Prism improves illumination uniformity of working surface
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Introduction An optical mouse uses light to detect its position.
Normally, an optical mouse has a light source, such as a light
emitting diode (LED), for illuminating a working surface (e.g.,
a mouse pad or the surface of a table) through a lens. The
illumination beam must be as compact and uniform as possible
in order to reflect the surface’s differences. Unfortunately,
the illumination source that is widely used is an LED die,
which has a bond pad and an associated wire in the middle
of the die. Due to the bond pad and wire, the resulting image
from the illumination beam has an undesirable dark region
in the center. Because of this dark region, the illumination
uniformity of the working surface is unsatisfactory. A method
of improving the illumination uniformity of a working surface
is needed.
Description To improve the illumination uniformity of a
working surface, the use of a special prism is proposed. A
prism is comprised of a collimating surface, an initial beam-splitting
roof and a second beam-splitting roof, and an output face.
The collimating surface of the prism generally receives divergent
illumination beams from a light emitting diode. The collimating
surface of the prism converts the received beams into nearly
parallel collimated illumination beams. Inside the prism,
the collimated illumination beams are repeatedly reflected
from the initial beam-splitting roof and the second beam-splitting
roof. These multiple reflections lead to the splitting of
the collimated illumination beams into a plurality of overlapping
sub-beams. The initial beam-splitting roof and the second
beam-splitting roof then redirect the overlapping sub-beams
through an output face onto the general illumination area
at slightly different angles of incidence. As a result, an
image with high illumination uniformity is generated on the
working surface. Thus, a prism improves the illumination uniformity
of a working surface.
Additional information The collimating surface also transforms a light
flux with a dark region in the center into a light flux with
an angular dark region. Such transformation of the light flux
eliminates the undesired dark region in the center of the
illumination spot. The collimating functions can be performed
by various collimating surfaces such as a Fresnel lens or
mirror. The splitting and overlapping functions can be performed
by using refractive surfaces.
