Method for full-field measurement using dynamic laser doppler imaging

A method for Doppler imaging a full field to perform a measurement, characterized in that in a heterodyne system, a heterodyne frequency shift between two beams is performed, and the frequency shift is flexibly adjustable according to measurement requirements;

• a change of a measured physical quantity affects a phase value, and the phase value is extracted by the mathematical means of Fourier transform or wavelet transform, and then the measured physical quantity is deduced, used for a laser which includes a polarization controller, a spatial filter, a general beam splitter, a polarizer, a telecentric imaging system and a plane mirror, wherein said method includes the following steps:
• 1) turning on the laser and adjusting the laser to make an emitted laser beam's level perpendicular to the emitted surface of the laser;
• after passing through the polarizer, a lithium niobate crystal controlled by a driving power source and a quarter-wave plate, the laser beam is expanded by a spatial filter and divided into two beams of light by the beam splitter;
• one of the two beams of light irradiates on the reference material through a polarizer, and the other beam irradiates on the object also through another polarizer;
• next, a reference light and an object light are transmitted and reflected by another beam splitter respectively, and then transmitted through a polarizing beam later, making the reference light and the object light interfered, and an interferogram is collected by a CCD camera through an imaging system;
• a center of every part of the device is coaxial at the same height, and surface of a reference object and a measured object are perpendicular to the direction of the incident light;
• 2) adjusting the spatial filter to obtain a circular laser spot with a uniform intensity distribution, in which a spot size needs to cover a surface to be detected, so as to achieve a full field surface detection;
• 3) adjusting the quarter-wave plate and all the polarizers in the laser system, so that the two beams of the reference object and the measured object have different frequencies and their polarization's directions are perpendicular to each other, an orientation of the polarizer in front of the CCD camera is set at 45°, so that two perpendicular polarized lights transmitted through this polarizer are interfered;
• 4) putting a slight pressure on the measured object and setting the CCD camera in a continuous acquisition mode so as to observe interference fringes, and adjust an optical path to make the interference fringes clearly visible;
• 5) setting a sampling frequency, a sampling time, an image format and a resolution of the CCD camera;
• 6) turning on a power switch to drive a lithium niobate crystal to generate a heterodyne carrier frequency;
• 7) providing a continuous equivalent thrust on the measured object by means of a piezoelectric ceramics to make the measured object get a continuous bending deformation;
• and
• 8) controlling the CCD camera by a computer for sampling, and collecting a group of time series light intensity interferograms with the continuous deformation of the object;
• processing the time series light intensity interferograms, thus obtaining a three-dimensional data module which includes the time and spatial distribution of the continuous deformation of the measured object.