Abstract:
An optical mouse includes a base and a lens for forming an image on an image sensor package. An LED projects light into the field of view of the lens via a prism. The prism enables the LED to be set horizontally, which reduces optical losses. The prism and lens may form part of a single molding.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to optical devices, and in particular, to an optical mouse including an optical device.  
       BACKGROUND OF THE INVENTION  
       [0002]     An optical mouse has advantages over a computer mouse that relies on the physical rolling of a ball. A variety of optical mouses are widely available. However, the optical mouses in current commercial use have a number of disadvantages.  
         [0003]     One disadvantage is that the thickness or height from the work surface is typically larger when compared to roller-type computer mouses due to constraints imposed by the optical projection and imaging systems included therein. This increase in thickness can be uncomfortable and lead to fatigue for the user. Another disadvantage is that the optical design is unduly complex, typically requiring changes of direction on two or more reflecting surfaces. This leads to additional component and assembly costs.  
       SUMMARY OF THE INVENTION  
       [0004]     In view of the foregoing background, an object of the present invention is to overcome or reduce the problems associated with optical computer mouses.  
         [0005]     This and other objects, advantages and features in accordance with the present invention are provided by an optical device which in use is positioned on a surface and illuminates an area of the surface under the device. The device may comprise a light source and a light guide for projecting light from the light source onto the area. The light guide may preferably comprise a body of optically transparent material formed to provide a prism having an input face facing the light source, and an output face spaced from and at an angle to the input face. The light from the light source may be refracted onto the surface by the prism without undergoing reflection.  
         [0006]     Another aspect of the present invention is directed to an optical mouse comprising a base adapted for sliding across a surface. The base may include an aperture. The optical mouse may further comprise a light source, an optical assembly arranged to deviate light from the light source through the aperture to impinge on the surface, and a lens mounted in the mouse to view the light projected on the surface. An image sensor may be positioned to receive an image from the lens. The optical assembly is as defined above.  
         [0007]     Preferred features of the invention will be apparent from the following description and from the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     An embodiment of the invention will now be described, by way of example only, with reference to the drawings, in which:  
         [0009]      FIGS. 1 and 2  are side views of a prism illustrating the principles of bending light in accordance with the present invention;  
         [0010]      FIG. 3  is a cross-sectional side view of a computer mouse in accordance with one embodiment of the present invention; and  
         [0011]      FIGS. 3   a  and  3   b  show in greater detail and to an enlarged scale selected areas of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]     Referring to  FIG. 1 , the present invention is based on the light-bending properties of prisms to allow light from a light source, such as an LED, to be diverted without making use of a reflection. In this embodiment, a prism  10  has an input face  12  and an output face  14 . The input face  12  is positively curved to provide light-gathering capability. The input face  12  may be spherically curved, for example.  
         [0013]     The rays shown in  FIG. 1  represent the output from an LED (not shown in  FIGS. 1 and 2 ) arranged at an angle with respect to the input face  12 . The LED has a typical conical output with a semi-angle between 13° to 22°.  
         [0014]     The output face  14  is arranged at an angle with respect to the input face  12 . Thus, light passing through the input face  12  undergoes bending, and passage through the output face  14  provides further bending.  
         [0015]     In this example, the LED axis is horizontal. The input face  12  is tilted between 25° to 45°, and is preferably 32° as shown. The output face  14  is flat or very slightly curved and is substantially vertical, preferably in the range 0 to −5°.  
         [0016]     The preferred forms rely on the principle of a prism, set where:  
                 ⅆ   D       ⅆ   I       =   0           (     Eq   .           ⁢   1     )             
 
 D is the deviation angle and I is the angle of incidence of the axial ray. The advantage of this is to have the same overall ray bending even though the light is incident off-axis due to the conical LED output. 
 
         [0017]     The prism  10  is most suitably formed as an integral unit by molding an optical plastic material. The prism  10  acts solely to deflect the light beam without any internal reflection. Thus, the surfaces of the prism  10  other than the input  12  and output  14  surfaces do not need to be of optical quality.  
         [0018]     The prism  10  not only provides the desired angle change for the light beam, but also forms the beam into the desired shape, i.e., with a plane or slightly curved wavefront as indicated at  18 .  
         [0019]      FIG. 2  illustrates an alternative prism that provides the same effects as in  FIG. 1 , but with a different geometry. In  FIG. 2 , the input face  12  is vertical (0°) and is positively curved. The output face  14  is flat and is tilted to illuminate at the desired distance between −20° and −32°, with −22° being shown in the example. Here again, the light is deviated to the desired location without being subject to a reflection.  
         [0020]     In general terms, the desired result may be achieved with a prism in which the input face has an angle from −10° to +60°, and the output face has an angle from −60° to +10°, with one of the two being substantially vertical (e.g., within ±5°).  
         [0021]      FIGS. 1 and 2  show the prism assembly as rotationally symmetrical. However, a non-symmetrical form may be preferred depending on the area to be illuminated.  
         [0022]     Turning now to  FIG. 3 , one specific form of an optical mouse is illustrated using a prism assembly as in  FIG. 1 . The mouse has a base  20  including ribs  22  upon which it can be slid across a surface, such as a mouse pad  24 . The base  20  defines an aperture  26 .  
         [0023]     An LED  28  is mounted horizontally, i.e., parallel to the mouse pad  24  to provide light  32  via a prism  30  to impinge on the mouse pad  24  via the aperture  26 . A lens  34  focuses an image of the illuminated surface via an optical aperture  36  onto the image surface  38  of an image sensor package  40 .  
         [0024]     The LED  28  is mounted on an extension  42  of the prism  30 . To collect the maximum amount of light, the LED  28  is placed close to the prism  30 , leaving a gap  52  ( FIG. 3   a ) of 0.1 to 0.4 mm.  
         [0025]     In this embodiment, the prism  30  and extension  42  are molded integrally with the lens  34 , and are spaced from the lens  34  by a leg  44  of the molding, which also includes a downwardly projecting rib  48  acting as an optical baffle between the prism  30  and the lens  34 . The aperture  36  is formed in a member  46  that may also be suitably formed as a plastic molding, and is shaped to cooperate with a formation on the lens molding such that these two parts fit together.  
         [0026]     The optical molding  30 ,  34 ,  42 ,  44  makes use of a prism  30  of the type shown in  FIG. 1 . The molding could equally incorporate a prism of the type shown in  FIG. 2 . However, the former is preferred since it is simpler and cheaper to mold.  
         [0027]     The image sensor package  40 , which may be any suitable sensor, such as a CMOS single-chip sensor, is mounted on a printed circuit board  50  that is parallel to the mouse base  20 . The PCB  50  mounts other electronic components (not shown), and in particular, the PCB  50  may provide a mounting for the mouse buttons (not shown). Suitable operation of the image sensor package to provide X and Y signals is known in the art and will not be described herein.  
         [0028]     Embodiments of the optical device provide a number of advantages. The mouse can be made with a low profile, which allows the light source and tracking sensor to be integrated with the button PCB. The efficient collection of light allows the use of a low power and low cost LED. Only two accurate optical surfaces are required, in comparison with three or more in the prior art. This further reduces costs. The prism angle may be chosen to allow relaxation of constructional tolerances.  
         [0029]     The optical device has been illustrated above as being incorporated within an optical mouse. However, those skilled in the art will realize that the principles of the invention can be incorporated in a number of different optical devices suitable for use in a number of different applications.  
         [0030]     These applications include the following, for example: a general motion sensor; detection of motion of an object on a person&#39;s skin (e.g., to enable shaving with a laser); surface shape detection; measuring presence of rain or dust on a surface, such as a car windshield; switches operable by waving of a hand near an appropriate sensor; checking the position of a moving object in an industrial context; a “virtual mouse” application, whereby hand movements are detected on a glass surface or even in three dimensions; detection of atmospheric changes; a proximity sensor for use as an alarm system or for parking safely; detection of wind speeds; and as a general detector to monitor rotational motion.