Hybrid optonumerical quasi Fourier transform digital holographic camera

Abstract

We present a novel hybrid digital holographic camera which shares most of the advantages of image-plane Fourier transform TV holography (TVH) and quasi-Fourier transform digital holography (QFTDH), whilst avoiding many of the drawbacks of both of them. As in TVH, it has a compact head where an objective lens is attached to accommodate objects of different sizes or placed at different distances; it is also free from aliasing artifacts produced by objects out of the field of view. As in QFTDH, the reconstruction of the object field (amplitude and phase) is accomplished by calculating just one fast Fourier transform (FFT) per hologram; light is spread over the sensor rather than being focused to produce an image, thus enabling the measurement in objects with very large radiance ranges. An optical imaging system (typically a zoom lens) selects the field of view and the working distance by projecting a reduced image of the object on the plane of a rectangular aperture. This image becomes the object for a lensless quasi-Fourier transform digital hologram, which is formed by making the light passing through the aperture to interfere with a reference beam diverging from its edge. This hologram is recorded with a video camera, digitized and numerically reconstructed by means of a single FFT. The function of the aperture is to crop the field of view to make the effective object size suitable to be recorded without aliasing on a sensor with a given pixel spacing; therefore, its size is determined by this spacing, the distance between the aperture and the sensor as well as by the wavelength of light.