Light intensity versus small aperture in two-wavelength contouring by analog difference holographic interferometry

Abstract

In two-wavelength contouring by difference holographic interferometry, the test object is illuminated holographically by real images of the master object belonging to the two different wavelengths. In this process, however, the illuminating holograms have to reconstructed together. The illuminations belonging to the other wavelengths are unwanted, but fortunately they are reconstructed with some direction shift. Thus they can be filtered out by a proper aperture in the Fourier plane of a lens. Because the shift of the corresponding spot in the Fourier plane is relatively small, the similarly small filtering aperture leads to a significant reduction of the master object wave intensities in the recording steps. This in practice may result in a low-quality master hologram and consequent poor holographic illumination as well. To overcome this, the paper suggests two techniques to increase the light intensity of the filtered master object waves. First, a shape change of the aperture is proposed, and its extension to an aperture system. Second, a special optimization of the optical arrangement geometry is suggested to maximize the applicable aperture size in the Fourier plane. The two techniques provide an order of magnitude of intensity increase, even up to more than half of the nonfiltered value.