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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 24 — Nov. 21, 2011
  • pp: 23901–23907
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Analysis and simulation of the phenomenon of secondary spots of the TDI CCD camera irradiated by CW laser

Ke Sun, Liangjin Huang, Xiang’ai Cheng, and Houman Jiang  »View Author Affiliations


Optics Express, Vol. 19, Issue 24, pp. 23901-23907 (2011)
http://dx.doi.org/10.1364/OE.19.023901


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Abstract

The phenomenon of secondary spots is observed in the experiment of TDI CCD camera irradiated by CW He-Ne laser. It is considered to be related to the scattering of the slit in front of the sensor and the reflection of the window of the TDI CCD chip. Additional experiments and ray tracing simulation are performed to study the mechanism of the secondary spots. The experimental and simulated results demonstrated that the scattering of the side walls of the slit is the main source of the secondary spots. Furthermore, the operation mode of rotary scanning provides the chance of scattering incident beam to the side wall of the slit. This paper will provide a preliminary hint to the optimum design of slit in camera to reduce the effects of the secondary spots to the image quality.

© 2011 OSA

1. Introduction

Time delay and integration (TDI) mode of CCD is a method of scanning in which a frame transfer device produces a continuous video image of a moving object by means of a stack of linear arrays aligned with and synchronized to the movement of the object to be imaged. As the image moves from one line to the next, the integrated charge moves along with it and integrates more and more. Then the TDI CCD provides increased sensitivity without the sacrifice of spatial resolution, and the effective integration time is increased by the number of TDI stage [1

1. U. Bastian and M. Biermann, “Astrometric meaning and interpretation of high-precision time delay integration CCD data,” Astron. Astrophys. 438(2), 745–755 (2005). [CrossRef]

,2

2. G. Hochman, Y. Yitzhaky, N. S. Kopeika, Y. Lauber, M. Citroen, and A. Stern, “Restoration of images captured by a staggered time delay and integration camera in the presence of mechanical vibrations,” Appl. Opt. 43(22), 4345–4354 (2004). [CrossRef] [PubMed]

]. So the TDI CCD camera finds its niches in remote sensing systems [3

3. G. Ming-hui, Z. Gui-jun, L. Lei, and R. Jian-yue, “Study on dynamic imaging on TDI CCD optical remote sensor of push-broom technology,” Proc. SPIE 76567B, (2010).

6

6. A. Lacan, F. M. Braéon, A. Rosak, F. Brachet, L. Roucayrol, P. Etcheto, C. Casteras, and Y. Salaiin, “A static Fourier transform spectrometer for atmospheric sounding: concept and experimental implementation,” Opt. Express 18(8), 8311–8331 (2010). [CrossRef] [PubMed]

], biochemistry analysis [7

7. J. M. Emory and S. A. Soper, “Charge-coupled device operated in a time-delayed integration mode as an approach to high-throughput flow-based single molecule analysis,” Anal. Chem. 80(10), 3897–3903 (2008). [CrossRef] [PubMed]

], inspection in semiconductor manufacturing [8

8. K. W. Tobin, “Inspection in Semiconductor Manufacturing,” in Webster’s Encyclopedia of Electrical and Electronic Engineering (Wiley & Sons, NY, 1999), vol. 10, pp. 242–262

] and industry detection [9

9. A. Asundi, M.R. Sajan, and L. Tong, “Dynamic photoelasticity using TDI imaging,” Opt. Lasers Eng . 38(1-2), 3–16 (2002). [CrossRef]

].

Due to the high sensitivity, the TDI CCD camera can easily be dazzled by intensive light such as the Sun or laser. But the dazzling effects on TDI CCD camera are rarely reported. Zhang and his associates studied the dazzling effect of repetitive short pulse laser on TDI CCD camera [10

10. Z. Zhang, X.- Cheng, R. Wang, T. Jiang, D.- Qiu, and Z.- Jiang, “Dong-dong Qiu, Zong-fu Jiang, “Dazzling effect of repetitive short pulse laser on TDI CCD camera,” Opt. Lasers Eng. 49(2), 292–296 (2011). [CrossRef]

]. In that experiment, the TDI CCD camera is dazzled by a picoseconds pulse laser with 20 Hz repetition frequency. The periodic dark fringes are observed and can be explained by periodic laser pulses scattered by dust particles in the view of field of camera.

Zhang’s experiment indicates that there are many special and interesting dazzling effects in TDI CCD camera worthy of the further study. And a well understanding of the physical phenomena leading to these dazzling effects on TDI CCD camera is a necessary step on the way to quantifying these effects and establishing new figures of performance and improvement criteria for camera systems [11

11. A. Durecu, P. Bourdon, and O. Vasseur, “Laser-dazzling effects on TV-camera: analysis of dazzling effects and experimental parameters weight assessment,” Proc. SPIE 6738, 67380L, 67380L-6 (2007). [CrossRef]

,12

12. A. Durecu, O. Vasseur, P. Bourdon, B. Eberle, H. Bursing, J. Dellinger, and N. Duchateau, “Assessment of laser-dazzling effects on TV-cameras by means of pattern recognition algorithms,” Proc. SPIE 6738, 67380J, 67380J-9 (2007). [CrossRef]

].

In this paper, we performed an experiment by using CW He-Ne laser to irradiate a TDI CCD camera and we found another special phenomenon in the output video image: there are two secondary spots on both sides of primary spot which is the image of laser along the scanning direction. Then we performed additional experiments and ray tracing simulations to study the mechanism of the phenomenon of secondary spots.

2. Experimental set-up

The experimental set-up is shown in Fig. 1
Fig. 1 Experimental set-up photograph of CW He-Ne laser irradiating TDI CCD camera
which is composed of the following components:

  • The output power of CW He-Ne laser is 3.2mW. The output diameter of the beam is 1mm, and the diameter of the laser spot on the first lens of camera optic is about 3mm.
  • A pair of polarizes are used to control the laser intensity for avoiding accidental damage to the CCD chip.
  • The Suruga Seiki’s KS402-180 rotary stage has a step accuracy of 0.004° and a positioning accuracy of 0.05°. The maximal rotational speed is 20°/s and it is fixed as 19.2°/s in our experiment.
  • The TDI CCD camera used in the experiment is a DALSA’s Piranha HS-41-02K30 camera containing a TDI CCD sensor with 2048×64 pixels and 5 selectable TDI stages (16,24,32,48 and 64 respectively). In this experiment, the stage number is fixed as 16. The pixel size is 13μm×13μm. The lens of the camera is Nikon AF 50 mm f/1.4D. The line rate is set as 1280Hz in order to match the rotational speed of camera.

During the experiment, the camera is fixed at the rotary stage and rotates with it. In this way, the TDI CCD camera scans the scenery and produces a continuous video image. As the laser is out of the field of view of the camera, the rotation stops and the video image is collected. For the elimination of influence of the light around, the room light is turned off during the experiment. Typical impacts of CW He-Ne laser on TDI CCD camera are illustrated in Fig. 2
Fig. 2 Impacts of CW He-Ne laser on TDI CCD camera. (a) Low irradiance level (b) High irradiance level
. There are three spots in the picture along the scanning direction (horizon direction in the image). The middle spot is the laser spot, called as primary spot, and the two spots on both sides of primary spot are called as secondary spots. The array of spots around the primary spot is confirmed as the diffracted light from pixels which is reflected back towards the sensor by the window of the CCD chip. These spots are not the focuses in this paper, and our attention should be concentrated on the secondary spots.

At a low level of laser irradiance, the primary spot and the two secondary spots make pixels saturated. And the two secondary spot are not centrosymmetrical to the primary spot. The distance between the center of the left secondary spot and that of the primary spot is about 140 pixels, while the distance between the center of the right spot and that of the primary spot is about 100 pixels. As the laser irradiance increases, the saturated areas of primary spot and two secondary spots spread over a large number of pixels along the linear array direction (vertical direction in the image), but the relative positions of the two secondary spots to the primary spot do not change.

3. Additional experiments and simulations

The appearance of secondary spots reveals a complicated structure of the camera. In order to analyze the mechanism of secondary spots, the structure of camera should be investigated in detail. Demounting the lens of camera, we find that there is a slit in front of the sensor, as shown in Fig. 3
Fig. 3 Photograph of the slit near the sensor
. Its size is 33mm(length) ×3mm (width) ×1mm (thickness). Figure 4 (a)
Fig. 4 (a) Simplified profile of the camera (b) photograph of scattered light (the red spots in the center) from the side wall of the slit
illustrates a simplified profile of the camera. Lacking of the detailed data of the actual lens, we use an equivalent single lens with the same focal length instead. After the lens, there is a slit in front of the sensor. Behind the slit is the window of the TDI CCD chip. We can imagine that at a moment of the rotation of the camera, incident beam with an off-axis angle of θ is focused to the window of the chip after passing the lens. Then it is reflected by the window and scattered by the side wall of the slit again. The side walls of the slit are so close to the sensor and the focused incident light is so intensive that the scattered light from the side wall is received by the sensor and saturates the pixels, as shown in Fig. 4 (b). So the secondary spots appear. After a careful observation, we find the pixel array is not in the middle of the chip but on the right (the isolation row is on the left of the chip). That means the pixel array is closer to the right side wall of the slit than that to the left side wall. It may be the reason for the asymmetry of the secondary spots to the primary spot.

According to the assumptions above, the side walls of the slit and the window of the chip are important to the secondary spots. Additional experiments and simulations have been performed to prove the assumptions. The direct method is to remove the slit. But for avoiding the expensive risk of damaging the camera, we use strips of black paper to cover the left side wall, the right side wall, the window and their different combinations respectively to study the impacts of different structures to the secondary spots. The experimental results are illustrated in Fig. 5
Fig. 5 Impacts of different combinations of structures covered by strips of black paper to the secondary spots (a) without covering (b) cover the right side wall of the slit (c) cover the left side wall of the slit (d) cover both of the side walls of the slit (e) cover the left part of the window (f) cover the right side wall and the left part of the window (g) cover the left side wall and the left part of the window (h) cover both of two side walls and the left part of the window
. Note that, for avoiding the disturbance of the normal work of the sensor, we only cover the left part of window where there is enough space to be covered because of the right offset of the pixel array in the chip, as illustrated in Fig. 4 (a).

From the experimental results, it is found that covering the side wall of the slit doesn’t eliminate the secondary spot, but enlarges the saturated area of the corresponding secondary spot along the linear array direction (vertical direction in the image). This is because the surface of the slit is treated by black anodize processing. The range of the scattered light from the side wall of the slit is smaller than that from black paper, so the secondary spot induced by black paper is larger than that induced by side wall of the slit.

However covering the left part of the window can eliminate the left secondary spot. It reveals that the window may be another source of the secondary spots. But which is the main source of the secondary spots, the side walls of the slit or the window? A further ray tracing simulation can provide the answer.

Figure 6
Fig. 6 The simulation profile of the camera to certify the source of the secondary spot
shows the ray tracing simulation profile of the camera to certify the source of the secondary spot. The camera optic is modeled as an equivalent lens and the slit is modeled as a block with a property of scattering. The black lines represent the incident rays. This simulation model can help us to certify the source of the secondary spots conveniently. Figure 7
Fig. 7 Simulated results of ray propergation near the sensor with different off-axis angles during the scanning process (a) 1.6° (b) 1.7° (c) 1.75° (d) 1.8°
shows simulated results of ray propergation with different off-axis angles during the scanning process which give us a clear illustration of the progress of the left secondary spot. When the off-axis angle is less than 1.6°, all the incident rays are reflected away by the window and there is no secondary spot. As the off-axis angle reaches 1.7°, a part of rays reflected from the window hit on the left side wall of the slit and they are scattered towards the pixel array again. The left secondary spot begins to appear. At the off-axis angle of 1.75°, the incident rays are scattered by the left side wall of the slit towards the sensor. The secondary spot appears. As the off-axis angle is greater than 1.8°, all the incident rays are covered by the slit and no rays can reach the pixel array. The process of the secondary spot stops. Figure 8
Fig. 8 Simulated results of ray propergation near the sensor with a strip of paper covering the left part of the window.
shows that paper covering the left part of window cuts off the path of the scattered rays from the left side wall of the slit towards the pixel array, so the left secondary spot disappears as illustrated in Figs. 5 (e)‑(h).

Now it is confirmed that the scattering of the side wall of the slit is the main source of the secondary spot since all the rays that contribute to the secondary spot come from the rays scattered from the side wall of the slit. The window lies on the path of the scattered rays. However, the operation mode of rotary scanning in our experiment provides the chance of scattering incident rays for the slit. The position of the incident rays on the slit changes with the rotation of the stage, and if the incident rays hits on the side wall of the slit, then they can be scattered towards the sensor. So the operation mode of the scanning is the other condition of secondary spot.

4. Summary

In this paper, the phenomenon of secondary spots of TDI CCD camera irradiated by CW He-Ne laser was observed. To reveal the mechanism of this phenomenon, additional experiments and ray tracing simulation were performed. The conclusion obtained from the experimental and simulated results was that the scattering of the side walls of the slit was the main source of the secondary spots. The rays contributing to secondary spots came from the rays scattered from the side walls of the slit. Meanwhile, the operation mode of rotary scanning in our experiment provides the chance of scattering incident beam for the side wall of the slit. We believe that the scattering of the side wall of the slit and the operation mode of rotary scanning are the two conditions to the phenomenon of the secondary spots.

The secondary spots may be the typical phenomenon of cameras with a slit in front of the sensor. The slit is so closed to the sensor that it should be well designed. This paper provides us a preliminary hint to the optimum design of slit in cameras to reduce the effects of the secondary spots to the image quality. Our further work is to improve the design of slit including improving the surface processing to make the surface of the slit “blacker” or rougher and optimizing the shape of the slit to reduce the irradiated area by laser.

Acknowledgements

This work is supported by the National Key Basic Research Program, China (Topic No. 1030110).

References and links

1.

U. Bastian and M. Biermann, “Astrometric meaning and interpretation of high-precision time delay integration CCD data,” Astron. Astrophys. 438(2), 745–755 (2005). [CrossRef]

2.

G. Hochman, Y. Yitzhaky, N. S. Kopeika, Y. Lauber, M. Citroen, and A. Stern, “Restoration of images captured by a staggered time delay and integration camera in the presence of mechanical vibrations,” Appl. Opt. 43(22), 4345–4354 (2004). [CrossRef] [PubMed]

3.

G. Ming-hui, Z. Gui-jun, L. Lei, and R. Jian-yue, “Study on dynamic imaging on TDI CCD optical remote sensor of push-broom technology,” Proc. SPIE 76567B, (2010).

4.

G. I. Vishnevsky, M. G. Vidrevitch, V. G. Kossov, O. P. Kourova, and M. V. Chetvergov, “Unpacked TDI CCD designed for large format optoelectronic systems for the Earth remote sensing,” Proc. SPIE 5944, 594409 (2005). [CrossRef]

5.

M. Iyenqar and D. Lange, “The Goodrich 3th generation DB-110 system: operational on tactical and unmanned aircraft,” Proc. SPIE 6209, 1–12 (2006).

6.

A. Lacan, F. M. Braéon, A. Rosak, F. Brachet, L. Roucayrol, P. Etcheto, C. Casteras, and Y. Salaiin, “A static Fourier transform spectrometer for atmospheric sounding: concept and experimental implementation,” Opt. Express 18(8), 8311–8331 (2010). [CrossRef] [PubMed]

7.

J. M. Emory and S. A. Soper, “Charge-coupled device operated in a time-delayed integration mode as an approach to high-throughput flow-based single molecule analysis,” Anal. Chem. 80(10), 3897–3903 (2008). [CrossRef] [PubMed]

8.

K. W. Tobin, “Inspection in Semiconductor Manufacturing,” in Webster’s Encyclopedia of Electrical and Electronic Engineering (Wiley & Sons, NY, 1999), vol. 10, pp. 242–262

9.

A. Asundi, M.R. Sajan, and L. Tong, “Dynamic photoelasticity using TDI imaging,” Opt. Lasers Eng . 38(1-2), 3–16 (2002). [CrossRef]

10.

Z. Zhang, X.- Cheng, R. Wang, T. Jiang, D.- Qiu, and Z.- Jiang, “Dong-dong Qiu, Zong-fu Jiang, “Dazzling effect of repetitive short pulse laser on TDI CCD camera,” Opt. Lasers Eng. 49(2), 292–296 (2011). [CrossRef]

11.

A. Durecu, P. Bourdon, and O. Vasseur, “Laser-dazzling effects on TV-camera: analysis of dazzling effects and experimental parameters weight assessment,” Proc. SPIE 6738, 67380L, 67380L-6 (2007). [CrossRef]

12.

A. Durecu, O. Vasseur, P. Bourdon, B. Eberle, H. Bursing, J. Dellinger, and N. Duchateau, “Assessment of laser-dazzling effects on TV-cameras by means of pattern recognition algorithms,” Proc. SPIE 6738, 67380J, 67380J-9 (2007). [CrossRef]

OCIS Codes
(040.1490) Detectors : Cameras
(040.1520) Detectors : CCD, charge-coupled device
(110.2960) Imaging systems : Image analysis
(120.4820) Instrumentation, measurement, and metrology : Optical systems
(140.3460) Lasers and laser optics : Lasers
(290.0290) Scattering : Scattering

ToC Category:
Detectors

History
Original Manuscript: June 15, 2011
Revised Manuscript: July 29, 2011
Manuscript Accepted: October 14, 2011
Published: November 10, 2011

Citation
Ke Sun, Liangjin Huang, Xiang’ai Cheng, and Houman Jiang, "Analysis and simulation of the phenomenon of secondary spots of the TDI CCD camera irradiated by CW laser," Opt. Express 19, 23901-23907 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-24-23901


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References

  1. U. Bastian and M. Biermann, “Astrometric meaning and interpretation of high-precision time delay integration CCD data,” Astron. Astrophys.438(2), 745–755 (2005). [CrossRef]
  2. G. Hochman, Y. Yitzhaky, N. S. Kopeika, Y. Lauber, M. Citroen, and A. Stern, “Restoration of images captured by a staggered time delay and integration camera in the presence of mechanical vibrations,” Appl. Opt.43(22), 4345–4354 (2004). [CrossRef] [PubMed]
  3. G. Ming-hui, Z. Gui-jun, L. Lei, and R. Jian-yue, “Study on dynamic imaging on TDI CCD optical remote sensor of push-broom technology,” Proc. SPIE 76567B, (2010).
  4. G. I. Vishnevsky, M. G. Vidrevitch, V. G. Kossov, O. P. Kourova, and M. V. Chetvergov, “Unpacked TDI CCD designed for large format optoelectronic systems for the Earth remote sensing,” Proc. SPIE 5944, 594409 (2005). [CrossRef]
  5. M. Iyenqar and D. Lange, “The Goodrich 3th generation DB-110 system: operational on tactical and unmanned aircraft,” Proc. SPIE 6209, 1–12 (2006).
  6. A. Lacan, F. M. Braéon, A. Rosak, F. Brachet, L. Roucayrol, P. Etcheto, C. Casteras, and Y. Salaiin, “A static Fourier transform spectrometer for atmospheric sounding: concept and experimental implementation,” Opt. Express18(8), 8311–8331 (2010). [CrossRef] [PubMed]
  7. J. M. Emory and S. A. Soper, “Charge-coupled device operated in a time-delayed integration mode as an approach to high-throughput flow-based single molecule analysis,” Anal. Chem.80(10), 3897–3903 (2008). [CrossRef] [PubMed]
  8. K. W. Tobin, “Inspection in Semiconductor Manufacturing,” in Webster’s Encyclopedia of Electrical and Electronic Engineering (Wiley & Sons, NY, 1999), vol. 10, pp. 242–262
  9. A. Asundi, M.R. Sajan, and L. Tong, “Dynamic photoelasticity using TDI imaging,” Opt. Lasers Eng. 38(1-2), 3–16 (2002). [CrossRef]
  10. Z. Zhang, X.- Cheng, R. Wang, T. Jiang, D.- Qiu, and Z.- Jiang, “Dong-dong Qiu, Zong-fu Jiang, “Dazzling effect of repetitive short pulse laser on TDI CCD camera,” Opt. Lasers Eng.49(2), 292–296 (2011). [CrossRef]
  11. A. Durecu, P. Bourdon, and O. Vasseur, “Laser-dazzling effects on TV-camera: analysis of dazzling effects and experimental parameters weight assessment,” Proc. SPIE 6738, 67380L, 67380L-6 (2007). [CrossRef]
  12. A. Durecu, O. Vasseur, P. Bourdon, B. Eberle, H. Bursing, J. Dellinger, and N. Duchateau, “Assessment of laser-dazzling effects on TV-cameras by means of pattern recognition algorithms,” Proc. SPIE 6738, 67380J, 67380J-9 (2007). [CrossRef]

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