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IOP Publishing

Optical Cryptosystems

Naveen K Nishchal

Chapter 3

Fully-phase image encryption

3.1 Introduction

A phase object is defined as a completely transparent object but has an optical thickness that varies from point to point. Such an object introduces phase difference between disturbances that pass through different parts of it. Consequently, the disturbances immediately behind the object and in the conjugate image plane produce the same amplitude at all points but show variations in phase from point to point. The human eye is sensitive to intensity only and cannot detect phase changes. Therefore, the field of view appears uniformly bright. The variations in optical thickness in the object cause variations in intensity in the image so that the phase object is rendered visible [1].

Imaging of phase objects has been a subject of considerable interest in the field of optics. In microscopy, there are several objects, which are largely transparent, thus absorbing little or no light. When light passes through such an object, the predominant effect is the generation of a spatially varying phase shift. For a number of applications, the spatial distribution of the phase is the only available and/or the only desirable information. In the ideal case, the phase object is absolutely invisible. Various techniques have been applied for the imaging and visualization of phase objects. The Zernike’s phase contrast method is a well-established technique for the visualization of phase perturbations. A fabricated phase contrast filter (PCF) is used for imaging the phase objects [2].

The use of phase has a longstanding history in optical image processing and is usually correlated with the use of coherent illumination. Invariably, a 2D image is encoded in the phase of an optical wavefront and decoded back again after processing using the phase contrast technique. The commercial availability of SLMs has widened the scope of complex-valued representation for computation applications. Phase encoding offers advantages in terms of computational efficiencies or light power efficiencies [3