Hyperspectral imaging (HI) technology allows us to obtain the spatial and spectral information of target objects, thus having important and various application prospects among remote sensing detection, military reconnaissance, and other fields. Recently, snapshot HI systems have been favored widely due to their advantages including the miniaturization and the lightweight, to maintain the mainstream of HI technology research. D. S. Jeon et. al. used a single diffractive optical element (DOE) instead of multiple complex elements to modulate the diffractive wavefront which greatly reduce the system volume and be significant application potential in portable equipment and small payloads1. However, the minimum line width of surface micro structure on the DOE is restricted by the current processing capability, which has a negative impact on the wavefront modulation. Consequently, the high-performance DOE design method considering for machinability is an urgent problem to be solved. We proposed a high-performance processable DOE design method which utilizes the physical model to constrain the minimum line width in the DOE surface. By using the end-to-end deep learning method, achieving processable design of DOE has almost no impact on its performance and imaging quality of hyperspectral imaging systems. Proposed model overcomes the processing problem of DOE from a design perspective without sacrificing their functionality. Comparing to the results without the constraint of the physical model, the proposed physical model has significant effects in constraining surfaces.
The phase measuring deflectometry is a powerful technique for the in-situ measurement of of complex optics. Its measurement accuracy is comparable with conventional interferometry, but with higher flexibility, stability and efficiency. The three main challenges in the deflectometric measurement, namely the position-angle uncertainty in calculating the pixel correspondences, height-slope ambiguity in specifying the normal vectors, and rank deficiency in surface reconstruction are analyzed. Some significant error factors and effective solutions are introduced. The measuring accuracy of complex surfaces can achieve a level of 100 nm RMS.
Deflectometry is a powerful measuring technique of complex optical surfaces. Usually a series of binary patterns or sinusoidal fringes are displayed on a screen, and correspondences are established between the screen and camera points according to their gray levels or phases. The image associated with a screen pixel is blurred due to the defocus and aberrations of the off-axis imaging system, and the calculated location of the correspondence point will in turn be biased. The space variant point spread function associated with the catadioptric system is analyzed based on the light field method, and the resulting blurring effect is then addressed using Wiener deconvolution algorithm. Henceforth the phase errors in the captured images can be compensated effectively. Experimental results are presented to demonstrate the feasibility and effectiveness of the proposed method.
The measurement of freeform optical surfaces is a challenging task in precision manufacturing. Those widely used instruments such as the point-scanning profiler and sub-aperture stitching interferometer are costly expensive and time consuming. The phase measuring deflectometry is a powerful measuring technique for complex optical surfaces. To image the measuring efficiency and reduce the number of the captured images, the modulating information can be reutilized in the second direction in the bi-directional phase shifting, so that totally only 6 images are needed. The tracing deviations caused by the form errors behave differently with those caused by the position errors. Then precise localization of the measured surface can be realized by error separation, so that detecting of feature points can be avoided. Experimental results demonstrate that the measurement error is below 150 nm.
In the phase measuring deflectometry, the phase error caused by the nonlinear intensity response, called the gamma distortion, can negatively affect the measurement quality of specular surfaces. Based on the generic exponential four-step phase-shifting fringe modal, this paper proposes a flexible and simple phase retrieval method to eliminate the phase errors without complex calibration or additional fringe patterns. The experimental results illustrate that the proposed method can accurately retrieve the phases from the distorted fringe patterns with the Gamma distortion, and the measurement precision can henceforth be improved.
Phase measuring deflectometry is a powerful in-situ measuring technique for complex specular surfaces. Its measuring accuracy depends on the quality of geometric calibration. An in-situ deflectometric measuring system is integrated into a single point diamond turning machine. An accurate self-calibration method is proposed to refine the positions of the camera and the screen. A world coordinate system is established by introducing a flat mirror without markers. The geometric positions are solved by minimizing the deviations of the traced screen pixels. The tracing deviations caused by the form errors behave differently with those caused by the position errors. Precise localization of the measured surface can be realized by error separation, so that detecting of feature points can be avoided. Experimental results demonstrate that the measurement error is below 300 nm.
The measurement of aspheric optics has attracted intensive attention in precision engineering, and efficient in-situ measurement technologies are required urgently. Phase measuring deflectometry is a powerful measuring method for complex specular surfaces. In this paper, an in-situ measurement method is developed based on the sub-aperture deflectometry. A complete measuring procedure is developed, including initial calibration, self-adaptive calibration, route planning, imaging acquisition, phase retrieval, gradient calculation, surface reconstruction and sub-aperture stitching. Several key points concerning the sub-aperture measurement are investigated, and effective solutions are proposed to balance the measuring accuracy and aperture, to overcome the height/slope ambiguity and to eliminate the stitching errors caused by point sampling and measuring errors. The measuring flexibility and stability can be greatly improved compared to the existing SCOTS measuring approach.
In recent years, the measurement of specular aspheric surface has attracted intensive attention in precision engineering. Phase measuring deflectometry is a powerful measuring technique, which could accurately measure specular surfaces. The software configurable optical test system and a four step phase shifting approach are applied to obtain the normal vectors of the measured surface. The geometric parameters are recalculated by optimization to improve the calibration accuracy. Then the surface is reconstructed using a optimization algorithm. The configuration parameters should be set according to specific surface shapes and measuring conditions. Numerical experiments demonstrate that good performance can be achieved using this method.
In order to obtain the scene information of a larger or 360 degrees field of view, it is necessary to accurately know the rotational angle of a camera around a fixed axis, which directly affects the quality of point cloud registration and integration. This paper presents a novel rotational angle calibration method by using an extra large viewing-angle camera and high precise checkerboard. The large viewing-angle camera is fixed and has the same imaging direction as the calibrated camera. The checkerboard is placed in front of the two cameras to determine their relative positions. The experimental results show that the error between the actual rotational angle and the calibration result is below 0.0959 degrees. The proposed calibration method can accurately and effectively obtain the rotational angle of a camera.
This paper presents a new Phase Measuring Deflectometry (PMD) method to measure specular object having discontinuous surfaces. A mathematical model is established to directly relate absolute phase and depth, instead of phase and gradient. Based on the model, a hardware measuring system has been set up, which consists of a beam splitter to change the optical path, and two LCD screens to display the same sinusoidal fringe patterns. By using model-based and machine vision method, system calibration is accomplished to provide the required parameters and conditions. The verification tests are given to evaluate the effectiveness of the developed system. The 3D shape of an artificial step having multiple specular surfaces and a concave mirror has been measured. Initial experimental results show that the proposed measurement method can obtain 3D shape of specular objects with discontinuous surface effectively.
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