STARR II

STARR II is a planned facility for the spectral and angular characterization of reflection, transmission and scattering of materials across the reflected solar wavelength spectrum (200 nm to 2500  nm).  STARR II will significantly extend the capabilities of the current NIST STARR facility for measurements at short-wave infrared wavelengths and for arbitrary illumination and viewing angles, with particular emphasis on scattering measurements needed by the environmental monitoring community.

The characterization of materials for reflection, transmission, and diffuse scattering over a range of wavelengths, incidence angles, and scattered angles is described by the bidirectional reflectance distribution function, or BRDF.  The BRDF and its transmission analogue, BTDF, can be used to extract information about the surface roughness and is also of intrinsic interest in applications such as computer rendering of realistic illuminated surfaces, quantifying scattering from highly absorbing black materials, and materials identification using BRDF signatures.

In the area of climate change monitoring, there is also a specific need for BRDF measurements of diffuse white targets or “plaques” that are incorporated into earth-observing satellites as calibration artifacts. Such satellites make spectrally-resolved measurements of the earth’s reflectance or albedo to determine the Earth radiation budget and monitor climate change. To make in-flight calibrations of the satellite spectroradiometer, a plaque of known reflectance is brought into the spectroradiometer field of view using the sun as a known, stable irradiance source. Because the spectrum of solar radiation reflected from the earth extends from 200 nm – 2500 nm, it is desirable to make the reference BRDF measurements of the plaques over the same wavelength range.

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 over a nominal spectral range of 200 nm – 2500 nm.  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 anisotropic samples.

A wide range of new technologies and techniques are being evaluated in the design of STARR II. Recently, we have demonstrated that a high-brightness, supercontinuum fiber laser source, coupled to a monochromator, can provide hundreds of microwatts of tunable, quasi-monochromatic light to illuminate a sample, compared to the sub-microwatt flux levels of a conventional lamp and monochromator-based source.  The use of a supercontinuum source in STARR II is expected to speed measurement times, expand the measurement range for angle-resolved BRDF, and allow the measurement of lower reflectance samples than are currently possible.

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, the use of a five-axis goniometer is envisioned, similar to that used in the current NIST Goniometric optical scatter instrument (GOSI) at the NIST Bidirectional optical scattering facility. This configuration allows selection of any incident and scattering angle combination over nearly a full hemisphere over the sample surface by rotating and angling the surface, while the source and detector remain fixed in a plane. Hemispherical scatter measurements, in which the light is incident from a fixed angle and total scatter over a hemisphere over the surface is measured, can be realized by integrating a set of measurements made with the goniometer. While hemispherical scattering measurements can also be made by using an integrating sphere to collect all of the scattered light from a surface, this technique does not reveal the angular distribution of the scattering.

Planning for STARR II is currently underway. Inquiries may be directed to Dr. Heather Patrick.