Patent Publication Number: US-9836164-B2

Title: Optical digitizer system with position-unique photoluminescent indicia

Description:
BACKGROUND 
     Users are increasingly demanding functionalities beyond merely recognizing a touch to the surface of the touch-sensitive device. Such other functionalities include handwriting recognition and direct note taking (using, for example, a stylus). Such functionalities are generally provided in so-called digitizing systems. 
     Digitizing systems having position-dependent indicia detected by an image sensor in a stylus are commercially available. Anoto Group AB sells a stylus that detects indicia printed on an opaque paper or cardboard substrate. Reference is made to US Patent Publication No. 2010/0001962 (Doray), which describes a multi-touch display system that includes a touch panel having a location pattern included thereon. 
     SUMMARY 
     Photoluminescent indicia disposed on a substrate uniquely are patterned such that they uniquely identify local areas of the substrate. Some parts of the indicia pattern have features that fluoresce in different wavelengths, a feature that may be utilized to uniquely identify local areas of the substrate. 
     In one embodiment a digitizer system is disclosed, the digitizer system comprising a substrate with position-unique photoluminescent indicia comprising a pattern of features that emit a plurality of wavelength combinations, the wavelength combinations uniquely defining local regions of the substrate; a stylus comprising an optical image sensor configured to sense the photoluminescent indicia; and, a processor configured to receive signals from the optical image sensor and provide location-specific positional signals based on the received signals. 
     In another embodiment, a substrate is described, the substrate having position-unique photoluminescent indicia, the indicia comprising a pattern of features that fluoresce in a plurality of wavelength combinations, the wavelength combinations uniquely defining local regions of the substrate. 
     These and other embodiments are described further herein. 
    
    
     
       BRIEF SUMMARY OF DRAWINGS 
       Embodiments described herein may be more completely understood in consideration of the following detailed description in connection with the accompanying drawings, in which: 
         FIG. 1 a    shows a digitizer and display system. 
         FIG. 1 b    shows a further embodiment of a digitizer and display system. 
         FIG. 1 c    shows a further embodiment of a digitizer and display system. 
         FIG. 1 d    shows an embodiment of a digitizer absent a display system. 
         FIG. 2 a    shows a magnified drawing of a prior art position-unique indicia pattern. 
         FIG. 2 b    shows a magnified drawing of a 6×6 position-unique indicia pattern with a plurality of wavelength combinations. 
         FIG. 2 c    shows a magnified drawing of a 5×5 position-unique indicia pattern with a plurality of wavelength combinations. 
         FIG. 2 d    shows a filter that could be used conjunction with a position-unique indicia pattern with a plurality of wavelength combinations. 
         FIG. 3 a    shows a cross section of the end portion of a stylus. 
         FIG. 3 b    shows a cross section of the end portion of dual-source stylus. 
         FIG. 3 c    shows a cross section of the end portion of dual-source stylus with a coaxial optical path and light guide. 
         FIG. 3 d    shows a cross section of the end portion of dual-source stylus with a coaxial optical path, dichroic mirror, and two image sensors. 
         FIG. 3 e    shows a cross section of the end portion of a stylus similar the one shown in  FIG. 3D , with two image sensors having filters. 
         FIG. 4 a    shows a magnified cross section view of a portion of a digitizer and display system. 
         FIG. 4 b    shows a magnified cross section view of a portion of a digitizer and display system with a dichroic reflector. 
         FIG. 4 c    shows a magnified cross section view of a portion of a digitizer and display system. 
         FIG. 4 d    shows a magnified cross section view of a portion of a digitizer and display system. 
         FIG. 4 e    shows a magnified cross section view of a portion of a digitizer and display system with a minimal overlay. 
         FIG. 4 f    shows a magnified cross section view of a portion of a digitizer and display system with stylus operating in reflective mode. 
         FIG. 4 g    shows a magnified cross section view of a portion of a digitizer and display system with indicia printed on the color filter of a display. 
         FIG. 5 a    shows an exemplary product construction. 
         FIG. 5 b    shows an exemplary product construction. 
         FIG. 5 c    shows an exemplary product construction. 
         FIG. 5 d    shows an exemplary product construction. 
         FIG. 5 e    shows an exemplary product construction. 
         FIG. 5 f    shows an exemplary product construction. 
         FIG. 6  shows an indicia-printed overlay on a display with a calibration indicium illuminated on the display. 
     
    
    
     In the figures, like reference numerals designate like elements, unless otherwise described. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     A digitizer system is described herein that includes a substrate having photoluminescent indicia that uniquely define an X-Y area of the substrate, and a pen or stylus with optical sensor that sense the indicia and determine based on the sensed indicia the position of the stylus relative to the substrate. 
     One pattern of indicia suited to embodiments described herein has been developed by, and is available from, Anoto Company AB, of Sweden. Anoto provides companies the ability to print small, opaque dots on paper in a pattern that uniquely defines each location on the paper. A stylus, that they also provide, is then used to sense portions of the pattern passing within the stylus field-of-view (FOV). The sensed pattern is then analyzed and the position of the stylus relative to the paper is computed. 
     Such analysis may comprise subjecting the sensed pattern to an image recognition algorithm whereby the sensed image is analyzed with a mathematical function that defines shapes or patterns, and/or with compared against a library of indicia. Within such a library are definitions of a large number of supported indicia and an indicator of the relative location of each indicium relative to other indicia. Identifying one indicium provides an indication of location on the surface of a digitizer. If digitizer locations have been predetermined relative to display coordinates, then indicia can be used to indirectly reference display coordinates. Identification of orientation (for example the rotation) of indicia relative to a stylus may also provide information about the orientation of the stylus. 
     As described further herein, the use of photoluminescent indicia, in some embodiments, allows for the substrate and the indicia to be highly light transmissive to human visible light, even to the point of being nearly transparent, such that it is well suited for use as a transparent overlay to be put on or incorporated into a display. Photoluminescent indicia could also be used on non-light-transmissive substrates, including opaque substrates (such as whiteboards or paper). A multi-wavelength pattern is also described herein. 
     Photoluminescent indicia also, in some embodiments, may improve the signal to noise ratio in a detection system. They may also provide improved handling of specular reflection from the substrate. In some embodiments, radiation from the excitation source can be reduced or eliminated at the detector by the use of an optical filter, thereby improving the detection of the photoluminescent qualities of the indicia. In particular, since photoluminescent indicia, in some embodiments, upon receiving excitation electromagnetic illumination (or radiation—the terms are used interchangeably herein) in a first wavelength range (usually in the form of ultraviolet (UV), visible, or infrared (IR) illumination), the indicia luminesce providing emitted electromagnetic radiation in one or more wavelength ranges different than the excitation range. 
     In some embodiments, signals associated with the excitation radiation source can be filtered out, thereby increasing the signal-to-noise ratio in the light received by the sensor from the luminescent indicia. The material of the substrate itself may also be selected, in some embodiments, to improve signal-to-noise ratios. For example, in a photoluminescent indicia detection system, the substrate with the indicia could be placed on an absorbing material, a diffusing material, or a transparent material in order to minimize excitation radiation returning to the sensor. 
     The material comprising the photoluminescent indicia may be selected based on properties of the substrate, as will be described more fully later. In some embodiments, the material may comprise photoluminescent inks available in the marketplace, or quantum dots (QDs). The photoluminescent material is configured to exhibit photoluminescence; in one embodiment the luminescent emissions provided by the indicia is primarily in the infra-red (IR) light wavelengths, and the “stimulating” excitation illumination is primarily in wavelengths shorter than those associated with IR. There may be some overlap in the light wavelength ranges used to excite the indicia as compared with that provided by the emission of the material, but in some embodiments less overlap is desirable. The photoluminescent material may be transmissive or transparent to visible light, and may be disposed on a substrate that itself is transmissive or transparent to visible light. 
     In some embodiments, the digitizer system may be configured to operate in one of several modes. In one mode, the stylus illuminates a field of view with light in a first illumination wavelength range that excites the photoluminescent indicia, and images are detected by an image detector, or sensor, that is responsive to emission wavelengths in a second indicia wavelength range that is substantially different from the first illumination wavelength range. In a second mode, the stylus provides illumination in a wavelength range, and the stylus detects non-photoluminescent images in the same wavelength range as the stylus illumination. In a third mode, the stylus may detect indicia radiating light in a first indicia wavelength range from a visible light emitting display. In some embodiments, the stylus image sensor and stylus processor may sense two or more wavelength ranges and discriminate between images in each wavelength range. 
       FIG. 1 a    shows stylus digitizer system  100  comprising stylus  120 , display  105 , digitizer panel  115  patterned with photoluminescent indicia, and electronic controller  130  that controls displayed images via link  135 . Controller  130 , comprised of various microprocessors and circuitry, controls displayed images via link  135 , or it may be communicatively coupled to another display-specific controller that controls displayed images. Display controller  130  receives signals from stylus  120  associated with the position of stylus  120  relative to digitizer panel  115 . Controller  130  may also serve as the system processor for a computer system, for example a portable computing system. Though controller  130  is shown as one box in  FIG. 1 , it may exist as discrete electronic sub-systems, for example, a first sub-system to interact with stylus  120 , and a second sub-system to interact with display  105 . Stylus  120  is communicatively coupled with controller  130  by link  124 . Link  124  may be a bundle of thin wires, but more preferably link  124  is a wireless radiofrequency (RF) link, and in such a construction stylus  120  and controller  130  include radios for back-and-forth communication, or, depending on implementation, one way communication. Such radios in one embodiment implement the Bluetooth™ communications protocol, or that defined by IEEE 802.11. 
     Another electronic subsystem could be configured to time the excitation source and the sensing unit (for example, stylus). For example, the excitation source could be pulsed (on/off) and the sensor capture timing set to correspond to the off state of the source. This configuration could be useful in some embodiments for indicia based on phosphorescent material or other photoluminescent material with a suitably long decay time. 
     In some embodiments, pulsed operation of the source/detection system would also mitigate motion induced artifacts in a moving stylus system. If the capture time were sufficiently short, the blurring of the indicia in the captured image would be minimal, possibly allowing for more accurate reading of the indicia. In addition, there may be a reduction in the rate of photoluminescent bleaching as compared to operation of the source in a continuous mode. Pulsed mode operation may also extend the operating time for battery powered stylus devices. 
     Stylus  120  has an optical image sensor that can detect patterns of light within its field of view (FOV). Stylus  120  detects light  5  emitted from photoluminescent indicia disposed on or within digitizer panel  115 . Stylus  120  may provide stimulating, or excitation, illumination in the form of excitation light  3  to illuminate indicia on digitizer panel  115 . In other embodiments, excitation illumination may come from sources other than those housed within the stylus (for example, LCD backlights and ambient light). Excitation light  3  may have a first wavelength range; indicia emitted light  5  (luminescence) has a second wavelength range. The first and second wavelength ranges in one embodiment do not overlap. In another embodiment the first and second wavelength ranges minimally overlap such that most of the excitation illumination energy is at different wavelengths than most of the indicia emitted energy. In another embodiment, the first and second wavelength ranges overlap. In yet another embodiment, emitted light  5  may be emitted from a visible display so no excitation light  3  may be required. In some embodiments, the second wavelength range of light  5  will comprise a plurality of wavelength combinations that may be discriminated from one another by the optical image sensor in stylus  120 . In most cases where indicia are fluorescent or phosphorescent, the first illumination wavelength range will be at shorter wavelengths than the second indicia wavelength range. The breadth of the first and the second wavelength ranges will be based on the nature of the photoluminescent indicia. The first and/or the second wavelength ranges may be beyond those associated with human ocular sensing. 
     Display  105  may be any type display including but not limited to electronically addressable displays such as liquid crystal displays (LCD), active matrix LCD (AMLCD), organic light emitting diode displays (OLED), active matrix organic light emitting diode displays (AMOLED), electrophoretic display, a projection display, plasma displays, or a printed static image. Display  105  is, in some embodiments, optional, as the digitizer may be used in applications where digitizer panel  115  is opaque. 
     Digitizer panel  115  is in one embodiment a transparent substrate, such as glass, polyethylene terephthalate (PET), polyethylene naphthalate (PEN) cellulose triacetate (TAC), or any suitable material. It may be fully light transmissive, partially light transmissive, or opaque. Preferably, it is highly light transmissive so as to allow a person to see the output of display  105 . Digitizer panel  115  may be one layer or may be comprised of multiple layers of various materials. Digitizer panel  115  includes, disposed upon or within it, photoluminescent indicia that uniquely define the substrate or some portion of the substrate, in two dimensions. Digitizer panel  115  may comprise more than one layer. For example, a durable layer may be used on the top surface. Anti-reflective (AR), anti-glare (AG), polarizing, color filtering, light reflecting, or dichroic optical layers may be included. Touch screen electrodes or resistive surfaces may be included, as well as adhesive layers used to laminate various layers of panel  115 . Panel  115  may be rigid or flexible. 
     Anoto Company AB, of Sweden, licenses software that allows companies to print opaque indicia in the form of ink on, for example, paper. Anoto also sells pens that recognize the indicia and thereby determine the X-Y coordinates of the pen relative to the printed paper. 
     Indicia and sensing techniques based on various patterns are further described in U.S. Pat. Nos. 5,051,763; 5,442,147; 5,477,012; 5,852,434; 6,502,756; 6,548,768; 6,570,104; 6,586,688; 6,663,008; 6,666,376; 6,667,695; 6,689,966; 6,722,574; and 7,622,182, each of which is hereby incorporated by reference in its entirety. Anoto is one company that has developed location unique indicia-based digitizer systems; other systems will be known by the skilled artisan, and inventions described herein may be applicable to many of them. 
     For digitizers that operate overlaid upon a display, indicia that produce indicia emitted light outside the visible range may be preferable. Indicia for such applications are preferably formed of any suitable material that provides emitted photoluminescence at wavelengths between 700 and 1000 nm. Such materials are readily available, as are IR filters and optical sensors that operate in this range. For example, Hamamatsu Photonics of Hamamatsu City, Japan, sells several charge-coupled device (CCD) optical sensors sensitive to IR wavelength ranges. For some applications, other wavelength ranges, such as longer IR wavelength ranges, may be preferable. 
     Any suitable photoluminescent material may be used for indicia. In one embodiment, a suitable indicia material comprises photoluminescent inks. Some example photoluminescent inks and dyes are available from QCR Solutions Corp, of Port St Lucie Fla. (see dyes including IRF820A and IRF940A). 
     In another embodiment, photoluminescent quantum dots may be embedded in a carrier material, such as a resin or liquid, to make a dye. Quantum dots, in some embodiments, luminesce when exposed to excitation light provided over a wider range of wavelengths from UV to IR, which is not true for many other luminescent materials. Thus, quantum dots may be particularly suited for, for example, system  102  ( FIG. 1 c    and  FIG. 4 d   ) where a quantum dot luminescent material can absorb energy from a white LCD backlight. Other luminescent materials may require a special backlight for system  102  that emits light in their specific absorption range. 
     There exists a wide variety of commercially available quantum dot options. Quantum dots can be selected as to provide indicia emitted light with a variety of wavelengths from the ultraviolet (for example, quantum dots comprised of ZnSe) through the visible (for example, quantum dots comprised of CdSe), and into the mid IR (&gt;2500 nm) (for example, comprised of PbSe. Quantum dots that provide indicia emitted light in the IR range may also be made of PbS, PbSe, or InAs. Quantum dots made of PbS having diameters from about 2.7 to 4 nm will provide indicia emitted light in the near IR range of wavelengths. Quantum dots with a core of InAs and a shell of higher band gap material, for example ZnSe or PbSe with an inorganic passivation shell of CdSe, may have improved photoluminescence quantum efficiencies. Quantum dots having core of CdTeSe with a ZnS shell are commercially available from Nano Optical Materials of Torrance, Calif. Quantum dots stabilized with a combination of thioglycerol and dithioglycerol have also been shown to improve stability of luminescence wavelength over time. Quantum dots may be purchased commercially from companies including Nano Optical Materials, NOM&#39;s parent company Intelligent Optical Systems (also of Torrance, Calif.), and Evident Technologies of Troy, N.Y. Evident Technology markets a PbS based dye with the name “Snake Eye Red 900” that may be formulated into an ink for printing. Life Technologies, of Grand Island, N.Y., and Nanosys, of Palo Alto, Calif. offer printable quantum dot solutions, for example anti-counterfeit inks containing quantum dots. 
     Indicia may be printed onto any suitable substrate, as mentioned above. Many known printing processes may be used, including flexographic, gravure, ink jet, or micro-contact printing. Photoluminescent material may be dispersed homogeneously into an optically clear resin and deposited on a substrate using methods such as flexographic or gravure printing. Gravure printing may be preferable for printing larger photoluminescent particles such as IRF820A. Flexographic printing may have cost advantages in high volume applications. Inkjet printing may be preferred where smaller volumes of customized patterns of indicia are desired. Inkjet printing of quantum dots is described in the articles: Small, A. C., Johnston, J. H. and Clark, N.,  Inkjet Printing of Water “Soluble” Doped ZnS Quantum Dots , European Journal of Inorganic Chemistry, 2010: pp. 242-247. 
     Printing of indicia in a thin layer is generally preferable in embodiments where indicia are printed on a substrate that overlays a display, because refraction of light through the indicia can be minimized Transparency can also be maximized by thinner layers and in some cases luminescence quenching can be reduced. Photoluminescent quantum dots can be printed in a monolayer using micro-contact printing. A monolayer of quantum dots maximizes transparency to visible and IR wavelengths. In doing so, visible light transmission is maximized along with reflection of indicia-generated IR light ( 5   a ,  6   a  in  FIG. 4 b , 4 d   ). 
     Monolayers of quantum dots may be printed using micro-contact printing methods described in the article  Direct Patterning of CdSe Quantum Dots into Sub -100  nm Structures  (Small, A. C., et. al., European Journal of Inorganic Chemistry (2010): pp. 242-247), and the article  Fabrication and Luminescence of Designer Surface Patterns with  β- Cyclodextrin Functionalized Quantum Dots via Multivalent Supramolecular Coupling  (Dorokhin, et. al., Institute of Materials Research and Engineering, Agency for Science, Technology and Research, ACS Nano (2010), 4 (1), pp. 137-142. The latter of these articles describes two methods of microcontact printing luminescent CdSe/ZnS core-shell quantum dots. In both methods, quantum dots are functionalized by coating with surface ligands of β-cyclodextrin that promote binding, and stabilize the quantum dots in a water-based colloidal suspensions. In both methods, polydimethylsiloxane (PDMS) stamps are used for micro-contact printing onto glass substrates. In one method, a substrate is first microcontact-printed with a pattern of adamantyl terminated dendrimeric material, then a colloidal suspension of functionalized quantum dots are exposed to the printed substrate, and quantum dots bind to the material printed on the substrate. In a second method, functionalized quantum dots are directly microcontact printed onto a dendrimer layer on a glass substrate. 
     Depending on the photoluminescent pattern required, microreplication can be used to make a tool with a negative of the desired pattern. The tool is then pressed and cured against a uniformly coated polymer layer to form consistent dents of the pattern in a polymer cured matrix. These dents can then be filled either directly with the a suitable photoluminescent material blended with a carrier or formulated into an ink using precision roll coating methods such as gravure printing, or, indirectly by using roll coating in combination with doctor-blading to remove excessive solution from areas other than within the dents. 
     Another way of patterning the substrate is to use a direct thermal printing process with a photoluminescent ink so that the ink resides within the well formed in the (typically polymeric) substrate by the thermal printing mechanism. This may have the added advantage of rapid single step digital processing that can provide custom patterns without, in some embodiments, the need for costly tool development. 
     Photoluminescent indicia can be created using either organic or inorganic dyes or pigments depending on the nature of the application. Organic dyes, such as CY7 which is available from Lumiprobe of Hallandale, Fla., provide several benefits such as high luminescence. However, the Stokes shift for these types of dyes is typically &lt;50 nm and the durability and light-fastness are often low, making them unsuitable for some applications. Carbon chains with conjugated bonds or aromatic rings are typical present in organic dyes, and are sometimes associated with nitrogen or sulfur atoms. For example, CY7 consists of a cyclohexane-bridged polymethyne chain. 
     Inorganic dyes, pigments, phosphors or other luminescent materials, such as the earlier mentioned Snake Eye Red provide another solution. The Stokes shifts for these materials can be relatively high due to the large bandwidth of the absorption curves compared to organic dyes. These materials consist of the cations of metal in an array with the non-metallic ions, such as lead-sulfide (PbS) in Snake Eye Red. 
     Turning now to  FIG. 1 b   , a stylus digitizer system  101  is shown, comprising stylus  120 , digitizer panel  115  patterned with photoluminescent indicia, display  105   a , and electronic controller  130  that receives position-related information from stylus  120  and controls displayed images via link  135 . Display  105   a  emits or reflects visible light, and display  105   a  is at least partially transmissive to stylus emitted excitation light  3  and to indicia emitted light  5 . Display  105 A may be a transparent OLED display, or static printed image, or other display type. 
     Turning now to  FIG. 1 c   , a stylus digitizer system  102  is shown, comprising stylus  120 , digitizer panel  115  with photoluminescent indicia, LCD  107 , backlight  108 , and electronic controller  130  that receives position-related information from stylus  120  and controls displayed images via link  145 . Stylus  120  detects patterns of light  5  emitted from indicia on digitizer panel  115 . Indicia emitted light  5  preferably has a different wavelength range than visible light emitted from backlight  108 . Indicia emitted light  5  preferably has a wavelength range including wavelengths sufficiently long such that they penetrate LCD  107  regardless of the On/Off state of the pixels in LCD  107 . For example, 950 nm light penetrates most LCDs regardless of pixel state. Indicia on digitizer panel  115  may be energized by excitation light from backlight  108 , so an indicia exciting light source on stylus  120  may be optional and not required. 
       FIG. 1 d    shows stylus digitizer system  103  comprising stylus  120 , digitizer panel  115  with photoluminescent indicia, and electronic controller  130 . Stylus  120  illuminates indicia on digitizer panel  115  with excitation light  3  and detects indicia emitted light  5 . Digitizer panel  115  may be printed with various graphics, or it may appear blank (as for e.g. a blank sheet of paper), and may be opaque. Stylus  120  may be combined with pen functionality (not shown in  FIG. 1 d   , described with respect to inking tip  52 ,  FIG. 4 ). Digitizer pane  115  may be a whiteboard, for example as used in a classroom, and the marker may be incorporated into the stylus, or the digitizer pane may comprise a screen onto which a display is projected. 
       FIG. 3 a    shows a cross sectional view of a portion of stylus  120 A. Stylus body  41  contains optional light source  34  that may emit excitation light  3  having a first illumination wavelength range. Indicia emitted light  5  having a second indicia wavelength range enters the tip of stylus  120 A and is passed through filter  43 . Filter  43  selectively passes at least some portion of the second wavelength range of indicia emitted light  5  while blocking light of the first wavelength range (that was emitted by light source  34 ). For example indicia may emit 800 nm to 1200 nm light and filter  43  may pass light of indicia wavelengths between 750 nm and 1200 nm while blocking light wavelengths below 750 nm. Lens  48  focuses indicia emitted light  5  to pass through aperture  33  then reflect from mirror  32  onto image sensor  45 . Lens  48  may be made of IR transparent, visible light blocking material so lens  48  can also perform the filtering function of filter  43 . Exemplary lens  48  is shown as a simple convex lens, but other lens configurations may be preferable. In some stylus configurations, lens  48  may be required to focus a wide range of wavelengths onto image sensor  53 . Where this is the case, lens  48  may be an achromatic lens. 
     Image sensor  45  may be any suitable sensor. Sensors based on charge-coupled devices (CCDs) and complementary metal-oxide-semiconductor (CMOS) technologies are used interchangeably in multiple areas of imaging, and may be suitable in this application. In some embodiments, light  5  may have a plurality of indicia wavelength and pattern combinations that uniquely define local regions of a substrate. In these embodiments, image sensor  45  may further include a color filter that passes selected indicia wavelengths of light  5  to some pixels and different wavelengths to other pixels of sensor  45 . 
     Image sensor  45  is connected to stylus processor  44  by conductors on printed circuit board (PCB)  46 . Light source  34  is connected to PCB  46   a  by link  36 . Stylus processor  44  controls image sensor  45  and light source  34  via conductors on PCB  46   a  and by link  36  respectively. In addition, the stylus processor controls collection of image information from image sensor  45  and communicating this to controller  130  via link  124 , (shown in  FIG. 1 a   ). Stylus  120  may also comprise additional components such as switches and a battery, (not shown). Stylus  120  may also comprise a probe  51  extending from the stylus that provides spacing for the optical components, and may activate a switch upon contact with the surface of the digitizer, causing certain stylus electronics to activate. Probe  51  may be made of solid plastic, or metal, and it may contain ink for writing on a surface with stylus  120 . Probe  51  may be optionally retractable. 
     The illumination wavelength range of light source  34  must provide radiation in a range that excites indicia to produce emission (luminescence) at a desired indicia wavelength and brightness. In one example, indicia emitted light luminesces due to stimulation by light in another, shorter wavelength range. In some embodiments it may be preferable that the excitation light and the indicia emitting light be minimally visible to the user. These criteria are met if light source  34  emits UV-A light, for example between 350 nm and 420 nm or if light source  34  emits near IR light, for example between 700 nm and 850 nm. Excitation light may instead, or in addition, be generated by other components, such as backlights in an LCD panel, or other illumination sources. 
     Stylus  120 B ( FIG. 3 b   ) is similar to stylus  120 A, except it has an additional light source  35 . This light source may be beneficially used in stylus embodiments that need to sense both photoluminescent indicia and traditional reflective indicia (the latter commercially available from Anoto). For example, in luminescent mode, processor  44  may select light source  34  to excite indicia with light in the 380 nM wavelength range that then luminesce and emit radiation in an indicia wavelength range centered on 850 nM. Image sensor  45  may be configured to detect images in the indicia wavelength range of 850 nM. In reflecting mode, processor  44  may select source  35  to illuminate indicia that do not luminesce. Source  35  would preferably emit illumination in the indicia wavelength range of 850 nM, so light of this wavelength range will be reflected differently from the indicia relative to their surrounding substrate, and the resulting contrast between indicia-reflected light and substrate-reflected light is detected by image sensor  45 . 
       FIG. 3 c    shows a simplified cross sectional view of a portion of stylus  120 C that has two light sources like stylus  120 A, but with an alternative construction including a common optical path for illumination light  3  and  6 , and indicia-emitted light  5 . Stylus body  41   c  contains light sources  34   c  and  35   c  that emit illumination light  3  light  6  respectively. Light sources  34   c  and  35   c  may have multiple emitters. In the example shown in  FIG. 3 c    axial view, each source comprises two LEDs. Also in the example, light from sources  34   c  and  35   c  is focused by a Fresnel lens  63  in front of each of the four LEDs. Light from sources  34   c  or  35   c  is focused through objective lens  48   c  and emitted from stylus  120 C, where it may illuminate field of view  62 . Light  5  within field of view  62  enters stylus  120 C through lens  48   c  and is focused through an aperture  33  in plate  56   c , then reflected on mirror  32 . From mirror  32 , light  5  passes to color filter  53   c  where light  5  within a selected wavelength range is passed through filter  53   c  to image sensor  45   c . The common optical path through lens  48   c  simplifies alignment of outgoing light  3  or  6  with incoming light  5 , and minimizes the required tip diameter of stylus  120 . 
     Various operating modes of stylus  120  are described elsewhere herein for illuminating indicia with various wavelengths of light and sensing images of photoluminescent or light-reflecting indicia. In a further mode, stylus  120  may detect images of indicia that radiate light in the visible wavelength range. For example, stylus  120  may sense images formed by pixels of a visible light emitting display, for example, as described in U.S. Pat. No. 7,646,377, which is hereby incorporated by reference in its entirety. Sensing visible images from a light emitting display such as an LCD, OLED, or projection display does not generally require illumination by a stylus light source, so sources  34  and  35  may be turned off by processor  44  when the stylus is in visible light sensing mode. Sensing light in the visible wavelength range requires that image sensor  45  be sensitive to visible wavelengths, and that incoming light  5  passes through filters so it reaches image sensor  45 . Any of sensors  45  may be sensitive to visible light. Any of filters  43  or  53 , or similar, may pass visible wavelengths. 
       FIG. 3 d    shows a simplified cross sectional view of a portion of stylus  120 D that is similar to stylus  120 C, but forward-facing light sources  34   c  and  35   c  are replaced by lateral-facing light sources with a light guide  61  that turns light  3  or light  6  by 90 degrees and focuses it toward objective lens  48   d . Light guide  61  may be coated with reflective material on its rear surface and on edges, with the exception of the edge where light from sources  34   d  and  35   d  enter light guide  61 . Fresnel lens facets  62  on light guide  61  may be used to focus light  3  and light  6  to uniformly illuminate the field of view  65 . Light  5  enters the end of stylus  120 D through lens  48   d  and is focused through aperture  33 , then reflected on mirror  32   d . From mirror  32   d , light  5  passes to color filter  53   d  and to image sensor  45   d .  FIG. 3 e    shows a simplified cross sectional view of a portion of stylus  120 E that is similar to stylus  120 D, but the single image sensor  45   d  is replaced by two image sensors  45   e  and  75 , having filters  53   e  and  73 , respectively. Light  5  enters the end of stylus  120 E through lens  48   e  and is focused through aperture  33  in aperture plate  56   d . Light in the IR wavelength range is then reflected on dichroic mirror  69  to filter  53   e  and image sensor  45   e . Filter  53   e  may separate IR light into different wavelength ranges so image sensor  45   e  can discriminate one wavelength range from another, or image sensor  45   e  may detect a monochrome image including a single range of IR wavelengths. Light  5   e  in the visible wavelength range passes through dichroic mirror  69  and visible light filter  73  to image sensor  75 . Filter  73  may separate visible light into different wavelength ranges so image sensor  75  can discriminate one wavelength range from another, or image sensor  75  may detect a monochrome image including the full range of visible wavelengths. Flexprint  77  connects image sensor  75  to PCB  46   c . Processor  44   e  uses image information from image detector  45   e  and  75  to resolve images that may include images in an IR wavelength range and/or images in a visible wavelength range. Dichroic mirror  69  may comprise the same material used for dichroic substrate  148 , described with respect to  FIG. 4 b   , separate image sensors may have advantages including higher potential special resolution, and availability of specific ICs that lack a color filter, but have advantages such as low cost and high imaging frame rate. 
       FIG. 2 a    shows an Anoto type indicium within the solid rectangle, comprising a pattern of opaque dots on a substrate, arranged on a virtual 6×6 matrix indicated by dotted lines. Each intersection of the matrix has one dot, and each dot is position-encoded into one of four positions above, below, left, or right of the intersection. This provides a coding system based on thirty-six digits of base four, so each 4-some combination of the indicia can represent as many as 4 36  unique codes. Permutations of codes are reduced to allow independent encoding of X and Y coordinates in each indicium. Permutations are also reduced by the requirement for redundancy that allows determination of the position of a partial surface, and to detect code sequences from any orientation. Even with such reductions, Anoto indicia can encode extremely large areas with position-unique indicia less than 2 mm in size, where each of the indicia uniquely defines a local region of a substrate. 
     Photoluminescent indicia described herein may use the same pattern of monochrome dots as used by Anoto, or other options may be preferable. For example, visible light transmissive photoluminescent materials may be used to make features of larger size that still have minimal optical visibility. Larger dots or other feature shapes may be easier to print, and material with larger photoluminescent particles may be used, while indicia size is maintained at less than 2 mm square. 
     Indicia features emitting distinguishable wavelengths of light provide additional coding alternatives. The base four coding system described above can be achieved with four wavelength combinations instead of four-quadrant dot placements or four symbol shapes. An example is shown in  FIG. 2 b   , where dots comprising photoluminescent nanoparticles with different emission wavelength ranges are used to uniquely define an indicium  190 , which together with other indicium may be used to uniquely define local areas of a substrate. A first indicia wavelength range may be centered on 850 nM and a second indicia wavelength range may be centered on 950 nM. Indicia may comprise dots, or other shapes, positioned in any of thirty-six positions. Each possible dot position has one of four features: a dot of the first indicia wavelength range  191 , a dot of the second indicia wavelength range  192 , a dot with indicia wavelengths from the first and second ranges  193 , or no dot. Other wavelength combinations, and other feature shapes or combinations of feature shapes may be used. For example, a four-position encoded pattern of dots may also have dots of two wavelength ranges, so each position in a 5×5 indicium has eight possible codes, resulting in more than 4 36  unique codes in a 5×5 matrix. 
       FIG. 2 c    shows an indicium  195  comprising a 5×5 array that comprise dots that luminesce in a plurality of wavelength combinations, the wavelength combinations uniquely defining local regions of the substrate. The 25 possible positions in the array have dots having one of four features: a dot of the first indicia wavelength range  191 , a dot of the second indicia wavelength range  192 , a dot with indicia wavelengths from the first and second ranges  193 , or no dot. The pattern is restricted to no dots at three corners, as shown; all other positions of the top, right, and bottom edges have dots that emit in at least one wavelength. This pattern provides easy recognition of indicia borders and angular orientation. Given feature-to-feature spacing of 0.3 mm, a repeating pattern of unique indicia can provide pattern and wavelength combinations uniquely defining local regions of a substrate area nearly 500,000 square meters. 
       FIG. 2 d    shows details of a portion of exemplary color filter  53  that is configured as an array of filter cells, using known color filter methods. Most color filters have arrays of cells that pass red, green, or blue light to pixels an image sensor. For example many color filters use a Bayer filter (U.S. Pat. No. 3,971,065) that has twice as many green filter cells as red or blue, often in a pattern of R, G, G, B or R, G, B, G. Filter  53  replaces one or more of the R, G, B filter cells with cells that pass light in the IR wavelength range. Filter  53  can pass up to four wavelength ranges to selected pixels of an image sensor, so the image sensor can discriminate among image features having different wavelength ranges, or colors. In one embodiment, filter  53  passes two wavelength ranges of visible light and two wavelength ranges of IR light to image sensor  45 , with coupled electronics that resolves a location based on signals from the image sensor. Such a filter, or one like it, may be advantageously used in conjunction with resolving indicia such as indicium  190  and  195 . 
     In one embodiment described with respect to  FIG. 2 b    and  FIG. 2 c   , four color filter cells in filter  53  comprise cells  112  and  113  that pass IR light centered on 850 nM and 950 nM respectively. And visible light cells  114  and  115  that pass light centered on wavelengths 500 nM and 600 nM respectively. IR cells  112  and  113  preferably pass light with a bandwidth of +/−50 nM. Visible cells  114  and  115  preferably pass light with a bandwidth of +/−100 nM. Other color filter layouts may be used, and other filter wavelength combinations may be used. In some embodiments, it may be sufficient to detect monochrome visible images, so filters  114  and  115  may both detect visible light from 450 nM to 700 nM, for example. In some embodiments, visible image detection may not be required, so all cells of filter  53  may pass one or more IR wavelength ranges. 
     Table 1 summarizes the operating modes of stylus  120 . Modes will be described with respect to components of stylus  120 C, although the modes apply to the other exemplary stylus configurations herein. Table 1 shows examples of components used in various combinations to read images from various media. 
     Stylus Mode 1—Photoluminescent Media 
     To read photoluminescent indicia, light source  35  is turned on and source  34  is off. The indicia image may be filtered through an IR-transparent color filter. For example, filter  43  or cells of a color separating filter such as cells  112  and/or  113  in filter  53 . The image is read from image sensor  45  by processor  44 . 
     Stylus Mode 2—Passive Media 
     To read light absorbing passive indicia on a light diffusing or reflective substrate such as paper, source  34  is turned on and source  35  is off, so light  3  emitted from stylus  120  is at a wavelength that will pass through stylus filters  43  or  53  to stylus image detector  45 . Light  5  reflected from indicia having an image formed by contrast between indicia features and the substrate, is received and filtered through IR-transparent color filter  43  or IR-transparent cells in filter  53 , and the IR image is read from image sensor  45  by processor  44 . 
     Passive indicia on a light transparent substrate may absorb light, or they may diffuse or reflect it. To read light from such media, source  34  is turned on and source  35  off, so light  3  emitted from stylus  120  is at a wavelength that will pass through stylus filters  43  or  53  to stylus image detector  45 . The illuminated indicia image light  5  is received and filtered, and the IR image is read from image sensor  45  by processor  44 . The image received from passive indicia on a transparent substrate may be a reverse image relative to passive media on an opaque substrate, depending on whether the indicia or the background reflect light  5  toward stylus  120 . 
     Stylus Mode 3—Visible Emitting Display 
     To read a light emitting visible image, (for example, indicia displayed on an LCD or OLED), source  35  and source  34  may be turned off. A displayed cursor may be used as a location-specific indicium, or pixels on a display may be tracked to detect movement as described in U.S. Pat. No. 7,646,377. In addition, the IR-measuring pixels of image sensor  45  may be read, to detect any time-varying IR signals that may be emitted from the display. Time-varying IR signals may be encoded to indicate which of several displays are in the field of view of stylus  120 , as described in U.S. patent application Ser. No. 13/454,066, which is hereby incorporated by reference in its entirety. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Stylus Component 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Image 
               
               
                   
                   
                 source 35 
                   
                 Image 
                 sensor 
               
               
                   
                 Image 
                 (excitation 
                 source 34 
                 sensor IR 
                 visible 
               
               
                 Mode 
                 being read 
                 light) 
                 (IR light) 
                 pixels 
                 pixels 
               
               
                   
               
               
                 1 
                 Photolumi- 
                 On 
                 Off 
                 Read 
                   
               
               
                   
                 nescent 
                   
                   
                 image 
               
               
                   
                 Indicia 
               
               
                 2 
                 Passive 
                 Off 
                 On 
                 Read 
               
               
                   
                 Indicia 
                   
                   
                 image 
               
               
                   
                 (reflective) 
               
               
                 3 
                 Visible 
                 Off 
                 Off 
                   
                 Read 
               
               
                   
                 emitting 
                   
                   
                   
                 image 
               
               
                   
                 display 
               
               
                   
               
            
           
         
       
     
     In one embodiment, multi-mode stylus  120  can operate in various modes as listed in Table 1. Manual switching of modes may be done with switches on the stylus or by interaction with an application and GUI on a display. Automatic switching among modes may also be supported, based on input received by stylus  120 . For example, given stylus  120  with filter  53  and image sensor  45  that can sense images in multiple wavelength ranges, and two illumination sources  34  and  35  of different wavelengths, images under different combinations of source illumination may be tested sequentially until a valid image is recognized. The following exemplary algorithm may be used to automatically switch stylus sensing modes:
     1. Stylus Mode 3 is activated, whereby sources  35  and  34  are turned off, then visible images are detected.
       1.1. Detected images are processed to recognize any supported indicia in the stylus FOV. If no supported indicium is detected, go to step 2, otherwise,   1.2. Any detected visible image is correlated with supported indicia or stylus-locator cursors, for example as, for example, described in U.S. Pat. No. 7,646,377.
           1.2.1. If matched, a display-referenced location is calculated and reported.
               1.2.1.1. Repeat from Step 1.1   
               1.2.2. If unmatched, report no stylus-related visible image is in the FOV.   
           
       2. Stylus Mode 1 is activated, whereby source  35  is turned on and source  34  is turned off, then IR images are detected.
       2.1. Any detected IR image is correlated with supported indicia patterns. If a positive correlation is determined, a luminescent digitizer location is calculated and reported.   2.1.1. Repeat Step 2 until no IR image is detected, then go to Step 1   
       3. Stylus Mode 2 is activated, whereby source  35  is turned off and source  34  is turned on, then IR images are detected.
       3.1. Any detected IR image is correlated with supported indicia patterns. If a positive correlation is determined, a passive digitizer location is calculated and reported.
           3.1.1. Repeat Step 3 until no IR image is detected, then Go to Step 1.   
           
       

     Referring now to  FIG. 4A , photoluminescent indicia  119  are deposited on the surface of visible light transmissive substrate  118 . Indicia may be deposited by printing a thin layer of photoluminescent material. For example photoluminescent quantum dots may be mixed with a printable medium, and printed using known methods. Indicia-printed substrate  118  is then laminated to durable overlay  116 , which may be glass, acrylic, or any suitable transparent working surface that will protect the indicia. Alternatively, indicia may be printed on the bottom surface of overlay  116 . 
     Adhesive  117  that binds substrate  118  to overlay  116  may be an optically clear adhesive (OCA) such as those sold be 3M Co. of St Paul, Minn., USA. Adhesive  117  may be replaced by an air gap, (providing indicia  119  have an environmentally protective coating) and air gap  109  may also be replaced with an OCA. 
     Illumination from stylus  120  comprises excitation light  3  and  3   a  which have a first wavelength range (for example, near UV-A or near IR). Light  3   a  that does not get absorbed by indicia  119  passes through substrate  118 , and is absorbed or reflected by layers below. Some of light  3  is absorbed by indicia  119  which excites indicia  119  such to radiate indicia emitted light  5  in various directions, including into the stylus housing which includes optical sensing electronics, as described earlier. Indicia emitted light  5 , in this embodiment, is primarily in a second wavelength range, in this case it is within the IR range. Visible display light  7  and  8  is emitted from display  105 . Some display light  8  that hits indicia  119  will pass through the indicia. Some of the display light  2  may also be absorbed by the indicia  119  and will excite the indicia causing it to luminesce, resulting in light  6 . Ambient light  9  may also cause photoluminescence in indicia  119 , resulting in indicia emitted light  6 . 
     Display  105  is shown as a planar cross section, but display  5  may include other shapes. For example, the display may comprise a rear projector and a rear projection panel. 
       FIG. 4B  shows a digitizer system identical to that in  FIG. 4 a   , except substrate  118  is replaced by dichroic substrate  148  that passes light of some the first wavelength range while reflecting light of the second wavelength range, where wavelengths of the first range are typically shorter than wavelengths of the second. For example, substrate  148  may be visibly transparent IR reflective material such as 3M Company&#39;s multilayer optical films (MOF) which are available commercially under the names Crystalline automotive films and Prestige Series residential window films. Dichroic reflective substrate  148  increases the efficiency of the system by re-directing indicia-generated light of the second wavelength range that would not otherwise have reached stylus  120 . 3M Company&#39;s PR90EX film may be used for the reflective substrate  148 . 
       FIG. 4C  shows a magnified cross section view of a portion of digitizer and display system  102  (see  FIG. 1 c   ). In this embodiment, indicia are excited by light radiation provided by backlight  108 . In other embodiments, however, the stylus could additionally include such a further illumination source. 
     If display  107  is a light emitting display such as a (transparent) OLED, backlight  108  may not be required. Where optional backlight  108  is used (for example, in combination with an LCD display), visible (white) display light  7  and  8  may be emitted from optional display backlight  108 . A portion of light  7  that is not absorbed by Indicia  119 A passes through substrate  118 , optically clear adhesive layer  117 , and through pixels of display  107 , to form a displayed image. Light from backlight  108 , may comprise wavelengths outside the visible spectrum. For example, near infrared (IR) light (700 nm-1000 nm) or near UV light (e.g. 350 nm-400 nm) may be emitted from the backlight to energize indicia on substrate  118 . Some of these wavelengths may be filtered out by layers above the indicia, (for example, the color filter of display  107 , which would be typical if  107  were an LCD), so minimal amounts of these wavelengths of light may reach the user. An LCD display comprising quantum dots energized by a backlight is described in US Patent Application No. 2008/0246388, “Infrared Display with Luminescent Quantum Dots.” 
       FIG. 4D  shows a magnified cross section view of a portion of digitizer and display system  103  (see  FIG. 1 d   ). The stylus, in this embodiment, is similar to that described with respect to  FIG. 4 b   , in that it includes an illumination source that excites the indicia. This embodiment shows the use of photoluminescent position-unique indicia in a whiteboard-type environment. A transparent overlay including photoluminescent indicia  119  and optically clear adhesive layer  117 , as well as substrate  156  (here, substantially light transmissive such that graphics on the surface of substrate  158  are visible through the stack comprising substrate  156  and OCA  117 ). Surface  159  of substrate  156  may be layered with a material, such as Tedlar® (available from DuPont), polypropylene, or other surface compatible with white-board applications, or it could be an anti-scratch, anti-reflective, anti-glare, polarizing, or color filtering layer. Substrate  156  may comprise a plurality of layers. In such embodiment, stylus  120  may include inks compatible with whiteboard applications, such that a user may write on the whiteboard while electronics in the stylus computes coordinates of the writing and provides these to a computer. 
       FIG. 4E  shows the use of photoluminescent indicia on an opaque or translucent substrate. Photoluminescent indicia  119  are deposited on the top surface of substrate  168 , which may be paper, cardboard, PET, PEN, glass, acrylic, or any material that supports indicia  119 . Indicia  119  are deposited by printing a thin layer of photoluminescent material. For example photoluminescent quantum dots may be mixed with a printable medium, and printed using known methods. Indicia-printed substrate  168  may then be covered with optional layer  167  that is transparent to stylus excitation light  3  and indicia emitted light  5 . Layer  167  may optionally be added to protect indicia. Layer  167  may be an anti-scratch layer, a durable layer comprising a polymer hardcoat, or a polymer hardcoat filled with silica particles, or a sheet of material such as PET may be laminated to substrate  168 . Surface  169  may have anti-reflective (AR), and/or anti-glare (AG) properties. Additionally, substrate  168  may be printed with a visible static image. While the pen showed in  FIG. 4 e    includes an illumination source to excite indicia within the stylus FOV, depending on the application, the stylus could sense indicia excited by other means, such as ambient light  9 . In such case, the stylus would not necessarily need an illumination source. 
       FIG. 4F  shows a somewhat different embodiment, where stylus  120  senses passive indicia that operate in a light absorbing or light reflecting mode, for example, as indicated in Table 1, Mode 2. In this embodiment, stylus  120  provides an illumination source (for example, light source  34  in  FIG. 3 c   ) that is reflected by substrate  178 , but not reflected by, or minimally reflected by the features of indicia  179 , for example the dots exemplified in  FIG. 2 a   . Indicia  179  preferably comprise a thin layer of IR-absorbing material; substrate  178  is preferably an IR-reflecting or IR transparent material; and the image sensor in stylus  120  senses the reflected light that has the same wavelength as the illumination source. An image is formed by contrast between reflected light and indicia-absorbed light. Thus this embodiment provides a “negative” image of the indicia, which may be desirable in some embodiments, for example to detect black (IR absorbing) indicia features printed on white paper. Alternatively, indicia features may reflect light of the illumination source  34  and the substrate may absorb stylus illumination, resulting in a “positive” image of indicia-emitted light on a dark background being detected by image sensor  45 . 
       FIG. 4G  shows a magnified cross section view of a portion of digitizer and display system similar to system  100  (see  FIG. 1A ), except the substrate portion of digitizer  115  is eliminated and indicia  119  are printed onto color filter  152  of LCD  105   g . Indicia  119  are in one embodiment optically transparent, and the features of indicia  119  (for example the indicia dot patterns similar to  190  or  195 ) may be aligned with the color cells of color filter  152 . As an alternative construction, indicia  119  may be printed onto the bottom surface of top polarizer layer  153 . Integration into an LCD offers the advantage of thinner construction and potential for better visible light transmission through LCD  105   g.    
     Electronically addressable display  105   g  (e.g., an LED, plasma, etc.) has display coordinates that uniquely identify the location of each pixel. Similarly, digitizer  115  ( FIG. 4A-4D ) includes a digitizer coordinate system wherein each location on the surface is identified by location-unique indicia  119 . Given the display is co-planar with a digitizer, digitizer coordinates can be aligned with display coordinates of such systems using a calibration process that associates each display pixel at a particular localized area with one or more of digitizer indicia  119  associated with that same localized area. After calibration, indicia  119  can be used to indirectly reference display pixel coordinates and vice versa. 
     Calibration may be performed at a manufacturing site, or by the end user of a device. In some “after market” embodiments, a user may assemble the digitizer  115  onto a display  105 . In such case, it will generally be necessary for the user to perform calibration. The calibration system described below allows a user of minimal skill to perform an accurate calibration. Two alternative calibration methods are described below. 
       FIG. 6  shows a view of display  105  and surface  115  with indicia  119 , which are in this drawing represented by position-unique number and letter symbols (used herein only for illustration purposes). In practical systems, indicia will typically comprise position-unique dot or line patterns, for example those described with respect to  FIG. 2 . Each of the indicia  119  are located at known digitizer coordinates on surface  115 . The X,Y coordinates of indicia  119  are designated as iX n , iY m . Coordinates of pixels on display  105  are designated as dX n , dY m . 
     A first calibration embodiment is described with respect to  FIG. 6 . In a calibration mode, display  105  radiates a calibration indicium  10  at a first localized area with a first indicia wavelength range that comprises visible light. In the example described with respect to  FIG. 6 , indicium  10  is a round spot that serves as an indicium for stylus  120  and may also serve as a user-visible cursor. Stylus  120 , or generically a sensor device or a sensing unit, is placed so calibration indicium  10  is in its field-of-view (FOV)  65 . Placement of the stylus may be done by a user aiming stylus  120  at visible indicium  10 . Stylus  120  senses indicium  10 , and stylus  120  also illuminates its FOV with light of a first illumination wavelength range and senses whatever photoluminescent indicia are also present with its FOV, which radiate light within a second indicium wavelength range. In other words, the photoluminescent indicia being sensed are also associated with a common first localized area. In some embodiments, the second wavelength range is not the same as the first. One or more digitizer indicia within the FOV will happen to be co-located with display indicium  10 . The digitizer will determine and note the coordinates of such co-located indicia. The co-located indicia are then associated with the display coordinates of the calibration indicium. These coordinates are stored and used to relate subsequent digitizer measurements to display coordinates. For example, in the example shown in  FIG. 6 , indicia r and s are in the FOV of stylus  120  but only s is co-located with indicium  10  so s is associated with the address of pixels forming indicium  10 . Calibration may continue with indicium  10  placed at a second predetermined location, and the above-described calibration procedure repeated. 
     A second embodiment of the calibration process is also described with respect to  FIG. 6 . In this embodiment stylus  120  does not sense radiation in the first indicium wavelength range emitted by indicium  10 , but only senses indicia-radiated light in a second indicia wavelength range that is emitted from digitizer indicia  119 . Indicia  119   a , upon exposure to excitation radiation, luminesce in a second indicia wavelength range in response to light energy from indicium  10 . 
     Display  105  activates indicium  10  having light in a first indicia wavelength range that may comprise visible light. Stylus  120  is placed so calibration indicium  10  is in its FOV  65 . Light from pattern  10  illuminates indicia  119   a , which is co-located with (i.e., in this case directly above) indicium  10 , and causes them to radiate indicia emitted light (luminescence) in a second indicia wavelength range that is different from the first wavelength range of indicium  10 . Stylus  120  senses indicia  119   a  in its FOV. Ambient light  9  is preferably limited to prevent ambient light  9  from being a source of excitation radiation to indicia  119  that are not illuminated by light from indicium  10 . 
     Display  105  is, in this second calibration embodiment, emitting no light except for the displayed indicium  10  that is centered at display coordinates dX 10 , dY 10 . (which correspond to the center of indicium  10 . Most of photoluminescent indicia  119  are dark because they have no excitation energy, with the exception that indicium  10  is illuminating indicia  119   a , causing it to luminesce. Stylus  120  with indicium  10  in its FOV senses indicia  119   a  at digitizer coordinates iX 10 , iY 10 . Thus the display coordinates dX 10 , dY 10 . are determined to be co-located with indicia coordinates iX 10 , iY 10 . Light sources  34  and  35  in stylus  120  are turned off during this procedure of embodiment 2. Display  105  may emit light in a time-modulated sequence and the stylus may demodulate the resulting time-modulated signal re-radiated from indicia  119   a.    
     If display  105  has higher resolution than indicia  119 , the location of indicium  10  may be adjusted in size until it minimally circumscribes a single indicium. This may increase the accuracy of aligning indicium  10  with digitizer indicium  119   a . Indicium  10  may also be incrementally adjusted in the X and/or Y direction as the illumination level of the excited indicium  119   a  is measured, to determine the exact location of the indicia relative to display  105 . 
     The procedures described in both the first and second calibration embodiments have a benefit that the calibration pattern must be in the stylus FOV, but it need not be in the center of the stylus FOV, so inaccurate placement of the stylus has minimal effect on calibration accuracy. Identification of orientation (for example the rotation) of indicia relative to a stylus may also provide information about the orientation of the stylus relative to a digitizer and a display. 
     Using either calibration method described above, calibration data is computed by way of a processor, and such calibration data may then be outputted to another computing device which may store such calibration data for future reference. For example, a computer may store the calibration data so the calibration routine need not be repeated each time the computer boots. 
     Stylus  120  may have an extendable tip  51  that makes contact with a digitizer surface. In Mode 2, the stylus is used with passive indicia, which may include indicia printed on paper. Where writing on paper may be preferable, an extendable/retractable inking tip  51  may be extended. In other modes, inking may not be desirable and a different (e.g. non-scratching) tip material such as Delryn plastic may be preferred. Stylus may have a plastic tip  71  that extends beyond lens  48 , and an ink-dispensing tip  51  that can be adjusted to extend beyond plastic tip  71 , or to retract so plastic tip  71  is the outermost point beyond lens  48 . 
     The digitizer panel can be integrated into a display stack-up in a variety of ways using a variety of rigid or flexible materials. One embodiment shown in  FIG. 5 a    comprises an overlay where the digitizer panel is attached to a device  205 , in an after-market application using self-wetting adhesive  201 . This panel is constructed by forming the photoluminescent indicia  202 , on one side of substrate  200  (in this case PET), and covering these indicia with said adhesive  201 . To attach the panel, the user may place the adhesive-side of the panel onto the device  205 . To improve the durability of the overlay, a hard-coat may be added to the side of the panel that faces the user. 
     Another embodiment shown in  FIG. 5 b    for an overlay in aftermarket applications may be constructed by first printing the indicia  202  on multilayer optical film (MOF)  203 , which reflects IR. In this construction, the device-facing adhesive  204  may be placed on the opposite side as the indicia since the emitted signals from the photoluminescent dyes do not pass through MOF  203 . To further protect the indicia from the user&#39;s interaction, a protective layer  200 , such as PET, may be adhered to the top of the MOF to cover the indicia, using adhesive  201 . 
     Another embodiment is an underlay where the digitizer panel is attached to a touch-sensitive screen including but not limited to those product offerings from 3M Touch Systems, as a surface-capacitive technology (SCT) screen or a projected-capacitive technology (PCT) screen.  FIG. 5 c    shows such an embodiment, where the photoluminescent indicia  202 , formed on a transparent substrate  200 , such as PET or MOF, covered with an optically-clear adhesive,  201 . This adhesive may then be attached to the back of the touch sensitive screen  205 , using a variety of methods known in the art. Another embodiment as shown in  FIG. 5 d    is to print the photoluminescent indicia  202  directly onto the touch sensitive screen  205 , either at the beginning, middle, or end of an already established process. 
     Another embodiment shown in  FIG. 5 e    is specific to a layered touch sensor, such as a PCT screen, as an underlay solution. In such an embodiment, the photoluminescent indicia  202  are disposed on the same layer  206  as the component or components including the matrix of electrodes, and then the screen may be integrated to as normal into other devices. 
     Yet another option is shown in  FIG. 5 f   , which shows the digitizer panel as an underlay between the cover lens of a touch sensitive screen  205  (in this case a visible light transmissive PCT screen) and the layer  206  that includes the matrix of electrodes, by forming photoluminescent indicia  202  on a visible-light-transparent substrate  200 , which could comprise materials such as PET or MOF, and adhering this to the cover glass using a variety of methods known to the art. The resultant stack can then be adhered to the digitizer panel using methods known in the art (for example, by laminating with optically clear adhesive  201 ). 
     Example 
     This example describes the set-up that may be used to demonstrate photoluminescent indicia that can be viewed at various stylus angles. It consists of an illumination source, a photoluminescent medium, and an image sensor placed behind a suitable filter. The illumination source comprised a light emitting diode (UVXTZ-400-15 supplied by BIVAR, Inc, of Irving, Calif.), powered with 20 mA of current and placed at a distance of about 3 cm from the substrate on which a luminescent dye was printed to form position-unique photoluminescent indicia. The diode had a spectral emission centered at about 400 nm. Light from the diode was incident on the printed fluorescent dye IRF820A and caused photoluminescent emission. The photoluminescent material comprised IRF820A, which was purchased from QCR Solutions Corp. of Port Saint Lucie, Fla. It came in a powder form with a quantum efficiency quoted at 0.2 for fluorescence between 700 nm and 1000 nm. 
     As an initial experiment, the fluorescent particles were dispersed at room temperature into OP2001 Matte Varnish resin at a concentration of 0.5% and manually deposited onto a small sample of MOF using a toothpick. The OP2001 Matte Varnish was purchased from the series for UV Flexo Varnishes of Nazdar Company of Shawnee, Kans. The MOF substrate comprised PR90EX manufactured by 3M Corporation of St. Paul, Minn. This particular MOF was high in transmission over the visible range and was highly reflective beyond 850 nm. An optical filter was placed in the path of the light entering a CCD image sensor that was used as an imaging tool in the stylus. The optical filter was a long pass filter, comprising Clarex NIR-75N, supplied by Astra Products of Baldwin, N.Y. This filter suppressed transmission of light wavelengths below 750 nm. As a result, specular reflection from the diode that obscured some indicia in was largely suppressed from reaching the image sensor, while IR light emitted by the fluorescent dye was detected by the image sensor, resulting in a clear image of position-unique photoluminescent indicia on the substrate. 
     In a further experiment, an illumination source comprised a light emitting diode (L750-04AU supplied by Marubeni America Corporation of Santa Clara Calif.), powered with 20 mA of current and placed at a distance of about 3 cm from the substrate onto which a fluorescent dye was printed to form position-unique photoluminescent indicia. The diode had a spectral emission centered at about 750 nm. Light from the diode was incident on the printed fluorescent dye EviDot Snake Eyes and caused photoluminescent emission. The photoluminescent material comprised EviDot Snake Eyes, which was purchased from Evident Technologies of Troy N.Y. It came in a liquid form of quantum dots in toluene with a quantum efficiency quoted at 0.3 for fluorescence between 400 nm and 1000 nm. As an initial experiment, the fluorescent particles were dispersed at room temperature into Integrity 1100D and manually deposited onto a small sample of PET using screen-printing techniques. The Integrity 1100D was purchased from Hexion Specialty Chemicals of Columbus Ohio by Evident Technologies. The PET substrate comprised ST505 manufactured by Dupont Teijin Films of Chester Va. This particular PET is a clear, heat stabilized polyester film which is pre-treated on both sides for improved adhesion. An optical filter was placed in the path of the light entering a CCD image sensor that was used as an imaging tool in the stylus. The optical filter was a long pass filter, comprising Clarex NIR-85N, supplied by Astra Products of Baldwin, N.Y. This filter suppressed transmission of light wavelengths below 850 nm. As a result, specular reflection from the diode that obscured some indicia in was largely suppressed from reaching the image sensor, while IR light emitted by the fluorescent dye was detected by the image sensor, resulting in a clear image of position-unique photoluminescent indicia on the substrate. These results were achieved at various stylus angles. 
     The term stylus used herein may include a device that may be moved relative to a digitizer surface. The stylus may have a shape similar to a pen, or a computer mouse, or any shape. Multiple styli may be used simultaneously on a digitizer surface as writing devices, cursor control devices, or game pieces, for example. The stylus may be moved manually or by a mechanical device or a machine. The digitizer surface may be planar, cylindrical, spherical, or any shape. 
     Unless otherwise indicated, all numbers expressing quantities, measurement of properties, and so forth used in the specification and claims are to be understood as being modified by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and claims are approximations that can vary depending on the desired properties sought to be obtained by those skilled in the art utilizing the teachings of the present application. Not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, to the extent any numerical values are set forth in specific examples described herein, they are reported as precisely as reasonably possible. Any numerical value, however, may well contain errors associated with testing or measurement limitations. 
     Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the spirit and scope of this invention, and it should be understood that this invention is not limited to the illustrative embodiments set forth herein. For example, the reader should assume that features of one disclosed embodiment can also be applied to all other disclosed embodiments unless otherwise indicated. It should also be understood that all U.S. patents, patent application publications, and other patent and non-patent documents referred to herein are incorporated by reference, to the extent they do not contradict the foregoing disclosure.