Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
  • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
  • G02B6/0033—Means for improving the coupling-out of light from the light guide
  • G02B6/0035—Means for improving the coupling-out of light from the light guide provided on the surface of the light guide or in the bulk of it
  • G02B6/004—Scattering dots or dot-like elements, e.g. microbeads, scattering particles, nanoparticles
  • G02B6/0041—Scattering dots or dot-like elements, e.g. microbeads, scattering particles, nanoparticles provided in the bulk of the light guide
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29D—PRODUCING PARTICULAR ARTICLES FROM PLASTICS OR FROM SUBSTANCES IN A PLASTIC STATE
  • B29D11/00—Producing optical elements, e.g. lenses or prisms
  • B29D11/00663—Production of light guides
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
  • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
  • G02B6/0065—Manufacturing aspects; Material aspects

Definitions

• This invention relates to processes for making light extraction structures, light extraction structure arrays, and/or light extraction structure array masterforms and, in other aspects, to light extraction structures, light extraction structure arrays, and/or light extraction structure array masterforms made thereby.
• This invention further relates to light guides comprising the light extraction structure arrays and to articles comprising the light guides.
• a variety of devices have been proposed for illuminating electronic displays and keypads. These devices include backlighting panels, front lighting panels, concentrators, reflectors, structured-surface films, and other optical devices for redirecting, collimating, distributing, or otherwise manipulating light.
• Passive optical components for example, lenses, prisms, mirrors, and light extraction structures
• Efficient use of light is particularly important in battery powered electronic displays and keypads such as those used in cell phones, personal digital assistants, and laptop computers. By improving lighting efficiency, battery life can be increased and/or battery sizes can be reduced.
• Prismatic films are commonly used to improve lighting efficiency and enhance the apparent brightness of a backlit liquid crystal display, and multiple light sources (for example, light emitting diodes (LEDs)) are commonly used for this purpose in keypads.
• LEDs light emitting diodes
• Lighting quality is also an important consideration in electronic displays and keypads.
• One measure of lighting quality for a backlit display or keypad is brightness uniformity. Because displays (and, to a somewhat lesser extent, keypads) are typically studied closely or used for extended periods of time, relatively small differences in the brightness can easily be perceived.
• a light scattering element for example, a diffuser
• scattering elements can negatively affect the overall brightness of a display or keypad.
• this invention provides a process for making a light extraction structure array or a light extraction structure array masterform.
• the process comprises providing a photoreactive composition, the photoreactive composition comprising (a) at least one reactive species that is capable of undergoing an acid- or radical-initiated chemical reaction, and (b) at least one multiphoton photoinitiator system.
• the reactive species is preferably a curable species (more preferably, a curable species selected from the group consisting of monomers, oligomers, and reactive polymers).
• At least a portion of the composition can be imagewise exposed to light sufficient to cause simultaneous absorption of at least two photons, thereby inducing at least one acid- or radical-initiated chemical reaction where the composition is exposed to the light.
• the imagewise exposing can be carried out in a pattern that is effective to define at least the surface of an array of light extraction structures, each of the light extraction structures having at least one shape factor, and the array of light extraction structures having a distribution that can be uniform or non-uniform. Generally, the distribution can be non-uniform and/or the shape factor of at least one light extraction structure can be different from that of at least one other light extraction structure.
• the composition can, optionally, be developed by removing the resulting exposed portion, or the resulting non-exposed portion, of the composition.
• at least a portion of the composition can be nonimagewise exposed to light sufficient to effect reaction of at least a portion of any remaining unreacted photoreactive composition.
• the areal density of the array of light extraction structures varies across the array and/or at least one shape factor varies across the array (more preferably, both areal density and at least one shape factor vary across the array: even more preferably, both areal density and height vary across the array; most preferably, areal density increases as the height of the light extraction structures increases across the array).
• At least some embodiments of the process of the invention meet the above- stated need for light extraction structure array fabrication processes that can satisfy the quality, cost, and/or performance requirements of a variety of different applications and also, in particular, provide efficient light guides enabling brightness uniformity and long battery life (or reduced battery size).
• Light extraction structure arrays made by the process of the invention can be suitable for use in numerous optical applications including, for example, in backlit displays and backlit keypads.
• Figure 2 is a scanning electron micrograph (a side view) of an embodiment of the light extraction structure array of the invention, which embodiment was produced by the process of the invention and is described in Example 1 below.
• average surface roughness means the average deviation between the actual surface profile of a light extraction structure and its average surface profile
• light extraction structure means a microstructure (having a length, width, and height of at least about one micrometer) that is capable of directing or distributing light (for example, a protruding or recessed microstructure that distributes light within and/or directs light from a light guide);
• photosensitizer means a molecule that lowers the energy required to activate a photoinitiator by absorbing light of lower energy than is required by the photoinitiator for activation and interacting with the photoinitiator to produce a photoinitiating species therefrom;
• shape factor in regard to a light extraction structure
• three-dimensional light pattern means an optical image wherein the light energy distribution resides in a volume or in multiple planes and not in a single plane.
• Patent No. 4,652,274, and acrylated oligomers such as those of U.S. Patent No. 4, 642,126); unsaturated amides (for example, methylene bis-acrylamide, methylene bis- methacrylamide, 1 ,6-hexamethylene bis-acrylamide, diethylene triamine tris- acrylamide and beta-methacrylaminoethyl methacrylate); vinyl compounds (for example, styrene, diallyl phthalate, divinyl succinate, divinyl adipate, and divinyl phthalate); and the like; and mixtures thereof.
• Suitable reactive polymers include polymers with pendant (meth)acrylate groups, for example, having from 1 to about 50 (meth)acrylate groups per polymer chain. Examples of such polymers include aromatic acid (meth)acrylate half ester resins such as SarboxTM resins available from Sartomer
• Suitable reactive polymers curable by free radical chemistry include those polymers that have a hydrocarbyl backbone and pendant peptide groups with free-radically polymerizable functionality attached thereto, such as those described in U.S. Patent No. 5,235,015 (AIi et al.). Mixtures of two or more monomers, oligomers, and/or reactive polymers can be used if desired.
• Preferred ethylenically-unsaturated species include acrylates, aromatic acid (meth)acrylate half ester resins, and polymers that have a hydrocarbyl backbone and pendant peptide groups with free-radically polymerizable functionality attached thereto.
• Suitable cationically-reactive species are described, for example, by Oxman et al. in U.S. Patent Nos. 5,998,495 and 6,025,406 and include epoxy resins.
• Such materials broadly called epoxides, include monomeric epoxy compounds and epoxides of the polymeric type and can be aliphatic, alicyclic, aromatic, or heterocyclic. These materials generally have, on the average, at least 1 polymerizable epoxy group per molecule (preferably, at least about 1.5 and, more preferably, at least about 2).
• the polymeric epoxides include linear polymers having terminal epoxy groups (for example, a diglycidyl ether of a polyoxyalkylene glycol), polymers having skeletal oxirane units (for example, polybutadiene polyepoxide), and polymers having pendant epoxy groups (for example, a glycidyl methacrylate polymer or copolymer).
• the epoxides can be pure compounds or can be mixtures of compounds containing one, two, or more epoxy groups per molecule.
• These epoxy-containing materials can vary greatly in the nature of their backbone and substituent groups.
• the backbone can be of any type and substituent groups thereon can be any group that does not substantially interfere with cationic cure at room temperature.
• Still other exemplary epoxy resins include epoxidized polybutadiene (for example, one available under the trade designation "POLY BD 605E”from Sartomer Co., Inc.,
• Other useful epoxy resins comprise copolymers of acrylic acid esters of glycidol (such as glycidyl acrylate and glycidyl methacrylate) with one or more copolymerizable vinyl compounds. Examples of such copolymers are 1 :1 styrene-glycidyl methacrylate and 1 : 1 methyl methacrylate-glycidyl acrylate.
• Useful epoxy-functional polymers include epoxy-functional silicones such as those described in U.S. Patent No. 4,279,717 (Eckberg et al.), which are commercially available from the General Electric Company. These are polydimethylsiloxanes in which 1-20 mole % of the silicon atoms have been substituted with epoxyalkyl groups (preferably, epoxy cyclohexylethyl, as described in U.S. Patent No. 5,753,346 (Leir et al.). Blends of various epoxy-containing materials can also be utilized.
• Suitable cationally-reactive species also include vinyl ether monomers, oligomers, and reactive polymers (for example, methyl vinyl ether, ethyl vinyl ether, tert-butyl vinyl ether, isobutyl vinyl ether, triethyleneglycol divinyl ether (RAPI-CURE DVE-3, available from International Specialty Products, Wayne, NJ), trimethylolpropane trivinyl ether, and the VECTOMER divinyl ether resins from Morflex, Inc., Greensboro, NC (for example, VECTOMER 1312, VECTOMER 4010, VECTOMER 4051, and VECTOMER 4060 and their equivalents available from other manufacturers)), and mixtures thereof.
• VECTOMER divinyl ether resins from Morflex, Inc., Greensboro, NC (for example, VECTOMER 1312, VECTOMER 4010, VECTOMER 4051, and VECTOMER 4060 and their equivalents available from other manufacturers)
• Blends in any proportion) of one or more vinyl ether resins and/or one or more epoxy resins can also be utilized.
• Polyhydroxy-functional materials such as those described, for example, in U.S. Patent No. 5,856,373 (Kaisaki et al.)
• epoxy- and/or vinyl ether- functional materials can also be utilized.
• Non-curable species include, for example, reactive polymers whose solubility can be increased upon acid- or radical-induced reaction.
• reactive polymers include, for example, aqueous insoluble polymers bearing ester groups that can be converted by photogenerated acid to aqueous soluble acid groups (for example, poly(4- tert-butoxycarbonyloxystyrene).
• Non-curable species also include the chemically- amplified photoresists described by R. D. Allen, G. M. Wallraff, W. D. Hinsberg, and L. L. Simpson in "High Performance Acrylic Polymers for Chemically Amplified
• the multiphoton photoinitiator system can be a one-component system that comprises at least one photoinitiator.
• Photoinitiators useful as one-component multi-photon photoinitiator systems include acyl phosphine oxides (for example, those sold by Ciba under the trade name IrgacureTM 819, as well as 2,4,6 trimethyl benzoyl ethoxyphenyl phosphine oxide sold by BASF Corporation under the trade name LucirinTM TPO-L) and stilbene derivatives with covalently attached sulfonium salt moeties (for example, those described by W. Zhou et al. in Science 296, 1106 (2002)).
• Other conventional ultraviolet (UV) photoinitiators such as benzil ketal can also be utilized, although their multi-photon photoinitiation sensitivities will generally be relatively low.
• the two-photon absorption cross-section of the photosensitizer is greater than about 1.5 times that of fluorescein (or, alternatively, greater than about 75 x 10 "50 cm 4 sec/photon, as measured by the above method); even more preferably, greater than about twice that of fluorescein (or, alternatively, greater than about 100 x 10 50 cm 4 sec/photon); most preferably, greater than about three times that of fluorescein
• Photoinitiators Suitable photoinitiators (that is, electron acceptor compounds) for the reactive species of the photoreactive compositions are those that are capable of being photosensitized by accepting an electron from an electronic excited state of the multiphoton photosensitizer, resulting in the formation of at least one free radical and/or acid.
• Such photoinitiators include iodonium salts (for example, diaryliodonium salts), sulfonium salts (for example, triarylsulfonium salts optionally substituted with alkyl or alkoxy groups, and optionally having 2,2' oxy groups bridging adjacent aryl moieties), and the like, and mixtures thereof.
• Suitable imide and methide anions include (C 2 F 5 SO 2 )IN", (C 4 F 9 SO 2 ) 2 N-, (C 8 F 17 SO 2 ) 3 C-, (CF 3 SO 2 ) 3 C-,
• Preferred sulfonium salts include triaryl-substituted salts such as triarylsulfonium hexafluoroantimonate (for example, SarCatTM SRlOlO available from Sartomer Company), triarylsulfonium hexafluorophosphate (for example, SarCatTM SR 1011 available from Sartomer Company), and triarylsulfonium hexafluorophosphate (for example, SarCatTM KI85 available from Sartomer Company).
• triarylsulfonium hexafluoroantimonate for example, SarCatTM SRlOlO available from Sartomer Company
• triarylsulfonium hexafluorophosphate for example, SarCatTM SR 1011 available from Sartomer Company
• triarylsulfonium hexafluorophosphate for example, SarCatTM KI85 available from Sartomer Company
• the reactive species itself can also sometimes serve as a solvent for the other components.
• the three components of the photoinitiator system are present in photochemically effective amounts (as defined above).
• the composition can contain at least about 5% (preferably, at least about 10%; more preferably, at least about 20%) up to about 99.79% (preferably, up to about 95%; more preferably, up to about 80%) by weight of one or more reactive species; at least about 0.01% (preferably, at least about 0.1%; more preferably, at least about 0.2%) up to about 10% (preferably, up to about 5%; more preferably, up to about 2%) by weight of one or more photosensitizers; optionally, up to about 10% (preferably, up to about 5%) by weight of one or more electron donor compounds (preferably, at least about 0.1%; more preferably, from about 0.1% to about 5%); and from about 0.1% to about 10% by weight of one or more electron acceptor compounds (preferably, from about 0.1% to about 5%) based upon the total weight of solids (
• Light source 12 can be any light source that produces sufficient light intensity to effect multiphoton absorption. Suitable sources include, for example, femtosecond near- infrared titanium sapphire oscillators (for example, those available from Coherent, Santa Clara, California, under the trade designation "MIRA OPTIMA 900-F") pumped by an argon ion laser (for example, those available from Coherent under the trade designation "INNOVA"). This laser, operating at 76 MHz, has a pulse width of less than 200 femtoseconds, is tunable between 700 and 980 nm, and has average power up to 1.4 Watts.
• femtosecond near- infrared titanium sapphire oscillators for example, those available from Coherent, Santa Clara, California, under the trade designation "MIRA OPTIMA 900-F
• argon ion laser for example, those available from Coherent under the trade designation "INNOVA”
• Another useful laser is available from Spectra-Physics, Mountain View, California, under the trade designation "MAI TAI", tunable to wavelengths in a range of from 750 to 850 nanometers, and having a repetition frequency of 80 megahertz, and a pulse width of about 100 femtoseconds (IxIO "13 sec), with an average power level up to 1 Watt.
• MAI TAI tunable to wavelengths in a range of from 750 to 850 nanometers, and having a repetition frequency of 80 megahertz, and a pulse width of about 100 femtoseconds (IxIO "13 sec), with an average power level up to 1 Watt.
• any light source for example, a laser
• any light source that provides sufficient intensity to effect multiphoton absorption at a wavelength appropriate for the multiphoton absorber used in the photoreactive composition can be utilized.
• Q-switched Nd:YAG lasers for example, those available from Spectra-Physics under the trade designation "QUANTA-RAY PRO"
• visible wavelength dye lasers for example, those available from Spectra-Physics under the trade designation "SIRAH” pumped by a Q-switched Nd:YAG laser from Spectra-Physics having the trade designation "Quanta-Ray PRO”
• Q-switched diode pumped lasers for example, those available from Spectra-Physics under the trade designation "FCBAR
• Optical system 14 can include, for example, refractive optical elements (for example, lenses or microlens arrays), reflective optical elements (for example, retroreflectors or focusing mirrors), diffractive optical elements (for example, gratings, phase masks, and holograms), polarizing optical elements (for example, linear polarizers and waveplates), dispersive optical elements (for example, prisms and gratings), diffusers,
• refractive optical elements for example, lenses or microlens arrays
• reflective optical elements for example, retroreflectors or focusing mirrors
• diffractive optical elements for example, gratings, phase masks, and holograms
• polarizing optical elements for example, linear polarizers and waveplates
• dispersive optical elements for example, prisms and gratings
• Exposure times generally depend upon the type of exposure system used to cause reaction of the reactive species in the photoreactive composition (and its accompanying variables such as numerical aperture, geometry of light intensity spatial distribution, the peak light intensity during the laser pulse (higher intensity and shorter pulse duration roughly correspond to peak light intensity)), as well as upon the nature of the photoreactive composition. Generally, higher peak light intensity in the regions of focus allows shorter exposure times, everything else being equal.
• Exemplary useful organic solvents include alcohols (for example, methanol, ethanol, and propanol), ketones (for example, acetone, cyclopentanone, and methyl ethyl ketone), aromatics (for example, toluene), halocarbons (for example, methylene chloride and chloroform), nitriles (for example, acetonitrile), esters (for example, ethyl acetate and propylene glycol methyl ether acetate), ethers (for example, diethyl ether and tetrahydrofuran), amides (for example, N-methylpyrrolidone), and the like, and mixtures thereof.
• alcohols for example, methanol, ethanol, and propanol
• ketones for example, acetone, cyclopentanone, and methyl ethyl ketone
• aromatics for example, toluene
• halocarbons for example, methylene chloride and
• An optional bake after exposure to light under multiphoton absorption conditions, but prior to solvent development, can be useful for some photoreactive compositions such as, for example, epoxy-type reactive species.
• Typical bake conditions include temperatures in a range of from about 40 0 C to about 200 0 C, for times in a range of from about 0.5 minutes to about 20 minutes.
• a nonimagewise exposure using actinic radiation can be carried out to effect reaction of the remaining unreacted photoreactive composition.
• Such a nonimagewise exposure can preferably be carried out by using a one-photon process.
• Geometric configurations can comprise such structural elements as a base, one or more faces (for example, that form a side wall), and a top (which can be, for example, a planar surface (for example, formed by truncation) or even a point).
• Such elements can be of essentially any shape (for example, bases, faces, and tops can be circular, elliptical, or polygonal (regular or irregular), and the resulting side walls can be characterized by a vertical cross section (taken perpendicular to the base) that is parabolic, hyperbolic, or linear in nature, or a combination thereof).
• Average surface roughnesses of ⁇ /2 (preferably, ⁇ /4; more preferably, ⁇ /10; most preferably, ⁇ /20) can be achieved (where ⁇ (lambda) is the wavelength of light for which the structure is designed; hereinafter the "operating wavelength").
• the fill factor of the arrays can be varied to control brightness and uniformity.
• the packing arrangement or distribution of the structures can be regular (for example, square or hexagonal) or irregular.
• the shape factors of the structures comprising the array can also vary throughout the array. For example, the heights can be varied according to the distance of a particular structure from a light source (to achieve uniform light extraction). To maintain continuously uniform light output (and minimize or eliminate bright spots), arrays exhibiting an irregular variation in shape factor and/or areal density can be prepared.
• the geometric configuration of at least one light extraction structure is selected from truncated cones, truncated aspheres, and combinations thereof, and/or the variation exhibited by the plurality is irregular across the plurality for at least one of areal density, shape factor, and principal axis.
• Such arrays can be useful, for example, for directing extracted light in multiple directions, in accordance with the variation in principal axis across the plurality of light extraction structures.
• Another preferred light extraction structure array comprises a plurality of light extraction structures having a non-uniform distribution, each of the light extraction structures having a geometric configuration, and the geometric configuration of at least one light extraction structure being a truncated asphere.
• the geometric configuration of each light extraction structure in the array is selected from aspheres, truncated aspheres, and combinations thereof.
• Such arrays can be useful, for example, for achieving uniform extracted light output without the appearance of discrete bright spots or lines (which can result from approaches that involve only a reduction in light extraction structure density in regions of the array that are relatively close to a light source).
• a replication tool such as a mold insert
• a mold insert can be prepared by using a light extraction structure array prepared as described above as a master. That is, another material can be placed against the master to prepare a mold insert having the negative image of the array. The master can then be removed, leaving a mold insert that can subsequently be used to prepare additional arrays. The mold insert will have cavities in the shape of the negative image of the array.
• a metal replication tool can be made from a master by electroplating or electroforming a metal, such as nickel, against the master and subsequently removing the master.
• a silicone replication tool can be made by curing a silicone resin against the master and subsequently removing the master.
• Coatings for example, reflective coatings of thin metal
• Coatings can be applied to at least a portion of one or more surfaces of the light guides (for example, to the interior or recessed surface of light extraction structures) by known methods, if desired.
• Individual light guide designs can, if desired, be evaluated without the need for actual fabrication by using suitable ray-tracing modeling software such as "ASAP” from Breault Research Organization, Inc., "Code V” and “Light Tools” from Optical Research Associates, Inc., “Rayica” from the Optica Software Division of i- Cyt Mission Technology, Inc., “Trace Pro” from Lambda Research, Inc., and “ZEMAX” from Zemax Development Corporation.
• the light guides of the present invention can be especially useful in backlit displays (for example, comprising a light source, a light gating device (for example, a liquid crystal display (LCD)), and a light guide) and keypads (for example, comprising a light source, an array of pressure-sensitive switches at least a portion of which transmits light, and a light guide).
• the light guides are useful as point to area or line to area back light guides for subminiature or miniature display or keypad devices illuminated with light emitting diodes (LEDs) powered by small batteries.
• Suitable display devices include color or monochrome LCD devices for cell phones, pagers, personal digital assistants, clocks, watches, calculators, laptop computers, vehicular displays, and the like.
• Other display devices include flat panel displays such as laptop computer displays or desktop flat panel displays.
• Suitable backlit keypad devices include keypads for cell phones, pagers, personal digital assistants, calculators, vehicular displays, and the like
• XP OmniCoat primer MicroChem Corp., Newton, MA
• the filtered solution was poured into a 5 cm x 5 cm (inside dimensions) area masked with a green gasket tape on the primed silicon wafer.
• the wafer was allowed to dry at room temperature over the weekend and was then placed in a forced air oven for 30 minutes at 65 0 C, followed by 90 minutes at 95 0 C , followed by 30 minutes at 65 0 C to afford a coated silicon wafer with a substantially solvent-free (hereinafter, "dry") coating thickness of approximately 300 ⁇ m.
• the backside of the wafer was cleaned with isopropyl alcohol to remove any debris.
• the wafer was then mounted on a porous carbon vacuum chuck (flatness greater thanl ⁇ m).
• a two-photon fabrication system was then activated to produce an optical signal that was stationary in the vertical position (the fabrication system was not activating the z-control to move the signal in the vertical direction).
• the signal was used as a detection mechanism to produce a reflection off of the wafer surface in conjunction with a confocal microscope system such that the only condition that would produce a confocal response would occur when the optical signal was focused on the surface of the wafer.
• Two-photon polymerization of the dry coating was carried out in the following manner, using a diode -pumped Ti:sapphire laser (Spectra-Physics, Mountain View, CA) operating at a wavelength of 800 nm, nominal pulse width of 80 fs, pulse repetition rate of
• the filtered solution was coated onto the silicon wafer by spin coating, followed by removal of solvent at 8O 0 C for 10 minutes, to yield a dry coating thickness of about 30 ⁇ m.
• the backside of the wafer was cleaned with isopropyl alcohol to remove any debris.
• the wafer was then mounted on a porous carbon vacuum chuck (flatness less than 1 ⁇ m).
• a two-photon fabrication system was then activated to produce an optical signal that was stationary in the vertical position (the fabrication system was not activating the z- control to move the signal in the vertical direction).
• the signal was used as a detection mechanism to produce a reflection off of the wafer surface in conjunction with a confocal microscope system such that the only condition that would produce a confocal response would occur when the optical signal was focused on the surface of the wafer.
• the system was aligned to the interface between the coating of photosensitive material and the wafer in the vertical direction.
• the system was then activated to scan the laser beam to polymerize the coating of photosensitive material to define the structures.
• the system was operated in an automated fashion to place light extraction structures (in the form of truncated and non-truncated aspheres) at prescribed positions (described below) using a software model that optimized the structure locations for optimum light extraction uniformity and efficiency. The distribution of the resulting array was non-uniform.
• a silicone (GE RTV 615 2-part silicone, General Electric Co, Waterford, NY) was cast on the master tool, which had sandblasting tape arranged around the light extraction structure array to form a dam about 3 mm thick corresponding to the outline of a final lightguide.
• the silicone was degassed in a vacuum oven for 15 minutes, a release liner was added on top of the silicone, and the silicone was cured at 8O 0 C for VA hours. The silicone was removed from the master tool to form a daughter tool.
• a dam about 3 mm high was constructed around the silicone daughter tool using sandblasting tape, and an ultraviolet (UV) curable acrylate (Photomer 6210, Cognis, Cincinnati, Ohio) was poured over the silicone daughter tool and degassed 15 minutes in a vacuum oven at 5O 0 C.
• the degassed construction was covered with a release liner and cured by passing under a UV light (H bulb, Fusion UV Systems, Gaithersburg, Maryland) 5 times at 12 centimeters per second (24 feet/minute).
• the resulting cured acrylate was separated from the silicone to form a microreplication tool.
• a dam was formed around the microreplication tool using sandblasting tape (about 2 mm), and the tool was filled with a silicone (GE RTV 615 2- part silicone).

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