Controlling several parameters in the digital holography reconstruction process

Recent developments in solid-state image sensors and digital computers have made it possible to directly record holograms by Charge Coupled Device (CCD) camera and numerical reconstruction of the object wave front by computer. Digital holograms recorded with a CCD array are numerically reconstructed in amplitude and phase through calculation of the Fresnel-Kirchhoff integral. Two methods are usually adopted to reconstruct digital holograms called Fresnel Transformation Method (FTM) and the Convolution Method (CM). In FTM, the reconstruction pixel increases with the reconstruction distance so that the size of image, in terms of number of pixels, is reduced for longer distances, limiting the resolution of amplitude and phase reconstruction. In CM, by contrast, the reconstruction pixel does not change, but remains equal to the pixel size of recording array. The CM is more appropriate for reconstruction at small distances whereas the FTM is useful for longer distances according to the paraxial approximation necessary to apply it. The flexibility offered by the reconstruction process in Digital Holography allows exploitation of new possibilities of application in different fields. Through the reconstruction process we will show that it is possible to control image parameters as focus distance, image size and image resolution. Those newly explored potentialities open further novel prospective of application of Digital Holography in single and multi-wavelengths operation either for display and metrological applications. We demonstrate the concept of controlling parameters in image reconstruction of digital holograms in some real situations for inspecting silicon MEMS structures.