WO2021224735A1

WO2021224735A1 - Optical film with discontinuous coating

Info

Publication number: WO2021224735A1
Application number: PCT/IB2021/053580
Authority: WO
Inventors: Jeremy O. SWANSON; Matthew S. COLE; Matthew S. Stay; Matthew E. Sousa; Matthew R.D. SMITH; Anthony M. Renstrom; Bharat R. Acharya; Lisa A. DENICOLA; Quinn D. Sanford; Jason S. Petaja
Original assignee: 3M Innovative Properties Company
Priority date: 2020-05-08
Filing date: 2021-04-29
Publication date: 2021-11-11

Abstract

A multilayer optical film includes an optical reflector and an array of discrete, spaced-apart optical bumps formed on the optical reflector. For substantially normally incident light and a visible wavelength range and an infrared wavelength range, the optical reflector may have an average optical reflectance of greater than about 30% in the visible wavelength range for at least a first polarization state, and a specular transmittance of greater than about 20% for at least one wavelength in the infrared wavelength range for each of the first polarization state and an orthogonal second polarization state. The optical bumps may have an average optical transmittance of greater than about 50% for each of the visible and infrared wavelength ranges for each of the first and second polarization states.

Description

OPTICAL FILM WITH DISCONTINUOUS COATING

Summary

In some aspects of the present description, a multilayer optical film is provided, including an optical reflector and an array of discrete, spaced-apart optical bumps formed on the optical reflector. For substantially normally incident light and a visible wavelength range extending from about 450 nm to about 600 nm and an infrared wavelength range extending from about 800 nm to about 1200 nm, the optical reflector may have an average optical reflectance of greater than about 30% in the visible wavelength range for at least a first polarization state, and a specular transmittance of greater than about 20% for at least one wavelength in the infrared wavelength range for each of the first polarization state and an orthogonal second polarization state. The optical bumps may have an average optical transmittance of greater than about 50% for each of the visible and infrared wavelength ranges for each of the first and second polarization states.

In some aspects of the present description, a multilayer optical film is provided, including an optical substrate and an array of discrete, spaced-apart optical bumps formed on the substrate. For substantially normally incident light and a visible wavelength range extending from about 450 nm to about 600 nm, and for an infrared wavelength range extending from about 800 nm to about 1200 nm, the optical bumps have an average optical transmittance of greater than about 50% for each of the visible and infrared wavelength ranges for each of the first and second polarization states. When the array of discrete, spaced-apart optical bumps side of the optical substrate is substantially uniformly illuminated with light having a visible wavelength in the visible wavelength range, the substrate reflects the illuminating light such that a contrast between light reflected from the optical bumps and light reflected from regions of the substrate between the optical bumps is less than about 5%. For a coverage of about 10% to about 40% of a surface area of the substrate by the optical bumps, the array of discrete, spaced-apart optical bumps side of the substrate has a coefficient of friction of between about 0.2 and about 0.6 when moving along a first direction against a structured surface including a plurality of linear prisms arranged along the first direction at a prism pitch of about 18 microns to about 24 microns and extending along an orthogonal second direction.

Brief Description of the Drawings

FIG. 1A is a side, cutaway view of a multilayer optical film with spaced-apart optical bumps in a discontinuous layer, in accordance with an embodiment of the present description;

FIG. IB is a side, cutaway view of a multilayer optical film with spaced-apart optical bumps in a continuous layer, in accordance with an embodiment of the present description;

FIG. 2A is an illustration showing normally incident light impinging on an optical reflector, in accordance with an embodiment of the present description;

FIG. 2B illustrates an optical reflector including a plurality of alternating polymeric layers, in accordance with an embodiment of the present description;

FIG. 3 illustrates light reflected from a surface of an optical film, in accordance with an embodiment of the present description; FIG. 4 illustrates the contrast of light reflected from the optical bumps versus light reflected from regions between optical bumps of an optical fdm, in accordance with an embodiment of the present description;

FIG. 5 provides a plot of percent coverage in area of optical bumps versus visible transmission for various optical fdms, in accordance with an embodiment of the present description;

FIG. 6 provides a plot of percent coverage in area of optical bumps versus specular transmission for various optical fdms, in accordance with an embodiment of the present description;

FIG. 7 provides a plot of percent coverage in area of optical bumps versus visible clarity for various optical fdms, in accordance with an embodiment of the present description; and

FIG. 8 provides a plot of percent coverage in area of optical bumps versus visible haze for various optical fdms, in accordance with an embodiment of the present description.

Detailed Description

In the following description, reference is made to the accompanying drawings that form a part hereof and in which various embodiments are shown by way of illustration. The drawings are not necessarily to scale. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present description. The following detailed description, therefore, is not to be taken in a limiting sense.

LCD manufacturers are turning to the incorporation of sensors (e.g., infrared sensors for fingerprint reading) and other technologies to improve user security on mobile devices. In addition, display manufacturers are working on reducing the thickness of display and mobile devices, and are thus transitioning from the use of lightguide plates to thinner lightguide films. Flowever, these hghtguide films, which are commonly made of polycarbonate and may have micro-replicated structures on both sides, can be incompatible with adjacent films. For example, they may cause physical damage to adjacent films and/or reduced visual performance.

According to some aspects of the present description, a discontinuous coating on a surface of an optical diffuser film may impart a surface roughness that lowers the coefficient of friction and eliminates or reduces damage to adjacent films. In some embodiments, the discontinuous coating may be substantially transparent to near infrared wavelengths, enabling infrared sensor functionality behind an LCD panel. In some embodiments, the discontinuous coating may include discrete raised features, such as rounded bumps. In some embodiments, the discrete features may be added to a substrate using a technique such as to flexographic printing (or similar printing process). In some embodiments, the selected printing process may be used to add functionality to an optical substrate by creating a plurality of raised transparent dots or bumps on the surface of the substrate. In some embodiments, other techniques or processes may be used to add the discrete features to a substrate.

The performance of the near infrared transparent optical film and the visibility of the pattern within the optical stack may be directly related to the quality of the printed material, in conjunction with the feature height and feature diameter. Features that are irregularly shaped or too tall may cause the printed pattern to become visible through the display, causing the sample to be rejected.

While the diameter of the printed features is largely controlled through the design of the print plate, thickness is primarily controlled via selection of the proper anilox roller. An anilox roller is a roller that has a multiplicity of engraved cells that act as a metering device for ink on the printing plate. The depth and width of the cells set the volume of ink transferred to the flexographic printing plates.

According to some aspects of the present description, a coating may be transparent to at least near infrared wavelengths, discontinuous (discrete bumps or structures) and controllably spaced across the surface of an optical substrate, such as a diffuser film or multilayer optical film. The coating pattern and individual feature characteristics (shape, size, and height) may be specifically controlled through the printing process (e.g., flexographic, gravure, inkjet, or screen printing). Controlling the size, shape, dimension, and uniformity of features may significantly affect the optical properties and functional performance of the final film. Using these coatings in conjunction with an optical film enables the film to be protected from negative interactions (e.g., sticking, scratching, etc.) with adjacent films in an LCD backlight unit, and without imparting curl to the film and without a significant negative impact to infrared transmission through the film.

According to some aspects of the present description, a multilayer optical film includes an optical reflector and an array of discrete spaced-apart optical bumps formed on the optical reflector. In some embodiments, the optical reflector may include a plurality of alternating first and second polymeric layers. In some embodiments, each of the polymeric layers may be less than about 500 nanometers (nm), or less than 400 nm, or less than 200 nm, thick. In some embodiments, each of the first polymeric layers may have a different index of refraction from each of the second polymeric layers. In some embodiments, the optical reflector may further include a metal layer. In some embodiments, the metal layer may include, but not be limited to, one or more of aluminum, silver, gold, and copper. In some embodiments, the optical reflector may include a plurality of alternating first inorganic layers and second inorganic layers deposed on an optically transparent substrate (e.g., polyethylene terephthalate, or PET), each of the first inorganic layers having a different index of refraction from each of the second inorganic layers.

In some embodiments, the array of discrete, spaced-apart optical bumps may form a regular array. In some embodiments, the regular array may include, but not be limited to, one or more of a square array, a rectangular array, a triangular array, a hexagonal array, and a circular array. In some embodiments, the array of discrete, spaced-apart optical bumps may form an irregular array. In some embodiments, the array of discrete, spaced-apart optical bumps may form a random array. In some embodiments, the discrete, spaced-apart optical bumps may have an average height of between about 0.2 microns and about 10 microns, or between about 0.5 microns and about 5 microns. In some embodiments, the discrete, spaced-apart optical bumps may have an average lateral size of between about 50 microns and about 200 microns, or between about 75 microns and about 150 microns. In some embodiments, the discrete, spaced-apart optical bumps may cover between about 10% and about 40% of the surface area of the optical reflector. In some embodiments, the discrete, spaced-apart optical bumps may form a discontinuous layer (i.e., the optical bumps are individually formed on the optical reflector and are not connected to each other). In some embodiments, the discrete, spaced-apart optical bumps may form a continuous layer (i.e., having substantially planar “land” portions in between and connecting the optical bumps).

For substantially normally incident light and a visible wavelength range extending from about 450 nm to about 600 nm (e.g., human-visible wavelengths) and an infrared wavelength range extending from about 800 nm to about 1200 nm, the optical reflector may have an average optical reflectance of greater than about 30%, or greater than about 40%, or greater than about 50%, or greater than about 70%, in the visible wavelength range for at least a first polarization state, and a specular transmittance of greater than about 20%, or greater than about 40%, or greater than about 60%, for at least one wavelength in the infrared wavelength range for each of the first polarization state and an orthogonal second polarization state. In some embodiments, the optical reflector may have an average optical specular reflectance of greater than about 30%, or greater than about 35%, or greater than about 40%, in the visible wavelength range for the at least the first polarization state.

In some embodiments, for substantially normally incident light in the visible wavelength range, the optical reflector may have an average optical reflectance of greater than about 50%, or about 60%, or about 70%, for the first polarization state, and an optical transmittance of at least 50%%, or about 60%, or about 70%, for the second polarization state.

In some embodiments, for substantially normally incident light and for the visible wavelength range, the optical reflector may have an average optical reflectance of greater than about 30%, or greater than about 40%, or greater than about 50%, or greater than about 70%, in the visible wavelength range for each of the first and second polarization states. The optical bumps may have an average optical transmittance of greater than about 50% for each of the visible and infrared wavelength ranges for each of the first and second polarization states.

In some embodiments, when the array of discrete, spaced-apart optical bumps side of the multilayer optical film is substantially uniformly illuminated with a white light from a substantially Lambertian light source oriented at an angle of about 30 degrees relative to a normal to the multilayer optical film, the multilayer optical film may reflect the illuminating light such that for light reflected at an angle of about 30 degrees relative to the normal to the multilayer optical film, the average contrast between light reflected from the optical bumps and light reflected from regions of the multilayer optical film between the optical bumps is less than about 5%, or less than about 4%, or less than about 3%, or less than about 2%.

In some embodiments, the optical bumps may have an average optical transmittance greater than about 50%, or about 55%, or about 60%, for each of the visible and infrared wavelength ranges for each of the first and second polarization states.

In some embodiments, increasing the percent coverage of the surface area of the optical reflector by the optical bumps from about 10% coverage to about 40% coverage may reduce the average total optical transmittance of the multilayer optical film by less than about 5%, or less than about 4%, or less than about 3%, in the visible wavelength range. That is, an increase in area coverage by the bumps, up to approximately 40% of the surface area of the optical reflector, may have a comparatively small effect on total optical transmittance. In some embodiments, increasing a percent coverage of the surface area of the optical reflector by the optical bumps from about 10% to about 40% may reduce the average specular optical transmittance of the multilayer optical fdm by less than about 5%, or less than about 4%, or less than about 3%, for a wavelength of about 940 nm. In some embodiments, increasing the percent coverage of the surface area of the optical reflector by the optical bumps from about 10% to about 40% may reduce an average optical clarity of the multilayer optical film by less than about 20%, or less than about 15%, or less than about 10%, in the visible wavelength range. In some embodiments, increasing the percent coverage of the surface area of the optical reflector by the optical bumps from about 10% to about 40% may increase the average optical haze of the multilayer optical film by less than about 5%, or less than about 4%, or less than about 3%, in the visible wavelength range.

In some embodiments, for a coverage ranging from about 10% to about 40% of the surface area of the optical reflector by the optical bumps, the array of discrete, spaced-apart optical bumps side of the multilayer optical reflector may exhibit a coefficient of friction of between about 0.2 and about 0.6 when moving along a first direction against a structured surface comprising a plurality of linear prisms arranged along the first direction at a prism pitch of about 18 microns to about 24 microns and extending along an orthogonal second direction. In some embodiments, for a coverage ranging from about 10% to about 40% of a surface area of the optical reflector by the optical bumps, the array of discrete, spaced-apart optical bumps side of the multilayer optical reflector may have a coefficient of friction of between about 1.0 and about 2.5 against a major surface of a polyethylene terephthalate (PET) film.

In some embodiments, when the array of discrete, spaced-apart optical bumps side of the multilayer optical reflector is substantially uniformly illuminated with light having a visible wavelength in the visible wavelength range, the multilayer reflects the illuminating light such that a variation in intensity of the reflected light is not less, or is less by no more than about 5%, or about 4%, or about 3%, or about 2%, as compared to a multilayer optical film that has the same construction except that is does not include the optical bumps.

According to some aspects of the present description, a multilayer optical film includes an optical substrate and an array of discrete, spaced-apart optical bumps formed on the substrate. In some embodiments, for substantially normally incident light and a visible wavelength range extending from about 450 nm to about 600 nm, and for an infrared wavelength range extending from about 800 nm to about 1200 nm, the optical bumps may have an average optical transmittance of greater than about 50%, or about 55%, or about 60%, for each of the visible and infrared wavelength ranges for each of the first and second polarization states. In some embodiments, when the array of discrete, spaced-apart optical bumps side of the optical substrate is substantially uniformly illuminated with light having a visible wavelength in the visible wavelength range, the substrate may reflect the illuminating light such that the contrast between light reflected from the optical bumps and light reflected from regions of the substrate between the optical bumps (substantially planar land portions) is less than about 5%, or less than about 4%, or less than about 3%. In some embodiments, for a coverage of about 10% to about 40% of the surface area of the substrate by the optical bumps, the array of discrete, spaced-apart optical bumps side of the substrate may have a coefficient of friction of between about 0.2 and about 0.6 when moving along a first direction against a structured surface including a plurality of linear prisms arranged along the first direction at a prism pitch of about 18 microns to about 24 microns and extending along an orthogonal second direction. In some embodiments, the optical substrate may include, but not be limited to, one or more of an optical diffuser, a reflective polarizer, a substantially optically specularly transparent substrate, and an absorbing polarizer.

Turning now to the figures, FIGS. 1A and IB provide alternate embodiments of a multilayer optical film with spaced-apart optical bumps according to the present description. FIG. 1A shows a multilayer optical film 100 which includes an optical substrate 10 (e.g., an optical reflector) and discontinuous layer 21 including an array of discrete, spaced-apart optical bumps 20. In the embodiment of FIG. 1 A, the optical bumps are spaced in an array, which may be a regular or irregular array, and disposed directly on the surface of optical reflector 10. In some embodiments, optical bumps 20 are rounded bumps which cover some percentage of the surface area of optical substrate 10, and which contribute to anti-wet-out performance (as well as the reduction of other unwanted optical effects, such as Newton’s rings) while remaining substantially transparent to at least some wavelengths of near infrared light. For the purposes of this document, the term “wet-out” may be defined as the unintended integration of two surfaces in contact, leading to unwanted optical effects. In some embodiments, the area coverage of the surface of optical substrate 10 may be between about 10% and about 40%.

FIG. IB shows an alternate embodiment of a multilayer optical film 100’, which includes an optical substrate 10 and continuous layer 2G featuring optical bumps 20’ separated by substantially planar land portions 22. The embodiment of FIG. IB differs from the embodiment of FIG. 1A primarily in that continuous layer 2G and optical bumps 20’ are formed as a single component which is disposed on optical substrate 10, rather than (as in FIG. 1A) having optical bumps 20 disposed directly on optical substrate 10. As with the embodiment of FIG. 1A, the embodiment of FIG. IB may, in some embodiments, have an area coverage of optical bumps 20’ on optical substrate 10 (including land portions 22 of continuous layer 2G) between about 10% and about 40%. FIGS. 5-8 provide additional information on the optimal area coverage percentage for optical bumps 20 to achieve the desired anti -wet-out behavior without a significant reduction in desired optical characteristic.

Throughout this document, the term “normally incident light” is used, referring to light that may impinge on a surface such as a display or optical substrate such that it is substantially perpendicular to the surface upon which it is impinging. FIG. 2A is an illustration showing normally incident light 30 impinging on a surface of an optical substrate 10. Of course, in reality, light may impinge on a surface from directions other than normally incident, at various angles off from normal (i .e., angles smaller or larger than 90 degrees relative to the surface of optical substrate 10). However, for the purposes of discussion, data and comparisons will often be discussed based on performance of and behavior related to normally incident light 30. In some embodiments, the optical substrate 10 (e.g., an optical reflector) may be constructed of a plurality of alternating polymeric layers. FIG. 2B illustrates an optical substrate 10’ which includes a plurality of alternating first polymeric layers 11 and second polymeric layers 12. In some embodiments, each of the first 11 and second polymeric layers 12 may be less than about 500 nm thick, or about 300 nm thick, or about 200 nm thick Optical substrate 10’ may include hundreds of alternating layers 11 and 12, for example about 500 layers, or about 300 layers, or about 100 layers, or about 50 layers total (including both layers 11 and 12). By carefully selecting the materials for the alternating polymeric layers 11 and 12, and specifically the index of refraction of polymeric layers 11 versus the index of refraction of polymeric layers 12, as well as the number of each type of layer, one can select and/or tune the optical properties of optical substrate 10’ (e g., to create an optical reflector which reflects or transmits light based on its wavelength, angle of incidence, and/or polarization type). For example, an optical reflector may be created with substantially reflects human-visible light (visible light) and substantially transmits one or more wavelengths of near infrared light. An optical substrate may also be configured such that it substantially transmits visible light of a first polarization type (e.g., light of a linear p polarization type) and substantially reflects light of a second, orthogonal polarization type (e.g., light of a linear s polarization type).

FIG. 3 provides an illustration of how light may be reflected from a surface of an optical film according to the present description. The multilayer optical film 100 of FIG. 3 may, for example, be the multilayer optical film 100 ofFIG. 1A, or the multilayer optical film 100’ ofFIG. IB. Multilayer optical film 100 includes optical bumps 20 in a discontinuous layer 21 (or, alternately, optical bumps 20’ of continuous layer 2G). In some embodiments, when the surface of multilayer optical film 100, including optical bumps 20, is substantially uniformly illuminated with white light 40 from a Lambertian light source 50 oriented at an angle Q1 which is about 30 degrees relative to normal 41, the multilayer optical film 100 reflects light 42 at an angle 02 which is about 30 degrees relative to normal to the surface of multilayer optical film 100, the average contrast between light reflected from optical bumps 20 and light reflected from the regions between the bumps (e.g., the surface of the optical substrate 10 between optical bumps 20) may be less than about 5%, or about 4%, or about 3%, or about 2%. FIG. 4 is an image showing the contrast of light reflected from the optical bumps 43 versus light reflected from regions between optical bumps 44 of an optical film 10, as described in the discussion ofFIG. 3.

FIGS. 5 through 8 include charts showing the measured performance of several optical characteristics of optical films according to the present description. Each of FIGS. 5 through 8 contain 4 charts, representing characteristics measured for four optical films with different ink volume values, measured in units of billion cubic microns per square inch of surface area of the anilox roller being used. The unit BCM/m², in practice often shortened to simply BCM, is used as a measure of the volume of ink or material that is transferred from the anilox roller to a substrate per square inch of substrate. Each of FIGS. 5-8 contain a chart labeled near the top as 1, 3, 6, and 10, representing optical films with 1BCM, 3BCM, 6BCM, and lOBCM respectively. In the flexographic printing industry, BCM is main driver in selecting the thickness (height of the bump above the substrate). A higher number BCM means a larger amount of ink was transferred onto the substrate. While a larger amount of ink/material may result in a slight increase in the diameter (or area coverage) because of the additional material transferred, a larger BCM number typically indicates the height of the feature. That is, a lOBCM substrate should have taller (or thicker) features that a 6BCM substrate, or a 3BCM substrate, or a 1BCM substrate.

Each of the four charts in a figure plot the performance of an optical characteristic (e g , visible transmission, visible haze, etc.) as the area coverage of optical bumps is increased from about 3% coverage to 100% coverage. Each of the four charts in each of FIGS. 5-8 shows a dashed box around an area from about 10% surface coverage (i.e., coverage of optical bumps) to about 40% surface coverage, indicating an area where the optical characteristic being measured shows acceptable optical performance (i.e., a “sweet spot” where percent coverage still gives acceptable optical performance). Each chart includes three plot lines, showing values measured for a transparent ink (lines with circles), a diffuse ink (lines with square), and an opaque ink (lines with diamonds). In several of the charts, the plots for transparent ink and diffuse ink substantially match, such that one line is superimposed on top of the other.

Turning now to FIG. 5, a plot of percent coverage in area of optical bumps versus the mean of visible transmission is provided. From the charts in FIG. 5, we can see that a 1 BCM substrate appears to provide the best overall performance relative for the mean visible transmission. Values of visible transmission remain fairly steady for all surface area coverage values up to 100 percent for transparent and visible ink, and opaque ink only shows a small percentage drop in visible transmission at 40% coverage, around a 3% reduction. Using the data measured, the proper configuration and coverage of optical bumps can be selected based on the technical requirements of the resulting optical substrate.

FIG. 6 provides a plot of percent coverage in area of optical bumps versus specular transmission as measured at a wavelength of 940 nm. These plots again illustrate a performance “sweet spot” in the area between 10% and 40% surface area coverage by optical bumps. FIG. 7 provides a plot of percent coverage in area of optical bumps versus visible clarity, and FIG. 8 provides a plot of percent coverage in area of optical bumps versus visible haze. As visible haze in this context is an undesired optical effect, the plots of FIG. 8 show increases in haze as the percent coverage of optical bumps is increased.

EXAMPLES

A first embodiment provides a matrix of transparent dots printed on one surface of a multilayer optical film. The base film for these articles was a near-infrared transmissive specular reflector multilayer optical film. The overprint varnish material was OP1028 Premium Gloss HS Overprint Varnish (mfg. by Nazdar Ink Technologies, Shawnee, KS).

A second embodiment provides a matrix of transparent dots printed on one surface of an optical film. The base film for these articles was polyester (PET) film, 0.003" Melinex 454 Film, commercially available from DuPont Teijin Films, Wilmington, DE. The overprint varnish material was OP 1028 Premium Gloss HS Overprint Varnish (mfg. by Nazdar Ink Technologies, Shawnee, KS).

A third embodiment provides a matrix of diffuse dots printed on one surface of an optical film. The base film for these articles was polyester (PET) film, 0.003" Melinex 454 Film, commercially available from DuPont Teijin Films, Wilmington, DE. The overprint varnish material was OP2018 Imprintable Matte UV Varnish (mfg. by Nazdar Ink Technologies, Shawnee, KS).

A fourth embodiment provides a matrix of opaque dots printed on one surface of an optical film. The base film for these articles was polyester (PET) film, 0.003" Melinex 454 Film, commercially available from DuPont Teijin Films, Wilmington, DE. The ink material was 9308 UV Flexo Ink (mfg. by Nazdar Ink Technologies, Shawnee, KS).

The printing process used to print the optical bumps utilized commercial 12” Reverse Angle Doctor Blade System (RADBS) flexographic printing deck manufactured by Retroflex Inc, Wrightstown, WI, US. The flexographic printing deck was set up using the following conditions. The Anilox Roll volume ranged from 1.0-10.0 Billion Cubic Micron per square inch (BCM), to control thickness of the discontinuously patterned features (fabricated and engraved by Interflex Laser Engravers, Spartanburg, SC). The printing plate used for these examples was a LUX ITP 60 0.067” thick photopolymer plates (available from MacDermid, Waterbury, CT) with an approximate designed area coverage of 3-73%, with an array of dots (imaged and developed via SGS Inc., Brooklyn Park, MN.) Dot arrays may be, but are not restricted to, a simple array consisting of a linear (square) packing of dots, a hexagonal array consisting of a hexagonal packing of dots, or a random array consisting of a non-uniform distribution of dots in a packing configuration that is neither linear nor hexagonal. The mounting tape used was E1060H mounting tape (available from 3M Company, St. Paul, MN).

The 14” wide substrate was first loaded onto the flexographic printing line and put under tension at approximately 1 pound per linear inch of tension. Next, the line was ran at 50 feet per minute to transport the web through the printing deck. After printing, the substrate with the printed coating was sent through the UV curing chamber from Xeric Web drying system, Neenah, WI. The arc bulb power was set to 30% relative to maximum bulb output. The cured coating on the substrate was subsequently wrapped up in roll formed into a wound roll.

The coating pattern and height is specifically designed to be highly specular in the near-infrared (NIR) yet have sufficient roughness to reduce film to film interference and lower the coefficient of friction. The printed features height ranged from 0.5 to 5 microns during experiments. Examples determine ranges of height and feature density which provide advantages in anti-wet out and coefficient of friction without adversely affecting the optical performance.

TEST METHODS

The total near-infrared transmission and diffuse near-infrared transmission were measured using an Ultrascan Pro spectrometer, available from Hunterlab (Reston, VA). The spectral range was evaluated from 350 nm to 1050 nm The near-infrared specular transmission was calculated by subtracting the diffuse near-infrared transmission from the total near-infrared transmission at a target wavelength of 940 nm.

The total visible transmission, haze, and clarity were determined using a Haze-gard Plus haze meter from BYK-Gardner USA (Columbia, MD). Visible spectral range is set by the instrument. Coefficient of friction is measured on an SP-2100 Slip/Peel Tester with coefficient of friction (COF) sled attachment available from IMASS, Inc. (Accord, MA). The test method used is a modification of ASTM-D1894 “Static and Kinetic coefficients of friction of plastic film and sheeting.” The sample for this measurement was 2.5 inches by 2.5 inches (63.5 mm by 63.5 mm) in dimension. The test procedure proscribed for slip coefficient of friction used 200g sled and 6 inches per minute velocity (152 mm/minute) and reported static peak value.

EXAMPLE DESCRIPTION

Each example below in Table 1 is printed with either a hexagonal array or randomized array of a clear ink using anilox rolls of varying ink volume, as commonly described with the acronym BCM (billion cubic microns / in²). BCM describes the volume of ink per square inch which can be stored on the engraved anilox roller. The examples vary by target area coverage, the clear ink type, and the substrate type (either a PET substrate or a near-infrared transmissive specular reflector multilayer optical fdm (NIR Film)). The comparative examples are printed as a continuous coating.

Table 1: Example Descriptions

Example 1 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 2 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 3 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 4 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 5 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 6 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 7 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 8 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 9 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 10 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 11 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 12 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 13 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 14 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 15 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar OP1028) on PET. Example 16 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Comparative Example 17 is continuously coated with a target area coverage of 100% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Comparative Example 18 is continuously coated with a target area coverage of 100% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Comparative Example 19 is continuously coated with a target area coverage of 100% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Comparative Example 20 is continuously coated with a target area coverage of 100% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 21 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via 5 BCM anilox roll using clear ink (Nazdar OP 1028) on NIR Film.

Example 22 is a randomized arrangement of printed features with a target area coverage of 4% and a controlled thickness via 5 BCM anilox roll using clear ink (Nazdar OP1028) on NIR Film.

Example 23 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar OP2018).

Example 24 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar OP2018).

Example 25 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 26 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 27 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 28 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 29 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 30 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 31 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 32 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 33 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 34 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar OP2018) on PET. Example 35 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 36 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 37 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Example 38 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Comparative Example 39 is continuously coated with a target area coverage of 100% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Comparative Example 40 is continuously coated with a target area coverage of 100% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar OP2018) on PET.

Comparative Example 41 is continuously coated with a target area coverage of 100% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Comparative Example 42 is continuously coated with a target area coverage of 100% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar OP1028) on PET.

Example 43 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 44 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 45 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 46 is a hexagonal array of printed features with a target area coverage of 3% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 47 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 48 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 49 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 50 is a hexagonal array of printed features with a target area coverage of 12% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 51 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 52 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 53 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar 9308) on PET. Example 54 is a hexagonal array of printed features with a target area coverage of 39% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 55 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar 9308) on PET. Example 56 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 57 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Example 58 is a hexagonal array of printed features with a target area coverage of 73% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Comparative Example 59 is continuously coated with a target area coverage of 100% and a controlled thickness via a 1 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Comparative Example 60 is continuously coated with a target area coverage of 100% and a controlled thickness via a 3 BCM anilox roll using clear ink (Nazdar 9308) on PET. Comparative Example 61 is continuously coated with a target area coverage of 100% and a controlled thickness via a 6 BCM anilox roll using clear ink (Nazdar 9308) on PET.

Comparative Example 62 is continuously coated with a target area coverage of 100% and a controlled thickness via a 10 BCM anilox roll using clear ink (Nazdar 9308) on PET. TEST MEASUREMENTS

Table 2: Test Measurements for the Examples

data not taken or measured Terms such as “about” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “about” as applied to quantities expressing feature sizes, amounts, and physical properties is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “about” will be understood to mean within 10 percent of the specified value. A quantity given as about a specified value can be precisely the specified value. For example, if it is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, a quantity having a value of about 1 , means that the quantity has a value between 0.9 and 1.1, and that the value could be 1.

Terms such as “substantially” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “substantially equal” is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “substantially equal” will mean about equal where about is as described above. If the use of “substantially parallel” is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “substantially parallel” will mean within 30 degrees of parallel. Directions or surfaces described as substantially parallel to one another may, in some embodiments, be within 20 degrees, or within 10 degrees of parallel, or may be parallel or nominally parallel. If the use of “substantially aligned” is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “substantially aligned” will mean aligned to within 20% of a width of the objects being aligned. Objects described as substantially aligned may, in some embodiments, be aligned to within 10% or to within 5% of a width of the objects being aligned.

All references, patents, and patent applications referenced in the foregoing are hereby incorporated herein by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control.

Descriptions for elements in figures should be understood to apply equally to corresponding elements in other figures, unless indicated otherwise. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.

Claims

What is claimed is:

1. A multilayer optical film comprising: an optical reflector; and an array of discrete spaced-apart optical bumps formed on the optical reflector, such that for substantially normally incident light and a visible wavelength range extending from about 450 nm to about 600 nm and an infrared wavelength range extending from about 800 nm to about 1200 nm: the optical reflector has an average optical reflectance of greater than about 30% in the visible wavelength range for at least a first polarization state, and a specular transmittance of greater than about 20% for at least one wavelength in the infrared wavelength range for each of the first polarization state and an orthogonal second polarization state; and the optical bumps have an average optical transmittance of greater than about 50% for each of the visible and infrared wavelength ranges for each of the first and second polarization states.

2. The multilayer optical film of claim 1, wherein the optical bumps form a discontinuous layer.

3. The multilayer optical film of claim 1, wherein the optical bumps form a continuous layer comprising land portions connecting the optical bumps.

4. The multilayer optical film of claim 1, wherein the optical reflector has an average optical specular reflectance of greater than about 30% in the visible wavelength range for the at least the first polarization state.

5. The multilayer optical film of claim 1, wherein for the substantially normally incident light, the optical reflector has an average optical reflectance of greater than about 50% in the visible wavelength range for the at least the first polarization state.

6. The multilayer optical film of claim 1, wherein for the substantially normally incident light, the optical reflector has an average optical reflectance of greater than about 70% in the visible wavelength range for the at least the first polarization state.

7. The multilayer optical film of claim 1, wherein for the substantially normally incident light, the optical reflector has a specular transmittance of greater than about 40% for at least one wavelength in the infrared wavelength range for each of the first and second polarization states.

8. The multilayer optical film of claim 1, wherein for the substantially normally incident light, the optical reflector has a specular transmittance of greater than about 60% for at least one wavelength in the infrared wavelength range for each of the first and second polarization states.

9. The multilayer optical film of claim 1, wherein the optical reflector comprises a plurality of alternating first and second polymeric layers, each polymeric layer less than about 500 nm thick.

10. The multilayer optical film of claim 1, wherein for the substantially normally incident light, the optical reflector has an average optical reflectance of greater than about 30% in the visible wavelength range for each of the first and second polarization states.

11. The multilayer optical film of claim 1 , wherein for the substantially normally incident light, the optical reflector has an average optical reflectance of greater than about 50% in the visible wavelength range for each of the first and second polarization states.

12. The multilayer optical film of claim 1, wherein for the substantially normally incident light, the optical reflector has an average optical reflectance of greater than about 70% in the visible wavelength range for each of the first and second polarization states.

13. The multilayer optical film of claim 1, wherein for the substantially normally incident light in the visible wavelength range, the optical reflector has an average optical reflectance of greater than about 50% for the first polarization state and an optical transmittance of at least 50% for the second polarization state.

14. The multilayer optical film of claim 1, wherein for the substantially normally incident light in the visible wavelength range, the optical reflector has an average optical reflectance of greater than about 70% for the first polarization state and an optical transmittance of greater than about 70% for the second polarization state.

15. The multilayer optical film of claim 1, wherein the optical reflector comprises a metal layer.

16. The multilayer optical film of claim 15, wherein the metal layer comprises one or more of aluminum, silver, gold and copper.

17. The multilayer optical film of claim 1, wherein the array of discrete spaced-apart optical bumps comprises a regular array.

18. The multilayer optical film of claim 17, wherein the regular array comprises one or more of a square array, a rectangular array, a triangular array, a hexagonal array, and a circular array.

19. The multilayer optical film of claim 1, wherein the array of discrete spaced-apart optical bumps comprises an irregular array.

20. The multilayer optical film of claim 1, wherein the array of discrete spaced-apart optical bumps comprises a random array.

21. The multilayer optical film of claim 1, wherein the optical bumps have an average height of between about 0.2 to about 10 microns.

22. The multilayer optical film of claim 1, wherein the optical bumps have an average height of between about 0.5 to about 5 microns.

23. The multilayer optical film of claim 1, wherein the optical bumps have an average lateral size of between about 50 to about 200 microns.

24. The multilayer optical film of claim 1, wherein the optical bumps have an average lateral size of between about 75 to about 150 microns.

25. The multilayer optical film of claim 1, wherein the optical bumps cover between about 10% to about 40% of a surface area of the optical reflector.

26. The multilayer optical film of claim 1, wherein when the array of discrete spaced-apart optical bumps side of the multilayer optical film is substantially uniformly illuminated with a white light from a substantially Lambertian light source oriented at an angle of about 30 degrees relative to a normal to the multilayer optical film, the multilayer optical film reflects the illuminating light such that for light reflected at an angle of about 30 degrees relative to the normal to the multilayer optical film, an average contrast between light reflected from the optical bumps and light reflected from regions of the multilayer optical film between the optical bumps is less than about 5%.

27. The multilayer optical film of claim 26, wherein the average contrast between light reflected from the optical bumps and light reflected from regions of the multilayer optical film between the optical bumps is less than about 4%.

28. The multilayer optical film of claim 26, wherein the average contrast between light reflected from the optical bumps and light reflected from regions of the multilayer optical film between the optical bumps is less than about 3%.

29. The multilayer optical film of claim 26, wherein the average contrast between light reflected from the optical bumps and light reflected from regions of the multilayer optical film between the optical bumps is less than about 2%.

30. The multilayer optical film of claim 1, wherein the optical bumps have an average optical transmittance of greater than about 50% for each of the visible and infrared wavelength ranges for each of the first and second polarization states.

31. The multilayer optical film of claim 1 , wherein increasing a percent coverage of a surface area of the optical reflector by the optical bumps from about 10% to about 40% reduces an average total optical transmittance of the multilayer optical film by less than about 5% in the visible wavelength range.

32. The multilayer optical film of claim 1, wherein increasing a percent coverage of a surface area of the optical reflector by the optical bumps from about 10% to about 40% reduces an average specular optical transmittance of the multilayer optical film by less than about 5% for a wavelength of about 940 nm.

33. The multilayer optical film of claim 1, wherein increasing a percent coverage of a surface area of the optical reflector by the optical bumps from about 10% to about 40% reduces an average optical clarity of the multilayer optical film by less than about 20% in the visible wavelength range.

34. The multilayer optical film of claim 1, wherein increasing a percent coverage of a surface area of the optical reflector by the optical bumps from about 10% to about 40% increases an average optical haze of the multilayer optical film by less than about 5% in the visible wavelength range.

35. The multilayer optical film of claim 1, wherein increasing a percent coverage of a surface area of the optical reflector by the optical bumps from about 10% to about 40% reduces an average specular optical transmittance of the multilayer optical film by less than about 3% in the visible wavelength range.

36. The multilayer optical film of claim 1, wherein for a coverage of about 10% to about 40% of a surface area of the optical reflector by the optical bumps, the array of discrete spaced-apart optical bumps side of the multilayer optical reflector has a coefficient of friction of between about 0.2 and about 0.6 when moving along a first direction against a structured surface comprising a plurality of linear prisms arranged along the first direction at a prism pitch of about 18 microns to about 24 microns and extending along an orthogonal second direction.

37. The multilayer optical film of claim 1, wherein for a coverage of about 10% to about 40% of a surface area of the optical reflector by the optical bumps, the array of discrete spaced-apart optical bumps side of the multilayer optical reflector has a coefficient of friction of between about 1.0 and about 2.5 against a major surface of a polyethylene terephthalate film.

38. The multilayer optical film of claim 1, wherein when the array of discrete spaced-apart optical bumps side of the multilayer optical reflector is substantially uniformly illuminated with light having a visible wavelength in the visible wavelength range, the multilayer reflects the illuminating light such that a variation in intensity of the reflected light is not less or is less by no more than about 5% as compared to a multilayer optical film that has a same construction except that is does not include the optical bumps.

39. The multilayer optical film of claim 38, wherein the variation in intensity of the reflected light is not less or is less by no more than about 4% as compared to a multilayer optical film that has a same construction except that is does not include the optical bumps

40. The multilayer optical film of claim 38, wherein the variation in intensity of the reflected light is not less or is less by no more than about 3% as compared to a multilayer optical film that has a same construction except that is does not include the optical bumps

41. The multilayer optical film of claim 38, wherein the variation in intensity of the reflected light is not less or is less by no more than about 2% as compared to a multilayer optical film that has a same construction except that is does not include the optical bumps

42. The multilayer optical film of claim 1, wherein the optical reflector comprises a plurality of alternating first inorganic layers and second inorganic layers deposed on an optically transparent substrate, each of the first inorganic layers having a different index of refraction from each of the second inorganic layers.

43. A multilayer optical film comprising: an optical substrate; and an array of discrete spaced-apart optical bumps formed on the substrate, such that for substantially normally incident light and a visible wavelength range extending from about 450 nm to about 600 nm and an infrared wavelength range extending from about 800 nm to about 1200 nm: the optical bumps have an average optical transmittance of greater than about 50% for each of the visible and infrared wavelength ranges for each of the first and second polarization states; wherein when the array of discrete spaced-apart optical bumps side of the optical substrate is substantially uniformly illuminated with light having a visible wavelength in the visible wavelength range, the substrate reflects the illuminating light such that a contrast between light reflected from the optical bumps and light reflected from regions of the substrate between the optical bumps is less than about 5%; and wherein for a coverage of about 10% to about 40% of a surface area of the substrate by the optical bumps, the array of discrete spaced-apart optical bumps side of the substrate has a coefficient of friction of between about 0.2 and about 0.6 when moving along a first direction against a structured surface comprising a plurality of linear prisms arranged along the first direction at a prism pitch of about 18 microns to about 24 microns and extending along an orthogonal second direction.

44. The multilayer optical film of claim 43, wherein the substrate comprises one or more of an optical diffuser, a reflective polarizer, a substantially optically specularly transparent substrate, and an absorbing polarizer.

45. The multilayer optical film of claim 43, wherein the contrast between light reflected from the optical bumps and light reflected from regions of the substrate between the optical bumps is less than about 4%.

46. The multilayer optical film of claim 43, wherein the contrast between light reflected from the optical bumps and light reflected from regions of the substrate between the optical bumps is less than about 3%.

47. The multilayer optical film of claim 43, wherein the contrast between light reflected from the optical bumps and light reflected from regions of the substrate between the optical bumps is less than about 2%.