# Patent

## Title
Projection optical system

## Abstract

Provided is a projection optical system including a first part which projects an image; a second part having an image forming device which forms an image using a light and emits the image to the first part, and having an axis angled toward an axis of the first part; a third part including at least one light source, which emits the light, and having an axis angled toward the axis of the second part; and a deflector which guides the light emitted from the third part toward the second part, wherein a pupil plane of the second part is located between the first part and the second part, and a cross-point between the axis of the first part and the pupil plane of the second part is separated from a cross-point between the axis of the second part and the pupil plane of the second part.

## Background
CROSS-REFERENCE TO RELATED PATENT APPLICATION
This application claims priority from Korean Patent Application No. 10-2008-0019320, filed on Feb. 29, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to an optical system, and more particularly, to a projection optical system which can be used to generate and project an image.
2. Description of the Related Art
In a projection optical system, a light generated by a light source is condensed, by a condensing lens providing homogeneous illumination, on a film and then is projected onto a screen via a lens. In order to form an image, a recent projection optical system (disclosed in Japanese Patent No. 2000-321529, and Japanese Patent No. 2005-092206) does not use the film but uses micromirror devices, such as a digital micromirror device (DMD), which are composed of a set of rotary mirrors. Such a projection optical system includes an illuminating optics such as the set of rotary mirrors for achieving the homogeneous illumination, and a projecting optics such as the lens for projecting the image onto the screen.
However, the projection optical system is disadvantageous in that, due to a relatively long travel distance of light within the projection optical system, a considerable loss of light occurs, and this requires an increase in optical power which may excessively heat the projection optical system. In order to prevent such excessive heat, the overall size of the projection optical system needs to be increased.
According to a projection system disclosed in U.S. Pat. No. 6,439,726, an illuminating optics and a projecting optics of the projection system are divided into first, second and third optical parts. The first and second optical parts have a common optical axis and together compose a projection lens. The second and third optical parts together compose the illuminating optics. An auxiliary illumination light, such as a backlight, is emitted from the third optical part, is illuminated on the second optical part including a micromirror device, forms an image pattern, is emitted from the projection system and then is projected onto the screen after having passed through the first optical part. At this time, by installing the third optical part at an angle to the first and second parts, an optical trajectory of the light, that is, a distance between a light source and the micromirror device is shortened so that loss of light is reduced and the required optical power is reduced. Accordingly, the size of the projection system can be reduced.
However, such a projection system has the following two disadvantages. First, in general, a compact projection system uses a light source, such as a gas-discharge lamp or a light emitting diode, which has great divergence. At this time, the light efficiency of a device, the overall size of the device, and image quality are important features. The higher requirements to the light efficiency along with application of high-aperture light sources necessitate great numerical aperture of the illuminating light beam falling on the micromirror plate and the great numerical aperture of the projection lens. Thus, an aperture diaphragm of the second optical part, wherein the aperture diaphragm is located between the first optical part and the second optical part, represents a pupil plane with respect to both the illumination light and a projection beam. A beam splitting in the pupil plane (with the numerical aperture that is equal to, or that exceeds, sinus of an inclination angle of the micromirror plate) or in its vicinity, minimizes and uniformizes beam vignetting. An acceptable distance depends on the numerical aperture of the lens, that is, the smaller the numerical aperture is, the further displacement of the beam splitting node from the pupil is possible. In general, such a distance does not exceed a diameter of the pupil. For efficient separation of an illumination part and a projecting part, an aberration in the pupil is also important. In other words, coincidence of the position and the size of the pupil for all points of the field is important. Also, pupil quality is important not only for the projecting part, but also for the illuminating part. In the case where the illuminating part has great pupil aberrations, the projection system demonstrates heterogeneity of vignetting across an image area (field), and increase in an ambient light component, such that optical devices overheat and image quality deteriorates. Thus, in order to achieve a uniform brightness and generate a high-quality image in the compact projection system, correction of the pupil of the second optical part is necessary for both the projection beam and illumination beam. By doing so, an adjustable numerical aperture of the second optical part is increased. In this case, an applied aperture is asymmetric with respect to the second optical part when the illumination beam reaches the second optical part at a great angle. On the other hand, the projection beam generally propagates symmetrically or almost symmetrically along an optical axis.
Second, in the compact projection system an image is projected a large diagonal distance, and there is a short distance between a projector and screen. However, the compact projection system has to project an image beyond an optical axis of the projection system for a spectators' convenience. This prevents the casting of spectators' shadows on the screen. For this purpose, the screen has to be placed above the optical axis of the projection system when the projector is located on a table, and the screen has to be placed below the optical axis of the projection system when the projector is located on a ceiling. In order to generate an image on a screen placed beyond the optical axis of the projection system, an asymmetric projection lens is used in a related art image projection apparatus. Here, the set of rotary mirrors is shifted in a vertical direction to an optical axis of the asymmetric projection lens and disposed. Such a disposal requires a significant increase in an adjustable vision field. An adjustable area of an object, that is, an area located in a plane of the set of rotary mirrors can exceed an effective vision field, which is normally equal to the size of the set of rotary mirrors, by two or three times. This seriously complicates lens design calculations, in terms of correction of distortions and aberrations. Thus, costs of calculations and manufacturing of the projection system increase, and the overall size of the projection system also increases.
SUMMARY OF THE INVENTION
Exemplary embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantages described above, and an exemplary embodiment of the present invention may not overcome any of the problems described above. Accordingly, it is an aspect of the present invention to provide a digital TV and a digital broadcasting system capable of actively controlling an application function with efficient use of a limited number of keys when operating an application program provided by a base broadcasting station by inputting a combination of suitable keys corresponding to the concerned application program, thereby controlling the application function, and a control method thereof.
The present invention provides a projection system which has a reduced size and is easy to manufacture by simplifying a design of a second optical part.
According to an aspect of the present invention, there is provided a projection optical system, including a first optical part which projects an incident image; a second optical part including an image forming device which forms an image by using a light, and emits the image to the first optical part, wherein the second optical part has an optical axis, and wherein the optical axis of the second optical part is angled toward an optical axis of the first optical part; a third optical part including at least one light source, which emits the light, wherein the third optical part has an optical axis, and wherein the optical axis of the third optical part is angled toward the optical axis of the second optical part; and a deflector which guides the light emitted from the third optical part toward the second optical part, wherein a pupil plane of the second optical part is located between the first optical part and the second optical part, and a first cross-point, which is between the optical axis of the first optical part and the pupil plane of the second optical part, is separated from a second cross-point, which is between the optical axis of the second optical part and the pupil plane of the second optical part.
An angle formed between the optical axis of the third optical part and the optical axis of the second optical part may be between 0° and 90°, and an angle formed between the optical axis of the first optical part and the optical axis of the second optical part may be between 0.3° and 15°.
A distance, by which the optical axis of the first optical part and the optical axis of the second optical part are separated on the pupil plane, may be between 2% and 50% of a pupil diameter of the second optical part.
The deflector may be one of a mirror, a prism, and an apparatus including both the mirror and the prism.
The image forming device may be a reflective-type image forming device, and a digital micromirror device (DMD) or a liquid crystal on silicon (LCOS) may be employed for the reflective-type image forming device.
The first optical part includes a first meniscus lens having a first concave side, which faces the second optical part; a first positive lens; and a first cemented including a first negative lens and a second positive lens, wherein the second positive lens is closer to the second optical part.
The second optical part includes a third positive lens; and a second cemented lens formed by gluing a second negative lens and a fourth positive lens, and having a structure in which the second negative lens is relatively closer to the image forming device.
A first cross-point, which is between the optical axis of the first optical part and the pupil plane of the second optical part, may be located at a position that is distant from a second cross-point, which is where the optical axis of the second optical part meets the pupil plane of the second optical part, by as much as 0.3 through 3.0 mm.
The third optical part may include a plurality of monochromatic light sources; a color composition device which combines a light beam from the plurality of monochromatic light sources; and a lens unit which adjusts a numerical aperture and a shape of a light beam combined by the color composition device.
According to the present invention, the projection optical system may have a reduced size, may be easy to design and manufacture, and may project an image on a screen placed beyond an optical axis.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:
FIG. 1 is a diagram of a projection optical system according to an exemplary embodiment of the present invention;
FIGS. 2 and 3 are diagrams of first and second optical parts of the projection optical system in FIG. 1; and
FIG. 4 is a diagram of a third optical part of the projection optical system in FIG. 1.



DETAILED DESCRIPTION OF THE INVENTION
The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. Like reference numerals in the drawings denote like elements. In the drawings, a size of each component can be exaggerated for clarity.
FIG. 1 is a diagram of a projection optical system 100 according to an exemplary embodiment of the present invention. The projection optical system 100 includes a first optical part 1, a second optical part 2, a third optical part 3, and a deflector 4.
The first optical part 1 includes at least one optical device so as to project an incident image.
The second optical part 2 forms an image by using an incident light, and emits the formed image to the first optical part 1. The second optical part 2 includes a reflective-type image forming device 6.
The third optical part 3 includes at least one light source 5, forms and emits a light.
The deflector 4 changes a light path so that the light emitted from the third optical part 3 is toward the second optical part 2. For example, the deflector 4 may be formed of one of an apparatus including a mirror, an apparatus including a prism, and an apparatus including both a mirror and prism.
The first optical part 1 and the second optical part 2 together compose a projecting optics by which the image, which is presented on a reflective surface of the reflective-type image forming device 6, is displayed on a screen (not shown). The second optical part 2 guides the light from the light source 5, wherein the light is formed by the third optical part 3 and redirected by the deflector 4.
Here, a pupil plane of the second optical part 2 is located between the first optical part 1 and the second optical part 2. The first optical part 1, the second optical part 2, and the third optical part 3 respectively have optical axes 7, 8, and 9. The optical axis 9 of the third optical part 3 is angled toward the optical axis 8 of the second optical part 2. For example, the optical axis 9 and the optical axis 8 form an angle between 0° and 90°. Also, the optical axis 8 of the second optical part 2 is angled toward the optical axis 7 of the first optical part 1. The angle formed between the optical axis 8 and the optical axis 7 may have a value between 0.3° and 15°. The optical axis 8 of the second optical part 2 is shifted in a vertical direction to the optical axis 7 of the first optical part 1 within an incident pupil plane of the first optical part 1. The shifted distance may be a value between 2% and 50% of a pupil diameter of the first optical part 1.
This value range is selected based on the following reasons. That is, if a distance between the optical axis 8 of the second optical part 2 and the optical axis 7 of the first optical part 1 is smaller than 2% of the pupil diameter, and if the angle formed between the optical axis 8 and the optical axis 7 is smaller than 0.3°, a positive effect for reducing the size of the projection optical system 100 having an effective vision field becomes negligible. Also, if the distance between the optical axis 8 of the second optical part 2 and the optical axis 7 of the first optical part 1 exceeds 50% of the pupil diameter, and if the angle formed between the optical axis 7 and the optical axis 8 is smaller than 15°, aberrations may not actually be compensated for.
When the angle formed between the optical axis 7 and the optical axis 8 is in a range between 0.5° and 3.0°, a cross-point between the optical axis 7 of the first optical part 1 and an incident pupil plane of the second optical part 2 may be located at a position that is distant from a cross-point where the optical axis 8 of the second optical part 2 meets the incident pupil plane, by as much as 0.3 through 3.0 mm.
In general, the projection optical system 100 is structured such that the third optical part 3 forms a predetermined-shaped light by using the light from the light source 5, and guides the predetermined-shaped light to the deflector 4. The deflector 4 guides the light formed by the third optical part 3 to the second optical part 2. The light, which has passed through the second optical part 2 in one direction, is reflected from the reflective-type image forming device 6. Here, the reflective-type image forming device 6 forms the image by using the incident light. Next, the light, which was formed to the image, passes through the second optical part 2 in an opposite direction, and reaches an entrance of the first optical part 1. The first optical part 1 projects the incident image on the screen (not shown).
As described above, the first optical part 1 and the second optical part 2 are arranged in a manner that the optical axis 7 of the first optical part 1 is angled toward the optical axis 8 of the second optical parts 2 by the angle between 0.3° and 15°. Such an arrangement shifts a projected image from the optical axis 7 of the first optical part 1, and provides a projection lens having an off-axis vision field. The pupil plane of the second optical part 2 is located between the first optical part 1 and the second optical part 2. The optical axis 7 of the first optical part 1 crosses the pupil plane of the second optical part 2 at a cross-point where the optical axis 8 of the second optical parts 2 crosses the pupil plane of the second optical part 2, by having a distance between 2% and 50% of a pupil diameter of the second optical part 2 with respect to a projection beam. Also, the optical axis 9 of the third optical part 3 and the optical axis 8 of the second optical parts 2 form an angle below 90 degrees. By doing so, the projection optical system 100 having a numerical aperture of more than 0.1 and projecting an image beyond an optical axis is provided.
In the projection optical system 100 according to an aspect of the present invention, the first optical part 1 and the second optical part 2 together compose the projection lens, and the optical axis 7 of the first optical part 1 and the optical axis 8 of the second optical part 2 is angled toward each other and are separated from each other. Accordingly, in an axially asymmetric projection system, a shift of a digital micromirror device (DMD) may be reduced so that a linear filed of the second optical part 2 is reduced, and light diameter of lenses composing the second optical part 2 is reduced. By simplifying a structure of the lens and reducing the light diameter without increasing the aberrations, a projector may be maintained considerably shifted from a normal which faces the screen and which is in a center of the image, and an adjustable numerical aperture of the second optical part 2 may be reduced.
That is, in order to easily project a shifted image, the projection optical system 100 according to the present invention partially performs centering for an object field of the second optical part 2, and also partially performs the centering for a space in the pupil plane so as to simplify a structure of the second optical part 2 so that the adjustable numerical aperture of the second optical part 2 may be reduced.
The projection optical system 100 according to the present invention provides a possibility about a projection system which has a great numerical aperture and which shifts an image by as much as 20% of a vertical dimension. The present invention provides the possibility about an optimized solution which satisfies two contradictory requirements, that is, a light efficiency and an image quality.
FIGS. 2 and 3 are diagrams of the first and second optical parts 1 and 2 of the projection optical system 100 in FIG. 1. Referring to FIGS. 2 and 3, the first optical part 1 is sequentially arranged toward the second optical part 2. The first optical part 1 includes a first meniscus lens 10 facing the second optical part 2 with its concave side, a first positive lens 11, and a first cemented lens 12 formed by gluing two lenses. The first cemented lens 12 may include a second positive lens 12-1 and a first negative lens 12-2, and may have a structure in which the second positive lens 12-1 is relatively closer to the second optical part 2. The second optical part 2 includes the reflective-type image forming device 6, a third positive lens 13, and a cemented lens 14 formed by gluing two lenses. The cemented lens 14 may include a second negative lens 14-1 and a fourth positive lens 14-2, and may have a structure in which the second negative lens 14-1 is relatively closer to the reflective-type image forming device 6. The digital micromirror device (DMD), or liquid crystal on silicon (LCOS) that is a reflective-type liquid crystal display (LCD) may be employed for the reflective-type image forming device 6.
FIG. 4 is a diagram of the third optical part 3 of the projection optical system 100 in FIG. 1. The third optical part 3 includes a plurality of monochromatic light sources 5, a color composition device 19 which combines lights from the plurality of monochromatic light sources 5, and a lens unit 20 which adjusts a numerical aperture and a shape of a light beam combined by the color composition device 19.
Each of the plurality of monochromatic light sources 5 may be a red light source, a green light source, and a blue light source. The color composition device 19 may be an X-cube used for splitting or combining of beams. Each of the plurality of monochromatic light sources 5 is disposed to face each surface of the color composition device 19.
The lens unit 20 adjusts the light beam combined and transmitted by the color composition device 19 so as to have a shape and numerical aperture requested by the projection optical system 100. The lens unit 20 may include a second meniscus lens 21 facing the color composition device 19 by its concave side, and a fifth positive lens 22.
A light condenser 18 may be arranged between the color composition device 19 and each of the plurality of monochromatic light sources 5. The light condenser 18 is an optical element which reduces a divergent angle of the lights from the plurality of monochromatic light sources 5, and uniformly condenses the light beam. For example, a compound parabolic concentrator such as a Winston concentrator or a focon may be employed as the light condenser 18. A relatively wider surface of the light condenser 18 is disposed so as to face each of the plurality of monochromatic light sources 5.
An aspect of the present invention has been described using a case having three colored (red, green, and blue) light sources, but the present invention is not limited thereto.
The aforementioned lenses of the projection optical system according to the present invention can be manufactured using glass or plastic. The manufacturing of the lenses may be possible by using a related art method.
The projection optical system according to the present invention can be applied to all types of projectors, projection television (TV) receivers, displays and other devices using front or rear projection in which compactness and a high optical efficiency are required.
While this invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The exemplary embodiments should be considered in a descriptive sense only and not for purposes of limitation. Therefore, the scope of the present invention is defined not by the detailed description of the present invention, but by the appended claims, and all differences within the scope will be construed as being included in the present invention.

## Claims

1. A projection optical system comprising:
a first optical part which projects an image;
a second optical part including an image forming device which forms an image using a light, and emits the image to the first optical part, wherein the second optical part has an optical axis, and wherein the optical axis of the second optical part is angled toward an optical axis of the first optical part;
a third optical part including at least one light source, which emits the light, wherein the third optical part has an optical axis, and wherein the optical axis of the third optical part is angled toward the optical axis of the second optical part; and
a deflector which guides the light emitted from the third optical part toward the second optical part,
wherein a pupil plane of the second optical part is located between the first optical part and the second optical part, and a first cross-point, which is between the optical axis of the first optical part and the pupil plane of the second optical part, is separated from a second cross-point, which is between the optical axis of the second optical part and the pupil plane of the second optical part.

2. The projection optical system of claim 1, wherein an angle formed between the optical axis of the third optical part and the optical axis of the second optical part is between 0° and 90°.

3. The projection optical system of claim 1, wherein an angle formed between the optical axis of the first optical part and the optical axis of the second optical part is between 0.3° and 15°.

4. The projection optical system of claim 1, wherein a distance, by which the optical axis of the first optical part and the optical axis of the second optical part are separated on the pupil plane, is between 2% and 50% of a pupil diameter of the second optical part.

5. The projection optical system of claim 1, wherein the deflector is one of a mirror, a prism, and an apparatus including both the mirror and the prism.

6. The projection optical system of claim 1, wherein the image forming device is a reflective-type image forming device.

7. The projection optical system of claim 6, wherein the reflective-type image forming device is one of a digital micromirror device (DMD) and a liquid crystal on silicon (LCOS).

8. The projection optical system of claim 1, wherein the first optical part includes:
a first meniscus lens having a first concave side which faces the second optical part;
a first positive lens; and
a first cemented lens including a first negative lens and a second positive lens, wherein the second positive lens is closer to the second optical part, and
wherein the first meniscus lens, the first positive lens and the first cemented lens are sequentially arranged in a direction toward the second optical part.

9. The projection optical system of claim 8, wherein the first cemented lens is formed by gluing the first negative lens and the second positive lens.

10. The projection optical system of claim 1, wherein the second optical part includes:
a third positive lens; and
a second cemented lens which includes a second negative lens and a fourth positive lens, wherein the second negative lens is closer to the image forming device, and
wherein the third positive lens and the second cemented lens are sequentially arranged in a direction toward the first optical part.

11. The projection optical system of claim 10, wherein the second cemented lens second cemented lens is formed by gluing the second negative lens and the fourth positive lens.

12. The projection optical system of claim 1, wherein, when the angle formed between the optical axis of the first optical part and the optical axis of the second optical part is between 0.5° and 3.0°, a first cross-point, which is between the optical axis of the first optical part and the pupil plane of the second optical part, is located at a position that is distant from a second cross-point, which is where the optical axis of the second optical part meets the pupil plane of the second optical part, by as much as 0.3 through 3.0 mm.

13. The projection optical system of claim 12, wherein the lens unit includes a second meniscus lens having a concave side facing the color composition device, and a fifth positive lens.

14. The projection optical system of claim 1, wherein the third optical part includes:
a plurality of monochromatic light sources;
a color composition device which combines a light beam from the plurality of monochromatic light sources; and
a lens unit which adjusts a numerical aperture and a shape of a light beam combined by the color composition device.

15. The projection optical system of claim 14, wherein the color composition device is an X-cube, each of the plurality of monochromatic light sources faces each surface of the X-cube, and a light condenser is arranged between the X-cube and each of the plurality of monochromatic light sources.

16. The projection optical system of claim 1, wherein the pupil plane of the second optical part is substantially perpendicular to optical axis of the second optical part and is angled toward the optical axis of the first optical part.

17. The projection optical system of claim 1, wherein a path of the light emitted by the second optical part to the first optical part omits the deflector.

18. The projection optical system of claim 1, wherein a position of the first cross-point is disposed closer to the deflector than a position of the second cross-point.