| The light-matter interaction consists of two processes: absorption and scattering.Turbid materials have strong scattering capability in comparison to their absorbingcapability, which include many samples closely related to people’s life, such as blood,soft tissue, pharmaceuticals, oil, ceramics, plastics, etc. Accurate analysis of turbidmaterials has attracted extensive research efforts because of possibility for wideapplications in various fields ranging from study of biological cell samples tomonitoring of food quality in industrial safety inspection.The optical properties of a homogeneous turbid sample can be characterized byits wavelength () dependence of absorption coefficient a, scattering coefficients and scattering anisotropy factor spectrum g On the basis of light-matterinteraction, the absorption coefficient of a turbid sample is mainly dominated by thetypes of molecules present in the sample while the scattering coefficient andscattering anisotropy factor spectrum are mainly determined by the morphology ofparticles in the sample that are of sizes similar to the wavelength. Therefore, accuratemeasurement of the three optical parameters and their spectral data of the turbidsample will greatly improve the accuracy of material analysis.The present methods for analysis of turbid samples generally need to use one ortwo integrating spheres to measure scattered light signals, which can be calculatedwith different theoretical models. The measured and calculated signals are comparedto iterate the calculations until a solution of the optical parameters can be achievedthat allows the matching of the two data sets. Because of its shortcomings, such asmeasurement complexity and high cost, existing methods are difficult to beimplemented at the instrumentation level, which hinder the practical application ofspectroscopic methods in analysis of turbid materials.In this thesis research, we have established a new instrumentation method whichcan determine the three optical parameter and their spectra for a homogeneous turbidsamples media. This new method allows the user to measure4light signals at a variable wavelength and determine inversely spectra of three optical parameters asa, s and g in real time. Compared with the existing spectrophotometrymethods and integrating-sphere based methods for measurement of turbid samples,the new method has several advantages. First, the experimental system employs alight measurement design that is simple to be implemented into a practical instrument.Secondly, the hardware and software part of the experimental system are integratedfor easier operation. Thirdly, the spectral measurement and parameter determinationcan be performed in a wide range of wavelengths from400nm to800nm and can becompleted within1hour. The light measurement design combines optical modulationtechnique with4photodiodes which collect transmitted and scattered light signalsfrom the turbid sample in different angular ranges. The measured signals consist ofcollimated transmitted light intensity, diffusely reflected light intensity and diffuselytransmitted light intensity in additional to a split-off incident light intensity signal.The measured signals are used to obtain the measured values of collimatedtransmittance, diffuse reflectance and diffuse transmittance, which are imported into aMonte Carlo simulation software with a perturbation algorithm for inverse calculationto speed up the process for determination of a, s and g at a variablewavelength. Using this new method of spectrophotometric analysis, accuratecharacterization of homogeneous turbid samples can be achieved with no integratingsphere and optical parameters can be determined rapidly.In this thesis, we will present and discuss the following results:1, A new method for determination of multiple optical parameters and theirspectra for turbid samples has been established which based on the measurement oflight signals in different angular ranges without the use of integrating sphere. Thediffuse reflectance, diffuse transmittance and collimated transmittance are obtainedfrom the measured light signals by obtaining the ratios of the intensity of diffuselyreflected light and intensity of the incident light, the ratio of the intensity of diffuselytransmitted light and intensity of the incident light, and the ratio of the intensity ofcollimated transmitted light and intensity of the incident light. With the collimatedtransmittance, we can obtain the attenuation coefficient tof the turbid sample by theBeer-Lambert law. A Monte Carlo simulation software has been developed based onthe radiative transfer theory and Fresnel formulas to calculate the diffuse reflectanceand diffuse transmittance. The calculated signals are compared to the measured ones in multiple iterations to obtain absorption coefficient μa, scattering coefficient μsandscattering anisotropy factor g of the sample at multiple wavelength, which provide thespectral data of these parameters.2, Measurement system for the spatial featured optical signals of turbid mediadefined in this paper has been designed and developed, which consists of an opticalsystem, a weak optical signal acquisition and amplification processing hardwarecircuit and digital filtering based on Fourier transform. The optical system consists ofbroad spectrum xenon light source, monochromator, optical modulator, and thesample holder. The output from the light source is fed into a monochromator to obtainan incident monochromatic beam of variable wavelength. The beam is furthermodulated by a mechanical chopper before incident on the measured sample. Thedetectors receive diffusely reflected, diffusely transmitted and collimated transmittedlight signals in additional to a portion of the incident light beam. The weak signalamplification circuit consists of the preamplifier circuit, the main amplifier, band-passfilter circuit and50Hz notch filter circuit. The preamplifier circuit converts the currentsignals from each of the detectors into voltage signals before the first-stageamplification. The functions of the band-pass filter circuit and50Hz notch circuit areto improve the SNR and dynamic ranges of the signals. The main circuit containsanother stage of the linear amplification for weak voltage signals. The transimpedance magnification of the circuit is designed to be of108-109(Ohm). The digitalfiltering algorithm based on Fourier transform consists of a high-speed multi-channelanalog-to-digital converter and a FFT transform software for digital filtering foraccurate extraction of the signals at the modulation frequency in real-time. Finally, weobtain the diffuse reflectance, diffuse transmittance and collimated transmittance ofthe measured sample before importing them into the inverse calculations.3, The optical parameters have been determined by importing the multiplemeasured signals into a Monte Carlo simulation software with a perturbationalgorithm for fast inverse calculations. The measured light signals are compared withthe calculated signals based on the Monte Carlo simulations. The optical parametersare optimized in multiple iterations by using the perturbation algorithm until thecalculated signals are matched to the measured ones. The final results of theoptimized parameters provide the absorption coefficient μa, scattering coefficient tμsand scattering anisotropy factor g of sample at the wavelength of the measured signals. By measuring the light signals in a broad spectrum, we can obtain the spectral data ofthe above parameters as a, s and g4,Validation of the experimental system has been made with the Intralipidsamples. We prepared the concentration of3%and6%Intralipid by mixed theconcentration of30%Intralipid and distilled water. Then we measured the opticalparameters and their spectra for two Intralipid samples of the two differentconcentration in the wavelength range of400nm to780nm. We will discuss the effectby changing the diffuse reflectance and diffuse transmittance.5, The new experimental system has also been used to measure and study of theoptical parameters for suspension samples of polystyrene microsphere in water. Weused different concentration of polystyrene microspheres whose diameters range from700nm to3500nm as the samples. Optical parameters of these samples have beendetermined for polystyrene microspheres with different size and differentconcentration.In this thesis we also discuss the existing problems for the experimental systemand inverse calculations for accurate determination of optical parameters and theirspectra of turbid samples. The main problems include:1,The current optical system istoo large to meet the application requirements of instrumentation field testing.2, Dueto the weak intensity of the incident light in the low end of spectrum near400nm ofthe xenon light source, the SNR of the measured signals is low.3, the Monte Carlobased inverse algorithm (iMC-GPU) requires further improvement in computerhardware to resolve the issue of crashing with two or more graphic cards. The futureresearch can be first focused on solving the above problems with more detailed andin-depth analysis of the optical and electronic designs to improve and reduce the sizeof the experimental system, measurement accuracy and inverse calculation speed.With these improvement, we believe the new method of spectrophotometricmeasurement developed through this thesis research can be applied for quantitativeanalysis of many turbid samples such as human blood, oil, drugs and foods. |
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