Telecentric f-theta lens system

Abstract: The present invention is directed to a telecentric f-­ ϑ lens system, the telecentric f-ϑ lens system comprises; a first lens group (FL) having an entrance pupil (EP), which inlcudes a first lens (L1) in the form of meniscus having a positive power, a second lens (L2) having a negative power, and third and fourth lenses (L3,L4) in the form of meniscus having a positive power, respectively; and a second lens group (RL) including a fifth lens having a positive power. Accordingly, the telecentric f-ϑ lens system has a flat image surface.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an f-ϑ lens system, and more particularly, it relates to a telecentric f-ϑ lens system which is applied to an optical beam scanner such as a laser printer. Description of the Prior Art An f-ϑ lens system satisfies: y = f·ϑ assuming that y represents the distance from an optical axis to a beam spot on an image formation surface to be scanned, f represents the focal length of the f-ϑ lens system and ϑ represents an angle of incidence of the beam upon the lens system. Therefore, when a laser beam is applied to a polygon mirror which is rotated at a constant angular velocity while imaging a beam reflected by the polygon mirror on an image formation surface through the f-ϑ lens system, the beam spot is moved on the image formation surface at a constant speed. In a conventional optical beam scanner, therefore, the polygon mirror is generally combined with the f-ϑ lens system in order to move the beam spot on the image formation surface at a constant speed. Further, the conventional optical beam scanner employs a telecentric optical system in order to prevent from misregistration. This is because all principal rays substantially vertically enter an image formation surface in the telecentric optical system, to cause extremely small misregistration even if the image formation surface is displaced in the direction of an optical axis, for example, as is well known in the art. Therefore, a telecentric f-ϑ lens system is employed in a laser printer etc. particularly requiring that a laser beam is incident upon the image formation surface in high accuracy. For example, Japanese Patent Laying-Open Gazettes Nos. 195211/1984 and 299927/1987 disclose telecentric f-ϑ lens systems of this type. Fig. 1 illustrates the structure of a conventional telecentric f-ϑ lens system, which is disclosed in the former gazette (Japanese Patent Laying-Open Gazette No. 195211/1984). As shown in Fig. 1, the conventional telecentric f-ϑ lens system is formed by a negative lens L₁₂ whose concave surface S₁₂ directed toward an entrance pupil EP, positive meniscus lenses L₁₃ and L₁₄ and a positive plano-convex lens L₁₅, and these lenses L₁₂ to L₁₅ are arranged in order from the entrance pupil EP side toward an image formation surface IS side. Figs. 2A and 2B show spherical aberration and astigmatism of the telecentric f-ϑ lens system shown in Fig. 1, respectively. A telecentric f-ϑ lens system disclosed in the latter gazette (Japanese Patent Laying-Open Gazette No. 299927/1987) is similar in structure to that shown in Fig. 1, except for that the f-ϑ lens system has been corrected for chromatic aberration. In the conventional telecentric f-ϑ lens system having the aforementioned structure, the curve of the astigmatism is generally inclined toward the negative side as shown in Fig. 2B. In order to obtain a flat image surface, therefore, it is necessary to also incline the curve of the spherical aberration toward the negative side. However, the curve of the spherical aberration is inclined toward the positive side in the conventional telecentric f-ϑ lens system, as shown in Fig. 2A. Thus, a flat image surface cannot be obtained in the conventional telecentric f-ϑ lens system. Further, higher resolution is generally expected for an optical beam scanner. To this end, it is necessary to reduce in the diameter of a beam spot. Thus, the F-number must be minimized. US-A-4 269 478 discloses another f-ϑ lens system which corresponds to the preamble of claim 1. SUMMARY OF THE INVENTION The present invention is directed to a telecentric f-ϑ lens system according to claim 1. Further, the present invention is directed to a scanning system according to claim 2. A principal object of the present invention is to provide a telecentric f-ϑ lens system which has an F-number of not more than 20, small spherical aberration, astigmatism and the like, and a flat image surface. Another object of the present invention is to provide a scanning system which can scan an optical beam on a recording surface with high resolution. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates the structure of a conventional telecentric f-ϑ lens system; Figs. 2A and 2B illustrate spherical aberration and astigmatism of the conventional f-ϑ lens sytem shown in Fig. 1, respectively; Fig. 3 illustrates an embodiment of a telecentric f-ϑ lens system according to the present invention; Figs. 4A, 4B and 4C illustrate spherical aberration, astigmatism and f-ϑ characteristic of a first example, respectively; Figs. 5A, 5B and 5C illustrate spherical aberration, astigmatism and f-ϑ characteristic of a second example, respectively; Figs. 6A, 6B and 6C illustrate spherical aberration, astigmatism and f-ϑ characteristic of a third example, respectively; Figs. 7A, 7B and 7C illustrate spherical aberration, astigmatism and f-ϑ characteristic of a fourth example, respectively; Figs. 8A, 8B and 8C illustrate spherical aberration, astigmatism and f-ϑ characteristic of a fifth example, respectively; Figs. 9A, 9B and 9C illustrate spherical aberration, astigmatism and f-ϑ characteristic of a sixth example, respectively; Figs. 10A, 10B and 10C illustrate spherical aberration, astigmatism and f-ϑ characteristic of a seventh example, respectively; Figs. 11A, 11B and 11C illustrate spherical aberration, astigmatism and f-ϑ characteristic of an eigth example, respectively; and Fig. 12 is a perspective view showing an optical system of the optical beam scanner. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3 illustrates an embodiment of a telecentric f-ϑ lens system according to the present invention. The f-ϑ lens system is formed by a rear lens group R L and a front lens group F L , which are arranged in order from an image formation surface IS side at a prescribed interval. The front lens group F L is formed by first, third and fourth meniscus lenses L₁, L₃ and L₄ having positive power and a second lens L₂ having negative power, as shown in Fig. 3. The first to fourth lenses L₁ to L₄ are arranged in order from an entrance pupil EP side at prescribed intervals, while all of concave surfaces S₁, S₃, S₅ and S₇ of the first to fourth lenses L₁ to L₄ are directed toward the entrance pupil EP. On the other hand, the rear lens group R L is formed by a fifth lens L₅ having positive power, and a convex surface S₁₀ of the fifth lens L₅ is directed toward an image formation surface IS. The f-ϑ lens system having the aforementioned structure satisfies: -0.65 < (r₁/f) < -0.25 (1) 0.4 < (d₈/f) < 1.16 (2) 1.61 < (f₅/f) < 3.5 (3) where r₁ represents the radius of curvature of a concave surface S₁ of the first lens L₁, d₈ represents the distance between the fourth and fifth lenses L₄ and L₅, f₅ represents the focal length of the fifth lens L₅, and f represents the focal length of the f-ϑ lens system. The inequality (1) shows the condition for correcting the spherical aberration and curvature of image surface. That is, when the value (r₁/f) is not less than -0.25, correction of the spherical aberration is so insufficient that the curve of astigmatism of the meridional image surface is also inclined toward the negative (-) side, whereby astigmatism at an end of a field angle is increased. When the value (r₁/f) is not more than -0.65, on the other hand, the spherical aberration is so excessively corrected and high-order meridional field curvature is increased. Both the inequalities (2) and (3) show conditions that the f-ϑ lens system has a telecentric property. That is, when the value (d₈/f) is not more than 0.4, not only the f-ϑ lens system cannot have the telecentric property but high-order meridional field curvature is arised. When the value (d₈/f) is not less than 1.16, on the other hand, the spherical aberration is excessively corrected and both of the meridional and sagittal image surfaces tend to be inclined toward the negative (-) sides. When the value (f₅/f) is not less than 3.5, both of the meridional and sagittal image surfaces are inclined toward the negative (-) sides, and when the value (f₅/f) is not more than 1.61, on the other hand, high-order astigmatism appears on the meridional image surface. In either case, therefore, it is difficult to implement a telecentric optical system which has small spherical aberration etc. When, to the contrary, an f-ϑ lens is formed by the first to fifth lenses L₁ to L₅ and the f-ϑ lens satisfies the inequalities (1) to (3) (Examples 1 to 8 described below), a telecentric f-ϑ lens which has the following characteristics can be obtained: (1) F-number is not more than 20; (2) spherical aberration, astigmatism etc. are small; and (3) image surface is flat. Example 1 Table 1 shows lens data of a telecentric f-ϑ lens system structured along Fig. 3. Referring to Table 1, symbol r i (i = 1, 2, ..., 10) represents radius of curvature of lens surfaces S i , such that r₁ represents the radius of curvature of the concave surface S₁ of the first lens L₁ and r₂ represents the radius of curvature of the convex surface S₂ of the first lens L₁ directed toward the image formation surface IS. Symbol d i (i = 1, 3, 5, 7, 9) represents thickness values of the first to fifth lenses L₁ to L₅, respectively. On the other hand, d i (i = 2, 4, 6, 8) represents spaces between pairs of the first to fifth lenses L₁ to L₅, respectively, such that d₂ represents the space between the first and second lenses L₁ and L₂, for example. Symbol n i (i = 1, 2, ..., 5) represents indexes of refraction of the first to fifth lenses L₁ to L₅ with respect to a line d . Symbol γ i (i = 1, 2, ..., 5) represents Abbe's numbers of the first to fifth lenses L₁ to L₅. The focal length f , the F-number F NO etc. of the telecentric f-ϑ lens having the aforementioned lens data are set as hown in Table 2. f F NO λ d₀ 100.00 20 623.8 mm 15.00 Referring to Table 2, d₀ represents the space between the entrance pupil EP and the concave surface S₁ of the first lens L₁. The focal length f₅ of the fifth lens L₅ is 320, although this is not shown in Tables 1 and 2. Thus, r₁/f = -0.634 d₈/f = 1.031 f₅/f = 3.20 and it is apparent that the telecentric f-ϑ lens system satisfies the inequalities (1) to (3). Figs. 4A, 4B and 4C illustrate spherical aberration, astigmatism and f-ϑ characteristic of the telecentric f-ϑ lens system having the aforementioned structure, respectively. The results shown in these figures have been obtained with respect to light of 632.8 nm in wavelength on the basis of the above data. In each of Fig. 4B and Figs. 5B, 6B, 7B, 8B, 9B, 10B and 11B, which are hereinafter described in detail, symbol S denotes a sagittal image surface and symbol M denotes a meridional image surface, respectively. The f-ϑ characteristic shown in each of Figs. 4C, 5C, 6C, 7C, 8C, 9C, 10C and 11C is a value showing scanning property provided by the following definition assuming that ϑ represents an angle of polarization and y represents a beam spot position from an optical axis Z on an image formation surface: Example 2 Table 3 shows lens data of a second example according to the telecentric f-ϑ lens system. The focal length f , the F-number F NO etc. of the telecentric f-ϑ lens system having the aforementioned lens data are set as shown in Table 4. f F NO λ d₀ 100.00 20 632.8 mm 15.00 The focal length f₅ of the fifth lens L₅ is 261.9, although this is not shown in Tables 3 and 4. Thus, r₁/f = -0.25 d₈/f = 0.787 f₅/f = 2.619 and it is apparent that the telecentric f-ϑ lens satisfies the inequalities (1) to (3). Figs 5A, 5B and 5C illustrate spherical aberration, astigmatism and f-ϑ characteristic of the telecentric f-ϑ lens having the aforementioned structure, respectively. The results shown in these figures have been obtained with respect to light of 632.8 nm in wavelength on the basis of the above data. Example 3 Table 5 shows lens data of a third example according to the telecentric f-ϑ lens system. The focal length f , the F-number F NO etc. of the telecentric f-ϑ lens system having the aforementioned lens data are set as shown in Table 6. f F NO λ d₀ 100.00 20 632.8 mm 15.00 The focal length f₅ of the fifth lens L₅ is 343.8, although this is not shown in Tables 5 and 6. Thus, r₁/f = -0.469 d₈/f = 1.16 f₅/f = 3.438 and it is apparent that the telecentric f-ϑ lens satisfies the inequalities (1) to (3). Figs 6A, 6B and 6C illustrate spherical aberration, astigmatism and f-ϑ characteristic of the telecentric f-ϑ lens having the aforementioned structure, respectively. The results shown in these figures have been obtained with respect to light of 632.8 nm in wavelength on the basis of the above data. Example 4 Table 7 shows lens data of a fourth example according to the telecentric f-ϑ lens system. The focal length f , the F-number F NO etc. of the telecentric f-ϑ lens system having the aforementioned lens data are set as shown in Table 8. f F NO λ d₀ 100.00 20 632.8 mm 15.00 The focal length f₅ of the fifth lens L₅ is 161.6, although this is not shown in Tables 7 and 8. Thus, r₁/f = -0.400 d₈/f = 0.400 f₅/f = 1.616 and it is apparent that the telecentric f-ϑ lens satisfies the inequalities (1) to (3). Figs 7A, 7B and 7C illustrate spherical aberration, astigmatism and f-ϑ characteristic of the telecentric f-ϑ lens having the aforementioned structure, respectively. The results shown in these figures have been obtained with respect to light of 632.8 nm in wavelength on the basis of the above data. Example 5 Table 9 shows lens data of a fifth example according to the telecentric f-ϑ lens system. The focal length f , the F-number F NO etc. of the telecentric f-ϑ lens system having the aforementioned lens data are set as shown in Table 10. f F NO λ d₀ 100.00 18.018 632.8 mm 15.00 The focal length f₅ of the fifth lens L₅ is 231.5, although this is not shown in Tables 9 and 10. Thus, r₁/f = -0.430 d₈/f = 0.713 f₅/f = 2.315 and it is apparent that the telecentric f-ϑ lens satisfies the inequalities (1) to (3). Figs 8A, 8B and 8C illustrate spherical aberration, astigmatism and f-ϑ characteristic of the telecentric f-ϑ lens having the aforementioned structure, respectively. The results shown in these figures have been obtained with respect to light of 632.8 nm in wavelength on the basis of the above data. Example 6 Table 11 shows lens data of a sixth example according to the telecentric f-ϑ lens system. The focal length f , the F-number F NO etc. of the telecentric f-ϑ lens system having the aforementioned lens data are set as shown in Table 12. f F NO λ d₀ 100.00 20.00 632.8 mm 10.00 The focal length f₅ of the fifth lens L₅ is 199.1, although this is not shown in Tables 11 and 12. Thus, r₁/f = -0.379 d₈/f = 1.054 f₅/f = 1.991 and it is apparent that the telecentric f-ϑ lens satisfies the inequalities (1) to (3). Figs 9A, 9B and 9C illustrate spherical aberration, astigmatism and f-ϑ characteristic of the telecentric f-ϑ lens having the aforementioned structure, respectively. The results shown in these figures have been obtained with respect to light of 632.8 nm in wavelength on the basis of the above data. Example 7 Table 13 shows lens data of a seventh example according to the telecentric f-ϑ lens system. The focal length f , the F-number F NO etc. of the telecentric f-ϑ lens system having the aforementioned lens data are set as shown in Table 14. f F NO λ d₀ 100.00 18.018 632.8 mm 10.00 The focal length f₅ of the fifth lens L₅ is 210.2, although this is not shown in Tables 13 and 14. Thus, r₁/f = -0.350 d₈/f = 0.961 f₅/f = 2.102 and it is apparent that the telecentric f-ϑ lens satisfies the inequalities (1) to (3). Figs 10A, 10B and 10C illustrate spherical aberration, astigmatism and f-ϑ characteristic of the telecentric f-ϑ lens having the aforementioned structure, respectively. The results shown in these figures have been obtained with respect to light of 632.8 nm in wavelength on the basis of the above data. Example 8 Table 15 shows lens data of a seventh example according to the telecentric f-ϑ lens system. The focal length f , the F-number F NO etc. of the telecentric f-ϑ lens system having the aforementioned lens data are set as shown in Table 16. f F NO λ d₀ 100.00 18.018 488 mm 10.00 The focal length f₅ of the fifth lens L₅ is 214.7, although this is not shown in Tables 15 and 16. Thus, r₁/f = -0.399 d₈/f = 0.983 f₅/f = 2.147 and it is apparent that the telecentric f-ϑ lens satisfies the inequalities (1) to (3). Figs 11A, 11B and 11C illustrate spherical aberration, astigmatism and f-ϑ characteristic of the telecentric f-ϑ lens having the aforementioned structure, respectively. The results shown in these figures have been obtained with respect to light of 488 nm in wavelength on the basis of the above data. The spherical aberration of the f-ϑ lens system is extremely small as understood from the spherical aberration diagrams (Figs. 4A, 5A, 6A, 7A, 8A, 9A, 10A and 11A), and the astigmatism thereof is also extremely small as understood from the astigmatism diagrams (Figs. 4B, 5B, 6B, 7B, 8B, 9B, 10B and 11B). Further, the f-ϑ lens system structured as above has an excellent f-ϑ characteristic, as understood from the f-ϑ characteristic diagrams (Figs. 4C, 5C, 6C, 7C, 8C, 9C, 10C and 11C). As shown in the spherical aberration diagrams and the astigmatism diagrams, the curves of the spherical aberration and astigmatism of the f-ϑ lens system are inclined toward the same direction. In Example 6, for example, the curve of the spherical aberration is inclined toward the negative side (see Fig. 9A), while the curve of the astigmatism is also inclined toward the negative side (see Fig. 9B). Thus, a flat image surface can be obtained, which is formed by putting together with least circles of confusion. Further, all the F-numbers F NO of the f-ϑ lens systems are not more than 20 in Examples 1 to 8, as hereinabove described. Therefore, when the aforementioned f-ϑ lens is applied to an optical beam scanner shown in Fig. 12, the beam spot diameter can be easily reduced to thereby improve resolution of the scanner. Fig. 12 is a perspective view showing an optical system of the optical beam scanner to which the aforementioned telecentric f-ϑ lens system is applied. Referring to Fig. 12, a laser beam B₁ outgoing from a laser tube 21 is incident upon an optical modulator system 25 which is formed by lenses 22 and 23 and an acoustic optical modulation element 24. In the optical modulator system 25, the beam B₁ enters the lens 22 at a prescribed angle to an optical axis of the lens 22 to be inclined by a prescribed angle from the horizontal plane (X-Y plane), so that a laser beam B₂ then enters the acoustic optical modulation element 24. This laser beam B₂ is on-off controlled on the basis of a signal which corresponds to an image to be recorded on a drum 30, which will be hereinafter described in detail. A laser beam B₃ outgoing from the acoustic optical modulation element 24 with inclination from the horizontal plane (X-Y plane) is corrected to a horizontal laser beam B₄ by the lens 23. The laser beam B₄ reflected by a mirror 26, and thereafter incident upon a polygon mirror 28 which is rotated at a constant angular velocity through a beam expander 27. Since a diameter of a laser beam B₅ reflected by the polygon mirror 28 is varied according to an area of the reflecting surface thereof, the polygon mirror 28 performs not only a deflector but also an apperture stop, so that an entrance pupil of a telecentric f-ϑ lens system 29, which is structed along Fig. 3, is located on the reflecting surface of the polygon mirror 28. The laser beam B₅ reflected by the polygon mirror 28 is imaged on the drum 30 through the telecentric f-ϑ lens system 29. Therefore, a laser beam B₆ applied onto the drum 30 through the polygon mirror 28 and the telecentric f-ϑ lens system 29 performs scanning in a direction parallel to the axis 30a of rotation of the drum 30, i.e., the so-called primary scanning direction X. The drum 30 is coupled with a driving part (not shown) to be rotated about the rotation axis 30a by the driving part for subscanning. As hereinabove described, the above telecentric f-ϑ lens system 29 having a small F-number can be applied to the optical system of the optical beam scanner shown in Fig. 12, whereby the beam spot diameter of the laser beam B₆ can be easily reduced to improve resolution of the optical beam scanner.

Claims: A f-ϑ lens system comprising: (a) a first lens group (FL) having an entrance pupil (EP), this first lens group including: (a-1) a first meniscus lens (L₁) having a positive power, said first lens (L₁) having a concave surface (S₁) which is directed to said entrance pupil (EP), (a-2) a second lens (L₂) having a negative power, (a-3) third and fourth meniscus lenses (L₃, L₄) each having a positive power, respectively, and (b) a second lens group (RL) including a fifth lens (L₅) having a positive power, the first through fifth lenses (L₁-L₅) being successively disposed in order from said entrance pupil (EP), characterized in that said f-ϑ lens system is telecentric and satisfies the following inequalities: -0.65 < (r₁/f) < -0.25 0.4 < (d₈/f) < 1.16 1.61 < (f₅/f) < 3.5, where r₁ is the radius of curvature of said concave surface (S₁) of said first lens (L₁), f is the focal length of the system, d₈ is the distance between said fourth and fifth lenses (L₄, L₅), and f₅ is the focal length of said fifth lens (L₅). A scanning system for scanning an optical beam (B) on a recording surface (30) comprising a telecentric f-ϑ lens system as said forth in claim 1, characterized by a light source (21) for generating an optical beam (B) and a deflector (28) for deflecting the optical beam (B) outgoing from said light source (21), wherein the telecentric f-ϑ lens system (29) images the optical beam (B) deflected by said deflector (28) on said recording surface (30).

Inventors: SHIROTA HIROYUKI DAINIPPON SCR, SHIROTA, HIROYUKI DAINIPPON SCREEN MFG. CO., LTD.
Applicants: DAINIPPON SCREEN MFG, DAINIPPON SCREEN MFG. CO., LTD.
Application year: 1989
Main classification: G02B13
IPC codes: G02B13