Lingqin Kong

Non-contact detection of oxygen saturation based on visible light imaging device using ambient light

A method that remotely measures blood oxygen saturation through two cameras under regular lighting is proposed and experimentally demonstrated. Two narrow-band filters with their visible wavelength of 660nm and 520nm are mounted to two cameras respectively, which are then used to capture two photoplethysmographic (PPG) from the subject simultaneously. The data gathered from this system, including both blood oxygen saturation and heart rate, is compared to the output of a traditional figure blood volume pulse (BVP) senor that was employed on the subject at the same time. Result of the comparison showed that the data from the new, non-contact system is consistent and comparable with the BVP senor. Compared to other camera-based measuring method, which requires additional close-up lighting, this new method is achievable under regular lighting condition, therefore more stable and easier to implement. This is the first demonstration of an accurate video-based method for non-contact oxygen saturation measurements by using ambient light with their respective visible wavelength of 660nm and 520nm which is free from interference of the light in other bands.

A novel optical readout infrared FPA imaging system with fiber reference channel

A novel fiber reference optical readout method was proposed in the bi-material micro cantilever infrared imaging system, which consists of an infrared imaging channel, an optical readout channel and a fiber reference channel. The fiber reference channel is used to monitor the intensity fluctuation of the light source, and provide a signal to correct the distortion of the infrared images from the optical readout channel. Comparing with the typical optical readout method without any references, the noise equivalent temperature difference (NETD) of such an infrared imaging system with the fiber reference optical readout method can be reduced by about 33% and edges of the IR images become clearer.

Imaging and image restoration of an on-axis three-mirror Cassegrain system with wavefront coding technology

Wavefront coding (WFC) technology is adopted in the space optical system to resolve the problem of defocus caused by temperature difference or vibration of satellite motion. According to the theory of WFC, we calculate and optimize the phase mask parameter of the cubic phase mask plate, which is used in an on-axis three-mirror Cassegrain (TMC) telescope system. The simulation analysis and the experimental results indicate that the defocused modulation transfer function curves and the corresponding blurred images have a perfect consistency in the range of 10 times the depth of focus (DOF) of the original TMC system. After digital image processing by a Wiener filter, the spatial resolution of the restored images is up to 57.14 line pairs/mm. The results demonstrate that the WFC technology in the TMC system has superior performance in extending the DOF and less sensitivity to defocus, which has great value in resolving the problem of defocus in the space optical system.

Angles-centroids fitting calibration and the centroid algorithm applied to reverse Hartmann test

In this paper, we develop an angles-centroids fitting (ACF) system and the centroid algorithm to calibrate the reverse Hartmann test (RHT) with sufficient precision. The essence of ACF calibration is to establish the relationship between ray angles and detector coordinates. Centroids computation is used to find correspondences between the rays of datum marks and detector pixels. Here, the point spread function of RHT is classified as circle of confusion (CoC), and the fitting of a CoC spot with 2D Gaussian profile to identify the centroid forms the basis of the centroid algorithm. Theoretical and experimental results of centroids computation demonstrate that the Gaussian fitting method has a less centroid shift or the shift grows at a slower pace when the quality of the image is reduced. In ACF tests, the optical instrumental alignments reach an overall accuracy of 0.1 pixel with the application of laser spot centroids tracking program. Locating the crystal at different positions, the feasibility and accuracy of ACF calibration are further validated to 10-6-10-4 rad root-mean-square error of the calibrations differences.

Double triangular prism filter based on the optical-readout method in a microelectromechanical infrared imaging system

This paper presents a novel filtering method with a double triangular prism in an optical-readout thermal imaging system. First, the working principle of this system is described in detail, followed by the analysis of sensitivity. Then, infrared images of hands are obtained. On the basis of the analysis, it is concluded that this filtering method, whose noise equivalent temperature difference (NETD) can reach 145 mK, is effective in obtaining high-quality images. Finally, comparing the filtering method with a knife-edge filter, we can draw the conclusion that the filtering method can effectively improve image quality (the value of NETD is less than that of a knife-edge filter).

Image quality improvement by the structured light illumination method in an optical readout cantilever array infrared imaging system

The structured light illumination method is applied in an optical readout uncooled infrared imaging system to improve the IR image quality. The unavoidable nonuniform distribution of the initial bending angles of the bimaterial cantilever pixels in the focal plane array (FPA) can be well compensated by this method. An ordinary projector is used to generate structured lights of different intensity distribution. The projected light is divided into patches of rectangular regions, and the brightness of each region can be set automatically according to the deflection angles of the FPA and the light intensity focused on the imaging plane. By this method, the FPA image on the CCD plane can be much more uniform and the image quality of the IR target improved significantly. A comparative experiment is designed to verify the effectiveness. The theoretical analysis and experimental results show that the proposed structured light illumination method outperforms the conventional one, especially when it is difficult to perfect the FPA fabrication.

Automatic segmentation of human depth map based on semantic segmentation of FCN and depth segmentation

Traditional 3D information acquisition of human body relies on either foreground extraction or threshold segmentation in a plain background. It is difficult to be applied directly in complex background. In this paper, a novel method is proposed on the basis of binocular vision, which combines the semantic segmentation of FCN with the depth segmentation to get the human body depth map. The depth map is obtained by binocular camera, and each point in the depth map corresponds to the point in the left camera image. The position of the human body is gained through semantic segmentation of the left camera image, then automatic depth segmentation can be conducted based on the depth of human body in the depth map. The final result is obtained by taking the intersection of the depth map segmentation result and the left camera image segmentation result. The results show that the segmentation precision is much higher than that of purely semantic segmentation of FCN, the segmentation accuracy has increased about 2%.

Differential computation method used to calibrate the angle-centroid relationship in coaxial reverse Hartmann test

A differential computation method is presented to improve the precision of calibration for coaxial reverse Hartmann test (RHT). In the calibration, the accuracy of the distance measurement greatly influences the surface shape test, as demonstrated in the mathematical analyses. However, high-precision absolute distance measurement is difficult in the calibration. Thus, a differential computation method that only requires the relative distance was developed. In the proposed method, a liquid crystal display screen successively displayed two regular dot matrix patterns with different dot spacing. In a special case, images on the detector exhibited similar centroid distributions during the reflector translation. Thus, the critical value of the relative displacement distance and the centroid distributions of the dots on the detector were utilized to establish the relationship between the rays at certain angles and the detector coordinates. Experiments revealed the approximately linear behavior of the centroid variation with the relative displacement distance. With the differential computation method, we increased the precision of traditional calibration 10-5 rad root mean square. The precision of the RHT was increased by approximately 100 nm.

Design of TMC wavefront coding system based on user defined surface mask plate

In wavefront coding optical system, with the traditional cubic mask plate (CMP) which owns several extension times of depth of focus (DOF), it is difficult to manufacture. With the symmetrical surface type mask plate which can be machine relatively easily, but it presents small multiples of the extension of DOF. In this paper, it makes a presentation of mask plate design by user defined surface(UDS) type where it has an easy mechanical process and several multiples of extension of DOF. It presents the analytic expression by calculation and its DLL data file which is adapted to ZEMAX. It also performs a simulation experiment based on the three mirror Cassegrain(TMC) wavefront system. The experiment results indicate that instead of the traditional mask plate, the UDS surface mask plate can obtain a larger ratio of extension of DOF and increase the valuable types of mask plate surfaces. What is more, it decreases the surface mechanical difficulty compared to the asymmetrical surface mask plate. The type of UDS surface has the unique design and the convenience manufacturing, which is of great value in both application and research.

Brightness checkerboard lattice method for the calibration of the coaxial reverse Hartmann test

The coaxial reverse Hartmann test (RHT) is widely used in the measurement of large aspheric surfaces as an auxiliary method for interference measurement, because of its large dynamic range, highly flexible test with low frequency of surface errors, and low cost. And the accuracy of the coaxial RHT depends on the calibration. However, the calibration process remains inefficient, and the signal-to-noise ratio limits the accuracy of the calibration. In this paper, brightness checkerboard lattices were used to replace the traditional dot matrix. The brightness checkerboard method can reduce the number of dot matrix projections in the calibration process, thus improving efficiency. An LCD screen displayed a brightness checkerboard lattice, in which the brighter checkerboard and the darker checkerboard alternately arranged. Based on the image on the detector, the relationship between the rays at certain angles and the photosensitive positions of the detector coordinates can be obtained. And a differential de-noising method can effectively reduce the impact of noise on the measurement results. Simulation and experimentation proved the feasibility of the method. Theoretical analysis and experimental results show that the efficiency of the brightness checkerboard lattices is about four times that of the traditional dot matrix, and the signal-to-noise ratio of the calibration is significantly improved.