## 4 Discussion  
The data on shock wave evolution into the long elastic wave in the polystyrene waveguide obtained using classical holographic interferometry and digital holography demonstrate good agreement despite some slight discrepancy caused by different sources of pulsed laser radiation used to generate initial shock waves in these experimental realizations. Due to this difference the wave profiles obtained using holographic interferometry and digital holography, although being similar in shape, differ in amplitude. The images shown in Figures 3a and 4a clearly show the presence of rapidly oscillating components indicated by yellow arrows. In the course of wave propagation in the waveguide, these linear short-wavelength components attenuate rapidly and are already scarcely visible in the images in Figures 3b and 4b, corresponding to wave patterns travelled for 10 cm in the waveguide. Since the wave pattern shown in Figure 3a was recorded at slightly smaller distance from the waveguide input than that shown in Figure 4a, short-wave oscillations in the former case had higher amplitudes. Additional small differences in the wave profiles shown in Figures 3a and 4a are due to the fact that the wave pattern in Figure 3a was recorded when this part of the waveguide was immersed in water, and additional disturbances propagating in surrounding water were superimposed on the resulting wave pattern in the waveguide (see [41] for details). Figure 4: Reconstructed profiles of strain waves in the polystyrene bar in the areas of 50–110 mm (**a**) and 100–160 mm (**b**) from the bar input. Yellow arrows in the graph (**a**) indicate fast-decaying short-wavelength oscillations. Figure 5: Initial stage of shock wave evolution after entering the polystyrene bar. (**a**) holographic interferogram; (**b**) set of digital holograms recorded during the first 15 us after shock wave generation; (**c**) phase distributions reconstructed from holograms (**b**). Note high amounts of rapidly oscillating components resulting in interference fringe breaks in (**a**,**b**) and phase wraps and singularities in (**c**). It is worth emphasizing major differences associated with obtaining and processing of experimental data in holographic interferometry and digital holography. The first and most obvious difference is the experimental procedure applied for the acquisition and analysis of the interference pattern. Classical holographic interferometry requires utilization of holographic recording materials; in most realizations, these are high-resolution photographic films or plates. The need for their chemical processing imposes restrictions on experiments in real time and does not allow for the prompt adjustment of experimental parameters in the course of experiment. On the contrary, digital holography easily allows for the automatization of the recording procedure and for synchronization of data processing workflow with specific steps of experiment, e.g. with waveguide displacement, change of the shock wave energy or of the delay between shock wave generation and hologram recording. Besides that, the recording of holograms of fast processes on photographic materials requires utilization of pulsed laser sources, while digital matrix photodetectors allow hologram recording using either pulsed [48; 49] or CW probe lasers. The latter case, though, necessitates the usage of fast digital cameras with the amplification and electro-optical shutter, providing registration of 8-12-bit images with exposure times of the order of nano- to microseconds, depending upon the particular process under study [26; 50]. Another fundamental difference when conducting experiments using digital holography and holographic interferometry is the ability of the latter technique to operate with highly distorted wavefronts.