The 1997 North American interagency Intercomparison of Ultraviolet Spectroradiometers including narrowband filter radiometers - Statistical Data Included

Journal of Research of the National Institute of Standards and Technology, Jan, 2002 by Kathleen Lantz, Patrick Disterhoft, Edward Early, Ambler Thompson, John DeLuisi, Jerry Berndt, Lee Harrison, Peter Kiedron, James Ehramjian, Germar Bernhard, Lauriana Cabasug, James Robertson, Wanfeng Mou, Thomas Taylor, James Slusser, David Bigelow, Bill Durham, George Janson, Douglass Hayes, Mark Beaubien, Arthur Beaubien

C = [summation over (i)][S.sub.i][[lambda].sub.i]/[summation over (i)][S.sub.i], (5.1)

where i indexes the signals [S.sub.i] and wavelengths [[lambda].sub.i], respectively, of those signals greater than 0.1 of the peak signal. Although baseline subtraction is not important for calculations of the centroid of laser lines because the light is monochromatic, to maintain consistency with the bandwidths determined by lamp emission lines, baseline subtraction was performed for spectral scans of laser light. A description of the procedure is given in Sec. 5.2.3.

For the high-resolution scans, normalization of the signals by the peak signal was straight-forward because there is no saturation of the signal. For the low-resolution scans, the peak signals obtained in the high-resolution scans and the optical densities of the filters were used to calculate the peak signals for the scans without the neutral-density filters. The optical density at 325 nm of a neutral-density filter was determined from the common wavelengths at which signals were measured for scans both with and without the filter.

The peak signals obtained in the high-resolution scans were used to normalize the signals from the lowresolution scans for the ASRC_RSS, NIST, NSF_SUV, and USDA_UIK instruments since there was no saturation. The peak signal for the SERC instrument was not as readily known because there is no filter centered at 325 nm. Therefore, the peak signal for each filter was obtained from the measured signal of the filter centered at the longest wave length that did not saturate. These peak signals were calculated by dividing the measured signal from the filter centered at 320.65 nm by the transmittance of that filter at 325 nm and multiplying by the peak transmittance of each filter.

5.1.3 Results and Discussion

The bandwidths of the instruments and the centroids of the laser line are most useful when included with those values obtained from the scans of the Hg, Cd, and Zn lamps. Therefore, the results from these determinations are shown in Figs. 5.3 and 5.4 in the next section, but to summarize, the bandwidths using the 325 nm line of the HeCd laser are close to the nominal values giving 0.53 nm for the ASRC_RSS instrument, 0.58 nm for EPA_101, 0.58 nm for EPA_114, 0.85 nm for the NIST instrument, 0.70 nm for the NSF_SUV instrument, 0.11 am for the USDA_UlK instrument, and 0.61 am for the YES_RSS instrument. These are given in Table 5.1 and the slit-scattering functions are given in Fig. 5.1. The shift in the signal at 325 nm for the EPA_101 and EPA_114 is due to several filter changes at that wavelength. From Fig. 5.1, the slit-scattering functions of the EPA_101, EPA_114, NIST, and USDA_UlK instruments are nearly triangular and symmetric about the peak wavelength. The wings of the slit function measured with the H eCd laser are chiefly determined by bulk and surface scattering of the spectrograph optics. The slit-scattering function was not determined for the YES_RSS instrument due to YES_RSS instrumental problems at the time.