# US 2019/0170988 A1 ยท Projection optical system, projection apparatus, and projection system > Agents: this data is available as JSON from the patentgrep API and MCP server โ€” see https://patentgrep.com/llms.txt - Number: US 2019/0170988 A1 - Document ID: US20190170988A1 - Status: Published application - Kind: A1 - Application number: 16/270,164 - Filed: 2019-02-07 - Published: 2019-06-06 - Earliest claimed: 2015-08-27 - Assignees: Ricoh Company, Ltd. - Applicants: Ricoh Company, Ltd. - Inventors: Yohei TAKANO; Hibiki Tatsuno; Yasuyuki Shibayama - Web page: https://patentgrep.com/patent/US20190170988A1 ## CPC - G02B 15/1421: {the first group being positive} - G02B 7/021: {for more than one lens} - G02B 13/16: for use in conjunction with image converters or intensifiers {, or for use with projectors, e.g. objectives for projection TV} - G02B 17/00: Systems with reflecting surfaces, with or without refracting elements - G02B 17/08: Catadioptric systems {(used in non-imaging applications G02B19/00)} - G02B 17/0852: {having a field corrector only} - G02B 17/0896: {with variable magnification or multiple imaging planes, including multispectral systems (systems with only refractive elements G02B15/14)} - G03B 21/142: {Adjusting of projection optics} - G03B 21/28: Reflectors in projection beam {(in illumination beam G03B21/2066)} - G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS - G03B: APPARATUS OR ARRANGEMENTS FOR TAKING PHOTOGRAPHS OR FOR PROJECTING OR VIEWING THEM; APPARATUS OR ARRANGEMENTS EMPLOYING ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ACCESSORIES THEREFOR (optical parts of such apparatus G02B; photosensitive materials or processes for photographic purposes G03C; apparatus for processing exposed photographic materials G03D) ## Abstract An optical system includes a reflective optical system on a magnification side along an optical path of the projection optical system and a refractive optical system on a reduction side along the optical path. The reflective optical system includes one reflective optical element having a power. The refractive optical system includes a front group on the magnification side and a rear group on the reduction side. The front group having, in order from the magnification side toward the reduction side, a first lens group with a positive or negative refractive power, a second lens group, and a third lens group with a positive refractive power. The rear group has a positive refractive power. The first lens group moves to the magnification side, and the second lens group and the third lens group move to the reduction side in a change in focus from a long distance to a short distance. ## Claims ### Claim 1 A projection optical system, comprising: a reflective optical system on a magnification side along an optical path of the projection optical system, the reflective optical system including at least one reflective optical element having a power; and a refractive optical system on a reduction side along the optical path, the refractive optical system including: a front group on the magnification side; and a rear group on the reduction side having a positive refractive power, wherein the front group includes a plurality of focusing lens groups that moves to at least one of the magnification side and the reduction side in a change in focus from a long distance to a short distance, the plurality of focusing lens groups include a first focusing lens group closest to the magnification side, a second focusing lens group second closest to the magnification side in the focusing lens group, and a third focusing lens group third closest to the magnification side, a distance between the first focusing lens group and the third focusing lens varies in the change in focus from the long distance to the short distance, and the rear group of the refractive optical system is fixed during the change in focus. ### Claim 2 (depends on 1) The projection optical system according to claim 1, wherein a first amount of movement of the first focusing lens is different from a second amount of movement of the second focusing lens in the change in focus from the long distance to the short distance. ### Claim 3 (depends on 1) The projection optical system according to claim 1, wherein the first focusing lens group has a positive refractive power. ### Claim 4 (depends on 1) The projection optical system according to claim 1, wherein the first focusing lens group moves to the magnification side in the change in focus from the long distance to the short distance. ### Claim 5 (depends on 1) The projection optical system according to claim 1, wherein the second focusing lens group has a negative refractive power. ### Claim 6 (depends on 1) The projection optical system according to claim 1, wherein the third focusing lens group has a positive refractive power. ### Claim 7 A projection optical system, comprising: a reflective optical system on a magnification side along an optical path of the projection optical system, the reflective optical system including at least one reflective optical element having a power; and a refractive optical system on a reduction side along the optical path, the refractive optical system including: a front group on the magnification side; and a rear group on the reduction side having a positive refractive power, wherein the front group includes three focusing lens groups that move to at least one of the magnification side and the reduction side in a change in focus from a long distance to a short distance, the three focusing lens groups include a first focusing lens group closest to the magnification side, a second focusing lens group, and a third focusing lens group third closest to the magnification side, a distance between the first focusing lens group and the third focusing lens varies in the change in focus from the long distance to the short distance, and the rear group of the refractive optical system is fixed during the change in focus. ### Claim 8 (depends on 7) The projection optical system according to claim 7, wherein the first focusing lens group has a positive refractive power. ### Claim 9 (depends on 7) The projection optical system according to claim 7, wherein the first focusing lens group moves to the magnification side in the change in focus from the long distance to the short distance. ### Claim 10 (depends on 9) The projection optical system according to claim 9, wherein the second focusing lens group has a negative refractive power. ### Claim 11 (depends on 1) The projection optical system according to claim 1, wherein โˆ’0.20