Artificial limiting of optical aperture obtains unambiguous speckle pattern
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Introduction In a conventional, coherent-light optical mouse,
it is necessary to detect the motion of a speckle pattern
by an image sensor. The conventional image sensor is comprised
of an array of square pixels. The size of a pixel determines
the pitch of the image sensor and its resolution. If the average
speckle size in the speckle pattern is smaller than the pixel
size, then during a small movement of the speckle pattern,
most speckles can stay within one pixel. When this is the
case, the image sensor does not detect the small motion of
most speckles. As a result, interpreting data read from the
image sensor becomes an ambiguous and difficult operation.
Such a speckle pattern is called ambiguous. Unfortunately,
the optical system of the mouse generates a speckle pattern
in which the average speckle size is smaller than the size
of the pixel in conventional semiconductor image sensors.
Therefore, a speckle pattern is often ambiguous. A method
of obtaining an unambiguous speckle pattern is needed.
Description To obtain an unambiguous speckle pattern, artificially
limiting an optical aperture is proposed. An optical sensing
assembly is comprised of at least one lens, one optical aperture
and one image sensor. For the given illumination wavelength,
an aperture size is selected to be smaller than the ratio
of the wavelength to the pitch or size of the pixel. The artificial
limiting of the optical aperture increases the average speckle
size in the speckle pattern. The average speckle size in the
speckle pattern becomes larger than the pixel size. As a result,
the image sensor detects any small movement of speckles of
this size. Thus, the artificial limiting of an optical aperture
obtains an unambiguous speckle pattern.
Additional information The working surface should preferably be illuminated
with a collimated beam from a coherent light source. For more
reliable detection of the computer mouse’s motion, two image
sensors should preferably be used. They should obtain two
speckle patterns from the same illumination point. The speckle
image data signals from the image sensors are then processed
by using cross-correlation analysis.
