Real-time double-exposure stroboscopic TV holography for the detection of very small vibrations: a comparison with other TVH techniques

Abstract

Conventional double-exposure stroboscopic TV holography (TVH) provides both the temporal resolution that characterizes stroboscopic techniques and the low sensitivity to environmental noise inherent to additive correlogram formation. This makes it very well suited for the analysis of vibrations, especially if they are non-sinusoidal, in industrial environments. Unfortunately, it also shares with other additive techniques a very low sensitivity to small object-phase changes between illumination pulses. In this communication, we present a novel technique for the enhancement of the sensitivity to small vibrations in double-exposure TVH. This technique is based in the modulation of the phase of the reference beam with a rectangular waveform in synchronism with the stroboscopic illumination whilst the secondary fringes are generated in real-time by sequential subtraction. Several variants are defined attending to the demodulation process (square-law detection or full-wave rectification) and to the illumination method (double pulse or double single-pulse). The performance of our technique is compared to well-established techniques as homodyne time-averaging and single-exposure stroboscopic TVH in terms of sensitivity, temporal resolution and immunity to environmental noise. The theoretical model of the new technique as well as its experimental implementation on a fibre-optic electronic speckle-pattern interferometer (ESPI) and some demonstrative practical results (showing vibrations with amplitudes down to 8 nm) are reported.