New Alternative Material for Reflectance Standards
PML researchers successfully utilized a new capability for measuring reflectance in the shortwave infrared (SWIR) to characterize diffuse reflectance materials. The results suggest a new material may serve as a promising alternative for diffuse reflectance standards.
Diffuse reflectance materials are frequently used as calibration artifacts, often called diffuse calibration plaques, for critical applications in remote sensing applications, such as irradiance to radiance transfer, ground truthing, and on-board calibration plaques in satellites. In recent years, the remote-sensing community has relied on sintered polytetrafluoroethylene (PTFE) as the standard material for diffusers. An ideal diffuser would not be non-fluorescing, immune to solarization that can alter reflectance properties, and its minimal spectral features in the SWIR. Additionally, the material should not attract particulates electrostatically and be easily cleaned, which can be problematic for diffusers used in the field for ground truthing measurements.
In contrast, ceramics are generally stable at high temperatures and in harsh environments. They are not believed to be subject to solarization and are easy to clean. While new ceramic materials have recently been developed to perform on the same level as PTFE, their reflectance properties in the SWIR have only been assessed using a directional-hemispherical geometry.
In 2010, NIST became the first facility in the world to measure bi-directional diffuse reflectance in the SWIR, from 1100 nm to 2500 nm, using a bi-directional geometry. This now added capability allows spectral coverage of the full solar reflective region 250 nm to 2500 nm for bi-directional diffuse reflectance. Thus, this new capability was recently applied to measuring the reflectance of the new ceramic materials. The researchers found that the reflectance of the ceramic material is comparable to sintered PTFE across the solar reflective region, but is less structured in the SWIR. This is significant because remote sensing instruments use sensors with large bandwidths, and their calibration depends on a “constant” reflectance value across the band for measurements with the lowest uncertainty.
These results were presented at CALCON Technical Conference on August 29, 2011, and the research was performed in collaboration with Mt. Baker Research LLC.