KR20080105935A - Convertible lens and hyperspectral imaging system having the same - Google Patents

Abstract

A convertible lens and hyperspectral imaging system is provided to allow a defect not shown on final image while having a simple structure compared with a conventional hyperspectral imaging system using a wide angle lens and a relay lens. In a hyperspectral imaging system, a hyperspectral filter(508) has the transparent window in optical axis and a converting lens is composed of a basis lens and the adapter lens. A normal image having the narrow angle of view is obtained in case that camera body and basis lens were aligned together, a hyperspectral image having the narrow angle of view is obtained in case that a hyperspectral filter is mounted ahead of the basis lens, and a hyperspectral image having the wide angle of view is obtained in case that a hyperspectral filter is mounted ahead of an adaptor lens.

Description

CONVERTIBLE LENS AND HYPERSPECTRAL IMAGING SYSTEM HAVING THE SAME}

1 is a conceptual diagram for understanding the angle of view of a conventional general camera.

2 is a conceptual diagram of a hypermultispectral imaging system of a first conventional embodiment in which an ultramultispectral filter is mounted in front of a lens;

3 is a conceptual diagram of an ultramultispectral imaging system of a second conventional embodiment in which an ultramultispectral filter is mounted between a lens and a camera body;

4 is a conceptual diagram of an ultramultispectral imaging system of a third conventional embodiment in which an ultramultispectral filter is mounted between a wide-angle lens and a relay lens;

5 is a conceptual diagram of an ultramultispectral imaging system of an embodiment of the present invention using a conversion lens.

6 is a graph showing the structure and the path of the light beam of the basic lens of the embodiment of the present invention.

7 is a modulation transfer function characteristic of the basic lens of the embodiment of the present invention;

8 is a distortion characteristic of the basic lens of the embodiment of the present invention.

9 is a graph showing the structure and the path of the light beam of the conversion lens of the embodiment of the present invention.

10 is a modulation transfer function characteristic of the conversion lens of the embodiment of the present invention.

11 is a distortion characteristic of a conversion lens of the embodiment of the present invention.

The present invention relates to a conversion lens having two angles of view and an ultra-multispectral imaging system using the same. More particularly, a lens with a long focal length that does not generate vignetting even when an ultramultispectral filter is mounted on the front of the lens A conversion lens and an ultra-multispectral imaging system using the conversion lens, characterized in that an ultra-multispectral filter is mounted in front of a long focal length lens, and an adapter lens is mounted in front of it to obtain a wide angle of view and no vignetting occurs. The purpose is to provide.

Currently, multi-spectral imaging sensors or hyperspectral imaging sensors are installed in satellites and are widely used for sorting crops or vegetation, diagnosing diseases caused by lack of moisture or certain pests. In addition, it is used to measure the distribution of plankton in the ocean, and image camera based image sensors are also used for cosmetic and dermatological diagnosis. The multispectral image sensor, however, only acquires images for a few predetermined wavelengths, thus limiting the amount of beneficial information that can be obtained. Therefore, a hyper-spectral imaging sensor is used for more precise diagnosis.

Unlike multispectral imaging systems, which rely primarily on rotating filter wheels, hyperspectral imaging sensors are known as diffraction gratings, acoustic-optical tunable filters (AOTFs), or liquid crystal variables. A Liquid Crystal Tunable Filter (LCTF) is used. Among them, LCTF is one of the most competitive technologies in the visible and near infrared range. Although the present specification describes a liquid crystal variable filter, the same technique may be applied to an ultra-multispectral imaging system using an acoustooptic variable filter.

와 렌즈의 초점거리 f_L에 의하여 카메라의 화각 2q는 수학식 1과 같이 주어진다.1 is a view for understanding a field of view (FOV) of a general image camera. The image camera includes a camera body 104 and a lens 106 including a CCD or CMOS image sensor 102 therein. The image acquisition process by the ideal image camera without distortion can be approximated with a pinhole camera. The nodal point N of the lens 106 corresponds to the position of this needle hole. The distance from the nodal point of the lens 106 to the image sensor 102 approximately coincides with the effective focal length f _{L of the} lens. In addition, the image sensor is a rectangular shape having a horizontal length of W and a vertical length of H, and W: H has a ratio of 4: 3, 16: 9, or 1: 1. The diagonal length of this image sensor

The angle of view 2q of the camera is given by Equation 1 by the focal length f _L of the lens.

According to the first exemplary embodiment of obtaining a hyperspectral image, a liquid crystal variable filter 208 is mounted in front of an image camera to obtain an image of a wavelength range selected by the liquid crystal variable filter as illustrated in FIG. 2. Since the liquid crystal variable filter 208 generally has the same mount as the filter of the camera lens 206, the liquid crystal variable filter 208 can be easily mounted on the camera. However, the liquid crystal variable filter has a considerable thickness L because it includes several liquid crystals and polarizing plates therein. In addition, the diameter A of the clear aperture CA of the liquid crystal variable filter is not very large, and as the diameter of the transparent window increases, the price of the liquid crystal variable filter steeply increases. When the liquid crystal variable filter whose diameter of the transparent window is not large enough for the angle of view of the lens is mounted in front of the camera, a vignetting phenomenon occurs in which the field of view is limited as if the view is seen through a cylindrical pipe. In order to prevent vignetting even if the liquid crystal variable filter is mounted in front of the lens, a long focal length lens having a narrow angle of view of the lens should be used. For example, in order to use a liquid crystal variable filter having a diameter A of 20 mm and a thickness L of 50 mm with a 1 / 2-inch CCD camera, the angle of view of the lens must be narrow to within 10. However, when the angle of view is narrowed, it is inconvenient to use because the distance between the multi-spectral imaging system and the subject must be sufficiently spaced to obtain an image of a subject having a certain size.

A second conventional embodiment for obtaining a wide angle of view while using an ultra-multispectral filter is shown in FIG. 3. In the second conventional embodiment, the ultra-multispectral filter 308 is mounted between the wide-angle lens 306 and the camera body 304. However, since the distance between the lens and the image sensor is very large due to the thickness of the ultra-multispectral filter, the lens 306 used in the second embodiment has a back vertex focal length (back) than the effective focal length f _L. flange) f _B must be a very long retrofocus type lens. In such a lens, as illustrated in FIG. 3, a front nodal point (FN) and a rear nodal point (RN) are usually far from each other. Vignetting does not occur in this manner, but a lens having an extremely large rear vertex focal length or a back flange compared to the effective focal length has a disadvantage in that it is very difficult to obtain and difficult to design.

On the other hand, in the third embodiment of the related art shown in FIG. 4, an ultra-multispectral filter is mounted behind a general wide-angle lens. An image plane 410 by the wide-angle lens is then formed in the ultra-multispectral filter 408, and the image plane 410 is again used by using a relay lens 418. 402). In such a structure, there is little concern about vignetting, and since a wide-angle lens having a general back flange can be used, there are many choices in constructing an ultra-multispectral imaging system. However, since the relay lens has a symmetrical structure having a large number of lens elements 414 and 416, the length of the relay lens increases, thereby increasing the size of the entire imaging system and increasing mechanical load to increase the overall load of the lens unit. . Therefore, it must be installed in a narrow place or it becomes an obstacle in constructing a mobile hyperspectral imaging system such as a car / aircraft. In addition, relay lenses are often quite expensive, making it difficult to build low-cost ultra-multispectral imaging systems.

An object of the present invention is to provide an ultra-multispectral imaging system using a conversion lens so that the overall size is not large and vignetting does not occur.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to FIGS. 5 to 11. 5 shows an ultramultispectral imaging system according to an embodiment of the present invention. In the embodiment of the present invention, a long-focus lens 506 in which no vignetting occurs, an ultra-multispectral filter 508 mounted in front of the lens, and an adapter capable of mounting and detaching in front of the ultra-multispectral filter ) Lens 520. This system configuration may look similar to the system configuration using the relay lens of FIG. 4, but there are differences in several ways.

First, the imaging system of FIG. 4 has only one field of view by the wide-angle lens 406, while the imaging system of FIG. 5 has a narrow field of view by the base lens 506 and a wide field of view when the adapter lens is mounted. Optionally available.

Secondly, the basic lens 506 of the present invention in construction is a focal lens capable of forming an image by itself and includes a plurality of lens elements and stops therein. have. However, the adapter lens 520 cannot form an image by itself as an afocal lens and does not have an aperture therein. However, both the wide-angle lens 406 and the relay lens 418 of the imaging system of FIG. 4 are imaging lenses, and are provided with a plurality of lens elements and apertures independently therein. In particular, a general relay lens is usually a finite-conjugate lens with a 1: 1 magnification of equal object distance and image distance. In particular, the adapter lens has an advantage that it can be implemented with a much smaller number of lens elements than the relay lens.

Thirdly, in the method of using a relay lens, an image of a subject is enclosed in the ultra-multispectral filter 410, and the image is transferred back to the image sensor using the relay lens. However, the ultra-multispectral filter having a complex structure such as a liquid crystal variable filter has a complicated internal structure, and a plurality of liquid crystals contained therein may have defects on its own, which can be easily identified with the naked eye. For example, when visually observed with the liquid crystal variable polyspectral filter in operation, the image is displayed in a specific wavelength as if wearing sunglasses, and if there is a liquid crystal defect inside the glass window as if it is stained You get a dirty image. Therefore, when the relay lens captures the image plane in the liquid crystal variable filter, the defect in the filter is transferred to the image sensor as it is, and the defect inside thereof is output as the final image. However, in the conversion lens of the present invention, since the adapter lens itself does not form an image, even if the light rays captured by the image sensor 502 pass through the ultra-multispectral filter 508, even if there is a defect inside the filter, The defect is not seen. This is the same phenomenon as if we didn't see it, even if the glasses were stained.

In this respect, it can be seen that the conventional embodiment using the relay lens and the embodiment of the present invention using the conversion lens are significantly different from each other.

6 illustrates an optical structure and a path of a light beam of a basic lens in a specific embodiment of the present invention. The basic lens of this embodiment is composed of an aperture and five lens elements from the object side. The first lens element, located just behind the iris, is a plano-convex lens element with the convex surface facing towards the object. The second lens element is a positive meniscus lens element with the convex surface facing towards the object. The third lens element is a biconcave lens element. The fourth lens element is a negative meniscus lens element, the concave surface of which faces upward. The fifth lens element is a biconvex lens element.

Table 1 shows a specific embodiment. The image sensor of the camera is assumed to be a 1 / 2-inch CCD sensor. Therefore, the length D in the diagonal direction of the image sensor is 8 mm. The mount of the camera is also assumed to be CS-mount. In addition, the ultra-multispectral filter is a liquid crystal variable filter operating in the visible light region, having a transparent aperture having a diameter of 20 mm, and the thickness L of the filter is 50 mm. The lenses used in the design all consist of optical glass that is readily available. The basic lens shown in Table 1 has an angle of view of 10 and an F-number of 2.0.

7 is a modulation transfer function of the basic lens of Table 1, and it can be seen that the VGA-type lens has sufficient resolution. 8 shows the distortion of this lens, and it can be seen that the distortion is very good, within 1%.

Meanwhile, FIG. 9 shows the optical structure and the light path of the entire conversion lens including the adapter lens. The optical path length is 75 mm assuming the ultra-multispectral filter is a glass plate having a refractive index of 1.5 and a thickness of 50 mm. Therefore, in FIG. 9, the multispectral filter is modeled as air having a thickness of 75 mm. The adapter lens consists of only three lens elements with no aperture. The first lens element located from the object side is a plano-concave lens element with a concave surface pointing upwards. The second lens element is a biconvex lens element, and the third lens element is a biconvex lens element. Table 2 shows a specific embodiment of the overall conversion lens including the adapter lens and the base element. This conversion lens has an angle of view of 35 and an F number of 2.8.

10 is a modulation transfer function of the conversion lens of Table 2, and it can be seen that the VGA-type lens has sufficient resolution. Fig. 11 shows the distortion of this lens, and it can be seen that the distortion is very good within 1%.

Preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings. However, the detailed description and the embodiments of the present invention are merely exemplary, and it is apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention.

The present invention is simpler in structure than the conventional multi-spectral imaging system using a wide-angle lens and a relay lens, the optical defect inside the ultra-multispectral filter is not visible in the final image, and a conversion lens capable of selecting two angles of view And it provides an ultra-multispectral imaging system using the same.

Claims (5)

In a multispectral imaging system, the hyperspectral imaging system includes a camera body including an image sensor therein, an ultramultispectral filter having a transparent window in an optical axis direction, a conversion lens including a basic lens and an adapter lens When the camera body and the primary lens are combined, a normal image having a narrow angle of view is obtained, and when an ultra-multispectral filter is mounted in front of the primary lens, an ultra-multispectral image having a narrow angle of view is obtained. An ultra-multispectral imaging system, characterized in that when an adapter lens is mounted in front of a spectroscopic filter, a hyperspectral image with a wide field of view is obtained. 2. The elementary lens of claim 1, wherein the basic lens consists of an aperture and five lens elements from the object side, wherein the first lens element immediately behind the aperture is a plano-convex lens element, with the convex surface at the object side. The second lens element is a positive meniscus lens element, with the convex side facing the object, the third lens element a biconcave lens element, and the fourth lens element negative. A meniscus lens element, wherein the concave surface faces upward, and the fifth lens element is a biconvex lens element. The hyperspectral imaging system according to claim 1, wherein the primary lens is given as in Table 1. Table 1

The method of claim 1, wherein the adapter lens consists of only three lens elements without an aperture, the first lens element from the object side is a plano-concave lens element, the concave surface facing upwards, And the third lens element is a biconvex lens element, and the third lens element is a biconvex lens element. The hyperspectral imaging system according to claim 1, wherein the conversion lens is given in Table 2. [Table 2]

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