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Laser damage measurements with multiple pulses at constant fluence (S-on-1 measurements) are of high practical importance for design and validation of high-power photonic instruments. Using nanosecond lasers, it was recognized long ago that single-pulse laser damage is linked to fabrication-related defects. Models describing the laser damage probability as the probability of encounter between the high fluence region of the laser beam and the fabrication-related defects are thus widely used. Nanosecond S-on-1 tests often reveal the "fatigue effect," i.e., a decrease of the laser damage threshold with increasing pulse number. Most authors attribute this effect to cumulative material modifications operated by the incubation pulses. We discuss the different situations that are observed upon nanosecond S-on-1 measurements that are reported in literature and speak in particular about the defects involved in the laser damage mechanism. These defects may be fabrication related or laser induced, stable or evolutive, cumulative or of short lifetime.

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