Patent Publication Number: US-8124217-B2

Title: Structured surfaces that exhibit color by rotation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from U.S. Patent Application No. 60/916,355, filed May 7, 2007 and U.S. Patent Application No. 61/001,240, filed Oct. 31, 2007, both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to an array of structures forming part of or formed upon a substrate wherein the color perceived when viewing the structures varies as the substrate is rotated or as the viewer rotates about the substrate. 
     BACKGROUND OF THE INVENTION 
     Optically variable color-shifting coatings are well known, in the form of color shifting ink, color shifting paint and multi-layer coatings deposited upon a substrate with the optical coating providing an observable color shift as the angle of incident light or viewing angle changes. 
     Color shifting pigments and colorants have been used in numerous applications, ranging from automobile paints to anti-counterfeiting inks for security documents and currency. Such pigments and colorants exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is shifted. Generally, in order to see a shift in color an observer shifts the angle of the substrate relative to the light source to create a color shifting effect. 
     It is an object of this invention to utilize known color shifting coatings in a novel and inventive manner so as to produce an object which exhibits varying color by rotation. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of this invention, an optically variable device comprises a substrate having an array of structures formed thereon, therein or supported thereby, wherein said structures are coated with an optically variable color-shifting coating, wherein each of the structures form a pyramidal structure and wherein each pyramidal structure has at least three slanted faces and wherein one or more than one color is seen when viewing the pyramids vary as substrate is rotated at least 30 degrees about an axis orthogonal to the substrate. 
     In accordance with a preferred embodiment of the invention the faces of the structures are substantially planar and/or may have a diffraction grating formed therein. 
     In accordance with a broad aspect of this invention an array of structures are provided formed within or upon a substrate where the structures have at least three upstanding walls, wherein each wall contacts and terminates at a location where it contacts an adjacent upstanding wall, and wherein the walls slant inward from a base formed of or on the substrate, wherein the walls are coated with a color shifting coating that exhibits a color shift with a change of viewing angle. 
     It should be understood that the apex of the pyramidal structure may be above or below the plane of the substrate; that is, the pyramidal structure may be raised or recessed, respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       Exemplary embodiments of the invention will now be described in conjunction with the figures in which: 
         FIG. 1  is an isometric drawing of a pyramidal cell in accordance with an aspect of this invention. 
         FIG. 2  is a drawing of a Model # 1  showing 25 views at different viewing angles each varying from the preceding one by 15 degrees and wherein color travel is seen as the model is rotated. 
         FIG. 3  is a drawing of a Model # 2  showing case 1 showing 25 views at different viewing angles each varying from the or preceding one by 15 degrees and wherein color travel is seen as the model is rotated. 
         FIG. 4  is a drawing of a Model # 2  showing case 2 showing 25 views at different viewing angles each varying from the or preceding one by 15 degrees and wherein color travel is seen as the model is rotated. 
         FIG. 5  is a drawing of a Model # 2  showing case 3 showing 25 views at different viewing angles each varying from the or preceding one by 15 degrees and wherein color travel is seen as the model is rotated. 
         FIG. 6   a  is a drawing showing an on axis view of Model # 2  case 4. 
         FIG. 6   b  is a drawing showing an off axis view of Model # 2  case 4. 
         FIG. 7  is a drawing of a Model # 2  showing case 4 showing 25 views at different viewing angles each varying from the or preceding one by 15 degrees and wherein color travel is seen as the model is rotated. 
         FIG. 8   a  is a drawing showing an on axis view of Model # 2  case 5. 
         FIG. 8   b  is a drawing showing an off axis view of Model # 2  case 5. 
         FIG. 9  is a drawing of a Model # 2  showing case 5 showing 25 views at different viewing angles each varying from the or preceding one by 15 degrees and wherein color travel is seen as the model is rotated. 
         FIG. 10  is a drawing illustrating model # 3  with a height/base ratio of 0.1, rotated in three different positions and at three different viewing angles. 
         FIG. 11  is a drawing illustrating model # 3  with a height/base ratio of 0.4, rotated in three different positions and at three different viewing angles. 
         FIG. 12  is a drawing illustrating model # 3  with a height/base ratio of 0.631, rotated in three different positions and at three different viewing angles. 
         FIG. 13  is a drawing illustrating model # 3  with a height/base ratio of 0.8, rotated in three different positions and at three different viewing angles. 
         FIG. 14  is a drawing illustrating model # 3  with a height/base ratio of 1.0, rotated in three different positions and at three different viewing angles. 
         FIG. 15  is a drawing showing a plan view of an array of three sided pyramids wherein the flat web is present between adjacent pyramids. 
         FIG. 16  is a drawing similar to that of  FIG. 15  wherein pyramids are shown that are upstanding, with pyramids of a different color representing inverted pyramids. 
         FIG. 17  is a drawing illustrating a model having the letter “A” defined within the web by having an absence of pyramids in this region. 
     
    
    
     DETAILED DESCRIPTION 
     In an aspect of this invention an array of pyramidal structures is provided upon a substrate wherein the structures are coated with a same optically variable color shifting special effect coating. The coating can be a color shifting ink, paint, or a multi-layer color shifting coating. Although standard pyramidal structures are shown in the following examples, frusto-pyramids or stepped pyramids or other pyramidal-like structures can be utilized to achieve novel color-shifting by rotation. Furthermore, in place of providing an array of upstanding pyramidal structures, coated, inverted, sunken pyramids can be formed in the substrate providing similar effects by rotation. 
     A surprising aspect of an embodiment of this invention is that when the same uniform thickness coating is applied to all faces of a uniform symmetric pyramidal structure, different color is seen when viewing facets or faces of the pyramid “head on” versus viewing side facets. For example the front face exhibits a different color than the side facets of the pyramid. As the pyramid is rotated about an axis orthogonal to the substrate, for example as little as 30 degrees, the colors of the front and side facets change, and upon further rotation, the side facets appear to take on the color of the front facets and vice versa so that different colors are seen. This effect is particularly pleasing when an array of such structures is provided as the effect is reproduced by each pyramid and the eye tends to integrate the effect as will be shown in subsequent figures. 
     Referring now to  FIG. 1 , a pyramidal unit cell having 4 upstanding slanted walls numbered  1  through  4  is shown upon a base supporting the unit cell. The unit cell may be hollow inside or may be a solid. This unit cell is the basic structure that will be described in accordance with this invention. However variants of this unit cell will also be described and encompass embodiments of this invention. 
     Modeling of Structured Surfaces that Exhibit Color By Rotation 
     A visual model of structured surfaces with an optically variable color shifting type coating design was modeled and simulated using a 3-D rendering software package with a custom lighting model using an optical coating design software program. The lighting model used assumes completely diffuse lighting conditions, so only the angle between the facet normals and camera location need to be considered to approximate the color observed by the viewer of the model. 
     The aforementioned modeling was used to investigate critical design parameters of a structured surface that exhibits a color by rotation effect. 
     The modeling was essentially focused on a micro replicated surface consisting of a simple pyramidal structure. Notwithstanding, more complicated geometries are possible and will also exhibit color by rotation effects. The modeling is used to identify key design parameters important from substrate and coating design perspective. 
     Coating Design 
     The coating design used to model the performance of the structure was an optical stack consisting of a reflector, dielectric, and absorber to simulate the performance of an optically variable color shifting coating on the structure. 
     The optical designs considered were the following:
         (1) Aluminum (100 mm)/Magnesium Fluoride (Variable Thickness)/Chromium (6 nm)   (2) Aluminum (100 nm)/Zinc Sulfide (Variable Thickness)/Chromium (6 nm)
 
Unit Cell
       

     Modeling was performed using the simple four sided pyramidal structure with a square base measuring 1 unit on each side as is shown in  FIG. 1 . The apex of the pyramidal structure was centered over the base, with the height above the base as a design variable. 
     The basic unit cell shown in  FIG. 1  has faces numbered ( 1 ) and ( 2 ) on the opposite sides of the pyramid structure. The faces ( 1 ) and ( 2 ) are defined to be in the machine direction. Accordingly, the machine direction refers to the direction of web travel. In most of the models presented, these faces will be assumed to generally have a greater coating thickness than faces ( 3 ) and ( 4 ) which are in the non-machine direction. 
     The relative coating thickness on each pyramid face is a complicated function of source and coating geometry. However, the optimal relative coating thicknesses will be considered in order to achieve the desired overall visual appearance in a product form. 
     Model # 1  presents a model that exhibits color by rotation effect wherein the pyramidal facets or faces have equal thicknesses of dielectric on each face. Hence the color of the coating is the same on each face if each face were to be viewed at the same angle. A color by rotation effect is observed on a pyramidal structure when there is no difference in dielectric coating thickness on each face of the structure. 
     In this instance, an MgF 2  layer was used for the dielectric spacer layer with a coating thickness of 360 nm which corresponds to a Green/Blue optically variable pigment (OVP) coating design. 
     The OVP face color is observed when viewing any face in a normal viewing position while the angle color is visible when viewing from non-normal position. In this case all face axes and angle axes are equivalent with each face having the same dielectric thickness. Model # 1  illustrates that a difference in the dielectric thickness among faces of the pyramid structure is not required to achieve a color by rotation effect. Stated more simply, all faces may be coated with a same thickness of coating to achieve the desired effect of color by rotation. 
     Table 1 below shows in a wire grid frame model a “Face View” and an “Angle View” of the pyramidal cell used. Below in the same table is a view of the “Unit Cell Model” with color shifting coating, wherein face view illustrates the face color, green, and the angle view illustrates a light blue seen from the same coated cell. The last row of cells in Table 1 shows a “Surface View of Array of Unit Cell Pyramids”. The array of pyramids that corresponds to the face view shows green pyramids whereas the array shown in angle view shows light blue pyramids. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Face View 
                 Angle View 
               
               
                   
               
             
            
               
                 Wire Frame Model 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Unit Cell Model 
                 
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Surface View of Array of Unit Cell Pyramids 
                 
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
         
       
     
       FIG. 2  shows views at different angles of color travel for model # 1  from 0 degrees, i.e. face view, to rotation through various angles in increments of 15 degrees and up to 360 degrees of rotation from the original face view at 0 degrees. 
     In contrast, model # 2  represents color by rotation with a coating of unequal thickness on adjacent faces. In this exemplary embodiment a different dielectric thicknesses is provided on different adjacent facets. Thus, facets or faces ( 1 ) and ( 2 ) are coated with a same thickness of coating and faces ( 3 ) and ( 4 ) are coated with a different same thickness of coating material. Although Model # 2  is a simulation representing an actual coating, such a coating with these design parameters can be coated with the proper source and masking geometry in a vacuum roll coater. In this embodiment, machine direction faces ( 1 , 2 ) have equal dielectric thicknesses while non-machine direction faces ( 3 , 4 ) have less dielectric due to oblique coating angles in machine. 
     In this embodiment, because adjacent pyramid faces have significantly different dielectric thicknesses, maximum color travel is observed every 90 degrees of rotation instead of every 45 degrees of rotation as was the case with model # 1  where all dielectric thicknesses are equal. Two different instances, exemplified by case 1, and case 2 of Model # 2 , are provided, wherein different coatings are applied in each. In model # 2  case 1, the ratio provided refers to the relative amount of coating deposited on each face compared to a nominal MgF2 design thickness. By way of example, for Model # 2  Case 1, the base design is Al/MgF2 (360 nm)/Cr (6 nm). Thus in this instance where the face  1  and face  2  ratio is 1, the following coating design is used: Al/MgF2 (360 nm)/Cr (6 nm). For face  3  and  4  where the ratio is 0.8 the following coating design is used: Al/MgF2 (288 nm)/Cr (6 nm). The ratio relates to the differing thickness in the MgF2 dielectric spacer layer. 
     
       
         
           
               
             
               
                 TABLE 2A 
               
               
                   
               
               
                 Model 2 Case 1 - Green/Blue (0.8) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Viewing Angle 
                 45 
                 Degrees 
               
               
                   
                 MgF2 Thickness 
                 360 
                 nm 
               
            
           
           
               
               
               
            
               
                   
                 Face (1) and (2) Ratio 
                 1.0 
               
               
                   
                 Face (3) and (4) Ratio 
                 0.8 
               
               
                   
                   
               
            
           
         
       
     
     The first case of Model # 2  used a ratio of 0.8 for the non machine direction faces. Case 2 shows a similar configuration using a 0.9 multiplier for the non-machine direction faces. 
     
       
         
           
               
             
               
                 TABLE 2B 
               
               
                   
               
               
                 Model 2 Case 2 - Green/Blue (0.9) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Viewing Angle 
                 45 
                 Degrees 
               
               
                   
                 MgF2 Thickness 
                 360 
                 nm 
               
            
           
           
               
               
               
            
               
                   
                 Face (1) and (2) Ratio 
                 1.0 
               
               
                   
                 Face (3) and (4) Ratio 
                 0.9 
               
               
                   
                   
               
            
           
         
       
     
     Decreasing the non-machine direction face ratio to 0.7 results in a shift to a reddish magenta color rather than blue at the 90 and 270 degree angles. 
                     TABLE 2C               Model 2 Case 3 - Green/Blue (0.7)                                                    Viewing Angle   45   Degrees           MgF2 Thickness   360   nm                             Face (1) and (2) Ratio   1.0           Face (3) and (4) Ratio   0.7                        
Model # 2  Observations
 
     Modifying the dielectric thicknesses of the off-axis faces ( 3 ) and ( 4 ) has a strong impact on the observed color when the substrate is rotated into a position where these faces dominate the field of view. (rotation angles of 90° and 270°) Changing the dielectric thickness of the off-axis faces ( 3 ) and ( 4 ) has a smaller impact on color when viewing opposing faces ( 1 ) and ( 2 ) because these faces are oblique, present a smaller cross section, and lose chromaticity due to higher angle viewing of the optical coating design. 
     The off-axis color on faces ( 3 ) and ( 4 ) varies strongly as the thickness ratio is changed because the dielectric thickness is changing. 
     Another import consequence of both on-axis and off-axis faces being observable in the same visual field is that when viewing the on-axis face, the off-axis face does make a significant contribution to the observed color. There are some combinations of dielectric thicknesses and off-axis face ratios that have a more appealing appearance because one or both of the following conditions may occur:
         (1) When viewing on-axis, the observed shift color from the off-axis facets coordinates well with the face color of the on-axis facets wherein the colors have a similar hue angle;   (2) When viewing off-axis, the observed shift color from the on-axis facets coordinates well with the face color of the off-axis facets.       

     The following examples illustrate the above conditions: 
     In Model  2  Case 4, the same unit cell is used, with a base MgF2 thickness of 480 nm which corresponds to a Magenta to Green OVP design. As in Case 1, a 0.80 ratio is used for the coating thickness on the off-axis faces. In this particular case, both conditions 1 and 2 are met. This is shown in  FIGS. 6   a  and  6   b . 
     
       
         
           
               
             
               
                 TABLE 2D 
               
               
                   
               
               
                 Model 2 Case 4 - Magneta to Green (0.8) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Viewing Angle 
                 45 
                 Degrees 
               
               
                   
                 MgF2 Thickness 
                 480 
                 nm 
               
            
           
           
               
               
               
            
               
                   
                 Face (1) and (2) Ratio 
                 1.0 
               
               
                   
                 Face (3) and (4) Ratio 
                 0.8 
               
               
                   
                   
               
            
           
         
       
     
     In this example, the on-axis view shows the Magenta design coordinating with the blue shift color from the off-axis faces. In the off-axis view, the green face color coordinates well with the green shift color from the magenta face. 
     Overall, Model  2  Case 4 shows a very strong color by rotation effect because viewing both on-axis and off-axis viewing result in coordinating colors which reinforce each other in both viewing axes. 
     The key parameters required to meet these requirements are dielectric index of refraction, dielectric thickness, on-axis to off-axis dielectric thickness, and unit cell design. Dielectrics with a low index of refraction, i.e. below about 1.6 such as MgF 2  are preferred because they exhibit a stronger color shift which is required to achieve coordinating colors with similar hue angles in the on-axis and off-axis orientations. High index materials may also be used to achieve color by rotation with functional but less dramatic results. 
     Model  2  Case 5 shown in  FIGS. 8   a ,  8   b  and  FIG. 9  shows an example of another case where coordinating colors are not realized, that is where colors of similar hue are not presented to the viewer at the on-axis orientation and the observed color is not quite as dramatic. In this case, coordinating colors are still present in the off-axis orientation resulting the in the reinforced gold color on the off-axis view. 
                     TABLE 2E               Model 2 Case 5 - Blue to Red (0.8)                                                    Viewing Angle   45   Degrees           MgF2 Thickness   270   nm                             Face (1) and (2) Ratio   1.0           Face (3) and (4) Ratio   0.8                        
Model # 3 : Optimization of Pyramid Height and Viewing Angles
 
     This model illustrated by  FIGS. 10 ,  11 ,  12 ,  13  and  14  investigates the potential impact of the height of the pyramid and viewing angle on product appearance. One of the parameters that must be determined is the target height of the surface structure. In this model, four pyramid heights and three viewing angles were simulated to investigate the impact on overall product appearance. 
                     TABLE 3               Model 3 - Magneta to Green (0.8)                                                Viewing Angle   Variable           MgF2 Thickness   480 nm           Face (1) and (2) Ratio   1.0           Face (3) and (4) Ratio   0.8                        
Viewing Angle Conclusions
 
     In a preferred embodiment of this invention, the most ideal unit cell height to base ratio has proven to be the “Golden Pyramid” ratio with a height to base ratio of 0.636. Of the ratios used in our simulations, this ratio showed the least amount of apparent color variation over a typical range of substrate viewing angles which ranged from 25 degrees to 65 degrees above the substrate plane. The 0.8 height to base ratio was also found to be acceptable. 
     Lower height to base ratios such as the case with 0.4 results in large hue changes as a function of viewing angle from the substrate plane. In this embodiment the extreme yellowing of the off-axis view as the viewing angle is increased. Larger height to base ratios such as 1.0 also started to show significant hue variation as a function of viewing angle from the substrate plane 
     Through simulations performed, we have found that the optimal height to base ratio appears to be in the range of 0.6 to 0.8. In this case, the assumption was that the off-axis faces were always receiving 80% of the coating that the on-axis faces were receiving. This is probably not feasible as the height to base ratio becomes small due to the coating geometry constraints. However, it is clear that there is an optimal height to base ratio of the unit cell that produces the least amount of visual variation as the viewing angle from the substrate plane is varied. 
     As a result of the modeling, we conclude that:
         1) To achieve a color by rotation effect on a pyramidal structure, a difference in MgF2 thickness on faces is not required. Color by rotation will be observable even with equal coating thicknesses. The maximum color travel will occur on the 45 degree axis in this case.   2) For unequal facet MgF2 thicknesses a stronger color shift is observed with maximum color travel on the 90 degree axis of rotation.   3) A stronger color by rotation effect will be observed in the situation where facet colors reinforce each other by having similar hue angles.   4) Model  3  provides information related to the optimal height to base ratio to provide optimal color travel when viewing the substrate at different angles of inclination.       

     The pyramidal array can be formed by embossing a flexible or rigid deformable substrate from an appropriate master. The master can be made by diamond cutting or other suitable micromachining techniques such as electron beam lithography, ion milling, or other microreplication techniques. We believe that techniques can be employed to make masters that could be used in an embossing process. In one embodiment the use of an anilox roller having indented pyramids and other shapes used in the printing trade as a template for making a positive nickel master by means of electro less nickel with a release layer, followed by growing a nickel daughter image from the nickel master which in turn is used to emboss a UV curable lacquer on a web to make positive pyramidal shapes. Information regarding anilox rollers can currently be found on the Internet at http://www.harperimage.com/anilox-specify. asp and http://www.appliedlaser. co. uk/anilox.htm. 
     In all of the embodiments described heretofore, it is preferred the dimensions of the pyramids are below eye resolution of about 100 microns. Thus the height of the pyramids is preferably below 100 microns. This is important from a security viewpoint because it would be non-obvious to an observer as to why one color shift is occurring. 
     In addition to providing visually appealing security coatings, embodiments of this invention use forms of linear coding by varying the heights of pyramids in a linear sequence to create an overt or covert readable “bar code” effect depending on the degree of modulation used. Overt images, symbols, words could be written into the pattern by geometrical variations to the unit cell. Alteration of cell height, orientation, cell size, facet angles could all be used to encode information.  FIG. 17  shows indicia in the form of a letter “A” provided by exposing the substrate in this region. 
     In an embodiment of this invention not shown in the drawings, a visible logo is formed by orienting a particular region of pyramids for forming a logo, at 45 degrees to other pyramids which serve as a contrasting background within the security device, and coating all of the pyramids with a same color shifting coating. In this manner, the pyramids forming the logo appear in a first color distinct from the color of the background pyramids. As the device is rotated the colors change, and at a particular angle, the colors of the two regions appear to switch. 
     Various other embodiments can be envisaged wherein indicia in the form of a logo or text can be encoded within the array of pyramids or between the pyramids. Regions of pyramids can have different geometries than others, thereby providing a visual distinction so as to define a logo or text. In a particular embodiment a security thread in which most of the area on the web has the same pyramid geometries but in certain regions some of the pyramids have different face angles. This would allow an image to appear when the device is rotated. Alternatively, some frusto-pyramids can be provided with flat tops, within a region, thereby defining a logo or indicia distinguishable from other regular shaped pyramids. Essentially what is required in all of these embodiments is a region, within a larger region of pyramids that is visually distinguishable in such a manner as to define indicia. 
     Heretofore, pyramids having flat planar surfaces have been described, however pyramids having one or more faces having diffractive grooves formed therein would provide additional effects to the color shifting described. For example pyramids having facial grooves extending from the base to the apex can be provided, and stepped pyramids can also be provided. The stepped pyramids can have steps at small diffractive widths or can be larger. These pyramids would provide interesting color/extinction effects as the faces move in rotation, especially with specular coatings on the faces. 
     Such pyramids would even provide interesting effects even with a mirror coating of, for example, aluminum. Aluminized pyramids can be provided in a selected pattern purposely dispersed within the optically variable coated pyramids so as to form a logo or indica that is distinguishable from the color shifting coated pyramids. 
     Various packing configurations of pyramids can be provided. For example  FIG. 15  shows an embodiment wherein planar spaces of the substrate are shown between upstanding pyramids, and in  FIG. 16 , upstanding and inverted pyramids are shown juxtaposed. 
     Pyramids can also be provided that are skewed relative to the machine direction shown in the figures. 
     The pyramid faces could also be etched to form diffractive surfaces as could the flat areas among the pyramid arrays.