Axially distributed sensing with a monoscopic imaging lens for single-shot distance measurements.

Using the image-based optical measurement method known as axially distributed sensing (ADS), the depth information of a scene is retrieved by evaluating images recorded with a camera positioned on multiple points along a common optical axis. We describe the design of a monoscopic lens that images the scene from two different on-axis points of perspective simultaneously onto a single camera. Designed for use in a territorial surveillance video system, this lens allows for capturing 3D scene information based on ADS with a single shot. We present the physical foundations of this approach and its implementation as a compact lens system and show the usability of the system prototype, supported by performance tests.

Estimation of the phase error in interferometric measurements by evaluation of the speckle field intensity.

Intensities recorded with CCD or CMOS sensors represent spatially averaged values of the intensity across the area of a pixel. In this work we investigate the influence of spatial averaging in interferograms on the evaluated phase from an object wave with well resolved, fully developed speckles. Based on an analytical description of the averaging process, a procedure is developed to create a quality map for the evaluated phase, in order to give an estimation of the expected error at each point. The proposed method uses only the local intensity distribution of the object wave for the qualification of the phase values. The theoretical results are tested and verified by means of numerically generated objective and subjective speckle fields.

Determination of large-scale out-of-plane displacements in digital Fourier holography.

A novel approach for the determination of large-scale out-of-plane displacements from digital Fourier holograms is presented. The proposed method is invariant to lateral object shifts. It is based on the determination of the scaling of the reconstructed image that occurs when the recording distance is changed. For a precise determination of the scaling factor, we utilize the Mellin transform. After the discussion of mathematical and computational issues, experimental results are presented to verify the theoretical considerations. The results show that displacements of at least up to 8.4% from the initial recording distance can be detected with this approach. The displacements could be determined with a deviation of typically less than 1.0% from the set values.

Improvement of accuracy in digital holography by use of multiple holograms.

Speckle pattern decorrelation reduces the accuracy of interferometric shape and deformation measurements. We introduce a technique for the reduction of speckle noise in digital holography. The method is not based on classical filtering techniques such as median filters. Instead it utilizes the shift theorem of the Fourier transform. For this method several holograms of the same object under test are recorded. The reconstruction leads to a set of object wave fields with different speckle patterns. A proper averaging procedure, taking into account the properties of the wrapped phases, leads to an improvement of the accuracy in the resulting phase difference. The theory of the applied method is described and our first results for technical components with an improvement of accuracy up to 1/57 of the wavelength are presented.

Correlation between intensity and phase in monochromatic light.

Analytical expressions to describe the phase gradient of monochromatic light by means of the three-dimensional intensity distribution are derived. With these formulas it is shown that the two-dimensional phase gradient in a plane can be completely determined from noninterferometric intensity measurements if the light propagates strictly in one direction. The analytical expressions are verified by means of numerical investigations on simulated speckle fields, and the results are discussed with respect to common deterministic phase retrieval approaches.

Miniaturized digital holography sensor for distal three-dimensional endoscopy.

A miniaturized sensor head for endoscopic measurements based on digital holography is described. The system was developed to measure the shape and the three-dimensional deformation of objects located at places to which there is no access by common measurement systems. A miniaturized optical sensor, including a complete digital holographic interferometer with a CCD camera, is placed at the end of a flexible endoscope. The diameter of the head is smaller than 10 mm. The system enables interferometric measurements to be made at speeds of as many as five reconstructions per second, and it can be used outside the laboratory under normal environmental conditions. Shape measurements are performed with two wavelengths for contouring, and the deformation is measured by digital holographic interferometry. To obtain full three-dimensional data in displacement measurements we illuminate the object sequentially from three different illumination directions. To increase the lateral resolution we use temporal phase shifting.