Advanced Hemispherical Reflectance Measurement of Heated Materials

Dynamic Bi-directional Reflectance Distribution Function (BRDF) of various solid targets (Zirconia, Aluminum and Steel) under conditions of rapid heating by a kilowatt class High Energy Laser (HEL) is measured with 3.5% accuracy. By integrating the measured BRDF over a hemisphere, the dynamic Directional-Hemispherical Reflectance can be extracted. In-band reflectance is sampled by photodiode detectors spatially dispersed in a hemispherical space, mechanically supported by a geodesic dome partially surrounding the target. The moving reflected lobe is imaged by a NIR camera monitoring a screen placed directly in the path of the reflected lobe. Camera frames, as well as detector signals are time stamped with IRIG-B standards to allow accurate processing of the data, as well as synchronization with diagnostics instrumentation and external events. During HEL engagement, rapid heating rates approaching 100ºC/sec are obtained. The BRDF of the target is extracted with msec temporal resolution, for interaction times as long as 120 sec. The heating caused by the absorption of the laser light by the target is numerically simulated by using the Optical Signature Code (OSC). Pyrometer and thermocouple readings show substantial agreement with the simulations, confirming the validity and accuracy of the code in modeling the laser heating of solid targets.