Chunyong Wang

Experimental measurement of the refractive index of biological tissues by total internal reflection

We discuss the refractive-index measurement of biological tissues by total internal reflection. The methodology of the measurement is illuminated comprehensively, and an experimental setup, combined with a data processing program, is developed correspondingly. Refractive indices of typical tissue samples are measured by use of the developed methodology. The agreement of our measurements with the reported results shows the validity of our scheme, which has the potential for being a simple, quick, and low-cost practical means for determining the refractive index of a turbid medium. Moreover, an empirical formula for evaluating the refractive index of Intralipid suspensions with different concentrations is also presented according to experimental measurements.

Monte Carlo simulation of Stokes vectors of polarized light scattering from two-dimensional random rough surfaces

A Monte Carlo model was established to simulate polarized scattering fields of two-dimensional rough surfaces based on the Kirchhoff approximation. Based on this model, numerical studies of the hemispherical distribution of Stokes vectors of scattered light from dielectric and metal rough surfaces were carried out. These surfaces have Gaussian distributions with correlation length of 3.1 µm and standard deviation varying between 0.1 and 0.6 µm. The results reveal that the V component of metal surfaces has peaks antisymmetric with the incident plane, whereas the V component of dielectric surfaces is almost zero. We consider that this property of the V component would provide a new method which could be used to distinguish the target material.

Influence of dead-time on detection efficiency and range performance of photon-counting laser radar that uses a Geiger-mode avalanche photodiode

Dead-time has a significant influence on the detection efficiency and range performance of a photon-counting laser radar system with a Geiger-mode avalanche photodiode. In this paper, a rapid universal recursive model of the detection probability of discrete time under various dead-times is proposed, which is verified with controlled parameters. Our model has the advantage of fast computing speed and unifies multi-trigger, single-trigger, and zero-dead-time models. The computing speed is 1 to 2 orders of magnitude faster than Gatts and Zhaos models under a short dead-time condition, with relative errors less than 0.001 and 10-14, respectively. Subsequently, the detection efficiency and range bias and precision with various dead-times are theoretically calculated and Monte Carlo simulated with different parameters. On the one hand, dead-time shorter than the end time of the target achieves better detection efficiency; however, this results in worse range performance. On the other hand, dead-time longer than the end time of the target maintains the detection efficiency at a low level but provides a better range performance. We discover that noise is the key reason for the periodic fluctuation of the detection efficiency and range performance versus different dead-times and the local optimum values of fluctuations occur when the dead-time is a few nanoseconds shorter or longer than 1, 1/2, 1/3, or even 1/4 of the end time of the target; further, this phenomenon becomes more evident when noise increases. Moreover, weaker noise level is crucial to the detection efficiency, and narrow pulse width and nearer target position in the range gate are important factors to improve precision.

A computational model for light transporting in biological tissues irradiated by converging laser beam

A semi-analytical method has been developed to calculate the spatially resolved diffuse light in layered biological tissues irradiated by converging laser beam based on the diffusion theory. Monte Carlo method is used to evaluate the correctness of the method, results show that they have good consistency and our method has higher computational efficiency. Numerical calculations disclosure same important features that are uniquely related to the propagation of the converging light in biological tissues. Those features are valuable to optimize the optical diagnosis and therapy.

The effect of smoke spatial distribution on pulse laser echo characteristics

A low false alarm rate in smoke has always been chasing after pulse laser ranging system. Smoke distributes particularly random in space so that different smoke distributions produce distinct interference echoes even if they are under same attenuation degree. Based on the theory of single scattering, in this paper the echo-element superposition model is established to study the relationship between smoke backscattering pulse and smoke thickness under the condition of same attenuation but different spatial distribution. The analysis shows that at a constant threshold, the pulse width of backscattered echoes in smoke are first broadened logarithmically by the thickness of the smoke and then compressed to zero with a negative exponential relationship, while a maximum width appears in the process; the peak of the backscattered echoes gradually decreases with the inverse of smoke thickness. In order to validate the model, the Monte Carlo method for photon tracing is used to simulate the model. The simulation results are in good agreement with the theoretical analysis. At the same time, an experimental device is set up in which pulse laser is transmitted in the smoke environment. Experimental results show that with the detection distance, one-way attenuation, transmitted pulse width of 25m, 60%, 20ns respectively, the pulse width of target echoes remains basically unchanged while smoke thickness varies, and the peak value and pulse width of the backscattering pulse are consisted with theoretical analysis within an error of 5%. This results can be used for laser ranging system in low-visibility environment for interference suppression and reducing false alarm rate, so as to improve system stability and anti-interference ability.

The effect of fog on the probability density distribution of the ranging data of imaging laser radar

This paper outlines theoretically investigations of the probability density distribution (PDD) of ranging data for the imaging laser radar (ILR) system operating at a wavelength of 905 nm under the fog condition. Based on the physical model of the reflected laser pulses from a standard Lambertian target, a theoretical approximate model of PDD of the ranging data is developed under different fog concentrations, which offer improved precision target ranging and imaging. An experimental test bed for the ILR system is developed and its performance is evaluated using a dedicated indoor atmospheric chamber under homogeneously controlled fog conditions. We show that the measured results are in good agreement with both the accurate and approximate models within a given margin of error of less than 1%.

Virtual experiment of optical spatial filtering in Matlab environment

The principle of spatial filtering experiment has been introduced, and the computer simulation platform with graphical user interface (GUI) has been made out in Matlab environment. Using it various filtering processes for different input image or different filtering purpose will be completed accurately, and filtering effect can be observed clearly with adjusting experimental parameters. The physical nature of the optical spatial filtering can be showed vividly, and so experimental teaching effect will be promoted.

Simulation on the particle flow in laser airborne particle counting sensor

Laser airborne particle counting sensor (LAPCS), based on light scattering of particle, is specially used in clean environment monitoring. LAPCS samples the air by a pump, and uses a laser illuminating the sampled air in the chamber, then counts the total number of scattering signal and its amplitude distribution, which can characterize the number of particles and size distribution. The structure of air-flow-path in LAPCS directly influences the flow of sampling air, the particle trajectories and velocity distribution in chamber that will influence the performance of LAPCS. In this paper, a finite element arithmetic based on Ansys Fluent14 software environment was developed to simulate the air flow and particle flow in LAPCS. Based on numerical calculations, velocity distribution of airflow and particle trajectories in chamber of LAPCS with different nozzles are presented intuitively. A few particles probably are disturbed outside the air-flow path and pass the photosensitive area many times, which can make the LAPCS iteration count. The results can provide a theoretical basis for optimizing design of the LAPCS.

The model of random signals generated by optical particle counter and the instrument improvement

In order to study and improve atmospheric and air pollution monitoring sensor, a new mathematical model of random signals is established based on measuring process of light scattering signals analyzed by laser particle counter which combines the high speed data acquisition card PCI-9812 and optical particles counting sensor. The measured random signals can be divided into stability constant part and random variation part. The performance of the instrument is improved by both this model and analytical methods. Statistical distributions of the amplitude of the standard particles with different diameters are studied by the original experiment and improved one. The resolving power of particle size could attain more than 90%. The results reveal statistical distributions match well with lognormal distribution with a natural number as an independent variable. The lognormal distribution plays an important role in describing the random fluctuation characteristics of random process in both theories and experiments. Furthermore, both normal and lognormal distribution fitting are applied in analyzing the experimental results and testified by chi-square distribution fit test and correlation coefficient for comparison.

Research on the effect of laser radar system parameters on the range image quality

The quality of the range image obtained by laser radar will directly influence the target profile reconstruction and the target identification, and the quality of the range image is mainly determined by the parameters of laser radar system. As a result, this paper concentrates on discussing the effect of laser radars parameters on the quality of range image. To evaluate the quality of range image quantitatively, two parameters, the mean squared error of range (R-MSE) and peak signal noise ratio of range (R-PSNR), are introduced and a simulation program is developed to study the influence of systems parameters on the range imaging quality. The results show that for a given pulse width, there has an optimal bandwidth and threshold level, that make the range image has better image quality. Above results have important significance to optimize the parameter design of laser radar.