Defect clusters in the bulk of large KDP crystals are revealed using a microscopic fluorescence imaging system and CW laser illumination. Exposure of the crystal to high power 355-nm, 3-ns laser irradiation leads to a significant reduction of the number of observed optically active centers. The initially observed defect cluster concentration is approximately 104-106 per mm3 depending on the crystal growth method and sector of the crystal. The number of defect clusters can be reduced by a factor of 102 or more under exposure to 355-nm laser irradiation while their average intensities also decreases. Spectroscopic measurements provide information on the electronic structure of the defects
