Thomas R. O'Brian

Ultraviolet radiometry with synchrotron radiation and cryogenic radiometry

The combination of a cryogenic radiometer and synchrotron radiation enables detector scale realization in spectral regions that are otherwise difficult to access. Cryogenic radiometry is the most accurate primary detector-based standard available to date, and synchrotron radiation gives a unique broadband and continuous spectrum that extends from x ray to far IR. We describe a new cryogenic radiometer-based UV radiometry facility at the Synchrotron Ultraviolet Radiation Facility II at the National Institute of Standards and Technology. The facility is designed to perform a variety of detector and optical materials characterizations. The facility combines a high-throughput, normal incidence monochromator with an absolute cryogenic radiometer optimized for UV measurements to provide absolute radiometric measurements in the spectral range from 125 nm to approximately 320 nm. We discuss results on photodetector characterizations, including absolute spectroradiometric calibration, spatial responsivity mapping, spectroreflectance, and internal quantum efficiency. In addition, such characterizations are used to study UV radiation damage in photodetectors that can shed light on the mechanism of the damage process. Examples are also given for UV optical materials characterization.

New ultraviolet radiometry beamline at the Synchrotron Ultraviolet Radiation Facility at NIST

We have constructed a new ultraviolet (UV) radiometry facility at the Synchrotron Ultraviolet Radiation Facility (SURF II) at the National Institute of Standards and Technology (NIST). The facility combines a high-throughput normal-incidence monochromator with an absolute cryogenic radiometer (ACR) optimized for UV measurements to provide absolute detector-based radiometric calibrations in the spectral range 125 nm to approximately 320 nm. The system can be configured for spectroradiometric calibration of photodetectors with completely automated mapping of detector spatial and angular responsivity. The facility can easily be adapted for other spectroradiometric measurements including transmittance and reflectance and the spectral range can be extended to about 50 nm.

ULTRAVIOLET SPECTRAL IRRADIANCE SCALE COMPARISON : 210 NM TO 300 NM

Comparison of the irradiances from a number of ultraviolet spectral irradiance standards, based on different physical principles, showed agreement to within their combined standard uncertainties as assigned to them by NIST. The wavelength region of the spectral irradiance comparison was from 210 nm to 300 nm. The spectral irradiance sources were: an electron storage ring, 1000 W quartz-halogen lamps, deuterium arc lamps, and a windowless argon miniarc.

Design, manufacture, and calibration of infrared radiometric blackbody sources

A radiometric calibration station (RCS) is being assembled at the Los Alamos National Laboratory (LANL) which will allow for calibration of sensors with detector arrays having spectral capability from about 0.4-15 micrometers. The configuration of the LANL RCS is shown. Two blackbody sources have been designed to cover the spectral range from about 3-15 micrometers, operating at temperatures ranging from about 180-350 K within a vacuum environment. The sources are designed to present a uniform spectral radiance over a large area to the sensor unit under test. THe thermal uniformity requirement of the blackbody cavities has been one of the key factors of the design, requiring less than 50 mK variation over the entire blackbody surface to attain effective emissivity values of about 0.999. Once the two units are built and verified to the level of about 100 mK at LANL, they will be sent to the National Institute of Standards and Technology (NIST), where at least a factor of two improvements will be calibrated into the blackbody control system. The physical size of these assemblies will require modifications of the existing NIST Low Background Infrared (LBIR) Facility. LANL has constructed a bolt-on addition to the LBIR facility that will allow calibration of our large aperture sources. Methodology for attaining the two blackbody sources at calibration levels of performance equivalent to present state of the art will be explained in the paper.

A method of realizing spectral-radiance and irradiance scales based on comparison of synchrotron and high-temperature black-body radiation

Improvements in synchrotron radiation and high-temperature black-body sources will provide a method of realizing scales of spectral radiance and irradiance with uncertainties comparable with those provided by absolute cryogenic radiometers. This paper discusses planned improvements in the synchrotron radiation facility at the National Institute of Standards and Technology (NIST), and how these improvements can be exploited to accurately measure the temperature of a high-temperature black body developed at the All-Russian Research Institute for Optophysical Measurements (VNIIOFI), thus allowing substantial improvement in radiometric scales.

Direct comparison of copper and gold freezing-point black-body primary standards from the NRLM and the NIST

Freezing-point black bodies used as primary standards of spectral radiance at two national standards laboratories are directly intercompared for the first time. The spectral radiance ratio is directly measured for a copper freezing-point black body from the National Research Laboratory of Metrology (NRLM), Japan, and a gold freezing-point black body from the National Institute of Standards and Technology (NIST), USA. Narrowband spectral radiance measurements at five wavelengths between 546 nm and 878 nm show the NRLM copper to NIST gold freezing-point temperature interval to be 20,39 K ± 0,12 K (2σ), in good agreement with the International Temperature Scale of 1990 (ITS-90) value of 20,44 K, and comfortably within the combined uncertainties assigned to the realization of the ITS-90 from each source.

Visible/infrared radiometric calibration station

Los Alamos National Laboratories has begun construction of a visible/infrared radiometric calibration station that will allow for absolute calibration of optical and IR remote sensing instruments with clear apertures less than 16 inches in diameter in a vacuum environment. The calibration station broadband sources will be calibrated at the National Institute of Standards and Technology (NIST) and allow for traceable absolute radiometric calibration to within +/- 3% in the visible and near IR (0.4-2.5 micrometers ), and less than +/- 1% in the infrared, up to 12 micrometers . Capabilities for placing diffraction limited images of for sensor full-field flooding will exist. The facility will also include the calibration of polarization and spectra effects, spatial resolution, field of view performance, and wavefront characterization. The configuration of the vacuum calibration station consists of an off-axis 21 inch, f/3.2, parabolic collimator with a scanning fold flat in collimated space. The sources are placed, via mechanisms to be described, at the focal plane of the off-axis parabola. Vacuum system pressure will be in the 10-6 Torr range. The broadband white-light source is a custom design by LANL with guidance from Labsphere Inc. The continuous operating radiance of the integrating sphere will be from 0.0-0.006 W/cm2/Sr/micrometers (upper level quoted for approximately 500 nm wavelength). The blackbody source is also custom designed at LANL with guidance from NIST. The blackbody temperature will be controllable between 250-350 degree(s)K. Both of the above sources have 4.1 inch apertures with estimated radiometric instability at less than 1%. The designs of each of these units will be described. The monochromator and interferometer light sources are outside the vacuum, but all optical relay and beam shaping optics are enclosed within the vacuum calibration station. These sources are to be described, as well as the methodology for alignment and characterization.

SURF III: The Next Generation Radiometric Storage Ring Facility at NIST

The National Institute of Standards and Technology in Gaithersburg, MD is upgrading the Synchrotron Ultraviolet Radiation Facility (SURF II) electron storage ring to serve as an absolute radiometric standard of unprecedented accuracy spanning the soft x-ray through infrared spectral regions. Major upgrades in the storage ring magnet system will substantially improve the uniformity of the magnetic field accelerating the stored electrons, and permit the new facility (SURF III) to operate at higher electron energies, extending the useful spectral range to at least 600 eV (about 2 nm). Parallel improvements in radiometric measurement of stored electron current will reduce the absolute spectral irradiance uncertainty in SURF III to about 0.5% or less from the infrared through far ultraviolet spectral range. A recently completed cryogenic electrical substitution radiometer will be used for high accuracy radiometry (0.1% order absolute uncertainty) exploiting SURF as a bright ultraviolet source. These developments will reduce the uncertainty in SURF-based ultraviolet radiometry by a factor of two to five, depending on wavelength and application. We will review the current states of ultraviolet radiometry at SURF II, discuss the SURF III storage ring upgrade, and outline the new radiometry programs planned for SURF III.

Configuration-dependent ac Stark shifts in calcium.

The energy shifts caused by intense laser radiation (108 to 1010 W/cm2) are investigated for several high-lying levels of calcium in order to determine the ac Stark shift of a loosely bound electron in a strong electromagnetic1field. Energy shifts are measured for the closely spaced levels 4s9s1S0, 4s8d1D2, 4s10s1S0, and 3d5s1D2 by using resonance ionization spectroscopy. The levels associated with Rydberg configurations show the expected ponderomotive energy shifts, but the 3d5s1D2 shift is approximately half as large as the ponderomotive shifts.