Maria A Velazco-Roa

Bio: Maria A Velazco-Roa is an academic researcher from Newcastle University. The author has contributed to research in topics: Scattering & Refractive index. The author has an hindex of 3, co-authored 4 publications receiving 158 citations.

Optical properties of apple skin and flesh in the wavelength range from 350 to 2200 nm

Abstract: Optical measurement of fruit quality is challenging due to the presence of a skin around the fruit flesh and the multiple scattering by the structured tissues. To gain insight in the light-tissue interaction, the optical properties of apple skin and flesh tissue are estimated in the 350-2200 nm range for three cultivars. For this purpose, single integrating sphere measurements are combined with inverse adding-doubling. The observed absorption coefficient spectra are dominated by water in the near infrared and by pigments and chlorophyll in the visible region, whose concentrations are much higher in skin tissue. The scattering coefficient spectra show the monotonic decrease with increasing wavelength typical for biological tissues with skin tissue being approximately three times more scattering than flesh tissue. Comparison to the values from time-resolved spectroscopy reported in literature showed comparable profiles for the optical properties, but overestimation of the absorption coefficient values, due to light losses.

Estimation of complex refractive index of polydisperse particulate systems from multiple-scattered ultraviolet-visible-near-infrared measurements

Abstract: A method to extract the complex refractive index of spherical particles from a polydisperse suspension at concentrations where multiple light-scattering effects are significant is presented. The optical constants are estimated from total diffuse reflectance and transmittance measurements and inverting the measurements using the radiative transfer equation (RTE) and the Mie theory for scattering by polydisperse spherical particles. The method is tested by applying it to three different polydisperse polystyrene suspensions and extracting the optical constants of polystyrene particles in the wavelength range of 450-1200 nm. The effect of particle size, concentration, and polydispersity on the estimated values of the optical constants is also discussed.

Complex refractive index of nonspherical particles in the visible near infrared region: application to Bacillus subtilis spores.

Abstract: A method is presented for the estimation of optical constants in the ultraviolet-visible-near-infrared (UV-Vis-NIR) region of nonspherical particles in a suspension at concentrations where multiple scattering is significant. The optical constants are obtained by an inversion technique using the adding-doubling method to solve the radiative transfer equation in combination with the single scattering theories for modelling scattering by nonspherical particles. Two methods for describing scattering by single scattering are considered: the T-matrix method and the approximate but computationally simpler Rayleigh-Gans-Debye (RGD) approximation. The method is then applied to obtain the optical constants of Bacillus subtilis spores in the wavelength region 400-1200 nm. It is found that the optical constants obtained using the RGD approximation matches those obtained using the T-matrix method to within experimental error.

Estimation of optical constants from multiple-scattered light using approximations for single particle scattering characteristics

Abstract: The inversion of multiple-scattered light measurements to extract the optical constant (complex refractive index) is computationally intensive. A significant portion of this time is due to the effort required for computing the single particle characteristics (absorption and scattering cross sections, anisotropy factor, and the phase function). We investigate approximations for computing these characteristics so as to significantly speed up the calculations without introducing large inaccuracies. Two suspensions of spherical particles viz., polystyrene and poly(methyl methacrylate) were used for this investigation. It was found that using the exact Mie theory to compute the absorption and scattering cross sections and the anisotropy factor with the phase function computed using the Henyey-Greenstein approximation yielded the best results. Analysis suggests that errors in the phase functions and thus in the estimated optical constants depend mainly on how closely the approximations match the Mie phase function at small scattering angles.

Postharvest quality of apple predicted by NIR-spectroscopy: study of the effect of biological variability on spectra and model performance. Postharvest Biol Technol

Abstract: Abstract The effect of cultivar, season, shelf-life and origin on the accuracy of near infrared (NIR) calibration models for the soluble solids content (SSC) and firmness of apple was studied based on a large spectral data set based on approximately 6000 apple fruit from different cultivars, origins, shelf-life exposure time and seasons. To interpret the variance in the spectra with respect to biological variability, functional analysis of variance (FANOVA) was used. From the FANOVA analysis it was concluded that the effects of cultivar, origin and shelf-life exposure time on the NIR spectra were all significant. The largest differences in the spectra were found around the water absorption peaks (970, 1170 and 1450 nm). External validations using independent data sets showed that the accuracy of the models increased considerably when more variability was included in the calibration data set. In general the RMSEP for predictions of the SSC were in the range 0.6–0.8 °Brix, while for Magness Taylor firmness it was 5.9–8.8 N, depending on the cultivar. It was shown that atypical data can lead to large validation errors. It is, therefore, important to collect a calibration data set which is sufficiently representative for future samples to be analyzed with the developed calibration models and to develop simple procedures for model adaptation during practical use.

Nondestructive Measurement of Fruit and Vegetable Quality

Abstract: We review nondestructive techniques for measuring internal and external quality attributes of fruit and vegetables, such as color, size and shape, flavor, texture, and absence of defects. The different techniques are organized according to their physical measurement principle. We first describe each technique and then list some examples. As many of these techniques rely on mathematical models and particular data processing methods, we discuss these where needed. We pay particular attention to techniques that can be implemented online in grading lines.