USOO6266 191B1 (12) United States Patent (10) Patent No.: US 6,266,191 B1 Abe (45) Date of Patent: *Jul. 24, 2001 (54) DIFFRACTIVE-REFRACTIVE ACHROMATIC 5,568,325 10/1996 Hirando et al. ...................... 359/785 LENS 5,790,321 * 8/1998 Goto ............ ... 359/571 5,818,632 * 10/1998 Stephenson . ... 359/566 (75) Inventor: Tetsuya Abe, Hokkaido (JP) 5,949.577 * 9/1999 Ogata ................................... 359/570 (73) Assignee: Asahi Kogaku Kogyo Kabushiki * cited by examiner Kaisha, Tokyo (JP) Primary Examiner Audrey Chang (*) Notice: This patent issued on a continued pros ASSistant Examiner Jennifer Winstedt ecution application filed under 37 CFR (74) Attorney, Agent, or Firm-Greenblum & Bernstein 1.53(d), and is subject to the twenty year P.L.C. patent term provisions of 35 U.S.C. (57) ABSTRACT 154(a)(2). A diffractive-refractive achromatic lens includes a refractive Subject to any disclaimer, the term of this lens System exhibiting longitudinal chromatic aberration patent is extended or adjusted under 35 that is Substantially proportional to wavelength Such that the U.S.C. 154(b) by 0 days. back focus of the refractive lens System decreaseS as the wavelength becomes shorter and a diffractive grating for (21) Appl. No.: 09/244,077 correcting the longitudinal chromatic aberration of the refractive lens System. The refractive lens having Such a (22) Filed: Feb. 4, 1999 chromatic aberration includes a positive lens having rela (30) Foreign Application Priority Data tively Small dispersion and a negative lens having relatively large dispersion. Further, the following condition (1) should Feb. 5, 1998 (JP) ................................................. 1O-O24789 be satisfied: (51) Int. Cl." .............................. G02B 27/44; G02B 5/18 (52) U.S. Cl. ........................... 359/566; 359/565; 359/569 0.005<ff-0.2 (58) Field of Search ..................................... 359/795, 566, where 359/569, 570, 571, 565, 16 f is a focal length of the entire lens System, and (56) References Cited f, is a focal length of the positive diffractive grating. U.S. PATENT DOCUMENTS 4,190.324 2/1980 Arai ..................................... 359/789 17 Claims, 15 Drawing Sheets U.S. Patent Jul. 24, 2001 Sheet 1 of 15 US 6,266,191 B1 U.S. Patent Jul. 24, 2001 Sheet 2 of 15 US 6,266,191 B1 U.S. Patent Jul. 24, 2001 Sheet 3 of 15 US 6,266,191 B1 U.S. Patent Jul. 24, 2001 Sheet 4 of 15 US 6,266,191 B1 þ'1:NA U.S. Patent Jul. 24, 2001 Sheet 5 of 15 US 6,266,191 B1 2'!:NA 0"|0- % U.S. Patent Jul. 24, 2001 Sheet 6 of 15 US 6,266,191 B1 0"|0- % ENIT–p ENIT-6··· ENIT-O---- U.S. Patent Jul. 24, 2001 Sheet 7 of 15 US 6,266,191 B1 U.S. Patent Jul. 24, 2001 Sheet 8 of 15 US 6,266,191 B1 U.S. Patent Jul. 24, 2001 Sheet 9 of 15 US 6,266,191 B1 U.S. Patent Jul. 24, 2001 Sheet 10 Of 15 US 6,266,191 B1 0"|0- % ENIT-?)– ENIT-6······· ENIT-O---- U.S. Patent Jul. 24, 2001 Sheet 11 of 15 US 6,266,191 B1 U.S. Patent Jul. 24, 2001 Sheet 12 Of 15 US 6,266,191 B1 U.S. Patent Jul. 24, 2001 Sheet 13 of 15 US 6,266,191 B1 NOILHO1S?0 Z'OZIO U.S. Patent Jul. 24, 2001 Sheet 14 of 15 US 6,266,191 B1 U.S. Patent Jul. 24, 2001 Sheet 15 of 15 US 6,266,191 B1 G08013008'9|+808'0|+w06914 US 6,266,191 B1 1 2 DIFFRACTIVE-REFRACTIVE ACHROMATIC persion and a negative lens having relatively large disper LENS sion. Further, the following condition (1) should be satisfied: BACKGROUND OF THE INVENTION 0.005<ff-0.2 (1) The present invention relates to an achromatic lens to be where used as a collimator lens, or an objective lens for a telescope. f is a focal length of the entire lens System, and Particularly, the present invention relates to a diffractive refractive achromatic lens that includes a refractive lens f, a is focal length of the positive diffractive grating. System provided with a diffractive grating. Preferably, the diffractive grating may be designed So that Conventionally, an achromatic lens has been well known the following condition (2) is satisfied: and used as a collimator lens or an objective lens. An example of the achromatic lens employs two thin lenses: one being a lens having large positive power that is made from crown glass having relatively low dispersion; and the other 15 where being a lens having Small negative power that is made from P and P are diffractive coefficients of second and fourth flint glass having relatively high dispersion. The resultant orders when the diffractive grating is expressed by the power is therefore positive, while the dispersion is neutral following optical path difference function d(h) ized. Since optical glass has dispersion Such that refractive indeX thereof increases with decreasing wavelength of light, a positive refractive lens has longitudinal chromatic aberra where tion where a back focus of the lens decreases as wavelength h is a height from the optical axis, and becomes shorter. The achromatic doublet corrects the chro 2 is a design wavelength of the diffractive grating. matic aberration of the positive lens by using a negative lens 25 Embodiments described in this specification includes that has opposite chromatic aberration. three groups. In a first group of the embodiments, the The conventional achromatic doublet is designed for refractive lens System includes a non-achromatic doublet. In correcting the longitudinal chromatic aberration with respect the Second and third groups of embodiments, the refractive to two different wavelengths, e.g., F-line (486 nm) and lens System consists of a combination of a refractive ach C-line (656 nm). That is, the light beams of F-line and C-line romatic lens and an additional lens. In each case, the are focused on Substantially the same focal point. diffractive grating could be formed on one Surface of the However, light beams having wavelengths except the C refractive lenses or provided as a separate diffractive ele and F-lines are not focused on the same focal point. ment. Particularly, the back focus of the doublet becomes remark In the first group of the embodiments, the doublet should ably larger in a range shorter than F-line. This remaining 35 have longitudinal chromatic aberration Such that the back focus of the refractive lens System decreases as the wave chromatic aberration is called as Secondary Spectrum. length becomes shorter. In Such a case, it is preferable that If the normal optical glasses are used for constituting the the refractive lens System Satisfies the following condition conventional type of the achromatic lens, it is impossible to correct the Secondary Spectrum Sufficiently. Because refrac (3); 40 tive index of glass increases as wavelength becomes shorter 1 (3) as described, while increasing degree of the flint glass is 0.02 larger than that of the crown glass in Short wavelength range. Then the chromatic aberration of the doublet is overcor rected (backfocus becomes too long) in the range shorter where than F-line. 45 In order to Suppress the Secondary Spectrum of the lon f is a focal length of the refractive lens System, gitudinal chromatic aberration with the conventional type of f is a focal length of an i-th lens of the refractive lens the achromatic lens, fluorite or anomalous dispersion glass System counted from the object Side, should be used. However, these materials are too expensive. v is an Abbe number of the i-th lens of the refractive lens 50 System counted from the object Side, and SUMMARY OF THE INVENTION m is a total number of lenses of the refractive lens System. It is therefore an object of the present invention to provide In the Second and third groups of the embodiments, the an achromatic lens, which is capable of correcting the refractive achromatic lens is corrected in chromatic aberra Secondary Spectrum of the longitudinal chromatic aberration 55 tion at two different wavelengths and an additional refractive without using expensive material Such as the fluorite or lens generates longitudinal chromatic aberration Such that anomalous dispersion glass. the back focus of the refractive lens System decreases as the For the above object, according to the present invention, wavelength becomes shorter. In Such a case, it is preferable there is provided a diffractive-refractive achromatic lens that that the refractive achromatic lens Satisfies the following includes a refractive lens System exhibiting longitudinal 60 condition (4), chromatic aberration that is Substantially proportional to wavelength such that the back focus of the refractive lens 1 (4) System decreases as the wavelength becomes shorter, and a positive diffractive grating for correcting the longitudinal fiXT, chromatic aberration of the refractive lens System. 65 The refractive lens System having Such a chromatic aber where ration includes a positive lens having relatively Small dis f is a focal length of the refractive achromatic lens, US 6,266,191 B1 3 4 f is a focal length of an i-th lens of the refractive FIGS. 2A through 2D are graphs showing Spherical achromatic lens counted from the object Side, aberration, lateral chromatic aberration, astigmatism and v is an Abbe number of the i-th lens of the refractive distortion, respectively, of the lens in FIG. 1; achromatic lens counted from the object Side, and FIG. 3 is a lens diagram of the diffractive-refractive m is a total number of lenses of the refractive achromatic achromatic lens according to a Second embodiment; lens.