Abstract:
This patent discloses methods for modifying cube corner retroreflective patterns by stretching a cube corner retroreflector. Stretching may either permanent through inelastic stretching usually under heat or elastic stretching using a controlled stretching mechanism.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to cube-corner retroreflective articles, particularly to such articles in which the cube-corner retroreflective elements are modified by stretching the article. 
   BACKGROUND OF THE INVENTION 
   Light rays from a true retroreflector will be directed back toward the source parallel to the original light rays. Several applications involving retroreflectors require that the light leaving the article diverge from the incident angel in order to be effective. For example, light from the hedlamps of an oncoming vehicle, reflected back toward the vehicle by a retroreflective sign, must diverge sufficiently as it leaves the sign to reach the eyes of the driver, who is positioned off-axis from the headlamp beam. In conventional cube-corner retroreflective articles, this cone-like spreading of retroreflected light is obtained through imperfections in the cube-cornerretroreflective elements (e.g. non-flatness of the faces, unintended tilting of the faces from their mutually perpendicular positions, etc.) and through diffraction caused because the retroreflected light exists through an aperture defined by the base edges of the three reflecting faces (see Stamm, U.S. Pat. No. 3,712,706). 
   However, the spreading of light from a cube corner has many important deficiencies; the cone of the retroreflected light is often too narrow for many uses that require reflected light to be seen farther off-axis; and the three-sided nature of the cube-corner reflective elements gives the retroreflected cone of light an undesirable asymmetric shape. Tanaka, U.S. Pat. No. 3,817,596 increases the divergence or spreading of light rays from the cube-corner retroreflective article by deliberately tilting the faces of the cube-corner retroreflective elements out of perpendicularity or orthogonality. 
   Although spreading of the light taught in U.S. Pat. No. 3,817,596 increases the observation angles from which the article may be seen by retroreflection, no effort is made to avoid the basic asymmetry that arises from the three-sided nature of a cube-corner retroreflective element. 
   Heenan, U.S. Pat. No. 3,833,285, changes the divergence or spreading of light from a cube-corner retroreflective article in a different manner, specifically by incorporating into the article a set of special cube-corner retroreflective elements arranged in a row. In each of these special cube-corner retroreflective elements, two of the faces intersect in a line that is aligned along the length of the row of elements and the dihedral angle at the intersection of the two faces is enlarged beyond the conventional 90 degrees, e.g., to 90° 30′ with the result that the light retroreflected by those elements within the row. It is contemplated that in different elements within the row the dihedral angle may be enlarged different amounts so as to spread light into an elongated pattern. The method of construction of the invention in U.S. Pat. No. 3,833,285 is to create a die (see Heenan, U.S. Pat. No. 3,833,285 column 4, line 58 through column 5, line 11) used to impress plastic or metal foil. 
   Appledorn, U.S. Pat. No. 4,775,219 describes how cube-corner retroreflective articles may be individually tailored so as to distribute light retroreflected by the articles into a desired pattern or divergence profile. The retroreflective article presented in U.S. Pat. No. 4,775,219 carry one one side an array of cube-corner retroreflective elements being formed by three intersecting sets of parallel V-shaped grooves, with at least one of the sets including, in a repeating pattern, a grove side angle hat differs from another groove side angle of the same set. As a consequence of the repeating variations in grove side angle, the array of cube-corner retroreflective elements is divided into repeating sub-arrays that each comprise a plurality of cube-corner retroreflective elements in a plurality of distinctive shapes. At least one of the distinctive shapes is for a non-orthogonal cube-corner retroreflective element, by which is meant that at least one face of the element is tilted at an angle that differs from the angle which would be required for all dihedral angles within the element to be orthogonal, such an element is still regarded as a cube-corner retroreflective element herein, because it closely resembles the ideal cube-corner retroreflective element in shape and function. The overall pattern or the light, the divergence profile for the article, comprises a summation of the different light patterns in which the distinctively shaped cube-corner retroreflective elements in a sub-array retroreflect incident light, and the individual distinctively shaped light patterns can be selected to give the overall pattern a desired shape or contour. The method of construction for the invention described in U.S. Pat. No. 4,775,219 is to scribe a plate (see Appledorn, U.S. Pat. No. 4,775,219 column 10, line 63 through column 11, line 12) that is used as a master for a mould from which the retroreflective article can be cast, embossed or otherwise molded. 
   Benjamin, W.O. U.S. Pat. No. 0,023,828 describes an invention that provides cube-corner retroreflective articles exhibiting features of rotational symmetry and entrance angularity like that of microsphere-based articles. Such features are achieved by orienting the cube-corner elements randomly so that typically essentially no two cube-corner elements are directly joined and typically no two adjacent elements are in the same orientation. Articles constructed in this manner may have enhanced brightness over microsphere-based articles due to the higher retroreflective efficiency of the cube-corner elements as compared to microspheres. 
   Rowland, U.S. Pat. No. 3,684,348 for “Retroreflective Material” was to provide a flexible retroreflective shaped material utilizing cube-corners which would conform and adhere to non-planar support surfaces. While this retroreflective material has functioned very well, it has been found that, if stretched during application to a support surface, such as wrapping a tape mask therefrom helically around bicycle handlebars, the cube-corners are distorted. This renders the cube-corner faces non-orthogonal to a degree resulting in significant loss of brightness. 
   Rowland, U.S. Pat. No. 3,992,080 for “Retroreflective Sheet Material with Controlled Stretch and Method of Making Same” provides a novel stretchable retroreflective sheet material utilizing cube-corner retroreflectors which is easily applied under tension to support surfaces and provides good retroreflection wen stretched a predetermined amount. This is readily attained in an elongated composite retroreflective sheet material with controlled stretch. The composite material comprising a first strip of transparent flexible synthetic resin having a multiplicity of minute cube-corner formations on one surface thereof bonded to a second strip of flexible backing material of lesser length than the first strip when in a relaxed condition with the cube-corner formations disposed adjacent the second strip. The composite retroreflective sheet material is puckered in the relaxed condition with the elimination of the puckered condition providing a visual measurement of the desired degree of elongation of the composite material to avoid distortion of the cube-corner formations. 
   In summary each modification of the cube-corner pattern requires a new mold, form or embossing element, the construction of which is costly and time consuming. Also, once constructed, the cube-corner article is not adaptable to changing conditions and requirements. 
   The present invention describes a less costly and time consuming method for obtaining useful cube-corner patterns by distorting cube-corner articles via inelastic stretching such that the cube-corner faces form different permanent angular relationships with each other within a cube-corner retroreflector to produce a desired pattern. Furthermore the cube-corner patterns may be elastically stretched such that the cube corner faces form different temporary angular relationships with each other within a cube-corner retroreflector to produce a temporary desired pattern. Furthermore the article may be heated to reduce the forces necessary for stretching the article. Furthermore the article may be heated without stretching. Heating without stretching has been observed to cause some retroreflective articles to approach true retroreflectors. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention there is provided a method for making a modified retro-reflective screen having a plurality of cube corner retro-reflectors, comprising the steps of: 
   providing a web having a plurality of cube corner retro-reflectors; and 
   applying tension in at least one direction to the web so as to change the shape of the cube corner retro-reflectors. 
   In accordance with another aspect of the present invention there is provided a method for providing various reflective image shapes from a retro-reflective screen having a plurality of cube corner retro-reflectors, comprising the steps of: 
   providing a web having a plurality of cube corner retro-reflectors; 
   controlling the amount of tension applied to the web in at least one direction so as to be able to adjust the shape of the plurality of cube corner retro-reflectors. 
   In still another aspect of the present invention there is provided a system for projecting a retro-reflective image comprising; 
   a retro-reflective screen having a plurality of cube corner retro-reflectors, comprising the steps of: 
   a tension applying device for applying a controlled amount of tension applied to screen in at least one direction so as to be able to adjust the shape of the plurality of cube corner retro-reflectors; and 
   a pair of projectors for projecting a stereoscopic image onto the retro-reflective screen for viewing by a viewer. 
   In still another aspect of the present invention there is provided a device for making a modified retro-reflective web having a plurality of modified cube corner retro-reflectors, comprising: 
   a first supply roll for holding a web of material having a plurality of cube corner retro-refectors; 
   a second take-up roll for receiving the web from the first supply roll; 
   a drive mechanism for moving the web at the first supply roll at a first velocity and the web at the second roll at a second higher velocity so apply a tension to the web so as to cause modified cube corner retro-refectors to be formed. 
   In another aspect of the present invention there is provided a device for making a modified retro-reflective screen having a plurality of cube corner retro-reflectors, comprising: 
   a holding mechanism for holding a sheet of material having a plurality of cube corner retro-reflectors; and 
   a tension applying mechanism for providing a tension on the sheet in at least one direction to so as to change the shape of the cube corner retro-reflectors. 
   In yet still another aspect of the present invention there is provided a device for making a sheet having a plurality of modified cube corner retro-reflectors, the device designed to be placed in an oven, comprising: 
   a holding mechanism for holding the sheet; and 
   a tension applying mechanism for placing tension on the sheet in at least one direction to so as to change the shape of the cube corner retro-reflectors. 
   These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims and by reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings in which: 
       FIG. 1  is a schematic diagram of an apparatus for making a continuous web in accordance with the present invention; 
       FIG. 2   a  is a cut sheet of a material for making a retroreflective screen in accordance with the present invention; 
       FIG. 2   b  is a schematic diagram of a device for used to make a retroreflective screen using the cut sheet of  FIG. 2   a;    
       FIG. 3  is a view similar to  FIG. 2   a  illustrating the cut sheet of  FIG. 2  mounted in the device of  FIG. 2   b;    
       FIG. 4   a  is a schematic diagram of a retroreflective viewing system made in accordance with the present invention; 
       FIG. 4   b  is top view of the system of  FIG. 4   a;    
       FIG. 5   a  is a frontal view illustrating how a prior art retroreflective screen returns light directly to the projection source; 
       FIG. 5   b  is a frontal view illustrating how the retroreflective screen of the present invention returns light to the observer&#39;s eyes; 
       FIG. 6   a  is a detail drawing of prior art unmodified cube-corner screen element; 
       FIG. 6   b  is a detail drawing of a modified cube-corner screen element in accordance with the present invention; 
       FIG. 6   c  is a schematic diagram illustrating how a prior art retroreflective screen returns light directly to the projection source; 
       FIG. 6   d  is a schematic diagram illustrating how the retroreflective 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a schematic diagram of a device  100  for making a modified retro-reflective screen having a plurality of cube corner retro-reflectors wherein the initial shape of said plurality of cube corner retro-reflectors. In  FIG. 1  there is illustrated a web  125  of made of a retro-reflective material, where the web  125  is made of Mylar. The web  125  has plurality of retro-reflectors each having a shape of a substantial triangle having a size ranging from about 100 microns to 500 microns.  FIG. 6   a  illustrates the initial shape of one of the cube corners retro-reflectors and  FIG. 6   b  illustrates the shape of modified cube corners retro-reflectors made in accordance with the present invention. The web  125  is preferably constructed so that the web  125  has a thickness in the range of about 0.1 mil. to 5 mil. The web  125  is heated by a heater  115  such that a sufficient amount of heat is applied to the web  125  so as to raise the temperature of the web  125  to a temperature in the range of about 110° to 130° centigrade as the web  125  is being transported from web supply roll  160  on a supply spindle  150 , which provides a web having a plurality of cube corner retro-reflectors, around the supply idling roller  145 , through the heating station  116 , around the take up idling roller  110  to take up roll  165  on the take up spindle  170 . The web supply roller  160  produces a velocity V 1  of web  125  that is less than the take up velocity V 2  of the web  125  at take up roller  165 . This applies tension in at least one direction to the web  125  so as to change the shape of said cube corner retro-reflectors on web  125 . The amount of said tension to be applied is determined so as to provide a predetermined retro-reflective image shape when the tension of web  125  material is removed, the web  125  stretches elastically between the heater  115  and the supply roll  160 . The web  125  also stretches elastically between the heater  115  and the take up roll  165 . However the web  125  stretches uniformly both elastically and inelastically in the heated region  120  as heat is applied to the web  125  during the application of said tension to the web  125 . A sufficient amount of heat is applied so as to raise the temperature of the web  125  to a temperature in the range of about 110° to 130° centigrade. This inelastic stretch is set into the web  125  so that the angles of said cube corner retro-reflector are changed for minimum eye crosstalk (incidental image going to the wrong eye) in the range of about 1% to 5% as the web passes around the take up idling roller  110 . A retroreflective pattern sensing device  130  may be provided so that the shape of said retro-reflective pattern is monitored as tension is being applied. The device  130  may monitor the amount of stretch, which can be related to the actual shape of the cube corner retro-reflectors. Supply velocity V 1  of the web  125  at roller  145  and the velocity V2 of the web  125  at roller  110  are controlled in such a manner that tension is applied in an increasing matter until said predetermined retro-reflective image is obtained. 
     FIG. 2   b  shows another device  210  for making a modified retroreflective screen having a plurality of cube corner retro-reflectors in accordance with the present invention where the initial shape of the plurality of cube corner retro-reflectors is in the shape of a substantial triangle having a size ranging in about 100 microns to 500 microns. In  FIG. 2   a , a sheet  225  of retro-reflective material is illustrated for use in device  210 , wherein the sheet  225  is made of Mylar, and has a thickness between about 0.1 mil. To 5 mil., Referring back to  FIG. 2   b , the sheet  225  is placed in device  210  which is heated in an oven  255  so that a sufficient amount of heat is applied so as to raise the temperature of the sheet  225  to a temperature in the range of about 110° to 130° centigrade as the sheet  225  is being stretched by the tension spring  205  of device  210 . In particular, one end  207  of the sheet  225  is attached to the spring loaded tension bar  215  and the other end  247  of the sheet  225  is attached to the pivot bar  235 . A tension spring  205  is attached to the spring loaded tension bar  215  and also to the base plate  230 . The pivot bar  235  is attached to the base plate  230  through a pivot pin  250  in such a manner that the pivot bar  235  is free to rotate about the pivot pin  250 . The tension spring  205  is mounted under sufficient tension such that when the assembled system  245  is heated in an oven  255  to said temperature the sheet  225  stretches both elastically and inelastically to a predetermined length define by the placement of the left stop  200  and right stop  210 . The system  245  is then removed from the oven  255  and allowed to cool. When the tension is removed from the sheet  225  by removing the tension spring  205 , the inelastic stretch of the sheet  225  will change the angles of said cube corner retro-reflector are changed in the range of about of 1% to 5% and produce the desired optical spread of the retroreflected image. 
   Referring to  FIG. 3  there is illustrated another method for making a modified retro-reflective screen having various reflective image shapes from a retro-reflective screen having a plurality of cube corner retro-reflectors. In this method a sheet  320  having a plurality of cube corner retro-reflectors is provided in device  302 . A controlled amount of tension is applied to the sheet  320  in at least one direction so as to be able to adjust the shape of said plurality of cube corner retro-reflectors where the initial shape of said plurality of cube corner retro-reflectors is in the shape of a substantial triangle having a size ranging in about 100 microns to 500 microns as illustrated in  FIG. 6   a . Sheet  320  is made of an retroreflective material, and in the embodiment illustrated the sheet  320  is made of Mylar, having a thickness between 0.1 mil. To 5 mil. The sheet  320  is elastically stretched and tension is maintained at a particular desired tension so as to provide a desired retro-reflective image area at room temperature. One end  317  of the sheet  320  is attached to the motor driven tension bar  315  and the other end  327  of the sheet  320  is attached to the pivot bar  335 . A linear drive member  305  is attached to the motor driven tension bar  315  and also to the linear drive motor  300 . The linear drive motor  300  is also attached to the base plate  325 . The pivot bar  335  is attached to the base plate  325  through a pivot pin  340  in such a manner that the pivot bar  335  is free to rotate about the pivot pin  340 . Likewise the motor driven tension bar  315  is connected to the linear drive member  305  through a drive pivot pin  310  in such a manner that the motor drive tension bar  313  is free to rotate about the drive pivot pin  310 . The forgoing arrangement equalizes the tension applying force across the width of the sheet  320 . The linear drive member  305  is connected to the linear drive motor  300 . The base of the linear drive motor  300  is attached to the base plate  325 . The linear drive motor  300  provides sufficient force to control the amount of tension applied to the sheet  320  in at least one direction so as to be able to adjust the shape of said plurality of cube corner retro-reflectors. This tension is maintained at a particular desired tension so as to provide a desired retro-reflective image area. Furthermore it is possible to vary the tension so as to modify the shape of said retro-reflective image area while said screen is in use. 
     FIGS. 4   a  and  4   b  illustrate a system  400  in accordance with the present invention for projecting a retro-reflective image comprising a retro-reflective screen having a plurality of cube corner retro-reflectors. In system  400  a tension applying device/motor  300  is used for applying a controlled amount of tension to screen in at least one direction so as to be able to adjust the shape of said plurality of cube corner retro-reflectors. A pair of projectors  450  are provided for projecting a stereoscopic image onto retro-reflective screen  320  for viewing by a viewer  440 . Like numbers indicate like parts and operation as previously described. Some elements are not illustrated for purposes of clarity. Retroreflective sheet  320  is placed under tension and elastically stretched by a linear drive motor  300  that is attached to a base plate  325 . One end of the sheet  320  is attached to the linear drive motor device  300  and the other end of the sheet  320  is attached to the base plate  325  to create the image viewing system  330 . Viewer  440  observes images on the sheet  320  generated by the right eye projector  450  and the left eye projector  435 . Tension is applied to the sheet  320  such that the retroreflected pattern due to the right eye projector  450  is seen only by the right eye of the viewer  440  and the retroreflected pattern due to the left eye projector  435  is seen only by the left eye of the viewer  440  as illustrated by  FIGS. 6   c  and  6   d  as described later herein. 
   For image projection onto a cube-corner screen, the screen effectively forms an “image” of the exit pupil of the projection lens back onto the projection lens exit pupil. When a ray of light enters a 3-sided, cube-corner element whose internal surfaces are reflective and intersect at a 90 degree angle, the entering light reflects off the cube&#39;s internal walls until exiting on the same heading that brought the light ray into the cube. Cube-corner screen material has a surface composed of many such small cube-corner elements. The crosshatching in  FIG. 5   a  illustrates the reflected image area produced with respect to the projector  40 . 
   By selectively modifying the cube-corner&#39;s internal angles of intersection from 90 degrees, the light ray&#39;s exit path from the cube-corner is made to differ from the entrance path, causing the trajectory of the reflected returning rays to deviate from the conventional retro-reflective condition. For a modified cube-corner screen comprising a plurality of such modified cube-corners, light reflected from the modified screen no longer forms an “image” of the projector exit constrained to the immediate area of (and radially surrounding) the projection lens assembly  60  referred to above. Instead, the modified screen forms a reflected “image” of the image exiting the projector  40  to an area around the projection lens assembly  60 . The nature of the expanded area of reflection can be controlled by the deviation of cube-corner dihedral angles from 90 degrees. In the preferred embodiment, the angles are modified so the exiting light rays form a horizontally elongated “image” of the projector exit pupil in the plane of the projector lens exit pupil as best illustrated by reference to  FIG. 5   b . As can be seen, that the reflected image of a modified cube-corner forms an elongated image area  70  (indicated by crosshatching in the illustration) centered on the projector lens assembly  60  as opposed to the generally circular reflective image area  72  of the standard cube-corner. The circular area of a prior art cube-corner display screen is not seen in a manner suitable for the observer, as the reflected image is reflected toward the associated projector. In contrast to the use of a modified cub-corner reflective screen made in accordance with the present invention, the projector  40  is positioned adjacent the head  48  so that a portion of the reflected image area  70  for the projector will cover the adjacent eye  44  so that the eye  44  can easily view the reflected image form the single adjacent projector. 
     FIG. 6   a  shows an element from a conventional cube-corner retroreflective screen, which provides a reflected image area illustrated by  FIG. 6   c .  FIG. 6   b  shows an element from a modified cube-corner retro-reflective screen made in accordance with the present invention which would be expected to provide the reflected image results illustrated in  FIG. 6   d . U.S. Pat. No. 4,775,219 teaches the preparation of tooling for the production of the type of modified cube-corner reflector element shown in  FIG. 6   b . The element of  FIG. 6   b  differs from the conventional element of  FIG. 6   a  in that the angles α are wider than the 90 degree angles β of  FIG. 6   a . The deviation of angles α from 90 degrees causes a deviation from true retroreflection such that the aforementioned horizontal elongation of the retroreflection of the “image” of the projector lens assembly  60  is achieved. 
     FIG. 6   c  illustrates how a screen of conventional cube-corner retroreflective material  80  returns a projected light beam  82  substantially to the projection lens  41 .  FIG. 6   d  illustrates that images projected onto a screen  14  of modified retroreflective cube-corner screen material are reflected as sufficiently elongated image areas that cover the eye closest to each projector  38 ,  40  without intersecting the other eye thus minimizing crosstalk of left and right images. Light from the modified retro-reflective screen  14  thus is returned by the screen directly to the eyes  44  of the observer  16 . 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
   PARTS LIST 
   
       
         14  modified retro-reflective screen 
         16  observer/observer&#39;s head 
         38 , 40  image projectors 
         41  projection lens 
         44  observer eyes 
         48  observer head 
         60  lens assembly 
         70  circular reflective image area 
         72  round image area 
         80  regular retroreflector screen 
         82  light beams 
         101  take up velocity V 2   
         110  take up idling roller 
         112  headband 
         115  heater 
         116  heating station 
         120  heated region 
         125  web 
         130  pattern sensing device 
         135  Supply velocity V 1   
         145  supply idling roller 
         150  supply spindle 
         160  supply roll 
         165  take up roll 
         170  take up spindle 
         200  left stop 
         205  tension spring 
         207  One end of the sheet  225   
         210  right stop 
         215  spring loaded tension bar 
         220  movement into the oven 
         225  sheet 
         230  base plate 
         235  pivot bar 
         240  base plate 
         245  assembled system 
         247  other end of the sheet  225   
         250  pivot pin 
         255  oven 
         300  linear drive motor 
         305  linear drive member 
         310  drive pivot pin 
         313  motor drive tension bar 
         315  motor driven tension bar 
         317  One end of the sheet  320   
         320  sheet 
         325  base plate 
         327  other end of the sheet  320   
         330  image viewing system 
         335  pivot bar 
         340  pivot pin 
         400  system 
         435  left eye projector 
         440  viewer 
         450  right eye projector