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Optical Technology High-Accuracy Cryogenic Radiometry Spectral Radiometry of Sources Low Background Infrared Metrology Infrared Optical Properties of Materials Non-Linear Optical Measurements Optical Scattering from Surfaces Near-Field Scanning Optical Microscopy Time-Resolved Infrared Spectroscopy Infrared Spectroscopic Chemical Imaging High-Resolution Ultraviolet Spectroscopy |
Division Contact: Gerald T. Fraser The interaction of an object with incident light determines how a human observer perceives that object. The reflected light from an object is generally categorized into spectral (color) and spatial (appearance) properties. We are currently developing measurement facilities to determine the most important color and appearance properties of objects. The color of an object perceived by a human observer is the visual sensation produced by light reflected by the object with wavelengths from 380 nanometers to 780 nanometers. As part of our project on measurement science for optical reflectance and scattering, we are establishing a program to provide calibrated reflectance color standards with a target ΔEab < 0.5 for all colors, where ΔEab represents the magnitude of the color difference in the CIELab color scale. A reference instrument for reflectance color measurements with this target as the primary specification is being assembled. Currently, agreement between commercial instruments used to measure the color of an object falls short of the requirements of industry. Standards calibrated by the reference instrument will help to define and improve the uncertainties of commercial instruments, thereby improving the agreement between them. Specular gloss is the perception of the mirror-like appearance of a surface. For many products, their specular gloss is the most important attribute for their final acceptance. Recently, we have completed development of a new reference goniophotometer for measuring specular gloss at the 20º, 60º, and 85º geometries, as defined by ASTM and ISO documentary standards, and are using a new primary standard. In addition, this instrument is capable of performing bi-directional luminous reflectance and transmittance measurements at angles from 0º to 85º in compliance with ASTM recommendations. Contact: Maria
Nadal Spectrophotometry is concerned with the reflectance and transmittance of light by materials, which provides information on their properties in the intended application or on their composition. We have reference instruments for both reflectance and transmittance, which serve to maintain and disseminate the national scales for these measurements. Both operate over a wavelength range from the ultraviolet to near-infrared (200 nanometers to 2500 nanometers). Reflectance measurements can be performed on neutral, non-fluorescent materials in both bi-direction and directional-hemispherical geometries, while transmittance measurements can be performed on non-fluorescent materials at normal incidence. In addition to the reference instruments, we also have commercial instruments for performing measurements with larger uncertainties but shorter measurement times. These instruments can measure both regular transmittance and directional-hemispherical reflectance. Contact: David W. Allen Photometry is the science of radiometric measurement of the response function of human vision. The candela, one of the seven base units of the International System of Units, is the basic unit of photometry. We have established a new detector-based candela that has improved the accuracy of these measurements by a factor of two. This improvement has become the basis for improved accuracy of all other photometric measurements offered by the division. Improved levels of accuracies are needed in industry to ensure the production of higher quality products so that U.S. companies can compete in the growing international market. Accurate light measurements are essential to the production of various lighting products such as light bulbs, discharge lamps, lighting fixtures, automobile headlights, and aircraft lamps. Accurate light measurements also are essential in the production of information displays such as cathode ray tubes, flat-panel displays, light-emitting diodes, and various other optical components. Another unit in photometry is the lumen, which is the measure of the total light output of lamps. The lumen is especially important to the lighting industry where millions of lamps are produced every week. We recently established a new lumen unit based on the detector-based candela that provides the highest accuracy levels of lumen measurements. Contact: Yoshi
Ohno Primary Optical Watt Radiometer We base many of our radiometric measurement scales, such as detector spectral responsivity, photometry, radiance, and irradiance, on a high-accuracy cryogenic radiometer (HACR). Cryogenic radiometers work on the principle of electrical substitution, where the temperature rise in a receiving cavity caused by optical heating is reproduced by electrical heating. Electrical substitution measurements tie the optical watt to electrical standards. The HACR measures the optical power of single laser lines with an uncertainty of 0.02 percent or better and calibrates transfer detectors to disseminate a spectral responsivity scale. We continue to improve the HACR base measurements and the optical transfer devices in different wavelength regions. Optical detector calibrations from the ultraviolet through the infrared are based on HACR measurements and are used in environmental, industrial, defense, and space applications. Cryogenic radiometry reduces the base uncertainty in the optical calibrations transferred to these customers. Contact: Joe Rice Spectral Radiometry of Sources Accurate calibrations of measuring instruments are necessary to monitor man-made changes in the environment, for instance, in determining the terrestrial solar spectral irradiance. Calibrated sources also are used in many different industrial and scientific applications. The current spectral radiance and spectral irradiance scales are derived from the absolute radiometric temperature determination of the freezing-temperature of gold. Work is in progress to base the two scales on detectors calibrated for absolute spectral responsivity traceable to the cyrogenic radiometer. The resulting uncertainties in spectral irradiance are expected to be a factor of 2 to 5 lower than the uncertainties based on the current scale. The scales will also be extended to 2,500 nanometers with the new scale realization. An effort is also under way to find transfer sources with better long-term stability than the currently issued standards. A new facility for spectral irradiance calibrations is under construction. The new facility will be capable of fully automated spectral irradiance calibrations from 250 nanometers to 2,500 nanometers needed to meet the increasing demand for calibrations. Contact: Howard
W. Yoon Our research and development programs span many activities associated with the measurement of optical radiation, covering the spectrum from 200 nanometers in the ultraviolet to the far infrared. Included are spectral radiance and irradiance measurements for many varied applications, such as manufacturing process control, remote sensing of the Earth's environment, and defense needs. We take demanding problems, such as the spectrophotometric measurement of dense optical media, and develop the detector metrology to perform the measurements and relate them to the stable, U.S. radiometric measurement base. Emphasis is placed on solid-state photodiode metrology and its application to all areas of radiometry, especially calibration services. We have several well-equipped laboratories for optical measurements in the ultraviolet, visible, and infrared spectral regions available for use, and we are developing new facilities to enable scientists and engineers to conduct research on detector improvements, detector applications, and optical properties of materials. One of these new facilities is the Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (SIRCUS) laboratory. This facility creates intense, uniform radiation fields from the UV to the IR for radiometric calibrations of radiance meters, irradiance meters, CCD cameras and other imaging systems. Powerful tunable lasers that are coupled to integrating spheres generate these radiation fields. Contact: Steve Brown Low Background Infrared Metrology The project team serves the national needs for low background infrared (LBIR) measurements. An absolute cryogenic radiometer (ACR) has been built and characterized as a national standard for LBIR measurements. The NIST LBIR calibration facility, which houses the ACR in a cryogenic chamber, has been serving customers since 1989, providing calibrations for radiance temperature measurements of blackbodies. Spectral capability to provide calibrations of cryogenic detectors also has been added to this facility. An active program to build new transfer standards radiometers called BXR-1 and BXR-2 is under way. These radiometers will be used for the calibration of test chambers in various missile sensor test facilities around the country serving the needs of the Ballistic Missile Defense Organization. Radiometers using high-Tc superconductor material properties are being developed for use in the calibration program. Contact: Raju
Datla Infrared Optical Properties of Materials This project is designed to address the need for spectral infrared optical properties of materials in a comprehensive way. We have established a Fourier Transform (FTIR) Spectrophotometry Laboratory that serves as the measurement facility for transmittance and reflectance measurements in the infrared spectral range of 1 micrometer to 100 micrometers, with particular emphasis on the 2-micrometer to 20-micrometer region. The facility consists of several commercial FTIR instruments coupled to a number of custom specialized accessories for handling a wide variety of sample types and controlling the three critical parameters of measurement: beam geometry (both illumination and viewing), beam polarization, and sample temperature. Additional facilities that support the optical properties effort include FTIR-microscope and CO2-laser-based infrared bidirectional reflectance distribution function laboratories. Standard Reference Materials (SRMs) developed in this facility, both currently available and under development, include a wavelength/wavenumber, neutral density transmittance, diffuse reflectance (high and low), and specular reflectance (high) SRM. Measurement services for specular and diffuse, reflectance, and transmittance are available. Input to this and related NIST programs is provided by the NIST-Industry Optical Properties of Materials Consortium. Contact: Leonard
Hanssen Non-Linear Optical Measurements We use non-linear optical methods to measure properties of interfaces that may be found at the surfaces of materials, in thin-film systems, or buried in layered materials. An example is the technique of sum frequency generation (SFG) in which two laser pulses at different frequencies combine to produce light at their sum frequency with an efficiency that depends on the broken symmetry at the interface. With femtosecond laser pulses, SFG provides a time-resolved optical diagnostic uniquely sensitive to interface structure. Measurements include spectroscopic characterization of electronic structure at buried epitaxial interfaces, ultrafast monitoring of carrier dynamics at semiconductor interfaces, assessment of the structure and quality of thin films, and vibrationally resonant SFG of organic films such as self-assembled monolayers. Resources include femtosecond laser sources for generating ultrafast pulses in the infrared through ultraviolet and instrumentation for spectral, directional, and polarization analyses of surface-generated optical signals. Contact: Kim Briggman Optical Scattering from Surfaces We are developing polarized light scattering methods for improving the optical characterization of defects in materials and for characterizing stray light in optical systems. Applications include the characterization of light scattering by particles, subsurface defects, and roughness on or near silicon wafers, magnetic media, optical mirrors, gratings, dielectric layers, paint coatings, and patterned devices. We make measurements on customer-supplied materials and on model systems we develop to establish the validity of theoretical models and the limits of application of those models. Recent experimental results have verified theoretical predictions that the polarization of light scattered into non-specular directions can indicate the source of light scattering or quantify two sources of scatter. These results have improved defect sensitivity and classification in a number of manufacturing applications. Resources include a goniometric optical scatter instrument with ultraviolet and visible laser light sources for measurement of bidirectional reflectance distribution function with polarization analysis and out-of-plane scattering capabilities; a scanning multidetector polarized light scattering instrument; expertise in first principles modeling of light-scattering materials and surfaces; and clean room facilities for handling optical samples. Contact: Thomas
Germer Near-Field Scanning Optical Microscopy We are using near-field scanning optical microscopy (NSOM) as a quantitative technique for non-invasive optical measurements on previously inaccessible length scales. So far, we have achieved lateral resolution on the order of 20 nanometers with this technique, and vertical resolution of less than 1 nanometer may be possible. We are building well-characterized microscopes and small light sources and are working on methods to determine the resolution of commercially available near-field microscopes. At a fundamental level, this means understanding the mechanisms that generate contrast in different materials and modeling the fields around small light sources as they interact with various materials and surface features. We collaborate with other NIST researchers interested in applying near-field microscopy to problems in chemical, optical, and semiconductor technology. Applications include mapping of optical properties of nanostructured materials, investigations of structure in biological membranes, and near-field measurements of photonic structures. Resources include NSOM measurements in the visible spectrum, NSOM tip characterization, theoretical modeling of probe-surface interactions and optical contrast mechanisms, and access to complementary scanning microscopies such as scanning electron microscopy and atomic force microscopy. Contact: Lori
Goldner Time-Resolved Infrared Spectroscopy We use ultrashort laser pulses to observe fast processes that occur in the condensed phase. We have developed unique femtosecond infrared spectroscopic techniques to study highly excited vibrational states, vibrational energy transfer, electron dynamics in solar cells, photochemical reactions, and the formation and rupture of hydrogen bonds. The measurement techniques help identify transient species and determine energy transfer rates, which serve to improve models of condensed phase chemistry. Our current collaborations with industry include measurements on catalytic systems and polymerization reactions. We also are developing sources of femtosecond pulses in the terahertz frequency region to be used to directly probe solid-state electron-phonon dynamics and liquid-phase hydrogen-bonding dynamics in model DNA and protein systems. Resources include femtosecond laser sources generating ultrafast pulses in the far infrared through ultraviolet, infrared and visible multielement detectors, and instrumentation for capturing transient spectra of samples with single laser pulses. Contact: Edwin
Heilweil Infrared Spectroscopic Chemical Imaging Many industrial quality control, forensic, and medical diagnostic labs have shown keen interest in developing rapid chemically sensitive imaging capabilities. A multichannel parallel imaging approach of using a step-scan FTIR instrument coupled to an infrared microscope and two-dimensional infrared array camera also can enable spatial determination and characterization of combinatorial library arrays of potentially new catalysts, materials, and inorganic species. In this vein, we are developing infrared spectroscopic imaging tools, data analysis methods, and standards for reliably capturing and identifying molecular components of spatially complex samples such as mixed polymers, diseased tissues, and supported catalysts. Contact: Edwin
Heilweil High-Resolution Ultraviolet Spectroscopy We use a high-resolution ultraviolet-laser molecular-beam spectrometer to measure the electronic spectra of organic and model biomolecules at a precision of approximately 2 parts in 1010 at 300 nanometers. We obtain information about electronic and vibrational energy flow, intermolecular and intramolecular forces, and isomerization processes to validate and guide the development of improved theories for electronic and vibrational relaxation. Additionally, our measurements provide critical tests of computational chemistry tools, such as molecular mechanics force fields and ab initio electronic structure theory, used in the design and development of new chemicals, pharmaceuticals, and materials. The spectrometer has capabilities for microwave ultraviolet double resonance, electrostatic field state focusing, and bolometric detection to aid the spectral identification and to provide additional diagnostics information. Efforts are also under way to use the unique laser source as part of a new sensitive optical diagnostics tool for characterizing the chiral purity of samples with potential applications to the chiral drug and chemical industry. Contact: David Plusquellic Continuous-Wave Terahertz Spectroscopy We are developing continuous-wave terahertz spectroscopy methods for plasma chemical diagnostics and for the investigation of the torsional force fields responsible for the flexibility of protein, polynucleotide, and polysaccharide backbones. Two terahertz development efforts are in progress, one based on compact solid-state photomixers and the other based on the distinctly different backward-wave oscillators (BWO). The photomixer system consists of a low-temperature-grown gallium arsenide photomixer, which is illuminated by two near-infrared lasers. The two lasers consist of a fixed-frequency near-infrared diode laser and a tunable titanium-sapphire laser. The near-infrared radiation is carried to the small mixer through a fiber optic cable, allowing the terahertz radiation to be generated in laboratories far removed from the near-infrared laser system, such as the one containing the plasma processing station. Terahertz power at the difference frequency between the two lasers is radiated from the photomixer. BWOs are electronic tubes that provide milliwatts of power, continuously tunable up to 1.1 terahertz. The BWOs are frequency stabilized to better than 100 kilohertz by offset locking to a low-power high-frequency-harmonic of a microwave frequency synthesizer, typically operating below approximately 100 gigahertz. With frequency doublers, output to two terahertz is possible. For both terahertz sources, the radiation is detected with a liquid helium cooled hot electron bolometer. Initial studies are concentrating on model biomolecules in vacuum, cryogenic matrices, and solids. Contact: David Plusquellic
Date
created:
August 17, 2001 |