The CCPR K2.c key comparison of spectral responsivity from 200 nm...
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The CCPR K2.c key comparison of spectral responsivity from 200 nm...
Home Search Collections Journals About Contact us My IOPscience The CCPR K2.c key comparison of spectral responsivity from 200 nm to 400 nm This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Metrologia 51 S336 (http://iopscience.iop.org/0026-1394/51/6/S336) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 176.9.124.142 This content was downloaded on 24/11/2014 at 12:22 Please note that terms and conditions apply. Bureau International des Poids et Mesures Metrologia Metrologia 51 (2014) S336–S343 doi:10.1088/0026-1394/51/6/S336 The CCPR K2.c key comparison of spectral responsivity from 200 nm to 400 nm Lutz Werner Physikalisch-Technische Bundesanstalt, Berlin, Germany E-mail: [email protected] Received 9 June 2014, revised 25 September 2014 Accepted for publication 30 September 2014 Published 20 November 2014 Abstract The CCPR K2.c key comparison of spectral responsivity from 200 nm to 400 nm was carried out in the framework of the CIPM Mutual Recognition Arrangement by 14 participating national metrology institutes. The key comparison was piloted by the Physikalisch-Technische Bundesanstalt (PTB). The comparison principle, measurements, analysis and results including a procedure to deal with the issue of inconsistent data are described. Keywords: key comparison, radiometry, detectors, spectral responsivity (Some figures may appear in colour only in the online journal) 1. Introduction The Mutual Recognition Arrangement (MRA) for national measurement standards and for calibration and measurement certificates issued by the national metrology institutes (NMIs) was signed in 1999. The objectives of the MRA were to establish the degree of equivalence of national measurement standards maintained by NMIs and to provide for the mutual recognition of calibration and measurement certificates issued by NMIs. Under the MRA the metrological equivalence of national measurement standards is determined by a set of key comparisons chosen and organized by the Consultative Committees of the CIPM working closely with the Regional Metrology Organizations (RMOs). At its meeting in March 1997, the Consultative Committee for Photometry and Radiometry (CCPR) identified several key comparisons in the field of optical radiation metrology. One of these was the CCPR K2.c key comparison of spectral responsivity in the spectral range from 200 nm to 400 nm. The Physikalisch-Technische Bundesanstalt (PTB), the NMI of Germany, was asked to be the pilot of this key comparison. The technical protocol of the key comparison was drawn up by a small working group comprising PTB (convenor), BIPM, MSL, NPL, NIST, and NRC. Laboratories from 14 NMIs including the pilot laboratory took part in this comparison. These were LNE-Cnam (France), NMIA (Australia), MIKES (Finland), IO-CSIC (Spain), MSL (New Zealand), NIM (China), NIST (United 0026-1394/14/06S336+8$33.00 States of America), NMIJ AIST (Japan), VSL (Netherlands), NPL (United Kingdom), NRC (Canada), PTB (Germany) pilot, NMC-A*STAR (Singapore), and VNIIOFI (Russian Federation). 2. Comparison artefacts and principle of the comparison Three types of transfer detectors based on two types of photodiodes were used as comparison artefacts: (i) single element photodiode detectors based on windowless SUV100 PtSi/n-Si Schottky photodiodes manufactured by the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland, (ii) single element photodiode detectors and (iii) three element reflection trap detectors, both based on windowless Hamamatsu S5227 1010 Si pn junction photodiodes. The latter two detector types, henceforth referred to as Si-photodiode-based detectors, were only used in the wavelength region from 250 nm to 400 nm because their spectral responsivity can alter due to exposure in the short-wavelength ultraviolet (UV) spectral range [1–3]. The PtSi-photodiodebased detectors were used in the whole wavelength region of this comparison ranging from 200 nm to 400 nm as they were thought to be less sensitive to UV exposure [4]. However, because of the poorer results of the PtSi-photodiode-based detectors in comparison with those of the Si-photodiodebased detectors, the former detectors were only used in the S336 © 2014 BIPM & IOP Publishing Ltd Printed in the UK L Werner Metrologia 51 (2014) S336 wavelength range from 200 nm to 240 nm to calculate the degrees of equivalence (DoE) and the key comparison reference value (KCRV). Each detector was equipped with a Pt100 temperature sensor to monitor the detector temperature during the measurements. Because of the large temperature coefficient of the PtSi photodiodes and the possibility of large differences in laboratory temperature between the participants and the pilot, these photodiodes were mounted on a brass block which could be warmed or cooled by water. Thus, the temperature of the PtSi photodiodes could be set independently of the laboratory temperature. The comparison was organized in a star form, carried out in three rounds with three groups of participants and using sets of transfer detectors each consisting of three PtSi photodiodes, three Si photodiodes and one Si-photodiode-based trap detector. The PtSi photodiodes showed a severe malfunction twice at NIM after transportation, thus, a fourth round of the comparison was added to allow NIM to repeat their measurements as the only participant in this round. To check the stability of the transfer detectors and to provide the linkage between the participants, the pilot performed its measurements in each round before the transfer detectors were sent to the participants, and in addition, after the last return of the transfer detectors. The final measurements of the pilot form an additional round comprising only these measurements of the pilot without any measurements of participants. 3. Measurements The measurements were performed from 2004 to 2007. Each participating NMI i determined the spectral responsivity si, j of their transfer detectors j in round k in order to perform the comparison on the spectral responsivity. The mean relative standard uncertainty u rel,cal, i of the spectral responsivity of the transfer detectors as stated by the participants is shown in figure 1. For reasons of clarity, only the average over the values for the PtSi photodiodes in the wavelength range from 200 nm to 240 nm and over the values for the Si-photodiodebased detectors in the wavelength range from 250 nm to 400 nm is given for each participant. The transfer detectors were compared by the pilot with reference detectors from the pilot at each wavelength before they were sent to the participants in each round and after their last return. For this, the ratios R of the photocurrent of the pilot’s reference detectors and the photocurrent of each transfer detector j when irradiated by the same source at the same wavelength were measured by the pilot delivering ratio Rj, k. These photocurrent ratios are equal to the ratios of the spectral responsivity of the corresponding detectors at the corresponding time. They were therefore used to calculate the spectral responsivity of the reference detector based on the results of participant i obtained in round k and the results of the pilot which were obtained before the measurements of the participant (i.e. in round k) and after the measurements of the participant (i.e. in round k + 1). Because of the large differences between PtSi photodiodes and Si photodiodes, different reference detectors were used: a PtSi photodiode as reference detector for PtSi-photodiodebased transfer detectors and a Si-photodiode-based trap detector as reference detector for Si-photodiode-based transfer detectors. The reference detectors were compared against a group of secondary detector standards of PTB in each phase of the comparison to check the stability of their spectral responsivity. It turned out that the detectors were not stable. The time dependent spectral responsivity of the Si-photodiode-based reference could be well fitted using a linear drift model with a wavelength dependent relative annual drift rate between −0.24% and −0.07%. The temporal change of the spectral responsivity of the PtSi photodiode reference was smaller and less regular and was described by a more common approach. It is assumed that the responsivity change of the reference detector during the measurements of a participant lies with equal probability somewhere in the interval limited by the values determined by the surrounding measurements of the pilot. The responsivity change of the reference detector during the measurements of a participant is then the mean value of the limits of this interval. As a consequence, this leads to a stepwise drift. The stepwise drift between subsequent rounds depends on the wavelength and the round and ranges between 0% and 0.13% with an overall mean of about 0.04%. 4. Analysis The calculation of the DoE and the KCRV are performed in agreement with the CCPR-G2 guidelines [5] and are based on the method of the weighted mean (with cut-off) which has been extended for non-stable artefacts. Due to the large differences between PtSi photodiodes and Si photodiodes regarding the magnitude, spectral dependence, temperature dependence, and temporal drift of the spectral responsivity, these two types of photodiodes were treated separately within the data analysis. The differences between the single element detectors and the trap detectors made up of Si photodiodes were rather small compared to the differences between the two types of photodiodes. Therefore, the detectors made up of Si photodiodes are summarised as Si-photodiode-based detectors and treated together in the data analysis. The data analysis is, unless specified otherwise, identical for both types of detectors and is briefly described below. In the first step, the results on the spectral responsivity of the reference detectors xi, j, k and xi, j, k + 1 are averaged over the set of transfer detectors of each type: M xi, j, k = Rj, k si, j xi, k = (1 / M ) and ∑x j=1 xi, j, k + 1 = Rj, k + 1si, j (1) and S337 i, j , k L Werner Metrologia 51 (2014) S336 12% 2.0% LNE-Cnam Relative standard uncertainty IO-CSIC 10% MIKES 1.5% MSL VSL 8% NIM 6% NIST 1.0% NMIA NMIJ 4% NPL 0.5% NRC PTB 2% NMC-A*STAR 0% VNIIOFI 0.0% 200 250 200 250 300 350 400 Wavelength / nm Figure 1. Mean relative standard uncertainty u rel,cal, i of the participants’ determination of the spectral responsivity of the PtSi-photodiode- based and Si-photodiode-based transfer detectors in the wavelength ranges from 200 nm to 240 nm and from 250 nm to 400 nm, respectively. The left part of the figure shows the wavelength range from 200 nm to 250 nm in an extended range of the relative standard uncertainty. M xi, k + 1 = (1 / M ) ∑ xi, j , k + 1 (2) j=1 M is the number of transfer detectors of one type, i.e. M is equal 3 and 4 for the PtSi photodiodes and the Si-photodiodebased detectors, respectively. It is assumed in the following that the temporal drift of all detectors (i.e. reference and transfer detectors) of one type is very similar. The spectral responsivity xi of the reference detector as obtained by the transfer from the transfer detectors calibrated by participant i at time ti is then xi = (1 − τi, k ) xi, k + τi, kxi, k + 1 (3) In case of the Si-photodiode-based detectors, drifting linearly with time, it is τi, k = (ti − tkP ) / (tkP+ 1 − tkP ) (4) with tkP and tkP+ 1 the time at which the pilot performed its measurements of the ratios Rj, k and Rj, k + 1 before and after the measurements of the participant in round k, i.e. in round k and k + 1, respectively. In case of the PtSi-photodiode-based detectors, described by a stepwise drift with time, xi is calculated as the mean of xi, k and xi, k + 1 which corresponds to τi, k = 0.5. Thus, N values of the spectral responsivity xi of the reference detector, derived from the N participants’ results, are obtained and used to calculate a reference value. A separate reference value is calculated for each wavelength. The relative standard uncertainty of xi is given by u rel (xi ) = 2 2 urel,cal, i + u rel,T, i (5) with u rel,cal, i — the mean relative standard uncertainty of the spectral responsivity of the transfer detectors determined by participant i. u rel,T, i is the relative standard uncertainty of the transfer comprising contributions arising from the measurements of the pilot, deviations from the assumed temporal behaviour of the transfer detectors, and averaging over the transfer detectors of one participant. The measurement results on the Si-photodiode-based and the PtSi-photodiode-based detectors are analysed using the linear drift model and the stepwise drift model, respectively, which are described in the appendix. The estimate yi (i = 1, … , N ) of the time dependent spectral responsivity of the reference detector forms the time dependent KCRV. The DoEs are expressed by the deviation di = xi − yi of the ith participant’s result from the KCRV and the expanded (k = 2) uncertainty of this deviation U (di ) = 2u(di ). In addition to the drift model based analysis, all results have been analysed with the common method of the weighted mean for comparison. The use of the above models requires the consistency of the set of data which was checked using the chi-squared test. The chi-squared test failed at a considerable number of wavelengths. Therefore, a procedure to achieve consistency and to exclude outlying results was developed by the pilot and discussed and agreed by all participants. The procedure is based on the CCPR-G2 Guidelines [5] and uses the following steps to achieve consistency: (1)Perform the chi-squared test. The test fails if the observed 2 = ∑ N w d 2 chi-squared value χobs i = 1 i i is greater than 2 χ(0.05) (ν ) being the critical value of the chi-squared distribution, with the corresponding degrees of freedom ν, at the 5 % significance level. (The weights wi are defined in the Appendix.) (2)If the chi-squared test fails and there are participants with di / U (di ) >2, then exclude the corresponding results as potential outliers from the calculation of the tentative KCRV. S338 L Werner Metrologia 51 (2014) S336 Relative deviation from KCRV 20% 2.5% 10% LNE-Cnam 2.0% IO-CSIC 1.5% MIKES MSL 1.0% 0% VSL NIM 0.5% -10% NIST 0.0% -20% NMIA -0.5% NMIJ -1.0% NPL NRC -1.5% -30% PTB NMC-A*STAR -2.0% -40% VNIIOFI -2.5% 200 250 200 250 300 350 400 Wavelength / nm Figure 2. Relative deviation Di from the KCRV. The left part of the figure shows the wavelength range from 200 nm to 250 nm in an extended range of the relative deviation. (3)If the chi-squared test fails and there are no participants with di / U (di ) >2 or if the chi-squared test still fails after the exclusion of potential outliers in step 2, then use the Mandel-Paule method [6] by applying an additional 2 under the ‘interlaboratory variance’, that is by adding uadd square root in equation (5), so that the data set will be forced to pass the chi-squared test. (4)If the Mandel-Paule method has been applied in step 3 after excluding potential outliers in step 2, check whether these outliers can be reincluded after applying the Mandel-Paule method. For this, check whether the condition di / U (di ) >2 still remains when the additional 2 of step 3 is applied. If it interlaboratory variance uadd remains the potential outliers remain excluded, otherwise apply the Mandel-Paule method without excluding these results. If the interlaboratory variance that has to be applied now is not considerably larger than that obtained in step 3, the potential outliers will not be excluded and the latter interlaboratory variance has to be applied. Otherwise, the potential outliers have to be excluded and the interlaboratory variance of step 3 has to be applied. Because of the poorer results of the PtSi-photodiode-based detectors in comparison with those of the Si-photodiodebased detectors, only the Si-photodiode-based detectors were used to calculate the KCRVs and the DoEs in the wavelength range from 250 nm to 400 nm. In this wavelength range, the results of the PtSi-photodiode-based detectors are calculated for information only. In the wavelength range from 200 nm to 240 nm the KCRVs and the DoEs are based on the PtSiphotodiode-based detectors. 5. Results and discussion In the wavelength region from 200 nm to 240 nm where PtSi-photodiode-based detectors were used, an outlier was excluded at two wavelengths, the Mandel-Paule method had to be applied at another two wavelengths and the Mandel-Paule method was applied after excluding one outlier at a fifth wavelength. In the wavelength region from 250 nm to 400 nm where Si-photodiode-based detectors were used, five outliers were excluded at three wavelengths in total and the Mandel-Paule method had to be applied at a fourth wavelength. The results of the PtSi-photodiodebased detectors in the wavelength range from 250 nm to 400 nm which were not used for the calculation of the KCRV and DoEs showed a much higher number of wavelengths with inconsistency than the Si-photodiode-based detectors. Outliers were excluded at three wavelengths, the Mandel-Paule method was applied at two wavelengths and the Mandel-Paule method together with excluding outliers was applied at four wavelengths, leaving only seven wavelengths with consistent data. The linear drift model applied for the Si-photodiode2 based detectors reduces the observed chi-squared value χobs by about 20% compared to that obtained with the common weighted mean without the linear drift model and thus assuming stable artefacts. This shows that the drift of the Si-photodiode-based detectors is significant and well handled by the linear drift model. The effect of the stepwise drift model which was used for the PtSi-photodiode-based detectors is less significant. The stepwise drift model was nevertheless used because otherwise the uncertainty contribution assigned to the stability of the detectors would have increased. The DoEs are a very important outcome of a key comparison. The relative deviation Di = di / xi from the KCRV is shown in figure 2. It is obvious that both the relative differences Di and the participants’ reported relative standard uncertainties u rel,cal, i shown in figure 1 strongly increase with decreasing wavelength below 250 nm. This is a result of increased experimental difficulties in the short-wavelength UV, e.g. the small available radiant power associated with S339 L Werner Metrologia 51 (2014) S336 4.5 LNE-Cnam IO-CSIC 4.0 MIKES 3.5 MSL VSL |di / U(di)| 3.0 NIM 2.5 NIST NMIA 2.0 NMIJ 1.5 NPL NRC 1.0 PTB 0.5 NMC-A*STAR VNIIOFI 0.0 200 250 300 350 400 Wavelength / nm 2.0 LNE-Cnam 1.8 IO-CSIC 1.6 MIKES MSL |di / U(di)| 1.4 VSL NIM 1.2 NIST 1.0 NMIA 0.8 NMIJ 0.6 NPL NRC 0.4 PTB NMC-A*STAR 0.2 VNIIOFI 0.0 200 250 300 350 400 Wavelength / nm Figure 3. Ratio di / U (di ) of the deviation from the KCRV and its expanded uncertainty (k = 2) displayed over its full range (top) and a reduced range (bottom). the small responsivity of the PtSi-photodiode-based detectors which are used in this wavelength range. The uncertainties claimed by a participant are considered to be supported by the key comparison if the ratio di / U (di ) of the deviation from the KCRV and its expanded uncertainty (k = 2) is smaller than 1. Figure 3 shows that 239 of the 277 values in total of this ratio are smaller than 1, whereas, 15 values are larger than 1.5 indicating the existence of underestimated uncertainties or overlooked biases in the measurement results of the participants. Figure 4 shows the relative standard deviation of the differences di of the participant’s results from the KCRV for this key comparison. The strong increase of the standard deviation with a decreasing wavelength below 250 nm is once more an evidence of the increased experimental difficulties in the short-wavelength UV already mentioned above. Figure 4 shows that the standard deviation of the PtSi-photodiode-based detectors in the wavelength range from 250 nm to 400 nm is larger than that of the Si-photodiode-based detectors by a factor of up to 3 in this key comparison. This and the large number of wavelengths with outliers and inconsistencies requiring the application of the Mandel-Paule method support the decision to use only the Si-photodiode-based detectors for the calculation of the KCRV and the DoEs and to exclude the PtSi-photodiodebased detectors in the spectral region from 250 nm to 400 nm. Since the previous key comparison CCPR-K2.b of the spectral responsivity in the wavelength range from 300 nm to 1 000 nm was performed with Si-photodiode-based single element detectors and trap detectors, both key comparisons can be compared well in the overlapping spectral region. Figure 4 shows that the standard deviation of CCPR-K2.c is between 20% and 50% smaller than that of CCPR-K2.b which indicates a considerable improvement of the calibration capabilities in this spectral region since the time of the measurements for CCPR-K2.b in the years 2000 and 2001. The issue of inconsistent data and outliers has been addressed by the particular procedure described above. S340 L Werner Metrologia 51 (2014) S336 1.6% K2.c (PtSi) 14% 1.4% K2.c (PtSi, not used) 12% 1.2% K2.b (Si) 10% 1.0% 8% 0.8% 6% 0.6% K2.c (Si) 4% 0.4% 2% 0% Relative standard deviation Relative standard deviation 16% 0.2% 200 250 300 350 400 0.0% Wavelength / nm Figure 4. Relative standard deviation of the differences of the participant’s results from the KCRV for the PtSi-photodiode-based detectors in the spectral region from 200 nm to 240 nm (full squares) and the Si-photodiode-based detectors in the spectral region from 250 nm to 400 nm (full circles). For comparison with the preceding CCPR-K2.b, the same relative standard deviation for the Si-photodiode-based detectors in the CCPR-K2.b (full triangles) is shown in the spectral region from 300 nm to 400 nm. In addition, the relative standard deviation of the differences from the reference value based on the PtSi-photodiode-based detectors of the CCPR-K2.c in the spectral region from 250 nm to 400 nm (blank squares) is shown. These latter results are not used for the calculation of the KCRV and DoEs and are given for information only. (The data in the wavelength range from 250 nm to 400 nm are plotted for the right vertical axis.) The approach is not based on a statistical model but rather includes several steps and procedures. Nonetheless, all cases of inconsistent data and potentially outlying results in this key comparison were handled by this procedure as consistently as possible. The obtained results were also compared to those of a Bayesian method based on a statistical model [7]. This comparison indicates that the procedure applied in this analysis excluded only the strongest outliers and that in some cases additional results might have been identified as being not in accordance with their quoted uncertainties. However, the applied procedure had been set up with the intention of avoiding any possibility of excluding a nonoutlying result. 6. Conclusions Laboratories of 14 NMIs took part in the key comparison of the spectral power responsivity of detectors in the spectral range from 200 nm to 400 nm. The comparison was carried out using three types of transfer detectors based on PtSi/n-Si Schottky photodiodes and Si pn junction photodiodes showing different temporal drift of the spectral responsivity. The calculation of the DoE and the KCRV were based on the method of the weighted mean (with cut-off) which was extended for non-stable artefacts drifting stepwise or linear with time. A comparison with the preceding key comparison in the overlapping spectral range from 300 nm to 400 nm showed a considerable improvement of the calibration capabilities in this spectral region. The inconsistency of the data at some wavelengths required the development and application of a particular procedure to achieve consistency. Though all cases of inconsistency and potentially outlying results could be handled as consistently as possible, alternative approaches might have been applied as well, leading to different conclusions in some cases. Hence, we also conclude that there is an urgent need for a harmonized, statistically well-founded treatment of inconsistent data in key comparisons. Acknowledgment The author would like to thank Clemens Elster and Alfred Link for their helpful short lectures and discussions on statistics and the introduction to Bayesian model averaging, Alfred Link for the data analysis with a Bayesian method based on a statistical model which served as a check for the results on the exclusion of outliers, Peter Meindl for encouraging and helpful discussions, and Katrin Vogel, Peter Meindl, Josef Breuer, Marianne Fleischer-Bartsch, and Sylvia Ludwig for performing the large number of measurements PTB had to perform as the pilot of this comparison. The author would like to thank all participants for their cooperation and their patience. Appendix The model for linear drift of the spectral responsivity of a detector is x = a + bt with c = (a, b )T being the unknown quantities of the linear trend in vector form, i.e., a is the value of the spectral responsivity at time t = 0 and b is the rate of its change. The following N + 1 equations are available to estimate the unknown quantities: S341 xi = a + bti (i = 1, … , N ) ) L Werner Metrologia 51 (2014) S336 xN + 1 = b (A.1) ti is the time at which the ith participant determined the spectral responsivity of its transfer detectors, and xN + 1 is the rate of change of the spectral responsivity of the reference detector as determined by the pilot. These equations can be written compactly in matrix form x = Ac (A.2) x = (x1, … , xN , xN + 1)T (A.3) and ⎛1 ⎜⋮ A =⎜ ⎜1 ⎝0 t1 ⎞ ⋮⎟ ⎟ tN ⎟ 1⎠ (A.4) xi = a + Bk i = 1, … , N xN + k = Bk k = 1, … , 4 x = (x1, … , xN , xN + 1, … , xN + 4 )T (A.6) c = (a, B1, B2, B3, B4 )T (A.7) and 1 0 1 0 0 1 0 ⋮ 0 1 0 0 0 0 ⋮ 0 0 1 0 0 0 0⎞ 0 0⎟ ⎟ 0 0⎟ 1 3 ⎟ 4 4⎟ ⋮ ⋮⎟ 1 0⎟ 0 0⎟ 0 0 ⎟⎟ 1 0⎟ 0 1⎠ (A.10) u rel,adj (xi ) = 2 2 u rel,cal,adj, i + u rel,T, i (A.11) u rel,adj (xi ) differs from u rel (xi ) defined in equation (5) by the replacement of u rel,cal, i by u rel,cal,adj, i taking into account the cut-off in the uncertainty according to the policy of the CCPR Working Group on Key Comparisons (WG-KC) to limit the weight of the measurement results of an individual participant [5]. The solution is c = (AT WA)−1 AT Wx (A.12) with the covariance matrix The index k in the first of the upper two equations denotes the round of the measurements of participant i. The xN + k are the shift of the spectral responsivity during the jth round of measurements of participants with respect to its value at time t = t0 as determined by the pilot. Again these equations can be written compactly in matrix form (A.2) with ⎛1 ⎜1 ⎜ ⎜1 ⎜1 ⎜ A = ⎜⋮ ⎜1 ⎜0 ⎜0 ⎜ ⎜0 ⎝0 −2 wi = (xiu rel,adj (xi ) ) with (A.5) (A.9) The weighting W is a diagonal matrix with the diagonal elements The model for stepwise drift of the spectral responsivity of a detector is y = a + Bk. a is the value of the spectral responsivity at time t = t0 and Bk is the shift of the spectral responsivity during the kth round of measurements of participants with respect to its value at time t = t0. The N + 4 equations available to determine the unknown quantities a and Bk are χ 2 = (x − Ac)T W (x − Ac) with in the third round. The fourth participant took part with one detector in the third round and with three detectors in the fourth round as it is in the case of NIM that had to repeat parts of its measurements because of broken detectors. The estimation of the unknown quantities was obtained by applying weighted least square fit, which means, the estimation minimizes the sum of squared weighted differences between data and model: T (c) = (AT WA)−1 AT WUx[(AT WA)−1 AT W] cov (A.13) The covariance matrix Ux has the diagonal elements u2 (xi ) = (xiu rel (xi ) )2 with u rel (xi ) defined in equation (5). The off-diagonal elements are zero since the measurement results of the participants are not correlated in this comparison. The estimates for the spectral responsivity of the reference detector yi = y(ti ) at time ti (i = 1, … , N ) and for the drift are summarized in y = (y1, … , yN , … , yN + k )T with K equal 1 or 4 in case of the linear drift or the stepwise drift, respectively. The yi with i > N give the estimates of the drift parameters b or Bk. The yi (i = 1, … , N ) form the time dependent KCRV. Vector y is given by y = Αc (A.14) The corresponding covariance matrix is (A.8) Uy = Acov (c) AT . (A.15) The DoE di = xi − yi and U (di ) = 2u(di ) are taken from the vector d = (d1, … , dN , … , dN + K )T and the matrix Ud with the elements u2(di ) at the diagonal. It is The example given for A indicates that the first two participants performed their measurements in the first round, the third participant in the second round, and the last participant d = x − y, (A.16) Ud = Ux + Uy − Ux, y − Uy, x (A.17) S342 L Werner Metrologia 51 (2014) S336 and Ux, y = Uy, x = A(AT WA)−1 AT WUx . 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