JP2018045249A - Optical element and optical system having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical element capable of reducing wavelength dependence of the transmissivity and changes in reflectance due to changes of film thickness in an absorber layer.SOLUTION: An absorber layer 13 is formed over a substrate 11. The transmissivity of the absorber layer 13 varies depending on the position of the substrate 11. The absorber layer 13 also contains a first material and a second material each of which the absorption coefficient having predetermined wavelength dependence. The extinction coefficient of the absorber layer 13 is 0.5 or less in a range of wavelength 400-700nm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical element used in an optical system such as a digital camera.

  As one of optical elements used in an optical system such as a digital camera, a gradation type ND (Neutral Density) filter having a transmittance distribution in an optical surface is known. By using a gradation-type ND filter (hereinafter referred to as “GND filter”), it is possible to arbitrarily control the brightness of the image and to improve the variation in the sharpness of the outline of the out-of-focus image (blurred image).

  As a characteristic required for the GND filter, the wavelength dependency of the transmittance is small.

  Patent Document 1 describes a GND filter in which the wavelength dependency of transmittance is reduced by forming absorption layers made of different metals on the front surface and the back surface of a substrate, respectively.

JP 2009-288294 A

  However, in the case of the GND filter described in Patent Document 1, since a metal film having a large extinction coefficient is used as the absorption layer, if the transmittance distribution is formed by changing the film thickness of the absorption layer, the GND filter The reflectance changes significantly depending on the position. When the change in reflectivity is large, it is difficult to realize a low reflectivity over the entire GND filter, and there is a problem in that unnecessary light that causes flare and ghosts is generated.

  An object of the present invention is to provide an optical element that can reduce both the wavelength dependence of the transmittance and the reflectance change due to the change in the transmittance of the absorption layer.

  The optical element of the present invention is an optical element comprising a substrate and an absorption layer having different transmittance according to the position on the substrate, and the absorption layer has an absorption coefficient at a wavelength of 700 nm. And a second material having an absorption coefficient at a wavelength of 400 nm greater than that at a wavelength of 700 nm, and the extinction coefficient of the absorption layer is 0. 005 or more and 0.5 or less.

  According to another aspect of the present invention, there is provided an optical element including a substrate and an absorption layer having different transmittance according to a position on the substrate, wherein the absorption layer includes the first material and the second material. The first material is titanium oxide having an extinction coefficient of 0.005 to 0.5 at a wavelength of 400 nm to 700 nm, and the second material is extinguished at a wavelength of 400 nm to 700 nm. Niobium oxide or tantalum oxide having an extinction coefficient of 0.005 or more and 0.5 or less.

  Another optical element of the present invention is an optical element comprising a substrate and an absorption layer having different transmittance depending on the position on the substrate, and the absorption layer has an absorption coefficient at a wavelength of 400 nm and a wavelength of 700 nm. And a second material having an absorption coefficient at a wavelength of 400 nm greater than the absorption coefficient at a wavelength of 700 nm, and the extinction coefficient of the absorption layer is It is characterized by being 0.5 or less at a wavelength of 400 nm to 700 nm.

  According to another aspect of the present invention, there is provided an optical element including a substrate and an absorption layer having different transmittance according to a position on the substrate, wherein the absorption layer includes the first material and the second material. The first material is titanium oxide having an extinction coefficient of 0.5 or less at a wavelength of 400 nm to 700 nm, and the second material is extinguished at a wavelength of 400 nm to 700 nm. It is characterized by being niobium oxide or tantalum oxide having an attenuation coefficient of 0.5 or less.

  Another optical element of the present invention includes a substrate, a first absorption layer that includes a first material that absorbs part of incident light and has an absorption coefficient at a wavelength of 400 nm that is smaller than the absorption coefficient at a wavelength of 700 nm, and incident light. A second absorption layer including a second material that absorbs a part of the first material and has a second absorption material having an absorption coefficient at a wavelength of 400 nm that is larger than the absorption coefficient at a wavelength of 700 nm, wherein the first absorption layer and the first absorption layer The transmittance of at least one of the two absorption layers differs depending on the position on the substrate, and the extinction coefficients of the first absorption layer and the second absorption layer are both 0 at wavelengths of 400 nm to 700 nm. 0.005 or more and 0.5 or less.

  Another optical element of the present invention includes a substrate and a first absorption layer that includes a titanium oxide that absorbs part of incident light and has an extinction coefficient of 0.005 to 0.5 at a wavelength of 400 nm to 700 nm. And a second absorption layer containing niobium oxide or tantalum oxide that absorbs part of incident light and has an extinction coefficient of 0.005 to 0.5 at a wavelength of 400 nm to 700 nm. The transmittance of at least one of the first absorption layer and the second absorption layer varies depending on the position on the substrate.

  ADVANTAGE OF THE INVENTION According to this invention, the optical element which can reduce both the wavelength dependence of the transmittance | permeability by the change of the transmittance | permeability of an absorption layer, and the change of a reflectance is realizable.

3 is a schematic diagram of a GND filter according to Embodiment 1. FIG. 6 is a diagram illustrating a film thickness distribution of an absorption layer of a GND filter according to Example 1. FIG. It is a figure which shows the wavelength dependence of the reflectance of the GND filter of Example 1, and the transmittance | permeability. 6 is a schematic diagram of a GND filter according to Embodiment 2. FIG. 7 is a film thickness distribution of the absorption layer of the GND filter of Example 2. FIG. It is a figure which shows the wavelength dependence of the reflectance of the GND filter of Example 2, and the transmittance | permeability. 6 is a schematic diagram of a GND filter according to Embodiment 3. FIG. 10 is a film thickness distribution of an absorption layer and a phase compensation layer of a GND filter of Example 3. It is a figure which shows the phase shift of the transmitted wave front of the GND filter of Example 3. FIG. It is a figure which shows the wavelength dependence of the reflectance of the GND filter of Example 3, and the transmittance | permeability. 10 is a schematic diagram of a GND filter according to Embodiment 4. FIG. 4 is a film thickness distribution of an absorption layer and a phase compensation layer of a GND filter of Example 4. It is a figure which shows the phase shift of the transmitted wave front of the GND filter of Example 4. It is a figure which shows the wavelength dependence of the reflectance of the GND filter of Example 4, and the transmittance | permeability. It is a figure which shows an example of the wavelength dependence of an extinction coefficient. It is a figure which shows an example of the transmittance | permeability distribution. It is the schematic of the GND filter of the modification 1. It is the schematic of the GND filter of the modification 2. 1 is a cross-sectional view of an optical system and a schematic view of an imaging apparatus. It is a figure which shows the example of the relationship between a film thickness and the transmittance | permeability. It is the schematic of the GND filter which simplified the GND filter of FIG. FIG. 22 is an example of an admittance trajectory diagram in the GND filter of FIG. 21. FIG. 22 is an example of an admittance trajectory diagram in the GND filter of FIG. 21.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In each figure, the same parts are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a schematic diagram showing a GND filter 10 as an optical element of the present invention. The GND filter 10 of the present embodiment includes a substrate 11 and an absorption layer 13 that absorbs part of incident light.

  In the present embodiment, the absorption layer 13 includes a first film 131 made of a first material and a second film 132 made of a second material. Both the first material and the second material have an extinction coefficient of 0.5 or less.

  The film thickness of the absorption layer 13 changes in an in-plane direction that is perpendicular to the surface normal of the substrate 11. Thereby, the absorption layer 13 has different transmittances depending on the position on the substrate.

  Further, as shown in FIG. 1, a surface layer 14 may be further provided. In this case, the absorption layer 13 is provided between the substrate 11 and the surface layer 14.

  In addition, as shown in FIG. 1, an intermediate layer 12 may be further provided between the absorption layer 13 and the substrate 11. In addition, the same layer as these may be laminated | stacked on the back surface 11a of the board | substrate 11, and an antireflection film may be provided (not shown).

[About Absorbing Layer]
In general, in the GND filter, since the wavelength dependency of the transmittance changes due to the change in the transmittance of the absorbing layer, it is difficult to reduce the wavelength dependency of the transmittance regardless of the transmittance of the absorbing layer. In addition, since the reflectance is different in each region where the transmittance of the absorbing layer is different, it is difficult to keep the reflectance low regardless of the transmittance of the absorbing layer. However, the GND filter 10 can reduce both the change in the wavelength dependency of the transmittance and the change in the reflectance due to the change in the transmittance of the absorption layer by satisfying the formulas (1) and (3) described later.

First, equation (1) will be described. In order to reduce the change in the wavelength dependency of the transmittance due to the change in the transmittance of the absorption layer 13, the GND filter 10 of the present embodiment uses the absorption coefficient α ₁ (λ) of the first material and the second material. Both absorption coefficients α ₂ (λ) satisfy the following conditional expression (1).
α ₁ (400) <α ₁ (700), α ₂ (400)> α ₂ (700) (1)

  Here, the numerical value in the parenthesis in the above formula (1) represents the wavelength of light in nanometers (nm). That is, equation (1) indicates that the first material has an absorption coefficient at a wavelength of 400 nm that is smaller than the absorption coefficient at a wavelength of 700 nm, and the second material has an absorption coefficient at a wavelength of 400 nm that is greater than the absorption coefficient at a wavelength of 700 nm. Represents. The absorption coefficient α (λ) is a value given by α (λ) = 4πk (λ) / λ, where k (λ) is an extinction coefficient depending on the wavelength.

  Here, the reason why the change in the wavelength dependency of the transmittance due to the change in the transmittance of the absorption layer 13 can be reduced by satisfying the formula (1) will be described.

  As an example of the relationship between the wavelength dependence of the transmittance and the film thickness, consider the case where light is incident on a single thin film made of titanium oxide having an absorption coefficient as shown in FIG. FIG. 20 shows the change in transmittance when the thickness of a single thin film made of titanium oxide is 10 nm, 100 nm, and 300 nm.

FIG. 20 shows that the transmittance decreases as the film thickness increases. This is because when the intensity of incident light is I ₀ and the film thickness of a film having an absorption coefficient α (λ) is t, the intensity I of transmitted light transmitted through the absorption film is expressed by the following equation (2): Because it is given by.
I = I ₀ · exp (−α (λ) · t) (2)

  Further, it can be seen from FIG. 20 that as the transmittance decreases, the transmittance changes greatly depending on the wavelength of incident light. Such a change in the wavelength dependency of the transmittance changes exponentially according to the wavelength dependency of α (λ), as can be seen from the equation (2). This means that when the film thickness of the absorption layer 13 changes, the wavelength dependency of the transmittance of the absorption layer 13 changes according to the wavelength dependency of the absorption coefficient of the material forming the absorption layer 13.

  Therefore, in order to reduce the change in the wavelength dependency of the transmittance due to the change in the transmittance of the absorption layer 13 in the GND filter 10, the wavelength dependency of α (λ) may be reduced.

  Since the first material satisfies the above-described formula (1), it absorbs more light on the long wavelength side than light on the short wavelength side. In addition, since the second material satisfies the above-described formula (1), it absorbs more light on the short wavelength side than light on the long wavelength side. In such a first material and a second material, in at least part of the wavelength from 400 nm to 700 nm, one absorption coefficient increases with respect to the wavelength, and the other absorption coefficient decreases with respect to the wavelength. (Hereinafter referred to as a different wavelength band).

  In such a wavelength band with the opposite sign, the absorption coefficient of the first material and the absorption coefficient of the second material have a relationship in which the wavelength dependence cancels each other. Therefore, the wavelength dependence of the absorption coefficient of the absorption layer 13 can be reduced as a whole, and a flat absorption coefficient can be obtained.

  In addition, even if both the first material and the second material satisfy the formula (1), the wavelength band in which the absorption coefficient increases or decreases in the wavelength range from 400 nm to 700 nm (hereinafter, the wavelength with the same sign). (Referred to as a band).

  However, the absorption coefficient of the absorption layer 13 in this embodiment is obtained by adding the absorption coefficients of the first material and the second material according to the film thickness ratio of the first material and the second material in the absorption layer. Has a value. Therefore, the wavelength dependency of the absorption coefficient of the absorption layer 13 in the wavelength band of the same sign is larger than the one having the larger wavelength dependency of the absorption coefficient in the wavelength band of the first material and the second material. There is no.

  Therefore, satisfying equation (1) makes it possible to reduce the wavelength dependency of the absorption coefficient of the absorption layer 13, and as a result, reduces the change in the wavelength dependency of the transmittance due to the change in the transmittance of the absorption layer 13. can do.

Next, equation (3) will be described. The extinction coefficient of the absorption layer 13 satisfies the following conditional expression at wavelengths of 400 nm to 700 nm.
0 <k ≦ 0.5 (3)

  Since the absorption layer 13 absorbs at least part of the incident light, it has an extinction coefficient greater than zero. In addition, the smaller the extinction coefficient of the absorption layer 13, the smaller the change in reflectance due to the change in the transmittance of the absorption layer 13. Equation (3) finds an appropriate range of k in view of the above.

  Here, regarding the reason why the change in reflectance due to the change in the transmittance of the absorption layer 13 can be reduced as the extinction coefficient of the absorption layer 13 is smaller, the GND filter having a configuration in which the absorption layer of the GND filter 10 is simplified. This will be described in detail by taking 10a as an example.

  FIG. 21 shows a schematic diagram of the GND filter 10a. In the GND filter 10a, an intermediate layer 2, an absorption layer 3, and a surface layer 4 are formed in order from the substrate 1. The intermediate layer 2 is composed of two layers of films 21 and 22. The absorption layer 3 has a film thickness distribution and is formed of a film made of a single material having a predetermined extinction coefficient. The surface layer 4 is composed of three layers of films 41, 42 and 43. That is, unlike the GND filter 10, the GND filter 10a has an absorption layer formed of a single material. Therefore, although the GND filter 10a is strictly different from the GND filter 10, the gist of the following description is not affected.

  22 and 23 are admittance trajectory diagrams in the GND filter 10a.

Admittance is a value represented by the ratio between the magnetic field strength and the electric field strength in the medium, and the refractive index of the medium is numerically equivalent to the admittance when the admittance Y _{0 in} free space is used as a unit. In the following description, admittance η is treated with Y ₀ as a unit. Further, the admittance trajectory diagram is a diagram expressing film characteristics using the concept of equivalent admittance. Equivalent admittance refers to admittance when the entire system in which a thin film is added on a substrate is replaced with a single substrate having equivalent characteristics.

  The details of the equivalent admittance and the admittance trajectory are described in the document “Mr. Lee Masanaka, translated by ULVAC, Inc.,“ Optical thin film and film formation technology ””.

  FIGS. 22A, 22B, and 22C show admittance trajectory diagrams in the GND filter 10a when the extinction coefficient of the absorption layer 3 is 0.218. 22A, 22 </ b> B, and 22 </ b> C show the case where light is incident from the air side at a position where the transmittance is 100%, 79%, and 10% in such a GND filter 10 a. It is an admittance trajectory map. In the GND filter 10a, the transmittance is lower as the absorption layer 3 is thicker. That is, FIG. 22A shows an admittance trajectory at a position where the thickness of the absorption layer 3 is 0, and the thickness of the absorption layer 3 increases from FIG. 22B to FIG. 22C.

First, how to read the diagram will be described using the admittance trajectory diagram of FIG. 22A as an example. In FIG. 22, the horizontal axis represents the real part Re (η) of the admittance η, the vertical axis represents the imaginary part Im (η) of the admittance, the x mark in the figure represents the admittance of the substrate 1, and the ◯ mark represents the air admittance. Represents each. When the refractive index of the substrate 1 is N _sub , the admittance η _sub = N _{sub of the} substrate 1 is obtained. On the other hand, when there is light absorption, the admittance at that time is equivalent to the complex refractive index N-ik. Here, N is a refractive index and k is an extinction coefficient.

  The trajectory in FIG. 22A shows a change in equivalent admittance when the films 21, 22, 3, 41, 42, and 43 are sequentially formed on the substrate 1. The end point of the trajectory when the film 43 (final layer) is formed represents the final equivalent admittance, and the Fresnel coefficient and the reflectance can be calculated from this equivalent admittance and air admittance (= 1). . When the equivalent admittance is equal to the admittance of air, the reflectance is zero.

  FIG. 22A shows the case where the film thickness of the absorption layer 3 is 0. However, when the film thickness of the absorption layer 3 increases and the transmittance decreases, the admittance trajectory becomes as shown in FIGS. ). When the film thickness of the absorption layer 3 is sufficiently thick, the equivalent admittance from the substrate 1 to the absorption layer 3 is substantially equal to the complex refractive index of the absorption layer 3 when light is incident from the air side.

  When the film thickness of the absorption layer 3 shown in FIG. 22A is 0 and the film thickness of the absorption layer 3 shown in FIG. 22C is sufficiently thick, the equivalent admittance has an extinction coefficient k = It has changed by an amount corresponding to 0.218.

  Thus, when the extinction coefficient of the absorption layer 3 is 0.218, when light enters the GND filter 10a from the air side, the equivalent admittance from the substrate 1 to the absorption layer 3 is as shown in FIGS. It changes within the range of c).

  Next, FIG. 23 shows an admittance trajectory diagram in the GND filter 10a when the extinction coefficient of the absorption layer 3 is 0.5. FIGS. 23A, 23B, and 23C show the case where light is incident from the air side at a position where the transmittance is 100%, 79%, and 10% in such a GND filter 10a. It is an admittance trajectory map. As shown in FIG. 23C, when the thickness of the absorption layer 3 is sufficiently thick, the equivalent admittance from the substrate 1 to the absorption layer 3 is substantially equal to the complex refractive index of the absorption layer 3.

  Comparing the case where the film thickness of the absorption layer 3 shown in FIG. 23 (a) is 0 and the case where the film thickness of the absorption layer 3 shown in FIG. 23 (c) is sufficiently thick, the equivalent admittance has an extinction coefficient k = It has changed by an amount corresponding to 0.5. Comparing FIG. 22 and FIG. 23 with respect to the change in equivalent admittance from the substrate 1 to the absorption layer 3 due to the change in the film thickness of the absorption layer 3, it can be seen that the change shown in each figure of FIG. 22 is smaller. . This is because the extinction coefficient of the absorption layer 3 is smaller in the case shown in FIG. 22 than in the case shown in FIG.

  When the change in equivalent admittance from the substrate 1 to the absorption layer 3 due to the change in the film thickness of the absorption layer 3 is small, the final change in equivalent admittance when the film is formed up to the film 43 (final layer) is also small. On the contrary, when the extinction coefficient of the absorption layer 3 becomes larger than 0.5, the change of the equivalent admittance becomes larger than that shown in FIG. A small change in equivalent admittance from the substrate 1 to the absorption layer 3 due to a change in the thickness of the absorption layer 3 means that a change in reflectance due to a change in the thickness of the absorption layer 3 is also small. Therefore, as the extinction coefficient of the absorption layer 3 is smaller, the change in reflectance due to the change in the thickness of the absorption layer 3 can be reduced.

  The same applies to the case where the absorption layer 13 includes the first material and the second material as in the GND filter 10. If the extinction coefficient of the absorption layer 13 is small, the change in the transmittance of the absorption layer 13 is changed. It is possible to reduce the change in reflectivity due to. Therefore, by satisfying Expression (3), the change in reflectance due to the change in the transmittance of the absorption layer 13 in the GND filter 10 can be reduced.

  In the present embodiment, the absorption layer 13 is formed of the first film 131 made of the first material and the second film 132 made of the second material. At this time, the extinction coefficient of the absorption layer 13 changes at the interface between the first film 131 and the second film 132, and both the first film 131 and the second film 132 have the above-described characteristics. If the expression (3) is satisfied, the absorption layer 13 satisfies the expression (3).

  Specifically, the extinction coefficient of the absorption layer 13 in the present embodiment is equal to the extinction coefficient of the first material in the first film 131 and the extinction coefficient of the second material in the second film 132. Equal to the coefficient. In this case, both the extinction coefficient of the first material and the extinction coefficient of the second material may be within the range of the expression (3).

  As described above, the GND filter 10 satisfies the above expressions (1) and (3). Thereby, both the wavelength dependency of the transmittance and the change of the reflectance due to the change of the transmittance of the absorption layer 13 can be reduced.

In addition, the film thickness of the absorption layer 13 can be made thin, so that the extinction coefficient of the absorption layer 13 is large. Thereby, when forming the absorption layer 13 by vapor deposition or sputtering, the film-forming time can be shortened. Furthermore, the thinner the film thickness of the absorption layer 13, the less the phase shift of the transmitted wavefront caused by the absorption layer 13. Therefore, the range of the formula (3) is preferably set to the range of the following formula (3a).
0.005 ≦ k ≦ 0.5 (3a)

More preferably, the range of the formula (3) is set to the range of the following formula (3b).
0.05 ≦ k ≦ 0.4 (3b)

In the GND filter 10 of the present embodiment, the absorption layer 13 includes the first film 131 and the second film 132. Each film constituting the absorption layer 13 has the following conditional expression (4). It is preferable to satisfy.
| ΔN _abs | <0.25 (4)

However, ΔN _abs is a refractive index difference with respect to light having a wavelength of 550 nm between adjacent films among the films constituting the absorption layer 13. By satisfy | filling Formula (4), the refractive index difference in the interface of the films | membranes which comprise the absorption layer 13 can be made small, and the change of a reflectance can be reduced more.

Moreover, it is preferable that each film | membrane which comprises the absorption layer 13 satisfy | fills the following conditional expression (5).
| Δk _abs | <0.2 (5)

However, Δk _abs is an extinction coefficient difference with respect to light having a wavelength of 550 nm between adjacent films among the films constituting the absorption layer 13. By satisfy | filling Formula (5), the extinction coefficient difference in the interface of the films | membranes which comprise the absorption layer 13 can be made small, and the change of a reflectance can be reduced more.

Furthermore, in order to further reduce the change in the wavelength dependence of the transmittance due to the change in the thickness of the absorption layer 13, the absorption coefficient α ₁ (λ) of the first material and the absorption coefficient α ₂ (λ of the _second material). The film thicknesses of the first film 131 and the second film 132 are preferably adjusted in accordance with the wavelength dependency of Therefore, when the film thickness of the first film 131 at the position where the film thickness of the absorption layer 13 is maximum is t ₁ and the film thickness of the second film 132 is t ₂ , the GND filter 10 has the following conditional expression: Meet.
−1.5 ≦ (a ₁ / a ₂ ) · (t ₁ / t ₂ ) ≦ −0.7 (6)

However, a ₁ is a coefficient of λ when the absorption coefficient α ₁ (λ) of the _first material is linearly approximated with respect to the wavelength λ by the least square method in the wavelength band of the opposite sign. Further, a ₂ is a coefficient of λ when the absorption coefficient α ₂ (λ) of the _second material is linearly approximated with respect to the wavelength λ by the least square method in the wavelength band of the opposite sign. That is, a ₁ is the slope of the approximate line of α ₁ (λ) in the wavelength band of the different sign, and a ₂ is the slope of the approximate line of α ₂ (λ) in the wavelength band of the different sign.

When the first film 131 and the second film 132 satisfy the expression (6), it is possible to increase the effect of canceling the wavelength dependence of the mutual absorption coefficient in the wavelength band with the opposite sign. Therefore, the change in the wavelength dependency of the transmittance due to the change in the thickness of the absorption layer 13 can be further reduced. More preferably, the formula (6) should be in the following range.
−1.1 ≦ (a ₁ / a ₂ ) · (t ₁ / t ₂ ) ≦ −0.9 (6a)

  Further, by making the ratio of the film thickness of the first film 131 and the second film 132 constant regardless of the film thickness of the absorption layer 13, a change in the wavelength dependency of the transmittance due to the film thickness change of the absorption layer 13 is achieved. Can be further reduced.

When forming the first film 131 and the second film 132 with the film thickness ratio as described above, it is preferable that the first material and the second material satisfy the following conditional expression.
−10 <a ₁ / a ₂ <−0.1 (7)

  By satisfying Expression (7), the film thickness ratio between the first film 131 and the second film 132 does not become extremely large, and the first film 131 and the second film 132 are formed by vapor deposition or the like. In this case, the film thickness can be easily adjusted.

  Next, specific materials that can be used as the first material and the second material will be described. FIG. 15A shows the wavelength dependence of the extinction coefficient of each material of titanium oxide, niobium oxide, and tantalum oxide, and FIG. 15B shows the wavelength dependence of the absorption coefficient.

  As shown in FIG. 15B, it can be seen that the formula (1) is satisfied by selecting titanium oxide as the first material and selecting niobium oxide or tantalum oxide as the second material.

  In this case, the entire wavelength band from a wavelength of 400 nm to 700 nm corresponds to a wavelength band having a different sign. 15A that the extinction coefficient of each material of titanium oxide, niobium oxide, and tantalum oxide is 0.5 or less at a wavelength of 400 to 700 nm. That is, by forming the absorption layer from these materials, the extinction coefficient of the absorption layer satisfies the formula (3). However, the first material and the second material are not limited to the above, and a material satisfying the formula (1) may be selected as appropriate so that the extinction coefficient of the absorption layer 13 satisfies the formula (3). .

  Note that in the case where titanium oxide is used as the first material, the first film 131 is preferably disposed on the absorption layer 13 on the side closest to the substrate 11. Titanium oxide tends to change its oxidation state under high temperature, high humidity, or ultraviolet irradiation. However, by disposing the first film 131 made of titanium oxide on the absorption layer 13 on the side closest to the substrate 11, the film made of titanium oxide can be protected by another film. Changes in the oxidation state can be suppressed.

Furthermore, when the first material is titanium oxide, it is preferable to satisfy the following condition instead of equation (7) in order to suppress changes in transmittance and reflectance due to changes in the oxidation state.
−10 <a ₁ / a ₂ ≦ −1 (7a)

  By satisfying Expression (7a), the film thickness of the first film 131 can be made thinner than the film thickness of the second film 132. Thereby, the change of the reflectance and the transmittance | permeability by the change of the oxidation state of a titanium oxide can be suppressed.

  In the present embodiment, the example in which the absorption layer 13 has a two-layer structure including the first film 131 and the second film 132 is shown, but the present invention is not limited to this. The absorption layer 13 only needs to contain the first material and the second material satisfying the expressions (1) and (3). For example, the absorption layer 13 may further include another film. Moreover, it is preferable that each film | membrane which forms the absorption layer 13 satisfy | fills Formula (4) and (5). Furthermore, by designing the film thickness ratio of each film according to the wavelength dependence of the absorption coefficient of each film forming the absorption layer 13, the change in the wavelength dependence of the transmittance due to the change in the film thickness of the absorption layer 13 can be further improved. Can be reduced.

[Surface layer and intermediate layer]
Next, the surface layer 14 and the intermediate layer 12 will be described.

  In general, in an optical element having an absorption layer such as a GND filter, the reflectance is different between when light is incident from the air side and when light is incident from the substrate side. This is because when there is an absorption layer, the Fresnel coefficient at each interface of the optical element differs depending on the incident direction of light. The GND filter 10 includes an intermediate layer 12 and a surface layer 14 in order to reduce the reflectance for both the case where light enters from the air side and the case where light enters from the substrate side.

  The surface layer 14 mainly has a function of reducing the reflectance between the absorption layer 13 and air. The surface layer 14 is composed of a single or a plurality of thin films. By increasing the number of thin films constituting the surface layer 14, the refractive index can be adjusted, the antireflection band can be expanded, the incident angle dependency can be reduced, and the polarization dependency can be reduced. The surface layer 14 is not limited to a uniform laminated film, and a layer containing hollow fine particles may be provided, or a structure having a fine uneven structure on the surface may be provided.

  Further, the intermediate layer 12 mainly has a function of reducing the reflectance between the substrate 11 and the absorption layer 13. The intermediate layer 12 is composed of a single or a plurality of thin films. By increasing the number of thin films constituting the intermediate layer 12, the refractive index can be adjusted, the antireflection band can be expanded, the incident angle dependency can be reduced, and the polarization dependency can be reduced.

  Since the GND filter 10 satisfies the above equation (3), the change in reflectance due to the change in the transmittance of the absorption layer 13 is small. Therefore, when light is incident from the air side, the reflectance can be reduced regardless of the transmittance of the absorption layer 13 while having a simple configuration in which the surface layer 14 having a uniform film thickness is provided. Similarly, when light is incident from the substrate side, the reflectance can be reduced regardless of the transmittance of the absorption layer 13, although the structure is simply provided with the intermediate layer 12 having a uniform film thickness.

The surface layer 14 is preferably is configured to include a film having a refractive index N ₁₄ satisfies the following conditional expression (8).
1 <N ₁₄ <N _{abs, sur} (8)

However, N _{abs, sur} is the refractive index of the film closest to the surface layer 14 among the films constituting the absorption layer 13. By satisfying Expression (8), it is possible to further reduce the change in equivalent admittance from the air to the absorption layer 13 when light is incident from the substrate side. As a result, the change in reflectance due to the change in the thickness of the absorption layer 13 can be further reduced.

The intermediate layer 12 is preferably is configured to include a film having a refractive index N ₁₂ satisfies the following conditional expression (9) or (9a).
N _sub <N ₁₂ <N _{abs, int} (where N _sub <N _{abs, int} ) (9)
N _sub > N ₁₂ > N _{abs, int} (where N _sub > N _{abs, int} ) (9a)

Here, N _{abs, int} is the refractive index of the film constituting the absorption layer 13 that is closest to the intermediate layer 12, and N _sub is the refractive index of the substrate 11. That is, satisfying the formula (9) or (9a) indicates that the intermediate layer 12 includes a film having a refractive index between N _{abs, int} and N _sub . By satisfying Expression (9) or (9a), the change in equivalent admittance from the substrate 11 to the absorption layer 13 when light is incident from the air side can be further reduced. As a result, the change in reflectance due to the change in transmittance of the absorption layer 13 can be further reduced.

[Phase difference of transmitted wave front due to film thickness distribution of absorbing layer]
The GND filter 10 of the present embodiment has an absorption layer 13 having a film thickness distribution. Therefore, it is conceivable that a phase shift corresponding to the film thickness of the absorption layer 13 occurs in the transmitted wavefront of the light transmitted through the absorption layer 13. In order to compensate for such a phase shift of the transmitted wavefront, phase compensation layers 32 and 43 having a film thickness distribution may be further provided as in the GND filter 30 shown in FIG. 7 or the GND filter 40 shown in FIG. good.

  FIG. 8 illustrates the film thickness distribution of the absorption layer 34 and the phase compensation layer 32 in the GND filter 30 having the phase compensation layer 32, where the vertical axis is the film thickness and the horizontal axis is normalized by the radius of the GND filter 30. Indicates the position in the plane. As shown in FIG. 8, the thickness of the absorption layer 34 increases from the center of the GND filter 30 toward the periphery, while the thickness of the phase compensation layer 32 changes so as to decrease from the center toward the periphery. ing. That is, the film thickness of the phase compensation layer 32 increases in the opposite direction to the direction in which the film thickness of the absorption layer 34 increases.

In order to satisfactorily correct the phase shift of the transmitted wavefront, it is preferable to change the thickness of the phase compensation layer 32 so as to satisfy the following conditions.
| ΔOPD / λ | ≦ 0.30 (10)

  Here, λ is the wavelength of light, and ΔOPD is the difference between the optical path length at the position where the thickness of the absorption layer 34 is the smallest and the optical path length at the position where the thickness of the absorption layer 34 is thicker than that position. That is, ΔOPD is an optical path length difference based on the position where the absorption layer 34 is thinnest. In the GND filter 30, the difference between the optical path length at the position indicated by the normalized radius 0 (the central portion of the GND filter 30) and the optical path length at other positions corresponds to ΔOPD. The optical path length referred to here is an amount defined by the sum of the product of the refractive index and the physical film thickness of each layer stacked on the substrate 31.

  By satisfying Expression (10), ΔOPD due to the film thickness distribution of the absorption layer 32 can be compensated as shown in FIG.

  Further, since the complex refractive indexes of the absorption layer and the phase compensation layer are different, it is difficult to match the admittance of the phase compensation layer and the admittance of the absorption layer regardless of the film thickness of the absorption layer. Therefore, the reflectivity also changes depending on the change in the thickness of the phase compensation layer.

  When the phase compensation layer is arranged closer to the substrate than the absorption layer, the change in reflectivity due to the change in the thickness of the phase compensation layer and the absorption layer is less due to the light coming from the substrate side than when light is incident from the air side. When incident, it tends to increase. On the other hand, when the absorption layer is arranged closer to the substrate than the phase compensation layer, the change in reflectance due to the change in film thickness of the phase compensation layer and the absorption layer is less from the air side than when light is incident from the substrate side. When light enters, it tends to increase.

  Thus, the tendency of the reflectance change due to the change in the film thickness of the absorption layer and the phase compensation layer differs depending on the position where the phase compensation layer is disposed and the light incident direction. In general, it is easier to reduce the reflectance when incident on the substrate side than when reducing the reflectance when incident on the air side. Accordingly, in order to reduce the reflectivity when light is incident from the air side and the reflectivity when light is incident from the substrate side, it is preferable to dispose the phase compensation layer between the substrate and the absorption layer. .

In addition, when the phase compensation layer 32 is disposed at a position adjacent to the substrate 31 as in the GND filter 30, it is preferable that the following conditional expression is satisfied.
| N _sub −N _cmp | <0.10 (11)

N _sub is the refractive index of the substrate 31 with respect to light having a wavelength of 550 nm, and N _cmp is the refractive index of the phase compensation layer 32 with respect to light having a wavelength of 550 nm.

By satisfying Expression (11), it is possible to reduce the reflectance at the interface between the substrate 31 and the phase compensation layer 32, and as a result, it is possible to reduce the change in reflectance due to the change in the thickness of the phase compensation layer. . More preferably, the range of the formula (11) is set to the following range.
| N _sub −N _cmp | <0.05 (11a)

As shown in FIG. 11, when the phase compensation layer 43 is disposed adjacent to the absorption layer 44, it is preferable that the following conditional expression is satisfied.
| N _{abs, c} −N _cmp | <0.15 (12)

However, N _{abs, c} is a refractive index with respect to light having a wavelength of 550 nm of a film adjacent to the phase compensation layer 43 among films constituting the absorption layer 44. By satisfying Expression (12), the reflectance at the interface between the phase compensation layer 43 and the absorption layer 44 can be reduced, and as a result, the change in reflectance due to the change in film thickness of the phase compensation layer 43 can be reduced. Can do. More preferably, the range of the formula (12) is set to the following range.
| N _{abs, c} −N _cmp | <0.10 (12a)

[Production method]
Next, a method for manufacturing the GND filter 10 in the present embodiment will be described.

An example of a method for forming the absorption layer 13 is vapor deposition. For example, by depositing Ti ₃ O ₅ at an appropriate oxygen partial pressure, a thin film made of titanium oxide having the characteristics shown in FIG. 15 can be formed. Further, by depositing Nb ₂ O ₅ in a vacuum, a thin film made of niobium oxide having the characteristics shown in FIG. 15 can be obtained. Further, by depositing Ta ₂ O ₅ in a vacuum, a film made of tantalum oxide having the characteristics shown in FIG. 15 can be obtained.

  Further, a film thickness distribution can be formed by generating a temperature gradient in the substrate 11 during vapor deposition or by inserting a mask between the target and the substrate 11.

  The method for forming the absorption layer 13 is not limited to vapor deposition, and may be selected according to the characteristics of the first material and the second material. Examples of other methods for forming the absorption layer 13 include sputtering, plating, and spin coating.

  The substrate 11 can be made of glass, plastic, or the like. The shape of the substrate 11 may be not only a flat plate but also a convex lens or a concave lens. When the GND filter 10 is arranged in the optical system of an image pickup apparatus such as a digital camera, the space for arranging the GND filter can be omitted by making the substrate 11 into a lens shape. For example, the optical system of the image pickup apparatus is small in size. Can be achieved.

[Transmittance distribution of GND filter]
The GND filter 10 of the present embodiment has a transmittance distribution corresponding to the film thickness distribution of the absorption layer 13. Various shapes can be used as the transmittance distribution of the GND filter 10. For example, the transmittance distribution may be formed concentrically as shown in FIGS. 16 (a) and 16 (b), or the transmittance changes in one direction as shown in FIGS. 16 (c) and 16 (d). Such a configuration may be adopted. In addition to these, there are various transmittance distribution shapes depending on the application, but this embodiment can be applied to any transmittance distribution shape.

  Hereinafter, each GND filter of this embodiment will be described in each example.

[Example 1]
The GND filter 10 as an optical element in the first embodiment is as shown in FIG. 1, and includes an intermediate layer 12, an absorption layer 13, and a surface layer 14 in order from the substrate 11. The absorption layer 13 includes a first film 131 and a second film 132.

  Table 1 shows the details of each film constituting the GND filter 10. In Table 1, n is a refractive index for light having a wavelength of 550 nm, k is an extinction coefficient for light having a wavelength of 550 nm, and d is a physical film thickness of the thin film. The same applies to the following embodiments.

  In the GND filter 10, the intermediate layer 12 is composed of four thin films, and the surface layer 14 is composed of five thin films.

  The first film 131 is made of titanium oxide, and the second film 132 is made of niobium oxide. The extinction coefficients of the titanium oxide and niobium oxide in this example are as shown in FIG. It can be seen that the extinction coefficient of the absorption layer 13 satisfies the equation (3) by forming the absorption layer 13 using the titanium oxide and niobium oxide shown in FIG. Moreover, as shown in FIG.15 (b), it turns out that Formula (1) is satisfy | filled by combining a titanium oxide and a niobium oxide. The first film 131 and the second film 132 are formed at a film thickness ratio of 1: 2 regardless of the film thickness of the absorption layer 13.

  FIG. 2 shows the film thickness distribution of the first film 131 and the second film 132 constituting the absorption layer 13. At the thickest position of the absorption layer 13, the first film 131 has a thickness of 333 nm, and the second film 132 has a thickness of 666 nm.

  FIG. 3 shows the wavelength dependence of the reflectance and transmittance of the GND filter 10. 3A shows the reflectance when light enters from the air side, FIG. 3B shows the reflectance when light enters from the substrate side, and FIG. 3C shows the transmittance. 3A, 3 </ b> B, and 3 </ b> C, the solid line indicates the thickness of the absorption layer 13 is 0 nm, the dotted line indicates the thickness of 50 nm, the broken line indicates the thickness of 100 nm, and the alternate long and short dash line indicates the thickness of 200 nm. The long broken line represents the case where the film thickness is 1000 nm.

  3A and 3B, the GND filter 10 has a reflectance of 4% or less both in the case of incident on the air side and in the case of incident on the substrate side. In particular, in the vicinity of 550 nm where the specific visibility is high, the reflectance is 2% or less regardless of the transmittance of the absorbing layer 23. Thus, it can be seen that the change in reflectance due to the change in transmittance of the absorption layer 13 is small. Further, from FIG. 3C, the change in the wavelength dependence of the transmittance due to the change in the transmittance of the absorption layer 13 is small, indicating a flat transmittance.

[Example 2]
FIG. 4 shows a schematic diagram of the GND filter 20 as an optical element in the second embodiment. The details of each film constituting the GND filter 20 are shown in Table 2. The GND filter 20 includes an intermediate layer 22, an absorption layer 23, and a surface layer 24 in order from the substrate 21, similarly to the GND filter 10 of the first embodiment.

  In the GND filter 20 according to the second embodiment, the absorption layer 23 includes a first film 231 made of titanium oxide and a second film 232 made of tantalum oxide. That is, the GND filter 20 is different from the GND filter 10 in that the second film 232 is made of tantalum oxide.

  The extinction coefficient of the tantalum oxide in this example is as shown in FIG. It can be seen that the extinction coefficient of the absorption layer 23 satisfies the equation (3) by forming the absorption layer 23 using the tantalum oxide and the titanium oxide shown in FIG. Further, as shown in FIG. 15B, it can be seen that the formula (1) is satisfied by combining the titanium oxide and the tantalum oxide. In the GND filter 20, the first film 231 and the second film 232 are formed at a film thickness ratio of 1: 1 regardless of the film thickness of the absorption layer 13.

  In the GND filter 20, the intermediate layer 22 is composed of four layers, and the surface layer 24 is composed of three layers.

  FIG. 5 shows the film thickness distribution of the first film 231 and the second film 232 constituting the absorption layer 23. At the thickest position of the absorption layer 23, the first film 231 has a thickness of 500 nm, and the second film 232 has a thickness of 500 nm.

  FIG. 6 shows the wavelength dependence of the reflectance and transmittance of the GND filter 10. 6A shows the reflectance when light enters from the air side, FIG. 6B shows the reflectance when light enters from the substrate side, and FIG. 6C shows the transmittance. 6A, 6 </ b> B, and 6 </ b> C, the solid line indicates the case where the thickness of the absorption layer 23 is 0 nm, the dotted line is 50 nm, the broken line is 100 nm, the alternate long and short dash line is 200 nm, and the long broken line is 1000 nm. Represents.

  6 (a) and 6 (b), the GND filter 20 has a reflectance of 4% or less both in the case of incidence on the air side and in the case of incidence on the substrate side. In particular, in the vicinity of 550 nm where the specific visibility is high, the reflectance is 2% or less regardless of the transmittance of the absorbing layer 23. Thus, it can be seen that the change in reflectance due to the change in transmittance of the absorption layer 23 is small. Further, from FIG. 6C, the change in the wavelength dependence of the transmittance due to the change in the transmittance of the absorption layer 23 is small, indicating a flat transmittance.

[Example 3]
FIG. 7 shows a schematic diagram of the GND filter 30 as an optical element in the third embodiment. Table 3 shows details of each film constituting the GND filter 30. The GND filter 30 includes a phase compensation layer 32, an intermediate layer 33, an absorption layer 34, and a surface layer 35 in order from the substrate 31. The GND filter 30 according to the present embodiment is different from the GND filter 10 according to the first embodiment in that a phase compensation layer 32 is provided. The structure of the absorption layer 34 is the same as that of the GND filter 10 of the first embodiment, and the absorption layer 34 of the GND filter 30 in the present embodiment is formed of a first film 341 made of titanium oxide and a niobium oxide. It consists of the second film 342.

  FIG. 8 shows the film thickness distribution of the absorption layer 34 and the phase compensation layer 32 of the GND filter 30 in this embodiment, and FIG. 9 shows the amount of phase shift of the transmitted wavefront. By designing the film thickness distribution of the phase compensation layer 32 as shown in FIG. 8, it is possible to compensate the phase shift amount of the transmitted wavefront due to the film thickness distribution of the absorption layer 34 as shown in FIG.

  FIG. 10 shows the wavelength dependence of the reflectance and transmittance of the GND filter 30. 10A shows the reflectance when light enters from the air side, FIG. 10B shows the reflectance when light enters from the substrate side, and FIG. 10C shows the transmittance. 10A, 10B, and 10C, the solid line represents the case where the thickness of the absorption layer 34 is 0 nm, the dotted line is 50 nm, the broken line is 100 nm, the alternate long and short dash line is 200 nm, and the long broken line is 1000 nm. Represents.

  10 (a) and 10 (b), the GND filter 30 has a reflectance of 4% or less both in the case of incidence on the air side and in the case of incidence on the substrate side. In particular, in the vicinity of 550 nm where the relative luminous sensitivity is high, the reflectance is 2% or less regardless of the transmittance of the absorbing layer 34, and it can be seen that the change in reflectance due to the change in the transmittance of the absorbing layer 34 is small. . Further, from FIG. 10C, the change in the wavelength dependency of the transmittance due to the change in the transmittance of the absorption layer 34 is small, indicating a flat transmittance.

[Example 4]
FIG. 11 shows a schematic diagram of a GND filter 40 as an optical element in the fourth embodiment. Table 4 shows details of each film constituting the GND filter 40. The GND filter 40 includes an intermediate layer 42, a phase compensation layer 43, an absorption layer 44, and a surface layer 45 in order from the substrate 41. The GND filter 30 according to the present embodiment is different from the GND filter 30 according to the third embodiment in that the phase compensation layer 43 is disposed between the intermediate layer 42 and the absorption layer 44.

  The structure of the absorption layer 44 is the same as that of the GND filter 10 of the first embodiment, and the absorption layer 44 of the GND filter 40 in this embodiment is formed of a first film 441 made of titanium oxide and a niobium oxide. A second film 442 is formed.

  FIG. 12 shows the film thickness distribution of the absorption layer 44 and the phase compensation layer 43 of the GND filter 40 in this embodiment, and FIG. 13 shows the phase shift amount of the transmitted wavefront. By designing the film thickness distribution of the phase compensation layer 43 as shown in FIG. 12, it is possible to compensate the phase shift amount of the transmitted wavefront due to the film thickness distribution of the absorption layer 44 as shown in FIG.

  FIG. 14 shows the wavelength dependence of the reflectance and transmittance of the GND filter 40. FIG. 14A shows the reflectance when light enters from the air side, FIG. 14B shows the reflectance when light enters from the substrate side, and FIG. 14C shows the transmittance. In each of FIGS. 14A, 14B, and 14C, the solid line indicates the case where the thickness of the absorption layer 44 is 0 nm, the dotted line is 50 nm, the broken line is 100 nm, the alternate long and short dash line is 200 nm, and the long broken line is 1000 nm. Represents.

  14A and 14B, the GND filter 30 has a reflectance of 4% or less both in the case of incident on the air side and in the case of incident on the substrate side. In particular, in the vicinity of 550 nm where the relative visibility is high, the reflectance is 2% or less regardless of the transmittance of the absorbing layer 44, and it can be seen that the change in reflectance due to the change in the transmittance of the absorbing layer 44 is small. . Further, from FIG. 14C, the wavelength dependency of the transmittance due to the change in the transmittance of the absorption layer 34 is small, indicating a flat transmittance.

[Modification 1]
Next, the GND filter 50 as an optical element in Modification 1 will be described. In the above-described embodiment, the example in which the absorption layer is formed by the first film made of the first material and the second film made of the second material is shown, but the present invention is not limited to this. In this modification, a case will be described in which the absorption layer is formed from a single film made of a resin in which a granular first material and a second material are dispersed.

  FIG. 17A shows a schematic diagram of the GND filter 50. The GND filter 50 includes an intermediate layer 52, an absorption layer 53, and a surface layer 54 in order from the substrate 51. FIG. 17B shows an enlarged view of the absorption layer 53 in the region indicated by the dotted line in FIG. Unlike the embodiment described above, the absorption layer 53 in the present modification is made of a medium in which a granular first material 531 and a granular second material 532 are dispersed in a resin 533.

  In this case, the extinction coefficient of the absorption layer is obtained by calculating the absorption coefficient α (λ) from the light absorption amount of the absorption layer and using the relational expression α (λ) = 4πk (λ) / λ from the absorption coefficient. be able to.

  When the absorption layer is formed from a single film as in this modification, the first material and the second material satisfy the above-described formula (1), and the extinction coefficient of the absorption layer is the above-described formula (3 ). Thereby, both the change in the wavelength dependency of the transmittance due to the change in the transmittance of the absorption layer and the change in the reflectance can be reduced.

Furthermore, in order to further reduce the change in the wavelength dependency of the transmittance due to the change in the thickness of the absorption layer, the absorption coefficient α ₁ (λ) of the first material and the absorption coefficient α ₂ (λ) of the _second material. The concentration of the first material and the second material in the absorption layer may be adjusted in accordance with the wavelength dependency of the wavelength. That is, the transmission due to the change in the thickness of the absorption layer 13 is obtained by replacing t ₁ in the expressions (6) and (6a) with the concentration of the first material in the absorption layer and t ₂ as the concentration of the second material in the absorption layer. The change in the wavelength dependency of the rate can be further reduced.

  Further, when the surface layer is further provided, by satisfying the formula (8), the change in reflectance due to the change in the thickness of the absorption layer can be further reduced as in the above-described embodiment. Even when an intermediate layer is further provided, by satisfying (9) or (9a), a change in reflectance due to a change in the thickness of the absorption layer can be further reduced as in the above-described embodiment. .

Here, the absorption layer of this modification is different from the absorption layer of the above-described embodiment, and the absorption layer is formed of a single film. Therefore, N _{abs, sur} in equation (8) is the same value as N _{abs, int} in equation (9) or (9a).

[Modification 2]
Next, the GND filter 60 as an optical element in Modification 2 will be described. In the above-described embodiment and the first modification, the example in which the absorption layer including the first material and the second material satisfying the formula (1) is formed on one surface of the substrate is shown. It is not limited to this. In this modification, a case will be described in which the first absorption layer containing the first material is formed on one surface of the substrate and the second absorption layer containing the second material is formed on the other surface of the substrate. .

  FIG. 18 shows a schematic diagram of the GND filter 60 in the present modification. The GND filter 60 includes an intermediate layer 62a, a first absorption layer 63a, and a surface layer 64a in order from the substrate 61 on one surface of the substrate 61. Further, on the other surface of the substrate 62, an intermediate layer 62b, a second absorption layer 63b, and a surface layer 64b are sequentially provided from the substrate 61. Both the first absorption layer 63a and the second absorption layer 63b change in film thickness, and the transmittance changes according to the position on the substrate.

  The first absorption layer 63a and the second absorption layer 63b both satisfy the expression (3). Thereby, both the change in the wavelength dependency of the transmittance due to the change in the transmittance of the first absorption layer 63a and the second absorption layer 63b and the change in the reflectance can be reduced.

  In this modification, both the first absorption layer 63a and the second absorption layer 63b have different thicknesses, and the transmittance changes according to the position on the substrate. It is sufficient that the transmittance of at least one of 63a and the second absorption layer 63b is changed according to the position on the substrate. As a result, a transmittance distribution can be formed in the GND filter 60.

  The first absorption layer 63a may form a thin film made of the first material by vapor deposition or the like as in the above-described embodiment, or the first material or the second material may be formed on the resin as in the first modification described above. These materials may be dispersed. Similarly to the first absorption layer 63a, the second absorption layer 63b may be a thin film made of the second material by vapor deposition or the like, or may be formed by dispersing the second material in a resin. Good.

[Optical system]
Next, an optical system as an embodiment of the present invention will be described.

  FIG. 19A is a cross-sectional view of the optical system 70 in the present embodiment. The optical system 70 has lenses as a plurality of optical elements. Light from the object passes through the optical system 70 and forms an image on the imaging surface IP.

  Here, at least one of the plurality of lenses of the optical system 70 is one of the GND filters of the first to fourth embodiments described above.

  The GND filters of Examples 1 to 4 reduce the wavelength dependency of the transmittance and the change of the reflectance due to the change of the transmittance of the absorption layer. For this reason, it is possible to suppress coloring, ghosting and flare on the image, and to obtain a high-quality image.

  The optical system 70 is a coaxial rotationally symmetric optical system, and in such an optical system, a concentric transmittance distribution as shown in FIGS. 16A and 16B is preferable. In addition, as shown in FIGS. 1, 4, 7, and 11, by providing a region where the absorption layer is not formed at the center of the GND filter, it is possible to suppress a decrease in the amount of transmitted light due to the GND filter. In that case, the light beam transmitted through the central portion of the GND filter is not subjected to transmittance modulation by the GND filter. Therefore, when the imaging apparatus having the optical system 70 has a phase difference type automatic focusing mechanism, automatic focusing can be performed using a light beam that has passed through the center of the GND filter.

When the transmittances at distances r ₁ and r ₂ (r ₁ <r ₂ ) from the center of the optical surface are T (r ₁ ) and T (r ₂ ), T (r ₁ ) ≧ T (r ₂ ) When a GND filter having a transmittance distribution as described above is disposed in the optical system 70, a high-quality blurred image can be obtained due to the apodization effect.

  In addition, when at least one such GND filter is disposed on the light incident side and the light emission side of the stop SP, an apodization effect can be effectively obtained even for off-axis light beams, and the entire area of the screen can be obtained. On the other hand, a high-quality image can be obtained.

  Moreover, by providing the area | region where the absorption layer is not formed in the center part of a GND filter, it can suppress that a blurred image becomes too small, improving the blurred image by an apodization effect.

On the other hand, when a GND filter having a transmittance distribution satisfying T (r ₁ ) ≦ T (r ₂ ) is arranged in the optical system 70, the peripheral light attenuation of the image can be corrected.

  Next, an image pickup apparatus having the optical system 70 of this embodiment will be described.

  FIG. 19B shows a digital camera 80 as the imaging apparatus of the present embodiment. The digital camera 80 has the optical system 70 of the above-described embodiment in the lens unit 82. An imaging element 83 such as a CCD or a CMOS sensor is disposed in the main body 81 on the imaging plane IP of the optical system 70.

  Since the digital camera 80 includes the optical system 70, it is possible to suppress coloration, ghost, and flare on the image, and a high-quality image can be obtained.

  Note that FIG. 19B shows an example in which the main body portion 81 and the lens portion 82 are integrated, but the present invention may be applied to a lens device that can be attached to and detached from the imaging device main body. Such a lens device is used as an interchangeable lens for a single-lens camera, for example. In this case, FIG. 19B can also be regarded as a state in which the lens device 82 having the optical system 70 is attached to the imaging device main body 81.

  The optical system of the present invention can also be applied to an imaging apparatus such as a digital camera or a lens apparatus (interchangeable lens) that can be attached to and detached from the imaging apparatus main body. For example, the optical system of the present invention may be applied to binoculars, a microscope, and the like.

  The preferred embodiments and examples of the present invention have been described above, but the present invention is not limited to these embodiments and examples, and various combinations, modifications, and changes can be made within the scope of the gist.

  Finally, Table 5 summarizes the values in Examples 1 to 4.

10, 20, 30, 40 GND filter 11, 21, 31, 41 Substrate 13, 23, 34, 44 Absorbing layer

Claims (26)

An optical element comprising a substrate and an absorption layer having different transmittance according to the position on the substrate,
The absorbing layer is
A first material having an absorption coefficient at a wavelength of 400 nm smaller than the absorption coefficient at a wavelength of 700 nm;
A second material having an absorption coefficient at a wavelength of 400 nm larger than the absorption coefficient at a wavelength of 700 nm,
An extinction coefficient of the absorption layer is 0.005 or more and 0.5 or less at a wavelength of 400 nm to 700 nm. An optical element comprising a substrate and an absorption layer having different transmittance according to the position on the substrate,
The absorbing layer is
Including a first material and a second material;
The first material is a titanium oxide having an extinction coefficient of 0.005 or more and 0.5 or less at a wavelength of 400 nm to 700 nm,
The optical material, wherein the second material is niobium oxide or tantalum oxide having an extinction coefficient of 0.005 or more and 0.5 or less at a wavelength of 400 nm to 700 nm. An optical element comprising a substrate and an absorption layer having different transmittance according to the position on the substrate,
The absorbing layer is
A first material having an absorption coefficient at a wavelength of 400 nm smaller than the absorption coefficient at a wavelength of 700 nm;
A second material having an absorption coefficient at a wavelength of 400 nm that is larger than the absorption coefficient at a wavelength of 700 nm,
An extinction coefficient of the absorption layer is 0.5 or less at a wavelength of 400 nm to 700 nm. An optical element comprising a substrate and an absorption layer having different transmittance according to the position on the substrate,
The absorbing layer is
A single layer comprising a first material and a second material;
The first material is a titanium oxide having an extinction coefficient of 0.5 or less at a wavelength of 400 nm to 700 nm,
The optical element, wherein the second material is niobium oxide or tantalum oxide having an extinction coefficient of 0.5 or less at a wavelength of 400 nm to 700 nm. The absorption layer has a first film containing the first material and a second film containing the second material,
3. The optical element according to claim 1, wherein a thickness of at least one of the first film and the second film varies depending on a position on the substrate. When the refractive index difference at a wavelength of 550 nm between adjacent films among the films constituting the absorption layer is ΔN _abs ,
| ΔN _abs | <0.25
The optical element according to claim 5, wherein the following conditional expression is satisfied. When the extinction coefficient difference at a wavelength of 550 nm between adjacent films among the films constituting the absorption layer is Δk _abs ,
| Δk _abs | <0.2
The optical element according to claim 5, wherein the following conditional expression is satisfied. In at least a part of the wavelength band from a wavelength of 400 nm to a wavelength of 700 nm, one absorption coefficient of the first material and the second material increases with respect to the wavelength, and the other absorption coefficient increases with respect to the wavelength. Decrease,
The coefficient of λ when the absorption coefficient of the first material is linearly approximated to the wavelength λ by the least square method in the partial wavelength band is a ₁ , and the _first coefficient is calculated by the least square method in the partial wavelength band. The coefficient of λ when linearly approximating the absorption coefficient of the material of 2 with respect to the wavelength λ is a ₂ , the thickness of the first film at the position where the thickness of the absorption layer is maximum is t ₁ , when the thickness of the second layer was t _2,
−1.5 ≦ (a ₁ / a ₂ ) · (t ₁ / t ₂ ) ≦ −0.7
The optical element according to claim 5, wherein the following conditional expression is satisfied. The first material is titanium oxide;
In at least a part of the wavelength band from a wavelength of 400 nm to a wavelength of 700 nm, one of the first material and the second material has an absorption coefficient that increases with respect to the wavelength, and the other absorption coefficient increases with respect to the wavelength. Decreased,
The coefficient of λ when the absorption coefficient of the first material is linearly approximated to the wavelength λ by the least square method in the partial wavelength band is a ₁ , and the _first coefficient is calculated by the least square method in the partial wavelength band. When the coefficient of λ when linearly approximating the absorption coefficient of the material of 2 with respect to the wavelength λ is a ₂ ,
−10 <a ₁ / a ₂ ≦ −1
The optical element according to claim 5, wherein the following conditional expression is satisfied. The first material is titanium oxide;
10. The optical element according to claim 5, wherein in the absorption layer, the first film is disposed closer to the substrate than the second film. The optical element further has a surface layer,
The absorbing layer is disposed between the substrate and the surface layer;
Of the films constituting the absorbing layer, when the refractive index at a wavelength of 550 nm of the film closest to the surface layer is _{Nabs, sur} ,
The optical element according to claim 1, wherein the surface layer includes a film having a refractive index greater than 1 and smaller than _{Nabs, sur at} a wavelength of 550 nm. The optical element further includes an intermediate layer between the substrate and the absorbing layer,
Among the films constituting the absorption layer, when the refractive index at a wavelength of 550 nm of the film closest to the intermediate layer is _{Nabs, int} , and the refractive index at a wavelength of 550 nm of the substrate is N _sub ,
12. The optical element according to claim 1, wherein the intermediate layer includes a film having a refractive index between _{Nabs, int} and N _{sub at} a wavelength of 550 nm. The thickness of the absorption layer varies depending on the position on the substrate,
The optical element according to claim 1, further comprising a phase compensation layer having a thickness increasing in a direction opposite to a direction in which the thickness of the absorption layer increases. The phase compensation layer is disposed at a position adjacent to the substrate;
When the refractive index at a wavelength of 550 nm of the substrate is N _sub and the refractive index of the phase compensation layer at a wavelength of 550 nm is N _cmp ,
| N _sub −N _cmp | <0.10
The optical element according to claim 13, wherein the following conditional expression is satisfied. The phase compensation layer is disposed at a position adjacent to the absorption layer,
Of the films constituting the absorption layer, when the refractive index at a wavelength of 550 nm of the film adjacent to the phase compensation layer is N _{abs, c} and the refractive index at a wavelength of 550 nm of the phase compensation layer is N _cmp ,
| N _{abs, c} −N _cmp | <0.15
The optical element according to claim 13, wherein the following conditional expression is satisfied.   The reflectance of the optical element when light is incident from the absorption layer toward the substrate at a position where the transmittance of the absorption layer is the lowest is 4% or less for each wavelength from 400 nm to 700 nm. The optical element according to claim 1, wherein the optical element is an optical element.   The reflectance of the optical element when light is incident from the substrate toward the absorbing layer at a position where the transmittance of the absorbing layer is the lowest is 4% or less with respect to each wavelength from 400 nm to 700 nm. The optical element according to claim 1, wherein the optical element is an optical element.   18. The optical element according to claim 1, wherein regions having the same transmittance of the absorption layer are distributed concentrically.   The optical element according to claim 18, wherein the absorption layer is not formed at the center of the concentric circle. A substrate,
A first absorption layer that includes a first material that absorbs part of incident light and has an absorption coefficient at a wavelength of 400 nm that is smaller than the absorption coefficient at a wavelength of 700 nm;
A second absorption layer that includes a second material that absorbs part of incident light and has an absorption coefficient at a wavelength of 400 nm that is larger than the absorption coefficient at a wavelength of 700 nm;
An optical element comprising:
The transmittance of at least one of the first absorption layer and the second absorption layer varies depending on the position on the substrate,
The optical element, wherein extinction coefficients of the first absorption layer and the second absorption layer are both 0.005 or more and 0.5 or less at a wavelength of 400 nm to 700 nm. A substrate,
A first absorption layer that includes titanium oxide that absorbs part of incident light and has an extinction coefficient of 0.005 to 0.5 at a wavelength of 400 nm to 700 nm;
A second absorption layer containing niobium oxide or tantalum oxide that absorbs part of incident light and has an extinction coefficient of 0.005 to 0.5 at a wavelength of 400 nm to 700 nm;
The optical element according to claim 1, wherein the transmittance of at least one of the first absorption layer and the second absorption layer varies depending on a position on the substrate.   The optical element according to any one of claims 1 to 21, wherein the substrate is a lens.   23. An optical system comprising a plurality of optical elements, wherein at least one of the plurality of optical elements is the optical element according to any one of claims 1 to 22.   An optical system comprising: a diaphragm; and an optical element according to any one of claims 1 to 22 disposed at least one each on a light incident side and a light emitting side of the diaphragm.   An imaging apparatus comprising: an imaging element; and the optical system according to claim 23 or 24.   A lens apparatus comprising the optical system according to claim 23 or 24, wherein the lens apparatus is detachable from an imaging apparatus main body.