Patent Publication Number: US-2005139744-A1

Title: Optical data recognition

Description:
The present invention relates to an improved optically readable surface and in particular, though not exclusively, to an improved mouse mat.  
      Mouse mats have long been used to provide a substrate over which a computer mouse can be moved. The mouse mat is typically provided with a roughened upper surface over which the ball of the mouse can roll without skipping or slipping. By eliminating such slippage the movement of the mouse over the mouse mat is accurately translated into corresponding movement of a cursor on a computer screen.  
      Recent developments in the field of computer peripherals have seen the introduction of optical computer mice. In an optical mouse the mouse ball and its associated movement sensors are replaced by a light source, typically a light emitting diode (LED) and an optical sensor, both of which are provided in the underside of the mouse.  
      It has been observed that the performance of an optical mouse can be affected by changes in the surface over which it is used. This problem has been particularly noticeable when using an optical mouse on a mouse mat having a printed image thereon.  
      According to the present invention there is provided a substrate for an optical position measurement sensor, the substrate having an image presented in a frequency modulated manner and a randomly textured surface overlying said image.  
      The frequency modulated image prevents spatio-temporal aliasing of an optical position measurement sensor used in conjunction therewith, while the random texture of the surface enables said sensor to register portions of the image of uniform colour or image density, or of colours incompatible with any light source of the sensor. It will be appreciated that the term image encompasses, by way of non limiting example, such visually perceptible representations as random or repeating patterns, pictorial representations, photographic images, text, alphanumeric characters, cartoon images and maps. It will be understood that the term random texture is intended to cover a texture which does not embody a repeated pattern of any form.  
      In one embodiment the substrate comprises an image bearing layer and an overlying transparent layer, wherein the image bearing layer is provided with an image presented in a frequency modulated manner and the transparent layer is randomly textured. The transparent layer may comprise a layer of a plastics material. Alternatively the transparent layer may comprise a film of varnish. Such a film may have a thickness of around 5 microns. In yet a further embodiment the surface on which the image is presented may be textured. It will thus be appreciated in such an embodiment that a separate surface layer overlying the image is omitted. Preferably the image is a printed image. In describing the overlying layer as transparent, it will be understood that the this layer may be clear or tinted, but not to such an extent that the image is obscured.  
      In an alternative embodiment the substrate may comprise a single layer having on one side an image printed in a frequency modulated manner and on the other side a random texture. In such an embodiment the textured surface may be formed on the layer, for example during manufacture thereof, prior to application of the image. In an alternative embodiment the texture may be applied to the layer after the application of the image.  
      The substrate may be incorporated into a mouse mat. A mouse mat incorporating such a substrate may be used in conjunction with both optical mice and more conventional roller ball mice. The textured surface is able to interact with the ball of a conventional mouse, while the combination of the optical properties of the printed image and the textured surface enable optical mice to register movement over the substrate.  
      In a preferred embodiment the image bearing layer is printed in a stochastic manner. The substrate may be incorporated into another item, for example a mouse mat. The random texture preferably has a mean depth of around 0.5 mm. That is to say the mean distance between high and low points of said surface is no more than 0.5 mm.  
      According to an alternative embodiment of the present invention there is provided a substrate for an optical sensor, the substrate having an image presented in a frequency modulated manner wherein the image is randomly textured.  
      According to a second aspect of the present invention there is provided a method of forming a substrate for an optical position measurement sensor, the method comprising the steps of: 
          providing an image presented in a frequency modulated manner; and     applying a random texture to the image.        

      The image is preferably provided on a substrate such as, for example, a layer of plastics material or paper. The step of applying the random texture to the image may comprise the step of texturing the image. In an alternative embodiment the step of applying the random texture may comprise the step of applying a transparent layer over the image and applying a random texture to the transparent layer. The transparent layer may comprise, for example, a thin layer of plastics material or a layer of lacquer or varnish applied to the image Application of the random texture to the transparent layer may be achieved by any appropriate means including, for example, urging the image and transparent layer against a former. Such a texturing operation may take place during an injection moulding operation. In such an embodiment the application of texture to the transparent layer may result in both the image and transparent layer being textured. Alternatively the application of texture may result in only the transparent layer being textured.  
      In yet a further embodiment the step of applying the random texture to the image may comprise the step of applying transparent layer having a randomly textured surface over the image.  
      According to a third aspect of the present invention there is provided a method of forming a substrate for an optical position measurement sensor, the method comprising the steps of: 
          providing a transparent layer of plastics material;     applying an image presented in a frequency modulated manner to the transparent layer; and     applying a random texture to the layer.        

      In a preferred embodiment the image is provided on one side of the layer and the random texture applied to the other side of the layer.  
      According to a fourth aspect of the present invention there is provided a method of forming a substrate for an optical position measurement sensor, the method comprising the steps of: 
          providing a transparent layer of plastics material, said layer having a random texture; and     applying an image presented in a frequency modulated manner to the transparent layer.        

    
    
      Embodiments of the present invention will now be described with reference to the accompanying drawings in which:  
       FIG. 1  shows a perspective view of a mouse and a mouse mat;  
       FIG. 2  shows an example of halftone printing;  
       FIG. 3  shows an example of stochastic printing;  
       FIG. 4  shows a cross-sectional view of two components of a first embodiment of the present invention;  
       FIG. 5  shows a cross-sectional view of a mouse mat incorporating the components of  FIG. 4 ;  
       FIG. 6  shows a cross-sectional view of another mouse mat incorporating the components of  FIG. 4 ;  
       FIG. 7  shows a cross-sectional view of a component of a second embodiment of the present invention;  
       FIG. 8  shows a cross-sectional view of a mouse mat incorporating the component of  FIG. 7 ; and  
       FIG. 9  shows a cross-sectional view of another mouse mat according to an embodiment of the present invention. 
    
    
      Referring firstly to  FIG. 1  there is shown a mouse mat, generally designated  10 , and a mouse  12 . The mouse mat is provided with a printed image  14  incorporated therein.  
      The mouse mat  10  may be manufactured by any appropriate means. For example the mouse mat  10  may comprise a foamed synthetic rubber base upon which there is provided respectively a sheet carrying the printed image  14  and a layer of clear plastic overlying the image sheet. Alternatively the mouse mat  10  may be manufactured in an injection moulding operation as described in the present applicant&#39;s co-pending international patent application WO 01/03918 (PCT/GB00/02625).  
      It has been observed that optical computer mice can experience problems when used in conjunction with printed image mouse mats  10  of the type shown. Movement of the mouse  12  over the mouse mat  10  is sometimes not accurately translated into movement of a screen cursor. This phenomenon has been observed when the optical mouse  12  is moved over both the primary image bearing portions  16  of the mat  10  and the relatively uncluttered background portions  18  of the mat  10 . The reasons for these problems will now be discussed.  
      Dealing firstly with the problems associated with the background portions  18  of the mat  10 , these by and large stem from the uniformity thereof. The background portions may typically comprise either unprinted areas corresponding to the base colour of the image carrying sheet, or areas printed in a single uniform colour. Either of these embodiments tend to prevent an optical mouse from operating as there is no discernible “pattern” to read.  
      The problems associated with the primary image bearing portions  16  of the mat  10  stem from the manner in which the image  14  is printed. Typically the image sheets  14  are printed in a manner known as halftone printing or screening. In halftone screening the image to be printed is broken up into an array of dots. The dots are arranged on the printed page in regular lines and columns however their size is varied to recreate the image. The varying of the dot size is commonly referred to as Amplitude Modulation. An enlarged close up representation of an image printed in halftone is shown in  FIG. 2 . From a distance the human eye cannot register each individual halftone dot and hence the overall effect perceived by the viewer is that of coloured areas of varying intensity.  
      The provision of the halftone dots in a regular array can lead to perceived image imperfections by virtue of an effect known as aliasing. Spatial aliasing occurs when two linear arrays or meshes with closely matched periodicity are superimposed and viewed in front of a light background. Temporal aliasing occurs when a regularly occurring event is viewed only briefly and at regular intervals. If the intervals have almost the same period as that of the recurring vent then temporal aliasing occurs. These two effects combined result in spatio-temporal aliasing. This is achieved when a recurring event, which incorporates movement of an object with a pronounced regular pattern, is viewed briefly and at regular intervals. If the speed of movement is such that, between each viewing interval, the object moves approximately the distance equal to the pattern repetition distance, an appearance of slow or even reverse motion is created.  
      The optical sensor of an optical mouse typically scans at a rate of between 100 to 200 frames per second. Thus if the mouse is moved at such a speed that results in the scanning rate being the same or equal to an integer number of dot separations, spatio-temporal aliasing occurs and the mouse sensor does not accurately interpret the movement of the mouse as a whole.  
      The present invention seeks to eliminate the possibility of spatio-temporal aliasing being experienced by the optical sensor of the mouse by utilising images printed in a stochastic manner. Stochastic printing recreates an image using randomly positioned dots rather than uniformly positioned dots of varying size. In stochastic printing an image is created using very fine micro dots which are much smaller than conventional halftone dots. The micro dots are dispersed in a random manner by varying their frequency. Hence the term frequency modulated can be used to describe stochastic printing.  
      As described above, in halftone printing the dots are regularly spaced on a grid. In contrast, stochastic printing dot placement is modulated by the image to be printed, with dots being placed irregularly at a frequency that depends on the image grey value. In halftone printing the size of the dot depends upon image density value, while in stochastic printing the fixed size dots are arranged closer together or farther apart to represent different shades of colour.  
       FIG. 3  shows the same image as  FIG. 2  but rendered in a stochastic manner. It will be appreciated that the elimination of dots placed in a regular grid formation greatly reduces the opportunity for spatio-temporal aliasing to occur at the mouse optical sensor. With regard to the aforementioned problem with plain or unpatterned backgrounds, the present invention addresses this problem by providing a randomly textured and optically transparent layer over the image. The random texture of this layer creates features of light and shadow which are discernible by the mouse optical sensor. Aliasing is prevented by the random nature of the texture.  
      Referring now to  FIG. 4  there is shown a cross sectional view of two layers  24 ,  26  which may be utilised so as to construct a mouse mat  20  as illustrated in the cross-sectional views of  FIGS. 5 and 6 . The layers  24 ,  26  of  FIG. 4  comprise a stochastic image bearing layer  24  and a surface layer  26 . The surface layer  26  is shown with a flat, untextured upper surface  27 , which is to say the surface which faces away from the image bearing layer  24 . In such an embodiment texture is applied to the upper surface  27  dining manufacture of a mouse mat  20  as will be described below. It will be understood that in an alternative embodiment the surface layer  26  may be provided in a pre-textured form.  
      Referring now to  FIG. 5  there is shown a cross-sectional view of a mouse mat, generally designated  20 , comprising a base  22 , a stochastic printed image bearing layer  24  and a surface layer  26  having a randomly textured surface  27 . The mat  20  is formed in an injection moulding operation as described in the above referenced International patent application. The image bearing and surface layers  24 , 26  may both manufactured from plastics materials. In an alternative embodiment the image bearing layer  24  may be manufactured from paper and the surface layer  26  of a plastics material. In such an embodiment the image bearing layer  24  may comprise paper having a weight of 150 GSM and the surface layer  26  may comprise clear polypropylene having a thickness of between 12 to 25 microns, preferably around 20 microns. While in this embodiment the surface layer  26  is described as being clear, it will be appreciated that the layer  26  may be colour tinted.  
      The layers  24 , 26  are laid up in a mould which is configured so as to impart the random texture to the surface  27 . This is achieved by providing a corresponding randomly textured surface on a portion of the mould which, in use, faces the surface layer  26 . The random texture of the mould may be achieved by, for example, a spark erosion process. Plastic is subsequently injected into the mould to form the base  22 . The heat and pressure in the mould incorporate the layers  24 ,  26  to the base  22  and further cause the outer surface  27  of the surface layer  26  to adopt the texture provided in the mould. In the embodiment of  FIG. 5  it will be appreciated that the only the surface layer  26  is randomly textured, with the image bearing layer  24  retaining the substantially planar appearance it embodied prior to moulding of the mouse mat  20 .  
       FIG. 6  shows an alternative embodiment of a mouse mat  20  wherein both the surface layer  26  and, to a certain extent, the image bearing layer  24  are both textured. Such a configuration may occur where the surface layer  26  is particularly thin. The use of a thin surface layer is advantageous in order that the quality of the image is not degraded by the textured surface layer  26 .  
      Referring now to  FIG. 7  the is shown an alternative embodiment of a single layer or sheet  36  which may be used to manufacture a mouse mat such as that generally designated  38  in  FIG. 8 . The sheet  36  is manufactured from a plastics material such as, for example, polypropylene. On one side of the sheet  36  there is provided a film of ink  40  defining an image or indicia which are printed directly onto the sheet  36 . As before, the embodiment of  FIG. 7  shows the outer surface  42  of the sheet  36  to be flat and un-textured.  FIG. 8  shows a mouse mat  38  incorporating the printed sheet  36  of  FIG. 7 . As in the embodiment described above, the mat  38  includes a base  44  of a plastics material which has been injected into a mould containing the printed sheet  36 . A texture has been applied to the sheet surface  42  during the moulding process.  
       FIG. 9  shows a cross-sectional view of an alternative embodiment of a more conventional mouse mat, generally designated  28 . As before the mat  28  comprises a base  30 , a stochastic printed image bearing layer  32  and a randomly textured surface layer  34 . The base  30  comprises a foamed synthetic rubber material such as, for example neoprene. The image bearing layer  32  may comprise paper, alternatively a plastics material of a layer of ink as described above. In the case of paper or plastic, the image bearing layer  32  may be attached to the base  30  by any appropriate means such as, for example, adhesive. The surface layer  34  is a plastics material and is provided with a random textured pattern. As described above this texture may be preformed on the surface layer  34  or alternatively applied to the layer  34  during construction of the mat  28 . It will be appreciated that in the embodiment where the surface layer is preformed with a texture, then such a surface layer will be thicker than for a surface layer which is adapted to receive a the texture after it has been associated with the image bearing layer. A pre-textured surface layer may typically have a thickness of between 100 to 200 microns. The surface layer  34  may be attached to the image bearing layer with adhesive also.  
      As will be readily understood from observing  FIGS. 2 and 3 , stochastic printing provides a clearer, sharper and more detailed image than that which can be achieved by halftone printing. As a result of this stochastic printed mouse mats are perceived to be more visually appealing than halftone printed mats. The texturing of the layer overlying the image ensures that the mats, while being suitable for optical mice, can be used in conjunction with conventional ball mice. The textured surface additionally assists the optical sensor of the mouse to “see”, and hence register movement over, colours which might otherwise be problematic. Optical mice typically utilise a red LED to illuminate the surface over which the mouse is traversing. This has the effect of making areas of red, orange and yellow tone almost disappear against a white background. The texture, as described above, creates features of light and shadow which are discernible by the mouse optical sensor irrespective of this apparent colour loss.  
      A further factor which has been found to affect optical mouse performance is that of “flying height” of the mouse. The flying height refers to the distance between the mousing surface and a datum plane of the optical sensor within the mouse. A typical optimum flying height for a mouse optical sensor is between 2.3 mm to 2.4 mm. This height is typically regulated by the provision of low friction pads on the underside of the mouse body which, in use, bear upon the mouse mat. Tests have indicated that mouse performance deteriorates rapidly if the flying height is exceeded by 0.5 mm or more. To this end the texture of the surface layer advantageously has a depth of no more than 0.5 mm so as to ensure that an acceptable flying height is maintained.  
      The foregoing examples refer specifically to mouse mats, however it will be understood that the combination of a stochastic printed image with a textured surface may be used in other instances where optical position measurement is used.