Compact and stable phase stepping interferometer for shape and full field displacement measurement in static and in
“real time” operation mode is presented. Double symmetrical illumination of the object in two orthogonal planes with
diode lasers, emitting in NIR (790 nm and 830 nm), through a four-exposure reflective holographic optical element
(Denisiyk’s volume reflection holograms of a reference plane) is applied. Phase stepping is introduced simply by precise
increments of the diode lasers current. The proposed system is very stable against external noise, produced by vibrations,
temperature changes, air flows, as well as against the influence of object’s “rigid body” motion, as the compact and low
weight interferometer can be stably fixed directly onto the measured construction.
A compact and stable phase-stepping four-channels one-beam interferometer for full field displacement measurement in static and “real time” operation mode can be built through incorporation of a four-exposure reflection holographic optical element, which reconstructs four reference planes under illumination with two pairs of laser diodes positioned in the horizontal and the vertical planes. Such a multi-channel system is prone to misalignment errors and their estimation is crucial for its successful operation. The present work gives analysis of the error due to misalignment in illumination directions of the laser sources for the case of the out-of-plane (normal) displacements under double symmetrical illumination. First, analysis of the misalignment error observed in the experimental data is provided. Second, computer simulation of the system was made for estimation of the out-of-plane (normal) component of the displacement vector at each point of the tested object, and quantitative analysis of the errors caused by the misalignment was performed.
The paper presents a holographic optical element (HOE) for a phase-stepping digital electronic speckle pattern
interferometry with double symmetrical illumination of the object both in vertical and horizontal planes for precision
full-field displacement measurement of objects under loading . More specifically, the proposed HOE allows for
simultaneous reconstruction of four virtual and parallel to the HOE surface reference planes for off-axis illumination
with two pairs of diode lasers emitting at two different wavelengths. In this way we transform a two-beam interferometer
into a multiple beam interferometer with four separate channels. The HOE is constructed as a sandwich structure of two
reflection (Denisuyk type) holograms of a diffuse metal screen illuminated at 30 degrees to the normal. Each of the
holograms comprises two holographic reflection records made successively on a single holographic plate. Recording is
performed in the visible part of the spectrum, and through additional chemical treatment the spectral maximum of the
developed holograms is shifted into the IR region to achieve correct reconstruction for illumination at 790 nm and 830
nm. Super high resolution silver halide emulsion with resolution over 6000 line/mm is used for recording of holograms.
The reconstructed four reference beams interfere with the beam reflected from the object, thus forming four independent
optical channels for two laser beams at 790 nm with S and P polarizations, as well as for two S and P polarized beams at