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CN-121994745-A - Terahertz transmission liquid crystal super-surface rapid imaging method and system

CN121994745ACN 121994745 ACN121994745 ACN 121994745ACN-121994745-A

Abstract

The invention discloses a terahertz transmission liquid crystal super-surface rapid imaging method and a system, which relate to the technical field of terahertz super-surface imaging, and the method is provided with a self-adaptive stop criterion, can dynamically evaluate acquired measurement data in the acquisition process (or after the acquisition is completed) and judge whether to continue acquisition according to data consistency and/or reconstruction stability; when the stopping condition is met, the subsequent coding measurement can be stopped in advance, so that redundant overhead caused by continuous acquisition after the imaging quality enters a platform period is avoided, and the total acquisition time is shortened. On the premise of ensuring that the terahertz frequency band image reconstruction quality meets the requirement, the equivalent imaging speed is obviously improved, and the rapid imaging requirement under a dynamic scene can be met.

Inventors

  • YANG CHUANG
  • LI JIANAN
  • PENG MUGEN

Assignees

  • 北京邮电大学

Dates

Publication Date
20260508
Application Date
20260331

Claims (10)

  1. 1. A terahertz transmission liquid crystal super-surface rapid imaging method is characterized by comprising the following steps of: Calculating an effective contrast threshold according to the equivalent transmission amplitude of the steady-state high transmission and the equivalent transmission amplitude of the steady-state low transmission of the liquid crystal super-surface transmission unit; Searching the shortest waiting time meeting the effective contrast threshold according to the switching condition of the current frame relative to the previous frame between preset minimum waiting time and maximum fixed residence time to obtain single-frame waiting time corresponding to the current frame; Detecting transmission responses corresponding to each space coding pattern by a single pixel detector, and after each space coding pattern is loaded, dynamically updating the response according to the corresponding single frame waiting time and acquiring a scalar value to obtain a measurement sequence, wherein the transmission responses are generated by a liquid crystal super-surface transmission unit under the action of bias voltage; Triggering one-time stop criterion inspection according to a preset inspection interval, taking the latest Q frame in the currently accumulated collected P frame scalar value as a verification window, and taking the rest P-Q frames as a training part; performing data consistency verification and/or reconstruction stability verification, triggering early stop if the verification is passed, stopping loading of a subsequent space coding pattern, and performing single-pixel imaging based on a measurement sequence acquired up to the current moment and outputting a result to complete terahertz transmission liquid crystal super-surface rapid imaging; Predicting a verification window based on a temporary imaging result, and calculating the difference between a predicted value and a true value of the verification window; reconstruction stability verification includes calculating a difference between a current provisional imaging result and a last provisional imaging result.
  2. 2. The method for rapid imaging of terahertz transmissive liquid crystal super-surface according to claim 1, wherein the effective contrast threshold is calculated as: Wherein the method comprises the steps of Is an effective contrast threshold; a constant greater than 0; Is contrast; And The equivalent transmission amplitude of the steady state high transmission and the equivalent transmission amplitude of the steady state low transmission of the liquid crystal super surface transmission unit are respectively.
  3. 3. The method for rapidly imaging a super-surface of a terahertz transmissive liquid crystal according to claim 2, wherein the specific method for searching for the shortest waiting time satisfying the effective contrast threshold according to the switching condition of the current frame relative to the previous frame between a preset minimum waiting time and a maximum fixed residence time includes: Determining a high liquid crystal super surface transmission unit and a low liquid crystal super surface transmission unit according to the switching condition of the current frame relative to the previous frame, and establishing a dynamic response model of the average transmission state of a high liquid crystal super surface transmission unit group and a low liquid crystal super surface transmission unit group: Wherein the method comprises the steps of A dynamic response value for the average transmission state of the high liquid crystal super surface transmission unit group; A dynamic response value for the average transmission state of the low liquid crystal supersurface transmission cell group; the target transmission state of the high-liquid crystal super-surface transmission unit group of the current frame is set; the average transmission state of the high-liquid crystal super-surface transmission unit group of the current frame at the previous moment is obtained; e is a natural constant, t represents time; A liquid crystal rising response time constant; The target transmission state of the low-high liquid crystal super-surface transmission unit group of the current frame is set; The average transmission state of the low-liquid crystal super-surface transmission unit group of the current frame at the previous moment is obtained; a response time constant for the liquid crystal drop; To be used for In order to meet the criterion of the effective contrast threshold, searching for the shortest time meeting the criterion between the preset minimum waiting time and the maximum fixed residence time, and determining the shortest time as the single frame waiting time corresponding to the current frame.
  4. 4. The terahertz transmission liquid crystal super-surface rapid imaging method according to claim 1, wherein the difference between the predicted value and the true value of the verification window is represented by a normalized residual, and the expression is: Wherein the method comprises the steps of Normalized residual error for verifying predicted value and true value of window; to verify the true value of the window; In order to verify the predicted value of the window, A measurement matrix corresponding to the verification window; is a temporary imaging result; When (when) And if the data consistency verification is smaller than the first set threshold, judging that the data consistency verification is passed, and if the data consistency verification is not passed, judging that the data consistency verification is not passed.
  5. 5. The terahertz transmission liquid crystal super-surface rapid imaging method according to claim 1, wherein when reconstruction stability verification is performed, if the difference between the current temporary imaging result and the last temporary imaging result is smaller than a second set threshold value, the reconstruction stability verification is judged to pass, otherwise, the reconstruction stability verification is judged not to pass.
  6. 6. The terahertz transmissive liquid crystal subsurface rapid imaging method according to claim 1, further comprising an overdrive operation: Applying an overdrive voltage higher than the operating voltage at the early stage of switching the coding pattern And continue And the time is used for enabling the liquid crystal to approach the target state at a faster equivalent speed in the initial stage so as to reduce the single-frame waiting time.
  7. 7. A system for implementing the terahertz transmissive liquid crystal subsurface rapid imaging method described in any one of claims 1 to 6, comprising: The device comprises a transmitting module, a receiving module and a receiving module, wherein the transmitting module is used for generating and transmitting terahertz electromagnetic waves, and the terahertz electromagnetic waves are collimated and then are incident to an optical path where an imaging object and a liquid crystal super surface are located; the liquid crystal super-surface modulation module is used for carrying out spatial modulation on the transmitted terahertz wave under the action of bias voltage to generate a preset spatial coding pattern, so that an imaging object correspondingly generates different transmission responses under different spatial coding patterns of the transmitted terahertz wave, and a measurement sequence required by single-pixel measurement is formed; The bias voltage control module is used for providing bias voltage for the liquid crystal super-surface modulation module and controlling time sequence switching of the bias voltage so as to realize loading of the space coding pattern; The receiving and collecting module is used for carrying out single-pixel detection on the transmitted terahertz signals and collecting measuring sequences corresponding to different space coding patterns; The system comprises a liquid crystal super-surface transmission unit, a data processing module, a measurement sequence and a corresponding space coding pattern information, wherein the data processing module is used for calculating an effective contrast threshold according to the steady-state high-transmission equivalent transmission amplitude and the steady-state low-transmission equivalent transmission amplitude of the liquid crystal super-surface transmission unit, searching the shortest waiting time meeting the effective contrast threshold between preset minimum waiting time and maximum fixed residence time according to the switching condition of a current frame relative to a previous frame to obtain single-frame waiting time corresponding to the current frame, receiving the measurement sequence and the corresponding space coding pattern information, triggering one-time stop criterion inspection according to a preset inspection interval, constructing a measurement model, executing image reconstruction, inverting to obtain two-dimensional distribution information of an imaging object in an imaging area, and realizing single-pixel imaging result output.
  8. 8. The terahertz transmission liquid crystal super-surface rapid imaging system according to claim 7, wherein the liquid crystal super-surface modulation module is of a periodic unit array structure and comprises a plurality of liquid crystal super-surface transmission units, each liquid crystal super-surface transmission unit sequentially comprises an upper glass substrate, an upper metal layer resonance structure, a liquid crystal layer, a lower metal layer resonance structure and a lower glass substrate along the incident direction, wherein the upper metal layer resonance structure and the lower metal layer resonance structure are arranged opposite to each other and form a transmission type resonance unit together with the liquid crystal layer, and the equivalent electromagnetic response of the transmission type resonance unit is changed by regulating the equivalent dielectric constant of liquid crystal through voltage so as to modulate the transmission coefficient.
  9. 9. The terahertz transmission liquid crystal super surface rapid imaging system according to claim 8, wherein when bias voltage is applied to the liquid crystal super surface modulation module, bias voltage is applied to each liquid crystal super surface transmission unit according to a preset spatial coding pattern, so that the array presents corresponding transmission coefficient spatial distribution, wherein different spatial coding patterns correspond to different transmission distributions for subsequent single-pixel measurement and image reconstruction.
  10. 10. The terahertz transmissive liquid crystal super-surface rapid imaging system according to claim 8, wherein the electrodes of the array are divided into row electrodes and column electrodes, the row electrodes and the column electrodes are crossed and correspond to form bias loading points of each liquid crystal super-surface transmissive unit, and the control module establishes potential differences between the selected row electrodes and the selected column electrodes through a row/column gating and level superposition mode, so that bias voltage loading and space coding state switching of the corresponding liquid crystal super-surface transmissive units are realized.

Description

Terahertz transmission liquid crystal super-surface rapid imaging method and system Technical Field The invention relates to the technical field of terahertz super-surface imaging, in particular to a terahertz transmission liquid crystal super-surface rapid imaging method and system. Background Terahertz waves (Terahertz, THz) generally refer to electromagnetic waves having frequencies in the range of 0.1-10 THz. The terahertz wave detector has unique physical properties that terahertz waves can penetrate through most nonpolar materials such as ceramics, plastics and the like to a certain extent, has excellent nondestructive detection capability, and has low photon energy and no ionization damage to biological tissues, so that the terahertz wave detector has important application prospects in the fields of safety inspection, medical imaging, industrial flaw detection and the like. Traditional terahertz imaging systems mostly employ mechanical scanning or phased array antenna technology. The phased array technology can realize rapid beam scanning, but relies on a large number of transmit-receive (T/R) channels and complex radio frequency front ends, and the problems of remarkable link loss, high system integration difficulty, high hardware cost and the like are often faced in a high frequency band such as 140 GHz. In order to break through the limitation, a feasible path is provided for computational imaging (such as single-pixel imaging), the technology carries out coded modulation on an incident field through a spatial modulator, a single-pixel detector is used for collecting projection measurement values, and then a compressed sensing algorithm is combined to realize target image reconstruction. The scheme greatly reduces the dependence of the system on the high-performance radio frequency front end while ensuring the imaging capability. The liquid crystal super surface has the advantages of continuous adjustability, low power consumption, easy integration and the like, and can generate various spatial coding modes (spatial coding patterns) by changing equivalent electromagnetic response through electric control, so that the liquid crystal super surface becomes an important candidate scheme for terahertz single-pixel imaging. Taking single pixel imaging as an example, instead of relying on a two-dimensional detector array, the acquisition is repeated multiple times with a "single pixel detector", with each acquisition preceded by a different spatial encoding applied to the incident field by a spatial modulator. In each encoding state, the single-pixel detector measures the "overall weighted sum" (a scalar value) of the scene transmission information under the spatial encoding, which is equivalent to projecting the two-dimensional scene information onto a numerical value. After repeated multiple times using different codes, a set of measurements is obtained. As long as the coding mode is properly designed, the measured value contains enough independent information, and then the two-dimensional image can be reversely deduced from the projection by using an algorithm. The usual mathematical model is: Wherein the method comprises the steps of Representing the reflected/transmitted/scattered intensity (spread as a vector) of the object to be imaged on the two-dimensional grid,Is a measurement sequence acquired with different spatial codes,In order to measure the matrix of the device,The weighting coefficients of the pixels for the spatial coding mode once correspond to each line of,Noise and systematic errors. The model has the meaning that the coding state determines what weighting is performed, the measured value is a weighting result, and the image can be reversely solved by multiple measurements. Reconstruction if understood by the conventional sampling theoremTypically requiring an independent measurement number on the order of the number of pixels (e.gSecondary). Many target images are structured, for example, the target area is not large in duty cycle, the image is composed mainly of edges/contours, or the image can be represented with a small number of coefficients in some transform domains. This "information sparseness/compressibility" allows for a matrix to be measuredWith some randomness or low correlation, the image can be restored with less measurement times than the number of pixels by compressed sensing ideas. Engineering, common reconstruction methods introduce a priori constraints to stabilize solutions, such as Total Variation (TV) regularization. The visual sense of TV is to encourage the image to remain smooth in most areas while allowing significant changes (edges) in a small number of positions, thus being particularly effective for "blocky/sharp" targets and also suppressing particle artifacts due to noise. In practical use, the higher the sampling rate (measurement number/pixel number) is, the smaller the noise is, the better the reconstruction quality is generally, and when the samplin