Calibration of the Cryogenic Target Optical Shadowgraphic Characterization System at LLE

Reflection and refraction of light by a cryogenic target produce rings on a shadowgraph image that characterize the ice surface position. Many different views give a 3-D ice-layer representation and the global ice-layer roughness. Calibration of the image resolution and accuracy is needed to measure the 1-\textit{$\mu $}m rms ice layers required for ignition. The position of the outer edge and bright ring can be resolved to $\sim $0.1-pixel rms (just over 0.1 \textit{$\mu $}m) in LLE shadowgraphs. Pincushion distortion because of lens aberration can add a several tenths of a micron $n$ = 2 artifact to the measurements. Using precision patterns to align the system minimizes the optical distortion and characterizes it for image correction. Comparison between shadowgraphic and high-precision atomic force microscopy 3-D characterizations of a sapphire sphere shows the accuracy of the 3-D analysis. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.