Light-blocking patches improve illumination uniformity over pixels of image sensor
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Introduction Ideally, the illumination intensity of the light
focused by a lens onto an optical image sensor should be uniform.
However, the intensity of light off the axis from a lens is
less than the light on the axis by a factor of cos4 theta,
where the angle theta is the angle from the lens axis to the
off-axis point. Light from the lens is incident upon the optical
image sensor and therefore will not have a uniform intensity.
A method of improving illumination uniformity over the pixels
of an image sensor is needed.
Description To improve illumination uniformity over the
pixels of an image sensor, the use of light-blocking patches
is proposed. The surface of each pixel of an image sensor
is marked into identical square-shaped sites. On each pixel,
a certain number of these square-shaped sites are coated with
light-blocking patches. The number of light-blocking patches
on each pixel is not the same. The number is highest on pixels
close to the center of the light spot on the surface of the
image sensor. In addition, the number dramatically reduces
in radial directions from the center of the light spot. The
number of light-blocking patches on each pixel is calculated
so that the exposed pixel area is maximal on the periphery
and reduces as a function of cos4 theta in directions from
the periphery to the center of the image sensor. The value
of the output pixel signal is proportional to the exposed
pixel area. Therefore, the growth of the illumination of the
entire surface of the pixel in directions from the periphery
to the center of the light spot is compensated by the reduction
in the area of the exposed surface of the pixel. Thus, light-blocking
patches improve illumination uniformity over pixels of an
image sensor.
Additional information The light-blocking patches are formed during
the fabrication of an optical image sensor by depositing an
opaque material through a corresponding mask. In optical mice,
the optical image sensor is conventionally illuminated by
light reflected from the working surface through a lens and
a square-shaped exit pupil.
