Application of two-dimensional wavelet transform to hologram analysis: visualization of glass fibers in a turbulent flame

In different fiberization processes, glass fibers are drawn out in a turbulent flame. High-speed in-line holography is applied to visualize glass fibers. However, the random spatial distribution of temperature induces strong local variations of the refraction index. Consequently, the reconstructed images are altered. The direct analysis of the diffraction patterns recorded by the holographic plate can offer an alternative solution. The diffraction process can be interpreted as a convolution with a wavelet family of functions. The scale parameter a is related to the distance between the object and the plane of observation. The 3-D location of a fiber element and its orientation (theta) are estimated by searching for the parameters a and (theta) , which yield a maximum modulus of the wavelet transform. The results are compared with those obtained from a conventional optical reconstruction. The application of the wavelet transform improves the SNR in the image and enables the 3-D fiber location to be determined more accurately (plus or minus 50 micrometers for the axial coordinate estimation).