Experimental investigation of high-energy laser reflections from melting metallic targets for laser safety assessments.

We measured the reflections of a high-energy laser (HEL) from melting metallic targets in order to perform a laser safety assessment and calculate nominal ocular hazard distances (NOHDs). The experimental setup consisted of an observation screen capturing the reflections and imaging them with two cameras: one with a large field-of-view recording the entire screen and one with a narrow field of view recording only the central part of the screen. The screen is first placed at a distance of 2.4 m and then 4.8 m from the target. A calibration procedure is described and used to obtain the reflected power values for each pixel of the camera. First, the power in different reflection directions and solid angles, that is the radiant intensity, with respect to the orientation of the HEL beam, is averaged over five targets per screen distance. The ratio of the radiant intensities between the two distances in different directions was calculated to be greater than 1, indicating that the measurement was not in the far field. We estimate that the intensity decreases with a 1/r1.25 dependency in the worst-case scenario instead of the classical 1/r2, where r is the distance from the target. The power distributions in different apertures were then plotted as histograms for the two distances and compared. Finally, we propose a methodology to extrapolate these measurements to HEL with higher output powers. A methodology to calculate NOHD from the extrapolated data that complies with laser safety rules and regulations and takes into account the findings from the measurement, is presented. Using this methodology, we calculated a NOHD of 560 m from our melting steel targets assuming a 100 kW laser.