JPH0812324B2 - Telecentric fθ lens - Google Patents

Description

The present invention relates to an fθ lens system used in an optical scanning device such as a laser printer.

[Conventional technology]

Most of the fθ lenses used in conventional optical scanning devices are
Since it was not telecentric, a scanning spot position error was likely to occur. Further, for example, as disclosed in Japanese Patent Laid-Open No. 59-195211, even if it is telecentric, the deviation from fθ is large and the linearity of scanning cannot be said to be sufficient. Moreover, since the chromatic aberration is not corrected, it is necessary to select and use only one specific wavelength with a filter or the like even when using a light source that emits light containing a large number of spectra.

[Problems to be solved by the invention]

For this reason, in the conventional telecentric f.theta. Lens system as described above, the output from the light source cannot be fully utilized, and the scanning device is inevitably low in efficiency.

Therefore, an object of the present invention is to provide a telecentric f.theta. Lens system that can improve the linearity of scanning while maintaining the telecentricity, correct the chromatic aberration well, and form an efficient and excellent scanning device.

[Means for solving problems]

The telecentric fθ lens system according to the present invention comprises a first
As in the first embodiment shown in the figure, the front group G having, in order from the entrance pupil EP side, a negative lens component L _FN having a concave surface facing the entrance pupil EP and a positive lens component L _FP arranged on the image side thereof. _{F 2} and a rear lens group G _R composed of a positive lens component L _RP arranged near the image plane. And the negative lens component in the front group G _F
The Abbe number of L _FN is ν _FN , the Abbe number of the positive lens component L _{FP in} the front group G _F is ν _FP , the Abbe number of the positive lens component as the rear group G _R is ν _R , and the combined focal length of the entire system the f, and the focal length f _F of the front group G _F, a focal length of the rear group G _R when the _{f R, ν FN <35 (} 1) ν FP> 45 (2) ν R> 55 (3 ) 0.8 <f _F /f<1.3 (4) 1.8 <f _R /f<2.8 (5) Each condition is satisfied.

[Action]

As described above, the negative lens component and the positive lens component constitute the front lens group in order from the entrance pupil side, and the rear lens group for telecentricity on the image side is arranged near the image plane. The conditions (1) to (3) provide good chromatic aberration correction, and the conditions (4) and (5) allow strict maintenance of telecentricity.

The condition of the expression (1) is a condition for simultaneously correcting the lateral chromatic aberration and the axial chromatic aberration. If the condition is deviated, the axial chromatic aberration can be corrected but the It becomes difficult to satisfactorily correct the chromatic aberration of magnification generated from the positive lens component.

The condition of the expression (2) is for correcting the chromatic aberration of magnification while maintaining good coma aberration when the negative lens component in the front lens group satisfies the condition of the expression (1). If this condition is not satisfied, the refractive powers of the negative lens component and the positive lens component in the front group will become too strong, making it difficult to perform good aberration correction.

The condition of the expression (3) is to reduce the burden on the front group as much as possible with respect to the correction of lateral chromatic aberration. If this condition is not satisfied, the burden of chromatic aberration correction on the front group will increase, and the number of lenses forming the front group will increase, making it difficult to make the configuration simple.

The condition of the expression (4) is a condition for arranging the rear group near the image plane to achieve telecentricity and reducing the burden of aberration correction as the rear group. If the lower limit of this condition is not satisfied, the power load on the front lens group will increase, making it difficult to correct aberrations in the front lens group and increasing the overall length. On the other hand, when the value exceeds the upper limit, the power load on the rear group increases, which causes coma aberration, and it becomes difficult to form the rear group with a single lens component. If the upper limit of this condition is 1.1 or less, better correction becomes possible.

The condition of the expression (5) is a condition for maintaining good telecentricity. If the upper limit of this condition is exceeded, the chief ray incident on the image plane will not be vertical, and the position of the scanning spot will become unstable with respect to displacement of the image plane in the optical axis direction, which may hinder accurate scanning. Occurs. On the other hand, when the value goes below the lower limit, coma aberration occurs and it is difficult to configure the rear group with a single component.

In the configuration of the present invention as described above, in order to adjust the characteristic of fθ in a state where the telecentricity by the rear group is maintained, the radius of curvature of the entrance pupil side lens surface of the positive lens component as the rear group is R _a , and the positive lens is When the radius of curvature of the image-side lens surface of the component is R _b , it is desirable that the condition of | R _a |> | R _b | If this condition is not satisfied, the distortion becomes positively large and deviates from the characteristic of fθ, and the linearity of scanning deteriorates.

〔Example〕

In the first embodiment according to the present invention, as shown in the optical path diagram of FIG. 1, a negative lens component L _FN composed of one negative lens and a positive lens component L composed of two positive lenses are arranged in this order from the entrance pupil EP side. _FP
Telecentric f where the front lens group G _F is composed of and and the rear lens group G _R is composed of the positive lens component L _RP arranged near the image plane I.
Theta lens system. The positive lens component as the rear group is a plano-convex lens having a plane on the entrance pupil side.

In the optical path diagram, only the optical paths of the axial luminous flux (solid line) and the luminous flux with the maximum angle of view (broken line) are shown.

In the second embodiment, as shown in FIG. 2, the front lens group G _F is composed of a negative lens component L _FN composed of two negative lenses and a positive lens component L _FP composed of two positive lenses. is there.

In the third embodiment, as shown in FIG. 3, the front lens group has substantially the same structure as the second embodiment, and the rear lens group is composed of a biconvex positive lens component.

In each of the above-described embodiments, the chromatic aberration is corrected for the center wavelength λ ₁ = 488 nm, the short wavelength side λ ₂ = 476.5 nm, and the long wavelength side λ ₃ = 496.5 nm.

Table 1, Table 2 and Table 3 below show the specifications of the first, second and third embodiments, respectively.

In the table, the numbers at the left end represent the order from the object side, the refractive index is the value at the wavelength of 488 nm, and the Abbe number is the d-line (λ = 5
87.6 nm). Further, d ₀ is the distance from the entrance pupil to the apex of the frontmost lens surface.

Aberration diagrams for the first, second, and third examples are shown in FIGS. 4, 5, and 6, respectively. Each aberration diagram shows spherical aberration, astigmatism, and distortion for λ ₁ = 488 nm as a reference wavelength. Short-wavelength light λ ₂ = 476.5n
Spherical aberration, astigmatism, and chromatic aberration of magnification are shown for each wavelength in order to show the correction state of chromatic aberration for m and long wavelength light λ ₃ = 496.5 nm. The distortion is a percentage of the displacement amount with respect to fθ in order to express the linearity of the scanning spot.

From each aberration diagram, it is apparent that in each of the examples, various aberrations are well corrected, and especially chromatic aberration and distortion are well corrected.

Generally, when using an optical system that has not been corrected for chromatic aberration, even if a laser that can simultaneously oscillate light of multiple wavelengths, such as an argon laser, is used as the light source, only a single wavelength can be oscillated by using a single wavelength oscillation or a filter. Had to be selected and only a single wavelength of energy was available. When an optical system in which chromatic aberration is corrected is used, the wavelength range of the correction range can be effectively used. Which wavelength range is to be corrected for chromatic aberration is determined by the output of each oscillation wavelength from the light source, the sensitivity characteristics of the photosensitive material, the light receiver, and the like. FIG. 7 shows an example of each spectral intensity in the simultaneous oscillation of multiple wavelengths of the argon laser and the relative sensitivity ratio of the photosensitive material. In each of the above-described embodiments, an argon laser having the characteristics shown in FIG. 7 is used as a light source, and a photosensitive material having the characteristics shown is used. Therefore, the central wavelength λ ₁ = 48
The energy from the light source can be used twice as effectively as compared to the case where only 8 nm is used.

It should be noted that such a method of correcting chromatic aberration according to the present invention is also effective when considering the chromatic aberration generated in the optical system up to the fθ lens system.

〔The invention's effect〕

As described above, according to the present invention, it is possible to achieve a telecentric fθ lens system in which the linearity of scanning is excellent while the telecentricity is maintained and the chromatic aberration is well corrected. Further, since the chromatic aberration is made over a relatively wide wavelength range, the light in the wide wavelength range from the light source can be efficiently collected and scanned, so that the efficiency as a scanning device can be improved.

[Brief description of drawings]

FIG. 1 is an optical path diagram showing a lens structure of a first embodiment according to the present invention, FIG. 2 is an optical path diagram showing a lens structure of a second embodiment according to the present invention, and FIG. 3 is a third embodiment of the present invention. FIG. 4 is an aberration diagram of the first embodiment, FIG. 5 is an aberration diagram of the second embodiment, and FIG. 6 is an aberration diagram of the third embodiment. FIG. 7 is a diagram illustrating an emission spectrum from a laser light source used in the use of each example according to the present invention and a sensitivity characteristic of a photosensitive material. [Explanation of Signs of Main Parts] EP ...... Entrance pupil G _F ...... Front group G _R …… Rear group L _FN …… Negative lens component L _{FP in} front group …… Positive lens component L _{RP in} front group ... Positive lens component in rear group

Claims (3)

[Claims] 1. A front lens group having, in order from the entrance pupil side, a negative lens component having a concave surface facing the entrance pupil side and a positive lens component arranged on the image side thereof, and a positive lens component arranged near the image plane. And the Abbe number of the negative lens component in the front group is ν _FN , the Abbe number of the positive lens component in the front group is ν _FP , and the Abbe number of the positive lens component in the rear group is ν
_R , the combined focal length of the entire system is f, and the focal length of the front group is
f _F , where f _R is the focal length of the rear group, ν _FN <32 ν _FP > 45 ν _R > 55 0.8 <f _F /f<1.3 1.8 <f _R /f<2.8 A telecentric fθ lens characterized in that Wherein said 0.8 <f instead of conditional expression _{F /f<1.3,} telecentric fθ lens Claims paragraph 1, wherein the system satisfies the following conditional expression. 0.8 <f _F / f <1.1 3. When the radius of curvature of the entrance pupil side lens surface of the positive lens component of the rear group is R _a and the radius of curvature of the image side lens surface of the positive lens component is R _b , the following conditions are satisfied: The telecentric fθ lens according to claim 1 or 2. | R _a | ＞ | R _b |