Abstract:
The imaging device has an imaging surface and a sensor, wherein the imaging surface is illuminated and reflects at least some of the illumination to the sensor to detect an image. The imaging device has a width and an optical path passing therethrough, wherein the optical path exhibits distortion as a result of the width of the device and the nature of the optical path. The imaging device includes an optical element which in use compensates for the distortion by generating a magnification profile across a tangential plane of the device at the sensor.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to optical navigation devices such as, for example, optical mice. 
       BACKGROUND OF THE INVENTION 
       [0002]    Computer devices are becoming ever smaller and full computing functionality can be found on phones and smart phones and other personal digital assistants (FDA). As the computer devices become smaller, the various features of the computer devices may also become smaller. This includes a desire for smaller input systems for the user to enter inputs into the device. One such input system is an optical navigation device. Many computer devices, large and small, are equipped with optical navigation devices. However, with the smaller computer devices, minimizing the size of the optical navigation device can often be restrictive and problematic. 
         [0003]    A number of devices have been proposed which offer thin optical navigation devices. These have had some success but the design has generally not addressed all of the problems associated with thin imaging devices. One such outstanding problem is associated with the positioning of the sensor. Ideally the sensor is located as close to an imaging surface of the imaging device as is possible. This can be achieved by de-centering the sensor. However, this may then give rise to distortion such as perspective distortion, in the image generated at the sensor. In addition, such de-centering of the sensor can cause problems with shielding the sensor from any stray light which enters the imaging surface. 
         [0004]    In professional and architectural photography, perspective control lenses exist. These may require considerable space for the design which incorporates the ability to tilt and de-center the input lens to correct for any perspective effect. However, the requirement of greater space may be undesirable in small-scale optical navigation devices. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the present invention to overcome at least some of the problems associated with the prior art as discussed above. It is a further object of the present invention to provide an optical navigation device having a thin sensor which may have less distortion than previous devices. 
         [0006]    According to one aspect of the present invention there is provided an imaging device of the type having an imaging surface and a sensor, wherein the imaging surface is illuminated and reflects at least some of the illumination to the sensor to detect an image. The imaging device has a predetermined width and an optical path passing therethrough, wherein the optical path exhibits distortion as a result of the width of the device and the nature of the optical path. The imaging device includes an optical element which in use compensates for the distortion by generating a magnification profile across a tangential plane of the device at the sensor. 
         [0007]    The present invention offers a number of benefits which may include a reduction to the perspective distortion and stray light shielding while maintaining a required thickness. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Reference will now be made, by way of example, to the accompanying drawings, in which: 
           [0009]      FIG. 1  is a cross-sectional diagram of an imaging device for an optical navigation device, in accordance with the prior art; 
           [0010]      FIG. 2  is a schematic diagram illustrating an optical navigation device, in accordance with an embodiment of the present invention; 
           [0011]      FIG. 3  is a cross-sectional diagram of a first imaging device for the optical navigation device, in accordance with an embodiment of the present invention; and 
           [0012]      FIG. 4  is a cross-sectional diagram of a second imaging device for the optical navigation device, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    The present invention relates to an imaging device associated with an optical navigation device. The optical navigation device may be a mouse of small-scale which is intended to be operated via frustrated total internal reflection (F-TIR) to recognize the movement of a finger on an imaging surface. This type of mouse is herein referred to as a finger mouse. 
         [0014]      FIG. 2  shows an example of a finger mouse  200  in accordance with an embodiment of the present invention. The finger mouse includes a base  202 ; an imaging device shown generally at  204 ; and an LED and sensor, both not shown. The top surface  206  of the imaging device  204  includes an imaging surface  208  which is positioned at a predetermined location on the surface  206 . It will be appreciated that the position of the imaging surface will depend on the optical path or paths that light takes from the LED to the sensor. In addition, the position of the sensor relative to the imaging device will depend on the optics forming the imaging device. This will be discussed in more detail below. 
         [0015]      FIG. 1  shows a conventional imaging device  100  having a sensor  102 . An imaging surface  104  gives rise to a specific pattern  106  after illumination by an appropriate light source, for example an LED. The pattern  106  is reflected by mirror  108  onto the sensor. Due to the thinness of the imaging device the resultant image on the sensor exhibits significant distortion, as is shown at  110 . This distortion is clearly unacceptable as it can significantly impact the accuracy of operation of the sensor and ultimately the imaging device. The distortion is due to the angle of the optical axis to the normal in the object plane and gives rise to what is known as perspective distortion. Perspective distortion is a pattern with parallel lines along the direction of the distortion that become convergent in the image plane. 
         [0016]    Perspective distortion is essentially a warping or transformation of an object that differs significantly from what the object would look like with a normal focal length, this is typically worse with a narrow field of view. The  FIG. 1  arrangement also gives rise to light pollution at the sensor, despite the existence of opaque mask  112 , as there are direct routes for ambient light to enter the imaging device and then pass to the sensor. This is another undesirable aspect of the prior art arrangement. The present invention seeks to overcome perspective correction without changing the thickness of the imaging device. 
         [0017]    Referring to  FIG. 3 , a degree of improvement on perspective distortion  300  can be achieved by tilting the mirror  302  and moving the position of sensor  304 . Within the confines of the width of the imaging device (the distance from the imaging surface  308  and the sensor  304 ), the mirror can be tilted to be between 0° and 30° from the vertical. The limiting factor of the degree of tilt on mirror  302  is the relative position of the sensor  304  and the opaque mask  310 . The mirror can be tilted in as much as the reflected beams from the mirror are not cut off by the mask  310 . The effect of tilting the mirror and moving the sensor extends the optical path of the reflected path from the imaging surface. 
         [0018]    The movement of the mirror and the sensor has a compensation effect on the perspective distortion. This is caused by the fact that the tilt of the mirror and the position of the sensor give rise to different magnification along the tangential plane of the optical drawing. The three points (shown on the sensor in FIG.  3 ) each present a different level of magnification. It will be appreciated that the points are shown for ease of comprehension of the optical drawing but in fact the light beam is a continuum and as such the magnification will vary over the continuum rather than on a point by point basis. 
         [0019]    The distance between the imaging surface  308  and sensor  304  in the present embodiment is in the region of 2.5 mm. This distance is the thickness of the imaging device and can vary between 2 mm and 3 mm. Ideally, the thickness is not generally greater than 5 mm. Moving the sensor has a further advantage in that the sensor is completely protected by the mask and no stray or ambient light can reach the sensor. 
         [0020]    Referring to  FIG. 4 , a second embodiment of the present invention is presented. The embodiment includes an imaging surface  400 , sensor  402  and, a mask  404 . In addition, the imaging device includes an ellipsoidal mirror  406 . The ellipsoidal mirror has a substantially flat portion  408  and a curved portion  410 . The ellipsoidal mirror may be tilted as shown and described with reference to  FIG. 3 , or may be used in a “non-tilted” orientation. The choice will depend on the particular optical design. The sensor  402  is in a similar position to that shown in the  FIG. 3  embodiment. The ellipsoidal mirror reflects light from the imaging surface onto the sensor. 
         [0021]    Due to the fact that some of the reflection occurs on a flat surface and some of the reflection occurs on a curved surface the final image on the sensor has less perspective distortion than would otherwise exist. This is due to the fact that the mirror has two different radii of curvature. The two radii of curvature are significantly different from one another which gives rise to a compensation for any stretch effect that would otherwise be evident due to the angles within the imaging device. Use of the two different radii of curvature enables the imaging device to be kept to a small size but still achieve minimal perspective distortion. In addition, the mirror (either tilted or not) gives rise to a variable magnification profile across the tangential plane of the drawing which effects the compensation to the perspective distortion. If the mirror is in a tilted orientation there will be a great degree of variability in the magnification profile, which may be desirable in certain situations. 
         [0022]    The mirror center is designed such that it does not correspond to the center of the optical path of the light from the imaging surface to the sensor. This has further advantages in that the equivalent radius of curvature is different along the vertical axis of the mirror, and so the magnification linking the size of the object and the image is different along this axis, correcting the perspective distortion. The ellipsoidal mirror is one example of a mirror having different radii of curvature. It will be appreciated that different shapes of mirror may be equally relevant provided the difference in radii of curvature is sufficient to compensate for the stretch effect in such a thin device. 
         [0023]    Where the mirror is an ellipsoid, the shape of the ellipsoidal mirror is determined by the equation of an ellipse. The shape of the mirror and the relative position of the sensor compensate for the perspective distortion that would otherwise exist in an equivalent prior art optical path. This is brought about by the varying magnification profile across the tangential plane of the drawing. As the sensor is located under the opaque mask  404  the sensor is completely protected by the mask and no stray or ambient light can reach the sensor. 
         [0024]    The resultant image produced by the  FIG. 4  embodiment is shown at  412 . Comparing with the original pattern  414  it can be seen that the perspective distortion is minimized in image  412  relative to the prior art. 
         [0025]    In use in an optical navigation system, the imaging device enables compensation for and/or reduction of any perspective distortion thereby improving the operation of the imaging device when used as a finger mouse. The image produced at the sensor  402  will have considerably less perspective distortion than in prior art systems and is substantially equivalent to the original pattern that illuminates the imaging surface  400 . 
         [0026]    The present embodiments thus provide a more accurate imaging device which is still capable of retaining the required thinness of generally less than 5 mm and desirably 3 mm or less. The imaging device can be formed from a single piece molding as shown in  FIG. 2 . The molding includes each of the individual optical elements shown in  FIG. 4 , namely the illumination optics (not shown); the imaging surface  400 ; the mirror  406 ; and the sensor  402 . 
         [0027]    The imaging device could alternatively be made in other appropriate ways with different optical elements which produce the same optical effect. The imaging device may also be made from a number of different elements, rather than a single molding. The technique for forming the imaging device may include techniques other than molding, such as replication, stamping, embossing or machining. The optical device is typically made from Polycarbonate (such as Lexan (™)), acrylics such as PMMA (polymethyl methacrylate), glass, Polyethylene or PVB (Polyvinyl butyral). The sensor is of any appropriate type and may be a CMOS sensor having an array of pixels for measuring reflected light at different locations to produce an image such as image  412 . 
         [0028]    The illumination source is for example an LED which may be of any appropriate type and may generate a source in the “optical” or non-optical ranges. Accordingly, reference to optics and optical are intended to cover wavelengths which are not in the human visible range. The illuminating optics which take the illumination from the source to the imaging surface may be of any appropriate type, for example as is shown in co-pending application reference P115041.GB.01 (incorporated herein by reference). 
         [0029]    The imaging device is intended for use in an optical navigation device; however it will be appreciated that the imaging device could be used in any appropriate device, for example fingerprint reader or Lab-on-chip/Bio-Optical sensor systems (which detect chemi-fluorescence for medical and/or bio-testing applications). 
         [0030]    The optical navigation device may be used in any suitable devices such as a mobile or smart telephone, other personal or communications devices, a computer, a remote controller, access modules for doors and the like, a camera or any other suitable device.