Optical filtering apparatus and method

Details Optical filtering apparatus and method Optical filtering apparatus and method

2000-05-25

2003-02-25

Juba, Jr., John (Department: 2872)

Optical: systems and elements

Diffraction

Using fourier transform spatial filtering

C359S558000, C359S900000, C349S017000, C349S024000

Reexamination Certificate

active

06525879

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to an optical filtering apparatus and method which perform optical filtering (spatial frequency filtering).
2. Description of the Prior Art
Optical filtering is a representative parallel optical computing technique. In this case, a Fourier spectrum of an input image is changed by using a spatial frequency filter.
Representative optical filtering techniques are low-pass filtering and high-pass filtering. Generally, low frequency components of an image spectrum correspond to a brief image structure, and edges and fine structure concentrate in a high frequency components. A low-pass filter passes only low frequency components, thus removing noise of high frequency components. A high-pass filter passes only high frequency components for the purpose of extracting image boundaries and enhancing the fine structure. Further, a band-pass filter which passes only a predetermined spatial frequency band is utilized in image compression and image analysis.
Conventionally, in optical filtering, a filter having a two-dimensional transmittance distribution is used as a spatial frequency filter.
FIG. 18
shows an example of the conventional optical filtering method. In this method, input image light
1
is Fourier-transformed by a lens
2
. A filter
4
having a two-dimensional transmittance distribution is provided on a Fourier transform surface of the lens
2
. A part of Fourier spectrum
3
Fourier transformed of the input image light
1
is passed through the filter
4
, and a transmission spectrum
5
is Inverse-Fourier-transformed by a lens
6
. Thus output image light
7
is obtained.
In low-pass filtering, the filter
4
, as shown as a filter
4
L in
FIG. 19A
, has a central round region corresponding to a low frequency spectrum of the Fourier-transformed image
3
as a light transmitting portion
4
a,
and the other region corresponding to a high frequency spectrum as a light shield portion
4
b.
Only the low frequency spectrum of the Fourier-transformed image
3
is passed through the filter
4
.
In high-pass filtering, the filter
4
, as shown as a filter
4
H in
FIG. 19B
, has a central round region corresponding to a low frequency spectrum of the Fourier-transformed image
3
as a light shield portion
4
c,
and the other region corresponding to a high frequency spectrum as a light transmitting portion
4
d.
Only the high frequency spectrum of the Fourier-transformed image
3
is passed through the filter
4
.
When the low-pass filtering and the high-pass filtering are simultaneously performed, as shown in
FIG. 20
, for example, the input image light
1
is divided into two light waves by a half mirror
8
. Input image light
1
L passed through the half mirror
8
is Fourier-transformed by a lens
2
L, and a low frequency spectrum
5
L of a Fourier-transformed image
3
L passes through the filter
4
L. The Fourier-transformed image
3
L is Inverse-Fourier-transformed by a lens
6
L. Thus the low frequency
7
L is obtained. On the other hand, input image light
1
H reflected by the half mirror
8
is further reflected by a mirror
9
, and Fourier-transformed by a lens
2
H. A high frequency spectrum
5
H passes through the filter
4
H, and Inverse-Fourier-transformed by a lens
6
H. Thus the high frequency reconstructed image light
7
H is obtained. The original input image can be reconstructed by combining the low frequency reconstructed image light
7
L and the high frequency reconstructed image light
7
H.
However, since the above-described conventional optical filtering method passes a predetermined frequency component and cuts other frequency components, the cut frequency components are lost on the filter output side. Accordingly, the original input image cannot be reconstructed.
That is, in a case where the filter
4
in
FIG. 18
is a low-pass filter as the filter
4
L in
FIG. 19A
, the high frequency spectrum of the Fourier-transformed image
3
is lost on the output side. On the other hand, in a case where the filter
4
is a high-pass filter as the filter
4
H in
FIG. 19B
, the low frequency spectrum is lost on the output side.
Accordingly, in a case where the low-pass filtering and the high-pass filtering are simultaneously performed or in a case where an original input image is reconstructed, it is necessary to provide two filters
4
L and
4
H, two Fourier transform lenses and Inverse-Fourier transform lenses, and an optical system to divide the input image light
1
into two optical waves, as shown in FIG.
20
. This complicates the filtering apparatus and increases the apparatus in size.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above circumstances, and enables selective or simultaneous execution of mutually-complementary low-pass filtering and high-pass filtering and the like, by using a common medium, without losing respective frequency components of Fourier spectrum on the output side, further enables reconstruction of original input image with ease.
According to an aspect of the present invention, the optical filtering apparatus has: a birefringent medium that modulates polarization of a Fourier-transformed image passed therethrough, in accordance with a two-dimensional birefringent distribution, formed in accordance with a spatial frequency distribution of the Fourier-transformed image; and a polarization device provided in an optical path of light passed through the birefringent medium.
Further, according to another aspect of the present invention, the optical filtering method includes the steps of: passing a Fourier-transformed image of an input image through a birefringent medium where a two-dimensional birefringent distribution corresponding to a spatial frequency distribution of the Fourier-transformed image is formed, so as to modulate polarization of the Fourier-transformed image in accordance with the birefringent distribution; and extracting, by a polarization device analyzer, a polarized light component in a desired or predetermined orientation from light passed through the birefringent medium.
In accordance with the present invention as described above, as a filtering medium, a birefringent medium where a two-dimensional birefringent distribution is formed is used in place of a filter having a two-dimensional transmittance distribution. A spatial frequency filter is formed with the birefringent medium and a polarization device analyzer.
As the birefringent medium, an electrically addressed type spatial light modulator, an optical storage medium having an optical storage layer exhibiting photo-induced birefringence on at least one surface side, on which the two-dimensional birefringent distribution is recorded, or the like, can be used. As the polarization device, a analyzer (analyzer), a polarizing beam splitter or the like can be used.
For example, in a case where low-pass filtering and high-pass filtering are selectively or simultaneously performed, in the birefringent medium, the formed birefringent distribution has a central round region corresponding to a low frequency spectrum of Fourier-transformed image in an orientation of 45° to a predetermined orientation (0°), and the other region corresponding to a high frequency spectrum is in the orientation of 0°.
In this arrangement, when the Fourier-transformed image 0° polarized from an input image passes through the birefringent medium, the polarization of the low frequency spectrum is rotated 90°, to an orientation of 90°, while the polarization of the high frequency spectrum is not rotated, still in the orientation of 0°.
Accordingly, if an analyzer is provided in the optical path of light passed through the birefringent medium and the orientation of the analyzer is adjusted to 90°, only 90° polarized component can be extracted through the analyzer. In this manner, the low-pass filtering is performed.
Further, the high-pass filtering is performed by adjusting the orientation of the same analyzer to 0° so as to extract only 0° polarized component of the light pass

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