Adaptative reconstruction distance in a lensless digital holographic otoscope

Abstract

We are developing a Digital Optoelectronic Holographic System (DOEHS) for measurements of shape and deformations acoustically induced of the human tympanic membrane (TM) in the clinic. Such measurements will be used to perform quantitative diagnosis of the middle-ear. The DOEHS platform consists of laser-delivery illumination (IS), optical head (OH), image-processing computer (IP), and positioning arm (PS) subsystems. Particularly, the OH of the DOEHS has been configured as an in-line, lensless, holographic arrangement to quantify deformations of the TM when it is subjected to controlled sound excitation. Holographic information is recorded by a digital camera and reconstructed numerically by the Fresnel integral approximation. Accurate measurements are achieved when TM samples are imaged within the depth of focus of the OH, which requires the selection of the optimal numerical reconstruction distance. In this paper, we discuss an experimental approximation to identify the best reconstruction distance based on the position of the reference beam (RB) within the OH and its associated phase factor within the Fresnel integral approximation. Results of these investigations are being used to further optimize the OH design of the DOEHS system, and to improve the numerical reconstruction algorithms used. Representative experimental measurements are shown.