Fig. 1. Typical particle trajectory image. (a) 2D movement; (b) 3D movement

Fig. 2. Arc trajectory model in polar coordinates

Fig. 3. Image processing flow chart for gray level fitting

Fig. 4. Synthetic straight trajectory images with different parameters

Fig. 5. Results of parameters measurement for trajectories in Fig. 4. (a) Result of image with no noise；(b) result of image with noise level of 5; (c) result of image with noise level of 10

Fig. 6. Synthetic arc trajectory images

Fig. 7. Recognition results of parameters measurement for trajectories in Fig. 6. (a) Result of image with no noise；(b) result of image with noise level of 3; (c) result of image with noise level of 5

Fig. 8. Experiment system for σ and z calibration

Fig. 9. Trajectory images when the target moves at speed of 30 mm/s (enhanced image)

Fig. 10. Relationship between z and σ

Fig. 11. Images of jet experiment (enhanced image). (a) Original image; (b) image reconstructed by gray－level fitting method

Fig. 12. 3D velocity results for Fig. 11. (a) 3D velocity field; (b) top view