JP2010048997A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multifocal lens having higher performance. <P>SOLUTION: The imaging lens includes: a diffractive optical element 1 in which a diffraction lattice is formed whose cross-section is made into a rectangular rugged shape by a first optical member and a second optical member alternately arranged so as to be a concentric shape with an optical axis as the center on a transparent substrate; and a refracting optical element 2. The first optical member and the second optical member are a combination in which their refractive indexes and Abbe's numbers are respectively different to incident light in a prescribed wavelength band, and also the value of ¾n<SB>1</SB>(λ)-n<SB>2</SB>(λ)¾×d/λ is made substantially equal in the above frequency band. Further, the refracting optical element 2, with diffraction light including at least a pair of one-dimensional or more of high order diffraction light which is emitted from the diffraction light element 1 as element incident light, gives refracted light obtained by refracting the refracting light serving as the element incident light in such a manner that the focal length of the +L-dimensional refraction light being the highest order diffraction light on the light condensation side is made longest and the focal length of the -L-dimensional diffraction light being the highest order diffraction light on the emission side is made shortest. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an imaging lens that expresses two or more focal points with respect to incident light in a predetermined wavelength band (hereinafter referred to as a multifocal lens).

  In digital imaging devices such as digital cameras and surveillance cameras that use imaging devices such as CCDs and CMOS sensors, objects and landscapes at multiple distances, such as infinitely distant images and near images, can be used as subjects. Image processing is performed on the captured image.

  However, if a single lens is used for a subject in a distant landscape and a subject in a near subject, it is difficult to obtain a high-resolution image for each.

  However, for example, when a bifocal lens having two focal lengths is used, it is possible to obtain high-resolution images in each. As an example of the bifocal lens, for example, there is a multifocal lens using a diffraction action described in Patent Document 1.

  Further, as prior art documents related to a diffractive optical element using a diffractive action, for example, there are Patent Document 2, Patent Document 3 and Patent Document 4.

JP 2006-139246 A JP-A-9-127321 Japanese Patent Laid-Open No. 11-04808 JP 2002-72082 A

  However, since the multifocal lens described in Patent Document 1 is configured as a single lens in which lens portions having different focal lengths are arranged concentrically and repeatedly as a single lens, when the lens is used, There is a problem that light transmitted through the annular zones having different focal lengths becomes stray light at the other focal length.

  The invention relating to the diffractive optical element described in Patent Document 2, Patent Document 3 and Patent Document 4, for example, is intended to increase the diffraction efficiency of a light beam having a desired order (or design order), that is, As is clear from the fact that light beams having diffraction orders other than the desired order (or design order) are regarded as having an adverse effect on the optical system, there is no idea of utilizing the diffractive action for multiple focal points. .

  In particular, what is required for a multifocal lens of an imaging system requires a function having a certain diffraction efficiency at least for light in the visible light region. This is because a light (image) signal having an equivalent light quantity level to the incident wavelength light (color information) needs to be given to the signal processing system at the subsequent stage.

  Moreover, since it is inconvenient in performing an image processing signal that the level of the light amount differs for each focus, it is preferable that the light amount level is the same even at different focal points.

  Therefore, an object of the present invention is to provide a multi-focal lens with higher performance. Specifically, it is an imaging lens that develops two or more focal points, and an object is to provide an imaging lens that can use at least light transmitted through an effective region of the imaging lens as an effective light beam at each focal point. And It is another object of the present invention to provide an imaging lens capable of providing an optical signal having an equivalent light amount level at each focal point to incident signal having a wavelength in the visible light region to a subsequent signal processing system. It is another object of the present invention to provide an imaging lens that can also improve chromatic aberration, which is a problem when a short focal length is expressed by a diffractive lens.

An imaging lens according to the present invention is an imaging lens that develops two or more focal points with respect to incident light in a predetermined wavelength band, a diffractive optical element that expresses diffracted light, and a refractive optical element that expresses refracted light. The diffractive optical element has a concavo-convex shape with a rectangular cross section by a first optical member and a second optical member that are alternately arranged concentrically around the optical axis on a transparent substrate. A diffraction grating is formed, and the first optical member and the second optical member have different refractive indexes and Abbe numbers with respect to incident light in the wavelength band, and the first optical member has a different refractive index. The refractive index for an arbitrary wavelength λ included in the wavelength band is n ₁ (λ), the refractive index for the arbitrary wavelength λ included in the wavelength band of the second optical member is n ₂ (λ), and When the thickness of the diffraction grating is d _{| N 1 (λ) -n 2} (λ) | · the value of d / lambda is substantially equal in the wavelength range, the refractive optical element is at least one set of primary or emitted from the diffractive optical element When the diffracted light including higher-order diffracted light is element incident light and the highest-order diffracted light among the diffracted lights is ± L-order diffracted light, the + L-order diffracted light on the collecting side has the longest focal length and diverges. Refracted light obtained by refracting the diffracted light as the element incident light is expressed so that the focal length of the -L order diffracted light on the side becomes the shortest.

  The diffractive optical element is a diffraction grating that expresses 0th-order diffracted light that is transmitted in a straight line and a set of first-order or higher-order diffracted light with respect to incident light in a predetermined wavelength band. A diffraction grating having substantially the same diffraction efficiency in the predetermined wavelength band may be formed.

  The diffractive optical element is a diffraction grating that expresses ± first-order diffracted light with respect to incident light in a predetermined wavelength band, and the diffraction efficiency of the ± first-order diffracted light is substantially in the predetermined wavelength band. A diffraction grating equal to may be formed.

  The diffractive optical element is a diffraction grating that expresses 0th-order diffracted light and ± 1st-order diffracted light that are transmitted in a straight line with respect to incident light in the predetermined wavelength band, and includes the 0th-order diffracted light and the ± 1st order diffracted light. A diffraction grating in which the diffraction efficiency of the folded light is substantially equal in the predetermined wavelength band may be formed.

  The predetermined wavelength band may be in a range of 430 to 660 nm.

  Further, either one of the first optical member or the second optical member may be constituted by an optical multilayer film in which two or more kinds of materials are laminated in parallel on a transparent substrate.

Also, the optical multilayer film includes a Si0 ₂ film and _{the Ta} 2 _{O 5} film may be stacked alternately.

  In addition, either the first optical member or the second optical member may be composed of a naphthalene compound or a fluorene compound.

  According to the present invention, diffracted light including at least one set of first-order or higher-order diffracted light using the characteristics of a binary type diffractive lens including a diffraction grating having a rectangular cross section, and diffracting each diffracted light Since the diffracted light having substantially the same efficiency is generated from the diffractive optical element and the generated diffracted light is refracted by the refractive optical element, the light transmitted through the effective region of the lens is effective at each focal point. In addition to being able to be used as a light beam, it is possible to provide an imaging lens that exhibits multifocality in which the amount of diffracted light connecting each focal point is substantially equal while reducing chromatic aberration. Note that “substantially equal in terms of light quantity” means a case where the diffraction efficiency of each diffracted light or the difference in light quantity of diffracted light at an arbitrary wavelength λ is within 20%.

  FIG. 1 is an explanatory view showing an example of a multifocal lens according to the present invention. The multifocal lens 100 shown in FIG. 1 is configured by combining a diffractive optical element 1 that expresses diffracted light and a refractive optical element 2 that expresses refracted light.

  Further, FIG. 2 shows a configuration example of the diffractive optical element 1. FIG. 2A is a schematic plan view of the diffractive optical element 1. FIG. 2B is a schematic cross-sectional view (A-A ′ cross-sectional view) of the diffractive optical element 1. The diffractive optical element 1 shown in FIG. 2 is formed by disposing at least a first optical member 11 and a second optical member 12 on a transparent substrate 13 alternately in a concentric manner with the optical axis as a center. .

  As the transparent substrate 13, a substrate having a flat surface such as glass or plastic is used.

  As shown in FIG. 2B, the first optical member 11 has a rectangular cross-sectional shape, and forms a convex portion on the transparent substrate 13. Hereinafter, the convex portion formed by the first optical member 11 may be referred to as the convex portion 11. In the example shown in FIG. 2, the convex portions 11B, 12C, and 12E whose planar shapes are ring-shaped are arranged concentrically around the convex portion 11A having a circular planar shape. The first optical member 11 may be, for example, an optical multilayer film composed of a plurality of layers formed in parallel with the substrate surface of the transparent substrate 13. The material, type, and number of layers of the film are selected so as to obtain a desired diffraction efficiency.

  The second optical member 12 is at least a transparent substrate by a filler having an Abbe number and a refractive index different from those of the first optical member 11 (here, limited to a refractive index for light incident in a desired wavelength band). The first optical member 11 on the first optical member 11 is formed so as to fill the concave portion, that is, the concave portion, thereby forming the concave portion on the transparent substrate 13. Hereinafter, the recess formed by the second optical member 12 may be referred to as the recess 12. FIG. 2 shows an example in which not only the concave portions between the convex portions but also the portions above the convex portions are filled. For example, an enethiol-based material is used as the filler. Further, when the first optical member 11 and the second optical member 12 are formed with the same thickness, either may be referred to as a convex portion or a concave portion, and either one is a convex portion and the other is a concave portion. do it. That is, when the thickness of the convex portion is the same as the thickness of the concave portion, the first optical member 11 can constitute a concave portion, and the second optical member 12 can constitute the convex portion.

  Although omitted in the example shown in FIG. 2, the surface opposite to the surface on which the transparent substrate 13 of the uneven layer formed by the first optical member 11 and the second optical member 12 is located (on the filler side). A cover having a flat surface made of glass or resin may be provided on the surface).

  The diffractive optical element 1 expresses diffracted light by causing the concavo-convex layer formed by the first optical member 11 and the second optical member 12 to act as a diffraction grating. As in this embodiment, the diffractive optical element 1 including an uneven diffraction grating having a rectangular cross-sectional shape is generally called a binary diffractive lens.

  The diffraction efficiency in one direction (for example, + 1st order diffracted light) of the binary type diffractive lens has a characteristic that it is smaller than the diffraction efficiency of the Fresnel type diffractive lens. This is because the diffracted light is in the other direction (for example, -1st order diffracted light). ) Is also emitted by a constant light quantity. The present invention realizes a multifocal lens by using the diffracted light emitted in a plurality of directions.

  In the diffractive lens, the diffraction efficiency and the diffraction angle change depending on the phase difference and pitch interval (grating period) of light generated by the unevenness which is a diffraction grating. In the binary type, the diffraction efficiencies of the + k order diffracted light and the −k order diffracted light at least in the high order diffracted light (± k order diffracted light (k is a natural number of 1 or more)) are the same.

The multifocal lens of the present invention includes the diffractive optical element 1 as a binary type diffractive lens, so that at least one set of diffracted light (± n-order diffracted light (n is an integer of 1 or more)) whose light amount is not substantially zero. ) Is expressed. In the binary type diffractive lens, the diffraction efficiency at each wavelength is determined by the difference (Δn) between the effective refractive index n of the convex portion and the concave portion. In the present invention, the following conditions are determined with respect to the refractive indexes of the convex portions and concave portions of the diffractive optical element 1. That is, with respect to an arbitrary wavelength λ included in a desired wavelength band, the thickness of the diffraction grating (here, the thickness of the convex portion 11 (strictly speaking, the height of the convex portion of the concave and convex portions)). When d, the condition is that | n ₁ (λ) −n ₂ (λ) | · d / λ is substantially equal. Here, n ₁ (λ) represents the refractive index of the convex portion with respect to the wavelength λ, and n ₂ (λ) represents the refractive index of the concave portion with respect to the wavelength λ. Further, the term “substantially equal” as used herein refers to a case where the variation of this value is within 0.2. Further, “substantially zero” regarding the amount of light means a range where the diffraction efficiency is 5% or less. In addition, regarding the combination of the optical members constituting the convex portion and the concave portion, the Abbe number and the refractive index of one optical member (here, limited to the refractive index with respect to light incident in a desired wavelength band) are the other optical member. It is assumed that the value is higher than that.

  By the way, the refractive index n has a property (wavelength dispersion characteristic) that varies depending on the wavelength λ. For this reason, it is preferable to devise the material or the like of the diffractive optical element 1 so that the ratio of the wavelength to the difference in refractive index (Δn) between the convex portion and the concave portion is constant in a desired wavelength band. The diffractive optical element 1 adjusted so that the ratio of the wavelength with respect to the difference (Δn) in the visible light wavelength band 440 nm to 670 nm is constant is presented in the examples described later (see Examples 1 to 3).

  The refractive optical element 2 may function as a convex lens (refractive lens). More specifically, it is only necessary to be able to emit refracted light that is refracted so that the diffracted light emitted from the diffractive optical element 1 has two or more focal points on the optical axis. That is, if the highest order diffracted light emitted from the diffractive optical element 1 is ± L order diffracted light, the refractive optical element 2 has the longest focal length of −L order diffracted light and the focus of + L order diffracted light. Each diffracted light may be refracted so that the distance becomes the shortest. In the present invention, one set of high-order diffracted light (± k-order diffracted light (k is a natural number of 1 or more)) that is collected toward the optical axis is called + k-order diffracted light, and the other (divergent one). Is called -k order diffracted light.

  FIG. 3 shows an example of image formation by the multifocal lens 100 in which the diffractive optical element 1 and the refractive optical element 2 are combined. In FIG. 3, when collimated light is incident, the diffractive optical element 1 develops ± first-order diffracted light, and the refractive optical element 2 refracts the ± first-order diffracted light so that two focal points are formed on the optical axis. Is shown. In the example shown in FIG. 3, since only the first-order diffracted light is expressed, the focal length of the −1st-order diffracted light is the longest, and the focal length of the + 1st-order diffracted light is the shortest.

  The multifocal lens 100 may be assembled in the imaging system so that the diffractive optical element 1 and the refractive optical element 2 maintain a predetermined positional relationship. Further, as shown in FIG. 4, the multifocal lens 100 may further include an aperture 3. In the example shown in FIG. 4, an example in which the diaphragm 3 is provided between the diffractive optical element 1 and the refractive optical element 2 is shown, but the present invention is not limited to this. For example, the diffractive optical element 1 and the refractive optical element 2 can be configured such that the order in which light is incident is reversed. The multifocal lens 100 may further include a fixing member (not shown) that fixes the diffractive optical element 1 and the refractive optical element 2 in a predetermined positional relationship.

  Hereinafter, a more specific configuration example of the diffractive optical element 1 in the multifocal lens 100 of the present invention will be described using examples. Hereinafter, as in the example shown in FIG. 2, the first optical member 11 will be described as a convex portion, and the second optical member 12 will be described as a concave portion.

Example 1.
The first embodiment is an example in which the bifocal diffractive optical element 1 is realized. FIG. 5 is an explanatory diagram showing an example of diffracted light emitted from the diffractive optical element 1 of the present embodiment. In the example shown in FIG. 5, it is shown that the diffractive optical element 1 expresses + 1st order diffracted light and −1st order diffracted light with substantially the same diffraction efficiency with respect to incident light (parallel light). Note that 0th-order diffracted light (straight forward transmitted light) is not expressed.

As shown in FIG. 5, when the + 1st order diffracted light and the −1st order diffracted light are to be expressed with substantially the same diffraction efficiency, the concavo-convex layer (the convex portion 11 and the concave portion 12) so as to satisfy the following expression (1). May be formed. In the formula (1), n ₁ (λ) represents the refractive index of the convex portion 11 with respect to the wavelength λ as described above. n ₂ (λ) indicates the refractive index of the concave portion 12 with respect to the wavelength λ as described above. d shows the thickness of the convex part 11 (refer FIG. 6). In addition, m should just be an integer greater than or equal to 0.

{N ₁ (λ) −n ₂ (λ)} × d = (2m + 1) λ / 2 Formula (1)

That is, what is necessary is just to design the refractive index of the convex part 11 and the recessed part 12, and the thickness of the convex part 11 so that the wavelength of emitted light may shift | deviate a half wavelength with the convex part 11 and the recessed part 12. FIG. However, as described above, the refractive index n has a property (wavelength dispersion characteristic) that varies depending on the wavelength λ. Therefore, in this embodiment, the refractive index n ₂ (λ) of the filler used is refracted. The ratio n ₁ (λ) and the Abbe number are adjusted so that the ratio of the wavelength to the refractive index difference (Δn) is constant in the desired wavelength band.

FIG. 7 is an explanatory view showing a design example of the material of each layer constituting the convex portion 11 (optical multilayer film) of this embodiment. In the design example shown in FIG. 7, an optical multilayer film in which 20 layers of 41 nm thick SiO ₂ films and 198 nm thick Ta ₂ O ₅ films are alternately stacked on one surface of a quartz glass substrate 11 is formed. This is an example. For example, each layer is sequentially formed with the material and film thickness shown in FIG. 7 to form an optical multilayer film. Then, the formed optical multilayer film is processed into an uneven shape spreading concentrically as shown in FIG. Each layer may be formed using, for example, a vacuum deposition method or a sputtering method. The processing of the optical multilayer film into the concavo-convex shape may be performed, for example, by photolithography and etching. The lens diameter in this embodiment is about 10 mm. The plate thickness of the quartz glass substrate 11 is 0.5 mm. The refractive index at a wavelength of 589nm of Si0 ₂ layers and _Ta 2 _{O 5} layer is 1.47 and 2.19 respectively, the Abbe number is respectively 58 and 21.

  The convex portion 11 is filled with an enthiol-based material having a refractive index of 1.71 at a wavelength of 586 nm and an Abbe number of 34 as a filler so as to fill at least the space between the convex portions 11. Then, the bifocal diffractive optical element 1 is manufactured by bonding the filler to the other quartz glass substrate (cover) using the filler as an adhesive. In the bifocal diffractive optical element 1 manufactured in this way, the + 1st order diffracted light and the −1st order diffracted light are expressed with respect to the incident parallel light, and the focal lengths of the diffracted lights are about +1000 mm and −1000 mm, respectively. It becomes. The + 1st order diffracted light functions as the output light of the convex lens, and the −1st order diffracted light functions as the output light of the concave lens.

  In FIG. 8, the calculated value of the effective refractive index with respect to each wavelength of the convex part 11 and the concave part 12 in the diffractive optical element 1 (bifocal diffractive optical element 1) of a present Example is shown. As shown in FIG. 8, in the diffractive optical element 1 of the present example, the difference in refractive index between the convex part 11 (multilayer optical film) and the concave part 12 (filler) in the visible light wavelength band 440 nm to 670 nm is relative to the wavelength. You can see that they are proportional.

  FIG. 9 shows the calculated values of diffraction efficiency for each wavelength of the diffractive optical element 1 (bifocal diffractive optical element 1) of this example. As shown in FIG. 9, in the diffractive optical element 1 of the present embodiment, it can be seen that the diffraction efficiencies of the + 1st order diffracted light and the −1st order diffracted light are substantially equal in the visible light wavelength band of 440 nm to 670 nm. . Further, the diffraction efficiency of each of the converging + 1st order diffracted light and the divergent −1st order diffracted light is about 40%, and the 0th order diffracted light that travels straight does not express almost any amount of light (the 0th order diffractive efficiency is substantially zero I understand).

  The diffracted light (+ 1st order diffracted light and −1st order diffracted light) expressed from such a diffractive optical element 1 is refracted by the refractive optical element 2 as shown in FIG. A multifocal lens can be realized.

Example 2
Next, a second embodiment of the present invention will be described. The present embodiment is an example for realizing the trifocal diffractive optical element 1. FIG. 10 is an explanatory diagram showing an example of diffracted light emitted from the diffractive optical element 1 of the present embodiment. In the example shown in FIG. 10, it is shown that the diffractive optical element 1 expresses + 1st order diffracted light, −1st order diffracted light, and 0th order diffracted light with substantially the same diffraction efficiency with respect to incident light (parallel light). Has been.

  As shown in FIG. 10, in order to express the + 1st order diffracted light, the −1st order diffracted light, and the 0th order diffracted light with substantially the same diffraction efficiency, for example, as shown in the design example shown in FIG. An optical multilayer film) may be configured. FIG. 11 is an explanatory view showing a design example of the material of each layer constituting the convex portion 11 (optical multilayer film) of the present embodiment. The design example shown in FIG. 11 is an example in the case of using an enethiol-based material having a refractive index of 1.65 at a wavelength of 589 nm and an Abbe number of 31 as a filler.

In the design example shown in FIG. 11, an optical multilayer film in which 20 layers of a Ta ₂ O ₅ film having a thickness of 34 nm and a SiO ₂ film having a thickness of 205 nm are alternately stacked is formed on one surface of the quartz glass substrate 11. This is an example.

  FIG. 12 shows the calculated values of diffraction efficiency for each wavelength of the diffractive optical element 1 (trifocal diffractive optical element 1) manufactured according to the design example shown in FIG. As shown in FIG. 12, in the diffractive optical element 1 of the present embodiment, the diffraction efficiencies of the + 1st order diffracted light, the −1st order diffracted light, and the 0th order diffracted light that appear in the visible light wavelength band of 440 nm to 670 nm are substantially equal. I understand that. Also, it can be seen that the diffraction efficiencies of the converging + 1st order diffracted light, the diverging -1st order diffracted light, and the straight forward 0th order diffracted light are about 30%.

  FIG. 13 shows an example of image formation by the multifocal lens 100 in which the diffractive optical element 1 and the refractive optical element 2 of this embodiment are combined. As shown in FIG. 13, since the diffracted light (+ 1st order diffracted light, −1st order diffracted light, and 0th order diffracted light) expressed from the diffractive optical element 1 of the present embodiment is refracted by the refractive optical element 2, substantially. By obtaining equal diffraction efficiency, it is possible to realize a multifocal lens having three focal points with substantially equal amounts of diffracted light.

Example 3 FIG.
Next, a third embodiment of the present invention will be described. In this embodiment, the trifocal diffractive optical element 1 is realized by a convex portion that is not an optical multilayer film. In this embodiment, a naphthalene-based or fluorene-based material is used as the member of the convex portion 11 (first optical member 11). More specifically, a material obtained by adding acrylic (naphthalene-6-yl acrylate) to naphthalene on one surface of a quartz glass substrate 11 having a thickness of 0.5 mm is spin-coated to a thickness of 6.3 μm. The film is formed. This material has a refractive index of 1.56 and an Abbe number of 49 at a wavelength of 589 nm. As the filler, an enthiol-based material having a refractive index of 1.53 at a wavelength of 589 nm and an Abbe number of 29 may be used.

  In FIG. 14, the calculated value of the diffraction efficiency with respect to each wavelength of the diffractive optical element 1 (trifocal diffractive optical element 1) of the present embodiment is shown. As shown in FIG. 14, in the diffractive optical element 1 of the present embodiment, the difference in effective refraction between the convex portion 11 and the concave portion 12 at each wavelength is proportional to the wavelength. It can be seen that the diffraction efficiencies of the zero-order diffracted light and the zero-order diffracted light are substantially equal in the visible light wavelength band of 440 nm to 670 nm. Also, it can be seen that the diffraction efficiencies of the converging + 1st order diffracted light, the diverging -1st order diffracted light, and the straight forward 0th order diffracted light are about 30%.

  Thus, a multifocal lens having three focal points with substantially the same amount of light can be realized without using a multilayer optical film as an optical member combined with a filler.

  Note that a plurality of sets of diffracted light can be expressed by adjusting the phase difference and pitch interval (grating period) of light generated by the convex portion 11 and the concave portion 12 functioning as a diffraction grating. By expressing a plurality of sets of diffracted light, a multifocal lens with three or more focal points can be realized.

  By the way, in each said Example, in order to implement | achieve a multifocal lens using the characteristic of a binary type diffractive lens, the example of the diffractive optical element 1 which makes diffraction efficiency constant within the range of a desired wavelength was shown, The chromatic dispersion characteristic seen in a diffractive lens changes the focal length depending on the wavelength, and also appears as chromatic aberration. In particular, chromatic aberration becomes noticeable when trying to achieve a short distance focus.

  In order to reduce this chromatic aberration, the present invention recommends multifocalization by a combination of a diffractive lens and a refractive lens. This is because, if a diffractive lens and a refractive lens are combined to realize multifocalization, it is possible to give superiority in terms of chromatic aberration as compared with the case where multifocalization is realized using only a diffractive lens.

For example, when an attempt realize f = 100 cm as the focal length of the imaging lens, and the diffractive optical element 1 having a focal distance _f 1 = 200 cm, by combining the refractive optical element 2 having a focal length _f 2 = 200 cm It is feasible. If the diffractive optical element 1 and the refracting optical element 2 are used in combination as in the configuration of the present invention, the chromatic aberration can be realized less than when a diffractive lens having a focal length f = 100 cm is used. It means you can do it.

  The present invention provides, for example, an optical lens mounted on a digital camera device, an information processing terminal, or the like that captures a normal subject at a standard distance and also captures a close subject at a distance closer to the normal subject. It can be applied to an imaging system.

It is explanatory drawing which shows the structural example of the multifocal lens 100 by this invention. 2 is an explanatory diagram illustrating a configuration example of a diffractive optical element 1. FIG. It is explanatory drawing which shows the example of the image formation by the multifocal lens 100 which combined the diffractive optical element 1 and the refractive optical element 2. FIG. It is explanatory drawing which shows the other structural example of the multifocal lens. It is explanatory drawing which shows the example of the diffracted light radiate | emitted from the diffractive optical element 1 of a 1st Example. It is explanatory drawing which shows the relationship between the refractive index of the convex part 11 and the recessed part 12, and the height of the convex part 11 in the diffractive optical element 1 of a 1st Example. It is explanatory drawing which shows the example of a design of the material of each layer which comprises the convex part 11 (optical multilayer film) in the diffractive optical element 1 of a 1st Example. It is a graph which shows the wavelength dispersion characteristic of the refractive index of the convex part 11 and the recessed part 12 in the diffractive optical element 1 of a 1st Example. It is a graph which shows the diffraction efficiency with respect to each wavelength of the diffractive optical element 1 of a 1st Example. It is explanatory drawing which shows the example of the diffracted light radiate | emitted from the diffractive optical element 1 of a 2nd Example. It is explanatory drawing which shows the example of a design of the material of each layer which comprises the convex part 11 (optical multilayer film) in the diffractive optical element 1 of a 2nd Example. It is a graph which shows the diffraction efficiency with respect to each wavelength of the diffractive optical element 1 of a 2nd Example. The example of the image formation by the multifocal lens 100 which combined the diffractive optical element 1 and the refractive optical element 2 of a 2nd Example is shown. It is a graph which shows the diffraction efficiency with respect to each wavelength of the diffractive optical element 1 of a 3rd Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Imaging lens 1 Diffractive optical element 11 1st optical member (convex part)
12 Second optical member (concave)
13 Transparent substrate 2 Refractive optical element 3 Aperture

Claims (8)

An imaging lens that develops two or more focal points for incident light in a predetermined wavelength band,
A diffractive optical element that expresses diffracted light; and a refractive optical element that expresses refracted light;
The diffractive optical element is
A diffraction grating having a concavo-convex shape with a rectangular cross section is formed by the first optical member and the second optical member alternately arranged concentrically around the optical axis on the transparent substrate,
The first optical member and the second optical member have different refractive indices and Abbe numbers for incident light in the wavelength band, and are included in the wavelength band of the first optical member. N ₁ (λ), the refractive index for any wavelength λ included in the wavelength band of the second optical member is n ₂ (λ), and the thickness of the diffraction grating is When d, the value of | n ₁ (λ) −n ₂ (λ) | · d / λ is substantially equal in the wavelength band,
The refractive optical element uses at least one set of first-order or higher-order diffracted light emitted from the diffractive optical element as element incident light, and the highest-order diffracted light among the diffracted lights is ± L next time. If it is a folded light, the refracted light that refracts the diffracted light as the element incident light so that the focal length of the + L order diffracted light on the collecting side is the longest and the focal length of the −L order diffracted light on the diverging side is the shortest. An imaging lens characterized by that. The diffractive optical element is a diffraction grating that expresses zero-order diffracted light that is transmitted in a straight line and a set of first-order or higher-order diffracted light with respect to incident light in a predetermined wavelength band, and each diffraction efficiency is The imaging lens according to claim 1, wherein substantially equal diffraction gratings are formed in the predetermined wavelength band. The diffractive optical element is a diffraction grating that expresses ± first-order diffracted light with respect to incident light in a predetermined wavelength band, and diffraction efficiency of the ± first-order diffracted light is substantially equal in the predetermined wavelength band. The imaging lens according to claim 1, wherein a diffraction grating is formed. The diffractive optical element is a diffraction grating that expresses 0th-order diffracted light and ± 1st-order diffracted light that are transmitted in a straight line with respect to incident light in a predetermined wavelength band, and includes the 0th-order diffracted light and the ± 1st-order diffracted light. The imaging lens according to claim 2, wherein diffraction gratings having diffraction efficiency substantially equal in the predetermined wavelength band are formed. The imaging lens according to any one of claims 1 to 4, wherein the predetermined wavelength band is in a range of 430 to 660 nm. Either one of said 1st optical member or said 2nd optical member is comprised by the optical multilayer film by which two or more types of materials are laminated | stacked in parallel with a transparent substrate. The imaging lens according to any one of the above. The optical multilayer film, Si0 ₂ film and the Ta ₂ O ₅ film and the imaging lens according to claim 6 which are alternately laminated. 6. The imaging lens according to claim 1, wherein either the first optical member or the second optical member is made of a naphthalene compound or a fluorene compound. .

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