective master or replica approximately normal to its plane.
• the master is preferably manufactured with a groove-forming machine that employs a direct machining technique that comprises forming a pattern of grooves that mutually intersect to form cube-corner elements.
• the grooved substrate is referred to as a master from which a series of replicas may be formed.
• Examples of direct machining techniques include flycutting, milling, grinding and ruling such as described in U.S. Pat. Nos. 4,588,258 (Hoopman) and 3,712,706 (Stamm), which disclose single or multiple passes of a machine tool having two opposing cutting surfaces for cutting grooves to form cube-corner optical faces in a substrate.
• Any substrate suitable for forming directly machined groove sets may be employed in the method of making the master of the invention. Suitable substrates should machine cleanly without burr formation, exhibit low ductility and low graininess and maintain dimensional accuracy after groove formation.
• a variety of machinable plastics or metals may be utilized. Suitable plastics comprise thermoplastic or thermoset materials such as acrylics or other materials. Machinable metals include aluminum, brass, electroless nickel alloys, and copper. Preferred metals include non-ferrous metals. Preferred machining materials are typically chosen to minimize wear of the cutting tool during formation of the grooves.
• the diamond tools suitable for directly machining the groove sets are of high quality such as diamond tools that can be purchased from K&Y Diamond (Mooers, NY) or Chardon Tool (Chardon, OH).
• suitable diamond tools are scratch-free within 10 mils of the tip, as can be evaluated with a 2000X white light microscope.
• the tip of the diamond has a flat portion ranging in size from about 0.00003 inches (0.000762 mm) to about 0.00005 inches (0.001270 mm).
• the surface finish of suitable diamond tools preferably have a roughness average of less than about 3 nm and a peak to valley roughness of less than about 10 nm.
• the surface finish can be evaluated by forming a test cut in a machinable substrate and evaluating the test cut with a micro- interferometer, such as can be purchased from Wyko (Tucson, AZ), a division of Veeco.
• the method of making a master in accordance with the present invention is directed toward direct machining methods, the tooling and retroreflective sheeting could be derived from a master manufactured by other methods as well provided that the precise formation of reflective geometric elements results in the occurrence of such moire-like pattern.
• the resulting cube corner elements in plan view may have other shapes that are non-triangular including but not limited to trapezoids, rectangles, quadrilaterals, pentagons, or hexagons.
• a plurality of V- shaped grooves are formed in a substrate (e.g. metal plate).
• cutting angle refers to the relative orientation of the groove sets with respect to each other.
• Gaoove spacing refers to the distance (e.g. in the x-direction) between the nadir of a groove and the nadir of an adjacent parallel groove, the nadir being the lowest point of the groove.
• Gaove depth refers to the distance (e.g. in the y-direction) between the top surface of the substrate (e.g. plate) and the nadir of the groove.
• “Groove position” refers to the two-dimensional position of the groove, (e.g. x, y coordinates) within the master.
• the third dimension e.g. z-direction
• the groove position is determined by the groove spacing and the groove depth.
• “Groove half-angle” refers to the angle formed on either side of the V-shaped groove. The groove half angles are measured relative to a reference plane that is aligned with the cutting direction and normal to the plane of the substrate.
• the "groove angle” is the sum of adjacent half angles sharing the same nadir.
• the V-shaped grooves are formed with a diamond-tooling machine that is capable of forming each groove with fine precision.
• a diamond-tooling machine that is capable of forming each groove with fine precision.
• Such machines typically include a laser interferometer- positioning device.
• a suitable precision rotary table is commercially available from AA Gage (Sterling Heights, MI); whereas a suitable micro-interferometer is commercially available from Zygo Corporation (Middlefield, CT) and Wyko (Tucson, AZ) a division of Veeco.
• the precision i.e.
• the groove spacing and groove depth is typically at least as precise as +/- 750 nm preferably at least as precise as +/- 500 nm, more preferably at least as precise as +/- 250 nm, and even more preferably at least as precise as +/- 100 nm.
• the resolution i.e. ability of groove forming machine to detect current axis position
• the precision is typically at least about 10% of the precision.
• the resolution is at least +/- 10 nm. Over short distances (i.e. 10 adjacent parallel grooves), the precision is approximately equal to the resolution.
• the temperature of the process is maintained within +/- 0.1°C and preferably within +/- 0.01°C.
• all the grooves in the master are preferably initially rough cut to a depth about 10 microns shallower than the final depth and then finish cut in alternating directions.
• the first groove 1, 1 i.e. first set, first groove
• the second groove is skipped and the third groove 1, 3 is finish cut in the same manner except in the opposite direction.
• the fourth groove was skipped and the fifth groove 1, 5 was finish cut in the first direction, etc. until the last groove at the bottom of the plate was formed.
• the alternating (i.e. skipped even numbered) grooves were finish cut from bottom to top in the same manner.
• the second and third groove sets are then cut in the same fashion.
• the retroreflective replicas thereof exhibit a moire-like pattern.
• the master is made from a suitable substrate such that the master itself is retroreflective (e.g. transparent plastic)
• the master will also exhibit this moire-like pattern.
• the pattern is replicated during the manufacture of the tooling. Further, the pattern is then replicated again when retroreflective sheeting is formed from such tooling.
• the surface of the retroreflective sheeting is substantially the same as the master from which it was derived.
• the reflected electric field of the light is theorized to only differ by the position of the cubes, the sum of the fields from individual cubes giving rise to the observed moire-like pattern
• the occurrence of this moire-like pattern was not readily evident in larger cubes, having a groove spacing of 0.01 inches (0.254 mm) for example.
• the methods and articles of the present invention are primarily directed to relatively small cubes, having a groove spacing (i.e. pitch) in the range of 0.0005 inches (0.0127 mm) to 0.007 inches (0.1778 mm). Further, it is surmised that the severity of the occurrence of this moire-like pattern tends to increase as the pitch decreases.
• the invention is most useful for methods of making masters and corresponding articles wherein the groove spacing is less than 0.004 inches (0.1016 mm), and preferably less than 0.0035 inches (0.0889 mm).
• the lateral dimension of the elements i.e. the size of the elements as measured between opposing sides or features
• the lateral dimension of the elements is preferably less than 0.004 inches (0.1016 mm) and more preferably less than 0.0035 inches (0.0889 mm).
• the occurrence of such pattern can be diminished or eliminated by introducing a certain magnitude and frequency of groove position variability into the method of making the master wherein the variability is intentional and controlled. Accordingly, in order to consistently produce articles having such moire-like pattern, such variation is substantially reduced.
• the cube-comer elements of the master, tooling and sheeting are substantially the same size throughout the array or sub-array, meaning that the smallest cube is at least 85% and preferably at least 90% of the size in comparison to the largest cube.
• the active aperture of each cube in the array is substantially the same.
• the effective area i.e.
• active aperture for a single or individual cube comer element may be determined by, and is equal to, the topological intersection of the projection of the three cube corner surfaces on a plane normal to the refracted incident ray with the projection of the image surfaces of the third reflection on the same plane.
• One procedure for determining effective aperture is discussed for example by Eckhardt, Applied Optics, v. 10 n. 7, July 1971, pp. 1559-1566.
• Straubel U.S. Patent No. 835,648 also discusses the concept of effective area or aperture.
• the grooves are formed with a diamond tool in the substrate such that the groove spacing, groove depth, and groove angle are uniform throughout each groove. The magnitude of groove position variation is equal to the precision of the groove-forming machine.
• the master itself may be useful as a retroreflective article such as in the case wherein grooves are formed in a suitable transparent plastic substrate.
• the master itself is not retroreflective such as in the case of employing a metal plate as the substrate.
• a master tool of suitable size for forming retroreflective sheeting a plurality of toolings (also referred to as tiles) are formed by electroplating the grooved surface of the master to form negative copies, subsequently electroplating the negative copies to form positive copies, electroplating the positive copies to form a second generation negative copies, etc. Electroplating techniques are generally known, such as described in U.S. Patent Nos. 4,478,769 and 5,156,863 to Pricone et al. A master tool of the desired size can then be assembled by tiling such toolings together.
• the tiling process may introduce groove position variation at the interface between seamed tiles. Accordingly, it is preferred that the sub-array having the moire-like pattern is greater than about 1/4" (0.635 cm) in order that the moire-like pattern is not diminished as a result of tiling. In the case of relatively small security markings, it is preferred that the tile is approximately equal to the size of the intended marking to insure that the pattern is not interrupted through the marking.
• the retroreflective sheet is preferably manufactured as an integral material, i.e. wherein the cube-comer elements are interconnected in a continuous layer throughout the dimension of the mold, the individual elements and connections therebetween comprising the same material.
• the surface of the sheeting opposing the microprismatic surface is typically smooth and planar, also being referred to as the "land layer".
• This land layer typically has a thickness ranging from about 0.001 inches (25 microns) to about 0.006 inches (150 microns) and is preferably at least 0.002 inches (50 microns) to 0.003 inches (75 microns).
• Manufacture of such sheeting is typically achieved by casting a fluid resin composition onto the master tool and allowing the composition to harden to form a sheet.
• Polycarbonates are particularly suitable because of their toughness and relatively higher refractive index, which generally contributes to improved retroreflective performance over a wider range of entrance angles. Provided that the sheeting is reflective to the extent that the moire-like pattern is observable under the prescribed viewing conditions, less transparent resins are also suitable for non-retroreflective uses such as security markings.
• a specular reflective coating such as a metallic coating can be placed on the backside of the cube-comer elements.
• the metallic coating can be applied by known techniques such as vapor-depositing or chemically depositing a metal such as aluminum, silver, or nickel.
• a primer layer may be applied to the backside of the cube-comer elements to promote the adherence of the metallic coating.
• a seal film can be applied to the backside of the cube-corner elements; see, for example, U.S. Pat. Nos. 4,025,159 and 5,117,304. The seal film maintains an air interface at the backside of the cubes that enables total internal reflection at the interface and inhibits the entry of contaminants such as soil and/or moisture.
• Colorants e.g. pigments and/or dyes
• ultraviolet light absorbers e.g., light stabilizers, free radical scavengers, antioxidants, and other additives
• processing aids such as antiblocking agents, releasing agents, slip agents, and lubricants
• the articles of the invention may further comprise an additional "color layer" for increasing daytime visibility or conspicuity.
• Such color layers comprise at least one dye or pigment dissolved or dispersed in a polymeric matrix.
• a preferred color layer, particularly for conspicuity comprises daylight fluorescent dye (i.e. a dye that emits visible light upon exposure to visible light) dissolved in a polymeric matrix.
• the polymeric matrix is preferably substantially transparent to visible light, particularly to light of the wavelengths emitted by the dye and light of the wavelengths that cause the dye to fluoresce.
• the fluorescent dye is typically selected based on the desired color, solubility with the polymeric matrix and stability in the polymeric matrix.
• the dye in the color layer will preferably consist essentially of thioxanthene, thioindigoid, isovaleranthrone, a napthalamide, benzoxazole coumarin, perylene, and/or perylene imide dyes.
• the color layer may also contain coloring agents such as pigments or other dyes in addition to those described above to adjust the color and appearance of the article.
• the moire-like patterned sheeting is also useful in a wide variety of retroreflective safety devices including articles of clothing, construction work zone vests, personal flotation devices (e.g. life jackets), rainwear, logos, patches, promotional items, luggage, briefcases, book bags, backpacks, rafts, canes, umbrellas, animal collars, truck markings, trailer covers and curtains, etc.
• the patterned sheeting may be adhered, sewn, or welded (e.g. heat, radio frequency, ultrasonic) onto such safety devices.
• the sheeting comprising the moire-like pattern is useful in the manufacture of various documents such as driver's licenses, license plates, passports, employee identification cards, military badges, etc. Due to the precision required to manufacture such sheeting, the pattern is difficult to counterfeit, the presence thereof useful for authenticating such articles.
• the sheeting be employed in a manner that does not detract from the legibility. It is surmised to employ the sheeting as a border or frame, position small stickers of such sheeting beneath a transparent cover film or on the outer surface of the article.
• the inventive sheeting, described herein is employed as a transparent overlay to protect documents from tampering, the articles may be made as described in U.S. Patent No. 5,743,981.
• the master was removed from the groove-forming machine.
• Toolings were made from the master by nickel electroforming the master as described in U.S. Patent Nos. 4,478,769 and 5,156,863. Multigenerational positive and negative copies were formed such that the tools had substantially the same degree of precise cube formation as the master.
• An electroformed negative tool was used to impart the pattern of the tool onto a polycarbonate film having a thickness of approximately 200 microns and having an index of refraction of about 1.59.
• the negative tool was observed to have a moire-like pattern, as depicted by Fig. 2.
• the tool was used in a compression molding press with the pressing performed at a temperature of approximately 375°F to 385°F, a pressure of approximately 1600 psi, and a dwell time of 20 seconds.
• the resulting retroreflective sheeting was evaluated in the same manner as Comparative Example A.
• the light that retroreflected from the sheeting was observed to exhibit the moire-like pattern.
• the coefficient of retroreflection, R A was measured as recommended by CIE publication No. 54 at -4° entrance, 0° orientation, 0.2° observation.
• the average of 9 spot reading over the entire area of the sample was 1085 candela per lux per square meter with a standard deviation of 37 candela per lux per square meter.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Optical Elements Other Than Lenses (AREA)
• Illuminated Signs And Luminous Advertising (AREA)
• Road Signs Or Road Markings (AREA)

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• 2002
  • 2002-06-11 US US10/167,091 patent/US6935756B2/en not_active Expired - Lifetime
• 2003
  • 2003-04-24 WO PCT/US2003/012982 patent/WO2003104859A1/en not_active Ceased
  • 2003-04-24 KR KR1020047020113A patent/KR100979149B1/ko not_active Expired - Fee Related
  • 2003-04-24 EP EP03724254A patent/EP1512036B1/en not_active Expired - Lifetime
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