HOE-based ESPI systems

Electronic speckle pattern interferometry (ESPI) is a full-field measurement technique, capable of displaying vibrational mode shapes. Two electronic speckle pattern interferometers using reflection holographic optical elements (RHOEs) are presented. In the first ESPI system the RHOE is designed to create the speckled reference beam. 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. It is demonstrated that the HOE-based system can be used for vibration measurements. Phase shifting can be implemented for fringe analysis. In the second ESPI system a reflection holographic optical element of a flat diffusely reflecting surface serves a dual purpose. On reconstruction, a diffuse beam of laser light is produced to act as a reference beam in the ESPI system. Undiffracted light passing through the RHOE serves to illuminate the object. This system is not completely insensitive to in-plane displacement but the illumination and observation directions can be made nearly collinear. The systems are compared in terms of flexibility in their adjustment, sensitivity, suitability and limitations for different applications. The introduction of holographic optical elements in ESPSI systems gives the advantage of using high aperture optical elements at relatively low cost. Both systems are suitable for out-of-plane vibration studies. The results obtained are promising for future applications of RHOEs in alternative laser Doppler vibrometry systems.