Abstract. Non-ideal angular response of a spectroradiometer is a well-known
error source of spectral UV measurements and for that reason
instrument specific cosine error correction is applied. In this paper,
the performance of the cosine error correction method of Brewer
spectral UV measurements in use at the Finnish Meteorological
Institute (FMI) is studied. Ideally, the correction depends on the
actual sky radiation distribution, which can change even during one
spectral scan due to rapid changes in cloudiness. The FMI method has
been developed to take into account the changes in the ratio of direct
to diffuse sky radiation and it derives a correction coefficient for
each measured wavelength. Measurements of five Brewers were corrected
for the cosine error and the results were compared to the reference
travelling spectroradiometer (QASUME). Measurements were performed
during the RBCC-E (Regional Brewer Calibration Center – Europe)
X Campaign held at El Arenosillo, Huelva (37∘ N,
7∘ W), Spain, in 2015. In addition, results of site audits of
FMI's Brewers in Sodankylä (67∘ N, 27∘ E) and Jokioinen
(61∘ N, 24∘ E) during 2002–2014 were studied. The results
show that the spectral cosine error correction varied between 4 and
14 %. After that the correction was applied to Brewer UV spectra the relative differences
between the QASUME and the Brewer diminished even by
10 %. The study confirms that the method, originally developed for
measurements at high latitudes, can be used at mid-latitudes as
well. The method is applicable to other Brewers as far as the required
input parameters, i.e. total ozone, aerosol information, albedo,
instrument specific angular response and slit function are available.
Erratum: “Experimental measurement of ionization chamber angular response and associated magnetic field correction factors in MR‐linac” [Med. Phys. 47(4), 1940‐1948 (2020)]
