Out-of-plane Vibration Analysis with a Transmission Holographic Optical Element Based Electronic Speckle Pattern Interferometer

A simple electronic speckle pattern interferometer (ESPI) using a transmission holographic optical element (THOE) is presented. The THOE is designed to create a speckled reference beam in the interferometer. It is a transmission hologram of a diffusely transmitting glass plate A specific requirement for the fabrication of the THOEs is for them to be recorded at one wavelength at which the recording material is photosensitive and reconstructed using a near infrared laser diode which can be current modulated for phase shifting purposes. A partially reflective glass plate provides illumination of the object along the normal to its surface, ensuring that the system is sensitive only to out-of-plane displacement of the object. The intensity of the object beam can be controlled by using reflective glass plates with different reflection coefficients. It is demonstrated that the HOE based system can be used for vibration measurements and modal analysis. A big advantage of the system is its simplicity