Catherine E. Towers

Time efficient color fringe projection system for 3D shape and color using optimum 3-frequency selection

We present a novel color fringe projection system to obtain absolute 3D shape and color of objects simultaneously. Optimum 3-frequency interferometry is used to produce time efficient analysis of the projected fringes by encoding three fringe sets of different pitch into the primary colors of a digital light projector and recording the information on a 3-chip color CCD camera. Phase shifting analysis is used to retrieve subwavelength phase information. Absolute phase across the field is calculated using the 3-frequency method independently at each pixel. Concurrent color data is also captured via the RGB channels of the CCD. Thus full-field absolute shape (XYZ) and color (RGB) can be obtained. In this paper we present the basis of the technique and preliminary results having addressed the issue of crosstalk between the color channels.

Simple calibration of a phase-based 3D imaging system based on uneven fringe projection

Phase-based fringe projection metrology systems have been widely used to obtain the shape of 3D objects. One vital step is calibration, which defines the relationship between the phase and depth data. Existing calibration methods are complicated because of the dependence of the relationship on the pixel position. In this Letter, a simple calibration procedure is introduced based on an uneven fringe projection technique, in which the relationship between phase and depth becomes independent of the pixel position and can be represented by a single polynomial function for all pixels. Therefore, given a set of discrete points with a known phase and depth in the measuring volume, the coefficient set of the polynomial function can be determined. A white plate having discrete markers with known separation is used to calibrate the 3D imaging system. Experimental results demonstrate that the proposed calibration method is simple to apply and can build up an accurate relationship between phase and depth data.

Method of excess fractions with application to absolute distance metrology: theoretical analysis

The method of excess fractions (EF) is well established to resolve the fringe order ambiguity generated in interferometric detection. Despite this background, multiwavelength interferometric absolute long distance measurements have only been reported with varying degrees of success. In this paper we present a theoretical model that can predict the unambiguous measurement range in EF based on the selected measurement wavelengths and phase noise. It is shown that beat wavelength solutions are a subset of this theoretical model. The performance of EF, for a given phase noise, is shown to be equivalent to beat techniques but offers many alternative sets of measurement wavelengths and therefore EF offer significantly greater flexibility in experimental design.

Snapshot color fringe projection for absolute three-dimensional metrology of video sequences

We present a method to obtain simultaneous three-dimensional shape and color information from a single captured image using a composite red, green, and blue (RGB) projected fringe pattern. Previous attempts at single snapshot shape and color metrology have suffered from either poor dynamic range or have been highly dependent on the color of the artifact. An optimum multiwavelength process has been employed to maximize the dynamic range of the shape data and give absolute depth information independently at each pixel. Fringe processing is via a Fourier transform process with algorithms introduced to obtain RGB color texture. Simulated and experimental data demonstrate the algorithm’s robustness in the vicinity of surface discontinuities. Experimental results from a human hand show the applicability to dynamic scenes.

Multiwavelength interferometry: extended range metrology.

We present an optimized method for multiwavelength interferometry that allows measurements beyond the largest beat wavelength. The approach exploits wavelength coincidence between two beat wavelengths in order to measure unambiguously over an extended range. Performance of the approach has been validated both through simulations and experimentally by means of a fiber interferometer for four measurement wavelengths. Initial results have demonstrated 1/200th of a fringe phase resolution, giving absolute metrology over 18.16 mm, or a dynamic range of 1 part in 2.4x10(6). With improved phase resolution the method has the potential to range over >100 m using femtosecond laser frequency comb sources.

Three-dimensional particle imaging by defocusing method with an annular aperture

We propose an annular-aperture-based defocusing technique for three-dimensional (3D) particle metrology from a single camera view. This simple configuration has high optical efficiency and the ability to deal with overlapped defocused images. Initial results show that an uncertainty in depth of 23 microm can be achieved over a range of 10 mm for macroscopic systems. This method can also be applied in microscopy for the measurement of fluorescently doped microparticles, thus providing a promising solution for 3D flow metrology at both macroscales and microscales.

Infrared video tracking of Anopheles gambiae at insecticide-treated bed nets reveals rapid decisive impact after brief localised net contact.

Long-lasting insecticidal bed nets (LLINs) protect humans from malaria transmission and are fundamental to malaria control worldwide, but little is known of how mosquitoes interact with nets. Elucidating LLIN mode of action is essential to maintain or improve efficacy, an urgent need as emerging insecticide resistance threatens their future. Tracking multiple free-flying Anopheles gambiae responding to human-occupied bed nets in a novel large-scale system, we characterised key behaviours and events. Four behavioural modes with different levels of net contact were defined: swooping, visiting, bouncing and resting. Approximately 75% of all activity occurred at the bed net roof where multiple brief contacts were focussed above the occupant’s torso. Total flight and net contact times were lower at LLINs than untreated nets but the essential character of the response was unaltered. LLINs did not repel mosquitoes but impacted rapidly: LLIN contact of less than 1 minute per mosquito during the first ten minutes reduced subsequent activity; after thirty minutes, activity at LLINs was negligible. Velocity measurements showed that mosquitoes detected nets, including unbaited untreated nets, prior to contact. This is the most complete characterisation of mosquito-LLIN interactions to date, and reveals many aspects of LLIN mode of action, important for developing the next generation of LLINs.

Method of excess fractions with application to absolute distance metrology: analytical solution

Multiwavelength interferometry provides a solution to a number of applications in metrology for the measurement of optical path differences longer than the source wavelength. To this day, the method of excess fractions (EF) has proved to provide very long, unambiguous measurement ranges with the highest reliability for a given set of wavelengths and level of phase noise. This is achieved because EF combines the individual phase values in an equivalent least-square problem and evaluates the correspondence for all possible solutions. However, this procedure can be slow for a number of applications. In this paper, an analytical solution for EF is presented that allows the direct calculation of the unknown integer fringe order. It is shown that this solution is consistent with the other phase unwrapping approaches as beat wavelength or Chinese remainder theorem-based solutions, but moreover, it can be understood as a unified representation and solution of the fringe order problem.

Three-dimensional particle imaging by wavefront sensing

We present two methods for three-dimensional particle metrology from a single two-dimensional view. The techniques are based on wavefront sensing where the three-dimensional location of a particle is encoded into a single image plane. The first technique is based on multiplanar imaging, and the second produces three-dimensional location information via anamorphic distortion of the recorded images. Preliminary results show that an uncertainty of 8 microm in depth can be obtained for low-particle density over a thin plane, and an uncertainty of 30 microm for higher particle density over a 10 mm deep volume.

Robust color and shape measurement of full color artifacts by RGB fringe projection

A novel method is presented to simultaneously measure shape and color information of artifacts containing color features. The technique operates by projecting composite red, green, and blue fringe patterns onto the surface of colorful objects. Theoretical analysis proves that there is no crosstalk between color channels during phase calculation by phase-shifting algorithm when three fringe patterns with the same fringe number are coded into red, green, and blue channels to form a composite RGB fringe pattern image. The color channel giving the maximum modulation depth at each pixel is used to measure shape information. Since three color channels are used, color information of the object surface can be extracted with high dynamic range from the same fringe pattern images. Using the recently developed color fringe projection system, composite RGB fringe patterns are projected onto colorful objects to test the proposed method. The experimental results show that the range of colors that can be measured and that shape and color information of colorful objects can be reliably obtained.