JPH0527205A - Optical low-pass filter and photographic system having the same - Google Patents

Abstract

PURPOSE:To offer the optical low-pass filter which provides excellent low-pass effect even when the stop aperture of the photographic system is reduced below two cycles of a phase grating and the photographic system which has the optical low-pass filter by properly setting the shape, pitch, etc., of the phase grating which is sectioned in a trapezoid shape. CONSTITUTION:The optical low-pass filter 1 is constituted by arraying the phase grating which is sectioned in the trapezoid shape continuously and linearly at the same pitch P. Then a projection type prism part is provided opposite the phase grating at the center part 1e of the opposite surface from the surface where the phase grating in the trapezoid shape is formed so that the refraction angle or of luminous flux passing through a slanting surface 1f1 of the phase grating at the center part 1e of the optical low-pass filter 1 is smaller than the refraction angle thetara of luminous flux passing through a slanting surface 1g of the phase grating in other areas.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Generally, in a video camera using a solid-state image pickup device having a discrete pixel structure, image information 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 color tone that the subject does not have occurs. That is, the frequency components (frequency components exceeding the Nyquist frequency) that cannot be sampled by the imaging system cannot be reproduced as image information, which causes so-called waveform distortion (aliasing), moire fringes, false colors, etc. Come on. Therefore, conventionally, an optical low-pass filter is arranged in a part of the photographing system to limit the high spatial frequency component of the subject.

Conventionally, various types of optical low-pass filters and photographing systems having the same have been proposed.
Among them, for example, Japanese Patent Publication No. 52-22247 proposes an optical low-pass filter using a phase grating. In this publication, phase gratings having a trapezoidal wave-shaped cross section are continuously arranged in a one-dimensional direction, and the shapes and pitches of the flat top portion and the inclined surface at that time are specified to obtain a good low-pass effect.

Further, in Japanese Patent Publication No. 56-2936, when an optical low-pass filter composed of a phase grating is provided and used in a part of the photographing system, a predetermined low-pass effect can be obtained even when the aperture diameter of the diaphragm is reduced. I am proposing a photography system that does this. That is, in the publication, the aperture shape of the diaphragm is an elongated aperture shape whose longitudinal direction is the cycle direction of the phase grating so that at least two cycles of the phase grating are included in the diaphragm aperture even in the case of the minimum diaphragm. This makes it possible to obtain a constant low-pass effect even when the aperture is made small.

[0005]

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

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

On the other hand, in the above-mentioned Japanese Patent Publication No. 56-2936, it is assumed that a certain low-pass effect cannot be obtained unless the light flux passes through at least two rounds of the phase grating, which corresponds to at least two cycles of the phase grating. The aperture shape of the diaphragm is configured to be elongated in the period direction of the phase grating so that the light beam enters the region. Therefore, there is a problem that the configuration of the diaphragm device becomes complicated.

Further, in this publication, no consideration was given to the low-pass effect when the light flux passes through the region of two phases or less of the phase grating.

According to the present invention, when an optical low-pass filter composed of a phase grating is used, the diaphragm aperture is narrowed so that the light beam passes through a region corresponding to two cycles or less of the phase grating, that is, various diaphragm values are used. It is an object of the present invention to provide an optical low-pass filter which can always obtain a constant low-pass effect even when changed to, and which enables photographing with good optical performance, and an imaging system having the optical low-pass filter.

[0010]

The optical low-pass filter of the present invention is an optical low-pass filter in which phase gratings having a trapezoidal wave-shaped cross section are continuously arranged in the one-dimensional direction at the same pitch P. The trapezoidal wave-shaped phase grating is formed so that the refraction angle of the light beam passing through the inclined surface of the phase grating at the center of the filter is smaller than the refraction angle of the light beam passing through the inclined surface of the phase grating in other regions. It is characterized in that a convex prism portion is provided on a surface opposite to the surface at a position facing the phase grating at the central portion.

Further, as an image pickup system having an optical lowpass filter of the present invention, an optical lowpass filter in which phase gratings having a trapezoidal wave-shaped cross section are continuously arranged in the same pitch P in a one-dimensional direction is one of the image pickup systems. When the optical low-pass filter is installed in the section, the optical low-pass filter passes through the inclined surface of the phase grating in the area where the refraction angle of the light beam passing through the inclined surface of the phase grating facing the central area including the aperture center of the aperture of the imaging system is different. A convex prism portion is provided on the surface opposite to the surface on which the trapezoidal wave-shaped phase grating is formed so as to be smaller than the refraction angle of the luminous flux. It is characterized by that.

[0012]

1 (A) and 1 (B) are 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 an optical low pass filter of the present invention, and FIG. It is a principal part schematic diagram of the imaging system which has the optical low pass filter of this invention.

In the figure, reference numeral 1 is an optical low-pass filter, which is formed by arranging phase gratings having a trapezoidal wave-shaped cross section in the one-dimensional direction at the same pitch P. 2 is a shooting system,
Reference numeral 3 is a diaphragm. Optical low-pass filter 1 has aperture 3
It is located in the vicinity. Reference numeral 4 is a solid-state image pickup device including a CCD or the like.

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

The phase grating constituting the optical low pass filter is integrally molded and manufactured, and its sectional shape is shown in FIG.
As shown in (1), the trapezoidal portions 1a having a trapezoidal wave shape are arranged in a one-dimensional direction at a pitch P continuously on the surface of the parallel plate substrate 1b. In the figure, the trapezoidal portion 1a has a top flat portion 1c with a width D, and C1 is the center line of the optical low-pass filter 1. C2 indicates the center of the aperture of the diaphragm 3 of the photographing system 2.

Reference numeral 1e designates a central region when the phase gratings (trapezoidal portion 1a) are arranged, and two regions on the left and right on which the center line C1 is located.
It corresponds to the space area formed by the inclined surface 1f1 of one trapezoidal portion 1a1.

Inclined surface 1f of trapezoidal portion 1a1 of central region 1e
The refraction angle θ _r of the light flux passing through 1 is 1 g
A convex prism 101 having a center line C1 as its apex is provided on the surface opposite to the surface on which the trapezoidal wave-shaped phase grating is formed so as to be smaller than the refraction angle θ _ra of the light flux passing through.

In this embodiment, the optical low pass filter 1 is used.
Is arranged in the photographing system so that its center line C1 substantially coincides with the center C2 of the aperture of the diaphragm 3.

FIG. 2 shows an example in which the optical low-pass filter 1 is arranged in the photographing system 2 so that the center C2 of the aperture of the diaphragm 1 coincides with the center line C1 of the optical low-pass filter 1.

In this embodiment, the refraction angle θ _r of the light beam passing through the inclined surface 1f1 of the phase grating in the central region 1e is the refraction angle θ _{ra of} the light beam passing through the inclined surface 1g of the phase grating in the other region.
By making the aperture diameter of the photographing system 2 smaller than 2 cycles of the phase grating, a constant low-pass effect can be easily obtained.

In particular, in this embodiment, the shape of the optical low-pass filter is specified as described above, and the posture when the optical low-pass filter is arranged in the photographing system is specified so that the subject brightness is high and the diaphragm 3 is small. Is effective to refract the passing light flux even when the phase grating is equal to or less than two cycles, so that a predetermined low-pass effect can be easily obtained while preventing deterioration of optical performance.

In addition, in the present invention, in order to obtain a good low-pass effect while preventing the deterioration of the optical performance by disposing an optical low-pass filter in the photographing system, it is preferable to satisfy the following conditions.

(A) The convex prism 10 of the light flux which is vertically incident on the substrate of the optical low-pass filter.
The angle of refraction of the light beam passing through the inclined surface 1f and the inclined surface 1f1 of the phase grating adjacent to the central region is θ _r ,
The next diffraction angle is θ _f , the angle between the inclined surface of the phase grating and the substrate is θ _a , the angle between the inclined surface of the convex prism and the substrate is θ, and the refractive index of the material of the phase grating is n, When the wavelength used is λ and θ _r = (n-1) (θ _a −θ) θ _f = sin ⁻¹ (λ / P) 0.4 <θ _r / θ _f <1.1 (1) Satisfy the following conditions.

[0024] When the aperture stop and below the lower limit of the condition (1) narrowing down below example 2 period of the phase grating, the angle of emergence theta _r the principle of geometrical optics is determined, the injection angle theta _r at this time This is not good because it becomes significantly smaller than the first-order diffracted light θ _{f of the} phase grating, and the low-pass effect deteriorates.

If the upper limit of conditional expression (1) is exceeded, conversely the exit angle θ _r of the first-order diffracted light becomes larger than the first-order diffraction angle θ _f of the phase grating and the resolving power is obtained when the diaphragm of the photographing system is narrowed down to 2 cycles or less. Is greatly reduced, which is not good.

(B) A refraction angle of a light beam which is perpendicularly incident on the substrate of the optical low-pass filter and which passes through an inclined surface of the phase grating other than the central region is θ _ra , and a flat top portion of the phase grating is θ _ra . The width is D, and the first-order diffraction angle of the phase grating is θ
_f , where θ _a is the angle between the inclined surface of the phase grating and the substrate, and θ _ra = (n−1) θ _a 0.15 <D / P <0.35 ‥‥‥‥‥ 2) Satisfy the conditions of 1.1 <θ _ra / θ _f <1.6 ‥‥‥‥‥‥‥‥ (3).

Next, conditional expressions (2) and (3) will be described with reference to the diagram showing the spectral intensity of the optical low-pass filter in FIG. In the figure, f _c indicates the cutoff frequency due to the low pass effect.

Conditional expressions (2) and (3) are related to the shape and characteristics of the phase grating other than the central region, and when the principle of wave optics is mainly followed, the 0th-order light and the 1st-order diffracted light are obtained at the target wavelength λ. The conditions under which the spectral intensities are substantially equal as shown in FIG. 5 (A) are defined.

By making the spectral intensities of the 0th-order light and the 1st-order diffracted light substantially equal to each other, there is almost no 0th-order light intensity for obtaining the same low-pass effect, and only the 1st-order diffracted light exists. The period of the phase grating is made smaller than that shown. As a result, a constant low-pass effect can be obtained even in the case of a small diaphragm in which the diaphragm aperture has two cycles or less of the phase grating.

Here, as shown in FIG. 5A, three spectra of equal intensity and two spectra of equal intensity in FIG. 5B are compared with an arbitrary cutoff frequency f _c . The interval between the ends of the spectra of the two spectra (first-order and the first-order spectra interval) is f _c / 2, and the interval between the ends of the spectra of the three spectra (first-order and first-order spectra interval) is 2f _c / 3. Then, the MTF can be set to 0 at the cutoff frequency f _c . That is, the period of the phase grating can be reduced by 25% when the three spectra are equal.

The conditional expressions (2) and (3) are set in consideration of the above points. If the lower limit of conditional expression (2) is exceeded, the width D of the flat top portion becomes too narrow, whereas the width of the inclined surface becomes wider, and the spectrum of the 1st-order diffracted light becomes larger than that of the 0th-order light, and imaging is performed. It is not good because the resolution of the system decreases. Also, if the width D of the flat top portion becomes too narrow, 3
It is not good because the spectral characteristics of the phase grating of the angular wave become closer and the change of the low-pass effect with respect to the wavelength change becomes large.

If the width D of the flat top portion becomes too wide beyond the upper limit of the conditional expression (2), the spectrum intensity of the 0th order light with respect to the 1st order diffracted light becomes large and the low pass effect is deteriorated, which is not preferable. Further, when the diaphragm is narrowed down to two cycles or less of the phase grating, the area ratio in the diaphragm opening diameter of the flat top portion becomes too large and the low pass effect is deteriorated, which is not preferable.

If the lower limit of conditional expression (3) is exceeded, the refraction angle θ _ra
Is too small compared to the first-order diffraction angle θ _f , the intensity of the first-order diffracted light is weakened with respect to the 0th-order light, and the low-pass effect is deteriorated, which is not preferable.

When the upper limit of conditional expression (3) is _{exceeded, the} refraction angle θ _{ra is exceeded.}
Is too large compared to the first-order diffraction angle θ _f , the intensity of the first-order diffracted light becomes too strong for the 0th-order light, and the resolving power of the photographing system is deteriorated.

(C) The photographing system has a solid-state image sensor, and 45 ° <where 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. k <75 ° ··························· (4) Arrange so as to satisfy the condition.

If the angle k is too small beyond the lower limit of conditional expression (4), the resolving power in the vertical direction of the photographing system will be greatly reduced, which is not preferable. On the other hand, when the value exceeds the upper limit, the resolution is improved but the low pass effect in the vertical scanning direction is lost, which is not good.

FIG. 6 is a sectional view of the essential parts of Embodiment 2 of the optical low-pass filter of the present invention. This embodiment shows a case where the trapezoidal portion 1a of the phase grating is formed so that the concavo-convex shape is reversed as compared with FIG.

Also in this embodiment, the same effect as that of the first embodiment is obtained by completely reversing the shape of the phase grating and the like so as to satisfy the above-mentioned conditional expressions.

FIG. 4 is an explanatory view showing a spectrum emission angle and a spectrum intensity ratio when an optical low-pass filter according to a first embodiment of the present invention described later and a conventional optical low-pass filter described later are used. The figure shows a case where the light beam is incident with various changes from two cycles of the phase grating to 0.25 cycle.

In the conventional optical low-pass filter, it is 1 when the aperture opening is narrowed down to 0.75 period or less of the phase grating.
Only the spectrum of the second-order diffracted light becomes, and the spectrum emission angle of the first-order diffracted light becomes larger than that in the case of two cycles, and the resolving power of the imaging system is greatly reduced.

On the other hand, in the present invention, even if the diaphragm aperture is 0.75 period or less of the phase grating, the spectral exit angle of the 1st-order diffracted light is about 75% as compared with the case where it is 2 periods. Even if the light disappears, the same low-pass effect as in the case of two cycles can be obtained.

Next, the relationship between specific numerical examples of the optical low-pass filter of the present invention, the numerical examples of the conventional optical low-pass filter described above, and the conditional expressions described above will be shown.

Numerical Example 1 Numerical Example 2 Numerical Example 3 P 1.88 1.88 1.88 A 0.47 0.376 0.564 θ 0.0208 ° 0.0293 ° 0.0123 ° n 1.49 1.49 1.49 λ 0.00055 0.00055 0.00055 θ _a 0.0463 ° 0.0463 ° 0.0463 ° K 60 ° 65 ° 55 ° θ _r / θ _f 0.744 0.496 0.992 D / P 0.25 0.2 0.3 θ _ra / θ _f 1.35 1.35 1.35 K 60 ° 65 ° 55 °

[0044]

According to the present invention, by setting each element of the optical low-pass filter as described above, when the optical low-pass filter is arranged in a part of the photographing system, the diaphragm aperture is made small. For example, even if the light flux passes through a region corresponding to two cycles or less of the phase grating, that is, even if the aperture value is changed variously, a constant low-pass effect can be easily obtained, and the density of the ND filter can be reduced. In addition, it is possible to achieve an optical low-pass filter capable of reducing the number of sheets to be used and having good optical performance with good blur and an imaging system having the optical low-pass filter.

[Brief description of drawings]

FIG. 1 is a front view and a side view of a first embodiment of an optical low-pass filter of the present invention.

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

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

FIG. 4 is a comparison diagram of the spectral intensities of the numerical example 1 of the optical low-pass filter of the present invention and the conventional optical low-pass filter.

FIG. 5 is an explanatory diagram of a spectrum of an optical low pass filter.

FIG. 6 is a cross-sectional view of the essential parts of Embodiment 2 of the optical low-pass filter of the present invention.

[Explanation of symbols]

1 Optical low-pass filter 1a Trapezoid 1b substrate 1c Top flat part 1e Central area 2 Shooting system 3 aperture 4 image sensor C1 center line C2 center of opening

Claims (6)

[Claims] 1. An optical low-pass filter in which phase gratings having a trapezoidal wave-shaped cross section are continuously arranged in the one-dimensional direction at the same pitch P, and the light passes through the inclined surface of the phase grating at the center of the optical low-pass filter. The phase grating at the center of the surface opposite to the surface on which the trapezoidal wave-shaped phase grating is formed so that the refraction angle of the light flux becomes smaller than the refraction angle of the light flux passing through the inclined surface of the phase grating in the other region. An optical low-pass filter, characterized in that a convex prism portion is provided at a position opposed to. 2. When an optical low-pass filter in which a phase grating having a trapezoidal wave-shaped cross section is continuously arranged in the one-dimensional direction at the same pitch P is provided in a part of the image-taking system, the optical low-pass filter is used. Of the trapezoidal waveform so that the refraction angle of the light beam passing through the inclined surface of the phase grating facing the central region including the aperture center of the stop is smaller than the refraction angle of the light beam passing through the inclined surface of the phase grating in a different region. An imaging system having an optical low-pass filter, characterized in that a convex prism portion is provided at a position opposite to the phase grating in the central region on the surface opposite to the surface on which the phase grating is formed. . 3. A refraction angle of a light beam, which is perpendicularly incident on the substrate of the optical low-pass filter and which has passed through the inclined surface of the phase grating adjacent to the convex prism and the central region, is θ _r , The first-order diffraction angle of the grating is θ _f , the angle between the inclined surface of the phase grating and the substrate is θ _a , the angle between the inclined surface of the convex prism and the substrate is θ, and the material of the phase grating is When the refractive index is n and the wavelength used is λ, and θ _r = (n−1) (θ _a −θ) θ _f = sin ⁻¹ (λ / P), 0.4 <θ _r / θ _f <1. 3. An image pickup system having an optical low-pass filter according to claim 2, wherein the condition 1 is satisfied. 4. A refraction angle of a light beam which is perpendicularly incident on the substrate of the optical low-pass filter and which passes through an inclined surface of the phase grating other than the central region is θ _ra , and a width of a flat top portion of the phase grating. , D, the first-order diffraction angle of the phase grating is θ _f ,
When the angle between the inclined surface of the phase grating and the substrate is θ _a and θ _ra = (n−1) θ _a , 0.15 <D / P <0.35 1.1 <θ _ra / θ _f An imaging system having an optical low-pass filter according to claim 2 or 3, wherein the condition <1.6 is satisfied. 5. The imaging system has a solid-state image sensor, and 45 ° <k when the angle formed by the phase grating direction of the optical low-pass filter and the horizontal scanning direction of the fixed image sensor is k. The photographing system having the optical low-pass filter according to claim 2, wherein the photographing system is arranged so as to satisfy the condition of <75 °. 6. A photographing system having an optical low-pass filter according to claim 2, wherein the concave and convex shapes of the optical low-pass filter are arranged so as to be opposite to the substrate. .