Laser induced damage in optical material in nanosecond regime is widely attributed to local precursors in range of
nanometer to micrometer size. The damage precursors nature strongly depends on materials (coatings, non linear
crystals, substrates,..), breakdown location (bulk, surface, interface) and irradiation parameters (wavelength, pulse
duration...). The weakness of knowledge on parameters as sizes, densities and natures of precursors, let think that the
choice of the diagnostic method which reveals laser damage has to be adapted to each situation of irradiation.
Concerning the LIDT determination, destructive methods are usually involved: we can cite full size test using the "real"
final configuration of irradiation, raster scan method using a focused laser beam allowing laboratory test and statistic
approach allowing study with different beam sizes in order to probe the material homogeneity in terms of precursors.
This multi-scale approaches give relevant information on material properties regarding high power laser irradiation.
In order to investigate the laser damage initiation mechanisms, it appears necessary to involve non-destructive
diagnostics. These diagnostics permit to highlight modifications linked to precursors before material breakdown. The
main difficulty here is the local character of the diagnostic added to the low density of initiating center. A multi-scale
approach is thus also well adapted to the non-destructive case. Interest of diagnostics as local fluorescence and
photothermal deflexion both correlated with LIDT results will be discussed. To illustrate the purpose, examples on non
linear crystals and coatings will be shown.