Detector-Based Calibration Method for High-Accuracy Solar UV Measurements

A novel method for calibrating the absolute responsivity of solar UV spectroradiometers has been developed and tested. The method is based on calibrated filter radiometers constructed from a detector, a precision aperture, a band-pass filter and devices for temperature stabilization. The filter radiometers utilize a trap detector with very low reflectance. The filter and the detector can therefore be characterized separately. As an example the detector-based calibration is compared at 312 nm wavelength with lamp-based calibration by measuring the irradiance of a deuterium lamp with both the filter radiometer and the lamp-calibrated spectroradiometer. The agreement between the results is at the level of 1%, well within the estimated uncertainties of both methods.     

THE EFFECT OF CHANGES OR DIFFERENCES IN ROBERTSON-BERGER RADIOMETER RESPONSIVITY ON SOLAR ULTRAVIOLET-B MEASUREMENT

Two long-term solar UV measurement campaigns, in the USA and in Austria, using Robertson-Berger radiometers found opposite trends for measured levels of ultraviolet-B radiation reaching the earth's surface. This could be a consequence of the method used to calibrate the radiometers. Changes or differences in responsivity were compensated for by adjusting the sensitivity of the field radiometers to match the output of a reference radiometer.
This radiometer intercomparison procedure has been evaluated in terms of the normalization wavelength to which the Robertson-Berger effective irradiance refers. There are small differences in spectral responsivities apparent in the radiometers used in the USA campaign, which require the field radiometers to be normalized at different wavelengths to match the response of the reference radiometer. This normalization wavelength is shown to depend on the time-averaged spectrum experienced by the instruments during the intercomparison. As a result there are substantial interradiometer variations in their calculated response to solar radiation when the measured spectral distribution is different from the spectrum used for the radiometer intercomparison procedure.     

STANDARDS FOR UV INSTRUMENTS AND RISK OF EXISTING DEVICES

Abstract— Different methods for measurements of UV-radiation of various light sources are quantitatively compared: direct measurements, based on detectors of known responsivity, as well as indirect measurements, based on a spectral irradiance calibration. The optical arrangement of an accurate spectroradiometric equipment, and the structure and parameters for a new type of cosine corrected detector for the measurement of actinic radiation are described, too. The measurements of actinic radiation with commercial devices are affected by great relative errors.
A standard for natural irradiance will allow a comparison of weighted irradiances and threshold irradiations with natural insolation and with artificial UV radiation. The risk of existing irradiation apparatuses will be calculable by this comparison.     

A LABORATORY INVESTIGATION OF TWO ULTRAVIOLET SPECTRORADIOMETERS

Abstract— Spectral measurements of the solar UV irradiance at the earth's surface depend critically on the characteristics of the spectroradiometers and the procedures used in their calibration. Two UV spectroradiometers were subjected to a series of laboratory tests to investigate which factors were most significant in limiting the reliability of the absolute irradiance measurements. Three independent standards of spectral irradiance were scanned by both instruments under a range of bench conditions. The results were consistent to within about 3%, most of the uncertainty being due to scattered light in the laboratory. An UV laser was used to determine the slit function of each spectrometer and the influence of internal stray light. Significant departures from the ideal cosine and azimuth responses were measured by a xenon lamp. Both spectroradiometers were kept indoors throughout the experiments. The relevance of these laboratory results is discussed with respect to the task of measuring solar UV radiation in the field.     

A COMPARISON OF DOSIMETERS USED FOR SOLAR ULTRAVIOLET RADIOMETRY

Radiometric measurements of terrestrial sunlight using three different types of broad-band dosimeters were compared with equivalent integrated quantities obtained from simultaneous spectroradiometric measurements. Measurements were made at Durham, UK (55 degrees N) during one day in mid-summer and one day in the autumn. By this means it was possible to encompass a wide range of ultraviolet irradiances. There was close agreement between UV-A irradiance measured using a broad-band radiometer and determined spectroradiometrically over the whole range of irradiances when allowance was made for the spectral sensitivity of the UV-A radiometer. The agreement between erythemally-effective irradiance determined spectroradiometrically and the response of a Robertson-Berger meter showed some non-linearity due to the mismatch between the erythema action spectrum and spectral response of the sensor. There was a similar disparity in agreement between erythemally-effective dose determined spectroradiometrically and the response of polysulphone film for similar reasons. Nevertheless it is concluded that if these latter two dosimeters are calibrated using sunlight, or a solar simulator, as the source, they can yield data which are sufficiently reliable for many applications.     

Spectral Mismatch Effect of Ultraviolet Radiometers in Actual UV-C Measurement

The management of radiant exposure to ultraviolet (UV) radiation, especially in the wavelength range from 100 nm to 280 nm (i.e. UV-C), is important for virus inactivation or photobiological safety. Recently, many commercial UV radiometers have been used to measure UV-C irradiance for industrial and public applications. The accuracy of the four types of commercial UV radiometers was investigated by comparing the reference irradiance values obtained from the spectral irradiance standard. It was found that the displayed values of the UV radiometers have discrepancies, such that the measured value can be more than twice the actual value in a certain case. The spectral mismatch between the calibration and test sources is a major factor in the discrepancies in the UV-C measurements. With spectral mismatch correction, most corrected values show a tendency to improve the result to approaching the reference values within 20%. Users need to provide spectral information about the source and radiometer used for UV-C measurement.     

A selective Pt-CdS photodiode to monitor erythemal flux

The design and potential benefit of a solar ultraviolet (UV) radiometer reporting a maximum instantaneous flux of erythemally weighted heterogeneous energy is considered. The proposed device is electronically peak detecting; the user would ideally 'point and paint' the sun to find a localized maximum. A projected exposure time can be calculated from an instantaneous reading of erythemally weighted flux for a given minimal erythemal dose (MED) specified by the user. This calculation, though not necessarily providing a true exposure time, may be useful and informative in that it serves as a more 'recognizable' measure of erythemal flux and introduces a custom scale for each individual via their MED. Erythemal flux is calculated as the weighted integral sum [symbol: see text]j(lambda,t) epsilon(lambda) d lambda, where j (lambda, t) is the instantaneous angular integrated spectral irradiance accepted by human skin. This instrument proposal uses a single interference filter over a Pt-CdS photodiode; the interference filter is offered as a nominal design transmittance. The simulated response of the selective photodiode has a near-linear relation to the effective irradiance. Test inputs for evaluation purposes and to elucidate a transducer response are constructed from a spline interpolation of the World Radiation Center (WRC) spectrum and classic transmittance models. Our desired erythemal flux is offered in interconvertible UV Indexes (UVIs) as a function of zenith angle and atmosphere, characterized by elevation, ozone path, and turbidity.     

Production of certified reference materials for mycotoxins: IRMM’s view on the assessment of uncertainties

Analytical difficulty and the economic importance of controlling mycotoxin levels in food and feed led the Community Bureau of Reference (BCR) to prepare a series of certified reference materials (CRM) for various mycotoxins. Because of the wide acceptance of these CRM and the need to ensure the comparability and traceability of measurements in the future it is necessary to prepare and certify new batches of mycotoxin reference materials (RM). In the following text two different approaches for evaluation of the characterisation uncertainty of CRM will be compared using the certification of aflatoxin M₁ (AfM₁) in milk powder as an example. The conventional approach is based on evaluation of characterisation exercise data; the alternative approach is based on measurement uncertainties of the employed analytical methods. Because laboratories are using totally different approaches to estimate the measurement uncertainties, combination of the uncertainties obtained from the participating laboratories was not recommended. Therefore, a new integrated approach for assessment of the measurement uncertainties of the analytical methods on the basis of additional data collected during the characterisation exercise will be described. The conventional approach was found to be the most appropriate and economical approach to evaluate the characterisation uncertainty as a characterisation exercise must be performed anyway to establish the property values of candidate (C)RM, irrespective of whether or not reliable measurement uncertainties can be provided by the laboratories. An integrated approach for assessment of measurement uncertainties based on additional characterisation data as applied here to enable use of an uncertainty-based approach provides more information but is too time-consuming and cost-intensive to become common practice.     

On the importance of spectral responsivity of Robertson-Berger-type ultraviolet radiometers for long-term observations

A system to determine the spectral responsivity of ultraviolet (UV) radiometers has been developed and is routinely operated at the Central Ultraviolet Calibration Facility, at the National Oceanic and Atmospheric Administration. The instrument and the measurement methodologies are described. Results of measurements from thermally controlled broadband UV radiometers of the Robertson-Berger (R-B)-type are described. Systematic differences in the spectral response curves for these instruments have been detected. The effect of these differences on the field operation of UV-B radiometers has been studied by calculating the instrumental response from modeled UV spectra. The differences of the weighted spectral UV irradiances, measured by two radiometers with different spectral response functions, caused by the daily variation in the position of the sun were estimated for fixed values of total ozone, altitude and albedo, and for cloud-free conditions. These differences increase with the solar zenith angle and are as large as 8%. Larger differences in the instrumental response may be produced by ozone variations. Thus, care must be taken when analyzing data from R-B radiometers and comparing results from different instruments. Routine cycling of UV-B radiometers in operative networks without a careful determination of the spectral responsivity, or small drifts of the spectral responsivity, may strongly affect the accuracy of UV radiation measurements and produce an erroneous trend. Because of the possible differences among radiometers, it would not be practical to derive the long-term behavior of UV radiation without routine and thorough characterization of the spectral responsivities of the instruments.     

Modified calibration procedures for a Yankee Environmental System UVB-1 biometer based on spectral measurements with a brewer spectrophotometer

The calibration of the erythemal irradiance measured by a Yankee Environmental System (YES) UVB-1 biometer is presented using two methods of calibration with a wide range of experimental solar zenith angles (SZAs) and ozone values. The calibration is performed through simultaneous spectral measurements by a calibrated double-monochromator Brewer MK-III spectrophotometer at "El Arenosillo" station, located in southwestern Spain. Because the range of spectral measurements of the Brewer spectrophotometer is 290-363 nm, a previously validated radiative transfer model was used to account for the erythemal contribution between 363 and 400 nm. Both methods are recommended by the World Meteorological Organization and we present and discuss here a wide range of results and features given by modified procedures applied to these two general methods. As is well established, the calibration factor for this type of radiometric system is dependent on atmospheric conditions, the most important of which are the ozone content and the SZA. Although the first method is insensitive to these two factors, we analyze this behavior in terms of the range used for the SZA and the use of two different mathematical approaches for its determination. The second method shows the dependence on SZA and ozone content and, thus, a polynomial as a function of SZA or a matrix including SZA and ozone content were determined as general calibration factors for the UV radiometric system. We must note that the angular responses of the YES radiometer and Brewer spectroradiometer have not been considered, because of the difficulty in correcting them. The results show in detail the advantages and drawbacks (and the corresponding associated error) given by the different approaches used for the determination of these calibration coefficients.     

Validation of Ozone Monitoring Instrument UV Satellite Data Using Spectral and Broadband Surface Based Measurements at a Queensland Site

This research reconstructed and validated the broadband UVA irradiances derived from discrete spectral irradiance data retrieved from the Ozone Monitoring Instrument (OMI) satellite from 1 January to 31 December 2009. OMI data at solar noon were compared to ground‐based spectral irradiances at Toowoomba (27°36′ S 151°55′ E), Australia, at 310, 324 and 380 nm for both cloud‐free and all sky conditions. There was a strong relationship between the ground‐based UV spectroradiometer data and satellite‐based measurements with an R² of 0.89 or better in each waveband for cloud‐free days. The data show an overestimate of the satellite‐derived spectral irradiances compared to the ground‐based data. The models developed for the subtropical site data account for this overestimation and are essential for any data correlation between satellite‐ and ground‐based measurements. Additionally, this research has compared solar noon broadband UVA irradiances evaluated with a model and the discrete satellite spectral irradiances for the solar noon values of cloud‐free days to those measured with a ground‐based UVA radiometer. An R² of 0.86 was obtained confirming that for cloud‐free days the broadband UVA can be evaluated from the OMI satellite spectral irradiances.     

Fiber‐Optic Probes for Small‐Scale Measurements of Scalar Irradiance

A new method for producing fiber‐optic microprobes for scalar irradiance (=fluence rate) measurements is described. Such fine‐scale measurements are important in many photobiological disciplines. With the new method, it is possible to cast spherical 30–600 μm wide light integrating sensor tips onto tapered or untapered optical fibers. The sensor tip is constructed by first casting a clear polymethyl methacrylate (PMMA) sphere (~80% of the size of the final probe tip diameter) onto the optical fiber via dip‐coating. Subsequently, the clear sphere is covered with light diffusing layers of PMMA mixed with TiO₂ until the fiber probe exhibits a satisfactory isotropic response (typically ±5–10%). We also present an experimental setup for measuring the isotropic response of fiber‐optic scalar irradiance probes in air and water. The fiber probes can be mounted in a syringe equipped with a needle, facilitating retraction of the spherical fiber tip. This makes it, e.g. possible to cut a hole in cohesive tissue with the needle before inserting the probe. The light‐collecting properties of differently sized scalar irradiance probes (30, 40, 100, 300 and 470 μm) produced by this new method were compared to probes produced with previously published methods. The new scalar irradiance probes showed both higher throughput of light, especially for blue light, as well as a better isotropic light collection over a wide spectral range. The new method also allowed manufacturing of significantly smaller scalar irradiance microprobes (down to 30 μm tip diameter) than hitherto possible, and such sensors allow minimally invasive high‐resolution scalar irradiance measurements in thin biofilms, leaves and animal tissues.     

Recent developments with light-exposure apparatus and measurement of radiation of their light sources

New techniques and devices which were recently developed in order to avoid various drawbacks of current light-exposure apparatus are introduced. The spectral distribution of the open flame carbon arc approximates more closely to that of sunlight by modifications of the filter system.The life of carbon arcs was lengthened more than two fold. The long-life xenon arc lamp minimized the reduction in output of radiation.The slanted holder improved the locational variance of radiation falling on a specimen.The instrumental methods for measuring light dosage are explained.     

An innovative method for dead time correction in nuclear spectroscopy

All nuclear spectroscopy systems, whether measuring charged particles, X-rays, or gamma-rays, exhibit dead time losses during the counting process due to pulse processing in the electronics. Several techniques have been employed in an effort to reduce the effects of dead time losses on a spectroscopy system including live time clocks and loss-free counting modules. Live time extension techniques give accurate results when measuring samples in which the activity remains roughly constant during the measuring process (i.e., the dead time does not change significantly during a single measurement period). The loss-free counting method of correcting for dead time losses, as introduced by HARMS and improved by WESTPHAL (US Patent No. 4,476,384) give better results than live time extension techniques when the counting rate changes significantly during the measurement. However, loss-free counting methods are limited by the fact that an estimation of the uncertainty associated with the spectral counts can not be easily determined, because the corrected data no longer obeys Poisson statistics. Therefore, accurate analysis of the spectral data including the uncertainty calculations is difficult to achieve. The Ortec® DSPEC ^PLUS implements an improved zero dead time method that accurately predicts the uncertainty from counting statistics and overcomes the limitations of previous loss-free counting methods. The uncertainty in the dead-time corrected spectrum is calculated and stored with the spectral data (Patent Pending). The GammaVision-32® analysis algorithm has been improved to propagate this uncertainty through the activity calculation. Two experiments are set up to verify these innovations. The experiments show that the new method gives the same reported activity and associated uncertainties as the well-proven Gedcke-Hale live time clock. It is thus shown that over a wide range of dead times the new ZDT method tracks the true counting rate as if it had zero dead time, and yields an accurate estimation of the statistical uncertainty in the reported counts.     

SOME PROBLEMS IN THE ABSOLUTE MEASUREMENT OF GERMICIDAL ULTRAVIOLET RADIATION: THE USE OF ''PEN RAY'' LAMPS AS A CALIBRATION STANDARD

Abstract— Problems of absolute measurement of the dose rate of ultraviolet radiation of germicidal lamps in energetic units were studied. Irradiance at 254 nm generated by three different Pen Ray SC-1 low-pressure mercury lamps was measured independently in different laboratories using different instruments: (a) Westinghouse SM-600 Meter, (b) General Electric Germicidal Meter, and (c) large-surface thermopile with a Bäckström filter. These lamps were then used as secondary standards of absolute irradiance at wavelength 254 nm and compared with the Latarjet dosimeter and the International Light IL-254 Germicidal Photometer. Mutual agreement of calibration coefficients of three calibrated Pen Ray SC-1 lamps was roughly within ±5 per cent error. This calibration uncertainty indicates limits of the usefulness of Pen Ray lamps as standards. A direct radiometric calibration using an FT-16 Schwarz-Hilger vacuum thermopile, with interference filter NB-254 or UVR-250, was in agreement with the above comparison. On the basis of above radiometric calibration absolute D₃₇ lethal doses were determined equaling 2·7. J/m² for the bacteriophage T2 and 11·0 J/m² for the bacteriophage φX-174, the values being read from exponential survival curves.     

Optical properties and redox state of silicate glass melts

Advanced experimental techniques to measure the spectral absorption and oxidation state of silicate glass melts are described. Exemplary results of the characterisation of industrial glass melts by these two methods are presented. The technological impact of the high-temperature optical properties and oxidation state of glass melts on radiative heat transfer and on the final glass colour is discussed.     

Stand-off Raman detection using dispersive and tunable filter based systems

Small, transportable Raman systems are being developed for stand-off Raman measurements at intermediate ranges (e.g. <20 m) for planetary measurements. Four variations of stand-off Raman systems are described that use a small telescope for light collection that is either fiber-optic or lens-coupled to a detection system. The performance of an acousto-optic tunable filter for wavelength selection and spectral imaging is tested by comparing signal-to-noise ratio and throughput to similar measurements using a conventional spectrograph, and by measuring a variety of organic and inorganic mineral samples at distances up to 15 m. We also determine optimal ICCD gate widths for acquiring remote Raman spectra under high ambient light conditions.     

PHOTON COUNTING SPECTRAL ANALYZING SYSTEM OF EXTRA-WEAK CHEMI- AND BIOLUMINESCENCE FOR BIOCHEMICAL APPLICATIONS

Abstract— The systematic study of the method for measuring the spectral distribution of extra-weak light signals, such as from biochemical systems and living tissues involving chemi- and bio-energized processes, led to the design and construction of a new type of spectrometer, called the filter spectral analyzer system, incorporated with the high-sensitivity photon counting technique. Experimental tests to examine the operational characteristics of this filter spectral analyzer system and comparisons of the signal-to-noise ratio with a conventional grating spectrometer were made by accomplishing the spectral analysis of low-level light sources. The measurement of the spectra of very weak chemilumines-cence accompanying various kinds of chemical and biological processes, including autoxidation and enzymatic reaction were performed successfully.     

SOLAR SPECTRAL IRRADIANCE IN THE VISIBLE AND INFRARED REGIONS

Abstract— The analytical formulas previously developed for estimating the spectral irradiance reaching the ground in the ultraviolet are extended into the visible and infrared(350–3000 nm). This approach has two distinct features: (1) all physical inputs for calculating the direct irradiance are given in analytical form, and (2) the diffuse spectral irradiance (skylight) is calculated using dimensionless ratios which relate it to the direct irradiance. In common with other approaches, the global spectral irradiance for arbitrary ground reflectivity is calculated from the sum of the direct and diffuse spectral irradiances and a divisor which depends upon the ground and air spectral reflectivities. The global spectral irradiance on a tilted surface may also be calculated in terms of the above quantities and two angles. As in the case of the ultraviolet, the formulas presented are intended for photobiological applications.