Television – Camera – system and detail – Optics
Reexamination Certificate
2000-05-16
2004-11-16
Garber, Wendy R. (Department: 2612)
Television
Camera, system and detail
Optics
C348S374000
Reexamination Certificate
active
06819361
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an imaging system utilizing a solid-state imaging device and, more particularly, to a method of constructing the transparent window of the solid-state imaging device using an optical low pass filter. The invention integrates the optical low pass filter into the solid-state imaging device to simplify the configuration of the imaging system, reducing its size and manufacturing cost. The present invention provides a solid-state imaging device having excellent frequency characteristic and a method for constructing the same by using a phase grating as the optical low pass filter.
2. Description of the Related Art
In a charge coupled device (CCD) image sensor currently widely used as an image sensor or a CMOS image sensor that has been using since 90s, the image sensors configured of light receiving elements are two-dimensionally arranged to convert input images into electrical signals.
FIG. 1
shows the configuration of a conventional camcorder or digital camera system. A motion picture or still picture
1
to be imaged is focused by an optical lens arrangement
3
and then passes through an optical low pass filter
2
to enter a light receiving element constructed on the surface of an image sensor
4
. The optical lens arrangement
3
consists of appropriate optical lenses such as concave lens and convex lens in order to focus the input image
1
on the imaging device
4
. The optical lens arrangement
3
or optical low pass filter of
FIG. 1
usually includes an UV filter or IR filter for respectively blocking ultraviolet rays or infrared rays contained in the input image
1
. The UV or IR filter is generally constructed in a manner that an appropriate material is coated on a lens or a transparent substrate. To restore the image inputted to the solid-state imaging device to the original state in the imaging system of
FIG. 1
, it is required that the optical low pass filter
2
has a cutoff frequency that is one-half the sampling spatial frequency.
FIG. 2
shows an ideal sampling in case where the repetitive period of the light receiving element is X in direction x and Y in the direction y in the two-dimensional image sensor. If an image having the spatial frequency spectrum of
FIG. 3A
is imaged using the two-dimensional sensor having the spatial sampling characteristic of
FIG. 2
, the sampled image has the spatial frequency spectrum of
FIG. 3B
in which the original image's spatial frequency spectrum is repeated. In
FIG. 3B
, the frequency spectrum of the sampled image has a repetitive period corresponding to the reciprocal of the sampling interval, that is, 1/X in the x-direction and 1/Y in the y-direction. Accordingly, to restore the image inputted to the two-dimensional image sensor to the original state, it is required that an optical low pass filter which passes the spectrum corresponding to one period starting from the starting point but cuts off a spatial frequency higher than this.
As described above, to restore the image inputted to the solid-state imaging device to the original state, it is the most ideal that the optical low pass filter
2
of
FIG. 1
has the cutoff frequency that is one-half the sampling spatial frequency. Here, the sampling spatial frequency corresponds to the reciprocal of the repetitive period of the light receiving element of the solid-state imager. That is, in the two-dimensional light receiving element arrangement of
FIG. 2
,
f
s
=
1
d
and
f
c
=
f
s
2
=
1
2
d
,
where d is X in the x-direction and Y in the y-direction. Here, f
s
represents the sampling frequency and f
c
represents the cutoff frequency of an ideal optical low pass filter.
FIG. 4
shows the spatial frequency transfer characteristic of the optical lens arrangement. The frequency band defined by a dotted line in
FIG. 4
is the frequency transfer function of an ideal optical low pass filter. The maximum transfer frequency of the lens, f
m
, is 2(NA/&lgr;). Here, NA represents the numerical aperture of the lens and &lgr; represents the wavelength of incident light. Though the lens functions as a kind of optical low pass filter, its maximum cutoff frequency, f
m
, is usually considerably higher than the ideal cutoff frequency, f
c
, of the low pass filter as shown in FIG.
4
. The frequency transfer characteristic of the lens can approximate to the straight line of
FIG. 4
to be mathematically modeled, and the difference between the approximate value indicated by the straight line and the actual transfer characteristic becomes smaller as f
m
becomes larger than f
c
.
FIG. 5A
is a perspective view showing the appearance of a conventional solid-state imaging device, and
FIG. 5B
is a cross-sectional view showing the conventional solid-state imaging device, taken along the line A—A of FIG.
5
A. In this conventional solid-state imaging device, the covers
51
and
52
of the solid-state imaging device chips
53
and
54
are configured of a transparent glass plate because input light should be transmitted through the covers
51
and
52
, that is, transparent window, to a light receiving device placed on the surface of the solid-state imaging device chip.
FIGS. 6A
,
6
B and
6
C illustrate conventional optical low pass filters utilizing a double refraction plate, which are currently widely used as an optical low pass filter in the conventional imaging system. Referring to
FIG. 6A
, an input beam incident on one surface of the double refraction plate is split into two beams, having a distance, d
n
, therebetween, while it passes through the double refraction plate. The relation among the thickness and refraction index of the double refraction plate and the distance, d
n
, satisfies the following equation:
d
n
=
t
(
n
e
2
-
n
o
2
)
2
n
e
n
o
where t is the thickness of the double refraction plate, n
e
is the extra-ordinary refraction index and n
o
is the ordinary refraction index. As shown in
FIG. 6B
, the conventional optical low pass filter utilizing the double refraction plate is constructed in such a manner that an x-directional double refraction plate and a y-directional double refraction plate lie in piles to enable beam splitting in the x-direction and the y-direction. An IR removal filter is generally inserted between the two double refraction plates.
In the operation of the conventional optical low pass filter utilizing the double refraction plate, the input beam, vertically incident on the surface of the filter, is split into two beams at the x-directional double refraction plate, and each of these two beams is further split into two beams at the y-direction double refraction plate. Thus, one input beam is split into four beams, arriving at the light receiving element of the solid-state imager. That is, the optical low pass filter using the double refraction plate functions as a 4-beam splitter as shown in FIG.
6
C. By splitting one input beam into four beams, an image having a higher spatial frequency is converted into a lower spatial frequency before sampling of the solid state imager.
The general optical transfer characteristic function of 2-plate type double refraction plate is equals to the magnitude of the absolute value of the cosine function with the period of
1
d
n
when it is Fourier-transformed. That is, the transfer function has a value proportional to abs(cos(2&pgr;×f×d
n
)) where f is spatial frequency and d
n
is the distance between the beams split by the double refraction plate, shown in FIG.
6
A. The optical transfer function of an image which passes through the optical lens to reach the double refraction plate filter is obtained by multiplying the transfer function of the lens shown in
FIG. 4
by the transfer function of the double refraction plate.
In case where the double refraction plate is applied to the conventional imaging system utilizing the solid-state imager, larger loss generates in the transfer function in a spatial frequency band lower than the cutoff frequency than in
Kim Shi Ho
Ko Chun Soo
Lee Jae Chul
Lim Sung Woo
Oho Yong Ho
Dykema Gossett PLLC
Garber Wendy R.
Havit Co., Ltd
Hernandez Nelson D.
LandOfFree
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