Fiber-optic microprobes for determining irradiance and scalar irradiance were used for light measurements in sandy sediments of different particle size. Intense scattering caused a maximum integral light intensity [photon scalar irradiance, E0(400 to 700 rim) and E0(700 to 880 nm)] at the sediment surface ranging from 180 % of incident collimated light in the coarsest sediment (250 to 500 mum grain size) up to 280 % in the finest sediment ( 1 mm in the coarsest sediments. Below 1 mm, light was attenuated exponentially with depth in all sediments. Light attenuation coefficients decreased with increasing particle size, and infrared light penetrated deeper than visible light in all sediments. Attenuation spectra of scalar irradiance exhibited the strongest attenuation at 450 to 500 ran, and a continuous decrease in attenuation coefficent towards the longer wavelengths was observed. Measurements of downwelling irradiance underestimated the total quantum flux available, i.e. scalar irradiance, by > 100 % throughout the sediment. Attenuation coefficents of scalar irradiance, downwelling irradiance and upwelling irradiance were, however, similar in deeper sediment layers where the light field became more diffuse. Our results demonstrate the importance of measuring scalar irradiance when the role of light in photobiological processes in sediments, e.g. microbenthic photosynthesis, is investigated.