JPS6183517A - Low-pass filter - Google Patents

Abstract

PURPOSE:To obtain a low-pass filter which decreases the loss of a low-frequency component by disposing the 2nd and 3rd double refracting plates in such a position where the crystal axes thereof within the plane intersecting orthogonally with the optical axes thereof are respectively about 45 deg. and 60 deg. with the crystal axis of the 1st double refracting plate. CONSTITUTION:Three sheets of the double refracting plates 8, 9, 10 are superposed in the optical axis direction and the 2nd plate 9 and 3rd plate 10 among these plates are so disposed that the crystal axes thereof within the plane orthogonal with the optical axes thereof are about 45 deg. and 60 deg. with the crystal axis of the plate 8.

Description

[Detailed Description of the Invention] Technical Field The present invention relates to an endoscope viewing device (hereinafter referred to as LS).
This relates to low-pass filters used in applications such as low-pass filters.

Conventional technology An endoscope viewing device is constructed as shown in FIG. Moiré occurs when an image is formed on the incident end surface of the image guy 146, which obstructs observation.
In the endoscope described in Publication No. 1028, a phase filter 7 is used to cut high frequency components of the kite 2,
Moiré was being removed. However, phase filter 7
However, if the diameter of the chisel-turn hole of the phase filter 7 is made smaller to compensate for the insufficient removal of high-frequency components, low-frequency components are also canted, resulting in a reduction in image quality. That is, the phase filter 7 plays a role in the first
As shown in Figure 7, <MTF (absolute value of response function)
Because the curve is gentle, moiré may not be removed sufficiently, and to compensate for this, the cutoff frequency fc is set to M.
By making TF' small, M T
The disadvantages were that the F was reduced and the resolution contrast was poor.

Purpose: In view of the above-mentioned problems, the present invention aims to provide a low-noise filter with less loss of low frequency components.

Summary The low-nomise filter according to the present invention has three birefringent plates stacked in the optical axis direction, and the crystal axes of the second and third birefringent plates in the plane orthogonal to the optical axis are the second and third birefringent plates. By arranging them at approximately 45° and 60° with respect to the crystal axis of one birefringent plate, the light ray separation angle is approximately the same as the arrangement of the fine nozzles on the end face of the image guide. I am trying to make them similar.

EXAMPLES Below, the present invention will be described in detail based on the examples shown in FIGS. and a third birefringent plate, wherein the crystal axes of the second birefringent plate 9 and the third birefringent plate 10 in a plane orthogonal to the optical axis are relative to the crystal axis of the first birefringent plate, respectively. They are arranged so as to form approximately 45° and 60° (Fig. 2).

Since the low-pass filter according to the present invention is constructed as described above, the incident light ray is first divided into two equal-intensity light rays by the first birefringent plate 8 in a plane perpendicular to the optical axis (the third The light is then divided into four equal-intensity rays by a second birefringent plate 9 (FIG. 4), and further divided into eight unequal-intensity rays by a third birefringent plate 1°. In addition, in FIGS. 3 to 5, the short lines passing through the black circles indicate the polarization direction of the light beam. Further, a, , and c represent the amount of separation of light rays by the first to third birefringent plates 8.9.10, respectively. Also, the intensity of the rays marked with double circles in Figure 5 is much larger than that of the rays marked with black circles, and the ratio is approximately 14:1, and d represents the distance between the rays of double circles. There is. That is, it is the double circle portion that forms the light beam separation pattern, and the second birefringence plate 9 is provided to balance the light intensity of the two polarized light components (double circle portion). Therefore, it is not important that the rays are shifted by b as shown in FIG. Therefore, the second birefringent plate 9
You can choose the thickness as you like as long as it's not too thick, but if possible, it's better to use a thin layer. Furthermore, the crystal axis of the third birefringent plate 10 is 60° because the line segments connecting the centers of adjacent fibers of the image guide 6 are approximately 60" apart from each other as shown in FIG. This is to make the beam separation nozzle approximately similar to the fiber arrangement nozzle on the end face of the image guide.

Therefore, as shown in FIG. 6, if the distance RfflP between the centers of adjacent fins of the image guide 6 is defined as By selecting the respective thicknesses of P6 and arranging them between the imaging lens 5 and the image guy 16 as shown in FIG. As a result, images without moiré can be obtained.

However, the direction of the crystal axis of the first birefringent plate 8 is assumed to be parallel to the straight line connecting the fiber centers of the incident end surface of the image guide 6, as shown in FIG. The low frequency filter according to the present invention is composed of a birefringent plate, so the loss of low frequency components is extremely high. There is almost no deterioration in image quality.

Note that the low-no-miss filter according to the present invention is used in the imaging lens 5.
or in the imaging lens 5, take into account the magnification of the optical system and make sure that the amount of separation on the image plane of the imaging lens 5 satisfies the above equation (1) or (2) for the entire groove. Just do it. or,
The directions of the crystal axes of the first to third birefringent plates 8.9.degree. 10 may be as shown in FIG. 9, and the separation of the light beams in this case is as shown in FIG. 10. Further, as the material of the first to third birefringent plates 8°9.10, other materials can be used instead of quartz as long as they have birefringence at.

Next, consider generalizing the above equation (2).

First, as shown in FIG. 11, if the angle of the crystal axis of the third birefringent plate 100 in the plane orthogonal to the optical axis with respect to the crystal axis of the first birefringent plate 8 is θ, then d in this case. and ψ (the angle formed between the straight line connecting the two double circles and the horizontal direction) shown in FIG. 5 is expressed by the following equation (3).

If c and θ are selected so that the equation (2) is satisfied, a low-noise filter that satisfies this will have better performance than a low-noise filter that satisfies the above equation (2).

Practically speaking, it is sufficient to select a+tzct'' to cover all the grooves for d and ψ defined by equation (3).

The range of this formula (4) is approximately determined as follows.

The M T F curve of a birefringent filter has a shape roughly as shown in Figure 12, and M T F' = cos 2
It can be approximately expressed as πfp. In this case, when the cutoff is reduced to this extent, moiré is almost invisible. Further, the condition of 1ψ1 is an error range that takes into account variations in the fiber diameter in the image guy P6. In other words, if there are variations in fiber diameter, the angle formed by the line segment connecting the centers of adjacent fibers will be 6.
This is because it does not become 0°.

FIG. 13 shows a second example of the use of the low-noise filter according to the present invention, in which it is used in combination with the phase filter 11, and the removal of high frequency components is sufficient.
Great moiré removal effect.

As shown in FIG. 14, the phase filter 11 is formed by depositing a fine phase film on a transparent substrate so as to occupy approximately 1/2 of the total filter area and having a phase difference of 172 times the wavelength of light. It is what it is. Incidentally, the phase filter may be constructed by depositing a minute phase film on any one of the first to third birefringent plates 8, 9, and 10.

Figure 15 is a third usage example of the low-no-miss filter according to the present invention. The low-pass filter of the present invention is installed between the image guide 2 and the imaging lens 5 in an electronic endoscope 1' in which a solid-state image sensor 12 is arranged behind the image guide 2. It is made by arranging it. In this case, the value of P in the above formula (1) or (2) is image guide -2
Let us take the distance between the centers of each adjacent fiber. Reference numeral 13 is a quartz crystal rotor filter described in Japanese Patent Publication No. 57-15369, etc., and the addition of this filter increases the moiré removal effect. Also, 14 is a camera control unit, 15id monitor TV-'
There is C'.

Effects of the Invention As described above, the low-noise filter according to the present invention has the practically important advantage of almost no loss of low frequency components.

[Brief explanation of the drawing]

FIG. 1 is a side view of an embodiment of the low/miss filter according to the present invention, and FIG. 2 shows the birefringent plate 8, which constitutes the above embodiment.
Figures 9.10 and 9.10 are diagrams showing the angles of the crystal axes, Figures 3 to 5 are diagrams showing the light beam separation effect by the birefringent plates 8 and 9.10, respectively, and Figure 6 is a diagram showing the angle of the end face of the image guy P6. FIG. 7 is a diagram showing an example of the use of the above embodiment. FIG. 8 is a diagram showing the direction of the crystal axis of the first birefringent plate 8 and the direction of the crystal axis of the first birefringent plate 8. phinoζ of the incident end face of
- Figure 9 is a diagram showing other possible angles of the crystal axes of the first to third birefringent plates 8, 9.10, Figure 10 is the case of Figure 9. FIG. 11 is a diagram showing the angle tθ of the crystal axis of the third birefringent plate 10, FIG. 12 is a graph showing the MTF curve of the birefringent filter, FIG. FIG. 14 is an enlarged view of the main part showing the arrangement of the micro phase film of the phase filter 11, FIG. 15 is a diagram showing the third usage example of the above embodiment, FIG. 16 is a diagram showing an example of the use of a conventional low-noise filter, and FIG. 17 is a graph showing an MTF curve of a phase filter. 3...LS, 5...imaging lens, 6...image guy)-'', 8,9.10.1.birefringent plate, 11...phase filter, 12...solid-state imaging device. 1st figure 2nd figure... 14th figure, 15th figure, 16th figure, fc f

Claims (4)

[Claims] (1) Three birefringent plates are stacked in the optical axis direction, and the crystal axis of the first birefringent plate is in the plane orthogonal to the optical axis of the second and third birefringent plates. Low-pass filters arranged at approximately 45° and 60° to the axis, respectively. (2) The low-pass filter according to claim (1), which satisfies the following conditions. (1/3)P<d<(2/3)P (1/3)P<a<(2/3)P 50°<|ψ|<70° or 110°<|ψ|<130
°However, P is the distance between the centers of adjacent fibers of the image guide, a is the amount of separation of the light beam by the first birefringent plate, and d is the distance between the high intensity parts of the light beam separated by the low-pass filter of the present invention. , ψ is the angle formed between the straight line connecting the high-strength parts and the horizontal direction. (3) The low-pass filter according to claim (1), characterized in that it is combined with a phase filter. (4) The low-pass filter according to claim (1), wherein the low-pass filter is disposed between an image guide and a solid-state image sensor.