JP2017187729A - Optical element, optical system, imaging apparatus and lens device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical element capable of reducing a change of a reflection rate when the permeability of an absorber layer changes.SOLUTION: A substrate 11 includes thereon: an absorber layer 13 with different permeability according to the position on the substrate 11; and a surface layer 14 including a plurality of thin films. In the surface layer 14, a refractive index of a first film 14c arranged in a position furthest from the substrate 11 is 1.05 or more and 1.4 or less at a wavelength 550 nm. A second film 14b having higher refractive index than the substrate 11 and a third film 14a having lower refractive index than the second film 14b are alternately laminated in two layers or more in total, in a position closer to the substrate 11 than the first film 14c of the surface layer 14. An extinction coefficient of the absorber layer 13 is 0.5 or less at wavelength 400 nm to 700 nm.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).

  A characteristic required for a GND filter is low reflectance. This is because when the GND filter is used in an optical system such as an imaging device, the reflected light from the GND filter causes flare and ghost. In order to sufficiently reduce the reflectance, it is effective to provide a film having a low refractive index on the most surface side of the antireflection layer.

  In Patent Document 1, by alternately laminating a film made of metal (absorption layer) and a film made of metal oxide on a substrate, and providing a layer made of a polymer with a refractive index of 1.23 on the most surface side, A GND filter with reduced reflectivity is described.

JP 2009-288295 A

  However, in the case of the GND filter described in Patent Document 1, since a metal having a large extinction coefficient is used as the absorption layer, if the transmittance distribution is formed by changing the thickness of the absorption layer, the GND filter is positioned at the position of the GND filter. Accordingly, the reflection conditions change remarkably. Therefore, even if a layer having a low refractive index made of a polymer is provided on the most surface side, it has been difficult to achieve a low reflectance over the entire GND filter.

  The objective of this invention is providing the optical element which can reduce the change of the reflectance by the change of the transmittance | permeability of an absorption layer.

  An optical element of the present invention includes a substrate, a surface layer including a plurality of thin films, and an absorption layer having a transmittance that varies depending on a position on the substrate provided between the surface layer and the substrate. The first film disposed in the surface layer at a position farthest from the substrate has a refractive index of 1.05 or more and 1.4 or less than the first film in the surface layer. Also, at a position close to the substrate, a second film having a higher refractive index than the substrate at a wavelength of 550 nm and a third film having a lower refractive index than the second film at a wavelength of 550 nm are alternately two layers in total. The extinction coefficient of the absorption layer is 0.5 or less at a wavelength of 400 nm to 700 nm.

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

It is a schematic diagram of a GND filter. It is an admittance trajectory when the extinction coefficient of the absorption layer is 0.218. It is an admittance trajectory diagram when the extinction coefficient of the absorption layer is 0.5. It is a figure which shows an example of the wavelength dependence of an extinction coefficient and an absorption coefficient. It is the schematic of a GND filter in case an absorption layer has a 1st absorption film and a 2nd absorption film. It is the schematic of a GND filter in case the absorption layer is formed with the resin which disperse | distributed the microparticles | fine-particles formed with the material which has a light absorptivity. It is a figure which shows an example of the transmittance | permeability distribution. It is a figure which shows the wavelength dependence of the reflectance of the GND filter of Example 1, and the transmittance | permeability. 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. 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. It is a figure which shows the wavelength dependence of the reflectance of the GND filter of Example 4, and the transmittance | permeability. 1 is a cross-sectional view of an optical system and a schematic view of an imaging apparatus.

  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, a surface layer 14, and an absorption layer 13 that absorbs part of incident light. The absorption layer 13 is disposed between the surface layer 14 and the substrate 11.

  The surface layer 14 is composed of at least three thin films. That is, the surface layer 14 includes a first film 14c, a second film 14b having a higher refractive index than the substrate 11 at a wavelength of 550 nm, and a third film having a lower refractive index than the second film 14b at a wavelength of 550 nm. 14a. The first film 14c is disposed at a position farthest from the substrate 11 in the surface layer 14, and has a refractive index of 1.05 or more and 1.4 or less at a wavelength of 550 nm. In addition, the second film 14b and the third film 14a are alternately stacked in a total of two or more layers in the surface layer 14 at a position closer to the substrate 11 than the first film 14c.

  The absorption layer 13 has an extinction coefficient of 0.5 or less at a wavelength of 400 nm to 700 nm, and part of the light incident on the GND filter 10 is absorbed by the absorption layer 13. Moreover, the thickness of the absorption layer 13 changes according to the position on the substrate 11. Thereby, the absorption layer 13 has different transmittances depending on the position on the substrate.

  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).

[Effect of reducing reflectivity]
In general, when the transmittance of the absorption layer changes, the reflectance also changes. Therefore, it has been difficult to reduce the reflectance for regions having different transmittances. However, in the GND filter 10, when the absorption layer satisfies the formula (2) described later and has the above-described surface layer, the change in reflectance due to the change in the transmittance of the absorption layer can be reduced. The reason for this will be described with reference to FIGS.

  2 and 3 are admittance trajectory diagrams of the GND filter 10 when the GND filter 10 includes the intermediate layer 12. FIG. 2 shows an admittance orbit when the extinction coefficient of the absorption layer 13 is small, and FIG. 3 shows an admittance orbit when the extinction coefficient of the absorption layer 13 is larger than that of FIG. The intermediate layer 12 is composed of two thin films, and the thin films 12 a and 12 b are laminated in order from the side closer to the substrate 11.

The admittance is the value represented by the ratio of the field strength and the field strength in the medium, when the admittance Y ₀ of the vacuum units, the refractive index of the medium is the admittance and equality is numerically. 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 ””.

  FIG. 2 shows an admittance trajectory diagram in the GND filter 10 in the case where the extinction coefficient of the absorption layer 13 is 0.218. 2A, 2B, and 2C are admittance trajectory diagrams when light is incident from the air side at positions where the transmittance of the GND filter 10 is 100%, 79%, and 10%. It is. In the GND filter 10, the transmittance becomes lower as the absorption layer 13 is thicker. That is, FIG. 2A shows an admittance trajectory at a position where the thickness of the absorption layer 13 is 0, and the thickness of the absorption layer 13 increases from FIG. 2B to FIG. 2C.

First, taking the admittance trajectory diagram of FIG. 2A as an example, how to read the diagram will be described. In FIG. 2, the horizontal axis represents the real part Re (η) of admittance η in units of Y ₀ , and the vertical axis represents the imaginary part Im (η) of η. Each mark represents air admittance. When the refractive index of the substrate 11, _{N sub,} admittance of the substrate 11 in which the _{Y 0} units is equal to _{N sub.} 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.

  In the trajectory of FIG. 2A, thin films 12a and 12b, an absorption layer 13 as intermediate layers, and a third film 14a, a second film 14b, and a first film 14c as surface layers are formed in order from the substrate 11. Shows the change in equivalent admittance. The end point of the trajectory when the first film 14c 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. 2A shows the case where the thickness of the absorption layer 13 is 0. However, when the thickness of the absorption layer 13 increases and the transmittance decreases, the admittance trajectory becomes as shown in FIGS. ). As the thickness of the absorption layer 13 increases, the equivalent admittance from the substrate 11 to the absorption layer 13 changes so as to spiral, and finally converges to a point substantially equal to the complex refractive index of the absorption layer 13.

  When the thickness of the absorption layer 13 shown in FIG. 2A is 0 and the thickness of the absorption layer 13 shown in FIG. 2C is sufficiently thick, the equivalent admittance is the extinction coefficient k. Is changed by an amount corresponding to 0.218.

  Thus, when the extinction coefficient of the absorption layer 13 is 0.218, the equivalent admittance from the substrate 11 to the absorption layer 13 changes as shown in FIGS. 2A to 2C, the change in the equivalent admittance from the substrate 11 to the first film 14c is smaller than the change in the equivalent admittance from the substrate 11 to the absorption layer 13. You can see that This is because the surface layer 14 is provided on the upper surface of the absorption layer 13.

  The surface layer 14 of the GND filter 10 has a structure in which the second film 14b and the third film 14a are alternately stacked. As described above, when the second film 14b and the third film 14a are alternately laminated, a V-shaped trajectory is drawn on the admittance trajectory diagram as shown in FIGS. 2 (a) to 2 (c). .

  Here, consider a case where the equivalent admittance from the substrate 11 to the absorbing layer 13 changes from FIG. 2A to FIG. 2B due to the change in the transmittance of the absorbing layer 13. At this time, the admittance trajectory of the third film 14a is shorter in the case shown in FIG. 2B than in the case shown in FIG. On the other hand, the admittance trajectory of the second film 14b is longer in the case shown in FIG. 2B than in the case shown in FIG.

  That is, the admittance trajectories of the third film 14a and the second film 14b change so as to compensate for each other's change. Thereby, the amount of change in equivalent admittance due to the change in transmittance of the absorption layer 13 is reduced.

In addition, a film having a small refractive index is provided as the first film 14c of the surface layer 14 in order to make the final equivalent admittance close to the admittance of air. In order to reduce the reflectance, the refractive index of the first film 14c is preferably small. Therefore, the refractive index _{n s} of the first film 14c satisfy the following condition at a wavelength 550nm (2).
_{1.05 ≦ n s ≦ 1.4 (1} )

If n _s is greater than 1.4, it is difficult to sufficiently reduce the change in reflectance due to change in transmittance of the absorption layer 13. Further, when _{n s} it is smaller than 1.05, production of the first film 14c becomes difficult.

In order to further reduce the change in reflectance due to the change in the transmittance of the absorption layer 13, the range of the formula (1) is preferably set to the following formula (1a).
1.05 ≦ n _s ≦ 1.3 (1a)

  Next, FIG. 3 shows an admittance trajectory diagram in the GND filter 10 when the extinction coefficient of the absorbing layer 13 is large and the extinction coefficient is 0.5. 3 (a), 3 (b), and 3 (c) show the case where light is incident from the air side in such a GND filter 10 where the transmittance is 100%, 79%, and 10%. It is an admittance trajectory map. As shown in FIG. 3C, when the thickness of the absorption layer 13 is sufficiently thick, the equivalent admittance from the substrate 11 to the absorption layer 13 is substantially equal to the complex refractive index of the absorption layer 3.

  Comparing the case where the thickness of the absorption layer 13 shown in FIG. 3A is 0 and the case where the thickness of the absorption layer 13 shown in FIG. 3C is sufficiently thick, the equivalent admittance is the extinction coefficient k. Is changed by an amount corresponding to 0.5. Comparing FIG. 3 and FIG. 2 with respect to the change in equivalent admittance from the substrate 11 to the absorption layer 13 due to the change in the transmittance of the absorption layer 13, it can be seen that the change shown in each diagram of FIG. 3 is larger. . This is because the extinction coefficient of the absorption layer 13 is larger in the case shown in FIG. 3 than in the case shown in FIG.

When the extinction coefficient of the absorption layer 13 is larger than 0.5, the change in equivalent admittance becomes larger than that shown in FIG. In this case, even if the surface layer 14 is provided, it becomes difficult to sufficiently reduce the reflectance. Therefore, in order to reduce the change in reflectance due to the change in the transmittance of the absorption layer 13, the extinction coefficient k of the absorption layer 13 satisfies the following conditional expression (2) at wavelengths from 400 nm to 700 nm.
0 <k ≦ 0.5 (2)

  When the extinction coefficient k of the absorption layer 13 satisfies the formula (2) and the surface layer 14 is provided on the upper surface of the absorption layer 13, a change in reflectance due to a change in the transmittance of the absorption layer 13 can be reduced.

In addition, the thickness of the absorption layer 13 can be made thinner as the extinction coefficient of the absorption layer 13 is larger. Thereby, when forming the absorption layer 13 by vapor deposition or sputtering, the film-forming time can be shortened. Furthermore, as the thickness of the absorption layer 13 is thinner, the phase shift of the transmitted wavefront due to the absorption layer 13 can be reduced. Therefore, the range of the formula (2) is preferably set to the range of the following formula (2a).
0.005 ≦ k ≦ 0.5 (2a)

More preferably, the range of the formula (2) is set to the range of the following formula (2b).
0.05 ≦ k ≦ 0.4 (2b)

  In addition, the larger the absolute value of the difference in refractive index between the second film 14b and the third film 14a, the greater the effect of reducing the amount of change in equivalent admittance due to the change in transmittance of the absorption layer 13. Therefore, the absolute value of the difference in refractive index between the second film 14b and the third film 14a adjacent to each other is preferably 0.4 or more.

  In order to further reduce the amount of change in equivalent admittance due to the change in the transmittance of the absorption layer 13, it is preferable to stack four or more third films 14a and second films 14b alternately.

Furthermore, when light enters the GND filter from the air side, the following conditional expression (3) must be satisfied at a wavelength of 550 nm in order to further reduce the change in reflectance due to the change in the transmittance of the absorption layer 13. preferable.
| Re (η _sub ) / Y ₀ −N _{abs, int} | <0.25 (3)

Here, η _sub is the equivalent admittance from the substrate 11 to the intermediate layer 12, and N _{abs, int} is the refractive index of the film closest to the intermediate layer 12 in the absorption layer 13. The Re and (eta _sub) are divided by _{Y 0} is for handling units of Re (eta _sub) as _{Y 0.}

Expression (3) indicates that the real part of the equivalent admittance from the substrate 11 to the intermediate layer 12 in the GND filter 10 is a value close to the refractive index of the absorption layer 13. By satisfy | filling Formula (3), the change of the equivalent admittance from the board | substrate 11 by the change of the transmittance | permeability of the absorption layer 13 to the absorption layer 13 can further be reduced. As a result, the change in reflectance due to the change in transmittance of the absorption layer 13 can be further reduced. The range of the formula (3) is more preferably the following formula (3a).
| Re (η _sub ) / Y ₀ −N _{abs, int} | <0.15 (3a)

  In the GND filter 10, the intermediate layer 12 that satisfies the expression (3) is provided. However, the expression (3) can also be obtained by setting the refractive index of the absorption layer 13 and the refractive index of the substrate 11 to be close to each other. The same effect as satisfying the above can be obtained.

  In the above description, the reflectance when light is incident on the GND filter 10 from the air side has been described. However, in the case of having the absorption film 13 that absorbs part of the incident light like the GND filter 10, the reflectance is different between the case where the light is incident from the air side and the case where the light is incident from the substrate 11 side. This is because the Fresnel coefficient at each interface of the GND filter 10 varies depending on the incident direction of light when the absorption layer 13 is present.

  In order to reduce the reflectance with respect to the light incident from the substrate 11 side, it is preferable to provide the intermediate layer 12 between the absorption layer 13 and the substrate 11 as shown in FIG.

Intermediate layer 12, when the refractive index of the substrate 11 was set to _{N sub,} it is preferable that the refractive index is configured with a film of values between _{N abs, int} and _{N sub.} Furthermore, it is preferable to have alternately a film having a higher refractive index than the substrate 11 and a film having a lower refractive index than the film. Thereby, even when light enters from the substrate 11 side, the change in reflectance due to the change in the transmittance of the absorption layer 13 can be reduced.

In order to reduce the change in equivalent admittance from the air to the absorption layer 13, it is preferable to satisfy the following conditional expression (4) at a wavelength of 550 nm.
| Re (η _air ) / Y ₀ −N _{abs, sur} | <0.25 (4)

Here, η _air is an equivalent admittance from air to the surface layer 14. N _{abs, sur} is the refractive index of the film closest to the surface layer 14 among the films constituting the absorption layer 13. When the absorption layer 13 is formed of a single film, N _{abs, sur} and N _{abs, int} are equal. On the other hand, when the absorption layer 13 is composed of two absorption films as will be described later, N _{abs, sur} and N _{abs, int} are different values.

Satisfying the expression (4) indicates that the real part of the equivalent admittance from the air to the surface layer 14 in the GND filter 10 is a value close to the refractive index of the absorption layer 13. Thereby, the change of the equivalent admittance from the air to the absorption layer 13 due to the change in the transmittance of the absorption layer 13 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. The range of the formula (4) is more preferably the following formula (4a).
| Re (η _air ) / Y ₀ −N _{abs, sur} | <0.15 (4a)

  Further, by satisfying both of the expressions (3) and (4), the reflection due to the change in the transmittance of the absorption layer 13 in both cases where light is incident from the air side and light is incident from the substrate 1 side. The change in rate can be reduced.

Furthermore, when a region where the thickness of the absorption layer 13 is 0 is provided at the center of the GND filter 10 as shown in FIG. 1, the absorption layer 13 is formed between the intermediate layer 12 and the surface layer 14 in the region. Will not be. In this case, it is preferable to design the surface layer 14 and the intermediate layer 12 so as to satisfy the following formula (5).
| Re (η _air ) / Y ₀ −Re (η _sub ) / Y ₀ | <0.3 (5)

  By satisfying Expression (5), the reflectance can be reduced even in the region where the thickness of the absorption layer 13 is zero.

[About materials]
Next, materials constituting the GND filter 10 will be described.

  As the substrate 11, glass, plastic, or the like can be used. 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 imaging apparatus such as a digital camera, the space for arranging the GND filter can be saved by making the shape of the substrate 11 into a lens shape, and the optical system can be downsized. Can do.

The 1st film | membrane 14c should just have the refractive index which satisfy | fills Formula (1) mentioned above. As a material satisfying the formula (1), for example, there is magnesium fluoride (MgF ₂ ). In addition, by using a film in which voids are formed (hereinafter referred to as a hollow film), a film having a lower refractive index than a uniform film made of magnesium fluoride can be obtained. This is because the effective refractive index of the film can be reduced by the air contained in the air gap.

When a hollow membrane is used as the first membrane 14c, it is preferable to form a hollow membrane by combining fine particles containing magnesium fluoride or silica (SiO ₂ ). This is because magnesium fluoride or silica is a material having a low refractive index, so that the effective refractive index can be reduced without excessively increasing the voids when forming the hollow membrane. The fine particle structure may be a hollow particle or a solid particle. Moreover, porous particles may be used. The particle diameter of the fine particles is preferably 40 nm or less. This is because if the particle size of the fine particles becomes too large, light scattering by the particles occurs.

  The absorption layer 13 should just have the extinction coefficient which satisfy | fills Formula (2) mentioned above in wavelength 400nm to 700nm. Examples of the material satisfying the formula (2) include metal oxides such as titanium oxide, niobium oxide, and tantalum oxide. FIG. 4A shows the wavelength dependence of the extinction coefficient of titanium oxide, niobium oxide, and tantalum oxide. As shown in FIG. 4A, the extinction coefficients of these materials satisfy the above-described formula (2) at wavelengths of 400 nm to 700 nm.

  Moreover, you may comprise the absorption layer 13 combining several metal oxides. The wavelength dependency of the transmittance of the GND filter 10 can be reduced by combining the two materials having different absorption coefficient wavelength dependencies to form the absorption layer 13. Thereby, when the GND filter 10 is used for the optical system or the like of the imaging apparatus, coloring of the image by the GND filter 10 can be reduced.

FIG. 5 shows the GND filter 10 when the absorption layer 13 includes a first absorption film 13a and a second absorption film 13b as an example in which the absorption layer 13 is formed by combining two materials. The first absorption layer 13a includes a first material that satisfies the following conditional expression (6a). The second absorption layer 13a contains a second material that satisfies the following conditional expression (6b).
α ₁ (400) <α ₁ (700) (6a)
α ₂ (400)> α ₂ (700) (6b)

Here, α ₁ (λ) is the absorption coefficient of the first material, and α ₂ (λ) is the absorption coefficient of the second material. Here, the numerical values in parentheses in the formulas (6a) and (6b) represent the wavelength of light in units of nanometers (nm). That is, Expression (6a) represents that the absorption coefficient of the first material at a wavelength of 400 nm is smaller than the absorption coefficient at a wavelength of 700 nm. Equation (6b) indicates that the absorption coefficient of the second material at a wavelength of 400 nm is larger than the absorption coefficient at a wavelength of 700 nm.

  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.

The reason why the wavelength dependency of the transmittance can be reduced by satisfying the expressions (6a) and (6b) will be described. When the intensity of incident light is I ₀ and the thickness of a film having an absorption coefficient of α (λ) is t, the intensity I of transmitted light transmitted through the absorption film is given by the following equation (7). .
I = I ₀ · exp (−α (λ) · t) (7)

  From equation (7), it can be seen that the wavelength dependence of the intensity of transmitted light originates from the wavelength dependence of the absorption coefficient. Therefore, in order to reduce the wavelength dependency of the transmittance, the wavelength dependency of α (λ) may be reduced.

  As described above, the first material absorbs more light on the long wavelength side than light on the short wavelength side. The second material 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 the wavelength bands with different signs, the absorption coefficient of the first material and the absorption coefficient of the second material are in 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 the first material and the second material satisfy the expressions (6a) and (6b), the wavelength band in which the absorption coefficient increases or decreases in the wavelength range from 400 nm to 700 nm (hereinafter, both) (Referred to as wavelength bands of the same sign).

  However, the absorption coefficient of the absorption layer 13 is obtained by adding the absorption coefficients of the first material and the second material according to the ratio of the thickness of the first absorption film 13a and the thickness of the second absorption film 13b. 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, by satisfying the equations (6a) and (6b), the wavelength dependency of the absorption coefficient of the absorption layer 13 can be reduced, and as a result, the wavelength dependency of the transmittance of the absorption layer 13 is reduced. Can do.

  FIG. 4B shows the wavelength dependence of the absorption coefficient of each material shown in FIG. As shown in FIG. 4B, the titanium oxide is selected as the first material, and niobium oxide or tantalum oxide is selected as the second material, so that the equations (6a) and (6b) are expressed. You can see that it meets. 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.

  Further, the combination of materials that can reduce the wavelength dependency of the transmittance of the absorption layer 13 is not limited to the above, and a combination of materials satisfying the formulas (6a) and (6b) is selected according to the characteristics. It ’s fine.

  In FIG. 5, the absorption layer 13 is configured by laminating a first absorption film 13a and a second absorption film 13b. The extinction coefficient of the absorption layer 13 at this time changes at the interface between the first absorption film 13a and the second absorption film 13b, but the first absorption film 13a and the second absorption film 13b If both satisfy the above formula (2), the absorption layer 13 satisfies the formula (2).

  Specifically, the extinction coefficient of the absorption layer 13 in FIG. 5 is equal to the extinction coefficient of the first material in the first absorption film 13a, and the extinction coefficient of the second material in the second absorption film 13b. It is equal to the decay 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 formula (2).

  Moreover, the absorption layer 13 is not limited to a uniform film formed by vapor deposition or sputtering. As shown in FIG. 6, the absorption layer 13 can also be formed by dispersing fine particles 132 formed of a light-absorbing material in a resin 131. FIG. 6 is an enlarged view of a part of the absorption layer 13.

  In this case, the extinction coefficient of the absorption layer 13 is calculated by calculating the absorption coefficient α (λ) from the amount of absorption and the thickness of the absorption layer 13, and from the absorption coefficient, α (λ) = 4πk (λ) / λ It can be obtained using an equation. The extinction coefficient thus obtained should satisfy the above-described formula (2) at wavelengths of 400 nm to 700 nm.

  Further, when the absorption layer 13 is formed as shown in FIG. 6, the first material and the second material satisfying the above-mentioned formulas (6a) and (6b), respectively, may be made fine particles and dispersed in the resin. good. Thereby, the wavelength dependence of the absorption coefficient of the absorption layer 13 can be reduced, and as a result, the wavelength dependence of the transmittance of the absorption layer 13 can be reduced.

[About the phase difference of the transmitted wavefront due to the change in the thickness of the absorbing layer]
The GND filter 10 has an absorption layer 13 whose thickness varies depending on the position on the substrate. Therefore, it is conceivable that a phase shift corresponding to the 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, the thickness depends on the position on the substrate, such as the GND filter 30 shown in FIG. 10A or the GND filter 40 shown in FIG. You may further provide the phase compensation layers 32 and 43 which are changing.

  FIG. 10B illustrates the thicknesses of the absorption layer 34 and the phase compensation layer 32 in the GND filter 30 having the phase compensation layer 32. The vertical axis represents the thickness, and the horizontal axis represents the radius of the GND filter 30. Indicates the position in the normalized plane. As shown in FIG. 10B, the thickness of the absorption layer 34 increases from the center to the periphery of the GND filter 30, while the thickness of the phase compensation layer 32 decreases from the center to the periphery. Has changed.

  That is, the thickness of the phase compensation layer 32 increases in the direction opposite to the direction in which the thickness of the absorption layer 34 increases. The thickness of the absorption layer 34 is the sum of the thickness of the first absorption film 34a and the thickness of the second absorption film 34b.

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 conditional expression (8).
| ΔOPD / λ | ≦ 0.30 (8)

  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 thickness of the absorption layer 34 is the 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 products of refractive indexes and thicknesses of the respective layers stacked on the substrate 31.

  By satisfying Expression (8), ΔOPD due to a change in the thickness of the absorption layer 32 can be compensated as shown in FIG.

  In addition, 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 layer regardless of the thickness of the absorption layer. Therefore, the reflectivity also changes depending on the thickness change of the phase compensation layer.

  When the phase compensation layer is arranged closer to the substrate than the absorption layer, the change in reflectance due to the change in the thickness of the phase compensation layer and the absorption layer is less from the substrate side than when light is incident from the air side. Tends to be large when light is incident. 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 the thickness of the phase compensation layer and the absorption layer is less on the air side than when light is incident from the substrate side. Tends to be large when light enters from.

  Thus, the tendency of the change in reflectance due to the change in the thickness of the absorption layer and the phase compensation layer differs depending on the position where the phase compensation layer is disposed and the incident direction of light. 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 (9) is satisfied.
| N _sub −N _cmp | <0.10 (9)

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 Equation (9), the reflectance at the interface between the substrate 31 and the phase compensation layer 32 can be reduced, and as a result, the change in reflectance due to the change in the thickness of the phase compensation layer can be reduced. it can. More preferably, the range of the formula (9) is set to the following range.
| N _sub −N _cmp | <0.05 (9a)

In addition, as shown in FIG. 12, when the phase compensation layer 43 is disposed at a position adjacent to the absorption layer 44, it is preferable that the following conditional expression (10) is satisfied.
| N _{abs, c} −N _cmp | <0.15 (10)

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 (10), 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 the thickness of the phase compensation layer 43 is reduced. be able to. More preferably, the range of the formula (10) is set to the following range.
| N _{abs, c} −N _cmp | <0.10 (10a)

[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. 4 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. 4 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. 4 can be obtained.

  Further, when the absorption layer 13 is formed by vapor deposition, the thickness of the absorption layer 13 can be changed by generating a temperature gradient in the substrate 11 or 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 material. Examples of other methods for forming the absorption layer 13 include sputtering, plating, and spin coating.

  After forming the absorption layer 13 in this way, the surface layer 14 is formed by vapor deposition or sputtering. At this time, when the above-described hollow film is used as the first film 14 c of the surface layer 14, a film other than the first film 14 is laminated by vapor deposition or sputtering, and then the hollow film is formed using a sol-gel method or the like. It ’s fine. First, a hollow membrane can be formed by applying a raw material solution for a hollow membrane by a spin coating method or the like and heating it with an electric furnace or a hot plate.

[Transmittance distribution of GND filter]
The GND filter 10 according to the present embodiment has a transmittance distribution corresponding to the thickness 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. 7A and 7B, or the transmittance changes in one direction as shown in FIGS. 7C and 7D. 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, the characteristics of the GND filter of the present embodiment will be described in each example.

[Example 1]
The characteristics of the GND filter as the optical element in the first embodiment will be described. Table 1 shows details of each film constituting the GND filter of Example 1.

  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 thickness of the thin film. The same applies to the following embodiments.

  In the GND filter of this embodiment, the surface layer is composed of three layers and the intermediate layer is composed of four layers.

  Table 1 shows that in the surface layer, the first film satisfies the formula (1). In addition, the second film and the third film are alternately stacked in total two layers.

  The absorption layer is composed of two absorption films in order to reduce the wavelength dependency of the transmittance. The first absorption film is made of titanium oxide, and the second absorption film is made of tantalum oxide. The extinction coefficients of the titanium oxide and tantalum oxide in this example are as shown in FIG. It can be seen that the extinction coefficient of the absorbing layer satisfies the formula (2) by forming the absorbing layer using the titanium oxide and tantalum oxide shown in FIG.

  Both the thickness of the first absorption film and the thickness of the second absorption film change depending on the position on the substrate. The ratio of the thickness of the first absorption film to the thickness of the second absorption film is 1: 1. The thickness of the absorption layer is 1000 nm at the thickest position. That is, at this position, the thickness of the first absorption film and the thickness of the second absorption film are both 500 nm.

  FIG. 8 shows the wavelength dependence of the reflectance and transmittance of the GND filter of this embodiment. 8A shows the reflectance when light enters from the air side, FIG. 8B shows the reflectance when light enters from the substrate side, and FIG. 8C shows the transmittance. 8A, 8 </ b> B, and 8 </ b> C, the solid line indicates that the thickness of the absorption layer 13 is 0 nm, the dotted line indicates that the thickness is 50 nm, the broken line indicates that the thickness is 100 nm, and the alternate long and short dash line indicates that the thickness is 200 nm. The long broken line represents the case where the thickness is 1000 nm.

  As shown in FIGS. 8A and 8B, the GND filter of the present embodiment has a reflectance of 4% or less in both the case of air side incidence and the case of substrate side incidence. In particular, in the vicinity of 550 nm where the specific luminous sensitivity is high, the reflectance is 2% or less regardless of the transmittance of the absorbing layer. Thus, it can be seen that the change in reflectance due to the change in transmittance of the absorption layer is small. Further, as shown in FIG. 8C, the change in the wavelength dependency of the transmittance due to the change in the transmittance of the absorption layer is small, indicating a flat transmittance. This is because the absorption layer has the first absorption film and the second absorption film that satisfy the expressions (6a) and (6a), respectively.

[Example 2]
Next, a GND filter as an optical element in Embodiment 2 will be described. Table 2 shows the details of each film constituting the GND filter of Example 2.

  In the GND filter of Example 2, the surface layer is composed of five thin films and the intermediate layer is composed of four thin films. In Example 2, the refractive index of the first film is lower than that in Example 1. Further, the third embodiment is different from the first embodiment in that the third film and the second film are alternately laminated in a total of four layers.

  The absorption layer is composed of two absorption films in order to reduce the wavelength dependency of the transmittance. The first absorption film is made of titanium oxide, and the second absorption film 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 absorbing layer satisfies the formula (2) by forming the absorbing layer using the titanium oxide and niobium oxide shown in FIG.

  Both the thickness of the first absorption film and the thickness of the second absorption film change depending on the position on the substrate. The ratio of the thickness of the first absorption film to the thickness of the second absorption film is 1: 2. The thickness of the absorption layer is 1000 nm at the thickest position. That is, at this position, the thickness of the first absorption film is 333 nm, and the thickness of the second absorption film is 667 nm.

  FIG. 9 shows the wavelength dependence of the reflectance and transmittance of the GND filter in this embodiment. FIG. 9A shows the reflectance when light enters from the air side, FIG. 9B shows the reflectance when light enters from the substrate side, and FIG. 9C shows the transmittance. In each of FIGS. 9A, 9B, and 9C, the solid line indicates the thickness of the absorbing layer is 0 nm, the dotted line indicates the thickness of 50 nm, the broken line indicates the thickness of 100 nm, the alternate long and short dash line indicates the thickness of 200 nm, The long broken line represents the case where the thickness is 1000 nm.

  As shown in FIGS. 9A and 9B, the GND filter of this example has a reflectance of 1.5% or less both in the case of air side incidence and in the case of substrate side incidence. . That is, in this embodiment, the reflectance can be further reduced as compared with the first embodiment. This is because the refractive index of the first film is reduced and the number of stacked layers of the second film and the third film is increased.

[Example 3]
Next, a GND filter as an optical element in Embodiment 3 will be described. FIG. 10A shows a schematic diagram of the GND filter 30 of the present embodiment. 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 side closer to the substrate 31. That is, the GND filter 30 according to the present embodiment is different from the second embodiment in that the phase filter layer 32 is provided. Table 3 shows details of each film constituting the GND filter 30 of Example 3.

  In the GND filter 30 according to the third embodiment, the surface layer is composed of five layers and the intermediate layer is composed of four layers.

  The absorption layer 34 is composed of two absorption films in order to reduce the wavelength dependency of the transmittance. The first absorption film 34a is made of titanium oxide, and the second absorption film 34b 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 34 satisfies the equation (2) by forming the absorption layer 34 using the titanium oxide and niobium oxide shown in FIG.

  Both the thickness of the first absorption film 34a and the thickness of the second absorption film 34b change according to the position on the substrate. The ratio of the thickness of the first absorption film 34a to the thickness of the second absorption film 34b is 1: 2. The thickness of the absorption layer 13 is 1000 nm at the thickest position. That is, at this position, the thickness of the first absorption film 34a is 333 nm, and the thickness of the second absorption film 34b is 667 nm.

  FIG. 11 shows the wavelength dependence of the reflectance and transmittance of the GND filter 30 in this embodiment. FIG. 11A shows the reflectance when light enters from the air side, FIG. 11B shows the reflectance when light enters from the substrate side, and FIG. 11C shows the transmittance. In each figure of FIG. 11 (a), (b), (c), the solid line has a thickness of 0 nm, the dotted line has a thickness of 50 nm, the broken line has a thickness of 100 nm, the alternate long and short dash line has a thickness of 200 nm, The long broken line represents the case where the thickness is 1000 nm.

  As shown in FIGS. 11A and 11B, the GND filter 30 of the present embodiment 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 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. .

[Example 4]
Next, a GND filter as an optical element in Embodiment 4 will be described. FIG. 12A shows a schematic diagram of the GND filter 40 of the present embodiment. 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 side closer to the substrate 41. That is, in the GND filter 40 of this embodiment, the position of the phase compensation layer 43 is different from that of the third embodiment. Table 4 shows details of each film constituting the GND filter 40 of Example 4.

  In the GND filter 40 according to the fourth embodiment, the surface layer is composed of five layers and the intermediate layer is composed of four layers.

  The absorption layer 44 is composed of two absorption films in order to reduce the wavelength dependency of the transmittance. The first absorption film 44a is made of titanium oxide, and the second absorption film 44b 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 absorbing layer 44 satisfies the formula (2) by forming the absorbing layer 44 using the titanium oxide and niobium oxide shown in FIG.

  Both the thickness of the first absorption film 44a and the thickness of the second absorption film 44b vary depending on the position on the substrate. The ratio of the thickness of the first absorption film 44a to the thickness of the second absorption film 44b is 1: 2. The thickness of the absorption layer 44 is 1000 nm at the thickest position. That is, at this position, the thickness of the first absorption film 44a is 333 nm, and the thickness of the second absorption film 44b is 667 nm.

  FIG. 13 shows the wavelength dependence of the reflectance and transmittance of the GND filter 40 in this embodiment. FIG. 13A shows the reflectance when light enters from the air side, FIG. 13B shows the reflectance when light enters from the substrate side, and FIG. 13C shows the transmittance. 13A, 13 </ b> B, and 13 </ b> C, the solid line indicates that the thickness of the absorption layer 44 is 0 nm, the dotted line indicates that the thickness is 50 nm, the broken line indicates that the thickness is 100 nm, and the alternate long and short dash line indicates that the thickness is 200 nm. The long broken line represents the case where the thickness is 1000 nm.

  As shown in FIGS. 13A and 13B, the GND filter 40 of this embodiment has a reflectance of 4% or less both in the case of air side incidence and in the case of substrate side incidence. In particular, in the vicinity of 550 nm where the relative luminous sensitivity is high, the reflectance is 2% or less irrespective 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 44 is small. .

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

  FIG. 14A 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 change in reflectance due to the change in the transmittance of the absorption layer. Therefore, generation of ghosts and flares can be suppressed, and a high-quality image can be obtained.

  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. 7A and 7B is preferable. In addition, as shown in FIGS. 1, 10 and 12, 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.

  In addition, by providing a region in which the absorption layer is not formed at the center of the GND filter, it is possible to obtain a large blurred image while improving the blurred image due to the 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. 14B 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, generation of ghosts and flares can be suppressed, and high-quality images can be obtained.

  14B shows an example in which the main body portion 81 and the lens portion 82 are integrated, 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. 14B 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.

DESCRIPTION OF SYMBOLS 10 GND filter 11 Substrate 13 Absorption layer 14 Surface layer 14a 3rd film | membrane 14b 2nd film | membrane 14c 1st film | membrane

Claims (21)

An optical element having a substrate, a surface layer including a plurality of thin films, and an absorption layer having a different transmittance depending on a position on the substrate provided between the surface layer and the substrate,
The refractive index of the first film disposed at the position farthest from the substrate in the surface layer is 1.05 or more and 1.4 or less,
A second film having a refractive index higher than that of the substrate at a wavelength of 550 nm and a refractive index lower than that of the second film at a wavelength of 550 nm are positioned closer to the substrate than the first film in the surface layer. A total of two or more third films are laminated alternately,
An extinction coefficient of the absorption layer is 0.5 or less at a wavelength of 400 nm to 700 nm.   The optical element according to claim 1, wherein an absolute value of a difference in refractive index between the second film and the third film adjacent to each other is 0.4 or more. The optical element further includes an intermediate layer between the substrate and the absorbing layer,
Of the films constituting the absorbing layer, when the refractive index of the film closest to the intermediate layer is N _{abs, int} , and the refractive index of the substrate is N _sub ,
The optical element according to claim 1, wherein the intermediate layer includes a film having a refractive index of a value between N _{abs, int} and N _sub . When the equivalent admittance from the substrate to the intermediate layer is η _sub and the vacuum admittance is Y ₀ , at a wavelength of 550 nm,
| Re (η _sub ) / Y ₀ −N _{abs, int} | <0.25
The optical element according to claim 3, wherein the following conditional expression is satisfied. The optical element has a region where the absorption layer is not formed between the intermediate layer and the surface layer,
When the equivalent admittance from the substrate to the intermediate layer is η _sub , the equivalent admittance from the air to the surface layer is η _air , and the vacuum admittance is Y ₀ , at a wavelength of 550 nm,
| Re (η _sub ) / Y ₀ −Re (η _air ) / Y ₀ | <0.3
The optical element according to claim 3, wherein the following conditional expression is satisfied. When the equivalent admittance from the air to the surface layer is η _air , the vacuum admittance is Y ₀ , and the refractive index of the film closest to the surface layer among the films constituting the absorption layer is _{Nabs, sur} , the wavelength is 550 nm. In
| Re (η _air ) / Y ₀ −N _{abs, sur} | <0.25
The optical element according to claim 1, wherein the following conditional expression is satisfied.   The optical element according to claim 1, wherein the first film has a gap formed therein.   The optical element according to any one of claims 1 to 7, wherein the first film contains silica or magnesium fluoride. The absorption layer includes a first material having an absorption coefficient at a wavelength of 400 nm that is smaller than the absorption coefficient at a wavelength of 700 nm;
The optical element according to claim 1, further comprising: a second material having an absorption coefficient at a wavelength of 400 nm that is larger than an absorption coefficient at a wavelength of 700 nm.   The optical element according to any one of claims 1 to 9, wherein the thickness of the absorption layer changes in accordance with a position on the substrate. A phase compensation layer that changes in accordance with the position on the substrate;
The optical element according to claim 10, wherein the thickness of the phase compensation layer increases in a direction opposite to a direction in which the thickness of the absorption layer increases. When the difference between the optical path length at the position where the thickness of the absorption layer is the smallest and the optical path length at the position where the thickness of the absorption layer is thicker than the position is ΔOPD and the wavelength of light is λ,
The thickness of the phase compensation layer is
| ΔOPD / λ | ≦ 0.30
The optical element according to claim 11, wherein the optical element is changed so as to satisfy the following conditional expression. The phase compensation layer is disposed at a position adjacent to the substrate;
When the refractive index of the substrate is N _sub and the refractive index of the phase compensation layer is N _cmp ,
| N _sub −N _cmp | <0.10
The optical element according to claim 11, 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 of the film adjacent to the phase compensation layer is N _{abs, c} and the refractive index of the phase compensation layer is N _cmp ,
| N _{abs, c} −N _cmp | <0.15
The optical element according to claim 11, wherein the following conditional expression is satisfied.   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 15, wherein the absorption layer is not formed at the center of the concentric circle.   The optical element according to claim 1, wherein the substrate is a lens.   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 claim 1.   An optical system comprising: a diaphragm; and at least one optical element disposed on each of a light incident side and a light emitting side of the diaphragm.   An image pickup apparatus comprising an image pickup element and the optical system according to claim 18.   A lens apparatus comprising the optical system according to claim 18, wherein the lens apparatus is detachable from the imaging apparatus main body.

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