Time of Flight Spectroscopy of Photon Migration in Turbid Media

Sammanfattning: Light interacting with biological matter is dominated by light scattering; biological matter is turbid. In order not to harm the natural structure of biological matter, visible light can be used to investigate it, and to avoid the light being absorbed from e.g. blood and water, an optical window in the longer wavelength region of the visible spectrum is used, that is, red light. One can actually check the validity of this optical window in a very simple way with nothing but a normal flash light and a finger tip. Putting the finger tip onto the flash light, it can be seen what light is mostly let through, and indeed, the finger appears red! Spectralon®, which is a type of plastic, is the material we know of with the highest diffuse reflectance, i.e. the most scattering material that has come to our attention. A first aim for this thesis was to investigate Spectralon® in terms of scattering and absorption coefficients, which is directly related to the number of events of scattering and absorption, respectively. These single events of scattering and absorption were recorded using Time-of-Flight Spectroscopy (TOFS). The system used to measure these properties of Spectralon® in short consists of a laser, optical fibers, i.e. one source fiber and one detection fiber, and a detector. In order to diagnose the response of the system, i.e. how fast it is, so called Impulse Response Function (IRF) measurements are made. These IRF measurements are made in connection to each sample measurement, and in order not to harm the detector with the light coming from the laser, as well as attempting to fill the entire cross-section of the detection fiber with light, some material must be put in between the source and the detection fiber. That is, in one aspect, this material might be seen as a pair of shades for the detector, which in turn is our eyes. A second aim for this thesis was finding a suitable material for this very important job. These investigations were carried out both theoretically by running computer simulations, but foremost experimentally by systematically testing different materials and then comparing the results . The number of materials were narrowed down to four; black-printed white writing paper, ready black paper, black-sprayed Teflon® tape, and black masking tape. As one of the first measurements in the world, the both the scattering coefficient and the absorption coefficient as a function of wavelength were measured for Spectralon®. The results agree with the theory on how the material should behave; the absorption coefficient increases with the wavelength and the scattering coefficient decreases in the same region. As for finding a suitable IRF material, it could be concluded from the experiments that the black-printed paper, already used today by the Biophotonics group, despite the porous nature of paper, indeed is a suitable material for this type of measurement.