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Low-contrast target detection using polarized Streak Tube Imaging Lidar
Optik 124 (2013) 2674–2678
Contents lists available at SciVerse ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Low-contrast target detection using polarized Streak Tube Imaging Lidar Jianfeng Sun ∗ , Tianjiao Wang, Xuefeng Wang, Jing S. Wei, Q. Wang National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, 150080 Harbin, China
a r t i c l e
i n f o
Article history: Received 30 March 2012 Accepted 1 August 2012
Keywords: Streak Tube Imaging Lidar Polarization imaging Target detection Degree of polarization
a b s t r a c t Streak Tube Imaging Lidar (STIL) is provided with the characteristics of high resolution, high detection sensitivity, and wide field of view (FOV). It is a new type laser radar, and has actual application. For the low-contrast condition, image segment becomes difficult to STIL, which will influence on the detection/recognizing rate. In order to resolve the problem, a new polarized STIL imaging system is presented. Its structure is designed, which is included a polarizer and a Wollaston prism to the transmitting and receiving system, respectively. The polarized STIL can simultaneously collect the two polarization images. Through image processing and computing, the intensity and DOP images with high resolution are obtained, which can provide the rich information of targets. The far distance imaging experiments that the part target is hidden in the clutter are carried out. The results state that the polarization images have high-contrast, and it is easy to segment target from the background. © 2012 Elsevier GmbH. All rights reserved.
1. Introduction Laser radar can collect the intensity image and range image simultaneously, which can be used to detecting and recognizing the interesting objects [1,2]. The intensity image reflects the target reflectivity for the light, therefore, the target and clutter can usually be segmented relying on the characteristic of laser radar. However, for some conditions, the reflectivity of the target and clutter is so similar, named low-contrast, the detecting target will become difficult. In order to resolve the problem, the polarization laser radar is presented [3–6], and this technique has attracted a lot of interests due to its ability to perform the target detection and recognition. In recently years, the different types of laser radar are presented, in which, Streak Tube Imaging Lidar (STIL) is attracted by researchers because of its characteristic [7–9], such as highresolution, high detection sensitivity, and wide field of view (FOW). For the far distance detecting, if the clutter around the target or camouflaged scene, the intensity image is usually low-contrast, it is difficult to discriminate the target. So, it is necessary to research the polarimetric STIL (P-STIL) in order to improve the ability of detection or recognizing. For the imaging polarimetry, in order to get different polarimetric information including degree of polarization (DOP) [10], azimuth of polarization, depolarization index and phase variation,
∗ Corresponding author. E-mail address: hit [email protected] (J. Sun). 0030-4026/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2012.08.076
some methods have been studied to measure Muller matrix of targets or Stokes parameters of scattered light. However, they are often difficult to control accurately and measurement steps are complex. DOP is an important information for laser radar, which reflects the target polarization characteristic. Therefore some methods, such as dual-rotating-retarder technique (DRRT) [9], reducing the number of measurements, are appeared. However, for the single detector, the technique still needs to move the optical elements. Gleckler introduces the imaging polarimetry in multi-slits Streak Tube Imaging Lidar system (MS-STIL) [7,10] by making only two independent measurements for capturing polarimetric data one time, this system was simple, but the imperfections in relative spacing between fibers of the fiber plane results in gaps and ripples on the multiple-slit end, which finally results in the distortion of image [8]. Comparatively, more clear images can be acquired by single-slit Streak Tube Imaging Lidar (STIL), which provides excellent solutions for high-resolution three-dimensional laser radar imagery [9]. In this paper, the polarimetric imaging system of STIL is designed, and the corresponding computing of polarization angles is finished. Using STIL instrument and other polarization and optics elements, P-STIL is built. For acquiring two images of the separated two beams with the orthogonal states simultaneously, we took off the slit in front of streak tube photocathode, and as well the transmitted beam of illuminating targets was transformed into fan beam horizontally by a cylinder lens. Through the two images, the intensity and DOP images of different targets were obtained, which are complementary and different. Presentation of the DOP
J. Sun et al. / Optik 124 (2013) 2674–2678
2675
Fig. 1. Experimental principle diagram of imaging polarimetry. The equations of Mueller matrix of linear polarizer and /4 wave plate, respectively, are denoted as [7,10].
and intensity in the fusion image enhances the visual contrast of the targets with respect to the other clutters. The detection or recognizing rate was improved by using P-STIL. 2. Polarization system design For the polarization STIL that its system chart is shown as Fig. 1, the polarization state generator (PSG) which consist of the linear polarizer and /4 wave plate is added between laser and transmitting optics, and the polarization state analyzer (PSA) which is composed of Wollaston prism and /4 wave plate is added between the streak tube detector and the receiving optics.
⎡
1
cos(2)
sin(2)
0
⎤
⎢ cos(2) cos2 (2) cos(2) sin(2) 0 ⎥ 1⎢ ⎥ Mp = ⎢ ⎥ 2 ⎣ sin(2) cos(2) sin(2) sin2 (2) 0⎦ 0
⎡
0
1
0
0
0
0 0
⎤
⎢0 cos2 (2˛) cos(2˛) sin(2˛) − sin(2˛) ⎥ ⎢ ⎥ Mq = ⎢ ⎥ sin2 (2˛) cos(2˛) ⎦ ⎣ 0 cos(2˛) sin(2˛) 0
− cos(2˛)
sin(2˛)
(1)
(2)
In receiving system, the Wollaston prism can split the returned light into two orthogonal linear polarization light, so it can be seen as two polarizers, which can be denoted by Mueller matrix Mp1 and Mp2 , respectively. The Mueller matrix of /4 wave plate in receiving system is denoted by Mq1 . At lastly, the streak tube detector will receive two beams, which is shown as S1 and S2 :
S1 = Mq1 · Mp1 · Sout
Because the streak tube detector can only detect the light strength, the first element in S1 and S2 is corresponding to the light strength, which are denoted as I⊥ and I// . In the paper, we consider the linear polarized light to finish the polarization experiments. So, the /4 wave plate will remove from the system. Setting the angle between the optics-axis of the linear polarizer and x-axis is 90◦ , and relying on its Mueller matrix, Sin can be obtained:
⎡ 1 −1 ⎢ −1 1 Sin = Mp · S = ⎣ 0 0 0
0
In which, is the angles of the optics axis of linear polarizer and x-axis, and ˛ is the angles of the fast-axis of wave plate and x-axis. The laser transmits a pulse light, and its Stokes vector S is: S = (S0 , S1 , S2 , S3 )T
In order to obtain the DOP for the laser radar, J.J. Gel thinks that using the elements of Mueller matrix can compute the DOP of target scattering light. However, in the actual application, measuring all the elements in Mueller matrix is so difficult because of the complex system design. According to analyzing a lot of experiments data, only the diagonal elements can affect the DOP, so the Mueller matrix is simplified, and it is denoted as:
⎡
Mtarget
1
0
0
⎢ 0 M11 0 ⎢ =⎢ ⎣ 0 0 M22 0
0
0
0
⎤
⎥ ⎥ ⎥ 0 ⎦
(5)
⎞
S3
0
Sout
(9)
0 Similarly, the returned light will go through Wollaston prism, and it can be seen as two linear polarizers, which angles are 90◦ and 0◦ , respectively. According to their Mueller matrix, the expression of I⊥ and I// can be shown as:
⎧ I ⎪ ⎨ I⊥ = 0 (1 + M11 )
⎪ ⎩ I// = I0 (1 − M11 )
(10)
2
At last, the total light strength of scattered light and DOP can be expressed as:
M33
So, after the target scattering, the Stokes vector of scattered light is shown as: Sout = Mtarget · Sin
0
⎛ 1 ⎞ ⎜ −M11 ⎟ ⎟ = Mtarget · Sin = I0 ⎜ ⎝ 0 ⎠
2
0
0
S0
in which, the first element is the light strength in the target, so I0 = (1/2)(S0 − S1 ). The Stokes vector of scattered light is
The S expression will be as Sin bellowed through PSG: (4)
0
0
⎤
⎛
⎛ 1 ⎞ ⎜ ⎟ 0 0 ⎥ ⎜ S1 ⎟ 1 ⎜ −1 ⎟ ·⎜ (S − S1 ) ⎟= 0 0 ⎦ ⎝ S2 ⎠ 2 ⎝ 0 ⎠ 0
0
(8)
(3)
Sin = MPSG · S = Mq · Mp · S
(7)
S2 = Mq1 · Mp2 · Sout
(6)
⎧ ⎨ Itotal = I⊥ + I// = I0 ⎩ DOP =
I⊥ − I// I⊥ + I//
= M11
(11)
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J. Sun et al. / Optik 124 (2013) 2674–2678
Fig. 4. One CCD image including two streak tube images.
Fig. 2. Polarization optics on elliptically polarized light.
the streak tube photocathode. At last, a 12 bits CCD camera captured the phosphor images at high spatial resolution. One experimental CCD image was shown in Fig. 4. The CCD camera was connected to a computer. The images of parallel and vertical polarization can be obtained at each laser pulse, which intensity is denoted by I⊥ and I// , respectively. 3.1. Indoor experiments
Fig. 3. The picture of polarization STIL.
When the incidence light is linear polarization, the equation of DOP is only included M11 element, which can state that DOP reflect the response characteristic of linear polarized light to target. Based on above analysis, the polarization system is completed, and its chart is shown in Fig. 2. 3. Polarization imaging experiments The polarization imaging instrument is shown as Fig. 3. The light source uses a diode pumped Nd:YAG pulse laser. The output wavelength is 532 nm, 8 ns width, 29 mJ each pulse and 3 mrad divergence angle. Spacious distribution of laser energy follows the gauss distribution. The polarization optics at the emission just consisted of a linear polarizer oriented with its axis at 90◦ with respect to the x-axis, and laser beams became linear polarized through the linear polarizer. The laser beams are expanded into 10◦ fan beam horizontally by a cylinder lens to illuminate the targets. The transmitting and receiving system was collinear, the distance between them was 30 cm. The returned light from targets was collected by a ˚70 mm optical telescope. The polarization optics just consisted of a Wollaston prism which splits the returned light into two beams with an 5◦ angular separation between the orthogonal states – parallel and vertical polarization. Since collecting the two beams one time, we remove the single-slit in front of streak tube photocathode. Through focus lens, the two beams were properly received on
In order to validate the imaging polarimetry of various kinds of targets in the same scene, the experiment is finished, and the targets are include (1) black iron plate, (2) black smooth wood board and (3) a yellow one, (4) a color magazine, (5) porous metal plate and (6) a steel ruler, and (7) the background is white foam which generates nearly diffuse reflection. The result is shown in Fig. 5. Fig. 5(a) is the scene picture, and using I⊥ image that is the polarization-maintaining component and I// image that is the depolarized component, the intensity image is synthesized, as Fig. 5(d). Based on the DOP equation, the DOP image is computed with its values ranging 0 (black) for completely depolarized return to 1 (white) for completely polarization-maintaining return. Compared with Fig. 5(d) and (e), we can find that paper and white foam background have high intensity values, but very low DOP values (∼4%), because a higher order of multiple scatting occurs after interaction of the transmitted light with such targets which reduces the polarization. Also, We can observe that black materials have very low reflectivity, but in particular, have higher DOP values (50–68%) than other man-made targets including metallic objects (∼24%), because black surfaces has less multiple scattering and a higher polarization from the first scatter [14]. Based on the analysis above, it is attracting to combine intensity and polarization information on a single image using HSI color-coding. Fusion image can be obtained by merging the two images into HSI colored image, the DOP is mapped to hue and the total intensity is mapped to lightness in the HSI color system, as shown by the color map in Fig. 5(f). The fusion image can display total intensity and degree of polarization simultaneously, obviously it is easier to identify and classify the targets. This example clearly shows that DOP data as a complement to intensity data reveals unique information of observed objects, enhancing their contrast. The above experiment states that P-STIL can improve the contrast for some low-contrast intensity image, special some metal
Fig. 5. The experiment of various kinds of targets in the same scene.
J. Sun et al. / Optik 124 (2013) 2674–2678
2677
Fig. 6. The experiment of the target cover by the branches.
Fig. 7. The experiment of outdoor target covered by hurst.
material targets. The experiment of two different material targets is validated, and the scene picture is shown in Fig. 6(a). The metal material target1 and target2 are cover by the branch which can be regard as clutter. For the intensity image, the contrast of target and clutter is so similar because of similar reflectivity, and it is difficult to segment the target from the clutter. Using P-STIL, the target is very clear in the DOP image, which improves the contrast comparing with intensity image. The fusion image combines the intensity and the DOP information. The targets have high DOP denoting by green color, and the branches, which depolarizes the light, appears in yellow. 3.2. Outdoor experiment We choose the outdoor building as interesting target that is far away from P-STIL about 400 m, and the clutter is the hurst. The scene picture is shown in Fig. 7. The image pixels are 120 × 512, which are high resolution image, as Fig. 7(b). Comparing with intensity image and DOP image, P-STIL can still improve the contrast in DOP image, even though for far distance targets. For the intensity image, the clutter and the part of target covered by the clutter is so very similar, and for the DOP image and fusion image, the part of target become very clear, as in Fig. 7(e) and (f). Based on the experiments results, P-STIL can penetrate the camouflage, such as
the mesh object, the branch or the brushwood, and collecting the high contrast images, which is easy to improve the detection and recognition rate. 4. Conclusion P-STIL is an effective method to improve the low-contrast image, which can increase the detection and recognition rate. The target material is metal or cement, and the clutter is wood, P-STIL can collect the high contrast image which is easy to segment the target from the background. Through the outdoor experiments, PSTIL can collect the high contrast image of building and woods. The results state that the P-STIL designed can be applied to the remote detection and recognition. Acknowledgement This work is sponsored by National Nature Science Foundation, Contract No.60901046. References [1] J.H. Shapiro, Target-reflectivity theory for coherent laser radar, Appl. Opt. 21 (18) (1982) 3398–3407.
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[2] B.A. Hutchinson, R.L. Galbraith, B.L. Stann, Simulation-based analysis of range and cross-range resolution requirements for the identification of vehicles in radar imagery, Opt. Eng. 42 (9) (2003) 2734–2745. [3] H. Chen, L.B. Wolff, Polarization phase-based method for material classification and object recognition in computer vision, in: IEEE Conf. on Computer Vision and Pattern Recognition, San Francisco, 1996, pp. 128–135. [4] W.G. Egan, M.J. Duggin, Synthesis of optical polarization signatures of military aircraft, SPIE 4481 (2002) 188–194. [5] C.S.L. Chun, F.A. Sadjadi, Polarimetric laser radar target classification, Opt. Lett. 30 (14) (2005) 1806–1808.
[6] Y.N. Xu, Y. Zhao, Y. Zhang, L.P. Liu, C.F. Jin, X.D. Sun, Imaging lidar for occluded target recognition, SPIE 7382 (2009), 738220-1–738220-7. [7] A.D. Gleckler, Multiple-slit streak tube imaging lidar (MS-STIL) applications, Proc. SPIE 4035 (2000) 266–278. [8] A. Gelbart, B.C. Redman, R.S. Light, C.A. Schwartzlow, A.J. Griffis, Flash lidar based on multiple-slit streak tube imaging lidar, Proc. SPIE 4723 (2002) 9–18. [9] J.F. Sun, Q. Wang, 4-D image reconstruction for streak tube imaging lidar, Laser Phys. 19 (3) (2009) 502–504. [10] A.D. Gleckler, A. Gelbart, Three-dimensional imaging polarimetry, SPIE 4377 (2001) 175–185.
Contents lists available at SciVerse ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Low-contrast target detection using polarized Streak Tube Imaging Lidar Jianfeng Sun ∗ , Tianjiao Wang, Xuefeng Wang, Jing S. Wei, Q. Wang National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, 150080 Harbin, China
a r t i c l e
i n f o
Article history: Received 30 March 2012 Accepted 1 August 2012
Keywords: Streak Tube Imaging Lidar Polarization imaging Target detection Degree of polarization
a b s t r a c t Streak Tube Imaging Lidar (STIL) is provided with the characteristics of high resolution, high detection sensitivity, and wide field of view (FOV). It is a new type laser radar, and has actual application. For the low-contrast condition, image segment becomes difficult to STIL, which will influence on the detection/recognizing rate. In order to resolve the problem, a new polarized STIL imaging system is presented. Its structure is designed, which is included a polarizer and a Wollaston prism to the transmitting and receiving system, respectively. The polarized STIL can simultaneously collect the two polarization images. Through image processing and computing, the intensity and DOP images with high resolution are obtained, which can provide the rich information of targets. The far distance imaging experiments that the part target is hidden in the clutter are carried out. The results state that the polarization images have high-contrast, and it is easy to segment target from the background. © 2012 Elsevier GmbH. All rights reserved.
1. Introduction Laser radar can collect the intensity image and range image simultaneously, which can be used to detecting and recognizing the interesting objects [1,2]. The intensity image reflects the target reflectivity for the light, therefore, the target and clutter can usually be segmented relying on the characteristic of laser radar. However, for some conditions, the reflectivity of the target and clutter is so similar, named low-contrast, the detecting target will become difficult. In order to resolve the problem, the polarization laser radar is presented [3–6], and this technique has attracted a lot of interests due to its ability to perform the target detection and recognition. In recently years, the different types of laser radar are presented, in which, Streak Tube Imaging Lidar (STIL) is attracted by researchers because of its characteristic [7–9], such as highresolution, high detection sensitivity, and wide field of view (FOW). For the far distance detecting, if the clutter around the target or camouflaged scene, the intensity image is usually low-contrast, it is difficult to discriminate the target. So, it is necessary to research the polarimetric STIL (P-STIL) in order to improve the ability of detection or recognizing. For the imaging polarimetry, in order to get different polarimetric information including degree of polarization (DOP) [10], azimuth of polarization, depolarization index and phase variation,
∗ Corresponding author. E-mail address: hit [email protected] (J. Sun). 0030-4026/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2012.08.076
some methods have been studied to measure Muller matrix of targets or Stokes parameters of scattered light. However, they are often difficult to control accurately and measurement steps are complex. DOP is an important information for laser radar, which reflects the target polarization characteristic. Therefore some methods, such as dual-rotating-retarder technique (DRRT) [9], reducing the number of measurements, are appeared. However, for the single detector, the technique still needs to move the optical elements. Gleckler introduces the imaging polarimetry in multi-slits Streak Tube Imaging Lidar system (MS-STIL) [7,10] by making only two independent measurements for capturing polarimetric data one time, this system was simple, but the imperfections in relative spacing between fibers of the fiber plane results in gaps and ripples on the multiple-slit end, which finally results in the distortion of image [8]. Comparatively, more clear images can be acquired by single-slit Streak Tube Imaging Lidar (STIL), which provides excellent solutions for high-resolution three-dimensional laser radar imagery [9]. In this paper, the polarimetric imaging system of STIL is designed, and the corresponding computing of polarization angles is finished. Using STIL instrument and other polarization and optics elements, P-STIL is built. For acquiring two images of the separated two beams with the orthogonal states simultaneously, we took off the slit in front of streak tube photocathode, and as well the transmitted beam of illuminating targets was transformed into fan beam horizontally by a cylinder lens. Through the two images, the intensity and DOP images of different targets were obtained, which are complementary and different. Presentation of the DOP
J. Sun et al. / Optik 124 (2013) 2674–2678
2675
Fig. 1. Experimental principle diagram of imaging polarimetry. The equations of Mueller matrix of linear polarizer and /4 wave plate, respectively, are denoted as [7,10].
and intensity in the fusion image enhances the visual contrast of the targets with respect to the other clutters. The detection or recognizing rate was improved by using P-STIL. 2. Polarization system design For the polarization STIL that its system chart is shown as Fig. 1, the polarization state generator (PSG) which consist of the linear polarizer and /4 wave plate is added between laser and transmitting optics, and the polarization state analyzer (PSA) which is composed of Wollaston prism and /4 wave plate is added between the streak tube detector and the receiving optics.
⎡
1
cos(2)
sin(2)
0
⎤
⎢ cos(2) cos2 (2) cos(2) sin(2) 0 ⎥ 1⎢ ⎥ Mp = ⎢ ⎥ 2 ⎣ sin(2) cos(2) sin(2) sin2 (2) 0⎦ 0
⎡
0
1
0
0
0
0 0
⎤
⎢0 cos2 (2˛) cos(2˛) sin(2˛) − sin(2˛) ⎥ ⎢ ⎥ Mq = ⎢ ⎥ sin2 (2˛) cos(2˛) ⎦ ⎣ 0 cos(2˛) sin(2˛) 0
− cos(2˛)
sin(2˛)
(1)
(2)
In receiving system, the Wollaston prism can split the returned light into two orthogonal linear polarization light, so it can be seen as two polarizers, which can be denoted by Mueller matrix Mp1 and Mp2 , respectively. The Mueller matrix of /4 wave plate in receiving system is denoted by Mq1 . At lastly, the streak tube detector will receive two beams, which is shown as S1 and S2 :
S1 = Mq1 · Mp1 · Sout
Because the streak tube detector can only detect the light strength, the first element in S1 and S2 is corresponding to the light strength, which are denoted as I⊥ and I// . In the paper, we consider the linear polarized light to finish the polarization experiments. So, the /4 wave plate will remove from the system. Setting the angle between the optics-axis of the linear polarizer and x-axis is 90◦ , and relying on its Mueller matrix, Sin can be obtained:
⎡ 1 −1 ⎢ −1 1 Sin = Mp · S = ⎣ 0 0 0
0
In which, is the angles of the optics axis of linear polarizer and x-axis, and ˛ is the angles of the fast-axis of wave plate and x-axis. The laser transmits a pulse light, and its Stokes vector S is: S = (S0 , S1 , S2 , S3 )T
In order to obtain the DOP for the laser radar, J.J. Gel thinks that using the elements of Mueller matrix can compute the DOP of target scattering light. However, in the actual application, measuring all the elements in Mueller matrix is so difficult because of the complex system design. According to analyzing a lot of experiments data, only the diagonal elements can affect the DOP, so the Mueller matrix is simplified, and it is denoted as:
⎡
Mtarget
1
0
0
⎢ 0 M11 0 ⎢ =⎢ ⎣ 0 0 M22 0
0
0
0
⎤
⎥ ⎥ ⎥ 0 ⎦
(5)
⎞
S3
0
Sout
(9)
0 Similarly, the returned light will go through Wollaston prism, and it can be seen as two linear polarizers, which angles are 90◦ and 0◦ , respectively. According to their Mueller matrix, the expression of I⊥ and I// can be shown as:
⎧ I ⎪ ⎨ I⊥ = 0 (1 + M11 )
⎪ ⎩ I// = I0 (1 − M11 )
(10)
2
At last, the total light strength of scattered light and DOP can be expressed as:
M33
So, after the target scattering, the Stokes vector of scattered light is shown as: Sout = Mtarget · Sin
0
⎛ 1 ⎞ ⎜ −M11 ⎟ ⎟ = Mtarget · Sin = I0 ⎜ ⎝ 0 ⎠
2
0
0
S0
in which, the first element is the light strength in the target, so I0 = (1/2)(S0 − S1 ). The Stokes vector of scattered light is
The S expression will be as Sin bellowed through PSG: (4)
0
0
⎤
⎛
⎛ 1 ⎞ ⎜ ⎟ 0 0 ⎥ ⎜ S1 ⎟ 1 ⎜ −1 ⎟ ·⎜ (S − S1 ) ⎟= 0 0 ⎦ ⎝ S2 ⎠ 2 ⎝ 0 ⎠ 0
0
(8)
(3)
Sin = MPSG · S = Mq · Mp · S
(7)
S2 = Mq1 · Mp2 · Sout
(6)
⎧ ⎨ Itotal = I⊥ + I// = I0 ⎩ DOP =
I⊥ − I// I⊥ + I//
= M11
(11)
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J. Sun et al. / Optik 124 (2013) 2674–2678
Fig. 4. One CCD image including two streak tube images.
Fig. 2. Polarization optics on elliptically polarized light.
the streak tube photocathode. At last, a 12 bits CCD camera captured the phosphor images at high spatial resolution. One experimental CCD image was shown in Fig. 4. The CCD camera was connected to a computer. The images of parallel and vertical polarization can be obtained at each laser pulse, which intensity is denoted by I⊥ and I// , respectively. 3.1. Indoor experiments
Fig. 3. The picture of polarization STIL.
When the incidence light is linear polarization, the equation of DOP is only included M11 element, which can state that DOP reflect the response characteristic of linear polarized light to target. Based on above analysis, the polarization system is completed, and its chart is shown in Fig. 2. 3. Polarization imaging experiments The polarization imaging instrument is shown as Fig. 3. The light source uses a diode pumped Nd:YAG pulse laser. The output wavelength is 532 nm, 8 ns width, 29 mJ each pulse and 3 mrad divergence angle. Spacious distribution of laser energy follows the gauss distribution. The polarization optics at the emission just consisted of a linear polarizer oriented with its axis at 90◦ with respect to the x-axis, and laser beams became linear polarized through the linear polarizer. The laser beams are expanded into 10◦ fan beam horizontally by a cylinder lens to illuminate the targets. The transmitting and receiving system was collinear, the distance between them was 30 cm. The returned light from targets was collected by a ˚70 mm optical telescope. The polarization optics just consisted of a Wollaston prism which splits the returned light into two beams with an 5◦ angular separation between the orthogonal states – parallel and vertical polarization. Since collecting the two beams one time, we remove the single-slit in front of streak tube photocathode. Through focus lens, the two beams were properly received on
In order to validate the imaging polarimetry of various kinds of targets in the same scene, the experiment is finished, and the targets are include (1) black iron plate, (2) black smooth wood board and (3) a yellow one, (4) a color magazine, (5) porous metal plate and (6) a steel ruler, and (7) the background is white foam which generates nearly diffuse reflection. The result is shown in Fig. 5. Fig. 5(a) is the scene picture, and using I⊥ image that is the polarization-maintaining component and I// image that is the depolarized component, the intensity image is synthesized, as Fig. 5(d). Based on the DOP equation, the DOP image is computed with its values ranging 0 (black) for completely depolarized return to 1 (white) for completely polarization-maintaining return. Compared with Fig. 5(d) and (e), we can find that paper and white foam background have high intensity values, but very low DOP values (∼4%), because a higher order of multiple scatting occurs after interaction of the transmitted light with such targets which reduces the polarization. Also, We can observe that black materials have very low reflectivity, but in particular, have higher DOP values (50–68%) than other man-made targets including metallic objects (∼24%), because black surfaces has less multiple scattering and a higher polarization from the first scatter [14]. Based on the analysis above, it is attracting to combine intensity and polarization information on a single image using HSI color-coding. Fusion image can be obtained by merging the two images into HSI colored image, the DOP is mapped to hue and the total intensity is mapped to lightness in the HSI color system, as shown by the color map in Fig. 5(f). The fusion image can display total intensity and degree of polarization simultaneously, obviously it is easier to identify and classify the targets. This example clearly shows that DOP data as a complement to intensity data reveals unique information of observed objects, enhancing their contrast. The above experiment states that P-STIL can improve the contrast for some low-contrast intensity image, special some metal
Fig. 5. The experiment of various kinds of targets in the same scene.
J. Sun et al. / Optik 124 (2013) 2674–2678
2677
Fig. 6. The experiment of the target cover by the branches.
Fig. 7. The experiment of outdoor target covered by hurst.
material targets. The experiment of two different material targets is validated, and the scene picture is shown in Fig. 6(a). The metal material target1 and target2 are cover by the branch which can be regard as clutter. For the intensity image, the contrast of target and clutter is so similar because of similar reflectivity, and it is difficult to segment the target from the clutter. Using P-STIL, the target is very clear in the DOP image, which improves the contrast comparing with intensity image. The fusion image combines the intensity and the DOP information. The targets have high DOP denoting by green color, and the branches, which depolarizes the light, appears in yellow. 3.2. Outdoor experiment We choose the outdoor building as interesting target that is far away from P-STIL about 400 m, and the clutter is the hurst. The scene picture is shown in Fig. 7. The image pixels are 120 × 512, which are high resolution image, as Fig. 7(b). Comparing with intensity image and DOP image, P-STIL can still improve the contrast in DOP image, even though for far distance targets. For the intensity image, the clutter and the part of target covered by the clutter is so very similar, and for the DOP image and fusion image, the part of target become very clear, as in Fig. 7(e) and (f). Based on the experiments results, P-STIL can penetrate the camouflage, such as
the mesh object, the branch or the brushwood, and collecting the high contrast images, which is easy to improve the detection and recognition rate. 4. Conclusion P-STIL is an effective method to improve the low-contrast image, which can increase the detection and recognition rate. The target material is metal or cement, and the clutter is wood, P-STIL can collect the high contrast image which is easy to segment the target from the background. Through the outdoor experiments, PSTIL can collect the high contrast image of building and woods. The results state that the P-STIL designed can be applied to the remote detection and recognition. Acknowledgement This work is sponsored by National Nature Science Foundation, Contract No.60901046. References [1] J.H. Shapiro, Target-reflectivity theory for coherent laser radar, Appl. Opt. 21 (18) (1982) 3398–3407.
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