Tunable-Laser-based Facility for Spectral Irradiance and Radiance
Responsivity Calibrations using Uniform Sources (SIRCUS)

Introduction

Detectors are calibrated for spectral radiant power responsivity at NIST on the Spectral Comparator Facility (SCF) using a lamp-monochromator system to tune the wavelength of the excitation source. Silicon detectors can be calibrated in the visible spectral region with combined standard uncertainties at the 0.1 % level. However, quantities of interest are often the amount of light falling on a surface (irradiance) or the amount of light emitted from a source (radiance). Because of the low flux in the lamp-monochromator system, instruments cannot be directly calibrated for irradiance or radiance responsivity and more complicated approaches must be taken that increase the uncertainty in the measurements to the 0.5 % level or greater. In addition, the uncertainty in spectral radiant power responsivity calibrations increases dramatically in both the UV and the IR spectral regions.

The SIRCUS facility was designed to address these two limitations in NIST detector calibration services; that is, to reduce the uncertainty in irradiance and radiance responsivity calibrations to the 0.1 % level and to expand the spectral range where these uncertainty levels are achievable. The facility expands on previous work at NIST and the National Physical Laboratory (NPL) in the U.K. beginning in the mid-1980's using tunable lasers and silicon-based pyrometers traceable to cryogenic radiometers to determine the melting and freezing points of primary metal blackbodies with uncertainties approaching or exceeding those achievable using more conventional technologies [1-5].

This facility supports the NIST Optical Technology Division within the Physics Laboratory. The Optical Technology Division is responsible for maintaining two fundamental Systeme Internationale (SI) units, the unit for temperature, the kelvin, above 1234.96 K, and the unit for luminous intensity, the candela. SIRCUS has been used for the detector-based realization of the radiance temperature scale and a new realization of the candela. SIRCUS supports programs with other government agencies, among them NASA, NOAA and USGS, as well as programs NIST has established with defense and environmental space-based remote sensing companies. It has been used to calibrate space-flight sensors directly, and to calibrate field instruments that participate in ground validation of satellite data.

SIRCUS uses continuously tunable lasers across the UV, visible, and much of the infrared spectrum to perform system-level spectral irradiance and radiance responsivity calibrations of electro-optical sensors, radiometers, and spectrometers.

Figure 1. This figure shows the current wavelength coverage by the SIRCUS facilities at NIST, using several different tunable lasers and nonlinear optics. The wavelength coverage is being further extended.

There are two separate SIRCUS facilities, separated by spectral coverage: the UV-Vis-NIR SIRCUS and the IR-SIRCUS. The UV-Vis-NIR SIRCUS covers the spectral range from 200 nm to 1000 nm. Continuous spectral coverage is provided in the IR SIRCUS from 700 nm to 5.3 µm; discrete lasers extend the spectral coverage to 10 µm. The two facilities are described in greater detail at links below.

In addition, the U.S. standard for optical power, the Primary Optical Watt Radiometer (POWR) is located adjacent to SIRCUS. Radiometers under test are calibrated against reference irradiance detectors that transfer the optical power scale either directly from POWR or from a recently introduced portable cryogenic radiometer that is directly traceable to POWR. Finally, there is a portable, table-top, tunable laser system, complete with integrating spheres and transfer standard detectors, called Traveling SIRCUS. Traveling SIRCUS has been sent to NASA, NOAA, and USGS sites to characterize instruments in support of NIST's Environmental Remote Sensing Program.

The irradiance responsivity of the reference detector is derived from the power responsivity and the area of a precision aperture attached to the front of the detector. The area of the precision aperture is measured on a dedicated facility, the NIST Aperture Area Facility [6], with uncertainties less than 100 ppm. The short calibration chain from the primary cryogenic radiometer to the reference detector responsivity ensures that the uncertainty in the detector responsivity is minimized. Lasers ultimately determine the spectral coverage available on SIRCUS while the uncertainties achievable are determined by the quality of the reference standard irradiance detectors. Because of the high power and wavelength stability of the source, calibrations can be made with uncertainties as low as, or better than, the best optical power measurements. In more advanced applications, large aperture instruments, e.g., telescopes, and detector array-based imaging systems, requiring tests in uniform, monochromatic fields, have been characterized.

References:

Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) facility at NIST,

Brown, S.W., Eppeldauer, G.P., Rice, J.P., Zhang, J., and Lykke, K.R.,

Proc. SPIE 5542, 363 (2004).

NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources (210 kB) 

Brown, S.W., Eppeldauer, G.P., and Lykke, K.R.,

Metrologia 37, 579-582 (2000).

Realization of a spectral radiance responsivity scale with a laser-based source and Si radiance meters (152 KB)

Eppeldauer, G.P., Brown, S.W., Larason, T.C., Racz, M., and Lykke, K.R.,

Metrologia 37, 531-534 (2000).