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Spectral irradiance measurements of tungsten lamps with filter radiometers in the spectral range 290 nm to 900 nm
Metrologia
Abstract
A method of measuring the absolute spectral irradiance of quartz-halogen-tungsten lamps is described, based on the known responsivity of a filter radiometer, the components of which are separately characterized. The characterization is described for the wide wavelength range essential for deriving the spectrum of a lamp, from 260 nm to 950 nm. Novel methods of interpolation and measurement are implemented for the spectral responsivity of the filter radiometer. The combined standard uncertainty of spectral irradiance measurements is less than 1.4 parts in 102 from 290 nm to 320 nm (ultraviolet B) and 4 parts in 103 from 440 nm to 900 nm (visible to near-infrared). As an example, the derived spectral irradiances of two lamps measured at the Helsinki University of Technology (HUT, Finland) are presented and compared with the measurement results of the National Institute of Standards and Technology (NIST, USA) and the Physikalisch-Technische Bundesanstalt (PTB, Germany). The comparisons indicate that the HUT spectral irradiance scale is between those of the NIST and the PTB in the wavelength range 290 nm to 900 nm. The long-term reproducibility of the spectral irradiance measurements is also presented. Over a period of two years, the reproducibility appears to be better than 1 part in 102.
... Kübarsepp et al [6] described a suitable method based on a multiband filter radiometer that can be used for accurate measurements of the spectral irradiance over a wide wavelength range of (290-900) nm. Kärhä et al [7] described a method for spectral irradiance measurement using a compact filter radiometer in the (380-900) nm range. ...
... R i (λ) includes the spectral responsivity R d (λ) of the detector and the filter transmittances τ i (λ) of each channel, as expressed in equation (3): When the spectral distribution of the source to be measured is smooth and continuous, there is no absorption peak, and the physical model of the source radiation can be described by polynomials. If the radiator is approximated as a physical model of blackbody radiation, it can be approximately expressed using the modified Planck's blackbody radiation law [6]. In this study, a 5th-order polynomial was used to describe the radiation model of the source. ...
... We assume that the spatial uniformity is wavelength-independent. The transmittance of the filters should be measured every six months unless an earlier visual inspection reveals any damage to the filter surfaces [6]. The spatial uniformity was measured at 300 nm. ...
We developed a new source, having uniform spectral radiance, for a large-aperture UV integrating sphere with pressed polytetrafluoroethylene coating. Since lamp panel systems have high uncertainty due to a low signal-to-noise ratio at 250 nm, to transfer the spectral radiance scale from 250 to 350 nm to the source, we developed and calibrated a filter radiometer based on a solar blind phototube. The spectral radiance was obtained via a model of the source and a recursive estimation of the model parameters. A standard FEL lamp validated the usage of the filter radiometer. Finally, the filter radiometer to calibrate the new source. The results show that the radiance of the source at 250 nm is five times the top-of-the-atmosphere spectral irradiance, and the expanded uncertainty is 4.7%.
... The excellent performance of silicon trap-detector based filter radiometers leads to the true detector-based realization of the spectral irradiance scale at several NMIs [4,5]. This method measures the spectral irradiance generated by a tungsten halogen lamp directly using a large number of filter radiometers operating at different wavelengths. ...
... As the results are calculated based on the spectral responsivity of the radiometer, the spectral transmittance of the filters and the aperture area without using a blackbody, the method greatly simplifies the realization process of a spectral irradiance scale. 4 Author to whom any correspondence should be addressed. ...
... The spectral power distribution of a tungsten lamp can be presented as a modified Planck's radiator at temperature T [4]: ...
An automatic multi-wavelength filter radiometer (MWFR) was developed at the National Metrology Centre (NMC) of Singapore to realize the spectral irradiance scale in the wavelength range from 250 nm to 1600 nm. The UV–VIS range (250 nm to 900 nm) is covered by a silicon trap detector with 17 filters, while the near IR range (900 nm to 1600 nm) is covered by an InGaAs photodiode with 7 filters. A complete run of measurements at all 24 wavelength points takes only 12 min. The spectral irradiance scales of NMC and MIKES/TKK (Finland) in the spectral range from 280 nm to 900 nm were compared successfully using this instrument. Measurement results of three standard lamps calculated using MIKES/TKK's algorithm showed excellent repeatability and very good agreement with their assigned values. It is concluded that, compared with the conventional monochromator based spectroradiometers, a properly designed MWFR with sufficient spectral coverage is a good alternative instrument for spectral irradiance comparison owing to its faster speed and better short-term reproducibility.
... One disadvantage of high-temperature blackbodies is that they are quite expensive and large, and may not be feasible for smaller NMIs. Using detector-based methods, the spectral irradiance scale can also be realized in tungsten halogen incandescent lamps [5][6][7][8]. In contrast to black bodies, incandescent lamps are not perfect Planckian radiators. ...
... The effective emissivity of a lamp is further influenced by the transmittance of the glass bulb, the absorption of the halogen or other filling gas, and possible impurities of the filament. The emissivity of an incandescent tungsten lamp is typically modeled using an Nth-degree polynomial [5,7,[11][12][13]. The degree N used typically varies between 3 and 7. Polynomial emissivity models require several spectral irradiance data points to avoid oscillations between the wavelengths of the measured points. ...
... As can be seen, the interpolated values are well within the expanded uncertainties of the TKK calibration. The spectral structure of the difference is similar for both FEL and DXW lamps, and it is caused mainly by the seventh degree interpolation polynomial used in the earlier calibrations [7]. The relatively high degree of the polynomial causes oscillation within uncertainties that has been noted in several intercomparisons, e.g., CCPR-K1.a ...
We have developed a physical model for the spectral irradiance of 1 kW tungsten halogen incandescent lamps for the wavelength range 340 – 850 nm . The model consists of the Planck’s radiation law, published values for the emissivity of tungsten, and a residual spectral correction function taking into account unknown factors of the lamp. The correction function was determined by measuring the spectra of a 1000 W, quartz-halogen, tungsten coiled filament (FEL) lamp at different temperatures. The new model was tested with lamps of types FEL and 1000 W, 120 V quartz halogen (DXW). Comparisons with measurements of two national standards laboratories indicate that the model can account for the spectral irradiance values of lamps with an agreement better than 1% throughout the spectral region studied. We further demonstrate that the spectral irradiance of a lamp can be predicted with an expanded uncertainty of 2.6% if the color temperature and illuminance values for the lamp are known with expanded uncertainties of 20 K and 2%, respectively. In addition, it is suggested that the spectral irradiance may be derived from resistance measurements of the filament with lamp on and off.
... The irradiance scale of UV measurements is traceable to the Aalto University, which is the national standards laboratory for optical quantities in Finland (Kübarsepp et al. 2000), where the primary standard of the FMI is annually calibrated. The calibration scale was transferred to the whole time series after 1990, which makes this time series one of the longest homogeneous spectral UV time series measured in the Arctic. ...
... The highest summer R g values were measured in 2002 (May and June) and 2003 (June and July), and indicated summers with low cloudiness. The lowest summer R g values were measured in June of 2000, 2004and 2008, and in July of 2001and 2010 The ratio of monthly mean erythemallyweighted and UVA (ERY/UVA) daily doses were calculated for June-August (Fig. 16). As the erythemal action spectrum gives more weight to short UVB wavelengths than UVA wavelengths, the changes in total ozone should be seen as changes in erythemally weighted UV dose rates. ...
Laurila T. 2016: Radiation measurements at the Pallas-Sodankylä Global Atmosphere Watch station — diurnal and seasonal cycles of ultraviolet, global and photosynthetically-active radiation. Boreal Env. Res. 21: 427–444. We present time series of solar ultraviolet radiation (UVR), global radiation, and photosyn-thetically-active radiation (PAR) measurements from the Pallas-Sodankylä Global Atmosphere Watch (GAW) station at Sodankylä for the period 1990–2013. Measurements were performed both in an open area and in the surrounding boreal forest, which had a total leaf area index of 3.6. We present a method to homogenize multifilter radiometer UV time series. Using this method, the relative mean differences between the multifilter radiometer and the reference spectroradiometer were 5% for UVB and erythemally-weighted dose rates, for all sky conditions and solar zenith angles (SZA) smaller than 60° during the period 2008–2012. Our results show that daily doses measured in the forest were four times smaller than those measured in the nearby open area. Maximum UVB (280–320 nm) and UVA (320–400 nm) daily doses of 75.0 kJ m –2 and 1.74 MJ m –2 , respectively, were measured during
... The realization of the spectral irradiance scale can be done using two methods, one based on the source and the other on the detector [1][2][3]. This second methodology was employed in this work because to be simpler and less costly and have compatible uncertainties applications for both a National Metrology Institute (NMI) and for industrial applications. ...
... The idea is to construct the spectral irradiance scale using the black body model to FEL lamp and considering the calculated current to a filter radiometer. To realize the scale it was used an interactive method in MATLAB platform as described in reference [1][2][3]. The measured spectral irradiance is given by the following equation: ...
This paper presents the preliminary results of the realization of absolute spectral irradiance scale at INMETRO in the ultraviolet, visible and infrared regions using filter radiometers as secondary standards. In the construction of these instruments are used, at least, apertures, interference filters and a trap detector. In the assembly of the trap detectors it was necessary to characterize several photocells in spatial uniformity and shunt resistance. All components were calibrated and these results were analyzed to mount the filter radiometer.
... The realization of the spectral irradiance scale at MIKES is described in Refs. [1][2][3] with detailed uncertainty budgets. The scale is based on absolutely characterized filter radiometers that consist of a trap detector made of three silicon photodiodes, interchangeable filters, and a precision aperture. ...
... The characterization of the filter radiometers is described in Refs. [2] and [3]. ...
A bilateral comparison of the spectral irradiance scales between MIKES (Finland) and NIMT (Thailand) was carried out at 22 wavelengths between 290 nm and 900 nm. MIKES acted as the pilot and link to the results of the key comparison CCPR-K1.a. The spectral irradiance values measured by NIMT generally agree with the key comparison reference value within the expanded uncertainty. The only exceptions are results at wavelengths 300 nm, 450 nm and 500 nm, where the ratios between the degree of equivalence (DoE) and the expanded uncertainty of DoE (k = 2) are 1.0, 1.4 and 1.2, respectively.
Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.
The final report has been peer-reviewed and approved for publication by the CCPR, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).
... The method of realization of the value of spectral irradiance is developed according to existing laboratory conditions, with respect to many already proved, detector based methods of realization of spectral irradiance scale [4][5][6]. ...
... The basic idea of this experiment was to use the transfer spectroradiometer to measure the relative spectral distribution of the standard lamp, instead of the set of filter-radiometers and interpolation based on adopted preliminary values of spectral emissivity of tungsten [5]. ...
Realization of the scale of spectral responsivity of the detectors in the Directorate of Measures and Precious Metals (DMDM) is based on silicon detectors traceable to LNE-INM. In order to realize the unit of spectral irradiance in the laboratory for photometry and radiometry of the Bureau of Measures and Precious Metals, the new method based on the calibration of the spectroradiometer by comparison with standard detector has been established. The development of the method included realization of the System of Spectral Comparisons (SSC), together with the detector spectral responsivity calibrations by means of a primary spectrophotometric system. The linearity testing and stray light analysis were preformed to characterize the spectroradiometer. Measurement of aperture diameter and calibration of transimpedance amplifier were part of the overall experiment. In this paper, the developed method is presented and measurement results with the associated measurement uncertainty budget are shown.
... The level of precision decreases along the chain of traceability, as the uncertainty of the high level standards is inherited to lower levels. Therefore, primary and transfer standards are maintained by NMIs [ The TKK method for measuring absolute spectral irradiance is based on filter radiometers whose components are characterized separately and the modified Planck's radiation law [34, 35]. The calibration of the filter radiometers is traceable to the absolute cryogenic radiometer via spectral responsivity scale, making the detectors crucial components of the filter radiometers. ...
... All detectors are currently used for calibration purposes covering the wavelength range between 200 nm to 1650 nm. In order to extend the detector-based spectral irradiance scale [34, 35] of TKK to the near infrared wavelength region, a Ge photodiode and a trap detector consisting of three Ge photodiodes have been constructed and characterized [P1]. Both detectors are shown in The spectral responsivity of large area Ge photodiodes in a trap detector configuration has been studied earlier by Stock et al. [17]. ...
... Many national metrology institutes (NMIs) have adopted the practice of realizing an absolute detectorbased spectral irradiance scale [1][2][3][4][5] using cryogenic radiometers as the primary standard because it offers a relatively short traceability chain and low uncertainties compared with the traditional source-based method. Several different calibration techniques for the absolute spectral irradiance responsivity of filter radiometers are commonly in use [6 -10]. ...
... While in most of the radiometric temperature measurements radiometers that measure the radiance of blackbody sources are used, the spectral irradiance measurements are usually an essential part of their spectral radiance responsivity calibration [9]. Significant improvements in the spectral irradiance scales have been made, especially in the shortwave IR region [3,4]. ...
Independent methods for measuring the absolute spectral irradiance responsivity of detectors have been compared between the calibration facilities at two national metrology institutes, the Helsinki University of Technology (TKK), Finland, and the National Institute of Standards and Technology (NIST). The emphasis is on the comparison of two different techniques for generating a uniform irradiance at a reference plane using wavelength-tunable lasers. At TKK's Laser Scanning Facility (LSF) the irradiance is generated by raster scanning a single collimated laser beam, while at the NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources (SIRCUS), lasers are introduced into integrating spheres to generate a uniform irradiance at a reference plane. The laser-based irradiance responsivity results are compared to a traditional lamp-monochromator-based irradiance responsivity calibration obtained at the NIST Spectral Comparator Facility (SCF). A narrowband filter radiometer with a 24 nm bandwidth and an effective band-center wavelength of 801 nm was used as the artifact. The results of the comparison between the different facilities, reported for the first time in the near-infrared wavelength range, demonstrate agreement at the uncertainty level of less than 0.1%. This result has significant implications in radiation thermometry and in photometry as well as in radiometry.
... Trap detectors can be equipped with precision apertures and used in overfilled conditions to measure the irradiance or illuminance of a light source with an omnidirectional radiation pattern. Typically, a passband filter and a precision aperture are used when measuring incandescent lamps or black-body radiators [5][6][7]. Recently, new measurement methods for LED lamps have been introduced, where trap detectors are used to measure illuminance of LED lamps without filters [8,9]. This has become possible, because with LEDs all radiation is within the silicon detector spectral range, but dust protection of the photodiodes is then again needed when operating in normal laboratory conditions [9]. ...
According to our experimental results, a nitrogen flow used to prevent dust and moisture entering a detector may influence measurements performed with trap detectors in overfilled conditions. A stable light source was measured with a wedged trap detector with 4 mm aperture, and the nitrogen flow rate was varied. The nitrogen flow was found to have the largest effect of up to 0.8% on the responsivity of the detector at around 1.0 l min ⁻¹ flow rate. The effect of nitrogen flow can be removed down to 0.02% by an added crossflow which removes the nitrogen out of the optical axis. In another experiment, the effect was removed almost completely by changing the flowing gas from nitrogen to synthetic dry air. We also present measurement results that indicate the responsivity changes with nitrogen to be smaller than 0.05% with underfilled beam geometry, even without the added crossflow. Based on simulations, the nitrogen flow through the detector forms a gradient-index type gas lens in front of the detector increasing the effective aperture area and thus the responsivity. In the underfilled measurement geometry there is no light close to the aperture edge which could be refracted inside the detector. Finally, we consider methods to ensure that the responsivity changes due to the gas flow remain below 0.05% in overfilled measurement geometry, without compromising the cleanliness of the detector with too small gas flow rate.
... The FMI Brewer spectroradiometers are calibrated every second or third month using 1 kW lamps in the laboratory . The primary calibration lamps are calibrated yearly at the National Standard Laboratory MIKES-Aalto (Heikkilä et al., 2016b;Kübarsepp et al., 2000). The quality assurance of the measurements includes corrections for temperature dependence and cosine error (Lakkala et al., 2008;Mäkelä et al., 2016;Lakkala et al., 2018), and data are submitted to the European UV database (Heikkilä et al., 2016a). ...
The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor (S5P) satellite was launched on 13 October 2017 to provide the atmospheric composition for atmosphere and climate research. The S5P is a Sun-synchronous polar-orbiting satellite providing global daily coverage. The TROPOMI swath is 2600 km wide, and the ground resolution for most data products is 7.2×3.5 km2 (5.6×3.5 km2 since 6 August 2019) at nadir. The Finnish Meteorological Institute (FMI) is responsible for the development of the TROPOMI UV algorithm and the processing of the TROPOMI surface ultraviolet (UV) radiation product which includes 36 UV parameters in total. Ground-based data from 25 sites located in arctic, subarctic, temperate, equatorial and Antarctic areas were used for validation of the TROPOMI overpass irradiance at 305, 310, 324 and 380 nm, overpass erythemally weighted dose rate/UV index, and erythemally weighted daily dose for the period from 1 January 2018 to 31 August 2019. The validation results showed that for most sites 60 %–80 % of TROPOMI data was within ±20 % of ground-based data for snow-free surface conditions. The median relative differences to ground-based measurements of TROPOMI snow-free surface daily doses were within ±10 % and ±5 % at two-thirds and at half of the sites, respectively. At several sites more than 90 % of cloud-free TROPOMI data was within ±20 % of ground-based measurements. Generally median relative differences between TROPOMI data and ground-based measurements were a little biased towards negative values (i.e. satellite data < ground-based measurement), but at high latitudes where non-homogeneous topography and albedo or snow conditions occurred, the negative bias was exceptionally high: from -30 % to -65 %. Positive biases of 10 %–15 % were also found for mountainous sites due to challenging topography. The TROPOMI surface UV radiation product includes quality flags to detect increased uncertainties in the data due to heterogeneous surface albedo and rough terrain, which can be used to filter the data retrieved under challenging conditions.
... The FMI Brewer spectroradiometers are calibrated every second or third month using 1 kW lamps in the laboratory . The primary calibration lamps are calibrated yearly at the National Standard Laboratory MIKES-Aalto Kübarsepp et al., 2000). The quality assurance of the measurements includes corrections for temperature dependence and cosine error (Lakkala et al., 2008;Mäkelä et al., 2016;Lakkala et al., 2018) and 140 data are submitted to the European UV data base . ...
The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor (S5P) satellite was launched on 13 October 2017 to provide the atmospheric composition for atmosphere and climate research. The S5P is a sun-synchronous polar-orbiting satellite providing global daily coverage. The TROPOMI swath is 2600 km wide, and the ground resolution for most data products is 7.2 x 3.5 km2 (5.6 x 3.5 km2 since 6 August 2019) at nadir. The Finnish Meteorological Institute (FMI) is responsible for the development and processing of the TROPOMI Surface Ultraviolet (UV) Radiation Product which includes 36 UV parameters in total. Ground-based data from 25 sites located in arctic, subarctic, temperate, equatorial and antarctic areas were used for validation of TROPOMI overpass irradiance at 305, 310, 324 and 380 nm, overpass erythemally weighted dose rate/UV index and erythemally weighted daily dose for the period from 1 January 2018 to 31 August 2019. The validation results showed that for most sites 60–80 % of TROPOMI data was within ±20 % from ground-based data for snow free surface conditions. The median relative differences to ground-based measurements of TROPOMI snow free surface daily doses were within ±10 % and ±5 % at two thirds and at half of the sites, respectively. At several sites more than 90 % of clear sky TROPOMI data were within ±20 % from ground-based measurements. Generally median relative differences between TROPOMI data and ground-based measurements were a little biased towards negative values, but at high latitudes where non-homogeneous topography and albedo/snow conditions occurred, the negative bias was exceptionally high, from −30 % to −65 %. Positive biases of 10–15 % were also found for mountainous sites due to challenging topography. The TROPOMI Surface UV Radiation Product includes quality flags to detect increased uncertainties in the data due to heterogeneous surface albedo and rough terrain which can be used to filter the data retrieved under challenging conditions.
... A driving current of 100 mA (4.4 A/cm 2 ) was chosen to maximize the efficiency and thus the radiative recombination rate and to provide a reasonable light intensity for the spectroradiometer, still limiting the heating of the sample with the injected current to few kelvins. The calibration of the spectroradiometer is traceable to the spectral irradiance scale of the National Standards Laboratory of Finland [29]. Fig. 3 shows normalized emission spectra of the single junction samples measured at three different temperatures together with curves fitted according to (1) and (4). ...
A recently developed method to characterize the band gap energies of III-V optosemiconductors was utilized to determine temperature-invariant band gap features of multijunction solar cells. The method is based on measuring electroluminescent spectra of the solar cells at different temperatures. The normalized spectra reveal temperature-invariant energy values of the different junctions which are further converted to band gap energies. The method utilization requires a calibrated spectroradiometer and a temperature controlled mounting base for the solar cell under test, however, no knowledge about the absolute temperature of the cell under measurement. The method was tested on GaAs and GaInP solar cells that consist of single and dual junctions. The band gap energies were also derived from spectral response measurements. The differences of the determined band gap energies from the literature values were smaller than 1.1%. Compared with other band gap determination methods, the developed method yields temperature-invariant band gap characteristics; with a known uncertainty, that separated the different junctions in a multijunction device without individual biasing for the different junctions. In addition, a temperature-independent characterization parameter ensures that the operating conditions of the device under test do not affect the results.
... This method was suggested in the NBS Self-Study Manual (Shumaker 1979). It has also been used to analyse filter radiometer measurements of the irradiance produced by a lamp (Kübarsepp et al. 2000). ...
... A scanning monochromator, as typically used with an ESCR, is to slow and bulky for a portable system. An alternative is to use a series of narrow band-pass filters to build a filter radiometer [7][8][9][10][11]. A filter radiometer measures the integrated spectral irradiance in a series of narrow bands. ...
... This may be every 20 nm in the UV region and every 50 nm in the visible. Values between the discrete wavelengths are interpolated with complicated functions, such as modified Planck's law, high order polynomials, or cubic splines [13][14][15]. The deviations in the interpolation leave com plicated systematic spectral errors in the irradiance values [7]. ...
Spectral irradiance data are often used to calculate colorimetric properties, such as color coordinates and color temperatures of light sources by integration. The spectral data may contain unknown correlations that should be accounted for in the uncertainty estimation. We propose a new method for estimating uncertainties in such cases. The method goes through all possible scenarios of deviations using Monte Carlo analysis. Varying spectral error functions are produced by combining spectral base functions, and the distorted spectra are used to calculate the colorimetric quantities. Standard deviations of the colorimetric quantities at different scenarios give uncertainties assuming no correlations, uncertainties assuming full correlation, and uncertainties for an unfavorable case of unknown correlations, which turn out to be a significant source of uncertainty. With 1% standard uncertainty in spectral irradiance, the expanded uncertainty of the correlated color temperature of a source corresponding to the CIE Standard Illuminant A may reach as high as 37.2 K in unfavorable conditions, when calculations assuming full correlation give zero uncertainty, and calculations assuming no correlations yield the expanded uncertainties of 5.6 K and 12.1 K, with wavelength steps of 1 nm and 5 nm used in spectral integrations, respectively. We also show that there is an absolute limit of 60.2 K in the error of the correlated color temperature for Standard Illuminant A when assuming 1% standard uncertainty in the spectral irradiance. A comparison of our uncorrelated uncertainties with those obtained using analytical methods by other research groups shows good agreement. We re-estimated the uncertainties for the colorimetric properties of our 1 kW photometric standard lamps using the new method. The revised uncertainty of color temperature is a factor of 2.5 higher than the uncertainty assuming no correlations.
... The foreoptics consisted of a measurement head UV-J1002 from CMS-Schreder with teflon diffuser, connected to the spectrometer by a quartz fibre. The irradiance scale used in the calibration of the spectrometer is traceable to VTT MIKES, Aalto University, Finland [3]. In the UV chambers, the measurement head was placed on the bottom of the chamber with the optical axis in vertical orientation. ...
We report on a study focusing on UV exposure conditions in three different types of chambers used for accelerated ageing of materials. The first chamber is equipped with four 300-W UVA/UVB mercury vapour lamps (Ultra-Vitalux/Osram). The second chamber uses four 40-W UVA fluorescent lamps (QUV-340/Q-Lab). The third chamber is Weather-Ometer Ci3000+ from Atlas with a 4500-W xenon arc lamp. UV irradiance prevailing in each chamber was measured using Bentham DM150 double monochromator spectroradiometer. The results were compared to measurements of solar spectral UV irradiance at Jokioinen, Finland, with a Brewer MkIII double monochromator spectrophotometer. The spectral shapes of the exposing UV radiation in the different chambers were found to notably differ from each other and from the solar UV spectrum. Both spatial inhomogeneities and temporal variability caused by various factors, like the ageing of the lamps, were detected. The effects were found to strongly depend on wavelength of the exposing UV radiation. The findings of this study underline the necessity of careful characterization of the UV exposure conditions provided by the facilities used in accelerated testing of materials.
... The intensity calibration of the spectroradiometer was carried out using a calibrated FEL lamp, which is traceable to the spectral irradiance scale of the National Standards Laboratory of Finland. 14 The wavelength scale of the spectroradiometer was calibrated against spectral lines of a mercury argon calibration source. Figure 1 shows the normalized spectra of the blue and red LEDs measured at different temperatures. ...
Relative emission spectra of light-emitting diodes (LEDs) depend on the junction temperature. The high-energy region of the emission spectrum can be modelled with Maxwell-Boltzmann distribution as a function of energy and junction temperature. We show that according to the model and our experiments, the normalized emission spectra at different junction temperatures intersect at a unique energy value. The invariant intersection energy exists for many types of LEDs and can be used to determine the alloy composition of the material. Furthermore, the wavelength determined by the intersection energy can be used as a temperature invariant wavelength reference in spectral measurements.
... The relative spectral fluxes e (λ) of the measured LEDs and the V(λ) curve are shown in figure 3. The spectra were measured using a double monochromator scanning spectroradiometer with 1 nm bandwidth. The relative intensity scale of the spectroradiometer was calibrated against the traceable spectral irradiance of an FEL lamp [28], whereas the argon ion laser line at 457.94 nm and the helium-neon laser line at 632.82 nm were used to calibrate the wavelength scale in air. The calculated spectral mismatch correction factors, measured photocurrents and the corresponding illuminance values are given for the PQED and the reference photometer in table 1. ...
The production of incandescent light bulbs is bound to end, as incandescent lighting is being phased out globally in favour of more energy-efficient and sustainable solutions. Temporally stable light-emitting diodes (LEDs) are potential candidates to replace incandescent lamps as photometric source standards. However, traditional V(λ) filter based photometers may have large uncertainty when LEDs are measured instead of incandescent lamps. This is due to the narrow and complicated spectra of LEDs. When the spectra of LEDs are limited to the visible wavelength range, new silicon detector technology can be advantageously exploited in photometry. We present a novel method—based on the recently introduced Predictable Quantum Efficient Detector (PQED)—for the realization of photometric units which completely eliminates the need to use V(λ) filters. Instead, the photometric weighting is taken into account numerically by measuring the relative spectral irradiance. The illuminance values of a blue and a red LED were determined using the new method and a conventional reference photometer. The values obtained by the two methods deviated from each other by −0.06% and 0.48% for the blue and red LED, respectively. The PQED-based values have much lower standard uncertainty (0.17% to 0.18%) than the uncertainty of the values based on the conventional photometer (0.46% to 0.51%).
... There is a double monochromator Brewer Mk-III spectrophotometer at Jokioinen, whereas at Sodankylä, the surface UV irradiance is measured with a single monochromator Brewer Mk-II. Both Brewers are calibrated with 1000 W lamps traceable to the irradiance scale of Helsinki University of Technology [Kübarsepp et al., 2000]. They are scheduled to perform spectral scans at fixed solar zenith angles, and, additionally, at solar noon each day. ...
Ozone Monitoring Instrument (OMI) onboard the NASA EOS Aura spacecraft
is a UV/Vis spectrometer with a 2600 km wide swath capable of daily,
global contiguous mapping. Mission requirements include monitoring of
ozone and other trace gases, cloud pressure and reflectivity and
aerosols. OMI is the successor to the NASA TOMS instrument but with
8-fold better ground resolution (13 by 24 km in nadir) and wide spectral
coverage from 270 to 500 nm. The OMI measurements are used as inputs to
the radiative transfer model to estimate the ultraviolet (UV) radiation
reaching the Earth's surface. Noontime surface spectral UV irradiance
estimates are produced at four wavelengths (305, 310, 324, 380 nm).
Additionally, noontime erythemal dose rate and the erythemal daily dose
are estimated. The OMI surface UV algorithm inherits from the TOMS UV
algorithm developed by NASA/GSFC. We present the first OMI surface UV
product validation results. We have compared the OMI UV data with other
satellite UV products as well as ground based spectral UV measurement
data. The results imply improvements in the accuracy of the satellite UV
data thanks to the improved spatial resolution of the instrument.
... Therefore, in some cases an increase in measurement uncertainty for area is acceptable to improve the speed and efficiency of the aperture area measurements. Such an efficiency improvement is especially important for a method (Kubarsepp et al. 2000) where more than ten separate detectors (filter radiometers) are used to realize the spectral irradiance scale, as each of these filter radiometers would need a dedicated 3-mm-diameter aperture with known area. Another need for straightforward aperture area measurement comes from the study of mechanical stability of aperture diameter over time scales of several months after the drilling. ...
The use of an optical coordinate measuring machine (CMM) for the diameter measurement of optical apertures is described. The traceability and mechanical stability of the aperture areas are of importance for accurate photometric and radiometric measurements. Detailed evaluation of the measurement uncertainty for the aperture diameter is presented. High-accuracy mechanical CMM was used to confirm the validity of the optical CMM results. The difference between the contact and non-contact measurement was 0.1 µm for the mean diameter result. If the required standard uncertainty for the mean diameter is of the order of 1 µm, the optical CMM provides an efficient method for aperture area measurements.
... Our present detector-based scale of spectral irradiance covers the wavelength range between 290 and 900 nm [8]. In order to meet the needs of our customers, we are in the process of extending our measurement capabilities to the nearinfrared region. ...
We have developed and characterized new detectors based on germanium (Ge) photodiodes to be used in the wavelength region between 900 and 1650 nm. The effects of spatial uniformity, temperature and low shunt resistance on the spectral responsivity measurements are studied. Our results for the spatial uniformities show improvements as compared with earlier studies. The spectral reflectances of a Ge photodiode and trap detector are also studied, and the trap reflectance is found to be less than 10−4 at wavelengths longer than 900 nm. The results of this study show that with careful use, the large area Ge photodiodes can offer a cost-effective alternative as compared with InGaAs photodiodes of similar diameters.
... In addition, at least one of the standard lamps should be certified by an accredited calibration laboratory or by an NSL. HUT has introduced a detector-based calibration method to overcome the problems caused by instabilities of lamps [Kärhä et al. 1996, 2000, Kübarsepp et al. 2000]. In this method, stable narrowband filter radiometers calibrated against an absolute cryogenic radiometer are used for the basic realisation of the scale from 280 to 400 nm and to transfer the scale to the standard lamps. ...
Relative emission spectra of LEDs depend on the junction temperature. The high-energy region of the emission spectrum can be modelled with the joint density of states and the Maxwell-Boltzmann distribution as a function of energy and junction temperature. It can be shown that the normalized emission spectra at different junction temperatures intersect at a unique energy value. Thus the wavelength and the relative intensity of the intersection point do not depend on the junction temperature of the LED. The invariant intersection energy exists for all LEDs manufactured using the elements from groups III-V. The wavelength determined by the intersection energy can be used as a temperature invariant wavelength and relative intensity reference in spectral measurements.
Spectral irradiance measurement system was developed to measure the spectral irradiance of optical sources in the wavelength range from 250[nm] to 1600[nm]. Our system is composed of source system, fore-optics, monochromator system, optical detector system, and automatic control system. Optical detector system with PMT, Si, InGaAs, and IR enhanced InGaAs detectors is used to measure the wide spectrum of optical sources in ultraviolet visible, and infrared wavelength regions. Spectral irradiance of the 1[kW] quartz-halogen tungsten lamp was measured and compared in the wavelength range from 250[nm] to 1600[nm]. The differences between our results and those reported by NIST are below 3[%], 3.5[%], and 5[%] in the wavelength range of 450?700[nm], 700?1600[nm], 250?400[nm], respectively.
The two Brewer spectrophotometers of the Finnish Meteorological Institute at
Jokioinen and Sodankylä have been operated according to the highest
levels of the WMO∕GAW (World Meteorological Organization∕Global Atmosphere Watch) recommendations with rigorous quality control and
quality assurance. The calibration of the instruments is based on annual
recalibrations of primary standard lamps in the VTT MIKES Metrology National Standards
Laboratory in Finland and an exhaustive measurement program with measurements of
standard and working lamps in the on-site optical laboratories. Over the
years, the maintenance of the calibration has produced data sets of
approximately 2000 lamp scans for both instruments. An extensive
re-examination of the lamp measurements and the response of the
spectrophotometers was carried out. The primary standard lamps were
found to age on an average rate of 0.3 % per burn. The responsivity at
wavelength 311 nm was found to exhibit both long-term and short-term
changes. The overall long-term change was declining. In addition, abrupt
changes of as large as 25 % were detected. The short-term changes were
found to fluctuate on time frames shorter than the interval between the
measurements of the primary standard lamps. This underlines the importance of
the use of more frequently measured working standard lamps.
The performance of an automatic multi-wavelength filter radiometer (MWFR) in the near IR range is evaluated using a three-element In GaAs trap detector and nine narrow-band interference filters from NMC (Singapore) and MIKES (Finland) in a joint experiment conducted by researchers from both labs. Comparisons of detector spectral responsivity and filter spectral regular transmittance in the same wavelength range are also carried out. The maximum relative deviation of the measured photocurrent using MWFR and of the calculated photocurrent using known spectral irradiance values from a standard lamp is less than 1.8 % with an expanded uncertainty of 1.9 % (k = 2). From the promising results obtained from this and earlier work, it is concluded that a direct realisation of the spectral irradiance scale in the wavelength range of 300 nm to 1600 nm is possible using the MWFR.
Five different types of LED lamps were aged for 35 months at room temperature and for six months at the temperatures of 45℃ and 60℃. The lifetimes for the lamps were predicted from the luminous flux measurement results. The long-term tests show that early predictions of lifetime can be pessimistic. The results at higher temperatures show that ageing can be accelerated by modest heating. On the average, heating to 45℃ reduced the lifetime by a factor of 1.35 and heating to 60℃ by a factor of 2.36. An alternative accelerated method to test and analyse ageing is proposed. During the ageing of 35 months, the changes in correlated colour temperatures were smaller than 4.3%.
A photometer used for on-line product testing of light-emitting diode (LED) buoy lanterns is calibrated for illuminance responsivity using two different methods. The first method is based on absolute calibration of the photometer with the CIE standard illuminant A light source, com- bined with spectral correction factors (SCFs) calculated from the mea- sured spectral responsivity of the photometer and relative spectra of the LED lanterns. The second method is based on direct comparison with a characterized reference photometer using the LED lanterns as light sources. Comparison of the resulting correction factors shows that both methods agree within 1%. However, the second method includes geo- metrical aspects and LED characteristics that caused problems. These problems are discussed and the reasons for recommending the first method are given. © 2004 Society of Photo-Optical Instrumentation Engineers.
Methods used for the establishment and dissemination of primary radiometric quantities are reviewed together with the performance and use of filter radiometers, which are dominating advances in optical radiation measurement. The possibility is also discussed of using many of these advances in other metrological sectors, such as in thermal metrology to improve the realization and dissemination of the kelvin.
Narrow-band filter radiometers at 248 nm, 313 nm, 330 nm and 368 nm wavelengths were used to compare calibration facilities of spectral (irradiance) responsivity at HUT, NPL and BNM–INM. The results are partly in agreement within the stated uncertainties. Use of demanding artefacts in the intercomparison revealed that the wavelength scales of the participating institutes deviate more than expected. Such effects cannot be seen in typical intercomparisons of spectral responsivity or spectral transmittance, where spectrally neutral samples are used.
Results of filter radiometer characterization with a wavelength-tunable Ti : sapphire laser in the wavelength band around 900 nm are presented. The effect of interference between the reflections from filter surfaces in the case of coherent laser light was studied and reduced with a special filter design with antireflection coatings. Measuring the responsivity as a function of wavelength over a very narrow band was used to reveal the remaining interference effects. Uncertainty analysis and test results indicate that filter radiometers can be characterized with a relative standard uncertainty of 9×10−4 using the scanning method. The results agree well with more conventional monochromator-based measurements.
The uncertainty of a primary spectral irradiance scale based on filter radiometers (FRs) is studied by analysing the propagation of uncertainties and covariances through a spectral interpolation process, when a modified Planck's radiation law is fitted to the measurement data. The advantage of performing the uncertainty analysis in optimizing the selection of the FR wavelengths is demonstrated. We also estimate the effect that correlations in the FR signals have on the uncertainty of the fitted spectral irradiance values. In the case of correlated input data, the results of the uncertainty propagation are found to be within two practical limits: uncertainty values in the FR data and the values obtained by uncertainty propagation with uncorrelated FR signals.
The results of a comparison of spectral irradiance scales between the National Institute of Standards
and Technology (NIST, USA), and the Metrology Research Institute, Helsinki University of Technology (HUT,
Finland), are presented. The comparison was carried out by measuring irradiances of two sets of 1 kW FEL-type lamps in the spectral range 290 nm to 900 nm. One set had calibration from the NIST 1992 source-based scale and the other from the HUT detector-based scale. The spectral irradiance values measured by the HUT are slightly higher than those given by the NIST: the differences are less than 1% in the visible/near-infrared wavelength region and up to 3.3% in the ultraviolet. The relative expanded uncertainty (k = 2) of the comparison varies between 3.1% to 1.0% from 290 nm to 900 nm, respectively.
UV radiation conditions in a UV chamber equipped with four 300-W mercury vapour lamps have been characterized to assess the exposure received by the specimens aged in the chamber. Spectrally resolved measurements were performed on UV radiation emitted by new lamps and lamps operated for 4850 h in the chamber. An intensity decrease of as much as 75% with a strong wavelength dependency was detected. By linking the measurements with independent broadband measurements of the UV radiation field emitted by a single lamp, the UV dose rate distribution on the specimen plane of the chamber was derived. A method for true exposure accumulation estimation, accounting for the fading of the lamps, was developed. Accumulated UV radiation exposures on material samples in the chamber were derived and compared to long-term (1995–2007) average UV exposures received in natural sunshine at Jokioinen, Finland (lat. 60.4°N, long. 23.5°E, 104 m a.s.l.). Acceleration factors for the exposure in the UV chamber were derived in terms of doses integrated over different wavelength ranges. The acceleration factor was found to depend strongly on the wavelength range of the dose considered, presenting an extra challenge to the assessment of the artificial material exposure duration.
The Dutch-Finnish Ozone Monitoring Instrument (OMI) on board the NASA EOS Aura spacecraft is a nadir viewing spectrometer that measures solar reflected and backscattered light in a selected range of the ultraviolet and visible spectrum. The instrument has a 2600 km wide viewing swath and it is capable of daily, global contiguous mapping. The Finnish Meteorological Institute and NASA Goddard Space Flight Center have developed a surface ultraviolet irradiance algorithm for OMI that produces noontime surface spectral UV irradiance estimates at four wavelengths, noontime erythemal dose rate (UV index), and the erythemal daily dose. The overpass erythemal daily doses derived from OMI data were compared with the daily doses calculated from the ground-based spectral UV measurements from 18 reference instruments. Two alternative methods for the OMI UV algorithm cloud correction were compared: the plane-parallel cloud model method and the method based on Lambertian equivalent reflectivity. The validation results for the two methods showed some differences, but the results do not imply that one method is categorically superior to the other. For flat, snow-free regions with modest loadings of absorbing aerosols or trace gases, the OMI-derived daily erythemal doses have a median overestimation of 0-10%, and some 60 to 80% of the doses are within +/-20% from the ground reference. For sites significantly affected by absorbing aerosols or trace gases one expects, and observes, bigger positive bias up to 50%. For high-latitude sites the satellite-derived doses are occasionally up to 50% too small because of unrealistically small climatological surface albedo.
The use of high-temperature blackbody (HTBB) radiators to realize primary spectral irradiance scales requires that the operating temperature of the HTBB be accurately determined. We have developed five filter radiometers (FRs) to measure the temperature of the National Research Council of Canada's HTBB. The FRs are designed to minimize sensitivity to ambient temperature fluctuations. They incorporate air-spaced colored glass filters and a Si photodiode detector that are housed in a cell whose temperature is controlled to ±0.1°C by means of annular thermoelectric elements at the front and rear of the cell. These wideband filter radiometers operate in four different wavelength bands. The spectral responsivity measurements were performed in an underfill geometry for a power-mode calibration that is traceable to NRC's cryogenic radiometer. The spectral temperature sensitivity of each of these FRs has been measured. The apertures for these FRs were cold-formed by swaging machine-cut apertures onto precision dowel pins. A description of the filter radiometer design, fabrication and testing, together with a detailed uncertainty analysis, is presented. We derive the equations that relate the spectral irradiance measured by the FRs to the spectral radiance and temperature of the HTBB, and deal specifically with the change of index of refraction over the path of the radiation from the interior of the HTBB to the FRs. We believe these equations are more accurate than recently published derivations. Our measurements of the operating temperature of our HTBB working at temperatures near 2500 K, 2700 K and 2900 K, together with measurements using a pyrometer, show agreement between the five filter radiometers and with the pyrometer to within the estimated uncertainties. yes yes
The quality assurance of the two Brewer spectrophotometers of the Finnish Meteorological Institute is discussed in this paper. The complete data processing chain from raw signal to high quality spectra is presented. The quality assurance includes daily maintenance, laboratory characterizations, calculation of long term spectral responsivity, data processing and quality assessment. The cosine correction of the measurements is based on a new method, and included in the data processing software. The results showed that the actual cosine correction factor of the Finnish Brewers can vary between 1.08–1.13 and 1.08–1.12, respectively, depending on the sky radiance distribution and wavelength. The temperature characterization showed a linear temperature dependence between the internal temperature and the photon counts per cycle, and a temperature correction was used for correcting the measurements. The long term spectral responsivity was calculated using time series of several lamps using two slightly different methods. The long term spectral responsivity was scaled to the irradiance scale of the Helsinki University of Technology (HUT) for the whole measurement time periods 1990–2006 and 1995–2006 for Sodankylä and Jokioinen, respectively. Both Brewers have participated in many international spectroradiometer comparisons, and have shown good stability. The differences between the Brewers and the portable reference spectroradiometer QASUME have been within 5% during 2002–2007.
The quality assurance of the two Brewer spectrophotometers of the Finnish Meteorological Institute is discussed in this paper. The complete data processing chain from raw signal to high quality spectra is presented. The quality assurance includes daily maintenance, laboratory characterizations, calculation of long-term spectral responsivity, data processing and quality assessment. The cosine correction of the measurements is based on a new method, and is included in the data processing software. The results showed that the actual cosine correction factor of the two Finnish Brewers can vary between 1.08–1.13 and 1.08–1.12, respectively, depending on the sky radiance distribution and wavelength. The temperature characterization showed a linear temperature dependence between the instruments' internal temperature and the photon counts per cycle, and a temperature correction was used for correcting the measurements. The long-term spectral responsivity was calculated using the time series of several lamps using two slightly different methods. The long-term spectral responsivity was scaled to the irradiance scale of the Helsinki University of Technology (HUT) for the whole of the measurement time-periods 1990–2006 and 1995–2006 for Sodankylä and Jokioinen, respectively. Both Brewers have participated in many international spectroradiometer comparisons, and have shown good stability. The differences between the Brewers and the portable reference spectroradiometer QASUME have been within 5% during 2002–2007.
An intercomparison of spectral irradiance measurements by 12 national laboratories has been carried out between 1987 and 1990. The intercomparison was conducted under the auspices of the Comite Consultatif de Photometrie et Radiometrie (CCPR) of the Comite International des Poids et Mesures, and the National Institute of Standards and Technology (NIST) served as the pilot laboratory. The spectral range of the intercomparison was 250 to 2400 nm and the transfer standards used were commercial tungsten-halogen lamps of two types. The world-wide consistency of the results (one standard deviation) was on the order of 1% in the visible spectral region and 2 to 4% in the ultraviolet and infrared portions of the spectrum. The intercomparison revealed no statistically significant differences between spectral-irradiance scales based on blackbody physics and absolute detector radiometry.
Using the National Institute of Standards and Technology high-accuracy cryogenic radiometer (HACR), we have realized a scale of absolute spectral response between 406 and 920 nm. The HACR, an electrical-substitution radiometer operating at cryogenic temperatures, achieves a combined relative standard uncertainty of 0.021%. Silicon photodiode light-trapping detectors were calibrated against the HACR with a typical relative standard uncertainty of 0.03% at nine laser wavelengths between 406 and 920 nm. Modeling of the quantum efficiency of these detectors yields their responsivity throughout this range with comparable accuracy.
A high-accuracy spectrometer has been developed for measuring regular spectral transmittance. The spectrometer is an automated, single-beam instrument that is based on a grating monochromator, reflecting optics, and an averaging sphere detector unit with a silicon photodiode. The uncertainties related to wavelength calibration, detector nonlinearity, system instability, beam displacement, polarization, stray light, interreflections, and beam uniformity are determined for the visible spectral range from 380 to 780 nm. A total uncertainty of 3 × 10⁻⁴ (1σ) is estimated for transmittance measurements of homogeneous neutral-density filters. The uncertainty of the wavelength scale is 0.06 nm. As a specific application, calibration of V(λ)-correction filters is studied. To verify the accuracy of the transmittance measurements, a comparison of the measured and predicted transmittances of a sample of high-purity fused silica is made, revealing agreement at the 5 × 10⁻⁴ level.
A six-element polarization-independent transmission trap detector with coaxial input and output beams has been constructed and full characterized. The measured optical parameters are compared with their values, predicted by Fresnel equations. Measured transmittances are in agreement with the predicted values within 2 × 10⁻⁵ in the wavelength region from 450 to 650nm. The spectral responsivity of the transmission trap detector is in agreement with the predicted values within 0.035% at 543.5- and 633.0-nm vacuum wavelengths. The spatial uniformity of the responsivity is ±0.03% across the active area of approximately 5 × 6 mm², measured with a laser beam of 1-mm diameter. The angular uniformity of the transmission trap detector is better than ±0.01% for ±3° rotation around two perpendicular axes.
Nonlinearities of the responsivity of various types of silicon photodetectors have been studied. These detectors are based on photodiodes with two sizes of the active area (10 × 10 mm² and 18 × 18 mm²). The detector configurations investigated include single photodiodes, two reflection trap detectors, and a transmission trap detector. For all devices, the measured nonlinearity was less than 2 × 10⁻⁴ for photocurrents up to 200 μA. The diameter of the measurement beam was found to have an effect on the nonlinearity. The measured nonlinearity of the trap detectors depends on the polarization state of the incident beam. The responsivity of the photodetectors consisting of the large-area photodiodes reached saturation at higher photocurrent values compared with the devices based on the photodiodes with smaller active area.
We improve the methods used to interpolate the responsivity of unbiased silicon photodetectors in the near-ultraviolet region. This improvement is achieved by the derivation of an interpolation function for the quantum yield of silicon and by consideration of this function in the interpolation of the internal quantum efficiency of photodiodes. The calculated quantum-yield and spectral-responsivity values are compared with measurement results obtained by the study of a silicon trap detector and with values reported by other research groups. The comparisons show agreement with a standard deviation of 0.4% between our measured and modeled values for both the quantum yield and the spectral responsivity within the wavelength region from 260 to 400 nm. The proposed methods thus extend the predictability of the spectral responsivity of silicon photodetectors to the wavelength region from 260 to 950 nm. Furthermore, an explanation is proposed for the change in the spectral responsivity of silicon photodiodes that is due to UV radiation. In our improved quantum efficiency model the spectral change can be accounted for completely by the adjustment of just one parameter, i.e., the collection efficiency near the SiO2/Si interface.
The Système International des Unités (SI) base unit for photometry, the candela, has been realized by using absolute detectors rather than absolute sources. This change in method permits luminous intensity calibrations of standard lamps to be carried out with a relative expanded uncertainty (coverage factor k = 2, and thus a 2 standard deviation estimate) of 0.46 %, almost a factor-of-two improvement. A group of eight reference photometers has been constructed with silicon photodiodes, matched with filters to mimic the spectral luminous efficiency function for photopic vision. The wide dynamic range of the photometers aid in their calibration. The components of the photometers were carefully measured and selected to reduce the sources of error and to provide baseline data for aging studies. Periodic remeasurement of the photometers indicate that a yearly recalibration is required. The design, characterization, calibration, evaluation, and application of the photometers are discussed.
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 × 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.
A detector-based absolute scale for spectral irradiance in the 380–900-nm wavelength region has been developed and tested at the Helsinki University of Technology (HUT). Derivation of the scale and its use for photometric and colorimetric measurements are described. A thorough characterization of a filter radiometer, constructed from a reflection trap detector, a precision aperture, and a set of seven temperature-controlled bandpass filters, is presented. A detailed uncertainty analysis of the scale indicates a relative standard uncertainty of approximately 0.2% throughout most of the wavelength region. The standard uncertainties obtained in measurements of correlated color temperature and luminous intensity of three Osram Wi41/G tungsten–halogen lamps are 2 K and 0.3%, respectively. The spectral irradiance scale is compared with the HUT luminous intensity scale. The agreement of the results at the 0.1% level is well within the combined standard uncertainty of the two scales.
A new determination of the spectral emissivity of tungsten has been performed in order to supply more reliable data for the use of the tungsten striplamp as a standard source of radiation. A great part of the data, used up to now for this purpose, has been obtained by extrapolation or by combination of the results of measurements carried out by different observers and relevant to tungsten of diverging origin and manufacture. The values of the emissivity of tungsten which are adopted by the principal standard laboratories show considerable differences.
The present measurements have been carried out on straight-rolled tungsten strip, as used in striplamps, of well-defined composition, texture and surface condition. The emissivity was determined by comparing the spectral radiant intensity of the wall with that of a hole in a tubular blackbody to which the tungsten ribbon was shaped. The manufacture of the blackbody was based on a calculation given in a previous paper (Physica 20 (1954) 669. The accuracy achieved in the present measurements is shown to be much better than that in the earlier measurements. The results are given (Fig. 6) as a function of wavelength in the region 0.23–2.7 μ, for temperatures ranging from 1600°K to 2800°K.
A method developed at the Helsinki University of Technology (HUT) for calibrating standard lamps of spectral irradiance in UV-A and UV-B regions is presented. The method is based on a compact filter radiometer that is characterized absolutely to measure spectral irradiance. The filter radiometer can be brought to laboratories where accurate calibrations are needed. The method overcomes some of the instability problems encountered when using lamps as transfer standards, which makes it also useful in intercomparison compaigns. Test measurements are presented where two standard lamps issued by the National Institute of Standards and Technology (NIST), USA, are compared with the HUT scale held by the radiometer. The results suggest that the agreement between the two scales (HUT and NIST) is approximately 1%-2% in the wavelength region from 300 nm to 400 nm.
High-power quartz-halogen lamps have been in use as transfer standards for a long time. However, even the best lamps available are not stable enough to match the increasing requirements stemming from the progress of measuring techniques. In particular, long-term stability in the ultraviolet-B region has to be improved. In order to achieve higher stability, it is necessary to change the operating conditions from constant-current to a detector-controlled system. The new OMTec transfer-standard system, consisting of a control unit including power supply and a lamp detector unit, is described. Its main features are: long-term stability with a deviation of the spectral radiant flux from the calibration values of less than 3 × 10-5 h-1 during nearly 400 hours of operating time throughout the spectral range from the ultraviolet-B to the near-infrared; continuous check on stability by monitoring and viewing the measured detector photocurrents; reduction of the required seasoning time by a factor of three thus enhancing the lifetime of the lamp; storage of lamp and calibration data in the lamp detector unit for external use.
The facilities of the Metrology Research Institute at the Helsinki University of Technology, and methods for characterisation of optical detectors for spectral radiant intensity and irradiance responsivity, are described. The instrumentation for such characterisations includes a reference spectrometer with a number of auxiliary set-ups, and equipment for the spectral irradiance measurements with a filter radiometer based on a trap detector. The methods of realising the spectral responsivity scales based on an absolute cryogenic radiometer in house are addressed. The procedures and results of characterisation of a multipoint measuring system of photosynthetically active radiation, by employing the available facilities, are briefly described. The absolute irradiance responsivity of the device is determined by using a photometric lamp, whose spectral irradiance has been measured with the filter radiometer. The combined standard uncertainty of this set of calibrations is 3.6% at the 1σ level. The uncertainty is caused almost completely by the multipoint measuring system.
A description is presented of an upgraded trap-detector-based realization of the units of luminous intensity (candela) and illuminance (lux) at the Helsinki University of Technology (HUT). The realization is accomplished using a reference photometer, a light source and a distance-measurement system. A thorough characterization is presented of the reference photometer, consisting of a reflection trap detector, a temperature-controlled V(λ) filter and a high-precision aperture. The maintenance of the units is described. An updated uncertainty budget of the realization is given. Two of the three main uncertainty components of our earlier realizations have been significantly decreased. The uncertainty analysis indicates a relative expanded uncertainty of 2.2 × 10-3 for the realization of the candela and 1.8 × 10-3 for that of the lux. The HUT has participated in three international measurement comparisons, whose results are reviewed. According to the results, the HUT candela deviates by +4.0 × 10-3 from the candela of the Swedish National Testing and Research Institute with an expanded uncertainty of 10-2, -2.7 × 10-3 from that of the National Physical Laboratory (UK) with an expanded uncertainty of 5.6 × 10-3 and -3.3 × 10-3 from the world mean with an expanded uncertainty of 5.9 × 10-3.
Photometric units in Australia are currently derived from power measurements made with a roomtemperature electrical-substitution radiometer. This basis has been transferred to a cryogenic radiometer. The cryogenic radiometer was used to calibrate the absolute responsivity of a four-element, planar, transmission trap detector (measuring the transmission loss each usage), at laser wavelengths. Modelling of the quantum defect then provided a reference for absolute spectral responsivity throughout the photometric wavelength range. This was used to measure the responsivity of a number of photometers at the mercury green line and to calculate correction factors for CIE Illuminant A. The candela so derived essentially agrees with our existing unit, maintained on a set of lamps. The trap detector was also used to determine the polarization properties of the monochromator.
The spectral irradiance of lamp standards can be stabilized within a limited spectral range using a suitable selective detector to control the irradiance of the lamps. The capability of one commercial lamp facility incorporating a computer-controlled power supply was tested in an international comparison. A set of three lamps and the detector-stabilized power supply were circulated between the Physikalisch-Technische Bundesanstalt (PTB, Germany), the Helsinki University of Technology (HUT, Finland) and the Swedish National Testing and Research Institute (SP, Sweden) for the comparison, EUROMET project No. 475.
A standard of illuminance responsivity has been realized on a set of photopically-corrected detectors. It is based on the use of a cryogenic radiometer for the measurement of radiant power in terms of electrical standards and on a spectral responsivity scale established on silicon photodiodes in a "trap" configuration to evaluate the photopic response of the detectors. The spectral responsivity of the photometers was obtained from the trap detector by comparison using a spectrophotometer beam which underfilled both. The spatial responsivity of the photometers was found to be non-uniform by up to 1 % and this is a significant source of uncertainty in the realization. The measurements of aperture area used visual identification of the edges and were corrected for a bias in the visual technique that was demonstrated by checks with a similar-sized ring gauge robust enough for contact measurement.
An optical method for calibration of the aperture area is described and studied both theoretically and experimentally. A spatially uniform, known irradiance is formed over the aperture by overlapping identical, parallel laser beams centred at constant spacing in an orthogonal lattice. The ratio of the throughput power and irradiance gives the area of the aperture. The method has several advantages compared with previous methods: it measures the area of the aperture directly, the shape of the aperture is not limited to a circle, it is relatively inexpensive to establish, it does not damage the edges of the aperture and the calibration set-up is similar to that for the actual use of the aperture. It is estimated that the relative standard uncertainty is in calibration of a circular 3 mm diameter aperture. The results that the present method gave for one aperture have been compared with the result of a mechanical calibration at the National Physical Laboratory (UK). The relative difference between the results was , with a combined standard uncertainty of .
Originally developed for application with laser beams, trap detectors are widely used as transfer standards in modern monochromator radiometry. However, a non-optimized beam focus applied to trap detectors can result in significant spectral errors. This paper shows that a defocused and improperly aligned beam entering a Si reflection trap detector can cause an apparent modification of the measured relative spectral responsivity. This is true especially in the ultraviolet (UV) region of the spectrum, where the measured relative differences are of the order of several percent due to the significant influence of the direct band-band transitions of Si on the reflectance of the Si photodiode. The spectral range from 248 nm to 600 nm was investigated in detail. Calculated curves, simulated by a computerized geometrical model, were fitted to the measured data. The geometrical model uses experimental spectral reflectance data.
The National Institute of Standards and Technology (NIST) candela is maintained via standard photometers which have shown long-term stability of better than 0.1 % per year. The detector-based method has allowed us to reduce the uncertainties of calibrations and to expand the range of calibration capabilities. There have been several new developments in photometry at the NIST. A high-illuminance calibration facility has been developed. Four temperature-controlled standard photometers, tested for linearity and thermal effects at high illuminances, are used with a high-pressure xenon arc source to provide the illuminance scale at levels up to 100 klx. A flashing-light photometric unit (lx s) has been realized using four standard photometers equipped with current integrators, and using two independent methods. Calibration services for illuminance meters at high illuminance levels and flashing-light photometers are now available at the NIST. Ongoing new projects include the development of a detector-based luminous-flux calibration facility using a 2.5 m integrating sphere, realization of the total spectral radiant-flux scale, and the development of a reference spectroradiometer for colorimetry of displays. Calibration services are planned in these areas of photometry and colorimetry in the near future.
A new design for a filter radiometer based on a trap detector, a set of temperature-controlled filters, and a precision aperture is described. This filter radiometer can be used to realize high-accuracy scales for illuminance, luminous intensity, and spectral irradiance. The new filter radiometer is an improved version of our earlier designs. It has been improved in such a way that the filter can be easily and reliably changed. This makes it more suitable for spectral irradiance measurements, where lamps usually have to be measured at several wavelengths. The results of the characterization of the filter radiometer equipped with a lambda filter using two different methods are presented. The results are in agreement at the level of 0.3%.
We have developed measurement systems for radiometric realization of the units of luminous intensity and illuminance. The advantages of the trap-detector-based systems include a low level of detector-filter interreflections and good spatial homogeneity. Evaluation of known uncertainty components gives a total uncertainty of 0,6% (2 σ).
An Absolute Radiation Detector (ARD) has been designed and constructed. After evaluation it will replace the existing UK primary national standard cryogenic radiometer, with an improved uncertainty. The ARD has been designed to measure black-body and laser radiation with an uncertainty approaching 10-5. From these measurements it will be possible to determine the fundamental constant, the Stefan-Boltzmann constant, confirming the radiometer as an absolute detector, and link this determination to the SI unit of luminous intensity, the candela, and other detector-based radiometric units. Thus source- and detector-based optical units will be tied to an invariant physical quantity ensuring their long-term stability. This paper describes some of the design considerations to enable the ARD to meet its objectives.
This paper describes an optical power calibrator whose overall calibration uncertainty is less than 10-4for an optical power of 0.13 mW. The laser light source of the system operates at a wavelength of 543.5 nm, being close to the wavelength at which the candela is defined, 555 nm. A stable optical power is achieved by stabilizing the intensity and the frequency of a green He-Ne laser. The optical power is detected by a cryogenic absolute radiometer based on the principle of electrical substitution radiometry. It can be employed to measure optical power up to 0.5 mW in the visible and near infrared region with a 3-0 uncertainty of about 5 x 10-5. The factors limiting the overall uncertainty of the calibrator are analyzed: the conductance fluctuations of the temperature sensor in the absorption cavity and the beam scatter are found to be the most significant error sources. Limited absorptivity of the cavity (0.999 98) and the background radiation cause additional uncertainty. The system is controlled by a microcomputer with self-check and autocalibration features.
Introduction Photodetectors constructed of silicon photodiodes are in widespread use in optical radiometry. Along with single photodiodes, configurations known as trap defectors 1,2 have been developed and implemented during the past two decades. Reflection trap detectors have been successfully used, e.g., in comparison measurements of optical power scales, 3,4 for establishing spectral responsivity scales, 5,6 and in realization of detector-based illuminance 7 and spectral irradiance scales. 8,9 Recently, a new branch of trap detectors for precision radiometry, known as transmission trap detectors, has been introduced 10,11 and characterized. 12,13 Measurement uncertainties at the level of 1 3 10 24 are achievable in precision radiometry with detectors based on silicon photodiodes. To reach such an accuracy, one must take into account