CN106405571A - 2D aperture imaging algorithm inhibiting influence of object thickness in Terahertz single frequency point - Google Patents

Abstract

The invention relates to a 2D aperture imaging algorithm inhibiting influence of the object thickness in a Terahertz single frequency point, and the algorithm is applied to reconstruction of double-direction-dimension scattering characteristic distribution of the object during Terahertz near-distance imaging. Distance tangent planes in parallel with an observation plane are used to carry out phase compensation on all strong scattering points of an object, imaging results in all the distance tangent planes are gathered and averaged in a weighted manner, and negative influence of difference among distance-dimension information of strong scattering points of the object on 2D aperture imaging is weakened. In the double-direction-dimension imaging system of the Terahertz single frequency point, the accuracy of phase compensation in the distance direction of the target influences the imaging effect directly, and, the algorithm of the invention can be applied more widely and higher in applicability.

Description

The two-dimentional aperture imaging algorithm of suppression target thickness impact under Terahertz single-frequency point

Technical field

The present invention relates to a kind of imaging technique, particularly to two of suppression target thickness impact under a kind of Terahertz single-frequency point Dimension aperture imaging algorithm.

Background technology

THz wave：THz wave (THz waves) refers generally to frequency 0.1THz-10THz (wavelength is 3mm-30pm) In the range of electromagnetic wave, its frequency range is in macroelectronics to the region of microcosmic photonic propulsion transition, has the physics of uniqueness Characteristic and important researching value.

Terahertz imaging：Terahertz imaging is one of important applied field of THz wave.Safety based on THz wave Property, high-resolution and the features such as penetrability, terahertz imaging has the advantage of uniqueness：Compare infrared and optical imagery, Terahertz becomes As the non-polar materials such as clothing, plank and plastics can be penetrated：Compare X radio frequency and microwave imaging, terahertz imaging is often The image of more high spatial resolution can be obtained：Compare x-ray imaging, it is right that the terahertz imaging of milliwatt magnitude power is generally acknowledged that Human body is harmless.According to whether having THz source, the THz imaging technology being applied to human body safety check can be divided into active and quilt Two kinds of dynamic formula.Passive type imaging forms human body surface image using the THz wave that bolometer detects human-body emitting.Due to The THz wave of human body radiation is very faint, and often contrast is relatively low for the imaging results of passive type imaging, and image is not clear.And Active imaging then typically utilizes antenna to launch the other THz wave of milliwatt, and THz wave penetrates human body surface clothes, passes through The terahertz signal of measurement reflection can get human body surface image.Not only picture contrast is high for active imaging, and passes through The method of synthetic aperture, can obtain higher azimuth resolution.Active THz imaging technology can be used for airport, The in-plant human-body safety detection in the places such as iron station.

Synthetic aperture：Synthesis aperture imaging system passes through to switch electronic switch change dual-mode antenna list in a direction order First position forms synthetic aperture, forms synthetic aperture in another one-dimensional square by mechanical scanning, radar transmit-receive receiver is big In synthetic aperture, the amplitude of target scattering field and phase place are recorded, then by being focused at imaging to scattering field data Reason reconstruct original image.

Closely it is imaged：The division of closely remote imaging is relevant with the radiation field division of antenna, and the radiation field of antenna is by near To far generally can be divided into sensing closely, radiation closely area (Fresnel region) and far-field region.Define the Net long wave radiation that d is antenna Bore length, λ is the electromagnetic wavelength of aerial radiation, and R is the distance of observation station and antenna, and the nearby region of antenna meetsBecause this main application scenarios of invention algorithm is human body safety check, being therefore approximately considered is closely to be imaged.For mouth Electrical path length is the antenna of d, is about using synthetic aperture imaging time space resolution ratioThe geometric representation being closely imaged is as schemed Shown in 2.In closely imaging algorithm, dimension of adjusting the distance carries out the important step that phase error correction is imaging, its correction Accuracy directly affect the performance of imaging.Routine closely in imaging algorithm distance dimension phasing be to be preferable based on target Planar target it is assumed that i.e. impact point be in same parallel to XOY plane of vision in section, according to this thinking pair The document that distance dimension carries out phase compensation mainly has：

(1)Yinsheng Zhang,Jungang Yang,Wei An.Single frequency two- dimensional synthetic aperture imaging system and imaging algorithm.Journal of National University of Defense Technology.2014；

(2)David Sheen,Douglas McMakin,Thomas Hall.Near-field three- dimensional radar imaging techniques and applications.Applied Optics.2010；

(3)Shichao Li,Chao Li,Wei Liu,et al.Study of Terahertz Super resolution Imaging Scheme With Real-Time Capability Based on Frequency Scanning Antenna.IEEE Transaction Terahertz Science and Technology,2016.

Above-mentioned this several documents are all to carry out two-dimensional direction dimension synthetic aperture imaging using single-frequency point, and its imaging performance limits to Can neglect with respect to the distance between plane of vision in acquiescence body the target no ideal situation of thickness or body target thickness Situation about slightly disregarding.Therefore, it is necessary to proposing a kind of is all to have certain thickness body target, for suppressing for conventional target Body target thickness causes the Terahertz single-frequency point two dimension synthetic aperture algorithm that imaging performance declines.

Content of the invention

The present invention be directed to routine closely imaging algorithm cause when body target has difference in thickness imaging effect decline Problem it is proposed that under a kind of Terahertz single-frequency point suppression target thickness impact two-dimentional aperture imaging algorithm, using multiple put down Row in plane of vision apart from section to all target strong scatterings click through line phase compensate, set all under section Imaging results are weighted average treatment again, thus improving body target strong scattering point, distance ties up the difference of information to imaging to each other The negative effect causing.In the two-dimensional direction dimension imaging system of Terahertz single-frequency point, the enterprising line phase to target range direction The accuracy compensating directly affects imaging effect.

The technical scheme is that：Under a kind of Terahertz single-frequency point, the two-dimentional aperture imaging of suppression target thickness impact is calculated Method, specifically includes following steps：

1) target's center's plane be XOY, parallel and with target's center plan range R₀Place sets up plane of vision X'OY', Plane of vision central point z=-R₀；The strong scattering point coordinates of body target is that (x, y, Δ z), wherein Δ z are on distance dimension direction Impact point deviates the distance apart from section z=0 for the default objects；Dual-mode antenna is in z=-R₀Plane of vision on, receive and dispatch sky The coordinate of line is (x', y' ,-R₀)；

2) the Terahertz single frequency point signal that emitter sends is：s_t(t)=e^j2πft, wherein f is carrier frequency, reception antenna The echo-signal receiving strong scattering point is：

Wherein c is the light velocity under vacuum state, and g (x, y) is target strong scattering point (x, y, the scattering coefficient of Δ z)；X, y are Coordinate on acquiescence z=0 target XOY plane for the strong scattering point, Δ z is (x, y) place strong scattering point in distance dimension and default value z= 0 actual deviation value；X', y' are the coordinates of transceiver on plane of vision；

Down-converted is carried out to transmission signal and receipt signal, eliminates time variable t, obtain：

3) Terahertz safety check imaging, image scene is in short range, by step 2) through frequency conversion echo-signal warp After crossing sphere Wave Decomposition, obtain changing to the echo of wave-number domain from transform of spatial domain, echo is expressed as：

WhereinFor wave-number domain frequency；k_x'、k_y'、k_z'It is that X in wave-number domain, Y, the wave-number domain frequency of Z-direction are divided Amount；

4) the Z-direction coordinate setting strong scattering point target is distributed in (- △ z_max,△z_max) interval interior, i.e. △ z ∈ (- △ z_max, △z_max), using multiple apart from plane compensating image alone, then gather all imaging results and be weighted average mode, carry out A series of Pixel-level apart from tangent plane imaging results are superimposed, and wherein m-th phase compensation apart from section is：

z_m∈(-Δz_max,Δz_max), m=1 ..., M；z_mFor cutting of introducing during one of dimension phase compensation for distance Plane, revised distance is tieed up compensating factor and is

According to a series of figures obtaining image after section carries out phase compensation using bidimensional inverse Fourier transform As the magnitude relationship of entropy to determine weighting weights, the target picture being obtained apart from section inverting according to m-th is：

Weighting weight w=[w₁,w₂,...,w_M]^T, meetw_m∈(0,1)；

Then obtaining optimum image according to M different reconstruction result and its weighting weights is：

Described step 3) in image scene be in short range, now transmitting antenna transmission signal reaches objective plane Wavefront form retains the characteristic of spherical wave.

Described step 4) according to each compensate plane compensation after image image entropy magnitude relationship, to determine power Value, specially：

Set m width image and have N kind gray scale, and the probability between various gray scale is respectively p₁,p₂,...,p_N, then root According to Shannon's theorems, the image entropy of image is expressed as：

The total image entropy of M width image is：

The weighting weights of correspondence image are：

The beneficial effects of the present invention is：Under Terahertz single-frequency point of the present invention, the two-dimentional aperture of suppression target thickness impact becomes As algorithm, compensated by phase place is come using the superposition of multiple Pixel-level apart from tangent plane, can effectively suppress body target thick Degree, to the impact caused by imaging algorithm, can more accurately reconstruct the distribution of strong scattering point target scattering properties.

Brief description

Fig. 1 is the flow chart of two-dimensional imaging algorithm of the present invention；

Fig. 2 is body target of the present invention and plane of vision spatial distribution schematic diagram；

The geometric representation that Fig. 3 is closely imaged for the present invention；

Fig. 4 is the three-dimensional image figure of impact point of the present invention；

Fig. 5 is the section figure along direction position for the impact point of the present invention；

Fig. 6 is the reconstructed image being provided without weighted average process；

Fig. 7 adopts the reconstructed image of weighted average process for the present invention.

Specific embodiment

Proposed by the present invention is a kind of single-frequency point two being used for based on Terahertz frequency range and suppressing body target thickness to impact Dimension synthetic aperture imaging algorithm.As shown in Figure 1 the comprising the following steps that of this algorithm：

1st, such as Fig. 2 is body target and plane of vision spatial distribution schematic diagram.Plane of vision X'OY' is located at z=-R₀Place, mesh Mark plane is XOY, and two planes are parallel, i.e. X'OY' //XOY；The strong scattering point coordinates of body target be (x, y, Δ z), wherein Δ z is that impact point deviates the distance apart from section z=0 for the default objects on distance dimension direction；It is in z=-R₀Observation put down On face, the coordinate of dual-mode antenna is (x', y' ,-R₀).Then the distance of antenna to impact point is：

2nd, the Terahertz single frequency point signal that emitter sends is：s_t(t)=e^j2πft, wherein f is carrier frequency.Reception antenna The echo-signal receiving strong scattering point is：

Wherein c is the light velocity under vacuum state, and g (x, y) is target strong scattering point (x, y, the scattering coefficient of Δ z)；X, y are Coordinate on acquiescence z=0 target XOY plane for the strong scattering point, Δ z is (x, y) place strong scattering point in distance dimension and default value z= 0 actual deviation value；X', y' are the coordinates of transceiver on plane of vision X'OY'.

Down-converted is carried out to transmission signal and receipt signal, eliminates time variable t, obtain：

3rd, this invention is mainly used in Terahertz safety check imaging, and its image scene is in short range, therefore image field Scape is in in-plant position.The form that the signal of now transmitting antenna transmitting reaches the wavefront of objective plane cannot be with remote Scene is equally approximately plane wave, and wavefront retains the characteristic of spherical wave, therefore will process (as shown in Figure 3) as spherical wave.

Existing multiple optimization identification method all using be that spherical wave is decomposed into plane in two orthogonal directions The mode of ripple superposition.To the surface integral in above-mentioned echo, after plane wave superposition launches, then along X, Y-direction is mapped to ripple Number field, can obtain changing to the echo of wave-number domain from transform of spatial domain, echo is expressed as after sphere Wave Decomposition：

WhereinFor wave-number domain frequency；k_x',k_y',k_z'It is X in wave-number domain, Y, the wave-number domain frequency of Z-direction is divided Amount.

X, Y, the wave-number domain frequency range of Z-direction is：

Wherein L_x,L_yIt is respectively the effective aperture length (as shown in Figure 2) in plane of vision X and Y-direction for the transceiver.

4th, phase offset on the dimension of distance present in echo-signal direction is compensated.Carry out two-dimentional Fourier's inversion again Change, thus realizing the function that target echo signal reconstructs target scattering characteristics distribution.

Tie up in imaging in the two-dimensional direction based on single-frequency point, the high-resolution that two-dimensional direction is tieed up is by the effective length of synthetic aperture Contribution, and merely with single point-frequency signal, apart from tieing up, resolution capability is limited it is believed that no distance dimension differentiates energy for it due in imaging Power.There is the grid deviation Δ z related to distance dimension and constant R in echo-signal₀, in order to eliminate its impact to imaging, Need the phase offset that they cause is compensated.Due to just with single point-frequency signal it is impossible to obtain distance dimension information, The grid deviation Δ z that therefore actual body target range dimension exists lead to acquiescence merely with detection plane and target's center's plane away from From R₀The phase compensation carrying out cannot eliminate existing phase offset, also can introduce new phase offset, causes the performance being imaged Decline, or even failure.This invention is directed to target to be measured in practice and generally has certain thickness difference, leads to the distance of scattering point to be believed Cease the phenomenon having differences, in order to suppress its impact to imaging it is considered to target has certain thickness information, if strong scattering point The Z-direction coordinate of target is distributed in (- Δ z_max,Δz_max) interval interior, i.e. Δ z ∈ (- Δ z_max,Δz_max).Using multiple distances Plane compensating image alone, then gather all imaging results and be weighted average mode, carry out a series of apart from tangent plane imaging The Pixel-level superposition of result, carries out Image Reconstruction.Wherein m-th phase compensation apart from section is：

z_m∈(-Δz_max,Δz_max), m=1 ..., M；z_mFor cutting of introducing during one of dimension phase compensation for distance Plane.Revised distance ties up compensating factor

According to a series of figures obtaining image after section carries out phase compensation using bidimensional inverse Fourier transform As the magnitude relationship of entropy to determine weighting weights, the target picture being obtained apart from section inverting according to m-th is：

Weighting weight w=[w₁,w₂,...,w_M]^T, meetw_m∈(0,1)；

According to the magnitude relationship of the image entropy of image after each compensation plane compensation, to determine weights.

If m width image has N kind gray scale, and the probability between various gray scale is respectively p₁,p₂,...,p_N, then basis Shannon's theorems, the image entropy of image is expressed as follows：

The total image entropy of M width image is：

The weighting weights of correspondence image are：

The target picture being obtained according to each inverting weighting weights corresponding with it obtain final image：

Using the mode of multiple Pixel-level apart from tangent plane superposition, phase place is compensated and can effectively suppress body mesh Mark thickness is to the impact caused by imaging algorithm.Compared to the image not using weighted mean mode to process, can be more accurately Reconstruct the distribution of strong scattering point target scattering properties.

Fig. 6 is to be provided without the image that weighted average is processed, and Fig. 7 is the image being processed using weighted average.Than Apparent compared with rear apparent Fig. 7.

Claims (3)

1. the two-dimentional aperture imaging algorithm that under a kind of Terahertz single-frequency point, suppression target thickness affects is it is characterised in that concrete wrap Include following steps：

1) target's center's plane be XOY, parallel and with target's center plan range R₀Place sets up plane of vision X'OY', and observation is flat Face central point z=-R₀；The strong scattering point coordinates of body target is that (x, y, Δ z), wherein Δ z are impact points on distance dimension direction Deviate the distance apart from section z=0 for the default objects；Dual-mode antenna is in z=-R₀Plane of vision on, the seat of dual-mode antenna It is designated as (x', y' ,-R₀)；

2) the Terahertz single frequency point signal that emitter sends is：s_t(t)=e^j2πft, wherein f is carrier frequency, and reception antenna receives Echo-signal to strong scattering point is：

$s_r\left(x^{\′},y^{\′},t\right)=\∫\∫g\left(x,y\right)e^{j2\mathrm{\π}f\left(t-\frac{2R\left(x^{\′},y^{\′},R_0\right)}{c}\right)}dxdy$

Wherein c is the light velocity under vacuum state, and g (x, y) is target strong scattering point (x, y, the scattering coefficient of Δ z)；X, y are to dissipate by force Coordinate on acquiescence z=0 target XOY plane for the exit point, Δ z is (x, y) place strong scattering point in distance dimension and default value z=0 Actual deviation value；X', y' are the coordinates of transceiver on plane of vision；

Down-converted is carried out to transmission signal and receipt signal, eliminates time variable t, obtain：

$s_r(x^{\′},y^{\′})=s_r(x^{\′},y^{\′},t)e^{-j2\mathrm{\π}ft}=\∫\∫g(x,y)e^{-j2\mathrm{\π}f\left(\frac{2R(x^{\′},y^{\′},R_0)}{c}\right)}dxdy;$

3) Terahertz safety check imaging, image scene is in short range, by step 2) through frequency conversion echo-signal through ball After the Wave Decomposition of face, obtain changing to the echo of wave-number domain from transform of spatial domain, echo is expressed as：

$\begin{array}{c}{S}_{FT2}\left(k_{x^{\′}},k_{y^{\′}}\right)=\∫\∫s_r\left(x^{\′},y^{\′}\right)e^{-{\mathrm{jk}}_{x^{\′}}{x}^{\′}}{e}^{-{\mathrm{jk}}_{y^{\′}}{y}^{\′}}{\mathrm{dx}}^{\′}{\mathrm{dy}}^{\′}\\ =\∫\∫g\left(x,y\right)\left(\∫\∫e^{-{\mathrm{jk}}_{x^{\′}}x-{\mathrm{jk}}_{y^{\′}}y-{\mathrm{jk}}_{z^{\′}}\left(\mathrm{\Δ}z+R_0\right)}{\mathrm{dx}}^{\′}{\mathrm{dy}}^{\′}\right)dxdy\end{array}$

$k_{z^{\′}}=\sqrt{4k^2-k_{x^{\′}}^2-k_{y^{\′}}^2}$

WhereinFor wave-number domain frequency；k_x'、k_y'、k_z'It is the wave-number domain frequency component of X in wave-number domain, Y, Z-direction；

4) the Z-direction coordinate setting strong scattering point target is distributed in (- △ z_max,△z_max) interval interior, i.e. △ z ∈ (- △ z_max,△ z_max), using multiple apart from plane compensating image alone, then gather all imaging results and be weighted average mode, carry out one Series is superimposed apart from the Pixel-level of tangent plane imaging results, and wherein m-th phase compensation apart from section is：

$S(k_{x^{\′}},k_{y^{\′}},z_m)=S_{FT2}(k_{x^{\′}},k_{y^{\′}})e^{{\mathrm{jk}}_{z^{\′}}{R}_0}{e}^{{\mathrm{jk}}_{z^{\′}}{z}_m}$

z_m∈(-Δz_max,Δz_max), m=1 ..., M；z_mTie up the section introducing during phase compensation for one of for distance, Revised distance ties up compensating factor

According to a series of image entropies obtaining image after section carries out phase compensation using bidimensional inverse Fourier transform Magnitude relationship determining weighting weights, the target picture being obtained apart from section inverting according to m-th is：

$g_m(x,y)=\∫\∫S(k_{x^{\′}},k_{y^{\′}},z_m)e^{{\mathrm{jk}}_{x^{\′}}x}{e}^{{\mathrm{jk}}_{y^{\′}}y}{\mathrm{dk}}_{x^{\′}}{\mathrm{dk}}_{y^{\′}}$

Weighting weight w=[w₁,w₂,...,w_M]^T, meetw_m∈(0,1)；

Then obtaining optimum image according to M different reconstruction result and its weighting weights is：

$\hat{g}(x,y)=\underset{m=1}{\overset{M}{\Σ}}{w}_m{g}_m(x,y).$

2. the two-dimentional aperture imaging algorithm that under Terahertz single-frequency point, suppression target thickness affects according to claim 1, it is special Levy and be, described step 3) in image scene be in short range, now transmitting antenna transmission signal reaches objective plane Wavefront form retains the characteristic of spherical wave.

3. the two-dimentional aperture imaging algorithm that under Terahertz single-frequency point, suppression target thickness affects according to claim 2, it is special Levy and be, described step 4) in the image entropy of image after plane compensation is compensated according to each magnitude relationship, to determine power Value, specially：

Set m width image and have N kind gray scale, and the probability between various gray scale is respectively p₁,p₂,...,p_N, then according to perfume (or spice) Agriculture theorem, the image entropy of image is expressed as：

The total image entropy of M width image is：

The weighting weights of correspondence image are：