Optical and numerical reconstruction of images from sound-wave holograms

Sound-wave holograms are constructed with sound waves of 10 kHz to 20 kHz by two holographic techniques; the coherent background and the electronic reference methods. The acoustical fields, which are scanned by a microphone and displayed by a lamp, are recorded on films as sound-wave holograms. In the process of reconstructing images from sound-wave holograms, optical and numerical reconstructions are investigated. In the optical reconstruction the images are reconstructed by illuminating the reduced hologram with laser light. The images reconstructed from grating-like holograms constructed in sampling-type holography are discussed. The numerical reconstruction of images is conducted by computer calculation of the digital Fresnel transform of sound-wave holograms using the FFT (fast Fourier transform) method. The analog sound-wave holograms recorded on films are divided into meshes of 64 × 64 cells and each cell is digitized into three levels according to the transparency of the film. The images reconstructed by the optical and the numerical methods are compared and discussed. Theoretical analyses are conducted to explain the experimental procedures and the obtained results.