Optical Filter, Optical Element, and Imaging Device

Abstract

To provide an optical filer having reflectance that depends on incident direction of light.SOLUTION: An optical filter is provided, consisting of a first layer, an absorptive layer, and a second layer laminated in order from the substrate side. The optical filter satisfies a condition expressed as: |R1-R2|≥10[%], where R1 [%] represents a reflectance for 550 nm-light entering from the side opposite the substrate, and R2 [%] represents a reflectance for the 550-nm light entering from the substrate side.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an optical filter that reflects a part of incident light and transmits the other part.

Background Art

[0002] As such an optical filter, a so-called half mirror is used. Patent Document 1 discloses a photographing apparatus provided with a half mirror as a front cover that covers internal members from the front side. A subject can perform imaging while checking their expression and pose by viewing their own image reflected by the half mirror.

[0003] Furthermore, a half mirror is also used as a cover that covers a surveillance camera so that the presence and orientation of the camera cannot be seen from the outside, and a half mirror cover having a curvature functions as a surveillance mirror itself.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Generally, for a half mirror, the reflectance (transmittance) for light incident from the front surface side and the reflectance (transmittance) for light incident from the back surface side are substantially the same. For this reason, in a half mirror cover for a surveillance camera, if the reflectance for light incident from the subject side is high, the reflectance for light incident from the camera side also becomes high. 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 ghosting and flare.

[0006] The present invention provides an optical filter having different reflectances depending on the incident direction of light.

Means for Solving the Problem

[0007] An optical filter as one aspect of the present invention includes a , as an anti-reflection layer first layer, having light absorption an absorption layer, and , as an anti-reflection layer a second layer is composed of arranged adjacent to each other in order from the side of the substrate. Each of the first and second layers is made of a material having an attenuation coefficient of 0.02 or less at a wavelength of 550 nm. When the reflectance when light of wavelength 550 nm is incident from the side opposite to the substrate is R1 [%] and the reflectance when incident from the substrate side is R2 [%], R2 - R1 ≧ 10 It is characterized by satisfying the above condition. An optical element having the above optical filter and an imaging device using the optical element also constitute other aspects of the present invention.

Advantages of the Invention

[0008] According to the present invention, it is possible to realize an optical filter whose reflectance varies depending on the incident direction of light and which functions as a reflection film for incident light from one side and as an antireflection film for incident light from the other side.

Brief Description of the Drawings

[0009] [Figure 1] Cross-sectional view of an optical element provided with an optical filter which is an embodiment of the present invention. [Figure 2] Cross-sectional view for explaining the reflection / transmission characteristics of the above optical filter. [Figure 3] Diagram showing the reflectance characteristics and transmittance characteristics of the above optical filter. [Figure 4] Diagram showing the refractive index and attenuation coefficient of the materials used in the optical filters of Examples 1 to 5. [Figure 5] Diagram showing the reflectance characteristics and transmittance characteristics of the optical filters of Examples 1 to 5. [Figure 6] Diagram showing the refractive index and attenuation coefficient of the absorption materials K2 and K3 used in the optical filters of Examples 6 to 10. [Figure 7] A diagram showing the reflectance characteristics and transmittance characteristics of the optical filters of Examples 6 to 10. [Figure 8] A diagram showing the refractive index and extinction coefficient of the materials used in the optical filters of Examples 11 to 17. [Figure 9] A diagram showing the reflectance characteristics and transmittance characteristics of the optical filters of Examples 11 to 17. [Figure 10] A diagram showing the refractive index and extinction coefficient of the materials used in the optical filters of Examples 18 to 23. [Figure 11] A diagram showing the reflectance characteristics and transmittance characteristics of the optical filter of Example 18. [Figure 12] A diagram showing the reflectance characteristics and transmittance characteristics of the optical filter of Example 19. [Figure 13] A diagram showing the reflectance characteristics and transmittance characteristics of the optical filter of Example 20. [Figure 14] A diagram showing the reflectance characteristics and transmittance characteristics of the optical filter of Example 21. [Figure 15] A diagram showing the reflectance characteristics and transmittance characteristics of the optical filter of Example 22. [Figure 16] A diagram showing the reflectance characteristics and transmittance characteristics of the optical filter of Example 23. [Figure 17] A schematic diagram of imaging using the optical element of the example. [Figure 18] A schematic diagram of the digital camera of the example. [Figure 19] A schematic diagram of the surveillance camera of the example.

BEST MODE FOR CARRYING OUT THE INVENTION

[0010] Hereinafter, examples of the present invention will be described with reference to the drawings. FIG. 1 shows an optical element 100 which is an example of the present invention. The optical element 100 includes a substrate 01 formed of a light-transmissive material and an optical filter 10. The optical filter 10 is composed of an antireflection layer 02, an absorption layer 03, and a reflection layer (enhanced reflection layer or reflection-increasing layer) 04 laminated in order from the substrate 01 side to the air 05 side. The enhanced reflection layer 04 is in contact with air 05 (refractive index 1). The refractive index of the substrate 01 is greater than the refractive index of the air 05.

[0011] The antireflection layer 02 and the enhanced reflection layer 04 are each composed of one or more thin films. Note that the number of layers of the antireflection layer 02 and the enhanced reflection layer 04 can be appropriately selected for adjusting the refractive index, expanding the use band, reducing the incident angle dependence, and reducing the polarization dependence. Further, the absorption layer 03 is composed of one or more materials having light absorption properties.

[0012] FIG. 2 shows an optical element 100' in which the antireflection layer 02 and the enhanced reflection layer 04 shown in FIG. 1 are not provided. The absorption layer 03 provided on the substrate 01 is in contact with air 05 (refractive index 1).

[0013] FIG. 3 shows the relationship between the reflectance and the film thickness of the absorption layer 03 and the relationship between the transmittance and the film thickness of the absorption layer 03 in the optical element 100' when the incident light has wavelengths λ = 450, 550, and 650 nm. The absorption layer 03 has a refractive index n = 2.35 and an extinction coefficient k = 0.05 regardless of the wavelength of the incident light.

[0014] As can be seen from the reflectance graph in FIG. 3, the reflectance when incident from the air 05 side is larger than the reflectance when incident from the substrate 01 side regardless of the film thickness. Also, the reflectance varies depending on the wavelength of the incident light. On the other hand, as can be seen from the transmittance graph in FIG. 3, the transmittance is equal regardless of the light incident direction. Further, the transmittance decreases as the film thickness of the absorption layer 03 increases.

[0015] From the above, by using a material having light absorptivity, an optical filter can be fabricated in which the reflectance is different 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 that of the optical element 100′, the reflectance varies depending on the wavelength of the incident light, and moreover, the antireflection effect is small. Therefore, in this embodiment, as in the optical element 100 (optical filter 10) shown in FIG. 1, an antireflection layer 02, which is a first layer, is disposed between the absorption layer 03 and the substrate 01, and a reflection-enhancing layer 04, which is a second layer, is disposed between the absorption layer 03 and the air 05. The antireflection layer 02 mainly functions as an antireflection film for incident light from the substrate 01 side, and the reflection-enhancing layer 04 mainly functions as a reflection film for incident light from the air 05 side.

[0016] The optical filter 10 of this embodiment is used for light having a wavelength of 420 to 680 nm in the visible light region. In the optical filter 10, let the reflectance when light with a wavelength of 550 nm is incident from the substrate 01 side (hereinafter referred to as the substrate-side reflectance) be R2 [%], and the reflectance when it is incident from the side opposite to the substrate 01 (hereinafter referred to as the air-side reflectance or the joint member-side reflectance) be R1 [%]. At this time, |R1 - R2| ≧ 10 [%] (1) It is desirable to satisfy the relationship. The optical filter 10 of this embodiment is different from a general half mirror in that the reflectance varies depending on the incident direction of light. Thereby, the optical filter 10 functions as an antireflection film for incident light from one side and as a reflection film for incident light from the other side.

[0017] Note that as the numerical range of the formula (1), it is more preferable to set it in order from the following formula (1)′ to formula (1)″. 80 ≧ |R1 - R2| ≧ 15 [%] (1)′ 60 ≧ |R1 - R2| ≧ 18 [%] (1)″ The optical filter 10 includes an antireflection layer 02 which is the first layer laminated in order from the substrate 01 side, an absorption layer 03, and an antireflection enhancement layer 04 which is the second layer. When the absorption layer 03 is composed of m (a plurality of) films, when the attenuation coefficient of the j-th film (hereinafter referred to as the absorption j film) from the substrate 01 side among the m films is kj and the physical film thickness is dj,

[0018]

Equation

[0019] It is desirable to satisfy the following conditions. 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 reflectance R1 and R2.

[0020] Also, the refractive index nj of the absorption j film is 1.8≦nj≦4.2 (3) It is desirable to satisfy the following conditions. In a dielectric material having an attenuation coefficient, the refractive index of a material that can be easily produced by a dry film formation method such as vapor deposition or sputtering generally falls within this range.

[0021] In addition, 2.0≦nj≦3.6 (3)′ It is more preferable to satisfy.

[0022] Furthermore, the attenuation coefficient kj of the absorption j film is 0.05≦kj≦4.00 (4) It is desirable to satisfy the following conditions. If the attenuation coefficient is less than 0.05, it is necessary to control the amount of oxygen introduced into the dry film formation apparatus with high precision, and moreover, since the amount of oxygen also depends on the amount of film-forming substance adhering inside the apparatus, it is difficult to form a film with good reproducibility with an attenuation coefficient less than 0.05. Also, if the attenuation coefficient is greater than 4.0, the film thickness required to realize the necessary characteristics becomes very thin and film thickness control becomes difficult, which is not preferable.

[0023] Incidentally, 0.10 ≦ kj ≦ 3.60 (4)' It is more preferable to satisfy.

[0024] The material of the substrate 01 is glass, plastic, etc., and is not particularly limited. Also, the shape of the substrate 01 may be a flat plate shape, or may be a convex shape, a concave shape, etc., and may be a curved surface shape other than a spherical surface.

[0025] As a method for forming the absorption layer 03, dry film formation methods such as vapor deposition and sputtering are desirable, but wet film formation methods such as electroplating and spin coating may also be used. Further, the material of the absorption layer 03 may satisfy the above-described conditions (conditional expressions (2) and (4)) regarding the attenuation coefficient k, and examples thereof include an oxygen-deficient TiO2 film, Nb2O5 film, Ta2O5 film, ITO film, Cr film, etc.

[0026] As a method for forming the first layer and the second layer, dry film formation methods such as vapor deposition and sputtering are desirable, but wet film formation methods such as electroplating and spin coating may also be used. The film materials constituting the first layer and the second layer are preferably made of materials having no light absorption. According to this, light absorption generated in the optical filter of the present embodiment can be suppressed, and a high transmittance can be realized. The attenuation coefficient at a wavelength of 550 nm of the film materials 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.

[0027] Also, when the reflectance R1 is larger than the reflectance R2 (R1 - R2 ≧ 10 [%]), 0 ≦ R2 / R1 ≦ 0.5 (5) It is desirable to satisfy the resulting condition. Since the reflectances R1 and R2 have values of 0 or more, they do not fall below the lower limit of the formula (5). As the reflectance R1 becomes smaller beyond the upper limit of the formula (5), it becomes difficult to confirm its own image using the air-side reflected light of the optical filter 10.

[0028] Incidentally, 0 ≦ R2 / R1 ≦ 0.4 (5)' It is more preferable to satisfy

[0029] 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 condition. Since the reflectances R1 and R2 have values of 0 or more, they do not fall below the lower limit of the formula (6). As the reflectance R2 becomes smaller beyond the upper limit of the formula (6), it becomes difficult to confirm its own image using the substrate-side reflected light of the optical filter 10.

[0030] In addition, 0 ≦ R1 / R2 ≦ 0.4 (6)' It is more preferable to satisfy

[0031] When the refractive index difference between the outermost layer on the side opposite to the absorption layer 03 in the antireflection layer 04 which is the second layer and the medium in contact with the outermost layer is defined as dn, dn ≦ 0.50 (7) It is desirable to satisfy the condition. As the refractive index difference dn becomes larger beyond the upper limit of the formula (7), it becomes difficult to sufficiently reduce the reflection generated at the interface using the interference effect.

[0032] In addition, dn ≦ 0.40 (7)' It is more preferable to satisfy

[0033] The second layer may be closely bonded to the optical element via a bonding member and arranged at the bonding interface so that the optical filter is not exposed to the atmosphere. Compared with the case where the second layer is in contact with air, the refractive index difference between the substrate side and the outermost layer on the side opposite to the substrate of the optical filter and the member adjacent to the outermost layer becomes smaller, so that a further antireflection effect can be obtained.

[0034] When the refractive indices of the substrate, the optical element, and the bonding member at a wavelength of 550 nm are ns, ne, and na, respectively, |ns - na| ≦ |ne - na| (8) It is desirable to satisfy the following conditions. By satisfying this, the refractive index difference between the outermost layer of the optical filter and the adjacent member can be made small, and a further antireflection effect can be obtained.

[0035] Specific examples are shown below. However, these are merely examples, and the examples of the present invention include those other than these.

[0036] Table 1 shows the film configurations of the optical filters of Examples 1 to 5. In each example, the material of the substrate 01 is a glass material S1 having no light absorption. The material of the antireflection layer 02, which is the first layer, is a dielectric material H1. The high-reflection layer 04, which is the second layer, is a multilayer film in which dielectric materials M1, H1, and L1 are laminated in order from the absorption layer 03 side. The number of layers (2) of the high-reflection layer 04 as a multilayer film is larger than the number of layers (1) of the antireflection layer 02. The dielectric materials M1, H1, and L1 are all materials having no light absorption. The refractive indices of the glass material S1 and the dielectric materials M1, H1, and L1 for each wavelength are shown in FIG. 4(a).

[0037] The material of the absorption layer 03 is an absorption material K1 having light absorption. The refractive index and extinction coefficient of the absorption material K1 for each wavelength are shown in FIG. 4(b). The extinction coefficient of the absorption material K1 at λ = 550 nm is 0.34.

[0038] FIGS. 5(a) to 5(d) show the air-side reflectance, substrate-side reflectance, and transmittance of the optical filters of Examples 1 to 5 for each wavelength. As can be seen from these figures, the difference between the air-side reflectance and the substrate-side reflectance of the optical filters of Examples 1 to 5 at λ = 550 nm is 10% or more, and the conditional expression (1) (further (1)') is satisfied.

[0039] Also, the film thicknesses of the absorption material K1 in Examples 1 to 5 are 50 nm, 100 nm, 150 nm, 200 nm, and 250 nm, respectively, as shown in Table 1, and kj × dj satisfies the conditional expression (2).

[0040]

Table 1

[0041] Table 2 shows the film configurations of the optical filters of Examples 6 to 10. Also in these examples, similar to Examples 1 to 5, the material of the substrate 01 is the glass material S1. The antireflection layer 02, which is the first layer, is a multilayer film in which dielectric materials H1, M1, and H1 are laminated in order from the substrate 01 side. The high-reflection layer 04, which is the second layer, is a multilayer film in which dielectric materials M1, H1, M1, H1, and M1 are laminated in order from the absorption layer 03 side. The number of layers (5) of the high-reflection layer 04 as a multilayer film is larger than the number of layers (3) of the antireflection layer 02.

[0042] The absorption layer 03 is a multilayer film in which absorption materials K3 and K2 are laminated in order from the antireflection layer 02 side. The refractive index and extinction coefficient of the absorption materials K2 and K3 for each wavelength are shown in FIGS. 6(a) and (b), respectively. The extinction coefficient of the absorption material K2 at λ = 550 nm is 0.19, and the material extinction coefficient of the absorption K3 at λ = 550 nm is 0.26.

[0043] FIGS. 7(a) to (d) show the air-side reflectance, substrate-side reflectance, and transmittance of the optical filters of Examples 6 to 10 for each wavelength. As can be seen from these figures, the difference between the air-side reflectance and the substrate-side reflectance of the optical filters of Examples 6 to 10 at λ = 550 nm is 10% or more, satisfying the conditional expression (1) (furthermore (1)').

[0044] Also, the film thicknesses of the absorption 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 × dj satisfies the conditional expression (2).

[0045] [Table 2]

[0046] Table 3 shows the film configurations of the optical filters of Examples 11 to 17. In these examples, the material of the substrate 01 is a glass material S2 that has no light absorption. The antireflection layer 02, which is the first layer, is a multilayer film in which dielectric materials H2, M2, and H2 are laminated in order from the substrate 01 side. The high-reflection layer 04, which is the second layer, is a multilayer film in which dielectric materials M2, H2, M2, H2, and M2 are laminated in order from the absorption layer 03 side. The number of layers (5) of the high-reflection layer 04 as a multilayer film is larger than the number of layers (3) of the antireflection layer 02. Both the dielectric materials M2 and H2 are materials that have no light absorption. The refractive indices of the glass material S2 and the dielectric materials M2 and H2 for each wavelength are shown in Fig. 8(a).

[0047] The material of the absorption layer 03 is an absorption material K4 that has light absorption. The refractive index and extinction coefficient of the absorption material K4 for each wavelength are shown in Fig. 8(b). The extinction coefficient of the absorption material K4 at λ = 550 nm is 0.20.

[0048] Figs. 9(a) to ( g ) show the air-side reflectance, substrate-side reflectance, and transmittance of the optical filters of Examples 11 to 17 for each wavelength. As can be seen from these figures, the difference between the air-side reflectance and the substrate-side reflectance of the optical filters of Examples 11 to 17 at λ = 550 nm is 10% or more, satisfying the conditional expression (1) (furthermore (1)′).

[0049] Also, the film thicknesses of the absorption material K4 in Examples 11 to 17 are 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, and 600 nm, respectively, as shown in Table 3, and kj × dj satisfies the conditional expression (2).

[0050]

Table 3

[0051] Table 4 shows the film structure of the optical filter of Example 18. The optical filter of this example is composed of an antireflection layer, which is the first layer, an absorption layer, and an antireflection layer, which is the second layer, laminated in order from the substrate side made of glass material S2. The optical filter of this example has the characteristic that the reflectance R2 when incident from the substrate side is larger than the reflectance R1. Dielectric materials M3, H3, and L3 are all materials without light absorption, and the refractive indices for each of these wavelengths are shown in Fig. 10(a).

[0052] The material of the absorption layer is an absorption material K5 having light absorption. The refractive index and attenuation coefficient for each wavelength of the absorption material K5 are shown in Fig. 10(b). The attenuation coefficient of the absorption material K5 at a wavelength of 550 nm is 3.33.

[0053] Fig. 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 Fig. 11, the optical filter of Example 18 satisfies the conditional expression (1).

[0054]

Table 4

[0055] Table 5 shows the film structure of the optical filter of Example 19. The configuration of the optical filter of this example is the same as that of Example 18.

[0056] The material of the absorption layer is an absorption material K6 having light absorption. The refractive index and attenuation coefficient for each wavelength of the absorption material K6 are shown in Fig. 10(c). The attenuation coefficient of the absorption material K6 at a wavelength of 550 nm is 1.24.

[0057] Fig. 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 Fig. 12, the optical filter of Example 19 satisfies the conditional expression (1).

[0058]

Table 5

[0059] Table 6 shows the film structure 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.

[0060] The material of the absorption layer is an absorption material K6 having light absorbency, and the attenuation coefficient at a wavelength of 550 nm is 1.24.

[0061] 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 the conditional expression (1).

[0062] [Table 6]

[0063] Table 7 shows the film structure of the optical filter of Example 21. The configuration of the optical filter of this example is the same as that of Example 18.

[0064] The material of the absorption layer consists of absorption materials K7 and K8 having light absorbency. The refractive index for each wavelength of absorption materials K7 and K8 and the average attenuation coefficient of the two layers are shown in Fig. 10(d). The average attenuation coefficient of the two layers at a wavelength of 550 nm is 0.29.

[0065] 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 the conditional expression (1).

[0066] [Table 7]

[0067] Table 8 shows the film structure of the optical filter of Example 22. The configuration of the optical filter of this example is the same as that of Example 18.

[0068] The material of the absorption layer is composed of absorption materials K7 and K8 having light absorption properties, and the average attenuation coefficient of the two layers at a wavelength of 550 nm is 0.29.

[0069] FIG. 15 shows the air-side reflectance, substrate-side reflectance, and transmittance of the optical filter of Example 22 for each wavelength. As can be seen from FIG. 15, the optical filter of Example 22 satisfies conditional expression (1).

[0070] [Table 8]

[0071] Table 9 shows the film configuration of the optical filter of Example 23. The optical filter of this example is composed of an antireflection layer, which is the first layer, an absorption layer, an antireflection layer, which is the second layer, a bonding member S3, and an optical element S4, laminated in order from the substrate side made of glass material S2. The optical element S4 is in close contact with the second layer via the bonding member S3, and is an optical element in which the optical filter is disposed at the bonding interface. The refractive indices of the glass material S2, the bonding member S3, and the optical element S4 at a wavelength of 550 nm satisfy conditional expression (8). Further, in order to reduce reflection at the interface between the bonding member S3 and the optical element S4, a layer having an antireflection effect may be provided at the interface between the bonding member S3 and the optical element S4.

[0072] The material of the absorption layer is an absorption material K6 having light absorption properties, and the attenuation coefficient at a wavelength of 550 nm is 1.24.

[0073] FIG. 16 shows the bonding member-side reflectance, substrate-side reflectance, and transmittance of the optical filter of Example 23 for each wavelength. As can be seen from FIG. 16, the optical filter of Example 23 satisfies conditional expression (1).

[0074] [Table 9]

[0075] Various numerical values in each example are summarized in Table 10. The transmittance is the value at a wavelength of 550 nm.

[0076]

Table 10

[0077] In Examples 11 to 23, wavelength dispersion dependence occurs in the transmittance. The correction of the color balance of the captured image caused thereby may be performed by the imaging device as needed. shown in Figure 17 It may also be performed by the imaging device. Further, the change in the color balance due to the optical filter 10 may be shown in Figure 17 configured to cancel out by the transmittance wavelength characteristics of the antireflection film 20. When the chromaticity coordinates in the CIE1976L*a*b* color space are a* and b*, either one of the chromaticity coordinates a* and b* of the transmitted light of the optical filter 10 and the antireflection film 20 may be configured to have different signs from each other.

[0078] In Examples 1 to 22, the cases where the material of the surface layer on the air side is L1 (refractive index 1.38), M1 (refractive index 1.45), M2 (refractive index 1.47), or L3 (refractive index 1.38) were shown, but a layer containing a fluorine-based material may be added to their surfaces. As the fluorine-based material, OF-SR, OF-210, etc. manufactured by Canon Optron Co., Ltd. can be used. By adding such a layer, the antifouling property of the lens surface can be enhanced.

[0079] FIG. 17 shows a state of imaging using the optical element 100 having the optical filter 10 of any one of Examples 1 to 23 described above. The optical element 100 is disposed between the subject 400 and the 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.

[0080] In the optical element 100, the optical filter 10 is provided on the surface of the substrate 01 on the subject 400 side, and the antireflection film 20 is provided on the surface on the imaging optical system 200 side. The antireflection film 20 may be a general antireflection film, and the film configuration and material are not limited. Further, the antireflection film 20 may be absent and the substrate 01 may be exposed to the atmosphere.

[0081] Since the optical filter 10 functions as a reflective film for the light incident from the subject 400, the image of the subject 400 is reflected on the optical filter 10. Therefore, the subject 400 or a third party can perform imaging with the imaging element 300 through the imaging optical system 200 while checking the expression, pose, etc. of the subject 400.

[0082] The light reflected by the imaging optical system 200 and the imaging element 300 enters the antireflection film 20. A part of the light that has entered the antireflection film 20 passes through the antireflection film 20 and enters the optical filter 10 through the substrate 10. The optical filter 10 functions as an antireflection film for the light incident from the side opposite to the subject 400. Therefore, it is possible to reduce unnecessary light that enters from the side opposite to the subject 400, is reflected by the optical filter 10, and returns to the imaging element 300, and suppress the occurrence of ghosting and flare.

[0083] Alternatively, an image display element may be arranged instead of the imaging optical system 200 and the imaging element 300. In this case, it can be configured as a smart mirror that can display character information, a photographed image, a composite image, etc. superimposed on the image reflected on the optical filter 10 from the subject 400 side. When an image display element is arranged, if the reflectance on the side opposite to the subject 400 of the optical filter 10 is high, flare or the like occurs and it becomes difficult to recognize a clear image. Also, when visually recognizing obliquely without facing the image display element directly, a plurality of mirror images are shifted due to the reflected light from the optical filter 10 and are visually recognized. According to the optical filter 10 of the embodiment, since the reflected light is suppressed, it is possible to visually recognize a clear image.

[0084] Note that the optical filter 10 may be arranged on either the subject 400 side or the imaging optical system 200 side from the substrate 01 according to the magnitude relationship of the reflectances on the substrate 01 side and the opposite side of the optical filter 10.

[0085] The optical filter of the embodiment may be used in an apparatus that recognizes an image or the like by reflected light. Examples of such apparatuses include HUD (Head Up Display), HMD (Head Mounted Display), a prompter, and the like. By reducing the reflection on the side opposite to where the reflected light is visually recognized, unnecessary light such as flare can be reduced, and the visibility of the reflected light can be improved. Further, it may be used in an image acquisition apparatus using coaxial epi-illumination. According to this, while the illumination light can be favorably reflected by the optical filter of the embodiment, the reflection on the image acquisition apparatus side can be reduced, so that it is possible to suppress the reflection of the image acquisition apparatus and the surrounding environment, flare due to ambient light, and the like.

[0086] FIG. 18 shows a digital camera 500 as an imaging apparatus of the embodiment. The digital camera 500 is composed of a lens unit 501 and a camera body unit 502. Note that the lens unit 501 may be integrally formed with the camera body unit 502, or may be detachable (replaceable) with respect to the camera body unit 502.

[0087] The optical element 503 provided on the subject side surface of a substrate (not shown) with any one of the optical filters 10 of Embodiments 1 to 23 is disposed at the subject side end portion of the lens unit 501. Thereby, the subject can confirm its own image reflected on the front surface of the lens unit 501. Further, unnecessary light generated by reflection within the lens unit 501 or the camera body unit 502 enters the optical filter 503 from the substrate side, and does not return to the inside of the lens unit 501 or the camera body unit 502 due to the function as an antireflection film of the optical filter 503. For this reason, the occurrence of ghosting and flare, that is, the degradation of the captured image can be suppressed.

[0088] From the perspective of the subject checking their own appearance, the shape of the substrate 01 provided with the optical filter 10 in Examples 1 to 23 is preferably convex toward the subject side (the incident side with a high reflectance of the optical filter). This makes the image of the subject's appearance larger when viewed from the subject side, facilitating the confirmation of expressions, poses, etc. Also, the substrate 01 preferably has a shape with an optical power (refractive power) of zero or very small. This can greatly suppress the aberration generated in the substrate 01 and can suppress the influence on the imaging performance of the lens unit 501.

[0089] Since it is attached to the subject end of the lens unit 501 of the digital camera 500, the diameter of the substrate 01 is preferably 10 mm to 300 mm. In particular, for a digital camera used for taking portraits etc., the diameter is preferably 50 to 150 mm. For a video camera for imaging videos, since the diameter of the camera lens itself is large, 100 to 300 mm is preferred. At this time, from the perspective of checking the facial expression and the overall appearance of the subject within the range of the distance between the subject and the camera 500 being 0.5 to 5.0 m, the radius of curvature of the convex surface is preferably between 200 mm and ∞.

[0090] Also, from the perspective of suppressing the aberration that degrades the captured image, the thickness of the substrate 01 is preferably 0.2 mm or more and 8.0 mm or less. In particular, since it is a filter attached to the subject end of the lens, it is more preferably 1.5 mm or more and 5.0 mm or less.

[0091] FIG. 19 shows a surveillance camera 600 as a photographing apparatus of an 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 covering it.

[0092] Any of the optical filters 10 in Examples 1 to 23 is provided on the outer surface of the dome-shaped substrate in the cover element 602. Therefore, the reflectance of the outer surface (optical filter 10) of the cover element 602 becomes high, and the outside world is reflected on the outer surface of the cover element 602, making it difficult to tell the direction of the camera unit 601 arranged inside the cover element 602 from the outside world. As a result, it is possible to relieve the vigilance and sense of tension of monitoring targets such as passers-by located in the outside world.

[0093] Furthermore, with respect to the light incident from the camera unit 601 to the cover element 602, reflection is prevented by the optical filter 10, so that deterioration of the monitoring image due to ghosts and flares caused by unnecessary light reflected by the cover element 602 can be suppressed.

[0094] Each of the embodiments described above is merely a representative example, and various modifications and changes can be made to each embodiment when implementing the present invention.

Explanation of Reference Numerals

[0095] 01 Substrate 02 First layer 03 Absorbing layer 04 Second layer 05 Air 10 Optical filter 100 Optical element

Claims

1. An optical filter provided on a substrate, which is composed of a first layer as an antireflection layer, an absorption layer having light absorptivity, and a second layer as an antireflection layer, which are arranged adjacent to each other in order from the side of the substrate, each of the first and second layers is composed of a material having an attenuation coefficient of 0.02 or less at a wavelength of 550 nm, when the reflectance when light with a wavelength of 550 nm is incident from the side opposite to the substrate is R1 [%] and the reflectance when light with a wavelength of 550 nm is incident from the side of the substrate is R2 [%], R2 - R1 ≧ 10 An optical filter characterized by satisfying the above conditions.

2. 0 ≦ R1 / R2 ≦ 0.5 The optical filter according to claim 1, characterized by satisfying the above conditions.

3. When the number of films constituting the absorption layer is m, the attenuation coefficient of the j-th film from the side of the substrate among the films is kj, and the physical film thickness is dj [nm], 【Equation 1】 The optical filter according to claim 1 or 2, characterized by satisfying the above conditions.

4. When the refractive index of the j-th film is nj, 1.8 ≦ nj ≦ 4.2 The optical filter according to claim 3, characterized by satisfying the above conditions.

5. 0.05 ≦ kj The optical filter according to claim 3 or 4, characterized by satisfying the above conditions.

6. The optical filter according to any one of claims 1 to 5, characterized in that the number of films constituting the first layer is larger than the number of films constituting the second layer.

7. When the difference between the refractive index of the film farthest from the absorption layer in the second layer and the refractive index of the medium adjacent to the film on the side opposite to the absorption layer is defined as dn, 0 ≦ dn ≦ 0.40 The optical filter according to any one of claims 1 to 6, characterized in that it satisfies the condition.

8. An optical filter according to any one of claims 1 to 7, and An optical element characterized by having the substrate.

9. The optical element according to claim 8, characterized by having an optical element joined to the second layer.

10. The second layer and the optical element are joined to each other via a joining member, When the refractive indices of the substrate, the optical element, and the joining member at a wavelength of 550 nm are ns, ne, and na, respectively, |ns - na| ≦ |ne - na| The optical element according to claim 9, characterized in that it satisfies the condition.

11. The optical element according to any one of claims 8 to 10, characterized in that the thickness of the substrate is 0.2 mm or more and 8.0 mm or less.

12. The optical element according to any one of claims 8 to 11, characterized in that the substrate is convex toward the side of the optical filter.

13. An optical filter according to any one of claims 1 to 7, and An imaging device characterized by having an imaging element that images a subject through the optical filter.

Images