JP2013174818A - Optical filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical filter that enables adjustment of light amounts in both color photography and monochromatic photography in order to obtain an excellent image.SOLUTION: An optical filter 12 includes a light wavelength attenuation film 22 that attenuates transmittance of visible light wavelength and infrared light wavelength on one face of a transparent substrate 21 of the optical filter 12, and an infrared light wavelength shielding film 22 that shields infrared light wavelength is formed on part of the other face of the transparent substrate 21. The optical filter 12 is arranged in an optical imaging system so as to enable adjustment of light amounts in both color photography and monochromatic photography, thereby contributing to miniaturization of an imaging apparatus.

Description

  The present invention relates to an optical filter mounted on a camera that performs color photography and monochrome photography such as a surveillance camera.

  Usually, an imaging device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) sensor is used in a photographing system such as a digital still camera or a video camera. An aperture device and an infrared cut filter are provided to adjust the amount of light incident on the image sensor.

  The aperture device adjusts the aperture diameter by driving the aperture blades according to the luminance of the subject. As the luminance of the subject increases, the aperture diameter decreases to reduce the amount of light incident on the image sensor. However, if the aperture diameter becomes too small, the image quality deteriorates due to hunting phenomenon or light diffraction.

  Furthermore, in recent years, the sensitivity of the image sensor has been improved, and there is an increasing demand for suppressing the amount of light. Therefore, an ND (Neutral Density) filter is disposed in the vicinity of the diaphragm blades to suppress the amount of light while maintaining a large aperture diameter. Specifically, the aperture diameter is gradually reduced as the luminance of the subject increases. However, when the aperture diameter becomes a certain size, the aperture diameter is maintained at that size, and an ND filter is provided on the optical path. The amount of light that is inserted and incident on the image sensor is adjusted. The ND filter is inserted into the optical path by being adhered to the diaphragm blades and driven together with the diaphragm blades or by being independently driven by another driving means without being directly adhered to the diaphragm blades.

  In addition, with the recent improvement in sensitivity of image pickup devices, there is an increasing demand for ND filters having a high optical density so that the aperture diameter does not become too small. The ND filter is formed by alternately forming a dielectric layer and a light absorption layer on a substrate through a vacuum deposition method or a sputtering method, or kneading a dye such as a dye or a pigment having light absorption on the substrate, or It is manufactured by coating on a substrate.

  In recent years, demands for ND filters that are smaller, lighter, and workable to an arbitrary shape have been increasing year by year. In order to meet this demand, synthetic resin substrates have been used.

  On the other hand, the infrared cut filter is used by inserting it into the optical path to prevent unnecessary infrared light wavelength from entering the image sensor when the amount of visible light wavelength is sufficient during color photography. The As a result, an image having substantially the same color as the color felt by the human eye can be obtained without recognizing light having an infrared light wavelength that cannot be detected by the human eye.

  In monochrome photography using light of infrared light wavelength, monochrome photography using light of infrared light wavelength is possible by retracting the infrared cut filter from the optical path. Infrared cut filter kneads an infrared light absorber on glass or resin substrate, coats the infrared light absorber on the substrate, or alternates multiple layers of dielectric and absorption layers by vacuum evaporation method or sputtering method etc. It is produced by laminating.

  In recent years, a substrate made of a synthetic resin has been used for an infrared cut filter as well as an ND filter for the purpose of thinning.

JP 2002-107509 A JP 2007-219210 A

  In a general imaging optical system, an ND filter that adjusts the amount of transmitted visible light and an infrared cut filter that blocks light having an infrared light wavelength are provided separately.

  However, in response to the recent miniaturization of imaging devices, there is an increasing demand for miniaturization of the imaging optical system. In order to meet this demand, Patent Document 1 discloses an ND film and an infrared ray on both surfaces of a single substrate. An optical filter provided with a cut film is disclosed.

  Patent Document 2 discloses a technique for forming an ND film on a substrate having a characteristic of absorbing infrared light wavelengths.

  In conventional camera applications, it is sufficient for the ND filter to have a substantially uniform transmittance in the visible light wavelength region. However, in applications such as surveillance cameras, monochrome photography using infrared light wavelengths may be performed in addition to color photography using visible light wavelengths. In monochrome photography, in order to adjust the amount of light at the infrared light wavelength, an optical filter that restricts the transmittance at the infrared light wavelength substantially uniformly is required.

  The optical filters of Patent Documents 1 and 2 are inserted into the optical path during photographing using visible light wavelengths, and adjust the amount of visible light wavelengths and shield infrared light wavelengths. Can get a good picture. However, when photographing using light having an infrared light wavelength, the optical filter is retracted from the optical path, and thus the amount of light having the infrared light wavelength cannot be adjusted.

  Furthermore, the filter in Patent Document 2 has a problem that if the light in the infrared light region is sufficiently shielded, the substrate becomes thick, which is contrary to the demand for downsizing that has been demanded in recent years. ing.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical filter that satisfies the demand for miniaturization, can adjust the light amount in both color photography using a visible light wavelength and monochrome photography using an infrared light wavelength, and can obtain a good image. There is.

  In order to achieve the above object, an optical filter according to the present invention includes a light wavelength attenuation film having a substantially uniform transmittance from a visible light wavelength to an infrared light wavelength on one surface of a transparent substrate, and the other surface of the transparent substrate. An infrared wavelength blocking film is formed on a part of the film.

  By using the optical filter according to the present invention, it is possible to provide an optical filter capable of adjusting the light amount in both color photography using a visible light wavelength and monochrome photography using an infrared light wavelength.

  The optical filter of the present invention has both functions of an ND filter and an infrared cut filter, and can contribute to downsizing of the imaging optical system.

1 is a configuration diagram of an imaging optical system of Example 1. FIG. It is a block diagram of an optical filter. It is a spectral characteristic figure of a light wavelength attenuation film. It is a spectral characteristic figure of an infrared-light wavelength shielding film. It is a block diagram of an optical filter. It is a spectral characteristic figure of an infrared-light wavelength transmission film. 6 is a configuration diagram of an optical filter of Example 2. FIG. It is a spectral characteristic figure of a visible light wavelength attenuation film. It is a spectral characteristic of an infrared light wavelength attenuation film. It is the graph which showed the relative sensitivity of the image pick-up element. 6 is a configuration diagram of an optical filter according to Embodiment 3. FIG. It is a block diagram of the optical filter of a modification.

  The present invention will be described in detail based on the embodiments shown in the drawings.

  FIG. 1 is a configuration diagram of an image pickup optical system according to the first embodiment. An image pickup device 7 including a lens 1, a light amount adjusting device 2, lenses 3 to 5, a low-pass filter 6, and a CCD is sequentially arranged along an optical axis L. Is arranged. The output of the image sensor 7 is connected to the filter drive unit 9 via the light amount control unit 8.

  In the light amount adjusting device 2, a pair of aperture blades 11 a and 11 b are movably attached to the aperture blade support plate 10. Further, in the vicinity of the diaphragm blades 11a and 11b, an optical filter 12 made of, for example, an ND filter for adjusting the amount of light passing through the openings formed by the diaphragm blades 11a and 11b is detachable with respect to the optical axis L. Is provided. The optical filter 12 is driven by a filter driving unit 9.

  The diaphragm blades 11a and 11b can freely advance and retreat in a direction perpendicular to the optical axis L, and are formed by the diaphragm blades 11a and 11b by driving the diaphragm blades 11a and 11b by a driving unit (not shown) according to the light amount. Adjust the size of the opening. The optical filter 12 has a different area covering the opening formed by the aperture blades 11a and 11b according to the light wavelength used for photographing.

  FIG. 2 shows a film configuration diagram of the optical filter 12 according to the first embodiment. On one surface of the transparent substrate 21 made of PET, substantially uniform or substantially the same in the visible to infrared wavelength region as shown in FIG. An optical wavelength attenuating film 22 having excellent transmission characteristics is formed on the entire surface. On the other hand, as shown in FIG. 4, the region A on the other surface of the transparent substrate 21 transmits light having a visible light wavelength (λ = 400 to 700 nm) and has an infrared light wavelength (λ = 700 to 1200 nm). An infrared wavelength shielding film 23 that shields against light is formed.

  In the region A where the infrared light wavelength shielding film 23 is formed by forming the light wavelength attenuation film 22 and the infrared light wavelength shielding film 23 having the above-described spectral characteristics on both surfaces of the transparent substrate 21, At the same time as adjusting the amount of light in the visible light region, it is possible to block light of the infrared wavelength. In the region B where the infrared light wavelength shielding film 23 is not formed, the amount of visible light wavelength can be adjusted.

  The optical filter 12 is incorporated in the imaging optical system shown in FIG. 1, and is arranged so that the area A having the infrared light wavelength shielding film 23 covers the opening formed by the diaphragm blades 11a and 11b when performing color photography. The On the other hand, when performing monochrome photography, the region B that does not have the infrared light wavelength shielding film 23 is arranged so as to cover the opening.

  As the transparent substrate 21 of the optical filter 12, a substrate that is transparent at least in the wavelength region from the visible light to the infrared light wavelength is used. Specifically, glass, polyester, norbornene, polyether, acrylic, styrene, PES (polyethersulfone), polysulfone, PEN (polyethylene naphthalate), PC (polycarbonate), polyimide resin, etc. And a synthetic resin substrate. In this embodiment, a synthetic resin substrate that can be thinned is used, and specifically, PET (polyethylene terephthalate) having a plate thickness of 75 μm is used.

  In the first embodiment, PET is used for the transparent substrate 21, but in consideration of film stress due to film formation of the light wavelength attenuation film 22 and infrared light wavelength shielding film 23, deformation due to thermal stress, spectral change due to moisture, etc. Those having a high transition temperature Tg, a high flexural modulus, and a low water absorption are preferred. Specifically, norbornene-based and polyimide-based resins are one of the optimum materials.

  If the transparent substrate 21 is too thick, the image quality may be adversely affected by light scattering within the transparent substrate 21. The plate thickness t is preferably as thin as possible within the range where rigidity can be maintained, preferably 10 μm ≦ t ≦ 100 μm, and more preferably 25 μm ≦ t ≦ 75 μm.

The optical wavelength attenuation film 22 can obtain an arbitrary transmittance by alternately laminating a plurality of dielectric layers and light absorption layers. In this embodiment, an Al ₂ O ₃ film is used as the dielectric layer, and a TiO _x film is used as the light absorption layer. However, not limited to these materials, SiO _2, MgF ₂ as the dielectric layer, the light absorbing layer Ni, Cr, Nb, Ta, Ti, Zr, Si, Ge or the like of a metal or an alloy, oxide, etc. Can also be used.

  In forming the optical wavelength attenuating film 22, first, the transparent substrate 21 is fixed to a film forming jig, a vapor deposition mask capable of forming a film on a desired region on the transparent substrate 21 is set, and this is attached to a vapor deposition umbrella of a vapor deposition device (not shown). Attach to. The vapor deposition umbrella is rotated at an arbitrary speed so that the film thickness and the temperature of the transparent substrate 21 do not change depending on the mounting position of the substrate 21.

When the temperature and pressure in the vapor deposition chamber reach the predetermined temperature and pressure, the hearth liner in which the first layer of vapor deposition material Al ₂ O ₃ is stored is heated to open the shutter, and after the predetermined film thickness is reached Close the shutter. Next, the hearth liner in which TiO _{x as} the second layer vapor deposition material is stored is heated to form a predetermined film thickness in the same manner as the Al ₂ O ₃ film.

Then, after an arbitrary number of layers are alternately stacked on the Al ₂ O ₃ film and the TiO _x film, an MgF ₂ film is formed as the outermost layer. The reason why the outermost layer is an MgF ₂ film is that MgF ₂ has a small refractive index and has an antireflection effect, and in order to further improve the antireflection effect, the film thickness is λ // in the optical film thickness (n · d). It is about 4. Here, λ is the light wavelength that becomes the center of the corresponding light wavelength region.

  The light wavelength attenuating film 22 can also be obtained by kneading a light absorbent into a synthetic resin substrate or coating a resin in which a light absorbing material is dispersed on the substrate. The use of interference with the light absorption layer has better spectral characteristics and can be made thin.

The infrared light wavelength shielding film 23 of this embodiment is formed by alternately laminating a plurality of layers of SiO ₂ films, which are low refractive materials, and TiO ₂ films, which are high refractive materials. In addition, MgF ₂ or the like can be used as the low refractive material, and Ta ₂ O ₅ , ZrO ₂ , Nb ₂ O ₅ or the like can be used as the high refractive material, and Al having an intermediate refractive index as necessary. A layer of ₂ O ₃ , MgO or the like may be provided. The infrared light wavelength shielding film 23 thus formed can obtain a spectrum that transmits light of visible light wavelength and reflects light of infrared light wavelength by adjusting the optical film thickness of each layer. The infrared light wavelength shielding film 23 in this example has an average transmittance of 90% or more and an average reflectance of 5% or less at a light wavelength (λ = 450 to 650 nm), and a light wavelength (λ = 750 to 900 nm). The average light transmittance is 3% or less.

  The method for forming the infrared light wavelength shielding film 23 is substantially the same as the method for forming the light wavelength attenuation film 22 described above, although the vapor deposition material is different. The infrared light wavelength shielding film 23 can also be obtained by coating a resin in which an infrared light wavelength absorber is dispersed on a substrate. However, as shown in the present embodiment, using the multiple interference of thin films having different refractive indexes, the transition region between the transmission band and the non-transmission band becomes steep, and the transmittance of the transmission band can be maintained higher. An infrared light wavelength shielding film 23 with good color balance can be formed.

  In this embodiment, the light wavelength attenuating film 22 and the infrared light wavelength shielding film 23 are both formed by a vacuum deposition method, but they are formed by a sputtering method, an ion plating method, an ion assist method, or the like. It is possible to select a film formation method suitable for the purpose and conditions.

  Further, as shown in FIG. 5, at least the infrared region as shown in FIG. 6 is formed in the region B where the infrared light wavelength shielding film 23 is formed on the surface of the transparent substrate 21 on which the infrared light wavelength shielding film 23 is formed. An infrared light wavelength transmission film 24 having spectral characteristics that transmits light of a light wavelength may be formed.

In this embodiment, the infrared light wavelength transmission film 24 obtains desired spectral characteristics by utilizing multiple interference of thin films having different refractive indexes, similarly to the infrared light wavelength shielding film 23. SiO _{2 is} used as the low refractive material, and TiO ₂ is used as the high refractive material. However, similarly to the infrared light wavelength shielding film 23, MgF ₂ , Al ₂ O ₃ , MgO, Ta ₂ O ₃ , ZrO ₂ , Nb _{2 are used.} O ₅ or the like can also be used. The infrared light wavelength transmission film 24 is formed by a vacuum deposition method in the same manner as the light wavelength attenuation film 22 and the infrared light wavelength shielding film 23, but a sputtering method, an ion plating method, an ion assist method, etc. It is also possible to form a film. Here, it is optimal that the film thickness of the infrared light wavelength transmission film 24 is an optical film thickness substantially equal to that of the infrared light wavelength shielding film 23.

  By doing so, the focus of the photographic light can be adjusted regardless of which of the region A having the infrared light wavelength shielding film 23 and the region B having the infrared light wavelength transmission film 24 of the optical filter 12 is disposed on the aperture opening. The position does not shift greatly. Therefore, a good image can be obtained even if an image is taken using any one of the region A having the infrared light wavelength shielding film 23 and the region B having the infrared light wavelength transmission film 24.

  FIG. 7 shows a configuration diagram of the optical filter 12 of the second embodiment. The same members as those of the first embodiment are denoted by the same reference numerals, and description thereof is omitted. A visible light wavelength attenuation film 31 that attenuates light of visible light wavelength to a substantially uniform or substantially the same level is formed in the area A on one surface of the transparent substrate 21 made of PET, and an infrared light wavelength is applied to the area B. Infrared light wavelength attenuating films 32 for substantially uniformly attenuating the light are respectively formed. On the other hand, in the region A on the other surface of the transparent substrate 21, an infrared light wavelength shielding film 23 that shields light having an infrared light wavelength is formed as in the first embodiment.

  The optical density (OD) of the visible light wavelength attenuation film 31 at the visible light wavelength is different from the optical density of the infrared light wavelength attenuation film 32 at the infrared light wavelength. In the second embodiment, the optical density of the infrared light wavelength attenuation film 32 is different. The density is smaller than the optical density of the visible light wavelength attenuation film 31. Here, the optical density (OD) is represented by OD = Log (1 / T), where T is the transmittance.

  In Example 2, the spectral characteristics of the visible light wavelength attenuation film 31 and the infrared light wavelength attenuation film 32 are as shown in FIGS. 8 and 9, respectively, and the optical densities thereof are 1.5 and 0.55, respectively. It is said that. In the second embodiment, the optical density of the infrared light wavelength attenuation film 32 is made smaller than the optical density of the visible light wavelength attenuation film 31. This is because, as shown in FIG. 10, the relative sensitivity of a general imaging device 7 incorporated in an imaging apparatus that performs color photography is higher in the sensitivity of the visible light wavelength than in the infrared light wavelength. is there. Depending on the sensitivity of the image sensor and the optical system to be incorporated, the optical density of the infrared wavelength attenuation film 32 may be higher.

  Similarly to the first embodiment, the infrared light wavelength transmission film 24 may be formed in the region B of the surface of the transparent substrate 21 having the infrared light wavelength shielding film 23. At this time, the infrared wavelength transmission film 24 is formed on the opposite surface of the transparent substrate 21 so as to face the infrared wavelength attenuation film 32.

  By incorporating such an optical filter 12 in the imaging optical system, it is possible to adjust the optimal amount of light regardless of whether the imaging element 7 has a sensitivity of visible light wavelength or infrared light wavelength. Become.

  FIG. 11 is a configuration diagram of the optical filter 12 in the third embodiment, and the same members as those in the first and second embodiments are denoted by the same reference numerals. In the optical filter 12 of the third embodiment, a light wavelength attenuation film 22 is provided on a part of one surface of the transparent substrate 21, and an infrared light wavelength shielding film 23 is provided on a part of the other surface of the transparent substrate 21. Or the area | region F which does not restrict | limit the transmittance | permeability of an infrared-light wavelength or both is provided. The region F has an area sufficient to cover the opening formed by the diaphragm blades 11a and 11b.

  When the optical filter 12 shown in FIG. 11 is incorporated in the imaging optical system and color imaging is performed, the region is used when the amount of light used for imaging can be controlled only by adjusting the aperture diameters of the aperture blades 11a and 11b. C is arranged on the optical axis. Further, when the amount of visible light is large and the aperture diameter is equal to or smaller than a certain size, the region D is arranged on the optical axis.

  On the other hand, when performing monochrome photography, if the amount of light used for photography can be controlled only by adjusting the aperture diameter of the aperture blades 11a and 11b, the region F is arranged on the optical axis, and the amount of light is large and the aperture diameter is constant. If it is less than or equal to, the region E is arranged on the optical axis.

  Further, in the optical filter 12 of the modified example shown in FIG. 12, the visible light wavelength attenuation film 31 and the infrared light wavelength attenuation film 32 are provided in parallel on each part of one surface of the transparent substrate 21. An infrared light wavelength shielding film 23 is provided on a part of the other surface of the transparent substrate 21 at a position corresponding to the region G of the visible light wavelength attenuation film 31 and the region H of the transparent part.

  When the optical filter 12 is incorporated in the imaging optical system and color photography is performed, the region H is arranged on the optical axis when the light amount can be adjusted only by the diaphragm blades 11a and 11b. In the case of a light amount with which the opening diameter of 11b is a certain value or less, the region G is disposed on the optical axis.

  On the other hand, when performing monochrome photography, if the light amount can be adjusted only with the diaphragm blades 11a and 11b, the region J is arranged on the optical axis L, and the light amount is such that the aperture diameter of the diaphragm blades 11a and 11b is less than a certain value. The region I is arranged on the optical axis L.

  The optical filter 12 according to the third embodiment is configured as shown in FIG. 11 or FIG. 12, but may have a region that does not limit the transmittance of visible light wavelength or infrared light wavelength. It is not limited to the form.

  Further, similarly to Example 1, an infrared light wavelength transmission film 24 is formed in a region where the infrared light wavelength shielding film 23 is not formed on the surface of the transparent substrate 21 having the infrared light wavelength shielding film 23. Also good. It is more preferable that the infrared light wavelength transmission film 24 has an optical film thickness substantially equal to that of the infrared light wavelength shielding film 23 because the focus adjustment at the time of switching between color photography and monochrome photography is facilitated.

  The imaging optical system incorporating the optical filter 12 shown in the third embodiment can adjust the light amount in a wider range in both color photography and monochrome photography, and can obtain a more optimal image.

  When the amount of visible light wavelength is sufficient during the daytime, the regions C, D, G, and H having the infrared light wavelength shielding film 23 of the optical filter 12 cover the openings formed by the apertures 11a and 11b. The color shooting is performed. On the other hand, when the amount of visible light wavelength is insufficient, the regions E, F, I, and J where the infrared wavelength blocking film 23 of the optical filter 12 is not formed are arranged so as to cover the opening. Monochrome shooting using infrared light wavelengths is performed.

  When the optical filter 12 according to the third embodiment is used, either the region where the light wavelength used for photographing is not attenuated or the region where it is attenuated is arranged on the optical axis L in accordance with the amount of light wavelength used for photographing. .

  In this way, by adjusting the region of the optical filter 12 that affects the optical axis L, a color image close to the color perceived by the human eye can be obtained in photographing using the visible light wavelength, and infrared light can be obtained. An image having a good image quality can be obtained even in monochrome photography using a wavelength.

DESCRIPTION OF SYMBOLS 1, 3-5 Lens 2 Light quantity adjusting device 6 Low pass filter 7 Image sensor 8 Light quantity control part 9 Filter drive part 11a, 11b Aperture blade 12 Optical filter 21 Transparent substrate 22 Light wavelength attenuation film 23 Infrared light wavelength shielding film 24 Infrared Light wavelength transmission film 31 Visible light wavelength attenuation film 32 Infrared light wavelength attenuation film

Claims (9)

  Forming a light wavelength attenuation film having a substantially uniform transmittance from the visible light wavelength to the infrared light wavelength on one side of the transparent substrate, and forming an infrared light wavelength shielding film on a part of the other surface of the transparent substrate. A characteristic optical filter.   2. The optical filter according to claim 1, wherein the optical filter has a region that does not restrict either visible light wavelength, infrared light wavelength, or both.   The light wavelength attenuation film has a visible light wavelength attenuation film and an infrared light wavelength attenuation film in a direction parallel to the one surface of the transparent substrate, and the infrared light wavelength shielding film faces the visible light wavelength attenuation film. The optical filter according to claim 1, wherein the optical filter is formed on the other surface of the transparent substrate.   The optical filter according to claim 3, wherein the infrared light wavelength shielding film is not formed on the other surface of the transparent substrate facing the infrared light wavelength attenuation film.   5. The optical filter according to claim 3, wherein the visible light wavelength attenuation film and the infrared light wavelength attenuation film have different optical densities with respect to corresponding wavelengths.   6. The infrared light wavelength transmission film that transmits at least light having an infrared light wavelength is provided on the other surface of the transparent substrate facing the infrared light wavelength attenuation film. The optical filter according to item 1.   The optical filter according to claim 1, wherein the transparent substrate is a synthetic resin substrate.   The optical filter according to claim 1, wherein the optical filter is an ND filter.   9. A light amount adjusting device having an aperture device for forming an aperture and an optical filter according to claim 1 for adjusting an amount of light on an optical axis, comprising a drive unit for switching a region for controlling the transmittance of the optical filter. A light amount adjusting device characterized by the above.

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