Irradiance Responsivity Calibrations Research Articles

Pyroelectric detector-based method for low uncertainty spectral irradiance and radiance responsivity calibrations in the infrared using tunable lasers.

The standard uncertainty of detector-based radiance and irradiance responsivity calibrations in the short-wave infrared (SWIR) traditionally has been limited to around 1% or higher by the poor spatial uniformity of detectors used to transfer the scale from radiant power. Pyroelectric detectors offer a solution that avoids the spatial uniformity uncertainty but also introduces additional complications due to alternating current (AC) measurement techniques. Herein, a new, to the best of our knowledge, method for low uncertainty irradiance responsivity calibrations in the SWIR is presented. An absolute spectral irradiance responsivity scale was placed on two pyroelectric detectors (PED) at wavelengths λ from 500 to 3400 nm. The total combined uncertainty (k=1) was ≈0.28% (>1000nm), 0.44% (900 nm), and 0.36% (≈950nm and <900nm) for PED #1 and 0.34% (>1000nm), 0.48% (900 nm), and 0.42% (≈950nm and <900nm) for PED #2. This was done by utilizing a demodulation technique to digitally analyze the time-dependent AC waveforms, which obviates the use of lock-in amplifiers and avoids associated additional uncertainty components.

The linearization of the relationship between scene luminance and digital camera output levels

The linearization of the relationship between scene luminance and digital camera output levels

Absolute irradiance responsivity calibration using diode lasers emitting at three wavelengths for tricolor laser applications

An absolute irradiance responsivity calibration system with diode laser light sources at three wavelengths is presented. This system is based on an optical detector with a silicon photodiode and a uniform light source combined with an integrating sphere. Experimental validation of the calibration system was performed with red, green, and blue lasers, and its utility was successfully demonstrated at all tested laser wavelengths. The expanded uncertainty of the absolute irradiance responsivity has been analyzed to be 0.55–0.57% (k = 2). It is expected that this system can be used to provide validation of tricolor laser-based applications such as displays and lighting.

Invited Article: Advances in tunable laser-based radiometric calibration applications at the National Institute of Standards and Technology, USA.

Recent developments at the National Institute of Standards and Technology's facility for Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (SIRCUS) are presented. The facility is predicated on the use of broadly tunable narrow-band lasers as light sources in two key radiometric calibration applications. In the first application, the tunable lasers are used to calibrate the spectral power responsivities of primary standard detectors against an absolute cryogenic radiometer (ACR). The second function is to calibrate the absolute radiance and irradiance responsivities of detectors with uniform light sources, typically generated by coupling the laser light into integrating spheres. The radiant flux from the uniform sources is determined by the ACR-calibrated primary standard detectors. Together these sources and detectors are used to transfer radiometric scales to a variety of optical instruments with low uncertainties. We describe methods for obtaining the stable, uniform light sources required for low uncertainty measurements along with advances in laser sources that facilitate tuning over broader wavelength ranges. Example applications include the development of a detector-based thermodynamic temperature scale, the calibration and characterization of spectrographs, and the use of a traveling version of SIRCUS (T-SIRCUS) to calibrate large aperture Earth observing instruments and astronomical telescopes.

Monochromatic measurements of the JPSS-1 VIIRS polarization sensitivity.

Polarization sensitivity is a critical property that must be characterized for spaceborne remote sensing instruments designed to measure reflected solar radiation. Broadband testing of the first Joint Polar-orbiting Satellite System (JPSS-1) Visible Infrared Imaging Radiometer Suite (VIIRS) showed unexpectedly large polarization sensitivities for the bluest bands on VIIRS (centered between 400 and 600 nm). Subsequent ray trace modeling indicated that large diattenuation on the edges of the bandpass for these spectral bands was the driver behind these large sensitivities. Additional testing using the National Institute of Standards and Technology's Traveling Spectral Irradiance and Radiance Responsivity Calibrations Using Uniform Sources was added to the test program to verify and enhance the model. The testing was limited in scope to two spectral bands at two scan angles; nonetheless, this additional testing provided valuable insight into the polarization sensitivity. Analysis has shown that the derived diattenuation agreed with the broadband measurements to within an absolute difference of about 0.4% and that the ray trace model reproduced the general features of the measured data. Additionally, by deriving the spectral responsivity, the linear diattenuation is shown to be explicitly dependent on the changes in bandwidth with polarization state.

Traceability of solar UV measurements using the Qasume reference spectroradiometer.

One major objective of the European Joint Research Project "Traceability for surface spectral solar ultraviolet (UV) radiation" was to reduce the uncertainty of spectral UV measurements. The measurement instrument used for this work was the portable UV European reference spectroradiometer Qasume. The calibration uncertainty of this instrument was decreased and validated by a comparison of direct calibrations against a primary standard for spectral irradiance, a high temperature blackbody radiator, and against a reference detector using a spectrally tunable laser as a monochromatic source. The spectral irradiance responsivity of the reference detector is traceable to the primary standard of optical power, realized through a cryogenic radiometer, and to the SI unit of meter. The measuring technique was improved by the construction of a new reference spectroradiometer, QasumeII. An improved input optics removes the dependences of the measured solar irradiance on the angle of incident for solar zenith angle smaller than 75deg. Moreover, a hybrid photon detection system enables continuous tracking of the instrument's responsivity changes. For both spectroradiometer systems an uncertainty budget was calculated. The improvements have reduced the measurement uncertainties of solar spectral UV irradiance measurements from 4.8% in 2005 to 2.0% (k=2) in the spectral region above 310nm. The largest sources of uncertainty were the absolute spectral irradiance responsivity calibration, the angular response uncertainty, and the instrument stability using the hybrid detector, which were reduced from 3.6% to 1.1%, from 1.2% to 0.6%, and from 0.65% to 0.4%, with respect to the situation prior to the project. The new instrument was validated during a four month intercomparison relative to the Qasume reference. The mean ratio of the solar irradiance scans between the two reference spectroradiometers has an offset of +0.7% and a standard deviation of ±1.5% for a wavelength greater than 305nm, which is well within the combined uncertainty of 3.7% calculated from the uncertainties of the two systems.

Validation of short-pulse-laser-based measurement setup for absolute spectral irradiance responsivity calibration.

This paper describes the validation process of mode-locked lasers in the "tunable lasers in photometry" (TULIP) setup at Physikalisch-Technische Bundesanstalt (PTB) regarding spectral irradiance responsivity calibrations. Validation has been carried out in the visible spectral range, 400-700 nm, with two different photometer heads and in the long wavelength range, 690-780 nm, with a filtered radiometer. A comparison of the results against those from two different validated measurement setups has been carried out for validation. For the visible spectral range, the comparison is conducted against the data obtained from a lamp-based monochromator setup for spectral irradiance responsivity calibrations and against the photometric values (integral quantity) measured at the photometric bench setup of PTB. For the long wavelength range, comparisons against results from two different lamp-based monochromator measurement setups were made. Additionally, the effect of different radiation bandwidths on interference oscillations has been determined for a filter radiometer without a diffuser. A procedure for the determination of the optimum bandwidth of the setup for the respective measurement device is presented.

反射式拼接CCD相机非均匀性定标与校正

According to the radiometric calibration experiments in laboratory,the reflector-based mosaic CCD camera's image non-uniformity caused by vignetting phenomenon was corrected.Firstly,the image un-uniformity reasons of reflector-based mosaic CCD camera are analyzed,and the irradiance deviation received by different CCD pixels in focal plane is the essential reason for images' non-uniformity.Then,the CCD pixels' irradiance responsivity calibration experiment and the whole camera's non-uniformity calibration experiment were carried out in the laboratory,and the relationship between each pixel's receiving irradiance and camera's incident radiance was built.Lastly,a correction algorithm is put forward based on the above relationship to calculate CCD pixels' correction factors,and the correction factors were used to correct the vignetting image.The correct experiment indicates that,within a certain range of incident radiance,the non-uniformity of original calibration images areabove 16%,and after correction,the values decreased to 1% or less.Besides,the correction factors were used to correct the images of outdoor experiments and it works very well.

Spectral responsivity-based calibration of photometer and colorimeter standards

Several new generation transfer- and working-standard illuminance meters and tristimulus colorimeters have been developed at the National Institute of Standards and Technology (NIST) [1] to measure all kinds of light sources with low uncertainty. The spectral and broad-band (illuminance) responsivities of the photometer (Y) channels of two tristimulus meters were determined at both the Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) facility and the Spectral Comparator Facility (SCF) [2]. The two illuminance responsivities agreed within 0.1% with an overall uncertainty of 0.2% (k = 2), which is a factor of two improvement over the present NIST photometric scale. The first detector-based tristimulus color scale [3] was realized. All channels of the reference tristimulus colorimeter were calibrated at the SIRCUS. The other tristimulus meters were calibrated at the SCF and also against the reference meter on the photometry bench in broad-band measurement mode. The agreement between detector- and source-based calibrations was within 3 K when a tungsten lamp-standard was measured at 2856 K and 3100 K [4]. The color–temperature uncertainty of tungsten lamp measurements was 4 K (k = 2) between 2300 K and 3200 K, which is a factor of two improvement over the presently used NIST source-based color temperature scale. One colorimeter was extended with an additional (fifth) channel to apply software implemented matrix corrections. With this correction, the spectral mismatch caused color difference errors were decreased by a factor of 20 for single-color LEDs.

A new method for spectral irradiance and radiance responsivity calibrations using kilohertz pulsed tunable optical parametric oscillators

Continuous-wave (CW) tunable lasers have been used for detector calibrations, especially for spectral irradiance and radiance responsivity, for many years at the National Institute of Standards and Technology (NIST) and other national metrology institutes. These CW tunable lasers, however, are expensive and difficult to automate. To address these issues, we developed a new method for spectral irradiance and radiance responsivity calibrations using relatively low cost, fully automated kilohertz pulsed tunable optical parametric oscillators (OPOs). The new method is based on measurements of the total energy of a pulsed OPO train using two synchronized current integrators (also called charge amplifiers) to measure the total integrated electric charges from a test detector and a standard detector, respectively. The absolute expanded uncertainty of this method is estimated to be 0.05% (with a coverage factor of k = 2) for spectral irradiance responsivity calibrations, and the dominant uncertainty contribution is from the reference trap detector.

Stray light characterization of an InGaAs anamorphic hyperspectral imager

Compact hyperspectral sensors potentially have a wide range of applications, including machine vision, quality control, and surveillance from small Unmanned Aerial Vehicles (UAVs). With the development of Indium Gallium Arsenide (InGaAs) focal plane arrays, much of the Short Wave Infra-Red (SWIR) spectral regime can be accessed with a small hyperspectral imaging system, thereby substantially expanding hyperspectral sensing capabilities. To fully realize this potential, system performance must be well-understood. Here, stray light characterization of a recently-developed push-broom hyperspectral sensor sensitive in the 1 microm -1.7 microm spectral regime is described. The sensor utilizes anamorphic fore-optics that partially decouple image formation along the spatial and spectral axes of the instrument. This design benefits from a reduction in complexity over standard high-performance spectrometer optical designs while maintaining excellent aberration control and spatial and spectral distortion characteristics. The stray light performance characteristics of the anamorphic imaging spectrometer were measured using the spectral irradiance and radiance responsivity calibrations using uniform sources (SIRCUS) facility at the National Institute of Standards and Technology (NIST). A description of the measurements and results are presented. Additionally, a stray-light matrix was assembled for the instrument to improve the instrument's spectral accuracy. Transmittance of a silicon wafer was measured to validate this approach.

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.

Comparison of absolute spectral irradiance responsivity measurement techniques using wavelength-tunable lasers

Independent methods for measuring the absolute spectral irradiance responsivity of detectors have been compared between the calibration facilities at two national metrology institutes, the Helsinki University of Technology (TKK), Finland, and the National Institute of Standards and Technology (NIST). The emphasis is on the comparison of two different techniques for generating a uniform irradiance at a reference plane using wavelength-tunable lasers. At TKK's Laser Scanning Facility (LSF) the irradiance is generated by raster scanning a single collimated laser beam, while at the NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources (SIRCUS), lasers are introduced into integrating spheres to generate a uniform irradiance at a reference plane. The laser-based irradiance responsivity results are compared to a traditional lamp-monochromator-based irradiance responsivity calibration obtained at the NIST Spectral Comparator Facility (SCF). A narrowband filter radiometer with a 24 nm bandwidth and an effective band-center wavelength of 801 nm was used as the artifact. The results of the comparison between the different facilities, reported for the first time in the near-infrared wavelength range, demonstrate agreement at the uncertainty level of less than 0.1%. This result has significant implications in radiation thermometry and in photometry as well as in radiometry.

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.

Investigation of comparison methods for UVA irradiance responsivity calibration facilities

A new method is described for comparing calibration facilities for broadband ultraviolet (UV) meters. The method is validated in a successful pilot comparison between five European laboratories performing UVA irradiance responsivity calibrations. Participating laboratories calibrated a broadband UVA detector using their ordinary procedures that differed both by calibration methods and radiation sources used. The pilot laboratory calculated different reference values for each laboratory based on the specified calibration methods and prior knowledge of the calibrated artefact. The results were in agreement within ±5%, which demonstrates a factor of two improvement in the consistency of the results as compared with earlier intercomparisons.

System-level calibration of a transfer radiometer used to validate EOS radiance scales

A number of transfer radiometers have been developed by the National Institute of Standards and Technology (NIST) with the support and funding of the National Aeronautics and Space Administration's (NASA's) satellite sensor calibration programmes. The transfer radiometers travel to NASA satellite builder and calibration facilities and validate the radiance scale of calibration sources at those facilities. One of these instruments, the Earth Observing Systems (EOS) Visible Transfer Radiometer (VXR), was calibrated for spectral responsivity in different facilities at NIST using two types of integrating sphere sources (ISSs): an incandescent lamp-illuminated ISS and a laser-illuminated ISS. The laser-illuminated ISS results in reduced uncertainties and a greater dynamic range in radiance responsivity. We will discuss the calibration procedures using the two types of source, compare results of calibrations of two of the VXR filter channels in the two NIST facilities, and suggest additional irradiance and radiance responsivity calibrations that are now possible using the new laser-illuminated facility.

Spectral power and irradiance responsivity calibration of InSb working-standard radiometers

New, improved-performance InSb power-irradiance meters have been developed and characterized to maintain the National Institute of Standards and Technology (NIST) spectral responsivity scale between 2 and 5.1 mum. The InSb radiometers were calibrated against the transfer-standard cryogenic bolometer that is tied to the primary-standard cryogenic radiometer of the NIST. The InSb radiometers serve as easy-to-use working standards for routine spectral power and irradiance responsivity calibrations. The spectral irradiance responsivities were derived from the spectral power responsivities by use of the measured area of the apertures in front of the InSb detectors.

NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources

A laser-based facility has been developed to provide high-flux, monochromatic, Lambertian radiation over the spectral range 0.2 µm to 18 µm. The facility was designed to reduce the uncertainties in a variety of radiometric applications, including irradiance and radiance responsivity calibrations. The operational characteristics of the facility are discussed and the results of detector responsivity calibrations over the spectral range 0.406 µm to 0.920 µm are presented.

Realization of a spectral radiance responsivity scale with a laser-based source and Si radiance meters

A spectral radiance responsivity (SRR) scale derived from the spectral irradiance responsivity scale has been realized on the newly developed NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources (SIRCUS). The scales were directly derived from the primary standard cryogenic radiometer using transfer standard irradiance trap detectors and laser-based uniform sources. Silicon radiance meters of standards quality have been developed to hold the SRR scale. The radiance meters were characterized and optimized for size-of-source effects. The optimized meters were calibrated for SRR against the monochromatic uniform source. In addition to the SIRCUS calibrations, preliminary tests were also made on the Spectral Comparator Facility (SCF) of the NIST. These relative SRR tests were made in both imaging and non-imaging measurement modes. The results of the different SRR calibrations are compared and evaluated.