JPH0727113B2 - Objective lens system with temperature compensation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is used at a finite magnification of a copying machine or the like in which a convex lens made of a plastic lens and a concave lens are used together to reduce fluctuations in optical characteristics due to temperature changes. The present invention relates to a symmetrical objective lens suitable for use in

(Prior Art) Conventionally, a lens system used in a copying machine or the like is replaced with an optical glass in order to meet the demand for cost reduction.
The use of plastic has been considered.

However, since the plastic material has a change in the refractive index due to temperature and has a large thermal expansion, the optical performance of the lens system is remarkably changed only by replacing the glass material with the plastic material. In particular, in a lens system used in a copying machine or the like, since the object surface and the photosensitive material surface (imaging surface) are fixed, the focal length of the lens system changes due to temperature changes, and the conjugate distance changes greatly. As a result, the set photosensitive material surface and the actual image forming surface that should coincide with it are greatly separated, which makes practical application difficult.

As a conventional technique, as a means for compensating for a change in optical characteristics due to temperature, there is a method of changing a lens interval according to temperature in order to correct a movement of a focus.
-202507, 57-202508, etc. are described.

However, if the lens interval is changed as described in the above-mentioned known technique in order to correct the movement of the focal point due to the temperature change, the structure of the lens barrel becomes complicated and a large space is required, resulting in a high cost. Therefore, it is not possible to obtain the merit of making the lens plastic and making it cheaper.

Further, if the lens interval is changed in order to correct the movement of the focal point due to the temperature change, the focal point position can be corrected, but the aberration such as the field curvature is deteriorated, and thus the imaging performance is deteriorated. Also occurs. Furthermore, it is difficult to maintain the matching accuracy of the optical axes of the lenses when changing the lens spacing.

(Problems to be Solved by the Invention) The present invention is free from the drawbacks and problems described above.
It is intended to provide an objective lens using a plastic material.

Generally, when a plastic material is used for a lens having a positive power, the refractive index of the plastic material becomes smaller as the temperature rises, and the radius of curvature becomes larger due to thermal expansion. Smaller). Therefore, if the imaging lens system includes a positive power plastic lens, the plastic lens is
It acts to increase the focal length of this imaging lens system when the temperature rises. Also, when a plastic material is used for a lens of negative power, the refractive index of the plastic material becomes smaller as the temperature rises, and the radius of curvature becomes larger due to thermal expansion, so the negative power becomes smaller,
Therefore, if the imaging lens system includes a negative power plastic lens, the plastic lens acts to reduce the focal length of the entire imaging lens system when the temperature rises.

Therefore, it is possible to use a positive power plastic lens and a negative power plastic lens together as part of the lens system to compensate for the effect of changing the focal length.

(Means for Solving Problems) According to the present invention, in order to solve the above problems, an objective lens including a plurality of lens elements, at least a part of which uses a plastic lens, The objective lens system is constructed by appropriately selecting and combining the refractive index of these plastic lens materials at room temperature, the temperature change coefficient at room temperature, and the focal length at room temperature as well. The objective lens system is capable of compensating for the deterioration of

(Embodiment) FIG. 1 shows a basic configuration of an embodiment of an objective lens system according to the present invention. In the figure, the objective lens system can be used, for example, as a lens for a copying machine, and in order from the object side, both convex surfaces r ₁ and r ₂ are a positive meniscus first lens and a convex surface r. ₃ , a front lens group consisting of three lenses of a negative meniscus second lens facing r ₄ and a positive meniscus third lens facing convex surfaces r ₅ and r ₆ ,
Disposed symmetrical relative to the rear in the arranged aperture of the third lens, fourth, fifth and sixth group six lens formed by the group after the sixth lens having a surface r ₇ ~r ₁₂ Consists of.

Tables 1 to 5 show five examples in the case where the present invention is applied to a lens for a copying machine with F = 6.3 and f = 100 mm, and all of them are the first lens, the second lens and the fifth lens. As the sixth lens, a lens made of a plastic material as shown in each table is used. That is, glass such as LLF2, BaLF6, SF11, and SF4 is used as the material of the third lens and the fourth lens, and the first lens, the sixth lens, the second lens, and the fifth lens are used as products for the respective products. Name "Sebian MAS2
0 ”, acrylic, and trade name“ Panlite AD-550
3 ", plastic name" Sebian MAS20 "are used in appropriate combination.

Here, F: lens aperture ratio f: focal length of entire lens system r: radius of curvature of lens surface d: lens thickness or air gap Nd: refractive index of lens to d-line νd: Abbe number of lens The coefficient of temperature change of the refractive index of the lens is × 10 ^-5 / ° C α: The coefficient of linear expansion of the lens is × 10 ^-5 cm / cm / ° C f ₁ : The room temperature of the first and sixth lenses alone (20 Focal length f ₂ at the temperature f): focal length of the second and fifth lenses alone at room temperature (20 ° C.) N ₁ : refractive index of the first and sixth lenses at room temperature (20 ° C) N ₂ : second And the refractive index of the fifth lens at room temperature (20 ℃) The temperature change coefficient of the refractive index of the first and sixth lenses, the unit of which is ×
10 ^-5 / ℃ The temperature change coefficient of the refractive index of the first and sixth lenses, the unit of which is ×
At 10 ^-5 / ℃, To satisfy N ₁ , N ₂ , f ₁ , f ₂ , Select.

By doing so, the first and sixth lenses made of a plastic material having positive power are
Since the refractive index decreases and the radius of curvature increases due to thermal expansion, it acts to increase the focal length of the entire objective lens system, and the second and fifth lenses made of a plastic material having negative power also increase in temperature. At times, the refractive index becomes smaller, the radius of curvature becomes larger, and the focal length of the entire system becomes smaller.

The above equation is intended to compensate for the shift of the focal point movement due to the temperature change of the objective lens system of the present invention by skillfully utilizing the above relation. In the above formula, if the upper limit of the formula is exceeded, the conjugation distance becomes longer when the temperature rises, and if the lower limit of the formula is exceeded, the conjugation distance becomes shorter when the temperature rises. When f changes, the image forming position largely deviates from a predetermined image forming surface (focal plane at room temperature).

The amount of movement of the imaging position due to temperature fluctuations is within the operating temperature range.
It is necessary to keep it within about half of the resolution depth, but normally F
= About 6.3, the resolution depth required for a lens for a copying machine used at an imaging magnification of about 1 × is about 3 to 4 mm.
It is necessary to keep the amount of movement of the imaging position due to temperature fluctuations to about 1.5 to 2 mm.

Tables 1 to 5 show the calculation results of the above equation in the lens system having the optical values shown in the table. According to this, the result when the lens focus processing is 100 mm and the temperature variation is 30 ° C. The movement of the image position is within ± 1.6 mm, which is within about half of the above resolution.

In addition, with respect to the examples of Tables 1 to 5 The value of is within the range of -0.998 and -0.721, but the installation and use environment of the copying machine can be sufficiently practical even if the temperature change from room temperature is about 20 ° C, and the above value is the usual value for optical design. ± 10%
The conditional expression in the claims is set to include the variation range of.

FIGS. 2 (A) to 6 (A) are magnifications at room temperature (20 ° C.) × 1 of each example of Tables 1 to 5 based on the present invention.
Aberration diagrams and Fig. 2 (B) to Fig. 6 (B)
Shows aberration diagrams of each example of Tables 1 to 5 based on the present invention at a magnification of 1 at 50 ° C., and the variation of aberration is slight even if the temperature varies by 30 ° C. Is clear.

(Effect of the Invention) As described above, according to the present invention, a part of the lens having positive power and negative power in the objective lens system configured by a plurality of lens elements is used as a plastic lens element, and the temperature fluctuation of each lens is achieved. Since the focal point movement due to the change of the focal length due to is compensated, the lens barrel does not need to have a complicated structure as in the conventional example, the cost is low, and the temperature compensation is performed without substantially deteriorating the optical performance. It is possible to provide an objective lens system capable of doing so. In the slit exposure type copying machine lens, there is an increasing number of cases in which a portion of the lens in the direction perpendicular to the longitudinal direction of the slit where light does not pass is processed into a strip shape in order to save space. Regarding this, when a conventional glass lens is used, a method has been adopted in which a lens having a large lens diameter outside the lens system is polished into a circular shape and then cut to obtain a strip shape. However, when a plastic material is used as in the present invention, it can be molded into a desired shape, the cutting step can be omitted, and in some cases, the lens barrel can be integrally molded. Also from this point, it is possible to obtain an extremely high implementation effect such as providing an objective lens system that achieves cost reduction.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram of an objective lens system in which a plastic lens having positive and negative powers of the present invention is used as a constituent element, and FIGS. 2 to 6 (A) are respectively Table 1 according to the present invention. -Aberration diagrams of each example in Table 5 at the same magnification at normal temperature and (B) of Figs. 2 to 6 are Table 1 according to the present invention.
FIG. 6 is an aberration diagram of each example in Table 5 at the same magnification at 50 ° C. _{r 1, r 2, r 3} , r 4, ~r 11, r 12: the curvature of the first to sixth lens radius,
_{d 1, d 2, d 3} ... d 11: lens thickness or air spacing,, f: the focal length of the entire lens system, ▲ ▼: lens conjugate distance of magnification when the entire system.

Claims (2)

[Claims] 1. A front lens group comprising three positive lens elements, a negative second lens element, and a positive third lens element in order from the object side, and a rear lens element disposed behind the third lens element. In an objective lens system that is composed of a rear group composed of fourth, fifth and sixth lenses and is arranged symmetrically with respect to each other, and uses a plastic material for the first, second, fifth and sixth lenses, The refractive index of the plastic materials used for the 1st and 6th lenses at room temperature is N ₁ , and the temperature change coefficient of the refractive index is The focal length of the first and sixth lenses at room temperature is f ₁ , the refractive index of the plastic materials used for the second and fifth lenses at room temperature is N ₂ , and the temperature change coefficient of the refractive index is When the focal length of the second and fifth lenses at room temperature is f ₂ , A temperature-compensated objective lens system characterized by satisfying the following conditions. 2. A front lens group comprising a positive meniscus first lens element having a convex surface directed toward the object side, and a negative meniscus second lens element and a positive meniscus third lens element in three groups, three lenses in order from the object side. , A rear group of the fourth, fifth, and sixth lenses, which is disposed behind the third lens system and is symmetrical with respect to the diaphragm, and includes the first, second, fifth, and sixth lenses. In an objective lens system using a plastic material for the lens, the refractive index of the plastic material used for the first and sixth lenses at room temperature is N ₁ , and the temperature change coefficient of the refractive index is The focal length of the first and sixth lenses at room temperature is f ₁ , the refractive index of the plastic materials used for the second and fifth lenses at room temperature is N ₂ , and the temperature change coefficient of the refractive index is When the focal length of the second and fifth lenses at room temperature is f ₂ , A temperature-compensated objective lens system characterized by satisfying the following conditions.