JP2017146561A - Diffractive optical element, optical system, and image capturing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffractive optical element that is easy to manufacture and capable of preventing image quality degradation due to rainbow flare, and to provide an optical system and an image capturing device.SOLUTION: In order to clear the above problem, an optical system is provided, which includes an optical surface with a fine periodic structure having a certain pitch caused by machining marks, and satisfies a predetermined conditional expression (1), where p (μm) represents the certain pitch, w (μm) represents a wavelength of a light ray of a shortest wavelength in a wavelength range in which the optical system is used, and ω' (°) represents an incident angle of a light ray having a maximum field angle among all the rays passing an optical axis on the optical surface.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a diffractive optical element, an optical system, and an imaging apparatus.

  Conventionally, a so-called cutting method is known as a method of processing an optical surface of an optical element or a mold thereof. The cutting method is a method in which a tool (cutting tool) is used to cut an optical surface of an optical element or a mold thereof to process it into a desired shape. When an optical surface or the like is processed by a cutting method, an unintended fine structure may be formed due to the feed pitch (processing pitch) of the cutting tool. Such a fine structure has the same periodic structure as the diffraction grating according to the feed pitch. Therefore, the light incident on the optical surface is diffracted by the fine structure of the surface and becomes flare. Since the diffraction angle of the diffracted light depends on the wavelength of the incident light, the light incident on the optical surface becomes iridescent diffracted flare (hereinafter referred to as “rainbow flare”) that is dispersed for each wavelength.

  When the rainbow flare is incident on the image plane, the image quality is greatly deteriorated, for example, the color tone becomes unclear and it becomes difficult to obtain a subject image with a clear outline. Therefore, it is required to process the optical surface so that an unintended fine structure does not remain on the optical surface. As such a method, for example, it is conceivable to polish the surface after cutting the optical surface. It is also conceivable to cut the optical surface using a tool having a large radius of curvature (R). According to these methods, the unevenness of the surface formed by the processing traces of the cutting tool can be reduced, and light incident on the surface can be prevented from being unintentionally diffracted.

  However, the above two methods cannot be applied when processing an optical surface having an annular structure such as a diffractive surface of a diffractive optical element or a Fresnel surface of a Fresnel lens. The diffractive surface is generally provided with a blazed grating having a sawtooth cross section (see, for example, Patent Document 1). The diffractive surface takes out diffracted light of a specific order from the light incident by the diffraction grating and uses the light diffractive action to provide wavelength dispersion and asphericity different from the light refracting or reflecting action. The optical surface to be used. In the method of polishing the surface after the above-described cutting process, the shape of the blazed grating changes, and a desired diffraction effect cannot be obtained. In addition, it is difficult to form a blazed grating having a desired shape by a method using a cutting tool having a large curvature. This is because it is necessary to form a blazed grating with sharp edges in order to efficiently extract a diffracted light of a predetermined order, but it is difficult to form such a blazed grating with a tool having a large curvature radius. . The same is true for the Fresnel surface.

  On the other hand, if the bite feed pitch is made as small as possible, the unevenness caused by the processing marks can be reduced, and the amount of rainbow flare can be reduced. However, reducing the tool feed pitch leads to an increase in machining time, leading to a reduction in machining accuracy and an increase in machining cost.

JP-A-4-248501

  Therefore, an object of the present invention is to provide an optical system, an imaging apparatus, and an optical element processing method that can be easily processed at the time of manufacture and can prevent image quality deterioration due to rainbow flare.

In order to solve the above problems, an optical system according to the present invention is an optical system including an optical surface having a fine periodic structure having a predetermined pitch caused by a processing mark, and the predetermined pitch is p (μm). And the wavelength of the shortest ray in the wavelength range of use of the optical system is w (μm),
Of the light rays passing through the optical axis of the optical surface, the following conditional expression (1) is satisfied, where ω ′ (°) is the incident angle of the light ray having the maximum field angle.
1.0> (p / w) × sin ω ′ (1)

  In order to solve the above problems, an imaging apparatus according to the present invention converts the optical system according to the present invention and an optical image formed by the optical system on the image plane side of the optical system into an electrical signal. And an image pickup device.

Moreover, in order to solve the said subject, the processing method of the optical element which concerns on this invention is a processing method which processes an optical element using a cutting tool, Comprising: Let the feed pitch of the said cutting tool be p (micrometer), the said The wavelength of the shortest light beam in the use wavelength range of the optical element is w (μm), and the incident angle of the light beam having the maximum field angle among the light beams passing through the optical axis of the optical surface is ω ′ (°). When the following conditional expression (1) is satisfied, the cutting tool is brought into contact with the optical surface, and the optical surface is cut by the cutting tool.
1.0> (p / w) × sin ω ′ (1)

  According to the present invention, it is possible to provide an optical system and an imaging apparatus that can be easily processed at the time of manufacture and can prevent deterioration in image quality due to rainbow flare.

It is a schematic diagram for demonstrating the rainbow flare resulting from the light ray which passes the optical axis of a diffraction surface. It is a schematic diagram for demonstrating the rainbow flare resulting from the light ray which passes off the optical axis of a diffraction surface. It is a schematic diagram which shows a telephoto type optical system. It is a schematic diagram which shows a retrofocus type optical system. 2 is a lens cross-sectional view illustrating a configuration of an optical system of Example 1. FIG. In the optical system of Example 1, it is the spot diagram of the rainbow flare which originated in the diffraction surface, and the light ray which passed the optical axis of the diffraction surface. 6 is a spot diagram of a rainbow flare generated due to a diffractive surface and a light beam passing through the optical axis of the diffractive surface in the optical system of Example 2. FIG. 6 is a lens cross-sectional view illustrating a configuration of an optical system according to Example 3. FIG. In the optical system of Example 3, it is the spot diagram of the rainbow flare which originated in the diffraction surface, and the light ray which passed the optical axis of the diffraction surface. In the optical system of Example 4, it is the spot diagram of the rainbow flare which originated in the diffraction surface, and the light ray which passed the optical axis of the diffraction surface.

  Hereinafter, embodiments of an optical system and an imaging apparatus according to the present invention will be described in order.

1. Optical system The optical system according to the present invention is an optical system including an optical surface having a fine periodic structure having a predetermined pitch caused by a processing mark. The optical system according to the present invention satisfies conditional expression (1) described later, and preferably satisfies conditional expressions (2) to (4). In the optical system according to the present invention, by satisfying these conditional expressions, the rainbow flare light does not enter the image plane even when the rainbow flare is generated by the fine periodic structure caused by the processing mark. It is possible to prevent the deterioration of the image quality due to. Hereinafter, after describing the configuration of the optical system according to the present invention, conditional expressions that the optical system according to the present invention should satisfy or preferably satisfy will be described.

1-1. Optical surface First, the optical surface will be described. In the present invention, the optical surface refers to a surface having an optical action such as refraction, reflection or diffraction with respect to incident light. The shape of the optical surface is not particularly limited, and may be any shape such as a spherical surface, an aspherical surface, or a flat surface. The optical surface may be a diffractive surface in which a diffraction grating is formed on a surface such as a spherical surface, an aspherical surface, or a flat surface. The optical surface may be a so-called Fresnel surface. The present invention is particularly suitable for an optical surface having an annular structure such as a diffractive surface or a Fresnel surface. This is because, on an optical surface having such an annular structure, the processing trace cannot be removed by a technique such as polishing, and the fine periodic structure resulting from the processing trace tends to remain. In the following, the case where the optical surface is a diffractive surface will be described as an example. However, the matter described below is not limited to the above-described fine period caused by processing marks, regardless of the shape of the optical surface listed above. As long as it has a structure, the same effects are obtained.

  For example, when the optical surface is a blazed diffraction surface, the diffraction surface can be expressed by the following equation.

Φ（ｈ）は、位相差関数であり、
ｈは、同径方向における光軸からの長さであり、
Ｃ１、Ｃ２、Ｃ３、Ｃ４は、任意の係数であり、
ｍは、回折次数であり、
λは、設計波長である。

Φ (h) is a phase difference function,
h is the length from the optical axis in the same radial direction,
C1, C2, C3, and C4 are arbitrary coefficients,
m is the diffraction order,
λ is the design wavelength.

  In the above formula (1), an arbitrary wavelength can be set as the design wavelength. The design wavelength is preferably an arbitrary wavelength within the use wavelength range of the optical system. For example, when the use wavelength range of the optical system is a visible light wavelength range (for example, 380 nm or more and 780 nm or less), a light beam having an arbitrary wavelength within the visible light wavelength range can be set as the design wavelength. Moreover, when the optical system uses wavelengths in the near infrared to infrared region, any wavelength within these wavelength regions can be set as the design wavelength.

1-2. Next, the fine periodic structure will be described. In this invention, the said fine periodic structure means the fine uneven structure formed in the surface resulting from a process trace. For example, when an optical surface of an optical element is processed by a cutting method, a cutting trace of a cutting tool such as a cutting tool may remain on the optical surface. In the cutting method, the optical surface is cut into a predetermined shape while rotating the work surface while bringing the cutting tool into contact with the optical surface and moving the cutting tool little by little from the outer periphery toward the inner periphery. Therefore, cutting marks (processing marks) are periodically formed in a groove shape at a predetermined pitch on the optical surface. In the present invention, this cutting trace, that is, a fine uneven structure resulting from the machining trace is referred to as a fine periodic structure. When the optical surface is the diffractive surface described above, if the diffractive surface is processed by a cutting method, an unintended fine periodic structure caused by such a processing mark apart from the diffraction grating structure defined by the above formula (1) Is formed. And when the said optical surface is a diffraction surface, as above-mentioned, a process trace cannot be removed by methods, such as grind | polishing the surface. This is because when the surface is polished, the shape of the diffraction grating intentionally provided on the surface changes, and a desired diffraction effect cannot be obtained.

  Therefore, in the present invention, even when light incident on the fine periodic structure caused by the processing trace is diffracted and rainbow flare is generated, the following conditional expression (1) is satisfied, so that processing during manufacturing is easy. Therefore, it is possible to prevent deterioration in image quality due to rainbow flare. Hereinafter, conditional expression (1) and conditional expression (2) to conditional expression (4) will be described in order.

1-3. Conditional expression 1-3-1. Conditional expression (1)
First, conditional expression (1) will be described. The optical system according to the present invention preferably satisfies the following conditional expression (1). The said conditional expression (1) is a conditional expression regarding an axial ray. In the case where the optical system includes a plurality of optical surfaces having a fine periodic structure having a predetermined pitch due to processing marks, the following conditional expression (1) may be satisfied with respect to at least one of the optical surfaces. The same applies to other conditional expressions.

  Conditional expression (1): 1.0> (p / w) × sin ω ′

  In the conditional expression (1), p (μm) is the pitch of the fine periodic structure, and as described above, the pitch (predetermined pitch) corresponds to the feed pitch of the cutting tool during processing. w (μm) is the wavelength of the shortest light beam in the wavelength range used by the optical system, and ω ′ (°) is the incident angle of the light beam having the maximum field angle among the light beams passing through the optical axis of the optical surface. It is.

  By satisfying conditional expression (1), even if the axial ray enters the optical surface having the fine periodic structure and the axial ray is diffracted to generate a rainbow flare, the rainbow flare light is generated. Is not incident on the image plane, it is possible to prevent deterioration in image quality due to rainbow flare. This point will be described in detail below with reference to FIG.

FIG. 1 is a diagram schematically showing a relationship between rainbow flare light and an image plane in an optical system including an optical surface having the fine periodic structure resulting from a processing mark. Optics generally configured by combining a plurality of lenses, whatever the optical system, as shown in FIG. 1, it can be represented by simply modeled single lens L ₀ thin. Here, in FIG. 1, an alternate long and short dash line represents the optical axis l, and IMG represents an image plane. The angle formed by the optical axis l and the straight line oy ′ corresponds to the half angle of view ω (°) of the optical system. Further, in FIG. 1, a line represented by a thin solid line indicates an optical path of a light beam incident on the optical system. In FIG. 1, the diffracted light related to the on-axis light beam diffracted by the fine periodic structure is schematically shown by a plurality of arrows.

  The diffraction angle (θ ′) of the diffracted light diffracted by the fine periodic structure can be expressed by the following formula (A).

  Formula (A): sin θ ′ = w ′ / p

  In the above formula (A), w ′ is the wavelength of incident light, and p (μm) is the pitch of the fine periodic structure.

  Here, as is clear from the equation (A), the diffraction angle (θ ′) of the diffracted light depends on the wavelength of the incident light incident on the fine periodic structure, and the diffraction angle (θ ′) depends on the wavelength of the incident light. The shorter, the smaller. As is apparent from FIG. 1, if the diffraction angle (θ ′) is larger than the half angle of view (ω) of the optical system, the diffracted light does not enter the image plane. Accordingly, when the light beam having the shortest wavelength in the use wavelength range of the optical system is w (μm), the diffraction angle (θ ′) of the diffracted light is less than the half field angle (ω) of the optical system. If the rainbow flare occurs, the rainbow flare light does not enter the image plane.

  From the above, it is required that θ ′> ω as a condition that the rainbow flare light does not enter the image plane for the axial ray. Here, since ω <90, the relationship of sin θ ′> sin ω holds. Therefore, based on the formula (A), the following formula (B) is derived as a condition that the rainbow flare light does not enter the image plane.

  Formula (B): 1> (p / w) × sin ω

  Here, in the optical system, if the incident angle of the light beam having the maximum field angle out of the light beams passing through the optical axis of the optical surface matches the half field angle of the optical system, the above formula (B) By satisfying the above, it is possible to prevent the rainbow flare light from entering the image plane. However, depending on the position where the optical surface having the fine periodic structure is arranged in the optical system, the incident angle of the light with the maximum field angle among the light beams passing through the optical axis of the optical surface, and the half of the optical system. The angle of view may be different. Conditional expression (1) is obtained by replacing the half field angle (ω) with the incident angle (ω ′) of the light beam having the maximum field angle among the light beams passing through the optical axis of the optical surface. It is. That is, by satisfying conditional expression (1), it is possible to prevent the rainbow flare light caused by the axial ray from entering the image plane regardless of the arrangement of the optical surface in the optical system.

  Here, the rainbow flare light has a certain spread in the image plane. Therefore, it is more preferable to satisfy the following conditional expression (1-a). By satisfying the conditional expression (1-a), it is possible to prevent the rainbow flare light from entering the image plane even when the rainbow flare light has a certain spread on the image plane with respect to the axial ray. it can.

  Conditional expression (1-a): 0.8> (p / w) × sin ω ′

1-3-2. Conditional expression (2)
Next, conditional expression (2) will be described. The optical system according to the present invention more preferably satisfies the following conditional expression (2). Conditional expression (2) is a conditional expression related to off-axis rays. In the following conditional expression (2), “p”, “w”, and “ω ′” are the same as those in the conditional expression (1), and thus description thereof is omitted here.

  Conditional expression (2): 0.5> (p / w) × sin ω ′

  By satisfying conditional expression (2), not only on-axis rays but also off-axis rays, even if the rainbow flare occurs, the rainbow flare light does not enter the image plane. Can be prevented. This point will be described in detail below with reference to FIG.

Figure 2 is a schematic diagram showing by simply modeled single lens L ₀ thin the optical system as in FIG. 2, dashed line represents the optical axis l, an angle formed between the optical axis l and the off-axis light rays of the principal ray l ₀ is the omega _0.

When off-axis rays are incident on the optical system at an incident angle ω ₀ , the diffraction angle (θ ₀ ′) of the diffracted light by the fine periodic structure and the exit angle (θ) when the incident light is emitted without being diffracted The relationship shown in the following formula (B) is established. In the following formula, m is the diffraction order, and “w” and “p” are the same as above.

Formula (B): sin θ ₀ ′ −sin θ = (m × w) / p

Here, as a condition that the diffracted light of the off-axis light beam does not enter the image plane, the diffraction angle (θ ₀ ′) of the −1st-order diffracted light beam of the off-axis light beam is required to be smaller than “−ω ₀ ”. From this, the following formula (C) is derived.

Formula (C): sin (−ω ₀ ) −sin ω ₀ <−w / p

  Then, when the above equation is modified, the following equation (D) is derived.

Formula (D): 0.5> (p / w) × sin ω ₀

Here, the incident angle (ω ₀ ) of the axial ray incident on the optical system and the ray having the maximum field angle among the rays passing through the optical axis of the optical surface in the optical surface having the fine periodic structure. Is different from the incident angle (ω ′). Therefore, in the above equation (D), the above equation (2) is derived by replacing “ω” with “ω ′”. That is, by satisfying conditional expression (2), it is possible to prevent rainbow flare light from off-axis rays from entering the image plane regardless of the arrangement of the optical surface in the optical system.

  However, in a general imaging optical system, the pupil tends to be small with respect to off-axis rays, and the amount of rainbow flare light itself that causes rainbow flare tends to decrease. Therefore, there is a case where it is not necessary to exclude the rainbow flare light generated by the light incident on the image plane at the maximum angle of view out of the light passing through the optical axis of the optical surface. This is because the amount of light is small and does not affect the image quality. Therefore, if the conditional expression (1), preferably the conditional expression (1-a) described above is satisfied, even if rainbow flare occurs, it is possible to sufficiently suppress deterioration in image quality.

1-3-3. Conditional expression (3)
Next, conditional expression (3) will be described. The optical system according to the present invention preferably satisfies the following conditional expression (3).

  Conditional expression (3): 1.0 ≧ ω ′ / ω

  Here, “ω ′” and “ω” are as described above. The incident angle (ω ′) of the light beam having the maximum field angle among the light beams passing through the optical axis of the optical surface having the fine periodic structure caused by the processing marks is the half field angle (ω) of the optical system. If the relationship of conditional expression (3) is satisfied, even if rainbow flare occurs, rainbow flare light does not enter the image plane by satisfying conditional expression (1) above. It is possible to satisfactorily prevent image quality deterioration. In other words, in an optical system including an optical surface in which a processing mark such as a diffractive surface cannot be removed by a method such as surface polishing, the optical surface is arranged at a position satisfying the conditional expression (3), thereby forming a processing mark. It becomes easy to suppress the diffracted light diffracted by the unintended fine periodic structure resulting from being incident on the image plane, and to suppress deterioration in image quality due to rainbow flare.

1-3-4. Conditional expression (4)
In the optical system according to the present invention, it is more preferable that the following conditional expression (4) is satisfied from the viewpoint that the effect obtained by the conditional expression (3) becomes better.

  Conditional expression (4): 0.8> ω ′ / ω

1-4. Arrangement of optical surface When the optical system is divided into a plurality of groups, the incident angle of the light beam having the maximum angle of view among the half field angle (ω) of the optical system and the light beam passing through the optical axis of the optical surface. (Ω ′) does not necessarily match. In particular, such a tendency becomes remarkable in an optical system in which the refractive power arrangement is asymmetric before and after the stop, such as a telephoto type optical system and a retrofocus type optical system.

  For example, in the rear group of the telephoto type optical system, the incident angle (α) of the light beam having the maximum field angle is larger than the half field angle (ω) of the optical system (see FIG. 3). Therefore, in the telephoto type optical system, that is, in order from the object side, it is composed of a front group having a positive refractive power and a rear group having a negative refractive power, which is larger than the focal length of the entire optical system. In an optical system having a short distance from the final lens surface to the image plane of the optical system, it is preferable that the front group includes the optical surface.

  On the other hand, in the rear group of the retrofocus type optical system, the incident angle (α) of the light beam having the maximum field angle is smaller than the half field angle (ω) of the optical system (see FIG. 4). Therefore, in a retrofocus type optical system, that is, in order from the object side, it is composed of a front group having negative refractive power and a rear group having positive refractive power, which is larger than the focal length of the entire optical system. In an optical system having a long distance from the final lens surface to the image plane of the optical system, the optical surface is preferably included in the rear group.

  As described above, depending on the arrangement of the optical surface in the optical system, the incident angle (ω ′) of the light beam having the maximum field angle among the light beams passing through the optical axis of the optical surface varies. It is preferable that the optical surface is disposed at a position satisfying 3) or conditional expression (4).

2. Next, an embodiment of an optical element processing method according to the present invention will be described. The processing method of the optical element according to the present invention is applicable to an optical element that does not have an annular structure on the optical surface, but in the present embodiment, an annular surface is applied to an optical surface such as a diffractive optical element or a Fresnel lens. An optical element having a structure will be described as an example.

  The processing method of the optical element of this embodiment is a method of processing the optical surface of the optical element by a so-called cutting method. At this time, the feed pitch of the cutting tool is p (μm), the wavelength of the shortest light beam in the use wavelength range of the optical element is w (μm), and among the light beams passing through the optical axis of the optical surface, When the incident angle of the light beam having the maximum angle of view is ω ′ (°), the cutting tool is brought into contact with the optical surface so as to satisfy the following conditional expression (5), and the optical surface is brought into contact with the cutting tool. An annular structure represented by the above-described phase difference function formula or the like is formed on the optical surface while cutting by the above.

  Conditional expression (5): 1.0> (p / w) × sin ω ′

  When an optical surface of an optical element is processed by a cutting method, the optical surface is processed while feeding a cutting tool at a feed pitch that satisfies the above-described conditional expression (1). ) Are periodically formed in a groove shape at a predetermined pitch, as described above, even if the rainbow flare is generated by diffracting the light incident on the fine periodic structure caused by the processing marks, the rainbow flare Can be prevented from entering the image plane. Therefore, when forming an annular structure on the optical surface, it is possible to suppress deterioration in image quality due to rainbow flare or the like caused by the processed marks without using a special method for removing the processed marks after forming the annular structure. be able to.

  However, as described above, the processing method of the optical element is also suitable for an optical element that does not have an annular structure on the optical surface. That is, according to the processing method, for example, even when a spherical lens or the like is processed by a cutting method, the image quality due to rainbow flare caused by the processing mark is removed without removing the processing mark by a method such as polishing after processing the spherical surface. Can be suppressed.

3. Next, an embodiment of an imaging apparatus according to the present invention will be described. The imaging apparatus according to the present embodiment includes the optical system and an imaging element that is provided on the image plane side of the optical system and converts an optical image formed by the optical system into an electrical signal. Can do. Since the imaging apparatus according to the present invention includes the optical system described above, even if rainbow flare occurs when a fine periodic structure resulting from a processing mark is included, the rainbow flare light does not enter the image plane, so image quality by rainbow flare Can be prevented. Therefore, even when using an optical system that easily retains the fine periodic structure due to processing marks, such as an optical system including an optical surface having an annular structure such as a diffractive surface or a Fresnel surface, the deterioration of the image quality is prevented. can do.

  Next, an Example is shown and this invention is demonstrated concretely. However, the present invention is not limited to the following examples.

  As shown in FIG. 5, the optical system according to the first embodiment is a telephoto type optical system including a front group GFt having a positive refractive power and a rear group GRt having a negative refractive power in order from the object side. is there. Specifically, the front group GFt is composed of a single positive lens, and the rear group GRt is composed of a negative meniscus lens having a convex shape on the image plane side in order from the object side, and a positive lens. The object side surface of the positive lens constituting the front group GFt is a diffractive surface. In addition, “IMG” shown in the drawing represents an image plane. Specific lens surface data, diffraction surface data, and specifications are as follows. In the lens surface data, “surface number” is the lens surface number counted from the object side, “r” is the radius of curvature of the lens surface, “d” is the refractive index with respect to the d-line (wavelength λ = 587.6 nm), “Ν” indicates the Abbe number with respect to the d-line. In the diffraction surface data, “m” represents a diffraction order, and “C2,” “C3,” and “C4” represent diffraction coefficients when the diffraction surface is represented by the above phase difference function. Since these matters are the same in the other embodiments, the description thereof will be omitted below.

(1) Lens surface data surface number rdn ν Remarks
1 48.60 12.00 1.51633 64.1 Aperture / Diffraction surface
2 162.00 70.00
3 -29.50 3.00 1.62041 60.3
4 -106.20 0.50
5 -150.00 10.00 1.62004 36.3
6 -60.50 64.75

(2) Diffraction surface data surface number m C2 C3 C4 Normalized radius
1 1 -2.62E + 00 1.30E-03 2.40E-07 1.000

(3) Original focal length 199.8mm
F number 5.6
Half angle of view ω 6.12 °

  In the optical system of Example 1, the first surface was a diffractive surface, and the feed pitch (p) of the cutting tool when processing the diffractive surface was 3.00 μm. In addition, the wavelength (w) of the shortest light beam in the use wavelength range of the optical system and the incident angle (ω ′) of the light beam having the maximum field angle among the light beams passing through the optical axis of the first surface, conditions The value of Formula (1) is as follows, respectively.

p 3.00μm
w 0.436μm
ω '6.12 °

Conditional expression (1): (p / w) × sin ω ′ = 0.73

  Further, FIG. 6 shows the rainbow flare light of the axial ray generated due to the diffractive surface in the optical system of Example 1, the axial ray, and the light ray of 70% and 10% of the maximum field angle. Shows a spot diagram.

  In the optical system of the first embodiment, since the stop is disposed on the first surface having the diffractive surface, the incident angle (ω ′) of the light beam having the maximum field angle among the light beams passing through the center of the diffractive surface, that is, the optical axis. ) Is equal to the half angle of view (ω) of the optical system. Therefore, as shown in FIG. 4, the rainbow flare light by the axial ray is distributed outside the maximum field angle (100%), and it can be seen that the rainbow flare light does not enter the image plane. Therefore, by setting the feed pitch of the cutting tool when processing the diffractive surface to satisfy the above conditional expression (1), even if rainbow flare occurs, the image quality deteriorates due to the rainbow flare light. It can be suppressed. For off-axis rays, some of the rainbow flare light may be incident on the screen, but the feed pitch of the cutting tool can be set to a relatively large value, thereby shortening the machining time and improving the machining accuracy. be able to.

  Next, the optical system of Example 2 will be described. The optical system of Example 2 is the same as the optical system of Example 1 except that the feed pitch of the cutting tool when machining the first surface is 1.60 μm. Accordingly, the description of the configuration of the optical system, lens surface data, diffraction surface data, and specifications of Example 2 is omitted here, and only the values of p, w, ω ′ and the numerical value of conditional expression (1) are shown below. .

p 1.60μm
w 0.436μm
ω '6.12 °

Conditional expression (1): (p / w) × sin ω ′ = 0.39

  In addition, in the optical system of Example 2 shown in FIG. 7, the rainbow flare light and the axial light beam due to the light beam with the maximum field angle generated due to the diffraction surface, and the light beam with the field angle of 70% and 10% of the maximum field angle. Shows a spot diagram.

  In the case of the optical system according to the second embodiment, all rainbow flare light including rainbow flare light other than light beams having a maximum field angle (not shown) is distributed outside the spot position having the maximum field angle. Compared with Example 1, although the feed pitch of the cutting tool when processing the first surface is small, since no rainbow flare light is incident on the image plane, a clear, clear and high-contrast image can be obtained.

  As shown in FIG. 8, the optical system of Example 3 is a retrofocus type optical system including a front group GFr having a negative refractive power and a rear group GRr having a positive refractive power in order from the object side. is there. Specifically, the front group GFr includes, in order from the object side, a negative meniscus lens having a convex shape on the object side and a positive meniscus lens having a convex shape on the object side. The rear group GRr includes a positive lens convex on the image side. The object side surface of the positive lens constituting the rear group GRr is a diffractive surface and an aspherical surface. Further, “S” shown in the drawing represents an aperture stop, and “IMG” represents an image plane. Specific lens surface data, diffraction surface data, aspheric surface data, and specifications are as follows. However, the aspheric surface data is an aspheric coefficient and a conic constant when the aspheric surface is expressed by the following equation.

Here, the aspheric surface is defined by the following equation.
z = ch ² / [1+ {1- (1 + k) c ² h ² } ^1/2 ] + A4h ⁴ + A6h ⁶ + A8h ⁸ + A10h ¹⁰ ...
(Where c is the curvature (1 / r), h is the height from the optical axis, k is the conic coefficient, A4, A6, A8, A10... Are aspheric coefficients of each order)

(1) Lens surface data surface number rdn ν Remarks
1 73.30 3.00 1.51633 64.1
2 14.60 10.00
3 28.20 8.00 1.80518 25.4
4 52.00 7.40
5-1.00 aperture
6 -100.00 10.00 1.62041 60.3 Diffractive surface / Aspherical surface
7 -20.30 50.0

(2) Diffraction surface data
Surface number m C2 C3 C4 Normalized radius
6 1 -1.63E + 01 6.00E-02 0.00E + 00 1.000

(3) Aspheric data surface number A4 A6
6 -1.70E-05 2.50E-08

(4) Original focal length 35.5mm
F number 5.0
Half angle of view ω 34.20 °

  In the optical system of Example 1, the thickness was 0.91 μm. In addition, the wavelength (w) of the shortest light beam in the use wavelength range of the optical system and the incident angle (ω ′) of the light beam having the maximum field angle among the light beams passing through the optical axis of the first surface, conditions The value of Formula (1) is as follows, respectively.

p 0.91μm
w 0.436μm
ω '25.88 °

Conditional expression (1): (p / w) × sin ω ′ = 0.91

  Further, in FIG. 9, in the optical system of Example 3, the axial rainbow flare light and axial light generated due to the diffractive surface, and the light beams having the angle of view of 70% and 10% of the maximum field angle. A spot diagram is shown.

  The optical system according to the third embodiment includes the front group having negative refractive power on the object side with respect to the stop, so that the incident light beam having the maximum field angle among the light beams passing through the optical axis of the diffractive surface included in the rear group. The angle (ω ′) is smaller than the half field angle (ω) of the optical system. Further, as shown in FIG. 9, the rainbow flare light by the axial ray is distributed outside the maximum field angle (100%), and it can be seen that the rainbow flare light does not enter the image plane. Therefore, by making the feed pitch of the cutting tool when processing the diffractive surface satisfy the above-mentioned conditional expression (1), even if rainbow flare occurs, it is possible to suppress deterioration in image quality due to the rainbow flare light. Is possible. For off-axis rays, some of the rainbow flare light may be incident on the screen, but the feed pitch of the cutting tool can be set to a relatively large value, thereby shortening the machining time and improving the machining accuracy. be able to.

  Next, an optical system according to Example 4 will be described. The optical system of Example 4 is the same as the optical system of Example 3 except that the feed pitch of the cutting tool when processing the sixth diffractive surface is 0.48 μm. Therefore, description of the configuration of the optical system, lens surface data, diffractive surface data, aspherical data, and specifications of Example 4 is omitted here, and the values of p, w, ω ′ and conditional expression (1) are as follows. Only numerical values are shown.

p 0.48μm
w 0.48μm
ω '25.88 °

Conditional expression (1): (p / w) × sin ω ′ = 0.48

    In addition, in the optical system of Example 4 shown in FIG. 10, the rainbow flare and the axial ray due to the light beam with the maximum field angle generated due to the diffraction surface, and the light beam with the angle of view of 70% and 10% of the maximum field angle. A spot diagram is shown.

  In the case of the optical system according to the fourth embodiment, all rainbow flare light is distributed outside the spot position having the maximum angle of view, including rainbow flare light other than light rays having a maximum angle of view (not shown). That is, since no rainbow flare is incident on the image plane, a clear and clear image with a high contour can be obtained.

  According to the present invention, it is possible to provide an optical system and an imaging apparatus that can be easily processed at the time of manufacture and can prevent deterioration in image quality due to rainbow flare.

GFt front group (telephoto type)
GRt rear group (telephoto type)
GFr front group (retro focus type)
GRr rear group (retro focus type)
S Aperture stop IMG Image surface l Optical axis

Claims (10)

An optical system including an optical surface having a fine periodic structure having a predetermined pitch caused by processing marks,
The predetermined pitch is p (μm),
W (μm) is the wavelength of the shortest wavelength in the wavelength range of use of the optical system,
Of the light rays passing through the optical axis of the optical surface, the incident angle of the light ray with the maximum field angle is ω ′ (°),
An optical system characterized by satisfying the following conditional expression (1).
1.0> (p / w) × sin ω ′ (1) The optical system according to claim 1, wherein the following conditional expression (2) is satisfied.
0.5> (p / w) × sin ω ′ (2) When the half angle of view of the optical system is ω (°),
The optical system according to claim 1 or 2, wherein the following conditional expression (3) is satisfied.
1.0 ≧ ω ′ / ω (3) The optical system according to claim 3, wherein the following conditional expression (4) is satisfied.
0.8> ω ′ / ω (4)   5. The optical system according to claim 1, wherein the optical surface includes an annular structure formed at a pitch different from a pitch of the fine periodic structure. In order from the object side, it is composed of a front group having negative refractive power and a rear group having positive refractive power,
The distance from the final lens surface of the optical system to the image plane is longer than the focal length of the entire optical system,
The optical system according to claim 5, wherein the rear group includes the optical surface. In order from the object side, it is composed of a front group having a positive refractive power and a rear group having a negative refractive power,
The optical total length of the optical system is shorter than the focal length of the entire optical system,
The optical system according to claim 5, wherein the front group includes the optical surface.   An optical system according to any one of claims 1 to 7, and an image pickup device that converts an optical image formed by the optical system into an electrical signal on an image plane side of the optical system. An imaging apparatus characterized by the above. An optical element processing method for processing an optical element using a cutting tool,
The feed pitch of the cutting tool is p (μm),
W (μm) is the wavelength of the shortest wavelength in the wavelength range of use of the optical element,
Of the light rays passing through the optical axis of the optical surface, the incident angle of the light ray with the maximum field angle is ω ′ (°),
A processing method of an optical element, wherein the cutting tool is brought into contact with the optical surface so as to satisfy the following conditional expression (5), and the optical surface is cut by the cutting tool.
1.0> (p / w) × sin ω ′ (5)   The processing method of the optical element according to claim 9, wherein an annular structure is formed on the optical surface while cutting the optical surface of the optical element with the cutting tool.

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