Developement of the method for realization of spectral irradiance scale featuring system of

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PUBLISHED BY IOP PUBLISHING FOR SISSA RECEIVED: July 14, 2010 ACCEPTED: September 8, 2010 PUBLISHED: October 5, 2010 Developement of the method for realization of spectral irradiance scale featuring system of spectral comparisons V. Skerovic, a,1 V. Zarubica, a L. Zekovic, b I. Belca b and M. Aleksić a a Directorate of measures and precious metals, Optical radiation Metrology department, Mike Alasa 14, 11000 Belgrade, Serbia b Faculty of Physics, Department for Applied physics and metrology, Studentski trg 12-16, 11000 Belgrade, Serbia E-mail: vladanskerovic@dmdm.rs ABSTRACT: 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. KEYWORDS: Spectrometers; Photon detectors for UV, visible and IR photons (solid-state) 1 Corresponding author c 2010 IOP Publishing Ltd and SISSA doi:10.1088/1748-0221/5/10/p10001

Contents 1 Introduction 1 2 System of Spectral Comparisons 1 3 Method of Derivation of the Value of Spectral Irradiance 3 4 Measurements 4 4.1 Spectral responsivity measurements 4 4.1.1 Spectral responsivity of filter-radiometer 7 4.2 Determining the aperture size of the filter-radiometer 7 4.3 Characterization of the transfer spectroradiometer 8 5 Measurement uncertainty budget 10 6 Conclusions 11 1 Introduction The traceability of radiometric quantities in the Directorate of measures and precious metals is established by means of standard detectors calibrated for the spectral responsivity at LNE-INM [1]. Realization of the spectral irradiance unit is based on the comparison method [2], where the value of spectral responsivity is transferred to filter radiometers and a spectroradiometer (used as a wavelength tuneable-filter radiometer). A filter radiometer is then used for spectral irradiance unit realization, whereas a spectroradiometer is used for derivation of the relative spectral irradiance of the FEL 1000 lamp [3]. 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 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]. The overall experiment was realized within preparation for EURAMET.PR-K1A Comparison of Spectral Irradiance [7]. 2 System of Spectral Comparisons The system of spectral comparisons (SSC) was designed on the principle of spectral comparisons, where the value was assigned to the object by its comparison with reference standard. It means 1

Photometric bench Hg QHT D2 FEL 1000 Monochromator Aperture Standard Detector on Linear Stage Condenser Optical System Spectroradiometer Filter radiometer Figure 1. Measurement setup for realization of Spectral Irradiance Based on the System of Spectral Comparisons. that the system works in the same or similar way for measurements of various optical radiation quantities (spectral transmittance /reflectance, detector spectral responsivity, spectral irradiance). The SSC (figure 1) consists of a monochromatic radiation source, an optical system, a motion system for changing the object of measurement and a detection system consisting of a transimpedance amplifier and a scanner DVM(digital voltmeter) [8]. The light source, including QHT (quartz halogen tungsten) and D2 (deuterium source), has an optical system that projects the image of the source on the entrance slit of the monochromator. A mercury lamp is used for wavelength calibration of the system. A filter wheel with order sorting filters and shutter is positioned in front of the entrance slit. The monochromator (1/4 meter focal length) has three interchangeable diffraction gratings, covering a spectral range from 200 nm to 2500 nm. The exit slit is square (1 mm 1 mm, for spectral irradiance measurement). The exit optical system can be modified, depending on the application, and it can be either collimating (quartz lens objective) or condensing (quartz lens objective or combination of plane and spherical mirror). An aperture is used to define beam geometry and to reduce scattered light. The optical path of about 0.5 m enables beam geometry to be well defined. A linear stage is used to change the detectors in the focal plane of the system. 2

3 Method of Derivation of the Value of Spectral Irradiance The theoretical basis of the method for determining the spectral irradiance by means of absolute radiometers, used in this experiment, is the one presented by Boivin in 1980 [9]. The practical formula used in measurement is I = A d (r 0 /r) 2 S(λ) E(λ) dλ (3.1) where I is measured filter-radiometer photocurrent, A d is aperture area of the filter-radiometer, r and r 0 are the distance on which the measurement is made and the distance (0.5 m) for which the irradiance is defined respectively, the S(λ) is the spectral responsivity of the filter-radiometer, and E(λ) is spectral irradiance of the standard lamp [10]. The assumption is that the shape of spectral responsivity function of the filter-radiometer is spectrally symmetrical around λ max (peak wavelength of detector responsivity). When S(λ) = S(λ max ) S rel (λ) and E(λ) = E(λ max ) E rel (λ), the formula (3.1) becomes: I = A d (r 0 /r) 2 S(λ max ) E(λ max ) S rel (λ) E rel (λ) dλ, (3.2) from where the E(λ max ) can be determined. The values of E rel (λ) are obtained from measurements with the transfer spectroradiometer. The methodology of derivation of spectral irradiance is shown in a block diagram (figure 2). The SSC was used to calibrate the spectroradiometer for spectral responsivity by comparison with the standard detector at 23 wavelengths. The spectroradiometer wavelength scale was calibrated using a Mercury lamp. The slit function of the spectroradiometer was determined independently for each of the 23 wavelengths. The slit function was determined as relative spectral responsivity of the spectroradiometer in the range of ± 10 nm from center wavelength used for calibration. The data were used to calculate integral relative spectral responsivity of the spectroradiometer at each wavelength for spectral range with non-zero responsivity, as for the filter radiometer. The spectroradiometer was a modified EG&G 550 spectroradiometer. The entrance optics was a small integrating sphere and a quartz lens that projects the image on the entrance slit. The monochromator consisted of a filter wheel with order sorting filters and a 1200 g/mm concave diffraction grating. The bandwidth of the monochromator was 10 nm. The Hamamatsu H6780-04 photomultiplier was used as the detector. The integrating sphere entrance was positioned in such a way that all the flux (irradiating the reference detector) entered the sphere. The spectroradiometer was then moved to the photometric bench for relative calibration of the FEL lamp. Absolute calibration of the lamp was performed with the filter radiometer (V(λ)), whose absolute spectral responsivity was determined by comparison with standard detectors in the primary spectrophotometric system (figure 2), [11]. The filter radiometer was a modified LMT Photometric detector, the diffuser replaced with a precision aperture. The aperture was calibrated by the method of direct comparison in the length department of the Directorate of Measures and Precious Metals [12]. Linearity measurements were performed by double aperture method [13, 14]. Cut-off filters were used at 600 nm and 400 nm to perform stray light analysis. 3

Figure 2. Methodology of derivation of spectral irradiance. Figure 3. Spectrophotometric system for detector comparisons. SISFRMS-, light source; SFRMS measurement station; RD, monitor detector; PN, detectors under test on translation stage. 4 Measurements 4.1 Spectral responsivity measurements The measurement of absolute responsivity of the filter radiometer was performed on the primary spectrophotometric system [11], (figure 3). 4

Figure 4. Simplified scheme of the transimpedance amplifier. Detectors were automatically exchanged in the path of the light beam. Each time, signals from the main and compensation (monitor) detectors were measured simultaneously, ten times per each detector. The average value of those readings was taken into account. At every wavelength, measurements were preformed for the sample and the reference detector, and the measurement of the dark current was made and correction applied before each. The photocurrent was measured by means of a specially constructed transimpedance amplifier [15, 16] and the scanner digital voltmeter. The transimpedance amplifier (figure 4) was calibrated by means of the KEITHLEY 263 low current source calibrator and the HP 3458 digital voltmeter [17]. The measurement uncertainty of the calibration of the transimpedance amplifier on 1MΩ range was 2.5 10 4 relative, mainly resulting from the uncertainty of the low current calibration source. The reproducibility of measurements was determined on the sample of 30 independent measurements of each sample for every wavelength. In uncertainty evaluation, the uncertainty components due to beam divergence and bandwidth effect were taken as negligible [18]. Nonuniformity of the detectors was not analysed. Nonlinearity of the system was determined by the double aperture method [14]. The uncertainty budget of the comparison method of detector spectral responsivity is presented in table 1. The overall uncertainty of reproducing the value of spectral responsivity of detectors was evaluated from the uncertainty of the method of comparisons and the uncertainty of the LNE-INM calibration of the standard detector (which amounts 0.6%, k=2). Prior to that, mutual comparisons of reference detectors were made to prove measurement uncertainty of reproducing the value of spectral responsivity. The method of mutual comparisons is based on comparison measurements between calibrated standards, where one is used as a standard and the other is used as a calibration artefact. For example, results of the comparison of two detectors is calculated as the relative deviation of the results of measurements for the second detector s responsivity, using the first one as a standard, from its values given in certificate [2]. 5

Table 1. The uncertainty budget of the comparison method of detector spectral responsivity. Uncertainty component Uncertainty Uncertainty (Type A) (Type B) Reproducibility (detector No 1) 2 10 4 Reproducibility (detector No 2) 2 10 4 Calibration of transeimpedance amplifier (detector No 1) 2.5 10 4 Calibration of transeimpedance amplifier (detector No 2) 2.5 10 4 DVM uncertainty 2 10 4 Nonlinearity 2 10 4 Wavelength accuracy 0.2 10 4 Stray light 10 8 Standard uncertainty (1σ) 2.8 10 4 4.5 10 4 Combined standard uncertainty u m (1σ) 5.3 10 4 0.01 0.005 0 400 500 600 700 800 900 1000-0.005-0.01-0.015-0.02 Figure 5. Average deviations of the results for the detectors spectral responsivity, from the values given in LNE-INM certificate, in the function of wavelengths. Mutual comparisons of three standard detectors (SR1, SR2, SR3 - Hamamatsu S1337-1010BQ photodiodes) were performed by comparing each to each, in both directions (measurement sequence), meaning that six different comparisons were made. For each of these comparisons, five measurement cycles were made at 23 wavelengths with ten readings at every wavelength. Calculations were made at each point of measurement. The key result of the measurements was the deviation of the results for the detectors spectral responsivity, obtained from measurements, from the values given in LNE-INM certificate [1]. The standard deviation of all mutual comparison measurements, for repeated measurements for each pair of detectors, was calculated and given as measure of reproducibility of measurements. The average reproducibility for all comparison measurements was below 0.1%, which complies with the results of evaluation of the measurement uncertainty of the comparisons method (table 1). In figure 5, the average deviations of the results for the detectors spectral responsivity (for all three detectors) from the values given in LNE-INM certificate after the latest calibration, in the function of wavelengths, are shown. Figure 5 shows that deviations of the results for the detectors spectral responsivity (obtained 6

Figure 6. Spectral responsivity of filter-radiometer. from measurements) from the values given in LNE-INM certificate are within the limits of measurement uncertainty of LNE-INM calibration of 0.6% (k = 2) and 1.0% for wavelengths above 900 nm. The average deviations (figure 5) show regularity in their spectral characteristics, which leads to the conclusion that some systematic effects have influence on the measurement results. The noticed systematic effects were not either explained or included in the measurement uncertainty of the method, evaluated and given in table 1. An additional component of measurement uncertainty due to noticed systematic effects was calculated from the results of mutual comparisons [19], as standard deviation of the results of mutual comparisons for the entire group of detectors and it is 0.25%, for k=1. 4.1.1 Spectral responsivity of filter-radiometer The filter radiometer was a modified LMT Photometric detector, where the diffuser was replaced with a precision aperture. Measurements were made on the primary spectrophotometric system (figure 3), by comparisons with three reference detectors. The average values from these measurements were taken as the result for spectral responsivity. The measurement results are presented in figure 6, as a respective V(λ) curve. 4.2 Determining the aperture size of the filter-radiometer The aperture area was calculated from the size of the aperture diameter. Measurements of the aperture diameter were made with Carl Zeiss measuring microscope [12]. The measurements were made around the aperture space on angular positions of every 15 degrees. An average value was taken as the result for the aperture diameter. The standard deviation of the results from all angu- 7

lar positions was taken as a measurement uncertainty of determining the aperture diameter. The average diameter is d sr =6.56026 mm, and σ sr= 0.000852. The measurement uncertainty of the aperture u A was obtained as the partial differential of the formula for aperture area size, and it equals 2σ sr. The aperture area size was A d = 0.3380119 cm 2 and u A = 0.001704 (1σ). All type B uncertainty components in diameter measurements were too small compared to the calculated standard deviation and hence neglected. 4.3 Characterization of the transfer spectroradiometer The spectroradiometer was a modified EG&G 550 spectroradiometer. The diffuser on the entrance of the spectroradiometer was replaced with a small integrating sphere. The detector, together with the detection electronics, was changed, and the Photomultiplier Hamamatsu H6780-04 was then used as the detector with transimpedance amplifier and a scanner digital voltmeter. The spectroradiometer wavelength scale was calibrated using a Mercury lamp together with the wavelength scale of the SSC in the setup shown in figure 1. The Mercury lamp was put in front of the entrance optics of the SSC, so that the wavelength calibrations of monochromator (SSC) and spectroradiometer were preformed simultaneously. In that way, both wavelength scales were matched. The uncertainty of the wavelength scale of the monochromator of the SSC was found to be 0.2 nm. The wavelength setting of the spectroradiometer was corrected to match the wavelength scale of the monochromator of the SSC. The uncertainty of setting of the wavelength of the spectroradiometer was 0.5 nm. This procedure was repeated for all wavelengths of interest for calibration. The slit function of the spectroradiometer was determined in the measurement setup shown in figure 1. The slit-function was measured as relative spectral responsivity of the spectroradiometer around central wavelength, by comparison with reference detector. In the measurement setup (figure 1), the reference detector was positioned in the light beam with step-motor driven linear stage. Spectroradiometer integrating sphere entrance was positioned in such a way that all the flux (irradiating the reference detector) entered the sphere. Measurements were made by setting the fixed wavelength of the spectroradiometer, and scanning with SSC over the range of ± 15 nm from central wavelength used for calibration. Results of measurements of the slit-function at the wavelength setting of 550 nm are shown in figure 7. The values represented as zero are of the order of 0.001. Figure 7 shows that the slit function is almost perfectly symmetrical, and that existing nonsymmetry is the result of wavelength uncertainty of spectroradiometer of 0.5 nm. The general idea of making the measurements of the slit-function of spectroradiometer is to be able to calculate absolute spectral responsivity of the spectroradiometer by numerical integration over the bandpass. Also, the data can be used to make the corrections for bandwidth effects [20]. Due to the near perfect triangular bandpass function (figure 7), and considering the fact that, in this experiment, the spectroradiometer was used only for relative spectral distribution measurements, none of these corrections were made. The assumption is that, with symmetrical triangular bandpass function, the bandwidth effects in the measurement of relative spectral distribution with the spectroradiometer can be neglected in this experiment. 8

Figure 7. The slit function of the spectroradiometer at the wavelength of 550 nm. The spectroradiometer nonlinearity was determined by the adapted double aperture method [14]. The double aperture was placed in front of the entrance of the integrating sphere (a part of the entrance optics of the spectroradiometer). All together, they were placed on the photometric bench. The lamps of nominal electrical powers ranging from 1000 W to 60 W were used for illumination. The signal level was changed, according to the inverse square law, by changing the distance from the source on the photometric bench. The approximation was made that the use of the integrating sphere annulled any spatial differences in the illumination of the entrance slit of the spectroradiometer. In the signal range of interest, the measured nonlinearities were of the order of 0.2%. Stray light analysis was preformed using cut-off filters at 600 nm and 400 nm. The stray light level was found to be of the order of 0.2%, which is in compliance with manufacturer specifications of stray light 0.3% [21]. The results of the measurement of the spectroradiometer relative spectral responsivity in the SSC facility are presented in figure 8. At each wavelength, 20 measurements were made and the average value of those readings was taken into account. At each wavelength, the measurement of the dark current was made and correction applied. All the results were normalised at 555 nm. Discontinuities on the relative spectral responsivity characteristic were the result of the change in the order of the sorting filters. The spectroradiometer was then moved to the photometric bench for relative calibration of the FEL lamp. Absolute calibration of the lamp was preformed with filter radiometer (V(λ)). Positions on the photometric bench (distance from the lamp) for both the spectroradiometer and filter-radiometer were chosen in such a manner that the signal level was similar to the signal level in spectral responsivity measurements. This was done in order to minimize the effects of nonlinearity. The distance of filter radiometer from the lamp filament was 2.5 m. Measurements were repeated 30 times and standard deviation was calculated for every wavelength. This standard deviation was taken as a direct component of measurement uncertainty due to reproducibility of measurements. 9

Figure 8. Relative spectral responsivity of transfer spectroradiometer. Figure 9. Results of realization of spectral irradiance scale. The standard lamp was an old type of the FEL1000 lamp manufactured by Hoffman Engineering Corporation. The electrical parameters of the lamp were controlled with the state- of-the-art electrical equipment [22]. The DC source of the stability of 1 10 5 was used for the lamp power supply. The measurement of the electrical current was preformed with a digital voltmeter (5 ppm accuracy) and a standard resistor. The lamp voltage was measured directly on the lamp to provide control of the lamp stability [22]. The results of the calibration of the lamp for spectral irradiance are shown in figure 9. The value of the irradiance at 550 nm was 0.07883 W m 2 nm 1. 5 Measurement uncertainty budget The measurement uncertainty analysis [10] was based on the measurement equation (3.2), and it included the analysis of both direct and indirect components of measurement uncertainty. The results and analysis presented previously in this paper were used for the uncertainty budget estimation [19]. The uncertainty budget is given in table 2. All uncertainties are relative values. 10

Table 2. The uncertainty budget of realization of the spectral irradiance scale. Uncertainty component Uncertainty [%] Type A d aperture area 0.1704 A r distance from the source 0.0100 B S(λ) responsivity of filter radiometer Signal measurement (standard detector) 0.02 10 4 A Signal measurement (filter radiometer) 0.02 10 4 A Gain (channal 1) 0.025 10 4 B Gain (channal 2) 0.025 10 4 B DVM 0.02 10 4 B Nonlinearity 0.02 10 4 B Wavelenght 0.002 10 4 B Stray light 10 8 B Calibration of standard detector (LNE-INM) 0.3 B Mutual comparisons (deviation for the group of the detectors) 0.25 A Combined standard uncertainty u S 0.4287 E (λ) relative spectral distribution of the lamp Relative spectral responsivity of spectroradiometer 0.4287 A Signal measurement 0.0632 A Spectroradiometer nonlinearity 0.2 B Stray light in spectroradiometer 0.2 B Wavelenght 0.01 B Bandwidth neglected B Combined standard uncertainty u E 0.5175 Lamp parameters Power supply stability 0.02 B Electrical measurements 0.01 B Standard resistor 0.04 B Lamp stability 0.05 B Standard uncertainty u FEL 0.068 Combined standard uncertainty (k=1) 0.7416 6 Conclusions This paper presents the current status of the research and development process in realization of the scale of spectral irradiance in the Directorate of measures and precious metals. The experiment was performed within the arrangement for Euramet K1.a key comparison of spectral irradiance measurement. All the results should be interpreted in that sense. The objective of this paper was to evaluate the adequacy of the method. The new method based on calibration of spectroradiometer by comparison with standard detector has been established to realize spectral irradiance scale. Within the process of development, also, the new methods for determination of the influential parameters have been established. 11

The modified double aperture method was realized in order to determine non-linearity of the spectroradiometer. The method of comparison has been used for determination the slit function of the spectroradiometer. In this paper, all the methods are explained and the results of measurements are presented. Results of spectral irradiance measurements presented in this paper are similar to the results reported in the CIPM Key Comparison K1-a Spectral Irradiance 250 nm to 2500 nm, [4]. The uncertainty analysis shows that the main components of uncertainty originate from the uncertainty of spectral responsivity scale. The evaluated measurement uncertainty corresponds to measurement uncertainties from the CIPM Key Comparison [23], which leads to a conclusion that the realized method is appropriate. The uncertainty analysis refers to main sources of uncertainties giving directions for some future improvements in the method. References [1] Calibration certificate N o 1180-Ra-07, LNE-INM, 2007. [2] V. Skerovic, P. Vukadin and V. Zarubica, Mutual comparison of detectors spectral responsivity to prove stated measurement uncertainty, in Newrad Conference 2005, 17 19 October 2005, Davos, Switzerland. [3] J. Metzdorf, A Sperling, S Winter, K.-H. Raatz and W. Möller, A new FEL-type quartz-halogen lamp as an improved standard of spectral irradiance, Metrologia 35 (1998) 423. [4] CIPM Key Comparison K1-a Spectral Irradiance 250 nm to 2500 nm, Final Report, NPL, 23rd January 2006. [5] T Kubarsepp, P Karha, F Manoocheri, S Nevas, L. Ylianttila and E. Ikonen, Spectral irradiance measurements of tungsten lamps with filter radiometers in spectral range 290 nm to 900 nm, Metrologia 37 (2000) 305. [6] C. Carreras, A. Corrons, Absolute spectroradiometric and photometric scales based on an electrically calibrated pyroelectric radiometer, Applied Optics 20 (1981) 1174. [7] Draft Protocol for the Euramet K1.A Comparison of Spectral Irradiance, NPL, 2008. [8] V. Skerovic, V. Zarubica, Realization of Spectral Irradiance Scale Featuring System of Spectral Comparisons, in Newrad Conference 2008, 13 16 October 2008, Daejeon, Korea. [9] L.P. Boivin, Calibration of incadescent lamps for spectral irradiance by means of absolute radiometers, Appl. Opt. 19 (1980) 2771. [10] B.K. Tsai, B.C Johnson, Evaluation of uncertainties in fundamental radiometric measurements, Metrologia 35 (1998) 587. [11] V. Skerovic, P. Vukadin, V. Zarubica and L. Zekovic, Realization of primary spectrophotometric system, Meas. Sci. Technol. 19 (2008) 085103. [12] C. Zeiss, User manual Universal-Messmikroskop, Jena. [13] L.P. Boivin, Automated absolute and relative spectral linearity measurements on photovoltaic detectors, Metrologia 30 (1993) 355. 12

[14] K.D. Mielenz and K. Eckerle, Spectrophotometer linearity testing using the double-aperture method, Appl. Opt. 11 (1972) 2294. [15] G. Eppeldauer and J.E. Hardis, Fourteen-decade Photocurrent measurements with Large-area silicon photodiodes at room temperature, Appl. Opt. 30 (1991) 3091. [16] G. Eppeldauer, Optical Radiation Measurement with Selected Detectors and Matched Electronic Circuits between 200 nm and 20 µm, NIST Technical Note 1438, (2001). [17] G. Eppeldauer, Traceability of NIST photocurrent measurements to electrical standards, in Newrad Conference 2008, 13 16 October 2008, Daejeon, Korea. [18] T.C. Larson, S.S. Bruce and A.C. Parr, Spectroradiometric Detector Measurements, NIST Special Publication 250-41, February 1998. [19] Guide of the Expression of Uncertainty in Measurements, International Organization for Standardization, Genève, Switzerland (1993). [20] E.R. Woolliams, M.G. Cox, P.M. Harris and H.M. Pegrum, Correcting for bandwidth effects in monochromator measurements, in Newrad Conference 2005, 17 19 October 2005, Davos, Switzerland. [21] Instruction manual Model 550/555 spectroradiometer system, EG&G electro-optics, (1976). [22] P. Vukadin, V. Skerovic, V. Zarubica, Tracing the values of light quantities in Federal bureau of measures and precious metals, publication CIE x020 2001: proceedings of the CIE Expert Symposium 2001 on Uncertainty Evaluation Methods for Analysis of Uncertainties in Optical Radiation Measurement, January 2001, Vienna, Austria. [23] E.R. Woolliams, N.P. Fox, M.G. Cox, P.M. Harris and N.J. Harrison, The CCPR K1-a key comparison of spectral irradiance from 250 nm to 2500 nm: measurements, analysis and results, Metrologia 43 (2006) 98. 13