Advanced Hemispherical Reflectance Measurement of Heated Materials

Measurements of Total Hemispherical Reflectance under high temperature conditions are needed for accurate assessment of lethality functions of the ABL program. During engagement by high power lasers, the surface temperature of solid targets changes rapidly causing drastic variations in the absorption and reflection characteristics. Current hemispherical reflectance measurement systems cannot perform under such fast heating rates (100 degrees per second). SARA’s proposed methodologies to address this shortcoming will be addressed by investigating alternative techniques aimed at measuring Total Hemispherical Reflectance. In particular, we will demonstrate the possibility of measuring the Bi-directional Reflectance Distribution Function (BRDF) of a target under the fast heating rates of interest. Hardware and methodologies to perform such measurement will be presented and the BRDF of representative military targets (4130 steel and 6061 Aluminum) will be performed using an in-house two-kilowatt CW Nd:YAG laser capable of irradiating samples at 10 kW/cm2 and producing sample heating rates of hundreds of degrees per second. Software to derive the Total Hemispherical Reflectance from BRDF measurements will be developed. The results will be applied to the design of a subscale device in Phase II and scalability to full deployment in a field environment for a Phase III device will be demonstrated.