Atomic force microscopy was used to determine in-situ the nm-scale morphological changes that occur on dielectric optical coatings as a result of laser illumination. Of particular interest is a process called laser conditioning in which the damage threshold of the films is increased by a factor of 2 to 3 when the film is first illuminated at fluences below the damage threshold. The optical coating studied was a highly refractive dielectrical multilayer mirror (HR) consisting of many alternating quarter-wave layers of HfO2 and SiO2 at 1.06 micrometers . The top layer was a (lambda) /2 SiO2 overcoat. Laser beam specifications were: 1.06-micrometers wavelength, 8- to 10-ns pulsewidth. Laser beam spot sizes ranging from 85 micrometers to 1.4 mm in diameter. The maximum scan range of the AFM was 80 micrometers . A survey of the as-deposited surface shows mostly hillocks approximately 200 nm in diameter and 10 nm in height. The predominant surface irregularity is micrometers -scale domes associated with the well known nodule defects. Laser illumination causes nodule defects to be easily ejected from the coating surface. Further damage may propagate from the resulting craters. These nodule defects therefore determine the damage threshold of the film. Using the AFM we have shown that for illumination at fluences below the nodule ejection threshold we observe a decrease in the surface roughness of the nodule defects and hillock structure of the as-deposited film. The subtle changes in these surface features may be an indication that the film is being mechanically stabilized, thus providing the observed conditioning effect.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.