Optical configurations for oceanographic applications of eHolography

Abstract

eHolography (electronic or digital holography) has been used for observing and measuring particle size and distribution in biomedicine, sedimentology and oceanology. In these areas, eHolography possesses advantages over other imaging techniques in terms of its capability for recording live organisms (e.g. plankton) or events (e.g. sediment erosion) in a non-disturbed environment. Compared with classical holography, an eHoIogram is recorded digitally on an electronic imaging sensor and reconstructed numerically, to produce a three-dimensional map of the scene. In this paper, we present our progress in developing an electronic Holographic Camera for high-resolution imaging, both subsea and in air (eHoloCam). Many optical configurations have been successfully used in classical (photographic) holography, including a back-illuminating inline beam with and without a separated reference beam, and side/front illuminated object with an off-axis reference beam. For eHolography, it is necessary to reexamine these and other optical geometries with specific reference to the constraints of reduced resolution and recording area of current CCD or CMOS imaging sensors. For example, using a divergent beam in recording back-illuminated in-line eHolograms can give an improvement in image resolution over the more usual collimated beam geometry. Furthermore, when using an off-axis beam in eHolography considerable limitations are encountered in obtaining useable holograms, such as the need to record at small reference beam angles. In this paper, we will discuss the various geometries we have used to overcome these difficulties.