JPH06250081A - Chromatic aberration compensating element - Google Patents

Abstract

PURPOSE:To provide a chromatic aberration compensating element capable of compensating an on-axis chromatic aberration generated in a positive lens and restricting the change of a spherical aberration to be small even when a light beam having a wavelength not contiguous to is changed over for use. CONSTITUTION:A divergent wave front having an excess spherical aberration is generated to the incidence of a parallel light beam having a wavelength shorter than a reference wavelength and a convergent wave front having a few spherical aberration is generated to the incidence of a parallel light beam having a wavelength longer than the reference wavelength. Concretely, an element scarsely having the refractive power is formed by sticking together both a positive and a negative lenses having no difference of a refractive index and different dispersions and the stuck surface r2 is made an aspherical surface whose absolute value of the radius of curvature gets smaller as the surface is separated from the optical axis.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an element for correcting chromatic aberration of an optical system, and more particularly to a chromatic aberration correcting element used in combination with an aspherical single lens in which aberrations other than chromatic aberration are corrected.

[0002]

2. Description of the Related Art In recent years, objective lenses for optical disks have been
A single lens with aspherical surfaces on both sides has been used for weight reduction. However, chromatic aberration cannot be corrected with an aspherical single lens.

The emission wavelength of a semiconductor laser used as a light source of an optical disk device shifts due to a change in output power or a change in temperature. For this reason, if the chromatic aberration of the objective lens is not corrected, the focal position of the light beam may change due to the wavelength shift, and errors may occur in reading and writing of information.

In order to solve this problem, the present inventors have
The present invention invented a chromatic aberration correction element in which two or three glass lenses are bonded together (Japanese Patent Laid-Open No. 3-155514 and Japanese Patent Laid-Open No. 3-1555).
55515). By using this chromatic aberration correction element in combination with an aspherical single lens, a lens that is not affected by wavelength fluctuations can be provided in a relatively small size.

These chromatic aberration correction elements are arranged in almost afocal portions of the optical system, and have the function of changing parallel light into divergent light or convergent light depending on the wavelength so as to cancel the axial chromatic aberration of the objective lens. There is.

A lens whose chromatic aberration is to be corrected is generally a positive lens whose spherical aberration is corrected at a single wavelength. The focal length of the positive lens is short when the wavelength is short at a wavelength near visible light, and is long when the wavelength is long. Therefore, in order to cancel the axial chromatic aberration and prevent the focus position from moving, the light flux entering the positive lens may be divergent light for short wavelength light and convergent light for long wavelength light.

[0007]

However, although the optical system using the above-described conventional chromatic aberration correcting element can correct axial chromatic aberration, spherical aberration changes due to wavelength change, so that the range of wavelength change is particularly large. When is large, it was not possible to maintain good performance at both wavelengths before and after the change.

The spherical aberration of the positive lens corrected at the reference wavelength becomes under for short wavelength light with a high refractive index and over for long wavelength light with a low refractive index. This is the change in spherical aberration with wavelength.

On the other hand, when the parallel light is changed to divergent light or converging light by the action of the conventional chromatic aberration correcting element, this change changes the object point at infinity from the positive lens side to a finite distance. Since this is equivalent to that, the spherical aberration changes. Due to this change, when the incident light on the positive lens becomes divergent light, the spherical aberration becomes under, and when it becomes convergent light, it becomes over. This is the change in spherical aberration generated by the action of the chromatic aberration correction element.

Since changes in spherical aberration due to these two factors appear in the same direction, they cannot be corrected by an optical system using a conventional chromatic aberration correcting element.

When the used wavelength band is narrow such that the change of the oscillation wavelength of the semiconductor laser is small, the change of spherical aberration is also small, so there is little problem, but the change of wavelength in a wider range, for example, near infrared semiconductor laser ( (780 nm) and visible red semiconductor laser (680 nm), or He-Ne laser (633 nm) and YAG laser SHG wave (532 nm) such as SHG waves (532 nm). If, for example, is used, the change in spherical aberration also becomes large, so some measure is required.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and in the case of correcting the axial chromatic aberration generated in the positive lens and switching the light beams of wavelengths which are not close to each other. Another object of the present invention is to provide a chromatic aberration correction element capable of suppressing a change in spherical aberration to be small.

[0013]

In order to achieve the above object, a chromatic aberration correcting element according to the present invention generates a divergent wavefront having an excessive spherical aberration with respect to the incidence of a parallel light beam having a wavelength shorter than the reference wavelength. However, it is characterized in that it generates a converging wavefront having spherical aberration that is under the incidence of a parallel light flux having a wavelength longer than the reference wavelength.

[0014]

Embodiments of the chromatic aberration correction element according to the present invention will be described below.

In order to correct the amount of change in spherical aberration due to the wavelength of the positive lens and the spherical aberration generated by the incidence of divergent light and convergent light on the positive lens with the chromatic aberration correction element, a surface having a chromatic aberration correction effect is used. The shape may be such that spherical aberration is generated. Therefore, the chromatic aberration correction element of the present invention generates a divergent wavefront having an excessive spherical aberration with respect to the incidence of a parallel light flux having a wavelength shorter than the reference wavelength, and has a divergence wavefront with respect to the incidence of a parallel light flux having a wavelength longer than the reference wavelength. It is configured to generate a focused wavefront with under spherical aberration.

As a concrete type of the chromatic aberration correction element, there are a refraction type constructed by bonding a positive lens and a negative lens using two kinds of materials having substantially no difference in refractive index and different dispersion at the reference wavelength. A diffractive type is conceivable in which at least one of the light incident and exit end faces is formed in a stepwise manner with a plane perpendicular to the optical axis as a concentric ring zone with respect to the optical axis. Then, in the case of the refraction type, the pasted surface, and in the case of the diffraction type, the absolute value becomes smaller as the radius of curvature of the base curve, which is a macroscopic curve of the surface formed in a stepwise manner, decreases. With such an aspherical surface, the spherical aberration described above can be generated.

Generally, low-order spherical aberration has a form of a fourth-order function with respect to the incident height. Therefore, if the surface of the color correction element is a surface having a fourth-order asphericity, the spherical aberration change is almost the same. Can be corrected. However, when an aspherical single lens is used as the positive lens to be corrected, if the aspherical surface of the chromatic aberration correction element is an aspherical surface close to a spheroidal surface having a positive conic constant, high-order spherical aberration change components It can be corrected including.

The aspherical surface close to the above-mentioned spheroid is
When the amount of deviation ε (h) from the spheroid at the point of distance h from the optical axis is expressed by equation (1), in the case of refraction type at all distances h within the effective maximum radius of the passing light beam. In
In the case of the formula (2) and the diffraction type, it is desirable that the condition of the formula (4) is satisfied.

[0019]

## EQU1 ## ε (h) = ΔX (h) -Ch ² / (1-√ (1- (1 + K) C ² h ² )) ... (1) | ε (h) | <λ / ΔnMAX ... (2 ) | Ε (h) | <λ / (n-1) (4) where ΔX (h) is the sag amount of the aspherical surface, C is the paraxial curvature,
K is the conic constant, λ is the maximum usable wavelength, ΔnMAX is the absolute value of the refractive index difference in the state where the difference in the refractive index of the media before and after the bonding surface is the largest in the used wavelength band, and n is the refractive index. Is.

In the case of a diffractive chromatic aberration correction element, the displacement amount of the base curve in the optical axis direction at a point at a distance h from the optical axis is ΔX (h), and from the optical axis of the surface formed in a stepwise manner. The displacement amount ΔX ′ (h) of the point at the distance h of is given by the equation (3).

[0021]

## EQU2 ## ΔX '(h) = (mλ0 / (n-1)) Int ((ΔX (h) / (mλ0 / (n-1))) + 0.5) (3) where m is an integer , N is the refractive index, λ0 is the wavelength using the chromatic aberration correction element or any one wavelength within the wavelength range,
Int (x) is a function that gives an integer that does not exceed x.

Expression (2) is a condition that the optical path length difference is 1λ or less when a refraction type chromatic aberration correction element is used. Similarly, Expression (4) is used when a diffraction type chromatic aberration correction element is used. ,
It is a condition that the optical path length difference is 1 λ or less, and if it exceeds the upper limit, the wavefront aberration rms value exceeds 0.1 λ and cannot be used for optical information recording / reproducing.

FIG. 1 is a lens diagram of a positive objective lens which is corrected by the chromatic aberration correction element of Example 1-3. The specific numerical configuration is shown in Table 1. In the table, NA is aperture ratio, f is focal length, ω is half angle of view, fb is back focus, r is radius of curvature, d is lens thickness or air gap, n
i is the refractive index at the wavelength inm, and ν is the Abbe number. In addition,
The first and second surfaces are objective lenses having aspherical surfaces on both sides, and the third and fourth surfaces are cover glasses for optical disks.

The aspherical surface is defined by a distance X from the tangent plane of the aspherical vertex of a coordinate point on the aspherical surface having a height Y from the optical axis, a curvature (1 / r) of the aspherical vertex C, and a cone. Coefficients K, 4th, 6th, 8
Next, the tenth-order aspherical surface coefficients are represented by the following formulas with A4, A6, A8, and A10.

[0025]

[Equation 3] X = (CY ² / (1 + √ (1- (1 + K) C ² Y ² ))) + A4Y ⁴ + A6Y ⁶ + A8Y ⁸ + A10Y ¹⁰

Table 2 shows the conical coefficient and the aspherical coefficient. Fig. 2 shows spherical aberration SA, sine condition SC, wavelength 780n.
7 shows the chromatic aberration exhibited by spherical aberration at m, 680 nm.

[0027]

[Table 1] NA = 0.55 f = 3.00 ω = 1.4 ° fb = 1.088 Surface number rd n588 ν n780 n680 1 1.894 2.200 1.49700 81.6 1.49282 1.49461 2 -4.186 1.088 3 ∞ 1.200 1.58547 29.9 1.57346 1.57834 4 ∞

[0028]

[Table 2]

[0029]

EXAMPLE 1 FIG. 3 shows an optical system in which the refractive chromatic aberration correction element according to Example 1 of the present invention is combined with the objective lens shown in FIG. In the chromatic aberration correction element of Example 1, the bonding surface r2 is an elliptical surface, and ε (h) is 0 within the effective diameter. The specific numerical configuration of this optical system is shown in Table 3. The first to third surfaces are chromatic aberration correction elements, and the fourth and fifth surfaces are
The surface is the objective lens, and the sixth and seventh surfaces are the cover glass of the optical disk. In this example, the second, fourth, and fifth surfaces are aspherical surfaces, and their aspherical surface coefficients are shown in Table 4. Figure 4
Shows spherical aberration and chromatic aberration due to this configuration.

[0030]

[Table 3] FNO = 1: 0.9 f = 3.00 ω = 1.4 ° fb = 0.00 Surface number r d n588 ν n780 n680 1 ∞ 2.000 1.75500 52.3 1.74523 1.74940 2 -4.400 1.000 1.76182 26.5 1.74404 1.75132 3 ∞ arbitrary 4 1.894 2.200 1.49700 81.6 1.49282 1.49461 5 -4.186 1.088 6 ∞ 1.200 1.58547 29.9 1.57346 1.57834 7 ∞

[0031]

[Table 4] 4th surface 5th surface 2nd surface K = -0.5800 K = 0.0000 K = 0.2500 × 10 A4 = 0.7540 × 10 ^-3 A4 = 0.3250 × 10 ^-1 A6 = -0.3670 × 10 ^-4 A6 =- 0.1000 x 10 ^-1 A8 = 0.2800 x 10 ^-4 A8 = 0.2000 x 10 ^-2 A10 = -0.3600 x 10 ^-4 A10 = -0.1820 x 10 ^-3

FIG. 5 shows a configuration similar to that of the first embodiment.
An optical system in which the bonding surface r2 is a spherical surface is shown, and FIG. 6 shows spherical aberration and chromatic aberration by the optical system in FIG. 4 and 6
By comparing with, it can be understood that by changing the bonding surface from a spherical surface to an elliptical surface, the amount of change in spherical aberration due to wavelength fluctuation is reduced.

[0033]

Second Embodiment FIG. 7 shows an optical system in which a diffractive chromatic aberration correction element is combined with the objective lens shown in FIG. As shown in FIGS. 8 (a) and 8 (b), the diffractive chromatic aberration correction element is formed by forming a plane perpendicular to the optical axis in a stepwise manner as concentric ring zones with respect to the optical axis. ing.

Table 5 shows the configuration of an optical system in which the diffractive chromatic aberration correction element of Example 2 is combined with the objective lens shown in FIG. In this chromatic aberration correcting element, the base curve, which is a macroscopic curve of the stepwise formed surface r1, is a quaternary aspherical surface. FIG. 9 shows spherical aberration and chromatic aberration due to this configuration, respectively.

In this example, the first, third and fourth surfaces are aspherical surfaces, and their aspherical surface coefficients are shown in Table 6.

[0036]

[Table 5] FNO = 1: 0.9 f = 3.00 ω = 1.4 ° fb = 0.00 Surface number r d n588 ν n780 n680 1 -104.400 1.000 1.51633 64.1 1.51072 1.51315 2 ∞ Arbitrary 3 1.894 2.200 1.49700 81.6 1.49282 1.49461 4 -4.186 1.090 5 ∞ 1.200 1.58547 29.9 1.57346 1.57834 6 ∞

[0037]

[Table 6] Third surface Fourth surface First surface K = -0.5800 K = 0.0000 K = 0.0000 A4 = 0.7540 × 10 ^-3 A4 = 0.3250 × 10 ^-1 A4 = -0.3400 × 10 ^-3 A6 = -0.3670 × 10 ^-4 A6 = -0.1000 x 10 ^-1 A8 = 0.2800 x 10 ^-4 A8 = 0.2000 x 10 ^-2 A10 = -0.3600 x 10 ^-4 A10 = -0.1820 x 10 ^-3

[0038]

[Embodiment 3] Table 7 shows the chromatic aberration correction element of Embodiment 3 shown in FIG.
The structure of an optical system combined with the objective lens shown in FIG.
The base curve, which is a macroscopic curve of the stepped surface r1 of the chromatic aberration correction element, is an elliptical surface, and ε (h) is 0 within the effective diameter. FIG. 10 shows spherical aberration and chromatic aberration due to this configuration, respectively. In this example, the first,
The third and fourth surfaces are aspherical surfaces, and their aspherical surface coefficients are shown in Table 8.
Is shown in.

[0039]

[Table 7] FNO = 1: 0.9 f = 3.00 ω = 1.4 ° fb = 0.00 Surface number rd n588 ν n780 n680 1 -104.400 1.000 1.51633 64.1 1.51072 1.51315 2 ∞ Arbitrary 3 1.894 2.200 1.49700 81.6 1.49282 1.49461 4 -4.186 5 ∞ 1.200 1.58547 29.9 1.57346 1.57834 6 ∞

[0040]

[Table 8] 3rd surface 4th surface 1st surface K = -0.5800 K = 0.0000 K = 0.2000 × 10 ⁺⁴ A4 = 0.7540 × 10 ^-3 A4 = 0.3250 × 10 ^-1 A6 = -0.3670 × 10 ^-4 A6 = -0.1000 × 10 ^-1 A8 = 0.2800 × 10 ^-4 A8 = 0.2000 × 10 ^-2 A10 = -0.3600 × 10 ^-4 A10 = -0.1820 × 10 ^-3

FIG. 11 shows a configuration similar to that of the second and third embodiments.
The spherical aberration and chromatic aberration when the base curve of the surface formed in a step shape is a spherical surface are shown. By comparing with FIGS. 9 and 10, it can be understood that the fourth-order aspherical surface and the elliptic surface make it possible to suppress the wavelength variation of spherical aberration smaller than that in the case of a spherical surface.

FIG. 12 shows a positive single lens having aspherical surfaces on both sides which is to be corrected by the chromatic aberration correcting element of Example 4-5. Specific numerical configurations are shown in Tables 9 and 10. Spherical aberration of this lens alone and its wavelength 633n
The chromatic aberration represented by spherical aberration at m and 532 nm is shown in FIG.
Shown in.

[0043]

[Table 9]

[0044]

[Table 10]

[0045]

[Embodiment 4] FIG. 14 is an explanatory diagram of an optical system in which a refractive chromatic aberration correction element according to Embodiment 4 of the present invention is combined with the objective lens shown in FIG. Specific numerical configurations are shown in Tables 11 and 12. The bonding surface r2 of the chromatic aberration correction element is an elliptical surface, and ε (h) is 0 within the effective diameter. FIG. 15 shows spherical aberration and chromatic aberration with this configuration.

[0046]

[Table 11] FNO = 1: 0.9 f = 3.29 ω = 1.7 ° fb = 0.00 Surface number r d n588 ν n633 n532 1 ∞ 0.800 1.74077 27.8 1.73541 1.74959 2 2.280 2.000 1.74100 52.7 1.73804 1.74567 3 ∞ arbitrary 4 2.180 2.250 1.54358 55.6 1.54151 1.54680 5 -6.250 1.332 6 ∞ 1.200 1.58547 29.9 1.58156 1.59194 7 ∞

[0047]

[Table 12] 4th surface 5th surface 2nd surface K = -0.3265 K = 0.0000 K = 0.6000 A4 = -0.2263 × 10 ^-2 A4 = 0.1670 × 10 ^-1 A6 = -0.5014 × 10 ^-3 A6 = -0.5080 × 10 ^-2 A8 = -0.7162 × 10 ^-5 A8 = 0.8000 × 10 ^-3 A10 = -0.3 194 × 10 ^-4 A10 = -0.4848 × 10 ^-4

FIG. 16 shows an optical system having the same structure as that described above, in which the bonding surface r2 of the chromatic aberration correction element is spherical.
Reference numeral 7 indicates the spherical aberration. By adopting the elliptical surface, the shapes of spherical aberrations at both wavelengths can be made close to each other and the amount of spherical aberrations can be reduced as a whole.

[0049]

[Embodiment 5] FIG. 18 is an explanatory view of an optical system in which a diffractive chromatic aberration correction element according to Embodiment 5 of the present invention and the objective lens shown in FIG. 12 are combined. Shown at 11 and 12. In this example, the base curve of the step-shaped surface of the chromatic aberration correction element is an elliptical surface, and ε (h) is 0 within the effective diameter. The spherical aberration and chromatic aberration due to this configuration are shown in FIG.

[0050]

[Table 13] f = 3.29 ω = 1.7 ° fb = 0.00 Surface number r d n588 ν n633 n532 1 ∞ 2.000 1.51633 64.1 1.51462 1.51900 2 41.000 arbitrary 3 2.180 2.250 1.54358 55.6 1.54151 1.54680 4 -6.250 1.341 5 ∞ 1.200 1.58547 29.9 1.58156 1.59194 6 ∞

[0051]

[Table 14] 3rd surface 4th surface 1st surface K = -0.3265 K = 0.0000 K = 0.2450 x 10 ⁺³ A4 = -0.2263 x 10 ^-2 A4 = 0.1670 x 10 ^-1 A6 = -0.5014 x 10 ^-3 A6 = -0.5080 x 10 ^-2 A8 = -0.7162 x 10 ^-5 A8 = 0.8000 x 10 ^-3 A10 = -0.3194 x 10 ^-4 A10 = -0.4848 x 10 ^-4

FIG. 20 shows spherical aberration and chromatic aberration when the base curve of the stepwise surface of the chromatic aberration correction element is spherical with the same configuration as in the above-mentioned fifth embodiment. By comparing FIG. 19 and FIG. 20, it can be understood that the variation of the spherical aberration can be suppressed by making the base curve an elliptical surface.

[0053]

As described above, according to the present invention, not only the axial chromatic aberration of the converging lens due to the fluctuation of the wavelength can be corrected but also the fluctuation of the spherical aberration can be suppressed. Thus, it is possible to suppress the change in the performance of the optical system due to the change in wavelength.

Therefore, a lens whose chromatic aberration has not been corrected can be used in an optical information recording device using two wavelengths relatively distant from each other and an information reading device using a light emitting diode or a white light source, and the optical system can be made compact. be able to.

[Brief description of drawings]

FIG. 1 is a lens diagram of a positive objective lens that is corrected by a chromatic aberration correction element of Example 1-3.

FIG. 2 is a diagram of spherical aberration and chromatic aberration of the objective lens shown in FIG. 1 alone.

FIG. 3 is a lens diagram of an optical system in which the lens shown in FIG. 1 is combined with the refraction type chromatic aberration correction element of Example 1 in which the bonding surface is an elliptical surface.

FIG. 4 is a spherical aberration and chromatic aberration diagram of the optical system shown in FIG.

5 is a lens diagram of an optical system in which the objective lens shown in FIG. 1 and a refraction type chromatic aberration correction element having a spherical bonding surface are combined.

FIG. 6 is a diagram of spherical aberration and chromatic aberration of the optical system shown in FIG.

FIG. 7 is a lens diagram of an optical system in which the lens shown in FIG. 1 and a diffractive chromatic aberration correction element are combined.

8A and 8B are explanatory views showing a configuration of a stepped surface of the diffractive chromatic aberration correction element, wherein FIG. 8A is a side view and FIG. 8B is a plan view.

9 is a diagram of spherical aberration and chromatic aberration in the case where the diffractive chromatic aberration correction element of Example 2 in which the base curve of the stepped surface is an elliptical surface is used in the optical system shown in FIG.

FIG. 10 is a spherical aberration diagram and a chromatic aberration diagram when the diffractive chromatic aberration correction element of Example 3 in which the base curve of the stepped surface is a fourth-order aspherical surface is used in the optical system shown in FIG. 7.

FIG. 11 is a diagram of spherical aberration and chromatic aberration when a diffractive chromatic aberration correction element in which the base curve of the stepped surface is a spherical surface is used in the optical system shown in FIG. 7.

FIG. 12 is a lens diagram of a positive objective lens that is corrected by the chromatic aberration correction elements of Examples 4 and 5.

13 is a diagram of spherical aberration and chromatic aberration of the objective lens shown in FIG. 12 alone.

FIG. 14 is a lens diagram of an optical system in which the objective lens shown in FIG. 12 and a refraction type chromatic aberration correction element having a spherical bonding surface are combined.

15 is a diagram of spherical aberration and chromatic aberration of the optical system shown in FIG.

16 is a lens diagram of an optical system in which the lens shown in FIG. 12 is combined with a refraction type chromatic aberration correction element of Example 4 in which the bonding surface is an elliptical surface.

17 is a diagram of spherical aberration and chromatic aberration of the optical system shown in FIG.

FIG. 18 is a lens diagram of an optical system in which the lens shown in FIG. 12 and a diffractive chromatic aberration correction element are combined.

FIG. 19 is a spherical aberration diagram and a chromatic aberration diagram in the case where the diffractive chromatic aberration correction element of Example 5 in which the base curve of the stepped surface is an elliptical surface is used in the optical system shown in FIG.

20 is a diagram of spherical aberration and chromatic aberration in the case where a diffractive chromatic aberration correction element in which the base curve of the stepped surface is a spherical surface is used in the optical system shown in FIG.

Claims (5)

[Claims] 1. A divergent wavefront having an excessive spherical aberration with respect to incidence of a parallel light flux having a wavelength shorter than a reference wavelength,
A chromatic aberration compensating element, which generates a condensed wavefront having spherical aberration which is under the incidence of a parallel light flux having a wavelength longer than the reference wavelength. 2. A positive lens and a negative lens, which are made of materials having substantially no difference in refractive index and different dispersions at the reference wavelength, are bonded to each other, and the bonding surface has a radius of curvature that is farther from the optical axis. The chromatic aberration correction element according to claim 1, wherein the chromatic aberration correction element is an aspherical surface having a small absolute value. 3. The bonding surface is an aspherical surface close to a spheroid having a positive elliptic constant, and a deviation amount ε (h) from the spheroid at a point of a distance h from the optical axis is (1 The chromatic aberration correction element according to claim 2, wherein the condition of the expression (2) is satisfied at all distances h within the effective maximum radius of the light flux passing through when expressed by the expression (4). ε (h) = ΔX (h) -Ch ² / (1-√ (1- (1 + K) C ² h ² )) ... (1) | ε (h) | <λ / ΔnMAX ... (2) where ΔX (h) is the amount of sag on the aspherical surface, C is the paraxial curvature, K is the conical constant, λ is the maximum usable wavelength, ΔnMAX is the difference in the refractive index of the media before and after the bonding surface within the usable wavelength band. Is the absolute value of the refractive index difference in the largest state. 4. A diffractive chromatic aberration compensating element in which at least one of the light incident and exit end faces is stepwise formed by using a plane perpendicular to the optical axis as a concentric ring zone with respect to the optical axis. The base curve, which is a macroscopic curve of the surface formed in a shape, is an aspherical surface whose absolute value decreases as the radius of curvature thereof moves away from the optical axis, and the base curve in the optical axis direction at a distance h from the optical axis. The displacement amount ΔX ′ (h) at a point of a distance h from the optical axis of the stepwise surface is given by the equation (3), where the displacement amount of the base curve is ΔX (h). A chromatic aberration correction element. ΔX '(h) = (mλ0 / (n-1)) Int ((ΔX (h) / (mλ0 / (n-1))) + 0.5) (3) where m is an integer and n is The refractive index, λ0, is a wavelength using the chromatic aberration correction element or any one wavelength within the wavelength range, and Int (x) is a function that gives an integer not exceeding x. 5. The base curve is an aspherical surface close to a spheroid having a positive conic constant, and the deviation amount ε (h) from the spheroid at a point of a distance h from the optical axis is (1). The chromatic aberration correction element according to claim 4, wherein when expressed by the formula, the condition of the formula (4) is satisfied at all distances h within the effective maximum radius of the passing light beam. ε (h) = ΔX (h) -Ch ² / (1-√ (1- (1 + K) C ² h ² )) ... (1) | ε (h) | <λ / (n-1) ... (4 However, C is a paraxial curvature, K is a conic constant, and (lambda) is the maximum usable wavelength.