Patent Description:
Various techniques of extending a depth of field have been proposed (see PTLs <NUM> to <NUM>).

<CIT>
discloses an imaging device for use with an endoscope, the imaging device comprising: a lens arrangement operable to receive light from a scene captured by the endoscope and to form an image of the scene using the received light; an image sensor operable to capture the image of the scene formed by the lens arrangement; a birefringent device positioned along an optical path between the endoscope and the image sensor, wherein the birefringent device comprises birefringent material arranged in a plurality of concentric rings, and wherein the birefringent material of each of the concentric rings is configured such that the polarisation directions of an ordinary ray and an extraordinary ray of light from the scene which travels through the birefringent material are different for at least two of the plurality of concentric rings; and an image processor operable to process the captured image to generate an output image. <CIT> discloses a camera head equipped with a photographing lens attached to an eyepiece of an endoscope for photographing, wherein the photographing lens has a meniscus lens with a concave surface facing at least an object side, and has positive refractive power in order from the object side, a first lens having negative refractive power, a second lens having negative refractive power, a third lens having positive refractive power, and a fourth lens having positive refractive power, and the following conditions (<NUM>) and (<NUM>) are satisfied. (<NUM>) <NUM><L/f<<NUM>; (<NUM>) <NUM><f1/f<<NUM> where L is the axial air conversion length from the first surface of the first lens to the image plane, f is the focal length of the entire photographing lens system, and f1 is the focal length of the first lens. <CIT> discloses an optical system comprising: a lens unit comprising a plurality of lenses, wherein an out-of-focus point spread function of the lens unit includes an intensity distribution with a ring-shaped side peak at a radial distance to a centre point; and a birefringent device in an optical path of the optical system, wherein the birefringent device is adapted to selectively attenuate the ring-shaped side peak in the out-of-focus point spread function of the lens unit. <CIT> discloses a control unit configured to generate a corrected image based on an image of the subject captured and a function for adjusting the blur amount of the image, wherein the function is calculated according to conditions related to observation of the subject. <CIT> discloses an imaging system comprising: an imaging lens unit, an imaging detector, and a birefringent element located between said imaging lens unit and said imaging detector, the system thereby providing in-focus imaging of objects located at both near-field and far-field ranges. <CIT> and <CIT> both disclose a medical image processing apparatus comprising: a control unit configured to perform control so as to generate a corrected image by executing image processing on an image obtained by imaging a subject image of a patient acquired via an optical system including a birefringent mask; and an acquisition unit configured to acquire information regarding an application situation of high-pressure steam sterilization processing on the optical system, wherein the control unit controls the image processing according to the application situation of the high pressure steam sterilization processing.

The invention is defined in the independent claim. Optional embodiments of the invention are described in the dependent claims. Although medical imaging equipment, for example, is demanded to be increased in resolution, the increase in resolution can cause a decrease in a depth of field.

It is desirable to provide an imaging optical system and an imaging apparatus that make it possible to extend a depth of field while suppressing a decrease in resolution performance.

An imaging optical system according to one embodiment of the present disclosure includes: an aperture stop; an image-forming optical system that causes an image to be formed toward an imaging plane of an image sensor; and an optical phase modulator that includes a substance having a birefringence index, and gives two pupil functions to the image-forming optical system. The following conditional expressions are satisfied: <MAT> <MAT> where.

An imaging apparatus according to one embodiment of the present disclosure includes: an imaging optical system; and an image sensor disposed at an image formation position of the imaging optical system. The imaging optical system includes the above imaging optical system according to one embodiment of the present disclosure.

In the imaging optical system or the imaging apparatus according to one embodiment of the present disclosure, while the predetermined conditions are satisfied, the optical phase modulator gives the two pupil functions to the image-forming optical system. Thus, the imaging optical system or the imaging apparatus extends a depth of field while suppressing a decrease in resolution performance.

In the following, description is given of embodiments of the present disclosure in detail with reference to the drawings. It is to be noted that the description is given in the following order.

An increase in resolution is a great advantage for medical imaging equipment. One reason for this is that an increase in precision of surgery is expectable by becoming able to see tissues that have not been visible at typical resolutions, such as fine blood vessels, nerves, and lymph nodes. However, the increase in resolution can cause a decrease in a depth of field. Even a slight shift from a focus position causes blurred appearance. Shallowness of the depth of field thus gives stress to a surgeon. In addition, there is also a possibility of increasing burden of operations, such as increasing frequency of focus corrections. Examples of the medical imaging equipment include a surgical microscope, an endoscope, and a surgical field camera. These pieces of equipment are desired to achieve extension of a depth of field concurrently with an increase in resolution.

In general, a depth of field in an optical system is extendable by simply darkening a F-number. However, a drop in limiting resolution and a drop in an amount of light exert an influence on image quality itself, due to an increase in noise, etc. Hence, there have been demands for development of a depth extension optical system having image quality equivalent to that of a normal optical system not subjected to extension of a depth of field.

PTL <NUM> (<CIT>) proposes a technique of extending a depth of field by imparting spherical aberration to an image-forming optical system. However, this technique assumes a bright F-number as a precondition, and is effective only in a case where the limiting resolution of the image-forming optical system has a sufficient margin with respect to a Nyquist frequency of a sensor. An optical system to be used in medical imaging equipment has resolution performance that is close to a diffraction limit, because the F-number is dark. Causing the spherical aberration impairs the limiting resolution.

The techniques disclosed in PTL <NUM> (<CIT>) and PTL <NUM> (<CIT>) enable depth extension by using a WFC (Wave Front Coding) technique including a filter with a concavo-convex structure for optical phase modulation near an aperture stop of an optical system. However, in consideration of, for example, medical reliability demanding resistance to high-temperature sterilization, such a physical structure raises a concern for aged deterioration and is not suitable. In addition, AR (Anti Reflection) coating or the like is also difficult to apply, which presumably makes it difficult to take measures against flare and ghost.

PTL <NUM> (<CIT>) proposes a phase modulator including a birefringence index substance, and proposes a technique of using this to enable depth extension as compared with a normal optical system. However, PTL <NUM> does not clearly illustrate a specific application range or an optimum design solution.

An imaging optical system according to one embodiment includes an aperture stop, an image-forming optical system, and an optical phase modulator. The image-forming optical system causes an image to be formed toward an imaging plane of an image sensor. The optical phase modulator is an optical device that includes a substance having a birefringence index, and gives two pupil functions to the image-forming optical system, by applying respective different phase modulations to two pieces of polarized light in an orthogonal relationship.

<FIG> schematically illustrates a configuration example of the optical phase modulator in the imaging optical system according to one embodiment of the present disclosure. <FIG> schematically illustrates another configuration example of the optical phase modulator in the imaging optical system according to one embodiment.

The optical phase modulator in the imaging optical system according to one embodiment is, a depth extension device (BM: Birefringent Mask), and has an effect of extending a depth of field of the image-forming optical system. The imaging optical system according to one embodiment is an EDOF (Extended Depth of Focus) optical system having an extended depth of field by being equipped with the BM. The technique of the device itself of the BM is disclosed in PTL <NUM> described above, for example, as well. The technology according to the present disclosure is related to the WFC technique.

The BM is an optical device having no refractive power, and includes an optical device substrate and a birefringent layer formed on a surface of the optical device substrate. For example, like a BM <NUM> illustrated in <FIG>, the BM includes a glass substrate <NUM>, and a BM layer <NUM> serving as a birefringent layer formed on a surface of the glass substrate <NUM>.

In addition, the BM may have a structure in which a birefringent layer is sandwiched between two optical device substrates like a sandwich. For example, like a BM 10A illustrated in <FIG>, a structure in which the BM layer <NUM> serving as a birefringent layer is formed between two glass substrates <NUM> and <NUM> may be used. Using the sandwich-type structure makes it easy to apply AR coating or the like onto the glass substrate <NUM>, for example, in the structure illustrated in <FIG>, which reduces concerns for ghost and flare.

<FIG> schematically illustrates a configuration example of concentric pattern regions of the BM <NUM> in the imaging optical system according to one embodiment.

Unlike other optical phase modulators, the BM <NUM> does not have a structure like a concavo-convex shape for achievement of its effect. The BM layer <NUM> includes concentric pattern regions, as illustrated in <FIG>, and has alignment in which a relative angle of birefringence anisotropy between the adjacent pattern regions is <NUM>°. <FIG> illustrates an example in which, a first concentric pattern region A1, a second concentric pattern region B1, and a third concentric pattern region A2 are formed in order from the center. However, the number of concentric pattern regions is not limited to three, and two or four or more may be formed.

Optical characteristics of the BM <NUM> are described further with reference to <FIG>. <FIG> schematically illustrate the optical characteristics of the optical phase modulator (the BM <NUM>). <FIG> schematically illustrates a configuration example of an imaging optical system according to Comparative Example (a normal optical system <NUM>). <FIG> schematically illustrates a configuration example of the imaging optical system (EDOF optical system) <NUM> according to one embodiment.

The imaging optical system according to Comparative Example illustrated in <FIG> includes an aperture stop St and an image-forming optical system <NUM>. The imaging optical system according to Comparative Example illustrated in <FIG> is the normal optical system <NUM> not including the BM <NUM> as a component.

The imaging optical system <NUM> according to one embodiment illustrated in <FIG> is, as compared with the configuration of the normal optical system <NUM>, the EDOF optical system including the BM <NUM> near the aperture stop St. It is to be noted that, although <FIG> illustrates the configuration example in which the BM <NUM> is disposed on an imaging plane Sip side with respect to the aperture stop St, a configuration in which the BM <NUM> is disposed on an object side with respect to the aperture stop St may also be used. In addition, although <FIG> illustrates the configuration example in which the image-forming optical system <NUM> is disposed on the imaging plane Sip side with respect to the aperture stop St, some optical systems of the image-forming optical system <NUM> may be disposed on the object side with respect to the aperture stop St. An imaging apparatus equipped with such an imaging optical system <NUM> includes an image sensor disposed at a position of image formation by the imaging optical system <NUM>, and an image processor <NUM> that performs, on an image captured by the image sensor, image processing using deconvolution derived from a PSF (Point Spread Function, point spread function) to be described later.

(A) of <FIG> illustrates an example of refractive indices applied to orthogonal two pieces of polarized light (X-polarized light and Y-polarized light) in the BM <NUM>. (B) of <FIG> illustrates an example of phase modulation applied to each of the X-polarized light and the Y-polarized light by the BM <NUM>. (C) of <FIG> illustrates, for each of the X-polarized light and the Y-polarized light, a through-focus MTF (Modulation transfer function) for a plurality of spatial frequencies. (D) of <FIG> illustrates, for each of the normal optical system <NUM> and the imaging optical system (EDOF optical system) <NUM> according to one embodiment, the through-focus MTF for the plurality of spatial frequencies.

For example, assume that, in the BM <NUM>, the first concentric pattern region A1 and the third concentric pattern region A2 have alignment in a Y direction, and the second concentric pattern region B1 has alignment in a X direction, as illustrated in <FIG>. In this case, as illustrated in (A) of <FIG>, in the first concentric pattern region A1 and the third concentric pattern region A2, a refractive index of n is applied to the Y-polarized light, whereas a refractive index of n + Δn is applied to the X-polarized light, because the BM <NUM> has a birefringence index. On the other hand, conversely in the second concentric pattern region B1, a refractive index of n + Δn is applied to the Y-polarized light, and a refractive index of n is applied to the X-polarized light.

This phenomenon causes refractive indices different between the first and third concentric pattern regions A1 and A2 and the second concentric pattern region B1 to be applied to each of the X-polarized light and the Y-polarized light. Therefore, as illustrated in (B) of <FIG>, a phase of transmitted light is shifted between the first and third concentric pattern regions A1 and A2 and the second concentric pattern region B1, in accordance with a distance when the light is transmitted through the BM <NUM> (a thickness of the BM <NUM>). The phase shift (retardation) is expressed by a parameter of
"(retardation) = Δn × (the thickness of the BM)". As a guide, the retardation is roughly about λ/<NUM> with respect to a dominant wavelength λ.

Light that has passed through the BM <NUM> has different wavefronts between the X-polarized light and the Y-polarized light, and is caused to form an image by the image-forming optical system <NUM>. Because of having different wavefronts between the X-polarized light and the Y-polarized light, as illustrated in <FIG>, a light ray Lb of the X-polarized light and a light ray La of the Y-polarized light respectively form images at positions Pb and Pa before and after an image formation position P1 of the normal optical system <NUM>. The through-focus MTF of each of the X-polarized light metethe Y-polarized light is as illustrated in (C) of <FIG>.

Image formation performance of the EDOF optical system as a whole is an average value of the X-polarized light and the Y-polarized light. The through-focus MTF obtained by averaging the through-focus MTF of each of the X-polarized light and the Y-polarized light in the EDOF optical system has a gentler shape than the through-focus MTF of the normal optical system <NUM>, as illustrated in (D) of <FIG>. However, the peak MTF decreases, because the wavefront in the EDOF optical system is not an ideal wavefront. In addition, in the EDOF optical system, the X-polarized light and the Y-polarized light having respective conjugate and different wavefronts form images. Therefore, it is possible to say that the EDOF optical system equipped with the BM <NUM> "have two pupil functions".

Here, assuming that respective wavefront aberrations in a first polarization state (e.g., the X-polarized light) and a second polarization state (e.g., the Y-polarized light) given by the phase modulation of the BM <NUM> are expressed by Ψ1(u, v) and Ψ2(u, v), they are in the following conjugate relationship: <MAT>.

The pupil functions P(u, v) of the EDOF optical system are given as follows in the first polarization state and the second polarization state. <MAT> <MAT>.

u and v are coordinates in the X direction and the Y direction on a pupil, and θ(u, v) is the wavefront aberration of the image-forming optical system <NUM> in a state of not being equipped with the BM <NUM>.

<FIG> illustrates, in comparison, optical characteristics of the imaging optical system according to Comparative Example (the normal optical system <NUM>) and optical characteristics of the imaging optical system (EDOF optical system) <NUM> according to one embodiment. <FIG> illustrates, as the optical characteristics, an example of the PSF and frequency characteristics of the MTF. <FIG> illustrates, in comparison, the through-focus MTF of the imaging optical system according to Comparative Example (the normal optical system <NUM>) and the through-focus MTF of the imaging optical system (EDOF optical system) <NUM> according to one embodiment. (A) of <FIG> illustrates an example of the through-focus MTF of the imaging optical system according to Comparative Example (the normal optical system <NUM>). (B) of <FIG> illustrates an example of the through-focus MTF of the imaging optical system (EDOF optical system) <NUM> according to one embodiment.

As illustrated in <FIG> and <FIG>, the MTF of the EDOF optical system has a lower peak value than that of the normal optical system <NUM>. Therefore, in an imaging apparatus according to one embodiment, it is desirable to create a deconvolution filter (inverse transform filter) for image processing on the basis of the PSF of the EDOF optical system, and perform calculation processing using the deconvolution filter on an image captured via the EDOF optical system in the image processor <NUM> (<FIG>). By performing this processing, it is possible to recover the MTF to the same level as that of the normal optical system <NUM>, as illustrated in (B) of <FIG>. In addition, the BM <NUM> is able to keep the high-frequency MTF at <NUM> or more at a just focus position. Therefore, if deconvolution processing for resolution recovery is applied, it is possible to achieve a limiting resolution comparable to that of the normal optical system <NUM> at the just focus position, as illustrated in (A) and (B) of <FIG>.

Described next are application examples of the imaging optical system <NUM> according to one embodiment to an imaging apparatus. It is to be noted that, in the following, substantially the same portions as the components of the imaging optical system <NUM> according to one embodiment described above are denoted with the same reference numerals, and description thereof is omitted as appropriate.

The imaging optical system <NUM> according to one embodiment is applicable to, for example, an endoscopic camera head of a rigid endoscope or the like, and a microscopic imaging camera unit. In addition, the imaging optical system <NUM> may be used as an optical system for capturing of an image formed by another afocal optical system or a substantially afocal optical system.

<FIG> schematically illustrates Application Example <NUM> of the imaging optical system <NUM> according to one embodiment to an imaging apparatus. <FIG> illustrates a configuration example in which the imaging optical system <NUM> according to one embodiment is applied to an endoscopic camera head <NUM>.

An endoscope <NUM> is, for example, a rigid endoscope or a fiber scope. An eyepiece <NUM> is attached to the endoscope <NUM>.

The endoscopic camera head <NUM> is attached to the eyepiece <NUM>. The endoscopic camera head <NUM> includes the imaging optical system <NUM> and an image sensor <NUM>. An image captured by the image sensor <NUM> is subjected to image processing using deconvolution derived from the point spread function in the image processor <NUM> (<FIG>).

<FIG> schematically illustrates Application Example <NUM> of the imaging optical system <NUM> according to one embodiment to an imaging apparatus. <FIG> illustrates a configuration example in which the imaging optical system <NUM> according to one embodiment is applied to a surgical microscopic imaging camera unit <NUM>.

A surgical microscope includes an eyepiece <NUM>, an image-forming optical system <NUM>, a prism <NUM>, a zoom system <NUM>, and an objective system <NUM>. This surgical microscope enables observation by the naked eye via the eyepiece <NUM>. The surgical microscopic imaging camera unit <NUM> is disposed on, for example, an optical path branched by the prism <NUM>.

The surgical microscopic imaging camera unit <NUM> includes the imaging optical system <NUM> and the image sensor <NUM>. The surgical microscopic imaging camera unit <NUM> is used to image an affected area via the surgical microscope. An image captured by the image sensor <NUM> is subjected to image processing using deconvolution derived from the point spread function in the image processor <NUM> (<FIG>).

Described below is a desirable configuration example of the imaging optical system <NUM> according to one embodiment.

In the imaging optical system <NUM> according to one embodiment, it is desirable to dispose, on the aperture stop St, a surface of the BM <NUM> to which the phase modulation is applied. However, due to various factors such as a measure against ghost and a mechanical structure of the aperture stop St, it is difficult to dispose the BM <NUM> at an ideal position. A position where the BM <NUM> is allowed to be disposed is defined, for example, by a conditional expression (<NUM>), on the basis of an angle of incidence of a light ray on the aperture stop St. <MAT> where.

<FIG> illustrates an overview of the central light rays and the peripheral light rays that pass through the imaging optical system <NUM> according to one embodiment. <FIG> illustrates an overview of the passage range of the central light rays and the passage range of the peripheral light rays in the optical phase modulator (the BM <NUM>) of the imaging optical system <NUM> according to one embodiment.

As illustrated in <FIG> and <FIG>, in the imaging optical system <NUM>, the peripheral light rays eccentrically enter the concentric pattern regions of the BM <NUM>. Therefore, the phase modulation by the BM <NUM> is not concentric for the peripheral light rays, and the MTF greatly changes as compared with the center. In an eccentric direction, the peak value of the MTF decreases at high frequency. Assuming that an allowable value for the decrease in the peak value of the MTF is a ratio between the central MTF and the peripheral MTF of <NUM>% or more, at a spatial frequency at which the central MTF is <NUM>% or more, this allowable value holds by satisfying the conditional expression (<NUM>).

It is desirable that the imaging optical system <NUM> according to one embodiment satisfy the following conditional expression. <MAT> where.

<FIG> illustrates an example of the through-focus MTF for a plurality of spatial frequencies of the imaging optical system <NUM> according to one embodiment. <FIG> illustrates, as a preferable example, the through-focus MTF in a case where the retardation of the BM <NUM> falls within the range of the conditional expression (<NUM>) (Re = λ/<NUM>*<NUM>), and, as an unpreferable example, the through-focus MTF in a case where the retardation falls outside the range of the conditional expression (<NUM>) (Re = λ/<NUM>*<NUM>).

The retardation of the BM <NUM> is optimum within the range of the conditional expression (<NUM>). One reason for this is that, if the retardation exceeds the upper limit of the conditional expression (<NUM>), focus positions of the X-polarized light and the Y-polarized light are separated too much in a front-back direction, and the through-focus MTF has two peaks depending on the spatial frequency, as illustrated in <FIG>. In such a state, the focus position is not definable in the first place, which results in difficulty of focusing by electronic calculation, such as AF (autofocus). In addition, if the retardation falls below the lower limit of the conditional expression (<NUM>), it is difficult to exert a desired effect of the BM <NUM>. The wavelength is about λ = <NUM> in a case of visible light, and λ in the conditional expression (<NUM>) is set to any wavelength at which maximum depth extension is desired.

In a case of applying the endoscopic camera head <NUM> illustrated in <FIG> to a rigid endoscope, for example, it is desirable that the imaging optical system <NUM> be a medium telephoto lens with a focal length of <NUM> or more in <NUM> conversion. Satisfying a conditional expression (<NUM>) corresponds to the imaging optical system <NUM> being such a medium telephoto lens. <MAT> where.

<FIG> illustrates an overview of occurrence of flare and ghost in an imaging optical system 101A according to one embodiment. <FIG> illustrates a configuration example that suppresses occurrence of flare and ghost in an imaging optical system 101B according to one embodiment.

The imaging optical system 101A illustrated in <FIG> illustrates, for example, a configuration example to be applied to the endoscopic camera head <NUM>. On the object side of the imaging optical system 101A, an endoscope cover glass <NUM> and a camera head cover glass <NUM> are disposed. The camera head cover glass <NUM> is disposed to be tilted. In the imaging optical system 101A, an optical filter FL and a seal glass SG are disposed on an optical path between the image-forming optical system <NUM> and the image sensor <NUM>. In the imaging optical system 101A, the flat-shaped BM <NUM> is disposed perpendicular to an optical axis (not disposed to be tilted). On the other hand, in the imaging optical system 101B illustrated in <FIG>, the BM <NUM> is disposed to be tilted. The configuration of the imaging optical system 101B is similar to the imaging optical system 101A, except that the BM <NUM> is disposed to be tilted.

In a case where a flat plate like the BM <NUM> is disposed before the image-forming optical system <NUM>, and an optical device, such as the endoscope cover glass <NUM> and the camera head cover glass <NUM>, is present before or after the BM <NUM>, reflected light Lr reflected off the BM <NUM> and the optical device before or after the BM <NUM> can cause flare and ghost, as illustrated in <FIG>. Particularly in a case where it is not possible to apply AR coating to the BM <NUM>, the reflected light Lr reflected off the BM <NUM> causes flare and ghost, which can result in a decrease in resolution performance, and misidentification of a subject due to an artifact.

As a measure against this, it is desirable to dispose the BM <NUM> with such a tilt that lets the reflected light Lr causing ghost and flare out of an angle of view, as illustrated in <FIG>. In this case, it is desirable that the BM <NUM> be disposed to be tilted to satisfy the following conditional expression. <MAT> where.

In addition, it is desirable that the BM <NUM> be disposed to be tilted in a tilt direction that is opposite to a tilt direction of the optical device, such as the camera head cover glass <NUM>, disposed to be tilted on the object side with respect to the BM <NUM>.

The imaging optical system <NUM> to be applied to an endoscope or a surgical microscope is a telephoto lens in many cases. A lens to be used in these applications is preferably small, and it is desirable that the imaging optical system <NUM> satisfy the following conditional expression. <MAT> where.

In addition, in the imaging optical system <NUM> to be applied to an endoscope or a surgical microscope, it is preferable that, on the imaging plane Sip side with respect to the aperture stop St, a lens group farthest on the object side have positive power, and a lens group having negative power be disposed next. Thus, a telephoto-type configuration is achieved, making it possible to miniaturize the imaging optical system <NUM>.

<FIG> illustrates an example of the passage range of the central light rays and the passage range of the peripheral light rays in the optical phase modulator (the BM <NUM>) in a case where there is vignetting in the imaging optical system <NUM> according to one embodiment. <FIG> illustrates an example of the central light rays and the peripheral light rays in a case where there is vignetting in the imaging optical system <NUM> according to one embodiment.

As illustrated in <FIG>, in a case where an aperture Sta other than the aperture stop St is present in the imaging optical system <NUM>, vignetting (Vignetting) occurs particularly for the peripheral light rays. In this case, as illustrated in <FIG>, the passage range of the peripheral light rays in the BM <NUM> becomes smaller than the passage range of the central light rays, and the passage range of the peripheral light rays loses circular symmetry. To give the effect of the BM <NUM> similarly to the central light rays and the peripheral light rays, it is desirable that there is no vignetting for the peripheral light rays. One reason for this is that, if the phase modulation of the BM <NUM> loses circular symmetry, it is no longer possible to exert intrinsic performance of the BM <NUM>.

Therefore, it is desirable that a structure that blocks a light ray forming an image in an effective image circle of the imaging plane Sip after passing through the aperture stop St not be present between the aperture stop St and the imaging plane Sip. For example, even if the image-forming optical system <NUM> includes an aperture structure for cutting of flare and ghost other than the aperture stop St, it is desirable that a diameter of the aperture structure be sufficiently larger than an optical effective diameter when the light ray forming an image in the effective image circle passes through the aperture structure.

As described above, in the imaging optical system <NUM> and the imaging apparatus according to one embodiment, while the predetermined conditions are satisfied, the BM <NUM> gives the two pupil functions to the image-forming optical system <NUM>. This makes it possible to extend the depth of field while suppressing a decrease in the resolution performance.

The imaging optical system <NUM> according to one embodiment enables optimum depth of field extension with the resolution performance comparable to that of the normal optical system <NUM>. If an appropriate measure against ghost is taken in the imaging optical system <NUM> according to one embodiment, it is possible to suppress a decrease in contrast and an adverse effect of a double image or the like, enabling an improvement in the image quality.

In the imaging optical system <NUM> according to one embodiment, the BM <NUM> does not include a complicated concavo-convex structure, which makes it possible to provide a depth extension optical system with high medical reliability and manufacturability. It is expectable that using the imaging optical system <NUM> according to one embodiment for medical imaging equipment enables efficient surgical operation with high precision.

It is to be noted that the effects described in the present specification are merely examples and not limitative, and other effects may be achieved.

Described next are specific numerical examples of the imaging optical system <NUM> according to one embodiment of the present disclosure. It is to be noted that, in the following, substantially the same portions as the components of the imaging optical system <NUM> according to one embodiment are denoted with the same reference numerals, and description thereof is omitted as appropriate.

Imaging optical systems <NUM> to <NUM> according to Examples <NUM> to <NUM> below each include the aperture stop St, the BM <NUM>, and the image-forming optical system <NUM>, in order from the object side toward the imaging plane Sip side. On an optical path between the image-forming optical system <NUM> and the imaging plane Sip, the optical filter FL and the seal glass SG are disposed.

In each of the imaging optical systems <NUM> and <NUM> according to Examples <NUM> and <NUM>, the BM <NUM> is disposed to be tilted. In the imaging optical system <NUM> according to Example <NUM>, the BM <NUM> is not disposed to be tilted. In each of the imaging optical systems <NUM> to <NUM> according to Examples <NUM> to <NUM>, the BM <NUM> includes the first concentric pattern region A1 and the second concentric pattern region B1.

In each of the imaging optical systems <NUM> to <NUM> according to Examples <NUM> to <NUM>, the image sensor <NUM> is disposed on the imaging plane Sip. An image captured by the image sensor <NUM> is subjected to image processing using an inverse transform filter in the image processor <NUM> (<FIG>).

<FIG> schematically illustrates the concentric pattern regions of the BM <NUM> and the diameter of the aperture stop St in the imaging optical system <NUM> according to Example <NUM>. <FIG> illustrates an overall configuration of the imaging optical system <NUM> according to Example <NUM>.

In the imaging optical system <NUM> according to Example <NUM>, the image-forming optical system <NUM> includes a first lens G1, a second lens G2, a third lens G3, a fourth lens G4, a fifth lens G5, and a sixth lens G6, in order from the object side toward the imaging plane Sip side. The first lens G1 and the second lens G2 are cemented to each other. The fourth lens G4 and the fifth lens G5 are cemented to each other. The cemented lens including the first lens G1 and the second lens G2 is a lens group having positive power. The third lens G3 is a lens group having negative power.

[Table <NUM>] shows basic lens data of the imaging optical system <NUM> according to Example <NUM>. In [Table <NUM>], "Si" indicates a surface number meaning an i-th surface counted from the object side. The surface number is affixed with an attribute of the surface. For example, "G1R1" indicates a lens surface on the object side of the first lens G1, and "G1R2" indicates a lens surface on the imaging plane Sip side of the first lens G1. Similarly, "G2R1" indicates a lens surface on the object side of the second lens G2, and "G2R2" indicates a lens surface on the imaging plane Sip side of the second lens G2. The same applies to other lens surfaces and optical surfaces. "ri" indicates a curvature radius (unit: mm) of the i-th surface counted from the object side. A portion where the value of "ri" is "∞" indicates the aperture stop St, a flat surface, or a virtual surface. "di" indicates an on-axis surface interval (unit: mm) between the i-th surface and an i+<NUM>-th surface counted from the object side. "ndi" indicates a refractive index for the d line (wavelength <NUM>) of a glass material or a material having the i-th surface on the object side. "vdi" indicates an Abbe number for the d line of a glass material or a material having the i-th surface on the object side. The same applies to lens data in other Examples below.

In addition, [Table <NUM>] shows values of a focal length (f) of the whole system, an aperture diameter (D) of the aperture stop St (see <FIG>), an open F-number (Fno), and an image height (IH) in the imaging optical system <NUM> according to Example <NUM>. In addition, [Table <NUM>] shows values of a radius (Ring1) of the first concentric pattern region A1 (see <FIG>), a radius (Ring2) of the second concentric pattern region B1 (see <FIG>), and retardation (Re) of the optical phase modulator (the BM <NUM>).

<FIG> illustrates the through-focus MTF of the imaging optical system <NUM> according to Example <NUM>. <FIG> illustrates the through-focus MTF related to the spatial frequency (<NUM> (Lp/mm)) at which the BM <NUM> exerts the strongest influence. <FIG> illustrates, as the through-focus MTF of the imaging optical system <NUM> according to Example <NUM>, characteristics in a case where image processing has not been performed (with depth extension (without image processing)), and characteristics in a case where the image processing using the inverse transform filter has been performed (with depth extension (with image processing)). In addition, <FIG> illustrates, as Comparative Example, the through-focus MTF of an optical system not including the BM <NUM> (without depth extension).

As illustrated in <FIG>, in the case where image processing has not been performed, the imaging optical system <NUM> according to Example <NUM> exhibits a great decrease in the peak of the through-focus MTF, as compared with the optical system not including the BM <NUM>. However, the use of the inverse transform filter to apply ideal deconvolution processing for recovery of the same resolution as the optical system not including the BM <NUM> has made it possible to return the peak of the through-focus MTF to a peak equivalent to that of the optical system not including the BM <NUM>, and to greatly extend the depth.

<FIG> illustrates the frequency characteristics of the MTF at the focus position of the imaging optical system <NUM> according to Example <NUM>. <FIG> illustrates, with a solid line, characteristics in the case where image processing has not been performed (with depth extension (without image processing)), as the frequency characteristics of the MTF of the imaging optical system <NUM> according to Example <NUM>. In addition, <FIG> illustrates, as Comparative Example, the frequency characteristics of the MTF of the optical system not including the BM <NUM> (without depth extension). In addition, <FIG> illustrates the frequency characteristics of the inverse transform filter to be used to apply the ideal deconvolution processing for recovery of the same resolution as the optical system not including the BM <NUM>. The same applies to the frequency characteristics of the MTF in other Examples below.

As illustrated in <FIG>, in the case where image processing has not been performed, the imaging optical system <NUM> according to Example <NUM> exhibits a decrease in the frequency characteristics of the MTF, as compared with the optical system not including the BM <NUM>. However, the use of the inverse transform filter to apply the ideal deconvolution processing for recovery of the same resolution as the optical system not including the BM <NUM> has made it possible to return the frequency characteristics of the MTF to frequency characteristics substantially equivalent to those of the optical system not including the BM <NUM>.

The imaging optical system <NUM> according to Example <NUM> has substantially the same configuration as the imaging optical system <NUM> according to Example <NUM>, but the aperture diameter (D) of the aperture stop St is varied from that in the imaging optical system <NUM> according to Example <NUM>. The imaging optical system <NUM> according to Example <NUM> indicates that changing the aperture diameter of the aperture stop St result in a change in the effect of the depth extension.

[Table <NUM>] shows basic lens data of the imaging optical system <NUM> according to Example <NUM>. In addition, [Table <NUM>] shows values of a focal length (f) of the whole system, an aperture diameter (D) of the aperture stop St (see <FIG>), an open F-number (Fno), and an image height (IH) in the imaging optical system <NUM> according to Example <NUM>. In addition, [Table <NUM>] shows values of a radius (Ring1) of the first concentric pattern region A1 (see <FIG>), a radius (Ring2) of the second concentric pattern region B1 (see <FIG>), and retardation (Re) of the optical phase modulator (the BM <NUM>).

<FIG> illustrates the through-focus MTF of the imaging optical system <NUM> according to Example <NUM>. <FIG> illustrates the through-focus MTF related to the spatial frequency (<NUM> (Lp/mm)) at which the BM <NUM> exerts the strongest influence. <FIG> illustrates, as the through-focus MTF of the imaging optical system <NUM> according to Example <NUM>, characteristics in a case where image processing has not been performed (with depth extension (without image processing)), and characteristics in a case where the image processing using the inverse transform filter has been performed (with depth extension (with image processing)). In addition, <FIG> illustrates, as Comparative Example, the through-focus MTF of an optical system not including the BM <NUM> (without depth extension). As illustrated in <FIG>, in the case where image processing has not been performed, the imaging optical system <NUM> according to Example <NUM> exhibits a great decrease in the peak of the through-focus MTF, as compared with the optical system not including the BM <NUM>. However, the use of the inverse transform filter to apply ideal deconvolution processing for recovery of the same resolution as the optical system not including the BM <NUM> has made it possible to return the peak of the through-focus MTF to a peak equivalent to that of the optical system not including the BM <NUM>, and to greatly extend the depth.

<FIG> illustrates the frequency characteristics of the MTF at the focus position of the imaging optical system <NUM> according to Example <NUM>. As illustrated in <FIG>, in the case where image processing has not been performed, the imaging optical system <NUM> according to Example <NUM> exhibits a decrease in the frequency characteristics of the MTF, as compared with the optical system not including the BM <NUM>. However, the use of the inverse transform filter to apply the ideal deconvolution processing for recovery of the same resolution as the optical system not including the BM <NUM> has made it possible to return the frequency characteristics of the MTF to frequency characteristics substantially equivalent to those of the optical system not including the BM <NUM>.

<FIG> schematically illustrates the concentric pattern regions of the optical phase modulator (the BM <NUM>) and the diameter of the aperture stop St in the imaging optical system <NUM> according to Example <NUM>.

<FIG> illustrates a configuration of the imaging optical system <NUM> according to Example <NUM>.

In the imaging optical system <NUM> according to Example <NUM>, the image-forming optical system <NUM> includes a first lens G1, a second lens G2, a third lens G3, a fourth lens G4, and a fifth lens G5, in order from the object side toward the imaging plane Sip side. Each lens is a single lens. The first lens G1 is a lens group having positive power. The second lens G2 is a lens group having negative power.

[Table <NUM>] shows basic lens data of the imaging optical system <NUM> according to Example <NUM>. In addition, [Table <NUM>] shows values of a focal length (f) of the whole system, an aperture diameter (D) of the aperture stop St (see <FIG>), an open F-number (Fno), and an image height (IH) in the imaging optical system <NUM> according to Example <NUM>. In addition, [Table <NUM>] shows values of a radius (Ring1) of the first concentric pattern region (see <FIG>), a radius (Ring2) of the second concentric pattern region (see <FIG>), and retardation (Re) of the optical phase modulator (the BM <NUM>).

[Table <NUM>] summarizes, for each of Examples, values related to the conditional expressions given above. It is apparent from [Table <NUM>] that Examples <NUM> and <NUM> satisfy the conditional expressions. Example <NUM> satisfy the conditional expressions except for the conditional expression (<NUM>).

The technology according to the present disclosure is not limited to the description of the embodiments and Examples described above, and various modifications may be made.

For example, the shapes and numerical values of the respective parts described above in Examples are mere examples of the implementation of the present technology, and the technical scope of the present technology should not be construed as being limited by these examples. While the predetermined conditions are satisfied, the optical phase modulator gives the two pupil functions to the image-forming optical system. This makes it possible to extend the depth of field while suppressing a decrease in the resolution performance.

Claim 1:
An imaging optical system (<NUM>, 101A, 101B) comprising:
an aperture stop;
an image-forming optical system (<NUM>, <NUM>) configured to cause an image to be formed toward an imaging plane of an image sensor (<NUM>); and
an optical phase modulator that includes a substance having a birefringence index, and gives two pupil functions to the image-forming optical system (<NUM>, <NUM>), characterized in that the following conditional expressions are satisfied: <MAT> <MAT> where
L: a distance between the aperture stop and the optical phase modulator;
D: an aperture diameter of the aperture stop;
w: a maximum angle of incidence of a principal light ray that enters the aperture stop;
λ: a wavelength of light; and
Re: phase retardation caused by birefringence of the optical phase modulator.