KR100272103B1 - Optical low pass filter - Google Patents

Abstract

PURPOSE: An optical low-pass filter is provided to decrease the aliasing distortion causing the degradation of image quality and to reduce the assembling clearance and manufacturing costs by using a single birefringent element with an optical axis. CONSTITUTION: The optical low-pass filter(30) has a single birefringent element(35) with an optical axis(35c) which is arranged under a predetermined angle(theta) with respect to a horizontal scanning direction of a solid picture generation apparatus provided in the video camera. An incident beam of the birefringent element(35) is divided into an ordinary ray and an extraordinary ray. That is, the incident beam is splitted into two beams while passing through the optical low-pass filter(30).

Description

Optical low pass filter

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical low pass filter, and more particularly, to an optical low pass filter which is employed in a video camera to reduce aliasing distortion during spatial sampling.

In general, a video camera includes a solid state image pickup device such as a charge coupled device (CCD) and a color filter, and spatially samples a color-specific image of a subject obtained through the color filter to produce a color video output. In this case, such spatial sampling causes an aliasing distortion that varies depending on color coding. Accordingly, in recent years, various methods for reducing such aliasing distortion have been sought in industrial video cameras, home video cameras, and still cameras.

1A is a diagram showing an example of an image element arrangement of a conventional single chip CCD. Where P_x represents the distance between the image elements R, G and B (1a) (1b) (1c) in the horizontal scanning direction. And P_y` represents the distance between image elements in the vertical scanning direction. When the color filter 1 is a stripe-shaped color filter in the longitudinal direction that is periodically repeated as shown in FIG. 1A, the color filter 1 may be represented as shown in FIG. 1B.

In the case of FIG. 1B, spatial sampling may be expressed as Equation 1 below. Where x and y are each coordinates of the horizontal and vertical scanning directions and m and n are integers.

In addition, Fourier transformation of the above-described equation gives equation (2). Where u and v represent spatial frequencies in the horizontal and vertical scanning directions.

Therefore, in the color filter 1 shown in FIG. 1B, the carrier components F_R, `` `F_G,` `` F_B`` of each of R, G, and B colors are represented by Equations 3, 4, and 5, respectively. Can be represented.

F_G` = `F_R`` e ^ {-j P_x` u`}

F_B` = `F_R``e ^ {-j`2`P_x` u}

Here, e ^ {-j `P_x` u`} and e ^ {-j `2`P_x` u`} in the carrier components F_G` and F_B` respectively indicate phase differences with respect to the carrier component F_R.

와

로 규격화 되어 있다.The carrier component as described above is represented by the spatial spectrum as shown in FIG. 1C. Where the abscissa f_x and the ordinate f_y` are

Wow

It is standardized as

In FIG. 1C the length of the arrow represents the amount of carrier component and the direction of the arrow represents the phase difference of the carrier component. The carrier component 2 whose spatial frequency is located at the coordinates (1,0) causes moire when the subject projected onto the solid-state image sensor has a long, thin black and white band in the vertical direction.

The carrier component (3) located at the coordinates (2/3, 0) causes a cross color of green (G) and magenta (Ma) for a subject having a rather narrow vertical stripe pattern, and coordinates (1/3). , The carrier component 4 located at 0) causes a cross color of green (G) and magenta (Ma) with respect to a subject having a slightly rough stripe pattern in the vertical direction.

In addition, the carrier component 5 located at coordinates (0, 1) causes moire for a subject having a narrow horizontal stripe pattern. On the other hand, the carrier component 6 located at coordinates (0, 1/2) causes flickering during scanning of a subject having a slightly coarse horizontal stripe pattern, but this flickering causes two flickering at the same time. Can be removed by scanning a horizontal line.

Meanwhile, the moire may be removed by a sample and hold operation in the signal processing method. Therefore, as a whole, the degradation of the image quality in the image element arrangement as shown in FIG. On the other hand, the dotted line of the carrier component that causes the aliasing distortion can be removed by electrical signal processing in the CCD. On the other hand, since the solid line cannot be removed by electrical signal processing, a light low pass filter is employed to reduce this.

2 is a schematic view of a conventional optical lowpass filter employed to reduce aliasing distortion.

Referring to the drawings, the conventional optical low pass filter 10 is composed of first and second birefringent elements 13 and 15 each having one optical axis. In this case, the optical axis 13a of the first birefringent element 13 is disposed to be inclined by θ with respect to the horizontal scanning direction of the solid state image pickup device as shown. The optical axis 15a of the second birefringent element 15 is disposed to be inclined by −θ with respect to the horizontal scanning direction of the solid state image pickup device.

When an unpolarized beam is incident on the first birefringent element 13, the beam is branched into an ordinary ray and an extra-ordinary ray. Each of the normal and abnormal light beams is incident on the second birefringent element 15, and branches back to the normal light beams and the abnormal light beams. Therefore, the incident beam passes through the light low pass filter 10 and is emitted as four outgoing light beams.

The brightness and position of each of the exiting light beams may include a thickness D1 of the first birefringent element 13, a thickness D2 of the second birefringent element 15, and each of the first and second birefringent elements 13, 15. It is determined by the angle 2θ formed by the optical axes 13a and 15a. Therefore, for each angle 2θ between the thicknesses D1 and D2 of the first and second birefringent elements 13 and 15, and the optical axes 13a and 15a, the first and second birefringent elements 13 and 15 are provided. The spatial response to the horizontal and vertical scanning directions of the light branched by each can be obtained.

In this case, the optical low pass filter 10 has a thickness D1 and D2 and an angle 2θ between the two optical axes 13a and 15a to be a predetermined value, so that an aliasing distortion, for example, for a spatial frequency at which a cross color is expected, is obtained. The reaction can be zeroed out. Therefore, the use of the optical low pass filter 10 as described above can reduce the aliasing distortion.

On the other hand, the optical low pass filter 10 as described above is frozen when the angle between the optical axis 13a of the first birefringent element 13 and the optical axis 15a of the second birefringent element 15 is out of a specific value. Since the earing distortion cannot be effectively reduced, the two birefringent elements 13 and 15 are precisely positioned so that the angle between the optical axes 13a and 15a of the first and second birefringent elements 13 and 15 has a specific value. Should be glued.

However, since it is difficult to bond so that the angle between the optical axes 13a and 15a of the two birefringent elements 13 and 15 becomes a specific value, it is difficult to manufacture the optical low pass filter 10 as described above. In addition, since the optical low pass filter 10 requires two birefringent elements, the manufacturing cost is high.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object thereof is to provide an optical low pass filter which is easy to manufacture and inexpensive.

1A shows an example of an image element arrangement of a conventional single chip CCD;

1B is a view showing a color filter used for the CCD of FIG. 1A;

1C is a spatial frequency spectrum of a carrier component generated when the color filter of FIG. 1B is employed;

Figure 2 is a perspective view schematically showing a conventional light low pass filter,

3 is a perspective view schematically showing a light low pass filter according to an embodiment of the present invention;

4 is a view schematically showing an example of an optical axis position of a birefringent element for obtaining a response to a spatial frequency of an optical low pass filter according to the present invention;

5 is a diagram illustrating an example in which the beam incident on FIG. 4 is divided into normal rays and abnormal rays;

6A is a graph schematically showing a frequency response in a horizontal scanning direction of an optical low pass filter according to the present invention;

6b is a graph schematically showing the frequency response in the vertical scanning direction of the optical low pass filter according to the present invention;

<Description of the symbols for the main parts of the drawings>

30 ... low pass filter 35 ... birefringent element

35C ... optical axis

In order to achieve the above object, the present invention provides an optical low pass filter which is employed in a video camera having a solid state image pickup device and a color filter to reduce aliasing distortion generated during spatial sampling. It has a single optical axis forming an angle of θ and the horizontal scanning direction of the solid-state imaging device, characterized in that consisting of a single birefringent element for splitting the incident light into a normal light and an abnormal light.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

3 is a perspective view schematically showing a light low pass filter according to an embodiment of the present invention.

Referring to the drawings, the optical low pass filter 30 according to the present invention preferably comprises a single birefringent element 35 having one optical axis. The incident surface 35a and the exit surface 35b of the birefringent element 35 are parallel to each other, and the optical axis 35c is arranged to form an angle of θ with the horizontal scanning direction of the solid state imaging element.

When an unpolarized beam is incident on the birefringent element 35, the light is branched into normal light and abnormal light and emitted. Therefore, the incident beam passes through the optical low pass filter 30 as described above and is converted into two output beams.

If the unpolarized beam is incident on the optical low pass filter 30, the brightness of each exiting light beam is approximately the same, and the position of each exiting light beam is the thickness D of the birefringent element 35 and the optical axis thereof is solid. It is determined by the angle θ formed in the horizontal scanning direction of the image pickup device.

Therefore, the horizontal and vertical scanning direction components of the outgoing light beam branched from the birefringent element 35 according to the thickness D of the birefringent element 35 and the angle θ between the horizontal scanning direction of the solid state image pickup device and the optical axis, that is, Each spatial response in this horizontal and vertical scanning direction can be obtained.

Hereinafter, the process of obtaining the spatial reaction will be described.

First, it is assumed that an optical low pass filter is provided as shown in FIG. 4 to obtain a spatial response to a general optical low pass filter having one or more optical axes, for example, two optical axes. That is, in two birefringent elements each having one optical axis, the first optical axis 36a forms an angle between the horizontal scanning direction and theta_1`, and the second optical axis 37a forms an angle between the first optical axis 36a and θ_2. If present, the light beams incident on the two birefringent elements are branched into four outgoing light beams o_1 o_2, `` `e_1 o_2,` `` o_1 e_2, `` `e_1 e_2`) and positioned as shown in FIG. 5. .

In FIG. 5, d_1` is a distance branched by the incident beam along the first optical axis 36a, and d_2` is a distance branched along the second optical axis 37a. Here, d_1` and `d_2` may be obtained by the thickness of each birefringent element and the angle between the incident beam and the optical axis. In this case, the brightness of each of the exit light beams may be represented by Equation 6.

If the signal of the incident beam is the same as Equation 7, e_1 = o_1`, the signal s_o (t) `of the emitted light beam for the horizontal scanning direction is expressed by Equation 8. Where d_1` and `d_2` are standardized values.

Then, by developing and solving Equation 8 and dividing by Equation 7, it becomes Equation 9.

In the above Equation 9, since f` means the spatial frequency in the horizontal scanning direction, it is changed to u` to differentiate it from the spatial frequency in the vertical scanning direction, and the response of the spatial frequency corresponding to the horizontal scanning direction (R_h (u)) Is given by Equation (10).

When the response of the spatial frequency R_v (v) corresponding to the vertical scanning direction is obtained in a similar manner to the process of obtaining the response of the spatial frequency R_h (u) corresponding to the horizontal scanning direction, Equation 11 is obtained.

For example, when the video camera includes a color filter as shown in FIG. 1B, when spatial sampling is performed, a decrease in image quality occurs mainly in the horizontal scanning direction. As described with reference to FIG. 1C, the coordinates (f_x, f_y) = coordinates (1 / Carrier component 3 (4) which causes a cross collar is located in the 3, 0) and coordinate (2/3, 0) position.

이 되도록 선택되고, u_2가

이 되도록 선택되면, 크로스 칼라가 발생되는 수평 스캐닝 방향의 공간주파수 위치에서의 공간 반응이 0이 되므로 화질을 저하시키는 크로스 칼라를 줄일 수 있다.As shown in FIG. 6A to be described later, a spatial frequency at which a spatial response becomes almost zero is called a first trap frequency u_1 and a second trap frequency u_2. And u_2 = 2 u_1, where u_1 is

Is selected, u_2

When selected so that the spatial response at the spatial frequency position in the horizontal scanning direction in which the cross color is generated becomes zero, it is possible to reduce the cross color which degrades the image quality.

To this end, the first and second trap points are set to u_1 and 2u_1 as described above in the horizontal scanning direction of the optical axis of the optical low pass filter 30 that satisfies the condition of Rh` (u_1) = Rh` (u_2) = 0. Can be obtained using Equation 12.

On the other hand, since the optical low pass filter 30 according to the present invention is a single birefringent element 35 having one optical axis, the optical low pass filter 30 corresponds to a case where there is no first optical axis 36a. Therefore, the angle formed between the first optical axis 36a and the horizontal scanning direction in Equation 12 is theta_1 = 0, and d_1 = 0 '. Therefore, substituting this into Equation 12 and replacing the separation distance d_2 between the normal light beam and the abnormal light beam branching along the second optical axis 37a with d` and theta_2 with theta`, as shown in FIG. The spatial response to the optical low pass filter 30 in which the optical axis 35c forms an angle of theta` with respect to the horizontal scanning direction can be obtained as shown in Equation (13).

Therefore, by using Equation 13, the angle formed by the optical axis 35c of the birefringent element 35 with respect to the horizontal scanning direction satisfying the condition that the spatial response becomes 0 in u_1` and u_2` satisfying u_2 = 2u_1` theta` satisfies Equation 14.

When the value of Equation 14 is substituted into Equation 13, a first trap frequency as shown in Equation 15 can be obtained.

와 일치하게 된다.If the theta` formed by the optical axis of the optical axis 35c of the birefringent element 35 according to the present invention with respect to the horizontal scanning direction satisfies Equation 14, separation of normal and abnormal rays by the birefringent element 35 If the distance d` is selected to satisfy the equation (15), the horizontal trap frequency u_1 is

To match.

Accordingly, the frequency response R_h (u) in the horizontal direction is such that the spatial response becomes almost zero at the first trap frequency u_1` and the second trap frequency u_2`, that is, 2u_1`, as shown in FIG. 6A. At the same time, the frequency response R_v (v) in the vertical direction is shown in FIG. 6B.

Therefore, if the frequency characteristics in the horizontal direction of the optical low pass filter according to the present invention are as shown in Fig. 6A, the reaction at the spatial frequency of (f_x, f_y) = (1/3, 0), (2/3, 0) It can be suppressed to be almost zero.

Therefore, even if a carrier component exists in these frequency domains, cross color is suppressed, so that the colorless subject can be colored. Therefore, by employing the optical low pass filter 30 as described above in the video camera, it is possible to reduce aliasing distortion due to spatial sampling.

The light low pass filter according to the present invention has been described by taking an example of a CCD having an image element array as shown in FIG. 1A, but is not limited thereto. Therefore, the optical lowpass filter according to the present invention may have a different thickness and optical axis direction so as to suppress aliasing distortion of, for example, a horizontal scanning direction, of a solid-state image pickup device having various image element arrangements in a video camera.

The optical low pass filter according to the present invention as described above can reduce the aliasing distortion causing the degradation of the image quality. In addition, since a single birefringent element having one optical axis is used, assembly tolerances and manufacturing costs can be reduced.

Claims (1)

An optical low pass filter employed in a video camera having a solid-state image pickup device and a color filter to reduce aliasing distortion generated during spatial sampling. The optical low pass filter consists of a single birefringent element having one optical axis forming an angle between the horizontal scanning direction and the angle of the solid-state image sensor, and splits incident light into normal light and abnormal light, And an angle θ formed between the optical axis of the birefringent element and the horizontal scanning direction of the solid-state imaging element satisfies the following equation.

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