JPS60239708A - Optical comb line filter - Google Patents

Abstract

PURPOSE:To attain comb line characteristics up to a high space frequency by selecting thickness and directions of optical axes of double refracting plates so that the total sum of optical path differences between ordinary rays and extraordinary rays passing the first double refracting plate and that passing the second double refracting plate are equal to each other. CONSTITUTION:Double refracting plates 7 and 8 are so arranged that planes including optical axes vertical to surfaces are orthogonal to each other, and a thickness d2 of the refracting plate 8 and a thickness d1 of the refracting plate 7 are so selected that a formula I is satisfied. The luminous flux made incident on the double refracting plate 7 are separated into ordinary rays L1 and extraordinary, rays L2, and an optical path difference l1 between both rays passing the double refracting plate 7 is given by a formula II. An optical path difference l2 between both rays passing the double refracting plate 8 is given by a formula III. Consequently, an optical path difference (l) between rays L1 and L2 after passage of both of the double refracting plate 7 and the double refracting plate 8 is l=l1+l2=0, and these rays are focused on the same image surface 12 DELTAW apart from each other. Thus, comb line characteristics are attained up to a high space frequency without influences of the F value of a lens and the selecting method of the image surface.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an optical comb filter using a birefringent plate such as quartz crystal.

[Technical background of the invention]

It is generally known that birefringent substances such as quartz can be used as optical comb filters, although they vary depending on their thickness, optical axis direction, etc. The principle is as described below. As shown in FIG. 1, the light ray 2 incident perpendicularly to the birefringent plate IK is separated into ordinary rays L0 and 14v rays LeK according to the direction of polarization due to the birefringence effect of the birefringent plate, and is emitted.

Here, the angle a formed by the ordinary light 11L and the extraordinary light 1i1L is the refractive index 'f of the birefringent plate 1 for the ordinary light; n0 is the refractive index for the extraordinary light incident perpendicularly to the optical axis; When the angle formed by the light is t-β, it is expressed by the following equation.

ta@α expressed by equation (1) is maximum when tanβ−n〆−, but when the birefringent plate l is made of quartz crystal, the difference between n and n is small, and β-degree is usually selected. The ordinary ray L0 and the extraordinary ray Le emitted from the birefringent plate 1 are rays separated by .

"Pi" L ΔW-d・ , , ・・・・・・・・・(2)xx,
+n.

FIG. 2 is a diagram showing an example in which a birefringent plate having the above characteristics is inserted into an imaging optical system. The birefringent plate 1 is arranged so that its surface is perpendicular to the optical axis 4 of the lens 3, and its direction is determined so that the optical axis is parallel to the plane of the paper. The light transmitted through the birefringent plate 1 is two parallel rays separated by k.

As a result, the images were originally supposed to be focused on one point by the lens 3, but instead they are focused on two points separated by the amount of light flux. Note that the reference numeral 5 indicates the imaging point of the ordinary ray Lρ, and the reference numeral 6 indicates the imaging point of the extraordinary ray Le.

The fact that the image forming point is separated by the axis when passing through the birefringent plate 1 in this way is expressed as a function of the KX axis in the light separation direction as shown below. Here, the delta function δ(x) indicates the input to the birefringent plate 1, and the output of the birefringent plate 1 relative to this is 'k h (X).Oh (x)−δ(X)+δ
(x jw ) , , , =, (3) Generally, it is known that a frequency response (MTF) can be obtained by taking the absolute value of a Fourier-transformed output of a delta function input.

In the above system, the spatial frequency efx in the X-axis direction.

If the MTF of the birefringent plate is H(fX), the MTF is expressed by the following formula.

H(fX)-IF(h(x))l-1cos(rjwr
fx)l -=・+=-(4) This equation (4) can be illustrated with the characteristics shown in (a) in Figure 4 and '&c,' and the birefringent plate l has a comb-shaped spatial frequency in the horizontal direction. It can be seen that it shows a response.

In a color imaging device that uses a single imaging device and obtains color signals by spatially modulating light in a specific wavelength range, if a component of the above modulation frequency exists in the input image, this component will be mixed into the color signal. color false signals occur. Conventionally, as a method of suppressing the generation of color false signals, a birefringent plate whose thickness is selected so that the response at the above modulation frequency is 0 is inserted into the optical system. Methods to reduce this are widely used. In addition, in an image sensor such as a CCD that captures spatial information discretely, among signals that exceed the Nyquist limit due to the sampling spatial frequency, all signals within the MTF limit of the image sensor are converted into moiré in the low frequency range. It is known to turn around. In order to reduce this moiré, a method is used in which several sets of birefringent plates and depolarization plates as described above are combined and used as an optical low-pass filter.

[Problems with background technology]

As mentioned above, a birefringent plate can be used as an optical comb-shaped filter, but the comb-shaped characteristics shown in Fig. 4 A can be obtained only when the lens 30F value (focal length/effective aperture) is large. , that is, only when the effect of defocus is small. Therefore, in situations 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 frequency sufficiently high for the lowest attenuation frequency of the comb-shaped filter, Then,
The birefringent plate 1 exhibits characteristics quite different from those of a comb filter. The reason will be explained below.

FIG. 3 shows an optical system that images a point light source on an optical axis at infinity. The light beam focused by the lens 3 passes through the birefringent plate l and forms an image. Here, for simplicity, consider that the effective aperture of the lens 3 is a square whose sides are parallel to the plane of the paper and one side is the focal length of 172◇Also, the birefringent plate l
Assume that is a crystal of thickness d, forming an angle with the surface, having an optical axis parallel to the plane of the paper. The birefringent plate 1 separates the incident light into an ordinary ray Lo and an extraordinary ray LeK.

However, since the birefringent plate I has a different refractive index for the ordinary ray LO and the extraordinary ray IMLe, the image planes of the image due to the ordinary ray L and the image due to the extraordinary ray are different. Here, if the refractive index of the birefringent plate 1 for ordinary light is n, and the refractive index for extraordinary light incident perpendicular to the optical axis is 't=ne, then the refraction of the birefringent plate 1 for the extraordinary ray is The distance ΔS of the image plane by the line LeK is expressed by the formula shown below, and when the extraordinary ray Le
When an image is captured on the image plane by If we take the axis parallel to the image plane and the paper plane as the X-axis and express it as a function on this X-axis, then if we subtract HL(fx), which is the MTF of the above optical system, we get the following equation. It will be as shown.

・・・・・・・・・(6) When the function shown in equation (6) above is illustrated, the characteristic line indicated by the opening in Figure 4 is obtained, which is different from the original comb-shaped characteristic line A. becomes. Such a phenomenon also occurs when the effective aperture of the lens 3 is circular or the like. Furthermore, the above MTF value changes depending on the difference in lens 30F value and how the image plane is selected.

Therefore, when the single birefringent plate 1 is used as an optical comb filter as described above, there is a drawback that a sufficient effect cannot be obtained depending on the situation.

In recent years, color imaging devices have been used that use an imaging element such as a COD that captures spatial information discretely and modulate incident light at a spatial frequency twice the sampling frequency of the element to obtain color information. In such a color imaging device, when the above-mentioned modulation frequency by the birefringent plate and frequency components at odd multiples thereof are included in the incident image, these components are mixed into the color signal and become a color false signal. If the birefringent plate has the original comb-shaped filter characteristic, the frequency that is an odd multiple of the lowest attenuation frequency becomes the attenuation frequency, which has a great effect of reducing the color false signal. However, as mentioned above, an optical comb filter using a single birefringent plate may exhibit characteristics different from those of an original comb filter depending on the situation. However, since the different characteristics deviate greatly from the original comb filter characteristics, there was a drawback that a sufficient reduction effect could not be obtained particularly for color artifacts caused by high spatial frequency components.

[Purpose of the invention]

In view of the above-mentioned drawbacks, an object of the present invention is to provide an optical comb-shaped filter that can obtain predetermined characteristics up to high spatial frequencies without being affected by the f-number of the lens or the method of selecting the image plane. There is a K.

[Summary of the invention]

The present invention includes one or more first birefringent plates having a first plane whitening optical axis perpendicular to the surface, and a second plane perpendicular to both the surface and the first plane. one or more 20W refracting plates each having an optical axis therein are disposed in parallel, and the total optical path difference between the ordinary light and the extraordinary light passing through the first birefringent plate and the second birefringent plate are arranged in parallel. said first so as to equalize the sum of optical path differences between ordinary light and extraordinary light passing through the birefringent plate.
The above object is achieved by selecting the thickness of the second birefringent plate and the direction of the optical axis.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings. Fifth
The figure is a perspective view showing an embodiment of the optical comb filter of the present invention. The optical comb filter is composed of a birefringent plate 7 and a birefringent plate 8 in close contact with each other. Reference numeral 9 indicates a projection line on each surface of the optical axis of the birefringent plate 7), and reference symbol 10 indicates a projection line on each surface of the optical axis of the birefringent plate 8. Birefringent plate 7
As shown by the projection line between and the optical axis of the birefringent plate 8,
Birefringent plate 7 and birefringent plate 8 are arranged so that planes including optical axes perpendicular to the surfaces thereof are orthogonal to each other.

Further, the thickness of the refracting plate 8 is selected so that the thickness of the refracting plate 7 satisfies the following formula.

However, to indicates the refractive index for ordinary light in the birefringent plate 7.8, and me indicates the refractive index for extraordinary light incident perpendicularly to the optical axis.

FIG. 6 is a diagram showing an imaging optical system using the optical comb filter of this embodiment shown in FIG. 5 above. Lens 11
An optical comb filter consisting of a birefringent plate 7.8 is arranged behind the birefringent plate 7.8 in such a way that the optical axis of the birefringent plate γ is parallel to the plane of the paper.

The light beam incident on the birefringent plate 7 has an ordinary ray L1 and an extraordinary ray,
separated into The optical path difference I11 between the ordinary ray ζ and the extraordinary ray Lt2 when they pass through the birefringent plate 7 is given by the formula shown below. By the way, the plane containing the optical axis perpendicular to the surface of the birefringent plate 7
Since the birefringent plate 7 and the birefringent plate 8 are arranged so that they are perpendicular to each other, Ll passes through the birefringent plate 8 as extraordinary light and L- passes as ordinary light. The optical path difference 4 between Ll and L when they pass through the birefringent plate 8 is given by the following equation.

Therefore, after passing through both the birefringent plate 7 and the birefringent plate 80, the optical path difference l between the two ends is t=Jj+m, -.9.

Therefore, the Ll portion that has passed through the comb filter of this embodiment.

are respectively formed on the same image plane 12 at points separated by h.

According to this embodiment, optical comb filter 1! - Consisting of two birefringent plates 7 and 8, so that the plane containing the optical axis perpendicular to the surface of the birefringent plates 7 and 8 is orthogonal to each other;
Since these birefringent plates 7.8 are arranged and the thickness dl of the birefringent plate 7 and the thickness of the birefringent plate 8 are selected so as to satisfy the above formula (7), Ray L0 and extraordinary ray L
e73 (It is possible to match the image planes for image formation, and to obtain the specified characteristics up to high spatial frequencies without affecting the characteristics of the optical comb filter depending on the F value of the lens 11 or the way the image plane is selected. Therefore, if the optical comb filter of this embodiment is used to reduce color false signals in color imaging devices and moiré in solid-state imaging devices,
It can be highly effective regardless of 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.
3. To the thickness of 14, the angles formed between the surface and the optical axis are both the same, but the birefringent plate 14 is 9 compared to the birefringent plate 13.
It is rotated by a degree. In addition, the code 15.16
are projection lines onto each plane of the optical axis of the birefringent plates 13 and 14.

Therefore, the light that passes through the birefringent plate 13 as an ordinary ray is
The birefringent plate 14 is transmitted as an extraordinary ray, and the birefringent plate 1
The light transmitted through the birefringence plate 14 as an extraordinary ray passes through the birefringent plate 14 as an ordinary ray. Therefore, the two birefringent plates 13
．． The two light beams separated (by step 14) form images at two points on the same image plane, and have the same effect as the previous embodiment.

Incidentally, there may be various variations in the thickness of the birefringent plate, the angle of the optical axis, the number of birefringent plates, etc., and the present invention is not limited to the above two embodiments. An optical comb-shaped filter may be formed by combining birefringent plates so that the imaging points of the passed extraordinary ray and ordinary ray are on the same image plane. Furthermore, the optical comb filter according to the present invention can be used not only alone, but also in combination with a depolarizing plate or the like to construct various specific spatial frequency filters.

〔Effect of the invention〕

As described above, according to the optical comb filter of the present invention, a plurality of birefringent plates are combined so that the imaging point of the ordinary ray and the imaging point of the extraordinary ray are on the same image plane. As a result, it is possible to obtain a predetermined comb-shaped characteristic up to a high spatial frequency without being affected by the F-number of the lens or how the image plane is selected.

[Brief explanation of drawings]

Figure 1 shows the amount of light emitted by the birefringent plate, Figure 2
The figure shows an example of an optical system including a conventional optical comb filter made of a single birefringent plate. Figure 4 is a characteristic diagram showing the spatial frequency response of the conventional optical comb filter in the optical system shown in Figure 3.
The figure is a perspective view showing one embodiment of the optical comb filter of the present invention, FIG. 6 is a diagram showing an example of an optical system including the optical comb filter shown in FIG. 5, and FIG. FIG. 3 is a perspective view showing another embodiment of the optical comb filter of FIG. 7.8.13.14...Birefringence plate 9.10.15.16...Projection line of optical axis 11...
Lens 12... Image surface agent Patent attorney Noriyuki Chika Figure 1 Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] one or more first birefringent plates having an optical axis in a first plane perpendicular to the surface, and a plane white optical axis perpendicular to both the surface and the first plane; and one or more second birefringent plates arranged in parallel, the total optical path difference between the ordinary light and the extraordinary light passing through the first birefringent plate and the second birefringent plate are arranged in parallel. An optical comb filter characterized in that the thicknesses and directions of the optical axes of the first and second birefringent plates are selected so that the sum of the optical path differences between ordinary light and extraordinary light passing through the refracting plates is equal. .