JP7693319B2 - Optical filter, optical element and imaging device - Google Patents

Description

The present invention relates to an optical filter that reflects part of the incident light and transmits the other part.

A so-called half mirror is used as the optical filter described above. Patent Document 1 discloses a photographing device that is provided with a half mirror as a front cover that covers the internal components from the front. The subject can view their own image reflected by the half mirror and check their facial expression and pose while taking the photograph.

Furthermore, half mirrors are also used as covers to cover surveillance cameras so that the presence and orientation of the camera cannot be seen from the outside, and the half mirror cover, which has a curvature, itself functions as a surveillance mirror.

JP 2016-38494 A

Generally, a half mirror has approximately the same reflectance (transmittance) for light entering from its front side and for light entering from its back side. For this reason, if a half mirror cover for a surveillance camera has a high reflectance for light entering from the subject side, it will also have a high reflectance for light entering from the camera side. For this reason, light that enters the camera from the subject side and is reflected by the camera is reflected by the half mirror cover, and this reflected light enters the camera again, causing ghosts and flares.

The present invention provides an optical filter whose reflectance varies depending on the direction of light incidence.

The optical filter according to one aspect of the present invention is composed of a first layer as a reflection enhancing layer , an absorption layer having light absorption properties , and a second layer as an anti-reflection layer, which are arranged adjacent to each other in this order from the substrate side, and each of the first and second layers is composed of a material having an extinction coefficient of 0.02 or less at a wavelength of 550 nm. When the reflectance when light having a wavelength of 550 nm is incident from the side opposite to the substrate is R1 [%] and the reflectance when light is incident from the substrate side is R2 [%],
R2-R1≧10
The optical filter is characterized in that it satisfies the following conditions: An optical element having the optical filter, and an imaging device using the optical element also constitute other aspects of the present invention.

The present invention makes it possible to realize an optical filter whose reflectance varies depending on the direction of light incidence, functioning as a reflective film for light incident from one direction and as an anti-reflective film for light incident from the other direction.

FIG. 1 is a cross-sectional view of an optical element provided with an optical filter according to an embodiment of the present invention. 3 is a cross-sectional view illustrating the reflection/transmission characteristics of the optical filter. FIG. 3A and 3B are diagrams showing reflectance and transmittance characteristics of the optical filter. FIG. 1 is a diagram showing the refractive index and extinction coefficient of materials used in the optical filters of Examples 1 to 5. FIG. 13 is a graph showing the reflectance and transmittance characteristics of the optical filters of Examples 1 to 5. FIG. 13 is a diagram showing the refractive index and extinction coefficient of the absorbing materials K2 and K3 used in the optical filters of Examples 6 to 10. FIG. 13 is a graph showing the reflectance and transmittance characteristics of the optical filters of Examples 6 to 10. FIG. 13 is a diagram showing the refractive index and extinction coefficient of materials used in the optical filters of Examples 11 to 17. FIG. 13 is a graph showing the reflectance and transmittance characteristics of the optical filters of Examples 11 to 17. FIG. 2 is a diagram showing the refractive index and extinction coefficient of materials used in the optical filters of Examples 18 to 23. FIG. 23 is a graph showing the reflectance and transmittance characteristics of the optical filter of Example 18. FIG. 23 is a graph showing the reflectance and transmittance characteristics of the optical filter of Example 19. FIG. 23 is a graph showing the reflectance and transmittance characteristics of the optical filter of Example 20. FIG. 23 is a graph showing the reflectance and transmittance characteristics of the optical filter of Example 21. FIG. 23 is a graph showing the reflectance and transmittance characteristics of the optical filter of Example 22. FIG. 23 is a graph showing the reflectance and transmittance characteristics of the optical filter of Example 23. 1 is a schematic diagram of an image pickup using an optical element according to an embodiment. FIG. 1 is a schematic diagram of a digital camera according to an embodiment. FIG. 1 is a schematic diagram of a surveillance camera according to an embodiment.

Below, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an optical element 100 according to an embodiment of the present invention. The optical element 100 is composed of a substrate 01 made of a light-transmitting material and an optical filter 10. The optical filter 10 is composed of an anti-reflection layer 02, an absorption layer 03, and a reflective layer (reflective layer or reflection-increasing layer) 04, which are stacked in this order from the substrate 01 side to the air 05 side. The reflective layer 04 is in contact with the air 05 (refractive index 1). The refractive index of the substrate 01 is greater than the refractive index of the air 05.

The anti-reflection layer 02 and the reflection-enhancing layer 04 are each composed of one or more thin layers. The number of layers of the anti-reflection layer 02 and the reflection-enhancing layer 04 can be appropriately selected to adjust the refractive index, expand the band of use, reduce the incidence angle dependency, and reduce the polarization dependency. The absorption layer 03 is composed of one or more layers of a material having light absorption properties.

Figure 2 shows an optical element 100' that does not have the antireflection layer 02 and the reflection-enhancing layer 04 shown in Figure 1. The absorption layer 03 provided on the substrate 01 is in contact with air 05 (refractive index 1).

Figure 3 shows the relationship between the reflectance and the thickness of the absorbing layer 03 in the optical element 100' and the relationship between the transmittance and the thickness of the absorbing layer 03 when the incident light has a wavelength λ of 450, 550, or 650 nm. The absorbing layer 03 has a refractive index n = 2.35 and an extinction coefficient k = 0.05, regardless of the wavelength of the incident light.

As can be seen from the reflectance graph in Figure 3, the reflectance when light is incident from the air 05 side is greater than the reflectance when light is incident from the substrate 01 side, regardless of the film thickness. The reflectance also differs depending on the wavelength of the incident light. On the other hand, as can be seen from the transmittance graph in Figure 3, the transmittance is the same regardless of the direction of light incidence. Furthermore, the transmittance decreases as the film thickness of the absorption layer 03 increases.

From the above, if a material having light absorption properties is used, an optical filter can be produced in which the reflectance differs between when light is incident from the air 05 side and when it is incident from the substrate 01 side. However, in a film configuration such as the optical element 100', the reflectance differs depending on the wavelength of the incident light, and the anti-reflection effect is small. For this reason, in this embodiment, as in the optical element 100 (optical filter 10) shown in FIG. 1, a first layer, the anti-reflection layer 02, is disposed between the absorption layer 03 and the substrate 01, and a second layer, the increased reflection layer 04, is disposed between the absorption layer 03 and the air 05. The anti-reflection layer 02 functions as an anti-reflection film mainly for incident light from the substrate 01 side, and the increased reflection layer 04 functions as a reflection film mainly for incident light from the air 05 side.

The optical filter 10 of this embodiment is used for light with wavelengths of 420 to 680 nm, which is in the visible light region. In the optical filter 10, the reflectance when light with a wavelength of 550 nm is incident from the substrate 01 side (hereinafter referred to as substrate side reflectance) is R2 [%], and the reflectance when light is incident from the opposite side to the substrate 01 (hereinafter referred to as air side reflectance or bonding member side reflectance) is R1 [%]. In this case,
|R1-R2|≧10[%] (1)
It is desirable to satisfy the following relationship. The optical filter 10 of this embodiment differs from a general half mirror in that the reflectance varies depending on the direction of incidence of light. As a result, the optical filter 10 functions as an anti-reflection film for light incident from one side and as a reflective film for light incident from the other side.

It is more preferable that the numerical range of formula (1) is set in the order of formula (1)' to formula (1)" below.
80≧｜R1-R2｜≧15 [%] (1)'
60≧｜R1-R2｜≧18[%] (1)″
The optical filter 10 includes an anti-reflection layer 02 as a first layer, an absorbing layer 03, and a reflective layer 04 as a second layer, which are stacked in this order from the substrate 01 side. When the absorbing layer 03 is composed of m (plural) films, the extinction coefficient of the j-th film from the substrate 01 side among the m films (hereinafter, referred to as the absorbing j film) is kj and the physical film thickness is dj,

It is desirable to satisfy the following condition. kj×dj is a coefficient that determines the transmittance of the absorption layer 03. If the sum of kj×dj is greater than 132, the transmittance becomes extremely small, and the optical element 10 cannot be used as an element that transmits light. On the other hand, if the sum of kj×dj is less than 8, it becomes difficult to increase the difference between the reflectances R1 and R2.

The refractive index nj of the absorbing film j is
1.8≦nj≦4.2 (3)
It is desirable to satisfy the following condition: The refractive index of a dielectric material having an extinction coefficient that can be easily produced by a dry film formation method such as vapor deposition or sputtering falls roughly within this range.

In addition,
2.0≦nj≦3.6 (3)′
It is more preferable to satisfy the following:

Furthermore, the extinction coefficient kj of the absorbing film j is
0.05≦kj≦4.00 (4)
It is desirable to satisfy the following condition. If the extinction coefficient is less than 0.05, the amount of oxygen introduced into the dry film forming apparatus must be controlled with high precision, and the amount of oxygen also depends on the amount of film forming material attached inside the apparatus, making it difficult to form a film with good reproducibility with an extinction coefficient of less than 0.05. Also, if the extinction coefficient is more than 4.0, the film thickness required to achieve the required characteristics becomes very thin, making film thickness control difficult, which is not preferable.

In addition,
0.10≦kj≦3.60 (4) '
It is more preferable to satisfy the following:

The material of the substrate 01 is glass, plastic, etc., and is not particularly limited. The shape of the substrate 01 may be a flat plate, a convex shape, a concave shape, etc., or a curved shape other than a spherical shape.

The method for forming the absorbing layer 03 is preferably a dry film formation method such as vapor deposition or sputtering, but may also be a wet film formation method such as plating or spin coating. The material for the absorbing layer 03 may be any material that satisfies the above-mentioned conditions related to the extinction coefficient k (conditional expressions (2) and (4)), and examples of the material include an oxygen-deficient _TiO2 film, _{a Nb2O5} film, a _Ta2O5 film, _an ITO film, and a Cr film.

As a method for forming the first layer and the second layer, a dry film formation method such as vapor deposition or sputtering is preferable, but a wet film formation method such as plating or spin coating may also be used. The film material constituting the first layer and the second layer is preferably made of a material that does not absorb light. This makes it possible to suppress the light absorption that occurs in the optical filter of this embodiment and achieve a high transmittance. The extinction coefficient at a wavelength of 550 nm of the film material constituting the first layer and the second layer is preferably 0.02 or less, more preferably 0.01 or less, and even more preferably 0.005 or less.

In addition, when the reflectance R1 is greater than the reflectance R2 (R1-R2≧10[%]),
0≦R2/R1≦0.5 (5)
It is desirable to satisfy the following condition. Since the reflectances R1 and R2 have values of 0 or more, they do not fall below the lower limit of formula (5). If the reflectance R1 is so small as to exceed the upper limit of formula (5), it becomes difficult to confirm one's own image using the air-side reflected light of the optical filter 10.

In addition,
0≦R2/R1≦0.4 (5) '
It is more preferable to satisfy the following:

On the other hand, when the reflectance R2 is greater than the reflectance R1 (R2-R1≧10[%]),
0≦R1/R2≦0.5 (6)
It is desirable to satisfy the following condition. Since the reflectances R1 and R2 have values equal to or greater than 0, they do not fall below the lower limit of formula (6). If the reflectance R2 is so small as to exceed the upper limit of formula (6), it becomes difficult to confirm the image of the optical filter 10 by using the light reflected on the substrate side.

In addition,
0≦R1/R2≦0.4 (6) '
It is more preferable to satisfy the following:

When the difference in refractive index between the outermost layer of the second layer, the reflective layer 04, opposite the absorbing layer 03 and the medium with which the outermost layer is in contact is dn,
dn≦0.50 (7)
If the refractive index difference dn is so large as to exceed the upper limit of the formula (7), it becomes difficult to sufficiently reduce the reflection occurring at the interface by using the interference effect.

In addition,
dn≦0.40 (7) ′
It is more preferable to satisfy the following:

The second layer may be tightly bonded to the optical element via a bonding member and disposed at the bonding interface so as not to expose the optical filter to the atmosphere. Compared to when the second layer is in contact with air, the difference in refractive index between the outermost layer on the substrate side of the optical filter and the surface layer opposite the substrate, and the member adjacent to the outermost layer, is smaller, so that a further anti-reflection effect can be obtained.

When the refractive indices of the substrate, the optical element, and the bonding member at a wavelength of 550 nm are n, n, and n, respectively,
|ns-na|≦|ne-na| (8)
By satisfying this condition, the difference in refractive index between the outermost layer of the optical filter and the adjacent member can be reduced, and an even greater anti-reflection effect can be obtained.

Specific examples are given below. However, these are merely examples, and the embodiments of the present invention also include other examples.

Table 1 shows the film configuration of the optical filters of Examples 1 to 5. In each example, the material of the substrate 01 is a glass material S1 that does not absorb light. The material of the anti-reflection layer 02, which is the first layer, is a dielectric material H1. The reflection-enhancing layer 04, which is the second layer, is a multilayer film in which dielectric materials M1, H1, and L1 are stacked in this order from the absorption layer 03 side. The number of layers (2) of the reflection-enhancing layer 04 as a multilayer film is greater than the number of layers (1) of the anti-reflection layer 02. All of the dielectric materials M1, H1, and L1 are materials that do not absorb light. The refractive indices for each wavelength of the glass material S1 and the dielectric materials M1, H1, and L1 are shown in Figure 4 (a).

The material of the absorption layer 03 is an absorption material K1 having light absorption properties. The refractive index and extinction coefficient of the absorption material K1 for each wavelength are shown in Figure 4 (b). The extinction coefficient of the absorption material K1 at λ = 550 nm is 0.34.

Figures 5(a) to (d) show the air-side reflectance, substrate-side reflectance, and transmittance for each wavelength of the optical filters of Examples 1 to 5. As can be seen from these figures, the difference between the air-side reflectance and substrate-side reflectance at λ = 550 nm of the optical filters of Examples 1 to 5 is 10% or more, satisfying conditional formula (1) (and further (1)').

The film thickness of the absorbing material K1 in Examples 1 to 5 is 50 nm, 100 nm, 150 nm, 200 nm, and 250 nm, respectively, as shown in Table 1, and kj x dj satisfies conditional formula (2).

Table 2 shows the film configuration of the optical filters of Examples 6 to 10. In these examples, the material of the substrate 01 is glass material S1, as in Examples 1 to 5. The first layer, the antireflection layer 02, is a multilayer film in which dielectric materials H1, M1, and H1 are stacked in this order from the substrate 01 side. The second layer, the enhanced reflection layer 04, is a multilayer film in which dielectric materials M1, H1, M1, H1, and M1 are stacked in this order from the absorption layer 03 side. The number of layers (5) of the enhanced reflection layer 04 as a multilayer film is greater than the number of layers (3) of the antireflection layer 02.

The absorbing layer 03 is a multilayer film in which the absorbing materials K3 and K2 are laminated in this order from the anti-reflection layer 02 side. The refractive indexes and extinction coefficients of the absorbing materials K2 and K3 for each wavelength are shown in Figures 6(a) and 6(b), respectively. The extinction coefficient of the absorbing material K2 at λ = 550 nm is 0.19, and the extinction coefficient of the absorbing material K3 at λ = 550 nm is 0.26.

Figures 7(a) to (d) show the air-side reflectance, substrate-side reflectance, and transmittance for each wavelength of the optical filters of Examples 6 to 10. As can be seen from these figures, the difference between the air-side reflectance and the substrate-side reflectance at λ = 550 nm of the optical filters of Examples 6 to 10 is 10% or more, satisfying conditional formula (1) (and further (1)').

The film thicknesses of the absorbing materials K2 and K3 in Examples 6 to 10 are 50 nm, 100 nm, 150 nm, 200 nm, and 250 nm, respectively, as shown in Table 2, and kj x dj satisfies conditional formula (2).

Table 3 shows the film configuration of the optical filters of Examples 11 to 17. In these Examples, the material of the substrate 01 is the glass material S2, which has no light absorption. The first layer, the anti-reflection layer 02, is a multilayer film in which the dielectric materials H2, M2, and H2 are stacked in order from the substrate 01 side. The second layer, the enhanced reflection layer 04, is a multilayer film in which the dielectric materials M2, H2, M2, H2, and M2 are stacked in order from the absorption layer 03 side. The number of layers (5) of the enhanced reflection layer 04 as a multilayer film is greater than the number of layers (3) of the anti-reflection layer 02. Both the dielectric materials M2 and H2 are materials that have no light absorption. The refractive indexes for each wavelength of the glass material S2 and the dielectric materials M2 and H2 are shown in Figure 8 (a).

The material of the absorption layer 03 is an absorption material K4 having light absorption properties. The refractive index and extinction coefficient of the absorption material K4 for each wavelength are shown in Figure 8 (b). The extinction coefficient of the absorption material K4 at λ = 550 nm is 0.20.

9(a) to ( g ) show the air-side reflectance, substrate-side reflectance, and transmittance for each wavelength of the optical filters of Examples 11 to 17. As can be seen from these figures, the difference between the air-side reflectance and the substrate-side reflectance at λ=550 nm of the optical filters of Examples 11 to 17 is 10% or more, satisfying conditional formula (1) (and further (1)').

The film thickness of the absorbing material K4 in Examples 11 to 17 is 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, and 600 nm, respectively, as shown in Table 3, and kj x dj satisfies conditional formula (2).

Table 4 shows the film structure of the optical filter of Example 18. The optical filter of this example is composed of a first layer, a reflection-enhancing layer, an absorption layer, and a second layer, an anti-reflection layer, which are stacked in this order from the substrate side made of glass material S2. The optical filter of this example is characterized in that the reflectance R2 when light is incident from the substrate side is greater than the reflectance R1. The dielectric materials M3, H3, and L3 are all non-light-absorbing materials, and their refractive indices for each wavelength are shown in Figure 10(a).

The material of the absorption layer is the light-absorbing material K5. The refractive index and extinction coefficient of the absorption material K5 for each wavelength are shown in Figure 10 (b). The extinction coefficient of the absorption material K5 at a wavelength of 550 nm is 3.33.

Figure 11 shows the air-side reflectance, substrate-side reflectance, and transmittance for each wavelength of the optical filter of Example 18. As can be seen from Figure 11, the optical filter of Example 18 satisfies conditional formula (1).

Table 5 shows the film configuration of the optical filter of Example 19. The configuration of the optical filter of this example is the same as that of Example 18.

The material of the absorbing layer is absorbing material K6, which has light absorbing properties. The refractive index and extinction coefficient of absorbing material K6 for each wavelength are shown in Figure 10 (c). The extinction coefficient of absorbing material K6 at a wavelength of 550 nm is 1.24.

Figure 12 shows the air-side reflectance, substrate-side reflectance, and transmittance for each wavelength of the optical filter of Example 19. As can be seen from Figure 12, the optical filter of Example 19 satisfies conditional formula (1).

Table 6 shows the film configuration of the optical filter of Example 20. The configuration of the optical filter of this example is the same as that of Examples 11 to 17.

The material of the absorption layer is the light-absorbing material K6, which has an extinction coefficient of 1.24 at a wavelength of 550 nm.

Figure 13 shows the air-side reflectance, substrate-side reflectance, and transmittance for each wavelength of the optical filter of Example 20. As can be seen from Figure 13, the optical filter of Example 20 satisfies conditional formula (1).

Table 7 shows the film configuration of the optical filter of Example 21. The configuration of the optical filter of this example is the same as that of Example 18.

The material of the absorbing layer is composed of absorbing materials K7 and K8 having light absorbing properties. The refractive indexes of the absorbing materials K7 and K8 at each wavelength and the average extinction coefficient of the two layers are shown in Figure 10(d). The average extinction coefficient of the two layers at a wavelength of 550 nm is 0.29.

Figure 14 shows the air-side reflectance, substrate-side reflectance, and transmittance for each wavelength of the optical filter of Example 21. As can be seen from Figure 14, the optical filter of Example 21 satisfies conditional formula (1).

Table 8 shows the film configuration of the optical filter of Example 22. The configuration of the optical filter of this example is the same as that of Example 18.

The material of the absorbing layer is composed of light-absorbing materials K7 and K8, and the average extinction coefficient of the two layers at a wavelength of 550 nm is 0.29.

Figure 15 shows the air-side reflectance, substrate-side reflectance, and transmittance for each wavelength of the optical filter of Example 22. As can be seen from Figure 15, the optical filter of Example 22 satisfies conditional formula (1).

Table 9 shows the film structure of the optical filter of Example 23. The optical filter of this example is composed of a first layer, a reflection-enhancing layer, an absorption layer, a second layer, an anti-reflection layer, a bonding member S3, and an optical element S4, which are stacked in this order from the substrate side made of glass material S2. The optical element S4 is closely bonded to the second layer via the bonding member S3, and is an optical element in which an optical filter is disposed at the bonding interface. The refractive indexes of the glass material S2, the bonding member S3, and the optical element S4 at a wavelength of 550 nm satisfy conditional expression (8). In addition, in order to reduce reflection at the interface between the bonding member S3 and the optical element S4, a layer having an anti-reflection effect may be provided at the interface between the bonding member S3 and the optical element S4.

The material of the absorption layer is the light-absorbing material K6, which has an extinction coefficient of 1.24 at a wavelength of 550 nm.

Figure 16 shows the bonding member side reflectance, substrate side reflectance, and transmittance for each wavelength of the optical filter of Example 23. As can be seen from Figure 16, the optical filter of Example 23 satisfies conditional formula (1).

Various numerical values for each example are summarized in Table 10. The transmittance is the value at a wavelength of 550 nm.

In Examples 11 to 23, the transmittance has wavelength dispersion dependency. The resulting color balance of the captured image may be corrected by the imaging device shown in FIG. 17 , if necessary. The change in color balance caused by the optical filter 10 may be offset by the transmittance wavelength characteristic of the antireflection film 20 shown in FIG. 17. When the chromaticity coordinates in the CIE1976L*a*b* color space are a* and b*, either the transmitted light of the optical filter 10 or the chromaticity coordinates a* and b* of the antireflection film 20 may be configured to have different signs.

In addition, in Examples 1 to 22, the material of the surface layer on the air side was L1 (refractive index 1.38), M1 (refractive index 1.45), M2 (refractive index 1.47) or L3 (refractive index 1.38), but a layer containing a fluorine-based material may be added to these surfaces. As the fluorine-based material, OF-SR or OF-210 manufactured by Canon Optron Inc. can be used. By adding such a layer, the stain resistance of the lens surface can be improved.

Figure 17 shows an image capture using an optical element 100 having an optical filter 10 according to any one of the above-mentioned Examples 1 to 23. The optical element 100 is disposed between a subject 400 and an imaging optical system 200. An imaging element 300 such as a CCD sensor or a CMOS sensor is disposed on the image plane of the imaging optical system 200.

In the optical element 100, an optical filter 10 is provided on the surface of the substrate 01 facing the subject 400, and an anti-reflection film 20 is provided on the surface facing the imaging optical system 200. The anti-reflection film 20 may be a general anti-reflection film, and there are no limitations on the film configuration or material. In addition, the anti-reflection film 20 may be omitted, and the substrate 01 may be exposed to the atmosphere.

The optical filter 10 functions as a reflective film for the light incident from the subject 400, so the subject 400's figure is reflected on the optical filter 10. Therefore, the subject 400 or a third party can capture an image using the imaging element 300 through the imaging optical system 200 while checking the facial expression, pose, etc. of the subject 400.

Light reflected by the imaging optical system 200 and the imaging element 300 is incident on the anti-reflection coating 20. A portion of the light incident on the anti-reflection coating 20 is transmitted through the anti-reflection coating 20, passes through the substrate 10, and is incident on the optical filter 10. The optical filter 10 functions as an anti-reflection coating for light incident from the opposite side of the subject 400. This reduces unnecessary light that enters from the opposite side of the subject 400, is reflected by the optical filter 10, and returns to the imaging element 300, thereby suppressing the occurrence of ghosts and flares.

In addition, an image display element may be arranged instead of the photographing optical system 200 and the image sensor 300. In this case, it is possible to configure it as a smart mirror that can display text information, photographed images, composite images, etc. superimposed on the image reflected by the optical filter 10 from the subject 400 side. When an image display element is arranged, if the reflectance of the optical filter 10 on the side opposite the subject 400 is high, flare, etc. will occur and it will be difficult to recognize a clear image. In addition, when viewing at an angle without facing the image display element, multiple mirror images will be seen out of alignment due to the reflected light from the optical filter 10. According to the optical filter 10 of the embodiment, the reflected light is suppressed, making it possible to view a clear image.

Depending on the magnitude relationship of the reflectance of the optical filter 10 on the substrate 01 side and the opposite side, the optical filter 10 may be placed on either the subject 400 side or the imaging optical system 200 side of the substrate 01.

The optical filter of the embodiment may be used in a device that recognizes images, etc., by reflected light. Examples of such devices include a head-up display (HUD), a head-mounted display (HMD), and a prompter. By reducing the reflection on the side opposite to the side where the reflected light is viewed, unnecessary light such as flare can be reduced and the visibility of the reflected light can be improved. The optical filter of the embodiment may also be used in an image acquisition device that uses coaxial epi-illumination. In this way, the optical filter of the embodiment can effectively reflect the illumination light while reducing reflection on the image acquisition device side, making it possible to suppress reflections of the image acquisition device and the surrounding environment, and flare due to ambient light.

Figure 18 shows a digital camera 500 as an image capturing device of the embodiment. The digital camera 500 is composed of a lens unit 501 and a camera body unit 502. The lens unit 501 may be configured integrally with the camera body unit 502, or may be detachable (replaceable) from the camera body unit 502.

An optical element 503 in which any one of the optical filters 10 of Examples 1 to 23 is provided on the subject-side surface of a substrate (not shown) is disposed at the end of the lens section 501 on the subject side. This allows the subject to see their own image reflected in front of the lens section 501. Furthermore, unwanted light generated by reflection within the lens section 501 or the camera body section 502 enters the optical filter 503 from the substrate side, and does not return to the lens section 501 or the camera body section 502 due to the optical filter 503's function as an anti-reflection film. This makes it possible to suppress the occurrence of ghosts and flares, i.e., degradation of the captured image.

The shape of the substrate 01 on which the optical filter 10 of Examples 1 to 23 is provided is preferably convex toward the subject side (the entrance side where the reflectance of the optical filter is high) from the viewpoint of allowing the subject to confirm his/her own appearance. This makes the subject's image larger when looking at the optical filter 10 from the subject side, making it easier to confirm facial expression, pose, etc. Furthermore, it is preferable that the substrate 01 has a shape with zero or very small optical power (refractive power). This makes it possible to keep the aberration occurring in the substrate 01 small and suppress the effect on the imaging performance of the lens section 501.

Since it is attached to the subject end of the lens section 501 of the digital camera 500, the diameter of the substrate 01 is preferably 10 mm to 300 mm. In particular, for digital cameras that take portraits, the diameter is preferably 50 to 150 mm, and for video cameras that take moving images, the diameter of the camera lens itself is large, so 100 to 300 mm is preferable. In this case, from the viewpoint of checking facial expressions and the overall appearance of the subject when the distance between the subject and the camera 500 is in the range of 0.5 to 5.0 m, the radius of curvature of the convex surface is preferably between 200 mm and infinity.

In order to suppress aberrations that deteriorate the captured image, it is preferable that the thickness of the substrate 01 is 0.2 mm or more and 8.0 mm or less. In particular, since it is a filter that is attached to the subject end of the lens, it is more preferable that the thickness is 1.5 mm or more and 5.0 mm or less.

Figure 19 shows a surveillance camera 600 as an image capturing device of the embodiment. The surveillance camera 600 is composed of a camera unit 601 including a lens and a cover element 602 as a dome-shaped optical element 100 that covers the camera unit 601.

The optical filter 10 of any of the first to twenty-three embodiments is provided on the outer surface of the dome-shaped substrate of the cover element 602. This increases the reflectivity of the outer surface (optical filter 10) of the cover element 602, and the outside world is reflected on the outer surface of the cover element 602, making it difficult to determine the orientation of the camera unit 601 arranged inside the cover element 602 from the outside. This can reduce the sense of caution and tension of the monitored person, such as a passerby in the outside world.

Furthermore, the optical filter 10 prevents reflection of light incident on the cover element 602 from the camera unit 601, thereby suppressing degradation of the surveillance image caused by ghosts and flares resulting from unwanted light reflected by the cover element 602.

The above-described embodiments are merely representative examples, and various modifications and variations are possible when implementing the present invention.

01 substrate 02 first layer 03 absorbing layer 04 second layer 05 air 10 optical filter 100 optical element

Claims (13)

An optical filter provided on a substrate,
The optical element is configured by a first layer serving as a reflection enhancing layer , an absorption layer having light absorption properties , and a second layer serving as an antireflection layer , which are disposed adjacent to each other in this order from the substrate side,
each of the first and second layers is made of a material having an extinction coefficient of 0.02 or less at a wavelength of 550 nm;
Let R1 [%] be the reflectance when light having a wavelength of 550 nm is incident from the side opposite to the substrate, and R2 [%] be the reflectance when light having a wavelength of 550 nm is incident from the side of the substrate,
R2-R1≧10
An optical filter characterized by satisfying the following conditions. 0≦R1/R2≦0.5
2. The optical filter according to claim 1, which satisfies the following conditions: When the number of films constituting the absorption layer is m, the extinction coefficient of the j-th film from the substrate side among the films is kj, and the physical film thickness is dj [nm],

3. The optical filter according to claim 1, which satisfies the following conditions: When the refractive index of the j-th film is nj,
1.8≦nj≦4.2
4. The optical filter according to claim 3 , which satisfies the following condition: 0.05≦kj
5. The optical filter according to claim 3 , which satisfies the following conditions: 6. The optical filter according to claim 1, wherein the number of films constituting the first layer is greater than the number of films constituting the second layer. When the difference between the refractive index of the film in the second layer that is farthest from the absorbing layer and the refractive index of the medium adjacent to the film on the opposite side to the absorbing layer is dn,
0≦dn≦0.40
7. The optical filter according to claim 1, which satisfies the following condition: An optical filter according to any one of claims 1 to 7 ;
and an optical element comprising the substrate. 9. The optical device of claim 8 , further comprising an optical element bonded to the second layer. the second layer and the optical element are bonded to each other via a bonding member,
When the refractive indices of the substrate, the optical element, and the bonding member at a wavelength of 550 nm are n, n, and n, respectively,
|ns-na|≦|ne-na|
10. The optical element according to claim 9 , which satisfies the following condition: 11. The optical element according to claim 8 , wherein the thickness of the substrate is 0.2 mm or more and 8.0 mm or less. 12. The optical element according to claim 8 , wherein the substrate has a convex shape facing the optical filter. An optical filter according to any one of claims 1 to 7 ;
and an image sensor for capturing an image of a subject through the optical filter.