STARR II

STARR II is a facility for measuring reflectance and scattering of materials in the ultraviolet to short-wave infrared wavelength range (230 nm to 2500 nm) that is currently under development.  STARR II employs new technologies, such as a tunable, supercontinuum laser-based light source and a robotic arm goniometer that will significantly extend NIST’s capabilities for reflectance measurements at short-wave infrared wavelengths and for arbitrary illumination and viewing angles.  STARR II will provide reference reflector calibrations for the environmental monitoring community, measurements of scattering and reflectance of materials for databases and model validation, and wavelength-resolved mirror reflectance measurements into the short-wave infrared.

Have you ever wondered why some objects appear glossy, and others diffuse?  Or thought about the complexity of characterizing the way light reflects from an object under different illumination and viewing angles and at different wavelengths in order to realistically render it in a computer simulation?  The characterization of materials for reflectance and scattering as a function of wavelength, polarization, incidence angles, and viewing angles is described by the bidirectional reflectance distribution function, or BRDF.  BRDF measurements are needed not only in visible light applications, but also in the ultraviolet and infrared wavelength regions.  For example, climate satellites make measurements of the earth’s reflectance or albedo over the entire reflected solar region, spanning approximately 250 nm to 2500 nm, to determine the Earth’s radiation budged and monitor climate change.  In order to help calibrate and validate the earth-viewing spectroradiometers on board, a calibrated  reference reflector, called a solar diffuser, is periodically brought into the spectroradiometer field of view.  Using the sun as a known illumination source, and the measured BRDF of the diffuser, this provides a source of known radiance to the instruments.

The current primary reflectance facility at NIST, STARR (Spectral Tri-function Automated Reflectance Reflectometer), has been in used since the mid-1990s.  It provides spectral BRDF, spectral specular reflectance, and spectral directional-hemispherical reflectance factor measurements from the ultraviolet to the short-wave infrared.  While the existing facility provides high accuracy, stable measurements, evolving measurement needs and improvements in technology are driving the need for a new spectral BRDF facility.  Among the needs to be addressed by STARR II are

• Improved measurement speed, on both a per angle and per wavelength basis
• Capability of measuring diffuse sample BRDF in the shortwave IR (SWIR) region, which is limited in the current STARR facility
• The addition of out-of-plane BRDF measurement capability, particularly important for the satellite and remote sensing community, who often employs out-of-plane viewing.

STARR II makes use of new technologies that were unavailable at the time of STARR’s commissioning.  A fiber-coupled, high-brightness broadband supercontinuum source has replaced a traditional lamp for the visible to shortwave infrared wavelength range.  Because the supercontinuum source can be very efficiently coupled to a monochromator, the tunable source power in STARR II has been shown to be 100 to 1000 times that in STARR.  The higher source power enables greater flexibility in the design of the receiver for measuring reflected and scattered light.  In STARR II, a single receiver can be used over the full wavelength range, removing the need to exchange and realign receivers for measurements at different parts of the spectrum.  The higher source power is also expected to speed data acquisition times, allow the measurement of lower reflectance samples, and allow far greater flexibility in measurement geometry.  For ultraviolet wavelengths, a tunable Ti:sapphire laser with frequency doubling and tripling provides tunable light from 230 nm to 500 nm.

The STARR II robotic arm goniometer

To fulfill the need for out-of-plane scatter measurements, in which the scattering angle is outside of the plane of incidence defined by the incident light and the surface normal, a robotic arm-based goniometer is under construction.  An industrial robot holds the sample and can tilt and rotate it such that, in combination with the angular motion of a receiver arm about the goniometer center, almost any combination of incident angle and in-plane or out-of-plane viewing angle can be reached.  Installation of the robotic arm goniometer is planned for late 2012.

Inquiries about STARR II may be directed to Dr. Heather Patrick.