阅读说明：本技术 宽带可见光反射器 (Broadband visible light reflector ) 是由 爱德华·J·基维尔 瑞安·T·法比克 蒂莫西·J·内维特 于 2016-12-12 设计创作，主要内容包括：本发明公开了宽带可见光反射器。特别地,描述了具有减小的轴向蓝色反射率的宽带可见光反射器。描述了在反射时呈现黄色的宽带可见光反射器。此类宽带可见光反射器可在背光源和显示器中使用。(The invention discloses a broadband visible light reflector. In particular, broadband visible light reflectors having reduced axial blue reflectance are described. Broadband visible light reflectors that appear yellow when reflected are described. Such broadband visible light reflectors may be used in backlights and displays.)

1. A broadband visible light reflector comprising:

a plurality of optical repeating units, each optical repeating unit comprising a first birefringent polymer layer and a second polymer layer,

wherein the broadband visible light reflector appears yellow when reflected, and

wherein the broadband visible light reflector provides a photopic weighted average R of at least 95% across the visible spectrum_hemi(λ), the photopic weighted average R_hemi(λ) is determined using the CIE 1931 photopic response function.

2. The broadband visible light reflector of claim 1, wherein the broadband visible light reflector appears blue in transmission.

Background

Polymeric multilayer optical films are formed by coextruding tens to hundreds of molten polymer layers and then orienting or stretching the resulting film. The microlayers have different refractive index characteristics and are sufficiently thin such that light is reflected at interfaces between adjacent microlayers. The broadband visible light reflector reflects all or substantially all of the visible spectrum and is useful for display and lighting applications.

Disclosure of Invention

In one aspect, the present description relates to broadband visible light reflectors. In particular, the broadband visible light reflector includes a plurality of optical repeat units, each optical repeat unit including a first birefringent polymer layer and a second polymer layer. The optical repeat units each have an optical thickness, and the optical thickness of any optical repeat unit does not exceed 220 nm.

In another aspect, the present description relates to a broadband visible reflector. The broadband visible light reflector includes a plurality of optical repeating units, each optical repeating unit including a first birefringent polymer layer and a second polymer layer. At normal incidence of light between 380nm and 430nm, the broadband visible reflector transmits no less than 30% of unpolarized light.

In yet another aspect, the present disclosure relates to a broadband visible light reflector. The broadband visible light reflector appears yellow when reflected. The broadband visible reflector provides a photopic weighted average R of at least 95% across the visible spectrum_hemi(λ), the photopic weighted average Rhemi (λ) is determined using the CIE1931 photopic response function.

Drawings

Fig. 1 is a schematic perspective view of a reflective film.

FIG. 2 is a graph illustrating calculated and measured hemispherical reflectivities for a multilayer polymeric reflective film.

Fig. 3 is a graph showing layer curves of the multilayer film of comparative example C1.

Fig. 4 is a graph showing the axial transmission of the multilayer film of comparative example C1.

Fig. 5 is a graph showing layer curves of the multilayer film of example 1.

Fig. 6 is a graph showing the axial transmission of the multilayer film of example 1.

Fig. 7 is a graph showing the relationship between photopically weighted hemispherical reflectivity and layer thickness, normalized to comparative example C1.

Detailed Description

Multilayer optical films (i.e., films that are at least partially arranged by microlayers of different refractive index to provide desired transmission and/or reflection characteristics) are well known. It is well known that such multilayer optical films are made by sequentially depositing inorganic materials in the form of optically thin layers ("microlayers") on a substrate in a vacuum chamber. Inorganic multilayer Optical films are described in textbooks such as h.a. moleod, Thin-Film Optical Filters (Thin-Film Optical Filters), second edition, Macmillan publishing co, (1986) and a.thelan, Design of Optical interference Filters (Design of Optical interference Filters), McGraw-Hill, inc. (1989).

Multilayer optical films have also been shown by coextrusion of alternating polymer layers. See, e.g., U.S. Pat. No. 3,610,729(Rogers), U.S. Pat. No. 4,446,305(Rogers et al), U.S. Pat. No. 4,540,623(Im et al), U.S. Pat. No. 5,448,404(Schrenk et al), and U.S. Pat. No. 5,882,774(Jonza et al). In these polymeric multilayer optical films, the polymeric materials are used primarily or exclusively in the preparation of the various layers. These polymeric multilayer optical films may be referred to as thermoplastic multilayer optical films. Such films are suitable for high-volume manufacturing processes and can be made into large sheets and rolls. The following description and examples relate to thermoplastic multilayer optical films.

Multilayer optical films include individual microlayers having different refractive index characteristics such that some light is reflected at interfaces between adjacent microlayers. The microlayers are sufficiently thin such that light reflected at the plurality of interfaces undergoes constructive or destructive interference in order to impart desired reflective or transmissive properties to the multilayer optical film. For multilayer optical films designed to reflect ultraviolet, visible, or near-infrared wavelengths of light, each microlayer generally has an optical thickness (physical thickness multiplied by refractive index) of less than about 1 μm. The layers can generally be arranged to be thinnest to thickest. In some embodiments, the arrangement of alternating optical layers may vary substantially linearly according to a layer count function. These layer curves may be referred to as linear layer curves. Thicker layers may also be included, such as skin layers at the outer surface of the multilayer optical film or Protective Boundary Layers (PBLs) disposed within the multilayer optical film to separate coherent groups of microlayers (referred to herein as "packets"). In some cases, the protective boundary layer can be the same material as the alternating layers of the at least one multilayer optical film. In other cases, the protective boundary layer may be a different material selected for its physical or rheological properties. The protective boundary layer may be on one or both sides of the optical packet. In the case of a single-packet multilayer optical film, the protective boundary layer can be on one or both outer surfaces of the multilayer optical film.

For the purposes of this description, the groupings typically change the thickness of the optical repeat unit monotonically. For example, a packet may monotonically increase, monotonically decrease, increase and be constant, or decrease and be constant, but not both. It should be understood that one or more layers that do not follow this pattern are not relevant to the overall definition or identification of a particular group of optical repeating layers as a packet. In some embodiments, it may be helpful to define a grouping as the largest discrete grouping of successive non-redundant layer pairs that collectively provide reflection over a particular subrange (e.g., the visible spectrum) of the spectrum of interest.

In some cases, the microlayers have thickness and refractive index values that provide 1/4 wavelength superimposition, i.e., the microlayers are arranged in optical repeat units or unit cells that each have two adjacent microlayers of the same optical thickness (f-ratio 50%), such optical repeat units can effectively reflect light by constructive interference, the wavelength λ of the reflected light being about twice the total optical thickness of the optical repeat units. Other layer arrangements are also known, such as multilayer optical films having 2 microlayer optical repeat units (with f-ratios other than 50%), or films where the optical repeat units include more than two microlayers. These optical repeat unit designs can be configured to reduce or increase certain higher order reflections. See, for example, U.S. Pat. Nos. 5,360,659(Arends et al) and 5,103,337(Schrenk et al). A thickness gradient of the optical repeat units along a thickness axis (e.g., z-axis) of the film can be used to provide a broadened reflection band, such as a reflection band that extends across the visible region of a human and into the near infrared region, such that the microlayer stack continues to reflect across the visible spectrum as the band shifts to shorter wavelengths at oblique angles of incidence. Sharpening band edges (i.e., wavelength transitions between high reflectance and high transmission) by adjusting the thickness gradient is discussed in U.S. Pat. No. 6,157,490(Wheatley et al).

In many applications, the reflective properties of the film can be measured as the "hemispherical reflectivity" R_hemiBy (λ), it is meant the total reflectance of a component (whether a surface, film, or collection of films) when light (the wavelength of which is a certain wavelength or a wavelength within the range of interest) is incident on the component from all possible directions. Thus, the component is illuminated with light incident in all directions (and all polarization states, unless otherwise specified) within a normally-centered hemisphere, and all light reflected into this same hemisphere is collected. The ratio of the total flux of reflected light to the total flux of incident light for the wavelength range of interest yields a hemispherical reflectivity, R_hemi(lambda). For the backlight recycling cavity, its R is used_hemi(λ) it is particularly convenient to characterize the reflector because light is typically incident on the inner surface of the cavity at all angles (whether it be the front, back or side reflectors). In addition, R is different from the reflectance of the normally incident light_hemi(λ) is insensitive to and takes into account the variation in reflectivity with angle of incidence, which is important for some components within the recycling backlight, such as prismatic films.

It will be appreciated that for a number of electronic display applications using backlights, and for backlights used for general and specific light applications, it may be desirable to form the backlight backplane to have a reflector film with high reflectivity characteristics. In fact, the hemispherical reflectivity spectrum R should be further understood _hemi(λ) is closely related to the light output effect of the backlight; r across the visible spectrum_hemiThe higher the value of (lambda), the higher the output of the backlightThe higher the output efficiency. This is particularly true for recycling backlights where other optical films may be configured at the backlight exit aperture to provide a collimated or polarized light output from the backlight.

Additional details of multilayer optical films and their related designs and constructions are discussed in U.S. Pat. No. 5,882,774(Jonza et al), U.S. Pat. No. 6,531,230(Weber et al), PCT publication WO 95/17303(Ouderkirk et al), and WO99/39224(Ouderkirk et al), and in the publication entitled "Giant Bireframing Optics in multilayerPolymer Mirrors" (Weber et al) (3 month 2000 "science, Vol 287). Multilayer optical films and related articles can include additional layers and coatings selected for their optical, mechanical, and/or chemical properties. For example, a UV absorbing layer may be added on the incident side of the film to protect the components from degradation by UV light. The multilayer optical film may be attached to the mechanical reinforcement layer using a UV curable acrylate adhesive or other suitable material. Such a reinforcing layer may comprise a polymer such as PET or polycarbonate, and may also include a structured surface that provides optical functions such as light diffusion or collimation (e.g., through the use of beads or prisms). Additional layers and coatings may also include a disorder resistant coating, a tear resistant layer, and a stiffening agent. See, for example, U.S. Pat. No. 6,368,699(Gilbert et al). Methods and apparatus for making multilayer optical films are discussed in U.S. patent 6,783,349(Neavin et al).

The reflective and transmissive properties of the multilayer optical film are a function of the refractive index of the respective microlayers and the thickness and thickness distribution of the microlayers. Each microlayer (at least at localized positions in the film) can pass an in-plane refractive index n_x、n_yAnd a refractive index n associated with a thickness axis of the film_zTo characterize. These indices represent the refractive indices of the subject material for light polarized along mutually perpendicular x, y and z axes, respectively. For ease of description in this patent application, unless otherwise specified, the x-axis, y-axis, and z-axis are assumed to be local Cartesian coordinates applicable to any point of interest on the multilayer optical film, wherein the microlayers extend parallel to the x-y plane, and wherein the x-axis is oriented in the plane of the film to maximize Δ n_xThe magnitude of (c). Thus, Δ n_yMay be equal toOr less than (but not greater than) Δ n_xThe magnitude of (c). In addition, the difference Δ n is calculated_x、Δn_y、Δn_zAt the beginning of selecting which material layer is composed of deltan_xIs determined to be non-negative. In other words, the difference in refractive index between the two layers forming the interface is Δ n_j＝n_1j–n_2jWherein j is x, y, or z, and wherein the layer numbers 1, 2 are selected such that n_1x≥n_2xI.e. Δ n_x≥0。

In practice, the refractive index is controlled by judicious choice of materials and processing conditions. The preparation method of the multilayer film comprises the following steps: a large number (e.g., tens or hundreds) of layers of two alternating polymers a, B are coextruded, optionally followed by passing the multilayer extrudate through one or more layer multiplication devices, followed by casting through a film die, and then stretching or otherwise orienting the extrudate to form the final film. The resulting film is typically composed of hundreds of individual microlayers whose thicknesses and refractive indices are tailored to provide one or more reflection bands in desired spectral regions, such as the visible or near infrared. To achieve high reflectivity with an appropriate number of layers, adjacent microlayers typically exhibit a difference in refractive index (Δ n) for light polarized along the x-axis _x) Is at least 0.05. In some embodiments, the materials are selected such that the refractive index difference for light polarized along the x-axis is as high as possible after orientation. If high reflectivity is desired for two orthogonally polarized lights, adjacent microlayers can also be prepared to exhibit a difference in refractive index (Δ n) for light polarized along the y-axis_y) At least 0.05.

The above-referenced' 774(Jonza et al) patent describes how, among other things, the refractive index difference (Δ n) between adjacent microlayers can be adjusted for light polarized along the z-axis_z) To achieve the desired reflection characteristics for the p-polarized component of obliquely incident light. To maintain high reflectivity of p-polarized light at oblique incidence angles, the z-axis refractive index mismatch Δ n between microlayers_zCan be controlled to be substantially smaller than the in-plane refractive index difference Deltan_xMaximum value such that Δ n_z≤0.5*Δn_xOr Δ n_z≤0.25*Δn_x. A z-axis index mismatch of magnitude zero or nearly zero produces such an interface between microlayers: the interface has a constant or nearly constant reflectivity for p-polarized light as a function of angle of incidence. In addition, the z-axis index mismatch Δ n can be controlled_zTo have a difference Δ n in refractive index compared to the in-plane_xOf opposite polarity, i.e. Δ n_z<0. This condition will result in an interface: the reflectivity of the interface for p-polarized light increases with increasing incidence angle, as is the case for s-polarized light.

Another design consideration discussed in' 774(Jonza et al) involves surface reflection at the air interface of a multilayer reflective polarizer. Unless the polarizer is laminated on both sides to an existing glazing component or another existing film with a clear optical adhesive, such surface reflection will reduce the transmission of light of the desired polarization state in the optical system. Thus, in some cases, it is useful to add an anti-reflection (AR) coating to the reflective polarizer.

Polymeric multilayer optical films as described herein can be highly reflective; for example, they may reflect more than 95%, 99%, or even 99.5% of visible light as measured at normal incidence. Visible light may be characterized as wavelengths between 400nm and 700nm, or in some cases between 420nm and 700 nm. In addition, polymeric multilayer optical films as described herein can be thin-in some cases, thinner than 100 μm, 85 μm, or 65 μm. In embodiments where the polymeric multilayer optical film includes a third optical packet, the film may be thinner than 165 μm.

Sometimes a skin layer is added. Typically this is done after layer formation and before the melt exits the film die. The multilayer melt is then cast through a film die onto a chill roll where it is quenched in the conventional manner for polyester films. The cast web is then stretched in different ways to achieve birefringence in at least one of the optical layers, resulting in either a reflective polarizer or mirror film in many cases, as has been described, for example, in U.S. patent publication No. 2007/047080 a1, U.S. patent publication No. 2011/0102891 a1, and U.S. patent No. 7,104,776(Merrill et al).

Broadband visible light reflectors are designed to provide broad coverage of the visible spectrum, reflecting light both axially and at an angle. For example, the broadband visible light reflector provides a photopic weighted average R of at least 95% across the visible spectrum_hemi(λ), the photopic weighted average Rhemi (λ) is determined using the CIE 1931 photopic response function. This means that the layer distribution needs to be designed such that there are layer pairs with optical layer thicknesses as low as 200nm or less in order to reflect blue light in the axial direction. These thin layers are considered essential to provide the reflector with a uniform appearance and high performance (i.e. to ensure that it is still a broadband reflector). However, these thin layers have the highest absorption for typical birefringent materials used for broadband visible reflectors. Furthermore, at an angle to the film, these thinnest layers either reflect only ultraviolet (invisible) light or become transparent to incident light and do not cause any reflection from the system. Surprisingly, the benefits of eliminating or minimizing these bluest layers (by using fewer layers or by providing a sharper layer profile slope) outweigh any losses due to axially transmitting a portion of the blue light. A layer of intermediate thickness (e.g., a green reflective layer) shifts its reflected wavelength at an angle to cover the bluer wavelengths. Accordingly, R can be observed when photopically weighted at visible wavelengths _hemiAn increase in (λ). This is particularly surprising since such films will have characteristics that can be described as blue leakage when inspected. Blue leakage refers to the fact that blue light is transmitted through the film. In transmission, the reflector appears blue. Upon reflection, such films will appear yellow. Despite being deficient in appearance, the examples provided herein show that such films can provide excellent performance on displays, with little relative importance to axial reflectance as compared to angular reflectance. By increasing R of photopic mean over the visible spectrum_hemi(λ) to capture this improvement in performance. In some embodiments, the optical thickness of any optical repeat unit is no less than 220nm (corresponding to a reflected axial wavelength of about 440 nm). In some embodiments, the optical of any optical repeat unitThe thickness is not less than 225 nm. In some embodiments, the broadband visible light reflector transmits no less than 20% of unpolarized light at normal incidence between 380nm and 430 nm. In some embodiments, the broadband visible light reflector transmits no less than 20% of unpolarized light at normal incidence between 380nm and 450 nm. In some embodiments, the broadband visible light reflector transmits no less than 30%, 40%, or 50% of unpolarized light at normal incidence between 380nm and 410 nm. These technical design criteria, i.e., minimizing or eliminating the bluest layer, may also provide performance comparable to or better than other high performance multilayer reflectors while reducing the necessary overall thickness of the broadband visible light reflector.