Retroreflecting article with contrast reduction layer

A retroreflective article is a structure that includes at least retroreflective elements. Retroreflective elements reflect incident light back in substantially the same direction. Examples of retroreflective elements include corner cube prism-shaped retroreflectors, bead-shaped retroreflectors, and the like.

In one aspect, the present description relates to retroreflective articles. In particular, the present disclosure relates to retroreflective articles having a light incident surface and including a retroreflective layer and a contrast-reducing layer positioned closer to the light incident surface than the retroreflective layer. The contrast-reducing layer reduces the near-infrared retroreflective efficiency of the retroreflective substrate by more than 50%.

<figref num="1">FIG. 2 is a schematic side view of a retroreflective layer;</figref><figref num="2">1 is a schematic side view of a retroreflective article; FIG.</figref>

Retroreflective articles are useful in many situations. For example, it may be prohibitively costly or impractical to provide direct and sustained illumination of signs or labels to facilitate readability or visibility. Instead, by using retroreflectors, light from a light source such as an automobile's headlights can provide "on-demand" illumination, allowing an observer to see and interpret the sign. to enable.

As transportation infrastructure becomes more complex, vehicles are gaining more driving autonomy. To perform tasks ranging from parking assistance, autonomous cruise control and lane departure warnings to fully autonomous operation and driving, including collision avoidance and traffic sign interpretation, for safe and effective operation. Sensing modules are increasingly being incorporated into vehicles.

To sense the world around the vehicle, the vehicle uses a set of sensors that emit one or more points of light. For example, lidar (optical radar) systems may use constellations of light points moving through their surroundings to detect potential obstacles or information objects. These interrogation light beams may use narrow wavelength bands, eg, 2-20 nm, or may use broad wavelength bands, eg, 100 nm or more.

A challenge with these types of systems is that objects with different levels of reflectance must be monitored and detected simultaneously, and it is difficult to calibrate or adjust the sensor system to the appropriate gain level. For example, vehicles must sense pedestrians at the same time as trees, other vehicles, and traffic signs. Traffic signs designed to be highly reflective may overwhelm sensor systems, making it very difficult to sense both reflective and non-reflective objects. Since the system must have sensitivity to detect both these types of objects simultaneously, it cannot be corrected simply by adjusting the gain in the detector. Additionally, if traffic signs contain encoded or other patterned information read by vehicle sensors, the correct level of gain is required to enable accurate detection and interpretation. all the more important.

FIG. 1 is a schematic side view of a retroreflective layer. Retroreflective layer 100 is shown with incident light ray 110 and retroreflected light ray 120 being retroreflected by the retroreflective layer. Retroreflective layer 100 has an optical structure that reflects light substantially toward its light source. In other words, any incident ray is reflected substantially 180° from its angle of incidence. For example, incident light ray 110 is retroreflected as retroreflected light ray 120 having substantially the same angle with respect to retroreflective layer 100 . Due to the geometry of the optical structure of the retroreflective layer, the retroreflected ray may be slightly translated from the incident ray. In other words, the incident light beam and the retroreflected light beam are substantially parallel, but need not coincide.

FIG. 2 is a schematic side view of a retroreflective article. Retroreflective article 200 includes retroreflective layer 210 , retardation layer 220 , and contrast-attenuating layer 230 .

Retroreflective layer 210 may be any suitable retroreflective layer or combination of layers. Suitable retroreflectors include metal-backed or air-backed prismatic (corner-cube) retroreflectors, metal-backed bead-shaped retroreflectors, and optionally, e.g. or bead-shaped retroreflectors partially embedded in a binder containing other reflective flake materials.

The retroreflective layer can be of any suitable size and may have elements of any suitable size. For example, the microreplicated prisms or beads used in the retroreflective layer may be micrometers in size (width or diameter), tens of micrometers in size, hundreds of micrometers in size, or millimeters in size. Furthermore, the size may be several centimeters. A plurality of different sizes and size distributions of beads may be utilized as appropriate and suitable for the application. Depending on the retroreflected wavelength of interest, a certain practical minimum May have a feature size.

For bead-shaped retroreflectors, glass beads are commonly used, but any substantially spherical material can be used. Materials may be selected based on durability, environmental robustness, manufacturability, refractive index, coatability, or any other physical, optical, or material properties. The beads may be partially covered with a reflective binder, including, for example, pearlescent or metallic flakes, or partially metallized by vapor coating, sputter coating, or any other suitable process. may In some embodiments, beads may be coated with a dielectric material. In some embodiments, a metal or metallized film may be laminated or otherwise attached to the bead surface. In some embodiments, the coating or layer may be a spectrally selective reflector. In some embodiments, the beads may form a light path through a non-reflective binder between the light incident surface of the retroreflector and the metal or metallized polymer film as the substrate. The binder may have any physical properties and may impart certain desired properties to the retroreflective layer. For example, the binder may include pigments or dyes that impart a coloring effect to the retroreflective article.

For prismatic retroreflectors, any suitable prismatic shape may be microreplicated or otherwise formed in a transparent medium (transparent for at least the wavelengths of interest). In some embodiments, a casting and curing microreplication process is used to form the prismatic surfaces. For example, Brightness Enhancing Film Film, BEF) can be used, but such prisms do not retroreflect over a very wide range of angles. Cube corners are widely used as a retro-reflecting prism shape, where each incident ray is reflected three times before being returned to the direction of incidence. Other surfaces with more facets may be used as prismatic retroreflectors. Any suitable microreproducible resin may be used, particularly resins that are applied in liquid or flowable form, then cured and removed from the tool. The tool can be formed by any suitable process, including etching (chemical or reactive ion etching), diamond turning, and the like. In some embodiments, the tool can be a collection of multiple parts that are fused, fused, or otherwise attached to cover the complete prismatic sheet surface pattern. Curing may be accomplished by applying heat or electromagnetic radiation. UV-curable resins, or resins that can be cured by uncommon ambient conditions, may be selected so that they do not inadvertently partially or fully cure during handling or pre-curing treatments. . In some embodiments, additive or subtractive manufacturing processes may be used to form either the tool surface for microreplication or the prism surface itself.

Retardation layer 220 may be any suitable retardation layer that selectively retards one of the orthogonal components of light to change the polarization of that component. In some embodiments, retardation layer 220 may be configured as a quarter-wave retarder. A quarter-wave retarder has a retardation characteristic of λ/4 for a particular wavelength of interest, λ. A quarter-wave retarder for light of a given wavelength converts that light from circularly polarized light to linearly polarized light, or vice versa. In some applications, a quarter-wave retarder may perform acceptably without having a perfect λ/4 retardation characteristic. In some applications, the use of achromatic retarders provides substantially 1 It may be enabled to maintain a /4 wavelength delay characteristic. In some embodiments, the quarter-wave retarder has substantially quarter-wave retardance over the entire near-infrared wavelength range, eg, 700-1400 nm. In some embodiments, a quarter-wave retarder has substantially a quarter-wave retardance across the entire visible wavelength band, eg, 400-700 nm. In some embodiments, the quarter-wave retarder has substantially quarter-wave retardance across both the near-infrared and visible regions.

In some embodiments, retardation layer 220 may provide substantially similar retardation characteristics over a wide range of angles of incidence. In some embodiments, the retardation characteristics should not vary by more than 10% in a cone with a half angle of 30 degrees, should not vary by more than 10% in a cone with a half angle of 45 degrees, or , shall not vary by more than 10% in a cone with a half angle of 60 degrees. For some applications, cones with half angles of 30 degrees, 45 degrees, or 60 degrees and variations of 20% or less may be acceptable.

Retardation layer 220 may comprise any suitable retardation material or materials. In some embodiments, retardation layer 220 includes or is a liquid crystal retarder. In some embodiments, retardation layer 220 comprises an oriented birefringent polymer film. A suitable thickness may be selected to obtain the desired retardance value according to the birefringence of the selected polymer set. In some embodiments, retardation layer 220 is a compensating film with low retardation (e.g., retardation of less than 100 nm) to enhance or preserve circularly polarized light over a wide range of angles for the wavelength or wavelength band of interest. or other additional films.

In some embodiments, retardation layer 220 may not be patterned. In some embodiments, it may be patterned, as shown in FIG. Retardation layer 220 may include at least first and second regions arranged in any spatial pattern, gradient, or any other arrangement. The first region and the second region differ at least in retardation of incident light. For example, in one embodiment, the first region may have a quarter-wave retardation characteristic for incident light of the first wavelength. At the same time, the second region may have substantially zero retardation properties for incident light of that first wavelength. In some embodiments, the second region may substantially absorb light at its first wavelength. In some embodiments, the second region may substantially depolarize light at the first wavelength. When patterned, delay layer 220 may encode or display information. The information may be human readable, machine readable, or both human and machine readable.

Retroreflective article 200 may allow certain sensor systems to operate with a high degree of fidelity. For example, sensors that detect circularly polarized light (eg, charge-coupled devices or CMOS used with filters that pass left-handed circularly polarized light) can be useful sensor configurations. For example, when queried with left-handed circularly polarized light, retroreflective article 200 will (depending on the construction and optics of retroreflective layer 210 and retardation layer 220) have a particular portion that retroreflects left-handed circularly polarized light. may be provided. They may be brightly visible or otherwise detectable by such a sensor arrangement. Elsewhere in the retroreflective article 200, the left-handed circularly polarized interrogation light may be depolarized, absorbed, or inverted to right-handed circularly polarized light. Such regions appear dark or are difficult to detect with such sensor configurations.

Although not limited to circularly polarized light, in some embodiments several potential advantages can be realized by utilizing such light. In particular, circularly polarized light tends to be rare in nature, reducing the probability of false positive signals or other interference. Furthermore, in contrast, the use of linearly polarized light in these applications results in a very high sensitivity to the angle of incidence, making the retroreflected light appear or Rapid and significant changes in intensity.

In some embodiments, retroreflective article 200 can be configured to operate in the near-infrared wavelength range. Certain sensor systems use near-infrared light to operate in wavelengths invisible to humans. In some embodiments, the retroreflective article 200 is configured as a retroreflective layer 210 that retroreflects near-infrared light and a quarter-wave retarder for at least one wavelength in the near-infrared wavelength range. Layer 220 may also be included.

Contrast-reducing layer 230 may be any suitable contrast-reducing layer, including layers that attenuate light by absorbing certain wavelengths. In some embodiments, broadband absorbers such as carbon black can be used. In some embodiments, selectively absorbing dyes or pigments may be used. In some embodiments, the use of dyes or inks that pass certain wavelength spectra (i.e., are transparent or highly transmissive in certain wavelength bands) but absorb light in other wavelength spectra. can be done. In some embodiments, a contrast-reducing layer (ie, a camouflage layer) that absorbs light in the visible spectrum but substantially transmits light in the near-infrared spectrum can be used. In some embodiments, substantially transmitting light means that 10%, 20%, 30%, 40%, 50%, 60%, 70%, or even 80% of the light is transmitted. On the other hand, for light that is absorbed, it means that less than 10%, 5%, or 1% is transmitted. In some applications it may be desirable to have a low visibility retroreflector to provide information to the detector without being distracting or confusing. In some embodiments, contrast-reducing layer 230 reduces light retroreflected from retroreflective article 200 to avoid overglow, blooming, or other detection artifacts that result from having too much contrast. may be reduced.

Contrast reduction layer 230 may also be or include any surface or bulk scatterer to attenuate light retroreflected along the expected cone of retroreflection angles. In other words, contrast-reducing layer 230 has a well-known retroreflective efficiency measurement, R_A.may be any suitable material or combination of materials to reduce the Retroreflectance (R_A.) is the coefficient (R_A.) standard test method-using the test criteria described in the observation angle 0.2⁰and approach angle 5⁰, i.e. 0.2/5⁰Angle can be measured.

Those embodiments in which contrast is reduced by scattering may extend the useful detection angle of the illuminated retroreflector, which may be useful for fast moving vehicles or difficult detection conditions. obtain.

In some embodiments, contrast reduction layer 230 is patterned. In some embodiments, the contrast-reducing layer 230 is patterned as an overlay or spatially similar to the retardation layer patterning. In some embodiments, specific regions of the contrast reduction layer may provide different levels of attenuation as part of the encoded information. In other words, the patterned contrast-reducing layer 230 may provide the grayscale information bits contained in the retroreflective article.

In some embodiments, retroreflective article 200 also includes visible-absorbing layer 240, particularly in applications where the patterned layer is to be invisible to humans. The visible absorbing layer can be placed closer to or further away from the plane of light incidence than the contrast-reducing layer. The visible absorbing layer absorbs light in the visible wavelength band while substantially transmitting near-infrared light for retroreflection. In some embodiments, the visible absorbing layer is a broadband visible absorber and appears black. In some embodiments, the visible absorbing layer may selectively absorb certain visible wavelengths to create a colored appearance. A colored appearance can be useful in some applications to blend the retroreflective article into its environment. In some embodiments, the visible absorbing layer may absorb at least 50%, 60%, 70%, 80%, or 90% of light in the visible band.

In some embodiments, the visible absorbing layer and the contrast-reducing layer may be combined into a single layer, in which one or both may be patterned. As an example, the visible absorbing layer may be dye or pigment spread throughout the combination layer, while the contrast-reducing layer may have patterning or spatial variations on the retroreflective article.

The retroreflective articles described herein can be useful for traffic control signs and directional/navigation infrastructure. In some embodiments, the retroreflective articles described herein can be useful as fixed signs. In some embodiments, these items may be or be included in temporary traffic control devices such as cones or flags or movement signs. In some embodiments, the articles may be used or incorporated into clothing or wearable articles such as high visibility vests, helmets, or other safety equipment. In some embodiments, the retroreflective article can be conformable, bendable, or foldable. In some embodiments, the articles may be attached to any type of vehicle, such as an automobile, motorcycle, aircraft, bicycle, quadcopter (drone), watercraft, or any other vehicle. In some embodiments, the articles are used for inventory control in warehouses, railroad yards, shipyards, or distribution centers, enabling automatic identification of contents, e.g., shelves, boxes, shipping containers, etc. can be

The retroreflective articles described herein can be of any suitable size, including small decals or stickers containing pressure sensitive adhesives to large, highly visible traffic signs. The retroreflective layer may be backed by a substrate that provides stiffness or easy adhesion (eg, pressure sensitive adhesion) without affecting the optics of the retroreflective article.

The following are exemplary embodiments according to the present disclosure.

Item 1. A retroreflective article having a light incident surface, comprising a retroreflective layer and a contrast-reducing layer positioned closer to the light incident surface than the retroreflective layer, wherein the contrast-reducing layer A retroreflective article that reduces the near-infrared retroreflective efficiency of a substrate by greater than 50%.

Item 2. 2. The retroreflective article of item 1, wherein the contrast-reducing layer reduces near-infrared retroreflective efficiency by absorption.

Item 3. 2. The retroreflective article of item 1, wherein the contrast-reducing layer reduces near-infrared retroreflective efficiency by scattering.

Item 4. 4. The retroreflective article of item 3, wherein the contrast-reducing layer comprises bulk scatterers.

Item 5. 4. The retroreflective article of item 3, wherein the contrast-reducing layer comprises surface scatterers.

Item 6. 2. The retroreflective article of item 1, wherein the retroreflective layer is patterned and includes at least first and second sections, wherein the retroreflectance is different in the first and second sections.

Item 7. The contrast-reducing layer is patterned and includes at least first and second sections, the first section absorbing greater than 80% of near-infrared light and the second section less than 80% of near-infrared light. The retroreflective article according to item 1, which absorbs

Item 8. item further comprising a visible-absorbing layer positioned closer to or further from the light-incident surface than the contrast-reducing layer, wherein the visible-absorbing layer absorbs at least 80% of the light in the visible band. 1. The retroreflective article according to 1.

Item 9. 2. The retroreflective article of item 1, wherein the retroreflective article is conformable.

Item ten. The retroreflective article according to item 1, wherein the retroreflective article is a tape.

Item 11. The retroreflective article according to item 1, wherein the retroreflective article is flexible.

Item 12. 2. The retroreflective article of item 1, wherein the retroreflective article exhibits a spatially varying pattern when interrogated by near-infrared light.