The two Brewer spectrophotometers of the Finnish Meteorological Institute at
Jokioinen and Sodankylä have been operated according to the highest
levels of the WMO
The Brewer spectrophotometers are used to measure total atmospheric column ozone and spectral irradiance at the UV part of the solar spectrum. The absolute calibration of a spectrophotometer is a crucial part of the measurement chain needed to obtain solar UV spectra with the lowest achievable uncertainty. In fact, maintenance of reliable absolute calibration may be considered as the most important requirement in UV spectroradiometry (Garane et al., 2006). The calibration is a challenging task due to several reasons: it has to be considered that the instrument measures each of the solar irradiance wavelengths separately, the signals at short wavelengths are orders of magnitude smaller than at longer wavelengths, the uncertainties differ at different wavelengths, the calibration lamps themselves are ageing and as a result their spectral output is changing whenever they are used, and multiple transfer standards need to be used.
Estimations on the uncertainties in measurements of solar UV(-B) irradiance range from 5 to 10 % (Gardiner, 1997). The dominating source of uncertainty is attributable to uncertainties in the calibration standards (Eleftheratos et al., 2014). Intercomparisons of the irradiance scales disseminated by the different National Metrology Institutes (NMIs) have varied from 2 to 5 % (Walker et al., 1991; Webb et al., 2003). While the uncertainties of the scales provided by NMIs are expected to decrease, as the institutes re-establish their scales by linking them to primary detector scales, the uncertainties related to the performance of transfer standard lamps and the calibration set-up remain the key components in the overall uncertainty.
Guidelines for quality control and quality assurance for solar UV irradiance
measurements have been prepared under the auspices of WMO
A review of the literature shows that the methods and algorithms differ amongst the stations maintaining spectrophotometers for measurements of solar spectral UV irradiance (Lam et al., 2002; Kazadzis et al., 2005; Garane et al., 2006, Lakkala et al., 2008; Simic et al., 2008; Schneider et al., 2008; Ialongo et al., 2008; Sabburg et al., 2001; Damiani et al., 2012). The measurements with Brewer #037 and Brewer #107 spectrophotometers at Sodankylä and Jokioinen respectively have followed the WMO recommendations, with stability checks every 3–4 weeks and measurements of standard lamps in laboratory (darkroom) every 6–8 weeks. In this study, the lamp measurements collected over the operational years of the spectrophotometers are looked into to examine the changes in the lamps and in the responsivities of the instruments. In addition to the long-term changes, the short-term variations and abrupt changes are of interest. Furthermore, the method adopted in the determination of the responsivity of the instrument is presented. Both the Level 1 responsivity used for online near real-time processing and the Level 2 responsivity used for offline post-processing of solar UV irradiance spectra are dealt with.
The Brewer spectrophotometer (Brewer, 1973; Bais et al., 1996) is a widely
used instrument to monitor solar UV radiation. The instrument is used to
collect photons of the incoming radiation through the entrance optics,
separate the incoming photons according to their wavelength by a
monochromator, and record the amount of photons at each wavelength. The
Finnish Meteorological Institute has operated Brewer spectrophotometers at
two meteorological sounding stations, one in Sodankylä
(67.37
The solar UV photons collected by a spectrophotometer are converted into the
principal measurand spectral irradiance, in units W m
The calibration of the Brewer spectrophotometers #037 and #107 is based on primary (reference) standard lamps regularly recalibrated in the National Standards Laboratory at VTT MIKES Metrology (hereinafter denoted as NSL) and regular lamp measurements in the on-site optical laboratories in Sodankylä and Jokioinen. Measurements of 1000 W standard lamps are used as a basis for the determination of the responsivity of the instruments. In addition to the primary standard lamps recalibrated regularly by NSL, several secondary and working standard lamps are used. The irradiance scale is transferred from the primary standard lamps to the secondary and working standard lamps to avoid unnecessary burning and consequent premature ageing of the primary standards. Secondary standard lamps are used to preserve the calibration provided by primary lamps and are therefore burned sparingly. Working standard lamps are used most frequently in the on-site laboratory measurements.
A portable lamp unit with 50 W lamps has been used both outdoors for stability checks and in laboratory for ancillary measurements. The outdoor lamp measurements are done in the evening, preferably after sunset. In case the measurements have to be made before sunset, separate solar UV scans are taken in between the lamp scans to ensure adequate sampling of the accumulating daily UV dose. The frequent measurements of the 50 W lamps are useful for detection of occasional sudden changes in the response of the instruments.
The first lamp measurements with a portable calibrator and 50 W lamps were
performed in June 1990 in Sodankylä and in January 1997 in Jokioinen. The
on-site optical laboratories were built in 1997, and the first scans of
1000 W lamps in the darkroom were made in June and October 1997 in Jokioinen
and Sodankylä respectively. Until the end of 2015, the lamp measurements
extending over 25 years for Brewer #037 has resulted in 1931 lamp scans on
325 days. For Brewer #107, a data set of 2012 lamp scans on 553 days has
accumulated. The procedure used for the scanning of the lamps has slightly
changed over the years. In Sodankylä, two different ways have been used
as follows:
6 June 1990–4 December 1997: upward–downward scan with 35 Å step 5 December 1997–present: upward scan with 5 Å step 7 January 1997–10 January 1997: upward scan 2900–3630 Å, step
10 Å 22 January 1997–25 June 1997: upward scan 2865–3630 Å, step 5 Å 26 June 1997–present: upward scan 2865–3650 Å, step 5 Å
In Jokioinen, three different ways have been used:
Figure 1 shows examples of dark count corrected raw counts produced and
printed out into files named XLjjjyy.107 and ULjjjyy.037 (jjj denoting the
Julian date, yy denoting the year) for lamp scans performed by Brewer #107
and Brewer #037 respectively in units counts cycle
Typical spectral irradiance measurements of a 1000 W standard lamp as raw counts per cycle. Brewer #037 measurement taken with primary standard lamp D24 on 20 March 2012. Brewer #107 measurement taken with primary standard lamp D01 on 26 January 2012.
In external lamp scans of Brewer #107, the slit mask is cycled through 30
oscillations for wavelengths shorter than 300 nm and through 20 oscillations
for wavelengths longer than 300 nm. More cycles are used at the lower
wavelengths to ensure acceptable statistics, i.e. collection of photons for
acceptable signal-to-noise ratio. The photon counts
Time series of the raw counts (in units counts per cycle) at 311 nm recorded by Brewer #037 spectrophotometer in measurements of a selection of 1000 W lamps. The lamps marked with an asterisk are primary standard lamps regularly recalibrated in VTT MIKES Metrology.
Time series of the raw counts (in units counts per cycle) at 311 nm recorded by Brewer #107 spectrophotometer in measurements of a selection of 1000 W lamps. The lamps marked with an asterisk are primary standard lamps regularly recalibrated in VTT MIKES Metrology.
The time series of the raw counts may be examined as such to examine the temporal development of the responsivity of the instrument. Figures 2 and 3 show plots of raw counts at 311 nm extracted from the measurements of a selection of 1000 W lamps. The wavelength of 311 nm was chosen to be used throughout this study since this wavelength was also measured in the earlier scans in the 1990s. The effects of the changes in the instrument's response and the ageing of the lamp are superimposed in the recorded counts per cycle. However, the figures may provide some preliminary information on the changes in responsivity, since the counts recorded from all the lamps follow the same pattern, no matter how frequently they are burned. It therefore appears that the ageing of the lamp has a notably smaller effect than the long-term change in the instrument's responsivity. There is a general downward tendency in the response of both instruments. In Brewer #107, three abrupt changes in the responsivity are obvious: the first in June 2004, the second in January 2010, and the third in August 2014. Short-term changes may also be seen in raw lamp counts of Brewer #037, although on a smaller scale.
The calibration of the Brewer spectrophotometers is based on an irradiance scale that was transferred to the on-site optical laboratories by primary standard lamps. Over the years 1990–1998, four different laboratories were used as suppliers of calibration of the primary standard lamps: Gigahertz-Optik GmbH (GH), Germany; Optronics Laboratories Inc. (OL), US; Radiation and Nuclear Safety Authority in Finland (STUK); and Statens Provningsanstalt (SP), Sweden. The years 1999–2001 denoted a period of transition during which the calibrations were ordered from SP, STUK, GH, and VTT MIKES Metrology (formerly: MIKES Helsinki University of Technology HUT), which includes the National Standards Laboratory (NSL) in Finland. Since December 2001, the calibrations have been ordered solely from NSL.
Information on the primary standard lamps used in calibration of Brewer #037 and #107.
The comparison measurements of spectral irradiance scales organized by the
Consultative Committee for Photometry and Radiometry (CCPR) in 1990, 1996,
and 2005 have shown differences between the irradiance scales provided by
different laboratories. Walker et al. (1991) reported differences of
2–4 % in the 1990 intercomparison in the UV wavelengths. A spread of
The primary standard lamps are 1 kW tungsten-filament incandescent halogen lamps of type DXW operated in vertical orientation in a distance of 50 cm of the focal plane of the diffuser. The calibration of the primary standard lamps at NSL is carried out by using the method for the realization of the detector-based spectral irradiance scale (Kübarsepp et al., 2000). The absolute responsivity of the used trap detector is traceable to the cryogenic electrical substitution radiometer of Statens Provningsanstalt, Sweden. In the 1990s, lamps manufactured by GH and OL were used. Currently, all the primary standard lamps regularly used in Jokioinen and Sodankylä are manufactured by GH. The lamps are labelled by a running number preceded by the letter “D”. Information on the lamps currently in use is compiled in Table 1.
Irradiance at 311 nm, as certified by the NSL, of the primary standard lamps used in calibration of Brewer #037 in Sodankylä (D22, D24, and D25) and Brewer #107 in Jokioinen (D01, D03, and D05). The filled circles denote parallel calibrations performed using Bentham DTMc300, extending over wavelengths 250–2100 or 250–2500 nm, traceable to another irradiance scale.
Figure 4 shows the temporal development of the radiative output of the lamps
at one selected wavelength, 311 nm, for six primary standard lamps currently
in use in Jokioinen and Sodankylä. Lamps D01, D03, and D05 seem to have
stayed within the calibration uncertainty throughout the time of their use.
Lamps D22, D24, and D25, on the contrary, seem to have lost some of their
intensity. In Sodankylä, however, the primary standard lamps are scanned
more frequently in the on-site darkroom than in Jokioinen. The primary lamps
have been measured in the on-site laboratories 1–3 times per year in
Jokioinen and 2–4 times per year in Sodankylä. The lamp may be expected
to age in some extent every time the lamp is burned. Examination on the
certified irradiances at 311 nm reveals overall drifts ranging from
Ageing rate of the primary standard lamps used in calibration of Brewer #037 in Sodankylä and Brewer #107 in Jokioinen, given by the NSL-certified irradiance at 311 nm as a function of the number of burning events.
The filled circles in Fig. 4 denote parallel calibrations performed using a Bentham DTMc300 spectroradiometer, extending over wavelengths 250–2100 or 250–2500 nm. The scale is traceable to MRI (Metrology Research Institute, Finland) for wavelengths below 900 nm, and to NPL (National Physical Laboratory, UK) for wavelengths above 900 nm. These certificates are not used for the calibration of Brewer spectrophotometers, but for other radiometers with a wavelength range extending to the infrared. However, they illustrate a difference between two irradiance scales. The difference is less than 1 % at 311 nm.
The instrument-specific spectral responsivity is needed to translate the raw
photon counts recorded by the spectrometer in spectral irradiance. The
formula connecting the irradiance
The responsivity of the spectrophotometer is determined for the day the (primary standard) lamp is measured in the on-site laboratory. Usually, at least three different 1000 W standard lamps are measured during the same day. After a recalibration of the primary standard lamps in NSL, several secondary and working standard lamps are measured in a row during a laboratory session of 1–2 days to transfer the irradiance scale from the newly recalibrated primary standard lamps to the secondary and working standard lamps. The responsivity may be based on one single lamp or on an average of two to three trusted lamps. The obtained responsivity is assumed to stay constant until the next scan of a standard lamp indicating a change in the responsivity of the instrument.
Responsivity of Brewer #037 as determined on the basis of the newly recalibrated primary standard lamps D22, D24, and D25 (discrete values) and the corresponding stepwise constant responsivity of type Level 1 used for near real-time processing of solar UV irradiance spectra.
Responsivity of Brewer #107 as determined on the basis of the newly recalibrated primary standard lamps D01, D03, and D05 (discrete values) and the corresponding stepwise constant responsivity of type Level 1 used for near real-time processing of solar UV irradiance spectra.
Figures 6 and 7 show the responsivity of Brewer #037 and #107 determined on the basis of measurements of three primary standard lamps in the on-site laboratories immediately after the recalibrations of the lamps in NSL. From the discrete points of the determined responsivity, the stepwise constant (Level 1) responsivity time series are formed. These kinds of responsivities are used in the near real-time processing of the solar UV measurements made by Brewer spectrophotometers #037 and #107. The operator may choose to fix the responsivity onto one lamp only or use an average of two to three lamps.
The stepwise Level 1 responsivity shown by Figs. 6 and 7 give insight into
the true changes in the responses of the instruments, cleaned of the effect
of the ageing of the lamps. The responsivity of Brewer #037 seems to
decline fairly steadily. The largest drop of the order of
In addition to the near real-time processing of solar UV irradiance measurements made by Brewer #037 and #107 spectrophotometers producing Level 1 data, the scans are retrospectively post-processed to produce Level 2 data. This allows the operator to account for even the small scale variations in responsivity that were neglected in the near real-time processing. In addition, it allows the operator to retrospectively view the behaviour of each individual lamp, separate the true changes in the instrument from the changes in the lamp, and choose the trusted lamps to serve as the basis of the determination of the responsivity for each period of time.
To demonstrate the determination of the Level 2 responsivity, the lamp measurements collected in 2015 with Brewer #107 are used and each phase in the process is described in the following sections. The phases are illustrated in Fig. 8.
Demonstration of the phases in the determination of the final responsivity of type Level 2 used for post-processing of solar spectral UV irradiance measured by Brewer #107 spectrophotometer. The dashed lines are plotted to guide the eye.
The primary standard lamps D01 and D05 were recalibrated in NSL on
19 May 2015. They were measured by Brewer #107 in the on-site darkroom in
Jokioinen on 23 June 2015 (Julian date 174
The irradiance scale may now be transferred to all the lamps measured on-site
on day 174
During 2015, lamps D01 and D05 have been measured in the on-site laboratory
also on 14 January 2015 (014
The working standard lamps D14 and D42 have been measured by Brewer #107
in the on-site laboratory on days 056
The discrete responsivity derived as an average of the lamps D14 and D42, calibrated against the average of the primary standard lamps D01 and D05, are next used to derive a time series of responsivity. The discrete points are connected with linear interpolation in time, resulting in a time series in the form of a polyline. The obtained time series, giving daily responsivity for Brewer #107, could already be used as such in post-processing of solar UV irradiance data. The time series is plotted with a thick grey line in Fig. 8.
The polyline-shaped time series of responsivity derived in Phase 2 contain sharp turning points. In most cases it may be assumed that in reality the changes are not that sudden in the responsivity of the instrument. The time series may therefore be filtered using a moving average with a window of suitable width. This is done here using a window of width of 31 days. The selection of the width depends on how small variations in the time series are to be retained and in which extent the sharp turns in the time series are to be smoothed away. The resulting time series is plotted with a thin black line in Fig. 8. This is considered a Level 2 responsivity time series that could be used for the post-processing of the solar spectral UV irradiance measurements collected during the year 2015.
The demonstration presented here is confined to the lamp measurement data
collected during 2015. The responses derived on the basis of the
measurements of the lamps D01 and D05 in the on-site laboratory on day
014
Level 2 responsivity of Brewer #037 at different wavelengths for the time period 1 January 1990–9 December 2015 used for post-processing of solar UV irradiance spectra.
Level 2 responsivity of Brewer #107 at different wavelengths for the time period 20 March 1995–7 April 2008 used for post-processing of solar UV irradiance spectra.
The Level 2 responsivity time series used in post-processing of the UV irradiance spectra measured by Brewer #037 and #107 are shown in Figs. 9 and 10 respectively. The time series for Brewer #037 is complete and extends over the whole operational lifetime of the instrument. For Brewer #107, work is under way to re-evaluate all the lamp scans taken since the beginning of 2008, to extend the existing Level 2 responsivity time series until the end of 2015.
The lamp measurement data collected over the operational years of Brewer #037 and Brewer #107 spectrophotometers include altogether 1931 and 2012 scans respectively. The data allow retrospective examination of the changes that occurred both in the lamps themselves and in the responsivity of the instruments. Looking into the raw counts obtained from the lamp measurements of Brewer #037 and Brewer #107 already revealed long-term declination and abrupt jumps in the responsivity. The features observed in the raw counts may be considered truly attributable to changes in the responsivity as all the lamps indicate the same behaviour, independent of the frequency at which they are burned. However, separation of the ageing of the lamps from the changes in the responsivity would be meaningful and should be possible by employing a linear mixed model, for instance.
Brewer #037 and Brewer #107 have both been frequently compared with the portable EU reference spectroradiometer QASUME, Quality Assurance of Spectral Ultraviolet Measurements in Europe (Bais et al., 2003). The comparisons provide external and additional evidence of the calibration stability of the instruments. During the QASUME site visits in 2002–2003, the ratio of the irradiance measured by Brewer #037 (Brewer #107) was found to be 1.05 in UVB and 1.03 in UVA (1.03 in UVB and 1.02 in UVA) (Gröbner et al., 2006). In addition, the site visits to Jokioinen in 2002 and 2003 proved that the performance of Brewer #107 was stable between the 2 consecutive years (Gröbner et al., 2005).
Regular recalibrations of the primary standard lamps enable identification of changes in the radiative output of the lamps. The temporal development of the primary standard lamps of Brewer #037 and Brewer #107 showed ageing in general. Individual differences between the lamps obviously exist in regard to their stability. The primary standard lamps used by Brewer #107 are burned more sparingly than the ones used by Brewer #037. This might be a feasible strategy when aiming at minimizing the ageing of the lamps. Following the temporal development of the radiative output of all the lamps is worthwhile since it also enables identification of the most stable lamps, which would serve well as frequently scanned working standard lamps.
The determination of the responsivity for both near real-time processing and
post-processing of solar spectral UV irradiance includes phases where the
operator has to make a choice between several alternatives. Up to the
discretion of the operator are, for instance, which primary standard
lamp(s) is to be used as the basis of the determination of the
responsivity of the instrument, which secondary
The strategy of using multiple primary standard lamps that are regularly and
rotationally recalibrated by NSL and multiple secondary
Two Brewer spectrophotometers have been used to monitor solar spectral UV
irradiance at Sodankylä and Jokioinen, Finland, for about 20 years
according to the guidelines defined by GAW
The large number of data accumulating from the lamp measurements and the multi-phase processing of the data call for carefully designed data management. Future work should also include determination of quantified criteria to assist the operator in making decisions on the various phases of the determination of the responsivity. This requires further research on the data, including experimentation with the different choices and evaluation of the consequences of each choice.
The research data underlying this study are available from the corresponding author on request.
The authors gratefully acknowledge the personnel in the Arctic Research Center in Sodankylä and at Jokioinen Observatory for taking care of the daily maintenance, the on-site quality control, and the lamp measurements made with Brewer spectrophotometers #037 and #107. Edited by: M. Zribi Reviewed by: two anonymous referees