JPH0968649A - Optical system for reading - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low cost optical system for reading in which an F number is made brighter, a half view angle ω is made wider, various aberrations are excellently compensated for over the wide temperature range of 0 deg.C to 65 deg.C and high contrast is obtained in a high spatial frequency region. SOLUTION: In order to reduce the cost, plastic lenses are used for a first lens L1 and a second lens L2 of an optical system constituted by three groups of six lenses. The fluctuation of the focal distance of the plastic lenses caused by the variation of the environmental temperature is excellently compensated for by the appropriate combination of positive and negative lenses. Moreover, by selecting and setting appropriate values for the refractive indexes of the shapes and materials used for six single lenses L1 to L2 and the Abbe constant, sphericality, astigmatism, distortion, coma and sine conditions are excellently compensated for while keeping a near 100% aperture efficiency to the vicinity of the peripheral section of the view angle and the balance between on and off axis aberrations in properly kept.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technical field of an image forming optical system using a plastic lens, and more specifically, it is used for a document reading section of a facsimile or a digital copying machine and various image scanners. The present invention relates to a reading optical system.

[0002]

2. Description of the Related Art A reading optical system used in a document reading section of a facsimile or a digital copying machine, or a reading optical system used in various image scanners is like a CCD in a state where image information to be read is reduced. The desired image information is converted into a signal by forming an image on a solid-state image sensor.

In general, such an optical system is required to have a high contrast performance in a high spatial frequency region on the image plane, and to be able to produce an aperture efficiency close to 100% to the peripheral portion of the angle of view and a low cost. In addition, at low temperature ~
Even if the environmental temperature fluctuates over a range from the normal temperature (standard) state to the high temperature state, it is usually required that the deterioration of the optical performance and the change in aberration correction due to the fluctuation do not occur.

It is a facsimile, a digital copying machine,
In the field of office equipment such as image scanners, there is a demand for downsizing of the device itself, and therefore it is general that the device itself is designed and manufactured in a smaller size. This is because a bright light source is used to illuminate, and therefore there is a situation in which temperature fluctuations inside the device due to heat generation from the light source become extremely large. In this case, the fluctuation range of the environmental temperature varies greatly depending on the difference in whether the area where the device is used is cold or warm and the season, but it is generally about 0 ° C at low temperature and at room temperature. It may be considered that it is 20 ° C at times and about 65 ° C at high temperature.

Needless to say, in order to provide the optical system at a low cost, it is advantageous to plasticize all or a part of the lenses constituting the optical system. Moreover, in the case of a plastic lens, since it is easy to make an aspherical surface, it is also advantageous for imparting high optical performance. However, the optical plastic material has a great disadvantage that it is significantly affected by the ambient temperature as compared with the optical glass material. Therefore, when designing a reading optical system, it is suitable for removing various effects caused by temperature. New treatment is needed.

For this reason, a technique for temperature compensation when using a plastic lens has already been proposed. For example, the technique disclosed in Japanese Patent Laid-Open No. 63-147122 is that. According to this conventional technique, temperature compensation is performed by utilizing a phenomenon in which the refractive index of the plastic material decreases as the environmental temperature rises and the radius of curvature of the plasticized lens increases due to thermal expansion during the temperature rise. It is a technology.

That is, when a lens having a positive refracting power is made plastic, the focal length of the plastic lens becomes large as the temperature rises to weaken the positive refracting power, and conversely, a lens having a negative refracting power is made plastic. Then, as the temperature rises, the focal length of the plastic lens decreases and the negative refracting power weakens. Therefore, the technology based on the fact that the positive plastic lens and the negative plastic lens are combined to compensate for the temperature fluctuation Is.

[0008]

However, the objective lens system disclosed in the above-mentioned Japanese Patent Laid-Open No. 63-147122 is, by using an optical plastic material for the four lenses in the two-group six-element structure, temporarily. Although the objective of obtaining a low-cost lens system has been achieved, the realized objective lens system has a dark F number (F / No.) Of F6.3, which makes it a bright light source for document illumination. I have a big problem that I have to use.

[0009] When such a problem occurs, the peripheral equipment of the lens system is often given an unexpected load.
In the objective lens system disclosed in the above publication, even if the cost of the lens itself can be reduced, there is a risk that the cost of the entire lens system device including peripheral devices will increase. This is a major drawback of the objective lens system described in the publication.

The present invention has been made in view of such circumstances, and an object of the present invention is to reduce the cost by using a plastic lens and to cause a change in environmental temperature when the plastic lens is used. The conventional optical system of this kind is designed to appropriately compensate for the variation of the focal length from the low temperature state to the high temperature state and to satisfactorily correct the deterioration of various aberrations caused by the variation of the focal length. The F / No. Is brighter than F4, and the reading optical system is capable of maintaining close to 100% aperture efficiency up to the peripheral part of the angle of view and achieving high contrast in the high spatial frequency range. is there.

[0011]

In order to achieve the above object, the invention according to claim 1 is a first lens which is a negative lens and a positive lens in order from the object side toward the image plane side. A first lens group consisting of two single lenses, a second lens and having a negative refracting power as a whole, a third lens which is a positive lens, a fourth lens which is a negative lens, and a fifth lens which is a positive lens. In a 6-element optical system composed of a second group consisting of three single lenses, which has a positive refracting power as a whole, and a third group consisting of only a sixth lens, which is a positive single lens, Both one negative lens and one positive lens in the system are formed as plastic lenses, and the focal length of the negative plastic lens for the e-line is f _P- , and the focal point of the positive plastic lens for the e-line is distance when the f _{P +,} ( ) - it is characterized in that it has configured to satisfy the 0.51 condition: _{- 0.52 <f P- / f P} + <.

According to a second aspect of the present invention, in addition to the conditional expression described in the first aspect, the combined focal length for the e-line of the entire optical system is f, and the focal point for the e-line of the first group is the focal point. The distance is f _1g , the focal length for the e-line of the second group is f _2g , the average refractive index for the e-line of each single lens having a negative refractive power is n ₋ , and each single lens having a negative refractive power is The average Abbe number related to the optical material is ν ₋ , the average refractive index for the e-line of each single lens having a positive refractive power is n ₊ , and the average Abbe number related to the optical material of each single lens having a positive refractive power is when the _{ν +, (2) - 2.6} <f 1g / f <- 2.2 (3) 1.4 <f 2g / f <1.7 (4) 0.003 <n + -n - <0.045 (5) 18.0 <ν + It is characterized in that it is configured so as to satisfy each conditional expression of −ν ₋ <23.0.

According to the third aspect of the invention, the first lens, which is a biconcave lens, and the second lens, which is a convex meniscus lens having a concave surface facing the object side, are arranged in this order from the object side toward the image plane side. A first group consisting of two single lenses, which has a negative refracting power as a whole, a third lens which is a biconvex lens, a fourth lens which is a biconcave lens, and a convex meniscus lens whose concave surface faces the object side. A second lens group consisting of three single lenses of 5 lenses and having a positive refractive power as a whole, and a positive lens group having only a sixth lens which is a convex meniscus lens having a concave surface facing the object side. In the 6-element optical system composed of 3 groups, the first lens and the second lens
Both the lens and plastic lens are formed,
Further, the focal length of the first lens, which is a plastic lens, with respect to the e-line is f _P− , and the second lens, which is a plastic lens, is
When the focal length of the lens with respect to the e-line is f _{P +} , it is characterized in that the conditional expression of (1 ′) −0.52 <f _P− / f _{P +} <− 0.51 is satisfied.

According to a fourth aspect of the invention, in addition to the conditional expression of the third aspect, the combined focal length for the e-line of the entire optical system is f, and the focal point for the e-line of the first group is The distance is f _1g , the focal length for the e-line of the second group is f _2g , the average refractive index for the e-line of each single lens having a negative refractive power is n ₋ , and each single lens having a negative refractive power is The average Abbe number related to the optical material is ν ₋ , the average refractive index for the e-line of each single lens having a positive refractive power is n ₊ , and the average Abbe number related to the optical material of each single lens having a positive refractive power is when the _{ν +, (2 ') -} 2.6 <f 1g / f <- 2.2 (3') 1.6 <f 2g / f <1.7 (4 ') 0.005 <n + -n - <0.045 (5') It is characterized in that it is configured so as to satisfy the respective conditional expressions 18.0 <ν ₊ −ν ₋ <21.0.

In the invention according to claim 5, the first lens which is a biconcave lens and the second lens which is a convex meniscus lens having a concave surface toward the object side are arranged in this order from the object side toward the image plane side. A first group consisting of two single lenses, which has a negative refracting power as a whole, a third lens which is a biconvex lens, a fourth lens which is a biconcave lens, and a convex meniscus lens whose concave surface faces the object side. Six lenses, which are composed of three single lenses of five lenses and have a positive refracting power as a whole, and a third group having a positive refracting power, which is composed of only the sixth lens which is a biconvex lens. In the optical system having the configuration, both the first lens and the second lens are formed as plastic lenses, and the focal length of the first lens, which is a plastic lens, with respect to the e-line is f _P− , and the plastic lens is a plastic lens. When the focal length of the second lens for the e-line is f _{P +} , it is configured to satisfy the conditional expression (1 ″) − 0.52 <f _P− / f _{P +} <− 0.51. is there.

According to a sixth aspect of the present invention, in addition to the conditional expression described in the fifth aspect, the combined focal length for the e-line of the entire optical system is f, and the focal point for the e-line of the first group is the focal point. The distance is f _1g , the focal length for the e-line of the second group is f _2g , the average refractive index for the e-line of each single lens having a negative refractive power is n ₋ , and each single lens having a negative refractive power is The average Abbe number related to the optical material is ν ₋ , the average refractive index for the e-line of each single lens having a positive refractive power is n ₊ , and the average Abbe number related to the optical material of each single lens having a positive refractive power is when the _{ν +, (2 ") -} 2.6 <f 1g / f <- 2.4 (3") 1.4 <f 2g / f <1.6 (4 ") 0.003 <n + -n - <0.045 (5") It is characterized in that it is configured so as to satisfy each of the conditional expressions 21.0 <ν ₊ −ν ₋ <23.0.

[0017]

In the present invention constructed as described above, 0 ° C to 20 ° C
It can be used in an environment temperature that changes from ℃ to 65 ℃, F / No. Is extremely bright with F4 and half angle of view ω is
Despite the wide angle of view of about 20 °, various aberrations and sine conditions are well corrected, and the aperture efficiency is 10
It is an issue to be solved to realize a reading optical system having a high contrast close to 0% in a high spatial frequency region at a low cost. To achieve this task, first, 6
By using two plastic lenses in the single-lens optical system, cost reduction is achieved, and the two plastic lenses are configured as one positive and one negative plastic lens.

Then, the fluctuation of the focal length due to the change of the environmental temperature when the plastic lens is used is appropriately compensated by the method of canceling it by this positive / negative combination, and the fluctuation of the focal length causes the fluctuation. It is now possible to satisfactorily correct the deterioration of various aberrations caused by. Furthermore, by selecting and setting appropriate values for the shapes of the 6 single lenses that make up the optical system, the refractive index of the materials used, and the Abbe number, various aberrations of spheres, astigmatism, distortion, coma, and sine conditions In order to realize a large-aperture, wide-angle reading optical system, it is possible to satisfactorily correct both the low-temperature state and the high-temperature state and to improve the balance of on-axis and off-axis aberrations. ing.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION As shown in FIGS. 1 and 2, the reading optical system according to claim 1 of the present invention is a negative lens in order from the object side toward the image plane side. Lens L
_{A first} group I having two single lenses, ₁ and a second lens L ₂ which is a positive lens, and having a negative refracting power as a whole;
A second lens unit having three positive lenses, a third lens L _{3 that} is a positive lens, a fourth lens L ₄ that is a negative lens, and a fifth lens L ₅ that is a positive lens, and has a positive refracting power as a whole.
This is an optical system having a _six- lens structure composed of a second lens unit III and a third lens unit III which includes only a positive single lens L ₆ .

Both one negative lens and one positive lens in this optical system are formed as plastic lenses,
Further, when the focal length of the negative lens, which is a plastic lens, with respect to the e-line is f _P- , and the focal length of the positive lens, which is a plastic lens, with respect to the e-line is f _{P +} , (1) -0.52 <f _P- / f It is characterized by being configured so as to satisfy the conditional expression _{P +} <-0.51.

The diaphragm S is provided between the third lens L ₃ and the fourth lens L ₄ . This reading optical system cancels the fluctuation of the focal length of one positive plastic lens that occurs when the environmental temperature changes from a low temperature state to a high temperature state, by the fluctuation of the focal length of one negative plastic lens. In this state, the change (fluctuation) of the focal length f of the entire optical system is suppressed.

In this case, the conditional expression (1) is for satisfactorily correcting the fluctuation of the focal length due to the temperature change of the two plastic lenses, and the value of f _P− / f _{P +} is the conditional expression ( If the upper limit of 1) is exceeded, fluctuations in the focal length of the plastic lens cannot be suppressed, and, for example, the focal length f of the entire optical system moves significantly positively at low temperatures, and conversely, f _P- /
If the value of f _{P +} exceeds the lower limit of the conditional expression (1), for example, the focal length f of the entire optical system moves negatively largely in a low temperature state. In any case, if the limit of the conditional expression (1) is exceeded, a good image forming performance on the image plane cannot be obtained.

On the other hand, in the reading optical system described in claim 2, in the reading optical system configured as in claim 1, the combined focal length of the whole optical system with respect to the e-line is f, The focal length of the group I with respect to the e-line is f _1g , and the second group is
The focal length of II for the e-line is f _2g , the average refractive index of the single lenses L ₁ and L ₄ having a negative refractive power is n ₋ , and the single lenses L ₁ and L having the negative refractive power are n ₋ . _The average Abbe number of the optical material of ₄ is ν _−, and the average refractive index of the single lenses L ₂ , L ₃ , L ₅ , and L ₆ having a positive refractive power to the e-line is n ₊ , and the positive refractive power is Each single lens L ₂ having,
The average Abbe number of the optical materials L ₃ , L ₅ and L ₆ is ν ₊
When a, (2) - 2.6 <f 1g / f <- 2.2 (3) 1.4 <f 2g / f <1.7 (4) 0.003 <n + -n - <0.045 (5) 18.0 <ν + -ν _- <it is characterized in that it has configured to satisfy the 23.0 made conditional expressions.

In this case, the conditional expression (2) defines the combined power of the first group I of the reading optical system, and f _1g /
When the value of f exceeds the upper limit of the conditional expression (2), the astigmatism becomes negative and becomes large, and conversely, when the value of f exceeds the lower limit of the conditional expression (2), the astigmatism becomes positive and large.

Conditional expression (3) defines the composite power arrangement of the second group II of the reading optical system, f _2g
When the value of / f exceeds the upper limit of conditional expression (3), spherical aberration becomes negative and large, and astigmatism becomes positive and large, and conversely, when the lower limit of conditional expression (3) is exceeded, spherical surface becomes conversely. Aberrations are positive and large, and astigmatism is negative and large. In any case, if the limit of the conditional expression (3) is exceeded, the balance of on-axis and off-axis aberrations will be greatly disturbed and coma flare will increase.

The conditional expression (4) is for defining the range of the refractive index of the positive lens and the negative lens in the optical system, and the value of n ₊ −n ₋ is the upper limit of the conditional expression (4). When it exceeds
The Petzval sum becomes too small and the image surface falls in the positive direction, and the field curvature becomes large. On the other hand, when the value of n ₊ −n ₋ exceeds the lower limit of the conditional expression (4), the Petzval sum becomes too large and the image surface tilts in the negative direction, so that the astigmatic difference becomes large. In any case, if the limit of conditional expression (4) is exceeded, the result is that good imaging performance cannot be obtained over the entire screen.

Conditional expression (5) is for satisfactorily correcting axial chromatic aberration, and when the value of ν ₊ −ν ₋ exceeds the upper limit of conditional expression (5), axial chromatic aberration is obtained. Is overcorrected and the chromatic aberration on the axis on the short wavelength side with respect to the main wavelength (reference wavelength) is positive and large, and conversely, the value of ν ₊ −ν ₋ is below the lower limit of conditional expression (5). If it exceeds, the axial chromatic aberration on the short wavelength side with respect to the dominant wavelength will be negative and large.

Next, the reading optical system described in claim 3 will be described. This reading optical system is one modification included in the reading optical system of claim 1. As shown in FIG. 1, this reading optical system includes a first lens L ₁ which is a biconcave lens and a second lens which is a convex meniscus lens having a concave surface facing the object side in order from the object side toward the image plane side. The first lens group I is composed of two single lenses L ₂ and L ₂ and has a negative refracting power as a whole, the third lens L ₃ which is a biconvex lens, the fourth lens L ₄ which is a biconcave lens, and the concave surface on the object side. The second lens unit II is composed of three single lenses including the fifth lens L ₅ which is a convex meniscus lens having a positive power, and has a positive refracting power as a whole, and the convex meniscus lens having a concave surface facing the object side. In a six-element optical system composed of a third lens group III having a positive refracting power consisting of only six lenses L ₆ , both the first lens L ₁ and the second lens L ₂ are formed as plastic lenses. In addition, the first is a plastic lens The focal length of the lens L ₁ with respect to the e-line is f _P- , and the second lens L ₂ is a plastic lens.
When the focal length with respect to the e-line is f _{P +} , it is characterized in that the conditional expression of (1 ′) −0.52 <f _P− / f _{P +} <−0.51 is satisfied.

In this case, the configuration in which the fluctuation of the focal length due to the use of the plastic lens is canceled by the fluctuation of the focal length of one positive and one negative plastic lens to suppress the fluctuation of the focal length f of the entire optical system, The same as in the reading optical system according to claim 1, and f _P− /
The reason for setting the conditional expression (1 ′) relating to f _{P +} and the meaning of determining the limit range are the same as in the case of the conditional expression (1) in the reading optical system of claim 1. Therefore, detailed description is omitted to avoid duplication.

On the other hand, in the reading optical system described in claim 4, in the reading optical system configured as in claim 3, the total focal length of the entire optical system with respect to the e-line is f, Let f _{1g be} the focal length of the first group I with respect to the e-line, and f _2g be the focal length of the second group II with respect to the e-line, and the average refraction of the single lenses L ₁ and L ₄ having negative refractive power with respect to the e-line. Each of the single lenses L ₁ and L having a refractive index n ₋ and a negative refractive power
_The average Abbe number of the optical material of ₄ is ν _−, and the average refractive index of the single lenses L ₂ , L ₃ , L ₅ , and L ₆ having a positive refractive power to the e-line is n ₊ , and the positive refractive power is When the average Abbe number related to the optical material of each single lens L ₂ , L ₃ , L ₅ , and L ₆ is ν ₊ , (2 ′) −2.6 <f _1g / f <− 2.2 (3 ′) 1.6 _{<f 2g / f <1.7 (} 4 ') 0.005 <n + -n - <0.045 (5') 18.0 <ν + -ν - < those characterized by being configured so as to satisfy the 21.0 made conditional expressions Is.

In this case, the conditional expression (2 ') relating to f _1g / f
The reason for setting and the meaning of determining the limit range are the same as in the case of the conditional expression (2) in the reading optical system of claim 2, and therefore detailed description is omitted to avoid duplication. Further, the reason for setting the conditional expression (3 ′) relating to f _2g / f and the meaning of determining the limit range are the same as in the case of the conditional expression (3) in the reading optical system according to claim 2, and therefore a detailed description will be given. Is omitted. Further, the reason for setting conditional expression (4 ′) related to n ₊ −n ₋ and the meaning of determining the limit range, and ν ₊ −ν ₋
The reason for setting the conditional expression (5 ′) and the meaning of determining the limit range are the same as those in the conditional expression (4) and the conditional expression (5) in the reading optical system according to claim 2, respectively. Detailed description is omitted.

Next, the reading optical system described in claim 5 will be described. This reading optical system is another modification included in the reading optical system of claim 1. As shown in FIG. 2, the reading optical system includes a first lens L ₁ which is a biconcave lens and a second lens which is a convex meniscus lens having a concave surface facing the object side in order from the object side toward the image plane side. The first lens group I is composed of two single lenses L ₂ and L ₂ and has a negative refracting power as a whole, the third lens L ₃ which is a biconvex lens, the fourth lens L ₄ which is a biconcave lens, and the concave surface on the object side. The fifth lens L ₅ , which is a convex meniscus lens directed to, and the second lens group II, which has a positive refracting power as a whole, and the sixth lens L, which is a biconvex lens.
Third group consisting of only ₆ and having positive refracting power as a whole
In the six-lens optical system configured by III, both the first lens L ₁ and the second lens L ₂ are formed as plastic lenses, and the e-line of the first lens L ₁ which is a plastic lens is further formed. _Where f _{P- is} the focal length of the second lens L ₂ which is a plastic lens and f _{P +} is the focal length of the second lens L ₂ which is a plastic lens, the conditional expression is (1 ″) − 0.52 <f _P− / f _{P +} <− 0.51 It is characterized by being configured to satisfy.

In this case, the structure in which the fluctuation of the focal length due to the use of the plastic lens is canceled by the fluctuation of the focal length of one positive and one negative plastic lens to suppress the fluctuation of the focal length f of the entire optical system, This is similar to the case of the reading optical system according to claims 1 and 3, and the reason for setting conditional expression (1 ″) related to f _P− / f _{P +} and the meaning of determining the limit range are the same as those in claims 1 and 3. This is the same as the case of the conditional expressions (1) and (1 ′) in the reading optical system, and therefore detailed description is omitted as in the case of claim 3.

On the other hand, in the reading optical system described in claim 6, in the reading optical system configured as in claim 5, the combined focal length of the whole optical system with respect to the e-line is f, Let f _{1g be} the focal length of the first group I with respect to the e-line, and f _2g be the focal length of the second group II with respect to the e-line, and the average refraction of the single lenses L ₁ and L ₄ having negative refractive power with respect to the e-line. Each of the single lenses L ₁ and L having a refractive index n ₋ and a negative refractive power
_The average Abbe number of the optical material of ₄ is ν _−, and the average refractive index of the single lenses L ₂ , L ₃ , L ₅ , and L ₆ having a positive refractive power to the e-line is n ₊ , and the positive refractive power is When the average Abbe number of the optical material of each of the single lenses L ₂ , L ₃ , L ₅ , and L ₆ is ν ₊ , (2 ″) −2.6 <f _1g / f <−2.4 (3 ″) 1.4 <F _2g / f <1.6 (4 ″) 0.003 <n ₊ −n ₋ <0.045 (5 ″) 21.0 <ν ₊ −ν ₋ <23.0 It is configured to satisfy each conditional expression. Is.

In this case, the conditional expression (2 ″) related to f _1g / f
The reason for setting and the meaning of the limit range determination are the same as in the case of the conditional expression (2) in the reading optical system of claim 2 as in the case of claim 4, and therefore detailed description will be omitted.
Further, the reason for setting the conditional expression (3 ″) related to f _2g / f and the meaning of determining the limit range are the same as in the case of the conditional expression (3) in the reading optical system of claim 2 as in the case of claim 4. it is similar to omit the detailed description. Further, n ₊ -n
_The reason why the conditional expression (4 ″) relating to ₋ and the meaning of the limit range determination, and the reason why the conditional expression (5 ″) relating to ν ₊ −ν ₋ and the meaning of the limit range determination are defined respectively.
Since this is the same as the case of the conditional expressions (4) and (5) in the reading optical system, detailed description of each is omitted.

[0036]

EXAMPLES Below, detailed data of each of the five examples, a table of focal lengths when temperature changes according to each example, and conditional expression parameters of each example are listed. Each data of the standard state, the low temperature state, and the high temperature state in the state in which the cover glass C (FIGS. 1 and 2) of the solid-state imaging device as described above is arranged on the image plane side is described.

The meanings of the symbols in each example are as follows. f: composite focal length of the entire optical system for the e-line f _1g : focal length of the first group I for the e-line f _2g : focal length of the second group II for the e-line f _P- : e-line of the negative plastic lens Focal length or the first lens L which is a plastic lens
Focal length of ₁ for e-line f _{P +} : Focal length of positive plastic lens for e-line, or second lens L that is a plastic lens
Focal length for ₂ e-line

F / No .: F number m: multiplication factor ω: half angle of view Y: object height r _i (i = 1 to 12): radius of curvature of i-th lens surface counted from the object side d _i (i = 1-12): counted from the object side the i-th axial distance (surface separation) n _i (i = 1 to 6): refractive index for the e-line of the optical material of the i-th single lens counted from the object side ν _i (i = 1 to 6): Abbe number related to the optical material of the i-th single lens counted from the object side n ₊ : Each single lens (convex lens) having a positive refractive power
Refractive index n ₋ of e-line: each single lens (concave lens) having negative refractive power
The average refractive index for the e-line [nu _+: Mean Abbe number according to the optical material of the single lens having a positive refractive power [nu _-: average Abbe number of the optical material of the single lens having a negative refractive power

R _ci (i = 1, 2): i counted from the object side
Th of the solid-state imaging device (CCD) radius of curvature d _c of the cover glass of: spacing of the cover glass of the solid-state imaging device (CCD) n _c: refractive index for the e-line of the optical material of the cover glass of the solid-state imaging device (CCD) ν _c : Abbe's number of the optical material of the cover glass of the solid-state imaging device (CCD) First, regarding Example 1 to Example 3, these Examples are examples in which the optical system configuration of FIG. 1 is adopted. And corresponds to claims 1 to 4. Detailed data for these examples are as follows. [Example 1]

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3] [Example 2]

[0043]

[Table 4]

[0044]

[Table 5]

[0045]

[Table 6] [Example 3]

[0046]

[Table 7]

[0047]

[Table 8]

[0048]

[Table 9] The value of the focal length when the temperature changes in each example and the conditional expression parameters of each example are as follows.

[0049]

[Table 10]

[0050]

[Table 11] As shown in the table of "focal length when temperature changes in each example", the reading optical systems according to these three Examples 1 to 3 are at each temperature change (low temperature state to high temperature state, the same applies hereinafter).
The variation of the focal length is extremely small, and in comparison with the conventional optical system of this type, the F / No. Is extremely bright as F4 and the half angle of view ω is a wide angle of view of 19.3 °. All the aberrations and the sine condition are well corrected, which fully shows how the reading optical system according to the present invention has excellent optical performance.

That is, the aberration corrections according to Examples 1 to 3 are as shown in the aberration diagrams of FIGS. 3 to 5 in the case of Example 1 and from FIG. 6 to in the case of Example 2. As shown in the aberration diagram of FIG. 8, in the case of Example 3, as shown in the aberration diagrams of FIGS. 9 to 11, in all cases, spherical, astigmatism, distortion, and coma are observed over a low temperature state to a high temperature state. Each aberration and sine condition are corrected well, and it is shown that the reading optical systems according to Examples 1 to 3 are extremely excellent in terms of aberrations.

The following two examples 4 and 5 are examples in which the optical system configuration of FIG. 2 is adopted and are examples corresponding to claims 1 and 2 and claims 5 and 6, respectively. Detailed data of each example are as follows. [Example 4] [Example 4]

[0053]

[Table 12]

[0054]

[Table 13]

[0055]

[Table 14] [Example 5]

[0056]

[Table 15]

[0057]

[Table 16]

[0058]

[Table 17] The reading optical systems according to the two Examples 4 and 5 also have extremely small fluctuations in the focal length when the temperature changes, as shown in the above "Table of focal length when temperature changes in each Example". Moreover, compared to this type of conventional optical system, the F / No. Is extremely bright with F4 and the half angle of view ω is 19.3.
In spite of the wide angle of view of °, various aberrations and sine conditions are well corrected, and how excellent the reading optical system according to the present invention is an optical system. Tells.

That is, the aberration correction situations of Examples 4 and 5 are as shown in the aberration diagrams of FIGS. 12 to 14 in the case of Example 4, and in FIGS. Figure 1
As shown in the aberration chart of No. 7, in all cases, the spherical, astigmatic, distortion, and coma aberrations and the sine condition were satisfactorily corrected over a low temperature state to a high temperature state, and the results are shown in Examples 4 and 5. It is shown that the reading optical system is an optical system excellent in terms of aberration.

Although the above description is based on the five embodiments, the present invention is not limited to this, and various modifications can be made without departing from the scope of the invention. For example, it is possible to make the optical surface of the plastic lens in the optical system aspherical to make it easier to correct aberrations and provide high optical performance.

[0061]

As described above, the present invention is achieved by using two plastic lenses in an optical system having six lenses to achieve cost reduction and using the plastic lenses. The change in the focal length due to changes in the ambient temperature is satisfactorily corrected by the proper combination of positive and negative lenses, and the F / No. Is extremely bright with F4 and the half angle of view ω is about 20 °. Despite being angular, spherical, astigmatism, distortion, coma, and various sine conditions are well corrected, and the on-axis and off-axis aberrations are well balanced, and the aperture efficiency is also improved. Is 10
The excellent effect that it is possible to realize a reading optical system having a high contrast in the high spatial frequency region close to 0% is achieved.

[Brief description of drawings]

FIG. 1 is an optical configuration diagram showing an example of the entire system configuration of a reading optical system according to the present invention, showing a state in which a cover glass C of a solid-state image sensor such as a CCD is arranged on the image plane side. Show.

FIG. 2 is an optical configuration diagram showing another example of the overall configuration of the reading optical system according to the present invention, in which a cover glass C of a solid-state image pickup device such as a CCD is arranged on the image plane side. Indicates the status.

FIG. 3 is an aberration diagram at room temperature (20 ° C.) according to the reading optical system of Example 1 of the present invention, and the meanings of symbols used in the aberration diagram are as follows. SA is spherical aberration, SC is sine condition, Ast is astigmatism, Dist is distortion, and Co is
ma indicates coma aberration, respectively. Further, in the spherical aberration diagram, the spherical aberration is shown by a solid line and the sine condition is shown by a broken line, and in the astigmatism diagram, the sagittal ray is shown by a solid line and the meridional ray is shown by a broken line. In the aberration diagram, e-line (546.07 nm), d-line (587.56 nm), and F-line (486.13 nm) are rays.

FIG. 4 is an aberration diagram at a low temperature (0 ° C.) related to the reading optical system according to the first embodiment of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

5 is an aberration diagram at a high temperature (65 ° C.) according to the reading optical system of Example 1 of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

FIG. 6 is an aberration diagram at room temperature (20 ° C.) according to the reading optical system of Example 2 of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

FIG. 7 is an aberration diagram at a low temperature (0 ° C.) according to the reading optical system of Example 2 of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

FIG. 8 is an aberration diagram at a high temperature (65 ° C.) related to the reading optical system of Example 2 of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

9 is an aberration diagram at room temperature (20 ° C.) according to the reading optical system of Example 3 of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

FIG. 10 is an aberration diagram at a low temperature (0 ° C.) related to the reading optical system according to the third embodiment of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

11 is an aberration diagram at a high temperature (65 ° C.) according to the reading optical system of Example 3 of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

FIG. 12 is an aberration diagram at room temperature (20 ° C.) according to the reading optical system of Example 4 of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

FIG. 13 is an aberration diagram at a low temperature (0 ° C.) related to the reading optical system of Example 4 of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

FIG. 14 is an aberration diagram at a high temperature (65 ° C.) related to the reading optical system according to the fourth embodiment of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

FIG. 15 is an aberration diagram at room temperature (20 ° C.) according to the reading optical system of Example 5 of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

FIG. 16 is an aberration diagram at a low temperature (0 ° C.) related to the reading optical system of Example 5 of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

17 is an aberration diagram at a high temperature (65 ° C.) related to the reading optical system of Example 5 of the present invention, and the symbols used in the aberration diagram have the same meanings as in FIG.

[Explanation of symbols]

I 1st group II 2nd group III 3rd group L ₁ 1st lens L ₂ 2nd lens L ₃ 3rd lens S Aperture L ₄ 4th lens L ₅ 5th lens L ₆ 6th lens C Cover glass r ₁ ~ r ₆ radius of curvature of first lens L ₁ to sixth lens L ₆ d _{1 to} d ₁₂ surface spacing of first lens L ₁ to sixth lens L ₆ r _c1 , r _c2 radius of curvature of cover glass C d _c cover glass C spacing

Claims (6)

[Claims] 1. A first lens, which is a negative lens, and a second lens, which is a positive lens, are arranged in this order from the object side toward the image plane side.
A single lens group consisting of a single lens element having a negative refracting power as a whole; a single lens element; a third lens element which is a positive lens element; a fourth lens element which is a negative lens element; and a fifth lens element which is a positive lens element. In a 6-element optical system consisting of a second group having a positive refracting power as a whole and a third group consisting of only a positive single lens, a single lens in the optical system. Both the negative lens and one positive lens are formed as plastic lenses, and the focal length of the negative plastic lens for the e-line is f _P- , and the focal length of the positive plastic lens for the e-line is f _{P +} . At this time, the reading optical system is configured so as to satisfy the conditional expression (1) -0.52 <f _P- / f _{P +} <-0.51. 2. A composite focal length for the e-line of the entire optical system is f, a focal length for the e-line of the first group is f _1g , and a focal length for the e-line of the second group is f _2g. The average refractive index for the e-line of each single lens having a refractive power is n ₋ ,
The average Abbe number of the optical material of the single lens having a negative refractive power [nu _- and then, for each single lens having a positive refractive power e
When the average refractive index with respect to a line is n ₊ and the average Abbe number related to the optical material of each single lens having a positive refractive power is ν ₊ , (2) -2.6 <f _1g / f <-2.2 (3) 1.4 <f _2g / f <1.7 (4) 0.003 <n ₊ −n ₋ <0.045 (5) 18.0 <ν ₊ −ν ₋ <23.0 It is constituted so as to satisfy each conditional expression. 1. The reading optical system described in 1. 3. A single lens consisting of a first lens, which is a biconcave lens, and a second lens, which is a convex meniscus lens with a concave surface facing the object side, in that order from the object side to the image side, , A single lens having a negative refractive power, a third lens that is a biconvex lens, a fourth lens that is a biconcave lens, and a fifth lens that is a convex meniscus lens having a concave surface facing the object side. And a third group having a positive refracting power and a third group having a positive refracting power consisting only of a sixth lens which is a convex meniscus lens having a concave surface facing the object side. in the optical system, in the both the first lens and the second lens is formed as a plastic lens, further, the focal length for the e-line of the first lens is a plastic lens f _P-, plastic lenses The focal length for the e line of the second lens when the f _{P +} that, (1 ') - for reading, characterized by being configured so as to satisfy the 0.51 condition: _{- 0.52 <f P- / f P} + < Optical system. 4. A composite focal length for the e-line of the entire optical system is f, a focal length for the e-line of the first group is f _1g , and a focal length for the e-line of the second group is f _2g. The average refractive index for the e-line of each single lens having a refractive power is n ₋ ,
The average Abbe number of the optical material of the single lens having a negative refractive power [nu _- and then, for each single lens having a positive refractive power e
When the average refractive index with respect to a line is n ₊ and the average Abbe number related to the optical material of each single lens having a positive refractive power is ν ₊ , (2 ') -2.6 <f _1g / f <-2.2 (3 ′) 1.6 <f _2g / f <1.7 (4 ') 0.005 <n ₊ −n ₋ <0.045 (5 ′) 18.0 <ν ₊ −ν ₋ <21.0 The reading optical system according to claim 3. 5. A single lens consisting of a first lens, which is a biconcave lens, and a second lens, which is a convex meniscus lens having a concave surface facing the object side, in that order from the object side to the image side, , A single lens having a negative refractive power, a third lens that is a biconvex lens, a fourth lens that is a biconcave lens, and a fifth lens that is a convex meniscus lens having a concave surface facing the object side. And a third group having a positive refracting power composed only of a sixth lens which is a biconvex lens. both lens and the second lens is formed as a plastic lens, further, the focal length for the e-line of the first lens f _P- is a plastic lens, for e line of the second lens is a plastic lens When a point distance was _{f P +, (1 ")} - 0.52 <f P- / f P + <- 0.51 condition: reading optical system characterized by being configured so as to satisfy the equation. 6. A composite focal length of the whole optical system with respect to the e-line is f, a focal length of the first group with respect to the e-line is f _1g , and a focal length of the second group with respect to the e-line is f _2g. The average refractive index for the e-line of each single lens having a refractive power is n ₋ ,
The average Abbe number of the optical material of the single lens having a negative refractive power [nu _- and then, for each single lens having a positive refractive power e
When the average refractive index with respect to the line is n ₊ and the average Abbe number related to the optical material of each single lens having a positive refractive power is ν ₊ , (2 ″) −2.6 <f _1g /f<−2.4 (3 ″) 1.4 <f _2g / f <1.6 (4 ″) 0.003 <n ₊ −n ₋ <0.045 (5 ″) 21.0 <ν ₊ −ν ₋ <23.0 The optical system for reading according to claim 5.