Measurement of out-of-plane displacement with a digital real-time holography

A digital real-time holographic measurement system is designed based on the principles of double-exposed digital holography in this paper. In order to adapt this system to the variation of specimen size, a concave lens is placed in the classic 4f system, and then the object light path of this system, which transverse magnification can be adjusted by regulating the positions of lens, is equivalent to the 4f system. The object spectrum arriving at charge-coupled device camera (CCD) can be obtained by the Fourier transformation, and the complex amplitude of object wave can be obtained by the Fourier inversion. The phase variation corresponding to the initial object wave when the specimen is deformed can be gained by computer, and furthermore, phase variation corresponding to any of the two different displacement status can also be obtained. That is to say, with the help of this system, we can get much more information than that obtained by classic holography. The system has been used to measure the out-of-plane displacement of PMM specimen containing cracks. The holograms quality obtained by this system is close to that obtained by classic holography. The holograms obtained by digital real-time holography are also compared well with those obtained by classic holography. The digital real-time holography described in this paper can also be adapted for the variation of projection size of the specimen, therefore this system can be used to measure the specimens those projection size are larger than that of area-array of CCD.