PATENT ABSTRACT
A dual light-source position detecting system for use in touch screens is provided that utilizes parallax to determine the position of an interposing object, and a prismatic film that is brightly retroreflective over a broad entrance angle to enhance to accuracy of the parallax determination of position. The position detecting system includes at least one camera positioned to receive light radiation traversing a detection area and that generates a signal representative of an image; two spaced-apart sources of light radiation, which may be LEDs, or IR emitters positioned adjacent to the camera for outputting light radiation that overlaps over at least a portion of a detection area, and a prismatic film positioned along a periphery of at least a portion of the detection area that retroreflects the light radiation from the two sources to the camera.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Application Nos. 61/285,684 filed Dec. 11, 2009, and 61/406,644 filed Oct. 26, 2010, both of which are incorporated herein by reference in their entireties. 
     FIELD OF THE INVENTION 
     The present invention relates generally to a system and method for detecting the position of an object within a touch screen or a position sensing system and a retroreflective or prismatic film used thereby. More specifically, the invention concerns a low profile position detecting system for use in touch screens or position sensing systems that employs a large spacing between source and detector in the plane of the screen, and a prismatic film that is brightly retroreflective at larger observation angles, and over a broad range of entrance angles. 
     BACKGROUND OF THE INVENTION 
     Some position detection systems related to touch screens sense the interruption of radiation (e.g., light) by an interposing opaque object (e.g., a finger, stylus, etc.). Such systems generally utilize radiation transmitters such as LEDs or IR emitters which are typically mounted in opposing corners of a same side of the touch screen. Each LED or IR emitter light source transmits a 90° fan-shaped pattern of light across the field of the touch screen, parallel to the viewing field surface. 
     A retroreflective sheeting material may be positioned around the perimeter of the active field of the touch screen, as disclosed in U.S. Pat. No. 4,507,557. The retroreflective sheeting material is generally arranged to reflect light received from the LED light sources back toward the originating source. Light incident on the front surface of the sheeting impinges on retroreflective elements, and is reflected back out through the front surface in a direction nominally 180 degrees to the direction of incidence. Digital cameras are located in the same opposing corners where the LED light sources are mounted to detect the retroreflected light that passes across the field of the touch screen and sense the existence of any interruption in this radiation by an opaque object. 
     One problem with the use of certain conventional retroreflective sheeting materials in touch screen applications and/or position detection systems is that dirt and/or moisture may penetrate the structure and adversely affect retro reflectivity of the retroreflective sheeting material. Another problem with conventional retroreflective sheeting material used in touch screen applications and/or position detection systems is difficulty in obtaining a uniform background throughout the area of interest (e.g., the detection area), against which the opaque object can be contrasted. Many conventional retroreflective sheeting material designs provide a non-uniform background and have portions, especially at or near the corner regions where the detected signal is very low. This makes it difficult to detect movement of the opaque object in such areas. 
     In operation, the position of the interposing object is typically determined by triangulation. When an interposing object such as a finger tip interrupts the pattern of light beams radiated from the LED light sources or IR emitters, a discrete shadow is created along a horizontal axis in the pattern of retroreflected light received by the two digital cameras. The digital cameras each generate a signal in which the discrete shadow registers as a dip in light intensity along a point of the horizontal axis of the camera field of view. A digital control circuit receives these digital camera signals and converts the horizontal position of the shadow into angles θ 1 , θ 2  whose vertex originates with the digital cameras. Because the digital cameras are separated a known distance D at opposite ends of a same side of the touch screen, the y coordinate of the interposing object can be computed by the digital control circuit using the formula y=D/(1/tan θ 1 +1/tan θ 2 ), and the x coordinate may be computed as x=y(1/tan θ 1 ). 
     BRIEF SUMMARY OF THE INVENTION 
     The embodiments of the present invention described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of the present invention. 
     While such prior touch-screen configurations are generally adequate for their intended purpose, the applicant has observed that such touch-screen configurations can only reliably detect a single touch at one time. A double touch will produce two dips in each camera signal, one for each object. Since it may not be clear which dip in the second camera signal corresponds to a given dip in the first camera signal, the resulting signal data may be ambiguous, making it impossible to determine with certainty the coordinate location of the two interposing objects. 
     A first aspect of the invention is the collocation of both source and detector in the plane of the touch-screen. This allows the dimension of the camera perpendicular to the plane of the screen to be minimized. The location of the source along the horizontal axis of the camera also maximizes parallax effects. 
     A second aspect of the invention relates to a position detection system having cameras that utilize parallax to unambiguously determine the position of an interposing object. To this end, the position detection system comprises a camera positioned to receive electromagnetic radiation traversing a detection area that generates a signal representative of an image; two spaced-apart sources of electromagnetic radiation positioned adjacent to said camera for outputting electromagnetic radiation that overlaps over at least a portion of a detection area, and a prismatic film positioned along a periphery of at least a portion of the detection area that retroreflects said electromagnetic radiation from said two sources to said camera. In such a configuration, the camera generates a double-image of any opaque, interposing object in the detection area which in turn allows a digital processor to make a parallax-based computation of the location of an object in the detection area based on the angle and distance of the object from the camera lens. If two cameras are mounted in opposing corners of a same side of the touch screen and two dual radiation sources are used in combination with these cameras, an unambiguous determination can be made of the location of two simultaneously interposing objects. Alternatively, if only a single touch capability is desired, then only a single camera in combination with a dual radiation source is necessary. 
     The applicant further observed that retroreflective properties of prior art prismatic films limit the accuracy of the parallax-based location computation. The accuracy of such computations increases with the distance of separation between two sources of electromagnetic radiation. However, prior art prismatic films have a limited range of observation angles for efficient retroreflection. Consequently, the farther apart the two sources are spaced, the dimmer or darker one or the other or both of the parallax images becomes, and the smaller the signal-to-noise ratio becomes. 
     Accordingly, a third aspect of the invention is the provision of a prismatic film that is brightly retroreflective over an unusually broad range of observation angles along the horizontal axis of the film. To this end, the prismatic film of the invention includes a plurality of triangular cube corner retroreflective elements having dihedral angle errors e 1 , e 2 , and e 3  such that e 1 ≈e 2 ≈0 and e 3 ≈0 Preferably, |e 1 | and |e 2 | are between about 0.02° and 0.20°. About half of the plurality of triangular cube corner retroreflective elements may have dihedral angle errors e 1  and e 2  between about 0.02° and 0.20°, and the remaining half of the plurality of triangular cube corner retroreflective elements have dihedral angle errors e 1  and e 2  between about −0.02° and −0.20°. Additionally, the triangular cube corner elements may be canted edge-more-parallel between about 8° and 20°. Finally, to further enhance retroreflectivity over a broad range of entrance angle, the prismatic film may include a metallized layer disposed over at least a portion of the retroreflective substrate. 
     In a still further aspect of the present invention a prismatic film, is described and includes an unpinned prismatic film having a retroreflective substrate including a plurality of triangular cube corner retroreflective elements; and wherein the pattern of retroreflected light has a horizontal spread greater than a vertical spread at entrance angles of 0° and 60°. In addition, the spread in the horizontal direction is 1.5 times greater than in the vertical direction. 
     A prismatic film in one or more of the foregoing embodiments wherein the triangular cube corners elements are canted between −10° and −6° and in a further embodiment between −15° and −6°. 
     In a still further exemplary embodiment of the presently described invention, a prismatic film, includes an unpinned prismatic film having a retroreflective substrate including a plurality of triangular cube corner retroreflective elements. The retroreflective cube corner elements have dihedral angle errors e 1 , e 2 , and e 3  such that e 1 ≈e 2 0 and e 3 ≈0, where the plurality of triangular cube corner elements are canted between −10° and −6°. 
     Other features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It is to be understood, however, that the detailed description of the various embodiments and specific examples, while indicating preferred and other embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by referring to the following more detailed description of the presently preferred exemplary embodiments of the invention in conjunction with the accompanying drawings, of which: 
         FIG. 1  is a schematic view of a touch screen system in accordance with aspects of the present invention; 
         FIG. 2  is a cross-sectional view of the touch screen system of  FIG. 1  in accordance with the present invention; 
         FIGS. 3 and 4  are cross-sectional views of prismatic film embodiments in accordance with the present invention; 
         FIG. 3   a  is a cross sectional view of a prismatic film embodiment in accordance with the present invention; 
         FIG. 5  is a plan view of prismatic film with an array of canted cube corner retroreflective elements as shown in  FIGS. 3 and 4 ; 
         FIG. 6  is a plan view of an exemplary canted cube corner retroreflective structure in accordance with aspects of the present invention; 
         FIG. 7  is a plan view of unpinned prismatic film of canted cube corner retroreflective elements as shown in  FIG. 6 ; 
         FIG. 8  is a cross-sectional view of  FIG. 5  taken along line  9 - 9 ; 
         FIG. 9  illustrates the dihedral angle errors e 1 , e 2 , e 3  present on the faces of the cube corners of the invention; 
         FIGS. 10A-10H  are spot diagrams of conventional cube corners canted edge more parallel at 0°, 16° and −8° for entrance angles (beta) of 0° and 60°; 
         FIGS. 11A-11D  are spot diagrams of cube corners of the invention canted edge more parallel at 0° and 16° for entrance angles (beta) of 0° and 60°; 
         FIGS. 12A ,  12 C,  12 E,  13 A,  13 C,  13 E are simulated light return patterns generated by unaberrated conventional cube corners canted edge more parallel at 0° and 16° for entrance angles (beta) of 0°, 60° and −8°; 
         FIGS. 12B ,  12 D,  12 F,  13 B,  13 D,  13 F are simulated light return patterns generated by cube corners aberrated according to the instant of the invention canted edge more parallel at 0° and 16° for entrance angles (beta) of 0°, 60° and −8°; and 
         FIG. 14  is an exemplary plot of camera signal for different screen sizes having a 16/9 aspect ratio. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is now illustrated in greater detail by way of the following detailed description which represents the best presently known mode of carrying out the invention. However, it should be understood that this description is not to be used to limit the present invention, but rather, is provided for the purpose of illustrating the general features of the invention. 
     One disadvantage of using arrays of prisms with positive cant in a touch-screen is the phenomenon of “sparkles.” At certain entrance angles, light from the LED can enter the prismatic film, bounce off of just two of the cube corner faces and return to the camera aperture. This creates a sharp “spike” in the camera signal at that particular angle. 
     The presence of “spikes” in the camera signal is undesirable. So for many applications, it is desirable to choose a cant for which sparkles only occur at entrance angles not seen in the touch-screen geometry. Typical touch-screens see entrance angles ranging out to about 60°. The following graph shows the location of sparkles for touch-screens with the retroreflective strip perpendicular to the plane of the screen. For cants of +7° and +15.5°, one can read off the location of the sparkles: 45° and 30°, respectively. It can be seen that no sparkles occur for the entrance angles of interest (0°-60°, if the cant ranges from about −19° to −0.5°. In a similar manner, prisms with cant=+15.5° give a sparkle at an entrance angle of about 30°. 
     If the retroreflective film is tilted slightly, so as to no longer be perpendicular to the plane of the touch-screen, the location of the sparkle can change somewhat. To illustrate this, the following graphs show the retroreflective efficiency of triangular cube corners with various cants, as a function of entrance angle (β) and orientation angle (ω s ). Overlaid in white are the angles at which a “sparkle” will occur. Overlaid in black are the angles encountered in the touch-screen geometry. The four black lines correspond to different tilts of the retroreflective film (−30°, −10, 10°, 30°. The graphs show that avoiding sparkles in the case of a tilted retroreflector requires a narrower range of cants. For example, a touch-screen with retroreflected film tilted 10° will avoid sparkles if the cant ranges from about −15° to about 0°. 
     U.S. Pat. No. 4,588,258 to Hoopman discloses retroreflective articles having a generally negative cant producing wide angularity, when using sets of matched pairs with the cube axes of the cubes in each pair being tilted toward one another. 
     For purposes of this application, certain terms are used in a particular sense as defined herein and other terms in accordance with industry accepted practice, such as current ASTM definitions, for example. 
     U.S. patent application Ser. No. 12/351,913, entitled “Retroreflector for use in touch screen applications and position sensing systems” filed Jan. 12, 2009 (having a common inventor and assigned to the same assignee as the present application) is hereby incorporated by reference herein as is necessary for a complete understanding of the present invention. 
     The term “cube” or “cube corner elements” (also “cube corner prisms” or “cube corners” or “cube corner retroreflective elements”) as used herein includes those elements consisting of three mutually intersecting faces, the dihedral angles of which are generally on the order of 90 degrees, but not necessarily exactly 90 degrees. 
     The term “cube shape” as used herein means the two-dimensional geometric figure defined by the projection of the cube perimeter in the direction of the principal refracted ray. For example, a triangular cube has a cube shape that is a triangle. 
     The term “dihedral angle error” as used herein refers to the difference between the actual dihedral angle and 90 degrees. Each cube corner element has three dihedral angle errors, e 1 , e 2 , and e 3 . For a canted cube corner with a cube shape that is an isosceles triangle, we adopt a convention whereby the label e 3  is assigned to the dihedral angle between the two faces with the same (but mirrored) shape. 
     The term “retroreflective substrate” as used herein means a thickness of a material having an array of either male or female cube corner elements formed on a second surface thereof. The first surface can be flat, or can be somewhat uneven in a pattern generally corresponding to the array of cube corner elements on the back surface. For male cube corner elements, the expression “substrate thickness” means the thickness of material on which the cube corner elements rest. For female cube corner elements, the expression “retroreflective substrate thickness” means the total thickness of material into which the female cube corner elements form cavities. 
     The term “cube axis” as used herein means a central axis that is the tri-sector of the internal space defined by the three intersecting faces of a cube corner element. The term “canted cube corner” as used herein means a cube corner having its axis not normal to the sheeting surface. Cant is measured as the angle between the cube axis and the sheeting surface normal. It is noted that when there is cant, a plan view normal to the sheeting surface shows the face angles at the apex not all 120 degrees. 
     The term “entrance angularity” as used herein means the angle between the illumination axis and the optical axis (retroreflector axis). The entrance angle is measured between the incident ray and the retroreflector axis. Entrance angle is a measure only of the amount by which an incident ray is angled to the retroreflector axis and is not concerned with the normal. 
     The term “face-more-parallel cant” (or “canted in a direction face-more-parallel or “canted in a face-more-parallel direction”) and “edge-more parallel cant” as used herein refer to the positioning of the cube relative to the principal refracted ray. When the angles between the cube faces and the principal refracted ray are not all equal to 35.26°, the cube is “face-more-parallel” or “edge-more-parallel” depending upon whether the face angle with respect to the principal refracted ray that is most different from 35.26° is respectively greater or less than 35.26°. In the case of sheeting or other retroreflectors for which the principal refracted ray is nominally perpendicular to the front surface of the retroreflector, then for face-more-parallel cubes the selected cube face will also be more parallel to the reflector front surface than will any face of an uncanted cube. 
     An exemplary position detection system  100  in accordance with aspects of the present invention is illustrated in  FIG. 1 .  FIG. 1  illustrates a plan view of a display  102  (e.g., a computer display, a touch screen display, etc.) having a screen area or viewing field  104  surrounded by a raised frame or border  106 . While shown in the context of a computer display, the position detection system  100  may be used in any type of optical position detection system. The inner surface of the border  106 , which is generally substantially perpendicular to the viewing field  104  of the display screen  102  is provided with a prismatic film (also referred to herein as retroreflective film  108 ). The prismatic film  108 , which is discussed in detail below, provides a retroreflective surface around at least a portion of the viewing field  104  (also referred to herein as a detection field). That is, the prismatic film  108  provides a surface that reflects radiation from an originating radiation source back toward the originating source. 
     The composition of the prismatic film  108  may be applied directly to the frame  106  through use of an adhesive or other attachment means, or it may be manufactured first in the form of an adhesive tape, which is subsequently applied to the inner surface of the border  106 . It is desirable to align the prismatic film in such a manner that a plane of maximum entrance angularity associated with the prismatic film is substantially parallel to the viewing field, the detection field and/or the display to optimize possible detection of an object in the area of interest. As discussed more fully below, the prismatic film  108  comprises a retroreflective film having multiple layers, wherein one of the layers includes a plurality of triangular cube corner retroreflective elements that reflect the incoming radiation. In an alternate embodiment, the film may include only a single layer that includes a plurality of triangular cube corner retroreflective elements. The triangular cube corners may have a negative cant ranging from between −10° and −6°. 
     The embodiments of the present invention described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of the present invention. 
     The position detection system  100  shown in  FIG. 1  further includes two sources of dual radiation  110 ,  112 , each of which includes two spaced-apart point sources (or substantially point sources)  111   a ,  111   b  and  113   a ,  113   b . The first source of dual radiation  110  may be disposed at one corner of the viewing field  104  and the second source  112  may be disposed at another corner of the viewing field  104 . In a preferred embodiment, the first source and second sources of dual radiation  110 ,  112  are mounted along a same side  114 . As shown in  FIG. 1 , side  114  may not be provided with the prismatic film  108 , which is provided on the other three sides of the display  102 . One of ordinary skill in the art will readily appreciate that the precise location of the dual radiation sources  110 ,  112  may vary depending a variety of design considerations, including environment, application, etc. Likewise, one of ordinary skill in the art will appreciate that the entire perimeter of the viewing field may be surrounded by prismatic film  108 . 
     The sources of dual radiation  110 ,  112  together illuminate the entire viewing field  104  with radiation, which extends in a direction parallel to the plane of the viewing field  104 . The sources of dual radiation may provide any desirable spectrum of electromagnetic radiation. That is, the sources of radiation may be selected to operate in any desired frequency range or at any desired wavelength. For example, the sources may be a source of infrared radiation, radio frequency radiation, visible light radiation, light emitting diode (LED), laser, IR emitter, etc. In a preferred embodiment, the point sources  111   a ,  111   b  and  113   a ,  113   b  of the dual radiation sources  110 ,  112  are infrared light emitting diodes. 
     The prismatic film  108  provided around the perimeter of the viewing field reflects the infrared radiation back toward the respective originating sources as is indicated by the arrows within the viewing field. Thus, for example, the dual rays of infrared radiation originating from the point sources  111   a ,  111   b  of source  110  extend outward to the side of the display screen and are reflected 180° to return to the source  110 , as shown in  FIG. 1 . Electromagnetic radiation is reflected backward toward its source by the prismatic film  108 . One or more of the layers that overlie the triangular cube corner retroreflective elements of the prismatic film  108  function to allow most of the infrared radiation through (e.g., a double-pass transmission of about 74% to about 100%) and substantially block visible light, which gives the film an appearance of darkness. These aspects of the invention will be further discussed below. 
     The point sources  111   a ,  111   b  and  113   a ,  113   b  of the dual radiation sources  110 ,  112  are symmetrically disposed along a horizontal axis H 1  on either side of lenses  115  and  116 , respectively (as is shown in  FIG. 2 ), for reasons which will become apparent hereinafter. The axis H 1  is coplanar with the longitudinal axis H 2  of the prismatic film  108 . Lenses  115  and  116  are further disposed in front of cameras  117 ,  118 , respectively. The lenses  115  and  116  focus the retroreflected radiation onto an image capturing device of the cameras  117  and  118 . The point sources  111   a ,  111   b  and  113   a ,  113   b  are positioned a distance x away from the lenses  115 ,  116  as indicated in  FIG. 2 . In the preferred embodiment, the distance x is preferably between about 1 and 6 millimeters for touch screens whose longest side ranges from between about 17 to 30 inches for reasons discussed in detail hereinafter. More preferably, the distance x is between about 2 and 4 millimeters for touch screens of such sizes. The cameras  117 ,  118  may be line-scan cameras and/or area-scan cameras. The image capturing device of the cameras  117 ,  118  may include a charge coupled device (CCD) sensor that is formed by an array of photosensitive elements (e.g., pixels). A line scan camera generally acquires an image on a single line of pixels. An area scan camera, like a conventional camera, includes a CCD sensor (usually rectangular in form) of pixels that generates two dimensional frames corresponding to length and width of the acquired image. Reflected radiation passes through corresponding lenses (e.g., lens  115  or lens  116 , depending on the location of the radiation source) and forms an image of an object detected by CCD sensor. The CCD sensor converts the detected radiation on a photo diode array to an electrical signal and outputs the measured amount. One single scanning line of a line scan camera may generally be considered as a one-dimensional mapping of the brightness related to every single point of an observed line. A linear scanning generates a line, showing on the Y axis the brightness of each point given in grey levels (e.g., from 0 to 255 levels for an 8-bit CCD sensor or from 0 to 1023 for a 10-bit CCD sensor). The outputs from the cameras  117 ,  118  can be processed by a control unit  119 . The control unit  119  includes a digital processor that processes the output signals received from the cameras into signals indicative of the X and Y coordinate position of the object  109  via a parallax algorithm. One of ordinary skill in the art will readily appreciate that a scan taken from an area camera will generate a two-dimensional mapping of the brightness related to every point of the observed area. 
     The operation of the position detecting system  100  is best understood with reference to  FIGS. 1 and 2 , which will be explained first with reference to dual radiation source  110 . Dual radiation source  110  emits two beams of infrared radiation from its pair of point sources  111   a ,  111   b . The viewing field  104  has a prismatic film  108  along three sides of the frame  106 , wherein both the point sources  111   a ,  111   b  are directed. The two different beams of infrared radiation generated by the point sources  111   a ,  111   b  strike an object  109  interposed within the viewing field  104  at different angles due to their 2× displacement from one another, creating two shadows of the object  109  located on either side of the object  109 . The two beams of infrared radiation striking the prismatic film  108  are reflected back to the line scan camera  117 . The infrared radiation passes through the lens  115  to the line scan camera  117 , which in turn focuses an image on the CCD of the camera that includes an image of the interposing object  109  and the shadows flanking it. 
     The amount separation between the object  109  and the flanking shadows along the horizontal axis H 1  is linearly proportional to the distance between the object  109  and the point light sources  111   a ,  111   b , being largest when the object  109  is closest to the point light sources  111   a ,  111   b  and smallest when the object  109  is farthest away. The amount of distance separation of the flanking shadows is also proportional to the distance 2× between the point light sources  111   a ,  111   b . The distance between the object  109  and the front of the lens  115  can be accurately computed if the distance 2× is known via a parallax algorithm from the amount of observed separation between the object  109  and its flanking shadows. The proportional line scan camera generates a corresponding line image corresponding to the image along the longitudinal axis H 2  of the prismatic film  108 , having a digitized brightness value that depends on the resolution of line scan camera for the various points along the line of the scanner. For any position in the line image that does not receive radiation a logical value 0 is generated. For example, if an opaque object  109 , such as a stylus or a human finger, enters the viewing field, a shadow is cast on the lens and the corresponding line scan camera, which results in very little or no charge being detected by the line scan camera for that particular pixel or area of pixels. In locations where radiation is detected, the radiation discharges a corresponding CCD sensor associated with the line scan camera, which generates a substantially higher signal value depending on the resolution of the line scan camera. The combination of the image of the object  109  and its flanking shadows generates a dip or trough in the image signal generated by the camera  117  (or even three discrete dips or troughs) whose breadth along the axis H 2  can be converted into a distance between the object  109  and the point radiation sources  111   a ,  111   b  by the digital processor of the control circuit  119  via a parallax algorithm (or a look-up table generated by such an algorithm). Additionally, the angle θ 1  can be determined by the digital processor of the control circuit  119  from the location of the midpoint of the dip (or group of dips) along the horizontal axis of the CCD of the camera  117 . Hence the location of a single interposing object  109  can be completely determined by a single camera  117  in combination with the dual radiation source  110  and the digital processor of the control circuit  119 . While the determination may initially be in polar coordinates, conversion to Cartesian X, Y coordinates is easily implemented. Moreover, if a combination of two dual radiation sources  110 ,  112  and line scan cameras  117 ,  118  are provided as are illustrated in  FIG. 1  and operated simultaneously, then the X and Y position of two simultaneously interposing objects may be unambiguously determined, as only one combination of a camera and dual light source is necessary to determine the X, Y coordinates of a single interposing object. 
     The prismatic film (also referred to herein as the retroreflective film)  108  will now be discussed. Referring to  FIG. 3 , an exemplary prismatic film  108  in accordance with aspects of the present invention is illustrated in cross-sectional view. The prismatic film  108  includes a first substrate  120  having a first surface  122  and a second surface  124 . The first surface  122  (also referred to as the front surface) of the prismatic film  108  is generally flat (and typically smooth). The second surface  124  is also generally flat and is secured to a second substrate  126 . 
     The second substrate  126  has a first surface  128  and a second surface  130 . As shown in  FIG. 3 , the first surface  128  of the second substrate  126  is generally flat (and typically smooth) and generally confronts the second surface  124  of the first substrate  120 . The second surface  130  of the second substrate  126  is also generally flat and is secured to a retroreflective substrate  132 . 
     The first and second substrates  120 ,  126  can be comprised of a material, such as a polymer that has a high modulus of elasticity. The polymer may be selected from a wide variety of polymers, including, but not limited to, polycarbonates, polyesters, polystyrenes, polyarylates, styrene-acrylonitrile copolymers, urethane, acrylic acid esters, cellulose esters, ethylenically unsaturated nitrites, hard epoxy acrylates, acrylics and the like, acrylic and polycarbonate polymers being preferred. Preferably, the first and second substrates are colored and/or have a dye distributed uniformly throughout the first and second substrates. In one embodiment, the first substrate  120  has a red dye distributed throughout and the second substrate  126  has a blue dye distributed throughout. In another embodiment, the first substrate  120  has blue dye distributed throughout and the second substrate  126  has a red dye distributed throughout. Both first and second substrates  120 ,  126  have dye distributed uniformly throughout. One of ordinary skill in the art will readily appreciate that aspects of the present invention include using any desirable color or combination of colors to obtain the desired functionality, aesthetic appearance, etc., discussed herein. For example, the substrates  120 ,  126  may have different colored dyes distributed throughout. See for example US published applications 20030203211 and 20030203212 (assigned to the same assignee as the present application) which are hereby incorporated by reference herein as is necessary for a complete understanding of the present invention. 
     The substrates are preferably chosen to be highly transparent in infrared wavelengths and non-transparent in visible light wavelengths, which will provide a substantially black appearance. The bright background provided by the film is preferably made to be as bright and uniform, as reasonably possible, to allow detection of an object  109  within the field of the prismatic film  108  (e.g., the viewing field  104 ). 
     The retroreflective substrate  132  has a first surface  134  and a second surface  136 . As shown in  FIG. 3 , first surface  134  is generally flat (and typically smooth) and generally confronts the second surface  130  of the second substrate  126 . The second surface  136  includes or otherwise defines a plurality of cube corner retroreflective elements  140  and may be confronted with an adhesive  143  for use in an application. The retroreflective substrate  132 , including the cube corner elements  140  formed therein, can be comprised of a transparent plastic material, such as a polymer that has a high modulus of elasticity. The polymer may be selected from a wide variety of polymers, including, but not limited to, polycarbonates, polyesters, polystyrenes, polyarylates, styrene-acrylonitrile copolymers, urethane, acrylic acid esters, cellulose esters, ethylenically unsaturated nitrites, hard epoxy acrylates, acrylics and the like, with acrylic and polycarbonate polymers being preferred. 
     The prismatic film of  FIG. 3   a , provides a signal layer film as opposed to the multiple layer film as provided in  FIGS. 3 and 4 . For convenience, similar reference numerals are used in the description of the  FIG. 3   a  embodiment. An exemplary prismatic film  108  in accordance with aspects of the present invention is illustrated in cross-sectional view. The retroreflective substrate  132  has a first surface  134  and a second surface  136 . As shown in  FIG. 3   a , first surface  134  is generally flat (and typically smooth). The second surface  136  includes or otherwise defines a plurality of cube corner retroreflective elements  140  and may be confronted with an adhesive  143  for use in an application. The retroreflective substrate  132 , including the cube corner elements  140  formed therein, can be comprised of a transparent plastic material, such as a polymer that has a high modulus of elasticity. 
     In another embodiment illustrated in  FIG. 4 , the first and second substrates  120 ,  126  may be replaced by a single substrate  150 . The substrate  150  has a single dye layer film to absorb the visible light with a front surface  152  and an opposing back surface  154 . The back surface  154  confronts the retroreflective substrate  132 , as discussed above with respect to the second substrate. The front surface  152  is generally smooth. In one embodiment, the substrate  150  is colored black. Benefits associated with a single dye layer are to make the overall film structure thinner and increase uniformity of transmission through the single dye layer  150 . 
     In one preferred embodiment, the retroreflective substrate  132 , including the cube corner elements formed therein, is made of acrylic, e.g., an acrylic material having an index of refraction of about 1.49. Of course, other suitable materials having a higher or lower index of refraction can be employed without departing from the scope of the present invention. The cube corner elements can be formed within or as an integral part of the substrate using, for example, any of the methods described in U.S. Pat. No. 6,015,214 (RE 40,700) and U.S. Pat. No. 6,767,102 (RE 40,455) (assigned to the same assignee as the present application) which are hereby incorporated by reference herein as is necessary for a complete understanding of the present invention. 
     As is described more fully below, the refractive index of the substrate, the size and canting of the cube corner elements may be selected to provide a desired retroreflectivity and uniformity. While the present invention is being described with respect to cube corner elements that are formed integrally as part of the substrate, it is to be appreciated that the present invention is applicable to cube corner elements that are formed separately (e.g., by casting or molding) from the substrate and bonded to the substrate. 
     The plurality of cube corner elements  140  are metallized  142  with a suitable metal, such as aluminum, silver, nickel, gold or the like. Such metallization can be provided by depositing (e.g., sputtering or vacuum depositing) a metal film over the surfaces of the cube corner elements. The metallized cube corner side of the substrate can be coated with or otherwise embedded in an adhesive  143  (forming, for example a product similar to conspicuity tape). The metallization of the cube corner elements allows the display to be cleaned and otherwise not susceptible to contaminants and/or moisture that may have deleterious effects on the retroreflectivity of the retroreflective film  108 . U.S. Pat. No. 7,445,347 (assigned to the same assignee as the present application) is hereby incorporated by reference herein as is necessary for a complete understanding of the present invention. 
     With reference now to  FIGS. 5-8  and continued reference to  FIG. 3 , the retroreflective film  108  includes a plurality of individual cube corner elements  140  ( FIG. 3 ) that are arranged in or otherwise formed as an array  200  ( FIG. 5 ). Each cube corner element  140  is formed by three substantially, but not completely perpendicular faces  202  that meet at an apex  204 . The faces intersect one another at dihedral edges  206 . The angles at the dihedral edges  206 , between the mutually intersecting faces  202  are commonly referred to as dihedral angles. In a geometrically perfect cube corner element, each of the three dihedral angles is exactly 90°. However, in the present invention, a specific pattern of errors is deliberately incorporated into two of the three dihedral angles in order to enhance the brightness of detected radiation retroreflected along the longitudinal axis of the prismatic film  108 , as will be described in detail hereinafter. 
     As depicted in  FIG. 6 , each cube corner element  140  has a triangular cube shape with three base edges  210 . In the present embodiment, each cube corner element  140  has an isosceles triangle cube shape, where two of the base edges (e.g., base edges having lengths a and b) are approximately the same length. Alternatively, one or more of the cube corner elements  140  can have a non-isosceles triangle cube shape. Because base edges  210  of cube corner element  140  are linear and in a common plane, an array of such is defined by intersecting sets of grooves. As shown in  FIG. 5 , each cube corner element  140  is defined by three V-shaped grooves  212 ,  214 ,  216 , which are each one member of three sets of grooves that cross the array  200  in an intersecting pattern to form matched pairs of cube corner elements. Normally all three sets of grooves are cut to the same depth (see, e.g., grooves  141  in  FIG. 4 ), but one or more sets of grooves may be offset vertically (i.e., cut shallow or deep with respect to the others). Also, one of the groove sets can be offset horizontally, causing the cube shape to differ from a triangle. Such cubes are still considered triangular cube corners and are within the scope of this invention. In the embodiment illustrated in  FIG. 6 , faces adjacent sides a and b have a half groove angle of about 38.5 degrees (e.g., 38.5211 degrees), while the face adjacent side c has a half groove angle of about 28.3 degrees (e.g., 28.2639 degrees). 
     The array  200  may be replicated several times over, for example in approximately square tiles of a desired size. In the preferred embodiment, such tiles provided in a linear arrangement as illustrated in  FIG. 7  whose longitudinal axis corresponds to the longitudinal axis of the strip of film  108  disposed around the border  106  of the position detection system  100  shown in  FIG. 1 . Sheeting with one tile or multiple tiles all having the same cube corner orientation is referred to as unpinned sheeting. 
     In prismatic films, a cube corner element is generally used with at least one other cube corner element as part of a matched pair and commonly is used with an array of such elements. Such an array is shown in  FIGS. 5-7 , and such a matched pair is shown in cross-section in  FIG. 8 . The cube corner elements illustrated in  FIGS. 6 and 8  and repeated in the arrays of  FIGS. 5 and 7  are preferably canted in the edge-more-parallel direction between about 8° and 24°, and are more preferably canted in the edge-more-parallel direction between about 12° and 20°. In a further exemplary embodiment the triangular cube corners elements are canted between −10° and −6° and in another embodiment between −15° and −6°. In the foregoing exemplary embodiments, each cube corner element is canted in the edge-more-parallel direction 15.5°. Additionally, each cube corner element preferably has a cube depth of between about 0.006 and 0.0055 and more preferably 0.002 and 0.0045 inches. In this exemplary embodiment, each cube corner element has a cube depth of 0.00325 inches. 
     As discussed above, one aspect of the present invention is directed to providing a retroreflective film that has a high brightness value. Accordingly, highly reflective prismatic sheeting is utilized to achieve this goal. However, the choice of prismatic sheeting potentially compromises the desire for uniformity. The geometry of a typical touch screen display is such that entrance angles range from 0 to 60 degrees. One of ordinary skill in the art will readily appreciate that this is a very large range over which to maintain uniform brightness with prismatic sheeting. Because observation angles also vary, particular care should be made in the selection of the cube geometry and size to achieve a combination of high brightness and good uniformity. 
     For prismatic sheeting applications, triangular cube corner prisms are most commonly used, because they can be directly machined into a substrate using conventional ruling or diamond turning techniques. An algorithm has been developed to simulate the signal brightness and uniformity as a function of geometry and size for isosceles triangular cube corners cut with equal groove depths. For these cube corners, the geometry and size are fully determined by two parameters: cube cant, and cube depth. One of ordinary skill in the art will readily appreciate that other types of triangular cube corners are possible, including for example, scalene triangles and bi-level or tri-level cutting of the groove sets. In these cases, it is not the cube cant/cube depth combination per se that determines signal brightness and uniformity, but rather the active aperture size for each direction of incident light. 
     The applicant has discovered that the brightness of the image of an interposing member  109  and the flanking shadows generated by the dual radiation sources  110 ,  112  can be enhanced if errors e 1 , e 2 , and e 3  of a particular pattern are deliberately incorporated into the normally 90° dihedral angles between the faces of the cube corners. 
     The pattern of errors e 1 , e 2 , and e 3  that forms part of this invention is best understood with respect to  FIG. 9 , which illustrates a single cube corner element having three triangular faces  202   a ,  202   b  and  202   c . As explained with respect to  FIG. 6 , these faces  202   a ,  202   b  and  202   c  intersect to form three substantially dihedral edges  206  and three substantially dihedral angles e 1 , e 2 , and e 3  which represent the angles between faces  202   b ,  202   c ;  202   a ,  202   c  and  202   a ,  202   b , respectively. In the cube corners of the invention, the dihedral angles each include a pattern of errors or departures e 1 , e 2 , and e 3  from the ideal 90° value such that e 1 ≈e 2 , and e 3 ≈0. Preferably, |e 1 | and |e 2 | are &gt;0.033° (or 2 minutes) and |e 3 |&lt;0.033° (or 2 minutes). More preferably, |e 1 | and |e 2 | are between about 0.035° and 0.10°, and between about 0.03° and 0.20° and |e 3 | is about 0°. In the preferred embodiment, |e 1 | and |e 2 | are both 0.063° (or 3.8 minutes) while |e 3 | is 0°. In a still further preferred embodiment, e 1  and e 2 ≈0; (e 1 +e 2 )/2&gt;0.03° and still more preferably (e 1 +e 2 )/2&gt;0.05°. In a further embodiment, e 3 &lt;0.03° and more preferably, where e 3 &lt;0.015°. In a further embodiment, |e 1 −e 2 &lt;0.06° and more preferably |e 1 −e 2 |&lt;0.03°. 
     One way such an error set may be achieved is by cutting the vee-grooves that form the opposing faces  202   c  of adjacent cube corner elements at an angle that has the effect of either increasing or decreasing the dihedral angles by 0.063°. These vee-grooves correspond to the horizontal vee-grooves of the cube corner array illustrated in  FIGS. 5 and 7 . However, such a technique would provide all of the cube corners with an error pattern having the same sign, i.e. e 1  and e 2  would all be either positive or negative, and the applicant has observed that the inclusion of both positive and negative assets of errors would advantageously reduce the sensitivity of film performance to dihedral angle variations that may arise during the manufacturing process. One method of achieving both positive and negative sets of dihedral angle errors is as follows. The cutter used to cut the horizontal grooves along the short sides of the triangles illustrated in  FIGS. 5 and 7  is tilted in one direction. This causes an increase in e 1  and e 2  in the cube corner elements on one side of the cutter, and a corresponding decrease in e 1  and e 2  in the cube corner elements on the other side of the cutter. This tilted cutter is used to cut every other groove. Then the substrate is rotated 180° and the missing grooves are cut. This provides the resulting cube corner array with alternating rows of cube corners where e 1  and e 2  are +0.063° and −0.063°, respectively. 
     A comparison of the spot diagrams illustrated in  FIGS. 10A-10H  with those of  FIGS. 11A-11D  illustrate that an array of cube corners canted 0°, 16° and −8° edge-more-parallel and having the error pattern e 1 , e 2 , and e 3  of the invention advantageously contains the spread of light as much as possible within the plane of the touch-screen. When a cube corner is exposed to a point source of light, each of the three faces of the cube corner generates two retroreflected spots as a result of the fact that some of the light reflected off each face is in turn reflected off each of the other two cube faces.  FIGS. 10A-10H  are spot diagrams of conventional cube corners canted at 0° and 60° for entrance angles (beta) of 0° and 60°, wherein all of the dihedral angles are 90° (i.e. e 1 =e 2 =e 3 =0°).  FIGS. 10A-10H  illustrate that, for all combinations of cant and entrance angle, all six of the retroreflected spots of the cube corner surfaces are accurately retroreflected 180° back to the point source of light such that they all converge onto the same x, y coordinates. By contrast, as is illustrated by  FIGS. 11A-11D , when a pattern of dihedral angle errors e 1 =6 min. e 2 =6 min. and e 3 =0 is introduced into the cube corners, the three faces of the cube corners do not retroreflect the six spots exactly 180° with respect to the point source, but instead retroreflect four of the six spots at divergent points (roughly 0.4°) along the x-axis. The remaining two spots are more compressed toward the x axis when the cube cant=16°. As the x-axis corresponds to the plane of the touch screen,  FIGS. 11A-11D  illustrate that cube corners incorporating the cant and dihedral angle error pattern of the invention advantageously contain the spread of light as much as possible within the plane of the touch-screen. 
       FIGS. 12A-12F  and  13 A- 13 F represent the anticipated retroreflected patterns of light when diffraction is taken into account.  FIGS. 12A ,  12 C,  12 E,  13 A,  13 C,  13 E are light pattern diagrams for conventional cube corners canted at 0° and 60° for entrance angles (beta) of 0° and 60°, wherein all of the dihedral angles are 90° (i.e. e 1 =e 2 =e 3 =0°).  FIGS. 12B ,  12 D,  12 F,  13 B,  13 D,  13 F illustrate the differences in the light pattern diagrams of such cube corners when a pattern of dihedral angle errors e 1 =6 min. e 2 =6 min. and e 3 =0 is introduced into them. In general, the retroreflected light is more concentrated along the x-axis, as is best seen with respect to  FIGS. 13B ,  13 D,  13 F. A comparison of these diagrams confirms the conclusions reached with respect to  FIGS. 10A-11D  and  FIGS. 11A-11D , i.e. that cube corners incorporating the cant and dihedral angle error pattern of the invention advantageously contain the spread of light as much as possible within the plane of the touch-screen. 
     As provided in  FIGS. 13B ,  13 D,  13 F the unpinned prismatic film includes a plurality of triangular cube corner retroreflective elements in which the light source when reflected produces a pattern of light having a horizontal spread greater than a vertical spread. The horizontal spread is at least 1.5 times greater at entrance angles of 0° and 60° and the total light return at 60° is at least 10% of light return at 0° and in certain cases greater than 30%. 
     Finally,  FIG. 14  is an exemplary plot of camera signal for different screen sizes having a 16/9 aspect ratio. As is evident in the graph, for screen sizes of 17 inches, 19 inches, 22 inches, 26 inches and 30 inches, the minimum signal strength at never falls below about 2.0 while the maximum signal strength may be as high as 30.0 over an observation angle of 90°. Hence the retroreflective material of the invention provides sufficient retroreflection over a 90° angle to generate an easily detectible signal in the cameras used in the preferred embodiment. 
     It will thus be seen according to the present invention a highly advantageous prismatic film for use with touch screen and position sensing systems has been provided. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it will be apparent to those of ordinary skill in the art that the invention is not to be limited to the disclosed embodiment, that many modifications and equivalent arrangements may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent structures and products. 
     Publications, patents and patent applications are referred to throughout this disclosure. All references cited herein are hereby incorporated by reference.