OPTICAL POSITION SENSING SYSTEM AND
OPTICAL POSITION SENSOR ASSEMBLY
 This application claims priority to U.S. Provisional PatentApplicationNo. 61/019,404, entitled "Optical Position Sensor With Miniature Sensor," which was filed on Jan. 7, 2008.
 The present invention relates generally to electronic sensors, and more particularly to optical position sensors, such as those used in connection with touch sensitive screens.
BACKGROUND OF THE INVENTION
 Optical position sensing systems, such as those used in connection with computer displays, office machinery, gaming equipment, etc., rely on a combination of line-scan or area image cameras, digital signal processing, front or back illumination and algorithms to determine a point of touch. Many optical position sensing systems use cameras, orientated along the touch screen surface so as to image the bezel. In this way, the system can track the movement of any object close to the surface of the touch screen by detecting variations in illumination emitted by an illumination source, such as an infrared light source.
 While cameras generally are more expensive than other types of detector devices that can be used in optical position sensing systems, such as photo-detectors (e.g., photo-diodes or photo-transistors), they allow greater accuracy for touch detection. As known in the art, cameras using both area scan or line scan sensors are typically expensive and too large in the dimensions which are critical to commercially viable small touch screens.
 Conventional optical position sensing systems use optical position sensors comprising multiple refractive elements (i.e., multiple element lens systems). Typically, these refractive elements are plastic or glass lenses. Lenses commonly used in optical sensors and other camera devices are typically designed for imaging applications. They are designed to have low image distortion when imaging a plane surface. Ideally, when light is transmitted and/or refracted onto a lens, all the rays of light are converged to a single point, resulting in a clear image. However, inmost lens systems light rays are diverted to different points due to lens imperfections and other influences. These influences are commonly called aberrations, and usually result in distorted images.  Conventional camera devices use multi-element lens systems because the use of multiple refractive elements makes it possible to correct and compensate for aberrations and image distortion over a single element lens system, increasing the clarity of the image. However, the use of multiple elements increases the overall size of the camera, especially the depth, and makes it more difficult to converge light rays at a single point. The problem is exacerbated in configurations where space is extremely limited. While distortion may be undesirable for imaging applications, this is not the case for optical position sensing. Thus, conventional position sensing systems do not require the primary benefit of a multilens system. Further, the increased size of multi-lens camera systems not only adds unwanted space to the overall system, but it also adds to the expense of manufacturing these systems.
 Additionally, when focusing the lens of conventional multi-element lens systems, manufacturers must physically move the lens elements relative to the body and sensor of the camera. Commonly this is done by a threaded lens barrel, and this results in a camera height which is set by the lens diameter. This is a difficult process given the relatively small amount of available space in a position sensing system. Additionally the multi element lenses and the focusing mechanisms are not mechanically robust, and sensitive to vibration. Unlike imaging applications, slight movement in the optical path causes significant position errors, even when no image degradation would result.
 In a retroreflective system, the triangle formed between the illumination source, the nearest point of the reflector, and the lens aperture, must subtend an angle less than the observation angle of the retroreflective material. Existing systems use low performance reflective materials such as beaded material, which compromise performance, and large screen sizes, so that the observation angle is large. Other known systems use expensive beam splitting optics, which are extremely susceptible to dust and contaminants blinding the camera. It is an objective of this invention to use high performance reflective material, on small screens, without expensive beam splitting optics, and with advantageous immunity to blinding from dust and other contaminants.
SUMMARY OF THE INVENTION
 The present invention provides an optical position sensing system including an improved optical position sensor assembly. The optical position sensing system includes a display, a bezel surrounding the display, at least one position sensor assembly for emitting radiation to cause illumination of the bezel and for generating data signals representing detected variations in said illumination, and a processor for processing the data signals to calculate a location of a touch relative to the display. Reflectors may be mounted to a face of the bezel that is perpendicular to a viewing area of the display. The reflectors may comprise retroreflective material, such as a prismatic film or tape. The at least one optical position sensor assembly may be mounted to the display or to an overlay that is positioned over the display. The optical position sensor assembly may therefore include one or more alignment features for mounting the optical position sensor assembly to a flat surface, such as the display or overlay.  Each optical position sensor assembly includes a body having a front face and a rear face and an opening therethrough. A lens holder is mounted to the body. The lens holder has a first side and a second side. The first side comprises an illumination window and the second side holds a single element aspherical lens, which may have an f-theta characteristic. The lens holder is mounted to the front face of the body such that the lens is aligned with the opening in the body. An optical sensor is mounted to the rear face of the body and is positioned such that it is aligned with the opening. A radiation source is positioned within the body above the lens holder and behind an illumination window. A light path separator is positioned between the illumination window and the imaging window, such that a path of radiation emitted by the radiation source is optically separated from a view path of the optical sensor. The light path separator may be a flexible printed circuit board that drives the radiation source. Alternatively, the light path separator may be an integral subcomponent of the lens holder or other component of the assembly. In