CROSS REFERENCE TO RELATED APPLICATION 
     Reference is made to commonly assigned, co-pending U.S. patent application Ser. No. 12/569,981 by Marcus et al. filed of even date herewith entitled “Apparatus For Controlling Peel Position In A Printer”, the disclosure of which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to measurement methods and apparatuses, and more particularly to a method and apparatus to maintain a desired peel location of a first web from a second web located in a printer. 
     BACKGROUND OF THE INVENTION 
     A typical multi-color dye donor web that is used in a dye transfer or thermal printer has a repeating series of three different rectangular-shaped color sections or patches such as a yellow color section, a magenta color section and a cyan color section. In addition, there may be a transparent colorless laminating section immediately after the color sections. 
     Each color section of the dye donor web consists of a dye transfer area which is used for dye transfer printing and a pair of opposite longitudinal edge areas alongside the dye transfer area which are often not used for printing. The dye transfer area may be about 152 mm wide and the two longitudinal edge areas may each be about 5.5 mm wide, so that the total web width is approximately 163 mm. 
     To make a multi-color image print using a thermal printer, a motorized donor web take-up spool draws a longitudinal portion of the dye donor web off a donor web supply spool in order to successively move an unused single series of yellow, magenta and cyan color sections over a stationary liner array (bead) of selectively heated resistive elements on a thermal print head between the supply and take-up spools. Respective color dyes within the yellow, magenta and cyan color sections are successively heat-transferred line-by-line, via the selectively heated resistive elements, onto a dye receiver medium such as a paper or transparency sheet or roll, to form the color image print. The selectively heated resistive elements often extend across the entire width of a color section, i.e. across the dye transfer area and the two longitudinal edge areas comprising that color section. 
     As each color section is drawn over the selectively heated resistive elements, it is subjected to a longitudinal tension particularly by the forward pulling force of the motorized donor web take-up spool. Since, the dye transfer area in the color section is heated by the resistive elements the web is weakened, making the web vulnerable to being longitudinally stretched if too much tension is applied. Consequently, too much longitudinal tension will stretch the donor web in the dye transfer area which in turn causes some creases or wrinkles to develop in the dye transfer area. As the dye donor web is pulled by the motorized donor web take-up spool over the selectively heated resistive elements, the creases or wrinkles tend to spread from a trailing (rear) end portion of a used dye transfer area at least to a leading (front) end portion of the next dye transfer area to be used. The line artifacts printed on the dye receiver medium, although they may be relatively short, are quite visible. This indicates that too much tension on the dye donor web will result in creases or wrinkles being created in an unused dye transfer area and line artifacts being printed on the dye receiver medium during the dye transfer process. 
     More significantly, as each color section is drawn over the selectively heated resistive elements too little tension will cause the web to be slack. Decreasing the tension further will cause more slackness in the web. This will result in improper peeling or delamination of the dye donor web from the receiver web. Improper peeling ranges from the peel position of the web shifting from the desired location at the peel bar to the extreme of not delaminating at all and causing a printer jam. When the peel location of the dye donor web is not at its desired location there is a high probability for defects to occur in the printed image. These defects include spot defects, creases, sticking defects and streaks. Many of these defects are due to the fact that the donor web will selectively stick to the receiver web at specific locations if there is not enough web tension to maintain the peel location. Spot defects are regions of low and high print density caused by micro folds in the donor sheet due to too little tension in the donor web. Sticking defects are due to the detachment of the dye layer in a thermal donor from the PET (polyethylene terephthalate) support and transfer of the dye layer to the receiver during the peel process following printing. This is a serious and unacceptable problem for the customer because it results in high density dye specs being scattered across the face of the receiver. 
     Thus, there is a need to maintain a desired peel location of a first web from a second web in thermal printers. The first web is usually a donor media (“dye donor web”) containing the colorants that are thermally transferred to the receiver media (second web). The receiver media is usually the final hardcopy print. The transfer process needs to be carefully controlled so that the correct amount of colorant(s) is transferred to produce a high quality image on the receiver material. During the printing process the two webs are brought in contact at the print head where thermal lamination occurs during the dye transfer process. After lamination, the two webs must be separated from each other in a controlled fashion. This separation is achieved by applying a known force to the laminated layers at a fixed location known as the peel location. This force is usually applied by tensioning the webs and forcing the webs to travel in different directions as they pass the desired peel location. The desired peel location is at a peel bar. If the force applied to the webs is insufficient then the webs will not separate at the desired peel location. When this occurs the quality of the print can be adversely affected and in severe cases the webs stay laminated together and cause the printer to jam. If the force applied to the web is too large then the webs may deform and introduce printing artifacts. 
     Different donor and receiver materials will have different binding forces when thermally laminated together and will therefore require different levels of separation force in order to ensure separation at the desired peel location in a thermal printer. Furthermore changing environmental conditions such as ambient temperature and humidity can also cause the binding forces to change for a given set of donor and receiver webs. Product variability resulting from material variations can also affect the binding forces. All sources of variation in the binding forces between a pair of donor and receiver webs will require different levels of separation force in order to ensure separation at the desired peel location in a thermal printer. 
     U.S. Pat. No. 6,315,471 by C. Hsieh and C. Chung entitled “Apparatus for Controlling Ribbon Tension in a Thermal Printer” describes an apparatus and method for controlling the tension on the web by pulse width modulation (duty cycle control) by monitoring the input and output diameters of the web on the supply and take-up reels and setting up a transforming table for varying the pulse width modulation as a function of web diameters to keep uniform tension on the ribbons (web). 
     U.S. Pat. No. 6,082,914 by G. Barrus and K. Moore entitled “Thermal Printer and Drive System for Controlling Print Ribbon Velocity and Tension” describes a thermal printer having a supply of media with a rotatable platen on which the media is moved for printing by a thermal printing head. A supply spindle supplies print ribbon from a supply spool mounted thereon, and a take-up spindle takes up the used print ribbon on a take-up spool. The spindles are each driven by a motor and controlled by a controller which detects the Back EMF (BEMF) of the motors, and calculates the velocity of the spindles, spool, and print ribbon to control each motor based on the BEMF. 
     Commonly assigned U.S. Pat. No. 6,859,221 by Z. J. Gao, R. F. Mindler and J. F. Corman entitled “Preventing Crease Formation In Donor Web In Dye Transfer Printer That Can Cause Line Artifact On Print” describes a method of preventing crease formation in a dye transfer area of a dye donor web that can cause a line artifact to be printed on a dye receiver during a dye transfer from the dye transfer area to the dye receiver in a dye transfer printer by controlling the heat distribution over the dye transfer area. 
     Commonly assigned U.S. Pat. No. 6,977,669 by Po-Jen Shih et al. entitled “Preventing Crease Formation in Donor Web in Dye Transfer Printer That Can Cause Line Artifact On Print” describes a thermal printer which employs the method described in commonly assigned U.S. Pat. No. 6,859,221. 
     U.S. Pat. No. 6,922,205 entitled “Color Thermal Printer And Color Thermal Printing Method” by M. Shusuke describes a thermal printer which conveys a recording sheet at a certain speed by keeping tension applied to a conveyor roller pair within a range designed not to influence the conveyance speed. 
     None of the prior art can ensure that the location of the peel location is correct and that the thermal printer is working at its designed print resolution. Thus there is a need for a thermal printer that includes a sensor system which can determine the actual peel location of the first web and the second web and to adjust the web tensions so that the peel location will be maintained at the desired peel location within desired tolerance limits for various combinations of thermal web media. Such a thermal printer will tolerate broader variations in manufacturing of the media which affect the tension requirements. Thus, the media may not need to have as tight manufacturing tolerances which would lead to less waste in media manufacturing. This thermal printer will also be able to accommodate for changing environmental conditions which change the peel force and will also result in decreased incidence of machine jams. With such a sensor system installed in a thermal printer, the web tension for new web materials can be automatically adjusted to enable for use in an existing printer. 
     SUMMARY OF THE INVENTION 
     The need is met according to the present invention by providing a method for maintaining a peel location for peeling a first web from a second web in a thermal printer. A preferred embodiment of the present invention comprises a method for maintaining a peel location of a first web from a second web in a thermal printer. An optical probe, that includes a light source and a photodetector, transmits light from the optical probe toward the first web. The web reflects a portion of the transmitted light onto the photodetector, which then outputs an electrical signal which is compared with a preselected signal level and the difference between them provides an indication as to how much adjustment the peel location requires. Adjusting the peel location may comprise changing environmental characteristics of the first web or the second web or adjusting a tension of the first or second web. The difference between the measured electrical signal levels is related to a physical distance of the first web from the desired peel location. The optical probe is pointed at one of the webs in a region after the desired peel location. 
     Another preferred embodiment includes a method for peeling a layer of media from a surface. The media typically is provided on a supply roll and is collected on a take-up roll by rotating the take-up roll via a drive motor. Between traveling from the supply roll and being taken up by the take-up roll the media can be adhered to a surface using a heat source. The media is then peeled from the surface also by using the take-up roll drive motor which rotates the take-up roll for drawing the media away from the surface. An optical probe proximate the peeled media includes a light source and at least one photodetector. By transmitting light from the light source toward the media the photodetector detects a portion of the transmitted light that is reflected by the media. In response, the photodetector outputs an electrical signal corresponding to an amount of reflected light detected. By monitoring an output level of the electrical signal the media can be peeled from the surface at a preferred peel location by adjusting a speed of the take-up roll drive motor in response to the output level of the electrical signal. The monitoring includes determining a difference between the output level of the electrical signal and a preselected electrical reference signal. This difference corresponds to a distance between the media and the optical probe. Therefore, the adjusting step includes increasing the distance between the media and the optical probe if so indicated by the difference in signal levels. The distance between the media and the optical probe indicates a peel location of the media being peeled from the surface. The preselected electrical reference signal represents about an ideal distance between the media and the optical probe. Adhering the media to the surface includes heating selected areas of the media against the surface. Different ones of the rollers can be rotated for moving the media. 
     Another preferred embodiment of the present invention includes a method to maintain a preselected peel location around a peel bar of a first web from a second web in a thermal printer. An optical probe is provided which includes a light source and at least one photodetector. The probe is faced toward the first web proximate the peel location. A load cell on the peel bar measures a web tension on the first web. Light is transmitted from the optical probe to the first web. A portion of the transmitted light is reflected off one or more surfaces of the first web onto the photodetector. Electrical output signal levels of the photodetector are measured when the first web is positioned at the preselected peel location to form a reference signal. Repetitively measuring the electrical output signal levels of the photodetector as a function of time and computing the difference between the measured electrical output signal levels and the reference signal while repetitively measuring a tension on the peel bar as a function of time provides information above which signal levels correspond to an ideal peel location. Having gathered this information, it is possible to adjust the tension of the first web in a manner corresponding to the difference between the measured electrical output signal levels of the photodetector and the reference signal while maintaining the tension on the peel bar in a safe range. Adjusting a speed of a roller, via controlling a drive motor speed, that takes up the first web is one way to accomplish this adjustment. 
     The present invention enables the design of improved thermal printers which provide improved performance in providing less printer defects and decreased incidence of machine jams. Furthermore the printer better accommodates for changing environmental conditions which change the peel force and better tolerates variations in manufacturing of the media which affect the tension requirements. 
     These, and other, aspects and objects of the present invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the present invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. The figures below are not intended to be drawn to any precise scale with respect to size, angular relationship, or relative position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
         FIG. 1  is plan view of a typical dye donor web including successive dye transfer areas. 
         FIG. 2  shows an overview schematic of a thermal printer according to one aspect of this invention. 
         FIG. 3A  shows the illumination and detection regions extending from the output end of an optical fiber probe according to one embodiment of this invention. 
         FIG. 3B  shows the output end of an optical fiber probe according to one embodiment of this invention. 
         FIG. 3C  shows the illumination and detection path of the optical fiber probe according to one embodiment of this invention. 
         FIG. 4  shows the detector response and the detector response ratio as a function of distance from a probe of the type shown in  FIG. 3 . 
         FIG. 5  shows a flow chart of the page printing process. 
         FIG. 6  shows an example sensor response ratio of a thermal web during printing of sequential donor patches under appropriate tension conditions. 
         FIG. 7  shows an example sensor response ratio of a thermal web during printing of sequential donor patches under inappropriate tension conditions. 
         FIG. 8  shows a flow chart of a process for controlling the peel location position during the printing of a donor patch in accordance with an embodiment of this invention. 
         FIG. 9  shows a flow chart of a process for controlling the peel location position during the printing of a donor patch in accordance with an alternate embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to methods and/or elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
       FIG. 1  depicts a typical multi-color dye donor web or ink ribbon  50  that is used in a dye transfer or thermal printer apparatus and method. The dye donor web  50  has a repeating series (only one completely shown) of three different rectangular-shaped color sections or patches such as a first color section  1  (usually yellow), a second color section  2  (usually magenta) and a third color section  3  (usually cyan). In addition, there is usually a transparent overcoat section or patch  4  (laminate) immediately after the third color section  3 . The repeating series of dye donor web sections is applied to the same area of a second web or receiver sheet  110  shown in  FIG. 2 . 
       FIG. 2  shows an overview schematic of a thermal printer  5  according to one preferred embodiment of this invention. A first web or dye donor web supply spool  10  is used to deliver a first web or dye donor web  50  past a guide roller  120  to a print head  30 . A second web or dye receiver sheet  110  gets thermally laminated to the first web  50  at the location at which the print head  30  and platen roller  100  come in close proximity while heat is applied to the print head  30 . It is desired for the webs to remain laminated over the print head to peel plate lamination distance  20  which terminates at the peel bar  40 . After passing peel bar  40  the dye donor web  50  passes peel guide roller  140  and is wound up on motorized donor web take-up spool  60 . Location  130   a  is the desired peel location of donor web  50  from receiver sheet  110 . If the tension on the donor web  50  is lower than desired the peel location may move to location  130   b . As the tension gets successively lower on donor web  50  the peel location will move to position  130   c , and at still lower tension on donor web  50  the donor web  50  will get caught between motorized capstan roller  80  and pinch roller  90  as indicated by position  130   d.    
     The peel bar  40  may also include an optional load cell  45  which is used to measure the tension of the donor web  50  as it passes over the contact region with the peel bar  40 . The purpose of peel bar web tension measuring load cell  45  is to keep the tension of the donor web within the safe level tension so that defects due to too much tension on the donor web  50  do not occur. This safe level tension is determined during initial printer set up. Too much tension on the donor web  50  causes an upward pressure on the peel bar  40  which is detected by the peel bar web tension measuring load cell  45 . Adjustment is then made to the speed of donor web take-up spool  60 . 
     During the printing process the receiver sheet  110  is driven forward by motorized capstan roller  80  and pinch roller  90  to the beginning of the first section  1  or patch of donor web  50 . The print head  30  is then lowered to make good thermal contact with donor web  50  and receiver sheet  110  over platen roller  100 . The donor web  50  and the receiver sheet  110  are then both transported at the same velocity while heat is applied to the webs by the print head  30 . When the printing of the first section  1  of donor web  50  is completed, by heat induced transfer of the donor web onto the receiver sheet  110  (the print), the webs are stopped and the print head  30  is raised. The receiver sheet  110  is then driven backward by motorized capstan roller  80  and pinch roller  90  to align the start of the printed region on the receiver sheet  110  with the beginning of the second section  2  of donor web  50 . The print head  30  is then lowered to make good thermal contact with donor web  50  and receiver sheet  110  over platen roller  100 . The donor web  50  and the receiver sheet  110  are then both transported at the same velocity while heat is applied to the webs by the print head  30 . When the printing of the second section  2  of donor web  50  is completed the webs are stopped and the print head  30  is raised. This process is repeated to transfer donor sections  3  and  4  to receiver sheet  110 . After printing section  4  of the donor web the print head  30  is raised and the print exits the printer. Not shown in  FIG. 2  are electrical connections and drive motors that are coupled to the motorized rollers, spools, etc., and for raising print head  30 . Although not all rollers shown in the figure are described as motorized, any of them can be so modified. Also not shown is a system controller that monitors and adjusts performance of all parts of the print system shown in  FIG. 2 . All these unillustrated components are well known to those skilled in the art. 
     An optical probe  70  which measures the distance of a web from the probe tip is installed in the printer with probe tip  270  facing the web. The optical probe  70  comprises a light source which transmits light to the web. Light is reflected from the web and the reflected light is incident on at least a pair of optical sensors which have different signal profiles as a function of the distance between the web and the sensors. The ratio of the two optical sensor signals is obtained, as explained below, and the ratio is dependent upon the distance from the web to the sensors. With an accurate measurement of the distance, i.e. at one of the peel locations  130 , the tension in the web can be adjusted so that the web can be brought back to the appropriate distance for the product. For example, a distance detected by the sensors may indicate that the donor web  50  is at position c or d (of  FIG. 2 ) and must be brought back to a preferred position at about a. The position of donor web  50  is controlled by adjusting the rotation speed, and therefore the tension, on donor web take-up spool  60 . 
     Details of an optical probe  70  embodiment are illustrated in  FIG. 3A ,  FIG. 3B  and  FIG. 3C  respectively. Commonly assigned U.S. Pat. No. 6,778,277, entitled “Apparatus And Method To Measure Film Motion In A Film Gate” by M. Marcus describes the principles of calibrating a reflective photonic probe, the disclosure of which is hereby incorporated by reference in its entirety. 
       FIG. 3A  shows the illumination and detection regions extending from the probe tip of an optical fiber probe according to a preferred embodiment of this invention.  FIG. 3B  shows the probe tip  270  of an optical fiber probe  70  according to a preferred embodiment of this invention. The probe tip  270  is the light output end of the optical probe  70 .  FIG. 3C  shows the illumination and detection path of the optical fiber probe according to a preferred embodiment of this invention. The optical probe tip  270  of the fiber optic probe  70  consists of an inner optical probe bundle  150  and an outer optical probe bundle  160 . The inner optical probe bundle  150  is composed of multiple illumination optical fibers  180  and multiple inner detection optical fibers  170  randomly bundled together. The outer optical probe bundle  160  is comprised of multiple outer detection optical fibers  190 . As shown in  FIG. 3C  the illumination fibers  180  in inner optical probe bundle  150  are coupled to light source  300  through optical fiber bundle  310 . As shown in  FIG. 3C  the inner detection optical fibers  170  in inner optical probe bundle  150  are coupled to photodetector (D 1 )  280  through optical fiber bundle  320 . Similarly the outer detection optical fibers  190  of outer optical probe bundle  160  are coupled to photodetector (D 2 )  290  through optical fiber bundle  330 . Signal conditioning electronics (not shown in  FIG. 3C ) are used to convert the electrical signal levels detected at photodetectors (D 1 )  280  and (D 2 )  290  indicative of the optical signal levels to electrical output signal levels indicative of a distance of web  50  from the optical probe tip  270 . 
       FIG. 3A  shows the illumination cone of light  200  being transmitted from the illumination optical fibers  180  as a function of distance d from the optical probe tip  270 . The illumination optical fibers  180  are illuminated with light source  300  typically in the NIR (near infra red) region of the spectrum. The outer detection optical fibers  190  are combined in optical fiber bundle  330  and the light passing through them is detected by photodetector (D 2 )  290 . The inner detection optical fibers  170  are also combined in optical fiber bundle  320  and light passing through them is detected by photodetector (D 1 )  280 . The illumination cone  200  of the inner optical probe bundle  150  is defined by the numerical aperture (NA) of the fibers making up the bundle with the maximum angles of illumination being indicated by illumination cone edge indicators  250 . The edge of the (detection cone) NA of the outer optical fiber bundle  160  is indicated by outer fiber bundle NA edge indicator  230 . In order for light to be incident on the first or second photodetector light must be reflected from a reflective surface facing the output end of the optical fiber probe  270  and be incident on the fibers at angles within the overlap of the cones defined by the NA of the illumination optical fibers  180  and the detection optical fibers  190 . In the configuration shown in  FIG. 2  the reflective surface is the donor web  50 . The NA of the outer fiber bundle fibers  190  limits the size of the overlap region between illumination fiber and the reflected light that can be coupled into the outer fiber bundle  160  and transmitted through optical fiber bundle  330  to photodetector (D 2 )  290 . Dashed line  240  indicates the edge of the light reflecting off of the donor web  50  which can be transmitted to photodetector (D 2 )  290 . Similarly dashed line  260  indicates the edge of the illumination light which can be reflected into outer detection fiber bundle  160 . 
     As shown in  FIG. 3A  first distance plane  210  is located at distance d 1  from the optical probe output end  270  and second distance plane  220  is located at distance d 2  from the optical probe output end  270 . At distances d less than d 1  no light illuminating donor web  50  from illumination optical fibers  180  will be transmitted down outer detection optical fiber to photodetector (D 2 )  290 . This is because there is no overlap of the illumination cone of illumination optical fibers  180  with the acceptance aperture, or detection cone, defined by the NA of optical fibers  190 . At distances between d 1  and d 2  the detected signal at photodetector (D 2 )  290  will increase monotonically as the distance between the optical probe output end  270  and the donor web  50  increases until a maximum is reached at distance d 2 . See  FIG. 4  for a graph of detected signal magnitudes. This is because the overlap area between the illumination cone of optical fibers  180  and the acceptance cone defined by the NA of optical fibers  190  is growing faster than the illumination cone of optical fibers  180  in this region. Beyond the distance d 2  the detected optical signal will decrease as a function of distance d, because the overlap area of the illumination cone of optical fibers  180  and the acceptance cone of optical fibers  190  remains constant in this region while the illuminated plane area continues to increase as a function of distance from the front surface of the optical probe  70 . As the illuminated plane area increases the amount of light per unit area on that plane decreases. Thus, when the overlap area remains constant as a function of increasing distance in this region the detected signal at the photodetector will decrease with increasing distance. 
       FIG. 4  shows a detector response for photodetector (D 1 )  280  and photodetector (D 2 )  290  and the detector response ratio as a function of distance from the output end  270  of an optical fiber probe  70  of the type shown in  FIG. 3A ,  FIG. 3B  and  FIG. 3C . The detector response is equivalent to the electrical output signal level, i.e. the voltage level, of the detector. Since the center to center spacing of the inner illumination fibers  180  and the inner detection fibers  170  is much closer than that between the inner illumination fibers  180  and the outer detection fibers  190  the detector response curve for photodetector (D 1 )  280  will reach a maximum at a much shorter distance from the probe surface than that for photodetector (D 2 )  290 . The amplitude of the two detector response functions D 1  and D 2  are dependent on the reflectivity of the dye donor web  50  which may vary for different patches  1 - 4  of the dye donor web  50 . The ratio of the 2 detector response functions also shown in  FIG. 4  is found to be independent of this reflectivity and is purely a function of distance of the dye donor web  50  from the output end  270  of the fiber optic probe  70 . The optical probe response function data shown in  FIG. 4  was obtained with an optical probe composed of 3.5 mil outer diameter optical fibers with NA=0.25. The diameter of the inner bundle  150  was 46 mil and the outer detection optical fibers were arranged in a ring with a diameter of 93 mil diameter. Distances labeled a and b in  FIG. 4  and  FIG. 7  correspond to locations a and b in  FIG. 2 . 
     In cases where the reflectivity of the web  50  is relatively constant as measured from patch to patch, it is not necessary to use a plurality of photodetectors in the optical probe  70 . In this case a single photodetector can be used and its electrical output signal, such as that for photodetector D 1  in  FIG. 4  can be used for control of peel location. 
     A flow chart of the page printing process  400 , as described above, is briefly illustrated in  FIG. 5 . At the beginning of the printing process, the first web  50  and second web  110  are brought in contact with each other and the first donor patch or section  1  on the first web  50  is aligned with corresponding print area on the receiver or second web  110  in Step  410 . After alignment, the print head is lowered and the first donor patch or section  1  is printed in Step  420 . The print head is then raised and the donor web  50  is peeled and advanced to the second patch or section  2 , and the receiver  110  is translated back to realign the print area with the second donor patch or section  2  in Step  430 . The second patch is then printed in Step  440  in the same manner as the first patch. The process repeats similarly to align and print the third and fourth patches on the print area of the receiver in Steps  450 - 480 . The printing process completes with Step  490 . After the fourth patch or section  4  is printed the print (receiver) is advanced and ejected from the printer. In the above embodiment of this invention, an optical probe  70  is mounted in a thermal printer  5  to monitor the donor web position  130  near the peel bar  40 . The position of the donor web  50  is determined by monitoring the electrical level of the ratio of the probe signals as shown in  FIG. 4 . This ratio is proportional to the distance between the optical probe  70  and the donor web  50 . By monitoring this ratio, variations in the donor web position  130  can be detected. 
     The repetitively measured ratio of the electrical output signal levels as a function of time during printing of four patches using normal print conditions is shown graphically in the plot in  FIG. 6 . The time period during which the first donor patch is printed is labeled  1 . The time periods for the printing of donor patches  2 - 4  are labeled with their corresponding numbers. The plot shows that the donor web position  130  during the printing of a patch is relatively constant with some variability around that position. The statistical mean of the measured ratio data values within a patch represents the average web position under normal printing conditions. Position signals labeled a and b in  FIG. 6  correspond to locations a and b in  FIG. 2 . Position signal a is an example of a reference signal, which we define as the measured probe signal for a web when measured at its desired peel location. In between the printing periods a magnitude of the position signal increases as the tension changes during print alignment. 
       FIG. 7  shows data collected when the web tension was deliberately decreased during printing. Position signals labeled a and b in  FIG. 7  correspond to locations a and b in  FIG. 2 . The data within the first patch shows that the position has deviated from the normal position. This deviation is denoted with an oval dashed line in  FIG. 7 . The deviation occurs during the first half of the patch printing duration, and then the position is observed to return to normal during the second half of the patch printing duration. If the tension is lowered further, greater deviations may occur. The shift in the measured position for the first patch indicates that the applied web tension was insufficient to peel the webs apart at the desired location, while the data for the subsequent patches implies that the tension was sufficient for those patches. This exemplifies the characteristic that different combinations of donor and receiver materials will require differing amounts of force to peel them apart. Consequently, measuring and monitoring the peel position can be used to detect abnormal printing situations, whether they arise from differing media combinations or changing printing conditions including mechanical or environmental conditions such as ambient temperature and humidity. 
     In a preferred embodiment of the present invention, the monitored detector position signal can be used as the basis of a negative feedback control loop to maintain the desired peel location during printing. A flowchart of the operation of a control loop for maintaining the peel position at the desired peel location during the printing process is shown in  FIG. 8 . The steps in  FIG. 8  describe the process  500  for controlling the peel position in the desired location during the printing of a patch or section of a print. To start printing the patch, the web tension is first set to an appropriate level in Step  510  by adjusting a drive motor that rotates take-up roller  60 . A system controller receives a signal output by the photodetector indicating a position of the web and adjusts the speed of the drive motor accordingly. The printing of the patch then begins in Step  520 . A portion of the image to be printed is sent to the print head and printed in Step  530 . After printing this portion of the patch, an the printer determines whether the patch printing is complete in Step  540 . If the printing of the patch is not yet complete, the current peel position is then evaluated at step  550  to determine whether the patch is peeling at the desired location in Step  560 . This is done by repetitively measuring the electrical output signal levels of the one or more photodetectors and computing the difference between the measured electrical output signal levels and the preferred position reference signal. If the peel position is consistent with the desired location, the next portion of the image is printed in Step  530 . If the determination in Step  560  indicates that the web is not peeling in the desired location, the system controller adjusts the drive motor voltage for roller  60  appropriately, and the web tension is adjusted in Step  570  to shift the peel position closer to the desired location. The magnitude of the adjustment is based on the difference between the measured electrical output signal levels and the preferred position reference signal, which difference is repetitively measured. The process then repeats with printing the next portion of the image in Step  530 . Eventually, when the print completion assessment in Step  540  indicates that the entire image of the patch has been printed, the patch printing process terminates in step  580 . 
     The thermal printer includes a controller (not shown) which is used to control web tension by regulating roller motor velocities, collecting sensor data from printer functions including photodetectors D 1  and D 2 . A comparator is used to determine the difference between the measured electrical output signal levels of the photodetector(s) and the preferred position reference signal, which can be stored in a controller memory. The comparator could be electronic or implemented as a software program in the controller. The tension on the donor web is then adjusted by the controller regulating roller motor velocities via a feedback loop based on the magnitude of the difference measured by the comparator. Motor speed control negative feedback loops are well known and are not described further. When the photodetectors sense that the donor web position  130  is closer to the photodetectors than a preferred position, a voltage or pulse width modulated duty cycle output to roller drive motors increases in response to the photodetectors, which increases the power to the roller motor controlling spool  60 , for example, thereby tightening the donor web and bringing its peel position closer to point a of the peel position as described above. Conversely, when the photodetectors sense that the donor web position  130  is further from the photodetectors and closer to the ideal position a described above, then a voltage or pulse width modulated duty cycle output to roller motors decreases, which decreases the power to the roller motor controlling spool  60 . A preferred embodiment of the present invention includes duty cycle control as described in U.S. Pat. No. 6,315,471, described above. 
       FIG. 9  shows a flow chart of a process for controlling the peel position during the printing of a donor patch in accordance with an alternate preferred embodiment of the present invention. In addition to the steps in the control loop shown in  FIG. 8 , steps  590  and  600  are added which insure that the tension on the donor web at the peel bar is in a safe regime. This is determined by comparing the measured tension with a previously determined safe peel bar tension level. The safe level tension level would be determined during the set up of the thermal printer. If the web tension is determined to be above the safe level in step  600  it is adjusted downward in Step  590  towards the safe level. 
     Although the discussion of the optical probe  70  up to now has described an optical fiber probe, it is understood that the optical probe  70  may also comprise a pair of LED/photodetector pair sensors such as the Honeywell HO1160 series or HOA1397 reflective pair, Optek OPB700 series or Fairchild QRB1133 optical sensors. Reflectivity compensated optical fiber probes are commercially available from Philtec as part of their RC 100 fiber optic sensor or from MTI as part of their 2100 photonic sensor series. 
     Alternative Embodiments 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. In particular, although the above discussions relate to maintaining the peel location in a printer, it is understood that the method and apparatus for maintaining a peel location applies to any peeling or separation process or device as applied between two or more substrates, sheets, or other media. The substrates may be bound together by adhesives, thermal processes or by any other method or technology. A substrate may exist naturally or by manufacture as an integrally formed single substrate that can be separated by peeling. Examples of such alternative peeling processes include solvent casting, compression rolling, thermal transfer and dry film photolithography and photoresist. 
     PARTS LIST 
     
         
           1  first color section 
           2  second color section 
           3  third color section 
           4  optional overcoat section 
           5  Thermal Printer 
           10  first web or dye donor web supply spool 
           20  Print head to Peel plate lamination distance 
           30  Print head 
           40  Peel bar 
           45  Load Cell 
           50  First web or dye donor web 
           60  first web take up spool 
           70  Optical probe 
           80  Motorized capstan roller 
           90  Pinch roller 
           100  Platen roller 
           110  Second Web or dye receiver sheet 
           120  Guide roller 
           130   a  Desired peel location 
           130   b  alternate peel location 
           130   c  alternate peel location 
           130   d  alternate peel location 
           140  Peel guide roller 
           150  Inner optical probe bundle 
           160  Outer optical probe bundle 
           170  Inner detection optical fiber 
           180  Illumination optical fiber 
           190  Outer detection optical fiber 
           200  Illumination cone 
           210  first distance plane 
           220  second distance plane 
           230  Outer fiber NA edge indicator 
           240  Outer fiber bundle reflected light edge indicator 
           250  Illumination cone edge indicator 
           260  Illumination fiber overlap edge indicator 
           270  Probe tip 
           280  First photodetector 
           290  Second photodetector 
           300  Light source 
           310  Light source optical fiber bundle 
           320  First detector optical fiber bundle 
           330  Second detector optical fiber bundle 
           400  Page printing flow chart 
           410  Step 
           420  Step 
           430  Step 
           440  Step 
           450  Step 
           460  Step 
           470  Step 
           480  Step 
           490  Step 
           500  Peel position control loop 
           510  Step 
           520  Step 
           530  Step 
           540  Step 
           550  Step 
           560  Step 
           570  Step 
           580  Step 
           590  Step 
           600  Step
A method for maintaining a peel location and for peeling a layer of media from a surface in a thermal printer. An optical probe, that includes a light source and a photodetector, transmits light from the optical probe toward a first web. The web reflects a portion of the transmitted light onto the photodetector, which then outputs an electrical signal which is compared with a preselected signal level and the difference between them provides an indication as to how much adjustment the peel location requires. Adjusting the peel location may comprise changing environmental characteristics of the first web or the second web (surface) or adjusting a tension of the first or second web. The difference between the measured electrical signal levels is related to a physical distance of the first web from the desired peel location.