JP2008216470A - Objective lens for imaging, imaging module, and method of designing objective lens for imaging - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce not only axial chromatic aberration but also chromatic difference of magnification to reduce chromatic aberration appearing on a screen as a whole, with respect to an objective lens for imaging having a diffraction grating. <P>SOLUTION: The objective lens for imaging which is provided with the diffraction grating on at least one face, and the diffraction grating satisfies ϕ(r)=M×(a1r<SP>2</SP>+a2r<SP>4</SP>+a3r<SP>6</SP>+a4r<SP>8</SP>+a5r<SP>10</SP>+a6r<SP>12</SP>+a7r<SP>14</SP>+a8r<SP>16</SP>+a9r<SP>18</SP>+a10r<SP>20</SP>) wherein: ϕ is a phase function; r is a length in a diametral direction of the objective lens; a1 to a10 are phase function coefficients; and -320≤a1≤-240 is true. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an imaging objective lens having a diffraction grating, an imaging module including the objective lens, and a method for designing the objective lens.

  A refractive optical system that refracts a light beam by a difference in refractive index is easy to handle and process, and thus has been widely used in the past. As an optical element other than the refractive optical system, a diffractive optical element (diffractive optical element: hereinafter, sometimes simply referred to as DOE) using a light diffraction phenomenon is also known. However, due to processing problems, diffractive optical elements have not been widely used.

  However, in recent years, techniques such as lithography techniques and ultra-precise processing have been remarkably advanced. If these techniques are utilized, a diffraction grating can be manufactured relatively easily.

As described above, the diffractive optical element exhibits a lens function based on the diffraction action of light. In general, when light enters the diffractive optical element, the light emitted by the diffraction action satisfies the following formula (1).
sinθ-sinθ '= mλ / d (1)
Where θ is the incident angle, θ ′ is the exit angle, λ is the wavelength of light, d is the grating spacing (grating pitch) of the diffraction grating, and m is the diffraction order.

Accordingly, by providing a plurality of gratings in a ring shape on one surface of the lens and appropriately setting the grating interval (grating pitch), the light incident on the diffractive optical element can be concentrated at one point. In other words, the diffractive optical element can have a lens action. Specifically, the radius of the jth grating is rj, the focal length of the diffraction grating is f, and the optical path difference between the central ray and the ray diffracted by the jth grating is an integral multiple of the wavelength of the incident light. Set as follows. As a result, the diffracted light and the undiffracted light strengthen each other. That is, the following expression (2) is satisfied.
√ (rj ² + f ² ) −f = jλ (2)

Further, when rj is increased with respect to the focal length, the ring radius rj of the grating is expressed by Expression (3).
rj = √ (2jλf) (3)

  By the way, the diffractive optical element also has different characteristics from the refractive optical element. The Abbe number of the diffractive optical element has a reverse dispersion characteristic of −3.45 that does not exist in a general glass glass material. That is, the Abbe number of a general glass glass material is νd = 64.1, whereas the Abbe number of a diffractive optical element is νd = −3.45. When a diffractive optical element is added to a normal single lens by utilizing the dispersion characteristics of the diffractive optical element, a hybrid lens (so-called achromatic lens) in which a diffractive action is added to a refractive action can be designed. This lens can also be applied to imaging (an optical system used in white, such as a photographing optical system or a visual observation optical system).

The above-described lens can be designed by distributing the power of the refractive lens and the diffractive optical element so as to satisfy the following expressions (4) and (5).
1 / f = 1 / fs + 1 / fdoe (4)
1 / fsvs + 1 / fdoeνdoe = 0 (5)
Here, fs is the focal length of the refractive lens, fdoe is the focal length of the diffractive optical element, f is the overall focal length, νs is the dispersion of the refractive lens, and νdoe is the dispersion of the diffractive optical element. By setting so as to satisfy the expressions (4) and (5), the axial chromatic aberration can be optimized (see Non-Patent Document 1).
Introduction to diffractive optical elements (supervised by: Japan Society of Applied Physics, Optical Design Research Group, Optical Society of Japan)

  However, when this lens is used as an image acquisition lens, it is necessary to correct lateral chromatic aberration as well as axial chromatic aberration. Even if only the axial chromatic aberration is optimized, the chromatic aberration of magnification that occurs depending on the screen height is deteriorated, so that the chromatic aberration of the entire screen is deteriorated. The axial chromatic aberration is an aberration that depends on the difference in focal length for each wavelength. The lateral chromatic aberration is an aberration that occurs depending on the height of the imaging screen.

  The present invention has been made to solve such problems, and relates to an imaging objective lens having a diffraction grating, which improves not only axial chromatic aberration but also lateral chromatic aberration, and improves chromatic aberration appearing on the screen as a whole. The purpose is to improve.

An imaging objective lens according to the present invention is an imaging objective lens provided with a diffraction grating on at least one surface, wherein the diffraction grating is
φ (r) = M × (a1r ² + a2r ⁴ + a3r ⁶ + a4r ⁸ + a5r ¹⁰ + a6r ¹² + a7r ¹⁴ + a8r ¹⁶ + a9r ¹⁸ + a10r ²⁰ )
Where φ is a phase function, r is a radial distance of the objective lens, a1 to a10 are phase function coefficients, and the phase difference function coefficient a1 is
−320 ≦ a1 ≦ −240
Satisfying, designed based on. Since not only axial chromatic aberration but also lateral chromatic aberration is improved, chromatic aberration appearing on the screen can be improved as a whole.

  The diffraction grating may include a plurality of concentric grooves. The plurality of grooves may be wider toward the center of the objective lens. The one surface provided with the diffraction grating is preferably an aspherical surface.

An imaging module according to the present invention is an imaging module including an objective lens that forms incident light on a two-dimensional imaging surface, and the objective lens has a diffraction grating on at least one surface,
φ (r) = M × (a1r ² + a2r ⁴ + a3r ⁶ + a4r ⁸ + a5r ¹⁰ + a6r ¹² + a7r ¹⁴ + a8r ¹⁶ + a9r ¹⁸ + a10r ²⁰ )
Where φ is a phase function, r is a radial distance of the objective lens, a1 to a10 are phase function coefficients, and the phase difference function coefficient a1 is
−320 ≦ a1 ≦ −240
Satisfying, designed based on.

An imaging objective lens design method according to the present invention is a method of designing an imaging objective lens having a diffraction grating on one surface, wherein the diffraction grating is
φ (r) = M × (a1r ² + a2r ⁴ + a3r ⁶ + a4r ⁸ + a5r ¹⁰ + a6r ¹² + a7r ¹⁴ + a8r ¹⁶ + a9r ¹⁸ + a10r ²⁰ )
Where φ is a phase function, r is a radial distance of the objective lens, a1 to a10 are phase function coefficients, and the phase difference function coefficient a1 is
−320 ≦ a1 ≦ −240
Satisfy, based on designing.

  According to the present invention, not only axial chromatic aberration but also lateral chromatic aberration can be improved, so that chromatic aberration appearing on the screen can be improved as a whole.

[First Embodiment]
Hereinafter, a first embodiment of the present invention will be described. FIG. 1 is a schematic diagram for explaining the configuration of the imaging module M1. FIG. 2 is an explanatory diagram for explaining the configuration of the imaging surface. FIG. 3 is an explanatory diagram for explaining the configuration of the objective lens.

  As illustrated in FIG. 1, the imaging module M <b> 1 includes a diaphragm 1, an objective lens 2, and an imaging device 3.

  The diaphragm 1 is a plate-like member having an opening on the imaging region of the imaging device 3. The diaphragm 1 adjusts the amount of light incident on the imaging device 3.

  The objective lens 2 focuses the light beam that has passed through the diaphragm 1 on the imaging surface of the imaging device 3. In the present embodiment, as will be described later, the objective lens 2 exhibits a diffractive action in addition to a refracting action. The chromatic aberration of the light imaged on the imaging surface of the imaging device 3 is generally improved by the diffractive action of the objective lens 2. The objective lens 2 is manufactured by molding a resin (molding) or the like.

  The imaging device 3 is a device in which a semiconductor imaging device such as a CCD (Charge Coupled Device) sensor or a CMOS (Complementary Metal Oxide Semiconductor) sensor is packaged. The imaging device 3 has a cover glass 3a and an imaging surface 3b. The imaging device 3 photoelectrically converts light imaged by the objective lens 2 and outputs an electrical signal.

  FIG. 2 is an explanatory view of the imaging surface 3b of the imaging device 3 as viewed from above. As shown in FIG. 2, an imaging region R <b> 1 is provided on the imaging surface 3 b of the imaging device 3. A plurality of pixels Px are two-dimensionally arranged in the imaging region R1. Note that the image information acquired by the imaging device 3 is image information with color. Accordingly, a band pass filter or the like that passes only a specific wavelength band is disposed on the pixel Px. Note that the distance between the center of the pixel Px1 and the center of the pixel Px2 is W1. The distance W1 corresponds to the arrangement interval (pixel pitch) of the plurality of pixels Px.

  As schematically shown in FIG. 1, the light beam passing through the aperture of the diaphragm 1 is imaged on the imaging surface 3 b of the imaging device 3 by the objective lens 2. In each pixel Px on the imaging surface 3b, a charge corresponding to the amount of incident light is generated. Then, an electrical signal is output from the imaging device 3, and color image information (color image) is configured by a subsequent processing mechanism.

  Here, FIG. 3 shows an explanatory diagram for explaining the configuration of the objective lens 2. FIG. 3A shows a schematic sectional view of the objective lens 2. FIG. 3B shows a shape of the diffraction grating provided on the light exit surface 2 b of the objective lens 2 in a partial cross-sectional view.

  As shown in FIG. 3A, the objective lens 2 is a single lens having a light incident surface (first surface) 2a and a light emitting surface (second surface) 2b. The light incident surface 2a of the objective lens 2 is a rotationally symmetric aspheric surface and has a shape that is recessed toward the center of the objective lens 2 as a whole. The light exit surface 2b of the objective lens 2 is a rotationally symmetric aspherical surface and has a shape protruding toward the center of the objective lens 2 as a whole. The objective lens 2 exerts a refracting action by the light incident surface 2a and the light emitting surface 2b, and forms an incident light beam on the imaging surface 3b.

Here, regarding the aspherical surface provided in the objective lens 2, the sag amount X (h) is shown in Expression (6). The sag amount X (h) is the length from point P to point Q, as schematically shown in FIG. Note that the point P is a coordinate point (for example, coordinate point P) where the perpendicular to the optical axis AX and the aspherical surface intersect. Point Q is a coordinate point where the optical axis AX and the aspherical surface intersect.
X (h) = Ch ² / (1 + √ (1− (1 + k) C ² h ² )) + A ₄ h ⁴ + A ₆ h ⁶ + A ₈ h ⁸ + A 1 ₀ h ¹⁰ + A 1 ₂ h ¹² (6)
C is the curvature on the optical axis of the aspherical surface (curvature near the point Q) (1 / r), k is the conic coefficient, A ₄ , A ₆ , A ₈ , A 1 ₀ , A ₁₂ are 4th order, 6 Next, 8th, 10th and 12th order aspherical coefficients.

  In the objective lens 2 in the present embodiment, a diffraction grating is provided on the light exit surface 2b. In other words, the light exit surface 2b functions as a diffractive surface exhibiting a diffractive action (hereinafter, the light exit surface 2b may be referred to as a diffractive surface 2b). As described above, the light emitting surface 2b also exhibits a bending action.

  FIG. 3B shows an explanatory diagram in which the diffraction surface 2b is partially sectioned. As shown in FIG. 3B, a plurality of grooves 5 are continuously formed on the diffractive surface 2b. The grooves 5 are arranged concentrically on the diffraction surface 2b with the optical axis AX shown in FIG. A plurality of annular zones 6 centering on the optical axis AX are continuously formed on the diffractive surface 2b by the adjacent grooves 5.

  The groove 5 is defined by a side surface 4a extending substantially parallel to the optical axis AX and a side surface 4b extending in a direction intersecting the optical axis AX. The annular zone 6 is also defined from the side surface 4a and the side surface 4b. In addition, the space | interval W2 in which the ring zone 6 is provided becomes large as it approaches the optical axis AX, and becomes narrow as it distances from the optical axis AX. Thereby, the objective lens 2 can be provided with a light condensing property.

  By providing the plurality of annular zones 6, the objective lens 2 has a function of correcting chromatic aberration. The plurality of grooves described above are formed by using a surface processing technique (a normal semiconductor process technique such as photolithography).

  In the present embodiment, a diffraction grating (a structure constituted by a plurality of annular zones 6 (grooves 5)) provided on the diffraction surface 2b of the objective lens 2 is designed using a phase difference function method.

In the phase difference function method, first, it is assumed that there is a diffraction grating on the light exit surface 2 b of the objective lens 2. Then, the wavefront phase conversion (addition of the optical path difference) represented by the equation (7) is performed on the light exit surface 2b assumed to have a diffraction grating, and the diffraction grating is set on the light exit surface 2b of the objective lens 2. .
φ (r) = M × (a1r ² + a2r ⁴ + a3r ⁶ + a4r ⁸ + a5r ¹⁰ + a6r ¹² + a7r ¹⁴ + a8r ¹⁶ + a9r ¹⁸ + a10r ²⁰ ) (7)
Here, φ is a phase function, r is a radial distance of the objective lens 2, and a1 to a10 are phase function coefficients.

  In the present embodiment, a1 satisfies the condition −320 ≦ a1 ≦ −240. As a result, not only axial chromatic aberration but also lateral chromatic aberration can be favorably corrected, and chromatic aberration of the entire acquired screen can be corrected appropriately. Note that a1 is a parameter for setting the focal length for each wavelength.

The reason why a1 is set so as to satisfy the above-described conditions will be described below using Examples 1 to 15. In addition, the value of a1 in Examples 1 to 15 is as described in Table 1.

Table 2 shows conditions other than a1 in Example 4. In other examples, only the value of a1 is different from that in Example 4.

  In Table 2, f is the focal length of the objective lens 2, Fno. Is the F number, λ is the design wavelength (usually 546 nm), r is the radius of curvature of the diffraction surface (unit: mm), and k is The conic coefficient, A4 is the fourth-order aspheric coefficient, d is the distance on the optical axis between the surfaces (thickness of the objective lens along the optical axis AX) (unit: mm), n is the refractive index at the design wavelength λ Corresponds to.

  In Table 2, surface number 1 corresponds to the light incident surface 2a of the objective lens 2, surface number 2 corresponds to the diffractive surface 2B of the objective lens 2, and surface number 3 corresponds to the surface of the cover glass 3a on the objective lens 2 side. The surface number 4 corresponds to the surface on the imaging surface 3b side of the cover glass 3A, and the surface number 5 corresponds to the imaging surface 3b.

  Chromatic aberration characteristics were evaluated for Examples 1 to 15 on the assumption of the conditions shown in Tables 1 and 2.

Table 3 shows the relative axial chromatic aberration values of Examples 1 to 15 and the MTF value at the center of the screen. The relative chromatic aberration is an aberration at the time of focusing of the objective lens, and is obtained from the relative chromatic aberration = the generated axial chromatic aberration ÷ the focal length. The MTF value at the center of the screen is the MTF value at the center of the imaging surface. Specifically, the MTF value at the center of the screen is acquired using pixels near the line CL shown in FIG.

  FIG. 4 is a graph showing the results of Table 3. In FIG. 4, the horizontal axis is a1, and the vertical axis is the MTF (%) at the center of the screen. When the value of a1 is in the range of −360 to −140 (in the case of Example 2 to Example 14), an MTF (Modulation Transfer Function) of 50% or more is obtained. Therefore, in this range, it can be said that the resolution is good.

  In addition, when a1 is in the range of −360 to −140, the case where the longitudinal chromatic aberration is minimized (example 14) is included. This is considered that the improvement of the MTF at the center of the screen is due to the correction of the longitudinal chromatic aberration by adding the phase difference.

  Table 4 shows the relative lateral chromatic aberration and the MTF value of 70% image height in Examples 1 to 15. For the MTF value with an image height of 70%, the MTF value is shown for each of the T direction (tangential direction) and the S direction (sagittal direction).

The relative magnification aberration is an aberration at the time of focusing of the objective lens, and is obtained from the relative chromatic aberration of magnification = generated chromatic aberration of magnification ÷ pixel arrangement interval (pixel pitch). Further, as schematically shown in FIG. 2, the MTF value at the image height of 70% is the MTF value near the line L70 at the position of 70% of the length from the line CL to the line L100, and near the line L70. Are obtained using the pixels.

  FIG. 5 shows the results of Table 4. In FIG. 5, the horizontal axis is a1, and the vertical axis is the relative magnification aberration.

  As shown in FIG. 5, in Examples 4 to 8 where a1 is in the range of −320 to −240, the occurrence of lateral chromatic aberration with respect to the pixel pitch is 50% or less.

  FIG. 6 also shows the results of Table 3. In FIG. 6, the horizontal axis is a1, and the vertical axis is MTF (%) at an image height of 70%. In Examples 4 to 8 in which a1 is in the range, an improvement is seen in the T direction of the MTF.

  In the case of the imaging module M1 as shown in FIG. 1, it is necessary to correct lateral chromatic aberration in addition to axial chromatic aberration. In the case where the diffraction grating is provided by focusing only on the axial chromatic aberration only in the optimum range, the chromatic aberration of the entire screen is deteriorated as the chromatic aberration of magnification is deteriorated. In the present embodiment, the diffraction grating is set in consideration of lateral chromatic aberration in addition to axial chromatic aberration. More specifically, the value of a1 is set to -320 to -240. As a result, the chromatic aberration of the entire screen can be set within a favorable range.

  Note that a1 is a parameter used when designing the diffraction grating, but a1 can also be obtained from the objective lens 2 itself having the diffraction grating. That is, by utilizing the following formulas (8) to (10), the value of a1 can be obtained in reverse from the designed objective lens. More specifically, the focal length of the objective lens is measured using the nodal slide method, the radius of curvature of the front and back surfaces of the objective lens is measured using Talysurf, and the phase difference function coefficient a1 is calculated. it can.

The focal length f of the objective lens is obtained from the following equation (8).
1 / f = 1 / fs + 1 / fdoe−Δ / (fs × fdoe) (8)
Here, fs is the focal length of the objective lens 2, fdoe is the focal length of the diffraction grating, and Δ is the distance between the principal point of the objective lens 2 and the principal point of the diffraction grating. Note that fs in equation (8) can be calculated by equation (9).
1 / fs = (n-1 ) (1 / r1-1 / r2) + (n-1) 2 d / (n × r1 × r2) ··· (9)
Also, fdoe in equation (8) can be calculated by equation (10).
1 / fdoe = − {2 × (a1 × λ / 2π) × λ} (10)
Further, Δ in the equation (8) can be calculated by (11).
Δ = r2 × d / {n × (r2−r1) + (n−1) × d} (11)
Here, n is the refractive index, r1 is the radius of curvature of the surface of the objective lens 2 (light incident surface 2a), r2 is the radius of curvature of the back surface of the objective lens 2 (diffractive surface 2b), and d is the optical axis of the objective lens 2. AX
, A1 is a phase difference function coefficient, and λ is a design wavelength.

  The focal length fdoe and the design wavelength λ can be obtained from the objective lens as a result. In addition, a1 can be obtained by using Expression (10). That is, by using Expression (10), the value of a1 can also be obtained in reverse from the resultant objective lens.

Finally, Table 5 shows specific examples of the minimum zone width and the number of steps of the diffraction surfaces of Examples 1 to 15. The minimum zone width is the smallest of the widths W2 of the zone 6. The width W2 of the annular zone 6 is preferably 20 μm or more in consideration of molding processing. Further, in order to suppress the occurrence of flare due to a shape error, the number of annular zones 6 (the number of grooves 5) is preferably 10 or less. Examples 4 to 8 all satisfy these conditions.

  The technical scope of the present invention is not limited to the above-described embodiment. The application of the imaging module is exemplified by a camera part of a mobile communication terminal (such as a mobile phone) and a camera part of an information processing apparatus (personal computer), but may be used for a camera of another device.

It is the schematic for demonstrating the structure of the imaging module M1 concerning 1st Embodiment. It is explanatory drawing for demonstrating the structure of an imaging surface. It is explanatory drawing for demonstrating the structure of an objective lens. It is a graph showing the relationship between a1 and the resolution (MTF) of a center part. It is a graph showing the relationship between a1 and a magnification chromatic aberration. It is a graph showing the relationship between a1 and the resolution (MTF) of a peripheral part.

Explanation of symbols

1 Aperture 2 Objective lens 3 Imaging device

Claims (6)

An objective lens for imaging provided with a diffraction grating on at least one surface,
The diffraction grating is
φ (r) = M × (a1r ² + a2r ⁴ + a3r ⁶ + a4r ⁸ + a5r ¹⁰ + a6r ¹² + a7r ¹⁴ + a8r ¹⁶ + a9r ¹⁸ + a10r ²⁰ )
Where φ is a phase function, r is a radial distance of the objective lens, a1 to a10 are phase function coefficients, and the phase difference function coefficient a1 is
−320 ≦ a1 ≦ −240
Satisfy,
Objective lens for imaging designed based on   The imaging objective lens according to claim 1, wherein the diffraction grating includes a plurality of concentric grooves.   The imaging objective lens according to claim 2, wherein the plurality of grooves become wider toward the center of the objective lens.   The imaging objective lens according to claim 1, wherein the one surface on which the diffraction grating is provided is an aspherical surface. An imaging module including an objective lens that forms an image of incident light on a two-dimensional imaging surface,
The objective lens has a diffraction grating on at least one surface,
The diffraction grating is
φ (r) = M × (a1r ² + a2r ⁴ + a3r ⁶ + a4r ⁸ + a5r ¹⁰ + a6r ¹² + a7r ¹⁴ + a8r ¹⁶ + a9r ¹⁸ + a10r ²⁰ )
Where φ is a phase function, r is a radial distance of the objective lens, a1 to a10 are phase function coefficients, and the phase difference function coefficient a1 is
−320 ≦ a1 ≦ −240
Satisfying, designed based on imaging module. A method of designing an imaging objective lens having a diffraction grating on one surface,
The diffraction grating,
φ (r) = M × (a1r ² + a2r ⁴ + a3r ⁶ + a4r ⁸ + a5r ¹⁰ + a6r ¹² + a7r ¹⁴ + a8r ¹⁶ + a9r ¹⁸ + a10r ²⁰ )
Where φ is a phase function, r is a radial distance of the objective lens, a1 to a10 are phase function coefficients, and the phase difference function coefficient a1 is
−320 ≦ a1 ≦ −240
The objective lens for imaging is designed based on satisfying the above.

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