The effects of illumination on the susceptibility of ferric borate are described, and the domain structures that occur in ferric borate are discussed. It is shown that the photo-induced changes agree with a simple domain wall model, the walls probably being of the Nee1 type. The time and temperature dependence of the photo-induced increases are consistent with a localized centre model in which the centres can exist in two energy states, each with a different anisotropy. The energy reqiiired to excite the centres is received indirectly from the photons, via Fe3+ ions.

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The development and improvement of oxygen sensors is continously matter of research because it is important to quantify oxygen concentrations in biological systems. The presented hybrid fiberoptical sensor combines the common advantages of fiber optic sensors e. g. flexible connection to the place of interest and potential separation between place of measurement and measuring device with the special advantage of the optical analysis that does not consume the analyte oxygen. The 02 sensing was tested by using 1 .) solutions with the dissolved oxygen indicator dye (tris (1 ,1O-phenantroline) ruthenium (II) chloride hexahydrate), and 2.) the 02 flux optode. The 02 flux optode consist of an oxygen permeable membrane (test membrane) combined with oxygen optodes to measure the P°2 difference across the membrane. The 02 flux can be calculated from the P°2 gradient across the test membrane and its permeability. The developed measuring system "FLOX" detects the phase shift between the sinusoidally modulated excitation light and the resulting fluorescence of the indicator. The phase shift is caused by the fluorescence lifetime that depends on the oxygen concentration. Many investigations have shown that measurements of lifetime are preferable to measurements of intensity because of higher stability. The FLOX system uses a blue light emitting diode (LED, ?pe45Onm) as light source together with optical filters and a photo multiplier tube (PMT) as detector. The coupling between the place of measurement and the FLOX is performed by a bifurcated fiber bundle in which single multimode fibers are statistically mixed. Its measuring end could be either immersed directly in an aqueous solution of the fluorophore or provided with a special sensor head for the measurement of the oxygen flux into a surface, for example into the surface of the human skin.First measurements demonstrated the applicability of the FLOX system to measure the 02 flux into human skin; for example an about 20-50% increase of the 02 flux after stop of the circulation could be clearly shown.KEYWORDS: fluorescence quenching, 02 flux sensor, oxygen supply, P°2 optode, phase modulation, lifetime, hybrid fiberoptical sensor, noninvasive sensing of skin microcirculation.

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