Common problems faced in optical comprehensive design experiment and going against the Washington Accord are pointed out. For resolving these problems, an instructional and innovative teaching scheme for Optics Comprehensive Design Experiment is proposed. We would like to understand the student that can improve the hands-on practical ability, theory knowledge understanding ability, complex problem solving ability, engineering application ability, cooperative ability after tracking and researching the student who have attended the class about Optical Comprehensive Design Experiment, We found that there are some problems on the course such as the experiment content vague, the student beginning less time, phase separation theory and engineering application, the experiment content lack of selectivity and so on. So we have made some improvements reference to the Washington Accord for the class teaching plan about Optical Comprehensive Design Experiment. This class must relevant to the engineering basic courses, professional foundation course and the major courses, so far as to the future study and work that which can play a role in inheriting and continuity to the students. The Optical Comprehensive Design Experiment teaching program requires students learning this course to have learnt basic courses like analog electronics technique, digital electronic technique, applied optics and computer and other related courses which students are required to comprehensively utilize. This teaching scheme contains six practical complex engineering problems which are respectively optical system design, light energy meter design, illuminometer design, material refractive index measuring system design, light intensity measuring system design and open design. Establishing the optional experiment and open experiment can provide students with a greater choice and enhance the students' creativity, vivid teaching experimental teachers and enriching contents of experiment can make the experiment more interesting, providing students with more opportunities to conduct experiment and improving students' practical ability with long learning time, putting emphasis on student's understanding of complex engineering problems and the cognitive of the process to solve complex engineering problems with actual engineering problems. Applying the scheme in other courses and improving accordingly will be able to ensure the quality of engineering education. Look forward to offering useful reference for the curriculum system construction in colleges and universities.
Camera calibration plays an important role in the field of machine vision and photogrammetry, and among the practical calibration methods, the one proposed by Zhang ZhengYou is higher accuracy and easily operated. However, this method needs to move the camera (or the planar target) to get three or more target images at different locations, and it is better to uniformly fill the calibration target in whole measurement volume to improve the calibrate precision. But manual movement and placement of the targets frequently will increase the difficulty in guaranteeing the uniform distribution of target. In view of this situation and according to the linear imaging model of the camera, a new camera calibration method based on the virtual planar targets is proposed in this paper. A liquid crystal display was used as a target plane, and the 2D target graphics were displayed on this LCD screen. Using TSai’s camera calibration method to get initial parameters, a serial of images of the virtual planar targets in different positions were captured with keeping the display position unchanged and are used to calculate the internal and external parameters of the camera by classic Zhang’s camera calibration method, and the new internal and external parameters would again guide the movement of virtual target. After several iterations, camera parameters can be obtained with high precision. The presented method is flexible and easy to operate, and it has been applied to calibrate different cameras and an actual 3D shape measurement system in our Lab. The comparison results of the transverse coordinates in plane calculated by this method and by Zhang’s camera calibration method shows that this proposed method is quite accurate and reliable.
Among temporal phase unwrapping methods based on structured light projection, tri-frequency heterodyne method, with the merits of less projected fringe, high precision and high reliability, has become a practical method in objects three-dimensional (3D) shape measurement. In this paper, a 3D shape measuring system was developed with a digital micromirror device (DMD) and synchronously trigged CCD camera. The 3D shape of a measured object was reconstructed from the deformed fringe patterns based on tri-frequency heterodyne method. The practical experiments were carried on some coins, and the results show that the system can restore their 3D shape on the tested partition with an accuracy of microns. This measurement system is prominent in 3D shape measurement of small or tiny objects, sample testing, and many other application fields.
Projector nonlinearity is a common problem for digital structured light-based three-dimensional (3-D) shape measurement techniques. A temporal-spatial binary encoding method is presented for the purpose of eluding it. We build a 3-D shape measurement scheme by combining our proposed method with phase measurement profiling. A standard sinusoidal fringe pattern is divided into more than two binary fringe patterns using specially designed temporal and spatial binary encoding rule based on intensity hierarchic quantification, and then are in-focus projected onto the measured object at a time sequence to reconstruct a frame phase-shifting fringe image. On account of the projected binary fringe pattern strictly consisting of zeros and ones, the influence of the projector nonlinearity on the measurement result can be effectively ruled out and simultaneously enables high-quality sinusoidality. In-depth investigations by theoretical analysis and experiments are conducted to demonstrate the performance of this method.
In principle, PMD needs the two components of the local surface gradient. Therefore a sequence of two orthogonal sinusoidal fringe patterns have to be displayed and captured separately. It is easy and convenient by using a digital display, but it will be much difficult to build a PMD system with mechanic gratings. In this paper, we present a novel phase-shift technique by using the cross fringe pattern, in which a one-dimensional N-phase shift allows for the acquisition of the two orthogonal phases, with only N exposures instead of 2N exposures. Therefore, it make PMD possible be implemented by a one-dimensional translation of the fringe pattern, instead of the common two-dimensional translation, which will be quite useful for certain applications.
Surface three-dimensional (3-D) shape information is needed for many fast processes such as structural testing of material, standing waves on loudspeaker cone, etc. Usually measurement is done from limited number of points using electrical sensors or laser distance meters. Fourier Transform Profilometry (FTP) enables fast shape measurement of the whole surface. Method is based on angled sinusoidal fringe pattern projection and image capturing. FTP requires only one image of the deformed fringe pattern to restore the 3-D shape of the measured object, which makes real-time or dynamic data processing possible. In our experiment the method was used for loudspeaker cone distortion measurement in dynamic conditions. For sound quality issues it is important that the whole cone moves in same phase and there are no partial waves. Our imaging resolution was 1280x1024 pixels and frame rate was 200 fps. Using our setup we found unwanted spatial waves in our sample cone.
A compact LED illumination based shape measurement system of glossy surfaces is presented. The system is
based on Phase Measuring Deflectometry (PMD). In this system the sinusoidal fringe pattern is formed using
photographic 35 mm film frame which is illuminated from behind using LED and diffuser. Beam splitter is used
to combine x- and y-direction fringe patterns, which are needed by PMD. Phase shifting is generated manually
using translational stages and micrometer actuators. Compared to digital fringes displayed on the screen, our
LED based setup can provide higher power, completely diffuse light and produce smoother sinusoidal fringes
with continuous intensity distribution. LED usage enables also pulsed illumination to freeze sample movement.
The resolution of the method is submicron level. Due to the compact size this setup is promising in the small
scale measurement field.
A method based on basic phase measuring deflectometry is proposed for testing the aspherical mirror. The method uses a reference screen in two different distances from the mirror under test. The sinusoidal, intensity-modulated patterns generated by the computer are displayed on the LCD screen, and the camera observes the patterns reflected off the testing mirror. The observed pattern appears distorted depending on the shape of the mirror. Using the phase-shifting technique, the original ray of every image point and its corresponding deflected ray can be constructed. Their intersection points and the surface normal are obtained. Then the mirror surface is reconstructed with high accuracy by numerically integrating the surface normals. The proposed method is robust against noise and can test the mirror full field. In this work, the method is introduced, and computer simulation and experimental results are shown.
Phase Measuring Deflectometry(PMD), which is aimed at testing specular free-form surfaces, has been developed in
recent years. Normally, two sets of sinusoidal fringe patterns are needed, i.e. horizontal and vertical fringe patterns. So it
will be time-consuming in PMD system while pursing high accuracy. Here we propose a Bi-color PMD system that will
produce one frame fringe consisting of two interlaced RGB format base color fringe patterns, i.e. vertical pattern and
horizontal pattern. The fringe patterns could be captured by color camera and the absolute phase will be got by
phase-shift technique. It could lead to a faster measuring process and make PMD technique more useful.
In this paper, a new wavefront measurement is proposed, which is based on active deflectormetry and phase-shift technique. The deflections of imaging rays caused by a phase object could be measured accurately with the phase-shift technique and a removable TFT flat panel to display both the horizontal and vertical sinusoidal intensity patterns respectively, and then the wavefront distribution could be calculated. When a phase object is placed between a display and a calibrated CCD camera, the intensity patterns will be distorted. The distortion can be measured, and another different distortion can be got by moving the display. Then the ray deflections can be measured as well as the gradients of phase shift caused by the object. Therefore the wavefront can be reconstructed. Experimental results show the feasibility of this method. Compared with other techniques, this technique is simpler, cheaper and more flexible.
The active stressed lap is the heart of polishing process. A novel non-contact optical method of dynamic deformation measurement and analysis of an active stressed lap is put forward. This method, based on structured illumination, is able to record full-field information of the bending and rotating stressed lap dynamically and continuously, while its profile is changed under computer control, and restore the whole process of lap deformation varied with time at different position and rotating angle. It has been verified by experiments that this proposed method will be helpful to the opticians to ensure the stressed lap as expected.
A Multi-Point velocity interferometer system for any reflector (VISAR) has been developed which can simultaneously measure velocity versus time histories of two to eight points by double delayed legs or two to sixteen points by single velocity sensitivity on a target or different objects during dynamic compression. A single-frequency laser beam is divided into two to eight or sixteen individual beams that are transmitted into an experimental device by incident fibers to illuminate measured points. Diffusely reflected laser beams from different measured points are separately collected by fiber detectors and guided by signal fibers into a common 'push and pull' interfering cavity with the same delay etalon to interfere. This not only simplifies the system structure and the experimental operation, but also eliminates the system error among measured points and makes the system almost as small in volume as that of a single-point VISAR. The Multi- Point VISAR possesses all the advantages of the traditional single-point VISAR as well as the temporal and the space resolution ability. We have used it to monitor velocity histories of several points on a target in a few explosion experiments, and good experimental results were obtained.