High power lasers at 1.06 micrometers wavelength like the French 'Megajoule' program require large area mirrors made of alternative layers of HfO2 and SiO2 with high damage threshold. It has ben already identified that nodular defects have a significant role in the activation of laser damage. The purpose of this paper is to study experimentally the role played by the size and the nature of the seed on the critical behavior of the nodular defects under laser irradiation. Artificial nodules have been grown with careful control of location and seed size. these nodules have been generated with diamond and silica seeds. Silica seeds of four different sizes have been used. A method which minimizes clusters of particles has been successfully used. With this method, we have obtained defects with a sharp size distribution. These samples have been tested on our automatic damage test bench in various modes. The thresholds between polluted and unpolluted mirrors are discussed according to the size and the nature of the seeds. The damage sites are observed with a scanning electron microscope. These experimental results are compared with a damage model.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.