An online fringe projection profilometry (OFPP) based on scale-invariant feature transform (SIFT) is proposed. Both rotary and linear models are discussed. First, the captured images are enhanced by “retinex” theory for better contrast and an improved reprojection technique is carried out to rectify pixel size while keeping the right aspect ratio. Then the SIFT algorithm with random sample consensus algorithm is used to match feature points between frames. In this process, quick response code is innovatively adopted as a feature pattern as well as object modulation. The characteristic parameters, which include rotation angle in rotary OFPP and rectilinear displacement in linear OFPP, are calculated by a vector-based solution. Moreover, a statistical filter is applied to obtain more accurate values. The equivalent aligned fringe patterns are then extracted from each frame. The equal step algorithm, advanced iterative algorithm, and principal component analysis are eligible for phase retrieval according to whether the object moving direction accords with the fringe direction or not. The three-dimensional profile of the moving object can finally be reconstructed. Numerical simulations and experimental results verified the validity and feasibility of the proposed method.
This paper discusses conventional synthetic-aperture method combined angular multiplexing in digital holography to increase the resolution and to enlarge the field of view at the same time. A structured illumination is used to realize angular multiplexing. A camera is moved by a motorized x-y stage, and scanning is performed at imaging plane. In this way we extend the band-pass for single hologram recording as well as obtain a greater sensor area resulting in a larger numerical aperture (NA). A larger NA enables a more detailed reconstruction combined with a smaller depth of field. Moreover, a phase map of the object is experimentally presented.
Wafer-level-optics now is widely used in smart phone camera, mobile video conferencing or in medical equipment that require tiny cameras. Extracting quantitative phase information has received increased interest in order to quantify the quality of manufactured wafer-level-optics, detect defective devices before packaging, and provide feedback for manufacturing process control, all at the wafer-level for high-throughput microfabrication. We demonstrate two phase imaging methods, digital holographic microscopy (DHM) and Transport-of-Intensity Equation (TIE) to measure the phase of the wafer-level lenses. DHM is a laser-based interferometric method based on interference of two wavefronts. It can perform a phase measurement in a single shot. While a minimum of two measurements of the spatial intensity of the optical wave in closely spaced planes perpendicular to the direction of propagation are needed to do the direct phase retrieval by solving a second-order differential equation, i.e., with a non-iterative deterministic algorithm from intensity measurements using the Transport-of-Intensity Equation (TIE). But TIE is a non-interferometric method, thus can be applied to partial-coherence light. We demonstrated the capability and disability for the two phase measurement methods for wafer-level optics inspection.
Transport-of-intensity equation (TIE) is an effective method for the quantitative phase analysis. Comparing with other quantitative phase imaging methods, TIE is dynamic, non-laser, and vibration insensitive. Based on the TIE technique, we designed and developed a system, including a phase quantitative imaging device and an application software, for real-time measurement of dynamic samples. In this paper, the structures of the system, especially the structure of the application software, are described. The performance and usage examples of the system are also demonstrated.
Digital holography microscopy (DHM) allows fast, nondestructive, high resolution and full-field 3D shape measurement of micro-objects. However, a drawback of many experimental arrangements of DHM is the requirement for a separate reference wave, which results in a measurement stability and interference fringe contrast decrease. In this paper, a common-path DHM (CDHM) is explored which only requires a single object illumination wave. Due to the fact that conventional phase unwrapping algorithms are not suitable for the complex and step surface of object, the dual wavelength linear regression phase unwrapping algorithm is introduced. By comparing two wrapped phase maps reconstructed at different wavelengths, the maps can be accurately unwrapped with straightforward and less timeconsuming. From the CDHM system and the phase unwrapping algorithm introduced, we experimentally obtained high quality depth profiles of micro-objects.
In the compact digital holoscope (CDH) measurement process, theoretically, we need to ensure the distances between the reference wave and object wave to the hologram plane exactly match. However, it is not easy to realize in practice due to the human factors. This can lead to a phase error in the reconstruction result. In this paper, the strict theoretical analysis of the wavefront interference is performed to demonstrate the mathematical model of the phase error and then a phase errors elimination method is proposed based on the advanced mathematical model, which has a more explicit physical meaning. Experiments are carried out to verify the performance of the presented method and the results indicate that it is effective and allows the operator can make operation more flexible.
Phase unwrapping is a process to reconstruct the absolute phase from a wrapped phase map whose range is (−π, π]. As the absolute phase cannot be directly extracted from the fringe pattern, phase unwrapping is therefore required by phasemeasure techniques. Currently, many phase unwrapping algorithms have been proposed. In this paper, four popular phase unwrapping algorithms, including the Goldstein’s branch cut method, the quality-guided method, the Phase Unwrapping via Max Flow (PUMA) method, and the phase estimation using adaptive regularization based on local smoothing method (PERALS), are reviewed and discussed. Detailed accuracy comparisons of these methods are provided as well.
A compact reflection digital holographic microscopy (DHM) system integrated with the light source and optical interferometer is developed for 3D topographic characterization and real-time dynamic inspection for Microelectromechanical systems (MEMS). Capability enhancement methods in lateral resolution, axial resolving range and large field of view for the compact DHM system are presented. To enhance the lateral resolution, the numerical aperture of a reflection DHM system is analyzed and optimum designed. To enhance the axial resolving range, dual wavelengths are used to extend the measuring range. To enable the large field of view, stitching of the measurement results is developed in the user-friendly software. Results from surfaces structures on silicon wafer, micro-optics on fused silica and dynamic inspection of MEMS structures demonstrate applications of this compact reflection digital holographic microscope for technical inspection in material science.
In this paper we present a new method to compensate for phase aberrations and image distortion with recording single digital hologram in digital holographic microscopy. In our method, tilt is removed from the abberrated phase map first. Then an area of interest (AOI) is generated by flood filled algorithm. By fitting AOI with discrete orthogonal Zernike polynomials, error phase map in the form of a series of Zernike polynomials is obtained. Final result can be calculated by subtracting the error phase map from the abberrated phase map. Through applying our method in microlens testing, phase aberrations and image distortion introduced by microscope objective are well suppressed.
The determination of numerical reconstruction distance is key to recover the wavefront at focal plane in digital holography. In this paper, we propose a new autofocus method based on angular spectrum method (ASM). The proposed method takes successive Fourier transform after 1<sup>st</sup> order spectrum selection, and then calculates the summation. It saves operations compared to classic autofocus functions. When an exhaustive z-axis search is performed, the proposed method obtains unimodality coincided with the results from four classic autofocus functions. Moreover, the proposed method is more time-effective, which is the optimal one for ASM.