55
Table 7.1 lists the BSRN wavelengths, maximum displacement from the nominal wavelength and the
maximum waveband (Full Width at Half Maximum) in order of priority.
Nominal
Wavelength
(nm)
Maximum
Displacement
(nm)
Maxi mum
Bandwidth
FWHM
(nm)
Wavelength
Mandatory
Absorption
2
412 2 6 Yes NO
862 4 6 Yes
32
500 3 6 Yes O , NO
(minor)
2
368 2 6 Yes NO
778 2 11
3
675 3 11 O
3
610 2 11 O
Table 7.1. Spectral passband information for BSRN aerosol optical depth measurements.
2
The absorption of NO is of minor consequence in remote or rural areas, but should be accounted
for when AOD measurements are made in urban areas.
The use of additional spectral bands, for the determination of atmospheric constituents or increased
information on aerosol optical properties, is encouraged. Aerosol properties, however, will only be
archived for these selected wavelengths at present.
The changing transmission and the rapid shifting of the central wavelength in interference filters that
have been used in sunphotometers have caused the rejection of large amounts of data and brought
even more data under suspicion. The manufacture of interference filters has improved over the last
decade. Filters produced, using ion deposition technology, should be more stable than those made
using older technologies. The use of these filters has not yet been proven, however. Several procedures
have been found to reduce filter degradation:
(1) Sealing the radiometer from the ambient environment. Filters have been found to degrade
more slowly when the instrument is temperature controlled and the humidity is controlled.
(2) Ensuring that the filters are exposed to solar radiation only during times when observations
are being made. This can be accomplished either by using a shutter to block radiation from
hitting the surface of the filter, or by turning the opening aperture away from the radiation
source.
(3) Filters have been found to degrade from the outer edge inward toward the centre. Therefore,
the use of oversize filters will prolong their useful life.
Even with these precautions, the transmission and waveband properties should be checked regularly.
The use of the “ratio-Langley” method (Section 7.4.2.2) provides a means of checking for variations
between wavelengths that may be caused by changes in the optical properties of the interference filters.
If changes do occur, the instrument should be carefully checked to determine the cause of the change
(multi-detector instruments may be showing changes in sensor responsivity). In cases, such as above,
and during times when the instrument is in an optical laboratory, the interference filters should be carefully
removed from the instrument and their optical properties measured using a high-quality bench
spectrometer. If changes in the bandpass are found so that it no longer meets the requirements set
out in Table 7.1, the filter must be replaced. The filter transmission may also vary with age. The principal
investigator must determine whether the change in transmission is enough to require the filter be changed.
The tolerance provided by most filter manufacturers is such that obtaining identical filters over a period
of years is unlikely. Therefore, if possible, spare filters should be purchased at the same time as the
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