RESUMO
An apparatus for direct determination of the spectral resolved scattering phase function of suspensions and emulsions is described. The system consists of a polychromatic xenon lamp as a light source and a spectrometer as a detector. Together with a stepper motor, the system enables spectrally and angularly resolved measurements in the range of 450 nm-950 nm and 10°-170°, respectively. A model for light propagation inside the cuvette in the regime of single scattering, which also takes the spectral dimension into account, was developed. A postprocessing algorithm applying the model and an extrapolation to the measured angular data allows a direct determination of the complete scattering phase function. By comparing measurements on polystyrene microspheres with Mie theory, the concept of the presented instrument was validated. Finally, the method was used to determine the scattering phase function of different types and brands of soybean oil emulsions such as Intralipid or Lipovenös. The measured scattering phase functions were then used to calculate the corresponding Legendre moments to an order of 20 and for wavelengths between 450 nm and 750 nm (available online), which besides the anisotropy factor also allow the determination of higher order factors such as gamma.
RESUMO
A multi-center study has been set up to accurately characterize the optical properties of diffusive liquid phantoms based on Intralipid and India ink at near-infrared (NIR) wavelengths. Nine research laboratories from six countries adopting different measurement techniques, instrumental set-ups, and data analysis methods determined at their best the optical properties and relative uncertainties of diffusive dilutions prepared with common samples of the two compounds. By exploiting a suitable statistical model, comprehensive reference values at three NIR wavelengths for the intrinsic absorption coefficient of India ink and the intrinsic reduced scattering coefficient of Intralipid-20% were determined with an uncertainty of about 2% or better, depending on the wavelength considered, and 1%, respectively. Even if in this study we focused on particular batches of India ink and Intralipid, the reference values determined here represent a solid and useful starting point for preparing diffusive liquid phantoms with accurately defined optical properties. Furthermore, due to the ready availability, low cost, long-term stability and batch-to-batch reproducibility of these compounds, they provide a unique fundamental tool for the calibration and performance assessment of diffuse optical spectroscopy instrumentation intended to be used in laboratory or clinical environment. Finally, the collaborative work presented here demonstrates that the accuracy level attained in this work for optical properties of diffusive phantoms is reliable.
RESUMO
We describe a fully automated setup which is based on measurements of the spatially resolved reflectance for the determination of the reduced scattering and absorption coefficients in semi-infinite turbid media. The sample is illuminated with a xenon light source in combination with a monochromator enabling the scan of the wavelength from 450 nm to 950 nm. Reflected light from the sample is detected with a CCD camera providing a high spatial resolution. The essential steps for signal processing including, e.g., the consideration of the optical transfer function and the correct treatment of the background subtraction, are presented. The solutions of the diffusion theory and of the radiative transfer theory are investigated regarding the exact detection and illumination geometry. Systematic errors caused by using the different theories for fitting the optical parameters are characterized. The system was validated using liquid phantoms which contain Intralipid 20% and ink, and the measurement range of the system is specified. Further, we carefully characterized the optical properties of Intralipid 20% in the wavelength range between 450 nm and 950 nm.
