JPS60242420A - Spatial frequency filter - Google Patents

Abstract

PURPOSE:To improve band limitation effect in a slanting direction by employing the basic constitution structured by stacking a birefringent plate which separates luminous flux at 45 deg. of a horizontal axis and birefringent plates which separate luminous flux horizontally and vertically. CONSTITUTION:The Savart plate S11 which separates the luminous flux at 45 deg. to the horizontal axis, Savart plate S12 which separates the luminous flux horizontally or vertically, and Savart plate S13 which separates the luminous flux at right angles to the luminous flux separation of the Savart plate S11. The angle between the luminous flux separation directions of the Savart plates S11 and S12 is 45 deg., so two pieces of luminous flux separated by the Savart plates S11 and S12 are separated in 1:1 light intensity proportion into four pieces of luminous flux, and the number of pieces of luminous flux passed through the Savart plate S13 is eight. Thus, sufficient band limitation effect is obtained even in the slanting direction, so components with low importance to the human eyes are removed, so that natural reproduced images are obtained when this filter is applied to a solid-state image pickup device, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a spatial frequency filter used in a system in which a parallel plane plate made of a birefringent material such as quartz is arranged in an imaging optical system.

[Technical background of the invention]

In recent years, solid-state imaging devices, fiberscopes, etc.
Systems have been developed that obtain image information by discretely sampling spatial information. However, this system has a Nyquist limit inherent to the sampling process. Of the information that exceeds the Nyquist limit, information within the response characteristic band determined by the imaging optical system and pixel aperture arrangement is reflected back into the low frequency range, interfering with correct information reproduction for signals within the Nyquist limit. There is a problem with this.

In order to solve the above problem, a birefringent plate such as quartz is used to construct a spatial frequency filter that has several optical frequency traps in the frequency range that exceeds the above-mentioned folding. There is a method to reduce the occurrence of false signals due to

FIG. 7 is an example of the above-mentioned spatial frequency filter, S1~
S3 is a Savart plate, and PI and PI are depolarization plates. FIG. 4(a) shows the configuration when in use, and FIG. 2(b) is an exploded view of each plate. The arrows on each plate indicate the direction in which the luminous flux is separated.

FIG. 8 shows a state in which the light beam is separated by the spatial frequency filter of FIG. V is vertical direction, H is horizontal direction, each O mark indicates the separation position of the minimum unit of luminous flux, ○ mark part 6
Arrows indicate polarization direction. Now, the light is Savart plate S1
Assume that the light is incident from the side. The incident light is separated into two beams by the Savart plate S1' according to the polarization direction. (
Figure 8(a)). The light emitted from the Savard plate S1 passes through the Savard plate S2 via the depolarization plate P1. As a result, the two light beams separated by the Tevar plate S are further separated into four light beams (FIG. 8(b)). This emitted light is passed through the Savart plate S3 via the depolarization plate P2,
Ultimately, the light beam is separated into eight beams as shown in FIG. 8(e). It is known that the Savart plate acts as a comb-shaped filter, but in the above filter, the Savart plate S
The spatial frequency filter has a characteristic obtained by multiplying the spatial frequency characteristics of each of the comb filters 1 to S3. The filter IOA shown in FIG. 10 also has similar characteristics. The same figure (a
) shows the structure of the entire filter, and FIG. 2(b) shows the beam separation direction of each of the Savart plates S1 to S4 used. It also exists in the spatial frequency filter 11 shown in FIG.

Figure (1) shows the overall structure of the filter, and Figure (b) shows the light beam separation direction of the Savart plates 81 and 82 used.

[Problems with background technology]

The spatial frequency filters shown in FIGS. 7 and 10 have trigonal points at the spatial frequencies shown by broken lines in FIG. 9. UH is a horizontal spatial frequency axis, and Uv is a vertical spatial frequency axis. In addition, the trough and g points indicated by broken lines are given the same reference numerals as the corresponding Savard plates.

When such a spatial frequency filter is provided in the optical system of a solid-state image pickup device using COD, spatial frequency information in an oblique direction, which is relatively less important to the human eye, is transmitted up to high frequency information.

Therefore, there is a problem in that the reproduced image looks unnatural. In addition, in order to obtain the multiplication characteristics of three Savart plates (comb-shaped filters), five birefringent plates are required in the example shown in Figure 7, and four birefringent plates are required in the example shown in Figure 10. It has the disadvantage of becoming. Furthermore, in the example shown in Figure 7, the depolarizing plate has wavelength characteristics, so if the wavelength range of the incident light is wide, it will have the characteristics described above, but if the wavelength range is narrow, the characteristics will differ depending on the wavelength. There are drawbacks. However, when light with a specified wavelength range is incident on the depolarizing plate, the polarization state of the separated light beam may not be in the desired state, resulting in insufficient frequency band limitation.

Furthermore, in color solid-state imaging devices, a method is often used that spatially modulates incident light in an oblique direction. If it is included in optical information,
There is a problem that color artifacts are reproduced, and the above-mentioned spatial frequency filter has the disadvantage that color artifacts cannot be sufficiently reduced.

Next, according to the spatial frequency filter 2JA shown in FIG. 11, four beam separation is obtained as shown in FIG. 12. In this separated luminous flux, the luminous flux at the spatial frequency position indicated by the circle is
The luminous flux is stronger than that at the position indicated by the Δ mark. In this filter, the diagonal direction has a trap point at a lower frequency than the horizontal and vertical directions, but as shown in Figure 13,
The drawback is that the filter does not have trap points in the horizontal direction.

[Purpose of the invention]

This invention was made to address the above-mentioned circumstances. Firstly, a birefringent plate with a small number of plates can exhibit a high band-limiting effect, and secondly, it can be used in diagonal directions that are less important for human vision. An object of the present invention is to provide a spatial frequency filter with improved band-limiting effect.

[Summary of the invention]

The present invention includes a Savart plate 811 that obtains luminous flux separation in a direction of 45° with respect to the horizontal axis, and a Savart plate S12 that obtains luminous flux separation in the horizontal or vertical direction, which is disposed on the lI side of this first birefringent plate. This Savard plate 812 is replaced by the Savard plate 81.
The above object is achieved by the Savart plate 81B which is sandwiched together with the Savard plate 1311 and separates the light beam in a direction perpendicular to the light beam separation direction of the Savard plate 1311.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, which is constructed by laminating three Savard plates S11 to sis.

Each of the Savart plates 811 to 813 has a light beam separation direction in the direction shown by an arrow in FIG.

FIG. 2 shows how the spatial frequency filter shown in FIG. 1 separates the light beam. When a light beam transmitted through one Savard plate is further transmitted through another Savard plate, the ratio of the amount of light incident on the second Savard plate to the amount of the separated light beam is as follows: If the beam separation direction is 0, then 5
iI12θ: Injury 2θ. In this embodiment, the angle formed by the light beam separation directions of the Savart plates 811 and 812 is 45
°. Therefore, the two light beams separated by the Savart plate S11 (FIG. 2(a)) are separated into four light beams (FIG. 2(b)) with a light intensity of 1:1. Even between Savart plate 812pS1:I, the beam separation direction is 45'
Therefore, the number of light beams transmitted through the Savart plate 813 is eight light beams (FIG. 2(C)).

According to the above-described spatial frequency filter, the Savart plate 81
The total characteristic is obtained by multiplying the characteristics of 1, 812, and 813 comb filters. As shown in FIG. 3, this comprehensive characteristic has a drag point at the spatial frequency indicated by the broken line.

Uv is the vertical spatial frequency axis, and UH is the vertical spatial frequency axis. In addition, the trap characteristic line 3* shown by the broken line,
3b, 4m, 4b, 5*.

The intersection of 5b and the axis is determined by the thickness of the Savart plate.

As mentioned above, according to the present invention, the characteristic line Ja.

3b, characteristic lines 4m, 4b, and characteristic line 5a.

Comprehensive characteristics of 3 sets of 5b 3 Sabal boards 811.8
12,813.

Moreover, according to this filter, in the horizontal direction, where the resolution of the human eye is good, there is no band limitation, and a characteristic is created that has trap points in the diagonal direction, which is not important to the human eye. , the separation direction of the luminous flux is 45° with respect to the horizontal axis
, a second Savard plate having a light beam separation direction perpendicular to the light beam separation direction of the first Savard plate, and a second Savard plate having a light beam separation direction perpendicular to the light beam separation direction of the first Savard plate. A third Savard plate having a light beam separation direction in the vertical or horizontal direction is provided between the second Savard plate to obtain the above characteristics.

Based on this basic principle, similar objectives and effects can be achieved even with the combinations shown in FIGS. 4(a) and 4(b). That is, the Savart plate 8211B2 in FIG. 4(a)
2,823 may be pasted together in order, or as shown in Figure 4 (
The Savart plates s31°832.833 of b) may be pasted together in order.

Of course, in this case as well, the thickness of the Savart plate with the light beam separation direction diagonally corresponds to the axis Uv in FIG.

In order to match the crossing points of the characteristic lines on the UH, they must be equal to each other.

In FIG. 5, a Savart plate 840 having a beam separation direction in the vertical direction is added to the embodiment shown in FIG. 1, and a trap characteristic line 6a is formed as shown in FIG.

6b is shown. Although it is possible to configure a filter with many trap points using more Savart plates, the present invention basically uses a first filter having a beam separation direction in the direction of 45 degrees with respect to the horizontal direction. a Savard plate, a second Savard plate having a light beam separation direction in a direction perpendicular to the light separation direction of the first Savard plate;
This first. There is no change in the structure in which the third Savard plate is placed between the second Savard plates and has a beam separation direction in the horizontal or vertical direction.

〔Effect of the invention〕

As explained above, according to the present invention, a comprehensive characteristic is obtained by multiplying the characteristics of one birefringent plate (Savart plate) and one comb-shaped filter, but in particular, a small number of birefringent plates , you can get a good frequency drag point. Moreover, since the present invention has a sufficient band-limiting effect even in oblique directions, components of low importance to the human eye are removed, and when used in solid-state imaging devices, etc., it is difficult to obtain natural reproduced images. I can contribute.

Furthermore, with the filters described in FIGS. 5 and 6, it is possible to have equivalent characteristics in the horizontal and vertical directions, and it is also possible to obtain spatial frequency characteristics that are approximately equidirectional. Furthermore, a color imaging device that spatially modulates incident light in an oblique direction;
In the optical system of a device with a checkered color filter,
If the plate thicknesses of both the comb-shaped filter that obtains luminous flux separation in the 45° or 1350° direction and the comb-shaped filter that obtains luminous flux separation in the horizontal direction are zero in response to the spatial modulation frequency in the diagonal direction, the color The effect of reducing false signals can be greatly improved.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of a configuration showing an embodiment of the present invention, Fig. 2 is an explanatory diagram of luminous flux separation of the spatial frequency filter of Fig. 1, and Fig. 3 is an explanatory diagram of spatial frequency characteristics of the spatial frequency filter of Fig. 1. 4 is an explanatory diagram showing another embodiment of the present invention, FIG. 5 is a configuration explanatory diagram showing still another embodiment, and FIG.
Spatial frequency characteristics explanatory diagram of the spatial frequency filter shown in Fig. 7.
The figure is an explanatory diagram of the configuration of a conventional spatial frequency filter, FIG. 8 is an explanatory diagram of the beam separation of the spatial frequency filter of FIG. 7, and FIG.
12 is an explanatory diagram of the beam separation of the spatial frequency filter of FIG. 11, and FIG. 13 is a response characteristic diagram of the spatial frequency filter of FIG. 11. 811-813, 821-823.8B1-S33. S
40...Savar board. Applicant's Representative Patent Attorney Takehiko Suzue Figure 1 Figure 2 Figure 3 Figure 4 (a) (b) (C) Figure 11 Figure 12 Figure 13 Figure 9 Amended Book of Proceedings Showa i9・1q 29th Manabu Shiga, Commissioner of the Japan Patent Office 1, Indication of the case, Patent Application No. 59-99612 2゜Name of the invention Spatial frequency filter 3, Person making the correction Relationship with the case Patent applicant (307) Toshiba Corporation ( (and 1 other person) 4. Agent 5, Voluntary amendment 6. Amendment 7. Contents of amendment (1) On page 2, line 10 of the specification, the phrase "beyond the fold" was changed to "``I'm going to make a comeback,'' he corrected. (2) “U
``H is vertical direction'' is corrected to ``UH is horizontal direction''. (3) In the first line of No. 11 of the specification, the phrase "of a small number of sheets" is corrected to "of a small number of sheets."

Claims (1)

[Claims] a first birefringent plate that obtains beam separation in the 45' direction with respect to the horizontal axis; and a second birefringent plate that has the same thickness as the first birefringent plate and obtains beam separation in a direction perpendicular to the direction of the beam separation. The basic structure consists of a birefringent plate and a third birefringent plate disposed between the first and second birefringent plates to obtain luminous flux separation in the horizontal or vertical direction. A spatial frequency filter characterized in that: