KR20000007615A - Superthin diffraction optical lens - Google Patents

Abstract

PURPOSE: A super thin diffraction optical lens is provided to solve a problem for using a glass type lens without a filter and reduce a cost of production and reduce number of lens and thin a lens system. CONSTITUTION: The super thin diffraction optical lens comprises meniscus type lens surfaces and a diffraction optical device surface. The meniscus type lens of concave non-spherical surface are formed to object direction and the diffraction optical device surface is formed in one of lens surface. The diffraction optical device surface corrects color number difference of optical beam and performs a function of optical low pass filter to attenuate high band pass characteristics of optical beam. The super thin diffraction optical lens also performs a function of absorption filter for an infrared ray.

Description

Ultra-Compact Diffractive Optical Lens

The present invention relates to an ultra-thin diffractive optical lens, and more particularly to an ultra-thin diffractive optical lens that can be thinned.

Recently, with the development of digital technology, the improvement of image compression and restoration technology, and the improvement of peripheral device technology of multimedia products, the camera lens employed in a personal computer (hereinafter referred to as a "PC") has been made thin and low cost. The situation is being developed continuously to. For example, as well as a multimedia computer video conferencing system, the spread of CCD cameras using a charge coupled device (hereinafter referred to as "CCD") as an image input means such as a personal digital assistant and an IMT-2000. This is spreading rapidly. In order to cope with this trend, CCD cameras are required to have excellent performance and excellent portability with an ultra-thin diffractive optical lens system.

Referring to FIG. 1, a thin lens according to the related art includes a meniscus-type first lens 2 having a negative refractive power and a second lens 4 having a positive refractive power. ), A third lens 6 having a positive refractive power, a fourth lens 8 having a negative refractive power, and an optical low pass filter through which a light beam of a low band is passed. Infra-Red Cut Filter (hereinafter referred to as " IRCF ") located between the OLPF 10 and the OLPF 10 to block light in the infrared region included in the light beam. And a CCD 12 for converting the light beam into an electrical signal. The material of the fourth lens 2 to 8 in the first lens is formed of glass, and when a plastic lens is used, chromatic aberration may be large due to the characteristics of the lens material. The first lens 2 is convex toward the object side to secure a wide viewing angle from the object side, and to secure a sufficient back focal length (hereinafter, referred to as "BFL") and provides negative refractive power. The meniscus-type lens which has is used. In addition, the second lens 4 uses a spherical lens having a positive refractive power to converge the light beam emitted from the first lens (2). Meanwhile, an aperture (Iris) 4 is installed between the first and second lenses 2 and 4 to adjust the amount of incident light. A third lens 6 having a positive refractive power and a fourth having a negative refractive power in order to correct image curvature or chromatic aberration generated by the first and second lenses 2 and 4. The lens 8 is configured as a pair. On the other hand, the OLPF 10 passes only the low band of the light beam to remove noise caused by the light beam. That is, the OLPF 10 is used to prevent the moiré pattern from occurring because the high band of the light beam acts as a noise on the grating structure of the CCD 12. In addition, the IRCF 16 blocks light in the infrared region included in the light beam. That is, since the CCD 12 is sensitive to the wavelength of the infrared region and the display characteristics are deteriorated, an IRCF 16 is used to filter light in the infrared region to prevent this. In addition, the CCD 12 converts light incident to itself into an electrical signal to form image information.

However, since a thin lens according to the prior art requires a pair of lenses to correct chromatic aberration and requires at least four lenses, a number of lenses is increased, which makes it difficult to reduce the thickness and weight. Since glass is made of glass, there is a problem that causes a cost increase. In addition, by employing a separate filter, the filter price occupies a greater proportion than the lens, which leads to a cost increase factor. In addition, in order to position IRCF and OLPF, sufficient BFL must be secured, resulting in a problem in that the length of the lens system becomes long and it is difficult to thin.

Accordingly, an object of the present invention is to provide an ultra-thin diffractive optical lens that can be thinned.

1 is a view showing a lens system for an image pickup device according to the prior art.

2 is a view showing an ultra-thin lens according to the present invention.

3 illustrates the principle of chromatic aberration correction.

<Description of Symbols for Main Parts of Drawings>

2: first lens 4: second lens

6: third lens 8: fourth lens

10 optical low pass filter 12,24 image pickup device

14,26: aperture 16: infrared absorption filter

22 diffraction optical lens

In order to achieve the above object, the ultra-thin diffractive optical lens according to the present invention is composed of aspheric meniscus lens surfaces concave toward the object side and has a diffraction optical element surface on at least one of the lens surfaces.

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

With reference to Figures 2 to 3 will be described a preferred embodiment of the present invention.

Referring to FIG. 2, the ultra-thin diffraction optical lens according to the present invention includes a diffraction optical lens 22 composed of meniscus lens surfaces S1 and S2 concave toward the object side, and a front end of the diffraction optical lens 22. A diaphragm 26 positioned to adjust the amount of light beams introduced from the object side and a CCD 24 for converting the light beams into electrical signals are provided. In the diffractive optical lens 22, meniscus lens surfaces S1 and S2 concave toward the object side are formed as an aspherical surface, and at least one diffraction optical element surface (hereinafter referred to as "DOE") is formed. It is formed to correct chromatic aberration, spherical aberration and distortion aberration. The diffraction optical lens 22 performs the function of OLPF because the DOE attenuates the high band (ie, high frequency component) transmission characteristics of the light beam. In addition, since the diffractive optical lens 22 uses a material that absorbs light in the infrared region, it performs the function of IRCF. In addition, the conditions of the focal length of the lens system (hereinafter referred to as “EFL”) and the radius of curvature CR1 and CR2 of the first lens surface S1 and the second lens surface S2 are shown in Equation 1 below. have.

Here, EFL means the focal length of the lens system, CR1 means the radius of curvature of the first lens surface, and CR2 means the radius of curvature of the second lens surface.

In addition, the diffraction optical lens 22 must satisfy the condition of Equation 2 below, in which a transmission characteristic for each wavelength is described below in order to filter the wavelength of the infrared region.

Here, T is the transmission characteristic of the light beam, λ means the wavelength of the light beam.

Further, in the DOE, diffraction occurs due to interference between the object light and the reference light, and the aspherical phase of the DOE when the diffraction occurs is shown in Equation 3 below.

Here φ (r) means the phase at height r. c1 to c4 mean coefficients of phase terms having an aspherical effect. In addition, referring to Equation 3, it can be seen that the phase of the DOE surface is changed at intervals of 2π. That is, the second lens surface S2 is formed so that the characteristics of the aspherical surface and the DOE are properly combined to correct chromatic aberration, spherical aberration and distortion, and attenuate the high band of the light beam to perform the function of OLPF. The principle in which chromatic aberration is corrected in the second lens surface S1 will be described with reference to FIG. 3. As shown in FIG. 3A, the light beams passing through the lens having positive refractive power are blue (hereinafter referred to as "B"), green (hereinafter referred to as "G"), and red. Imaging points are formed at different positions in the order of (Red; hereinafter referred to as "R"). As shown in (b) of FIG. 3, the light beams passing through the DOE are formed at different positions in the order of R, G, and B. FIG. 3 (a) and 3 (b), the incident light beam has chromatic aberration in which an imaging point varies depending on the color component. At this time, when the DOE is formed in the lens as shown in FIG. 3A to correct chromatic aberration, the color components of the incident light beam have the same imaging point, thereby correcting the chromatic aberration. On the other hand, the diffractive optical lens 22 must satisfy the following equation (4) to correct the function and chromatic aberration of the OLPF to prevent moire (Moire) generation.

Here, phi means DOE phase amount.

In addition, the second lens surface S2 is formed to have a positive refractive power so that the light beam corrected for chromatic aberration, spherical aberration, and distortion aberration proceeds to the CCD 24. Meanwhile, the first and second lens surfaces S1 and S2 are formed of aspherical lenses. An aspherical lens will be described. Aspherical lenses are defined by the aspherical formula shown in equation (5).

X is a Seg (Sag) value for the aspherical surface at height r from the optical axis, C is the curvature of the lens surface from the optical axis, K is a Conic constant, a1 to a4 is an aspheric coefficient.

In addition, the curvature radius, the center thickness and the refractive index of the lens system according to the present invention are shown in Table 1.

Curvature radius, center thickness and refractive index of lens system Lens surface Radius of curvature interval Refractive index Characteristic iris · 0.0610 S1 -1.2303 0.3636 1.4936 Aspheric surface S2 -0.4546 0.1033 DOE

In addition, the coefficients of the aspherical surface and the DOE surface for each lens surface according to the present invention are shown in Table 2.

Aspheric and DOE coefficients for each lens surface Lens surface K a1 (c1) a2 (c2) a3 (c3) a4 (c4) a5 (c6) S1 -5.5797E + 02 -3.1759E + 02 2.0990E + 02 -1.0100E + 05 2.5040E + 06 -2.4750E + 02 S2 (Aspherical Surface Factor) -1.7926E + 01 -2.5707E + 01 4.3977E + 02 -5.0593E + 03 2.8749E + 04 0.0000E + 00 S2 (DOE coefficient) 0.0000E + 00 -9.5319E + 02 2.8549E + 00 -3.3670E + 01 0.0000E + 01 0.0000E + 00

As an example of the formation of an aspherical lens, in order to form an S1 aspherical surface having a curvature radius of -1.2303, a predetermined position (i.e., a1 to a5 positions) on a spherical lens having the same radius of curvature is matched with the data shown in Table 2. Processing to form an aspherical lens.

As described above, the problem of introducing the lens of the glass material for correcting chromatic aberration using DOE is solved, and the use of a separate filter can be eliminated, thereby reducing the manufacturing cost. In addition, the number of lenses is reduced, making the lens system thinner.

As described above, by using the ultra-thin diffraction optical lens, DOE according to the present invention solves the problem of introducing the lens of the glass material, there is an advantage that can be eliminated the use of a separate filter can reduce the manufacturing cost.

In addition, the ultra-thin diffractive optical lens according to the present invention has an advantage of reducing the number of lenses to thin the lens system.

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)

An ultra-thin diffractive optical lens comprising a meniscus lens surface of an aspherical surface concave toward the object side and a diffractive optical element surface formed on at least one of the lens surfaces. The method of claim 1, And said diffractive optical element surface is formed to correct chromatic aberration of a light beam. The method of claim 1, And the diffractive optical element surface attenuates the high band transmission characteristics of the light beam to perform a function of an optical low pass filter. The method of claim 1, An ultra-thin diffractive optical lens, characterized in that the phase amount? Of the surface of the diffractive optical element satisfies a range between a lower limit value (0.0) and an upper limit value (30.0). The method of claim 1, An ultra-thin diffractive optical lens, characterized in that it further comprises an aperture positioned at the front of the diffraction optical lens to adjust the amount of light beams flowing from the object side. The method of claim 1, And said diffractive optical lens performs the function of an infrared absorption filter. The method of claim 1, The radius of curvature of the front side of the diffraction optical lens divided by the total focal length satisfies the range of the lower limit (0.5) and the upper limit (1.8), and the radius of curvature of the rear side of the diffraction optical lens divided by the total focal length is the lower limit ( 0.2) and an upper limit value (0.9) satisfying the ultra-thin diffractive optical lens.