JP5686554B2 - Optical system and optical apparatus having antireflection structure - Google Patents

Description

  The present invention relates to an optical system in which a fine antireflection structure is formed on at least one optical surface.

  In an optical element such as a lens formed using a light-transmitting medium (light-transmitting material) such as glass or plastic, as a surface treatment for reducing loss of transmitted light due to surface reflection, for example, an incident surface and An antireflection film is provided on at least one of the emission surfaces. As an antireflection film for visible light, a so-called multicoat, which is a multilayer film in which a plurality of dielectric thin films are stacked, is known. This multilayer film is formed by depositing a metal oxide or the like on the surface of a light-transmitting substrate by vacuum deposition or sputtering.

  A reflection reduction method is also known in which an antireflection structure in which a plurality of fine irregularities are formed at a predetermined pitch is provided on the surface of a translucent material. Patent Document 1 discloses that a light-transmitting plastic is injection-molded using a mold having a concave and convex shape with a period (pitch) of 400 nm or less and a depth (height) of 700 nm or less. A method of forming an antireflection structure by transferring a plurality of fine irregularities is disclosed.

JP 2003-43203 A

  However, the method for forming an antireflection structure disclosed in Patent Document 1 has a problem that it is difficult to sufficiently transfer the fine uneven shape formed on the mold to the molding target. This is because the molten plastic cannot be sufficiently filled even inside the extremely fine irregular shape with a period of 400 nm or less because the fluidity of the molten plastic at the time of injection molding is low.

  As one of the methods for solving such problems, there is a heat cycle molding method in which molten plastic is filled in a state where the temperature of the mold is raised above the glass transition point, and then the mold is cooled and released. . In this method, the fluidity of the molten plastic is increased because the temperature of the mold is high, so that the molten plastic is sufficiently filled into the fine uneven shape, and as a result, the transferability of the uneven shape can be improved. .

  However, such a heat cycle molding method has a problem that the time required for one molding becomes longer because the time required for heating and cooling the mold is added to the normal molding method.

  The present invention provides an optical system provided with an antireflection structure that avoids an increase in the time required to form the antireflection structure and has good antireflection performance.

An optical system as one aspect of the present invention has an optical surface including an antireflection structure in which a plurality of concave and convex portions are arranged,
P is the arrangement pitch of the concave and convex portions, λ _min is the shortest wavelength in the wavelength range of light incident on the optical surface, n _{1 is} the refractive index of the medium closer to the light incident side than the optical surface, and The refractive index of the medium on the light exit side is n ₂ , the maximum incident angle of the light ray incident on the position x on the optical surface is θ _1max (x), and the light ray that has _exited from the position x on the optical surface and reaches the image surface When the maximum emission angle is θ _2max (x),

It satisfies the following condition.

  According to the present invention, it is possible to avoid an increase in the time required for forming the antireflection structure and to suppress generation of unnecessary light such as ghosts and flares by the good antireflection performance of the antireflection structure. An optical system capable of doing this can be realized.

1 is a cross-sectional view illustrating a configuration of a scanning optical system that is Embodiment 1 of the present invention. Sectional drawing which shows the structure of the imaging optical system which is Example 2 of this invention. FIG. 6 is a diagram showing the reflectance characteristics of an antireflection structure in Example 2. The figure which shows the reflectance characteristic of the antireflection structure in a comparative example. The figure which shows the basic matter common to Example 1,2. The figure which shows the digital camera provided with the imaging | photography optical system of FIG.

  Embodiments of the present invention will be described below with reference to the drawings.

  First, prior to description of specific embodiments, basic matters common to the embodiments of the present invention will be described with reference to FIG.

  FIG. 5A shows an example of the optical element 2 having the antireflection structure 3 constituted by fine concavo-convex shapes. In this example, the antireflection structure 3 is formed on the optical surface (exit surface) on the exit side of the optical element 2. An antireflection structure may be formed on the optical surface (incident surface) on the incident side of the optical element 2.

  The antireflection structure 3 is configured by forming (repeatingly forming) a plurality of fine concavo-convex portions including one concave portion 3a and one convex portion 3b at a predetermined pitch. The “pitch” of the concavo-convex portion corresponds to the interval between the convex portions 3b (or between the concave portions 3a) in two adjacent concavo-convex portions.

  The optical element 2 is manufactured by a molding method using a mold such as injection molding or press molding using a light-transmitting medium such as plastic or glass as a material. As the mold in this case, for example, a mold in which a plurality of pores are formed on the surface by anodizing aluminum or an aluminum alloy can be used.

  The light 1 incident on the optical element 2 reaches the image plane 5 through the antireflection structure 3. At this time, since the pitch of the concavo-convex portion is set to be equal to or less than the diffraction limit, the emitted light 4 reaches the image plane 5 without being diffracted.

The pitch below the diffraction limit means a pitch at which no diffracted light is theoretically generated. For the pitch p _limit below the diffraction limit, the wavelength of the incident light 1 is λ, the refractive index of the medium on the light incident side of the exit surface is n1, and the refractive index of the medium on the light exit side of the exit surface is n2. When

... [Formula 1]
Is represented by

  However, in practice, if the pitch of the concavo-convex portions is less than the diffraction limit, the dimensions of the concave portions (portions for forming the convex portions 3b of the antireflection structure 3) formed on the surface of the mold (mold, etc.) are also very large. Therefore, the molten translucent medium cannot be sufficiently filled inside the recess. For this reason, the height of the convex portion 3b of the antireflection structure 3 becomes lower than the design value.

  The antireflection structure 3 having a cross-sectional shape as shown in FIG. 5A is equivalent to a single layer film having a low refractive index corresponding to the space filling rate of air and medium. For this reason, if the height of the convex portion 3b does not reach about one-fourth of the wavelength of the incident light 1, sufficient antireflection performance cannot be obtained, and unnecessary light such as ghost or flare is generated by the generated reflected light. There is a fear.

  Therefore, in the embodiment, as shown in FIG. 5B, the pitch (predetermined pitch) p of the concavo-convex portions is set so as to satisfy the following conditions.

... [Formula 2]
However, p is the pitch of the concavo-convex portions (in some cases, the average pitch), and λ _min is the shortest wavelength in the wavelength range of light incident on the optical system.

n ₁ is the refractive index of the medium on the light incident side of the optical surface on which the antireflection structure 3 is formed (the exit surface of the optical element 2 in the example of FIG. 5), and n ₂ is formed by the antireflection structure 3. This is the refractive index of the medium on the light exit side of the optical surface.

In addition, θ _1max (x) is the maximum incident angle of the light ray incident on the position x on the optical surface on which the antireflection structure 3 is formed, and θ _2max (x) is the optical surface on which the antireflection structure 3 is formed. Is the maximum exit angle of the light beam that exits from the position x and reaches the image plane 5.

  In this way, by setting the pitch p of the concave and convex portions to be larger than the diffraction limit (value on the left side of [Expression 2]), the diffracted lights 6 and 7 are generated. However, since the pitch p is set to be smaller than the value on the right side of [Expression 2], the diffracted lights 6 and 7 have a large exit angle (diffraction angle) that does not reach the image plane 5. The occurrence of 7 is not a problem.

  On the other hand, by setting the pitch p of the concavo-convex portion to be larger than the diffraction limit, the size of the concave portion on the surface of the mold for molding the antireflection structure 3 (optical element 2) is enlarged, and FIG. Compared with the case shown, the filling property of the translucent medium at the time of molding is improved. Therefore, even under the same molding conditions, the height of the convex portion 3b in the antireflection structure 3 can be brought close to or matched with the design value. Accordingly, it is possible to ensure a good antireflection performance (low reflectance) of the antireflection structure 3 while avoiding an increase in the time required for forming the antireflection structure 3, such as a ghost caused by reflected light, etc. Generation of unnecessary light can be suppressed.

  FIG. 5C shows an optical element 2 ′ having an antireflection structure 3 ′ as a comparative example with respect to the antireflection structure 3 shown in FIG. In this comparative example, the pitch p of the uneven portion is

... [Formula 3]
Has expanded to. As a result, the size of the concave portion of the mold can be further enlarged, but the emission angle (diffraction angle) of the diffracted light 6 ', 7' is reduced by increasing the pitch p, and the diffracted light 6 ', 7' is reduced. It reaches the image plane 5. Therefore, unnecessary light such as ghost and flare becomes a problem.

  That is, the antireflection structure 3 shown in FIG. 5B is characterized in that the pitch p is expanded beyond the diffraction limit within a range satisfying the condition of [Formula 2]. Thereby, while avoiding the adverse effect of the generated diffracted light, the reflectance of the antireflection structure 3 can be reduced, and an optical system in which generation of unnecessary light is not a problem can be realized.

  FIG. 1 shows the configuration of a scanning optical system that is Embodiment 1 of the present invention. This scanning optical system is used in an image forming apparatus (optical apparatus) such as a laser beam printer that forms an image on a photosensitive drum as an image surface 5 with a laser beam.

  In the figure, reference numeral 11 denotes a light source, for example, a semiconductor laser light source that emits light having a wavelength of 780 nm included in an infrared region (near infrared region). That is, the scanning optical system of the present embodiment is used for light in the infrared region.

  12 is a collimator lens, 13 is an aperture stop, 14 is a cylindrical lens, and 15 is a light deflecting member (for example, a polygon mirror). Laser light (incident light) 1 whose direction is changed by the light deflecting member 15 reaches the image plane 5 via a plurality of optical surfaces (lens surfaces) of the optical elements (lenses) 2a and 2b.

  The optical element 2a is manufactured by an injection molding method using a cycloolefin polymer (manufactured by ZEON Corporation: ZEONEX, refractive index at a wavelength of 780 nm is 1.524) which is a translucent medium. The exit surface of the optical element 2a is provided with an antireflection structure 3 formed of a cycloolefin polymer that is the same material as the optical element 2a. The antireflection structure 3 is configured by forming a plurality of concave and convex portions shown in FIG. 5B at a pitch p.

In this scanning optical system, the maximum incident angle of a light beam on the exit surface of the optical element 2a (the optical surface on which the antireflection structure 3 is formed) is 4 °, and the exit angle 58 from an arbitrary position x on the exit surface. Rays emitted above the angle do not reach the image plane 5. Therefore, in order to set the emission angle of the light beam from the position x to 58 ° or more,
λ _min = 780 nm
n ₁ = 1.524
θ _1max (x) = 4 °
n ₂ = 1.000
θ _2max (x) = 58 °
Was substituted, and the pitch p of the uneven portions was determined to be 800 nm.

  The method for producing the mold (mold) for molding the optical element 2a is not particularly limited, but in this example, it was produced by using an anodic oxidation method.

  First, an aluminum layer is formed on the surface of a normal mold used for injection molding. Thereafter, in the phosphoric acid aqueous solution, a direct current was passed to form micropores perpendicular to the surface of the aluminum layer, and the micropores were then enlarged by immersion in the phosphoric acid aqueous solution without being energized. When the surface of the obtained mold was observed with a scanning electron microscope, it was confirmed that fine holes having a diameter of about 600 nm were formed in a triangular arrangement having an average pitch of about 800 nm.

  Here, the reason why the pitch p in [Equation 2] is the average pitch is that the fine holes manufactured by this method have a slight dispersion rather than a perfect periodic structure. Thus, when the pitch is not a perfect periodic structure, the average pitch may satisfy the condition of [Formula 2].

The optical element 2a was produced by an injection molding method in which a cycloolefin polymer was injected into the mold thus produced. The molding conditions at this time were such that the mold temperature was 125 ° C. and the holding pressure during the injection of the cycloolefin polymer was 950 kg / cm ² . This is a normal molding method that does not use a special molding method (for example, a heat cycle molding method) for transferring fine uneven shapes, so that the time required for heating and cooling the mold is shortened. It can be avoided that the time required for molding the antireflection structure 3 (optical element 2a) becomes long.

  When the exit surface of the optical element 2a manufactured as described above was observed and measured with a scanning electron microscope and an atomic force microscope (AFM), a cylindrical protrusion having a diameter of about 600 nm and a height of about 140 nm ( A plurality of convex portions) were formed with an average pitch of about 800 nm. Moreover, the reflectance at a wavelength of 780 nm of the exit surface (antireflection structure 3) of the optical element 2a was a very low value of 0.2% or less.

  The height of the protrusion (convex portion) corresponds to the film thickness of the antireflection film, and when used at a wavelength of 780 nm, the height is preferably in the range of 80 nm to 250 nm.

  When the scanning optical system incorporating the optical element 2a was installed in a laser beam printer and evaluated, a high-quality image with almost no unnecessary light (noise) such as a ghost could be obtained.

  As shown in FIG. 1, the diffracted light 6 is generated because the pitch p of the concave and convex portions in the antireflection structure 3 is larger than the diffraction limit. However, since the pitch p is set to about 800 nm so as to satisfy the condition of [Equation 2], the direction of the diffracted light 6 is largely changed on the exit surface of the optical element 2a and does not reach the image plane 5. For this reason, the diffracted light 6 does not adversely affect the image.

  Further, although the amount of light (energy) of the emitted light 4 slightly decreases due to the generation of the diffracted light 6, this can be dealt with by increasing the light emission intensity of the light source 11 or adjusting the sensitivity of the photosensitive drum 5. Is possible.

  As described above, in this example, the pitch of the concavo-convex portions of the antireflection structure is set to be larger than the diffraction limit within a range where the diffracted light does not reach the image plane. Accordingly, it is possible to realize an optical system including an antireflection structure having a good antireflection performance while avoiding an increase in the time required for forming the antireflection structure.

In this embodiment, the case where an antireflection structure is provided on one of the plurality of optical surfaces included in the scanning optical system has been described. However, as an embodiment of the present invention, a plurality of optical surfaces are provided. Of these, an antireflection structure may be provided on at least one optical surface (that is, two or more optical surfaces).
[Comparative Example 1]
For comparison with Example 1, a mold having a recess pitch of 300 nm was manufactured so as to satisfy the condition of [Formula 1]. The mold manufacturing method is the same as in Example 1, but in anodizing, by setting the voltage when applying a direct current to a low value, fine holes with a pitch of about 300 nm and a diameter of about 230 nm are formed. Got the mold.

  Using the mold thus obtained, the optical element 2a was molded under the same molding conditions as in Example 1, and the emission surface thereof was observed. Cylindrical protrusions (convex portions) having a diameter of about 230 nm and a height of about 50 nm were formed at a pitch of about 300 nm. The reflectance of the exit surface at a wavelength of 780 nm was 3.3%.

  When the optical element 2a is arranged in the scanning optical system in the same manner as in Example 1 and the scanning optical system is evaluated using a laser beam printer, noise due to a strong ghost is generated and a good image cannot be obtained. It was.

  FIG. 2 shows a configuration of a photographing optical system used for light in the visible range with a wavelength of 400 to 700 nm. In the figure, incident light 1 reaches a solid-state image sensor disposed on an image plane 5 through a plurality of optical surfaces (lens surfaces) of optical elements (lenses) 2c, 2d, 2e, and 2f. The solid-state image sensor is constituted by a CCD sensor or a CMOS sensor.

  The optical element 2c is manufactured by an injection molding method using an acrylic resin (with a refractive index of 1.501 at a wavelength of 450 nm), and an antireflection structure 3 constituted by fine unevenness is formed on the emission surface. Is provided.

  In the photographing optical system shown in FIG. 2, the maximum incident angle of the light beam on the exit surface of the optical element 2c (the optical surface on which the antireflection structure 3 is formed) is 15 °. Then, light emitted from an arbitrary position x on the exit surface at an exit angle of 45 ° or more does not reach the image plane 5.

Therefore, in this embodiment, in order to set the exit angle of the light beam from an arbitrary position x on the exit surface of the optical element 2c to 45 ° or more, [Expression 2]
λ _min = 450 nm
n ₁ = 1.501
θ _1max (x) = 15 °
n ₂ = 1.000
θ _2max (x) = 45 °
Was substituted, and the pitch p of the uneven portion was determined to be 410 nm.

  In addition, when the wavelength used spreads to the area | region (wavelength range) with a certain width | variety, what is necessary is just to determine the pitch p of an uneven | corrugated | grooved part using the shortest wavelength among used wavelengths. This is because long wavelength light is emitted with a larger diffraction angle than short wavelength light.

  The method for producing the mold (mold) for molding the optical element 2c is not particularly limited, but in this example, it was produced by using the anodizing method in the same manner as in Example 1. When the surface of the obtained mold was observed with a scanning electron microscope, it was confirmed that fine holes with a diameter of about 300 nm were formed in a triangular arrangement with an average pitch of about 410 nm.

  The optical element 2c was produced by an injection molding method in which acrylic resin was injected into the mold thus produced. The molding conditions at this time were such that the mold temperature was 95 ° C., and the holding pressure during the injection of the acrylic resin was 80 MPa. This is a normal molding method that does not use a special molding method (for example, a heat cycle molding method) for transferring fine uneven shapes, so that the time required for heating and cooling the mold is shortened. It can be avoided that the time required for molding the antireflection structure 3 (the optical element 2c) becomes long.

  When the exit surface of the optical element 2c manufactured as described above was observed and measured with a scanning electron microscope and an atomic force microscope (AFM), a cylindrical protrusion having a diameter of about 300 nm and a height of about 100 nm ( A plurality of convex portions) were formed with an average pitch of about 410 nm. Further, the reflectance in the entire wavelength range of 400 to 700 nm on the exit surface (antireflection structure 3) of the optical element 2c was a low value of 1% or less as shown in FIG.

  Note that the height of the protrusion (convex portion) corresponds to the film thickness of the antireflection film and is preferably in the range of 60 nm to 200 nm when used in the visible range.

  When a photographing optical system incorporating the optical element 2c is mounted on an imaging device (optical apparatus) such as a digital camera and evaluated, a high-quality image with almost no unnecessary light (noise) such as flare and ghost can be obtained. did it.

  As described above, in this example, the pitch of the concavo-convex portions of the antireflection structure is set to be larger than the diffraction limit within a range where the diffracted light does not reach the image plane. Accordingly, it is possible to realize an optical system including an antireflection structure having a good antireflection performance while avoiding an increase in the time required for forming the antireflection structure.

  In the present embodiment, the case where an antireflection structure is provided on one optical surface among the plurality of optical surfaces included in the photographing optical system has been described. However, as an embodiment of the present invention, a plurality of optical surfaces are provided. Of these, an antireflection structure may be provided on at least one optical surface (that is, two or more optical surfaces).

  In the present embodiment, the case where the optical element 2c is manufactured by an injection molding method using an acrylic resin has been described. However, the manufacturing method of the optical element including the antireflection structure in the embodiment of the present invention is not limited thereto. It is not limited. For example, the optical element may be manufactured by a glass mold method using low-melting glass.

  FIG. 6 shows a digital still camera (imaging device) including the photographing optical system shown in FIG. In FIG. 6, reference numeral 20 denotes a camera body, 21 denotes a photographing optical system, 22 denotes a built-in camera body 20, and a solid-state imaging device (photoelectric conversion) such as a CCD sensor or a CMOS sensor that receives a subject image formed by the photographing optical system 21. Element).

  Reference numeral 23 denotes a memory for recording information corresponding to the subject image photoelectrically converted by the image sensor 22, and reference numeral 24 denotes a finder configured by a liquid crystal display panel or the like for observing the subject image formed on the solid-state image sensor 22. .

  As described above, by applying the imaging optical system shown in FIG. 2 to the imaging apparatus, an imaging apparatus having high optical performance can be realized.

Although the imaging apparatus has been described here, the optical system of the embodiment is not limited to the imaging apparatus, and may be used as an observation optical system for an observation apparatus (optical apparatus) such as binoculars.
[Comparative Example 2]
For comparison with Example 2, a mold having a recess pitch of 170 nm was manufactured so as to satisfy the condition of [Formula 1]. The manufacturing method of the mold is the same as that of the second embodiment. However, in anodization, by setting the voltage when direct current is applied low, fine holes with a pitch of about 170 nm and a diameter of about 125 nm are formed. Got the mold.

  The optical element 2c was molded under the same molding conditions as in Example 2 using the mold thus obtained, and the emission surface was observed. Cylindrical protrusions (convex portions) having a diameter of about 125 nm and a height of about 40 nm were formed at a pitch of about 170 nm. The reflectance in the visible region of the exit surface was as high as 2.1 to 3.2% as shown in FIG.

  When the optical element 2c was arranged in the photographing optical system in the same manner as in Example 2 and evaluated using the photographing optical system in an image pickup apparatus, ghost light was generated and a good image could not be obtained.

  Each embodiment described above is only a representative example, and various modifications and changes can be made to each embodiment in carrying out the present invention.

  By realizing an optical system including an antireflection structure having a good antireflection performance, an optical apparatus capable of obtaining a high-quality image can be provided.

2, 2a, 2c Optical element 3 Antireflection structure 3a, 3b Uneven portion

Claims (10)

Has an optical surface including an antireflection structure unevenness of several are arranged,
P is the arrangement pitch of the concave and convex portions, λ _min is the shortest wavelength in the wavelength range of light incident on the optical surface, n _{1 is} the refractive index of the medium closer to the light incident side than the optical surface, and The refractive index of the medium on the light exit side is n ₂ , the maximum incident angle of the light ray incident on the position x on the optical surface is θ _1max (x), and the light ray that has _exited from the position x on the optical surface and reaches the image surface When the maximum emission angle is θ _2max (x),

An optical system, characterized by satisfying the following condition. An optical surface including an antireflection structure in which a plurality of concave and convex portions are arranged;
P is the arrangement pitch of the concave and convex portions, and λ is the shortest wavelength in the wavelength range of light incident on the optical surface _min , The refractive index of the medium on the light incident side of the optical surface is n ₁ , The refractive index of the medium on the light exit side from the optical surface is n ₂ , The maximum incident angle of the light beam incident on the position x on the optical surface is θ _1max (X) The maximum exit angle of the light beam that exits from the position x on the optical surface and reaches the image plane is θ _2max (X),

Satisfying the conditions
The wavelength λ _min An optical system characterized in that diffracted light is generated when the light beam enters the optical surface at any incident angle.
The optical system according to claim 1, wherein the following condition is satisfied. 800 nm <p
The optical system according to any one of claims 1 to 3, wherein the following condition is satisfied. Optical system according to claim 1, any one of 4, characterized in that it comprises an optical element including an optical surface on which the antireflection structure is formed. The optical system according to claim 5 , wherein the antireflection structure is made of the same material as the optical element. The optical element is molded by using a mold, the mold is according to claim 5 or 6, characterized in that it has a plurality of pores formed aluminum or aluminum alloy by anodic oxidation on the surface Optical system. A deflecting member for deflecting the light from the light source, an optical system according to any one of claims 5 7, characterized in that it comprises said optical element deflected light by the deflecting member passes, the. The optical system according to any one of claims 1 to 8 , wherein a wavelength range of light incident on the optical system is a visible range or an infrared range. An optical apparatus comprising an optical system according to any one of claims 1 to 9.