WO2012116199A1 - Variable index light extraction layer and method of illuminating with same - Google Patents

Variable index light extraction layer and method of illuminating with same Download PDF

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Publication number
WO2012116199A1
WO2012116199A1 PCT/US2012/026349 US2012026349W WO2012116199A1 WO 2012116199 A1 WO2012116199 A1 WO 2012116199A1 US 2012026349 W US2012026349 W US 2012026349W WO 2012116199 A1 WO2012116199 A1 WO 2012116199A1
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WO
WIPO (PCT)
Prior art keywords
layer
light extraction
extraction layer
variable index
light
Prior art date
Application number
PCT/US2012/026349
Other languages
French (fr)
Inventor
David Scott Thompson
Kevin R. Schaffer
Zhaohui Yang
Audrey A. Sherman
Michael A. Meis
Encai Hao
Original Assignee
3M Innovative Properties Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to US201161446642P priority Critical
Priority to US61/446,642 priority
Application filed by 3M Innovative Properties Company filed Critical 3M Innovative Properties Company
Publication of WO2012116199A1 publication Critical patent/WO2012116199A1/en

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Classifications

    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B5/00Optical elements other than lenses
    • G02B5/02Diffusing elements; Afocal elements
    • G02B5/0205Diffusing elements; Afocal elements characterised by the diffusing properties
    • G02B5/0236Diffusing elements; Afocal elements characterised by the diffusing properties the diffusion taking place within the volume of the element
    • G02B5/0247Diffusing elements; Afocal elements characterised by the diffusing properties the diffusion taking place within the volume of the element by means of voids or pores
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B82NANOTECHNOLOGY
    • B82YSPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y20/00Nanooptics, e.g. quantum optics or photonic crystals
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/0001Light guides specially adapted for lighting devices or systems
    • G02B6/0011Light guides specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0033Means for improving the coupling-out of light from the light guide
    • G02B6/0035Means 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
    • G02B6/004Scattering dots or dot-like elements, e.g. microbeads, scattering particles, nanoparticles
    • G02B6/0043Scattering dots or dot-like elements, e.g. microbeads, scattering particles, nanoparticles provided on the surface of the light guide
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/0001Light guides specially adapted for lighting devices or systems
    • G02B6/0011Light guides specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0065Manufacturing aspects; Material aspects
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS, OR APPARATUS
    • G02B6/00Light guides
    • G02B6/0001Light guides specially adapted for lighting devices or systems
    • G02B6/0011Light guides specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0033Means for improving the coupling-out of light from the light guide
    • G02B6/0058Means for improving the coupling-out of light from the light guide varying in density, size, shape or depth along the light guide
    • G02B6/0061Means for improving the coupling-out of light from the light guide varying in density, size, shape or depth along the light guide to provide homogeneous light output intensity

Abstract

A variable index light extraction layer (100) suitable for use in optical applications is described. The variable index light extraction layer has first and second regions (140; 130) with different refractive indices, the regions being disposed such that for light being transported at a supercritical angle in an adjacent layer (120), the extraction layer can selectively extract the light in a predetermined way based on the geometric arrangement of the first and second regions. Also described are optical films and devices including the variable index light extraction layer, methods of making the extraction layer and methods of illuminating using the extraction layer.

Description

VARIABLE INDEX LIGHT EXTRACTION LAYER

AND METHOD OF ILLUMINATING WITH SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following U.S. Provisional Patent Applications, filed on even date herewith and incorporated by reference: "Front- Lit Reflective Display Device and Method of Front- Lighting Reflective Display" (U.S. Provisional Application No. 61,446,740) and "Illumination Device and Method of Front- Lighting Reflective Scattering Element" (U.S. Provisional Application No.

61/446,712).

FIELD

This application relates generally to optical films suitable for managing light in a predetermined way, with particular application to optical films used in combination with lightguides.

BACKGROUND

Illumination systems or devices, such as those used to illuminate objects or provide illumination in an electronic display system, utilize one or more optical layers for managing light emitted by one or more light sources. Often, the optical layers are required to have a desired optical transmittance, optical haze, optical clarity, or index of refraction. In many applications, the optical layers include a lightguide used in combination with an air layer and a light extraction layer such that light emitted by the light source(s) is transported within the lightguide, and the air layer and the extraction layer manage the light by supporting total internal reflection (TIR) and extraction of the light from the lightguide. A continuing need exists for optical films which are capable of managing light and are suitable for use in thin, flexible systems as well as in bulky systems.

SUMMARY

This application generally relates to transparent polymer films suitable for use in optical applications, methods for making the same, and methods for using the same to illuminate articles. More particularly, this disclosure relates to a variable index light extraction layer having regions of different properties such as refractive index, haze, transmission, clarity, or a combination thereof. The disclosed variable index light extraction layer can be incorporated into, or used in conjunction with, various kinds of display devices and general lighting systems.

In one aspect, this application describes a variable index light extraction layer having first and second regions, the first region including nanovoided polymeric material, the second region including the nanovoided polymeric material and an additional material, the first and second regions being disposed such that for light being transported at a supercritical angle in an adjacent layer, the variable index light extraction layer selectively extracts the light in a predetermined way based on the geometric arrangement of the first and second regions.

The variable index light extraction layer that can function as a high performance optical layer having optical properties tailored for different applications. For example, the first region can have a haze less than about 5% and a clarity greater than about 90%, and/or the layer can have a light transmittance of greater than about 90%. For another example, the layer can have a haze less than about 10% and a clarity greater than about 90%. The first and second regions can be continuous across a transverse plane of the layer, or they can be discontinuous, arranged in a pattern or randomly disposed. The variable index light extraction layer can be tailored to exhibit particular optical properties varying the relative areas of the first and second regions. For example, the second regions can include from about 5 to about 60% of an area across a transverse plane of the layer.

In another aspect, this application describes an optical film the variable index light extraction layer described above, disposed on a transparent substrate. The transparent substrate can include a lightguide. The optical film can have a haze less than about 10%, a clarity greater than about 85% and a light transmittance of greater than about 90%. Also described is an optical film including the variable index light extraction layer disposed on a reflective scattering substrate.

In another aspect, this application describes a method of preparing a variable index light extraction layer, including providing a nanovoided polymeric layer having a first refractive index; and printing an additional material on the nanovoided polymeric layer such that the additional material substantially penetrates the nanovoided polymeric layer, thereby forming a variable index light extraction layer including a first region including a portion of the nanovoided polymeric layer and a second region including another portion of the nanovoided polymeric layer and the additional material. The first and second regions are disposed such that for light being transported at a supercritical angle in an adjacent layer, the variable index light extraction layer selectively extracts the light in a predetermined way based on the geometric arrangement of the first and second regions.

In another aspect, this application describes an optical device including a light source in combination with a lightguide and the variable index light extraction layer.

In yet another aspect, this application describes a method of providing light, including providing a light source, lightguide and an optical film including the variable index light extraction layer of claim 1 ; and optically coupling the light source to the lightguide and the lightguide to the variable index light extraction layer, such that light emitted by the light source is transported within the lightguide by total internal reflection and selectively extracted from the lightguide by the variable index light extraction layer.

The above summary is not intended to describe each disclosed embodiment or every

implementation of this disclosure. The figures and the detailed description below more particularly exemplify illustrative embodiments. BRIEF DESCRIPTION OF DRAWINGS

In the following description, reference is made to the accompanying set of drawings that form a part of this disclosure and in which are shown various general and specific embodiments. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the invention. The following detailed description, therefore, is not to be taken in a limiting sense. The figures are schematic drawings and are not necessarily to scale.

FIG. 1 a shows a schematic cross section of an exemplary variable index light extraction layer. FIGS, lb-lc show schematic cross sections of an exemplary variable index light extraction layer disposed on a transparent adjacent layer.

FIG. 2 illustrates the variable index light extraction layer having refractive indices that can vary across a transverse plane of the layer.

FIG. 3 is a schematic cross-sectional view of a first region of the variable index light extraction layer.

FIG. 4a is a plan view of a variable index light extraction layer showing an exemplary geometric arrangement of the first and second regions.

FIG. 4b illustrates the refractive index profile for the variable index light extraction layer shown in FIG. 4a.

FIGS. 4c and 4d show profiles for selected optical properties % transmission and % clarity, respectively, for the variable light extraction layer shown in FIG. 4a.

FIGS. 5a and 5b show plan views of variable index light extraction layers showing exemplary geometric arrangements of the first and second regions.

FIG. 6 shows a schematic of an exemplary illumination device comprising the variable index light extraction layer in combination with a light source and a reflective scattering element.

FIG. 7 shows a schematic cross-sectional view of an exemplary optical film comprising the variable index light extraction layer.

FIG. 8 shows a roll of an optical film comprising a variable index light extraction layer disposed on a transparent substrate.

FIG.9a shows a reflective display device with a front light having no variable index extraction layer.

FIG. 9b shows a reflective display device with a front light including a variable index light extraction layer.

FIG. 10 shows a random 100 um gradient dot pattern for flexographic tool.

FIG. 1 1a shows a reflective display device with a front light having a variable index light extraction layer.

FIG. l ib shows a reflective display device with a front light having a variable index light extraction layer. FIG. 12a shows a Prometric image and axial luminance plot as a function of position for a reflective display device illuminated by a front light having a variable index light extraction layer.

FIG. 12b shows a Prometric image and axial luminance plot as a function of position for a reflective display device illuminated by a front light without a variable index light extraction layer.

DETAILED DESCRIPTION

In general, the variable index light extraction layer disclosed herein comprises at least two different areas or regions, wherein light of any angle incident upon the layer can be managed differently because the regions have different refractive indices. The variable index light extraction layer can be used in a variety of optical film constructions, assemblies and devices as described, for example, in "Front-Lit Reflective Display Device and Method of Front-Lighting Reflective Display" (Attorney Docket No. 66858US002) and "Illumination Device and Method of Front-Lighting Reflective Scattering Element" (Attorney Docket No. 67313US002), both filed on even day herewith.

The variable index light extraction layer is an optical layer that acts to extract light traveling in an adjacent layer at supercritical angles, while at the same time has little to no light scattering for subcritical angle light incident on the extraction layer. The variable index light extraction layer extracts light from an adjacent layer such as a transparent layer, and can deliver the extracted light to an article or element such that the article or element is illuminated. The variable index light extraction layer does not have features that significantly or functionally scatter light. Thus, when looking through the layer, as shown in FIG. 8, there is little distortion of images and objects on the opposite side of the layer. Ideally, the materials in first and second regions have different refractive indices, and both are highly transmissive with very low haze. The first and second regions in the variable index light extraction layer can be shaped and arranged to yield a layer with high clarity, low haze and high transmission when the layer is physically attached and optically coupled to a lightguide, reflective scattering element or reflective display.

The variable index light extraction layer allows the lightguide to be transparent exhibiting little to no haze and high clarity with and without illumination. This allows for viewing of images on a reflective display or of a graphic without significant reduction in resolution and contrast, and without visible optical artifacts generated by light scattered or diffracted by different regions. In traditional lightguides, extraction layers have light scattering features in order for light being transported within the lightguide by TIR (at angles equal to or greater than the critical angles) in a lightguide to be directed out of the lightguide. These light scattering features typically comprise diffuse reflective printed extraction dots or structures that are disposed on or are etched into the surface of the lightguide which cause significant reduction in the viewing quality when looking through the lightguide.

In addition to optical benefits, the variable index light extraction layer can be produced by relatively simple coating and printing techniques amenable to high speed, low cost manufacturing. This disclosure generally relates to polymeric optical films or layers that exhibit regions of high index-like optical properties and low index-like optical properties, or otherwise interact with the transmission, scattering, absorption, refraction or reflection of light. The regions of high index-like optical properties and low index-like optical properties vary across a transverse plane of the optical layer, that is, the optical layer is a variable index optical layer. Throughout this disclosure, the term "index" is often used in place of index of refraction or refractive index. The transverse plane of a variable index light extraction layer disclosed herein can be described as a plane that is parallel to at least one major surface of the layer.

FIG. 1 a shows a schematic cross section of an exemplary variable index light extraction layer 100. The extraction layer comprises first regions 140a and 140b, both regions comprising a nanovoided polymeric material. In some embodiments, the nanovoided polymeric material comprises a plurality of interconnected nanovoids as described in WO 2010/120422 Al (Kolb et al.) and WO 2010/120468 Al (Kolb et al.). The plurality of interconnected nanovoids is a network of nanovoids dispersed in a binder wherein at least some of the nanovoids are connected to one another via hollow tunnels or hollow tunnel- like passages. Nanovoided polymeric material comprising interconnected nanovoids have nanovoids or pores that can extend to one or more surfaces of the material.

The variable index light extraction layer comprises second region 130 disposed between first regions 140a and 140b. The second region comprises the nanovoided polymeric material and an additional material. In some embodiments, this additional material occupies at least a portion of the void volume of the nanovoided polymeric material. Throughout this disclosure, dashed lines in cross section and plan views are used to indicate general location of the first and second regions, however, these dashed lines are not meant to describe any sort of boundary between the regions.

FIG. lb shows a schematic cross section of an exemplary variable index light extraction layer disposed on a transparent adjacent layer. Optical film 105 comprises variable index light extraction layer 100 disposed on adjacent layer 120 which is a transparent substrate. Variable index light extraction layer 100 comprises first regions 140a and 140b, and second region 130 disposed between the first regions.

In general, an area or region is identified by the material it comprises in combination with the refractive index of the region. The first region comprises a nanovoided polymeric material and has a first refractive index. A first region is identified if substantially all of the region comprises the nanovoided polymeric material and if the region has a refractive index within ± 0.02 across a continuous transverse plane of the layer. Methods for determining the refractive index across a transverse plane of the layer are described below.

The second region comprises the nanovoided polymeric material and an additional material, and has a second refractive index that is different from the first refractive index by at least about 0.03. The nanovoided polymeric material is the same material in both the first and second regions. A material is considered an additional material if it is incorporated substantially within the variable index light extraction layer and causes a change in refractive index of the first region by at least about 0.03, for example, from about 0.03 to about 0.5, from about 0.05 to about 0.5, or from about 0.05 to about 0.25.

In some embodiments, the additional material is different from the binder used to form the nanovoided polymeric material. In some embodiments, the additional material is the same as the binder used to form the nanovoided polymeric material. A second region is identified if (i) all of the region comprises the nanovoided polymeric material, (ii) the region has a refractive index within ± 0.02 across a continuous transverse plane of the variable index light extraction layer, and (iii) the region has a refractive index that is different from that of the first region by at least about 0.03.

In some embodiments, the variable index light extraction layer can be made by combining an additional material with portions of the nanovoided polymeric material that has been formed into some desirable shape such as a layer. Enough of the additional material is combined with the nanovoided polymeric material such that the desired change in refractive index results, and which is at least about 0.03, for example, from about 0.03 to about 0.5, from about 0.05 to about 0.5, or from about 0.05 to about 0.25.

The variable index light extraction layer comprises the first and second regions disposed relative to each other such that for light being transported at supercritical angles in an adjacent layer, the variable index light extraction layer selectively extracts the light in a predetermined way based on the geometric arrangement of the first and second regions. As used herein, supercritical angles are angles that are equal to or greater than the critical angle for a given interface formed by the first region of the variable index light extraction layer and the adjacent layer, is determined by the refractive index difference between the first region and the adjacent layer. The critical angle is the smallest angle of incidence at which a light ray passing from one medium to another less refractive medium can be totally reflected from the boundary between the two.

Referring to FIG. lb, which is a simplified view of FIG. la, light represented by rays 150 and 160 are being transported within adjacent layer 120 by TIR. In this embodiment, the refractive index of first regions 140a and 140b are that much less than that of the adjacent layer which defines critical angle 9C as shown. Light traveling at a supercritical angle represented by ray 150 strikes an interface between adjacent layer 120 and first region 140b, and this angle of incidence for ray 150 is greater than 9C, which results in substantially all of the light being reflected at the interface.

Also in this embodiment, the refractive index of second region 130 is approximately equal to or greater than that of adjacent layer 120. In this circumstance there is no critical angle at the interface and the light represented by ray 160 passes through the interface between adjacent layer 120 and second region 130, thus being extracted from the adjacent layer into the second region 130.

Thus, for the embodiment shown in FIG. la and FIG. lb, the first and second regions are disposed relative to each other such that light being transported at supercritical angles in an adjacent layer can be extracted selectively by the variable index light extraction layer in a predetermined way based on the geometric arrangement of the first and second regions. FIG. lc shows the schematic cross section of optical film 105 with light at subcritical angles impinging on the adjacent layer. Light represented by rays 180 and 190 impinges at subcritical angles on surface 170 of adjacent layer 120, and the light travels essentially undeviated through layers 120 and 100. Light represented by ray 190 travels through first region 140b, and light represented by ray 180 travels through second region 130. There is little to no deviation of light travelling through the different regions of variable index light extraction layer 100. This results in an optical film, such as exemplary optical film 105, that has low haze and high clarity, such that when one looks through the optical film there is little to no distortion of images on the opposite side. The variable index light extraction layer can have any geometric arrangement of first and second regions to produce the desired extracted light pattern.

In general, the refractive index profile of the variable index light extraction layer may vary in any way, as long as the desired optical performance of the layer is obtained. FIG. 2 illustrates the variable index light extraction layer having refractive indices that can vary across a transverse plane of the layer. The refractive index profile shows a plot of distance d, which corresponds to a distance across a transverse plane of the layer, for the layer in plan view. FIG. 2 shows that at some initial position on the layer corresponding to do, the layer has first refractive index n i corresponding to the first region. Moving across the transverse plane of the layer, first refractive index nt is observed until reaching dj where the refractive index of the layer abruptly increases to n2 which corresponds to the second refractive index of the second region. Continuing to move across the transverse plane of the layer, the second refractive index ¾ is observed until reaching <¾ where the refractive index of the layer abruptly decreases to n i indicating a second first region.

The change in refractive index between two adjacent first and second regions having low and high indices, respectively,