SURFACE RETROREFLECTOR

Description

[0001] The invention relates to a planar reflection structure, e.g., a retroreflector, which has a retroreflective, microprismatic relief structure formed in a plastic material and retroreflects optical radiation incident from an incident side by means of total internal reflection.

[0002] A retroreflective structure reflects incident optical radiation largely in the direction from which the radiation is incident, regardless of the direction of incidence and the reflector's orientation. This is referred to as retroreflection. With plane mirrors, however, the reflected radiation depends on the mirror orientation, which is only rarely oriented perpendicular to the beam source. Flat retroreflectors have a large number of adjacent individual retroreflectors. Retroreflective films are a common implementation of flat retroreflectors. They find a wide variety of applications, particularly in traffic engineering, but also in security technology, for example, as security elements for authenticating valuable documents, as per US 2014 / 0226212 A1.Most traffic signs and license plates are also equipped with retroreflective structures to increase visibility at night or to improve security against counterfeiting (see US 5656360).

[0003] Generally, two variants of retroreflectors are widely used: Embedded high-refractive-index microspheres, which are mirrored on the underside, as described, for example, in US 4763985 and US 2009 / 0300953 A1. The optical principle here is based on so-called Lüneburg lenses, which reflect the incoming light towards the beam source. The other variant, in the field of which the invention is arranged, is based on relief structures made of flat surfaces, so-called microprism structures, which usually have the shape of cube corners. In this case, the incoming light is successively deflected by approximately 45° at two, usually even three (in the case of triple prism structures) mutually inclined flat surfaces of the structure and reflected back to the beam source. This geometry has the highest retroreflection efficiency for perpendicular light incidence; the efficiency decreases for increasingly oblique light incidence. The geometry of the structure is in Fig. 8A-D shown. Fig. 8A shows a schematic side view of the relief structure of a planar retroreflector with an optical path for retroreflection. The retroreflector is constructed as a film composite comprising a carrier film 1 and a cover film 2, into which a reflection structure 3 is embedded. On the incident side (relative to the incident radiation), the reflection structure 3 comprises a transparent plastic material 3a applied to the carrier film 1, in which a relief structure 3b consisting of flat surfaces is molded. Its geometry is designed such that incident radiation is deflected multiple times, so that it is ultimately retroreflected. Fig. 8B illustrates the path of retroreflection as it occurs in triangular surfaces of a triple prism, from whose juxtaposition the retroreflective relief structure 8b for the planar retroreflector can be constructed. Fig. 8C shows an arrangement of the triangular surface structure of Fig. 3b in top view. Fig. 8D shows an arrangement of square surfaces as cube corners in perspective form.

[0004] The retroreflective coefficient Ra of a retroreflector is defined in DIN 67520 as the ratio of the reflected luminous intensity in candelas to the illuminance in lux (the latter based on normal incidence). Ra values ​​for mirrored microprismatic structures, as described, for example, in WO 2014 / 117086 A1, are significantly higher than for retroreflectors based on microspheres. According to the standard, for white, at an angle of incidence of 5° and an observation angle of 0.2°, the minimum Ra values ​​for microsphere structures are 70 and for microprismatic structures 625. Therefore, microprismatic structures are preferred, especially when particularly good visual perceptibility is required. Furthermore, as known from WO 2017 / 121956 A1, their production can be carried out cost-effectively on films by duplication via an embossing process starting from a master structure.

[0005] WO 2014 / 117086 A1 describes the construction in which the relief surface is metallically mirrored and completely embedded in plastic. Fig. 8A the relief structure 3b, which is formed at the interfaces between the plastic material 3a and the cover layer 3c, is mirrored. The light is deflected for retroreflection at the mirrored flat surfaces of the relief structure 3b. The deflection and thus the retroreflection are therefore lossy. With a mirror coating with aluminum, for example, a maximum of half of the incident light is retroreflected. Another disadvantage is that such retroreflectors always appear metallically shiny due to the mirror layer required on the relief structure. For many applications, such as in traffic engineering (vehicle license plates, traffic signs), however, a white appearance is desired, at least in diffuse lighting. Sometimes it is (also) required that, in addition to the retroreflection, a background color or a structured background is visible over a wide viewing angle range.For certain applications, retroreflection in the infrared is required, while a structured background is visible or retroreflection should not occur in the visible. All of this is not possible with a metallically mirrored relief surface.

[0006] In the case of the aforementioned version known from US 6413615 B2, the microprismatic structures are not mirrored, but the deflection on the plane surfaces is achieved by total reflection. This shows Fig. 8E This principle is followed by the plastic reflectors colloquially known as "cat's eyes," which are used in traffic engineering, among other things. They have triple prisms molded onto a plastic plate, which are free on the rear side (relative to the direction of light incidence), i.e., adjacent to air. They reflect by total internal reflection. Through the rear interface to an air gap L (in Fig. 8E In the air gap (shown in black), the incident light undergoes total internal reflection at the deflection points. Air is required because of its refractive index n=1 to ensure total internal reflection. This relief structure is formed in a transparent plastic material with a refractive index of typically 1.5, e.g. in highly transparent bulk materials such as polymers (PET, UV varnishes, etc.) (n less than 1.6) or polycarbonate (n=1.58). Incident light falls via the transparent plastic material onto the relief structure. Since the medium adjacent to the rear in the air gap L has a refractive index n=1, retroreflection occurs by means of total internal reflection, which is almost loss-free. The retroreflection value is therefore very high. In addition, the flat retroreflector does not appear metallic. An additional advantage is that the transparent plastic material can be colored.Thus, under ambient conditions, an observer perceives not only the retroreflection but also the underlying color. The disadvantage, however, is that the relief structures require air pockets for the air gap or must be completely exposed on their backside. This is complex to manufacture and poses risks to long-term stability, especially outdoors under real environmental conditions.

[0007] WO 2018 / 217519 A1 further develops such a retroreflector operating with total internal reflection by additionally providing at least one double layer to influence the reflection properties depending on the wavelength. The at least one double layer is applied to a retroreflective structure with a flat surface and causes total internal reflection at the transition between the at least one double layer and an air gap.

[0008] US 2015 / 0173302 A1 proposes the use of a totally reflective element to improve plant illumination and thus increase plant growth. One embodiment uses a retroreflective prism structure, with retroreflection achieved either by a metallic coating or by total internal reflection.

[0009] A backlit license plate is known from WO 2008 / 121475 A1.

[0010] The invention is based on the object of creating a flat retroreflector with a retroreflective, microprismatic relief structure, particularly for traffic engineering applications, which has a non-metallic appearance while being easy to manufacture and having good long-term stability.

[0011] The invention is defined in the independent claims. The dependent claims relate to advantageous developments.

[0012] The invention is based on a planar retroreflector that uses total internal reflection to achieve a particularly high retroreflection value. A retroreflective, microprismatic relief structure is formed in a transparent plastic material. It has appropriately arranged flat surfaces, e.g., in the form of adjacent triple prism surfaces (triangular surfaces or cube corners). The surface of the relief structure is coated (at least in some areas) with a dielectric coating. A cover layer covers the dielectric coating on its rear surface (in the viewing direction). The refractive index of this cover layer is preferably lower than that of the dielectric coating. It can also be the same. The dielectric coating thus has a refractive index that is higher than or equal to that of the adjacent cover layer on the rear.If the refractive index of the dielectric coating is higher than that of the adjacent back cover layer, light incident on the relief structure from the higher-refractive coating strikes the material of the back cover layer and is totally reflected due to this refractive index jump. Reflection also occurs if the refractive index of the dielectric coating is equal to the refractive index of the back cover layer, since in this case, differences in the refractive index between the (front) plastic material and the coating are responsible for the reflection.

[0013] A key aspect of the concept is that the relief structure is coated with a high-index coating on the incident side, so that the refractive index jump occurs either with the back and / or front material. This allows reflection or even total internal reflection without an air gap, and the metallic appearance of mirrored flat surfaces is eliminated. The terms "incident side" and "back side" understandably refer to the direction of incidence of the radiation to be retroreflected.

[0014] For the inventive concept, it is not crucial which material the relief structure is formed from and which material covers the relief structure after its formation and coating with the dielectric, high-refractive-index material. It is equally possible to mold the relief structure in the material referred to above as the cover layer, then coat it, and subsequently apply the plastic material. This is merely a matter of more convenient production. Where it is stated below that the relief structure is formed in the plastic material and covered with the cover layer, this equally encompasses that the relief structure can also be molded in the cover layer, coated, and then filled with the plastic material.

[0015] The invention allows the realization of a film composite. This contains the plastic material, the relief structure with a high-refractive-index dielectric coating, and the cover layer on one film or between two films (counted from the incident side). This film composite can particularly preferably be provided with an adhesive lamination so that the retroreflective film composite can be adhered to desired substrates. Since the retroreflector still allows the color of the substrate to be recognized if the plastic material is transparent or semi-transparent, it can be used to make a surface retroreflective without losing the surface color. This is particularly advantageous for license plates or traffic signs, for example.

[0016] The refractive index of the dielectric coating is preferably at least 20%, particularly preferably at least 30%, at least 40% or even at least 50% higher than that of the adjacent material on the back (in the viewing direction). The dielectric coating is non-metallic and non-reflective. The dielectric coating is generally highly refractive, i.e. has a refractive index of at least 2.0, 2.2, 2.4 or higher. Its refractive index is preferably higher than that of the material on the back of the relief structure. It can also be the same. One or more of the following materials are particularly suitable as a highly refractive index material: ZnS, ZnO, ZnSe, SiNx, SiOx (with x<1, i.e. substoichiometric silicon oxide compounds), Cr2O3, Nb2O5, Ta2O5, TixOy and ZrO2.

[0017] A particularly preferred thickness for the coating is between 20 nm and 500 nm. The range from 40 nm to 140 nm has proven to be particularly favorable.

[0018] To improve total reflection in certain spectral ranges, it has proven advantageous for the coating to be a multilayer coating with alternating refractive indices, namely a trilayer coating consisting of a layer sequence of higher-refractive, lower-refractive, and higher-refractive material. Due to this multilayer design, infrared radiation can be retroreflected as optical radiation and / or the overall retroreflection value can be further improved.

[0019] In the field of security technology, ie forgery and copy protection, the retroreflector can also be used advantageously, in particular for securing valuable documents, etc. It can also be used very advantageously to produce the microimage structure for a microlens array.

[0020] The reflectance of the plane surfaces of the relief structure can be adjusted by selecting the material and the thickness of the dielectric coating. This allows the flat retroreflector to be designed in such a way that the background on which it is mounted remains recognizable to a certain degree. This avoids a metallic impression, as no reflective coating is used. This allows the retroreflector to reproduce, for example, the color impression of the background. This option is not available with conventional retroreflectors that operate with total internal reflection, nor with retroreflectors that have reflective coatings.

[0021] Embossed microprismatic structures, such as cube-corner structures or triangular surface arrangements, which are coated with at least one high-index layer, retain retroreflective properties even after the structure has been completely embedded. They can also be semi-transparent and thus backed with a color. These designs avoid the disadvantage of air inclusion while still retaining good reflective properties. They can be manufactured like the aforementioned metallic variant—without its metallic appearance. The advantage over this variant is not only the improved retroreflection, but also that the semi-transparent relief structure can be backed, for example, with a white color or with any textured background. The metallic appearance of conventional retroreflective films is avoided.

[0022] The invention is explained in more detail below using exemplary embodiments with reference to the accompanying drawings, which also disclose features essential to the invention. These exemplary embodiments are for illustrative purposes only and are not to be interpreted as restrictive. For example, a description of an embodiment with a large number of elements or components should not be interpreted to mean that all of these elements or components are necessary for implementation. Rather, other embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different exemplary embodiments may be combined with one another unless otherwise stated. Modifications and variations described for one of the exemplary embodiments may also be applicable to other exemplary embodiments.To avoid repetition, identical or corresponding elements in different figures are designated by the same reference numerals and are not explained more than once. The figures show: . Fig. 1A a vehicle license plate in which a retroreflective film is used, Fig. 1B a banknote having a security element with a retroreflector, Fig. 2A and 2B schematic sectional views through flat retroreflectors, Fig. 3 a sectional view similar to the Fig. 2A und 2B for a planar retroreflector with a trilayer coating, Fig. 4A and 4B, the spectral reflection properties of a retroreflector with a TiO 2 coating, Fig. 5A and 5B, and Fig. 6 the spectral reflection properties of a retroreflector with trilayer coating according to Fig. 3 and Fig. 7A and 7BSectional views similar to Fig. 2A for a flat retroreflector that has additional reflective areas that are designed differently.

[0023] Fig. 1A shows a vehicle registration plate which has a black imprint D in front of a retroreflective background H, which is particularly easy to recognize due to the retroreflective properties of the background H. The background H is caused by a retroreflective flat retroreflector, e.g. a film, which is subsequently Fig. 2A will be explained in more detail using examples. Fig. 1B shows a security feature S on a banknote B, whereby the security feature S also uses a flat retroreflector, as explained below.

[0024] Fig. 2A shows the flat retroreflector for the background H of the vehicle license plate of the Fig. 1A in a schematic sectional view. The retroreflector reflects radiation that is Fig. 2A from above. It is constructed on a transparent carrier film 1, onto which a transparent plastic material 3a is applied, into which a retroreflective, microprismatic reflection structure 3b is formed, for example embossed. A cover layer 3c and a cover film 2 cover the structure. Between the cover layer 3c and the plastic material 3a and thus on the interface of the relief structure 3b there is a dielectric, high-refractive index coating 3d. The relief structure 3b, together with the coating 3d, forms a reflection structure 3. The film composite can be applied to the carrier of the license plate of the Fig. 1A The imprint D is printed on the light-incident side of the film composite. It is particularly easy to see due to the retroreflective properties of the retroreflector.

[0025] The microprismatic relief structure 3b is embossed and preferably coated with UV varnish with a refractive index n 1 , which is the same as the plastic material 3a on the carrier film 1 (e.g., PET, PP, PE, PC, etc.). The high-index coating 3d with a refractive index n 2 and a thickness t is applied to the relief structure 3b. The coating 3d is preferably created by vacuum deposition. Finally, it is covered with the cover layer 3c with a refractive index n 3 .

[0026] Fig. 2B shows a similar design, with the refractive indices of the individual materials indicated. Furthermore, a coating is added between film 2 and cover layer 3c, which has an optical structure, for example, the imprint D or a printed image.

[0027] The optical paths in Fig. 2B illustrate the retroreflection at the coated semi-transparent interface of the relief structure 3b as well as the light paths with reflection at the structured background, e.g. the print D. Thus, in Fig. 2A und 2B Areas I and II were created, which differ in plan view. In Fig. 2A they are on the front, in Fig. 2B under the relief structure 3b.

[0028] In order to increase the degree of reflection, in principle, as in Fig. 3 shown, the dielectric, high-refractive coating 3d on the microprismatic relief structure 3 is designed as a multiple coating, in the form of a trilayer coating 3d1, 3d2, 3d3, wherein the layer sequence has alternating refractive indices. Fig. 3 shows the retroreflector in cross-section comprising the transparent plastic material 3a with refractive index n 1 , which is coated with three layers (3d1, 3d2, 3d3) of refractive indices n 2 , n 4 and n 5 and is located on the carrier film 1. The first and third layers 3d1, 3d3 are highly refractive, and the following applies to the refractive indices: n 2 > n 1 , n 2 > n 4 , n 5 > n 4 , n 5 > n 3 . These layers have the thicknesses t1, t2 and t3 respectively. The illustrated trilayer coating is covered with the cover layer 3c with refractive index n 3 and is protected by a cover film 2. Of course, the prints D etc. can also be applied here as in Fig. 2A und 2B be used.

[0029] Preferably, n 2 >n 1 . In embodiments, n 2 = n 1 . In this case, a difference between n 2 and n 3 is present and responsible for the reflection.

[0030] Fig. 4A shows the spectral reflection in the visible range in a microprismatic retroreflector in which the light incident from above is deflected and reflected three times by 45° at the highly refractive-index coated interface of the relief structure 3b. The materials 3a, 3c, between which the relief structure 3b is formed and between which the highly refractive-index coating 3d is located, have a refractive index n=1.5. The material of the highly refractive-index coating 3d in this example is TiO2. The reflection value has a maximum value of approximately 3.5% for a layer thickness t=50 nm. In comparison, an aluminum coating achieves an average reflection value of approximately 55% with triple reflection at alpha=45° each. This comparison results in an Ra value of approximately 70 for the retroreflector.

[0031] To explain the color properties of this coating in the CIE 1931 color space, the reflection spectra were convolved with the emission curve of a D65 standard lamp and the sensitivity of the human eye, and color coordinates X, Y, and Z were calculated. The D65 illumination corresponds approximately to daylight. The XYZ coordinates were then converted into color values ​​x, y and inserted into the color diagram of Fig. 4B These values ​​can be directly correlated with the human perception of color. The white point is also shown and labeled "WP." This triangle defines the color range that can typically be displayed on screens.

[0032] From this color diagram, it is clear that a TiO2 layer thickness of approximately 50 nm produces a neutral color (white). Furthermore, by selecting a thicker TiO2 layer, a predefined color tone for retroreflection can be achieved.

[0033] If higher retroreflection values ​​are required, as shown in Fig. 3 As explained, several layers with alternating refractive indices are applied to the microprismatic relief structure 3b. Fig. 5A und 5B show the spectral reflection in the visible range for three layers, which are embedded in materials 3a, 3c with refractive index n=1.5. Fig. 5A the first and third layers of ZnS; Fig. 5B These layers are made of TiO 2 . The middle layer consists of silicon dioxide (SiO 2 ). The layer thickness of ZnS (see Fig. 5A ) is t=60 nm, The layer thickness of TiO 2 (see Fig. 5B ) is t=50 nm. The different spectra refer to different layer thicknesses of SiO 2 in the range 80 nm ≤ t ≤ 160 nm. This shows that a retroreflector with a ZnS trilayer can achieve a maximum reflection value of about 14%. This value even increases to about 40% when using TiO 2. Therefore, microprismatic retroreflectors with a high-index trilayer coating 3d1, 3d2, 3d3 according to Fig. 3 Ra values ​​of several hundred. A neutral color value is achieved for ZnS as well as for TiO 2 at a SiO 2 layer thickness of t=80 nm. Furthermore, these structures have the advantage of not appearing metallic, and the substrate is visible due to the semi-transparency of the coating. It can therefore be structured, e.g., printed, as shown in the example in Fig. 2B The example of the trilayer coating compared to the single coating of Fig. 4 shows that the degree of retroreflection can be significantly increased by multi-coating. For applications requiring even higher reflectance values, this can be achieved by increasing the number of layers with alternating refractive indices.

[0034] Furthermore, the high-refractive coating can be selected so that reflection occurs predominantly in the infrared. Fig. 6 shows the spectral reflection of a microprismatic retroreflector, which in this example is coated with a trilayer consisting of 160 nm ZnS, various thicknesses of SiO 2, and 220 nm ZnS. For this coating, the maximum reflection value is in the near infrared at a wavelength of approximately 1500 nm. In the visible range, the retroreflection is significantly weaker. By selecting the layer thickness, the maximum retroreflection value can be adjusted to a given focal wavelength.

[0035] The microprismatic relief structure with high-refractive coating is arranged in further developments so that adjacent areas in the horizontal plane have different characteristics and thus exhibit different optical effects. Fig. 7A In addition to the microprismatic areas I, another area II is shown, which is a flat mirror surface 5 and therefore does not show any retroreflection. The area II of Fig. 7B includes a sawtooth structure 6, which is suitable for displaying motion effects and also spatial effects to a viewer. Furthermore, in some embodiments, regions II with holographic structures are also provided. Regions II can encode a motif, e.g., through their outline.

[0036] In further embodiments, metallized areas are located in sections below the 3d coating. These metallized areas can serve to enhance the effect in the aforementioned non-retroreflective areas. However, these additional metallized areas are not necessarily limited to these areas.

[0037] For production, a master of the relief structure 3b is created, preferably by diamond milling, diamond stylus, etching, or lithographic processes. For the latter, laser writing systems and, in particular, systems based on the two-photon absorption process can be used. The master thus produced is then copied electroplated or using a nanoimprinting process. Next, a multiple arrangement of the original pattern is applied to a matrix by hot stamping or nanoimprinting to create an embossing cylinder for subsequent duplication. Such embossing cylinders ultimately allow the continuous duplication of the original structure in UV varnish or by hot stamping on foils in a roll-to-roll process.

[0038] Finally, the relief structure embossed into the film is coated with a high-index coating or coating sequence. Sputtering, electron beam deposition, or thermal evaporation are preferred methods for this.

[0039] Particularly suitable materials for high refractive indexes are ZnS, ZnO, ZnSe, SiN x , Cr 2 O 3 , SiOx (with x<1, i.e. substoichiometric silicon oxide compounds), Nb 2 O 5 , Ta 2 O 5 , Ti x O x and ZrO 2 . If an alternating layer sequence is chosen, SiO 2 , MgF 2 or polymers can be used as low refractive index materials.

[0040] Additional metallization can be achieved in some embodiments by full-surface metallic vapor deposition, preferably with aluminum, chromium, etc., followed by demetallization. Alternatively, areas can also be printed with metallic coatings, e.g., supersilver.

[0041] The retroreflective structures described above are used in traffic engineering applications for improved recognition of license plates or traffic signs. Structures adapted for retroreflection in the infrared range are used in embodiments for communication between autonomous driving systems and marker points.

[0042] The planar retroreflectors described here are combined in various embodiments with known holographic structures or micromirror arrays. These structures provide additional optical effects that offer particular advantages in the authentication of, for example, license plates or valuable documents, ID cards, etc.

Claims

1. Reflective structure which has a retroreflective, microprismatic relief structure (3b) configured in a transparent plastic material (3a) and which retroreflects an optical radiation incident from an incidence side and through the plastic material (3a) by means of total reflection, wherein the microprismatic relief structure (3b) is coated with a dielectric coating (3d) and the dielectric coating (3d) is concealed on the rear with a cover layer (3c), wherein there is a difference between the refractive index (n2) of the dielectric coating (3d) and the refractive index (n3, n1) of the cover layer (3c) and / or of the plastic material (3a), characterized in that the plastic material (3a) and the cover layer (3c) respectively fill the microprismatic relief structure (3b), the coating (3d) is formed as a trilayer (3d1, 3d2, 3d3) with the layer sequence high refractive-low refractive-high refractive and the refractive index (n5) of the layer (3d3), adjacent to the cover layer (3c), of the layer sequence of the dielectric coating (3d) is greater than the refractive index (n3) of the cover layer (3c).

2. Reflective structure according to Claim 1, characterized in that the dielectric coating (3d) comprises at least one of the following materials: ZnS, ZnO, ZnSe, SiNx, Cr2O3, SiOx (with x<1, i.e. substoichiometric silicon oxide compounds), Nb2O5, Ta2O5, TixOy and ZrO2.

3. Reflective structure according to either one of Claims 1 or 2, characterized in that the coating (3d) has a layer thickness (t) of between 20 nm and 500 nm.

4. Reflective structure according to Claim 3, characterized in that the layer thickness (t) is between 40 nm and 150 nm.

5. Reflective structure according to any one of Claims 1 to 4, characterized in that retroreflected radiation is infrared radiation.

6. Reflective structure according to any one of Claims 1 to 5, characterized in that adjacent to the relief structure (3b) a holographic structure and / or a mirrored surface portion (5, 6) is provided.

7. Reflective structure according to any one of Claims 1 to 6, characterized in that under the coating (3d) or under the relief structure (3b) a structure imparting motif and / or colour is provided.

8. Reflective structure according to Claim 7, characterized in that the structure imparting motif and / or colour comprises a colour-printed layer.

9. Reflective structure according to any one of Claims 1 to 8, characterized in that under the coating (3d) or under the relief structure (3b) a metallization is provided in some regions.

10. Film composite comprising a carrier film (1) and a cover film (2) between which the retroreflector according to any one of Claims 1 to 9 is arranged.

11. Microimage structure for a microlens assembly, characterized in that it comprises a retroreflector according to any one of Claims 1 to 9.