The amount that the edges of one photograph overhang the other represents the offset between the images, and in the case of an optical computer mouse the distance it has moved. Place both photographs on a light table to make them transparent, and slide one across the other until their images line up. To understand how optical flow is used in optical mice, imagine two photographs of the same object except slightly offset from each other. Images of these surfaces are captured in continuous succession and compared with each other to determine how far the mouse has moved. These surfaces, when lit at a grazing angle by a light emitting diode, cast distinct shadows that resemble a hilly terrain lit at sunset. Optical mice use image sensors to image naturally occurring texture in materials such as wood, cloth, mouse pads and Formica. A simple binary-image version of digital image correlation was used in the 1980 Lyon optical mouse. The technology underlying the modern optical computer mouse is known as digital image correlation, a technology pioneered by the defense industry for tracking military targets. S5085 optical sensor IC die (CMOS sensor + driver) Other manufacturers soon followed Microsoft's lead, including Apple for their Pro Mouse, using components manufactured by Agilent (once they spun off from HP), and over the next several years mechanical mice became obsolete. These mice worked on almost any surface, and represented a welcome improvement over mechanical mice, which would pick up dirt, track capriciously, invite rough handling, and need to be taken apart and cleaned frequently. These mice used technology developed by Hewlett-Packard under their Agilent Technologies subsidiary (see below). Xerox's inventions were never massively commercially exploited, however, and optical mice would remain elusive in the personal computer market until Microsoft released the IntelliMouse with IntelliEye and IntelliMouse Explorer in 1999. A surface-independent coherent light optical mouse design was patented by Stephen B. This advance enabled the mouse to detect relative motion on a wide variety of surfaces, translating the movement of the mouse into the movement of the cursor and eliminating the need for a special mouse-pad. As computing power grew cheaper, it became possible to embed more powerful special-purpose image-processing chips in the mouse itself. Modern surface-independent optical mice work by using an optoelectronic sensor (essentially, a tiny low-resolution video camera) to take successive images of the surface on which the mouse operates. The optical sensor from a Microsoft Wireless IntelliMouse Explorer (v. The Kirsch and Lyon mouse types had very different behaviors, as the Kirsch mouse used an x-y coordinate system embedded in the pad, and would not work correctly when the pad was rotated, while the Lyon mouse used the x-y coordinate system of the mouse body, as mechanical mice do. Lyon of Xerox, used a 16-pixel visible-light image sensor with integrated motion detection on the same n‑type ( 5 µm) MOS integrated circuit chip, and tracked the motion of light dots in a dark field of a printed paper or similar mouse pad. Predictive algorithms in the CPU of the mouse calculated the speed and direction over the grid. One of these, invented by Steve Kirsch of MIT and Mouse Systems Corporation, used an infrared LED and a four-quadrant infrared sensor to detect grid lines printed with infrared absorbing ink on a special metallic surface. The first two optical mice, first demonstrated by two independent inventors in December 1980, had different basic designs: An early Xerox optical mouse chip, before the development of the inverted packaging design of Williams and Cherry
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