This head-to-head comparison was coordinated by the Optical Technology Division of the National Institute of Standards and Technology (NIST) and the Surface Radiation Research Branch (SRRB) of the National Oceanic and Atmospheric Administration (NOAA). Unfortunately, the NSF instrument's temperature controller broke at the beginning of the intercomparison. The wavelength of the monochromator in terms of micrometer steps was determined at the factory based on the wavelengths of Hg emission lines.
The angle of the car is adjusted for the latitude, and the azimuth is aligned with the Earth's pole. All the characterizations were performed outside the booklets by techniques developed at the previous Intercomparisons. For the high-resolution scans, normalization of the signals by the peak signals was straightforward because there was no saturation.
Background subtraction and calculation of the centroid and bandwidth for each line was performed as detailed in Sect. Only the bandwidths for single lines were taken as an indication of the bandwidth of the instrument at that wavelength. The actual centroids of the lines were calculated from the wavelengths and relative intensities of the emission lines for that particular model of Hg lamp and from the published values for Cd and Zn emission lines [7].
The bandwidths calculated from the measurements of singlet Cd, Hg and Zn lines and the HeCd line are plotted in figure. Similarly, the differences between the calculated and actual centroids of the Cd, Hg, Zn and HeCd lines are plotted in Fig.
There is a systematic trend for the centroid differences between the AES and EPA instruments that is consistent across sources. A glass slide was placed over the entrance port of the integrating sphere and the retroreflective beam from the center of the port defined the optical axis. The holder was leveled so that the front plane was perpendicular to the optical axis, and the lamp filament was centered on the optical axis and 50.0 cm from the entrance port.
The filament was centered on the optical axis at a distance of 50.0 cm from the entrance port to the center of the filament. The mount was leveled and offset so that the nipple was on the optical axis and the center of the filament was 50.0 cm from the entry port. The wavelength was calibrated by illuminating the entrance aperture of the integrating sphere with a Hg emission lamp.
The irradiance spectral responsivity R(\) of the spectroradiometer was determined during one day's measurements using standard secondary lamps. The main purpose of this experiment was to compare the spectral radiance levels of the participants with the NIST scale. Additional experiments are required to definitively determine the cause of the relatively finite differences between spectral irradiance rates for horizontal lamps.
The relative difference as a function of wavelength between the spectral irradiance of a lamp operated in a horizontal position and that of the lamp in the vertical position is shown in Fig. The relative difference for the NIST lamp is consistent with results obtained previously and detailed in [8].
M-881 Horizontal
Participants and luminaries are indicated in each panel, calibration sources are indicated in the legend in (c), and vertical lines are standard uncertainties. Second, with the exception of the NSF bulb, the spectral irradiance calibrations of the lamps matched the spectral irradiance measured with the NIST scale. On the other hand, it means that the discrepancy between the NIST scale and the participants' scales observed in the intercomparisons arises during the transfer of the scales from standard lamps, such as those measured in this experiment, to the lamps used in the Intercomparisons.
Measuring the spectral radiant responsivity (hereinafter simply referred to as responsiveness) of the instruments, both with the NIST standard lamps and with the participants' standard lamps, was the main characterization performed during the Intercomparison. As with the other Intercomparisons, these measurements determined the agreement between the spectral radiance scales, the temporal stability of the instruments, and the responsiveness of each instrument for the synchronized measurements of the Sun's ultraviolet radiation. The spectral irradiance of the 1000 W FEL-type NIST standard lamps, designated E-002 and E-004, was determined in the horizontal position as detailed in Sec.
A schedule of spectral scans of standard lamps is given in Table 5.1, together with the corresponding instrument temperatures. The spectral radiances of the standard lamps were fit by a cubic linear interpolation to the wavelengths of the signals. The standard uncertainties in the signals were the standard deviations of the mean, and these were propagated to the direct signals.
A comparison between these scales and the NIST spectral radiance scale is very important to assess the accuracy of the participants' scales. The relative difference between a participant's spectral radiance scale and the NIST scale is given by the relative difference between the responsiveness using the NIST standard lamp and the responsiveness using the participant's standard lamp, assuming that the responsiveness of the instrument remains the same. stable during the period between the two measurements. The spectral radiance scales of the lamps used by AES and EPA are generally within ±5% of the NIST scale.
The locations of the calibrations on which the USDA scale was based are indicated in the legends in (g) and (h).
Solar Irradiance
The ultimate goal of the intercomparison was to have all instruments measure solar ultraviolet irradiance simultaneously, which was achieved within a few days of the intercomparison. A0) was calculated from the measured signals S(\0) using a simplified measurement equation. E(Ao) = S(Ao)/a(Ao), (6.1) with the responsivity R(\0) for each instrument determined by outdoor scanning of the NIST standard lamp. Synchronized spectral scans of solar ultraviolet irradiance were initiated hourly and half hourly from wavelengths 290 nm to 340 nm in 0.2 nm steps with 3 s between each wavelength.
The stray light suppression of the instruments, shown in Figure 5.2, can result in relatively large signals at the shortest wavelengths. The stray light suppression of the AES instrument was sufficiently great that correction of the signals at the shortest wavelengths was not necessary. The days and times of the solar radiation responsivities used are shown in Table 6.2.
The responsivity of the EPA instrument was extrapolated to 325.2 nm using a third-order polynomial fit. Because the responsivities were not determined at all wavelengths of the synchronized solar scans, the responsivity at these wavelengths was calculated from natural cubic spline interpolations. Also, the absence of any spectral structure in the relative differences between the AES and EPA instruments, except at the longer wavelengths on day 172 for the EPA instrument, indicates good wavelength stability for these instruments.
The results can be understood from the atmospheric conditions at the time of the measurements. The value used to quantify the agreement between instruments was the standard deviation of the convolved irradiance divided by the mean irradiance at each wavelength, expressed as the relative standard deviation.
Conclusions
Spectral examinations of the emission lines of Cd, Hg and Zn lamps and the HeCd laser were carried out outdoors. Scattered light rejections of the instruments were consistent with those expected for single and dual monochromators and for interference filters. The greatest success of Intercomparison was the evaluation of techniques for determining the spectral response of instruments to radiation in the field.
The results showed that the participants' scales, with the exception of NSF, agreed with the NIST scale. In addition, the spectral irradiances of the participants' lamps in the horizontal orientation were reduced relative to those when the lamps were in the vertical orientation. Field calibration units produced by NIST and NOAA were used on the first day of intercomparison to measure the response of the EPA and SERC instruments.
The spectral irradiance responsivity of each instrument was determined with a NIST standard lamp operating in a field calibration unit at least three times outdoors during the Intercomparison. Responsibilities of the AES and EPA instruments were also determined using participants' lamps. The spectral irradiance scales for AES and EPA were within 5% of the NIST scale.
Instrument responses remained relatively stable for the AES and EPA instruments, while the SERC and USDA instrument responses were significantly less stable, particularly for the USDA 270 unit. A synchronized scan of solar irradiance from 290 nm to 340 nm was performed every half hour for the four days of the Intercomparison.
Appendix A. Attendees
The Department of Agriculture, the Environmental Protection Agency, and the National Science Foundation made this work possible. Operation of the National Science Foundation's Polar Ultraviolet Monitoring Network, along with participation in this Intercomparison, was funded through contract STF-M8871-01 from Antarctic Support Associates under the direction of Dr. Operation of the National Ultraviolet Monitoring Network, along with participation in this Intercomparison, was funded through the U.S.
Development of the Smithsonian Ultraviolet Scanning Radiometer, along with participation in this intercomparison, was funded by the U.S. Department of Agriculture's Cooperative State Research, Education, and Extension Service UV-B Radiation Monitoring Program through a contract under the direction of Drs.
