A FIBEROPTIC SCALAR IRRADIANCE MICROSENSOR - APPLICATION FOR SPECTRAL LIGHT MEASUREMENTS IN SEDIMENTS

The manufacturing of a new spherical fibreoptic microsensor is described. The microsensor measures scalar irradiance, i.e. the spherically integrated light at a point in space. The light collector of the probe was a 70-mu-m diffusing sphere cast on the tip of a 125-mu-m wide optical fibre tapered down to 15-20-mu-m diametre. The microsensor had an isotropic (+/- 10%) response from -160-degrees to +160-degrees over the whole spectral range from 400-900 nm in air as well as in water. The microsensor was coupled to a sensitive spectroradiometre and the spectral distribution of scalar irradiance in sediments was measured at 100-mu-m spatial resolution. Light was available for photosynthesis near the sediment surface at a higher intensity and a different spectral composition than could be expected from the illumination. By the combination of oxygen microelectrodes and the present fibre-optic microsensor it is now possible to study the depth distribution of microbenthic photosynthesis in relation to the available photosynthetically active radiation at less-than-or-equal-to 100-mu-m resolution.