Fig. 1. FLEET experiment schematic diagram

Fig. 2. Scaling calibration. (a) Original scale; (b) After threshold segmentation; (c) After tilt correction

Fig. 3. Air fluorescent filament images excited by femtosecond laser with different focal length and laser energy. (a)~(c) With 300 mm focus length and 1 mJ, 2 mJ and 3 mJ laser energy respectively; (d)~(f) With 1000 mm focus length and 1 mJ, 2 mJ and 3 mJ laser energy respectively

Fig. 4. Process for definition of effective area of fluorescent filament. (a) Projection directions; (b) One dimension curve of projection along x direction, I_y; (c) Projections of signal and background along y direction, I_s and I_n ; (d) I_sx curve before and after filtering in space frequency domain; (e) After space frequency filtering of I_sx and I_nx; (f) Effective area of fluorescent filament

Fig. 5. Geometric feature extraction of fluorescent filament. (a) Result of binarization; (b) Center line extraction

Fig. 6. Fluorescent filament SNR and length vs laser power and focal length. (a) SNR curve; (b) Filament length curve

Fig. 7. The normalized peak density of air fluorescent filament for different time delay under different pressures

Table 1. The maximum gray affected by laser power and focal length

Table 2. The laser power density at back focal plane with different laser power and focal length/(W/cm²)