Calibration of an array projector used for high-speed three-dimensional shape measurements using a single camera.

Geometric calibration of digital light processing projectors in single-camera, fringe-projecting 3D measurement systems have been studied assuming the projector is inverse pinhole modeled. Conversely, a high-speed multi-aperture array projector (MAAP) projecting aperiodic fringes is not dependent on a digital mirror device and cannot be pinhole modeled. With MAAP projection, a stereo camera setup is required. This paper presents a model-less method to calibrate a MAAP by direct measurement of its illumination field and re-enables 3D measurements with a single camera even with surface discontinuities present. Experimental proof of principle and preliminary measurement performance are shown.

Aberration correction in coherence imaging microscopy using an image inverting interferometer.

We present an imaging method with the ability to correct even large optical phase aberrations in a purely numerical way. For this purpose, the complex coherence function in the pupil plane of the microscope objective is measured with the help of an image inverting interferometer. By means of a Fourier transform, it is possible to reconstruct the spatially incoherent object distribution. We demonstrate that aberrations symmetric to the optical axis do not impair the imaging quality of such a coherence imaging system. Furthermore, we show that it is possible to gain an almost complete correction of remaining aberrations with the help of a reference measurement. A mathematical derivation is given and experimentally verified. To demonstrate the ability of our method, randomly generated aberrations with peak-to-valley values of up to 8 λ are corrected.

3D shape measurement of static and moving objects with adaptive spatiotemporal correlation.

Common correlation-based photogrammetric 3D shape measurement techniques evaluate either temporal or spatial features. Temporal approaches achieve high accuracies but are limited to the measurement of static objects. Spatial techniques can deal with moving objects but provide relatively inaccurate results. Our goal is to combine these methods in order to measure dynamic scenes that contain static and moving objects. Therefore, we present a spatiotemporal correlation that adapts its temporal and spatial support locally to the motion of the measured objects. In addition, our technique compensates motion by warping the correlated image regions temporally. Our approach is based on structured illumination of random patterns, which are well suited for dynamic scenes due to high possible frame rates. The proposed technique is tested with simulated data and real measurements.

Coherent two-beam interference fringe projection for highspeed three-dimensional shape measurements.

Two-beam interference is a fundamental and well-understood approach to create Fizeau's interference fringes. With a Mach-Zehnder interferometer, we utilize these two-beam interference Fizeau fringes for three-dimensional (3D) shape measurements. By introducing an acousto-optical deflector the phase of the interference fringes can be shifted with a rate of up to 200,000 Hz. When used in conjunction with highspeed cameras, this stereo-photogrammetric approach performs well for highspeed applications in comparison with the commonly used digital light processing projectors for stripe projection. Maximum speed and the achievable accuracy are discussed. Experiments and media substantiate the suitability, accuracy, and speed of this technique for very fast 3D shape measurements.

Just-noticeable differences for wavefront aberrations obtained with a staircase procedure.

PURPOSE: To explore the magnitude of aberrations that alter the subjective visual quality measurably. The suitability of a staircase procedure for threshold determination was investigated. METHODS: Spectacle correction (correction A) was achieved in 28 healthy eyes by an adaptive optics set-up, and additional astigmatism, coma, or trefoil was induced (correction B). Subjects observed a radial test pattern with noncyclopleged eyes using an artificial 5-mm pupil. They compared corrections A and B and identified the subjectively better correction. The magnitude of the aberration for each presentation varied according to the "Best PEST" staircase procedure. Just-noticeable difference (JND) was found after 30 presentations. Participants were divided into three groups, and the experiment was repeated under the following conditions: without modifications (n = 9), with a letter chart (n = 10), and with full correction of all aberrations (n = 9). RESULTS: Median JND was 0.091 µm for astigmatism, 0.059 µm for coma, and 0.108 µm for trefoil, with a large interindividual spread. Repeatability coefficient across the three aberrations was 0.095 µm. A significant correlation was found between JNDs and the residual aberrations (without the generated aberration) in astigmatism and coma. Differences between radial test pattern and letter chart and between spectacle correction and full correction were not significant. CONCLUSIONS: JNDs were lower for coma than for astigmatism and trefoil, depending on the magnitude of other residual aberrations, and showed large variations among subjects. Thus, the decision to correct more pronounced aberrations (eg, by wavefront-guided LASIK) should not depend on wavefront measurement results alone. A staircase procedure for determination of JNDs provides repeatable results.

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.

Partially coherent light-emitting diode illumination for video-rate in-line holographic microscopy.

The light of a light-emitting diode or a common thermal source, such as a tungsten filament lamp, is known to be quasi-incoherent. We generated partially coherent light of these sources with a volume of coherence in the micrometer range of 5-100 µm3 by spatial and spectral filtering. The corresponding degree of partial coherence was adapted for microscopic interference setups, such as a digital in-line holographic microscope. The practicability of the sources was determined by the spectral emittance and the resulting signal-to-noise ratio (SNR) of the detector. The microscale coherence in correlation with the SNR and its resolution for microscopy were analyzed. We demonstrate how low-light-level, non-laser sources enable holographic imaging with a video frame rate (25 frames/s), an intermediate SNR of 8 dB, and a volume of coherence of 3.4×10(4) µm3. Holograms of objects with a lateral resolution of 1 µm were achieved using a microscope lens (50×/NA=0.7) and a CCD camera featuring a 4-12 bit dynamic range.

Generalized phase evaluation for stereophotogrammetric correspondence assignment.

Phase-shifting fringe projection is the primary structured illumination method for high-accuracy, three-dimensional (3D) shape measurements in the fields of profilometry and stereophotogrammetry. Many different schemes for the phase evaluation and the phase-shifted fringe pattern design exist. Here we focus on the role of the phase evaluation in the context of stereophotogrammetry, where the nominal phase value itself is merely used as an image feature that can be exploited to establish a correspondence between the two camera views. Starting from the classical phase evaluation function, we will discuss its essential properties for a highly accurate correspondence mapping. Based on the findings, we generalize the classical function to derive a generalized phase value for a sequence of stereo images. An experimental comparison between a correspondence assignment using the classical phase evaluation function and a specifically chosen general phase evaluation function is given.

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.

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.

High-speed three-dimensional shape measurements of objects with laser speckles and acousto-optical deflection.

Many three-dimensional (3D) shape measurement techniques in stereophotogrammetry with temporal coded structured illumination are limited to static scenes because the time for measurement is too long in comparison to the object speed. The measurement of moving objects result in erroneous reconstructions. This is apparent to reduce measurement time to overcome this limitation, which is often done by increasing the projection rate for illumination while shrinking the amount of images taken for reconstruction. The projection rate limits most applications in its speed because digital light processing (DLP) projectors, which are widely used, bring a limited projection rate along. Our approach, in contrast, does not take a DLP. Instead we use laser speckles as projected patterns which are switched using an acousto-optical deflector. The projection rate is 10× higher than what the fastest stripe projection systems to our knowledge achieve. Hence, we present this uncommon but potential approach for highspeed (≈250 3Dfps= [3D measurements per second]), dense, and accurate 3D measurements of spatially separated objects and show the media that emphasizes the ability of accurate measurements while the objects under testing move.

High-speed pattern projection for three-dimensional shape measurement using laser speckles.

We propose a high-speed projection system that is able to project statistical speckle patterns at a rate of 500Hz. Its purpose is to generate structured light for a real-time photogrammetry stereo vision setup. As conventional digital light projector (DLP) projection setups are limited in their maximum projection rate to 250Hz for gray-value patterns, stripe projection systems are usually applied for real-time three-dimensional (3D) measurements. However, these techniques can only be used on steady surfaces as phase unwrapping has to be done. In contrast, the proposed setup is able to measure the shape of multiple spatially separated objects at once. We compare the speckle setup with a system using a DLP projector and with other fast 3D shape measurement setups, like the widely used stripe projection methods, qualitatively and quantitatively.

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.

Self-focusing without external electric field in BaTiO(3).

We report on the first observation, to our knowledge, of self-focusing without an external electric field in barium titanate crystals under cw laser beam irradiance. This effect we observed at an intensity of 0.2W/cm(2) on the 633nm wavelength regime in the case of ordinarily as well as extraordinarily polarized light.

Human face measurement by projecting bandlimited random patterns.

This article presents a fast and accurate method to measure human faces for medical applications. To encode an object point, several random patterns are projected. A correlation technique, which takes only the area of one pixel into account, is used to locate the homologous points. It could be shown that band limited random patterns are helpful for noise reduction. The comparison of the point cloud of a measured plane with an ideal one showed a standard deviation less then 50 mum. Furthermore a depth difference of 20 mum is detectable.

Image aberrations in optical three-dimensional measurement systems with fringe projection.

In optical shape measurement systems, systematic errors appear as a result of imaging aberrations of the lens assemblies in the cameras and projectors. A mathematical description of this effect is intended to correct the whole measurement area with a few independent coefficients. We apply the ideas of photogrammetry to one- and two-dimensional fringe projection techniques. We also introduce some new terms for close-range applications and telecentric objectives. Further, an algorithm for distance-dependent corrections is introduced. Also, we describe a new method with which to determine coefficients of aberration with an optimization-based method.

Stable self-focused beams in a photorefractive Bi12TiO20 crystal.

We present experimental results on the self-focusing of light beams in a photorefractive, optically active Bi12TiO20 crystal. Using circular polarization of the input HeNe laser beam we succeeded to create a stable self-focused, asymmetric light-beam over several hours. Depending on external parameters, like the external electric field and the intensity ratio, a transient symmetric state is observed.

Formation of micro-optical structures by self-writing processes in photosensitive polymers.

A local exposure of UV-sensitive polymers leads to a local curing. This corresponds to a saturable and irreversible nonlinear change of the refractive index, which evidently leads to a filamentation of the hardening polymer. This paper investigates the physical background of these effects and analyzes how the different influencing factors could be used for a steered, partly self-written formation of micro-optical structures. The structure formation is simulated on the basis of an iterative beam propagation method with consideration of a set of process parameters, e.g., the photoinitiator concentration or the exposure intensity. It is shown theoretically as well as experimentally that a variation of material- and exposure-specific process parameters gives opportunities for a controlled structure formation. The experimental realization of a configuration by use of a beam shaper within a UV contact exposure process is presented by means of the preparation of high-aspect-ratio conic structures.