Abstract: In chirped pulse amplification laser systems, the damage threshold of the final grating in the pulse compressor seriously limits the energy output. This paper proposes a measurement, by which the exact correlation between the local fluence and local damage characteristics can be established. This method collects the near-field intensity distribution of a cm-sized beam spot and its corresponding raster damage image through on-line monitoring and image processing, and then matches the pixels on the two images to each other. Finally the gray level in the damage image is converted in terms of damage density by microscope observation. It does not strictly require the spatial uniformity of the beam. Moreover, since the intensity fluctuation is taken into account, the damage densities versus the fluence can be extracted within one shot.