KR100536866B1 - Ultra-thin type mega-pixel level optical system for camera - Google Patents

Abstract

The present invention relates to an optical system for an ultra-thin megapixel camera, comprising: a first lens formed of an aspherical surface having a negative magnification having a convex surface on an object side and a concave surface on an image side; An aperture to selectively converge light from the first lens; A second lens and a third lens formed of a spherical surface having a positive magnification having a concave surface on the object side and a convex surface on the object side and sequentially arranged; A fourth lens formed of an aspherical surface having a positive magnification with convex surfaces formed on the object side and the image side, respectively; And an image pickup device for converting light passing through each lens into an electrical signal. As a result, the optical system is composed of two aspherical lenses and two spherical lenses, and the optical length is 17.80 mm, the aperture ratio is F / 2.8, and the angle of view is 53.8 °, so that the optical system can be made smaller and lighter. In addition, manufacturing costs can be reduced, and a 2 mega pixel mega pixel camera capable of obtaining a clear image over a wider area can be implemented.

Description

Optical system for ultra-thin megapixel camera {ULTRA-THIN TYPE MEGA-PIXEL LEVEL OPTICAL SYSTEM FOR CAMERA}

The present invention relates to an optical system for an ultra-thin megapixel camera, and more specifically, by configuring an optical system with two aspherical lenses and two spherical lenses, the optical system can be made smaller and lighter, and the manufacturing cost can be reduced. It relates to an optical system for ultra-thin mega pixel camera.

BACKGROUND OF THE INVENTION [0002] Digital still cameras, which are in the spotlight in recent years, are imaging devices which convert a still image formed by a lens into an electrical signal using an image pickup device, and record the converted electrical signal on an internal memory or a memory card. Such digital still cameras have been spotlighted for their real-time performance and ease of use as playback monitors, but have been pointed out that their quality is lower than that of general cameras. However, as the number of pixels increases and the resolution is improved with the development of the image pickup device, the resolution is closer to that of a general camera.

On the other hand, in recent years, as the performance of small communication devices such as mobile phones and PDAs is diversified, a digital still camera is mounted to store and transmit image data or to perform a video chat. In order to integrate a digital camera into such a small communication device, miniaturization of a digital camera must be preceded. Accordingly, attempts to narrow a focal length by stacking a plurality of lenses have been actively conducted. However, with the use of four or more lenses, the optical length of the lens is long, making the lens extremely thin.

The structure of the optical system for a digital still camera disclosed in Japanese Patent Laid-Open Nos. 3-63613, 10-213742, 10-293246, and the like is as follows. A digital still camera generally includes an optical system including five lenses and an image pickup device for converting a light source passing through each lens into an electrical signal. A low pass filter and a color filter are disposed between the lens and the image pickup device. have.

Here, each lens of the optical system is the first lens having a negative magnification, which is disposed closest to the object side and has a convex surface on the object side, and a second lens having a positive magnification with both convex surfaces on the object side and the image side. And a third lens having a negative magnification with concave surfaces on both sides, a fourth lens having a positive magnification, and a fifth lens having a positive magnification with convex surfaces on both sides.

In such an optical system, when the distance between the first lens and the second lens is reduced, it is difficult to obtain a sufficient postfocal distance, and when the postfocal distance is small, it is difficult to correct the aberration by increasing the magnification of the first lens. . On the other hand, when the distance between the first lens and the second lens is increased, the outer diameter of the first lens is increased, and the back focal length is increased, and thus the entire lens is enlarged.

In addition, when the distance between the second lens and the third lens is reduced, it is difficult to insert an aperture between the second lens and the third lens, and conversely, when the distance between the second lens and the third lens is increased, As the diameter increases and the position where the light ray of the optical axis passes is relatively high, it becomes difficult to correct the aberration of the optical axis.

As described above, the optical system requires a postfocal length of a predetermined length or more, but when the postfocal distance becomes longer than necessary, miniaturization becomes difficult as the entire optical system becomes longer. In addition, the distance between each lens should be spaced more than a predetermined distance for the correction of each aberration, the longer the distance between each lens, the longer the length of the optical system. In addition, conventionally, since five lenses are used, there is a disadvantage that the optical system is enlarged by the length of the lens itself even if the postfocal distance is adjusted. Therefore, an optical system having as many as five lenses has a problem that it is difficult to mount it in a small communication device.

Accordingly, it is an object of the present invention to reduce the number of lenses and at the same time arrange each lens so as to form an appropriate postfocal distance, so that not only the optical system can be made smaller and lighter, but also the manufacturing cost can be reduced. It is to provide an optical system for ultra-thin mega pixel camera.

According to one aspect of the present invention, there is provided an optical system for an ultra-thin megapixel camera, comprising: a first lens formed of an aspherical surface having a negative magnification having a convex surface on an object side and a concave surface on an image side; An aperture to selectively converge light from the first lens; A second lens and a third lens formed of a spherical surface having a positive magnification having a concave surface on the object side and a convex surface on the object side and sequentially arranged; A fourth lens formed of an aspherical surface having a positive magnification with convex surfaces formed on the object side and the image side, respectively; And an image pickup device for converting light passing through each lens into an electrical signal.

Here, the imaging device is preferably a CMOS sensor or a CCD sensor.

When the focal length that is the distance from the first lens to the image is f and the rear focal length that is the distance from the rear surface of the fourth lens to the image is B, it is preferable that 0.8 ≦ B / f ≦ 1.0.

If an aspherical surface is formed on the first lens and the fourth lens, the sag by the aspherical surface is X _a , and the sag by the reference spherical surface is X _o ,

X _a -X _o > 0 (first surface R1 of the aspherical first lens)

X _a -X _o > 0 (second surface R2 of the aspherical first lens)

X _a -X _o <0 (Seventh surface (R7) of aspherical fourth lens)

It is preferable to satisfy X _a -X _o <0 (the eighth surface R8 of the aspherical fourth lens).

If the optical length, which is the distance from the first lens to the fourth lens, is T, and the focal length is f, it is preferable that T / f ≦ 1.2 is satisfied.

The refractive index n1 and the dispersion νd of the first lens are 1.49 ≦ n1 ≦ 1.65, 28 ≦ νd ≦ 33,

The refractive index (n2) and the dispersion (νd) of the second lens are 1.80≤n2≤1.90, 23≤νd≤27,

The refractive index (n3) and the dispersion (νd) of the third lens is 1.70≤n3≤1.85, 42≤νd≤50,

It is preferable that the refractive index n4 and the dispersion vd of the fourth lens satisfy 1.45≤n4≤1.55 and 55≤νd≤70.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

The optical system for ultra-thin megapixel cameras can be miniaturized by constructing an optical system using two aspherical lenses and two spherical lenses having negative, positive, positive and positive magnifications. To reduce costs due to the decrease in numbers. In addition, it maintains a good aberration correction state and reduces distortion to have excellent performance.

As shown in FIG. 1, the ultra-thin megapixel camera optical system adjusts scattering and transmission of incident light and lens systems 10, 20, 30, and 40 formed by an aperture 50 and four lenses. And an optical filter 60 (IR cutting filter) for improving the performance of the optical system by suppressing color deformation of the image, and an imaging device 80 formed of a CMOS sensor or a CCD sensor for converting light into an electrical signal.

The lens systems 10, 20, 30, and 40 have a convex surface formed on the object side and a concave surface formed on the image side, and a first lens 10 having a negative magnification, and a concave surface formed on the object side. The second lens 20 and the third lens 30 having the positive magnification with the convex surface formed therein, and the fourth lens 40 with the positive magnification with the convex surface formed on the image side and the object side, respectively. Here, the first lens 10 and the fourth lens 40 are aspherical, and the second lens 20 and the third lens 30 are spherical. The magnification referred to in the present invention refers to a magnification for each lens surface, and may be defined as magnifying power or magnification ratio, which means an absolute difference between an actual object and an image size. For example, if the height of a real object is 1m but the phase is 2m, the magnifying power is 2. These magnifications are particularly denoted by negative magnifications on concave lenses and positive magnifications on convex lenses, where lenses may have two sides, in particular, convex and concave lenses on one side are called meniscus lenses. The sum of the two magnifications is the magnification of the overall lens.

The first lens 10 generally uses a meniscus-type lens that has a convex and negative refractive index on the object side in order to secure a wide viewing angle on the object side and a sufficient postfocal distance.

The second lens 20 is disposed at a predetermined distance from the first lens 10, and an aperture 50 is mounted between the first lens 10 and the second lens 20. Meanwhile, the third surface R3 of the second lens 20 is formed as a concave surface having a relatively small radius of curvature, and an outer portion of the concave surface is formed flat toward the first lens 10. The diaphragm 50 is arranged on the same line as the outer portion of the third surface R3 of the second lens 20.

Meanwhile, the second lens 20, the third lens 30, and the fourth lens 40 are sequentially disposed such that neighboring lenses and at least one side thereof contact each other. That is, the fourth surface R4, which is the image side surface of the second lens 20, and the fifth surface R5, which is the object side surface of the third lens 30, are closely disposed so that their outer portions contact each other. The sixth surface R6, which is an image side surface of the lens 30, and the seventh surface R7, which is an object side surface of the fourth lens 40, are closely disposed such that the optical axes thereof contact each other.

At the rear of the lens system, an optical low pass filter (OLPF) through which a low-band light beam passes between the fourth lens 40 and the image pickup device 80, or the sensitivity in the infrared wavelength region of the image pickup device 80 is lowered. An optical filter 60, such as an infrared absorption filter, may be installed for the purpose of approaching the visual sensitivity of the eye. On the other hand, a window glass 70 for protecting the imaging device 80 is mounted in front of the imaging device 80 including the CCD sensor and the CMOS sensor.

In general, a CCD (Charge Coupled Device) sensor is an integrated light emitting diode that receives light and converts it into an electrical signal, and converts the intensity of the light into an amount of charge to make an electron. In other words, the object is captured by the light and shade of light, and then converted into an electrical signal and stored. At this time, the CCD device receives the incident light at once and electronically filters the RGB primary colors or separates the RGB primary colors using a separate primary filter. The resolution of the digital camera is determined by such a CCD element, and the higher the resolution, the more precise an image can be expressed.

On the other hand, CMOS (Complementary Metal-Oxide Semiconductor) sensor is formed in a structure that uses one transistor for one light receiving element, because the light is directly converted into an electrical signal in the detected light receiving element due to the characteristics of the transistor Noise is generated. Due to such noise, the use of a CMOS sensor has been limited for expensive digital products, but recently, a method of designating a color according to the depth of penetration of light is determined using the fact that the depth of penetration into the silicon layer varies depending on the color of the light. Used CMOS sensor is in the spotlight. The CMOS sensor adopts three layers of RGB photosensitive layers to filter RGB primary colors, and determines the color according to whether each photosensitive layer reaches the light beam, and converts the determined color into an electric signal. With the advent of such CMOS sensors, resolution and vivid color can be displayed better than conventional CCDs.

In the lens system, the second lens 20 and the third lens 30 are formed of spherical lenses made of high refractive glass, and the first lens 10 and fourth lens 40 are formed of aspherical lenses made of plastic. Accordingly, by assembling the spherical lens and the aspherical lens in combination, the spherical aberration is corrected and the corrected state of the meridian curvature is improved by considering the proper distribution of magnification, and the total length of the optical system is 17.80 mm The aperture ratio is F / 2.8, and can be configured by selecting a clock angle of 53.8 °. Here, the first lens 10 and the fourth lens 40 are made of a plastic material of general refraction, so that mass productivity can be induced and a weight reduction effect can be obtained.

Meanwhile, the first lens 10, the second lens 20, the third lens 30, and the fourth lens 40 are designed to satisfy the following conditions 1,2 and 3.

<Condition 1>

0.8≤B / f≤1.0

Here, the focal length of the entire lens system is f, and the rear focal length B, which is the distance from the eighth surface R8 of the fourth lens 40 to the focal point, is referred to as B. FIG.

According to condition 1, the rear focal length B is shorter or equal to the focal length f, and this sufficient rear focal length B is a separate optical low pass filter (OLPF) between the third lens 30 and the image pickup device 80. Or an infrared absorption filter.

<Condition 2>

X _a -X _o > 0 (first surface R1 of the aspherical first lens 10)

X _a -X _o > 0 (second surface R2 of the aspherical first lens 10)

X _a -X _o <0 (7th surface R7 of the aspheric fourth lens 40)

X _a -X _o <0 (eighth surface R8 of the aspherical fourth lens 40)

2 (a) to 2 (b) show that when the height from the optical axis to the vertex of the spherical surface is Y in the curvature on the optical axis of the reference spherical surface is C (= 1 / R), Sag X _a , A graph showing a relationship with sag X _o by a sphere is disclosed. Here, when a line is drawn perpendicularly from the vertex on the optical axis of the first lens 10 and the fourth lens 40, Sag, which is the distance between the Y-axis and the curved surface of the lens, is X _a , and sag by the reference spherical surface is X a. _o

On the other hand, when forming an aspherical surface, the difference between X _a by an aspherical surface and X _o by _a spherical surface cannot be zero. This is because the values of X _a and X _o are the same so that the difference may be zero only in a sphere having the same radius of curvature as the reference sphere. When comparing (a) and (b) of FIG. 2, when X _a is larger than X _o , that is, when the radius of curvature of the aspherical surface is smaller than the radius of curvature of the reference sphere, X _a -X _o > 0. If X _a is smaller than X _o , that is, the radius of curvature of the aspherical surface is larger than the radius of curvature of the reference sphere, X _a -X _o <0. According to the condition 2, the first surface R1 and the second surface R2 of the first lens 10 form an aspherical surface smaller than the radius of curvature of the reference sphere, and the seventh surface of the fourth lens 40. R7 and the eighth surface R8 form an aspherical surface larger than the radius of curvature of the reference spherical surface.

<Condition 3>

T / f≤1.2

Here, the optical length, which is the distance from the first lens 10 to the fourth lens 40, is T, and the focal length from the first lens 10 to the image is f.

In general, the distortion aberration in the optical system is minimal when the image distance and the object distance are similar. Accordingly, the optical system corrects the distortion aberration by adjusting the shape of the lens and the arrangement of the refractive power and the position of the diaphragm 50. In the optical system, as in the conditions 1 and 3, the focal length f and the rear focal length B The ratio is set to be adjacent to 1, and the ratio of the optical electric field T to the focal length f is set to have a value adjacent to 1, so that the minimum distortion aberration is obtained.

On the other hand, in general, sag X _o of the spherical lens and sag X _a of the aspherical lens can be represented by the following equations (1) and (2), where the curvature on the optical axis of the reference sphere is C (= 1 / It is based on the case where the height in optics is Y in the plane of R).

[Equation 1]

[Equation 2]

Here, K is a conic constant that determines whether a sphere is a hyperbola, a parabola, or an ellipse, and AD, AE, AF, and AG are aspherical coefficients.

When using the above Equation 1 and Equation 2 to configure the above conditions 1,2,3, the optical length of the length from the first lens 10 to the image pickup device 80 is 17.80mm, An optical system with an aperture ratio of F / 2.8 and a field of view of 53.8 ° is formed.

The optical system is formed by using the equations 1 and 2, and the lens surfaces of the first lens 10, the second lens 20, and the third lens 30, the aperture 50, the optical filter 60, Table 1 shows the results of analyzing information such as a radius of curvature, a center interval, a refractive index, and dispersion of the imaging device 80.

Lens surface Radius of curvature Center interval Refractive index Dispersion Remarks First side (R1) 4.90 1.85 1.585 30.3 Aspheric surface 2nd side (R2) 10.41 0.664 Aspheric surface iris ∞ 1.42 Page 3 (R3) -2.74 0.45 1.847 23.8 Fourth side (R4) -44.0 0.03 Page 5 (R5) -35.5 3.1 1.773 49.6 Page 6 (R6) -4.30 0.05 Page 7 (R7) 6.40 2.7 1.49 57.9 Aspheric surface 8th page (R8) -66.99 2.0 Aspheric surface Optical Filters and Window Glasses ∞ 1.25 1.517 64.2 Imaging device ∞ 4.285

Here, K = 0.0, AD = 0.2057111E-02, AE = 0.2444475E-04, AF = 0.1417240E-04, AG = -0.8375848E-06 on the first surface R1,

K = 0.0, AD = 0.3031838E-02, AE = -0.3265619E-03, AF = 0.0000000E + 00, AG = 0.0000000E + 00 of the second side R2,

K = -0.93649, AD = 0.0000000E + 00, AE = 0.0000000E + 00, AF = 0.0000000E + 00, AG = 0.0000000E + 00 on the seventh surface (R7),

K = 0.0, AD = 0.7150568E-03, AE = -0.1177828E-04, AF = 0.0000000E + 00, AG = 0.0000000E + 00 on the eighth surface R8.

On the other hand, the resolution measured by the Projector (Profile Projector), the effective diameter, spherical system f focal length f of the entire lens system, rear focal length B, from the first first lens 10 of the optical system to the last fourth lens 40 Compared with the distance T, angle of view θ, and aperture ratio, the lens shape and lens arrangement of the optical system are

<Condition 1>

0.8≤B / f≤1.0

<Condition 2>

X _a -X _o > 0 (first surface R1 of the aspherical first lens 10)

X _a -X _o > 0 (second surface R2 of the aspherical first lens 10)

X _a -X _o <0 (7th surface R7 of the aspheric fourth lens 40)

X _a -X _o <0 (eighth surface R8 of the aspherical fourth lens 40)

<Condition 3>

It can be seen that T / f ≦ 1.2 is satisfied.

3 to 7 are graphs showing aberration characteristics of the present optical system, and FIGS. 3A to 3C show meridional surface curvatures on optical axes of 0.50, 0.71, and 1.00, and FIGS. 4A to 4C. (c) shows the curvature of the convex phase on the optical axis of 0.50, 0.71, 1.00, FIG. 5 is a graph showing the aberration about the optical axis, FIG. 7 shows chromatic aberration (solid line) and percent distortion (dotted line). Here, the field is divided into three by dividing a single object photographed by the optical axis, so that the performance can be examined according to the optical axis.

As shown in the graphs of FIGS. 3 to 7, when the meridian surface curve and the spherical surface curve are analyzed according to each field, the values of the phases are shown adjacent to the axis in almost all fields, and thus excellent performance in spherical aberration and chromatic aberration. It can be seen that an effective design has been made.

Thus, in this optical system, the optical length is 17.80 mm, the aperture ratio is F / 2.8, and the viewing angle (view angle) is formed at 53.8 degrees. In other words, as the optical length is formed to 17.80㎜, the optical system can be miniaturized and the exposure time is increased by setting the aperture ratio to F / 2.8, so that a sufficient amount of light is supplied through each lens. The difference in brightness around you can be eliminated, so you can get a clear image of the object. In addition, since the angle of view is formed at 53.8 °, it is larger than the angle of view of 46 °, which is the general angle of view of the standard lens, so that a large area can be taken and the focal length is short, thereby reducing the size of the subject. In particular, as the angle of view increases, the focal length is shortened, and the optical system is composed of four lenses, so that the camera can be made compact and lightweight, which not only realizes a mega pixel camera of 2 million pixels, but also reduces manufacturing costs. Can be.

As described above, according to the present invention, the optical system is composed of two aspherical lenses and two spherical lenses, and has an optical length of 17.80 mm, an aperture ratio of F / 2.8, and an angle of view of 53.8 °. In addition to reducing the size and weight, the manufacturing cost can be reduced, and a clear image can be obtained over a wider area.

1 is a cross-sectional view showing an arrangement of an optical system for an ultra-thin megapixel camera according to the present invention;

(A) to (b) is a cross-sectional view comparing the radius of curvature of the aspherical lens and the reference spherical lens of FIG.

3 is a graph showing the meridion plane curvature of the optical system of FIG. 1 on an optical axis of 0.50, 0.71, and 1.00;

4 is a graph showing the curvature of the spherical phase for the optical system of FIG. 1 on an optical axis of 0.50, 0.71, and 1.00;

5 is a graph showing aberrations for an optical axis;

6 is a graph illustrating refractive indices of meridians and curvatures of curvature according to focal lengths;

7 is a graph showing chromatic aberration and percent distortion.

Explanation of symbols on the main parts of the drawings

10: first lens 20: second lens

30: third lens 40: fourth lens

50: aperture 60: optical filter

70: window glass 80: image pickup device

Claims (7)

In the optical system for ultra-thin megapixel camera, A first lens formed of an aspherical surface having a negative magnification having a convex surface on an object side and a concave surface on an image side; An aperture to selectively converge light from the first lens; A second lens and a third lens formed of a spherical surface having a positive magnification having a concave surface on the object side and a convex surface on the object side and sequentially arranged; A fourth lens formed of an aspherical surface having a positive magnification with convex surfaces formed on the object side and the image side, respectively; And an imaging device for converting light passing through each lens into an electrical signal. The method of claim 1, The imaging device is an ultra-thin megapixel camera optical system, characterized in that the CMOS sensor. The method of claim 1, The imaging device is an ultra-thin megapixel camera optical system, characterized in that the CCD sensor. The method of claim 1, When the focal length, which is the distance from the first lens to the image, f, and the rear focal length, which is the distance from the rear surface of the fourth lens to the image, B, 0.8≤B / f≤1.0 Ultra-thin mega pixel camera optical system, characterized in that to satisfy. The method of claim 1, If an aspherical surface is formed on the first lens and the fourth lens, the sag by the aspherical surface is X _a , and the sag by the reference spherical surface is X _o , X _a -X _o > 0 (first surface R1 of the aspherical first lens) X _a -X _o > 0 (second surface R2 of the aspherical first lens) X _a -X _o <0 (Seventh surface (R7) of aspherical fourth lens) X _a -X _o <0 (eighth surface (R8) of the aspherical fourth lens) Ultra-thin mega pixel camera optical system, characterized in that to satisfy. The method of claim 1, If the optical length that is the distance from the first lens to the fourth lens is T, and the focal length that is the distance from the first lens to the image is f, T / f≤1.2 Ultra-thin mega pixel camera optical system, characterized in that to satisfy. The method of claim 1, The refractive index n1 and the dispersion νd of the first lens are 1.49 ≦ n1 ≦ 1.65, 28 ≦ νd ≦ 33, The refractive index (n2) and the dispersion (νd) of the second lens are 1.80≤n2≤1.90, 23≤νd≤27, The refractive index (n3) and the dispersion (νd) of the third lens is 1.70≤n3≤1.85, 42≤νd≤50, The refractive index (n4) and the dispersion (νd) of the fourth lens is 1.45≤n4≤1.55, 55≤νd≤70, the optical system for ultra-thin megapixel camera.