JPH04236585A - Image pickup optical device - Google Patents

Abstract

PURPOSE:To pick up a sharp image over a broad object distance range even when no focusing device is provided (or insufficient) in the image pickup optical system having an optical low pass filter. CONSTITUTION:The optical system is provided with an objective lens 1, a solid- state image pickup element 2 receiving an image formed by the objective lens 1, an optical low pass filter 3 whose spatial frequency limit effect is variable arranged between the objective lens 1 and the solid-state image pickup element 2, a detection circuit 11 detecting a focus state of the picked-up object by the objective lens 1 and a control circuit 12 receiving a signal from the detection circuit 11 and controlling the optical low pass filter 3 so as to weaken the spatial frequency limit effect in response to the deviation of the picked-up object from the focus state.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to an imaging optical system used in video cameras, endoscopes, etc., which is equipped with an optical low-pass filter for removing moiré caused by interference between an object image and an imaging means. This invention relates to an imaging optical system.

0002

[Prior Art] Usually, in video cameras and the like, an optical low-pass filter is placed in the imaging optical path in order to prevent moiré from occurring due to interference between an object image and an imaging means such as a solid-state imaging device. . An optical low-pass filter eliminates interference between the spatial frequency components of the object image and the sampling frequency of the solid-state image sensor by limiting the spatial frequency response of the objective lens and suppressing high spatial frequency components in the object image. It is something to do. This optical low-pass filter acts in the direction of reducing the sharpness of the object image, so if the object position changes and the object image on the image sensor becomes blurred, the sharpness of the image decreases significantly, impairing image quality. Cause.

0003

In order to correct the blurring of an object image, so-called focusing is usually performed by moving the position of the objective lens back and forth with respect to the image plane. However, in recent years, as imaging devices have become smaller, it has become increasingly difficult to implement a lens drive mechanism for focusing. Even without a focusing mechanism, the object image can be seen sharply within the depth of focus of the objective lens, so there is no practical problem within this range. However, in an imaging optical system equipped with an optical low-pass filter, the filter acts to reduce the sharpness of the image, so the range in which the image can be seen sharply without focusing is extremely narrow, and the range of use is limited. The problem is that it is extremely limited.

[0004] In view of the above-mentioned problems, the present invention provides an imaging optical system equipped with an optical low-pass filter that can produce sharp images over a wide object distance range even if it does not have (or is insufficiently equipped with) a focusing mechanism. The purpose is to provide an imaging optical system that can take pictures.

[0005]

[Means for Solving the Problems] An imaging optical system according to the present invention includes an objective lens, an imaging means for receiving an image formed by the objective lens, and a space disposed between the objective lens and the imaging means. an optical low-pass filter with a variable frequency limiting effect; a detection means for detecting a focused state of the object to be imaged by the objective lens; and a detection means for detecting a deviation from the focused state of the object to be imaged by receiving a signal from the detection means. The present invention is characterized by comprising a control means for controlling the optical low-pass filter so that its spatial frequency limiting effect is weakened accordingly.

[0006] The objective lens does not move at all for focusing, or even if it moves, it only has a range of movement that covers a portion of the entire range of object distances. Further, the detection means is an object distance detection means used for autofocus of a camera, etc., and is adapted to detect the distance to the object to be imaged.

[0007]

[Operation] According to the above configuration, when the object distance is outside the predetermined distance range determined by the depth of focus range of the objective lens or the focusable distance range, it is detected by the detection means and controlled. By means of weakening the spatial frequency limiting effect of the optical low-pass filter. In other words, when an object image becomes blurred due to being out of focus, this fact itself reduces the high frequency components of the spatial frequency of the object image, making moiré less likely to occur, but on the other hand, the resolution of the object image decreases, so an optical low-pass filter is used. By weakening the spatial frequency limiting effect, it is possible to prevent the resolution of the object image from decreasing too much and obtain the sharp image inherent to the objective lens.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in detail below based on one embodiment shown in the drawings. FIG. 1 is a schematic diagram showing the overall configuration of an electronic endoscope including an embodiment of the imaging optical system of the present invention, and FIG. 2 is a diagram showing the above embodiment. Because an endoscope has an objective lens, solid-state image sensor, and other components packed into its very thin tip, it is often impossible to provide a focusing mechanism that moves the objective lens. It is suitable as an object of application of the invention.

In FIGS. 1 and 2, 1 is an objective lens;
Reference numeral 2 denotes a solid-state image sensor that receives an image formed by the objective lens 1, and 3 an optical low-pass filter with variable spatial frequency limiting effect, which is disposed between the objective lens 1 and the solid-state image sensor 2. The optical low-pass filter 3 is constructed by forming transparent electrodes 5, 5 on the inner surfaces of two crystal plates 4, 4, respectively, and providing a twisted nematic liquid crystal layer 6 between them. It is assumed that the two crystal plates 4, 4 separate the ordinary light (indicated by a solid line) and the extraordinary light (indicated by a dotted line) in the same direction. Also, two crystal plates 4, 4
have the same thickness, and the thickness is the same as that of the optical low-pass filter 3.
It is determined appropriately depending on the degree of spatial frequency limiting effect.

Reference numeral 7 denotes a power source connected to the transparent electrodes 5, 5 via a switch 8. The liquid crystal layer 6 is turned off when the switch 8 is turned off.
When F is applied, no voltage is applied between the transparent electrodes 5, 5, so a twisted nematic arrangement is taken and optical rotation is achieved, but when the switch 8 is ON, a voltage is applied between the transparent electrodes 5, 5, so the twisted nematic arrangement is unraveled. The optical rotation is lost. The thickness of the liquid crystal layer 6 is determined so that the vibration plane of the incident light rotates by 90 degrees when the switch 8 is turned on.

The light passing through the objective lens 1 is separated into ordinary light and extraordinary light by a first crystal plate 4. As shown in FIG. 2, when the switch 8 is ON, the liquid crystal layer 6 has no optical rotation and the directions of the vibration planes of both lights do not change. It remains as it is. Therefore, since the separation distance between the two lights is expanded by the second crystal plate 4 and the two lights enter the solid-state image sensor 2, a spatial frequency limiting effect is produced in accordance with the separation distance.

Furthermore, as shown in FIG. 3, the switch 8 is turned off.
At F, the directions of the vibration planes of both lights change by 90 degrees due to the optical rotation of the liquid crystal layer 6, so the ordinary light and
The light that was abnormal light becomes extraordinary light and ordinary light for the second crystal plate 4, respectively. Since the thickness of the two crystal plates 4, 4 is the same, both lights separated by the first crystal plate 4 are merged into one beam again when exiting the second crystal plate 4, and solid-state imaging is performed. incident on element 2. Therefore, the light beam is only shifted laterally, and no spatial frequency limiting effect occurs.

Referring again to FIG. 1, reference numeral 9 denotes a signal processing circuit that converts the output signal from the solid-state image sensor 2 into a video signal, and 10
is a TV monitor that displays images in accordance with the video signal from the signal processing circuit 9. 11 is a detection circuit that detects defocus (blur) based on a signal from the signal processing circuit 9; 12;
is a control circuit that controls the switch 8 of the optical low-pass filter 3 based on the signal from the detection circuit 11. 13 is a light source device consisting of a light source lamp 14, a variable aperture 15 that changes the amount of light from the light source lamp 14, and a condenser lens 16 that collects the light from the light source lamp 14; 17 is a light source device that directs the light from the light source device 13 to the tip of the endoscope; A light guide 18 is an illumination lens that irradiates the object with light from the light guide 17.

The principle of detecting defocus (blur) using the signal from the signal processing circuit 9 will now be explained. For example, when an endoscope is inserted into a body cavity, video signals are output one after another from the signal processing circuit 9 while the solid-state image sensor 2 is being driven. A detection circuit 11 uses a portion of the image to detect the sharpness of the image. As the detection method, a method well known in the field of automatic focus detection etc. may be adopted. For example, output signals xi, xi+1 between mutually adjacent pixels
Calculate the value Σ|xi +xi+1 | which is the sum of the absolute value of the difference for all pixels used for defocus detection, and this value increases when the distance between the object and the endoscope tip changes. It can be determined whether the distance to the object is in front of or behind the in-focus position by determining whether the distance to the object is in front of or behind the in-focus position. Since the distance between the endoscope tip and the object changes frequently (for example, the inner wall of the stomach is constantly moving), this method is sufficient to detect defocus (blur).

Since this embodiment is constructed as described above, the detection circuit 11 uses a signal from the signal processing circuit 9 to determine the object distance based on the depth of focus range of the objective lens 1 or the range of distance that can be focused. When detecting that the distance is within a predetermined distance, the control circuit 12 turns on the switch 8 of the optical low-pass filter 3 in response to a signal from the detection circuit 11. Then, according to the above principle, the spatial frequency limiting effect of the optical low-pass filter 3 occurs, and moiré is removed. On the other hand, the detection circuit 11 is connected to the signal processing circuit 9
When it is detected by the signal from the detection circuit 11 that the object distance is outside the predetermined distance range determined by the depth of focus range of the objective lens 1 or the focusable distance range, the control circuit 12 is activated by the signal from the detection circuit 11. Turn off the switch 8 of the optical low-pass filter 3. Then, according to the above-mentioned principle, the spatial frequency limiting effect of the optical low-pass filter 3 is eliminated, so that the resolution of the object image is prevented from decreasing too much, and the sharp image inherent to the objective lens 1 can be obtained. Note that when the object image becomes a little blurred due to being out of focus, moiré is also removed because the high frequency component of the spatial frequency of the object image decreases due to this fact.

Next, when the image is out of focus, this is changed to the MTF (Modulation) of the optical low-pass filter 3.
It shows how much compensation can be made by changing the value of n Transfer Function.

FIG. 4 shows an enlarged view of the pixel arrangement of the solid-state image sensor 2, and the pitches of one pixel in the horizontal scanning direction and the vertical scanning direction are respectively PX and PY. The imaging method used here is called a color difference sequential method, and Cy
(cyan), Mg (magenta), Ye (yellow), G
(green) color filters are respectively attached, and a color mosaic filter is constructed using the array of color filters shown in FIG. 4 as a unit. Then, in odd-numbered fields, the two lines indicated by ■ are added and read out, and in the even-numbered fields, the two lines indicated by ■ are added and read out (References: CCD camera technology, written by Hiroo Takemura, radio Published by Gijutsusha).

Assuming that the interval between the double images formed by the optical low-pass filter 3 is S as shown in FIG. 2, in order to remove color moiré, S≒PX.

(1) is common. At this time, assuming that the objective lens 1 has no aberration, the MTF of the image in the horizontal scanning direction is equal to the MTF of the optical low-pass filter 3, as shown in A of FIG. Here, if the MTF of the optical low-pass filter 3 is ML, then ML = cos(πPX u)

(2) becomes. However, u is the spatial frequency in the horizontal scanning direction.

Next, it is assumed that the position of the object deviates from the focused state and the imaging light beam on the solid-state image sensor 2 formed by the objective lens 1 blurs into a circular shape with a diameter d, as shown in FIG. At this time, assuming that the blur is at the limit where it is not felt as image blur, that is, at the limit of the depth of field, the pixel scanning
P is the average pitch in the direction and vertical scanning direction.

002
0]

[Math 1]

[0021] And d=4P

Since a value of about (4) poses few practical problems, the properties of the optical system will be examined within this range. Depending on the subject, 2P≦d≦6P

It is known that it takes a value in the range of (5).

[0022] Here, the reason why there is an upper limit and a lower limit to the range of depth of field comes from a practical viewpoint. If the object has relatively few fine structures (for example, many objects observed with medical endoscopes have such structures), the observer cannot detect it even if the amount of defocus is quite large. Therefore, in such a case, the practical depth of focus becomes very deep. Conversely, when observing objects with many fine structures (objects observed with industrial endoscopes are more like this than with medical endoscopes), the amount of defocus is small and the observer is unable to can be detected, so the practical depth of focus is shallow. This is the reason why the upper and lower limits of depth of field are three times different.

[0023] Also, if the MTF when defocused to the extent that the diameter of the circle of confusion becomes d is Md, then
Md = 2J(πdu)/πdu
(6) becomes. However, J(πdu) is a first-order Bessel function. d=
A graph of Md in the case of 4P is shown in FIG. 5B. According to this, the cutoff frequency of Md is
1.22/4P=0.305/P=0.369/PX
It is given by (7).

One pixel of the solid-state image sensor 2 used recently is often long in the horizontal scanning direction, and PY = kPX

It can be expressed as (8). However, k=0.5~
It is about 1.0. If k=0.682, then PX=1.211P can be obtained from equations (8) and (3). Therefore, the cutoff frequency of ML is 0.413/P, as shown in FIG.

Now, when the object is at the limit of the depth of field,
If there is a spatial frequency limiting effect of the optical low-pass filter 3, the MTF, which is the sum of the spatial frequency limiting effect and the blur caused by out-of-focus, will be as shown in C in FIG. C is the product of A and B. However, if there is no spatial frequency limiting effect of the optical low-pass filter 3, then M
The TF becomes like B, and the MTF is improved by the difference between B and C, resulting in a sharper image.

The case where d=4P=3.30PX has been explained above, but the spatial frequency limiting effect of the optical low-pass filter 3 may be canceled at an object distance of about d=3P, which produces a sharper image. can be obtained. That is,
The cutoff frequency of Md when d=3P=2.48PX is 0.406/P, which is almost the same frequency as the cutoff frequency of the optical low-pass filter 3.
Even if the spatial frequency limiting effect of the optical low-pass filter 3 is eliminated, the moiré removal effect is sufficient.

Overall spatial frequency limitation when switching the diameter d of the circle of confusion (does not include the blurring effect caused by the optical low-pass filter 3) and the spatial frequency limitation effect of the optical low-pass filter 3 as the object distance changes The relationship with the effect is shown in solid lines A, B, C, and D in FIG. A is d=
When the spatial frequency limiting effect of the optical low-pass filter 3 is switched at 1.65PX, B is d=2.48P.
X is the case where the spatial frequency limiting effect of the optical low-pass filter 3 is switched, C is the case where the spatial frequency limiting effect of the optical low-pass filter 3 is switched at d=3.30PX, and D is the case where the spatial frequency limiting effect of the optical low-pass filter 3 is switched at d=4.13PX. The cases in which the spatial frequency limiting effect of the low-pass filter 3 is switched are shown. The value of d for switching the spatial frequency limiting effect of the optical low-pass filter 3 depending on the subject is:
It may be selected appropriately between 1.5PX and 6PX.

The size of the diameter d of the circle of confusion caused by the objective lens 1 can be easily calculated from the F number of the objective lens 1 if the object distance is known. When detecting the object distance using a light beam that does not pass through the optical low-pass filter 3, the object distance can be determined accurately, so the diameter d of the circle of confusion is calculated from this and the F number of the objective lens. When detecting the object distance using the light flux that has passed through the optical low-pass filter 3,
Since the sharpness of the object image includes the spatial frequency limiting effect by the optical low-pass filter 3, in this case, the object distance is calculated from the MTF value. That is, the MTF characteristic of the optical low-pass filter 3 can be naturally determined since its operating state is known, and if the Fourier transform of the image from the solid-state image sensor 2 is divided by the MTF of the optical low-pass filter 3, the MTF characteristic of the optical low-pass filter 3 can be determined. MTF of
The MTF of the object image excluding . Since the approximate object distance is known from the MTF of this object image, the diameter d of the circle of confusion can be determined. Based on the confusion circle d obtained as described above, the spatial frequency limiting effect of the optical low-pass filter 3 is switched as shown in FIG.

In the above example, the spatial frequency limiting effect of the optical low-pass filter 3 is switched in two stages, so
Although the spatial frequency limiting effect changes step by step,
By using an optical low-pass filter in which the light separation interval S changes continuously, as shown in Japanese Patent No. 268,570, the spatial frequency limiting effect can be changed continuously. That is, FIG. 8 is a sectional view showing such an optical low-pass filter 19, in which a liquid crystal layer 20 is sandwiched between two transparent electrodes 21, 21, and both transparent electrodes 21, 21 are connected to a variable resistor 22 (control circuit 12). The power source 7 is connected to the power source 7 via a Then, as shown by dotted lines E and F in FIG. 7, the spatial frequency limiting effect of the optical low-pass filter 19 may be gradually reduced as the diameter d of the circle of confusion increases. In this case, as shown in FIG. 9, at the point where the spatial frequency limiting effect is halved, that is, at the cutoff frequency where the spatial frequency limiting effect is the largest, the MTF value is the value at which the spatial frequency limiting effect is the largest ( As shown in Fig. 7, the point Q at which the value is 1/2 of the difference between the maximum value) and the value when the spatial frequency limiting effect is the smallest (minimum value) is 1.5PX ≦d≦6PX

If (9) is selected, the decrease in MTF due to defocus can be suppressed by reducing the spatial frequency limiting effect of the optical low-pass filter 19.

We have considered the horizontal scanning direction so far, but similarly in the vertical scanning direction, if the spatial frequency limiting effect of the optical low-pass filter is reduced as the defocus increases, the decrease in MTF can be suppressed. . In this case, 1.5P≦d≦6P

It is sufficient to select a point or point Q at which the spatial frequency limiting effect of the optical low-pass filter changes during (10). The optical low-pass filter may have a spatial frequency limiting effect also in the horizontal scanning direction, or may have a spatial frequency limiting effect only in the vertical scanning direction. In the latter case, the switching point of the spatial frequency limiting effect of the optical low-pass filter is 1.5PY ≦d≦6PY

You can choose between (11). Further, the spatial frequency limiting effect of the optical low-pass filter may be changed in both the horizontal scanning direction and the vertical scanning direction, or may be changed in only one direction.

Furthermore, when using the solid-state image sensor 2 without a color mosaic filter, the point where the spatial frequency limiting effect of the optical low-pass filter changes, or the point Q when the spatial frequency limiting effect changes continuously. ,
P≦d≦4P

A similar effect can be obtained by selecting between (12). This is because it is known that the limit of the depth of field is about d=2P to 2.5P. The value of d in this case is approximately 2/3 of that in the case of the solid-state image sensor 2 with a color mosaic filter.

Although the case of 0.5<k<1.0 has been described above, the case of 0.25<k<3.0 may actually occur. At that time, depending on the characteristics of the imaging optical system,
Equation (9) is, PX ≦d≦9PX

(13), formula (10) becomes P≦d≦9P

(14), formula (12) becomes 0.5P≦d≦6P

(15) is replaced. Also, regarding the vertical scanning direction, PY ≦d≦9PY

It is sufficient to select a point or point Q at which the spatial frequency limiting effect of the optical low-pass filter changes within the range of (16).

In short, the spatial frequency of the optical low-pass filter is adjusted as the object distance changes so that the moiré removal effect, which is caused by the product of the spatial frequency limiting effect caused by defocus and the spatial frequency limiting effect of the optical low-pass filter, becomes approximately constant. It is important to change the strength of the frequency limiting effect.

As a method for detecting defocus (blur), light for object distance detection such as infrared light is emitted from the illumination optical system, and the incident position of the light incident on the objective lens 1 is detected by the solid-state image sensor 2. There is also a method of detecting the distance to the object using the distance between the illumination lens 18 and the objective lens 1 as the base line length, as determined above. In the field of autofocus detection, this
This is called an active focus detection method.

As another example of the active focus detection method, an infrared light beam using a semiconductor laser or the like as a light source is projected onto an object from the tip of the endoscope a little distance from the objective lens 1, and the reflected light is detected. There is a method of detecting the distance to the object by capturing it with an imaging optical system and utilizing the fact that the image position on the solid-state image sensor 2 changes depending on the object distance.

In addition to inputting the output signal from the signal processing circuit 9 to the signal processing circuit 11, the light source device 13
A sensor for detecting the amount of light incident on the light guide 17 is disposed at the center, and its output signal is input to the signal processing circuit 11, so that the signal from the signal processing circuit 9 indicates that the image is largely blurred, and the sensor If the signal indicates that the amount of incident light is large, the object is at the far point, the signal from the signal processing circuit 9 indicates that the image is largely blurred, and the signal from the sensor indicates that the amount of incident light is small. If this indicates that the object is at the periapsis, a method of determining that the object is at the periapsis may be adopted. However, in this case, since the amount of light incident on the light guide 17 also changes depending on the reflectance of the object, it is desirable to use them together depending on the situation.

[0036] In addition to the above-mentioned examples, an optical low-pass filter with a variable spatial frequency limiting effect is made by superimposing at least two birefringent plates such as quartz plates in the optical axis direction.
An optical low-pass filter may also be used that rotates them relative to each other to change the birefringence effect, that is, the spatial frequency limiting effect.

Furthermore, as shown in FIG. 10, the rotationally symmetrical toric lens 23 made of a transparent elastic material such as transparent rubber is deformed by the force of the piezoelectric element 24, etc., and the shape of the point spread intensity distribution is changed, thereby changing the spatial frequency. The limiting effect may be changed, and as shown in FIG. 11, the shape of the point spread intensity distribution may be changed by deforming the diffraction grating filter 25 made of a transparent elastic material such as transparent rubber by the force of the piezoelectric element 24 or the like. It may also be one that changes the spatial frequency limiting effect.

Although the electronic imaging optical system has been described so far, the present invention can also be applied to an imaging optical system such as a fiberscope using an image guide. In that case, the spatial frequency limiting effect of the optical low-pass filter may be changed when d satisfies the above equation (15).

In addition, in the case of an electronic imaging optical system, if the defocus is so great that it cannot be compensated for only by reducing the spatial frequency limiting effect of the optical low-pass filter, the MTF characteristics of the aperture correction circuit of the signal processing circuit may be adjusted to a high frequency. It may also be possible to compensate for the decrease in MTF by lifting it.

In the examples so far, the spatial frequency limiting effect of the optical low-pass filter is variable, but the present invention is not limited to this, and the spatial frequency limiting effect can be changed by inserting and removing the optical low-pass filter in the optical path. But it's okay.

[Effects of the Invention] As explained above, the imaging optical system according to the present invention has a wide object distance range even when the imaging optical system is equipped with an optical low-pass filter and does not have a focusing mechanism (or is insufficient). It has the advantage of being able to take sharp images.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the overall configuration of an electronic endoscope including an embodiment of an imaging optical system of the present invention.

FIG. 2 is a diagram showing the above embodiment with the spatial frequency limiting effect of an optical low-pass filter.

FIG. 3 is a diagram showing a state in which the spatial frequency limiting effect of the optical low-pass filter is eliminated in the above embodiment.

FIG. 4 is an enlarged view showing the pixel arrangement of the solid-state image sensor of the above embodiment.

FIG. 5 is a diagram showing the MTF in the horizontal scanning direction of the image in the above embodiment.

FIG. 6 is a diagram showing the diameter of the blur of the imaging light beam on the solid-state imaging device due to the objective lens of the above embodiment.

FIG. 7 is a diagram showing the relationship between the diameter of the circle of confusion as the object distance changes and the spatial frequency limiting effect as a whole when the spatial frequency limiting effect of the optical low-pass filter is switched in the above embodiment.

FIG. 8 is a diagram showing another example of an optical low-pass filter.

FIG. 9 is a diagram showing the point at which the spatial frequency limiting effect of the optical low-pass filter is halved.

FIG. 10 is a diagram showing means for changing other spatial frequency limiting effects.

FIG. 11 is a diagram showing means for changing other spatial frequency limiting effects.

[Explanation of symbols]

1 Objective lens 2 Solid-state image sensor 3, 19 Optical low-pass filter 4
Crystal plates 5, 21 Transparent electrodes 6, 20 Liquid crystal layer 7 Power supply 8 Switch 9 Signal processing circuit 10 TV monitor 11 Detection circuit 12 Control circuit 13 Light source device 14 Light source lamp 15 Variable aperture 16 Condensing lens 17 Light guide 18 Illumination lens 22 Variable Resistor 23 Toric lens 24 Diffraction grating

Claims (1)

[Claims] 1. An objective lens, an imaging means for receiving an image formed by the objective lens, and an optical low-pass filter having a variable spatial frequency limiting effect disposed between the objective lens and the imaging means. a detection means for detecting a focused state of the object to be imaged by the objective lens; and a detection means for receiving a signal from the detection means and limiting the spatial frequency of the optical low-pass filter according to a deviation from the focused state of the object to be imaged. an imaging optical system comprising: a control means for controlling the effect to be weakened;