JP5016872B2 - Optical filter - Google Patents

Description

  The present invention relates to an optical filter used in a photographing optical system such as a video camera or a still camera.

  2. Description of the Related Art Conventionally, a photographing system such as a video camera or a digital still camera has been provided with a light amount diaphragm device for controlling the amount of light incident on a solid-state imaging device such as a CCD or CMOS sensor. This light amount diaphragm device has a structure in which the aperture is narrowed down as the field becomes brighter.

  Therefore, when shooting a clear or high-brightness subject, the aperture is in a so-called small aperture state, which is easily affected by the hunting phenomenon of the aperture, the light diffraction phenomenon, etc. is there.

  As countermeasures against this, the amount of light is controlled by disposing an ND (Neutral Density) filter in the vicinity of the stop, or by directly attaching an ND filter to the stop blade. In this way, even if the brightness of the object scene is the same, a contrivance is made so that the aperture of the diaphragm can be made larger.

  In recent years, the sensitivity of the image sensor has improved, and the density of the ND filter has been increased to further reduce the light transmittance. Improvements have been made to prevent the opening from becoming too small. A transparent substrate such as glass is also used as a substrate for forming the ND filter. However, in recent years, a substrate made of a plastic material has been used due to demands for processing into an arbitrary shape, miniaturization, and weight reduction. It is coming.

  As a general manufacturing method of the ND filter, a multi-layer film is formed on a transparent substrate such as plastic or glass by a vacuum deposition method, a sputtering method, or the like.

  In addition, there is an increasing demand for higher accuracy in ND filters for various reasons. Among them, reducing the spectral reflectance in the entire visible light wavelength region of approximately λ = 400 to 700 nm is one major issue. It has become. This is because with the recent increase in sensitivity and resolution of solid-state image sensors, there is a possibility that defects such as ghosts and flares may occur in the captured image even if the reflectance is reduced to the same level as before. This is because of the increase.

As a method of reducing the reflection of the ND filter, an antireflection film is generally formed on the substrate surface of the ND filter by a vacuum vapor deposition method or the like. However, when an antireflection film consisting of a single layer film is formed on the surface of the substrate, it is possible to suppress the surface reflection at a specific wavelength, but in a wavelength region other than that wavelength, reflection is not possible. There is a drawback that becomes larger. Therefore, for example, several types of thin films having different refractive indexes such as SiO ₂ , MgF ₂ , Nb ₂ O ₅ , TiO ₂ , Ta ₂ O ₅ , and ZrO ₂ are stacked to form a multilayer film as shown in Patent Document 1. Therefore, suppression of reflection in an arbitrary wavelength region is widely performed.

JP-A-8-0795902

  However, in the case of an antireflection film formed of a multilayer film as described above, reflection in a wider wavelength range can be reduced than in the case of an antireflection film formed of a single layer film, but again only in a specific wavelength region. The fact that reflection cannot be suppressed remains the same. In order to reduce reflection in a wider wavelength range, the number of layers is limited and the film design values are complicated to obtain arbitrary optical characteristics because the thin film materials that make up the multilayer film are limited. There are problems such as becoming. Furthermore, there are many drawbacks such as a large change in optical characteristics due to a change in incident angle of light and a change in polarization state.

  When light rays that have passed through the imaging optical system including the camera's light intensity adjustment device form an image on the surface of the solid-state image sensor, some of the light rays are reflected on the element surface, the surface of the rear lens group, and the lens barrel wall surface. There is a case of returning to the diaphragm side. The reflected light is re-reflected by the ND filter and re-enters the solid-state imaging device, thereby causing the above-described problem. In order to prevent this, the ND film surface to which the antireflection film is applied is usually arranged so as to be on the solid-state image sensor side. However, as in the case of the ND filter alone described above, the ND film is transmitted to the opposite side. Reflection at the interface between the substrate and the atmosphere cannot be prevented.

  Further, when the antireflection film as described above is formed simultaneously with the formation of an optical filter such as an ND filter formed only on one side, reflection of the surface provided with the antireflection film can be suppressed. . However, it is not possible to prevent reflection on the opposite surface through the substrate. That is, the light that has passed through the optical filter layer and further passed through the substrate is reflected again on the surface in contact with the atmosphere and returns to the incident side. As a result, although reflection varies slightly depending on the refractive index of the substrate and the like, when reflection is measured in the atmosphere, even if it is an optimum film design, reflection of approximately 4% remains theoretically.

  Therefore, there is a limit to the reflection that can be suppressed only by the antireflection film provided on the surface side. Therefore, a means for forming an antireflection film on both sides of the substrate is also conceivable. However, in the case of an optical filter such as an ND filter, a film configuration for considering antireflection in parallel with the function originally intended. Becomes complicated, the number of layers increases, and the film thickness increases. As a result, there is a possibility that the heat load generated in the film forming process becomes large, or the stress generated by the laminated film itself becomes large, thereby causing other problems such as warpage and cracks.

  In order to solve these problems, it is necessary to examine the material, thickness, etc. of the substrate, and the substrates that can be used are very limited.

  An object of the present invention is to provide an optical filter that solves the above-described problems, reduces the spectral reflectance, and reduces the occurrence of defects such as flare and ghost.

  Another object of the present invention is to provide an optical filter that can reduce defects caused by reflected light by reducing reflection in the entire desired wavelength region.

The technical feature of the optical filter according to the present invention for achieving the above object is that an ND layer is provided on one surface of a transparent substrate, an antireflection film is formed on the surface layer, and an upper layer of the antireflection film is provided. A non-reflective periodic layer made up of a number of anti-reflection structures is formed on the other surface of the transparent substrate, and the non-reflective periodic layer is reflected at intervals of a period shorter than the wavelength of light targeted for anti-reflection. The prevention structure is arranged.

  According to the optical filter of the present invention, the spectral reflectance can be reduced over the entire visible wavelength region, and adverse effects on images such as flare and ghost caused by reflected light can be reduced.

The present invention will be described in detail with reference to the illustrated embodiments.
FIG. 1 is a configuration diagram of a photographing optical system, in which a solid-state image pickup device 7 including a lens 1, a light amount diaphragm device 2, lenses 3 to 5, a low-pass filter 6, a CCD, and the like are sequentially arranged. In the light quantity diaphragm device 2, a pair of diaphragm blades 9 a and 9 b are movably attached to the diaphragm blade support plate 8. An ND filter 10 for reducing the amount of light passing through the substantially rhombus-shaped opening formed by the diaphragm blades 9a and 9b is bonded to the diaphragm blade 9a.

  However, in the case of the ND filter 10 having a low density, the reflectance tends to increase as the density decreases, and the possibility of various problems due to reflected light increases.

Empirically, if the ND filter 10 has a density of approximately 1.0 or less and a transmittance of 10% or more, a defect due to reflection is caused. Furthermore, when the concentration is 0.5 or less and the transmittance is about 31.6% or more, the possibility tends to be remarkably increased. The correlation equation between the density (D) and the transmittance (T) is D = Log ₁₀ (1 / T).

  For this reason, the ND filter 10 having a concentration of 1.0 or less often requires some reflection suppressing means such as forming an antireflection film on the back surface of the ND film in addition to the ND film. .

FIG. 2 shows a film configuration diagram of the ND film of the ND filter 10. Al ₂ O ₃ films 12 are laminated on the first, _third , fifth, seventh and ninth layers on the substrate 11 made of a transparent plastic material, and TixOy films 13 are alternately laminated on the _second , fourth, sixth, eighth and tenth layers. A total of eleven ND films 15 each having a MgF ₂ film 14 formed on the eleventh surface layer are formed.

This ND film 15 has sufficient problems such as the linearity of spectral transmittance in the visible light wavelength region, the film stress which is a concern due to the use of a plastic film for the substrate 11, and the problem of thermal stress generated as a whole film forming process. It is designed in consideration of. Design values such as the film thickness of each layer is different, any number layers or all of the Al ₂ O ₃ film 12 of the Al ₂ O ₃ film 12 may be replaced with the SiO ₂ film. Even when the SiO ₂ film or the Al ₂ O ₃ film 12 and the TixOy film 13 are stacked on each other, it is possible to produce the ND film 15 having substantially the same optical characteristics.

The outermost MgF ₂ film 14 has an optical film thickness of n × d (n: refractive index, d: physical film thickness), and is formed to a thickness of λ / 4 with λ = 540 nm. This outermost MgF ₂ film 14 is an antireflection film configured for the purpose of reducing the reflectivity of the ND film 15, and is selected as having a refractive index n of 1.5 or less in the visible wavelength range. Yes. In this embodiment, the MgF ₂ film 14 is used as the antireflection film. However, since the main purpose is to reduce the reflectance, any material having a low refractive index may be used. For example, a SiO ₂ film or the like is used. Even so, a substantially similar ND film 15 can be produced.

  As the substrate 11 in this embodiment, a transparent plastic material is used for reasons such as workability into an arbitrary shape. Specifically, Arton (manufactured by JSR), which is a norbornene-based resin that is excellent as a substrate material in terms of heat resistance, flexibility, and cost, has a thickness that does not include a non-reflective periodic structure described later. However, a 200 μm film is selected.

  In this embodiment, Arton (manufactured by JSR) is selected as the substrate 11, but the present invention is not limited to this, and other norbornene resins such as Zeonex and Zeonor (manufactured by ZEON Corporation, trade name) may be used. . Furthermore, various plastic substrates such as PMMA, polycarbonate, PET, PEN, PC, and PO polyimide resin other than norbornene resin can be used.

  In general, the material of the substrate 11 used as the ND filter 10 as in this embodiment has high heat resistance (glass transition point Tg), high bending elasticity, and further transparency in the visible light wavelength range. A material having a high water absorption rate is preferable. When the film is formed on the substrate 11 made of a thin plastic film like the ND filter 10 of the present embodiment, the norbornene-based resin is formed in consideration of the above-described heat resistance, bending elasticity, and cost factors. One of the most suitable materials.

In recent years, SWS (sub-wavelength grating) called “ Moth eye”, which has a periodic structure shorter than the wavelength of light, can be manufactured along with improvements in microfabrication technology used in semiconductors, MEMS (Micro Electro Mechanical System), etc. It has become. For example, when the eyelids are observed with the naked eye, the phenomenon of appearing black suggests that the reflection is suppressed, and the surface reflection is suppressed by SWS. This is because CG. It was discovered by measuring the reflectance of the concavo-convex structure of several hundred nm unit formed on the surface of the eye of Bernhard by Bernhard. In this SWS, most of the intensity of incident light is transmitted into the substance as 0th-order diffracted light, and hardly generates diffracted light other than the 0th-order, and can be generated into an arbitrary shape.

  FIG. 3 is a perspective view of a conical fine uneven periodic structure 21 which is one of SWS, and is generated on the surface for the purpose of reducing Fresnel reflection by applying the above-described features of SWS. Further, instead of the fine uneven periodic structure 21, a pyramid-shaped fine uneven periodic structure 22 as shown in FIG. 4 may be used. FIG. 5 shows a perspective view in the case where the non-reflective periodic layer 23 composed of a large number of conical fine uneven periodic structures 21 is formed on the surface on which the reflection of the substrate 11 is desired to be suppressed.

  In addition, as shown in FIG. 7, a non-reflective periodic layer 25 may be formed on the surface on which the reflection of the substrate 11 is to be suppressed, using a large number of inverted conical fine irregular periodic structures 24 as shown in FIG. . Thereby, it is possible to reduce reflection by using a distribution in which the refractive indices of the medium before and after the incident light are smoothly connected artificially.

  Various methods such as an injection molding method, a thermosetting resin molding method, and a 2P molding method using a UV curable resin can be considered for the production of these fine uneven periodic structures 21, 22, 24. It is made by injection molding. The injection molding method had a desired shape by filling the mold cavity composed of male and female molds with molten plastic material at high speed and high pressure with a screw, rapidly cooling and taking out from the mold. This is a method for obtaining a molded product.

  In the present embodiment, a non-reflective periodic layer 23 made of a conical fine uneven periodic structure 21 as shown in FIG. 3 is employed. Considering the application of the ND filter 10, the objective is to reduce the reflectance in the visible light wavelength region of approximately λ = 400 to 700 nm, and the ratio of height to period (aspect ratio) is 1 at a height of 250 nm and a period of 220 nm. It is designed to be the above.

  The pyramid-shaped fine uneven periodic structure 22 shown in FIG. 4 and the inverted conical fine uneven periodic structure 24 shown in FIG. 6 also have substantially the same reflection reduction effect although the design values are slightly different. it can. Therefore, if a certain condition is satisfied with respect to a target value such as a period and an aspect ratio, an optimum shape is selected based on various factors such as a certain constraint condition and the difficulty of manufacturing a fine uneven periodic structure. It is possible.

  In the case of the fine uneven periodic structure 21 shown in FIG. 3, the arrangement may be the square arrangement shown in FIG. 8 (a), the hexagonal arrangement shown in FIG. 8 (b), or the like. Since the exposed surface of the material 11 is small, it is said that the antireflection effect is high. However, in this embodiment, the square arrangement shown in FIG. 8A is used for the convenience of manufacturing the fine uneven periodic structure 21.

  FIG. 9 is a cross-sectional view of the ND filter 10 in the present embodiment. The non-reflective periodic layer 23 is formed on the substrate 11 as described above, and the ND film 15 is formed on the other surface of the substrate 11 by vacuum evaporation. Has been. The antireflection function can be improved even if the single-concentration type ND film 15 having a concentration of 0.6 is formed in any region of the film formation area. Further, like the ND filter 10 shown in FIG. 10, the non-reflective periodic layer 23 can be formed on both surfaces of the substrate 11.

  In this embodiment, the ND film 15 is formed on the surface. However, the present invention is not limited to this, and when an optical filter other than the ND filter 10 is generated, the target thin film is the ND film 15 shown in FIG. What is necessary is just to laminate instead of.

  In the present embodiment, the vacuum deposition method used for forming the ND film 15 can control the film thickness relatively easily, has very little scattering in the visible light wavelength region, and has less wavelength dependence of the spectral transmittance. It has the advantage that it can be controlled to a value. However, it is not limited to the vacuum deposition method, and it is possible to form a film by a film formation method such as sputtering method, IAD method, IBS method, ion plating method, cluster vapor deposition method, etc. An appropriate film forming method may be selected.

  FIG. 11 is a graph showing the spectral reflectance in the visible wavelength region of the surface of the ND film 15 of the manufactured ND filter 10, and the spectral reflectance is 0.5% or less in the entire visible wavelength region. For example, when an ND filter having the same film configuration as that of the ND filter 10 of the present embodiment and not including the non-reflective periodic layer 23 is compared, the spectral reflectance in the visible light wavelength region of the ND film 15 surface is 3% at the maximum. It will be about. Thus, in the ND filter 10 provided with the non-reflective periodic layer 23 according to the present embodiment, the reflectance is significantly reduced.

This is because the spectral reflectance at the boundary between the atmosphere and the ND film 15 and the boundary between the ND film 15 and the substrate 11 among the light rays incident on the ND filter 10 is suppressed by the MgF ₂ film 14 of the stacked ND film 15. The The reflectance at the boundary between the substrate 11 and the atmosphere, that is, the light exit surface, is reduced by the non-reflective periodic layer 23.

  In this embodiment, the surface side of the ND film 15 is the light incident side, and the surface on which the non-reflective periodic layer 23 is formed is the exit side. However, when the non-reflective periodic layer 23 is formed, this surface is Similar effects can be obtained on the incident side. Even in appearance, there is no occurrence of wrinkles or cracks, and a good ND filter 10 can be obtained.

  FIG. 12 is a perspective view of a light quantity diaphragm device using the ND filter 10 having such characteristics. The ND filter 10 moves in and out of the diaphragm base plate opening 32 as the diaphragm blades 9a and 9b are moved by the diaphragm driving unit 31. It is supposed to be free.

  By applying this light quantity diaphragm device to a video camera or digital still camera as shown in FIG. 1, it is possible to reduce problems caused by reflected light of the ND filter 10 such as ghost and flare.

  As shown in this embodiment, by forming the non-reflective periodic layer 23 on the surface on the opposite side of the ND film 15, reflection on all surfaces can be reduced, and the reflected light of the ND filter 10 such as ghost and flare is used. Defects can be reduced.

Further, since the spectral reflectance of the non-reflective periodic layer 23 is usually lower than the spectral reflectance of the MgF ₂ film 14 that is an antireflection film formed on the ND filter 10, the non-reflective period is contrary to the conventional case. By attaching the layer 23 side toward the solid-state imaging device 7, even better spectral reflectance characteristics can be obtained.

It is a block diagram of an imaging optical system. It is a film | membrane structure figure of ND film | membrane. It is a perspective view of a fine uneven | corrugated periodic structure. It is a perspective view of the fine uneven | corrugated periodic structure of a modification. It is a perspective view of the board | substrate which provided the fine uneven | corrugated periodic structure. It is a perspective view of the fine uneven | corrugated periodic structure of a modification. It is a perspective view of the board | substrate which provided the fine uneven | corrugated periodic structure. It is explanatory drawing of the example of arrangement | sequence of a fine uneven | corrugated periodic structure. It is sectional drawing of ND filter which has a fine uneven | corrugated periodic structure. It is sectional drawing of the ND filter which has another fine uneven | corrugated periodic structure. It is a graph of the spectral reflectance of an ND filter. It is a disassembled perspective view of the light quantity aperture apparatus which uses ND filter.

Explanation of symbols

10 ND filter 11 substrate 12 Al ₂ O ₃ film 13 TixOy film 14 MgF ₂ film 15 ND film 21, 22, 24 fine uneven periodic structure 23 and 25 non-reflective periodic layer

Claims (4)

An ND layer is provided on one surface of the transparent substrate, an antireflection film is formed on the surface layer thereof, and an antireflection film comprising a number of antireflection structures on the upper layer of the antireflection film and the other surface of the transparent substrate. An optical filter characterized in that a periodic layer is formed, and the antireflective periodic layer is arranged at intervals of a period shorter than the wavelength of light to be antireflected.   The optical filter according to claim 1, wherein the transparent substrate is a transparent plastic substrate. 3. The optical filter according to claim 1, wherein the maximum value of the concentration of the ND layer is 1.0 or less in a visible wavelength region.   An imaging optical system having a solid-state imaging device and a light quantity diaphragm device, wherein the optical filter according to any one of claims 1 to 3 is provided in the light quantity diaphragm device, and the non-reflective periodic layer of the optical filter. An imaging optical system, wherein the imaging optical system is attached toward the solid-state imaging device.

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