KR100343967B1 - Projection Lens System - Google Patents

Abstract

The present invention relates to a projection lens system that is compact and capable of chromatic aberration correction.

The projection lens system of the present invention has a first lens having a positive refractive power at the center and a negative refractive power at the periphery and a relatively large amount of the positive refractive power to cover most of the total refractive power of the projection lens system for correcting spherical aberration, wave front aberration and astigmatism. A second lens having refractive power, a third lens having positive refractive power and a fourth lens having negative refractive power for correcting astigmatism and image curvature, and the first lens, the third lens, and the fourth lens are designed as aspherical surfaces And a diffractive optical element formed on at least one of aspherical surfaces of the lens for chromatic aberration correction.

According to the present invention, by employing a diffractive optical element to correct chromatic aberration and spherical aberration, it is possible to realize a high resolution without increasing the number of lenses, and to reduce the manufacturing cost.

Description

Projection Lens System

The present invention relates to a projection optical system of the rear projection apparatus, and more particularly to a projection lens system capable of compact and chromatic aberration correction.

In recent years, as the demand for large screens and high-definition images increases as a display device, the spread of projection type devices that enlarge and project a small image by using a projection lens is rapidly spreading. Projection apparatuses are roughly classified into a front projection method and a rear projection method depending on the direction in which the image is projected on the screen. Among them, the rear projection is getting more attention due to the advantage of displaying a relatively bright image even in a bright environment. Projection Television (hereinafter referred to as Projection Television) is a typical rear projection apparatus. Projection TVs mainly use cathode ray tubes (hereinafter referred to as CRTs) or liquid crystal displays (hereinafter referred to as LCDs) as a light source for realizing a fire image. The reproduced image reproduced on the CRT or the liquid crystal panel is enlarged by the projection lens and then projected on the rear of the screen to be displayed in a large screen.

1A and 1B, a front and side internal structure of a projection TV using a conventional CRT as a light source is shown. The projection TV includes a CRT (2) for displaying an image corresponding to an image signal, a projection optical system (4) for magnifying and projecting the image displayed on the CRT (2), and an image projected from the projection lens system (4). A reflecting mirror 6 reflecting toward the screen, and a screen 8 for displaying an image projected by the projection lens system 4 in an enlarged manner. The CRT 2 is composed of red, green, and blue CRTs (2R, 2G, 2B), as shown in Fig. 1A, to display fire extinguishing images of red, green, and blue, respectively. The projection lens system 4 consists of red, green, and blue projection lens systems 4R, 4G, and 4B disposed on the output side of each of the red, green, and blue CRTs (2R, 2G, 2B). 2R, 2G, and 2B) enlarge and project the image from each. The screen 8 displays a large color image by reflecting the image projected by the red, green, blue projection lens system 4R, 4G, 4B of the reflector 6 and forming an image on the screen 8. Here, the projection lens system 4 needs to secure a performance capable of decomposing the scanning line on the CRT through chromatic aberration correction as well as spherical aberration, astigmatism, and distortion in order to realize high-definition high performance. To this end, each of the red, green, and blue projection lens systems 4R, 4G, and 4B is composed of a plurality of plastic lenses and glass lenses as shown in FIG.

In detail, each of the red, green, and blue projection lens systems 4R, 4G, and 4B includes the first and second lenses 10 and 12 having weak refractive power, respectively, and most of the projection lens systems, as shown in FIG. The third lens 14 which is responsible for refractive power, the fourth lens 16 having a weak positive refractive power, and the fifth lens 18 having a strong negative refractive power are provided. The first lens 10 corrects spherical aberration and the second lens 12 corrects coma and astigmatism. The first and second lenses 10 and 12 are made of plastic material. The fourth and fifth lenses 16 and 18 are made of a plastic material to perform a function of correcting astigmatism. The third lens 14 is composed of a glass-bonded lens to correct chromatic aberration. In other words, the third lens 14 corrects chromatic aberration by a combination of a lens having a positive refractive power and a lens having a negative refractive power.

As described above, the conventional projection lens system 4 is composed of the combined lens 14 and the number of lenses in order to correct chromatic aberration and various optical aberrations. Thus, the conventional projection lens system 4 is not only difficult to miniaturize by providing a plurality of lenses, but also causes a cost increase. Accordingly, there is a demand for a projection lens system capable of realizing high resolution and high brightness while reducing the number of lenses.

Accordingly, it is an object of the present invention to provide a projection lens system capable of realizing high resolution and high brightness while reducing the number of lenses.

1a and 1b is a front view and a side view schematically showing the configuration of a general rear projection device.

2 is a diagram showing a configuration of a conventional projection lens system.

3 is a diagram illustrating a configuration of a projection lens system according to an exemplary embodiment of the present invention.

4A and 4B show dispersion characteristics of light beams of a refractive lens and a diffractive optical element, respectively.

5A and 5B are graphs illustrating chromatic aberration correction characteristics of a projection lens system according to an exemplary embodiment of the present invention.

FIG. 6 is a graph showing a phase amount of a diffractive surface of the diffractive optical element shown in FIG. 3; FIG.

<Description of Symbols for Main Parts of Drawings>

2: cathode ray tube 4: projection lens system

6: reflector 8: screen

10, 20: first lens 12, 22: second lens

14, 24: third lens 16, 26: fourth lens

18: fifth lens 24A: diffractive optical element

28: refractive lens

In order to achieve the above object, the projection lens system according to the present invention is a first lens having a positive refractive power in the center and a negative refractive power in the peripheral portion for the correction of spherical aberration, wave front aberration and astigmatism, and most of the total refractive power of the projection lens system A second lens having a relatively large positive refractive power, a third lens having a positive refractive power and a fourth lens having a negative refractive power for correcting astigmatism and image curvature, a first lens and a third lens And the fourth lens is designed as an aspherical surface and includes a diffractive optical element formed on at least one of aspherical surfaces of the lens to correct chromatic aberration.

Other objects and features of the present invention other than the above object will become apparent from the description of the preferred embodiment with reference to the accompanying drawings.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to FIGS. 3 to 6.

3 illustrates a projection lens system according to an exemplary embodiment of the present invention. The projection lens system of FIG. 3 has a first lens 20 having a positive refractive power at the center and a negative refractive power at the periphery, a second lens 22 having a positive refractive power, and a diffraction on one side with a positive refractive power. A third lens 24 in which an optical element (DOE) is formed and a fourth lens 26 having negative refractive power are provided. The first lens 20 is made of a plastic aspherical lens to correct the spherical aberration. In addition, the first lens 20 has positive refractive power in the center portion and negative refractive power in the peripheral portion, thereby correcting coma and astigmatism. The second lens 22 is made of glass to cover most of the total refractive power of the projection lens system. The third and fourth lenses 24 and 26 are made of plastic lenses to have a function of correcting astigmatism and field curvature. Here, the third lens 24 has a structure in which the diffractive optical element 24A is formed on one side of the lens having positive refractive power to correct chromatic aberration.

In detail, the conventional refractive lens 28 allows the focal length of the blue (B) light beam to be shorter than the focal length of the red (R) light beam among the light beams having the color signal as shown in FIG. 4A. On the other hand, the diffractive optical element 24A allows the focal length of the red (R) light beam to be shorter than the focal length of the blue (B) light beam among the light beams having the color signal as shown in FIG. 4B. In other words, the color signal dispersion characteristics of the refractive lens 28 and the diffractive optical element 24A are opposite to each other. Accordingly, the third lens 24 including the plastic lens (that is, the refractive lens) having positive refractive power and the diffractive optical element 24A formed on the surface of the plastic lens with diffraction characteristics is distributed with opposite color signals. The characteristic is used to correct chromatic aberration. By this third lens 24, the projection lens system shown in Fig. 3 has good chromatic aberration correction characteristics as shown in Figs. 5A and 5B.

Data on curvature radius, spacing and refractive index of each lens surface that can be applied in the design of the projection lens system having the above-described configuration, and the characteristics of each lens surface are shown in Table 1.

Lens surface Radius of curvature interval Refractive index Cotton features S1 72.4300 8.6400 1.4935 Aspheric surface S2 80.9700 28.8000 Aspheric surface S3 73.6700 25.0000 1.5916 Spherical S4 -215.3300 20.0200 Spherical S5 -359.8100 8.0000 1.4938 Aspheric surface S6 -94.5 29.55 Diffraction optical plane S7 -49.5100 3.5000 1.4938 Aspheric surface S8 -47.0000 12.9400 1.4392 Refrigerant S9 infinity 14.1000 1.5551 plate S10 -350.0000 0.000 Ph

The coefficient values for determining the aspherical shape and the shape of the diffractive optical element surface of the projection lens system shown in FIG. 3 are shown in Table 2 below.

Lens surface K a1 / c1 a2 / c2 a3 / c3 a4 / c4 a5 / c5 S1 -1.3480E + 00 1.6080E-07 -8.4679E-10 1.5770E-13 1.3850E-17 -2.9262E-21 S2 5.0000E-02 5.3046E-07 -1.8278E-09 1.4642E-12 -7.9311E-16 2.5535E-19 S5 8.3325E + 01 -1.5942E-06 -6.9339E-10 3.800E-13 -6.0061E-16 2.0398E-19 S6 -14.987 9.66E-07 5.32E-10 -3.77E-13 -3.26E-17 0 S7 0.432533 -3.12E-06 1.09E-08 -1.90E-11 1.80E-14 -8.65E-18 S6 (Diffraction Optical Element Coefficient) -4.45E-04 0 0 0 0 0

Here, coefficient values for determining the shape of the first lens 20, the third lens 24, and the fourth lens 26 formed as aspherical surfaces are defined by an aspherical formula as shown in Equation 1 below.

X is a Seg value for an aspherical surface at height r from the optical axis, c is a curvature of the lens surface, K is a Conic constant, and a1 to a4 are aspherical coefficients. The aspherical phase amount of the diffractive optical element 24A generated by the interference between the object light and the reference light in the diffractive optical element 24A is determined by the following equation.

Here,? (R) represents the phase at the height r point from the optical axis. c1 to c4 mean coefficients of a phase term having an aspherical effect. The phase amount of the diffractive optical element 24A applicable to the present invention by the above Equation 2 has a characteristic of decreasing in proportion to the height r from the optical axis as shown in FIG. The phase amount characteristic graph of the diffractive optical element 24A shown in FIG. 6 is related to the leap number of the diffractive optical element 24A, and the diffraction optical element 24A is used to improve optical performance considering the diffraction efficiency and the processability of the lens. The optimal design of the phase amount in charge is required. To this end, the diffractive optical element 24A is formed such that a plurality of concentric grooves have rotational symmetry, and the pitch of the grooves becomes smaller from the center to the periphery. By combining the diffractive optical element 24A with a plastic aspherical lens, chromatic aberration, spherical aberration, distortion aberration, and the like can be corrected. Accordingly, it is not necessary to separately use a lens made of an expensive material that has a negative refractive power to increase beam dispersion for chromatic aberration correction, thereby reducing costs and miniaturizing the projection lens system. In addition, as the focal length of the projection lens system becomes shorter, the projection lens system becomes thinner. To this end, it is preferable to design the second lens 22, which is responsible for most of the total refractive power of the projection lens system, to have a large refractive power. However, when the refractive power is large, spherical aberration occurs, thereby increasing the refractive power of the second lens 22. There is a limit. Accordingly, by dividing the refractive power of the second lens 22 to the diffractive optical element 24A, the refractive power of the projection lens system can be increased to reduce the overall focal length, thereby making it thinner.

Further, the diffraction optical element 24A can improve the brightness of the projection lens system. In other words, it is possible to improve the brightness of the projection lens system by sharing the refractive power of the second lens 22 with the diffractive optical element 24A to reduce the focal length of the projection lens system. This is because the brightness of the projection lens system is inversely proportional to the square of F / # having a relationship proportional to the focal length f as shown in Equation 3 below.

Here, D represents the aperture of the lens. According to Equation 3, as the overall focal length of the projection lens system becomes smaller, F / # becomes smaller, and as a result, the brightness of the projection lens system inversely proportional to the square of F / # is improved, thereby enabling high brightness.

As described above, the projection lens system of the present invention employs the diffractive optical element 24A to correct chromatic aberration, spherical aberration, and the like, thereby improving optical performance without increasing the number of lenses. In addition, the projection lens system of the present invention employs the diffractive optical element 24A to share the refractive power, thereby reducing the overall focal length, thereby enabling thinning and high brightness. In addition, the projection lens system of the present invention compensates optical aberrations (spherical aberration, wave front aberration, astigmatism) by using the first lens 20 having an aspherical surface and positive refractive power at the center and negative refractive power at the periphery. As a result, the number of lenses can be reduced.

As described above, the projection lens system according to the present invention employs a diffractive optical element to correct chromatic aberration and spherical aberration, thereby realizing high resolution without increasing the number of lenses and reducing manufacturing costs. In addition, according to the projection lens system according to the present invention, by reducing the focal length by adopting the diffractive optical element, it is possible to realize a thinner and higher brightness. In addition, the projection lens system of the present invention can reduce the number of lenses by using a lens that can compensate for spherical aberration, wavefront aberration, astigmatism, so that the thickness can be reduced and manufacturing cost can be reduced.

Those skilled in the art will appreciate that various changes and modifications can be made without departing from the technical spirit of the present invention. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims.

Claims (7)

In the projection lens system, A first lens having positive refractive power at the center and negative refractive power at the periphery for correcting spherical aberration, wave front aberration and astigmatism, A second lens having a relatively large amount of refractive power to cover most of the total refractive power of the projection lens system; A third lens having a positive refractive power and a fourth lens having a negative refractive power for correcting astigmatism and image curvature, And the first lens, the third lens, and the fourth lens are designed as an aspherical surface and include diffractive optical elements formed on at least one of aspherical surfaces of the lens to correct chromatic aberration. The method of claim 1, A projection lens system, characterized in that one aspherical surface of the third lens having a positive refractive power of the lens formed as the diffractive optical element surface. The method of claim 1, The diffractive optical element is a projection lens system, characterized in that a plurality of concentric grooves are formed with rotational symmetry. The method of claim 3, wherein And a pitch of the groove is reduced so that the amount of phase decreases from the center of the diffractive optical element toward the periphery. The method of claim 1, And the second lens is made of glass, and the first lens, the third lens, and the fourth lens are made of plastic. delete delete