CN113267823A - Large-depth-of-field imaging lens for terahertz frequency band - Google Patents
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- 238000003384 imaging method Methods 0.000 title claims abstract description 123
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- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V8/00—Prospecting or detecting by optical means
- G01V8/10—Detecting, e.g. by using light barriers
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
- G01S17/89—Lidar systems specially adapted for specific applications for mapping or imaging
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- G—PHYSICS
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- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/4802—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
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Abstract
The utility model provides a big depth of field imaging lens for terahertz frequency channel, relates to terahertz imaging field, and the face type is aspheric surface cylinder or aspheric surface hyperboloid, and when using different many times of item coefficient combination, can obtain the non-curved surface lens of different face types, through the control to the different aperture department curvatures of lens, realizes the focus to the different degree of relevant position incident light to reach the purpose that increases the imaging lens depth of field. Therefore, the imaging lens suitable for different field angles and different depth of field ranges can be designed according to different practical use requirements. The field depth range of the terahertz imaging system is expanded by using a single imaging lens, the imaging depth of field of the system can be obviously improved while the composition of the imaging system is not changed by using the lens, and the lens has the advantages of simple system structure, easiness in implementation, reduction of optical elements and the like.
Description
Technical Field
The invention relates to the field of terahertz imaging, in particular to a large depth-of-field lens of a terahertz frequency band for nondestructive testing and human body security inspection.
Background
When an ideal optical system images an object in a three-dimensional space, a certain image plane can only image points of a point object on an object plane conjugated with the certain image plane, and for point objects outside the object plane, the image formed on the image plane is a scattered spot. If the speckles are small enough not to exceed the resolving power of the receiving system, the image formed by these speckles can still be seen as a sharp image, while conventional lens-based imaging systems allow for small speckles, and thus a significant disadvantage of such systems is that the image quickly blurs once out of focus. The imaging systems of terahertz nondestructive detection and human body security inspection have the problems, and the problem of defocus blur is caused by the fact that the depth of field of the systems is very small. The depth of field of the optical system is the maximum distance that an object can move back and forth in an object space when a clear image can be obtained in an image plane. Similarly, if the object is stationary, and a clear image is guaranteed on the image plane, the maximum distance that the image plane can be moved back and forth at its conjugate image plane is commonly referred to as the depth of focus. It can be seen that the depth of field and the depth of focus are uniform in nature, but the depth of field is an object-side parameter, and the depth of focus is an image-side parameter.
Terahertz (THz for short) waves can penetrate through non-metal and non-polar substances such as plastics, paper, clothes and the like, have lower photon energy compared with X radiation, almost do no harm to a human body, and have higher spatial resolution compared with millimeter waves, so that the Terahertz imaging technology has wide application prospects in various fields such as nondestructive testing, human body security inspection and the like. However, the depth of field of a common terahertz imaging system is very small, when an object with a certain depth is observed, only one part of the image is clear, and the out-of-focus part is fuzzy, so that an operator needs to continuously focus on the object to be detected to observe different layers clearly; when the human body is subjected to security check scanning, the human body imaging results which are clear at different distances from the security check instrument are difficult to obtain on the same image, which is very unfavorable for manual judgment and computer-aided processing judgment of the characteristics of the detected object. Therefore, the conventional terahertz imaging system has two defects, one of which is that a clear tomographic image cannot be formed outside the field depth range, and the other of which is not suitable for observing a three-dimensional object.
In view of the above, it is desirable to design an imaging system with a large depth of field to reduce the adverse effect on imaging quality due to the depth of field of the system. Therefore, the invention provides the aspheric large-depth-of-field imaging lens to achieve the purpose of increasing the depth of field of the terahertz imaging system, the lens can be manufactured by using a 3D printing technology according to the designed aspheric surface, and the material of the lens has smaller energy loss to terahertz waves, so that the terahertz wave imaging lens has outstanding application value and practicability.
Disclosure of Invention
In the active terahertz security inspection imaging and nondestructive testing system, the imaging system has a non-negligible influence on the imaging quality due to the depth of field. Aiming at the situation, the invention designs the large-depth-of-field imaging lens of the terahertz waveband, which meets the imaging function of a common lens, increases the depth-of-field range of the system and provides a new idea for the improvement of the performance of terahertz security inspection imaging and nondestructive testing systems.
According to the invention, on the basis of the imaging range required by the known terahertz imaging system, the structural design is carried out on the imaging lens with large depth of field, and the design scheme is optimized. The large depth of field lens is based on the principle that the curvature of the aspheric lens can be continuously changed from the center to the edge, so that light incident from different apertures of the lens can be gathered at different positions to form a focal line, and the purpose of increasing the depth of field of an imaging system can be realized by only one lens without influencing imaging. The invention provides an aspheric large-depth-of-field imaging lens, and provides a possibility for realizing high-quality and large-range terahertz security inspection human body imaging and nondestructive detection.
The technical scheme adopted by the invention is as follows:
the utility model provides a big depth of field imaging lens for terahertz frequency channel now, the face type is the aspheric surface hyperboloid, single big depth of field imaging lens of aspheric surface can be under the prerequisite of not changing imaging system any parameter to the object of awaiting measuring that has certain degree of depth or wait to examine the human body and clearly image apart from security inspection system certain distance range, can realize the formation of image to three-dimensional object even.
For a general surface type, it can be described by the following formula, where c is the curvature (corresponding to the radius), r is the radial coordinate in units of lens length, and k is the conic coefficient. The conic coefficients are less than-1 for hyperbolic curves, -1 for parabolas, -1 to 0 for ellipses, and 0 for spheres.
The general form of an aspheric surface is expressed as:
in the formula, a2，a4，a6+ … is a polynomial coefficient for a when detection of radius of curvature r of the lens is required, e.g. optical decentration detection2Taking 0, it can be derived that equation (1) is derived from a quadric surface, and thus equation (2) represents the degree of deviation of the high-order aspheric surface from the quadric surface.
When different multiple term coefficients are combined, aspheric lenses with different surface types can be obtained, and the focusing of incident light at corresponding positions in different degrees is realized by controlling the curvatures of different apertures of the lenses, so that the purpose of increasing the depth of field of the imaging lens is achieved. Therefore, the imaging lens suitable for different field angles and different depth of field ranges can be designed according to different practical use requirements.
The terahertz imaging system improves the surface type of the imaging lens in the terahertz imaging system to increase the depth of field of the terahertz imaging lens. The large-depth-of-field imaging lens is made of high-density polyethylene or polytetrafluoroethylene and other materials with high terahertz wave transmittance. The surface type may be an aspherical cylindrical surface or an aspherical hyperboloid.
The invention has the technical effects that:
the invention realizes the expansion of the field depth range of the terahertz imaging system by using a single imaging lens, the lens can also obviously improve the imaging field depth of the system while not changing the composition of the imaging system, has the advantages of simple system structure, easy realization, reduction of optical elements and the like, and provides a possibility for realizing high-quality and rapid terahertz security inspection human body imaging and nondestructive detection.
Drawings
Fig. 1 is an appearance diagram of an aspheric large depth-of-field imaging lens. Different from a spherical lens with constant curvature from the center to the edge of the lens, the curvature of the aspherical lens designed by the invention is continuously changed from the center to the edge, and when incident light enters from different apertures of the lens, the incident light can be focused to different degrees, so that objects at different object distances can be imaged in the same image plane, and the effect of large depth of field is realized. Therefore, the single aspheric imaging lens with large depth of field can realize the purpose of expanding the imaging depth of field on the premise of not increasing the optical elements of the imaging system. The aspheric surface large depth-of-field imaging lens provided by the invention has a hyperboloid surface as a selected surface, and corresponds to a conical coefficient k<-1, using a polynomial coefficient a2，a4，a6+ … controls the lens curvature.
Fig. 2 is a schematic diagram of optical paths of an aspheric large-depth-of-field imaging lens for imaging at different object distances. Fig. 2(a) and (b) are computer simulation diagrams of beam propagation in the Z direction at different viewing angles, that is, sectional views of different planes in a three-dimensional image, for describing the optical paths of the aspheric large-depth-of-field imaging lens imaged at different object distances, and the diagrams reflect the focusing ability of the lens designed according to the present invention to light rays at different object distances.
FIG. 3 is a simulation of the imaging results of the present invention. In fig. 3, (a) shows an object for imaging, which is a concentric square, the central square has a side length of 6mm, and is spaced from the adjacent square by 6mm, and the image has an overall side length of 20 mm. Fig. 3(b), (c) and (d) are simulation graphs of imaging results obtained by using the aspheric large-depth-of-field imaging lens of the invention and a common imaging lens at an alignment plane (object distance 250mm), a near view (object distance 240mm) and a far view (object distance 347mm), respectively.
Fig. 4 is a schematic view of an aspheric large-depth-of-field imaging lens in a terahertz human body security inspection imaging system. The large-depth-of-field imaging lens 4 is applied to a terahertz human body security inspection imaging system. The radiation source 1 is used for generating terahertz waves, flat-top light with uniformly distributed light intensity is formed after the terahertz waves pass through the collimating lens 2 and the light homogenizing system 3 to irradiate a person to be detected, the large-depth-of-field imaging lens 4 enables terahertz wave signals reflected or scattered by the person to be detected within a certain range from the terahertz human body security inspection imaging system to be focused on the terahertz detector 5, therefore, the person to be detected within the certain range from the terahertz human body security inspection imaging system can be clearly imaged on the premise that the distance between the terahertz detector 5 and the large-depth-of-field imaging lens 4 is not changed, and the purpose of expanding the depth of field of the terahertz human body security inspection imaging system is achieved. Among them, there are various choices of radiation sources, such as: a radiating horn antenna, an array antenna, a waveguide slot antenna, etc., without limiting the present invention.
In the figure: 1. terahertz source, 2 collimating lens, 3 dodging system, 4 aspheric surface large depth-of-field imaging lens and 5 terahertz detector
Detailed Description
Example 1, a single aspheric large depth of field imaging lens increases the depth of field range:
fig. 1 is an appearance diagram of an aspheric large depth-of-field imaging lens. Unlike a spherical lens having a constant curvature from the center to the edge of the lens, the aspherical lens designed by the present invention has a curvature that varies continuously from the center to the edge, and for a general surface type, can be described by the following formula,
where c is the curvature (corresponding to the radius), r is the radial coordinate in units of lens length, and k is the conic coefficient. The conic coefficients are less than-1 for hyperbolic curves, -1 for parabolas, -1 to 0 for ellipses, and 0 for spheres.
The general form of an aspheric surface is expressed as:
in the formula, a2，a4，a6+ … is a polynomial coefficient. When incident light enters from different apertures of the lens, the incident light can be focused to different degrees, so that objects at different object distances can be imaged in the same image plane, and the effect of large depth of field is achieved. Therefore, the single aspheric imaging lens with large depth of field can realize the purpose of expanding the imaging depth of field on the premise of not increasing the optical elements of the imaging system. The aspheric surface large depth-of-field imaging lens provided by the invention has a hyperboloid surface as a selected surface, and corresponds to a conical coefficient k<-1, using a polynomial coefficient a2，a4，a6+ … controls the lens curvature.
Fig. 2 is a schematic diagram of optical paths of an aspheric large-depth-of-field imaging lens for imaging at different object distances. Fig. 2(a) and (b) are computer simulation diagrams of beam propagation in the Z direction at different viewing angles, that is, sectional views of different planes in a three-dimensional image, for describing the optical paths of the aspheric large-depth-of-field imaging lens imaged at different object distances, and the diagrams reflect the focusing ability of the lens designed according to the present invention to light rays at different object distances. Fig. 2(a) is a cross-sectional view of the XOZ plane, i.e., a schematic view of focusing in the X direction, because the curvatures of the surfaces of the lenses from the center to the edge are different, the light emitted from different apertures of the lenses can be focused to different degrees, so that the on-axis points at different object distances can be focused to the same point in the same image plane. FIG. 2(b) is a cross-sectional view of the YOZ plane, i.e., a Y-direction shaping diagram, which can be focused on the same image plane by the designed lens of the present invention for the off-axis points at different object distances. The single aspheric large-depth-of-field imaging lens realizes imaging of objects at different object distances under a certain field angle.
In order to more clearly describe the increase of the lens of the invention to the depth of field, the imaging simulation after the radiation source radiates the terahertz wave is preferred in this embodiment, and fig. 3 is a simulation diagram of the imaging result of the invention. In fig. 3, (a) shows an object for imaging, which is a concentric square, the central square has a side length of 6mm, and is spaced from the adjacent square by 6mm, and the image has an overall side length of 20 mm. Fig. 3(b), (c), and (d) are simulation graphs of imaging results obtained by using an aspheric large-depth-of-field imaging lens and a common imaging lens at an alignment plane, a near view, and a far view, respectively. The specific imaging process is as follows:
the radiation source radiates terahertz wave beams which are transmitted in a free space at a certain divergence angle to irradiate an object shown in fig. 3(a), when the position of an image surface is fixed (the image distance is about 72mm), a clear image can be obtained only when a common imaging lens is in a small range (the minimum is 255mm and the maximum is 286mm) near an alignment plane, the aspheric large-depth-of-field imaging lens designed by the invention can obtain a clear imaging result at the alignment plane (the object distance is 250mm) and can also obtain a clear image of the object in an interval of the minimum is 240mm and the maximum is 347mm, and imaging simulation results are respectively shown in fig. 3(b), (c) and (d). Assuming that the size of one pixel of an actual detector is 0.6mm × 0.6mm, the straight-line distance between the center of the last surface of the imaging lens and the center of the detection surface is about 72 mm. Compared with the common imaging lens, when the aspheric large-depth-of-field imaging lens images the same object, the depth of field of the designed aspheric lens is 107mm through quantitative analysis, the depth of field of the common imaging lens (standard surface type) is 31cm, and the depth of field is increased by 76mm after the aspheric design, which is about 3 times that of the standard spherical lens. In this embodiment, the aperture of the aspheric large-depth-of-field imaging lens is 30mm × 30mm, and the size of the object plane is 20mm × 20 mm. The aperture of the aspheric large-depth-of-field imaging lens and the imaging range of the target object plane can be increased in combination with actual use requirements.
The above embodiment may also combine with the collimating lens to make the incident light parallel, thereby realizing long-focus depth imaging.
Example 2, application example:
based on the above embodiment, in another embodiment of the present invention, a terahertz human body security inspection imaging system is disclosed. The system comprises the aspherical large depth-of-field imaging lens 4 and other imaging devices in the above embodiments. The aspheric large-depth-of-field imaging lens is the same as that in the above embodiments, and will not be described here. Other imaging devices include: the terahertz source 1, the collimating lens 2, the dodging system 3, the terahertz detector 5 and the image processing device connected with the terahertz detector. The terahertz detector 5 is used for receiving the reflected terahertz wave signal and outputting terahertz wave amplitude information. And finally synthesizing a terahertz wave reflection image of the person to be detected by using a splicing algorithm based on region correlation or a splicing algorithm based on feature correlation. The reflection image represents the radiation intensity of the terahertz waves of all parts of the human body or other articles. For example, when a person hides a metal prohibited article in clothing, the intensity of terahertz wave radiation at the position of the article is in strong and weak contrast with other parts of the human body, and the terahertz wave radiation is represented as a gray difference on a reflected image, so that the detection of the concealed article is realized.
In this embodiment, this active terahertz is human security inspection imaging system now carries out homogenization treatment to terahertz wave beam earlier through even optical system 3 and then directly radiates large tracts of land facula to the human body on, terahertz wave signal that human reflection or scattering was collected through big depth of field imaging lens 4 of aspheric surface, receive by terahertz detector 5 at last and form the reflection image, make this security inspection imaging system have bigger detection range, not only do not have strict requirement to the position that the personnel of treating inspection stood, still can realize many people even dynamic security inspection scanning imaging, imaging speed is faster, the imaging range is wider, security inspection efficiency is higher. Meanwhile, the security inspection imaging system uses terahertz wave beams, so that the system can efficiently detect some potential nonmetal hidden contraband articles, dangerous articles and the like, and is safer compared with radioactive security inspection equipment such as X-rays in the prior art.
Claims (6)
1. A large depth of field imaging lens for a terahertz frequency band is characterized in that: the surface type is an aspheric cylindrical surface or an aspheric hyperboloid, and is described by the following formula for a general curved surface type:
wherein c is curvature, r is a radial coordinate taking a lens length unit as a unit, k is a conical coefficient, the conical coefficient is smaller than-1 for a hyperbolic curve, is-1 for a parabola, is between-1 and 0 for an ellipse, and is 0 for a spherical surface;
the general form of an aspheric surface is expressed as:
in the formula, a2，a4，a6And +. is a polynomial coefficient.
2. The large depth-of-field imaging lens for the terahertz frequency band as set forth in claim 1, wherein: the surface type is hyperboloid and corresponds to a conical coefficient k<-1, using a polynomial coefficient a2，a4，a6Control the lens curvature.
3. The large depth-of-field imaging lens for the terahertz frequency band as set forth in claim 1, wherein: the non-curved lens with different surface types is obtained by using different multi-term coefficient combinations, and the focusing of the incident light at the corresponding position in different degrees is realized by controlling the curvature of different apertures of the lens.
4. The large depth-of-field imaging lens for the terahertz frequency band according to any one of claims 1 to 3, characterized in that: the lens material is high density polyethylene or polytetrafluoroethylene.
5. A terahertz human body security inspection imaging system is characterized by comprising the large-depth-of-field imaging lens as claimed in any one of claims 1 to 4.
6. The terahertz human body security inspection imaging system according to claim 5, comprising a terahertz source, a collimating lens, an optical homogenizing system, a terahertz detector and an image processing device connected with the terahertz detector, wherein the terahertz beam is homogenized by the optical homogenizing system, then a large-area light spot is directly radiated onto a human body, and terahertz wave signals reflected or scattered by the human body are collected by the aspheric large-depth-of-field imaging lens and finally received by the terahertz detector to form a reflected image.
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