Abstract:
Described is a sensing device of a surface acoustic wave (SAW) touch panel having a new reflector columns and rows arrangement. As compared to the conventional design in the art where each of the reflector columns and rows are arranged from thinness to thickness, each of the arrangements of the reflector columns and rows herein is composed of a plurality of uniformly disposed reflectors having several sub-reflectors isolated with a gap or gaps.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. application Ser. No. 11/858,392, filed on 09/20/2007, which is herein incorporated by reference for all intents and purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a touch panel and particularly to a sensing device of a surface acoustic wave (SAW) touch panel in which the reflector columns and rows are each formed by uniformly arranged reflectors having a gap or gaps therein. 
         [0004]    2. Description of the Prior Art 
         [0005]    Surface acoustic wave (SAW) touch panel is a touch panel which determines a touch position thereon by detecting a vibration signal at a target position. Specifically, a transducer having a piezoelectric material therein is utilized to converse an electric signal into the vibration signal and whether the vibration signal is blocked from transmission by a touch at the touch position is judged for the touch position determination by referring to the received vibration signal, generally an output electric signal conversed from the received vibration signal, at the target position of the touch panel. 
         [0006]      FIG. 1A  is a schematic diagram of a structure of a conventional SAW touch panel. As shown in  FIG. 1A , the touch panel  10  comprises a screen area  11  and a reflecting area  12  having a sensing device  13  therein. The sensing device  13  has a first and second X-axis transducers  14   a,    14   b  and a first and second Y-axis transducers  15   a,    15   b.  The second X-axis and Y-axis transducers  14   b,    15   b  are used to receive vibration signals Signal_V  1  and Signal_V 2  conversed from input electric signals Signal_Ei 1  and Signal_Ei 2  emitted from the first X-axis and Y-axis transducers  14   a,    15   a,  respectively. In addition, the sensing device  13  also includes a first and second Y-axis reflecting units  16   a,    16   b  and a first and second X-axis reflecting units  17   a,    17   b.  Each of the first and second X-axis and Y-axis reflecting units  16   a,    16   b,    17   a,    17   b  includes a plurality of reflector r each having the reflecting-in-part and transmitting-in-part characteristic. In this case, the vibration signals Signal V 1  and Signal V 2  required for detecting a touch position P on the X- and Y-axes of the screen area  11  can proceed along each of the first and second X-axis and Y-axis reflecting units  16   a,    16   b,    17   a,    17   b.  In general, each of the reflectors r in the first and second X-axis and Y-axis reflecting units  16   a,    16   b,    17   a,    17   b  is a line layer printed on a glass substrate of the touch  10  and thus has a low cost. In addition, the reflectors r in the first and second X-axis and Y-axis reflecting units  16   a,    16   b,    17   a,    17   b  are arranged from thinnest to thickness (viewed from the proceeding directions of the vibrations Signal_V 1  and Signal_V 2 , respectively), respectively. This is simply because when the thinness to thickness configuration of the reflecting units  6   a,    16   b,    17   a,    17   b  is absent, the intensity of the vibration signals Signal_V  1  and Signal_V 2 , undesirably becomes smaller as the vibration signals Signal_V  1  and Signal_V 2  proceed longer along a single respective X- or Y-axis reflecting units  16   a,    16   b,    17   a,    17   b,  and thus the touch position sensing ability becomes weaker for the touch point P associated with the farer side of the single respective X- or Y-axis reflecting units  16   a,    16   b,    17   a,    17   b.  Therefore, the thinness to thickness configuration is provided to each of the reflecting units  16   a,    16   b,    17   a,    17   b  for compensation for this effect.  FIG. 1B  and  FIG. 1C  are waveform plots of Signal_Eo 1  and Signal_Eo 2  when the touch point P exists on and is absent from the SAW touch panel shown in  FIG. 1A , respectively. As shown in  FIG. 1B  and  FIG. 1C , Vy is the waveform of the output electric signal Signal_Eo 1  and corresponds to an X-axis coordinate of the touch point P on the SAW touch panel  10 . On the other hand, Vx is the waveform of the output electric signal Signal_Eo 2  and corresponds to a Y-axis coordinate of the touch point P. It can be seen that the output electric signal Vx has a longer signal span than that of the output electric signal Vy. This is because the vibration signal Signal_V 2  corresponding to the output electric signal Vx experiences a longer path than that of the vibration signal Signal_V 1  corresponding to the output electric signal Vy. In  FIG. 1C , there is a notch on the waveform of the output electric signal Vx and Vy, respectively, with which the touch position P may be determined. In addition, at the beginning of both the output electric signals Vy and Vx, there is a spike, which is resulted from the fact that the vibration signals Signal_V  1  and Signal_V 2  from the input electric signals Signal_Ei 1  and Signal_Ei 2  are directly received by the second X-axis transducer  14   b  and the second Y-axis transducer  15   b  via the second X-axis reflecting unit  17   b  and second Y-axis reflecting unit  16   b.    
         [0007]    However, the SAW touch panel  10  having the thinness to thickness configuration also has its demerits. Owing to the thinner arrangement portion of the reflectors at each of the reflecting units  16   a,    16   b,    17   a,    17   b,  the touch position P may sometimes associate with between two neighboring reflectors in a single reflecting units  16   a,    16   b,    17   a,    17   b.  In this case, the determination of the touch position P on the SAW touch panel  10  is not ideal enough. 
         [0008]    In this regard, the present invention sets forth a sensing device of a SAW touch panel, which may well overcome the problem encountered in the prior art. 
       SUMMARY OF THE INVENTION 
       [0009]    It is, therefore, an object of the present invention to provide a sensing device of a surface acoustic wave (SAW) touch panel, so as to overcome the problem encountered in the prior art. 
         [0010]    The objectives of the present invention can be achieved by the following technical schemes. The present invention proposes a surface acoustic wave (SAW) touch panel, which includes: a substrate for providing transmission of a SAW; a reflector array including a plurality of pairs of reflectors, each pair of reflectors determining a path on the substrate, respectively, wherein these reflectors include a plurality of dashed-line reflectors, each dashed-line reflector including a plurality of sub-reflectors spaced apart by at least a gap; at least one transmitter for generating a SAW; and at least one receiver for generating a signal based on the SAW transmitted by each path, wherein the physical total length of the pair of reflectors that transmit the SAW on each path determines the amount of the SAW transmitted on the path. 
         [0011]    The objectives of the present invention can further be achieved by the following technical schemes. The present invention proposes a method for configuring a reflector array of a surface acoustic wave (SAW) touch panel, which includes: providing a substrate for providing transmission of a SAW; determining the locations of a plurality of pairs of reflectors of the reflector array on the substrate; providing the reflector array based on the locations of the plurality of pairs of reflectors of the reflector array on the substrate, each pair of reflectors determining a path on the substrate, respectively, wherein these reflectors include a plurality of dashed-line reflectors, each dashed-line reflector including a plurality of sub-reflectors spaced apart by at least a gap; providing at least one transmitter for generating a SAW; providing at least one receiver for generating a signal based on the SAW transmitted by each path, wherein the physical total length of the pair of reflectors that transmit the SAW on each path determines the amount of the SAW transmitted on the path; and adjusting the total length of the gap of each dashed-line reflector based on the signal, so that the signal is maintained at a zero-value range during a detection period. 
         [0012]    The objectives of the present invention can further be achieved by the following technical schemes. The present invention proposes a method for configuring a reflector array of a surface acoustic wave (SAW) touch panel, which includes: providing a substrate for providing transmission of a SAW; determining the locations of a plurality of pairs of reflectors of the reflector array on the substrate; providing the reflector array based on the locations of the plurality of pairs of reflectors of the reflector array on the substrate, each pair of reflectors determining a path on the substrate, respectively, wherein these reflectors include a plurality of dashed-line reflectors, each dashed-line reflector including a plurality of sub-reflectors spaced apart by at least a gap; providing at least one transmitter for generating a SAW; providing at least one receiver for generating a signal based on the SAW transmitted by each path, wherein the physical total length of the pair of reflectors that transmit the SAW on each path determines the amount of the SAW transmitted on the path; and adjusting the locations of these reflectors based on the signal, so that the signal is maintained at a zero-value range during a detection period. 
         [0013]    The objectives of the present invention can further be achieved by the following technical schemes. 
         [0014]    The physical total length of each said dashed-line reflector does not include the lengths of all the gaps. 
         [0015]    In said reflector array, the closer a pair of reflectors is to the at least one transmitter and the at least one receiver, the longer the total length of all the gaps of the pair of reflectors. 
         [0016]    The magnitude of said signal is determined based on the length of the path and the physical total length of the pair of reflectors that transmit the SAW. 
         [0017]    The lengths of said reflectors are the same, wherein the length of each dashed-line reflector includes the lengths of all the gaps. 
         [0018]    The heights of said reflectors are the same. 
         [0019]    Separations between each reflector and its neighboring reflectors are equal. 
         [0020]    Compared to the prior art, the reflector array provided by the present invention includes a plurality of dashed-line reflectors, such that the SAW can pass through the gaps of the dashed-line reflectors without being obstructed, thereby reducing the difference in intensities between the SAWs transmitted on each reflecting path, and the separations between reflectors can be made equal. 
         [0021]    Since the reflectors in the first and second X-axis and Y-axis reflecting units of the sensing area of the SAW touch panel are uniformly arranged, the problem which a touch point can not be effectively sensed on the same touch panel associated with the thinly distributed reflectors can be overcome. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The above and other objects of the present invention will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein: 
           [0023]      FIG. 1A  is a schematic structure for illustrating how a touch position made on a conventional surface acoustic wave (SAW) touch panel is detected; 
           [0024]      FIG. 1B  is waveform plots of two output electric signals from the SAW touch panel shown in  FIG. 1A  when no touch input is impinged on the same, respectively; 
           [0025]      FIG. 1C  is waveform plots of two output electric signals from the SAW touch panel shown in  FIG. 1A  when there is a touch input impinged on the same; 
           [0026]      FIG. 2A  is a schematic structure for illustrating how a touch position on a SAW touch panel according to the presenting invention is detected; 
           [0027]      FIG. 2B  is waveform plots of two output electric signals from the SAW touch panel shown in  FIG. 2A  when no touch input is impinged on the same, respectively; and 
           [0028]      FIG. 2C  is waveform plots of two output electric signals from the SAW touch panel shown in  FIG. 2A  when there is a touch input impinged on the same, respectively. 
           [0029]      FIG. 3  is a flowchart illustrating a method for configuring the reflector array of the SAW touch panel according to the present invention; and 
           [0030]      FIG. 4  is a flowchart illustrating another method for configuring the reflector array of the SAW touch panel according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    The present invention is a sensing device of a surface acoustic wave (SAW) touch panel according to the present invention, and will be described taken in the preferred embodiments with reference to the accompanying drawings. 
         [0032]    Referring to  FIG. 2A , which a schematic structure for illustrating how a touch position on a SAW touch panel according to the present invention is detected. As shown, the SAW touch panel  20  is a rectangular device which may be measured with an X-axis and a Y-axis and has a screen area  21  and a reflecting area  22  at which a sensing device  23  is disposed. The sensing device  23  includes a first and second X-axis transducers  24   a  and  24   b  and a first and second Y-axis transducers  25   a  and  25   b.  The sensing area  23  further includes a first and second Y-axis reflecting units  26   a  and  26   b  and a first and second X-axis reflecting units  27   a  and  27   b.  The first and second Y-axis and X-axis reflecting units  26   a,    26   a,    27   a,    27   b  are vertically or horizontally arranged circumferentially with respect to the screen area  21 . The first and second Y-axis reflecting units  26   a  and  26   b  (also termed as the first and second reflecting columns herein) each include a first number of reflectors r while the first and second X-axis reflecting units  27   a  and  27   b  (also termed as the first and second reflecting rows herein) each include a second number of reflectors r. In addition, all or some of the reflectors r each have the transmitting-in-part and reflecting-in-part characteristic and each have a plurality of sub-reflectors r s  each separated from the neighboring one or ones among the plurality of sub-reflectors r s  with a gap g. The first and second X-axis reflecting units  27   a  and  27   b  and the first and second Y-axis reflecting units  26   a  and  26   b  are collectively called a reflector array. In addition, a reflector with at least one gap g in the form of a dashed line is called a dashed-line reflector. The lengths of these reflectors r are the same. The length of each dashed-line reflector includes the lengths of all the gaps. Compared to the dashed-line reflectors in the present invention, reflectors r used for reflecting vibration signals Signal V 1  and Signal V 2  in the prior art are solid-line reflectors. 
         [0033]    In real operation, an electric signal Signal_Ei 1  is inputted into the first X-axis transducer  24   a  of the SAW touch panel  20 , in which the electric signal Signal_Ei 1  is conversed into a vibration signal Signal_V 1 . The vibration signal Signal_V 1  thus obtained then proceeds along the first Y-axis reflecting unit  26   a  where the vibration signal Signal_V 1  is transmitted in part and reflected in part. The reflected portion of the vibration signal Signal_V 1  is then further reflected by a corresponding reflector r in the second Y-axis reflecting unit  16   b  and finally received by the second X_axis transducer  24   b  after a proceeding path of the reflected vibration signal portion Signal_V 1 , depicted in  FIG. 2A  as A 1 , in which the vibration signal portion Signal_V 1  is conversed into an output electric signal Signal_Eo 1 . Similarly but unconcurrently, an electric signal Signal_Ei 2  is inputted to the SAW touch panel  20  at the first Y-axis transducer  25   a,  in which the input electric signal Signal_Ei 2  is conversed into a vibration signal Signal_V 2 . The reflected portion of the vibration signal Signal_V 2  is then further reflected by a corresponding reflector r in the second X-axis reflecting unit  17   b  and finally received by the second Y_axis transducer  25   b  after a proceeding path of the reflected vibration signal portion Signal_V 2 , depicted in  FIG. 2A  as A 2 , in which the vibration signal portion Signal_V 2  is conversed into an output electric signal Signal_Eo 2 . Finally, the output electric signals Signal_Eo 1  and Signal_Eo 2  are relied upon to determine where the touch point P is located on the SAW touch panel  20  by referring to the input electric signals Signal_Ei 1  and Signal_Ei 2 . 
         [0034]    In the above, that the transducers  24   a  and  24   b  are operated at different time from that of the transducers  25   a  and  25   b  is made to prevent the vibration signals Signal_V  1  and Signal_V 2  from interfering with each other. Correspondingly, the first and second input electric signals Signal_Ei 1  and Signal_Ei 2  are supplied alternatively to the first X-axis and Y-axis transducers  24   a  and  25   a.  As such, any possible touch position on the SAW touch panel  20  can be continuously detected. 
         [0035]    In addition, the output electric signals Signal_Eo 1  and Signal_Eo 2  above mentioned have the waveforms Vy and Vx shown in  FIG. 2B , respectively. 
         [0036]    When a touch position P appears on and contacts with the screen area  21  of the SAW touch panel  20 , the proceeding paths of the first and vibration signals Signal_V 1  and Signal_V 2  associated with the touch position P are blocked, the first and second output electric signals Signal_V 1  and Signal_V 2  each have a decreased level Vy and Vx, respectively, shown in  FIG. 2C . By referring to the point of time the decreased levels Vy and Vx appears, a coordinate (X, Y) of the touch position P contacted with the screen area  21  of the SAW touch panel  20  can be determined. 
         [0037]    Since the sub-reflectors rs is present, the vibration signals Signal_V 1  and Signal_V 2  which may be reflected by the reflectors r located at a rear part of each of the first and second Y-axis and X-axis reflecting units  26   a,    26   a,    27   a,    27   b  (viewed from the directions that the vibration signals Signal_V 1  and Signal_V 2  outputted from the transducers  24   a  and  25   a,  respectively) remain at effective intensities. Namely, the vibration signals Signal_V 1  and Signal_V 2  reflected by the reflectors r located at the rear part of each of the first and second Y-axis and X-axis reflecting units  26   a,    26   a,    27   a,    27   b  (viewed from the same directions) do not decrease is simply because the reflectors r of each of the first and second Y-axis and X-axis reflecting units  26   a,    26   a,    27   a,    27   b  each have the gaps g and the vibration signals Signal_V 1  and Signal_V 2  can better transmit through a fore part of each of the first and second Y-axis and X-axis reflecting units  26   a,    26   a,    27   a,    27   b  to the rear part of the same. 
         [0038]    Therefore, in a best mode of the present invention, the SAW touch panel includes: a substrate for providing transmission of a SAW; a reflector array including a plurality of pairs of reflectors r, each pair of reflectors r determining a path on the substrate, respectively, wherein these reflectors r include a plurality of dashed-line reflectors, each dashed-line reflector including a plurality of sub-reflectors r s  spaced apart by at least a gap g; at least one transmitter (e.g. the first x-axis transducer  24   a  or the first y-axis transducer  25   a ) for generating a SAW; and at least one receiver (e.g. the second x-axis transducer  24   b  and the second y-axis transducer  25   b ) for generating a signal based on a SAW transmitted by each path, wherein the physical total length of the pair of reflectors r that transmit the SAW on each path determines the amount of the SAW transmitted on the path, wherein the physical total length of each dashed-line reflector does not include the lengths of all the gaps g, and the magnitude of the signal is determined based on the length of each path (since the longer the path, the more reflectors the signal has to pass through) and the physical total length of the pair of reflectors r that transmit the SAW. In an example of the present invention, all the reflectors r are dashed-line reflectors. In another example of the present invention, at least one reflector r is not a dashed-line reflector. For example, one or more reflectors at the end of the paths of the vibration signals Signal V 1  and Signal V 2  are solid-line reflectors. 
         [0039]    Compared to the prior art, since the reflectors r of the present invention have gaps g, the vibration signals Signal V 1  and Signal V 2  passing through the gaps g will not be obstructed and attenuated by reflectors r. Therefore, the size (length) of the gap g on each reflector r can be adjusted so as to allow the vibration signals Signal V 1  and Signal V 2  to maintain effective intensities when passing through each reflector. For example, on the paths of the vibration signals Signal V 1  and Signal V 2 , the reflectors r the signals pass through earlier (closer to the first x-axis transducer  24   a  or the first y-axis transducer  25   a ) have larger gaps g, whereas the reflectors r the signals pass through later (closer to the second x-axis transducer  24   b  or the second y-axis transducer  25   b ) have smaller gaps g. As a result, assuming that the emission intensities of the vibration signals Signal V 1  and Signal V 2  are the same, and the height of each reflector r is the same, the intensities of the vibration signals Signal V 1  and Signal V 2  after passing through each reflector r will be greater than the prior art. In other words, the total length of all the gaps g of the reflectors that are closer to the transmitters and/or the receivers is longer. 
         [0040]    Furthermore, the neighboring reflectors r of each of the first and second Y-axis and X-axis reflecting units  26   a,    26   a,    27   a,    27   b  may be arranged with an equidistance, such as a separation sep, that is, the separations sep between each reflector and its neighboring reflectors are the same, without losing the ability to detect the touch position P on the SAW touch panel  20 , owing to the provision of the sub-reflectors r s . In this manner, all the possible touch positions P on the SAW touch panel  20  can be located at the proceeding paths of the reflected portions of the vibration signals Signal_V 1  and Signal_V 2 , respectively. Accordingly, any possible touch position P on the SAW touch panel  20  can be well detected, as contrasted to the case in the prior art where some possible touch positions P may appear between the two neighboring proceeding paths A 1  or/and A 2  with a relatively larger separation and thus can not be perfectly detected. 
         [0041]    In a preferred embodiment, the separation sep of each of the neighboring reflectors of the first and second Y-axis and X-axis reflecting units  26   a,    26   a,    27   a,    27   b  is set to be equal. Each of the neighboring sub-reflectors r s  of each of the first and second Y-axis and X-axis reflecting units  26   a,    26   a,    27   a,    27   b  and a relationship of the gaps among each of the sub-reflectors r s  of the reflectors r of the first and second Y-axis and X-axis reflecting units  26   a,    26   a,    27   a,    27   b  are dependent upon a material forming each of the reflectors r. Further, any one of all the gaps g has an optimal relationship with the other gaps of the reflectors r in the first and second Y-axis and X-axis reflecting units  26   a,    26   a,    27   a,    27   b  obtained by experiment. 
         [0042]    In an embodiment of the present invention, the reflector array on the SAW touch panel can be arranged in such a way that the gaps g between each sub-reflectors r s  and the separations sep between each reflector r are configured according to penetration levels of the SAW with respect to various materials of the reflector. 
         [0043]    For example, a method for configuring the reflector array of the SAW touch panel according to the present invention is shown in  FIG. 3 . First, in step  310 , a substrate is provided for providing transmission of a SAW. In addition, as shown in step  320 , the locations of a plurality of pairs of reflectors r of a reflector array on the substrate are determined, and as shown in step  330 , the reflector array is provided based on the locations of the plurality of pairs of reflectors r of the reflector array on the substrate. Each pair of reflectors r in the reflector array determines a path on the substrate, respectively, and these reflectors r include a plurality of dashed-line reflectors (all or some of the reflectors are dashed-line reflectors), and each dashed-line reflector includes a plurality of sub-reflectors r s  spaced apart by at least a gap g. Further, in step  340 , at least one transmitter is provided for generating a SAW, and as shown in step  350 , at least one receiver is provided for generating a signal based on a SAW transmitted by each path, wherein the physical total length of the pair of reflectors r that transmit the SAW on each path determines the amount of the SAW transmitted on the path. Moreover, as shown in step  360 , the total length of the gap g of each dashed-line reflector is adjusted based on the signal so that the signal is maintained at a zero-value range during a detection period. For example, when the signal on a path is greater than the zero-value range, the total length of the gaps g of the pair of reflectors r on this path is increased. On the contrary, when the signal on a path is smaller than the zero-value range, the total length of the gaps g of the pair of reflectors r on this path is decreased. 
         [0044]    As another example, a method for configuring the reflector array of the SAW touch panel according to the present invention is shown in  FIG. 4 . First, in step  310 , a substrate is provided for providing transmission of a SAW. In addition, as shown in step  320 , the locations of a plurality of pairs of reflectors r of a reflector array on the substrate are determined, and as shown in step  330 , the reflector array is provided based on the locations of the plurality of pairs of reflectors r of the reflector array on the substrate. Each pair of reflectors r in the reflector array determines a path on the substrate, respectively, and these reflectors r include a plurality of dashed-line reflectors (all or some of the reflectors are dashed-line reflectors), and each dashed-line reflector includes a plurality of sub-reflectors r s  spaced apart by at least a gap g. Further, in step  340 , at least one transmitter is provided for generating a SAW, and as shown in step  350 , at least one receiver is provided for generating a signal based on a SAW transmitted by each path, wherein the physical total length of the pair of reflectors r that transmit the SAW on each path determines the amount of the SAW transmitted on the path. Moreover, as shown in step  460 , the location of each dashed-line reflector is adjusted based on the signal so that the signal is maintained at a zero-value range during a detection period. For example, when the signal on a path is greater than the zero-value range, the pair of reflectors r on this path are shifted towards the first x-axis transducer  24   a  or the first y-axis transducer  25   a  (e.g. removing and regenerating reflectors) to shorten the path. On the contrary, when the signal on a path is smaller than the zero-value range, the pair of reflectors r on this path are shifted towards the second x-axis transducer  24   b  or the second y-axis transducer  25   b  to lengthen the path. 
         [0045]    The detection period can be the period for detecting whether a touch exists as shown in  FIGS. 2B and 2C . 
         [0046]    In addition, each of the reflectors r has generally the form of a reflecting line layer made of ink. The reflecting line layer is fabricated on a transparent substrate (now shown), like the sensing device  23  by a printing method. In a preferred embodiment, the transparent substrate is a transparent glass substrate. In an example of the present invention, the height of each reflector r in the reflector array of the present invention is uniform, which can be manufactured all together by one common printing method. 
         [0047]    In addition, the first and second input electric signals Signal_Ei 1  and Signal_Ei 2  can be supplied by a single external signal source (now shown). At this time, a switch may be provided to switch alternatively the signal external signal source to be the first and second input electric signals Signal_Ei 1  and Signal_Ei 2 . In addition, each of the first and second input electric signals Signal_Ei 1  and Signal_Ei 2  takes the form of a signal consisting of bursts. 
         [0048]    In the prior art, when the height of the reflectors r is uniform, the intensity of the SAW being reflected exhibits a gradient. This is because the SAW is gradually attenuated when passing through each reflector r. The amount of attenuation varies with the materials and the heights of the reflectors r. The difference between the intensities of the reflected SAWs affects the level of density of the reflectors r. The greater the difference between the intensities of the reflected SAWs, the greater the difference in the densities of the reflectors r. With the dashed-line reflectors provided by the present invention, the difference between the intensities of the reflected SAWs is minimized; moreover, even the densities of the reflectors can be made uniform. 
         [0049]    It is readily apparent that the above-described embodiments have the advantage of wide commercial utility. It should be understood that the specific form of the invention hereinabove described is intended to be representative only, as certain modifications within the scope of these teachings will be apparent to those skilled in the art. Accordingly, reference should be made to the following claims in determining the full scope of the invention.