JPH0219821A - Optical low-pass filter - Google Patents

Abstract

PURPOSE:To excellently remove influences of aliasing with an image pickup element even when the element has a small aperture ratio only by causing the value of separating width of each optical member constituting an optical low-pass filter to satisfy specific conditions. CONSTITUTION:This optical low-pass filter separates incident light rays into two light beams and is constituted of 1st, 2nd, and 3rd optical members 1, 2, and 3 and values of the components of the light beam separating widths of the members 1, 2, and 3 projected in the scanning direction of an image pickup element are respectively P1, P2, and P3. When the minimum, maximum, and intermediate values of the values P1-P3 of the projected component of each light beam on the image pickup element are respectively designated to (a), (c), and (b), the values are caused to satisfy conditions of inequalities I and II. Therefore, influences of aliasing can be removed excellently even when the used image pickup element has a small aperture ratio only.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an optical low-pass filter, and in particular to a COD
The present invention relates to an optical low-pass filter suitable for an imaging device that discretely obtains images using an imaging element such as a MOS.

(Prior Art) In general, a video camera or the like using an imaging device such as a CCD or MOS having a discrete pixel structure spatially samples image information optically to obtain an output image.

In this case, if the subject contains high spatial frequency components higher than the sampling frequency, false signals such as structures and color tones that the subject does not have will be generated.

In other words, frequency components that cannot be collected by imaging equipment (frequency components that exceed the Nyquist frequency) cannot be reproduced as image information, and a phenomenon called waveform distortion (aliasing) occurs, causing moire fringes and false colors in the captured image. This causes the formation of

For this reason, conventionally, an optical low-pass filter is placed in a part of the photographing system to limit the high frequency components of the subject and eliminate the effects of aliasing.

Conventionally, many optical low-pass filters that utilize birefringence, such as a quartz plate, have been used.

FIG. 4 is an explanatory diagram of an optical low-pass filter using a conventional crystal plate and utilizing birefringence.

In the figure, 40 is an optical low-pass filter 41 for incident light, 42 is an ordinary ray, and 43 is an extraordinary ray.

In the same figure, incident light 1141 is divided into ordinary ray 42 and extraordinary ray 4.
A low-pass effect is obtained by separating the light beam into two light beams.

Here, if f is the spacing between the light beams separated by the crystal plate, or the spatial frequency, then the transfer function, so-called MTF(f), becomes a cosine function, such as MTF(f)-lcostcDf.

FIG. 5 is an explanatory diagram of the transfer function MTF(f) at this time. As shown in the figure, by controlling the thickness D of the crystal plate, a predetermined spatial frequency component included in the subject can be controlled.

FIG. 6 is an explanatory diagram of an image sensor having a stripe filter used in a single-chip video camera or the like.

As shown in the figure, many single-chip video cameras and the like have color filters arranged in stripes perpendicular to the scanning direction.

An image sensor having this stripe filter has advantages such as being easy to manufacture and being advantageous in increasing the number of pixels.

However, on the other hand, it has the disadvantage that aliasing is relatively easy to occur in the horizontal direction (scanning direction).

For this reason, the effects of aliasing are suppressed by applying the aforementioned optical low-pass filter of the crystal plate in the direction above the water.

Here, an image sensor such as a frame transfer (FT) type CCD, in which the charge product part of the image sensor is located outside the effective imaging screen, will be described.

In the case of this frame-transfer type COD, the aperture ratio (the ratio of the sampling pitch to the corresponding size of the photosensitive area in the cell) is close to 100%, so the transfer function (MTF) of the image sensor itself is low. , the image sensor itself has the effect of suppressing the effects of aliasing to some extent.

For this reason, the image sensor uses one optical low-pass filter 40 that utilizes the birefringence of a crystal plate, as shown in FIG. 4, to separate the incident light beam into two, an ordinary ray and an extraordinary ray, in the horizontal direction. The effects of aliasing can be sufficiently suppressed with just this method.

However, this frame transfer one-type CC
The size of image sensors such as D is large because the charge storage part is located outside the effective imaging screen, and when strong light is incident, many charged particles are generated, causing a phenomenon (blooming) that overflows to neighboring cells. there were.

For this reason, an image sensor such as an inter-line (!•L) type CCD, which is located in the charge storage section of the image sensor or within the effective imaging screen, is generally used.

However, since the charge storage part of the inter-line type image sensor such as CCD is located within the effective imaging screen, the aperture ratio is lower than that of the frame transfer type.
Since the aperture ratio is about 1/4, it is difficult to suppress the influence of aliasing in the image sensor itself.

FIG. 7 is an explanatory diagram showing a comparison of the transfer function MTF(f) (II) of a frame transfer type CCD and an inter line type CCD.

As shown in the figure, since the inter-line type CCD image sensor has a higher transfer function MTF (f) value than the frame transfer type, the image sensor itself has the effect of suppressing aliasing. becomes low, which makes aliasing more likely to occur.

Conventionally, in order to solve the above-mentioned problems, an optical low-pass filter consisting of two or more quartz birefringent plates is placed in a part of the photographing system.

FIG. 8 is a configuration diagram when an optical low-pass filter having two birefringent plates is used in an image sensor for an inter-line CCD.

In the figure, 81 and 82 are birefringent plates made of quartz. A phase plate 83 converts linearly polarized light into circularly polarized light, and is provided between birefringent plates 81 and 82.

As shown in the figure, an optical low-pass filter is constructed by sandwiching a phase plate between two birefringent plates 81 and 82.

In the figure, the incident light ray E is separated into two by a birefringent plate 81, linearly polarized light is converted into circularly polarized light by a phase plate 83, and further divided into two by a birefringent plate 82, resulting in four lights. A low-pass effect is obtained by separating the light beams.

FIG. 9 is an explanatory diagram of the transfer function MTF(f) value of the optical low-pass filter at this time.

As shown in the figure, the Nyquist frequency f of the image sensor is near the first trap frequency. The influence of aliasing can be suppressed by setting the MTF value of the optical low-pass filter to a sufficiently small value at a frequency twice (2fw).

In the same figure, the optical low-pass filter using these two birefringent plates shows the rise of the MTF curve near the frequency of 2f.
It is made smaller than the rise of the F curve.

Therefore, the MTF value of the optical low-pass filter becomes a small value, and the influence of aliasing can be suppressed.

However, the opening ratio is about 25%.

With a line-type CCD image sensor, the MTF value near the 2f frequency is still not small enough, and the 9th
urF(f) (part C shown in the explanatory diagram of a) (1st-
There is a swell in the MTF curve (between the second trap frequency), and the rise of the MTF curve after the second trap frequency is also too large.

For this reason, it cannot be said that aliasing was sufficiently suppressed, and there was a drawback that phenomena such as false color and false resolution in which the contrast was reversed occurred in the photographed images.

Furthermore, Japanese Patent Application Laid-Open No. 61-261988 proposes an optical low-pass filter using three crystal birefringent plates for an inter-line type CCD.

Figure 1(A) is a block diagram of the optical low-pass filter for inter-line CCD proposed in the same publication, and Figure 1(B) shows how the optical low-pass filter splits the light ray E. FIG.

In the same figure (A), the angles of the light rays separated with respect to the optical axes of the birefringent plates IOT and 102 and 103 and the scanning direction of the image sensor are respectively -45° and 0045°, and the separation width is t:
JT: By setting it to 1, a low-pass effect is obtained by separately obtaining light rays at seven points forming the vertices and center of the hexagon shown in Figure (B).

Figure 11 shows the MT of the optical low-pass filter at this time.
It is an explanatory diagram of F(f)(a).

As shown in the figure, near the second Traub frequency! 1ltT
Although the rise of the F curve can be said to be small, the rise of the MTFIIh line near the first trap frequency cannot be said to be small, and compared to the MTF curve of the part indicated by C° (between the first and second trap frequencies). There is a huge excitement.

For this reason, the MTF value is not a sufficiently small value and cannot sufficiently suppress aliasing in the scanning direction.

(Problems to be Solved by the Invention) The present invention solves the problem of the optical low-pass filter M T
(By setting li to a sufficiently small value.

To provide an optical low-pass filter capable of satisfactorily removing the influence of aliasing even in an image sensor having a stripe filter and having only a small aperture ratio, such as an inter-line type CCD.

(Means for solving the problem) An optical low-pass filter is placed in the optical path of incidence on the image sensor, and the optical low-pass filter separates the incident light beam into two, and divides the light beam into two beams. A first optical member whose projected component value in the scanning direction of the image sensor is Pi, and each light ray emitted from the first optical member is further separated into two, and the image is captured with a separation width of the light rays. a second optical member whose projection component in the scanning direction of the element has a value of 22;

a third optical member which further separates each light beam emitted from the second optical member into two and whose projection component of the separation width of the light beam in the scanning direction of the image sensor is P3; Values P1, P of the projection components of each of the light rays onto the image sensor
2. When the value of the minimum projected component of P3 is a, and the value of the maximum projected component is C1, the value of the intermediate projected component is b, 0.56 ≦ a / b ≦ 1 1 ≦ c / b ≦ 1.
The requirement is to satisfy condition 8.

(Embodiment) FIG. 1 is a block diagram of an optical low-pass filter according to an embodiment of the present invention.

In the figure, an optical low-pass filter 1 is a first optical member, and is formed from a birefringent plate 1b made of quartz crystal. Reference numeral 2 denotes a second optical member, which is formed of a phase plate 2a that converts linearly polarized light into circularly polarized light and a birefringent plate 2b made of quartz crystal. Reference numeral 3 designates a third optical member, which is composed of a phase plate 3a having the same function as the phase plate 2a and a birefringent plate 3b made of crystal.

In this embodiment, these three optical members 1 and 2.3 constitute an optical low-pass filter 10. Note that E is an incident light beam.

In this embodiment, the light i1E incident on the optical low-pass filter IO is separated into two by the birefringent plate 1b, and the separated light &aE is converted from linearly polarized light to circularly polarized light by the phase plate 2a, and the birefringent plate 1b converts the linearly polarized light into circularly polarized light. 2b. The birefringent plate 2b further separates the light beam E into two beams, one phase plate 3a converts the linearly polarized light into circularly polarized light, and the light beam E enters the birefringent plate 3b. The birefringent plate 3b further separates the light &iE into two beams each, resulting in a maximum of eight beams and a minimum of four beams in total, which pass through the optical low-pass filter lO to obtain a low-pass effect. The light is incident on the illustrated image sensor.

In this embodiment, the separation width of each birefringent plate 1b, 2b, 3b is set to Q1. Let Q2 and Q3 be the minimum separation width, a middle separation width, and C the maximum separation width, then the transfer function of an optical low-pass filter, the so-called MTF (
f) is MTF(f)-lcosπQ+f+cos ff Qf+cos *Qsfslcosπaf・cosπb
r+CO5πcr+e・(3) (a ≦b ≦C), and the first, second . This results in an MTF curve with a third trap frequency. Second
The figure shows the transfer function M of the optical low-pass filter at this time.
It is an explanatory diagram of TF (f) value.

In this embodiment, the values of each of these separation widths a, b, and c are set so as to satisfy the above-mentioned conditional expressions (1) and (2). The rise of the MTF curve can be made sufficiently small anywhere near the third trap frequency, that is, the MT of the optical low-pass filter
The effect of aliasing is removed by reducing the F value.

Here, if the sampling rate of the image sensor is 21 and b '' Pi / 2 (4), then the Nyquist frequency f8 of the image sensor doubled is 2fs -1 / Pi -1/ 2b ・-・(S
), which corresponds to the second trap frequency 1/2b of the MTF curve.

Also, the MTF (f) of the optical low-pass filter in Figure 2
As shown in the explanatory diagram of the values, there is a wide area (between the 1st and 3rd trap frequencies) where the MTF value is sufficiently close to 0 around the Nyquist frequency 2f, so it is a good optical low-pass with little effect of aliasing. You can get filters.

In conditional expressions (1) and (2), if the value of the minimum separation width a is too small, the lower limit of conditional expression (i) will be exceeded, and the MTF curve curve portion (second to second 3 trap frequencies) becomes too large, and the rise of the MTF curve near the second and third trap frequencies becomes too large, which is undesirable.

Also, if the maximum value of the separation width is too large, the upper limit of conditional expression (2) will be exceeded, and the rise in the part of the lock TF curve A (between the 42nd trap frequency) shown in Figure 2 will become too large. 1st. The rise of the MTF curve near the 21st-lap frequency becomes too large, making it difficult to provide an optical low-pass filler with good resolution.

FIGS. 3(A) and 3(B) are explanatory diagrams of an optical low-pass filter showing another embodiment of the present invention, and FIG.
) is a cross-sectional view of the optical low-pass filter in the direction perpendicular to the optical axis, and (B) is an IJI diagram showing how the optical low-pass filter divides the light beam, and the cross-sectional view is perpendicular to the incident light beam E. It is shown on a plane.

In FIGS. 3(A) and 3(B), 31 is the first optical member;
2 is a second optical member, 33 is a third optical member, and these optical members 31, 32, and 33 constitute an optical low-pass filter 30.

Further, these optical members 31, 32, and 33 are formed of birefringent plates 31b, 32b, and 33b made of crystal.

Note that 34 indicates the same position as the incident light 1aE.

In this embodiment, the angles between the optical axes of each of the birefringent plates 31b, 32b and 33b and the light rays separated with respect to the horizontal direction (scanning direction) of an image sensor (not shown) are -45°, O', and 4°, respectively.
It is set to 5″′.

In this example, it is the same as the incident ray E! ! The light beam at 134 is separated in the direction of 0 by the birefringent plate 31b.

The birefringent plate 32b further separates the light beams in the @ direction, and the birefringent plate 33b further separates them in the O direction, resulting in eight light beams with a low-pass effect.

In this embodiment, each birefringent plate 31b, 32b, 33b
(7) Each molecule 4 width 'l Q )I+Q 3z,Q.

Then, the two-dimensional transfer function of the optical low-pass filter, the so-called MTF (f, ``a)'', becomes MTF(F, , ra), and the horizontal MTF (f, , O) becomes MTF(f,
, mouth) Then 1For example, Q + = Q ff+/ 1 block, Q,
=Q.

If Q co = Q ff, l/ ff, the equation becomes exactly the same as conditional expression (3) in the previous embodiment, and a good optical low-pass filter with small aliasing effect that can be handled in the same way as in the previous embodiment can be achieved. are doing.

In this example, since the optical low-pass filter is configured without using the phase plate used in the previous example, the thickness of the optical low-pass filter can be made thin.
This provides an optical low-pass filter suitable for use in cameras and the like where mechanical space is limited.

In each of the above embodiments, a quartz birefringent plate or a combination of a phase plate and a quartz birefringent plate was used as the optical member constituting the optical low-pass filter.
Any optical member may be used as long as it can separate any light beam into two.

(Effects of the Invention) According to the present invention, the MTF value of the optical low-pass filter is adjusted to the value of the separation width of each optical member constituting the optical low-pass filter as described above in the vicinity of twice the Nyquist frequency of the image sensor. Conditional expressions (1) and (2) are satisfied, and the effects of aliasing can be effectively removed even if the image sensor has a stripe filter and has a small aperture ratio, such as an inter-line type COD. Optical low-pass filters with high optical performance can be coupled.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an optical low-pass filter according to an embodiment of the present invention, FIG. 2 is an explanatory diagram of the transfer function MTF (f) (1173) of the optical low-pass filter of the present invention, and
), (B) are block diagrams of an optical low-pass filter according to another embodiment of the present invention, FIG. 4 is an explanatory diagram of a conventional optical low-pass filter using a crystal plate, and FIG. 5 is an illustration of a conventional optical low-pass filter using a crystal plate. Figure 6 is an explanatory diagram of the image sensor having a stripe filter, and Figure 7 is the transfer function MTF of frame transfer 1 and inter-line COD. (f) An explanatory diagram showing a comparison of values. Figures 8 and 10 are configuration diagrams of optical low-pass filters conventionally used for inter-line type CCDs. Figures 9 and 1 are diagrams of inter-line type CCDs. This is an explanatory diagram of the transfer function MTF(f)(a) of an optical low-pass filter conventionally used for line-type CCDs. In the figure, 1.31 is the first member, 2 and 32 are the second members, 3. 3
3 is a third member, 2a, 3a are phase plates, lb, 2b, 3b
, 31b, 32b, and 33b are birefringent plates, and E is an incident light beam. Patent applicant Canon Co., Ltd.

Claims (1)

[Scope of Claims] An optical low-pass filter disposed in an incident optical path to an image pickup device, the optical low-pass filter separates an incident light beam into two beams, and divides the separation width of the light beam into two beams. A first optical member whose projection component value in the scanning direction is P1, and each light ray emitted from the first optical member is further separated into two, and the separation width of the light ray is the scanning direction of the image sensor. a second optical member whose projection component value is P2, and each light ray emitted from the second optical member is further separated into two, and the separation width of the light ray is divided into two in the scanning direction of the image pickup device. Consisting of a third optical member whose projected component value is P3,
Values P1, P2 of projected components of each of the light rays onto the image sensor
, P3, where a is the value of the smallest projected component, c is the value of the largest projected component, and b is the value of the middle projected component, 0.56≦a/b≦1 1≦c/b≦1. 8. An optical low-pass filter characterized by satisfying the following conditions.