JP2783691B2 - 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 an optical low-pass filter disposed in front of a solid-state image pickup device, and more particularly, to a solid-state image pickup device which suppresses high spatial frequency components of a subject which generates a signal and has a good effect. The present invention relates to a phase grating type optical low-pass filter capable of obtaining an image output.

[0002]

2. Description of the Related Art It is well known that a color solid-state imaging device having a predetermined sampling pitch generates a false signal for a subject having a high spatial frequency higher than a predetermined value. In order to suppress the false signal, an optical low-pass filter made of a quartz plate or a phase grating type optical low-pass filter is usually arranged in front of the solid-state imaging device.

[0003] Quartz optical filters can provide a relatively ideal low-pass effect, but are difficult to manufacture and have problems in cost.

On the other hand, a phase grating type optical low-pass filter has an advantage that it can be manufactured relatively inexpensively, although the optical performance is slightly inferior. For example, it has been proposed in Japanese Patent Publication No. 52-22247 and Japanese Patent Publication No. 56-2936. Along with being used in some video cameras.

[0005]

The phase grating type optical low-pass filter is disclosed in, for example, Japanese Patent Publication No. Sho 52-2.
As pointed out in Japanese Patent No. 2247, the low-pass effect is abruptly reduced when the stop is in a small stop state and the stop opening particularly extends within two periods of the phase grating.

This is because the phase grating type optical low-pass filter causes a low-pass effect by causing diffraction by a plurality of gratings, and the desired diffraction effect is no longer reduced in a small aperture state. Can be

For this reason, in a video camera using an optical low-pass filter of a phase grating, for example, a plurality of ND filters are attached to a part of the aperture so that the minimum aperture state is not less than two periods of the grating, However, since the density of the color was increased, not only the cost was affected but also the bokeh was reduced.

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide a phase grating type optical low-pass filter in which the low-pass effect does not decrease so much even when the aperture opening cuts in two periods of the phase grating.

[0009]

A feature of the present invention is that a phase grating type optical low-pass filter in which a plurality of prisms each having a convex or concave cross section are repeatedly arranged at a predetermined pitch is provided. The photographing optical system is arranged so as to be located in one of the prism portions having the center of the aperture opening and a concave portion or a convex portion formed by the prism portion adjacent to the prism portion.

[0010]

FIG. 1 shows a sectional view and an optical path diagram of a phase grating type optical low-pass filter according to the present invention. FIG. 2A is a front view of the optical low-pass filter, and FIG. 2B is a cross-sectional view of the stop and the optical low-pass filter.

As described above, the optical low-pass filter 1 of this embodiment forms a phase grating in which a large number of prisms having a trapezoidal cross section are repeatedly arranged at a pitch P. Then, C ₁ (the optical axis of the taking lens) aperture center of the stop 3 adjacent have arranged to come to some adjacent recessed region of a particular two prisms.

By doing so, in a state where the diaphragm 3 has a small F value (large aperture), diffraction by the wave optical phenomenon is caused by a plurality of phase gratings, and a desired low-pass effect can be obtained. When the diaphragm is stopped down, the low-pass effect due to diffraction is weakened, but instead of this diffraction effect, the above-mentioned geometrical optical refraction effect by the adjacent concave portion is dominant, and the point image is separated into two. Thus, the desired low-pass effect is maintained.

As shown in FIG. 1, the width of the top of the trapezoid is A, the width of the slope is B, the period is P, the primary diffraction angle of the phase grating is θf, the refraction angle of the slope is θr, and the phase grating is θr. Assuming that the refractive index is n, the design wavelength is λ (for example, e-line), and the angle between the slope portion and the flat portion of the phase grating is θ, the above-described first-order diffraction angle θf
And the refraction angle θr are respectively written as θf = sin (λ / p) θr = (n−1) θ. In order to provide a good low-pass effect even when the aperture becomes small, it is desirable to set the shift amount between the center C _{1 of the} aperture opening and the adjacent portion (vertex C _{2 of the} valley) where the trapezoidal lattices are adjacent to each other by D. | D | / B <0.33 (1) It is preferable to satisfy the following conditional expression.

It is more desirable to satisfy the following condition: 0.15 <A / P <0.35 0.5 <θr / θf <1.4

Conditional expression (1) shows that the trapezoidal adjacent portion C of the phase grating is
2 and the amount of deviation D between the aperture opening center C1 is limited.
When the stop aperture center C1 is located outside the range of this conditional expression, for example, at A (mountain) or C (slope) in FIG. The prism effect in the aperture opening is almost eliminated by passing through only the slope B, and the intensity of only one spectrum increases between the zero-order spectrum and the two primary spectra, so that the low-pass effect is greatly reduced. Resulting in.

On the other hand, when the stop aperture center C1 is located at B (valley) in FIG. 4 included in the conditional expression (1), the low-pass effect does not decrease in all cases where the stop aperture is less than two cycles.

Conditional expression (2) limits the ratio of the width A of the trapezoidal flat portion to the period P of the phase grating. In the region below the conditional expression, the area of the flat portion is reduced. Even if the low-pass effect is present at a specific wavelength where the characteristics are close to each other and the change of the low-pass effect with respect to the wavelength change is large, the low-pass effect is reduced at other wavelengths, which is not preferable. Further, in a region exceeding the conditional expression, the area of the plane portion becomes large, so that the spectrum of the zero-order light becomes too large with respect to the primary light when narrowing down to less than two periods of the phase grating.

Conditional expression (3) restricts the ratio between the diffraction angle θf of the phase grating and the refraction angle θr of the slope, and is governed by wave optics for more than two periods of the phase grating and geometric optics for less than two periods. In the region below the conditional expression, the diffraction angle θf
On the other hand, since the refraction angle θr of the slope becomes smaller, when the aperture is narrowed down to less than two periods, the exit angle of the primary spectrum becomes significantly smaller than the primary diffraction angle θf, so that the low-pass effect decreases, which is not preferable. In the region above the conditional expression, the diffraction angle θf
Since the angle of refraction θr of the slope becomes large, the exit angle of the secondary spectrum becomes large with respect to the primary diffraction angle θf when the aperture is narrowed down to less than two periods.

FIG. 3 is a perspective view of the video camera, and FIG.
Is a taking lens, 1 is an optical low-pass filter described above,
Reference numeral 3 denotes a stop, and 4 denotes a CCD which is a solid-state image sensor.

In this embodiment, the horizontal scanning direction of the CCD 4 and the longitudinal direction (perpendicular to the arrangement direction) and the horizontal scanning direction of the grating of the optical low-pass filter in order to provide a low-pass effect also in the scanning direction and the vertical direction. The optical low-pass filter is tilted so as to have a predetermined angle. Then, in order to provide a low-pass effect in the horizontal scanning direction and the vertical direction, it is desirable that the angle k is set to satisfy the condition of 45 ° <k <70 ° (4). In the region below the conditional expression, the resolution in the vertical direction is greatly decreased, which is not preferable. On the other hand, in the region above the conditional expression, the low-pass effect in the vertical direction is hardly obtained, which is not preferable.

Hereinafter, numerical examples of the optical low-pass filter according to the present invention will be described.

In this embodiment, each of the trapezoidal lattices has a convex shape. However, as shown in FIG. 5, the lattice of FIG. 1 is inverted (the convex portion has a triangular cross section). The effect as an optical low-pass filter is exactly the same as that of the filter of FIG. At this time, the center of the aperture (optical axis) C ₁
By arranging the grid so that matches the convex vertex,
The object of the present invention is achieved.

[0023]

[Outside 1]

[0024]

As described above, the optical low-pass filter of the present invention has an effect that a sufficient low-pass effect can be obtained even in a state where the conventional low-pass effect is reduced to less than two periods of the phase grating. . For this reason, the density of the ND filter provided in a part of the aperture used to prevent over-aperture can be made thinner as compared with the conventional one,
There is an advantage that only one ND filter needs to be used, and it is effective in improving the blurring effect and reducing the cost.

[Brief description of the drawings]

FIG. 1 shows a sectional view and an optical path diagram of an optical low-pass filter according to the present invention.

FIG. 2 is a front view and a sectional view of an optical low-pass filter according to the present invention.

FIG. 3 is a perspective view of a video camera using the optical low-pass filter of the present invention.

FIG. 4 is a diagram showing a spectrum intensity when the center of the aperture opening is changed.

FIG. 5 is a cross-sectional view of another optical low-pass filter according to the present invention.

[Explanation of symbols]

 1 Optical low-pass filter 3 Aperture

Claims (4)

(57) [Claims] 1. A phase grating type optical low-pass filter in which a plurality of prisms each having a convex or concave cross section are repeatedly arranged at a predetermined pitch. An imaging system comprising an optical low-pass filter, which is disposed so as to be located approximately at a concave portion or a convex portion formed by an adjacent prism portion. 2. The prism section has a slope, and when a shift amount between the center of the aperture opening and a vertex of the formed concave portion or convex portion is D, and a width of the slope is B, | D | / 2. An imaging system comprising the optical low-pass filter according to claim 1, wherein the following conditional expression is satisfied: B <0.33. 3. An imaging system comprising an optical low-pal filter according to claim 2, wherein said convex or concave section is trapezoidal. 4. The width of the flat portion of the trapezoid is A, the pitch is P, and the first-order diffraction angle of the optical low-pass filter is θ.
f, satisfying a conditional expression of 0.15 <A / P <0.35 0.5 <θr / θf <1.4, where θr is a refraction angle of light by the slope. 3. An imaging system having an optical low-pass filter.