Three-channel solar radiometer for the determination of atmospheric columnar water vapor

The design of a three-channel solar radiometer used to determine total columnar atmospheric water-vapor amounts is presented. The main channel is located in the 0.94-μm water-vapor band, and two other channels are located in adjacent nonabsorption regions of the solar spectrum and are used to remove scattering effects from the main channel. Water-vapor transmittance is determined by means of a modified Langley approach, and these transmittances are converted to columnar water vapor by means of a band model developed at the University of Arizona. Several cases are presented in which columnar water-vapor amounts are determined through the use of the instrument and method described here. These results are compared with sounding-balloon results. Tests of the method indicate that columnar water vapor may be retrieved with an uncertainty of less than 10%.     

Global ultraviolet spectra derived directly from observations with multichannel radiometers

Some general features of multichannel filter radiometers operating in the UV region of the solar spectrum are reviewed with emphasis on calibration problems that are due to incomplete knowledge of responsivity in the UV-B region. An alternative calibration procedure that is able to generate a full UV spectrum obtained by a constrained inversion method is presented. Accuracy of such spectra is assessed with simulated and with real data. A comparison between customary calibration and an alternative procedure is made in terms of monochromatic UV-B irradiance and CIE dose rate (CIE is the Commission Internationale de l'Eclairage) and indicates that irradiances are estimated within 8% accuracy with solar zenith angles as great as 60 degrees and that dose rates are within 6% for any solar zenith angle. The advantage of having an additional channel in the UV-B region is considered.     

Presence of terrestrial atmospheric gas absorption bands in standard extraterrestrial solar irradiance curves in the near-infrared spectral region

The solar irradiance curves compiled by Wehrli [Physikalisch-Meteorologisches Observatorium Publ. 615 (World Radiation Center, Davosdorf, Switzerland, 1985)] and by Neckel and Labs [Sol. Phys. 90, 205 (1984)] are widely used. These curves were obtained based on measurements of solar radiation from the ground and from aircraft platforms. Contaminations in these curves by atmospheric gaseous absorptions were inevitable. A technique for deriving the transmittance spectrum of the Sun's atmosphere from high-resolution (0.01 cm(-1)) solar occultation spectra measured above the Earth's atmosphere by the use of atmospheric trace molecule spectroscopy (ATMOS) aboard the space shuttle is described. The comparisons of the derived ATMOS solar transmittance spectrum with the two solar irradiance curves show that he curve derived by Wehrli contains many absorption features in the 2.0-2.5-μm region that are not of solar origin, whereas the curve obtained by Neckel and Labs is completely devoid of weak solar absorption features that should be there. An Earth atmospheric oxygen band at 1.268 μm and a water-vapor band near 0.94 μm are likely present in the curve obtained by Wehrli. It is shown that the solar irradiance measurement errors in some narrow spectral intervals can be as large as 20%. An improved solar irradiance spectrum is formed by the incorporation of the solar transmittance spectrum derived from the ATMOS data into the solar irradiance spectrum from Neckel and Labs. The availability of a new solar spectrum from 50 to 50 000 cm(-1) from the U.S. Air Force Phillips Laboratory is also discussed.     

Evaluation of the Applicability of Solar and Lamp Radiometric Calibrations of a Precision Sun Photometer Operating Between 300 and 1025 nm

Over a period of 3 years a precision Sun photometer (SPM) operating between 300 and 1025 nm was calibrated four times at three different high-mountain sites in Switzerland, Germany, and the United States by means of the Langley-plot technique. We found that for atmospheric window wavelengths the total error (2varsigma-statistical plus systematic errors) of the calibration constants V(0) (lambda), the SPM voltage in the absence of any attenuating atmosphere, can be kept below 1.6% in the UV-A and blue, 0.9% in the mid-visible, and 0.6% in the near-infrared spectral region. For SPM channels within strong water-vapor or ozone absorption bands a modified Langley-plot technique was used to determine V(0) (lambda) with a lower accuracy. Within the same period of time, we calibrated the SPM five times using irradiance standard lamps in the optical labs of the Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center, Switzerland, and of the Remote Sensing Group of the Optical Sciences Center, University of Arizona, Tucson, Arizona. The lab calibration method requires knowledge of the extraterrestrial spectral irradiance. When we refer the standard lamp results to the World Radiation Center extraterrestrial solar irradiance spectrum, they agree with the Langley results within 2% at 6 of 13 SPM wavelengths. The largest disagreement (4.4%) is found for the channel centered at 610 nm. The results of these intercomparisons change significantly when the lamp results are referred to two different extraterrestrial solar irradiance spectra that have become recently available.     

Thin-film polycrystalline silicon solar cells formed by flash lamp annealing of a-Si films

We have fabricated thin-film solar cells using polycrystalline silicon (poly-Si) films formed by flash lamp annealing (FLA) of 4.5-µm-thick amorphous Si (a-Si) films deposited on Cr-coated glass substrates. High-pressure water-vapor annealing (HPWVA) is effective to improve the minority carrier lifetime of poly-Si films up to 10 µs long. Diode and solar cell characteristics can be seen only in the solar cells formed using poly-Si films after HPWVA, indicating the need for defect termination. The actual solar cell operation demonstrated indicates feasibility of using poly-Si films formed through FLA on glass substrates as a thin-film solar cell material.     

Application of the SKYRAD Improved Langley plot method for the in situ calibration of CIMEL Sun-sky photometers

The in situ procedure for determining the solar calibration constants, originally developed for the PREDE Sun-sky radiometers and based on a modified version of the Langley plot, was applied to a CIMEL instrument located in Valencia, Spain, not integrated into AERONET. Taking into account the different mechanical and electronic characteristics of the two radiometers, the method was adapted to the characteristics of the CIMEL instrument. The iterative procedure for the determination of the solar calibration constants was applied to a 3-year data set. The results were compared with the two sets of experimental calibration constants determined during this period using the standard Langley plot method. The agreement was found to be consistent with the experimental errors, and the method can definitely also be used to determine the solar calibration constant for the CIMEL instrument, improving its calibration. The method can be used provided the radiometer is previously calibrated for diffuse radiance using a standard lamp.     

Statistical-uncertainty-based adaptive filtering of lidar signals

An adaptive filter signal processing technique is developed to overcome the problem of Raman lidar water-vapor mixing ratio (the ratio of the water-vapor density to the dry-air density) with a highly variable statistical uncertainty that increases with decreasing photomultiplier-tube signal strength and masks the true desired water-vapor structure. The technique, applied to horizontal scans, assumes only statistical horizontal homogeneity. The result is a variable spatial resolution water-vapor signal with a constant variance out to a range limit set by a specified signal-to-noise ratio. The technique was applied to Raman water-vapor lidar data obtained at a coastal pier site together with in situ instruments located 320 m from the lidar. The micrometeorological humidity data were used to calibrate the ratio of the lidar gains of the H(2)O and the N(2) photomultiplier tubes and set the water-vapor mixing ratio variance for the adaptive filter. For the coastal experiment the effective limit of the lidar range was found to be approximately 200 m for a maximum noise-to-signal variance ratio of 0.1 with the implemented data-reduction procedure. The technique can be adapted to off-horizontal scans with a small reduction in the constraints and is also applicable to other remote-sensing devices that exhibit the same inherent range-dependent signal-to-noise ratio problem.     

Spectral control of an alexandrite laser for an airborne water-vapor differential absorption lidar system

A narrow-linewidth pulsed alexandrite laser has been greatly modified for improved spectral stability in an aircraft environment, and its operation has been evaluated in the laboratory for making water-vapor differential absorption lidar measurements. An alignment technique is described to achieve the optimum free spectral range ratio for the two étalons inserted in the alexandrite laser cavity, and the sensitivity of this ratio is analyzed. This technique drastically decreases the occurrence of mode hopping, which is commonly observed in a tunable, two-intracavity-étalon laser system. High spectral purity (> 99.85%) at 730 nm is demonstrated by the use of a water-vapor absorption line as a notch filter. The effective cross sections of 760-nm oxygen and 730-nm water-vapor absorption lines are measured at different pressures by usingthis laser, which has a finite linewidth of 0.02 cm(-1) (FWHM). It is found that for water-vapor absorption linewidths greater than 0.04 cm(-1) (HWHM), or for altitudes below 10 km, the laser line can be considered monochromatic because the measured effective absorption cross section is within 1% of the calculated monochromatic cross section. An analysis of the environmental sensitivity of the two intracavity étalons is presented, and a closed-loop computer control for active stabilization of the two intracavity étalons in the alexandrite laser is described. Using a water-vapor absorption line as a wavelength reference, we measure a long-term frequency drift (≈ 1.5 h) of less than 0.7 pm in the laboratory.     

Radiometric Measurement Comparison Using the Ocean Color Temperature Scanner (OCTS) Visible and Near Infrared Integrating Sphere

As a part of the pre-flight calibration and validation activities for the Ocean Color and Temperature Scanner (OCTS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color satellite instruments, a radiometric measurement comparison was held in February 1995 at the NEC Corporation in Yokohama, Japan. Researchers from the National Institute of Standards and Technology (NIST), the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC), the University of Arizona Optical Sciences Center (UA), and the National Research Laboratory of Metrology (NRLM) in Tsukuba, Japan used their portable radiometers to measure the spectral radiance of the OCTS visible and near-infrared integrating sphere at four radiance levels. These four levels corresponded to the configuration of the OCTS integrating sphere when the calibration coefficients for five of the eight spectral channels, or bands, of the OCTS instrument were determined. The measurements of the four radiometers differed by −2.7 % to 3.9 % when compared to the NEC calibration of the sphere and the overall agreement was within the combined measurement uncertainties. A comparison of the measurements from the participating radiometers also resulted in agreement within the combined measurement uncertainties. These results are encouraging and demonstrate the utility of comparisons using laboratory calibration integrating sphere sources. Other comparisons will focus on instruments that are scheduled for spacecraft in the NASA study of climate change, the Earth Observing System (EOS).     

The 1997 North American Interagency Intercomparison of Ultraviolet Spectroradiometers Including Narrowband Filter Radiometers

The fourth North American Intercomparison of Ultraviolet Monitoring Spectroradiometers was held September 15 to 25, 1997 at Table Mountain outside of Boulder, Colorado, USA. Concern over stratospheric ozone depletion has prompted several government agencies in North America to establish networks of spectroradiometers for monitoring solar ultraviolet irradiance at the surface of the Earth. The main purpose of the Intercomparison was to assess the ability of spectroradiometers to accurately measure solar ultraviolet irradiance, and to compare the results between instruments of different monitoring networks. This Intercomparison was coordinated by NIST and NOAA, and included participants from the ASRC, EPA, NIST, NSF, SERC, USDA, and YES. The UV measuring instruments included scanning spectroradiometers, spectrographs, narrow band multi-filter radiometers, and broadband radiometers. Instruments were characterized for wavelength accuracy, bandwidth, stray-light rejection, and spectral irradiance responsivity. The spectral irradiance responsivity was determined two to three times outdoors to assess temporal stability. Synchronized spectral scans of the solar irradiance were performed over several days. Using the spectral irradiance responsivities determined with the NIST traceable standard lamp, and a simple convolution technique with a Gaussian slit-scattering function to account for the different bandwidths of the instruments, the measured solar irradiance from the spectroradiometers excluding the filter radiometers at 16.5 h UTC had a relative standard deviation of ±4 % for wavelengths greater than 305 nm. The relative standard deviation for the solar irradiance at 16.5 h UTC including the filter radiometer was ±4 % for filter functions above 300 nm.     

In situ measurements of H2O from a stratospheric balloon by diode laser direct-differential absorption spectroscopy at 1.39 microm

A distributed-feedback InGaAs laser diode emitting near 1.393 mum is used in conjunction with an optical multipass cell that is open to the atmosphere to yield ambient water-vapor measurements by infrared absorption spectroscopy. To obtain the high dynamic range for the measurements that is required for continuous water-vapor monitoring in the upper troposphere and the lower stratosphere, we used a simple circuit that combined differential and direct detection. Furthermore, the laser emission wavelength was tuned to balance the steep decrease in H(2)O concentration with altitude by sweeping molecular transitions of stronger line strengths. The technique was implemented by use of the Spectromètre à Diodes Laser Accordables (SDLA), a tunable diode laser spectrometer operated from a stratospheric balloon. Absorption spectra of H(2)O in the 5-30-km altitude range obtained at 1-s intervals during recent balloon flights are reported. Water-vapor mixing ratios were retrieved from the absorption spectra by a fit to the full molecular line shape in conjunction with in situ pressure and temperature measurements, with a precision error ranging from 5% to 10%.     

Ground-based differential absorption lidar for water-vapor and temperature profiling: methodology

A comprehensive formulation of the differential absorption lidar (DIAL) methodology is presented that explicitly includes details of the spectral distributions of both the transmitted and the backscattered light. The method is important for high-accuracy water-vapor retrievals and in particular for temperature measurements. Probability estimates of the error that is due to Doppler-broadened Rayleigh scattering based on an extended experimental data set are presented, as is an analytical treatment of errors that are due to averaging in the nonlinear retrieval scheme. System performance requirements are derived that show that water-vapor retrievals with an accuracy of better than 5% and temperature retrievals with an accuracy of better than 1 K in the entire troposphere are feasible if the error that results from Rayleigh-Doppler correction can be avoided. A modification of the DIAL technique, high-spectral-resolution DIAL avoids errors that are due to Doppler-broadened Rayleigh backscatter and permits simultaneous water-vapor and wind measurements with the same system.     

Determination of the Atmospheric-Water-Vapor Content in the 940-nm Absorption Band by Use of Moderate Spectral-Resolution Measurements of Direct Solar Irradiance

We have analyzed three methods that can be used to determine the integrated water vapor of the atmosphere in the 940-nm band by means of modeled and measured direct solar spectral irradiance. The experimental irradiance data were obtained with a commercial LI-COR 1800 spectroradiometer, based on a monochromator system, of high to moderate spectral resolution (6 nm) in the 300-1100-nm range. The modeled data are based on monochromatic approaches to determine atmospheric transmittance constituents; for those of water vapor we used the lowtran7 model. The first method is a curve-fitting procedure that makes use of the entire shape band absorption information to retrieve a unique water-vapor value. The second method makes use of the monochromatic approach of the absorption transmittance formula to determine the amount of water vapor at each wavelength of the absorption band, and the third method is the classic differential absorption technique suitably applied to our data. Spectral analysis showed the advantages and disadvantages of each method, such as problems linked to the various spectral resolutions of the experimental and the modeled data, the width of the spectral range used to define the water-vapor absorption band, and the dependence of the retrieval on the choice of the two selected wavelengths in the last-named technique. All these problems were considered so they could be avoided or minimized and the associated errors estimated. We used the methods to determine water-vapor values for the period from March to November 1995 at a rural station in Vallodolid, Spain, allowing for the evaluation of the differences in real monitoring conditions. Finally, the contribution of continuum absorption was also evaluated, yielding lower water-vapor values between 13 and 30%. These differences were considerably greater than those that were due to the problems that we have just enumerated.     

Multichannel radiometer calibration: a new approach

The error in irradiance measured with Sun-calibrated multichannel radiometers may be large when the solar zenith angle (SZA) increases. This could be particularly detrimental in radiometers installed at mid and high latitudes, where SZAs at noon are larger than 50 degrees during part of the year. When a multiregressive methodology, including the total ozone column and SZA, was applied in the calculation of the calibration constant, an important improvement was observed. By combining two different equations, an improvement was obtained at almost all the SZAs in the calibration. An independent test that compared the irradiance of a multichannel instrument and a spectroradiometer installed in Ushuaia, Argentina, was used to confirm the results.     

Noise-Optimized Silicon Radiometers

This paper describes a new, experimentally verified, noise analysis and the design considerations of the dynamic characteristics of silicon radiometers. Transimpedance gain, loop gain, and voltage gain were optimized versus frequency for photodiode current meters measuring ac and dc optical radiation. Silicon radiometers with improved dynamic characteristics were built and tested. The frequency-dependent photocurrent gains were measured. The noise floor was optimized in an ac measurement mode using photodiodes of different shunt resistance and operational amplifiers with low 1/f voltage and current noise. In the dark (without any signal), the noise floor of the optimized silicon radiometers was dominated by the Johnson noise of the source resistance. The Johnson noise was decreased and equalized to the amplified 1/f input noise at a 9 Hz chopping frequency and 30 s integration time constant, resulting in an equivalent root-mean-square (rms) photocurrent noise of 8 × 10⁻¹⁷ A. The lowest noise floor of 5 × 10⁻¹⁷ A, equal to a noise equivalent power (NEP) of 1.4 × 10⁻¹⁶ W at the 730 nm peak responsivity, was obtained at a 100 s integration time constant. The radiometers, optimized for ac measurements, were tested in a dc measurement mode as well. Performances in ac and dc measurement modes were compared. In the ac mode, a ten times shorter (40 s) overall measurement time was needed than in the dc mode (400 s) to obtain the same 10⁻¹⁶ A noise floor.     

Measurements of high resolution atmospheric water-vapor profiles by use of a solar blind Raman lidar

Presented here are preliminary results of measurements of atmospheric water-vapor profiles which were obtained by use of a solar blind Raman lidar. Interesting new features of the data gathered include high spatial resolution during daylight hours along with associated measurement errors.     

In situ calibration technique for UV spectral radiometers

A technique for calibrating spectral radiometers measuring global (2π sr) irradiance using solar irradiance at the top of the atmosphere as the absolute irradiance reference is reported. In addition to providing a calibration at all measured wavelengths, the technique provides a direct measure of the angular response of the radiometer. For instruments that can be used to measure the ultraviolet-B region, the calibration also provides an estimate of the ozone column amount.     

Development of n-type microcrystalline SiO<Subscript>x</Subscript>:H films and its application by innovative way to improve the performance of single junction µc-Si:H solar cell

Light trapping is one of the fundamental necessities of thin film based solar cell for its performance elevation. Back reflection of unused light of first pass is the key way to improve the light trapping phenomena. In this study we have reported the development of n-type hydrogenated microcrystalline silicon oxide (n-µc-SiO:H) layers of different characteristics. The deposition has been done by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. The detailed characterization of the films include the following: (1) electrical properties (2) optical properties like E₀₄ (3) structural studies which include crystalline fraction by Raman spectroscopy and grain size by X-ray diffraction measurement, FTIR spectroscopy, AFM and TEM studies. n-µc-SiO:H layer has been introduced as the n-layer of single junction p–i–n structure µc-Si solar cells. By various techniques the optimum use of n-µc-SiO:H layer for enhancing the performance of µc-Si:H solar cells has been done. It has been found that by using suitable bilayer of two different n-µc-SiO:H layers, it is possible to increase the solar cell performances. The maximum efficiency obtained without any back reflector is 8.44% that is about 8.9% higher than that obtained by using n-µc-Si:H layer as n-layer in the solar cells.     

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.     

Determination of the solar calibration constant for a sun-sky radiometer: proposal of an in-situ procedure

A new procedure is presented for determining in situ the solar calibration constant, i.e., the Sun-sky radiometer counts for a direct normal solar flux extrapolated to the top of the atmosphere. The method makes use of a modified version of the Langley plot based on the use of an inversion code of column-integrated aerosol size distribution, and it is ordinarily applied to calibrate Prede Sun-sky radiometers. To analyze how such an in situ method can work accurately, the technique has been applied to a five-month dataset obtained from measurements taken in Rome, Italy, by a Prede Sun-sky radiometer from 22 April to 5 November 2001. The precision of the in situ method has been estimated to within 1-2.5%, depending on the wavelength.