C L. Cromer

Realization of a scale of absolute spectral response using the National Institute of Standards and Technology high-accuracy cryogenic radiometer

Using the National Institute of Standards and Technology high-accuracy cryogenic radiometer (HACR), we have realized a scale of absolute spectral response between 406 and 920 nm. The HACR, an electrical-substitution radiometer operating at cryogenic temperatures, achieves a combined relative standard uncertainty of 0.021%. Silicon photodiode light-trapping detectors were calibrated against the HACR with a typical relative standard uncertainty of 0.03% at nine laser wavelengths between 406 and 920 nm. Modeling of the quantum efficiency of these detectors yields their responsivity throughout this range with comparable accuracy.

National Institute of Standards and Technology High-Accuracy Cryogenic Radiometer

A high-accuracy cryogenic radiometer has been developed at the National Institute of Standards and Technology to serve as a primary standard for optical power measurements. This instrument is an electrical-substitution radiometer that can be operated at cryogenic temperatures to achieve a relative standard uncertainty of 0.021% at an optical power level of 0.8 mW. The construction and operation of the high-accuracy cryogenic radiometer and the uncertainties in optical power measurements are detailed.

The NIST High Accuracy Scale for Absolute Spectral Response from 406 nm to 920 nm

We describe how the National Institute of Standards and Technology obtains a scale of absolute spectral response from 406 nm to 920 nm. This scale of absolute spectral response is based solely on detector measurements traceable to the NIST High Accuracy Cryogenic Radiometer (HACR). Silicon photodiode light-trapping detectors are used to transfer optical power measurements from the HACR to a monochromator-based facility where routine measurements are performed. The transfer also involves modeling the quantum efficiency (QE) of the silicon photodiode light-trapping detectors. We describe our planned quality system for these measurements that follows ANSI/NCSL Z540-1-1994. A summary of current NIST capabilities based on these measurements is also given.

THE NIST DETECTOR-BASED LUMINOUS INTENSITY SCALE

The Système International des Unités (SI) base unit for photometry, the candela, has been realized by using absolute detectors rather than absolute sources. This change in method permits luminous intensity calibrations of standard lamps to be carried out with a relative expanded uncertainty (coverage factor k = 2, and thus a 2 standard deviation estimate) of 0.46 %, almost a factor-of-two improvement. A group of eight reference photometers has been constructed with silicon photodiodes, matched with filters to mimic the spectral luminous efficiency function for photopic vision. The wide dynamic range of the photometers aid in their calibration. The components of the photometers were carefully measured and selected to reduce the sources of error and to provide baseline data for aging studies. Periodic remeasurement of the photometers indicate that a yearly recalibration is required. The design, characterization, calibration, evaluation, and application of the photometers are discussed.

Sources of Error in UV Radiation Measurements

Increasing commercial, scientific, and technical applications involving ultraviolet (UV) radiation have led to the demand for improved understanding of the performance of instrumentation used to measure this radiation. There has been an effort by manufacturers of UV measuring devices (meters) to produce simple, optically filtered sensor systems to accomplish the varied measurement needs. We address common sources of measurement errors using these meters. The uncertainty in the calibration of the instrument depends on the response of the UV meter to the spectrum of the sources used and its similarity to the spectrum of the quantity to be measured. In addition, large errors can occur due to out-of-band, non-linear, and non-ideal geometric or spatial response of the UV meters. Finally, in many applications, how well the response of the UV meter approximates the presumed action spectrum needs to be understood for optimal use of the meters.

The Detector-Based Candela Scale and Related Photometric Calibration Procedures at NIST

The candela, one of the SI base units, has been realized by using absolutely calibrated detectors rather than sources. A group of eight photometers was constructed using silicon photodiodes, precision apertures, and glass filters for V (λ) match. Their absolute spectral responsivities were calibrated against the NIST absolute spectral responsivity scale. The measurement chain has been significantly shortened compared with the old scale based on a blackbody. This resulted in improving the calibration uncertainty to 0.46% (2σ), a factor-of-2 improvement. This revision has made various photometric calibrations at NIST more versatile and flexible. Luminous intensities of light sources ranging from 10 -3 to 10 4 candelas are directly calibrated with the standard photometers, which have a linear response over that range. Illuminance meters are calibrated directly against the standard photometers. A luminance scale has also been realized on the detector base using an integrating sphere source. Total flux ranging from 10 -2 to 10 5 lumens can be measured in a 2 m integrating sphere using a photometer with a wide dynamic range. The revisions of the calibration procedures significantly improved the calibration uncertainty.

Comparison of the NIST high Accuracy Cryogenic Radiometer and the NIST scale of Detector Spectral Response

Two independent methods of measurement were used to determine the absolute spectral responsivity and external quantum efficiency of light-trapping silicon photodiode packages. These trap packages were calibrated first by the NIST High Accuracy Cryogenic Radiometer at laser wavelengths of 633 nm and 442 nm. They were also measured in the NIST Spectral Comparator Facility with working standards traceable to a 100% quantum efficient radiometer (QED-200). The two sets of measurements agree to better than 0,1% at 633 nm and 0,25% at 442 nm.

Calibration of a pyroelectric detector at 10.6 µm with theNational Institute of Standards and Technology high-accuracy cryogenicradiometer

The National Institute of Standards and Technology (NIST) is establishing an infrared detector calibration facility to improve radiometric standards at infrared wavelengths. The absolute response of the cryogenic bolometer that serves as the transfer standard for this facility is being linked to the NIST high- accuracy cryogenic radiometer (HACR) at a few laser wavelengths. At the 10.6-microm CO(2) laser line, this link is being established through a pyroelectric detector that has been calibrated against the HACR. We describe the apparatus, methods, and uncertainties for the calibration of this pyroelectric detector.

Power measurement standards for high-power lasers: comparison between the NIST and the PTB

We report the results of the first laser high-power measurement comparison between the Physikalisch-Technische Bundesanstalt (PTB, Germany), and the National Institute of Standards and Technology (NIST, USA). Laser power transfer standards were calibrated at both national standards laboratories between 82 W and 127 W at 1.06 µm and between 85 W and 554 W at 10.6 µm. Relative agreement between the standards of the two laboratories was demonstrated to lie between 5 × 10−3 and 7 × 10−3, which is well within the combined uncertainties.

Mode-locked lasers for high-accuracy radiometry

The use of a mode-locked laser as a source for high-accuracy calibration of transfer standards to a cryogenic radiometer is discussed. The critical issue is the relative response of the transfer standard to the modelocked laser light as compared with continuous-wave laser light. As a first test, we have measured the linearity of the response of silicon photodiode light-trapping detectors using mode-locked laser light at a wavelength of 532 nm.