Improvements in the design of thermal-infrared radiation thermometers and sensors.

A new design for thermal-infrared radiation thermometer and sensors is described. Critical optical elements, such as the field stop, Lyot stop, collimating lens, and detector, are placed inside a thermally stabilized assembly that is controlled using thermo-electric coolers and thermistors. The assembled radiation thermometer is calibrated using both variable-temperature fluid-bath and heat-pipe blackbodies from -45 °C to 75 °C and the use of a modified-Planck function and these blackbodies. The size-of-source effect both with and without the Lyot stop has been measured. This new design, during operations without the need for cryogenic cooling, demonstrates sub millikelvin temperature measurement resolution with few millikelvin, week-long stable operations while measuring room-temperature objects.

Calibration of night vision goggles: an SI-units-based gain measurement technique.

A gain measurement technique for the calibration of night vision goggles (NVG) is proposed and evaluated. This technique is based on the radiance measurements at the input and output of the NVG. In contrast to the old definition, which uses a non-International System of Units (SI) traceable luminance, the "equivalent luminance unit," the suggested technique utilizes the radiance quantities that are traceable to the SI units through National Institute of Standards and Technology (NIST) standards. Due to the implementation of the scaling coefficients originating from the NVG spectral responsivities, the same NVG gain is expected within both techniques. The suggested method was evaluated at the NIST night vision calibration facility and the experimental data were compared to the results obtained with a commercial NVG test set. The comparison of the radiometric quantities obtained using the two different methods indicated differences up to 15% due to different calibration conditions. However, at proper calibration, equal NVG gains within both the suggested and old gain definitions were measured for the goggles equipped with a filmless image tube. The NVG gain uncertainty analysis, including the effect of no-moon night sky radiation, was performed for goggle types A, B, and C.

Infrared spectral responsivity scale realization and validations.

An InSb working standard radiometer, first calibrated at the National Institute of Standards and Technology (NIST) in 1999 against a cryogenic bolometer, was recently calibrated against a newly developed low-noise-equivalent-power pyroelectric transfer standard detector. The pyroelectric transfer standard, which can operate at the output of a monochromator, holds the newly realized NIST spectral power responsivity scale between 1.7 and 14 µm with an uncertainty of 1% (k=2). The InSb working standard was also measured at the National Physical Laboratory (NPL) of the United Kingdom in 1999. The less than 2% spectral power responsivity disagreements obtained on the InSb working standard (both from the 1999 NIST and NPL comparison and also against the pyroelectric standard) validate the three independently realized power responsivity scales and verify the long-term stability of the InSb working standard. The InSb working standard was also used in irradiance measurement mode to validate the previously determined spectral irradiance responsivity of four narrowband InSb radiometers that were applied to calibrate IR target simulators. The uncertainty of the present spectral irradiance responsivity scale held by the InSb working standard is 2.5% (k=2) in the 2 to 5.2 µm wavelength range.

Standardization of Broadband UV Measurements for 365 nm LED Sources.

Broadband UV measurements are evaluated when UV-A irradiance meters measure optical radiation from 365 nm UV sources. The CIE standardized rectangular-shape UV-A function can be realized only with large spectral mismatch errors. The spectral power-distribution of the 365 nm excitation source is not standardized. Accordingly, the readings made with different types of UV meters, even if they measure the same UV source, can be very different. Available UV detectors and UV meters were measured and evaluated for spectral responsivity. The spectral product of the source-distribution and the meter's spectral-responsivity were calculated for different combinations to estimate broad-band signal-measurement errors. Standardization of both the UV source-distribution and the meter spectral-responsivity is recommended here to perform uniform broad-band measurements with low uncertainty. It is shown what spectral responsivity function(s) is needed for new and existing UV irradiance meters to perform low-uncertainty broadband 365 nm measurements.

Matrix-based color measurement corrections of tristimulus colorimeters.

For colorimetric imaging the tristimulus technique is still the best practical method to keep the measurement time within reasonable limits. However, the achievable color measurement uncertainties for special sources can be large. It is described how the systematic errors can be significantly reduced by using matrix-based color corrections and how the matrix elements can be optimized to obtain the smallest spectral mismatch errors for different light-source distributions. An approach for decreasing the systematic errors is to increase the number of the colorimeter channels (or filters) used for a measurement. Using five channels in a colorimeter is an optimum choice. Determination of the optimum matrices for the five channels is discussed. The correction matrices are designed such that the spectral mismatch errors of the realized functions are minimized relative to the CIE standard color matching functions for several selected test-source distributions. The optimum matrix depends on the (test) light source to be measured. Adaptive matrix values are determined by using the channel outputs and the spectral power distribution of color LEDs approximated with a simple approximation function. The systematic errors are evaluated for a number of colored and white LEDs. The noise propagation with the applied matrix corrections is investigated.

Thermodynamic-temperature determinations of the Ag and Au freezing temperatures using a detector-based radiation thermometer.

The development of a radiation thermometer calibrated for spectral radiance responsivity using cryogenic, electrical-substitution radiometry to determine the thermodynamic temperatures of the Ag- and Au-freezing temperatures is described. The absolute spectral radiance responsivity of the radiation thermometer is measured in the NIST Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) facility with a total uncertainty of 0.15% (k=2) and is traceable to the electrical watt, and thus the thermodynamic temperature of any blackbody can be determined by using Planck radiation law and the measured optical power. The thermodynamic temperatures of the Ag- and Au-freezing temperatures are determined to be 1234.956 K (+/-0.110 K) (k=2) and 1337.344 K(+/-0.129 K) (k=2) differing from the International Temperature Scale of 1990 (ITS-90) assignments by 26 mK and 14 mK, respectively, within the stated uncertainties. The temperatures were systematically corrected for the size- of-source effect, the nonlinearity of the preamplifier and the emissivity of the blackbody. The ultimate goal of these thermodynamic temperature measurements is to disseminate temperature scales with lower uncertainties than those of the ITS-90. These results indicate that direct disseminations of thermodynamic temperature scales are possible.

Facility for spectral irradiance and radiance responsivity calibrations using uniform sources.

Detectors have historically been calibrated for spectral power responsivity at the National Institute of Standards and Technology by 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, uncertainties increase dramatically when measuring an instrument's spectral irradiance or radiance responsivity. We describe what we believe to be a new laser-based facility for spectral irradiance and radiance responsivity calibrations using uniform sources (SIRCUS) that was developed to calibrate instruments directly in irradiance or radiance mode with uncertainties approaching or exceeding those available for spectral power responsivity calibrations. In SIRCUS, the emission from high-power, tunable lasers is introduced into an integrating sphere using optical fibers, producing uniform, quasi-Lambertian, high-radiant-flux sources. Reference standard irradiance detectors, calibrated directly against national primary standards for spectral power responsivity and aperture area measurement, are used to determine the irradiance at a reference plane. Knowing the measurement geometry, the source radiance can be readily determined as well. The radiometric properties of the SIRCUS source coupled with state-of-the-art transfer standard radiometers whose responses are directly traceable to primary national radiometric scales result in typical combined standard uncertainties in irradiance and radiance responsivity calibrations of less than 0.1%. The details of the facility and its effect on primary national radiometric scales are discussed.

Design and characterization of a photometer-colorimeter standard.

A photometer and tristimulus colorimeter has been developed at the National Institute of Standards and Technology (NIST) to realize a color scale. A novel construction was developed to implement the spectral-responsivity-based scale with small uncertainty. The new device can be used as a reference illuminance and luminance meter as well. Temperature-controlled filter combinations, with 5-8 layers in one package, are used to match the responsivity of a silicon tunnel-trap detector to the CIE color-matching functions with small spectral mismatch values (f1'). Design considerations to extend the tunnel-trap detector with replaceable single and double apertures and changeable filter combinations are described. The design and fabrication of the filter packages and the dependence of the f1' values on the thickness of the filter layers are discussed. The colorimeter was characterized for angular, spatial, and spectral responsivity. An improved preamplifier can convert current to voltage in an 11-decade dynamic range with 0.01% uncertainty.

Development of a Tunable LED-Based Colorimetric Source.

A novel, spectrally tunable light-source utilizing light emitting diodes (LEDs) for radiometric, photometric, and colorimetric applications is described. The tunable source can simulate standard sources and can be used as a transfer source to propagate photometric and colorimetric scales from calibrated reference instruments to test artifacts with minimal increase in uncertainty. In this prototype source, 40 LEDs with 10 different spectral distributions were mounted onto an integrating sphere. A voltage-to-current control circuit was designed and implemented, enabling independent control of the current sent to each set of four LEDs. The LEDs have been characterized for stability and dependence on drive current. The prototype source demonstrates the feasibility of development of a spectrally tunable LED source using LEDs with up to 40 different spectral distributions. Simulations demonstrate that such a source would be able to approximate standard light-source distributions over the visible spectral range-from 380 nm to 780 nm-with deviations on the order of 2 %. The tunable LED source can also simulate spectral distributions of special sources such as discharge lamps and display monitors. With this tunable source, a test instrument can be rapidly calibrated against a variety of different source distributions tailored to the anticipated uses of the artifact. Target uncertainties for the calibration of test artifacts are less than 2 % in luminance and 0.002 in chromaticity for any source distribution.