CN114674784A - Terahertz waveband light field imaging system and light field acquisition method - Google Patents
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Abstract
The invention discloses a terahertz waveband light field imaging system and a light field acquisition method. The imaging module is used for converting the spatial information of an imaging target in a scene into a projected two-dimensional image; the positioning adjustment module is used for controlling the position and the posture of the imaging module in space and the optical characteristics of a camera including but not limited to a diaphragm aperture and the distance between a focal plane and a lens group; utilize light imaging system's location adjustment module to realize the accurate positioning to imaging module, realize surveying the measurement to the careful ration of the inside complicated space complex refractive index distribution of object, simple imaging system relatively, this application light field imaging system's range of application is wider.
Description
Technical Field
The invention relates to the field of computational imaging and the technical field of terahertz imaging, in particular to a terahertz waveband light field imaging system and a light field acquisition method.
Background
Light field imaging is an emerging computational imaging modality.
The definition of the light field is "light flux distribution in space"; in the field of imaging applications, we usually describe the light field in space by means of a Plenoptic Function (Plenoptic Function).
According to the visual perception mode of human eyes to light, the light field in the space can be composed of seven-dimensional plenoptic function
Representing x, y, z representing three-dimensional coordinates of any point in space,
represents the direction of travel of the light, λ represents the wavelength of the light, and t represents time. In the case of recording a transient light field with a specific wavelength, lambda and t are fixed and constant, and the plenoptic function can be simplified into five dimensions
While neglecting attenuation, the intensity of a "ray" can be regarded as propagating along the ray direction unchanged independent of the observer distance, and the plenoptic function can be simplified into four dimensions
That is, each set of "rays" may be defined by a four-dimensional function
And (4) showing.
The light field imaging technology greatly breaks through the limitation of the traditional camera in the aspects of imaging capability and imaging flexibility by introducing new dimension information. Compared with the traditional two-dimensional integral imaging mode, the light field imaging can realize a plurality of imaging applications which cannot be realized by the four-dimensional light field by extracting and fusing redundant dimension information in the four-dimensional light field; compared with a coherent imaging mode with richer information content, the light field imaging does not depend on harsh laboratory environment, strict coherent light sources and complicated system construction, and the acquisition of similar information richness can be realized only by simpler integrated equipment.
By post-processing the four-dimensional light field, the reconstruction and reproduction of the two-dimensional slice of the four-dimensional light field (namely refocusing enhancement, full-focusing image synthesis and visual angle interpolation), depth estimation, three-dimensional scene measurement and reproduction based on the depth estimation, signal-to-noise ratio improvement and super-resolution imaging based on equivalent large-aperture synthesis and the like can be realized.
Terahertz waves are electromagnetic waves having a wavelength of 3000 μm to 30 μm and a frequency of 0.1THz to 10 THz. Because terahertz waves have good penetrability to dielectrics (such as wood, paper, ceramics, plastics, composite materials and the like); and because the photon energy is low (0.4-41meV) and is far below the threshold value causing ionization, the method also has good nondestructive property on most detected objects. Therefore, the imaging technology of the terahertz frequency band is widely applied to the fields requiring perspective imaging and nondestructive testing, such as cultural relics archaeology, biomedical science, industrial testing, safety inspection and the like.
An imaging target applying the terahertz imaging technology has complex spatial complex refractive index distribution, namely spatial refractive index distribution and spatial absorption rate distribution, in a terahertz waveband. These distributions are closely related to the shape, state, property and other attributes of the imaging target, and therefore are also the main quantitative probe measurement target for terahertz imaging.
At present, the imaging means of an object in a terahertz wave band mainly comprises two main categories, namely coherent and incoherent: the coherent means includes Synthetic Aperture Radar (Synthetic Aperture Radar), Tomography (Computed Tomography), Time-Domain Spectroscopy (Time-Domain Spectroscopy), etc., and most of the coherent means stay in the laboratory stage due to the large and complicated equipment, dependence on the laboratory environment, insufficient maturity of the component technology and the data processing algorithm, etc.; the incoherent means is a traditional integral imaging system which is composed of a catadioptric optical system and an energy detector comprising a microbolometer, a Schottky diode, a CMOS-micro-nano mechanical structure, a CMOS-antenna structure and a SiGe heterojunction diode, the information abundance degree of the incoherent means is far lower than that of the coherent imaging means, and the detailed and quantitative exploration and measurement of complex space complex refractive index distribution in an object cannot be realized.
Disclosure of Invention
In view of the above-mentioned problems in the background art, it is an important object of the present invention to provide a light field imaging system.
In order to achieve the above purpose, the specific technical scheme of the invention is as follows:
a terahertz waveband light field imaging system comprises:
at least one imaging module for converting spatial information in a scene into a projected two-dimensional image at a particular time and at a particular spatial position and pose; the imaging module includes:
an imaging lens group for focusing light energy from an imaging target in space on a target position;
the detector array is arranged at the position of an image plane conjugated by the imaging lens group for a certain imaging range, is fixedly arranged and is vertical to the optical axis of the imaging lens group; the system is used for converting the received light energy into a specific digital image signal according to the intensity distribution; and
the light modulation module is arranged near the imaging lens group and is fixed relative to the imaging lens group; the light source is used for modulating the intensity, the spectrum and the space spectrum of the light energy received by the detector array;
the light source module is used for providing pulse or continuous wave illumination with a specific wavelength or frequency spectrum, a specific energy distribution or an illumination pattern for an imaging target, and feeding back the information about the imaging target to the imaging module for receiving and imaging;
a positioning adjustment module for controlling the position and attitude of the imaging module in space, and camera optical characteristics including, but not limited to, a diaphragm aperture and a distance between a focal plane and a lens group; the positioning adjustment module includes:
the position positioning device is used for bearing and controlling the accurate position of the imaging module in space, namely the x, y and z positions under a Cartesian coordinate system;
the attitude positioning device is used for bearing and controlling the accurate attitude of each camera module in space, namely the direction and the pitching attitude under a Cartesian coordinate system;
the controllable diaphragm is used for adjusting the light transmission amount and the light transmission caliber of the imaging lens group of the imaging module and can be completely closed for setting the imaging module detector to be zero;
lens group control means for adjusting a relative distance between an imaging lens group and a sensor array in the imaging module;
the control processing module is respectively connected with the imaging module, the light source module and the positioning adjustment module and is used for operating the imaging module, the positioning adjustment module and the light source module through control signals and feedback signals so as to enable the imaging module, the positioning adjustment module and the light source module to work synchronously and orderly; and receiving parallax image information from the imaging module and parameter information fed back by the positioning adjustment module, and generating a four-dimensional light field according to arrangement and reconstruction.
Further, the distance between the imaging lens group and the detector satisfies that the imaging module can focus a point on the imaging target between the corresponding minimum imaging distance and the maximum imaging distance clearly and correspondingly on the detector array.
Further, the light modulation module comprises an attenuation sheet, a filter sheet, a fixed or adjustable spatial light modulation sheet.
Specifically, in an embodiment of the present invention, the light source module includes:
a light source for generating terahertz waves of specific wavelength or spectrum, specific energy and energy distribution, continuous waves or specific pulse width pulses for illuminating an imaging target;
the collimating and beam expanding optical path is used for changing the propagation direction, the propagation mode and the beam aperture of the terahertz waves generated by the light source so as to correctly and properly irradiate an imaging target and comprises a lens, a reflector and a paraboloid mirror element; and
the light source modulation module is used for changing the light intensity, energy distribution, illumination pattern and coherence of terahertz waves generated by a light source and comprises an attenuation sheet, a filter sheet, an interference component and a light beam homogenizer element.
Specifically, in the technical solution of the present invention, the control processing module includes:
the positioning attitude control is used for controlling the positioning adjusting module according to a set program or a mode of sending a control signal to the positioning adjusting module and receiving feedback so as to position the position and the attitude of each imaging module at a proper position;
the detector array control is used for controlling the integration time and the zero setting of the detector array according to a set program or user input, and the synchronous action among the detector array, the light source module and the positioning adjustment module;
the diaphragm control is used for controlling the action of the controllable diaphragm according to a set program or user input;
a lens group control for controlling the operation of the lens group control device according to a predetermined program or user input;
and data processing and storage are used for feeding back the received parallax images and corresponding positioning postures, detectors, diaphragms and lens group parameters to assemble according to rules and generate complete four-dimensional light field information.
The second purpose of the present invention is to provide a light field collecting method of a light field imaging system based on the terahertz waveband, which includes the following specific steps:
s1, firstly, placing an imaging target in an imaging range of the light field imaging system, and illuminating the imaging target in a proper mode and direction by a light source module according to requirements; then, according to the size and optical characteristics of the imaging target and the requirements on the aspect of post-processing of the light field, determining the acquisition parameters of the light field, namely the position, the posture and the optical parameter set of all parallax images contained in the acquired light field;
and S2, sending a control signal to the positioning adjustment module by the control processing module in the light field imaging system according to the set acquisition parameters, controlling the positioning adjustment module to bear all the imaging modules to be positioned at the specified positions in the specified postures and the specified optical parameters, and acquiring all the parallax images contained in the required light field information and the parameters attached to each parallax image in a simultaneous or time-sharing mode under the control of the synchronous signal.
In the technical scheme of the invention, in step S2, the position positioning device and the posture positioning device of the positioning adjustment module are sequentially connected in series and are respectively fixedly connected with each imaging module; under the control of a positioning attitude control signal, the position positioning device and the attitude positioning device bear each imaging module at a specified position and at a specified attitude, and a parallax image related to a target, namely a part of light field information, is acquired in a time-sharing or one-time mode; during this time, the controllable diaphragm controls the diaphragm aperture under the diaphragm control signal, and the lens group control device controls the distance between the lens group and the detector array under the lens group control signal.
In the technical solution of the present invention, the controlling of the positioning accuracy of the imaging module includes:
the spatial positioning precision of the position positioning device is higher than that of the imaging module at the minimum imaging distance, and the size of each pixel of the detector array of the imaging module is half of the corresponding size on the image plane, namely
Wherein, Δ L is the positioning error in each dimension, L is the minimum imaging distance, f is the image plane distance, and Δ p is the pixel size;
the attitude positioning precision of the attitude positioning device is higher than that of the imaging module at the maximum imaging distance, and the size of each pixel of the detector array of the imaging module corresponds to one half of the angular resolution in the image space, namely
Wherein Δ θ is the attitude angle error in each dimension, f is the image plane distance, and Δ p is the pixel size;
the position positioning device, the posture positioning device, the controllable diaphragm and the lens group control device of the positioning adjustment module have the adjustment capability on the aspects of the posture and the optical parameters of the imaging module, and jointly form the imaging range of the whole light field imaging system, namely the whole light field imaging system can effectively acquire the available position, the available posture and the effective acting distance of the parallax image.
In the technical scheme of the invention: in the positioning control, the imaging module can be abstracted to a pinhole camera from the mathematical description, namely when a coordinate system is established by taking the camera as an origin and the optical axis direction of the camera as the z-axis direction, a plane conjugated with the image plane of the pinhole camera in an object space is called as an imaging plane, a point of intersection of the imaging plane and the z-axis is called as a principal point, and a uv rectangular coordinate system is established on the imaging plane by taking the principal point as the origin; the projective transformation of the camera from object space to image plane satisfies:
wherein u and v are image plane coordinates, X, Y and Z are object plane coordinates, and f is a focal length;
when considering the case where the origin of the detector array is not at the exact center, the transformation equation becomes:
wherein u is0,v0Is the offset of the center of the sensor coordinates relative to the principal point.
Further, when the camera position and posture are introduced, the obtained light field needs to be transformed between the camera coordinate system and the world coordinate system, and the transformation formula is as follows:
Xcam＝R(Xworld-C)，
wherein, XcamRepresenting the camera coordinate system, XworldRepresenting a world coordinate system, R representing a rotation matrix of the camera, and C representing a displacement matrix of the camera; therefore, the parallax acquired by the imaging module can be converted into a part of the light field according to the position and posture information of the camera and the camera parameters.
Compared with the prior art, the invention has the beneficial effects that:
the invention solves the defect of low abundance of incoherent imaging information, realizes the accurate positioning of the imaging module by using the positioning adjustment module of the optical imaging system, realizes the delicate quantitative exploration and measurement of complex spatial complex refractive index distribution in an object, is relatively simple, and has wider application range of the optical field imaging system.
Drawings
FIG. 1 is a block diagram of the structural schematic of the functional modules of a light field imaging system;
FIG. 2 is a schematic view of an imaging module;
FIG. 3 is a schematic view of a light source module;
FIG. 4 is a schematic view of a positioning adjustment module;
fig. 5 is a block diagram schematically illustrating the structure of a control processing module.
Detailed Description
The following detailed description of embodiments of the invention refers to the accompanying drawings. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.
Embodiment 1 as shown in fig. 1 to 3, a terahertz waveband optical field imaging system includes:
at least one imaging module for converting spatial information in a scene into a projected two-dimensional image at a particular time and at a particular spatial position and pose; the imaging module includes:
an imaging lens group for focusing light energy from an imaging target in space on a target position;
the detector array is arranged at the conjugate image plane position of the imaging lens group in a certain specific imaging range, is fixedly arranged and is vertical to the optical axis of the imaging lens group; the system is used for converting the received light energy into a specific digital image signal according to the intensity distribution; and
the light modulation module is arranged near the imaging lens group and is fixed relative to the imaging lens group; the light source is used for modulating the intensity, the spectrum and the space spectrum of the light energy received by the detector array; including attenuation, filter, fixed or adjustable spatial light modulation slice and other elements;
for the scenery in the imaging range in the space, the light energy from the scenery passes through the aperture of the imaging lens group, is focused on different points of the sensor array through the focusing and conversion of the imaging lens group, and is converted into a specific digital image signal by the sensor array according to the intensity distribution of the received light energy, thereby completing the collection of a visual angle in the light field.
The distance between the imaging lens group and the sensor array, namely the imaging range of the sensor relative to the imaging lens group conjugate can be continuously and accurately adjusted by the lens adjusting module;
the acquisition time and the integration time of the sensor array can be controlled by a synchronous signal so as to keep synchronization with the light source module and the positioning adjustment module.
The light source module is used for providing pulse or continuous wave illumination with a specific wavelength or frequency spectrum, a specific energy distribution or an illumination pattern for an imaging target, and feeding back the information about the imaging target to the imaging module for receiving and imaging; the light source module includes:
a light source for generating terahertz waves of specific wavelength or spectrum, specific energy and energy distribution, continuous waves or specific pulse width pulses for illuminating an imaging target;
the collimating and beam expanding optical path is used for changing the propagation direction, the propagation mode and the beam aperture of the terahertz waves generated by the light source so as to correctly and properly irradiate an imaging target and comprises a lens, a reflector and a paraboloid mirror element; and
the light source modulation module is used for changing the light intensity, energy distribution, illumination pattern and coherence of terahertz waves generated by a light source and comprises an attenuation sheet, a filter sheet, an interference component and a light beam homogenizer element.
Collimated light or divergent light generated by the light source is modulated by the light source modulation module, then the light beam propagation direction, the propagation mode and the light beam caliber are converted by the collimation and beam expansion light path, the collimated light or the divergent light is irradiated on an imaging target in a proper mode, and the collimated light or the divergent light is received and imaged by the imaging module after carrying information related to the imaging target through interaction with the imaging target, including absorption, refraction, transmission, reflection and scattering. The emitting time of the light source can be controlled by a synchronous signal so as to keep synchronization with the imaging module and the positioning adjusting module.
A positioning adjustment module for controlling the position and attitude of the imaging module in space, and camera optical characteristics including, but not limited to, a diaphragm aperture and a distance between a focal plane and a lens group; the positioning adjustment module includes:
the position positioning device is used for bearing and controlling the accurate position of the imaging module in space, namely the x, y and z positions under a Cartesian coordinate system;
the attitude positioning device is used for bearing and controlling the accurate attitude of each camera module in space, namely the direction and the pitching attitude under a Cartesian coordinate system;
the controllable diaphragm is used for adjusting the light transmission quantity and the light transmission caliber of the imaging lens group of the imaging module and can be completely closed for setting zero of the imaging module detector;
lens group control means for adjusting a relative distance between an imaging lens group and a sensor array in the imaging module;
the control processing module is respectively connected with the imaging module, the light source module and the positioning adjustment module and is used for operating the imaging module, the positioning adjustment module and the light source module through control signals and feedback signals so as to enable the imaging module, the positioning adjustment module and the light source module to work synchronously and orderly; and receiving parallax image information from the imaging module and parameter information fed back by the positioning adjustment module, and generating a four-dimensional light field according to arrangement and reconstruction.
Specifically, in the technical solution of the present invention, the control processing module includes:
the positioning attitude control is used for controlling the positioning adjusting module according to a set program or a mode of sending a control signal to the positioning adjusting module and receiving feedback so as to position the position and the attitude of each imaging module at a proper position;
the detector array control is used for controlling the integration time and the zero setting of the detector array according to a set program or user input, and the synchronous action among the detector array, the light source module and the positioning adjustment module;
the diaphragm control is used for controlling the action of the controllable diaphragm according to a set program or user input;
a lens group control for controlling the operation of the lens group control device according to a predetermined program or user input;
and data processing and storage are used for feeding back the received parallax images and corresponding positioning postures, detectors, diaphragms and lens group parameters to assemble according to rules and generate complete four-dimensional light field information.
Inputting acquisition parameters of a light field to a data processing and storing module by a user or a program, wherein the acquisition parameters comprise spatial position, posture and camera parameters of each piece of corresponding parallax image in required light field information, then converting all the parameters into control signals and synchronous signals of positioning posture control, detector array control, diaphragm control and lens group control by the data processing and storing module, and sending the signals to an imaging module, a light source module and a positioning adjusting module through a software and hardware interface so that the imaging module, the light source module and the positioning adjusting module sequentially acquire the light field under the control of the signals; the acquired parallax images and the corresponding parameter information are transmitted back to the data processing and storage module through the software and hardware interface, and then are converted into light field slices according to the initial light field acquisition parameters and are fused into required complete light field information.
Embodiment 2 is a light field acquisition method of a light field imaging system based on the terahertz waveband, which includes the following steps:
s1, firstly, placing an imaging target in an imaging range of the light field imaging system, and illuminating the imaging target in a proper mode and direction by a light source module according to requirements; then, according to the size and optical characteristics of the imaging target and the requirements on the aspect of post-processing of the light field, determining the acquisition parameters of the light field, namely the position, the posture and the optical parameter set of all parallax images contained in the acquired light field;
and S2, sending a control signal to the positioning adjustment module by the control processing module in the light field imaging system according to the set acquisition parameters, controlling the positioning adjustment module to bear all the imaging modules to be positioned at the specified positions in the specified postures and the specified optical parameters, and acquiring all the parallax images contained in the required light field information and the parameters attached to each parallax image in a simultaneous or time-sharing mode under the control of the synchronous signal.
In the technical scheme of the invention, in step S2, the position positioning device and the posture positioning device of the positioning adjustment module are sequentially connected in series and are respectively fixedly connected with each imaging module; under the control of a positioning attitude control signal, the position positioning device and the attitude positioning device bear each imaging module at a specified position and at a specified attitude, and a parallax image related to a target, namely a part of light field information, is acquired in a time-sharing or one-time mode; during this time, the controllable diaphragm controls the diaphragm aperture under the diaphragm control signal, and the lens group control device controls the distance between the lens group and the detector array under the lens group control signal.
In the technical solution of the present invention, the controlling of the positioning accuracy of the imaging module includes:
the spatial positioning precision of the position positioning device is higher than that of the imaging module at the minimum imaging distance, and the size of each pixel of the detector array of the imaging module is half of the corresponding size on the image plane, namely
Wherein, Δ L is the positioning error in each dimension, L is the minimum imaging distance, f is the image plane distance, and Δ p is the pixel size;
the attitude positioning precision of the attitude positioning device is higher than that of the imaging module at the maximum imaging distance, and the size of each pixel of the detector array of the imaging module corresponds to one half of the angular resolution in the image space, namely
Wherein Δ θ is the attitude angle error in each dimension, f is the image plane distance, and Δ p is the pixel size;
the position positioning device, the posture positioning device, the controllable diaphragm and the lens group control device of the positioning adjustment module have the adjustment capability on the aspects of the posture and the optical parameters of the imaging module, and jointly form the imaging range of the whole light field imaging system, namely the whole light field imaging system can effectively acquire the available position, the available posture and the effective acting distance of the parallax image.
In the technical scheme of the invention: in the positioning control, the imaging module can be abstracted to a pinhole camera from the mathematical description, namely when a coordinate system is established by taking the camera as an origin and the optical axis direction of the camera as the z-axis direction, a plane conjugated with the image plane of the pinhole camera in an object space is called as an imaging plane, a point of intersection of the imaging plane and the z-axis is called as a principal point, and a uv rectangular coordinate system is established on the imaging plane by taking the principal point as the origin; the projective transformation of the camera from object space to image plane satisfies:
wherein u and v are image plane coordinates, X, Y and Z are object plane coordinates, and f is a focal length;
when considering the case where the origin of the detector array is not at the exact center, the transformation formula becomes:
wherein u is0,v0Is the offset of the center of the sensor coordinates from the principal point.
Further, when the camera position and posture are introduced, the obtained light field needs to be transformed between the camera coordinate system and the world coordinate system, and the transformation formula is as follows:
Xcam＝R(Xworld-C)，
wherein, XcamRepresenting the camera coordinate system, XworldRepresenting a world coordinate system, R representing a rotation matrix of the camera, and C representing a displacement matrix of the camera; therefore, the parallax acquired by the imaging module can be converted into a part of the light field according to the position and posture information of the camera and the camera parameters。
In a specific embodiment, the present embodiment provides a light field imaging system, which includes an imaging module, a light source module, a positioning adjustment module, and a control processing module.
As shown in fig. 2, in the present embodiment, the portion of the imaging module and the positioning adjustment module that controls the imaging module includes:
The detector array 202
Diaphragm controlled adjustable aperture stop 203
Spatial light modulation section 204
Band-pass filter 205
An attenuation sheet 206;
the lens group control device is driven by a stepping motor, and the imaging lens group 201 is an optical lens group made of a material with a certain refractive index for the terahertz waves and used for focusing the terahertz waves for imaging; the imaging lens group 201 can move and position along the optical axis direction under the control of the lens group control device, so that the detector array 202 is conjugated with planes of different depths in an object space; the detector array 202 is an area array photoelectric detector sensitive to terahertz wave bands and is used for converting received terahertz wave signals into electronic image signals; the adjustable aperture diaphragm 203 and the imaging lens group 201 are coaxial, and the position of the adjustable aperture diaphragm is near the imaging lens group 201, so that the light transmission aperture can be accurately changed under the control of diaphragm control, and the adjustable aperture diaphragm is used for adjusting the light transmission amount and the depth of field of the imaging module; the spatial light modulator 204 is an element having a certain spatial complex refractive index distribution, or a tunable spatial complex refractive index distribution, and utilizes the fourier transform property of the optical lens to modulate the received image information in the spatial frequency domain, including but not limited to intensity and phase; the band-pass filter 205 is a uniform sheet transparent to terahertz waves in a specific wavelength range and opaque to electromagnetic waves in other wavelength ranges, is positioned in front of the imaging lens group and is used for filtering stray light and improving the image signal-to-noise ratio, and the pass band of the band-pass filter is matched with the sensitive wavelength range of the detector array 202; the attenuation sheet 206 is a semitransparent uniform sheet with certain absorptivity for terahertz waves in a specific wavelength range, is positioned in front of the imaging lens group, and is used for adjusting the incident light intensity to match with the dynamic range and the maximum receiving power of the detector array 202;
in the imaging process of the imaging module, after the terahertz waves are attenuated by the attenuation sheet 206 and filtered by the filter sheet 205, the terahertz waves are focused by the imaging lens group 201, are modulated by the aperture diaphragm 203 and the spatial light modulation sheet 204, and are focused on the detector array 202 to form an image, and the light energy signals are converted into image signals by the detector array 202; in the case that the imaging module and the positioning adjustment module are subjected to sufficient aberration calibration and geometric calibration, the parallax image output by the imaging module can be regarded as an accurate slice of the light field at a specific position.
As shown in fig. 3, in the present embodiment, the light source module includes:
Diverging concave lens 302
Collimating convex lens 303
A mirror 304;
the light source 301 generates a terahertz light beam which is approximately collimated and has a small light beam size, and the frequency or the frequency spectrum of the terahertz light beam is matched with the sensitive wavelength range of the detector array 202; the divergence concave lens 302 is an optical concave lens made of a material having a certain refractive index for the terahertz waves, and is used for diverging the terahertz waves generated by the light source 301 to change the diameter of the light beams; the collimating convex lens 303 is an optical concave lens made of a material having a certain refractive index for the terahertz waves, and is used for re-collimating the terahertz waves diverged by the diverging concave lens 302 into approximate parallelism so as to obtain a proper beam diameter and power density; the reflector 304 is a plane reflection mirror made of a green material with high reflection to the terahertz wave, and is used for changing the terahertz wave beam direction and irradiating the collimated terahertz wave on the imaging target 305; in the light source module, the optical axes of the light source 301, the divergent concave lens 302 and the collimating convex lens 303 coincide; the middle point of the reflector 304 is approximately coincident with the optical axes of the first three, the aperture of the reflector 304 is matched with the aperture of the collimating convex lens 303, and the focus of the reflector 304 is approximately coincident with the virtual focus of the diverging concave lens; the light emission of the light source module 301 is controlled by the synchronization signal, and only when the synchronization signal arrives, a pulse is emitted;
in the process of illumination of the light source module, the approximately collimated small-diameter terahertz light beam generated by the light source 301 is diverged by the diverging concave lens 302, and then is collimated by the collimating convex lens to be a terahertz light beam with a larger aperture, and the collimated and expanded terahertz light beam is reflected by the reflector 304 and irradiated on the imaging target 305 to interact with the imaging target 305.
As shown in fig. 4, in the present embodiment, the portion of the positioning adjustment module for controlling the position and the posture includes:
Y-axis translation stage 402
Z-axis translation stage 403
Pitching table 404
A rotary table 405
An adapter 406;
the x-axis translation frame 401, the y-axis translation frame 402 and the z-axis translation frame 403 are three-dimensionally configured translation frames, are driven by a stepping motor to move and position, and can position the load at a specified spatial coordinate position in three dimensions with sufficient accuracy; 404 is a pitching platform, which is driven by a stepping motor to move and position, and can adjust the pitching attitude of the positioning load with sufficient precision; 405 is a rotary table, which is driven by a stepping motor to move and position, and the azimuth attitude of the positioning load can be adjusted with sufficient precision; in the positioning adjustment module, an x-axis translation frame 401, a y-axis translation frame 402, a z-axis translation frame 403, a pitching table 404 and a revolving table 405 are connected in series;
when the position and the posture of the camera are adjusted by the positioning adjustment module, the x-axis translation frame 401, the y-axis translation frame 402 and the z-axis translation frame 403 calculate the movement amount according to the conversion of the movement amount of the translation frame and the space coordinate of the world coordinate system in the geometric calibration in advance, and the load is positioned at the appointed position by respectively moving the translation frames by the corresponding movement amounts; similarly, the tilt table 404 and the turn table 405 are rotated according to a desired world coordinate system and a camera coordinate system, and an attitude adjustment amount is calculated, so that the camera reaches a specified attitude through the rotation motion of the tilt table 404 and the turn table 405.
Referring to fig. 5, in this example, the control processing module structure can be divided into three layers according to software, hardware and functions: an execution layer, a hardware layer, and an application layer.
The execution layer, i.e. the part outside the control processing module specifically used for implementing the light field imaging function, is characterized by not carrying out the generation and transmission of signals, and only converting the control signals into specific opto-mechanical system actions after receiving the control signals, and comprises an imaging module, a light source module and a positioning adjustment module;
hardware layers, namely circuits and interfaces for generating, transmitting and receiving control signals and image data in the processing control module, including a camera control circuit, an image acquisition circuit, a synchronization circuit, a positioning controller, an attitude controller, a diaphragm control circuit, a lens group control circuit for control purposes, USB2.0, RS-232 and ethernet interfaces for signal transmission, and a microcomputer for summarizing and distributing data and control signals;
the application layer is a software part which runs on the hardware layer and is used for controlling the hardware layer to generate signals meeting requirements, and the software part comprises a sensor zero setting program, an integration time control program, a geometric calibration program, an image acquisition and processing program, a synchronous signal generation and control program, a positioning control program, an attitude control program, a diaphragm control program and a lens group control program.
When the control processing module processes data, firstly, a user inputs light field parameters to be acquired, and the light field parameters are converted into specific acquisition parameters input into a hardware layer through a control program of an application layer; then the microcomputer with the hardware layer as the main carrier of the application layer converts the specific acquisition parameters into communication signals through a USB2.0 interface, an RS-232 interface and an Ethernet interface, and transmits the communication signals to other control circuits and synchronous circuits of the hardware layer; then each control circuit and each synchronization circuit convert the communication signals into specific control signals for controlling specific equipment of the execution layer to acquire specific parallax images; the acquired parallax images are converted into image information through an image acquisition circuit, transmitted back to the microcomputer through a hardware interface in a signal mode, converted into light field slices through an image acquisition and processing program of an application layer and fused into a final light field image.
Although the embodiments of this patent have been disclosed above, they are not limited to the applications listed in the specification and the examples, which are fully applicable to many areas of applicability of this patent, and additional modifications will readily occur to those skilled in the art. The invention is therefore not to be limited to the specific details and illustrations shown and described herein, without departing from the general concept defined by the claims and their equivalents.
Claims (10)
1. A terahertz waveband light field imaging system is characterized by comprising:
at least one imaging module for converting spatial information of an imaging target in a scene into a projected two-dimensional image; the imaging module includes:
an imaging lens group for focusing light energy from an imaging target in space on a target position;
the detector array is arranged at the conjugate image plane position of the imaging lens group for a specific imaging range, is fixedly arranged and is vertical to the optical axis of the imaging lens group; the system is used for converting the received light energy into a specific digital image signal according to the intensity distribution; and
the light modulation module is arranged near the imaging lens group and is fixed relative to the imaging lens group; the light source is used for modulating the intensity, the spectrum and the space spectrum of the light energy received by the detector array;
the light source module is used for providing pulse or continuous wave illumination with a specific wavelength or frequency spectrum, a specific energy distribution or an illumination pattern for an imaging target, and feeding back the information about the imaging target to the imaging module for receiving and imaging;
a positioning adjustment module for controlling the position and attitude of the imaging module in space, and camera optical characteristics including, but not limited to, a diaphragm aperture and a distance between a focal plane and a lens group; the positioning adjustment module includes:
the position positioning device is used for bearing and controlling the accurate position of the imaging module in space, namely the x, y and z positions under a Cartesian coordinate system;
the attitude positioning device is used for bearing and controlling the accurate attitude of each camera module in space, namely the direction and the pitching attitude under a Cartesian coordinate system;
the controllable diaphragm is used for adjusting the light transmission amount and the light transmission caliber of the imaging lens group of the imaging module and can be completely closed for setting the imaging module detector to be zero;
lens group control means for adjusting a relative distance between an imaging lens group and a sensor array in the imaging module;
the control processing module is respectively connected with the imaging module, the light source module and the positioning adjustment module and is used for operating the imaging module, the positioning adjustment module and the light source module through control signals and feedback signals so as to enable the imaging module, the positioning adjustment module and the light source module to work synchronously and orderly; and receiving parallax image information from the imaging module and parameter information fed back by the positioning adjustment module, and arranging and reconstructing the parallax image information and the parameter information to generate a four-dimensional light field.
2. The system of claim 1, wherein the distance between the imaging lens group and the detector is such that the imaging module can focus a point on the imaging target between the corresponding minimum imaging distance and the maximum imaging distance on the detector array clearly and correspondingly.
3. The optical field imaging system of the terahertz waveband according to claim 1, wherein the optical modulation module comprises an attenuator, a filter, a fixed or adjustable spatial light modulation sheet.
4. The terahertz waveband light field imaging system as claimed in claim 1, wherein the light source module comprises:
a light source for generating terahertz waves of specific wavelength or spectrum, specific energy and energy distribution, continuous waves or specific pulse width pulses for illuminating an imaging target;
the collimating and beam expanding optical path is used for changing the propagation direction, the propagation mode and the beam aperture of the terahertz waves generated by the light source so as to correctly and properly irradiate an imaging target and comprises a lens, a reflector and a paraboloid mirror element; and
the light source modulation module is used for changing the light intensity, energy distribution, illumination pattern and coherence of terahertz waves generated by a light source and comprises an attenuation sheet, a filter sheet, an interference component and a light beam homogenizer element.
5. The terahertz waveband light field imaging system as claimed in claim 1, wherein the control processing module comprises:
the positioning attitude control module is used for controlling the positioning adjustment module according to a set program or a mode of sending a control signal to the positioning adjustment module and receiving feedback so as to position the position and the attitude of each imaging module at a target position;
the detector array control is used for controlling the integration time and the zero setting of the detector array according to a set program or user input, and the synchronous action among the detector array, the light source module and the positioning adjustment module;
the diaphragm control is used for controlling the action of the controllable diaphragm according to a set program or user input;
a lens group control for controlling the operation of the lens group control device according to a predetermined program or user input;
and data processing and storage are used for feeding back the received parallax images and corresponding positioning postures, detectors, diaphragms and lens group parameters to assemble according to rules and generate complete four-dimensional light field information.
6. A light field acquisition method of a light field imaging system based on the terahertz waveband of any one of claims 1 to 5 is characterized by comprising the following specific steps:
s1, firstly, placing an imaging target in an imaging range of the light field imaging system, and illuminating the imaging target by a light source module according to needs; then, according to the size and optical characteristics of the imaging target and the requirements on the aspect of post-processing of the light field, determining the acquisition parameters of the light field, namely the position, the posture and the optical parameter set of all parallax images contained in the acquired light field;
and S2, sending a control signal to the positioning adjustment module by the control processing module in the light field imaging system according to the set acquisition parameters, controlling the positioning adjustment module to bear all the imaging modules to position at the target position in the required posture and optical parameters, and acquiring all the parallax images contained in the required light field information and the parameters attached to each parallax image in a simultaneous or time-sharing manner under the control of the synchronous signal.
7. The light field collection method of a terahertz waveband-based light field imaging system according to claim 6, wherein in step S2, the position locating device and the posture locating device of the positioning adjustment module are sequentially connected in series and are respectively fixedly connected to each of the imaging modules; under the control of a signal of positioning posture control, the position positioning device and the posture positioning device bear the weight of each imaging module to acquire a parallax image related to a target, namely a part of light field information, in a time-sharing or one-time mode at the target position in the posture; during this time, the controllable diaphragm controls the diaphragm aperture under the diaphragm control signal, and the lens group control device controls the distance between the lens group and the detector array under the lens group control signal.
8. The light field acquisition method of the terahertz wave band based light field imaging system according to claim 7, wherein the control of the positioning accuracy of the imaging module comprises:
the spatial positioning precision of the position positioning device is higher than that of the imaging module at the minimum imaging distance, and the size of each pixel of the detector array of the imaging module is half of the corresponding size on the image plane, namely
Wherein, Δ L is the positioning error in each dimension, L is the minimum imaging distance, f is the image plane distance, and Δ p is the pixel size;
the attitude positioning precision of the attitude positioning device is higher than that of the imaging module at the maximum imaging distance, and the size of each pixel of the detector array of the imaging module corresponds to one half of the angular resolution in the image space, namely
Wherein Δ θ is the attitude angle error in each dimension, f is the image plane distance, and Δ p is the pixel size;
the position positioning device, the posture positioning device, the controllable diaphragm and the lens group control device of the positioning adjustment module have the adjustment capability on the aspects of the posture and the optical parameters of the imaging module, and jointly form the imaging range of the whole light field imaging system, namely the whole light field imaging system can effectively acquire the available position, the available posture and the effective acting distance of the parallax image.
9. The light field acquisition method of the light field imaging system based on the terahertz waveband as claimed in claim 6, wherein during positioning control, the imaging module can be abstracted to a pinhole camera in a mathematical sense, that is, when a coordinate system is established with the camera as an origin and the optical axis direction of the camera as the z-axis direction, a plane conjugate to the image plane of the pinhole camera in an object space is referred to as an imaging plane, a point where the imaging plane intersects with the z-axis is referred to as a principal point, and a uv rectangular coordinate system is established on the imaging plane with the principal point as the origin; the projective transformation of the camera from object space to image plane satisfies:
wherein u and v are image plane coordinates, X, Y and Z are object plane coordinates, and f is a focal length;
when considering the case where the origin of the detector array is not at the exact center, the transformation equation becomes:
wherein u is0,v0Is the offset of the center of the sensor coordinates relative to the principal point.
10. The light field collection method of a terahertz waveband-based light field imaging system as claimed in claim 9, wherein when the camera position and posture are introduced, the obtained light field needs to be transformed between the camera coordinate system and the world coordinate system, and the transformation formula is:
Xcam＝R(Xworld-C)，
wherein, XcamRepresenting the camera coordinate system, XworldRepresenting a world coordinate system, R representing a rotation matrix of the camera, and C representing a displacement matrix of the camera; therefore, the parallax acquired by the imaging module can be converted into a part of the light field according to the position and posture information of the camera and the camera parameters.
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