Laser spectroscopic studies on scattering media - Application to food packages and wood materials

Abstract: In this project, an optical technique called gas in scattering media absorption spectroscopy (GASMAS) has been employed to investigate optical properties in scattering media such as food packages and wood materials. The GASMAS technique has been combined with a technique called frequency domain photon migration (FDPM) and from the integrated system the gas absorption signal and total optical path length have been assessed when measuring on milk packages. Measurements performed on empty packages with thin walls show that the total optical path length could be a good approximation of the gas absorption length, enabling gas concentration assessment through Beer-Lambert law in some cases. However, when a liquid is present in the package, its optical properties will affect both the absorption and total path length of the light. When performing headspace measurements on the milk package, it was shown that a decrease of the scattering coefficient in the milk resulted in a decrease in absorption path length. When replacing the milk content with water, a drastic decrease in both absorption path length and total optical path length could be observed. The integrated system was also was used to perform a longtime study on a sealed milk package. The GASMAS measurement mode was used to take data continuously over several days and sample measurements were performed with the setup working in the FDPM mode. From the results it could be concluded that the decrease of the absorption signal was due to the depletion of oxygen in the headspace of the package and no change of the optical properties in the milk could be observed. By fitting the absorption signal to a bacterial growth formula, it was then demonstrated that the GASMAS technique can be used to indirectly monitor the bacterial population by measuring the gas content in the headspace. The GASMAS technique was also employed for gas diffusion studies inside pine and mahogany wood samples. A fiber probe was developed consisting of two optical fibers for light delivery and collection, i.e., one fiber was coupled to the diode laser and the second one to a PMT. By inserting the probe into a hole of the wood sample and flushing with nitrogen into another hole at a distance of 10 mm, the gas diffusion process would occur. It was shown that the pine sample had a much faster gas exchange rate with the ambient air compared to the mahogany sample, providing a great potential of using the fiber probe for archaeological wood material studies.