The suitability of a commercially available Fourier transform spectrometer equipped with a fiber-coupled global entrance optic as a reference spectroradiometer for the measurement of spectral solar ultraviolet irradiance at ground level has been investigated. The instrument has been characterized with respect to the wavelength uncertainty, and a calibration of the spectral irradiance responsivity has been performed by using the calculable irradiance of a high temperature black-body radiator and by using a secondary irradiance standard lamp. The relative standard uncertainty of solar irradiance measurements in the wavelength range from 310 nm to 400 nm with this spectroradiometer, based on the described methodology, is 1.6% for solar zenith angles of less than 60°.
© 2014 Optical Society of America
A number of remote measuring sites are in operation for the measurement of the solar spectral irradiance at ground level in particular in the ultraviolet spectral range which is important for several effects e.g. on earth climate or human health . These irradiance measurements have to be traceable to the international system of units (SI) with low uncertainties to be reliable and comparable . The calibration of the instruments at these remote measuring sites is performed by using reference instruments which disseminate the radiometric units that are realized with high accuracy at the laboratories of the national metrology institutes e.g. of the Physikalisch-Technische Bundesanstalt (PTB). Hence, such reference instruments play an important role as travel standards for the quality control of UV monitoring [3–5]. So far, scanning spectroradiometers based on grating monochromators are in use as reference instruments (QASUME [6, 7],). There are a number of disadvantages connected with this type of radiometer. E.g. the sequential scanning of each solar spectrum needs several minutes and limits the temporal resolution of the radiometer. Furthermore, these instruments have a low throughput and therefore need photomultiplier tubes as detectors, and additionally the wavelength scale has to be calibrated and checked each day of use. The wavelength accuracy is essential especially in the wavelength range below 330 nm because of the steep decrease of solar UV irradiance over many orders of magnitude. The standard uncertainty of these instruments for solar irradiance measurements is around 2.3% to 4.4% depending on wavelength and solar zenith angle .
It is the intention of the European Metrology Research Project ENV03 to enhance the reliability of spectral solar UV radiation measurements at the Earth’s surface by optimizing existing reference instruments but also by developing new techniques and devices . Instruments should have a wavelength uncertainty of smaller than 50 pm, and furthermore fast spectroradiometers are required because of possible temporal variations of solar UV radiation due to varying atmospheric conditions e.g. moving clouds.
With regard to these demands, the usage of Fourier transform spectroradiometers may improve the dissemination of absolute irradiance scales due to the specific advantages of these instruments [9, 10]:
- • Fourier transform spectrometers (FTS) have a high throughput because of the circular aperture of these instruments (also known as Jacquinot advantage). Furthermore, there are no diffraction losses to higher-order spectra as it is the case in grating spectrometers. This results in a comparably high signal-to-noise ratio.
- • The whole spectrum is measured simultaneously which allows fast measurements.
- • FTS cover broad spectral ranges with high resolution and high wavenumber accuracy. Additionally, the spectral resolution can easily be adapted to the demands of the measurement.
- • Modern FTS often use integrated HeNe lasers for the measurement of the position of the moveable mirror of the FTS interferometer. This laser can be used for the wavenumber calibration . In this way, the wavenumber scale of the FTS is inherently traced to the SI with low uncertainties in contrast to scanning spectroradiometers whose wavelength scale has to be recalibrated on-site periodically against spectral lamps .
- • Spectral distortions caused by the instrument are often calculable and correctable after the measurement.
Fourier transform spectroradiometers are in use for the measurement of solar reference spectra with high wavelength resolution . In contrast to these facilities, the concept of this work is to evaluate the usability of a commercially available Fourier transform spectrometer as a reference spectroradiometer for the absolute measurement of solar UV irradiance at ground level. In a first step, the Fourier transform spectroradiometer has been thoroughly characterized, and the measurement uncertainties for solar UV irradiance measurements have been determined. The results of this first suitability study are presented within this publication. Future investigations will concern a direct solar UV irradiance comparison measurement between FTS and QASUME. Furthermore, a demonstration that the proposed methodology is sufficiently robust and reliable under routine operating conditions as a travel standard will be subject of coming investigations.
A Bruker VERTEX 80v Fourier transform spectrometer has been chosen for the purpose of this investigation. This type of spectrometer is designed for the mid IR region but can be used in the spectral range from 5 cm−1 up to the ultraviolet region at 50000 cm−1 with additional optical components which are an appropriate beam splitter and an adequate detector. The goal was to cover the spectral range from 35000 cm−1 (about 280 nm) to 25000 cm−1 (400 nm). The Vertex 80v uses a HeNe laser at 633 nm to monitor the position of the movable interferometer mirror and to determine the interferogram data sampling positions. The monochromatic beam of the HeNe laser is coupled into the interferometer collinear to the investigated radiation and generates an additional sinusoidal interferogram which is measured by a separate detector.
A fiber coupled global entrance optic has been fitted to the spectrometer to be able to perform measurements of the solar UV irradiance. The same entrance optic (UV-J1002-REG by CMS Ing. Dr. Schreder GmbH, Austria) as used with QASUME has been chosen. This enables a direct comparison of these two different types of spectrometers.
Several types of detectors have been considered to use within the FTS in the UV range. The instrument has a higher throughput compared to scanning instruments, and for this reason the use of semiconductor detectors instead of a photomultiplier tube was tried. This type of detector could simplify the dissemination of the irradiance scale because usually the spectral responsivity of semiconductor detectors is sufficiently stable in contrast to the responsivity of photomultiplier tubes. Two types of semiconductor detectors (Si and GaP delivered from Bruker and additionally a Si detector S8552 made by Hamamatsu) have been used. Additionally, a photomultiplier tube (Hamamatsu photosensor module H10723-210) has been applied. This enables to compare the properties of different types of detectors within the Fourier transform spectroradiometer but also to compare the properties of the FTS against the properties of the scanning instrument QASUME directly, independently of the detector type.
3. Traceability of the FTS wavenumber scale and spectral resolution
Traceability of solar UV irradiance measurements to SI units concerns the irradiance itself but also the wavenumber scale of the measured spectra. The wavenumber calibration of the FTS is realized by a built-in HeNe laser . As mentioned above, the HeNe laser beam is coupled into the interferometer nearly coaxial to the radiation under investigation and is used to determine the position of the movable FTS interferometer mirror. The knowledge of the absolute mirror position enables to calculate the wavenumber scale of the measured spectra also in the UV. The frequency (respectively the wavenumber or the wavelength) of unstabilized HeNe lasers has been added to the list of standard frequencies of the mise en pratique of the definition of the meter [13, 14] which gives advice for the practical realization of the SI unit meter . For this reason, an unstabilized HeNe laser radiating at a wavelength of 633 nm can be used as a primary standard of the SI unit meter. In this way, the whole wavenumber scale covered by the FTS from the ultraviolet to the far infrared range is traceable to the SI as it is. It should be mentioned that almost always spectral lamps that are commonly used for wavelength calibrations in grating spectrometers are not primary standards.
The wavenumber uncertainty of the FTS is determined by a few sources of uncertainty. At first, the HeNe laser beam may have a very slight misalignment with respect to the optical axis of the interferometer. This error can be corrected by performing a wavenumber calibration measurement with an external HeNe laser. The remaining wavenumber calibration uncertainty after the correction is about 0.1 cm−1. A further contribution arises from the dispersion of the refractive index of the ambient air. The deviation is up to 0.8 cm−1 for UV wavenumbers far from the wavenumber of the HeNe laser. But this error can be numerically corrected or also avoided by evacuating the spectrometer. However, the deviation is rather small and can be assumed as an additional uncertainty. Another wavenumber uncertainty arises from the significant diameter of the used aperture of the FTS (8 mm) that allows beams which are inclined to the optical axis to pass the interferometer. These inclined off axis beams lead to a wavenumber deviation which has to be corrected . This correction gives an additional uncertainty contribution of 0.4 cm−1 to 0.7 cm−1. Finally, the HeNe wavenumber itself has a small uncertainty with respect to the SI ((15798.018 ± 0.024) cm−1 ). However, this contribution is negligible. The combined wavenumber uncertainty of the FTS is about 0.4 cm−1 to 1.1 cm−1 dependent on the wavenumber which corresponds to a wavelength uncertainty of about 7 pm to 11 pm in the ultraviolet spectral range from 250 nm to 500 nm. This is nearly one order of magnitude below the requested uncertainty of 50 pm. As a result, the wavenumber scale of the FTS is inherently calibrated with respect to the SI with comparatively low uncertainties. If necessary, the wavelength uncertainty can be significantly reduced by applying a correction of the air dispersion and by reducing the amount of off axis beams by choosing smaller apertures.
The spectral resolution of the FTS is stated to be better than 0.2 cm−1. However, this high resolution was not needed for the purpose of a reference instrument for irradiance measurements. For this reason, a limited resolution of 10 cm−1 was typically used. This reduces the travel distance of the movable interferometer mirror and therefore enables faster measurements. A Blackman-Harris-4-Term apodization  was applied to the measured interferograms. This reduces signal leakage into side lobes and increases in that way the dynamic range of the spectrometer. However, the resolution after applying this apodization is reduced to about 20 cm−1. This correlates to a wavelength resolution of 0.32 nm at 400 nm or 0.18 nm at 300 nm which is well below the resolution of QASUME that is 0.8 nm . In order to enable a direct comparison of the obtained results with the properties of the scanning spectroradiometer QASUME, the resolution of the FTS has then been numerically reduced to the resolution of QASUME.
It should be mentioned that the resolution of an FTS can be freely chosen within a range that is limited by the maximum travel distance of the movable FTS interferometer mirror. So, the resolution is a parameter that competes against the spectrum acquisition time and the signal-to-noise ratio and can be chosen arbitrarily within a wide range depending on the demands and e.g. on atmospheric conditions.
4. Calibration of the FTS spectral irradiance responsivity
The spectral irradiance responsivity of the Fourier transform spectroradiometer including the global entrance optic and the different types of detectors has been calibrated for absolute solar UV irradiance measurements. This calibration has been carried out by using a black-body radiator HTBB pg3200 as a calculable irradiance source [17, 18] and additionally by using a secondary irradiance standard lamp (Fig. 1 and Fig. 2). The secondary irradiance standard - a 250 W tungsten halogen lamp - has been calibrated against the national irradiance standard of the PTB [19, 20]. The calibration has therefore been conducted in the same way as it has been done with the scanning spectroradiometer QASUME .
The measurement setup is sketched in Fig. 2. An aperture of 20 mm diameter was placed in front of the high temperature black-body radiator BB3200pg. The temperature of the black-body radiator has been determined by using a linear pyrometer LP3 (KE Technologie GmbH). The temperature stability of the high temperature black-body was 0.5 K/h. The LP3 was calibrated against a black-body radiator at the temperature of the freezing point of gold T90(Au) = 1337.33 K which is a fixed-point according to the International Temperature Scale ITS-90. Measurements have been performed at temperatures between 2781 K and 3010 K.
The spectral irradiance calibration has been performed with the Si and GaP detectors delivered from Bruker, with a silicon UV detector S8552 made by Hamamatsu, and finally with the Hamamatsu photomultiplier module H10723-210. Both semiconductor detectors delivered from Bruker have a small sensitive area of about 1 mm2 that hardly covers the radiation from the interferometer. For this reason, an additional windowless silicon photodiode Hamamatsu S8552 (10 × 10 mm2) has been applied in order to overcome this limitation. The measured spectral irradiance responsivities of the FTS with these three semiconductor photodiodes are shown in Fig. 3. The FTS with the GaP photodiode has a higher spectral irradiance responsivity with a maximum around 450 nm compared to the FTS with the Si photodiodes. However, it was found that the calibration of the irradiance responsivity is only reasonable at wavelengths down to about 360 nm when using these semiconductor detectors. These detectors have a much higher stability of the spectral responsivity compared to photomultiplier tubes. Unfortunately, this advantage cannot be applied below 360 nm even when using an FTS which has the advantage of a high throughput. The radiant power that reaches the detectors is too low. This is a consequence of the rather low throughput of the global entrance optic which is not optimized for a high throughput but for a preferably perfect angular response.
Consequently, a photomultiplier module (Hamamatsu H10723-210) has been put into operation. This photomultiplier tube has a circular sensitive area of 8 mm diameter with a bialkali photocathode. The detectivity of the photomultiplier tube is much higher than the detectivity of the semiconductor photodiodes which have been used. Hence, the spectral responsivity and the associated signal-to-noise ratio of the FTS are larger. However, a few details of the interferometer optics had to be redesigned. A little part of the HeNe laser radiation, which is used for the measurement of the movable interferometer mirror position, reaches the detector. This radiation contains modulated and not modulated parts. The modulated part generates a significant spectral line in the measured spectra if the detector is responding at the laser wavenumber of 15798 cm−1 (corresponding to a wavelength of 633 nm). Furthermore, an additional spectral line appears at the doubled wavenumber (i.e. around 316.5 nm) due to a little detector nonlinearity. This artifact partly disturbs the wanted solar UV spectra. Additionally, the laser radiation saturates the detector severely and makes it difficult to use a photomultiplier tube. Thus, the laser radiation has to be blocked which is possible with two different means. At first, a conveniently shaped light cover has been placed in the widespread beam behind the interferometer. This allows blocking most of the laser radiation, but also some parts of the wanted radiation are lost. Secondly, optical band-pass filters have been used. These filters should block the laser radiation at 633 nm and should be transparent in the wanted UV range at the same time which is not easy to realize. Two filters from Schott (UG11 and UG5) have been used for this purpose. UG11 suppresses the laser radiation nearly completely but is transparent only below about 390 nm. On the other hand, UG5 is transparent also up to 500 nm but does not completely block the laser radiation which generates a little spectral peak at 316.5 nm. Also the band-pass filters absorb a little part of the wanted solar radiation. The spectral transmission properties of both band-pass filters in combination with the spectral responsivity of the photomultiplier tube mainly determine the spectral irradiance responsivity of the FTS (Fig. 3).
In conclusion, the FTS equipped with the photomultiplier module and the global entrance optic can be calibrated at wavenumbers up to 36000 cm−1 (i.e. at wavelengths down to 280 nm). However, the advantage of using a more stable semiconductor detector cannot be applied. Consequently, the spectroradiometer in this configuration has to be recalibrated each day of use by using a portable irradiance standard (calibrator lamp) as it is also performed with the scanning spectroradiometer QASUME .
5. Uncertainty of the Radiometric Calibration Measurement
The radiometric calibration has been performed by using a high-temperature black-body radiator which is calculable according to Planck's formula. The spectral radiance dependent on the temperature T of the cavity is given by :
with the emissivity ε(λ) of the cavity, refractive index of air n, speed of light in vacuum c0, Planck constant h, and Boltzmann constant k. The spectral irradiance seen with the aperture of the global entrance optic (diameter 2r2) located at a distance d from the aperture of the black-body radiator (diameter 2r1) is given by:
Here, the geometry is taken into account with the factor G:
The spectral irradiance responsivity is then given by the ratio of the FTS signal IFTS and the spectral irradiance Eλ:
According to Eq. (4) the measurement uncertainty of the spectral irradiance responsivity is:
The contributions to the combined uncertainty of the calibration of the spectral irradiance responsivity are therefore caused by the uncertainty of the temperature of the black-body radiator u(T) = 0.85 K, the wavelength uncertainty u(λ), the uncertainty of the radius of the aperture in front of the black-body radiator u(r1) = 25 µm, and the uncertainty of the distance between the apertures of the black-body radiator and of the global entrance optic u(d) = 1.1 mm at a distance of d = 51.6 cm. The contribution of the uncertainty of the radius of the global entrance optic aperture can be neglected. In spectral ranges with low irradiance or low spectral responsivity the uncertainty is dominated by measurement noise uA. The goal of this work is to compare the FTS with the scanning spectroradiometer QASUME. For this reason, the resolution of the FTS during the calibration measurements has been reduced to the resolution of QASUME in order to perform a direct comparison. The correspondent uncertainty budget is shown in Table 1. The wavenumber dependence of this uncertainty is shown in Fig. 4.
The combined uncertainty is limited by the uncertainty of the temperature measurement of the black-body radiator and by the uncertainty of the optical geometry between 330 nm and 415 nm. Below 330 nm and above 415 nm the measurement is limited by measurement noise due to low available radiant power or a low spectral responsivity. The uncertainty due to noise can be reduced by increasing the number of measured interferograms. This can be done by extending the duration of the measurement if there are no systematic drifts or by reducing the resolution of the FTS at the same measurement duration because this reduces the measurement time for each interferogram and allows to gain more interferograms per time. Furthermore, the uncertainty could be optimized by using a black-body radiator with higher temperatures and by improving the uncertainty of the black-body temperature measurement. QASUME has been calibrated against a high-temperature black-body radiator at temperatures of more than 3000 K with temperature uncertainties of 0.5 K . Asuming this lower temperature uncertainty, the standard uncertainty of the FTS calibration could be slightly reduced in the wavelength range from 330 nm to 415 nm. Using a higher black-body temperature on the other hand will result in a larger spectral radiant power especially in the spectral range below 330 nm and could furthermore improve the uncertainty due to measurement noise. Table 2 shows a comparison of the combined measurement uncertainties obtained with the different types of detectors with the FTS. The best results have been obtained with the photomultiplier module in combination with the UG5 spectral filter.
A further calibration of the FTS has been performed by using a secondary irradiance standard tungsten halogen lamp. This standard has been calibrated against the national irradiance standard of the PTB. The obtained uncertainties are slightly larger in comparison to the calibration against the black-body radiator (Table 3 and Fig. 5). The lamps provide less spectral radiant power and are used at lower distances (30 cm). The spectral responsivities which have been achieved with both methods agree well within the stated expanded uncertainties when using semiconductor detectors. However, deviations of a few percent may occur in case of using the photomultiplier tube. The stability of the spectroradiometer including the photomultiplier module has been investigated by repeated calibrations of the spectral responsivity against a secondary irradiance standard in a period of two weeks. The optical setup was unchanged during these measurements. The stability of the spectral responsivity was better than 1% when leaving the instrument unchanged. However, a movement of the instrument to another room including the reconnection of the system leads to a deviation of a few percent. This indicates the need for recalibration measurements with portable transfer standard lamps in a short period before or after the solar irradiance measurements when using a photomultiplier detector. The contribution to the uncertainty of solar UV irradiance measurements due to the instrument instability can be reduced in this way.
6. Measurements of solar UV irradiance
Solar UV irradiance measurements have been performed by using the calibrated Fourier transform spectroradiometer equipped with the photomultiplier tube and a spectral filter. The same type of fiber coupled global entrance optic as used with QASUME has been chosen for use with the FTS to be able to compare these two different types of instruments directly. There are a number of measurement uncertainties connected to this entrance optic. The transmittance is dependent on the temperature of the optic. Furthermore, there are deviations from the cosine response dependent on the solar zenith angle (SZA) and the wavelength. Additionally, the transmittance depends on the azimuth angle. Within this investigation we have assumed that the entrance optics of both instruments, FTS as well as QASUME, have the same characteristics. Therefore we have expected the same measurement uncertainties caused by the entrance optic as reported in . However, the angular dependence of the entrance optic has been measured and can be assumed to be in the range reported in  for a SZA of not more than 60°.
The resulting uncertainty budget for solar irradiance measurements with the FTS is given in Table 4 and is illustrated in Fig. 6. The resolution has been reduced to the resolution of QASUME as it is reported in  to be able to compare the measurement uncertainties directly with the uncertainties obtained with QASUME. For the same reason, the measurements have been averaged over a period of 12 minutes which is a typical duration for the acquisition of a spectrum with QASUME. Considering the fact, that the FTS needs a regular recalibration against the secondary halogen lamp irradiance standard when using on a remote measuring site, the corresponding radiometric calibration uncertainty has been assumed in the uncertainty budget.
The aim of this suitability study was to evaluate the usability of a commercially available Fourier transform spectrometer for absolute surface spectral solar UV irradiance measurements with the farther goal of using this type of instrument as a reference spectroradiometer. The investigated Fourier transform spectrometer Bruker VERTEX 80v has a comparatively small wavelength uncertainty of less than 11 pm in the wavelength range from 250 nm to 500 nm. This is well below the 50 pm which are demanded to obtain an uncertainty of solar UV irradiance measurements of less than 2%. Therefore, the wavelength uncertainty will not limit the uncertainty of solar UV irradiance measurements. Furthermore, the wavelength scale is traceable to the SI via the built-in HeNe laser or via a wavenumber measurement of an external HeNe laser. This property is a big advantage compared with grating spectroradiometers that initially do not have a traceable wavelength scale.
The FTS in the chosen configuration (photomultiplier tube incl. spectral filter UG5) can be used in the spectral range from 300 nm to 500 nm for solar UV irradiance measurements. The relative standard uncertainty is around 1.6% in the spectral range from 310 nm to 400 nm and increases up to 7.4% at 500 nm. This is slightly less than the uncertainty of QASUME which has a relative measurement uncertainty around 2.3% to 3% for solar zenith angles of less than 50° and wavelengths from 310 nm to 400 nm . The spectral range of the FTS is determined by the responsivity of the used photomultiplier and the transmission range of the spectral filter. For this reason the spectral range for solar UV irradiance measurements was limited at 300 nm in this configuration. However, the FTS can be adapted to a different spectral range by choosing other types of photomultipliers and filters. The target uncertainty of the European Metrology Research Project ENV03 “SolarUV” is around 1% to 2% for solar UV irradiance measurements to resolve changes of solar UV irradiance which are in the order of 2% per decade . This target has been reached using the Fourier transform spectroradiometer.
The FTS is a fast instrument which measures around two spectra per second with the chosen spectral resolution of 10 cm−1. It should be mentioned that the interferograms of these spectra can be flexibly averaged after the measurement depending on the variability of the atmospheric conditions. This enables a maximization of the SNR and provides a high temporal resolution on the other hand. In contrast to this, such a postprocessing of data is not possible with scanning spectroradiometers where the measurement parameters have to be chosen in advance. Temporal variations of the solar irradiance will lead to strong distortions of the spectra with such scanning instruments. In contrast to these instruments, a Fourier transform spectroradiometer measures always complete spectra at each point in time and therefore enable to resolve temporal changes of solar UV spectra.
In conclusion, it has been shown that Fourier transform spectroradiometers are suitable for solar UV irradiance measurements with comparatively low measurement uncertainty and with high temporal and spectral resolution. Fourier transform spectroradiometers have a number of specific advantages and this can therefore improve the dissemination of radiometric units to remote measuring sites.
A direct comparison experiment between FTS and QASUME is subject of current investigations, and the results will be published shortly. A demonstration that the proposed methodology is sufficiently robust and reliable under routine operating conditions has to be subject of coming investigations.
The work leading to this study was partly funded by the EMRP ENV03 Project “Traceability for surface spectral solar ultraviolet radiation”. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. Furthermore, thanks to Peter Sperfeld, Klaus Anhalt and Lutz Werner from PTB for the providing of irradiance sources and their support.
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