Holger Babovsky

3D shape measurement of macroscopic objects in digital off-axis holography using structured illumination

We propose what we believe to be a novel approach to measure the 3D shape of arbitrary diffuse-reflecting macroscopic objects in holographic setups. Using a standard holographic setup, a second CCD and a liquid-crystal-on-silicon spatial light modulator to modulate the object wave, the method yields a dense 3D point cloud of an object or a scene. The calibration process is presented, and first quantitative results of a shape measurement are shown and discussed. Furthermore, a shape measurement of a complex object is displayed to demonstrate its universal use.

Enhanced resolution of microscopic objects by image inversion interferometry

We demonstrate in experiment that the resolution of a conventional light microscope can be enhanced by 26% with the help of an image inverting interferometer. In order to prove this statement, we measured the point spread function of the system as well as the resolution of two-point objects. Additionally, the contrast transmission function of the interferometric setup was measured and compared to the results gained with a conventional wide-field microscope. Using the interferometric system, the spatial frequencies near the cutoff-frequency were far better transmitted than by the conventional microscope. Finally, we demonstrate the improved resolution with the help of images of two-dimensional structures.

Stereophotogrammetric 3D shape measurement by holographic methods using structured speckle illumination combined with interferometry.

We present a unique combination of the numerical three-dimensional (3D) reconstruction of the shape of an object with interferometric deformation measurements. Two cameras record several holograms of an object that is illuminated by structured illumination. This illumination is realized by speckle patterns. To improve the image quality, an inplace speckle reduction technique is combined with the structured illumination to reduce the effect of disturbing subjective speckles which appear in the reconstructed images. Stereophotogrammetric methods are applied to extract the 3D surface information of the object out of the reconstructed images. Since the recording is done by holography and because stereophotogrammetry enables a pointwise correlation between the two views, it is possible to combine other holographic techniques with the reconstructed 3D shape. This is demonstrated by an interferometric deformation measurement of an object cooling down. The resulting interferometric fringes are mapped onto the reconstructed 3D surface. Hence, the proposed method enables automatic and dense matching of interferometric fringe-maps recorded by spatially separated holograms onto the surface of the object, which has not yet been realized by existing techniques.

Measurement of three-dimensional deformation vectors with digital holography and stereophotogrammetry.

We present a method to determine the three-dimensional (3D) deformation vectors of an arbitrary stressed object by combining stereophotogrammetry and digital holography in a setup with four cameras. The resulting data consists of a dense 3D point cloud, where every point is associated with a deformation vector. Our method is able to calculate the deformation without prior knowledge of the sensitivity vectors or the object surface. In the experimental setup only the base distance of the cameras needs to be known.

Effects on vision with glare after correction of monochromatic wavefront aberrations.

PURPOSE To investigate effects of optical aberration correction on vision with glare. METHODS Correction of aberrations up to the 6th Zernike order (closed-loop correction) was compared with conventional spectacle correction in 42 healthy eyes. To create these corrections, an adaptive optics system including a thin-film transistor (TFT) monitor for displaying optotypes with additional glare sources was used. Employing both corrections, visual acuity and contrast sensitivity (CS) were tested alternately with and without glare. Disability glare was computed as the difference between log CS without and with glare. Individuals were also asked to rate subjectively the quality of three images displayed on the TFT monitor. RESULTS Significant improvements of CS without and with glare were found with the closed-loop correction (0.147 and 0.198 log CS, respectively), whereas no significant difference in visual acuity was found in either correction. Correlations were determined between reduction of total root-mean-square error and increase of CS with glare (Pearson correlation coefficient r=0.42) and decrease of disability glare (r=-0.33). Visual acuity was correlated with the visual Strehl ratio based on the optical transfer function (r=0.46). Subjective comparison of the images showed improvements more clearly. Depending on the image, in 57% to 78% of the eyes, closed-loop correction was rated better than spectacle correction. The subjective glare effect was reduced as well. CONCLUSIONS Investigation of vision with glare seems to be a reasonable additional test to evaluate the visual outcome of a customized correction.

Subjective speckle suppression in laser-based stereo photogrammetry

Abstract. The use of objective speckles as patterns is of high interest for the ongoing development of stereo photogrammetry. The depth of focus of the projected speckle patterns, which can be found to be several meters, can hardly be matched by other projection principles. On the downside, the use of coherent light leads to subjective speckles generated by the rough surface of the object under test. This effect decreases the accuracy under which objects can be reconstructed. We show how laser-based stereo photogrammetry can be adjusted to increase the measurement accuracy of three-dimensional (3-D)-surface measurements while preserving the advantages of speckles projection. Therefore, we present a method to decrease the contrast of subjective speckles in the images by pixel-wise shifting the cameras orthogonally to their viewing direction and back shifting the taken images numerically, accordingly. This leads to an increase in 3-D-reconstruction quality, as seen in a decrease in standard deviation, peak-to-valley value and in an increase in the number of reconstructed points for measured test objects.

Digital holography for microscopic imaging and 3D shape measurement

Digital holography is used for a wide range of applications. A lot of techniques deal with holographic microscopy or the 3D shape measurement of objects. We present our approaches to these applications. To increase the resolution of a microscopic imaging system a method for aperture synthesis is applied, where the spatial frequency shift, the global phase differences and the amplitude ratios of the individual sections of the Fourier spectrum are measured by using an overlap between them. It is shown that this method can be performed out including sub-pixel accuracy. The experimental holographic setup uses tilted illumination beams realized by an LCoS SLM, which can be easily adapted to the numerical aperture of the microscope objective. For the 3D shape measurement of arbitrary diffuse-reflecting macroscopic objects a novel approach is demonstrated, which uses common digital holographic setup together with a second CCD and an LCoS to modulate the object wave. Our idea is to capture a series of holograms from multiple positions and to apply concepts of structured light photogrammetry, which deliver more accurate depth information. The method yields a dense 3D point cloud of a scene.

High-density three-dimensional measurements through multilayer perceptron calibration and statistical band-limited patterns

Calibration of optical metrology stereophotogrammetric systems is vital to obtain accurate and precise three-dimensional (3-D) measurements. Despite its importance, the work pipeline of intrinsic and extrinsic camera calibration still remains manually laborious with high technical complexity. The use of a multilayer perceptron neural network to calibrate an optical metrology stereophotogrammetric system utilizing a statistical band-limited pattern projection system is demonstrated. Highly accurate, highly precise, and highly dense 3-D surface reconstructions are obtained solely from homologous corresponding pairs without the need for intrinsic and extrinsic camera calibration. Measurement performance in the typical optical metrology sense, where 3-D measurements were evaluated with respect to length and surface gauges, is shown.

Stereophotogrammetric Image Field Holography

In conventional holography, it is difficult to gain access on the three-dimensional surfaces of objects under test [1]. In contrast, there are well established techniques like the stereophotogrammetry that reconstruct surfaces of objects in dense three-dimensional point-clouds by using at least two cameras and structured illumination [2]. Recent works demonstrated, that reconstructed digital holograms can be used as images in the stereophotogrammetrical determination of object surfaces resulting in a three-dimensional point-cloud of the object [3]. Additionally, it is possible to combine the point-cloud with holographic techniques, such as holographic interferometry or phase-shifting techniques [4]. Due to such a combination, full three-dimensional deformation vectors can be calculated, providing the user with a method of high precision deformation measurements [5]. To accomplish these calculations, four cameras are used to connect three point-clouds with each other, so that the deformations measured, depending on the sensitivity-vector of each camera, can be combined to a full three-dimensional vector.