Narrowband filter radiometer for ground-based measurements of global ultraviolet solar irradiance and total ozone

The ultraviolet narrowband filter radiometer (UV-RAD) designed by the authors to take ground-based measurements of UV solar irradiance, total ozone, and biological dose rate is described, together with the main characteristics of the seven blocked filters mounted on it, all of which have full widths at half maxima that range 0.67 to 0.98 nm. We have analyzed the causes of cosine response and calibration errors carefully to define the corresponding correction terms, paying particular attention to those that are due to the spectral displacements of the filter transmittance peaks from the integer wavelength values. The influence of the ozone profile on the retrieved ozone at large solar zenith angles has also been examined by means of field measurements. The opportunity of carrying out nearly monochromatic irradiance measurements offered by the UV-RAD allowed us to improve the procedure usually followed to reconstruct the solar spectrum at the surface by fitting the computed results, using radiative transfer models with field measurements of irradiance. Two long-term comparison campaigns took place, showing that a mean discrepancy of +0.3% exists between the UV-RAD total ozone values and those given by the Brewer #63 spectroradiometer and that mean differences of +0.3% and -0.9% exist between the erythemal dose rates determined with the UV-RAD and those obtained with the Brewer #63 and the Brewer #104 spectroradiometers, respectively.     

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.     

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.     

Charge-coupled device spectrograph for direct solar irradiance and sky radiance measurements

The characterization of a charged-coupled device (CCD) spectrograph developed at the Laboratory of Atmospheric Physics, Thessaloniki is presented. The absolute sensitivity of the instrument for direct irradiance and sky radiance measurements was determined, respectively, with an uncertainty of 4.4% and 6.6% in the UV-B, and 3% and 6% in the UV-A, visible and near-infrared (NIR) wavelength ranges. The overall uncertainty associated with the direct irradiance and the sky radiance measurements is, respectively, of the order of 5% and 7% in the UV-B, increasing to 10% for low signals [e.g., at solar zenith angles (SZAs) larger than 70 degrees ], and 4% and 6% in the UV-A, visible, and NIR. Direct solar spectral irradiance measurements from an independently calibrated spectroradiometer (Bentham DTM 300) were compared with the corresponding CCD measurements. Their agreement in the wavelength range of 310-500nm is within 0.5% +/- 1.1% (for SZA between 20 degrees and 70 degrees ). Aerosol optical depth (AOD) derived by the two instruments using direct Sun spectra and by a collocated Cimel sunphotometer [Aerosol Robotic network (AERONET)] agree to within 0.02 +/- 0.02 in the range of 315-870 nm. Significant correlation coefficients with a maximum of 0.99 in the range of 340-360 nm and a minimum of 0.90 at 870 nm were found between synchronous AOD measurements with the Bentham and the Cimel instruments.     

Automated multifilter rotating shadow-band radiometer: an instrument for optical depth and radiation measurements

The multifilter rotating shadow-band radiometer is a ground-based instrument that uses independent interference-filter-photodiode detectors and the automated rotating shadow-band technique to make spectrally resolved measurements at seven wavelength passbands (chosen at the time of manufacture between 350 nm and 1.7 μm) of direct-normal, total-horizontal, and diffuse-horizontal irradiances. This instrument achieves an accuracy in direct-normal spectral irradiance comparable with that of tracking radiometers, and it is more accurate than conventional instruments for the determination of the diffuse and total-horizontal spectral irradiances because the angular acceptance function of the instrument closely approximates the ideal cosine response, and because the measured direct-normal component can be corrected for the remaining angular acceptance error. The three irradiance components are measured with the same detector for a given wavelength. Together with the automated shadow-band technique, this guarantees hat the calibration coefficients are identical for each, thus reducing errors when one compares them (as opposed to measurements made with independent instruments). One can use the direct-normal component observations for Langley analysis to obtain depths and to provide an ongoing calibration against the solar constant by extrapolation to zero air mass. Thus the long-term stability of all three measured components can be tied to the solar constant by an analysis of the routinely collected data.     

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.     

Instrument self-shading in underwater optical measurements: experimental data

Self-shading error of in-water optical measurements has been experimentally estimated for upwelling radiance and irradiance measurements taken just below the water surface. Radiance and irradiance data have been collected with fiber optics that terminated with 1°, 18°, and 2π optics housed in the center of a disk that simulated the size of the instrument. Analysis of measurements taken at 500, 600, and 640 nm in lake waters have shown errors ranging from a few percent up to several tens of percent as a function of the size of the radiometer, the absorption coefficient of the medium, the Sun zenith, and the atmospheric turbidity. Comparisons between experimental and theoretical errors, the latter computed according to a scheme suggested by other authors, have shown absolute differences generally lower than 5% for radiances and lower than 3% for irradiances. Analysis of radiance measurements taken with 1° and 18° fields of view have not shown appreciable differences in the self-shading error. This finding suggests that correction schemes for self-shading error developed for narrow-field-of-view radiance measurements could also be applied to measurements taken with relatively larger fields of view.     

Spectral reflectance of silicon photodiodes

A precision spectrometer was used to measure the spectral reflectance of a silicon photodiode over the wavelength range from 250 to 850 nm. The results were compared with the corresponding values predicted by a model based on thin-film Fresnel formulas and the known refractive indices of silicon and silicon dioxide. The good agreement at the level of 2 x 10(-3) in the visible wavelength range verifies that the reflection model can be used for accurate extrapolation of the spectral reflectance and responsivity of silicon photodiode devices. In addition, characterization of the photodiode reflectance in the ultraviolet region improves the accuracy of the spectral irradiance measurements when filter radiometers based on trap detectors are used.     

Calibration and uncertainty estimation of erythemal radiometers in the argentine ultraviolet monitoring network

The erythemal radiometers of the Ultraviolet Monitoring Network of the Argentine Servicio Meteorológico Nacional were calibrated in an extensive in situ campaign from October 1998 to April 1999 with Austrian reference instruments. Methods to correct the influence of the location's horizon and long-term detector changes are applied. The different terms that contribute to the measurement uncertainty are analyzed. The expanded uncertainty is estimated to be +/- 10% at 70 degrees solar zenith angle (SZA) and +/-6% for a SZA of <50 degrees. We observed significant changes for some detectors over hours and days, reaching a maximum diurnal drift of +/-5% at a SZA of 70 degrees and a maximum weekly variation of +/-4%.     

Absolute spectral measurements of direct solar ultraviolet irradiance with a Brewer spectrophotometer

A methodology for the absolute calibration of spectral measurements of direct solar ultraviolet radiation, performed with a Brewer spectrophotometer is presented. The method uses absolute measurements of global and diffuse solar irradiance obtained practically simultaneously at each wavelength with the direct-Sun component. On the basis of this calibration, direct-Sun spectra, measured over a wide range of solar zenith angles at a high altitude site, were used to determine the extraterrestrial solar spectrum by applying the Langley extrapolation method. Finally this spectrum is compared with a solar spectrum derived from the airborne tunable laser absorption spectrometer 3 Space Shuttle mission, showing an agreement of better than +/-3%.     

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.     

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.     

Traveling reference spectroradiometer for routine quality assurance of spectral solar ultraviolet irradiance measurements

A transportable reference spectroradiometer for measuring spectral solar ultraviolet irradiance has been developed and validated. The expanded uncertainty of solar irradiance measurements with this reference spectroradiometer, based on the described methodology, is 8.8% to 4.6%, depending on the wavelength and the solar zenith angle. The accuracy of the spectroradiometer was validated by repeated site visits to two European UV monitoring sites as well as by regular comparisons with the reference spectroradiometer of the European Reference Centre for UV radiation measurements in Ispra, Italy. The spectral solar irradiance measurements of the Quality Assurance of Spectral Ultraviolet Measurements in Europe through the Development of a Transportable Unit (QASUME) spectroradiometer and these three spectroradiometers have agreed to better than 6% during the ten intercomparison campaigns held from 2002 to 2004. If the differences in irradiance scales of as much as 2% are taken into account, the agreement is of the order of 4% over the wavelength range of 300-400 nm.     

An algorithm to evaluate solar irradiance and effective dose rates using spectral UV irradiance at four selected wavelengths

The paper shows a semi-analytical method for environmental and dosimetric applications to evaluate, in clear sky conditions, the solar irradiance and the effective dose rates for some action spectra using only four spectral irradiance values at selected wavelengths in the UV-B and UV-A regions (305, 320, 340 and 380 nm). The method, named WL4UV, is based on the reconstruction of an approximated spectral irradiance that can be integrated, to obtain the solar irradiance, or convoluted with an action spectrum to obtain an effective dose rate. The parameters required in the algorithm are deduced from archived solar spectral irradiance data. This database contains measurements carried out by some Brewer spectrophotometers located in various geographical positions, at similar altitudes, with very different environmental characteristics: Rome (Italy), Ny Alesund (Svalbard Islands, Norway) and Ushuaia (Tierra del Fuego, Argentina). To evaluate the precision of the method, a double test was performed with data not used in developing the model. Archived Brewer measurement data, in clear sky conditions, from Rome and from the National Science Foundation UV data set in San Diego (CA, USA) and Ushuaia, where SUV 100 spectroradiometers operate, were drawn randomly. The comparison of measured and computed irradiance has a relative deviation of about +/-2%. The effective dose rates for action spectra of Erythema, DNA and non-Melanoma skin cancer have a relative deviation of less than approximately 20% for solar zenith angles <50 degrees .     

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.     

Method for correcting the wavelength misalignment in measured ultraviolet spectra

A new method for correcting the wavelength misalignment in measured UV spectra is presented. It is based on a comparison between measured irradiances and irradiances computed from a radiative transfer code for a set of given atmospheric and solar conditions (250 < Dobson units < 450, 30 degrees < solar zenith angle < 75 degrees ). Results of tests run with spectra recorded on a clear-sky day by two spectroradiometers in a French UV spectral network station are analyzed. Applying the method once reduces shift to less than 0.05 nm. The smoothing included in the method enables detection of aberrant irradiance values and then completion of an initial quality control of measured spectra. A technique for assessing the instruments' slit function is also presented. The key algorithms needed to build a computer code based on this method are given.     

Effect of topography on average surface albedo in the ultraviolet wavelength range

The reflectivity of the 22 km x 24 km region surrounding Sonnblick Observatory near Salzburg, Austria (3104-m altitude, 47.05 degrees N, 12.95 degrees E), was calculated with a three-dimensional albedo model. The average albedo of the region was calculated at 305 and 380 nm for different solar zenith angles, ground reflectances, and solar azimuth angles. To determine geometrical effects, we first carried out the simulations without taking account of the effects of the atmosphere. The ratio to the reflectivity of a corresponding flat surface area (area with the same ground characteristics) was always less than 1 and showed a decrease with increasing solar zenith angle and with diminishing ground reflectance. Even when the ground reflectance was 100%, the average albedo was less than 1. The effect of the atmosphere was then taken into consideration in these calculations and was found to diminish the reflected components. This diminishing effect was compensated for, however, by the scattered irradiance. Finally, simulations of real conditions (nonhomogeneous ground reflectivities) were performed for different snow lines in the Sonnblick region. The average albedos obtained when all the surroundings were covered with snow were 0.32-0.63 with low solar zenith angles and 0.38-0.77 with a 40 degrees solar zenith angle.     

Underwater light polarization and radiance fluctuations induced by surface waves

The underwater light field is an ever-changing environment. Surface waves induce variability in the radiance and the light's polarization. We examined the dependence of the polarization fluctuations associated with diffuse light (not including contribution from direct skylight) on the viewing zenith angle (30 degrees, 70 degrees, and 90 degrees), solar zenith angle (23 degrees -72 degrees), depth of 0.5-3 m, and light wavelength (380-650 nm) while observing within the azimuthal plane in the wind-wave direction. Polarization and radiance fluctuated with time. Light variability (presented by the coefficient of variation calculated over a series of fluctuations in the radiance and percent polarization, and by the standard deviation calculated over a series of fluctuations in the e-vector orientation) was highest at a viewing zenith angle of 70 degrees , depended positively on the solar zenith angle, and decreased with depth at viewing zenith angles of 30 degrees and 70 degrees . Additionally, the variability of the percent polarization was significantly higher than that of the radiance. The temporal light fluctuations offer possibilities, such as enhancing the detection of transparent and reflecting objects; however, they set constraints on the optimal underwater polarization vision by both animals and by the use of instruments.     

Estimation of absorption and backscattering coefficients from in situ radiometric measurements: theory and validation in case II waters

A model that relates the coefficients of absorption (a) and backscattering (b(b)) to diffuse attenuation (K(d)), radiance reflectance (R(L)), and the mean cosine for downward irradiance (mu(d)) is presented. Radiance transfer simulations are used to verify the physical validity of the model for a wide range of water column conditions. Analysis of thee radiance transfer simulations suggest that absorption and backscattering can be estimated with average errors of 1% and 3%, respectively, if the value of mu(d) is known with depth. If the input data set is restricted to variables that can be derived from measurements of upward radiance (L(u)) and downward irradiance (E(d)), it is necessary to use approximate values of mu(d). Examination of three different approximation schemes for mu(d) shows that the average error for estimating a and b(b) increases to approximately 13%. We tested the model by using measurements of L(u) and E(d) collected from case II waters off the west coast of Scotland. The resulting estimates of a and b(b) were compared with independent in situ measurements of these parameters. Average errors for the data set were of the order of 10% for both absorption and backscattering.     

Atmospheric correction of ocean color sensors: computing atmospheric diffuse transmittance

Using the reciprocal equation derived by Yang and Gordon [Appl. Opt. 36, 7887-7897 (1997)] for atmospheric diffuse transmittance of the ocean-atmosphere system, I examined the accuracy of an analytical equation proposed by Gordon et al. [Appl. Opt. 22, 20-36 (1983)] in computing the atmospheric diffuse transmittance for wavelengths from 412 to 865 nm for both a pure Rayleigh and a two-layer Rayleigh-aerosol atmosphere overlying a flat Fresnel-reflecting ocean surface. It was found that for viewing angles up to approximately 40 degrees , the analytical formula produces errors usually between 2% and 3% for nonabsorbing and weakly absorbing aerosols and for aerosol optical thicknesses tau(a) 相似文献