Recording and erasing method for a thermoreversible recording medium

Abstract: A conventional rewritable recording device utilizing a heat-sensitive recording medium (1) which is turned into a colored state when heated at a first predetermined temperature and turned into a colorless state when heated at a second predetermined temperature has a problem that the recording medium will not be turned into a completely colorless state depending on the history of recording and erasing, when, for example, no satisfactory heating to the second predetermined temperature is performed or when repeated recording and erasing have deprived the recording medium of its reversibility. In order to solve this problem, the invention device is provided with a data erasing means (7) wherein the energy applied to a heating means is varied so that the recording medium can be heated to the predetermined temperature. Furthermore, a recording medium, which has been used many times until reuse thereof is no longer possible, is discriminated and discarded, so that the recording medium incapable of recording is prevented from being used.

Description: Field of the Invention This invention relates to a recording and erasing method for recording an image on a thermoreversible recording medium and for erasing such a recorded image therefrom by controlling a quantity of energy applied thereto. Background of the Invention Up to now, efforts have been made to develop recording systems (see for example EP-A-0461606 or EP-A-0468237) which can repeatedly record and erase an image on and from a thermoreversible recording medium which becomes black, or transparent and colorless depending upon quantities of thermal energy applied thereto. Japanese patent laid-open publications No. Sho 57-77140 and Hei 2-188294 propose examples of thermographic materials for such a recording medium. The former Japanese publication exemplifies a thermoreversible recording medium comprising layers of thermoreversible material of a whitening group applied on the surface of a glass or plastic substrate. This material inverts its state at two transition temperatures t 1 and t 2 (t 1 < t 2 ). When heated above the temperature t 2 for a given period of time, the material becomes white. On the other hand, when heated above t 1 but below t 2 for a second given period of time, the material becomes transparent and colorless. Therefore, heating elements of a thermal head associated with an image to be recorded are heated above t 2 , while heating elements associated with an image to be erased are heated above t 1 but under t 2 . The latter Japanese publication discloses a thermoreversible medium including a thermoreversible material of a dye group. When the recording medium contains a dye whose transparency or color changes with temperatures, the medium can be repeatedly used for recording and erasing images such as letters and symbols thereon and therefrom, respectively, similarly to the foregoing thermoreversible medium of the whitening group. The principle of the recording system will be described hereinafter. When a first energy (h 1 ) is applied from a dynamic heat source such as a thermal head, the thermoreversible material is developed to form a first dark image (in black). The image is maintained as it is in a normal environment (temperature and humidity), but is erasable when a second energy (h 2 ) is applied thereto. When the first energy (h 1 ) is applied again, a second image can be formed. Thus, the recording and erasing can be performed repeatedly. Fig. 1 of the accompanying drawings is a schematic view showing the configuration of the foregoing recording medium 1, which comprises a protective film 14, a recording layer 15 including materials such as a dye, an agent for making an image visible/invisible and a binder, and a substrate 16. When the first large energy (h 1 ) of 200 to 300°C is applied onto the recording medium 1 for a short period of time, e.g. 1 to 3 ms, in the direction shown by an arrow A, a black image is formed on the recording medium 1, for example. Conversely, when the second small energy (h 2 ) of 80 - 160°C is applied to the recording medium 1 for a relatively long period of time, e.g. 5 ms to 2 sec, in the direction of the arrow A, the image is erased from the recording medium. Specifically, the recording layer 15 includes an agent for making the image visible/invisible which becomes acid and salt in response to an applied energy, and a leuco dye whose color changes with variations of acidity. Fig. 2 shows phenyl carbonate and organic amine salt as an example of the agent for making the image visible/invisible. Fig. 3 (a) shows a colorless leuco compound and Fig. 3 (b) shows a colored leuco compound. The agent for making the image visible/invisible becomes acid when it is heated above the temperature t 2 , so that lactone rings of the leuco dye are opened. Thus, the leuco dye becomes colored. When heated above the temperature t 1 but under the temperature t 2 , the agent for making the image visible/invisible changes to alkaline, so that the opened lactone rings are closed. Therefore, the leuco dye becomes colorless. This recording medium has characteristics as shown in Figs. 4 and 5. In Fig. 4, the abscissa represents a period of time for voltage supply, and the ordinate represents a recording density. From Fig. 4, it can be seen that the recording medium has the maximum recording density of 1.2 when the recording medium is applied with a voltage for approximately 3 ms. In Fig. 5, the abscissa denotes an erasing temperature and the ordinate a recording density after erasure. In this case, the recording medium is applied with the voltage for 3 ms (i.e. the state where the recording medium has a recording density of 1.2) and is then heated by a heat roller, a thermal head or the like. Fig. 5 shows that the recording medium is completely free from an image near 120°C to 150°C (i.e. the state where the recording medium is similar to that having the density 0.15 prior to the recording). The erasing characteristics are also shown in Figs. 6 and 7, which are obtained in a different manner. Fig. 6 shows a completely black pattern 41 formed by the thermal head on the recording medium 1. Fig. 7 shows the erasing characteristic of the recording system which erases the black pattern of Fig. 6. An energy of 1.0 mJ/dot and an energy of 0.6 mj/dot are applied to the recording medium in the direction shown by an arrow B for the recording and erasing, respectively. Referring to Fig. 7, it can be seen that the erasing is not complete at the beginning of the erasing process (i.e. about the first to 30th lines in the black image) and substantially after the 300th and succeeding lines of the black image. The head portion of the recorded image is not erased because the thermal head does not reach its effective temperature. This is because heating elements of the thermal head take a certain period of time to become effective even when thermal head is left at room temperature (without applying a voltage thereto for a while) and is heated under such a condition. The thermal head is not elevated to its effective temperature until the tenth line is being erased. In other words, the thermal head is unstable in its operation until it is sufficiently activated. The reason why the image is not erased in a portion following a 300th line is that the heating elements become too hot in the heated thermal head. Two kinds of energy are reserved in the thermal head. One is a part of the energy generated by the heating elements and the other is the energy which is used to erase a previous line and both energies remain accumulated around the heating elements. Both of these energies raise the temperature of the heating elements which are repeatedly heated for every line. Thus, the thermal head becomes too hot to erase the recorded image. Fig. 8 shows a comparison of erasing characteristics on a large recording medium of A4 size and a small recording medium of a card size. In Fig. 8, the abscissa represents the numerical order of a line to be erased, and the ordinate represents the recording density after erasure. The larger the recording medium, the more incomplete the erasure. The conventional recording and erasing system for the thermoreversible recording medium adopts a method in which energies are applied to the recorded image so as to make it invisible. In other words, the recorded image to be erased is heated at the temperature which is above t 1 but under t 2 as mentioned above. As described so far, the thermoreversible recording medium tends to vary its reflectance and recording density somewhat depending upon its recording and erasing history. In other words, the recording medium shows different degress of reflectance and recording densities at the recorded and erased areas and at the areas which have never been recorded and erased. Therefore, incompletely erased images sometimes remain vaguely on the recording medium in a manner such that they are faintly visible. Prior art recording and erasing systems suffer from the problem that erasure is somewhat incomplete. Furthermore, there are few recording media which are completely thermoreversible. Usually, the more often they are used, the poorer they become, and finally they will become unusable. During repeated use, the recording medium extensively undergoes physical and chemical changes so that it may become worn out. Furthermore, the recording medium may have its protective film and thermoreversible film damaged by heat and pressure applied thereto via the thermal head as a heating means. Therefore, the user has to determine whether or not the recording medium in use is still usable, and remove the unusable recording medium. If such a unusable recording medium is continuously used since the user is not aware of its reduced performance, either recording or erasing cannot be carried out thereon, which will be inconvenient to the user. Such determination on the performance of the recording medium will be troublesome to the user. Sometimes, the user might throw away a still usable recording medium, or recording might be performed to no avail on an unusable recording medium. Summery of the Invention This invention is intended to overcome the foregoing Problems encountered with prior art systems. It is an object of the invention to provide a recording and erasing method which can erase a previous image from a recording medium so that it is remarkably indistinct, and which can identify a used-up recording medium. This object is achieved by a method as outlined in the attached claims. Brief Description of the Drawings The principles of the present invention are shown Figs. 1 to 8. Fig. 1 shows the configuration of a thermoreversible recording medium 1 in film shape. Fig. 2 shows the structure of an agent for making an image visible/invisible constituting the thermoreversible recording medium. Fig. 3 shows the structure of dye used for the recording medium. Fig. 4 is a graph showing the relationship between a recording density and a voltage-supplying period. Fig. 5 is a graph similar to Fig. 4. Fig. 6 shows an area to be heated for recording and erasing processes on the recording medium. Fig. 7 is a graph showing recording densities of respective lines after the erasing process. Fig. 8 is a graph showing recording densities of respective erased lines. Fig. 9 is a schematic view of a recording and erasing system usable with the method according to the present invention. Fig. 10 shows the configuration of a control unit 5. Fig. 11 shows a first example of a line data and voltage-supplying pulse width table. Fig. 12 shows a second example of a line data and voltage-supplying pulse width table. Fig. 13 shows the configuration of another control unit 5. Fig. 14 shows the configuration of a further control unit 5. Fig. 15 is a timing chart showing the operation of the control unit of Fig. 14. Fig. 16 shows a third example of a line data and voltage-supplying pulse width table. Fig. 17 shows a fourth example of a line information and current-supplying pulse table. Fig. 18 shows the configuration of another recording and erasing system usable with the method according to the invention. Fig. 19 shows a fifth example of a line data and voltage-supplying pulse width table. Fig. 20 is a graph showing the relationship between an voltage-supplying pulse width and the number of lines. Fig. 21 shows a non-erased portion of a recorded image on the recording medium 1. Fig. 22 shows a recorded area and an area to be erased. Fig. 23 shows the configuration of means for generating erasing data. Fig. 24 shows the manner in which a heating head is controlled so that an area wider than the recorded area is heated for the erasure. Fig. 25 shows a further example of how to control the thermal head so that an area wider than the recorded area is heated for the erasure. Fig. 26 shows the configuration of still another recording and erasing system usable with the method according to the invention. Fig. 27 shows the relationship between a heating period for the recording and another heating period for the erasing in the recording and erasing system of Fig. 26. Fig. 28 shows the relationship between a recording means and an erasing means. Fig. 29 is a view similar to Fig. 28. Detailed Description of Preferred Embodiments The invention will be described hereinafter with reference to the drawing figures. Example 1: The recording and erasing system has the structure as shown in Fig. 9. The recording and erasing system is applicable to devices such as an information display, an electronic board and a message board used in a railway station. A thermoreversible recording medium 1 is repeatedly used for the recording and erasing processes, and is in the shape of a film in this example. The recording medium 1 extends around supports 2 and 4 in a manner such that one image area thereof is visible in the direction shown by an arrow C. The supports 2 and 4 are made of material like rubber, and are rotated either clockwise or counterclockwise by a drive source such as a motor, not shown. A heating means 3 comes into contact with the support 2 so as to heat the recording medium 1, thereby perform the recording or erasing thereon. The heating means 3 comprises a thermal head, and has a size substantially equal to the width of the recording medium 1. For instance, when a visible area of the recording medium 1 is approximately of A4 size, the heating means 3 includes approximately 2,500 heating elements (not shown). A control means 5 controls the recording and erasing operations. In operation, the recording and erasing system records an image based on data which are read by a word processor, a scanner or the like, and are transferred to the control means 5. Specifically, the control unit 5 sequentially transfers the image data to the thermal head 3, so that a voltage is applied to the heating elements for a given period of time. When heated, the heating elements provide the recording medium 1 with joule heat. Under this condition, the generated energy has a sufficiently high temperature so that the recording medium is elevated to a temperature above the second transition temperature t 2 mentioned above. Thereafter, the recording medium 1 is developed based on the image data. Then, the developed recording medium 1 is fed by one line in the direction D by a means such as a motor (not shown). Thereafter, the foregoing operation is repeated so as to record the image on the recording medium. To erase the recorded image, the data which are the same as those for the recording will be supplied to the thermal head 3 from the control unit 5 or from an external unit (not shown) so that the thermal head 3 is supplied with voltage and heated for a given period of time. In this case, the energy to be applied has a temperature above the first transition temperature t 1 but below the temperature t 2 . Thus, one line of the image is erased. This erasing process is repeated until the entire image is erased. The foregoing describes the basic recording and erasing operations. The control unit 5 plays a very important role in the recording and erasing system, and has the configuration as shown in detail in Fig. 10. An input terminal 6 receives image data from an external source, not shown. An erase data generating unit 7 outputs a signal, e.g. "1", so as to heat heating elements of the thermal head. A selector 8 supplies either the image or erasing data to a voltage supply control unit 9 (to be described later). In this example, the control unit 5 prepares erasing data therein. When the erasing data are supplied from the external source (not shown), both the erasing data generating unit 7 and the selector 8 will be dispensable. The voltage supply control unit 9 control clock pulses, latch pulses, voltage-supplying pulses, voltages and so on to be applied to the thermal head 3. A CPU 10 not only controls the control unit 5 but also transfers data on voltage-supplying pulse width or applied voltage to the voltage supply control unit 9. A ROM 11 stores programs for the control unit 5 and data on the voltage-supplying pulse width or applied voltage. To erase the recorded image, the selector 8 is set to a portion (Fig. 10) so as to transfer the erasing data to the voltage supply control unit 9 from the erasing data generating unit 7. Simultaneously, the CPU 10 designates an address in the ROM 11, so that data on the voltage-supplying pulse or applied voltage are transferred to the voltage-supplying control unit 9, which controls the thermal head 3 based on the received data. The ROM 11 has a table as shown in Fig. 11. To control the thermal head based on the voltage-supplying pulse width, the CPU 11 outputs voltage-supplying pulse width data associated with the address data (line data). The CPU 11 controls the thermal head based on an applied voltage in the similar manner. Further, it is possible to perform the foregoing control based on both the voltage-supplying pulse width and the applied voltage. In the table of Fig. 11, the width of the voltage-supplying pulse is gradually reduced from the first line and so on. The 30th and succeeding lines have the pulse width of 10 ms. A position away from the first line to increase the pulse width depends upon characteristics of the thermal head 3, and heat radiating performance of members around the thermal head 3. It is remarkably effective to apply greater energy to the first line of the image to be erased. In such a case, it is preferable to apply to the first line an energy which is 1.1 to 1.5 times as large as that applied to the remaining lines of the image. In this case, the foregoing heat radiating characteristics affect the determination of which line should be applied with greater energy. Example 2: In the example 1, the ROM 11 stores the operation sequence program and the data on the voltage-supplying pulse width or data on the voltage to be applied. In response to the designated address, the ROM 11 provides the CPU 10 with the program and the forgoing data. Then, the CPU 10 transfers the data to the voltage supply control unit 9. Alternatively, a ROM table 12 is provided for storing only the data on the voltage-supplying pulse width and a voltage to be applied as shown in Fig. 13. In response to the address designated by the CPU 10, the ROM 12 directly transfers the foregoing data to the voltage supply control unit 9. Example 3: It is acceptable to connect an output of a line counter 13 to the ROM table 12 so that the ROM table 12 outputs the data to the voltage supply control unit 9. In this case, the CPU does not designate the address. The line counter 13 receives data such as a reset signal and a clock signal, and outputs line data. In operation, prior to the erasing, the output of the line counter 13 is cleared to "0" by the reset signal. The line counter 13 is incremented by one (1) by a clock signal each time one line is erased. When the lines are erased as required, another reset signal is resupplied to the line counter 13 so as to clear its output to "0". The line counter 13 repeats this operation. The output "0" of the line counter 13 represents the first line in the line data. Specifically, when the ROM table 12 has the contents as shown in Fig. 12, a pulse having a 15-ms width for the first line is applied to the thermal head 3. For the second and succeeding lines, pulses of a 10-ms-width are applied to the thermal head 3. Example 4: It is also conceivable for the CPU 10 to calculate the voltage-supplying pulse width or voltage to be applied and to output data on these items without the provision of the ROM table. In this case, an empirical formula is derived from experiment data so as to calculate the voltage-supplying pulse width of voltage to be applied. Further, the values shown on the table may be stored in either a combination circuit or a sequential circuit instead of the ROM or RAM. A number of variations are possible without departing from the scope of this application. As shown in Fig. 16, the voltage-supplying pulse of the first line may be smaller in width than that of the second line. This measure is sometimes taken when there is no image to be erased in the first line but an energy is applied just for convenience. Conversely, even when there is an image portion to be erased in the first line, the energy applied in 1.3 ms is larger the energy applied in 10 ms for the sixth and succeeding lines, so that the image portion can be erased substantially completely. As shown in Fig. 17, voltage-supplying pulses having the widths of 20 ms and 15 ms may be alternately applied. Application of such pulses is effective to stabilize the temperature at the leading edge of the recording medium, so that substantially complete erasing can be accomplished. In this example, the pulses are controlled with respect to their widths when they are applied to the thermal head 3. Alternatively, the similar effect can be attained by controlling the number of pulses applied to the thermal head. Specifically, the number of pulses for respective lines is stored in the ROM table 12. The voltage supply control unit 9 controls pulses so that they are applied to the thermal head 3 according to the preset number. Example 5: Fig. 18 is a schematic view of a recording and erasing system according to a fifth example. The recording and erasing system may be applied to make a record of the balance on a prepaid card, for example. The recording and erasing system comprises a reusable recording medium 1, a support 2 serving as a platen roller, a thermal head 3 as long as the width of the recording medium 1, a voltage supply control unit 9, a ROM table 12, and a line counter 13, all of which function similarly to those mentioned in the foregoing examples. This example is characterized in that energy to be applied is controlled by checking at least the numerical order of a line to be erased. A recorded image is erased by applying energy in the same manner as that for recording an image. Otherwise, the recorded image is erased by applying energy as if a complete black image is recorded. In the former case, the recorded image is stored in the memory beforehand, and energies different from those for the recording are applied to the thermal head 3. Alternatively, erasing data are transferred to the thermal head 3 from the external source as is done when recording an image. For this purpose, the recording medium has to be very precisely advanced so as to minimize non-erased image portions. This is because the erasure should be carried out in complete agreement with the recorded image. In the latter case, the erase data are set to "1" so that the thermal head 3 can be heated by the energy for the erasure. The erasing process using the all-black pattern (Fig. 6) will be carried out as follows regardless of the type of image to be erased. In this case, the thermal head 3 is heated so as to apply a lower energy (second energy h 2 ) than the recording energy to the recording medium. As described with reference to Fig. 7, the longer the thermal head 3 is heated, the more incompletely the image will be erased because of energy accumulated in the heating elements. The present invention is aimed at overcoming this problem. The recording and erasing system includes at least a line counter 13 for checking the numerical order of a line to be erased. Based on an output from the line counter 13, energy to the thermal head 3 is gradually reduced. In this example, the thermal head 3 is not selectively but continuously heated for the all-black image pattern. Therefore, it is possible to reliably know the temperature increase of the thermal head 3 by checking the numerical order of a line to be erased. At least the line counter 13 and the voltage supply control unit 9 suffice for precise and reliable erasure. Specifically, Fig. 19 is the ROM table 12 showing the contents thereof, i.e. correspondence of the line data and the voltage-supplying pulse width which are output of the line counter. This table can be easily prepared through experiments or calculation. For example, the temperature of the thermal head 3 is designed to be within the erasing temperature range of the reusable recording medium 1 as shown in Fig. 20. The line counter 13 checks the numerical order of a line to be erased, which corresponds to a period of time after heating the thermal head 3, or positional data (i.e. distance). The foregoing period of time or positional data can also be derived by performing calculations in terms of the erasing cycle or the extent to which the motor is rotated. To reduce the memory capacity or make the circuitry compact, the contents of the ROM table 12 may be determined for every plurality of lines. Example 6: A sixth example will be described hereinafter. Insufficiently erased portions will be left if the image to be erased is in complete agreement with the recorded image. This phenomenon is caused by a number of factors. One of them is a positional shift between the recording medium carrying the image to be erased and the thermal head. Peripheral areas of the image are often left indistinctly visible. To overcome this positional shift, the recording medium should be moved in a precise relationship with the thermal head, which inevitably makes the recording and erasing system very expensive. A second factor is that since the thermal head takes time to become hot, the leading edge of the recording medium is not sufficiently heated at the initial stage. A third factor is that energy tends to leak from the opposite side edges of the recording medium, which are slow to become hot. The trailing edge of the image is sometimes left incompletely erased because of energy accumulated in the thermal head. The peripheral edge 41a of the recorded image tends to be left non-erased as shown in Fig. 21. It is also an object of the invention to overcome this problem inexpensively and reliably. The thermal head 3 is used for the erasure as in the foregoing examples. To erase the opposite side edges of the image completely, more heating elements are used than those for the recording. Specifically, when the thermal head 3 has 400 heating elements, the tenth to 350th heating elements (in the area A in Fig. 22) are selectively heated so as to form an image. To erase the image, the fifth to 355th heating elements are heated (in the area B in Fig. 22). Thus, the erase area 42 of the image is wider than the recorded image area 41 across the recording medium. When the ninth to 351st heating elements are heated to erase the image, i.e. one heating element is increased on each side edge of the image, the image can be erased to a sufficient extent. To erase the image perfectly, it is preferable to heat three or more heating elements beyond each side edge of the image. The number of heating elements to be heated depends upon the performance of the thermal head to be used, and is not limited to the above-mentioned values. Furthermore, it is also possible to vary the number of heating elements, e.g. one heating element on the right side and two heating elements on the left side. The recording and erasing system of this example has the configuration as shown in Fig. 23. The unit for preparing data to be input to the thermal head 3 comprises an input terminal 6, an erasing data generating unit 7 for issuing a "1" signal to heat the thermal head 3, a selector 8, a line memory 21, and an address control unit 22 for the line memory 21. An output from the line memory 21 is supplied to a voltage supply control unit 9. In operation, recording data are input to the input terminal 6 from an external source, and are transferred to the line memory 21 via the selector 8. In this case, the address control unit 22 determines an address to be input. Specifically, referring to Fig. 24, the address control unit 22 clears the line memory which is capable of storing 500 data (i.e. emits the signal "0" denoting non-heating). Next, the address control unit 22 sets an address 100 to be output, inputs the recording data, increments the input data, stores the recording data in the manner as shown in Fig. 24 (2), and transfers the recording data corresponding to the address 1 and succeeding addresses to the voltage supply control unit 9 in succession. To erase the recorded image, the selector 8 is set to its lower position, the line memory 21 is cleared, the address control unit 22 generates a value (i.e. 99 in this case) by subtracting one (1) from the address to which the head of the recording data are input, and the data corresponding to the signal "1" is sequentially stored in the line memory for the 99th and succeeding lines. The line memory stores the data "1" up to the end address +1 of the recording data. Therefore, the recording width +2 is equal to the erasing width. In this example, the area to be erased varies with the recording data. Alternatively, it is possible to determine the erasing area to be invariable. In such a case, since it is not necessary to derive an address from the recorded data, the foregoing mechanism will be simplified. For instance, the erasing data generating unit 7 and selector 8 may be dispensed with, so that the data on the signal "1" may be stored during the erasure. Further, both the line memory 21 and the address control unit 22 may be dispensed with, and the selector 8 is operated to select either the recording data from the input terminal 6 or the erasing data from the erasing data generating unit 7, so that the number of heating elements to be heated for the erasing is greater than the number of heating elements to be heated for the recording. Alternatively, heating means are separately provided for the recording and erasing. This arrangement is also as effective as those mentioned above. The erasing data from the input terminal 6 are generated so that the erasing area is larger than the recorded area. To prevent an insufficient erasure at the leading or trailing edge of the recording medium, the area to be erased starts at a position in front of the head of the image and terminates at a position beyond the end of the recorded image. Referring to Figs. 26 and 27, the recording and erasing system of this example comprises the thermoreversible recording medium 1, roller 2, thermal head 3, CPU 10, and a sensor 31 for detecting the leading edge of the recording medium 1. In operation, when the recording medium 1 is in the shape of a card (Fig. 25), the leading edge of the card is set to "0". To erase the image, the recording medium is heated at the timing A. To record the image, the recording medium is heated at the timing B. Then, the heating is finished at the timing D in the former case. Conversely, the heating is finished at the timing C to record the image. The relationships of these timings is 0 ≤ A < B < C < D. When the recording medium 1 is loaded into the recording and erasing system in the direction shown by an arrow, the sensor 31 detects the leading edge of the recording medium 1, and notifies this to the CPU 10. At the timing B, the CPU 10 commands the control unit 5 to heat the thermal head 3 until the timing C. At the timing C, the CPU 10 instructs to stop heating the thermal head 3. To erase the image, the recording medium 1 is loaded into the recording and erasing system. Detecting the leading edge of the recording medium 1, the sensor 31 notifies this to the CPU 10. At the timing A, the CPU 10 commands the control unit 5 to heat the thermal head 3, which is heated until the timing D. In this case, heating is controlled based on a period of time or a position after the detection of the leading edge of the recording medium, or a rotational extent of the motor. In this example, the thermal head 3 is used for both the recording and erasing processes. Alternatively, two heating units may be discretely used for the recording and erasing processes. Further, a heat roller may be used as a heating means for the erasing process. In the latter case, the heat roller may be continuously kept heated within the erasing temperature. Example 7: In this example, the recording and erasing system is characterized in that the width of the erasing unit is larger than the width or maximum recording width of the recording medium, and that heating units are discretely provided for the recording and erasing processes. For instance, Figs. 28 and 29 show the relationship between the thermal heads 3 for the recording and the heat rollers 51 for the erasing process, respectively. When the recording area of the thermal head 3 is wider than the erasing area of the heat roller 51, a remarkably wide area might be left insufficiently erased. This means that the recording medium is not reusable. The erasing units whose erasing areas are wider than the recording medium can assure sufficient erasure of the image therefrom. When the thermal head 3 has the recording width which is smaller than its own length, the erasing unit should have a width larger than the recording width. Here, the term "width of the erasing unit" represents a width of the recording medium which can be heated by the erasing unit. The foregoing description mainly relates to the relationship between the thermal head 3 and the heat roller 51. The recording and erasing processes can be effectively carried out by separate thermal heads 3 for the recording and erasing processes. The foregoing examples may be used in combination. The material of the recording medium is not limited to particular ones, but may be of materials such as organic compounds with low moleculars, dyes, high polymers refined by the phase-separation, crystalline high polymers refined by the phase-change, high polymeric liquid crystalys refined by the phase-transformation, thermochromics, polymer blends, and so on. Industrial Applicability The recording medium used with the present invention is applicable as a parking card, a prepaid card, a commuter ticket and so forth. Repeated use of such cards is very effective in the conservation of natural resources. Further, contents of previous recording will not be revealed when the recording medium is reused.

Claims: A method for repeatedly recording and erasing an image on and from a thermoreversible recording medium ( 1 ), comprising the steps of applying heat to said recording medium ( 1 ) by applying energy to a heating means ( 3 ), wherein the energy applied for recording is different from the energy applied for erasing, characterized by the step of, during erasing an area of said recording medium ( 1 ), decreasing the amount of energy applied to said heating means ( 3 ), wherein the maximum amount of energy is applied to a leading portion of said area to be erased. A method according to Claim 1, characterized in that said applying of energy to said heating means ( 3 ) is effected in units of lines. A method according to Claim 2, characterized in that said leading portion of said area to be erased consists of only one line. A method according to Claim 2, characterized in that said leading portion of said area to be erased consists of a plurality of lines. A method according to Claim 4, characterized in that the amount of energy being applied to one of said plurality of lines differs from the amount of energy being applied to a subsequent one of said plurality of lines. A method according to Claim 5, characterized in that the amount of energy applied to said subsequent one of said plurality of lines is gradually decreased compared to the amount of energy being applied to the preceding line. A method according to any one of the preceding Claims, characterized by the step of effecting a relative movement between said recording means ( 1 ) and said heating means ( 3 ) during recording and erasing. A method according to Claim 7, characterized in that said recording medium ( 1 ) and said heating means ( 3 ) have elongated shapes and the longitudinal extension of said recording medium ( 1 ) is arranged perpendicular to the longitudinal extension of said heating means ( 3 ) such that said relative movement is effected in units of a line. A method according to any one of Claims 1 to 8, characterized by applying the energy to said heating means ( 3 ) in form of pulses, wherein the width, the amplitude or the number of said pulses is varied in order to be able to vary the energy amount applied to said heating means ( 3 ). A method according to any one of Claims 2 to 9, characterized by the step of evaluating the amount of energy to be applied to the heating means ( 3 ) in correspondence with the actual line number either by accessing a ROM ( 11 ), a ROM table ( 12 ) or by calculating an empirical formula. A method according to any one of Claims 2 to 9, characterized by the step of using a line counter ( 13 ) for evaluating the amount of energy to be applied to the heating means ( 3 ) in correspondence with the actual line number by accessing a ROM table ( 12 ). A method according to any one of the preceding Claims, characterized in that an area ( 42 ) of said recording medium ( 1 ) used for erasing is greater than an area ( 41 ) of said recording medium ( 1 ) used for recording. A method according to Claim 12, characterized in that said heating means ( 3 ) comprises a plurality of heating elements, wherein the number of heating elements used for erasing is greater than the number of heating elements used for recording. A method according to Claim 12 or 13, characterized in that the number of lines used for erasing is greater than the number of lines used for recording.

Inventors: OHNISHI MASARU, YAMADA KEIKI, OHNISHI, MASARU, YAMADA, KEIKI, OHNISHI, MASARU MITSUBISHI DENKI KABUSHIKI KAISHA, YAMADA, KEIKI MITSUBISHI DENKI KABUSHIKI KAISHA
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: B41M5
IPC codes: B41M5