Allan W. Smith

Precise Measurement of Lunar Spectral Irradiance at Visible Wavelengths.

We report a measurement of lunar spectral irradiance with an uncertainty below 1 % from 420 nm to 1000 nm. This measurement uncertainty meets the stability requirement for many climate data records derived from satellite images, including those for vegetation, aerosols, and snow and ice albedo. It therefore opens the possibility of using the Moon as a calibration standard to bridge gaps in satellite coverage and validate atmospheric retrieval algorithms. Our measurement technique also yields detailed information about the atmosphere at the measurement site, suggesting that lunar observations are a possible solution for aerosol monitoring during the polar winter and can provide nighttime measurements to complement aerosol data collected with sun photometers. Our measurement, made with a novel apparatus, is an order of magnitude more accurate than the previous state-of-the-art and has continuous spectral coverage, removing the need to interpolate between filter passbands.

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.

Supercontinuum Sources for Metrology

Supercontinuum (SC) sources are novel laser-based sources that generate a broad, white-light continuum in single-mode photonic crystal fibres. Currently, up to 6 W of optical power is available, spanning the spectral range from 460 nm to 2400 nm. Advances in these sources promise polarized radiant flux with expanded spectral coverage down to 380 nm. We evaluate the use of SC sources for fundamental optical metrological applications.

THE COSMIC INFRARED BACKGROUND EXPERIMENT (CIBER): THE NARROW-BAND SPECTROMETER

We have developed a near-infrared spectrometer designed to measure the absolute intensity of the solar 854.2 nm Ca II Fraunhofer line, scattered by interplanetary dust, in the zodiacal light (ZL) spectrum. Based on the known equivalent line width in the solar spectrum, this measurement can derive the zodiacal brightness, testing models of the ZL based on morphology that are used to determine the extragalactic background light in absolute photometry measurements. The spectrometer is based on a simple high-resolution tipped filter placed in front of a compact camera with wide-field refractive optics to provide the large optical throughput and high sensitivity required for rocket-borne observations. We discuss the instrument requirements for an accurate measurement of the absolute ZL brightness, the measured laboratory characterization, and the instrument performance in flight.

The cosmic infrared background experiment (CIBER): The low resolution spectrometer

Absolute spectrophotometric measurements of diffuse radiation at 1 μm to 2 μm are crucial to our understanding of the radiative content of the universe from nucleosynthesis since the epoch of reionization, the composition and structure of the zodiacal dust cloud in our solar system, and the diffuse galactic light arising from starlight scattered by interstellar dust. The Low Resolution Spectrometer (LRS) on the rocket-borne Cosmic Infrared Background Experiment is a λ/Δλ ~ 15-30 absolute spectrophotometer designed to make precision measurements of the absolute near-infrared sky brightness between 0.75 μm <λ < 2.1 μm. This paper presents the optical, mechanical, and electronic design of the LRS, as well as the ground testing, characterization, and calibration measurements undertaken before flight to verify its performance. The LRS is shown to work to specifications, achieving the necessary optical and sensitivity performance. We describe our understanding and control of sources of systematic error for absolute photometry of the near-infrared extragalactic background light.

Low-background temperature calibration of infrared blackbodies

The Low Background Infrared (LBIR) facility at the National Institute of Standards and Technology (NIST) has performed ten radiance temperature calibrations of low-background blackbodies since 2001, when both the calibration facility and method of calibrating blackbodies were significantly improved. Data from nine of these blackbody calibrations are presented, showing a surprisingly large spread in blackbody performance. While some blackbodies performed relatively well, in no case did the measured radiance temperature agree with the temperature sensors in the blackbody core to within 0.3 K over the entire operating temperature range of the blackbody. Of the nine blackbodies reported, five showed temperature errors greater than 1 K at some point in their operating temperature range. The various sources of uncertainty, such as optical geometry and detector standard uncertainty, are presented with examples to support the stated calibration accuracy. Generic blackbody cavity design features, such as cavity thermal mass, cavity volume and defining aperture placement are discussed and correlated with blackbody performance. Data are also presented on the performance of the absolute cryogenic radiometers (ACRs) that are used as detector standards in the calibration of blackbodies. Recent intercomparisons of all the LBIR ACRs with a trap detector calibrated against the NIST primary optical power measurement standard show that ACRs used to calibrate blackbodies are suitable detector standards and contribute less than 0.02% uncertainty (k = 1) to radiance temperature measurements of the blackbody cavities.

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.

A sensitive, spatially uniform photodetector for broadband infrared spectrophotometry

We describe the design and performance of a liquid helium-cooled As:Si blocked-impurity-band photodetector system intended for spectrophotometry in the thermal infrared (2 to 30 mum) spectral region. The system has been characterized for spectral sensitivity, noise, thermal stability, and spatial uniformity, and optimized for use with a Fourier-transform infrared spectrophotometer source for absolute goniometric reflectance measurements. Its performance is evaluated and compared to more common detector systems used in this spectral region, including room-temperature pyroelectric and liquid-N(2)-cooled photoconductive devices.

Radiometrically deducing aperture sizes

The desire for high-accuracy infrared sources suitable for low-background seeker/tracker calibrations pushes the limits of absolute cryogenic radiometry and blackbody design. It remains difficult to calibrate a blackbody at irradiance levels below 1 nW cm−2 using electrical substitution radiometry. Ideally, the blackbody temperature should be chosen so that most of the emitted power lies in the spectral range of interest. This constraint frequently necessitates the use of small apertures (less than 1 mm diameter) to achieve the required reduction in power. However, the dimensions of small apertures are difficult to determine accurately. Also, the effects of diffraction and systematic problems such as aperture heating and light leaks become amplified. To diagnose these problems and to calculate diffraction effects, aperture dimensions must be known accurately. We describe a technique of radiometrically deducing the diameter of small apertures in the blackbody in situ utilizing the cryogenic blackbody calibration.

Radiometer standard for absolute responsivity calibrations from 950 nm to 1650 nm with 0.05% (k = 2) uncertainty

A near-IR radiometer standard with similar performance to silicon trap detectors has been developed to calibrate detectors and radiometers for absolute spectral power, irradiance and radiance responsivities between 950 nm and 1650 nm. The new radiometer standard is utilized at the Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) which is the reference calibration facility of NIST for absolute responsivity. The radiometer standard is a sphere detector with a unique geometrical arrangement and it can convert the radiant power responsivity scale of the primary-standard cryogenic radiometer into a reference irradiance responsivity scale. The 0.05% (k = 2) scale conversion uncertainty is dominated by the two largest uncertainty components of the radiometer: the spatial non-uniformity of responsivity of less than 0.05% in power mode and the 0.03% angular responsivity deviation from the cosine function in a 5° angular range in irradiance mode. These small uncertainty components are the results of a tilted input aperture (relative to the sphere axis) and four symmetrically positioned InGaAs detectors around the incident beam spot in the sphere. With the new radiometer standard, it is expected that a thermodynamic temperature uncertainty of 10 mK (k = 2) can be achieved at 157 °C, the freeze temperature of the In fixed-point blackbody.