JP2841937B2 - Imaging system with optical low-pass filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photographing system having an optical low-pass filter, and more particularly to a method for discretely obtaining an image using a camera tube or a camera plate (CCD) with a video camera, an electronic still camera or the like. The present invention relates to a photographing system having a suitable optical low-pass filter.

[0002]

2. Description of the Related Art Generally, in a video camera or the like using a solid-state imaging device having a discrete pixel structure, an image is optically spatially sampled to obtain an output image. In this case, if the subject contains a high spatial frequency component equal to or higher than the sampling frequency, a false signal such as a structure or a color tone that the subject does not have is generated. That is, frequency components that cannot be collected by the imaging system (frequency components exceeding the Nyquist frequency) cannot be reproduced as image information, and cause so-called waveform distortion (alias), moire fringes, false colors, and the like. Come. For this reason, conventionally, an optical low-pass filter is arranged in a part of a photographing system to limit a high spatial frequency component of a subject.

Various types of optical low-pass filters and photographing systems having the same have conventionally been proposed.
Of these, for example, Japanese Patent Application Laid-Open No. 63-313323 proposes an optical low-pass filter using a phase grating. In this publication, the phase grating has a trapezoidal wave-shaped cross section having a flat top portion and a bottom flat portion having a predetermined width, and the shape, pitch, and the like at that time are specified to obtain a good low-pass effect.

In Japanese Patent Publication No. 56-2936, when an optical low-pass filter composed of a phase grating is provided in a part of a photographing system and used, a predetermined low-pass effect can be obtained even when the aperture diameter of the diaphragm is reduced. We have proposed such a shooting system. That is, in the publication, even at the time of the minimum aperture, the aperture shape of the aperture is set to be an elongated aperture shape whose longitudinal direction is the periodic direction of the phase grating so that at least two periods of the phase grating are included in the aperture. Thus, a constant low-pass effect can be obtained even when the aperture is set to a small aperture.

[0005]

Generally, in order to obtain a constant low-pass effect using an optical low-pass filter comprising a phase grating, a light beam is passed through a region corresponding to a plurality of periods, for example, two periods of the phase grating. It needs to be diffracted. For this reason, the aperture of the photographing system cannot be reduced too small. For example, when the brightness of the subject is high and the aperture aperture needs to be small, the amount of light passing therethrough is reduced by using an ND filter, and the aperture diameter does not fall below a certain value. Like that.

However, the method of reducing the amount of transmitted light by using the ND filter deteriorates the blurring of the obtained image, and requires the ND filter to be attached to a part of the photographing system, which complicates the entire apparatus. There was a problem of coming.

On the other hand, Japanese Patent Publication No. 56-2936 discloses that a certain low-pass effect cannot be obtained unless the light beam passes through at least two rounds of the phase grating, which corresponds to at least two periods of the phase grating. The aperture shape of the stop is configured to be elongated in the period direction of the phase grating so that the light beam enters the region. For this reason, there has been a problem that the configuration of the diaphragm device becomes complicated.

In this publication, no consideration is given to the low-pass effect when a light beam passes through a region of two phases or less of the phase grating.

According to the present invention, when an optical low-pass filter comprising a phase grating is used, the aperture opening is made smaller so that the light beam passes through a region corresponding to two periods or less of the phase grating, that is, the aperture value is variously changed. Therefore, it is an object of the present invention to provide an imaging system having an optical low-pass filter that can easily obtain a constant low-pass effect even when it is changed and can perform imaging with good optical performance.

[0010]

[0011]

According to the present invention, there is provided an imaging system having an optical low-pass filter, wherein a trapezoidal section is arranged in a one-dimensional direction at a pitch P with a flat bottom section having a predetermined width interposed therebetween. When an optical low-pass filter composed of a phase grating is provided in a part of a photographing system, a central region composed of a flat bottom portion of the phase grating at the center of the optical low-pass filter substantially coincides with the center of the aperture of the diaphragm of the photographing system. When the width of the flat portion at the bottom of the central region is A, the following condition is satisfied: A / P <0.15１ (1)

[0012]

1 is a front view and a sectional view of an optical low-pass filter according to a first embodiment of the present invention, FIG. 2 is a schematic sectional view of the optical low-pass filter of the present invention, and FIG. 3 is an optical low-pass filter of the present invention. FIG. 3 is a schematic diagram of a main part of an imaging system having a filter.

In FIG. 1, reference numeral 1 denotes an optical low-pass filter, which has a trapezoidal wave-like phase grating in cross section. 2
Denotes a photographing system, and 3 denotes an aperture. The optical low-pass filter 1 is arranged near the stop 3. Reference numeral 4 denotes a solid-state imaging device including a CCD or the like.

The optical low-pass filter 1 is arranged such that the phase grating direction thereof is inclined by an angle k with respect to the horizontal scanning direction 4a of the solid-state imaging device 4.

The phase grating constituting the optical low-pass filter is integrally molded and manufactured.
As shown in the figure, the trapezoidal wave-shaped trapezoidal portions 1a are arranged in a one-dimensional direction at a pitch P with a pitch P sandwiching a bottom flat portion 1d of a predetermined width continuously on the surface of a parallel plate-like substrate 1b. . Here, “continuous” means that the width of the bottom flat portion 1d is 0.

In FIG. 1, the trapezoidal portion 1a is a flat top portion 1 having a width D.
c, and the trapezoidal portions 1a are arranged so as to sandwich a flat bottom portion 1d having a different width depending on the location.

C1 is the center line of the optical low-pass filter 1. C2 indicates the center of the aperture of the stop 3 of the photographing system 2.

The central portion of the optical low-pass filter 1 is located at the bottom flat portion 1d when the trapezoidal portions 1a are arranged. At this time, the center line C1 is the bottom flat portion 1d.
Is set to pass through the central part of Reference numeral 1e denotes a central region when the phase gratings (trapezoidal portions 1a) are arranged, and two left and right trapezoidal portions 1a1 and 1a2 where the center line C1 is located.
Corresponds to a space region formed by the inclined surfaces 1f1 and 1f2.

The width A of the bottom flat portion 1e1 of the central region 1e is smaller than the width C of the other bottom flat portion 1d. When the width A is 0, the bottom flat portion 1e1 does not exist (this corresponds to Example 2 in FIG. 4 described later). In this case, the two trapezoidal portions 1a1 and 1a2 of the central region 1e are arranged continuously. It will be. At this time, only the boundary line exists in the central region 1e.

In this embodiment, the optical low-pass filter 1 is used.
Are arranged in the photographing system such that the center line C1 substantially coincides with the opening center C2 of the stop 3.

In FIG. 2, the optical low-pass filter 1 is disposed in the photographing system 2 so that the center C2 of the aperture of the stop 1 is slightly displaced from the bottom flat portion 1e1 of the center region 1e and passes through a part of the inclined surface 1f1. Is shown as an example.

When the width A of the bottom flat portion 1e1 is small and substantially zero, an optical low-pass filter is arranged in the photographing system such that the boundary of the central region 1e substantially coincides with the opening center C2 of the stop 3. doing. The trapezoidal shape of the trapezoidal portion 1a, for example, the width of the bottom flat portion 1d and the inclination angle θ of the inclined surface 1f with respect to the substrate 1b are variously different depending on the location.

In this embodiment, as described above, the shape of the optical low-pass filter and the posture when the optical low-pass filter is arranged in the photographing system are specified, so that the subject brightness is high and the aperture 3 is small. Even in the following cases, the passing light beam is effectively diffracted, so that a predetermined low-pass effect can be easily obtained while preventing a decrease in optical performance.

In particular, in the present invention, when the optical low-pass filter is arranged in the photographing system, the ratio of the width A of the bottom flat portion 1d of the central region to the period P of the phase grating is set so as to satisfy the conditional expression (1). By setting, a good low-pass effect is obtained. If conditional expression (1) is not satisfied, when the aperture opening is narrowed down to less than two periods of the phase grating, particularly when it is narrowed down to 0.5 period or less, the ratio of the area of the bottom flat portion within the diameter of the aperture opening to the inclined surface is reduced. This is not good because only the spectral intensity of the zero-order light becomes too strong and the low-pass effect decreases.

In addition, in order to obtain a good low-pass effect while preventing a decrease in optical performance by arranging an optical low-pass filter in the photographing system in the present invention, the following conditions should be satisfied.

(A) When the amount of deviation between the center of the aperture of the stop and the center of the bottom flat portion of the central region of the optical low-pass filter is B, the optical low-pass filter is B / P <0.15 °. ‥‥‥‥‥ (2) The setting must be made to satisfy the following condition. Conditional expression (2)
When the aperture opening is narrowed to 0.5 cycle or less of the phase grating, the area ratio of only the inclined surface or the flat top portion of the phase grating increases, and as a result, the prism action for separating the light flux decreases, and the low-pass effect is reduced. Is not good because it decreases.

(B) The refraction angle of a light beam which is perpendicularly incident on the substrate of the optical low-pass filter and has passed through the inclined surface of the phase grating adjacent to the central region is θ _r , and the first-order diffraction angle of the phase grating Is θ _f , the angle between the inclined surface of the phase grating and the substrate is θ, the refractive index of the material of the phase grating is n, and the wavelength used is λ, and θ _r = (n−1) θ θ _f = sin ^{− When 1} (λ / P), the condition 0.5 <θ _r / θ _f <1.4 ‥‥‥‥‥‥ (3) should be satisfied.

[0028] When the aperture stop and below the lower limit of the condition (3) narrowed down in the following example one period of the phase grating, the injection angle theta _r is determined from the principles of geometrical optics. Injection angle theta _r at this time is significantly reduced with respect to first-order diffracted light theta _f of the phase grating, not good low pass effect is lowered.

[0029] Condition (3) exit angle of the first order diffracted light in the opposite exceeds the upper limit of not good resolution of increased and imaging system with respect to 1 order diffraction angle theta _f of the phase grating is lowered significantly.

(C) When the width of the bottom flat portion other than the center region is C, the width of the top flat portion of the phase grating is D, and the height from the bottom flat portion to the top flat portion is H

[0031]

(Equation 1)

If conditional expression (4) is not satisfied, the change of the low-pass effect with respect to the wavelength change becomes large, and it becomes difficult to obtain a good optical low-pass filter over the entire visible region.

(D) The photographing system has a solid-state image sensor. When the angle between the phase grating direction of the optical low-pass filter and the horizontal scanning direction of the fixed image sensor is k, 45 ° < k <70 ° ‥‥‥‥‥‥ (5) The arrangement is such that the following condition is satisfied.

If the angle k is too small below the lower limit of conditional expression (5), the resolving power of the photographing system is greatly reduced, which is not good. If the value exceeds the upper limit, the resolving power is improved, but the low-pass effect in the vertical scanning direction disappears, which is not good.

FIG. 4 is a front view and a sectional view of Embodiment 2 of the optical low-pass filter according to the present invention. In the present embodiment, a case where the trapezoidal portions 1a of the phase grating are continuously arranged, that is, a case where the width of the flat portion at the bottom is 0 is shown.

Also in the present embodiment, the same effect as in the first embodiment can be obtained by setting the shape of the phase grating and the like so as to satisfy the above-mentioned conditional expressions.

FIG. 5 is an explanatory diagram showing a spectrum emission angle and a spectrum intensity ratio when an optical low-pass filter according to Embodiment 2 of the present invention and a conventional optical low-pass filter described later are used. FIG. 3 shows a case where a light beam is incident with various changes from two periods of the phase grating to 0.25 period.

In the conventional optical low-pass filter, the spectral intensity of the first-order diffracted light is about １ ／ of that of the zero-order light when the aperture is 1.25 periods of the phase grating, and the low-pass effect is reduced. Also, in the 0.75 cycle, the spectral exit angle of the first-order diffracted light is about 1.6 times that in the case of two cycles, but the spectral intensity ratio between the first-order diffracted light and the 0th-order light is 1: 1 and the resolution of the photographing system is increased. Decreases. When the period is 0.5 cycles or less, only the spectral intensity of the zero-order light increases, so that the low-pass effect decreases.

As described above, in the conventional optical low-pass filter, when the aperture diameter is about two cycles or less of the phase grating, the spectral intensity changes greatly, and the low-pass effect and the resolving power are greatly reduced by the aperture value of the photographing system.

On the other hand, in the optical low-pass filter according to the present invention, even if the aperture is smaller than two periods of the phase grating, the change in the spectrum intensity is small, and the same low-pass effect as when the period is longer than two periods is obtained. be able to.

In each of the above embodiments, the same effect can be obtained even if the unevenness of the phase grating is completely reversed.

Next, the relationship between the first and second embodiments of the present invention, the above-mentioned various values relating to the shape of the conventional optical low-pass filter, and the above-described conditional expressions will be described.

(Example 1) (Example 2) (Conventional example) A = 0.188 A = 0 A = 0.47 B = 0.188 B = 0 B = 0 C = 0.47 C = 0.47 C = 0.47 D = 0.47 D = 0.47 D = 0.47 P = 1.88 P = 1.88 P = 1.88 θ = 0.0378 ° θ = 0.0378 ° θ = 0.0549 ° H = 0.00045 H = 0.00045 H = 0.00045 λ = 0.00055 λ = 0.00055 λ = 0.00055 n = 1.49 n = 1.49 n = 1.49 K = 60 ° K = 60 ° K = 60 ° A / P = 0.1 A / P = 0 A / P = 0.25 B / P = 0.1 B / P = 0 B / P = 0 θ _r / θ _f = 1.1 θ _r / θ _f = 1.1 θ _r / θ _f = 1.6 C / P = 0.25 C / P = 0.25 C / P = 0.25 D / P = 0.25 D / P = 0.25 D / P = 0.25 H / λ = 0.82 H /λ=0.82 H / λ = 0.82 K = 60 ° K = 60 ° K = 60 °

[0044]

According to the present invention, by setting each element of the optical low-pass filter as described above, the aperture opening can be reduced when the optical low-pass filter is arranged in a part of the photographing system for photographing. For example, even if the light beam passes through a region corresponding to, for example, two periods or less of the phase grating, that is, even if the aperture value is variously changed, a constant low-pass effect can always be easily obtained, and the density of the ND filter can be reduced. In addition, it is possible to achieve a photographing system having an optical low-pass filter capable of performing photographing with good optical performance with good blurring and reduced blurring.

[Brief description of the drawings]

FIG. 1 is a front view and a side view of Embodiment 1 of an optical low-pass filter according to the present invention.

FIG. 2 is a schematic cross-sectional view of a part of FIG.

FIG. 3 is a schematic diagram of a main part of a photographing system having an optical low-pass filter according to the present invention.

FIG. 4 is a front view and a side view of Embodiment 2 of the optical low-pass filter according to the present invention.

FIG. 5 is a comparison diagram of the spectral intensity between the optical low-pass filter according to the second embodiment of the present invention and the conventional optical low-pass filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical low-pass filter 1a Trapezoid part 1b Substrate 1c Top flat part 1d Bottom flat part 1e Center area 2 Imaging system 3 Aperture 4 Image sensor C1 Center line C2 Opening center

Claims (5)

(57) [Claims] An optical low-pass filter having a trapezoidal wave-shaped phase grating in which trapezoidal portions are arranged in a one-dimensional direction at a pitch P with a bottom flat portion having a predetermined width therebetween is provided in a part of an imaging system. The center area of the bottom of the phase grating at the center of the optical low-pass filter was arranged so as to substantially coincide with the center of the aperture of the aperture stop of the imaging system, and the width of the flat bottom of the center area was A. An imaging system having an optical low-pass filter, wherein the following condition is satisfied: A / P <0.15. 2. The condition of the optical low-pass filter being B / P <0.15, where B is a shift amount between the center of the aperture of the stop and the center of the bottom flat portion of the central region of the optical low-pass filter. 2. A photographing system having an optical low-pass filter according to claim 1, wherein the following conditions are satisfied. 3. A refraction angle of a light beam, which is perpendicularly incident on a substrate of the optical low-pass filter and passes through an inclined surface of a phase grating adjacent to the center region, is θ _r , and a first-order diffraction angle of the phase grating is θ _f , the angle between the inclined surface of the phase grating and the substrate is θ, the refractive index of the material of the phase grating is n, and the wavelength used is λ. θ _r = (n−1) θ θ _f = sin ⁻¹ 2. The imaging system having an optical low-pass filter according to claim 1, wherein a condition 0.5 <θ _r / θ _f <1.4 is satisfied when (λ / P). 4. The width of a bottom flat portion other than the center region is C, the width of a top flat portion of the phase grating is D, and the height from the bottom flat portion to the top flat portion is H. 4. The method according to claim 1, wherein a condition of 2 ≦ C / P ≦ 0.3 0.2 ≦ D / P ≦ 0.3 0.6 <H / λ <1.2 is satisfied. An imaging system with an optical low-pass filter. 5. The image pickup system has a solid-state image pickup device. When an angle between the phase grating direction of the optical low-pass filter and the horizontal scanning direction of the fixed image pickup device is k, 45 ° <k. 5. An imaging system having an optical low-pass filter according to claim 1, wherein the optical system is arranged so as to satisfy a condition of <70 °.