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Abstract
We have proposed a novel method for the spectral irradiance measurement of the standard lamp. The measurement has been realized by using national primary scale based on a large area WC-C fixed point blackbody for the first time. The wavelength range is from 450 nm to 1000 nm. The spectral irradiance of the standard lamp has been measured based on the traditional variable temperature blackbody and large area WC-C fixed point blackbody. The results of the two methods agree within the uncertainties. Compared with the traditional measurement method, the fixed point method can reduce the major component of the spectral irradiance measurement uncertainty related to the temperature measurement.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For absolute radiometry metrology, the blackbody as the primary standard source is still the most common way to realize the traceability [1,2]. Spectral irradiance measurement plays a key role in radiometry metrology and has received intense attention [3–10]. The accurate measurement of spectral irradiance is wildly used in many fields, such as solar spectral irradiance measurement [11–13], climate change monitoring [14] and lunar observation [15].
Spectral irradiance from 250 nm to 2500 nm is a subject of the CCPR K1-a Key Comparison [16], where tungsten halogen lamps are used as transfer standards. The lamps are also widely used for the spectral irradiance measurement, especially for calibration of all kinds of spectroradiometers. In many national metrology institutes [16], the lamps are measured by means of comparison with the variable high temperature blackbody, using a chain shown in Fig. 1(a). The spectral radiance of the blackbody, ${L_{BB}}(\lambda ,T)$, can be described by the Planck equation [17],
Fig. 1. Traceability chain for spectral irradiance measurement of standard lamp. (a) The common traceability chain for source based method. (b) The traceability chain for large area WC-C fixed point method.
The fixed point blackbodies used to calibrate the pyrometer usually have rather small cavity opening. They can not be used for spectral irradiance measurement directly. Therefore, we have to use the variable temperature blackbody in irradiance mode to calibrate the lamp. It enlarges the measurement uncertainty. Recently, for the purpose of spectral irradiance measurement, the large area tungsten carbide–carbon peritectic (WC-C) fixed point blackbody has been developed and investigated [19]. The melting temperature of WC-C fixed point is around 3021 K, which is close to the color temperature of the tungsten halogen lamps (about 3000 K). Therefore, this kind of fixed point is very useful for spectral irradiance application. In this paper, we use the large area WC-C fixed point blackbody to calibrate the spectral irradiance of the standard lamp directly, without using a variable temperature blackbody. The traceability chain for the fixed point method is shown in Fig. 1(b). This method can shorten the traceability chain and reduce the measurement uncertainty of spectral irradiance of the standard lamp related to the temperature measurement. This is the first time when the fixed point method is used for the spectral irradiance measurement of standard lamp. In the following, we will introduce the fixed point method for spectral irradiance measurement. In order to verify the fixed point method, we also measured the spectral irradiance of the same standard lamp by comparing it with the variable temperature blackbody.
2. Experimental schematics
The blackbodies BB3500M and BB3500MP, are shown in Fig. 2(a). For BB3500M, the inner diameter of the cavity is 38 mm. It is used as the variable temperature blackbody and also can be used as a small area fixed point blackbody, when containing a small fixed point cell. For BB3500MP, the inner diameter of the cavity is 59 mm. It is used, with a large fixed point cell, as the large area WC-C fixed point blackbody. The large WC-C cell is shown in Figs. 2(b) and 2(c). The diameter of the cell radiating cavity is 14 mm, which is large enough for the spectral irradiance measurement. The emissivity of the large area WC-C fixed point blackbody is 0.9997.
Fig. 2. (a) Photograph of blackbodies, BB3500M and BB3500MP. (b) Cross section of the large area WC-C fixed point cell. (c) Photograph of the large WC-C cell (left) and small cell (right).
The experimental schematics is shown in Fig. 3. BB3500M was a variable high temperature blackbody. BB3500MP was a large area WC-C fixed point blackbody. The spectral comparator and the pyrometer were placed on a translational optical platform. First, we measured the spectral irradiance of the standard lamp with the variable temperature blackbody. For that, we set the temperature of BB3500M at the temperature of approximately 3000 K. The temperature was measured by the pyrometer. Then, we moved the optical platform to the position of BB3500M (BB3500M stands in front of the integrating sphere). The spectral comparator was calibrated against the BB3500M. And then, we moved the optical platform to the position of the standard lamp and measured its spectral irradiance. The wavelength range was from 450 nm to 1000 nm, which was limited by the detector and the gratings. The distance between the lamp and the integrating sphere was 500 mm. A water-cooled aperture with area of 20.488 mm2 was put in front of the blackbody. The distance between the aperture and the integrating sphere was 434.24 mm. This method is marked as Method A. This is the common method to calibrate a standard lamp.
Fig. 3. Experimental schematics. The system includes two blackbodies, one standard lamp, one pyrometer and spectral comparator consisting of one integrating sphere, one double-grating monochromator and one Si detector.
3. Experimental results
For the fixed point method, we should investigate the WC-C cell and determine the temperature drop effect of the large cell first [19]. The large cell was compared with a small WC-C fixed point cell, whose cavity opening is as small as 3 mm. The small WC-C cell was installed inside the blackbody, BB3500M. Three full melt-freeze cycles of WC-C is shown in Fig. 4. The melting and freezing plateaux were realized by applying the furnace step of +15 K above and -15 K below the melting point of WC-C.
Fig. 4. Three full melt-freeze cycles of small WC-C cell, with cavity opening of 3 mm and installed inside the blackbody, BB3500M.
The large area WC-C cell was investigated with the same method. The large WC-C fixed point cell was installed inside the blackbody, BB3500MP. Two full melt-freeze cycles of the large WC-C cell are shown in Fig. 5. The melting and freezing plateaux were also realized by applying the furnace step of +15 K above and -15 K below the melting point of WC-C. Using the first derivative of the melting curve, which is shown in Fig. 6, the melting temperature of the cell was determined as the point of inflection of the melting plateau. Using the second derivative of the melting curve, we obtained the duration of the melting plateau [20,21]. The typical duration was about 15 minutes, which was long enough to calibrate the spectral comparator against the large WC-C cell blackbody at several typical wavelengths. In our experiment, we covered the wavelength range from 450 nm to 1000 nm.
Fig. 5. Two full melt-freeze cycles of large WC-C cell, with cavity opening of 14 mm and installed inside of the blackbody, BB3500MP.
Comparing the point of inflection temperatures for large and small cells, we obtained the difference between the melting temperatures of the cells, which associated with the temperature drop effect. We had done the cells comparison experiments in different three days. The temperature difference measured was -0.51 ℃, -0.50 ℃ and -0.46 ℃, respectively. The average difference was -0.49 ℃. Therefore, assuming that the average published value of the melting temperature of the small cell is 3020.65 K (2747.5℃) [22,23], we defined the melting temperature of the large cell WC-C fixed point blackbody as 3020.16 K (2747.01℃). In the following experiment, we used the melting plateau of the large WC-C cell as the primary standard. We could know the melting temperature exactly without the pyrometer.
The procedure of spectral irradiance measurement with the fixed point method was as following. The first melt-freeze cycle of the large cell was recorded by the pyrometer, and the duration of the melting plateau was defined. Because the plateau duration time was limited, we had to estimate the time interval for the spectral irradiance measurement and select the wavelength points to be measured. Further plateaux were used to calibrate the system. The beginning of each melt-freeze cycle was also recorded by the pyrometer, but once the melting plateau started, we moved the optical platform to the position of BB3500MP opposite to the integrating sphere, and calibrate the system. After the measurement, we moved the pyrometer back to the blackbody, and recorded the rest of the melting plateau. If we realized that the temperature was out of the melting plateau, it meant that the measurement was not correct. The measurement needed to be repeated several times. The melting curve recorded by the pyrometer, used for the spectral irradiance measurement, is shown in Fig. 7. The whole melting curve for spectral irradiance measurement with the fixed point method is shown in Fig. 7(a). The gap in the melting plateau corresponds to the time interval used for calibrating the system against the large WC-C fixed point blackbody. Figure 7(b) shows the melting curve used for the spectral irradiance measurement with bold line. The thin line is the melting curve used for defining the plateau duration. We calibrated the system three times. An aperture with the area of 18.861 mm2 was in front of the blackbody. The aperture diameter was so small that the integrating sphere received the radiation from the cell cavity only. There was no need to measure the blackbody temperature, because the melting temperature of the large WC-C cell was fixed and well known from the previous measurements. When calibration of the spectral comparator against the fixed point blackbody was completed, we moved the optical platform to the position of the standard lamp and measured the spectral irradiance of the lamp. The wavelength range was the same as for the Method A, from 450 nm to 1000 nm. The distance between the lamp and the integrating sphere was 500 mm and the distance between the blackbody aperture and the integrating sphere was 611.16 mm. The large area WC-C fixed point method is marked as Method B.
Fig. 7. (a) The whole melting curve for spectral irradiance measurement with the fixed point method. The gap in the melting plateau corresponds to the time interval used for calibrating the system against the large WC-C fixed point blackbody. (b) Melting curve (bold line) used for the spectral irradiance measurement, recorded by the pyrometer. The thin line is the melting curve used for defining the plateau duration.
For the uncertainty analysis, we have compared the two different methods. For Method A, the uncertainty budget for spectral irradiance measurement of the standard lamp is presented in Table 1. The uncertainty of temperature measurement of blackbody (represented by u4) is 0.636 K, the none-uniformity of blackbody (represented by u5) is 0.17 K, the instability of blackbody (represented by u6) is 0.2 K. For Method B, the uncertainty budget is presented in Table 2. The uncertainty of melting temperature of the fixed point blackbody is 0.4 K, the none-uniformity of the fixed point blackbody is 0.09 K, the instability of the fixed point blackbody is 0.144 K. From these tables, one can notice that, the fixed point method can reduce the measurement uncertainty related to temperature measurement significantly compared with the conventional method. Other components of the measurement uncertainties are almost the same for the two methods. Therefore, the fixed point method can reduce the spectral irradiance measurement uncertainty of the standard lamp.
Table 1. Uncertainty budget for spectral irradiance measurement of the standard lamp–Method A.
Table 2. Uncertainty budget for spectral irradiance measurement of the standard lamp–Method B.
The results of the spectral irradiance of the standard lamp measured with Method A and Method B are presented in Table 3. The table also presents the difference between the two methods. The results of the two methods agree with each other very well. From this table, we can notice that the results of the two methods agree within the uncertainties.
Table 3. Measurement results of spectral irradiance of the standard lamp.
4. Conclusion
In conclusion, we have realized a fixed point method for spectral irradiance measurement of the tungsten halogen lamp based on the large area WC-C fixed point blackbody from 450 nm to 1000 nm. This is the first try to measure the spectral irradiance of the standard lamp through national primary scale using this method. The large WC-C fixed point cell with cavity opening of 14 mm, which is specially used for spectral irradiance measurement, has been manufactured. The melting plateau of the large WC-C cell is used as the primary standard. The melting temperature can be determined exactly without the pyrometer. The measurement results agree very well with the traditional variable blackbody method. This fixed point method can shorten the traceability chain and reduce the measurement uncertainty related to temperature measurement significantly. It is meaningful for the spectral irradiance measurement. However, there are also some problems to solve. For instance, in order to obtain a better melting curve shape, we need to do more experiments to find the optimal position of the large cell inside the furnace. We also need to find a proper approximation of the melting plateau curve to reduce the temperature measurement uncertainty at different measurement time.
Funding
National Key Research and Development Program of China (2016YFF0200304); Quality Technology Capacity Improvement Program (ANL1909); Equipment Sharing Center for High–Precision Measuring Technologies in Photonics (www.ckp.vniiofi.ru) founded on the basis of VNIIOFI.
Acknowledgments
The authors thank Yandong Lin and Xiaofeng Lu for helpful discussion.
Disclosures
The authors declare no conflicts of interest.
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