Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 71)

Forgan, B.W ., 1988: Sun photom eter calibrations by the ratio-Langely m ethod. In Baselien Atm ospheric
18
Program (Australis) 1986, edited by B.W . Foragan and P.J. Fras er, pp 22 - 26, Bureau of M eteorology,
M elbourne, Australia.
59
which dom inates the extinction. Forgan using this observation has developed the ratio-Langley technique 18
to reduce the extrapolation error associated with norm al Langley calibrations.
2 1
For a wavelength pair 8 < 8 , where neither wavelength occurs in a region of strong absorption,
and the signals can be given as:
and
where i represents the ith atm ospheric attenuator.
The ratio of the two wavelengths can be given as (dropping the wavelength for clarity):
The sum of the optical depth differences is m ade up of a term com bining the constant differences in
attenuation due to m olecular scattering and gaseous absorption plus the difference due to aerosol
attenuation. The latter term is a function of the aerosol optical properties, prim arily the size distribution.
0
Using an Ångström size distribution, it can be shown that the bias error in the ratio of the I pair caused
0
by system atic trends in the AOD can be less than the sm allest bias for the individual I . Therefore,
the ratio can be used to obtain inform ation on the extraterrestrial constants of the wavelength pair even
when the individual extraterrestrial constants are found unacceptable by the rem oval of atm ospheric
effects. This can be accom plished in a m anner sim ilar to determ ining the individual extraterrestrial
constants by regressing the ratio against airm ass.
The ratio results can be utilized successfully for a variety of purposes:
(1) If a single wavelength is well-calibrated, the ratio m ethod can be used to successfully transfer
the calibration inform ation to other wavelengths on the sam e instrum ent.
(2) If well-known extraterrestrial constants are known for a pair of wavelengths, the ratio technique
will provide a m eans of checking the stability of the filters. By com bining a variety of filter
pairs, those filters with changing responsivity can be determ ined.
(3) The transfer of the calibration from one radiom eter to a second can be im proved through
the use of the ratio-Langley technique. The transfer of the extraterrestrial constant from one
instrum ent to another cannot be accom plished using sim ple procedures, except when the
radiom eters have identical optics and the centre wavelengths and passbands are perfectly
m atched. W hile m odern m anufacturing techniques used in the construction of com m ercially
available instrum ents provide the precision necessary to reproduce the optical geom etry,
the passband and wavelength sim ilarity of interference filters is unlikely. Rearranging the
above equation and again rem oving 8 and now t for clarity, it can be seen that:
Only if the instrum ents are identical does the )J term reduce to zero, so that the calibration transfer
is no longer airm ass dependent.