JPH11218612A - Optical low-pass filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the size and weight of an optical low-pass filter for efficiently attenuating a pseudo signal (moire form), i.e., multiple image distortion which occurs at the time of converting the video signal formed by a lens with a solid state image pickup element into an electric signal and is hardly removable by signal processing with electric circuits or the like and to reduce a production cost. SOLUTION: This optical low-pass filter 5 consists of plural sheets formed by bonding double refractive plates 1 of lithium niobate having high refractive indices to both main surfaces or one surface of a quartz crystal plate 2. While the deposition of antireflection films are needed for both main surfaces of the double refractive plates having the high refractive indices, the need for the antireflection films for the quartz crystal plate 2 is eliminated to reduce the production cost. The thickness of the double refractive plates 1 is reduced by using the lithium niobate varying in the crystal structure from the quartz crystal. The drastic reduction of the thickness over the entire part of the optical low-pass filter 5 is thus made possible by the combination with the quartz crystal plate 2.

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 constituted by laminating a plurality of birefringent plates and realizing a reduction in the thickness of the entire optical low-pass filter even when the beam separation width is increased.

[0002]

2. Description of the Related Art Optical low-pass filters are widely used in industrial video cameras, consumer video cameras, and home video cameras for the purpose of attenuating particularly high-frequency components of video signals formed by lenses of video cameras. I have. The image received by the lens of the video camera is converted into an electric signal by a solid-state image sensor (CCD). At the time of conversion, the image formed by the lens of the video camera includes 1/1 of the sampling frequency of the solid-state image sensor. When two or more high frequency components are included, a moire-like pseudo signal is generated, and it is difficult to remove the pseudo signal by signal processing such as an electric circuit.

An optical low-pass filter generally employs a crystal birefringent plate or a crystal birefringent plate and a crystal phase plate bonded together with an adhesive, and is removed by signal processing such as an electric circuit. The generation of the pseudo signal is greatly improved by arranging an optical low-pass filter while the video signal formed by the lens of the video camera reaches the solid-state imaging device.

[0004] In short, the improvement characteristic of a pseudo signal as an optical low-pass filter in a crystal birefringent plate is determined by the separation width of a light beam due to birefringence and the light beam separation pattern.
This separation width is proportional to the thickness of the (quartz) birefringent plate. As described above, the thickness and separation width of the optical low-pass filter depend on the refractive index of the birefringent plate, and when a single crystal plate of lithium niobate having a high birefringence is used for the birefringent plate, the optical low-pass filter is used. It is known to reduce the overall thickness.

[0005]

However, recent video cameras (especially home video cameras) have been reduced in size and weight.
With the rapid development toward cost reduction, these demands naturally cover the entire video camera, including the electronic circuit components and drive mechanism of the video camera. However, it is desired to satisfy the above-mentioned requirements to the extent possible.

As described in the prior art, current optical low-pass filters mainly include a combination of a plurality of crystal birefringent plates or a combination of a crystal birefringent plate and a crystal phase plate. Although the thickness of the optical low-pass filter can be further reduced by using a single crystal plate having a high birefringence index, for example, when a plurality of birefringent plates are attached to each other using lithium niobate, each plate is formed. It is necessary to form an anti-reflection film on the main surface of each plate to reduce unnecessary reflection between them, which increases the manufacturing cost and makes it difficult to obtain an adhesive with a refractive index that adjusts the refractive index of each plate. Issues.

[0007]

In order to solve the above-mentioned problem of requiring a large thickness of the optical low-pass filter itself and a large material cost due to the thickness, an example of the present invention employs both principal surfaces of a crystal phase plate ( The material of the crystal birefringent plate of the optical low-pass filter composed of three pieces of crystal birefringent plates bonded together or the crystal birefringent plate and a crystal phase plate attached to each other using adhesive. By using lithium niobate having a different crystal structure from quartz,
The thickness of the entire optical low-pass filter can be significantly reduced and reduced.

Further, by bonding lithium niobate and a quartz plate, the adhesive used for bonding the respective plates can be a conventional adhesive having a refractive index of 1.40 to 1.70. Although anti-reflection films are formed on both main surfaces of the lithium birefringent plate, anti-reflection films are not required on both main surfaces of the quartz plate, reducing manufacturing costs and miniaturizing and measuring the entire optical low-pass filter. I was able to.

[0009]

Embodiments of the present invention will be described below with reference to the accompanying drawings. In each drawing, the same reference numeral indicates the same object. FIG. 1 is a side view of an optical low-pass filter 5 of the present invention. A birefringent plate 1 is attached to both principal surfaces of a quartz (phase) plate 2, and the birefringent plate 1, the quartz (phase) plate 2,
1 shows an optical low-pass filter 5 having a birefringent plate 1 having a configuration of three sheets.

An antireflection film 4 (4) is provided on both main surfaces of the birefringent plate 1.
a, 4b) and a quartz plate that does not require the formation of the anti-reflection film 4 in the range of 1.40 to 1.70 in refractive index, generally an epoxy-based material having a refractive index of 1.52 or Are bonded together by an acrylic adhesive 3 to generate a relationship between input light and output light as shown in FIG.

FIG. 3B is a perspective view showing this state. The input light (incident light) is separated at the first birefringent plate 1 into two points, an ordinary ray (solid line) and an extraordinary ray (dotted line), and the rays separated into ordinary rays and extraordinary rays pass through the quartz plate as they are, Polarized light can be eliminated by passing through a quartz plate. Then, by passing another birefringent light beam, the two light beams passing through the quartz plate are further separated into an ordinary light beam and an extraordinary light beam (separated by four points), thereby blurring the input light and causing a moire phenomenon. It is possible to obtain output light in which the signal is attenuated. In this case, the separation width of the light beam from the birefringent plate changes depending on the cutting angle (θ: the angle between the main surface and the optical axis).

In the present invention, the birefringent plate 1 constituting the optical low-pass filter 5 employs lithium niobate having a crystal with a high birefringence, which is smaller than that of a generally used quartz birefringent plate 6. When the separation width efficiency of the light beam for attenuating the pseudo signal is viewed as a ratio between the thickness of the birefringent plate 1 and the refractive index of the birefringent plate 1, the use of the lithium niobate birefringent plate 1 makes the quartz birefringent plate better. Even if the plate thickness is smaller than that using 6, the light beam separation width can be obtained efficiently and the number of steps for forming the antireflection film 4 can be reduced. (In short, the thickness of the optical low-pass filter (FIG. 2) composed of the birefringent plate 6 made of quartz is defined as T ′, and the lithium niobate birefringent plate 1
Let T be the plate thickness of the optical low-pass filter using
<T 'relationship)

The above will be described with reference to a theoretical comparison between the quartz birefringent plate 6 and the lithium niobate birefringent plate 1. The design condition is a comparison of theoretical values when the design wavelength is set to 550 nm with the optical low-pass filter 5 of four-point separation in which both the horizontal separation width and the vertical separation width are 15 μm. The separation width of the birefringent plate used for the optical low-pass filter 5 can be obtained by the following equation (1).

D / t = ((ne ² −no ² ) × sin θ × cos θ) / (no ² sin ² θ−ne ² cos ² θ) (1) where no: refractive index of ordinary ray ne: refractive index of extraordinary ray d: separation width of birefringent plate, t: plate thickness of birefringent plate, θ: cutting angle (45 °)

When the design conditions are input into the above equation (1) and calculated, the d / t (separation width / thickness) of the lithium niobate birefringent plate 1 is 0.039438905, and the quartz birefringent plate 6
Is 0.005906986, the value of the separation width d (15 μm) of the birefringent plate is now assumed to be equal to the expression (1).
And the thickness t of the birefringent plate is calculated, the thickness of the lithium niobate birefringent plate is 0.380 mm, and the thickness of the quartz birefringent plate is 2.539 mm. At this time, the phase plate constituting the optical low-pass filter 5 uses the crystal (phase) plate 2.

As described above, the combination of the birefringent plate and the phase plate constituting the optical low-pass filter 5 uses a lithium niobate crystal plate whose crystal structure has a high birefringence. The thickness of the optical low-pass filter 5 can be made smaller than that of the crystal birefringent plate 6. In this embodiment, the lithium niobate crystal plate is described as using a birefringent plate, but potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (KDP) having a higher birefringence than quartz are used. Similar effects can be obtained if the material is crystalline such as ADP) and calcite (CaCO ₃ ).

In this embodiment, a birefringent plate is bonded to both principal surfaces of the quartz (phase) plate 2 to form a birefringent plate, a quartz (phase) plate.
Optical low-pass filter 5 composed of plate 2 and birefringent plate
Although all of the birefringent plates use lithium niobate crystal plates, a quartz plate can be used as one of the birefringent plates.

[0018]

According to the present invention, the number of steps for forming the antireflection film 4 on the main surface of the birefringent plate constituting the optical low-pass filter can be reduced, and the separation width of the birefringent plate can be efficiently increased even if the plate thickness is reduced. I got it. As a result, the plate thickness of the entire optical low-pass filter can be significantly reduced, and the size and weight of the optical low-pass filter can be reduced, so that the material can be reduced.

[Brief description of the drawings]

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

FIG. 2 is a side view of a conventional optical low-pass filter.

FIG. 3 is a perspective view illustrating a conceptual diagram of an optical low-pass filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Birefringent plate 2 Quartz plate 3 Adhesive 4 (4a, 4b) Antireflection film 5 Optical low-pass filter

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[Procedure amendment]

[Submission date] March 4, 1998

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0011

[Correction method] Change

[Correction contents]

FIG. 3B is a perspective view showing this state. The input light (incident light) is separated into two points, an ordinary ray (solid line) and an extraordinary ray (dotted line), by the first birefringent plate 1, and the ray separated into the ordinary ray and the extraordinary ray passes through the quartz plate as it is. Polarized light can be eliminated by passing through a quartz plate. After that, by passing another birefringent light, the two light beams passing through the quartz plate are further separated into an ordinary light beam and an extraordinary light beam (separated by four points), thereby blurring the input light and causing a moire phenomenon. It is possible to obtain output light in which the signal is attenuated. In this case, the separation width of the light beam from the birefringent plate changes depending on the cutting angle (θ: the angle between the normal to the principal surface and the optical axis).

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction contents]

D / t = ((ne ² −no ² ) × sin θ × cos θ) / (no ² sin ² θ + ne ² cos ² θ) (1) where no: refractive index of ordinary ray ne: refractive index of extraordinary ray d: separation width of birefringent plate, t: plate thickness of birefringent plate, θ: cutting angle (45 °)

Claims (6)

[Claims] 1. An optical low-pass filter comprising a plurality of crystal birefringent plates bonded to each other, wherein at least one of the birefringent plates is bonded to a single crystal plate having a crystal structure having a birefringence different from that of the crystal plate. An optical low-pass filter characterized by being laminated by using an agent. 2. An optical low-pass filter comprising a plurality of crystal birefringent plates and a plurality of crystal phase plates bonded together, wherein at least one of the birefringent plates has a single crystal having a crystal structure having a birefringence different from that of the quartz plate. An optical low-pass filter, wherein the plates are bonded by using an adhesive. 3. An optical low-pass filter comprising a plurality of crystal birefringent plates bonded to each other, wherein a crystal plate is disposed between the birefringent plates having a different birefringence from that of the crystal plate. An optical low-pass filter, wherein antireflection films are formed on both main surfaces of the birefringent plate and bonded with an adhesive. 4. An optical low-pass filter comprising a plurality of crystal birefringent plates and a plurality of crystal phase plates bonded to each other, wherein a crystal having a different birefringence from the crystal plate is provided between the birefringent plates. An optical low-pass filter comprising: a plate; and an anti-reflection film formed on both main surfaces of the birefringent plate and bonded by an adhesive. 5. The optical low-pass filter according to claim 1, wherein the crystals having different birefringences use lithium niobate. 6. The adhesive according to claim 1, wherein the adhesive used for bonding the respective plates has a refractive index of 1.40.
An optical low-pass filter using a filter having a size of from 1.70 to 1.70.