As line scanning short wave infrared (SWIR) hyperspectral imaging (HSI) is a growing field in the food industry, it is important to select efficient illumination designs to image contaminants with high contrast and low noise. Illumination systems can efficiently be compared and optimized through the use of ray tracing simulations. However, these simulations provide illumination patterns in absolute radiometric units while HSI systems typically provide relative measurements. To bridge this gap, a supercontinuum laser and monochromator setup was used in this study to calibrate a SWIR HSI imager in spectral radiometric units. For the radiometric calibration, an integrating sphere (IS) was illuminated with the monochromatic laser light, while both a high sensitivity photodiode and the hyperspectral camera were positioned at different ports of the IS to measure the diffuse light synchronously. For each spectral band, the radiance observed by the imager corresponding to a line was detected using image analysis, while the remainder of the image was used to sample the noise of the sensor. Laser power fluctuations were monitored using a power meter coupled with a thermal sensor, allowing for their correction. As these measurements were time consuming, while InGaAs based sensors are very sensitive to thermal drift, the dark current was sampled frequently to avoid noise time drifts. This approach allowed correcting for 6% of temporal noise fluctuations. A per-pixel linear radiometric model was fitted with an R2 of 0:94±0:3 and used to transfer the measured light distribution of a halogen spot with and without a diffuser into absolute radiometric units. This allowed comparing measurements with the results of ray tracing.
Near infrared spectroscopy offers a promising technological platform for continuous glucose monitoring in the human body. NIR measurements can be performed in vivo with an implantable single-chip based optical NIR sensor. However, the application of NIR spectroscopy for accurate estimation of the analyte concentration in highly scattering biological systems still remains a challenge. For instance, a thin tissue layer may grow in the optical path of the sensor. As most biological tissues allow only a small fraction of the collimated light to pass, this might result in a large reduction of the light throughput. To quantify the effect of presence of a thin tissue layer in the optical path, the bulk optical properties of tissue samples grown on sensor dummies which had been implanted for several months in goats were characterized using Double Integrating Spheres and unscattered transmittance measurements. The measured values of diffuse reflectance, diffuse transmittance and collimated transmittance were used as input to Inverse Adding-Doubling algorithm to estimate the bulk optical properties of the samples. The estimates of absorption and scattering coefficients were then used to calculate the light attenuation through a thin tissue layer. Based on the lower reduction in unscattered transmittance and higher absorptivity of glucose molecules, the measurement in the combination band was found to be the better option for the implantable sensor. As the tissues were found to be highly forward scattering with very low unscattered transmittance, the diffuse transmittance measurement based sensor configuration was recommended for the implantable glucose sensor.
A hyperspectral laser scatter imaging (HLSI) system based on a supercontinuum laser in combination with a
monochromator has been developed for contactless and non-destructive measuring the ripeness of Braeburn apples.
Reflectance images were obtained by a CCD camera at 91 different wavelengths ranging from 550 nm to 1000 nm and
transformed into reflectance profiles. A linear function was fitted to the logarithm (log10) of the extracted profiles,
resulting in an intercept and a slope. These two parameters were then correlated with apple ripeness parameters such as
firmness and soluble solids content (SSC) measured by the reference, destructive methods. Preliminary results showed
the potential of slope and intercept to be used as a ripeness indicator. Moreover, during fruit ripening, the new HLSI
measurement technique clearly showed the degradation of chlorophyll over time.
This paper presents an adaptation of the widely accepted Monte Carlo method for Multi-layered media
(MCML). Its original Henyey-Greenstein phase function is an interesting approach for describing how light scattering
inside biological tissues occurs. It has the important advantage of generating deflection angles in an efficient - and
therefore computationally fast- manner. However, in order to allow the fast generation of the phase function, the MCML
code generates a distribution for the cosine of the deflection angle instead of generating a distribution for the deflection
angle, causing a bias in the phase function. Moreover, other, more elaborate phase functions are not available in the
MCML code.
To overcome these limitations of MCML, it was adapted to allow the use of any discretized phase function. An
additional tool allows generating a numerical approximation for the phase function for every layer. This could either be a
discretized version of (1) the Henyey-Greenstein phase function, (2) a modified Henyey-Greenstein phase function or (3)
a phase function generated from the Mie theory. These discretized phase functions are then stored in a look-up table,
which can be used by the adapted Monte Carlo code.
The Monte Carlo code with flexible phase function choice (fpf-MC) was compared and validated with the original
MCML code. The novelty of the developed program is the generation of a user-friendly algorithm, which allows several
types of phase functions to be generated and applied into a Monte Carlo method, without compromising the
computational performance.
Food quality is critically determined by its microstructure and composition. These properties could be quantified noninvasively
by means of optical properties (absorption and reduced scattering coefficients) of the food samples. In this
research, a spatially-resolved spectroscopy setup based on a fiber-optic probe was developed for acquiring spatiallyresolved
diffuse reflectance of three sugar foams with different designed microstructures in the range 500 - 1000 nm. A
model for light propagation in turbid media based on diffusion approximation for solving the radiative transport equation
was employed to derive optical properties (absorption and reduced scattering coefficients) of these foams. The accuracy
of this light propagation model was validated on four liquid phantoms with known optical properties. The obtained
results indicated that the optical properties estimation was successfully validated on these liquid phantoms. The
estimated reduced scattering coefficients μs' of the foams clearly showed the effect of foaming time on their
microstructures. The acquired absorption coefficients μa were also in good agreement with the designed ingredients of
these sugar foams. The research results clearly support the potential of spatially-resolved spectroscopy for nondestructive
food quality inspection and process monitoring in the food industry.
In this study, a tool was developed to calculate the bulk optical properties for systems consisting of an absorbing medium
and polydisperse spherical particles that can scatter and/or absorb. The developed tool is based on the Mie-theory for
monodisperse-spherical absorbing and scattering particles in vacuum. First, the original Mie-theory was expanded to also
include physical (real part of refractive index) and chemical (aborption, imaginary part of refractive index) information
of the host medium. Secondly, the polydispersity of the spherical particles was taken into account. Since particle size
distributions (PSD) are typically continuous distributions and Mie-scattering properties can only be calculated for a
monodisperse system, the PSD is fractionated and Mie-scattering properties were calculated for each fraction. These
Mie-scattering properties are then combined with the weight for each fraction to derive bulk optical properties. As the
number of fractions is unknown and needs to be optimized for each calculation, the developed tool keeps on
fractionating until the desired properties (μabs, μsca and P11(cos(θ))) converge to stable values. This flexible tool
allows for the simulation of the bulk optical properties for a wide range of wavelengths, particle volume fractions,
complex refractive indices of both the particles and the medium and PSD's based on normal, lognormal, gamma,
bimodal and custom defined functions. This code was successfully validated for the case of a lognormal PSD of
scattering spheres in vacuum by comparing the simulated values to those reported in literature. The main novelties
of the developed program are the extension of Mie-theory simulations to the case of polydisperse scattering particles in
absorbing media and the automatic optimization of the number of PSD fractions needed to converge.
Accurate knowledge of the optical properties of turbid media in the light path is important in NIR absorption
spectroscopy of biological tissues where multiple scattering complexes the collected light signals due to the non-uniform
tissue architecture. Several approaches such as time resolved spectroscopy and spatially resolved spectroscopy have been
proposed to measure the bulk optical properties of turbid media. Among them, double integrating sphere (DIS)
measurements are recognized as the "golden standard" for in vitro optical properties measurement of turbid media
because of its high accuracy and robustness in different conditions. A DIS system is convenient to measure the in vitro
optical properties of turbid media like intralipid solutions and biological tissues, since it measures the diffuse reflectance
and transmittance simultaneously. However, DIS measurements have been mostly limited to the optical window region
(400-1000 nm) or suffered from low signal levels on the detectors due to the absorption by water in the NIR region. In
this study, we developed a DIS system for optical property measurement in the 1300-2350 nm region based on a novel
wavelength tunable spectroscopic setup which incorporates a high power broadband supercontinuum laser and a high
precision monochromator. With this system, optical properties of intralipid solutions were measured in the wavelength
region of 1300-2350nm.
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