JPH0677114B2 - Optical comb filter - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to an optical comb filter using a birefringent plate such as quartz.

[Technical background of the invention]

It is generally known that a birefringent substance such as quartz can be used as an optical comb filter although it depends on its thickness, optical axis direction and the like. The principle is as described below. As shown in FIG. 1, a ray 2 that is vertically incident on the birefringent plate 1 is separated into an ordinary ray L _o and an extraordinary ray L _e by the birefringent action of the birefringent plate, depending on the direction of polarization. Is ejected. Here, the angle α formed by the ordinary ray L _o and the extraordinary ray L _e is
If the refractive index of the birefringent plate 1 for ordinary light is n _o , the refractive index for extraordinary light incident perpendicularly to the optical axis is n _e , and the angle between the optical axis and the incident light is β, then the following formula is given.

It becomes the maximum when tan β = n _e / n _o shown in the equation (1), but when the birefringent plate 1 is a crystal, the difference between n _o and n _e is small,
Normally β = 45 degrees is selected. The ordinary ray L _o and the extraordinary ray L _e emitted from the birefringent plate 1 are rays separated by Δ _W.
W is represented by the following formula when β = 45 degrees and the thickness d of the birefringent plate.

FIG. 2 is a diagram showing an example in which a birefringent plate having the above characteristics is inserted in an image forming optical system. The birefringent plate 1 is arranged so that its surface is orthogonal to the optical axis 4 of the lens 3, and its direction is determined so that its optical axis is parallel to the paper surface. The light transmitted through the birefringent plate 1 delta _W two parallel spaced by rays L _o, since the L _e, originally, a light beam was supposed to be focused on one point by the lens 3 is delta _W only The image is formed at two points apart from each other. Reference numeral 5 is an image forming point of the ordinary ray L _o , and reference numeral 6 is an image forming point of the extraordinary ray L _e .

The fact that the image formation points are separated by Δ _W when the light passes through the birefringent plate 1 in this way is expressed as follows by a function having the x axis in the light separation direction. Here, the delta function δ (x) indicates the input to the birefringent plate 1, and the output of the birefringent plate 1 for this is h (x).

In general, the frequency response (MT
F) is known to be obtained. In the above system, x is the spatial frequency in the x-axis direction and H is the MTF of the birefringent plate.
Assuming (x), the MTF is expressed by the following equation.

H ((x) = | F [h (x)] | = | cos (πΔ _W x) |
(4) When this equation (4) is illustrated, the characteristics shown in (a) of FIG. 4 are obtained, and it can be seen that the birefringent plate 1 exhibits a comb-shaped spatial frequency response in the horizontal direction.

In a color imaging device that uses a single image sensor to spatially modulate light in a specific wavelength range to obtain a color signal, if the above-mentioned modulation frequency component exists in the input image, this component is mixed into the color signal. Then, a false color signal is generated. Conventionally, as a method of suppressing the generation of the color false signal, the color false signal is inserted by inserting a birefringent plate whose thickness is selected so that the response at the modulation frequency becomes 0. The method of reducing the is widely used. Also, in an image sensor that captures spatial information discretely, such as a CCD, all signals within the MTF limit of the image sensor, out of the signals that exceed the Nyquist limit due to the sampling spatial frequency, should be folded back to the low frequency range as moire. It has been known. In order to reduce this moire, a method is used in which several sets of the above birefringent plate and depolarizing plate are combined and used as an optical low pass filter.

[Problems of background technology]

As described above, the birefringent plate can be used as an optical comb filter, but the comb-shaped characteristic as shown in FIG. 4B is obtained by the F value (focal length / effective aperture) of the lens 3. Is large, that is, the influence of defocus is small. Therefore, in a situation where the effect of defocus cannot be ignored, such as when the F-number of the lens is small, or when an accurate comb-shaped response is required up to a sufficiently high frequency with respect to the lowest attenuation frequency of the comb-shaped filter, ,
The birefringent plate 1 comes to exhibit characteristics that are considerably different from the characteristics as a comb filter. The reason will be described below.

FIG. 3 shows an optical system for forming an image of a point light source on the optical axis at infinity. The light beam condensed by the lens 3 forms an image through the birefringent plate 1. Here, for the sake of simplification, the effective aperture of the lens 3 is considered to be a square in which one side having a side parallel to the paper surface is half the focal length. The birefringent plate 1 is a crystal having an angle of 45 degrees with respect to the surface, an optical axis parallel to the paper surface, and a thickness of d. The birefringent plate 1 separates the incident light into an ordinary ray L _o and an extraordinary ray L _e . However, the birefringent plate 1 is the refractive index with respect to ordinary rays L _o and the extraordinary ray L _e are different, the image plane of the image by the ordinary ray L _o image and the extraordinary ray L _e by different. Here, the refractive index for ordinary light of birefringence plate 1 is n _o, and the refractive index for the extraordinary light incident perpendicularly to the optical axis is n _e, the refractive index of the birefringent plate 1 for extraordinary ray L _e is Becomes Therefore, the distance Δ _S of the image plane between the ordinary ray L _o and the extraordinary ray L _e is expressed by the following equation.

At this time, when an image is captured on the image plane by the extraordinary ray L _e , the image formed is a square with points and one side Δ _S / 2. this,
Taking the axis parallel to the image plane and the paper plane as the x-axis and expressing it as a function on this x-axis, Becomes From this image, H ₁ (
When x) is obtained, it becomes as shown by the following equation.

When the function expressed by the above equation (6) is illustrated, the characteristic line shown in B of FIG. 4 is obtained, which is different from the original comb-shaped characteristic line a. Such a phenomenon also occurs when the effective aperture of the lens 3 is circular or the like. Further, the above MTF value changes depending on the difference in the F value of the lens 3 and how the image plane is selected. Therefore, when the single birefringent plate 1 is used as the optical system comb filter as described above, there is a drawback that a sufficient effect cannot be obtained depending on the situation.

2. Description of the Related Art In recent years, a color image pickup apparatus has been used in which an image pickup element that captures spatial information such as a CCD is used, and incident light is modulated at a spatial frequency that is half the sampling frequency of the element to obtain color information. In such a color image pickup device, when the above-mentioned modulation frequency by the birefringent plate and the frequency components of odd multiples thereof are included in the incident image, these components are mixed into the color signal to become a color false signal. If the birefringent plate has the original comb-shaped filter characteristic, a frequency that is an odd multiple of the lowest attenuation frequency becomes the attenuation frequency, so that the color false signal is greatly reduced. However, as mentioned above,
In an optical comb filter using a single birefringent plate, the characteristics may be different from the original comb filter depending on the situation, and further, in the high spatial frequency range, the different characteristics may be different. Since there is a large deviation from the comb filter characteristic of No. 3, there is a drawback that a sufficient reduction effect cannot be obtained for a color false signal due to a particularly high spatial frequency component.

[Object of the Invention] In view of the above-mentioned drawbacks, an object of the present invention is to provide an optical comb shape capable of obtaining a predetermined characteristic up to a high spatial frequency without being influenced by the F value of the lens and the way of selecting the image plane. To provide a filter.

[Outline of Invention]

The present invention is directed to one or more first birefringent plates having an optical axis in a first plane perpendicular to the surface, and a second plane perpendicular to both the surface and the first plane. One or more second birefringent plates each having an optical axis inside are arranged in parallel, and a sum of optical path differences between ordinary and extraordinary rays passing through the first birefringent plate, and The sum of the optical path differences between the ordinary ray and the extraordinary ray passing through the second birefringent plate is equal to achieve the above object.

Example of Invention

Embodiments of the present invention will be described below with reference to the drawings. Fifth
FIG. 1 is a perspective view showing an embodiment of the optical comb filter of the present invention. The optical comb filter is constituted by a birefringent plate 7 and a birefringent plate 8 in close contact with each other. Reference numeral 9 is a projection line for each surface of the optical axis of the birefringent plate 7, and reference numeral 10 is a projection line for each surface of the optical axis of the birefringent plate 8. As the optical axes of the birefringent plate 7 and the birefringent plate 8 are shown by the projection lines of each surface, the planes including the optical axes perpendicular to the surfaces are orthogonal to each other,
The birefringent plate 7 and the birefringent plate 8 are arranged. Further, the thickness d ₂ of the refraction plate 8 is selected so as to satisfy the following formula with respect to the thickness d ₁ of the refraction plate 7.

However, n _o is the refractive index for ordinary ray in the birefringent plate 7, 8, n _e is the refractive index for the extraordinary light incident perpendicularly to the optical axis.

FIG. 6 is a view showing an image forming optical system using the optical comb filter of this embodiment shown in FIG. Behind the lens 11, an optical comb filter composed of the birefringent plates 7 and 8 is arranged so that the optical axis of the birefringent plate 7 is parallel to the paper surface.

The light beam incident on the birefringent plate 7 is separated into an ordinary ray L ₁ and an extraordinary ray L ₂ . The optical path difference l ₁ between the ordinary ray L ₁ and the extraordinary ray L ₂ while passing through the birefringent plate 7 is given by the following equation.

By the way, the plane including the optical axis perpendicular to the surface is the birefringent plate 7.
Since the birefringent plate 7 and the birefringent plate 8 are arranged so as to be orthogonal to each other, L ₁ passes through the birefringent plate 8 as extraordinary light and L ₂ passes as ordinary light. L ₁ and L ₂ are birefringent plates 8
The optical path difference l ₂ between the two while passing through is given by the following equation.

Therefore, after passing through both the birefringent plate 7 and the birefringent plate 8,
The optical path difference l between L ₁ and L ₂ is l = l ₁ + l ₂ = 0. Therefore, L ₁ and L ₂ that have passed through the comb filter of this embodiment are on the same image plane 12
Then, images are formed at points separated by Δ _W.

According to this embodiment, the optical comb filter is composed of two birefringent plates 7 and 8, and the planes including the optical axis perpendicular to the surface are orthogonal to each other in the birefringent plate 7 and the birefringent plate 8. to, disposed these birefringent plates 7 and 8, and the thickness of the birefringent plate 7 d ₁
And the thickness d ₂ of the birefringent plate 8 are selected so as to satisfy the above expression (7), the image planes on which the ordinary ray L _o and the extraordinary ray L _e are imaged can be matched, It is possible to obtain a predetermined characteristic up to a high spatial frequency without the characteristic of the optical comb filter being influenced by the F value of the lens 11 and the method of selecting the image plane. Therefore, when the optical comb filter of the present embodiment is used for reducing a color false signal in a color image pickup device, reducing moire in a solid-state image pickup device, and the like, a high effect can be exhibited without being affected by the situation.

FIG. 7 is a perspective view showing another embodiment of the optical comb filter of the present invention. The optical comb filter is composed of two birefringent plates 13 and 14 in close contact with each other.
Although the thicknesses d ₁ and d ₂ and the angle formed by the optical axis and the surface are the same, the birefringent plate 14 is arranged so as to be rotated by 90 degrees with respect to the birefringent plate 13. Reference numerals 15 and 16 are birefringent plates 13 and 14
Is a projection line on each surface of the optical axis of. Therefore, the birefringent plate 13
The light transmitted as the ordinary ray passes through the birefringent plate 14 as the extraordinary ray, and the light transmitted through the birefringent plate 13 as the extraordinary ray passes through the birefringent plate 14 as the ordinary ray. Therefore, the two light beams separated by these two birefringent plates 13 and 14 are imaged at two points on the same image plane, and the same effect as the previous embodiment is obtained.

It should be noted that various modifications such as the thickness of the birefringent plate and the angle of the optical axis or the number of birefringent plates are conceivable, and the present invention is not limited to the above-mentioned two embodiments, in short, it passes an optical comb filter. The optical comb filter may be formed by combining the birefringent plates so that the image forming points of the extraordinary ray and the ordinary ray are on the same image plane. Further, the optical comb filter according to the present invention can be used not only alone but also in combination with a depolarizer or the like to form various specific spatial frequency filters.

〔The invention's effect〕

As described above, according to the optical comb filter of the present invention, the optical comb filter is formed by combining a plurality of birefringent plates so that an image point of an ordinary ray and an image point of an extraordinary ray are on the same image plane. Therefore, there is an effect that a predetermined comb-shaped characteristic can be obtained up to a high spatial frequency without being influenced by the F value of the lens and the method of selecting the image plane.

[Brief description of drawings]

FIG. 1 is a diagram showing separation of light by a birefringent plate, FIG. 2 is a diagram showing an example of an optical system including an optical comb filter made of a conventional single birefringent plate, and FIG. FIG. 4 is a diagram showing an image formation state by an optical system including an optical comb filter, FIG. 4 is a characteristic diagram showing a spatial frequency response of a conventional optical comb filter in the optical system shown in FIG. 3, and FIG. FIG. 6 is a perspective view showing an embodiment of the optical comb filter of the present invention, FIG. 6 is a view showing an example of an optical system including the optical comb filter shown in FIG. 5, and FIG. 7 is an optical system of the present invention. It is the perspective view which showed the other Example of the static comb filter. 7, 8, 13, 14 ... Birefringent plate 9, 10, 15, 16 ... Projection line of optical axis 11 ... Lens 12 ... Image plane

Claims (1)

[Claims] 1. One or more first birefringent plates having an optical axis in a first plane perpendicular to the surface, and a second birefringent plate perpendicular to both the surface and the first plane. One or more second birefringent plates having an optical axis in a plane and arranged in parallel, and a sum of optical path differences between ordinary and extraordinary rays passing through the first birefringent plate, An optical comb filter, wherein the sum of the sum of the optical path differences between the ordinary light passing through the second birefringent plate and the extraordinary light is zero.