WO2011142101A1 - 回折レンズ - Google Patents
回折レンズ Download PDFInfo
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- WO2011142101A1 WO2011142101A1 PCT/JP2011/002508 JP2011002508W WO2011142101A1 WO 2011142101 A1 WO2011142101 A1 WO 2011142101A1 JP 2011002508 W JP2011002508 W JP 2011002508W WO 2011142101 A1 WO2011142101 A1 WO 2011142101A1
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- light
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- diffraction
- diffractive lens
- diffraction grating
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/18—Diffraction gratings
- G02B5/1876—Diffractive Fresnel lenses; Zone plates; Kinoforms
Definitions
- the present invention relates to a diffractive lens that collects light using diffraction and an imaging apparatus using the diffractive lens.
- optical elements There are two types of optical elements that have the function of condensing light, one that uses refraction and the other that uses diffraction. It is also possible to combine optical elements having these functions.
- an optical element in which an optical element using refraction and an optical element using diffraction are combined is referred to as a diffractive lens.
- the diffractive lens is formed by providing a diffraction grating (grating) on the lens refracting surface, and the design parameters for adjusting the optical characteristics can be greatly increased. For this reason, there is an advantage that the number of lenses can be reduced while maintaining the optical performance.
- the diffractive lens is excellent in correcting aberrations of the lens such as curvature of field and chromatic aberration (displacement of image forming point due to wavelength). This is because the diffraction grating has a dispersibility (reverse dispersibility) opposite to the dispersibility caused by the optical material.
- the shape of the diffractive lens is configured by combining the base shape of the lens base on which the diffraction grating is provided, that is, the shape as a refractive lens, and the shape of the diffraction grating.
- FIG. 13A shows the aspherical shape Sb of the lens base
- FIG. 13B shows the shape Sp1 of the diffraction grating.
- the shape Sp1 of the diffraction grating is determined by a phase function represented by the following formula (1).
- ⁇ (r) is a phase function indicated by a curve Sp in FIG. 13B
- int means an integerization operator
- r is a radial distance from the optical axis
- ⁇ 0 is a design wavelength
- a i are phase coefficients.
- a phase step occurs in the shape Sp1 every time the phase crosses 2 ⁇ .
- the aspherical shape Sb has a diffraction grating, and the optical characteristics obtained when the optical path length difference based on the phase function Sp is given to the aspherical shape Sb are desired.
- the aspherical coefficient for determining the aspherical shape Sb and the phase coefficient for determining the phase function sp are simultaneously obtained so as to be level.
- the shape Sbp1 of the diffraction grating surface is determined by adding the shape Sp1 corresponding to the phase difference function to the aspherical shape Sb (see FIG. 13C).
- the height d of the phase step shown in FIG. 13C generally satisfies the formula (2).
- ⁇ is the used wavelength
- d is the step height of the diffraction grating
- n 1 ( ⁇ ) is the used wavelength ⁇ .
- It is a refractive index of the lens material which comprises the lens base
- the refractive index of the lens material is wavelength dependent and is a function of wavelength. If the diffraction grating satisfies Expression (2), the phase difference is 2 ⁇ on the phase function between the root and the tip of the phase step, and the optical path difference is an integral multiple of the wavelength with respect to the light of the used wavelength ⁇ . Become. For this reason, the diffraction efficiency of the q-order diffracted light with respect to the light of the used wavelength (hereinafter referred to as “q-order diffraction efficiency”) can be almost 100%.
- Japanese Patent Application Laid-Open No. 2004-228688 detects a pixel position with saturated luminance and removes a flare caused by diffracted light other than the designed order in an image processing apparatus using such a diffractive lens. And a method for estimating the intensity and removing the influence of flare by image signal processing.
- Patent Document 2 in a digital camera using a diffractive lens, if there is a saturated pixel in the first frame, the second frame is shot so that the pixel is not saturated. And a method of removing the influence of flare by performing arithmetic processing on the captured image of the second frame.
- Patent Documents 1 and 2 reduce the influence of flare by the arithmetic processing of the captured image, and do not remove the flare itself. For this reason, it cannot be said that it is a drastic measure.
- the image processing device disclosed in Patent Document 1 it is difficult to estimate the position and intensity of the flare, and if the estimation is inaccurate, the flare appearing in the obtained image may be more noticeable.
- the digital camera disclosed in Patent Document 2 it is necessary to perform shooting twice, and it is necessary to adjust sensitivity so that pixels are not saturated in the second shooting, which may impair the convenience of shooting. It is done.
- the inventor of the present application photographs an object with very high light intensity, if the ring pitch on the diffraction grating surface of the diffractive lens is reduced, the influence of diffracted light other than the above-described design order is In contrast, it has been found that flare light caused by diffracted light generated due to the influence of light shielding at the phase step portion is generated. This flare is called a diffraction-induced flare. It is not known that such diffraction-induced flare light is generated in a diffractive lens. Further, according to the inventors of the present application, it has been found that, under certain conditions, the diffraction-induced flare light may be one of the causes for reducing the quality of the captured image.
- the present invention has been made to solve such a problem, the purpose of which is the diffraction-induced flare, that is, the flare light caused by the diffracted light generated by the light shielding effect at the phase step portion, or It is an object of the present invention to provide a diffractive lens capable of suppressing flare light caused by diffracted light other than the designed order, and an imaging apparatus using the same.
- the diffractive lens of the present invention has a condensing function, and the diffractive lens has an aspherical surface or a surface provided with a diffractive grating along a spherical surface in the effective region, A circle formed by the nth phase step (n is an integer between 0 and n 0 ) counted from the optical axis side of the diffractive lens, having n 0 phase steps that are concentric around the optical axis.
- Radius r n of A, b, c, m are a> 0, 0 ⁇ c ⁇ 1, m> 1, and D n is an arbitrary value in the range of ⁇ 0.25 ⁇ d n ⁇ 0.25.
- the diffractive lens of the present invention has a condensing function, and the diffractive lens has an aspherical surface or a surface provided with a diffractive grating along a spherical surface in the effective region, Concentric with an optical axis as a center, a plurality of annular zones and a plurality of concentric phase steps positioned between the annular zones, respectively, and the nth annular zone is counted from the optical axis side
- the distance between the condensing point of the light transmitted and diffracted through the surface to be diffracted and the condensing point obtained by diffracting the light passing through the shadow region corresponding to the nth phase step counted from the optical axis side is n However, it increases with increasing n.
- the diffractive lens further includes an optical adjustment film that covers the diffraction grating.
- An image pickup apparatus includes any one of the above diffraction lenses and an image pickup element.
- the phase step is provided at a predetermined position, the flare light caused by diffraction spreads without interfering with each other, and the surface perpendicular to the optical axis on the condensing point of the diffractive lens To reach. For this reason, the intensity
- (c) is a ray tracing figure which shows the mode of condensing of the conventional diffraction lens. It is a figure which shows the profile of the phase function in the 1st Embodiment and the conventional diffraction lens, and the position of a phase level
- (d) Shows the result when b / b 0 0.10
- FIG. 1 It is typical sectional drawing which shows 4th Embodiment of the diffraction lens by this invention.
- (A) and (b) are a sectional view and a plan view of an embodiment of a laminated optical system according to the present invention, and (c) and (d) are other embodiments of the laminated optical system according to the present invention. It is sectional drawing and a top view. It is sectional drawing of embodiment of the imaging device by this invention.
- (A) to (c) are diagrams showing a method for deriving a diffraction grating surface shape of a conventional diffractive lens, (a) is a diagram showing a base shape, and (b) is a diagram showing a phase difference function.
- (C) is a figure which shows the surface shape of a diffraction grating. It is a figure which shows the mode of condensing of the incident light parallel to an optical axis in the conventional diffractive lens. It is a figure which shows the mode of the condensing of the light which injected diagonally with respect to the optical axis in the conventional diffractive lens. It is the spot diagram of the light ray just after permeate
- the light intensity distribution on the imaging surface when light having a wavelength of 0.538 ⁇ m is incident on a conventional diffractive lens at an angle of 60 degrees with respect to the optical axis is obtained as a result of wave calculation, and FIG. This is a case where there is no shadow area, and (b) shows the result when there is a shadow area.
- FIG. 14 schematically shows a cross section of a conventional diffractive lens 101.
- the diffractive lens 101 includes a lens base 10 and a diffraction grating 2 ′ provided on the surface 10 a of the lens base 10.
- the diffraction grating 2 ′ is configured by a phase step 2 a provided along a concentric circle with the optical axis 3 of the diffraction lens 101 as the center, and a ring-shaped annular zone 2 b sandwiched between the phase steps 2 a.
- each annular zone 2b has a shape obtained by synthesizing the base shape of the lens base 10 and the phase function, as described with reference to FIG. For this reason, the light emitted from the surface 2bs of the annular zone 2b captures, for example, the incident light beam 4 by the diffraction effect of the diffraction grating 2 in addition to the refraction effect on the lens system refractive surface including the surface 10a of the lens base 10. It can be converted into a light beam 6 that is focused on a point F on the imaging surface 5 of the element.
- the light emitted from the surface 2as of the phase step 2a has a discontinuous wavefront with respect to the wavefront of the light transmitted through each annular zone 2b and travels in all directions as stray light. It does not contribute substantially to the light collection. Therefore, in the region 6 'surrounding the surface 2as of the phase step 2a and the condensing point F, there is substantially no light that exits from the surface 2as and reaches the point F. It can be said that the region 6 ′ is a “shadow” region in the sense that there is no light contributing to condensing in the light flux that converges to the condensing point F.
- the region 6 ' is referred to as a shadow region 6'.
- FIG. 15 shows how the diffractive lens condenses when the incident light beam 4 is incident obliquely with respect to the optical axis of the diffractive lens. Since the incident light beam 4 is inclined with respect to the optical axis 3, the condensing point F on the imaging surface 5 deviates from the optical axis 3, and the width of the shadow region 6 'increases remarkably.
- FIG. 16 is a spot diagram of light rays immediately after being transmitted through the diffraction grating when light is incident on the diffractive lens in a set of two sheets at an angle of 60 degrees with respect to the optical axis in the actual design example.
- the simulation is performed on the assumption that no light is incident on the surface 2as (FIG. 14) of the phase step 2a.
- the light inclination direction coincides with the y-axis direction in FIG. 16, and the crescent-shaped portion shown in white in the circular area shown in FIG. 16 corresponds to the shadow area 6 '.
- the shadow region 6 ' has the largest width in the direction parallel to the y axis.
- shadow regions 6 ′ having a width of 9 ⁇ m within a radius of 0.72 mm on a straight line passing through the center of the circle and parallel to the y-axis.
- the actual width of the shadow region 6 ′ varies depending on the position, the following embodiment will be described with a fixed value (9 ⁇ m) fixed to make the comparison conditions uniform.
- FIG. 17 shows the result of calculating the light intensity distribution on the imaging surface of the light transmitted through the diffraction lens by wave calculation when the wavelength is 0.538 ⁇ m and the incident angle of the incident light beam 4 is 60 degrees with respect to the optical axis 3. Is shown. The light intensity intensity between the bottom and the top, that is, on the imaging surface is indicated by contour lines obtained by dividing 10,000.
- (a) shows a case where calculation is performed assuming that light having a wavefront continuous with light transmitted through the annular zone 6 is also present in the shadow region 6 ′, as will be described in detail below.
- (B) shows the calculation result of the conventional diffractive lens calculated on the assumption that the shadow region 6 ′ under the condition shown in FIG. 16 exists. As shown in FIG. 17B, a diffraction-induced flare 9 caused by the diffraction grating is generated around the condensing spot 8. The intensity level of the flare 9 is about 6/10000 of the peak of the focused spot 8 at the maximum.
- the flare 9 is considered to be caused by the presence of the shadow region 6 ′ as described above, in other words, due to the fact that the flare is substantially shielded by the phase step constituting the diffraction grating.
- the inventor of the present application pays attention to the existence of the shadow region in this way, and has come up with a structure that reduces the flare intensity caused by the diffracted light generated by the influence of the light shielding by the phase step when the diffractive lens is used.
- a diffractive lens and an imaging apparatus according to the present invention will be described.
- FIG. 1 is a cross-sectional view showing a first embodiment of a diffraction grating lens according to the present invention.
- the diffraction grating lens 51 of the first embodiment includes a lens base 10.
- the lens base 10 has a first surface 510a and a second surface 10b, and the diffraction grating 2 is provided on the first surface 10a.
- the diffraction grating 2 is provided on the first surface 10a.
- the diffraction grating 2 may be provided on the second surface 10b, and may be provided on both the first surface 10a and the second surface 10b. It may be.
- the base shape of the first surface 10a and the second surface 10b is an aspherical shape, but the base shape may be a spherical shape or a flat plate shape.
- the base shapes of both the first surface 51a and the second surface 51b may be the same or different.
- the surface on which the diffraction grating 2 is provided is preferably aspheric or spherical. This is because the traveling direction of incident light can be greatly changed by providing the diffraction grating 2 on an aspherical or spherical surface.
- the base shapes of the first surface 10a and the second surface 10b are respectively convex aspherical shapes, but may be concave aspherical shapes.
- one of the base shapes of the first surface 10a and the second surface 10b may be convex and the other may be concave.
- the “base shape” refers to the design shape of the surface of the lens base 10 before the shape of the diffraction grating 2 is applied. If a structure such as the diffraction grating 2 is not provided on the surface, the surface of the lens base 10 has a base shape. In the present embodiment, since the diffraction grating is not provided on the second surface 10b, the second surface 10b has an aspherical shape that is a base shape.
- the first surface 10a is configured by providing the diffraction grating 2 in a base shape, that is, an aspherical shape. Since the first surface 10a is provided with the diffraction grating 52, the first surface 10a of the lens base 10 is not aspherical in the state where the diffraction grating 2 is provided. However, since the diffraction grating 2 has a shape based on a predetermined condition as described below, by subtracting the shape of the diffraction grating 2 from the shape of the first surface 10a on which the diffraction grating 2 is provided, the first The base shape of the surface 10a can be specified. Since the base shape is a design shape, it is not necessary that the lens substrate 10 before the diffraction grating 2 is provided has a base-shaped surface.
- the diffraction grating 2 is provided along the base shape of the first surface 10 a in the effective area of the diffraction lens 51.
- the effective region 10e is a region where the diffractive lens 51 has a condensing function on the first surface 10a and the second surface 10b.
- the diffractive lens 51 refers to an area that contributes to forming an image on the imaging surface of the imaging element.
- the diffraction grating 2 has a plurality of concentric annular zones 2b around the optical axis 3 of the diffraction lens 51 and a plurality of phase steps 2a positioned between the plurality of annular zones 2b.
- the plurality of phase steps 2a are also located on concentric circles having different radii with the optical axis 3 as the center.
- each phase step 2a is expressed by the following formula (2).
- ⁇ is the used wavelength
- n 1 ( ⁇ ) is the refractive index of the lens material constituting the lens substrate at the used wavelength ⁇ . It is.
- the refractive index of the lens material is wavelength dependent and is a function of wavelength. If the diffraction grating satisfies Expression (2), the phase difference is 2 ⁇ on the phase function between the root and the tip of the phase step, and the optical path difference is an integral multiple of the wavelength with respect to the light of the used wavelength ⁇ . Become. For this reason, the diffraction efficiency of the q-order diffracted light with respect to the light of the used wavelength (hereinafter referred to as “q-order diffraction efficiency”) can be almost 100%.
- the diffraction lens 51 can have a practically sufficient condensing function. Specifically, if the height d of the phase step 2a satisfies the following expression (3), it can be said that the light condensing action sufficient for practical use can be obtained and the expression (2) is substantially satisfied.
- the surface 2bs of the annular zone 2b of the diffraction grating 2 provided on the first surface 10a of the lens base 10 is formed by superimposing the shape of the diffraction grating on the base shape as described above. For this reason, it has the effect
- action which condenses the incident light ray 4 on the point F on the image pick-up surface 5 of an image pick-up element by the refraction effect by the base shape, and the diffraction effect by the diffraction grating 2.
- the light emitted from the surface 2as of the phase step 2a does not contribute to the condensing at the condensing point F. For this reason, in the shadow region 6 'surrounding the surface 2as of the phase step 2a and the condensing point F, there is no light emitted from the surface 2as and reaching the point F, and a shadow region 6' is formed.
- the diffraction-induced flare is caused by the fact that the light that passes through the diffraction lens 51 and contributes to condensing is substantially blocked by the phase step 2b of the diffraction grating 2, and a shadow region is generated. Conceivable.
- the phase step 2b is formed at a specific position in order to suppress the occurrence of diffraction-induced flare. Specifically, as shown in FIG. 1, the position of each phase step 2a, when represented with a radius r n around the optical axis 3, the radius of the n th phase step 2a position from the center O r n meets the following formula (4).
- a and c are constants in the range of a> 0 and 0 ⁇ c ⁇ 1, and dn is an arbitrary value in the range of ⁇ 0.25 ⁇ d n ⁇ 0.25.
- n is an integer of 0 or more and n 0 or less, and the expression (4) is satisfied for all n.
- d n may be different for each phase step 2b. That is, d 1 , d 2 ,... Dn 0 may be different from each other.
- c corresponds to a constant term in the phase function and relates to the position where the phase step 2a starts.
- c 0.
- d n corresponds to the positional accuracy of the phase step 2a
- ⁇ 0.25 ⁇ d n ⁇ 0.25 indicates that the aberration caused by the position error is 0.25 wavelength or less.
- Equation (4) indicates that the position of the phase step 2a follows the root rule. That is, if the radius of the first phase step is 1, the nth radius is n 1/2 . For simplicity in the following description, the case c and d n is zero.
- phase difference function ⁇ (r) of the diffraction grating 2 satisfies the following formula (5).
- ⁇ (r n ) ⁇ (r n ⁇ 1 ) 2q ⁇ (5)
- q is an integer.
- FIG. 2 is a spot diagram of light rays immediately after being transmitted through the diffraction grating formed on the final surface when light is incident on the diffraction lens 51 of the present embodiment at an angle of 60 degrees with respect to the optical axis. Similar to the spot diagram of a conventional diffractive lens, the light inclination direction coincides with the y-axis direction in FIG. A shadow area 6 ′ shown in white in the circular area shown in FIG. 2 is an area that does not contribute to the focusing on the focal point F. There are 24 shadow regions 6 ′ having a width of 9 ⁇ m within a radius of 0.72 mm, and their positions follow the equation (4), that is, the root rule. The position of the crescent moon region in FIG. 2 is different from the position shown in FIG. Thereby, generation
- the influence of the shadow region 6 ′ on the light distribution on the imaging surface 5 can be estimated as follows using the Babinet principle.
- (Diffraction image amplitude distribution of light passing through shadow area) + (Diffraction image amplitude distribution of light passing through areas other than shadow areas) (Diffraction image amplitude distribution of light passing through all areas [shadow area + area other than shadow area]) (6)
- the “light passing through the shadow region” means light having a continuous intensity and wavefront with light passing through the region other than the shadow region. Therefore, the following relationship is obtained.
- (Diffraction image amplitude distribution of light passing through areas other than shadow areas) (Diffraction image amplitude distribution of light passing through the entire region)- (Diffraction image amplitude distribution of light passing through shadow area) (7)
- the flare 9 around the focused spot 8 does not exist in the diffraction image amplitude distribution of light passing through the entire region. Accordingly, the flare appearing in the diffraction image of the light passing through the region other than the shadow region 6 'is equivalent to the diffraction image of the light passing through the shadow region 6'.
- “light passing through the shadow region” means light having a continuous intensity and wavefront with light passing through a region other than the shadow region. In other words, if light having a wavefront continuous with light passing through a region other than the shadow region (that is, the annular zone) is defined by the shadow region 6 ′, it means light that is diffracted through the shadow region.
- “light passing through the shadow region” will be used in this sense.
- the shadow region 6 ′ is a cone region having the surface 2 as of each phase step 2 a as the bottom surface and the condensing point F of the diffraction lens 51 as the vertex. Specifically, triangles respectively connecting both ends of a line segment where the surface including the optical axis 3 of the diffractive lens 51 intersects the surface 2as of each phase step 2a, and the condensing point F of the diffractive lens 51, This is an area formed by rotating around the optical axis 3.
- the present invention does not directly take into account the diffracted light caused by the light transmitted through the end portion of the annular zone 2b wrapping around the shadow region 6 ′, but “passes through the shadow region having a phase opposite to this diffracted light.
- a diffraction image of light passing through the shadow region will be considered.
- the light condensing point on the imaging surface 5 of the light passing through the region other than the shadow region 6 ′ is F, and the light passing through the shadow region corresponding to the nth phase step 2ab counting from the center.
- phase difference ⁇ n is expressed by the following equation (12). ⁇ n ⁇ r n 2 ⁇ / (2f 2 ) (12)
- the light passing through the shadow region corresponding to each phase step 2a is collected at the point F ′ regardless of the wavelength (without aberration).
- the phase of the light passing through the shadow region and the diffracted light caused by the light transmitted through the end of the annular zone 2b wrapping around the shadow region 6 ' are opposite to each other, but the amplitude distribution is almost equal. For this reason, it can be said that the diffracted light caused by the light passing through the shadow region and the light transmitted through the end of the annular zone 2b wraps around the shadow region 6 'is also condensed at the point F' regardless of the wavelength.
- FIG. 3A is a ray tracing diagram showing how the diffractive lens 51 condenses light.
- Point F is a condensing point of the focused light beam 6 of the diffractive lens 51.
- the fact that the diffracted light (ray group 6a) of light passing through the shadow region is collected without aberration at the point F ′ is that the ray group 6a is at one point F ′ as shown in FIG. Equivalent to crossing.
- the ray group 6a passes through the point F 'and then reaches the imaging surface 5 without intersecting each other. On the imaging surface 5, the absence of the intersection of the light beam groups 6a means that there is little light interference in terms of wave optics.
- the diffracted light by the light beam group 6a spreads uniformly on the imaging surface 5 without intensifying interference, and the light intensity can be minimized. That is, it is possible to reduce the interference on the imaging surface of the flare that appears in the diffraction image of the light passing through the area other than the shadow area.
- This effect is obtained when the condensing point F ′ of light passing through the shadow region is closer to the diffraction lens 51 than the condensing point F of light passing through the region other than the shadow region, that is, ⁇ is a positive value.
- ⁇ is a positive value.
- Expression (13) holds when the position of the phase step 2a satisfies Expression (4). .
- the diffracted light of the light passing through the shadow region corresponding to the phase step 2a is collected at the point F ′ regardless of the position of the phase step 2a.
- transmits the surface 2bs which comprises the nth annular zone 2b counted from the optical axis 3 side, and the shadow area
- the distance from the condensing point F ′ of light passing through is constant regardless of the value of n.
- the light focused on the point F ′ is the first-order diffracted light diffracted by the light passing through the shadow region, and the zero-order diffracted light corresponds to the focused light beam 6.
- the ⁇ 1st order diffracted light as shown in FIG. 3B, there is a light beam group 6a ′ that converges at a point F ′′ that is symmetrical to the point F ′ with respect to the imaging surface 5.
- the pattern formed when the light beam group 6a ′ intersects the imaging surface 5 is the same as the pattern formed when the light beam group 6a intersects the imaging surface 5, the way of interference of light on the imaging surface 5 is the light beam group. 6a, interference due to the ⁇ 1st order diffracted light can be taken into consideration, so only the light group 6a of the 1st order diffracted light will be mentioned in this embodiment and the following embodiments.
- FIG. 3 (c) is a ray tracing diagram showing the state of light collection by a conventional diffractive lens.
- the diffractive lens of the conventional example the light beam group 6a does not converge at one point F '.
- the density of the light rays is sparse. Therefore, the interference of light on the imaging surface 5 increases. As a result, the flare 9 occurs as shown in FIG.
- the diffractive lens 51 of the present embodiment since the position of the phase step 2a is provided at a position satisfying the equation (4), as shown in FIG.
- the diffraction by 6a is condensed at a point F ′ closer to the diffraction lens 51 than the condensing point F of the diffraction grating 2 without depending on the wavelength. Further, after passing through the point F ′, they cross each other and reach the imaging surface 5 without increasing interference. That is, it is possible to reduce interference on the imaging surface of flare that appears in a diffraction image of light passing through an area other than the shadow area. For this reason, the diffraction-induced flare can be suppressed.
- the diffracted light of the light passing through the shadow region is not a focusless aberration.
- the generation of diffraction-induced flare when light parallel to the optical axis is incident is greatly suppressed, even when light is incident obliquely with respect to the optical axis, the intersection of rays on the imaging surface 5 is The interference of light is greatly suppressed.
- the optical phase shift amount generated at the phase step changes depending on the incident angle of light.
- diffracted light of an order other than the designed order is generated, which may cause flare.
- This kind of flare also has the same focusing behavior as the diffraction of light passing through the shadow region described above. Therefore, based on the above-described principle, the maximum intensity of flare caused by diffracted light of orders other than the designed order (so-called unnecessary diffraction orders) can be reduced under the same conditions.
- the diffractive lens 51 is designed by a design method different from the conventional one.
- conventional diffractive lens design using commercially available optical design software, multiple aspherical coefficients that define the aspherical shape of the diffractive lens are independent of the multiple phase coefficients that define the phase function of the diffraction grating.
- Each optimum value is obtained recursively so as to satisfy the desired optical performance by treating it as a parameter.
- the position of the phase step is determined according to the equation (4). Since the phase difference between adjacent phase steps is 2 ⁇ (or 2q ⁇ ), determining the position of the phase step means determining the phase function first.
- the parameters are determined so that desired optical characteristics can be obtained by using the base shape, for example, the aspherical coefficient of the function for determining the aspherical shape as a parameter.
- FIG. 4 schematically shows an example of the phase function sp ′ determined by the conventional diffractive lens and an example of the phase function Sp of the diffractive lens 51 of the present embodiment.
- R ′ 4 ...
- phase step positions r 1 , r 2 , r 3 , r 4 ... are determined according to the equation (4).
- the phase step of the diffractive lens 51 of the present embodiment is narrower in pitch than the conventional one as it goes from the center of the lens to the periphery.
- FIG. 5 shows a result obtained by calculating the light intensity distribution on the imaging surface when light having a wavelength of 0.538 ⁇ m is incident on the diffraction lens 51 at an angle of 60 degrees with respect to the optical axis.
- the light intensity intensity between the bottom and the top, that is, on the imaging surface is indicated by contour lines obtained by dividing 10,000.
- the conditions of the width and the number of shadow areas are the same as the conditions in FIG.
- the intensity level of the flare 9 due to diffraction caused by the diffraction grating generated around the condensing spot 8 is about 3/10000 of the peak of the condensing spot 8 at the maximum, which is halved compared to the conventional example. I understand that.
- a second embodiment of the diffractive lens according to the present invention will be described with reference to FIG.
- the diffraction lens 52 of the second embodiment is different from the diffraction lens 51 of the first embodiment in the position of the phase step 2a. Therefore, the position of the phase step 2a will be described in detail.
- each phase step 2a when represented with a radius r n around the optical axis 3, the radius r n of the n-th phase step 2a counted from the center O formula (15) Meet.
- a, b, c, m are constants in the range of a> 0, b> 0, 0 ⁇ c ⁇ 1, m> 1, and dn is ⁇ 0.25 ⁇ d for all n. Any value in the range of n ⁇ 0.25.
- n is an integer from 0 to n 0 and satisfies the formula (15) for all n.
- d n may be different for each phase step 2b. That is, d 1 , d 2 ,... Dn 0 may be different from each other.
- equation (15) includes the term b.
- c corresponds to a constant term in the phase function and relates to the position where the phase step 2a starts.
- c 0.
- d n corresponds to the positional accuracy of the phase step 2a
- ⁇ 0.25 ⁇ d n ⁇ 0.25 indicates that the aberration caused by the position error is 0.25 wavelength or less.
- c and d n is described as a zero.
- equation (15) is expressed by the following equation (16).
- FIG. 6 is a ray tracing diagram showing a state of condensing by the diffractive lens 52 of the second embodiment.
- the light ray 6a1 that passes through the shadow region corresponding to the phase step 2a close to the optical axis among the light ray group that passes through the shadow region is focused in the vicinity of the point F ′, but is located on the outer peripheral side.
- the light condensing point F′2 shifts to the diffractive lens side.
- the distance from the condensing point of the light passing through the region increases as n increases.
- the light beam groups 6a1 and 6b2 that pass through the surface 2as of the phase step 2a travel, the light beam groups 6a1 and 6a2 do not intersect on the imaging surface 5 as in the first embodiment.
- the distance between the light beams 6a1 and 6b2 is larger on the imaging surface 5 than in the embodiment. Therefore, the diffracted light by the light beam groups 6a1 and 6a2 spreads on the imaging surface 5 without increasing the interference, and flare can be suppressed more than in the first embodiment.
- the diffracted light of the light passing through the shadow region is not an aberration-free condensing.
- generation of diffraction-induced flare when light parallel to the optical axis is incident is greatly suppressed, there is little crossing of light rays on the imaging surface 5 even when light is incident obliquely with respect to the optical axis.
- the interference of light, that is, the occurrence of flare is greatly suppressed.
- FIG. 7A (a) to (f), (g) to (i) in FIG. 7B, (a) to (f) in FIG. 7C, and (g) to (i) in FIG.
- the figure shows the result of the light intensity distribution on the imaging surface obtained by wave calculation when 0.538 ⁇ m light is incident at an angle of 60 degrees with respect to the optical axis.
- the light intensity intensity between the bottom and the top, that is, on the imaging surface is indicated by contour lines obtained by dividing 10,000.
- the conditions of the width and the number of shadow areas are the same as the conditions in FIG.
- Table 1 below shows m and b in the above formula (15) used for obtaining the calculation results shown in these figures.
- b a value normalized by the upper limit b 0 was used.
- the flare 9 is generated around the focused spot 8, but the intensity of the flare 9 is higher than that of the conventional example (FIG. 17B). I understand that it is small. It can also be seen that the strength of flare 9 decreases as b / b 0 increases. For example, the intensity level of the flare 9 shown in (i) of FIG. 7B and (i) of FIG. 7D is about 1 to 2/10000 of the peak of the focused spot 8 at the maximum.
- FIG. 7E shows a value (flare energy ratio) obtained by dividing the value of b / b 0 and the energy of the flare 9 (in-plane integrated value of intensity) in the calculation results shown in FIGS. 7A to 7D by the energy of the focused spot 8. Shows the relationship.
- FIG. 7E shows the relationship between b / b 0 and the flare energy ratio for each m and s.
- the flare energy ratio decreases as b / b 0 increases in any curve.
- FIG. 7E shows that the flare energy ratio greatly depends on s and b / b 0, but the dependence on m is small.
- the value of b preferably satisfies the following formula (22), and more preferably satisfies the following formula (22 ′).
- b 0 satisfies the following formula (21).
- Figure 8 is an optical intensity distribution on the imaging surface in the case where is incident at an angle of 60 degrees to the optical axis of light of wavelength 0.538 ⁇ m to the diffraction lens 52, a d n for each n
- the calculation result of the light intensity distribution on the imaging surface in the case of giving randomly within a certain range is shown.
- (A) and (b) are values where d n is between ⁇ 0.25 and 0.25
- (c) and (d) are values where d n is between ⁇ 0.5 and 0.5. is there.
- Other conditions are the same as in FIG.
- the flare energy ratio is about twice the light intensity distribution of FIG. 8A in the light intensity distribution of FIG. Further, it can be seen that the flare energy ratio is about four times the light intensity distribution of FIG. 8B in the light intensity distribution of FIG. From these results, it is understood that the range of d n is preferably between ⁇ 0.25 and 0.25, and beyond this, flare energy increases remarkably.
- the diffractive lens 52 is also provided at a position where the phase step 2a satisfies the expression (15) as in the first embodiment, a shadow region corresponding to the phase step 2a is formed as shown in FIG.
- the focal point of the light 6a that has passed shifts more toward the diffraction lens 52 as the phase step 2a moves from the optical axis side toward the outer peripheral side. For this reason, in the imaging surface 5, since the light 6a which passed the shadow area
- the optical phase shift amount generated at the phase step changes depending on the incident angle of light.
- diffracted light of an order other than the design order (so-called unnecessary diffraction order) is generated, which may cause flare.
- This kind of flare also has the same focusing behavior as the diffraction of light passing through the shadow region described above. Therefore, based on the above-described principle, the maximum intensity of flare caused by diffracted light of orders other than the designed order can be reduced under the same conditions.
- the diffractive lens 52 first determines the position of the phase step, that is, the phase coefficient of the phase function so as to satisfy the equation (15).
- the parameters are determined so that desired optical characteristics can be obtained by using the aspherical coefficient of the function for determining the base shape, for example, the aspherical shape, as a parameter.
- b that can be adjusted in Expression (15) is included as a parameter.
- the phase coefficient of the phase function is determined using b as a parameter, and the aspherical coefficient can be determined while adjusting the value of b within the range of Equation (22). It is easy to find a design solution.
- the occurrence of flare can be suppressed by providing the phase step at the position satisfying the formula (4) or the formula (15).
- the light passing through the surface constituting the nth ring zone counted from the optical axis side and the light passing through the shadow region corresponding to the nth phase step counted from the optical axis side The distance from the condensing point is constant with respect to n, or increases as n increases. Therefore, the position of the phase step may be determined so as to satisfy such a relationship.
- the shape of the annular zone 2b of the diffraction grating 2 ′ is different from those of the first and second embodiments.
- the cross section of each annular zone 2b in the plane including the optical axis has a tip on the optical axis 3 side and a root on the outer peripheral side.
- the cross section of each annular zone 2b in the plane including the optical axis has a root on the optical axis 3 side and a tip on the outer peripheral side.
- each annular zone 2a is opposite to that of the eleventh and second embodiments.
- Other structures are the same as those in the first or second embodiment.
- the position of the phase step 2a satisfies Expression (4) or Expression (15) as in the first and second embodiments.
- the direction of the saw blade shape formed by the cross section of each annular zone 2a is to make the diffraction power positive or negative, or the medium between the lens base 10 on which the diffraction grating 2 'is formed and the adjacent medium.
- This is a design item determined by the relationship of the refractive index.
- the diffractive lens 53 is suitable when the refractive index of the medium 13 with which the diffraction grating 2 ′ is in contact is larger than the refractive index of the material constituting the lens base 10.
- the diffractive lens 53 is also provided at the position where the phase step 2a satisfies the relationship of the formula (4) or the formula (15), so that flare can be suppressed. it can.
- FIG. 10 is a cross-sectional view showing a fourth embodiment of a diffraction grating lens according to the present invention.
- the diffraction grating lens 54 shown in FIG. 10 includes, for example, the diffraction lens 51 of the first embodiment and the optical adjustment film 11 provided so as to cover the diffraction grating 2 provided on the diffraction lens 51.
- An optical adjustment film 11 is provided so as to completely fill the phase step of the diffraction grating 2.
- the diffractive lens 52 of the second embodiment or the diffractive lens 53 of the third embodiment may be used.
- the diffractive lens 51 is made of a first material having a refractive index n 1 ( ⁇ ) at the operating wavelength ⁇ .
- the optical adjustment film 11 is made of a second material having a refractive index n 2 ( ⁇ ) at the operating wavelength ⁇ .
- the phase step has a height d shown by the following (23).
- the operating wavelength ⁇ is a wavelength in the visible light region, and the expression (23) is substantially satisfied with respect to the wavelength ⁇ in the entire visible light region. “Substantially satisfied” means satisfying the relationship of the following expression (24), for example.
- the first material having a refractive index n 1 ( ⁇ ) having a wavelength dependency such that d is substantially constant in the visible light region may be combined with a second material having a refractive index n 2 (lambda), for example, the second material may be any low refractive index and high dispersion material than the first material.
- the refractive index of the second material is preferably smaller than the refractive index of the first material
- the Abbe number of the second material is preferably smaller than the Abbe number of the first material.
- the first material and the second material a composite material in which inorganic particles are dispersed in glass or resin to adjust the refractive index and wavelength dispersion may be used.
- n 2 ( ⁇ ) is larger than the refractive index n 1 ( ⁇ )
- d is a negative value.
- the diffraction lens 53 is used instead of the diffraction lens 51.
- the diffraction grating 2 is covered with the diffractive optical lens 54 and the optical adjustment film 54 of the present embodiment, which is different from the diffractive optical lens 54 of the first embodiment. If it is a layer, it can be said that the diffractive optical lens 54 and the diffractive optical lens 51 have the same structure.
- the refractive index n 2 ( ⁇ ) of the second material which is an optical material, is generally larger than 1. Therefore, the diffractive optics of the first embodiment is used.
- the height d of the phase step is increased.
- the light shielding width at the phase step portion is also increased, the generation of diffraction-induced flare is suppressed as in the first embodiment. Further, by satisfying Expression (23), flare caused by unnecessary-order diffracted light can be reduced over the entire use wavelength range.
- FIG. 11A is a schematic cross-sectional view showing an embodiment of an optical system according to the present invention
- FIG. 11B is a plan view thereof.
- the optical element 55 includes the diffractive lens 21 and the diffractive lens 22.
- the diffractive lens 21 is, for example, the diffractive lens 51 of the first embodiment, and is provided with the diffraction grating 2 having the structure described in the first embodiment.
- the diffraction lens 22 is provided with a diffraction grating 2 ′ having a shape corresponding to the diffraction grating 2.
- the diffractive lens 21 and the diffractive lens 22 are held with a predetermined gap 23 therebetween.
- FIG. 11 (c) is a schematic sectional view showing another embodiment of the optical system according to the present invention
- FIG. 11 (d) is a plan view thereof.
- the optical element 55 ′ includes the diffractive lens 21 ⁇ / b> A, the diffractive lens 21 ⁇ / b> B, and the optical adjustment film 24.
- the diffraction grating 2 having the structure described in the first embodiment is provided on one surface of the diffraction lens 21A.
- the diffraction grating 2 is also provided in the diffraction lens 21B.
- the optical adjustment film 24 covers the diffraction grating 2 of the diffraction lens 21A.
- the diffraction lens 21A and the diffraction lens 21B are held so that a gap 23 is formed between the diffraction grating 2 provided on the surface of the diffraction lens 21B and the optical adjustment film 24.
- the optical element 55 and the optical element 55 ′ on which the diffractive lenses are stacked also include the diffraction grating 2 provided with a phase step at a predetermined position. As described in the embodiment, generation of diffraction-induced flare is suppressed.
- FIG. 12 is a schematic cross-sectional view showing an embodiment of an imaging apparatus according to the present invention.
- the imaging device 56 includes a lens 31, a diffraction lens 33, a diaphragm 32, and an imaging element 34.
- the lens 31 includes a lens base 55.
- the first surface 55a and the second surface 55b of the lens base 55 have a known lens surface shape such as a spherical shape or an aspherical shape.
- the first surface 55a of the lens base 35 has a concave shape
- the second surface 55b has a convex shape.
- the diffractive lens 32 includes a base 36.
- the base shape of the first surface 36a and the second surface 36b of the base body 36 has a known lens surface shape such as a spherical shape or an aspherical shape.
- the first surface 36a has a convex shape
- the second surface 36b has a concave shape.
- the diffraction grating 2 described in the first embodiment is provided on the second surface 36b.
- the light from the subject incident from the second surface 35 b of the lens 31 is collected by the lens 31 and the diffraction lens 33, forms an image on the surface of the image sensor 34, and is converted into an electric signal by the image sensor 34.
- the imaging device 56 of the present embodiment includes two lenses, the number of lenses and the shape of the lenses are not particularly limited, and may be one, or may include three or more lenses. Good. Optical performance can be improved by increasing the number of lenses.
- the diffraction grating 2 may be provided in any lens among the plurality of lenses.
- the surface on which the diffraction grating 2 is provided may be arranged on the subject side, may be arranged on the imaging side, or may be a plurality of surfaces.
- the diaphragm 56 may not be provided.
- the imaging apparatus includes the diffractive lens provided with the diffraction grating described in the first embodiment, an image with little diffraction-induced flare light can be obtained even when shooting a strong light source. .
- the present invention in order to explain the flare generated by the diffraction grating, an example in which the light emitted from the surface provided with the diffraction grating irradiates the imaging surface of the imaging element is given.
- the present invention has been described.
- the present invention is not limited to the use for condensing light on the imaging surface of the image sensor, and the present invention can be applied to various optical systems.
- the diffractive lens of the present invention and an image pickup apparatus using the diffractive lens have a function of reducing flare light caused by diffraction, and are particularly useful as a high-quality lens and camera.
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Abstract
Description
本発明による回折レンズの第1の実施形態を説明する。
ここで、qは設計次数(1次の回折光の場合はq=1)であり、λは使用波長であり、n1(λ)は使用波長λにおけるレンズ基体を構成するレンズ材料の屈折率である。レンズ材料の屈折率は波長依存性があり、波長の関数である。式(2)を満たすような回折格子であれば、位相段差の根元と先端とで、位相関数上において位相差が2πとなり、使用波長λの光に対して、光路差が波長の整数倍となる。このため、使用波長の光に対するq次回折光の回折効率(以下、「q次回折効率」という。)を、ほぼ100%にすることができる。
φ(rn)-φ(rn-1)=2qπ (5)
ただしqは整数である。
(影領域を通過する光の回折像振幅分布)+
(影領域以外の領域を通過する光の回折像振幅分布)
=(全領域[影領域+影領域以外の領域]を通過する光の回折像振幅分布)
(6)
ここで、「影領域を通過する光」とは、影領域以外の領域を通過する光と強さや波面が連続した光を意味する。従って、次の関係が求まる。
(影領域以外の領域を通過する光の回折像振幅分布)
=(全領域を通過する光の回折像振幅分布)-
(影領域を通過する光の回折像振幅分布) (7)
(F’Pn- FPn)=-Δ・cosθn (8)
ただしθn=∠PnFO
δn=(1-cosθn) ・Δ (9)
位相差δn≒θn 2・Δ/2 (10)
θn≒rn/f (11)
ただしf=OF
δn≒rn 2 Δ/(2f2 ) (12)
Δ=2f2λ/a (13)
δn=nλ (14)
図1を参照しながら、本発明による回折レンズの第2の実施形態を説明する。第2の実施形態の回折レンズ52は、位相段差2aの位置が第1の実施形態の回折レンズ51と異なっている。このため、位相段差2aの位置について詳細に説明する。
0<b<b0 (20)
ただしb0は以下の式(21)を満たす。
0.05b0<b<b0 (22)
0.2b0<b<b0 (22’)
ただしb0は以下の式(21)を満たす。
図9を参照しながら、本発明による回折レンズの第3の実施形態を説明する。第3の実施形態の回折レンズ53では、回折格子2’の輪帯2bの形状が第1および第2の実施形態と異なっている。具体的には、第1および第2の実施形態では、光軸を含む平面における各輪帯2bの断面は、光軸3側に先端を有し、外周側に根元を有していた。これに対し、本実施形態の回折レンズ53において、光軸を含む平面における各輪帯2bの断面は、光軸3側に根元を有し、外周側に先端を有している。つまり、各輪帯2aの断面によって形成される鋸刃形状の方向が第11および第2の実施形態とは逆になっている。その他の構造は第1または第2の実施形態と同じである。特に、位相段差2aの位置は、第1および第2の実施形態と同様、式(4)または式(15)を満たしている。
図10は、本発明による回折格子レンズの第4の実施形態を示す断面図である。図10に示す回折格子レンズ54は、例えば、第1の実施形態の回折レンズ51と、回折レンズ51に設けられた回折格子2を覆うように設けられた光学調整膜11とを備える。回折格子2の位相段差を完全に埋めるように光学調整膜11が設けられている。第1の実施形態の回折レンズ51の替わりに、第2の実施形態の回折レンズ52または第3の実施形態の回折レンズ53を用いてもよい。
図11(a)は、本発明による光学系の実施形態を示す模式的断面図であり、図11(b)はその平面図である。光学素子55は、回折レンズ21と回折レンズ22とを備える。回折レンズ21は、例えば第1の実施形態の回折レンズ51であり、第1の実施形態で説明した構造を有する回折格子2が設けられている。回折レンズ22は、回折格子2と対応する形状を有する回折格子2’が設けられている。回折レンズ21と回折レンズ22とは所定の間隙23を隔てて保持されている。
図12は、本発明による撮像装置の実施形態を示す模式的断面図である。撮像装置56は、レンズ31と、回折レンズ33と、絞り32と撮像素子34とを備える。
2a 位相段差
2b 輪帯
4 入射光
5 撮像面
6、6a 光線
6’ 影領域
10 レンズ基体
10a 第1の表面
10b 第2の表面
10e 有効領域
51、52、53 回折レンズ
Claims (4)
- 集光作用を有する回折レンズであって、
前記回折レンズは、有効領域において、非球面または球面に沿って回折格子が設けられた面を有し、
前記回折格子は、前記回折レンズの光軸を中心とする同心円状に、複数の輪帯および前記複数の輪帯間にそれぞれ位置する同心円状の複数の位相段差を有し、
前記光軸側から数えてn番目の輪帯を構成する面を透過、回折する光の集光点と、前記光軸側から数えてn番目の位相段差に対応する影領域を通過する光が回折するとして求めた集光点との距離が、nに対して一定、または、nの増大とともに増加する、回折レンズ。 - 前記回折格子を覆う光学調整膜をさらに備える請求項1または2に記載の回折レンズ。
- 請求項1から3のいずれかに記載の回折レンズと、
撮像素子と
を備えた撮像装置。
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JP2000171704A (ja) * | 1998-12-09 | 2000-06-23 | Asahi Optical Co Ltd | 回折レンズの設計方法 |
JP2000249818A (ja) * | 1999-03-02 | 2000-09-14 | Konica Corp | 回折レンズ及びその設計方法 |
JP2001305323A (ja) * | 2000-04-20 | 2001-10-31 | Canon Inc | 回折格子を有する光学系、該光学系を有する撮影装置及び観察装置 |
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JP2000333076A (ja) | 1999-05-19 | 2000-11-30 | Asahi Optical Co Ltd | デジタルカメラにおけるフレア成分除去方法 |
JP4250513B2 (ja) | 2003-12-01 | 2009-04-08 | キヤノン株式会社 | 画像処理装置及び画像処理方法 |
EP1528797B1 (en) | 2003-10-31 | 2015-07-08 | Canon Kabushiki Kaisha | Image processing apparatus, image-taking system and image processing method |
US7156516B2 (en) * | 2004-08-20 | 2007-01-02 | Apollo Optical Systems Llc | Diffractive lenses for vision correction |
JP4077510B2 (ja) * | 2006-05-15 | 2008-04-16 | 松下電器産業株式会社 | 回折撮像レンズと回折撮像レンズ光学系及びこれを用いた撮像装置 |
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JP2000171704A (ja) * | 1998-12-09 | 2000-06-23 | Asahi Optical Co Ltd | 回折レンズの設計方法 |
JP2000249818A (ja) * | 1999-03-02 | 2000-09-14 | Konica Corp | 回折レンズ及びその設計方法 |
JP2001305323A (ja) * | 2000-04-20 | 2001-10-31 | Canon Inc | 回折格子を有する光学系、該光学系を有する撮影装置及び観察装置 |
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