TECHNICAL ACTIVITIES 1998 - NISTIR 6268

CURRENT DIRECTIONS

• Calibration Services. The Division provides calibration services and measurements in the areas of 1) radiance temperature, 2) spectroradiometric sources, 3) optical properties of materials, 4) photometry, and 5) spectroradiometric detectors. During the past year, the calibration staff worked very hard to reduce the time in progress from 180 days to 90 days in response to a request from customers for a more rapid turn-around in calibration services. The Division has completed an ambitious program to implement the International Quality Standard for Calibration Laboratories, the ISO Guide 25, in all of its calibration services during the past several years and has developed a protocol for an annual assessment of its quality system.

• Cryogenic Radiometry. The Division maintains an absolute High Accuracy Cryogenic Radiometer (HACR) with a combined relative standard uncertainty of 0.02% as the foundation for a radiometric measurement chain to maintain scales of spectral radiance and irradiance, photometry, and absolute detector responsively. A second, high-sensitivity cryogenic radiometer is the basis for the Low Background Infrared (LBIR) facility, which provides calibrations, research and development for high-sensitivity infrared sensors. At the Synchrotron Ultraviolet Radiation Facility (SURF), a new monochromator-based cryogenic radiometer has been established as a part of the effort to furnish a spectral radiance power scale based upon SURF's output and to serve as a calibration facility for transfer standard detectors. Development of new radiometers incorporating superconducting technology and high T_c materials is an important component of the cryogenic radiometry program. A second generation HACR is being developed to improve utility and sensitivity.

  Transfer-standard detectors used throughout the Division are calibrated with the cryogenic radiometers. The Division also develops transfer standard detectors to enable the high-accuracy radiometric scales to be propagated to other laboratories. Transfer standards are being developed at near-infrared and ultraviolet wavelengths that will substantially improve the calibration uncertainties in these areas.

• Photometry, Colorimetry, and Appearance. Photometry, the science of measuring light with the response function of an "average" human observer, is an integral part of the detector metrology program. The SI unit of luminous intensity, the candela, is maintained using a set of well-characterized, filtered detectors. This provides a direct link between the HACR and the candela and provides an alternate method, other than conventional lamps, for transferring calibrations of this unit to customers. The Division can offer photometric detector characterization to customers as a more direct and stable calibration procedure than the conventional use of lamps as standards. The Division has developed a total luminous flux scale based upon the new candela which should result in lower costs and better calibrations for the Division's customers.

  The physical measurement of appearance quantifies attributes of an object's interaction with light. Appearance is generally categorized into spectral (color) and spatial (gloss, texture, etc.) properties of reflected light. Physical measurements of source, object, and reflected light are weighted by CIE tristimulus functions or by standard illuminants for the computation of visual appearance and color. A goniophotometer is under development for the measurement of 20° , 60°, and 85° gloss, and research is underway to develop primary gloss standards. A gloss measuring instrument has been established and a haze measuring instrument is under development. In the area of colorimetry, the Spectral Tri-function Automated Reference Reflectometer (STARR) is being used to develop a measurement assurance program with industry standard color tiles and to perform research into the instrument attributes necessary for highly accurate colorimetry for future calibration services. The primary goals of the program are development of reference instruments and standards for current appearance measurement technologies and eventual development of new measurements and standards to more accurately capture visual appearance.

• Spectrophotometry. The Division maintains reference instruments for both spectral reflectance and transmittance, as well as specialized instruments for industrial optical measurement problems. The reference instrument for spectral reflectance produces a variety of measurement standards, while the two reference instruments for regular spectral transmittance continue to be refurbished after moving into a new laboratory. A specialized instrument for visual diffuse transmission density is fully operational, while an instrument for characterizing retroreflection is being designed. In addition to satisfying internal NIST needs, these facilities provide calibrations for a wide range of customers.

• Optical Scattering Metrology. Mechanisms by which material properties and surface topography affect the distribution and polarization of light scattered from surfaces are studied to develop sophisticated measurement methods for use in industry. A hemispherical scanning optical scatter instrument has been designed and constructed to perform as a working prototype of a scanning inspection instrument. The design was based upon knowledge acquired by using a previously constructed goniometric instrument and upon extensive theoretical calculations and simulations. By polarization discrimination, this instrument can be blind to microroughness, thereby increasing its sensitivity to particulate contamination or subsurface defects.

• Near-field Scanning Optical Microscopy (NSOM). NSOM is being developed as a quantitative technique for noninvasive, optical measurements. Its resolution is not limited by the wavelength of light, as in traditional diffraction-limited microscopes, but by the size of the sub-wavelength aperture or tip used as a probe. Well-characterized microscopes and compact light sources have been constructed and methods to determine resolution are being developed. This requires fundamental understanding of contrast mechanisms and modeling the fields around small light sources as they interact with materials and surface features. The Division collaborates with other NIST programs applying near-field microscopy to problems in chemical, biological, optical, and semiconductor technology.

• Nonlinear Spectroscopy at Interfaces. The nonlinear spectroscopy of Sum Frequency Generation (SFG) is uniquely sensitive to molecular structure at interfaces. Our new implementation of SFG relies on femtosecond lasers and nonlinear optics to generate ultrafast, spectrally-broad, IR pulses. These are mixed at the interface of interest with transform-limited picosecond visible pulses so that the entire SFG spectrum in the IR region of interest is produced and recorded on every laser shot, rapidly obtaining vibrationally-resonant, SFG spectra with high resolution and signal-to-noise. Spectroscopic measurement applications include characterization of electronic structure at buried epitaxial interfaces, assessment of the structure and quality of thin films, and vibrationally-resonant SFG of molecules at liquid interfaces, organic films such as self-assembled monolayers, and biological interfaces such as cell membranes or biomimetic membranes in physiological, aqueous solutions.

• Analytic Spectroscopy. Spectroscopic technology is increasingly important for applications in chemical analysis and detection, including atmospheric remote sensing, emissions monitoring, catalysis, industrial process control, forensic science, medical diagnostics, chemical manufacturing, and materials development. The Division has a vertically-integrated research and development effort to support this technology. The effort includes: (1) establishing and disseminating spectroscopic databases to facilitate choices of monitoring frequencies and inversion of measurements to extract concentrations; (2) developing quantum-mechanical Hamiltonians that provide convenient and concise representations of spectroscopic data and their validation; (3) making laboratory spectroscopic measurements to provide accurate frequency and intensity information for instrument calibration; (4) developing of new optical chemical-sensor technology in the microwave, infrared, and visible/UV spectral regions; (5) working with other government agencies to solve novel and important chemical analysis and detection problems; and (6) working with industry to transfer these technologies and to assess needs for new optical chemical analysis technologies, standards, and data.

• Terahertz Spectroscopy. Several new avenues of research are being pursued involving spectroscopy in the far-infrared spectral region and the use of two recently developed, distinctly different, frequency sources. One source is based on the use of an ultrafast photomixer that was developed at MIT's Lincoln Laboratories. It is a solid state device that generates broadly tunable radiation based on the frequency difference of two near infrared lasers. The power output is in the µW regime but it is sufficient for use as a spectroscopic source. The output of the diode lasers that drive this device can be coupled through fiber optics to remote locations. Current work is underway to couple this system to the plasma diagnostics cell in the Atomic Physics Division.

  A second area of research involves collaboration with the staff of the Applied Physics Institute in Nizhny Novgorod, Russia. In this work, backward wave oscillators (BWOs) manufactured in Russia are being used to generate radiation from 100 GHz to 700 GHz. The output power of these devices typically is around 10 milliwatts and is also broadly tunable over ~100 GHz to 200 GHz.

Mission  |  Organization  |  Current Directions  |  Technical Highlights  |  Future Directions

TECHNICAL ACTIVITIES 1998 - Contents

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Online: April 1999