Wenping Guo

A local curve-fitting method for the complex refractive index measurement of turbid media

An improved curve-fitting method for a simultaneous measurement of the real and imaginary parts of the complex refractive index of turbid media is demonstrated. Based on an introduction of angle-dependent light penetration into turbid media by Calhoun et al (2010 Opt. Lett. 35 1224), the fitting is performed on the rapid change part of the measured curve around the critical angle. Moreover, a technology to locate this part of the curve by differentiation is introduced. Experimental results on Intralipid solutions show that this method can significantly reduce the computation time as well as the fitting error between the measured curve and theoretical one.

A two-reflection divergent differentiating critical angle refractometer

A new critical angle refractometer (CAR) for high accuracy refractive index measurement of liquid has been developed. The instrument improves the accuracy by two reflections in an elongated parallelogram prism, and acquires the angular reflectivity without any angle scanning parts through introduction of a point source with a divergent beam and a charge coupled device. In addition, it employs a simple and robust measurement method that gets the critical angle by differentiating the angular reflectivity. Through investigating absorbing media with absorption index κ (the imaginary part of refractive index) from 0 to 10(-2.1), the theoretical calculation shows that the proposed two-reflection CAR would outperform the traditional one-reflection CAR on lowering the principal error from the differentiation method and improving the ability of getting the critical angle. By testing two typical liquids-salt-water solution and milk, the preliminary experiment indicates that this two-reflection divergent differentiating critical angle refractometer is feasible and of high accuracy.

Expanding the measurement range of a critical-angle refractometer through Fourier analysis

We present a practical algorithm based on the Fourier power spectrum for expanding the measuring range of a critical-angle refractometer (CAR). Refractive index (RI) information is extracted from the gradual change in the relative reflective ratio. The Abbe-type refractometer has a limited measurement range based on Snells reflective law. When the RI of a liquid sample is beyond the upper limit of the Abbe-type refractometer, it can be obtained by using the Fourier power spectrum of reflectivity. The maximal value of the Fourier power spectrum of the reflective ratio is associated with the RI of the liquid. Supported by theoretical analysis and experimental results using sugar solutions with RI that varied from 1.359 to 1.3766 tested through the reflective CAR, the algorithm is found to be computationally effective and robust to expand the measuring range of the Abbe-type refractometer.

Real part of refractive index measurement approach for absorbing liquid.

An algorithm based on use of a reflected refractometer to measure the real part of the refractive index (RI) for an absorbing liquid is presented. The absorption of liquid will blur the division between bright and dark regions on a Fresnel reflective curve. However, the reflective ratio at some incident angles that are less than the critical angle have little sensitivity to absorbability. Unlike common methods that extract RI from reflectivity in critical angle vicinity, the presented method acquires the real RI from reflective ratio at a subcritical angle. Supported by the theoretical analysis and experimental results on a reflected refractometer, we have achieved accuracy better than 3×10(-4) RIU on ink samples with absorption coefficient around 300  cm(-1). Additional tests on Alizarin yellow GG solutions prove that the subcritical algorithm is feasible and of high accuracy.

Underwater linear object detection based on optical imaging

Nowadays, more and more underwater electricity or communication cables and oil or gas pipelines have been installing. Equipment aging and damages to them have caused series of accidents, resulting in huge economic loss and environmental pollution. This paper proposes a long distance underwater linear object detection method based on range-gated optical imaging, which can help the maintenance and inspections of underwater cables and pipelines. The whole object detection algorithm can be divided into three stages: image enhancement, edge detection and object detection. In the image enhancement step, The system deals with the low contrast, blur and noises characteristics of underwater images by means of contrast normalization, median filtering, wavelet transform, and finally gets high quality images. Then, the Canny operator was used to extract objects edge features. Finally, for the emergence of noise edges, a robust algorithm named Random Sample Consensus was chosen to accurately detect linear object and estimate its parameters such as position and direction. This algorithm has been tested on the experimental data in the boat tank of Huazhong University of Science and Technology, collected with a range-gated imaging system. The results show that the algorithm can effectively detect underwater curved-linear objects, with the detection rate achieving 96%, and the effective detection range can be up to 5 times the length of the underwater decay.

Perovskite solar cells employing Al 2 O 3 scaffold layers

Perovskite-type solar cell represents the pursuit of the ultimate goal of clean and renewable energy. Recently, hydroxyethyl methyl halide perovskite lead has been identified as a promising solar absorber. Recently, the perovskite-type solar cell performance improved rapidly achieve efficiencies up to 15%. Now we showed a new structure for perovskite cells based on a sublimated methylammonium lead iodide perovskite layer that is sandwiched between a very thin electron conductive layers and a Al2O3 scaffold layer processed on a substrate consist of polymer using spinning-coating techniques which lead a high power conversion efficiencies solar cell.

A simple accurate algorithm for the critical angle refractometer

We present a fast, simple, sub-pixel algorithm on the critical angle refractometer to measure the refractive index of the liquid sample by determining the centroid of the light intensity of the relative reflective curve. The centroid algorithm utilizes a divergent fiber-coupled royal blue LED source to irradiate on the dielectric surface between the prism and the media, which generates the light intensity distribution of the reflectance facula. Instead of the critical angle pixel as the differential algorithm and the threshold algorithm, the sub pixel centroid algorithm is based on calculating the centroid value of the light intensity of the relative reflective curve. In some moderate turbid solutions, the centroid algorithm is less sensitive to the scattering and absorption than the differential algorithm and the threshold algorithm. It is possible to utilize the centroid point of the relative reflective curve to determine the refractive index. Supported by the theoretical analysis and experimental results on saline solutions, we can conclude that the proposed algorithm is effective to get the super resolution and meaningful to the refractive index measurement of the liquid. The critical angle refractometer with this centroid method is potential to be a high-accuracy, high-resolution, and reliable automatic refractometer.

An experimental calibration method for digital Abbe refractometer

An improved calibration method for digital Abbe refractometer is proposed. Based on Fresnel reflection theory, digital Abbe refractometer measures the index of refraction by processing bright-dark pattern images. For extreme environment applications, our team has developed a digital Abbe refractometer. By analyzing bright-dark pattern images, it shows optical aberration may reduce positioning accuracy on critical angle. The main work of this paper is to propose a new calibrate method for digital Abbe refractometer. An optical system is built to simulate the refractometer. A motorized micropositioning stage is inserted to precisely control the position of bright-dark boundary. An area CCD captures an image each time boundary displaced. Get the boundary through entire measuring range to form an image database. The database indicates the corresponding relations between bright-dark pattern image acquired by CCD camera and boundary position read by motorized stage. When measuring the refractive index of liquid, match its bright-dark pattern image to images in database, and get the boundary position from the nearest match. Compared to the former method of computing boundary position from images with aberration, the proposed method calibrate refractometer by large amount of experimental data, thus improve stability of the measurement.

Method of using area CCD to obtain the reflectivity curve

Reflectivity curve is always used for refractive index measurement. With the help of a mechanical scanning part, conventional method alters the incident angle of a collimated beam to obtain the reflectivity curve, but it has troublesome drawbacks that mechanical scanning is time consuming and would introduce mechanical error. In this paper, a proposed method exploits the reflection characteristic of divergent beam at the critical angle. Its principle using area CCD to obtain the reflectivity curve is demonstrated by simulation. The result shows that the proposed method can instantaneously obtain the reflectivity curve without mechanical errors.