High Accuracy Cryogenic Radiometer Research Articles

Improvements in the AIST Cryogenic Radiometer With Superconducting Thermometer

We are developing a cryogenic radiometer (CR) with a superconducting thermometer for the optical laser power calibration. This device consists of a niobium transition edge sensor and an electrical substitution heater on a silicon substrate. The device temperature is locked at the niobium transition temperature with a superconducting quantum interference device amplifier and a proportional-integral-derivative controller. The device successfully determines the absolute optical power in the range from 20 nW to 600 muW, with a fast response of 3 s. The equivalence measurement shows that the response difference between the substitution power and the simulated optical power is less than 0.0034%. These properties are very attractive for next generation of a high-accuracy CR

Intercomparison of the LBIR Absolute Cryogenic Radiometers to the NIST Optical Power Measurement Standard.

The Low Background Infrared calibration (LBIR) facility at the National Institute of Standards and Technology (NIST) presently maintains four absolute cryogenic radiometers (ACRs) which serve as standard reference detectors for infrared calibrations performed by the facility. The primary standard for optical power measurements at NIST-Gaithersburg has been the High Accuracy Cryogenic Radiometer (HACR). Recently, an improved radiometer, the Primary Optical Watt Radiometer (POWR), has replaced the HACR as the primary standard. In this paper, we present the results of comparisons between the radiometric powers measured by the four ACRs presently maintained by the LBIR facility to that measured by the HACR and POWR. This was done by using a Si photodiode light-trapping detector as a secondary transfer standard to compare the primary national standards to the ACRs maintained by the LBIR facility. The technique used to compare an ACR to the trap detector is described in detail. The absolute optical power measurements are found to be within 0.1 % of the primary standard for all the ACRs examined in this study.

Uncertainty Propagation for NIST Visible Spectral Standards.

Uncertainties in the NIST spectral standards for detectors and sources in the visible wavelength range are propagated from the high accuracy cryogenic radiometer measurements, taking correlations into account at every stage. Partial correlations between spectral values at different wavelengths, important for subsequent radiometric calculations, are estimated. Uncertainty propagation through fitting and through transfer spectral measurements is described in detail. Detector uncertainties are propagated through the spectral comparator facility for external calibrations and for internal photometric quantities. Uncertainties in spectral irradiance are derived for the detector-based temperature determination, then propagated through working standards to calibrated artifacts. Spectral irradiance calibrations are generally provided at a limited number of wavelengths. Interpolation, rather than fitting, is recommended for the interpolation of NIST-provided spectral irradiance values.

The realization of the NIST detector-based spectral irradiance scale

The realization of a detector-based spectral irradiance scale at the National Institute of Standards and Technology is described. The new scale is established using filter radiometers calibrated for absolute spectral power responsivity traceable to the NIST High-Accuracy Cryogenic Radiometer unlike the previous NIST spectral irradiance scales based upon the gold freezing-point blackbody. The radiance temperatures of a high-temperature blackbody (HTBB) operating near 3000 K are found using calibrated filter radiometers. The spectral irradiances of a group of 1000 W FEL lamps are assigned using the spectral irradiance of the HTBB determined using the knowledge of the geometric factors and the detector-based radiance temperatures. The detector-based spectral irradiance scale leads to a reduction in the uncertainties from the previous, source-based, spectral irradiance scales by at least a factor of two in the ultraviolet and visible wavelength regions, and also leads to a reduction in the uncertainties in the short-wave infrared wavelength region by at least a factor of two to ten, depending on the wavelength. Following the establishment of the spectral irradiance scale in the early 1960s, the detector-based spectral irradiance scale represents a fundamental change in the way that the NIST spectral irradiance scale is realized, and beginning in the calendar year 2001, all spectral irradiance sources are issued using the detector-based scale.

Realization of the National Institute of Standards and Technology detector-based spectral irradiance scale.

A detector-based spectral irradiance scale has been realized at the National Institute of Standards and Technology (NIST). Unlike the previous NIST spectral irradiance scales, the new scale is generated with filter radiometers calibrated for absolute spectral power responsivity traceable to the NIST high-accuracy cryogenic radiometer instead of with the gold freezing-point blackbody. The calibrated filter radiometers are then used to establish the radiance temperature of a high-temperature blackbody (HTBB) operating near 3,000 K The spectral irradiance of the HTBB is then determined with knowledge of the geometric factors and is used to assign the spectral irradiances of a group of 1,000-W free-electron laser lamps. The detector-based spectral irradiance scale results in the reduction of the uncertainties from the previous source-based spectral irradiance scale by at least a factor of 2 in the ultraviolet and visible wavelength regions. The new detector-based spectral irradiance scale also leads to a reduction in the uncertainties in the shortwave infrared wavelength region by at least a factor of 2-10, depending on the wavelength. Following the establishment of the spectral irradiance scale in the early 1960s, the detector-based spectral irradiance scale represents a fundamental change in the way that the NIST spectral irradiance scale is realized.

Long-term temporal stability of the National Institute of Standards and Technology spectral irradiance scale determined with absolute filter radiometers.

The temporal stability of the National Institute of Standards and Technology (NIST) spectral irradiance scale as measured with broadband filter radiometers calibrated for absolute spectral irradiance responsivity is described. The working standard free-electron laser (FEL) lamps and the check standard FEL lamps have been monitored with radiometers in the ultraviolet and the visible wavelength regions. The measurements made with these two radiometers reveal that the NIST spectral irradiance scale as compared with an absolute thermodynamic scale has not changed by more than 1.5% in the visible from 1993 to 1999. Similar measurements in the ultraviolet reveal that the corresponding change is less than 1.5% from 1995 to 1999. Furthermore, a check of the spectral irradiance scale by six different filter radiometers calibrated for absolute spectral irradiance responsivity based on the high-accuracy cryogenic radiometer shows that the agreement between the present scale and the detector-based scale is better than 1.3% throughout the visible to the near-infrared wavelength region. These results validate the assigned spectral irradiance of the widely disseminated NIST or NIST-traceable standard sources.

Responsivity calibration methods for 365-nm irradiance meters

Two detector-based responsivity calibration methods have been compared at the National Institute of Standards and Technology for ultraviolet irradiance meters in the 365-nm spectral region. Both methods are based on an electrical substitution high-accuracy cryogenic radiometer, but utilize different facilities and transfer standards. One facility is a monochromator-based spectral-power responsivity measuring system utilizing an aperture-scanning method, while the second is a tunable-laser and integrating-sphere source system using a light-trapping silicon transfer detector with a known aperture area. The first reported comparison of these two fundamentally different methods agreed to 1%-2% near the peak and long wavelength side of the bandpass curves which is comparable to their expanded (k=2) uncertainties.

The Candela and Photometric and Radiometric Measurements.

The national measurement system for photometric and radiometric quantities is presently based upon techniques that make these quantities traceable to a high-accuracy cryogenic radiometer. The redefinition of the candela in 1979 provided the opportunity for national measurement laboratories to base their photometric measurements on optical detector technology rather than on the emission from high-temperature blackbody optical sources. The ensuing technical developments of the past 20 years, including the significant improvements in cryogenic radiometer performance, have provided the opportunity to place the fundamental maintenance of photometric quantities upon absolute detector based technology as was allowed by the 1979 redefinition. Additionally, the development of improved photodetectors has had a significant impact on the methodology in most of the radiometric measurement areas. This paper will review the status of the NIST implementation of the technical changes mandated by the 1979 redefinition of the candela and its effect upon the maintenance and dissemination of optical radiation measurements.

An electrically substituted bolometer as a transfer-standard detector

An electrically substituted bolometer (ESB) has been developed at the National Institute of Standards and Technology (NIST) to serve as a portable transfer-standard detector over the wavelength range 200 nm to 20 µm. The ESB is designed to provide a direct transfer of the optical power scale from the NIST High Accuracy Cryogenic Radiometer (HACR) to the NIST spectral-comparator facilities in the infrared region, where silicon-photodiode trap detectors cannot be used and where the noise of pyroelectric detectors currently limits the uncertainty. The noise floor of the 8 mm diameter active-area ESB approaches 10 pW/Hz1/2 at 15 Hz, corresponding to a detectivity of the order of 1011 cm Hz1/2/W. This is an improvement by a factor of almost 1000 on that attainable from a similar-sized room-temperature electrically calibrated pyroelectric radiometer (ECPR). The ESB is linear from the noise floor to 1 mW (the power range of the HACR), whereas previous helium-cooled bolometers developed at the NIST have a similar noise floor but are non-linear above about 10 µW and so cannot be directly calibrated against the HACR. Because of its low noise, linearity over a wide dynamic range, and the spectral and spatial flatness of its response, the ESB is finding other applications beyond that which motivated its development.

Thermal-electrical modeling of absolute cryogenic radiometers

This study employs finite-element (FE) software to model the nonlinear thermal-electrical dynamic response of absolute cryogenic radiometers (ACRs). The high-accuracy cryogenic radiometer (HACR) and laser-optimized cryogenic radiometer (LOCR) have been analyzed to determine the input geometric and thermal parameters. Comparisons between the FE model and experimental results for the HACR and LOCR show an agreement in temperature within 2%. A proportional-integral-derivative (PID) temperature-control algorithm has been developed for both the receiver and the heat sink. The optimized control parameters are related to the thermal conductances and time constants of the radiometers. The LOCR receiver has the advantage of a reduction in measurement time over the HACR receiver; whereas the HACR heat sink offers the advantage of better temperature stability over the LOCR heat sink. The time constant of the HACR receiver can be significantly reduced without degrading the performance. The temperature stability of the LOCR heat sink can be improved by a factor of two by using a larger heat-sink heat capacity, C HS≥80 mJ/K. This research will help improve future design of ACRs.

Realization of an infrared spectral radiant power response scale on a cryogenic bolometer

The National Institute of Standards and Technology (NIST) has developed an infrared (IR) radiant power response scale for the spectral range from 2 µm to 20 µm. The scale has been realized on a cryogenic bolometer. The response of this bolometer has been determined by multiple ties to a primary standard, the NIST High Accuracy Cryogenic Radiometer (HACR). These transfers were made using three intermediate detectors over three different spectral regions. In addition, an independent method was used to determine the relative response of the bolometer over the entire spectral range. An IR spectral response scale over the entire 2 µm to 20 µm range was determined from these measurements, with the interlocking nature of the multiple independent measurements providing improved overall confidence and reduced uncertainties. The overall scale combined standard uncertainty for the entire spectral range of 2 µm to 20 µm is of the order of 0.25 %, with the typical combined standard uncertainty of an individual measurement made with the bolometer being ~0.8 %.

Developments for a New Spectral Irradiance Scale at the National Institute of Standards and Technology.

Recent developments for a new spectral irradiance scale realization at the National Institute of Standards and Technology have been targeted to reduce the present relative expanded uncertainties of 0.67 % to 4.34 % (coverage factor of k = 2 and thus a 2 standard deviation estimate) in the spectral irradiance scale to 0.17 % for the range from 350 nm to 1100 nm. To accomplish this goal, a suite of filter radiometers calibrated using NIST’s high accuracy cryogenic radiometer have been used to measure the temperature of a high-temperature black-body. A comparison of the filter radiometer calibrations with the spectral irradiance scale along with an evaluation of the black-body calibration technique have been performed. With the aid of a monochromator, the calibrated filter radiometers will then be utilized to calibrate primary and secondary spectral irradiance standard lamps at NIST.

Calibration of a pyroelectric detector at 106 µm with the National Institute of Standards and Technology high-accuracy cryogenic radiometer

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.

Angular effects in silicon photodiode responsivity comparisons

Effects of non-normal incidence and finite angular beam width on silicon photodiode responsivity comparisons are modelled and compared with measurements. These effects may be significant if the photodiodes being compared have different oxide layer thicknesses, particularly at short wavelengths. They may also be significant in the transfer of high-accuracy cryogenic radiometer measurements to monochromator-based measurements.

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.

Comparison of filter radiometer spectral responsivity with the NIST spectral-irradiance and illuminance scales

Narrowband filter radiometers are being used in the development of a new method of irradiance scale realization at the National Institute of Standards and Technology (NIST). To improve the accuracy of the current spectral-irradiance scale, we propose to eliminate several steps from the calibration process and to use the filter radiometers and a high-temperature black body in realizing the scale. Ultimately, the filter radiometers will be calibrated against a trap detector measured at the NIST High Accuracy Cryogenic Radiometer, which maintains an expanded uncertainty at 633 nm of 0,0266% for a coverage factor of k = 2. In this work, the filter and detector were calibrated as a unit using the NIST Detector Comparator Facility, and the aperture areas were measured by flux comparison with a uniform source. Six filter radiometers covering the spectral range 350 nm to 1100 nm were used to measure a known irradiance source; the measurements agreed with predictions to within 1%. Comparisons were also conducted with three NIST standard photometers, and similar agreement was observed. Since the irradiance source and the photometers were calibrated using independent methods, our results not only verify the calibration of the filter radiometers but also imply that the NIST scales of spectral irradiance and illuminance are consistent. Future plans include characterization of a high-temperature black body and the calibration of quartz-halogen irradiance lamps using this black-body source.

A Method of Realizing Spectral Irradiance Based on an Absolute Cryogenic Radiometer

A technique is presented for realizing spectral irradiance using a large-area, high temperature, uniform, black-body source and filter-radiometers that are calibrated using a High Accuracy Cryogenic Radiometer. The method will be studied by calibrating irradiance lamps with this new technique and comparing the results with those obtained by the method currently employed at the National Institute of Standards and Technology (NIST). Progress to date and preliminary results are presented. The ultimate goal of the programme is to reduce the measurement uncertainties in the spectral irradiance scales that are made available to industry by calibrating deuterium and tungsten-halogen irradiance lamps.

Intercomparison between the NIST LBIR Absolute Cryogenic Radiometer and an Optical Trap Detector

The goal of the Low Background Infrared (LBIR) calibration facility at the NIST is to provide the infrared user community with a measurement base for both broadband and spectral radiometric calibrations in a low background environment. The standard detector used in this facility is the Absolute Cryogenic Radiometer (ACR). The ACR is an electrical substitution radiometer which is capable of measuring between 20 nW and 100 μW of radiant power. By using an optical trap detector, an intercomparison with the ACR is made as part of a programme to monitor the long-term stability of the ACR. A comparison is also made between the optical trap detector and an in-house trap detector that was calibrated using the High Accuracy Cryogenic Radiometer (HACR).