The Reverse Hartmann test is developed rapidly, robustly, and accurately in measuring precision aspheric surface. The onaxis
design provides better control of the astigmatism in the test. We use an on-axis Hartmann test in reverse to measure
the aspheric optical mirrors. In the configuration, the LCD with a light pattern on the screen illuminates to the tested surface,
and a 2μm-thick pellicle beam splitter is employed to obtain the coaxial light model. An optical flat with 1/20λ surface
precision is used to calibrate the rays which pass through the external pinhole and image at the detector, and the data are
processed to obtain the direction vectors of arbitrary reflected rays. The surface gradients are determined by the spatial
equations of incident and reflected rays which have been calibrated. The shape of surface is finally reconstructed by
Zernike polynomial fitting. The experiments include measuring a 76.2mm off-axis parabolic mirror and a 76.2mm
spherical mirror. The experimental results show coaxial reverse Hartmann test system may allow for accurate
measurements with uncertainties in the micrometer range using cost-effective equipments.
In the research of optical synthetic aperture imaging system, phase congruency is the main problem and it is necessary to detect sub-aperture phase. The edge of the sub-aperture system is more complex than that in the traditional optical imaging system. And with the existence of steep slope for large-aperture optical component, interference fringe may be quite dense when interference imaging. Deep phase gradient may cause a loss of phase information. Therefore, it’s urgent to search for an efficient edge detection method. Wavelet analysis as a powerful tool is widely used in the fields of image processing. Based on its properties of multi-scale transform, edge region is detected with high precision in small scale. Longing with the increase of scale, noise is reduced in contrary. So it has a certain suppression effect on noise. Otherwise, adaptive threshold method which sets different thresholds in various regions can detect edge points from noise. Firstly, fringe pattern is obtained and cubic b-spline wavelet is adopted as the smoothing function. After the multi-scale wavelet decomposition of the whole image, we figure out the local modulus maxima in gradient directions. However, it also contains noise, and thus adaptive threshold method is used to select the modulus maxima. The point which greater than threshold value is boundary point. Finally, we use corrosion and expansion deal with the resulting image to get the consecutive boundary of image.
A method of two channel exercise electrocardiograms (ECG) signals denoising based on wavelet transform and independent component analysis is proposed in this paper. First of all, two channel exercise ECG signals are acquired. We decompose these two channel ECG signals into eight layers and add up the useful wavelet coefficients separately, getting two channel ECG signals with no baseline drift and other interference components. However, it still contains electrode movement noise, power frequency interference and other interferences. Secondly, we use these two channel ECG signals processed and one channel signal constructed manually to make further process with independent component analysis, getting the separated ECG signal. We can see the residual noises are removed effectively. Finally, comparative experiment is made with two same channel exercise ECG signals processed directly with independent component analysis and the method this paper proposed, which shows the indexes of signal to noise ratio (SNR) increases 21.916 and the root mean square error (MSE) decreases 2.522, proving the method this paper proposed has high reliability.
This paper proposes and experimentally demonstrates a new denoising and hole-filling algorithm through discrete points removal and bilinear interpolation based on the bi-material cantilever FPA infrared imaging system. In practice, because of the limitation of FPA manufacturing process and optical readout system, the quality of obtained images is always not satisfying. A lot of noise and holes appear in the images, which restrict the application of the infrared imaging system. After analyzing the causes of noise and holes, an algorithm is presented to improve the quality of infrared images. Firstly, the statistic characteristics such as probability histograms of images with noise are analyzed in great detail. Then, IR images are denoised by the method of discrete points removal. Second, the holes are filled by bilinear interpolation. In this step, the reference points are found through partial derivative method instead of using the edge points of the holes simply. It can detect the real points effectively and enable the holes much closer to the true values. Finally, the algorithm is applied to different infrared images successfully. Experimental results show that the IR images can be denoised effectively and the SNRs are improved substantially. Meanwhile, the filling ratios of target holes reach as high as 95% and the visual quality is achieved well. It proves that the algorithm has the advantages of high speed, great precision and easy implement. It is a highly efficient real-time image processing algorithm for bi-material micro-cantilever FPA infrared imaging system.
We describe the application of wavefront coding technique for infrared imaging system to control thermal defocus. For traditional infrared imaging system, athermalization is necessary to maintain imaging performance which may increase complexity and cost of the imaging system. Wavefront coding includes a phase mask at the pupil which can re-modulate wave front so as to produce an encoded image. After digital processing, the system is insensitive to defocus. In this paper, the combination of wavefront coding technique and infrared imaging system has been discussed. We report here the optic design of the wavefront coding IR system based on Zemax. The phase mask is designed to ensure that the modulation transfer function (MTF) is approximately invariant in the range of working temperature. Meanwhile, we designed three IR systems to put up contrast experiments. System one and two are designed to compare the influence before and after the insertion of phase mask. System three is designed to compare the imaging performance before and after reducing lens in wavefront coding IR system. The simulation results show that the infrared imaging system based on wavefront coding can control thermal defocus in a temperature varying from -60ºC to 60 ºC, at the same time the weight and cost of optical elements are reduced by approximately 40%.
This paper proposes a method to recover the pulse signal with the theory of lock-in amplifier and calculates the oxygen saturation. The pulse signal is obtained based on the method of Photoplethysmography (PPG). We use a LED as the light source and a photoelectric diode as the receiver to get a measured pulse wave. Because the pulse wave obtained by this method is easily disturbed by motion artifact, we use an electrocardiogram (ECG) signal to aid PPG measurement. Firstly, the ECG signal is processed by the Fast Fourier Transform (FFT) and get the heart rate. Secondly, with the value of heart rate, a typical noise free pulse waveform can be constructed. Finally, we use it as a reference input to get a recovered pulse wave by the theory of lock-in amplifier. Thus, the value of oxygen saturation can be calculated accurately through two recovered pulse waveforms of red (660nm) and infrared (940nm) light. Some volunteers were tested. The correlation coefficient between the experimental data and the data provided by a reference instrument is 0.98, proving that this method has high reliability and utility in motion.
Low order aberration was founded when focused Gaussian beam imaging at Kodak KAI -16000 image detector, which is integrated with lenslet array. Effect of focused Gaussian beam and numerical simulation calculation of the aberration were presented in this paper. First, we set up a model of optical imaging system based on previous experiment. Focused Gaussian beam passed through a pinhole and was received by Kodak KAI -16000 image detector whose microlenses of lenslet array were exactly focused on sensor surface. Then, we illustrated the characteristics of focused Gaussian beam and the effect of relative space position relations between waist of Gaussian beam and front spherical surface of microlenses to the aberration. Finally, we analyzed the main element of low order aberration and calculated the spherical aberration caused by lenslet array according to the results of above two steps. Our theoretical calculations shown that , the numerical simulation had a good agreement with the experimental result. Our research results proved that spherical aberration was the main element and made up about 93.44% of the 48 nm error, which was demonstrated in previous experiment. The spherical aberration is inversely proportional to the value of divergence distance between microlens and waist, and directly proportional to the value of the Gaussian beam waist radius.