Abstract:
One or more arrays of heating elements are configured with insulating regions to prevent the dissipation of heat to unintended regions of a thermochromic substrate. Methods include printing and arranging impressions on a two-sided substrate avoiding bleeding and other problems more-commonly associated with traditional two-sided thermal printing techniques. A simple and reliable thermal printing system is provided for use in ballot marking, including several mechanisms for receiving and detecting the orientation of a substrate within a thermal printing apparatus.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority under 35 U.S.C. §119 from U.S. provisional patent application No. 61/015,158 filed Dec. 19, 2007 entitled “Thermal Ballot Marking System” which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to the field of thermal printing. More specifically, the invention relates to thermal printing techniques for producing markings on multiple sides of a thermochromic medium. Particular embodiments of the invention are particularly useful in thermal printing applications to mark votes on a two-sided ballot during an election to provide visual evidence of voters&#39; selections. 
     2. Description of the Related Art 
     In general, thermal printing methods produce an image by selectively heating a thermochromic medium using a thermal printer head. Thermal printing is typically practiced by first applying a coating of thermochromic dye to a substrate, resulting in a thermochromic medium. Thermochromic dye is responsive to heat and changes its appearance when heat-activated, resulting in a marking. After an application of dye, the process continues by selectively applying heat to various regions of the thermochromic medium, thus leaving markings on the medium. 
     However, various problems exist in the field of thermal printing. For example, as heat naturally dissipates, bleeding may occur, thus marking unintended regions of the medium. In particular, a marking on one side of the printing medium may conduct through the substrate, resulting in an unintended marking on the opposite side. 
     Recent controversies surrounding the accuracy of electoral systems have led to the development of direct recording electronic (DRE) voting machines that, according to proponents, minimize the number of inadvertently spoilt ballots. Critics of DRE machines, however, challenge the resulting lack of a voter verifiable paper trail. Consequently, hybrid systems have been proposed in which an electronic voting machine, e.g. a computer with a touch sensitive display, assists voters in marking a paper ballot that may be inspected by the voter prior to optical scanning. In these systems the ballots are archived should the counting procedure be subsequently contested. 
     Thermal printing is an attractive technology for use in ballot marking systems on account of the simple and reliable printing mechanism and the relatively low resolution of typical ballot markings. However, as explained above, an unintended marking of a ballot due to the shortcomings of the current state of the art negates the advantages of employing a thermal printing system to record votes. 
     Furthermore, the majority of existing thermal printing systems are capable of printing only on a single side of each sheet of thermochromic media. Typically, a thermochromic coating is applied to only one side of the substrate. However, when marking printable media, a desire for readability and simplicity strongly motivates printing on both sides of a single sheet of thermochromic media. In the case of a ballot, limiting a printable media to a single sheet simplifies ballot handling and improves the integrity of the election by eliminating the possibility of a split ballot. Yet the length of many ballots, especially those with multiple voter initiatives, makes compressing all election issues onto a single side of a single sheet a difficult endeavor if readability is to not be compromised. The increased surface area available to two-sided printing systems strongly motivates the use of a printing system with two-side printing capability. 
     Various two sided thermal printing systems have been proposed, but typically either increase the complexity of the heating element control system or require a substrate of sufficient thickness and insulating capability as to prevent heat conduction from one side to the other. However, simpler approaches would be highly desirable for use in ballot marking. 
     Of course, any thermal printing system must ensure uniform thermal contact between the thermochromic ballot and the heating elements that produce the markings. Indeed, ensuring proper contact in the case of marking ballots presents special challenges. This is because the ballots are necessarily handled by voters themselves, who may or may not be familiar with thermal printing technology. Thus, there is a need for a simple yet reliable mechanism for ensuring uniform contact between the ballot and the heating elements after the ballot is received from the voter. 
     SUMMARY OF THE INVENTION 
     In some embodiments of the invention, a fixed size and layout of a printable medium allows the use of a fixed array of heating elements that remain stationary relative to the printing medium during printing, greatly simplifying the printing procedure. 
     In some embodiments of the invention, a system and apparatus are disclosed comprising one or more arrays of heating elements configured with alternating heating regions and insulating regions to selectively heat regions on a substrate treated with a thermochromic coating. 
     In some embodiments, two or more arrays of heating elements are disposed facing towards each other and are configured with a slot to accept a two-sided medium formed with thermochromic coatings on each side of a substrate. According to these embodiments, techniques are disclosed for printing on a two-sided medium, as well as techniques for designing a two-sided medium to mitigate the adverse effects typically associated with the conduction of heat away from the applied location and through the substrate, as well as other problems more-commonly associated with traditional two-sided thermal printing techniques. 
     In some embodiments, the two-sided medium is a voting ballot. Similarly, some embodiments of the invention provide simple and reliable thermal printing systems for use in ballot marking. The ballot markings are produced by a heating element array (HEA) in thermal contact with the ballot. 
     The invention also provides an apparatus for receiving and orienting a thermochromic medium. Likewise, methods of receiving and orientating a voting ballot are also disclosed that, while simple for the voter to operate, ensure reliable thermal contact between the ballot and the HEA. In particular, the receiving and orienting mechanisms are capable of accepting either folded or unfolded ballots in any orientation, greatly reducing the opportunities for voter error. In some embodiments, orientation indicators and/or sensors are employed to automatically tell a processor about the orientation of a ballot inserted into the voting machine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic side view of a heating element array (HEA) for marking a thermochromic medium according to some embodiments of the invention; 
         FIG. 1B  is a schematic isometric view of a single plate of a heating element array according to some embodiments of the invention; 
         FIG. 2A  is a visual representation of a ballot taking advantage of a technique for thermally marking a ballot on both sides according to some embodiments of the invention; 
         FIG. 2B  illustrates the steps of a method of designing a two-sided thermochromic medium according to some embodiments of the invention; 
         FIG. 2C  illustrates the steps for a method of designing a two-sided thermochromic medium and thermally printing marks on checkboxes according to some embodiments of the invention; 
         FIG. 3A  illustrates the steps for a method of designing a two-sided thermochromic medium according to some embodiments of the invention; 
         FIG. 3B  illustrates the steps for a method of designing a two-sided thermochromic medium and thermally printing marks on checkboxes according to some embodiments of the invention; 
         FIG. 3C  is a visual representation of a ballot taking advantage of an alternative technique for thermally marking a ballot on both sides according to some embodiments of the invention; 
         FIG. 4A  shows several configurations ensuring reliable contact between a substrate and a heating element array according to various embodiments of the invention; 
         FIG. 4B  is an isometric view of two printed circuit boards having heating elements disposed on one side of each board according to some embodiments of the invention; 
         FIG. 4C  shows front and rear views of a thermochromic medium used with the heating element apparatus shown in  FIGS. 4B and 4D ; 
         FIG. 4D  is a side view of two printed circuit boards having heating elements disposed on one side of each board with a thermochromic medium disposed therebetween according to some embodiments of the invention; 
         FIG. 5  is an isometric view of a voting machine according to some embodiments of the invention; 
         FIG. 6  illustrates a ballot labeling scheme and associated sensor positions for orienting a ballot according to some embodiments of the invention; and 
         FIG. 7  illustrates four possible ballot orientations capable of being identified by a thermal printing machine according to some embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Some embodiments of the invention comprise an apparatus with one or more arrays of heating elements configured with heating regions and insulating regions to selectively heat selected areas of a substrate treated with a thermochromic coating. 
     Heating Element Arrays 
       FIG. 1A  shows a heating element array (HEA) for marking a thermochromic medium according to the invention. In the presently preferred embodiment of the invention, the thermochromic medium is a ballot  500 . However, it will be readily apparent to those having ordinary skill in the art having the benefit of this disclosure that the invention may be used to mark a wide array of thermochromic media to yield a correspondingly wide array of printed materials. 
     The ballot  500  is positioned between the HEA  100  and an insulator  200 . In this and the following figures, the ballot  500 , HEA, and insulator  200  are shown with a small separation for clarity. In actual use, these components are preferably in direct thermal contact with one another. 
     According to presently preferred embodiments of the invention, the heating elements  121  and  122  within the HEA are surrounded by an insulating substrate  110 . The heating elements can be heated individually to mark the ballot  500  as desired. The insulating substrate  110  and insulator  200  help contain the applied heat to the immediate vicinity of the heating element such that the mark produced by an individual heating element  121  or  122  is limited to the dimensions of the heating element  121  or  122 , respectively. As such, it is easy to design a ballot or other thermochromically printed materials. 
     In some embodiments of the invention, the HEA is constructed using a printed circuit board (PCB). The heating elements may be either surface mount resistors or shaped traces. While shaped traces allow for more precise control of the shape of the marking produced on the ballot, surface mount resistors provide sufficient resolution for many applications. Notably, the commonly available “0201” surface mount package is capable of producing a marking as small as 0.002 in by 0.001 in. 
     Since the ballot  500  can only be marked when positioned near the heating element  121 , and  122 , the heating elements  121 , and  122  are preferably arrayed in a regular pattern and substantially cover the entire extent of the ballot  500 .  FIG. 1B  is an isometric view of a plate incorporating a heating element array  130  containing a matrix of heating elements, indicated by filled boxes, separated by rows and columns of insulating substrate, indicated by empty boxes. Using a matrix of heating elements, like the matrix illustrated in  FIG. 1B , allows the HEA to be used for many different printing layouts, and greatly simplifies the ballot design processes. 
     A heating element array using a matrix pattern of heating elements is particularly useful in printing applications in which the medium to be printed upon comprises a plurality of check boxes or the like. According to some embodiments of the invention, a method of designing a check box medium is provided to specifically match the design of the heating element array. Check box type applications typically associate the check boxes with some other text. For example, a voting ballot typically includes a list of candidates written in text with an associated checkbox logically placed proximate to the text to indicate one&#39;s vote for that candidate. The invention provides solutions for designing a check box medium to be used for ballot markings in two-sided thermal printing environment. 
     In the presently preferred embodiment of the invention, a ballot is designed for a thermal printing system such that voting choices are memorialized. According to some embodiments, a ballot is designed for a thermal printing voting machine, wherein the ballot and printer are configured such that the machine accurately marks the ballot irrespective of the orientation of the ballot within the machine. To accomplish this aspect of the invention, one or more ballot orientation indicators and one or more corresponding sensors are utilized as explained below. 
     Two-Sided Thermal Printing 
     The invention provides techniques for manufacturing a thermochromic medium by treating a substrate with a thermochromic coating on both sides and applying heat to on one side of the medium such that the heat will not produce a visible marking on the other side of the medium. The manufacturing techniques provide this functionality despite assuming that heat applied to one side of the medium will conduct through the substrate with a sufficient intensity to activate a thermochromic coating applied to the opposite side of the substrate. 
       FIG. 2A  shows a visual representation of a technique for thermally marking a ballot on both sides according to the invention. In this example, the thermochromic medium is described as a ballot  500  based on a paper substrate. However, it will be readily apparent to those possessing ordinary skill in the art that any thermochromic medium may benefit from the technique. 
     In this technique, a thermochromic coating is applied to selected regions on each side of the substrate in an exclusionary fashion. According to these techniques, no region of paper exists in which thermally sensitive coatings are applied to both sides of the paper. On the front of the ballot  500 , the sets of candidates running for several offices are press printed at left. The boxes  531  for indicating a voter&#39;s preference for a particular candidate are press printed in a vertical column at right. The entire column of boxes is backed by a thermochromic coating  521 . On the back of the ballot, a series of services are press printed below a set of press printed instructions, and a set of boxes  532  for indicating the voter&#39;s perceived importance of each service is press printed below each service. The sets of boxes, and portions of the printed services, are backed by a thermochromic coating. The thermochromic coatings applied to each side are offset from one another. That is, the region  522  opposing the coated region  521  on the front of the ballot is free of thermochromic coating, and the region  524  opposing the coated region  523  on the back of the ballot is free of thermochromic coating. This exclusionary approach to applying the thermochromic coating ensures that when marks are created on one side of the ballot, no unintended marks are produced on the opposing side of the ballot. 
       FIG. 2B  illustrates the steps of a method  201  of designing a two-sided thermochromic medium according to some embodiments of the invention. In some embodiments of the invention, the two-sided thermochromic medium is a voting ballot. However, it will be readily apparent to those having ordinary skill in the art having the benefit of this disclosure, that the method of designing a two-sided thermochromic medium is useful in a variety of other applications. 
     The method begins with providing a two-sided substrate at step  202 . The method  201  continues by establishing various queries and checkboxes at step  203 , wherein the checkboxes accept one or more indicia representing a response to the various queries. In some embodiments of the invention, two sets of queries and corresponding checkboxes are established, wherein the one set is positioned on the front of the two-sided thermochromic medium and the second set is positioned on the back of the two-sided printing medium. In the presently preferred embodiment the first checkbox region does not line up through the two-sided printing medium to a thermochromic coating in a second checkbox region. Rather, the queries and checkboxes are designed and established in an exclusionary manner such that an application of thermochromic coating in a first checkbox region does not line up through the two-sided printing medium to a second checkbox region. 
     It will be readily apparent to those having ordinary skill in the art having the benefit of this disclosure that a wide variety and number of sets of queries and corresponding checkboxes are able to be positioned in a similar exclusionary manner on a wide variety of positions on the medium. 
     At step  204  the method  201  continues with printing the two-sided substrate with the established queries and checkboxes. In the presently preferred embodiment of the invention, the queries and checkboxes are press printed. However, it will be readily apparent to those having ordinary skill in the art having the benefit of this disclosure that a variety of printing methods, now known or later developed, may be used to print the queries and checkboxes. 
     The method of designing a two-sided thermochromic medium  201  continues with treating the region of the substrate backing the checkboxes with a thermochromic coating at step  205 . 
       FIG. 2C  illustrates a similar method  201 ′ of designing a two-sided thermochromic medium and thermally printing marks on checkboxes according to some embodiments of the invention. According to  FIG. 2C , steps  202 ,  203 ,  204 , and  205  are repeated from the method  201  of  FIG. 2B . Additionally, the method continues with placing the treated printing medium in thermal contact with an array of heating elements at step  206  and applying heat to various checkboxes at step  207 , thus printing marks. 
     In some other embodiments of the invention, the methods  201  and  201 ′ further comprise providing at least one orientation indicator on the thermal printing medium. As explained below, orientation indicators are employed to detect the orientation of a thermal printing medium such that the thermal printing medium is marked accurately irrespective of its orientation relative to the HEA. According to these embodiments, the thermal printing system is more user-friendly. 
       FIGS. 2B and 2C  illustrate one way to designing a printable medium such that markings are not visible through the printable medium.  FIGS. 3A and 3B  illustrate another method of printing unintended marking through the use of masking. Similarly,  3 C illustrates a printable medium taking advantage of these masking methods. 
       FIG. 3A  illustrates the steps of a method  301  of designing a two-sided thermochromic medium using a substantially opaque masking according to some embodiments of the invention. In some embodiments of the invention, the two-sided thermal printing medium is a voting ballot. However, it will be readily apparent to those having ordinary skill in the art having the benefit of this disclosure, that the method of designing a two-sided thermochromic medium is useful in a variety of other applications. 
     The method begins with providing a two-sided substrate at step  302 . The method  301  continues by establishing various queries and checkboxes at step  303 , wherein the checkboxes accept one or more indications representing a response to the various queries. Two sets of queries and corresponding checkboxes are established, wherein the one set is positioned on the front of the two-sided printing medium and the second set is positioned on the back of the two-sided printing medium. 
     At step  304  the method  301  continues with printing the two-sided substrate with the established queries and checkboxes. In the presently preferred embodiment of the invention, the queries and checkboxes are press printed. However, it will be readily apparent to those having ordinary skill in the art having the benefit of this disclosure that a variety of printing methods, now known or later developed, may be used to print the queries and checkboxes. The step  304  of printing one or more checkboxes results in creating one or more “shadow areas” on the opposite side of the printable medium. The shadow areas comprise the area on the opposite side of the printable medium that is vulnerable to an unintended mark caused by heating the opposing checkbox area. 
     The method  301  continues with the step  305  of masking the first side shadow area and the second side shadow area with a substantially opaque coating. As used herein, the term “opaque” refers to a condition of a region on the printable medium in which a mark caused by the heating a thermochromic coating in that region is unnoticeable. In some embodiments, the opaque coating comprises dark ink. The benefit of this masking technique is that a user can simply treat the entirety of both sides with the thermochromic coating, resulting in an easier and less expensive coating process. Furthermore, the masking ensures that the marks coming through to an unintended side of the printing medium are not visible. 
     The method of designing a two-sided thermochromic medium  301  continues with treating the substrate with a thermochromic coating at step  306 . In some embodiments of the invention, at least the checkboxes are treated with the thermochromic coating. In some other embodiments of the invention the entire substrate is treated with the thermochromic coating thereby resulting in an easier treatment process. 
       FIG. 3B  illustrates a method  301 ′ of designing a two-sided thermochromic medium and thermally printing marks on checkboxes according to some embodiments of the invention. According to  FIG. 3B , steps  302 ,  303 ,  304 ,  305 , and  306  are repeated from the method  301  of  FIG. 3A . Additionally, the method continues with placing the treated printing medium in thermal contact with an array of heating elements at step  307  and applying heat to various checkboxes at step  308 , thus printing marks. 
     In some other embodiments of the invention, the methods  301  and  301 ′ further comprise providing at least one orientation indicator on the thermal printing medium. As explained below, orientation indicators may be employed to detect the orientation of a thermal printing medium such that the thermal printing medium may be marked accurately irrespective of its orientation within the HEA. According to these embodiments, the thermal printing system is more user-friendly. 
       FIG. 3C  shows a ballot resulting from the masking technique for thermally marking a printable medium on both sides according to the invention. Again, although the thermochromic medium is described as a ballot, it will be readily apparent to those possessing ordinary skill in the art that any thermochromic medium may benefit from the technique. In any region where thermal marking may occur on one side, an opaque ink is applied to the opposing side&#39;s shadow area to ensure that any unintentional thermal markings on the opposing side are masked. On both the front and back of the ballot shown in  FIG. 3C , the sets of candidates running for several offices are press printed at left, and voter preferences are thermally marked in a column of boxes at right. Because marks within these boxes may produce a marking on the opposing side of the ballot, dark-colored opaque vertical bars  561  and  562  are press printed in the shadow area. 
     In the above approaches, the ability to thermally mark both sides of the thermochromic substrate allows the ballot designer to utilize the increased surface area offered by both sides of the ballot. The invention presupposes that heat will in fact conduct through the paper and the invention prevents unintentional marking on the opposing side by the masking. Thus, according to some embodiments of the invention, the paper can be marked on both sides using heating elements on only one side of the paper. This significantly speeds and simplifies the printing process in that the paper need not be flipped, either automatically or by the voter, before printing on the second side. 
     Ballot and Heating Element Array Positioning Techniques 
       FIG. 4A  shows several heating element array (HEA) configurations ensuring reliable contact between a thermochromic medium and the HEA according to some embodiments of the invention. In the following examples, the thermochromic medium is described as a ballot, however, it will be readily apparent to those possessing ordinary skill in the art that any thermochromic medium may benefit from the technique. 
     1. HEA-Insulator Clamshell Around Ballot 
     In some embodiments of the invention, a ballot  403  is clamped between an HEA  404  and an insulator  405  connected by a hinge  406 . The insulator  405  ensures firm, uniform contact between the ballot  403  and the HEA  404  and minimizes the transfer of heat away from the marked regions, thereby reducing any bleeding of the marks produced. If the ballot  403  is formed of sufficiently insulative paper, this configuration allows for full-coverage marking on one side of the ballot  403 . If more conductive paper is used, this configuration allows for full-coverage, identical marking or exclusionary, distinct marking on opposing sides of the ballot  403 , as described above. 
     2. HEA Clamshell Around Ballot 
     In some embodiments of the invention, a ballot  407  is clamped between two HEAs  408 ,  409  connected by a hinge  410 . An insulating substrate  412 ,  413  is disposed between the heating elements  414 ,  415  within the HEAs  408 ,  409 . The insulating substrate minimizes conduction of heat away from the marked regions of the ballot  407 . If the ballot  407  is formed of sufficiently insulative paper, this configuration allows for full-coverage, distinct marking on both sides of the ballot  407 . If more conductive paper is used, this configuration allows for full-coverage, identical marking or exclusionary, distinct marking on opposing sides of the ballot  407 . 
     3. HEA Clamshell Around Ballot Folded Around Insulator 
     In some embodiments of the invention, a ballot  416  is folded around an insulator  417 . The folded ballot  416  is clamped between two HEAs  418  and  419  connected by a hinge  420 . Heating elements  422 ,  423  within the HEAs  418 ,  419  thus mark onto the exterior side of the folded ballot  416 . The insulator  417  prevents marking of one half of the folded ballot by the HEAs  422 ,  423  adjacent to the complementary half of the folded ballot  416 . The insulating substrate  417  between the heating elements  422 ,  423  within the HEAs  418 ,  419  minimizes conduction of heat away from the marked regions. If the ballot  416  is formed of sufficiently insulative paper, this configuration allows for full-coverage marking on the exterior side of the folded ballot  416 . If more conductive paper is used, this configuration allows for full-coverage, identical marking or exclusionary, distinct marking on both exterior and interior sides of the ballot  416 . In allowing for a folded ballot  416 , this configuration reduces the effective size of the ballot  416 , thus easing voter handling of the ballot  416  without reducing the markable area. Furthermore, if only the exterior side of the folded ballot  416  is marked, voter privacy can be enhanced by inverting the folded ballot  416  upon removal from the ballot marking device. 
     4. Insulator Clamshell Around Ballot Folded Around Two-Sided HEA 
     In some embodiments of the invention, a ballot  424  is folded around a two-sided HEA  425 . The two-sided HEA  425  may be constructed from a single insulating substrate, e.g. a PCB, with heating elements  426 ,  427  on either side, or from two single-sided HEAs abutted against one another with heating elements facing outward toward the interior side of the folded ballot  424 . The two-sided HEA thus marks onto the interior side of the folded ballot  424 . The folded ballot  424  is preferably clamped between two insulators  428 ,  429  that are connected by a hinge  430 . The insulators  428  and  429  ensure firm, uniform contact between the folded ballot  424  and the heating elements  426 ,  427  within the two-sided HEA  425  and minimize conduction of heat away from the marked regions. If the ballot  424  is formed of sufficiently insulative paper, this configuration allows for full-coverage marking on the interior side of the folded ballot  424 . If more conductive paper is used, this configuration allows for full-coverage, identical marking or exclusionary, distinct marking on both interior and exterior sides of the folded ballot  424 . In allowing for a folded ballot  424 , this configuration reduces the effective size of the ballot  424 , thus easing voter handling of the ballot  424  without reducing the markable area. Furthermore, if only the interior side of the folded ballot  424  is marked, voter privacy is enhanced. 
     5. HEA Clamshell Around Ballot Folded Around Two-Sided HEA 
     In some embodiments of the invention, a ballot is folded around a two-sided HEA  432 . The two-sided HEA  432  may be constructed from a single insulating substrate, e.g. a PCB, with heating elements  433 ,  434  on either side, or two single-sided HEAs abutted against one another with heating elements  433 ,  434  facing outward toward the interior side of the folded ballot  431 . The insulating substrates  435  and  436  positioned between the heating elements  433  and  434  of the HEA  432  minimize conduction of heat away from the marked regions. If the ballot  431  is formed of sufficiently insulative paper, this configuration allows for full-coverage marking on both the interior and exterior sides of the folded ballot  431 . If more conductive paper is used, this configuration allows for full-coverage, identical marking or exclusionary, distinct marking on both interior and exterior sides of the folded ballot  431 . In allowing for a folded ballot  431 , this configuration reduces the effective size of the ballot  431 , thus easing voter handling of the ballot  431  without reducing the markable area. Furthermore, if only the interior side of the folded ballot  431  is marked, voter privacy is enhanced. 
     It will be readily apparent to those having ordinary skill in the art and having the benefit of this disclosure, that the examples given above in (1)-(5) are merely some possible configurations. It will be obvious to ordinary practitioners that other configurations of heating element arrays are envisioned to accomplish a wide variety of design specific goals that may present themselves. For example,  FIG. 4B  is an isometric view of two PCBs  401  and  402  with heating elements  411 ,  421 ,  412 , and  422  disposed on one side of each board. The two PCBs  401  and  402  illustrated in  FIG. 4B  are disposed facing each other, such that a thermochromic medium  499  inserted between the PCBs is exposed to heating elements on each side. In some embodiments the heating elements are “nested” within the substrate or insulator. Furthermore, the two PCBs  401  and  402  have alternating rows of heating elements such that any one region on the medium placed between the PCBs is only heated by one of the PCBs  401  or  402 . 
     As shown in  FIG. 4C , the thermochromic medium  499  has checkboxes in rows on both the front and rear side disposed in an alternating fashion. In the presently preferred embodiment of the invention, the check boxes are individually treated with a thermochromic coating such that the check boxes are marked with the application of heat from a heating element. According to these embodiments, the invention utilizes the alternating rows of heating elements and the alternating rows of coated checkboxes to cause heat by one or more heating elements to make a marking without inadvertently marking a checkbox on the opposing side of the ballot. 
     Receiving Ballot and Determining Ballot Orientation 
     The invention further provides mechanisms for receiving an unmarked ballot from the voter and determining the orientation of the received ballot.  FIG. 5  shows a voting machine  560  according to some embodiments of the invention. In the presently preferred embodiment of the invention, the voting machine  560  includes a pair of opposing PCBs  564  and  565  with heating element arrays  566 . The PCBs also includes paper orientation sensors  563 , e.g. surface mount LED and receiver pairs, at each corner. In some embodiments, the PCBs are mechanically coupled to a pressing mechanism  561  and rear housing member  562 , with a spring providing a clamping action  567  forcing the PCBs against one another. 
     According to  FIG. 5 , a user inserts a ballot  570  in an opening  580  above the PCBs  564  and  565  and between the pressing mechanism  561  and the rear housing member  562 . The opening  580  is positioned such that the lower edge of the ballot comes to rest in a seam between the two PCBs which, initially, are in contact with one another. The voter then pulls a lever (not shown), mechanically connected to the pressing mechanism  561 , to separate the PCBs  564  and  565  in opposition to the spring. The ballot  570  falls downward between the PCBs  564  and  565 , and comes to rest against a stop  585 . The lever is released and the ballot  570  is firmly clamped by the PCBs  564  and  565 . One skilled in the art will appreciate that this approach is readily adapted to the ballot and HEA positioning schemes of  FIGS. 4A and 4D . 
     After the ballot is received from the voter, the orientation sensors  563  then inspect the ballot to confirm proper insertion and determine the orientation of the inserted ballot. The sensors detect the presence or absence of orientation indicators on the inserted ballot. The relative positions of the sensors match the relative positions of the locations on the ballot where orientation indicators may be present. 
       FIG. 6  shows a ballot labeling scheme and associated sensor positions for orienting a ballot according to the invention. In this scheme, orientation indicators are printed in several corners of the ballot. Specifically, orientation indicators  510  are printed in the lower left, upper left, and upper right corners of the front side of the ballot  500 , and orientation indicators are printed in the lower right, upper left, and upper right corners of the back side of the ballot, as shown. Here, “front”, “back”, “upper”, and “lower” are defined relative to the printed matter, e.g. a set of candidate&#39;s names and corresponding checkboxes, on the ballot. Also shown in  FIG. 6  are the four sensor positions a, b, c, and d, corresponding to the four corners of the ballot when in the inserted position. The indicators may be markings, as shown in  FIG. 6  or may be holes punched through the ballot. 
       FIG. 7  shows four possible ballot orientations. Table 1 shows the sensor readings a, b, c, and d corresponding to an improper ballot insertion and the four possible orientations of  FIG. 7 . Here, 1 corresponds to the detection of an orientation indicator and 0 corresponds to no detection of an indicator. The uniqueness of the rows indicates that each of the four orientations can be uniquely determined given the pattern of orientation indicators and arrangement of sensors shown in  FIG. 6 . It should be noted that the scheme described above is also tolerant of a single sensor failure—either “fail-on” or “fail-off”. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Sensor readings and corresponding orientations. 
               
             
          
           
               
                   
                 Sensor 
                   
               
             
          
           
               
                   
                 Orientation 
                 a 
                 b 
                 c 
                 d 
               
               
                   
                   
               
               
                   
                 No Ballot 
                 0 
                 0 
                 0 
                 0 
               
               
                   
                 Front Up 
                 1 
                 0 
                 1 
                 1 
               
               
                   
                 Back Up 
                 0 
                 1 
                 1 
                 1 
               
               
                   
                 Back Down 
                 1 
                 1 
                 1 
                 0 
               
               
                   
                 Front Down 
                 1 
                 1 
                 0 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     The ballot marking device may proceed with marking the ballot once the orientation of the ballot is determined. The manner in which the heating elements are addressed, however, is adapted to reflect the determined orientation of the ballot. For example, if the ballot marking device uses the ballot and HEA configuration of FIG.  4 A-( 2 ) and the inserted ballot is a one-sided ballot formed of paper sufficiently insulative as to permit one-sided marking, only one of the two HEAs will be addressed to mark the ballot. Specifically, only the HEA facing the printed side of the ballot is addressed during marking. If the inserted ballot is a two-sided ballot formed of paper sufficiently insulative as to permit one-sided marking, both HEAs are addressed and the marking pattern with which each HEA is addressed is based on the side of the ballot facing the HEA. 
     Additionally, the marking patterns with which the HEAs are addressed may need to be inverted vertically based upon the ballot orientation. If instead the ballot marking device uses the ballot and HEA configuration of FIG.  4 A-( 1 ), the pattern with which the HEA is addressed is inverted either horizontally, vertically, or horizontally and vertically based on the ballot orientation. If all ballot orientations are to be accepted using the configuration of FIG.  4 A-( 1 ), however, the ballot must be formed of paper sufficiently conductive as to permit marking on both sides of the paper from a single HEA on one side of the ballot. If more insulative paper is used, the voter may be instructed to insert the ballot in one of the two acceptable orientations. 
     Other Applications 
     One skilled in the art will appreciate that aspects of the invention, while described herein with reference to a ballot marking device, may find use in other applications. In particular, the techniques for manufacturing two-sided thermal printing paper described in  FIGS. 2 and 3  may be useful in applications in which the thermally printable area comprises a relatively small fraction of the entire paper area. The positioning schemes of  FIG. 4  and the orientation scheme of  FIGS. 6 and 7  may find use in most any thermal printing application in which a user inserts single sheets of paper for printing. 
     As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Likewise, the particular naming and division of the members, features, attributes, and other aspects are not mandatory or significant, and the mechanisms that implement the invention or its features may have different names, divisions and/or formats. Accordingly, the disclosure of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following Claims.