JPH0210403B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is a condensing lens for concentrating a coherent parallel beam of light such as a laser into a very small spot, especially a condensing lens with a long back focus compared to the focal length. The present invention relates to a so-called achromatic high-resolution condensing lens optical system that has a sufficient condensing effect even when light beams are used simultaneously. The configuration of the present invention includes, in order from the light incident side, a first lens that is a negative lens, a second lens that is a negative lens, a third lens that is a positive lens, a fourth lens that is a positive lens, a fifth lens that is a negative lens, and a positive lens. A lens consisting of six lenses consisting of the sixth lens, the second lens and the third lens being laminated, satisfying the following conditions,
The present invention is also a high-resolution condensing lens optical system characterized by a back focus that is 1.2 times or more the focal length. (1) 8.0>｜f ₁ |/f>3.0, f ₁ <0 (2) f/f ₂₃ <0.1 (3) 0.9> ₂₃・(d ₃ + d ₄ )/f>0.6 (4) 1.2> ₂₃・(r ₃ − r ₅ ) / (d ₃ + d ₄ )＞0.8 (5) n ₅ −n ₄ ＞0.2 However, f: Focal length of the entire system f ₁ : Focal length of the first lens f ₂₃ : Second lens and the combined focal length of the third lens ₂₃ : Average value of the refractive index of the second lens and the third lens d ₃ : Thickness of the second lens d ₄ : Thickness of the third lens r ₃ : Value of the light of the second lens Radius of curvature of the incident side surface r ₅ : Radius of curvature of the light exit side of the third lens n ₄ : Refractive index of the glass material of the fourth lens n ₅ : Refractive index of the glass material of the fifth lens The present invention focuses on back focus rather than focal length. In order to make the lens longer, a negative lens was placed as the first lens to create a so-called retrofocus type configuration in which a negative lens was placed in the front group and a positive lens was placed in the rear group. In addition, the second and third lenses are negative and positive lenses, respectively.
By laminating the sheets together to have an achromatic effect and a spherical aberration correction effect, and by adding a certain amount of thickness, a long back focus can be obtained.
The aim was to simultaneously shorten the overall length of the lens. In addition, two positive lenses are arranged in the fourth lens and the sixth lens so that the fourth lens to the sixth lens have a positive focal length as a whole, and a positive lens is arranged in the fourth lens and the sixth lens to correct chromatic aberration and spherical aberration. There are 5 negative lenses. Condition (1) relates to the focal length of the first lens. conditions
If the upper limit of (1) is exceeded, the refractive power of the first lens will become too weak, making it difficult to obtain a long back focus for the entire system, or it will be necessary to increase the overall length of the lens in order to obtain the desired back focus. This results in a disadvantage in terms of size. If the lower limit is exceeded, the refractive power of the first lens becomes stronger, which is advantageous for lengthening the back focus, but over-corrected spherical aberration and chromatic aberration occur, making it difficult to maintain high resolving power in the entire system. Become. Conditions (2) to (4) represent the conditions that the second lens and the third lens must satisfy, and condition (2) represents the conditions that the second lens and the third lens must satisfy.
Regarding the composite focal length of the lens. If condition (2) is violated, the combined focal length of the second and third lenses becomes short, the action as a positive lens becomes too strong, the effect of retrofocus weakens, and it becomes difficult to maintain backfocus for a long time. . Condition (3) relates to the thickness of the second lens and the third lens, particularly the average optical thickness of the second lens and the third lens. By giving a diverging effect to the light incident side surface of the second lens and satisfying condition (3), the back focus can be lengthened by pushing the principal point toward the image side, and the total length of the lens can be shortened. is intended. If the upper limit of condition (3) is exceeded, the second
The optical thickness of the lens and the third lens increases, making it easier to ensure a long back focus, but it also increases the overall lens length and outer diameter, and increases the occurrence of overcorrected spherical aberration. This is undesirable as it has disadvantages. On the other hand, if the lower limit is exceeded, it becomes difficult to maintain the back focus for a long time. Condition (4) is a condition for defining the relationship between the radii of curvature of the surfaces of the second lens and the third lens that are in contact with the air. Due to this condition (4), the light incident side surface _r3 of the second lens and the light exit side surface _r5 of the third lens are kept in a state in which their respective centers of curvature are close to each other, that is, in a concentric state. The third surface and the fifth surface each share the opposite aberration correction function, thereby maintaining a highly aberration correction state. If the upper limit of condition (4) is exceeded, the third surface becomes too tight compared to the fifth surface, and overcorrected spherical aberration remains. On the other hand, if the lower limit is exceeded, the fifth surface becomes too tight compared to the third surface, and undercorrected spherical aberration remains, which is undesirable. The above three conditions (2) to (4) work together to define the configuration of the second lens and the third lens, ensuring a long back focus compared to the focal length and maintaining high resolution performance. This contributes to achieving both. Condition (5) defines the difference in refractive index of the glass materials used for the fourth lens and the fifth lens. That is, by making the refractive index of the glass material of the fifth lens larger than the refractive index of the glass material of the fourth lens, an effect is created to overcorrect spherical aberration and chromatic aberration at the boundary between the fourth lens and the fifth lens. This is to function to satisfactorily correct the undercorrected spherical aberration and chromatic aberration that occur in the fourth and sixth lenses of the positive lens. If this condition (5) is violated, the overcorrection effect at the boundary between the fourth and fifth lenses will fade, and overall undercorrected spherical aberration and chromatic aberration will remain, making it difficult to maintain high performance. Become. Furthermore, in order to more highly correct chromatic aberration in the entire system, it is desirable that the Abbe number of the glass material used for the fourth lens be larger than the Abbe number of the glass material used for the fifth lens. Note that in some cases, a parallel plane plate made of glass, quartz, etc. may be inserted between the lens system and the image plane for the purpose of protecting the lens system. This does not depart from the spirit of the invention and is included in the present invention. Below, numerical values of Examples of the present invention are shown. here,
F _NO is the F number, f is the focal length of the entire system, f _B is the back focus (for Examples 4, 8, and 9, when the parallel plane plate is excluded), r is the radius of curvature, d is the lens thickness or lens spacing, n is the refractive index for the F-line, and v is the Abbe number.

[Example 1]

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[Example 2]

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[Example 3]

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[Example 4]

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[Example 5]

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[Example 6]

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[Example 7]

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[Example 8]

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[Example 9]

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【table】 [Brief explanation of drawings]

1st, 3rd, 5th, 7th, 9th, 11th, 13th, 15th, 1st
Figure 7 shows Examples 1, 2, 3, 4, and 7 of the present invention, respectively.
5, 6, 7, 8, 9 lens configuration diagram, 2nd, 4th,
Figures 6, 8, 10, 12, 14, 16, and 18 show Examples 1, 2, 3, 4, 5, 6, and 1 of the present invention, respectively.
7, 8, and 9 aberration diagrams.

Claims (1)

[Claims] 1. In order from the light incident side, a first negative lens, a second negative lens, a third positive lens, a fourth positive lens, a fifth negative lens, and A six-element lens consisting of a positive sixth lens, the second and third lenses being laminated together, satisfying the following conditions, and having a back focus of at least 1.2 times the focal length. Features a high-resolution condensing lens optical system. (1) 8.0>｜f ₁ |/f>3.0, f ₁ <0 (2) f/f ₂₃ <0.1 (3) 0.9> ₂₃・(d ₃ + d ₄ )/f>0.6 (4) 1.2> ₂₃・(r ₃ − r ₅ ) / (d ₃ + d ₄ )＞0.8 (5) n ₅ −n ₄ ＞0.2 However, f: Focal length of the entire system f ₁ : Focal length of the first lens f ₂₃ : Second lens and the combined focal length of the third lens ₂₃ : Average value of the refractive index of the second lens and the third lens d ₃ : Thickness of the second lens d ₄ : Thickness of the third lens r ₃ : Value of the light of the second lens Radius of curvature of the incident side surface r ₅ : Radius of curvature of the light exit side of the third lens n ₄ : Refractive index of the glass material of the fourth lens n ₅ : Refractive index of the glass material of the fifth lens