JP5554012B2 - Optical filter and imaging apparatus using the optical filter - Google Patents

Description

  The present invention relates to an optical filter suitable for use in a photographing system such as a digital video camera or a digital still camera, and an imaging apparatus using the optical filter.

  Currently, imaging devices such as video cameras and digital cameras are equipped with a light amount adjustment device for controlling the amount of light incident on a solid-state image sensor. Is narrowed down to a small size.

  However, when the aperture is too small, there is a problem that the image performance is deteriorated due to the influence of diffraction of light passing therethrough. Therefore, for example, a film-like ND (Neutral Density) filter is attached to the diaphragm blades so that the diaphragm aperture is not excessively reduced and the light quantity is attenuated while maintaining a predetermined size.

  As the ND filter, for example, one having a configuration in which the transmittance gradually increases toward the optical axis center as disclosed in Patent Document 1, or toward the optical axis center as disclosed in Patent Document 2. A structure having a continuously increasing transmittance is known.

  In general, the base material of such an ND filter has excellent optical properties such as light transmittance and turbidity of the material, and excellent durability, such as PET (polyethylene terephthalate) and PEN (polyethylene naphthalate). A film-like transparent resin substrate is used. Then, a light attenuating film in which a light-absorbing material and a material for reducing the reflectance are alternately laminated is formed on these transparent resin substrates by a vacuum deposition method or the like, and a dielectric layer is further formed on the outermost layer. By forming the body film, the reflectance is further reduced.

  However, in the vapor deposition process for manufacturing the ND filter, the surface temperature of the transparent resin substrate increases with the elapse of the vapor deposition time, and the ND filter may undergo a shape change such as wrinkles or undulations. In addition, PET and PEN base materials have birefringence and a phase difference occurs, which may affect the resolution of the imaging device.

  Therefore, in Patent Document 3, a norbornene-based resin having a glass transition temperature of 120 ° C. or higher is used as the transparent resin substrate on which the light attenuating film is vapor-deposited. It is disclosed to prevent the occurrence of. Patent Document 4 discloses that birefringence is reduced by using the same norbornene-based resin for the transparent resin substrate.

Japanese Patent No. 2592949 JP 2004-117467 A Japanese Patent Laid-Open No. 2004-37545 JP 2008-102363 A

  With the recent improvement in image quality of imaging devices, it has become an important issue to reduce the reflectance of the ND filter while reducing the influence of heat and birefringence. In particular, when the reflectance of the ND filter is high, the light beam reflected by the surface of the image sensor is reflected again by the surface of the ND filter and re-enters the image sensor, causing ghost, flare, and the like.

  In order to reduce this, Patent Documents 1 and 2 describe disposing an antireflection film on the surface layer of the ND filter, but this may not be sufficient.

  In Patent Documents 3 and 4, norbornene-based resins having high transparency, low haze values, and low birefringence are used to prevent deformation of the transparent resin substrate due to the influence of heat such as vapor deposition. . However, the reflectance can be reduced as the refractive index of the base material is higher as described later, but the refractive index of the transparent resin substrate using the norbornene-based resin is about 1.51 to 1.53. There is a limit to the reduction of reflectance only by forming an antireflection film on the surface of the material.

  The object of the present invention is to solve the problems of both birefringence and reflectivity by using a transparent resin substrate that is less susceptible to deformation due to heat during film formation and has low reflectivity and low birefringence. An optical filter and an imaging device using the optical filter are provided.

In order to achieve the above object, an optical filter according to the present invention is an ND filter that adjusts the amount of transmitted light by forming an inorganic hard film having at least a dielectric film on a transparent resin substrate. The transparent resin substrate is a polyester resin having a fluorene structure having a glass transition temperature of 120 ° C. or more, a refractive index of 1.6 or more, and a retardation value of 30 nm or less, and an ND film and the dielectric on the transparent resin substrate The inorganic hard film is formed by forming an antireflection film in the order of the film, the highest density as the ND filter is 1.5, and the reflectance at a wavelength of at least 550 nm which is the central wavelength in the visible light region Is 1% or less.

In addition, an imaging apparatus using the optical filter according to the present invention includes an imaging optical system, an optical filter that becomes an ND filter that is inserted into and removed from the imaging optical system and adjusts the amount of transmitted light, and imaging that converts an optical image into an electrical signal In the imaging device comprising the element, the optical filter is formed on a transparent resin substrate made of a polyester resin having a fluorene structure having a glass transition temperature of 120 ° C. or higher, a refractive index of 1.6 or higher, and a retardation value of 30 nm or lower. In addition, an inorganic hard film is formed by laminating an ND film and an antireflection film in this order, and the highest density as the ND filter is 1.5, and 1% at a wavelength of at least 550 nm , which is the central wavelength in the visible light region. It is characterized by the following reflectance.

  According to the optical filter and the imaging device using the optical filter according to the present invention, generation of wrinkles and undulations during film formation can be prevented, and since the birefringence is small, it is possible to cope with high image quality.

It is a block diagram of an imaging optical system. It is a chemical structural formula of fluorene. It is a chemical structural formula of an example of polyester which has a fluorene structure. It is the schematic of a vacuum evaporation system. It is the side view and top view of a vapor deposition jig. It is a top view of the transparent resin substrate of the state removed with the vapor deposition mask. It is explanatory drawing at the time of cutting out an ND filter. It is a cross-sectional block diagram and a transmittance distribution diagram of the ND filter. It is a film | membrane block diagram of a ND filter. It is a graph of the reflectance of the single layer antireflection film according to the refractive index of the substrate. It is explanatory drawing of the reflection prevention by interference of a single layer antireflection film.

The present invention will be described in detail based on the embodiments shown in the drawings.
FIG. 1 is a configuration diagram of a photographing optical system of the image pickup apparatus in the present embodiment. On the optical axis, a solid-state image pickup device 7 including a lens 1, a light amount diaphragm 2, lenses 3 to 5, a low-pass filter 6 having an infrared cut function, a CCD, and the like are sequentially arranged. In the light quantity diaphragm device 2 for adjusting the light quantity, the diaphragm blade support plate 8 is movably attached with diaphragm blades 9a and 9b, which are a pair of diaphragm members that form a diaphragm aperture, and the diaphragm blade 9a has an ND filter 10 mounted thereon. It is attached.

  In the light amount diaphragm device 2, the transmitted light amount in the visible light region passing through the substantially diamond-shaped opening formed by the diaphragm blades 9 a and 9 b can be attenuated by the ND filter 10 to be attenuated. The ND filter 10 can be inserted into and removed from the opening alone without being attached to the aperture blade 9a.

  In such a photographing optical system, the optical image that has passed through the lens 1 is adjusted in light amount through the light amount diaphragm device 2, formed on the surface of the solid-state image sensor 7, and converted into an electrical signal.

  With recent improvements in image quality of imaging devices, it is desired that the birefringence of the ND filter 10 be small. Birefringence is a phenomenon in which light passing through a material whose refractive index differs depending on the direction is separated into extraordinary and ordinary rays. Molecular birefringence inherent to the material and shearing force during molding process and the molten resin solidify. It largely depends on the residual stress generated when If this birefringence is large, the transmitted light is separated and the image forming point is shifted, so that there is a problem that the image forming performance is deteriorated.

  In general, the magnitude of birefringence can be represented by a retardation value (retardation). The retardation value is the difference in the speed of light caused by the above-described difference in refractive index. The retardation value Re [nm] is the extraordinary ray refractive index Ne, the ordinary ray refractive index No, and the birefringent material thickness. Is d [nm], it is calculated by the following equation (1).

Re = d · (Ne−No) (1)
In general, a polyethylene terephthalate (PET) film, which is a transparent resin substrate used for the ND filter 10, is usually stretched during film formation, and therefore has a large birefringence due to molecular orientation, and Re> 100 nm.

  However, with recent improvements in image quality of imaging devices, a transparent resin substrate having a low birefringence has been demanded.

  The glass transition temperature of the transparent resin substrate is preferably 120 ° C. or higher in consideration of heat received from the vapor deposition source during vapor deposition.

  As described above, the refraction of the transparent resin substrate and the glass transition temperature are important, and as a resin that satisfies the above-described conditions, the polyester resin having the fluorene structure shown in FIG. 2 is suitable, and FIG. 3 shows an example thereof. .

  In the present embodiment, by using a polyester resin having a fluorene structure shown in FIG. 3 for the transparent resin substrate, the retardation value Re can be reduced to 30 nm or less at a practical optical filter thickness of 25 to 200 μm. it can. Moreover, since the glass transition temperature of the polyester resin having a fluorene structure shown in FIG. 3 is 120 ° C. or higher, problems such as generation of wrinkles are avoided. At the same time, an excellent function as an optical filter can be expected due to the antireflection effect by having a high refractive index as will be described later.

FIG. 4 shows a schematic view of a vacuum vapor deposition apparatus for manufacturing the ND filter 10. A rotatable rotating dome 12 is provided in the vapor deposition chamber 11, and a vapor deposition jig 13 for holding a transparent resin substrate serving as a base material of the ND filter 10 is disposed on the rotary dome 12. In the vapor deposition chamber 11, a vapor deposition source 14 made of Al ₂ O ₃ and a vapor deposition source 15 made of TiOx are provided as vapor deposition sources for the ND film to be vapor deposited, and a vapor deposition source 16 made of MgF ₂ is deposited as the vapor deposition source of the antireflection film. It has been. Above the vapor deposition sources 14 to 16, a shutter 17 for depositing only the specific vapor deposition sources 14 to 16 is provided, and an optical monitor 18 for measuring the thickness of the vapor deposition film is provided above the vapor deposition chamber 11. Has been placed.

  5A is a cross-sectional view of the vapor deposition jig 13, and FIG. 5B is a plan view. A transparent resin substrate 21 made of a polyester resin having a fluorene structure as a substrate of the ND filter 10 is attached to the vapor deposition jig 13. In addition, an evaporation mask 24 having an opening 23 having a predetermined width is disposed through the spacer 22.

  Then, the vapor deposition jig 13 is disposed on the rotating dome 12 so that the film formation surface of the transparent resin substrate 21 faces the vapor deposition sources 14 to 16, and vapor deposition is performed together with the rotating dome 12 in order to equalize the substrate temperature and film thickness. Film formation is performed while the jig 13 is rotated.

In the vapor deposition step, first, the inside of the vapor deposition chamber 11 is kept in a vacuum state before film formation, and the vapor deposition jig 13 on which the transparent resin substrate 21 is set is heated to a predetermined temperature. When the predetermined pressure and temperature are reached, the vapor deposition source 14 is heated as the first layer of the ND film to evaporate Al ₂ O ₃ as the vapor deposition material and onto the transparent resin substrate 21 set in the vapor deposition jig 13. An Al ₂ O ₃ film having a predetermined thickness is formed. Thereafter, the vapor deposition sources 14 to 16 are switched as necessary to form an ND film and an antireflection film, which are inorganic hard films.

  The higher the concentration of the ND filter 10, the thicker the necessary film thickness, the longer the vapor deposition time, and the greater the radiant heat from the vapor deposition sources 14 to 16.

  In this embodiment, the surface temperature of the transparent resin substrate 21 in the ND filter 10 having a maximum density of about ND1.5 (transmittance of 3.2%) partially reaches nearly 120 ° C. If the temperature is exceeded, soot may be generated.

  The transparent resin substrate 21 is preferably formed as thin as possible. This is because when the ND filter 10 is incorporated in the light quantity diaphragm device 2 and used, the light quantity diaphragm device 2 can be downsized. Further, by making the ND filter 10 lightweight, it is possible to reduce power consumption for driving the ND filter 10 or the diaphragm blade 9a to which the ND filter 10 is attached. However, if the plate thickness of the transparent resin substrate 21 becomes too thin, it becomes difficult to handle the production of the ND filter 10, and the necessary plate thickness is 25 to 200 μm, preferably 50 to 100 μm.

FIG. 6 shows a plan view of the transparent resin substrate 21 made of a polyester resin having a fluorene structure with the vapor deposition mask 24 removed. As shown in FIG. 5A, an ND film having a concentration distribution in which the concentration continuously changes on the transparent resin substrate 21 by providing a gap between the transparent resin substrate 21 and the vapor deposition mask 24 via a spacer 22. 31 can be formed. Subsequently, the vapor deposition mask 24 is removed, and the vapor deposition jig 13 to which the transparent resin substrate 21 is attached is set on the rotating dome 12 of the vapor deposition chamber 11 again. Then, by heating the vapor deposition source 16, a MgF ₂ film having a thickness of λ / 4 (λ = 550 nm) is formed as an antireflection film as a dielectric film on the entire surface of the transparent resin substrate 21.

  Thereafter, if necessary, the transparent resin substrate 21 may be turned over and attached to the vapor deposition jig 13 to form an antireflection film on the back surface of the transparent resin substrate 21. After forming the above-described antireflection film 32, the ND filter 10 having gradation density can be obtained by cutting into the shape of the ND filter as shown in FIG.

  FIG. 8A is a schematic cross-sectional view of the ND filter 10 in this embodiment, and FIG. 8B is a transmittance distribution diagram in the cross-sectional area of FIG. In this embodiment, a gradation ND filter whose light transmittance continuously changes from a low density area A to a high density area D will be described. However, from a single density ND filter or a plurality of different density areas. The same effect can be obtained in the multi-density ND filter.

  The ND film 31 is not formed in the regions A to B, the ND film 31 having a gradation density whose transmittance changes continuously in the regions B to D, and the uniform concentration in the regions C to D. The ND film 31 is formed. Further, an antireflection film 32 which is a dielectric film having a uniform film thickness is formed on the outermost surface layer of the transparent resin substrate 21 in the regions A to D.

  The area A to B is a transparent area in which only the antireflection film 32 is formed. This transparent area has almost no light attenuation effect, but the imaging plane position caused by the thickness and refractive index of the ND filter 10. In order to prevent deviation, it is also used as a filter during actual photographing. The gradation density ND filter 10 of this embodiment has such a transparent region, but the same effect as that of this embodiment can be obtained even with a gradation density ND filter having no transparent region.

  In an optical filter used for a normal digital video camera or digital still camera, the reflectance in the range of 400 to 700 nm in the wavelength visible light region is preferably as low as possible. For this reason, in particular, in an ND filter having a transparent region, such as the ND filter 10 of the present embodiment, an antireflection film 32 is usually required in this transparent region.

FIG. 9 shows a film configuration diagram of the ND filter 10 in this embodiment. On the transparent resin substrate 21, eight layers of Al ₂ O ₃ films that are antireflection layers of dielectric films for reducing the reflectance and TiOx films that are light absorption layers for reducing the transmittance are alternately arranged. An ND film 31 that is a laminated inorganic hard film is laminated. Then, on the outermost layer on the TiOx film of the eighth layer, an antireflection film 32 made of MgF ₂ film, which is a low refractive material, has an optical film thickness n · d (n: refractive index, d: physical film thickness), The film is formed at λ / 4 (λ = 500 to 600 nm).

  The transmittance of the ND filter 10 varies depending on the total thickness of the TiOx films that are the second, fourth, sixth, and eighth light absorption layers, and the transmittance decreases as the film thickness increases. The value of x of TiOx is 0 to 2, and is adjusted according to desired optical characteristics.

  In this embodiment, as shown in FIG. 8A, the transmittance is continuously changed in the regions B to C by continuously changing the film thickness of each layer, and the transmission shown in FIG. It has a rate distribution.

In this embodiment, the MgF ₂ film is used for the outermost antireflection layer, but a low refractive material such as a SiO ₂ film can be used instead of the MgF ₂ film. In general, a vacuum deposition method is used to form these antireflection films 32, but the same effect can be obtained also by an ion plating method or a sputtering method. The material of the film, the number of layers, the film thickness, etc. are not limited to this.

FIG. 10 shows a graph of reflectivity when an antireflection film is formed on transparent resin substrates having different refractive indexes. The solid line shows the reflectance when a single layer MgF ₂ film having a thickness of λ / 4 is formed as the antireflection film 32 on the norbornene resin substrate having a refractive index of 1.53. The dotted line shows the reflectance when a similar antireflection film 32 is formed on a polyester resin substrate having a fluorene structure with a refractive index of 1.63. Incidentally, the refractive index of polyethylene terephthalate (PET) (not shown) is about 1.57, indicating a characteristic between the reflectances of the two substrates described above.

  In the ND filter 10 of this embodiment, a single-layer antireflection film 32 is used, and the reflectance can be reduced as the refractive index n of the transparent resin substrate 21 is higher as shown in FIG. As shown in FIG. 11, the reflected light A on the surface of the antireflection film 32 is offset by the reflected light B from the interface between the transparent resin substrate 21 and the antireflection film 32 due to interference. Therefore, when the difference in refractive index between the transparent resin substrate 21 and the antireflection film 32 increases and the reflected light B increases, the interference effect increases.

  The advantage of forming the single-layer antireflection film 32 is that the film formation can be completed in a shorter time than the multilayer antireflection film, and the influence of heat can be reduced. Furthermore, as shown in FIG. 8A, when the antireflection film 32 is entirely formed from the transparent region to the high concentration region, the total film thickness in the high concentration region becomes extremely large, and the film stress is increased. Generation of cracks can be prevented.

  However, the single-layer antireflection film 32 tends to have a higher reflectance than the multilayer antireflection film, and the refractive index of the transparent resin substrate 21 described above is important.

  As shown in FIG. 10, when a polyester resin having a fluorene structure with a refractive index of 1.6 or higher is used as compared with conventional resins such as PET and norbornene, the center wavelength in the visible light region is around 550 nm. The reflectance is 1% or less at a wavelength of.

  When the plate thickness d at this time is 100 μm, the retardation value Re is 23 nm, and as a result of photographing with the imaging optical system shown in FIG. 1, good imaging performance is obtained. In addition, there are different grades of polyester resins having such a fluorene structure, and when they are used, the retardation value Re can be reduced to 30 nm or less even at a practical ND filter thickness of 200 μm.

  In this embodiment, the ND filter has been described. However, the same effect can be obtained with an optical filter in which an inorganic hard film is formed on a transparent resin substrate made of a polyester resin having a fluorene structure. For example, application to an infrared cut filter, an ultraviolet cut filter, or the like is possible by appropriately changing the laminated film.

2 Light quantity diaphragm 9a, 9b Diaphragm blade 10 ND filter 11 Deposition chamber 12 Rotating dome 13 Deposition jig 14-16 Deposition source 17 Shutter 21 Transparent resin substrate 24 Deposition mask 31 ND film 32 Antireflection film

Claims (7)

In an optical filter that is an ND filter that adjusts the amount of transmitted light by forming an inorganic hard film having at least a dielectric film on a transparent resin substrate, the transparent resin substrate has a glass transition temperature of 120 ° C. or higher and a refractive index. Is a polyester resin having a fluorene structure having a retardation value of 30 nm or less, an ND film, and an antireflection film composed of the dielectric film on the transparent resin substrate in this order. An optical filter comprising a hard film, having a maximum density of 1.5 as the ND filter, and a reflectance of 1% or less at a wavelength of at least 550 nm , which is a central wavelength in a visible light region.   The optical filter according to claim 1, wherein the optical filter includes a region where the inorganic hard film is formed and a region where only the antireflection film is formed.   The optical filter according to claim 2, wherein the optical filter has a region where the light transmittance of the ND film continuously changes.   The optical filter according to any one of claims 1 to 3, wherein the antireflection film is a single layer and is formed on an outermost layer of the optical filter.   A light amount adjusting device using the optical filter according to any one of claims 1 to 4. An imaging apparatus comprising: an imaging optical system; an optical filter serving as an ND filter that is inserted into and removed from the imaging optical system to adjust the amount of transmitted light; and an imaging device that converts an optical image into an electrical signal. An inorganic hard layer is formed by laminating an ND film and an antireflection film in this order on a transparent resin substrate made of a polyester resin having a fluorene structure with a temperature of 120 ° C. or higher, a refractive index of 1.6 or more, and a retardation value of 30 nm or less. An optical filter is used, wherein the optical filter is characterized in that the maximum density as the ND filter is 1.5 and the reflectance is 1% or less at a wavelength of at least 550 nm which is a central wavelength in the visible light region. The imaging device that was.   The imaging apparatus according to claim 6, wherein the photographing optical system includes a diaphragm member that forms a diaphragm aperture, and the optical filter is attached to the diaphragm member to form a diaphragm aperture together with the diaphragm member. .