PROJECTION OPTICAL SYSTEM AND IMAGE PROJECTION DEVICE

• US 20120162753A1
• Filed: 12/06/2011
• Published: 06/28/2012
• Est. Priority Date: 12/27/2010
• Status: Active Grant

• Current Assignee: Ricoh Company Limited
• Original Assignee: Ricoh Company Limited
• Inventors: TATSUNO, Hibiki
• Granted Patent: US 8,717,671 B2
• Time in Patent Office: Days
• US Class Current: 359/364
• CPC Class Codes: G02B 17/08 Catadioptric systems used in non-imaging applications G02B19/00
• CPC Class Codes: G02B 17/0816 off-axis or unobscured systems in which not all of the mirrors share a common axis of rotational symmetry, e.g. at least one of the mirrors is warped, tilted or decentered with respect to the other elements
• CPC Class Codes: G03B 21/147 Optical correction of image distortions, e.g. keystone
• CPC Class Codes: G03B 21/28 Reflectors in projection beam in illumination beam G03B21/2066

Abstract

A projection optical system for projecting an image on a surface is provided. The image is an enlarged image of an image which is formed on an image forming element. The projection optical system includes a coaxial optical system having an optical axis; and a non-coaxial optical system including a rotationally asymmetric curved-surface mirror. The non-coaxial optical system does not share the optical axis with the coaxial optical system. The coaxial optical system includes a first lens having a positive refractive power and being an aspheric plastic lens; and a second lens having a negative refractive power and being an aspheric plastic lens. The first lens has a first refractive index distribution, and the second lens has a second refractive index distribution.

Claims

1. A projection optical system for projecting an image on a surface to be projected, the image being an enlarged image of a formed image which is formed on an image forming element, the projection optical system comprising:

a coaxial optical system having an optical axis; and

a non-coaxial optical system including a rotationally asymmetric curved-surface mirror, the non-coaxial optical system not sharing the optical axis with the coaxial optical system,

wherein the coaxial optical system and the non-coaxial optical system are arranged in this order on a light path from the image forming element to the surface,

wherein the coaxial optical system includes, at least, a first lens having a positive refractive power and being an aspheric plastic lens; and

a second lens having a negative refractive power and being an aspheric plastic lens,

wherein the first lens has a first refractive index distribution along a first direction from the center of the first lens to a peripheral portion of the first lens, and the second lens has a second refractive index distribution along a second direction from the center of the second lens to a peripheral portion of the second lens, and

wherein one of the first lens and the second lens is arranged at a position closest to the curved-surface mirror among the optical elements included in the coaxial optical system.

2. The projection optical system according to claim 1, wherein,

when, for the first lens, a first refractive index at the center of the first lens is N1, and the first refractive index at the peripheral portion of the first lens is N1_o, and when, for the second lens, a second refractive index at the center of the second lens is N2_i and the second refractive index at the peripheral portion of the second lens is N2_o, a sign of a first difference of the first refractive indices of the first lens Δ

N₁=N1_i−

N1_o along the first direction from the center of the first lens to the peripheral portion of the first lens is equal to a sign of a second difference of the second refractive indices of the second lens Δ

N₂=N2_i−

N2_o along the second direction from the center of the second lens to the peripheral portion of the second lens.

3. The projection optical system according to claim 2, wherein

the first lens and the second lens are independently reciprocable in a direction along the optical axis so as to adjust a focal point of the image being projected on the surface, and

wherein the focal point of the image being projected on the surface is adjusted by changing a spacing of at least three optical elements included in the projection optical system, the at least three optical elements including the first lens, the second lens, and the curved-surface mirror.

4. The projection optical system according to claim 3, wherein

the curved-surface mirror is a free-form surface mirror, and

wherein, when a horizontal direction of the surface is defined to be an X-axis direction and a vertical direction of the surface is defined to be a Y-axis direction, a curvature in the X-axis direction of the curved-surface mirror increases as a Y-axis coordinate value increases from a first Y-axis coordinate value corresponding to a first end portion of the curved-surface mirror, the first end portion being close to the optical axis of the coaxial optical system, to a second Y-axis coordinate value corresponding to a second end portion of the curved-surface mirror, the second end portion being separated from the optical axis of the coaxial optical system.

5. The projection optical system according to claim 2, wherein,

when a first distance from a first point, at which a first outermost light beam among first light beams passing through the first lens exits the first lens, to the optical axis is W₁, and when a second distance from a second point, at which a second outermost light beam among second light beams passing through the second lens exits the second lens, to the optical axis is W₂, (Δ

N₁/W₁) and (Δ

N₂/W₂) are substantially equal.

6. The projection optical system according to claim 1, wherein

a focal distance of the first lens and a focal distance of the second lens are substantially equal.

7. The projection optical system according to claim 1, wherein

the first lens and the second lens are arranged adjacent to each other.

8. The projection optical system according to claim 1, wherein

at least one of the first lens and the second lens has a shape such that a portion, through which light fluxes do not pass, is removed from a rotationally symmetric shape.

9. A projection optical system for projecting an image on a surface to be projected, the image being an enlarged image of a formed image which is formed on an image forming element, the projection optical system comprising:

a coaxial optical system having an optical axis; and

a non-coaxial optical system including a rotationally asymmetric curved-surface mirror, the non-coaxial optical system not sharing the optical axis with the coaxial optical system,

wherein the coaxial optical system and the non-coaxial optical system are arranged in this order on a light path from the image forming element to the surface,

wherein the coaxial optical system includes at least a group of lenses including a first lens and a second lens,

wherein the first lens has a first refractive power, a surface of the first lens close to the image forming element being an aspheric surface, and a surface of the first lens close to the curved-surface mirror being a spherical surface,

wherein the second lens has a second refractive power, a surface of the second lens close to the image forming element being a spherical surface, and a surface of the second lens close to the curved-surface mirror being an aspheric surface,

wherein the spherical surface of the first lens and the spherical surface of the second lens are joined,

wherein the first lens has a first refractive index distribution along a first direction from the center of the first lens to a peripheral portion of the first lens, and the second lens has a second refractive index distribution along a second direction from the center of the second lens to a peripheral portion of the second lens, and

wherein the second lens is arranged at a closest position to the curved-surface mirror among the optical elements included in the coaxial optical system.

10. A projection optical system according to claim 9, wherein

the first refractive power is a positive refractive power and the second refractive power is a negative refractive power.

11. A projection optical system according to claim 9, wherein

the first refractive power is a negative refractive power and the second refractive power is a positive refractive power.

12. An image projection device comprising:

an image forming element configured to form a formed image thereon in accordance with a modulated signal; and

a projection optical system that irradiates light from a light source to the image forming element and projects an image on a surface to be projected, the image being an enlarged image of the formed image which is formed on the image forming element, the projection optical system including;

a coaxial optical system having an optical axis; and

a non-coaxial optical system including a rotationally asymmetric curved-surface mirror, the non-coaxial optical system not sharing the optical axis with the coaxial optical system,

wherein the coaxial optical system and the non-coaxial optical system are arranged in this order on a light path from the image forming element to the surface,

wherein the coaxial optical system includes, at least, a first lens having a positive refractive power and being an aspheric plastic lens; and

a second lens having a negative refractive power and being an aspheric plastic lens,

wherein the first lens has a first refractive index distribution along a first direction from the center of the first lens to a peripheral portion of the first lens, and the second lens has a second refractive index distribution along a second direction from the center of the second lens to a peripheral portion of the second lens, and

wherein one of the first lens and the second lens is arranged at a position closest to the curved-surface mirror among the optical elements included in the coaxial optical system.

13. The image projection device according to claim 12, wherein

the projection optical system is an intermediate image system that forms a real image of the image forming element in between the coaxial optical system and the non-coaxial optical system.

14. The image projection device according to claim 12, wherein

a spacing between the image forming element and a lens being arranged at a position closest to the image forming element is fixed.

15. The image projection device according to claim 12, wherein,

when, for the first lens, a first refractive index at the center of the first lens is N1_i and the first refractive index at the peripheral portion of the first lens is N1_o, and when, for the second lens, a second refractive index at the center of the second lens is N2_i and the second refractive index at the peripheral portion of the second lens is N2_o, a sign of a first difference of the first refractive indices of the first lens Δ

N₁=N1_i−

N1_o along the first direction from the center of the first lens to the peripheral portion of the first lens is equal to a sign of a second difference of the second refractive indices of the second lens Δ

N₂=N2_i−

N2_o along the second direction from the center of the second lens to the peripheral portion of the second lens.

16. The image projection device according to claim 15, wherein

the first lens and the second lens are independently reciprocable in a direction along the optical axis so as to adjust a focal point of the image being projected on the surface, and

wherein the focal point of the image being projected on the surface is adjusted by changing a spacing of at least three optical elements included in the projection optical system, the at least three optical elements including the first lens, the second lens, and the curved-surface mirror.

17. The image projection device according to claim 16, wherein

the curved-surface mirror is a free-form surface mirror, and

wherein, when a horizontal direction of the surface is defined to be an X-axis direction and a vertical direction of the surface is defined to be a Y-axis direction, a curvature in the X-axis direction of the curved-surface mirror increases as a Y-axis coordinate value increases from a first Y-axis coordinate value corresponding to a first end portion of the curved-surface mirror, the first end portion being close to the optical axis of the coaxial optical system, to a second Y-axis coordinate value corresponding to a second end portion of the curved-surface mirror, the second end portion being separated from the optical axis of the coaxial optical system.

18. The image projection device according to claim 15, wherein,

when a first distance from a first point, at which a first outermost light beam among first light beams passing through the first lens exits the first lens, to the optical axis is W₁, and when a second distance from a second point, at which a second outermost light beam among second light beams passing through the second lens exits the second lens, to the optical axis is W₂, (Δ

N₁/W₁) and (Δ

N₂/W₂) are substantially equal.

19. The image projection device according to claim 12, wherein

a focal distance of the first lens and a focal distance of the second lens are substantially equal.

20. The image projection device according to claim 12, wherein

the first lens and the second lens are arranged adjacent to each other.