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.

Incandescent lamp-based facility for measuring the long-term radiometric stability of spectrographs.

The long-term temporal stability of a spectrograph is one of the most important characteristics affecting the spectrograph's radiometric performance. For many applications, from monitoring ocean color and lunar irradiance to laboratory irradiance measurement standards, the stability of a spectrograph is a primary factor in the overall measurement uncertainty and therefore is the major criterion for the suitability of the spectrograph as an optical-scale transfer standard. Here we report a facility built for testing the long-term radiometric stability of commercial, fiber-coupled spectrographs. The facility uses tungsten quartz-halogen irradiance standard lamps, type "FEL," of the National Institute of Standards and Technology (NIST) as light sources. To ensure the highest stability of these lamps during spectrograph tests, parameters such as lamp current, lamp voltage, and signals from an independent filter radiometer were continuously recorded to monitor any possible instability caused by such effects as lamp aging. Using this facility, we report the stability study of four spectrographs with spectral coverage from the UV to short-wave infrared over an interval of two months during which the lamp irradiance was stable to better than 0.02%. The tested spectrographs show good stability in general, ranging from 0.02% to 0.1% in the visible over a span of 11 days. For a longer two-month test, the variation in spectrograph responses increases by less than 0.1% with no discernable long-term drifts. In addition, we measured the response variation of two of the test spectrographs before and after they were sent to remote field locations and subjected to adverse environmental conditions. In this case, a larger response variation of up to 1.0% dependence on the wavelength was observed. We discuss the performance of the facility and the implications for using these spectrographs for several of NIST's remote sensing projects as radiometric transfer standards based on these stability measurements.

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.

On the fluorescence from integrating spheres.

Our recent study of the performance of integrating spheres shows prominent UV induced fluorescence features that are associated with contamination of the diffusing wall material by hydrocarbons. Because of multiple reflections of the radiation inside the integrating sphere, fluorescence is induced multiple times with each reflection of the incident radiation by the wall. Here, we report a simple theory on the fluorescence of integrating spheres developed from first principles. The results indicate a strong dependence of fluorescence on the reflectance of the diffusing material at both the excitation and fluorescence wavelengths as well as the geometry of the integrating sphere. Because of multiple reflection of the exciting radiation, a gain of more than an order of magnitude in fluorescence is possible compared with direct and single irradiation of a flat piece of the diffusing/fluorescing material. Applications of such fluorescence analysis for integrating spheres are discussed.

Ultraviolet characterization of integrating spheres.

We have studied the performance of polytetrafluoroethylene integrating spheres in the ultraviolet (UV) region with wavelengths as short as 200 nm. Two techniques were used for this study; first, the spectral throughput of an integrating sphere irradiated by a deuterium lamp was analyzed by a monochromator. Second, a UV laser beam was directed into an integrating sphere, and spectrally dispersed laser induced fluorescence was studied. Significant absorption and fluorescence features were observed in the UV region and attributed to the contamination in the integrating sphere. We demonstrate that integrating spheres are easily contaminated by environmental pollutants such as polycyclic aromatic hydrocarbons emitted from engine exhaust. Baking of the contaminated integrating sphere can reverse some but not all of the effects caused by contaminants. The implications for using integrating spheres for UV measurement are discussed.

Synchrotron radiation-based irradiance calibration from 200 to 400 nm at the Synchrotron Ultraviolet Radiation Facility III.

A new facility for measuring irradiance in the UV was commissioned recently at the National Institute of Standards and Technology (NIST). The facility uses the calculable radiation from the Synchrotron Ultraviolet Radiation Facility as the primary standard. To measure the irradiance from a source under test, an integrating sphere spectrometer-detector system measures both the source under test and the synchrotron radiation sequentially, and the irradiance from the source under test can be determined. In particular, we discuss the calibration of deuterium lamps using this facility from 200 to 400 nm. This facility improves the current NIST UV irradiance scale to a relative measurement uncertainty of 1.2% (k=2).

Stability of photodiodes under irradiation with a 157-nm pulsed excimer laser.

We have measured the stability of a variety of photodiodes exposed to 157-nm light from a pulsed excimer laser by using a radiometry beamline at the Synchrotron Ultraviolet Radiation Facility at the National Institute of Standards and Technology. The intense, pulsed laser light exposed the photodiodes, whereas the low-intensity, continuously tunable light from the synchrotron source measured changes in the characteristics of the photodiodes, such as in the responsivity and the reflectance from the surface of a photodiode. Photodiodes studied include both silicon pn-junction and Schottky-barrier types. Among these photodiodes, we found that the damage mechanism for photodiodes with SiO2-based passivating layers is mainly the buildup of SiO2-Si interface trap states. The interface trap state buildup is well known for other semiconductor devices and is generally recognized as a product induced by radiation with an energy more than the 9-eV SiO2 bandgap energy rather than the 7.9-eV energy of the 157-nm radiation. Based on the generation of interface trap states, a model is proposed to describe the dependence of detector responsivity on exposure to 157-nm radiation. We also observed slow recovery in some of the damaged photodiodes, confirming that some of the interface trap states are only semipermanent. Radiation damage induced by low-power continuous 157-nm synchrotron light was also studied. As for the other photodiodes with no SiO2 layers, measurement results support the assumption that the changes in responsivity are due mainly to the deposition of thin layers on the tops of the detectors during laser irradiation.

Quantum yield of the iodide-iodate chemical actinometer: dependence on wavelength and concentrations.

The quantum yield (QY) of the iodide-iodate chemical actinometer (0.6 M KI-0.1 M KIO3) was determined for irradiation between 214 and 330 nm. The photoproduct, triiodide, was determined from the increase in absorbance at 352 nm, which together with a concomitant measurement of the UV fluence enabled the QY to be calculated. The QY at 254 nm was determined to be 0.73 +/- 0.02 when calibration was carried out against a National Institute of Standards and Technology traceable radiometer or photometric device. At wavelengths below 254 nm the QY increased slightly, leveling off at -0.80 +/- 0.05, whereas above 254 nm the QY decreases linearly with wavelength, reaching a value of 0.30 at 284 nm. In addition, the QY was measured at different iodide concentrations. There is a slight decrease in QY going from 0.6 to 0.15 M KI, whereas below 0.15 M KI the QY drops off sharply, decreasing to 0.23 by 0.006 M KI. Calibration of the QY was also done using potassium ferrioxalate actinometry to measure the irradiance. These results showed a 20% reduction in QY between 240 and 280 nm as compared with radiometry. This discrepancy suggests that the QY of the ferrioxalate actinometer in this region of the spectrum needs reexamination.

Characterization of an ultraviolet and a vacuum-ultraviolet irradiance meter with synchrotron radiation.

We have constructed and characterized a simple probe that is suitable for accurate measurements of irradiance in the UV to the vacuum UV spectral range. The irradiance meter consists of a PtSi detector located behind a 5-mm-diameter aperture. The probe was characterized at various wavelengths ranging from 130 to 320 mm by use of continuously tunable synchrotron radiation from the Synchrotron Ultra-violet Radiation Facility III. We determined the irradiance responsivity by scanning a small monochromatic beam over the active area of the irradiance meter and measuring its response on a grid with regular spacing. The angular response was also determined and shown to be suitable for applications such as photolithography. In addition, we studied the radiation damage using a 157-nm excimer laser and found that the irradiance meter can endure more than 100 J/cm2 of 157-nm radiation before a noticeable change occurs in its responsivity. Many industrial applications such as UV curing, photolithography, or semiconductor chip fabrication that require accurate measurement of the irradiance would benefit from having such a stable, accurate LTV irradiance meter.