Raju V. Datla

Absolute detector quantum-efficiency measurements using correlated photons

An experimental system using correlated photons for radiometric purposes has been set up at the National Institute of Standards and Technology (NIST). We use pairs of correlated photons to produce spatial maps of the absolute efficiency of a photomultiplier photocathode at four wavelengths. We also compare this technique with measurements carried out by conventional means tied to existing radiometric standards. These initial comparisons show an average agreement within ~0,6% between the two methods. The results show that correlated photons can be a useful tool in radiometry and that the method holds enough promise to warrant further studies.

Measuring absolute infrared spectral radiance with correlated visible photons: technique verification and measurement uncertainty

An experimental system in which correlated photons for radiometric measurements were used has been set up at the National Institute of Standards and Technology. We use visible-IR pairs of correlated photons produced by means of optical parametric downconversion to measure the radiance of a high-temperature IR source at 3.415 and 4.772 mum in an intrinsically absolute manner (i.e., without requiring any externally calibrated radiometric standard). To our knowledge, this is the only radiometric method with which one measures radiance directly, instead of using radiant power and aperture geometry measurements to deduce radiance indirectly. This technique has an additional unusual characteristic: It allows absolute radiometric measurements of IR radiation to be made with high-quality visible detectors. We compare measurements made with this technique with radiance measurements made with conventional means tied to existing radiometric standards. These comparisons show an average agreement to within ~3% between the two methods. The results demonstrate an accuracy consistent with the estimated uncertainty of the currentmeasurements. This is the first time to our knowledge that this method has been used to provide absolute radiance measurements of a source that has been calibrated conventionally, revealing unexpected systematic effects and allowing estimates of the ultimate accuracy of this method. In addition, these measurements are further into the IR than any previous measurements of this process and have produced the highest thermally stimulated downconversion signal yet seen.

Characterization of an absolute cryogenic radiometer as a standard detector for radiant-power measurements

An active cavity radiometer of the electrical substitution type with a cone receiver that operates at 2-4 K has been developed for measuring radiant fluxes in the dynamic range of 20 nW to 100 microW within an uncertainty of +/-1% (2sigmalevel). It is a broadband absolute detector with a flat overall absorption efficiency that is >99% for radiation from the visible to long-wavelength IR. The system is designed based on thermal modeling and experimental measurements of concepts. It has been installed in the cryogenic chamber for low-background infrared radiation calibrations at the National Institute of Standards and Technology (NIST) for testing cryogenic blackbody sources, detectors, and optical components. Its time constant, responsivity, and nonequivalence error have been measured. They are in agreement with design predictions. Radiant power measurements of an amplitude-stabilized He-Ne laser beam with the radiometer and an industry standard photodiode detector, QED-200, have been intercompared and found to be in agreement. The intercomparison ratio of the measurements with the absolute cryogenic radiometer and QED-200 was 1.004 in the 75-100-microW range with an uncertainty of 0.5% (the 3sigma level).

Transmittance measurements for filters of optical density between one and ten

We have developed a facility for measuring the transmittance of optical filters at a wavelength of 1064 nm, using a Nd:YAG laser, a power stabilizer, and linear photodiode detectors. A direct measurement method was used for filters with optical densities (ODs) less than or equal to 4, and a reference substitution technique was used for filters with ODs as great as 10. The apparatus and data-acquisition system are described. Measurement results for a set of filters are presented. The expanded uncertainties for the measured OD and deduced absorption coefficient are determined through a detailed analysis of all the uncertainty components.

Thermal Modeling of Absolute Cryogenic Radiometers

This work consists of a detailed thermal modeling of two different radiometers operated at cryogenic temperatures. Both employ a temperature sensor and an electrical-substitution technique to determine the absolute radiant power entering the aperture of a receiver. Their sensing elements are different: One is a germanium resistance thermometer, and the other is a superconducting kinetic-inductance thermometer. The finite element method is used to predict the transient and steady-state temperature distribution in the receiver. The nonequivalence between the radiant power and the electrical power due to the temperature gradient in the receiver is shown to be small and is minimzed by placing the thermometer near the thermal impedance

Erratum: Statistical Interpretation of Key Comparison Reference Value and Degrees of Equivalence

Key comparisons carried out by the Consultative Committees (CCs) of the International Committee of Weights and Measures (CIPM) or the Bureau International des Poids et Mesures (BIPM) are referred to as CIPM key comparisons. The outputs of a statistical analysis of the data from a CIPM key comparison are the key comparison reference value, the degrees of equivalence, and their associated uncertainties. The BIPM publications do not discuss statistical interpretation of these outputs. We discuss their interpretation under the following three statistical models: nonexistent laboratory-effects model, random laboratory-effects model, and systematic laboratory-effects model.

Review Article: Uncertainty analysis of remote sensing optical sensor data: guiding principles to achieve metrological consistency

Climate change monitoring requires decades-long time-series radiometric measurements using multiple optical sensors in multiple platforms covering the globe. The problem of achieving traceability to SI units for these measurements is discussed. A major challenge is to determine the result of a measurement and its associated uncertainty using various calibration and validation processes. These processes are plagued by systematic (non-statistical) uncertainties that are not well understood. In particular, different, but in principle equivalent, SI traceable measurements may differ by more than would be expected from the uncertainties associated with the individual measurements. We propose a methodology based on the International Organization for Standardization (ISO) Guide to the Expression of Uncertainty in Measurement (GUM) for the analysis of uncertainties in such measurements along with consistency checking. This allows the measurement result and its associated uncertainty to evolve as new knowledge is gained from additional experiments, and it promotes greater caution in drawing conclusions in view of the sparse measurements. We use data from ongoing total solar irradiance measurements from various instruments in orbit to illustrate the principles.

An apparatus for infrared transmittance and reflectance measurements at cryogenic temperatures

A facility for measuring the optical properties of solid materials at cryogenic temperatures is being developed at the National Institute of Standards and Technology. A cryostat that houses four ur bolometric detectors and a six-position sample holder was designed and built. The bolometers operate near 5 K, and the sample temperature can he varied from 6 to 100 K. The beam from a Fourier transform spectrometer is directed to the cryostat by reflective optical components. The measurable wavelengths extend from 1μm to 1 mm, with appropriate sources and beamsplitters in the spectrometer as well as windows and detectors in the cryostat. The angle of incidence on the sample ranges from 7.5 to 60. The mechanical electrical, and optical designs are described in this paper. Initial measurement results at wavelengths from 2 to 30μm and a sample temperature of 10 K are presented.

High-optical-density out-of-band spectral transmittance measurements of bandpass filters

We have measured the out-of-band transmittance (attenuation) for several narrow-band filters using two Fourier-transform infrared spectrometers in the wavelength range of 2–25 μm. Band-rejection filters are used to eliminate the power transmitted in the main band of the filter under investigation. Neutral-density filters are used for the reference measurement to reduce the effect of nonlinearity. Results from different optical arrangements are compared and discussed. This study demonstrates the feasibility of using Fourier-transform infrared instruments to measure a spectral transmittance of 10−6.

Improved broadband blackbody calibrations at NIST for low-background infrared applications

The low-background infrared (LBIR) facility at the National Institute of Standards and Technology (NIST) has continued to develop its facilities and knowledge base to meet the needs of the infrared community. Improvements in refrigeration capability at the LBIR facility have made it possible to perform calibrations of infrared sources and detectors in a stable 17 K background environment as compared to a relatively unstable 25 K environment available until about two years ago. This, combined with improved power measurement instrumentation, allows measurements of 1 nW with a standard uncertainty of 1% due to repeatability and reproducibility effects. A brief overview will be given of the changes to the LBIR facility that led to these improvements. The higher sensitivity in power measurement capability and some of the methods being used to generate low-power beams have highlighted new measurement issues that had previously been relatively unimportant. These issues include aperture quality, background scene temperature stability, beam shuttering, diffraction, and the noise floor of power measurement hardware. Demonstrations of common problems encountered will be shown and guidelines will be given for developing infrared sources that not only meet the needs of the user but can also be well calibrated.