Forgan, B.W., 1988: Sun photometer calibrations by the ratio-Langely method. In Bas elien Atmospheric
18
Program (Australis) 1986, edited by B.W . Foragan and P.J. Fraser, pp 22 - 26, Bureau of Meteorology,
Melbourne, Australia.
59
which dominates the extinction. Forgan using this observation has developed the ratio-Langley technique
18
to reduce the extrapolation error associated with normal Langley calibrations.
21
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 atmospheric attenuator.
The ratio of the two wavelengths can be given as (dropping the wavelength for clarity):
The sum of the optical depth differences is made up of a term combining the constant differences in
attenuation due to molecular scattering and gaseous absorption plus the difference due to aerosol
attenuation. The latter term is a function of the aerosol optical properties, primarily 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 systematic trends in the AOD can be less than the smallest bias for the individual I . Therefore,
the ratio can be used to obtain information on the extraterrestrial constants of the wavelength pair even
when the individual extraterrestrial constants are found unacceptable by the removal of atm ospheric
effects. This can be accomplished in a manner similar to determining the individual extraterrestrial
constants by regressing the ratio against airmass.
The ratio results can be utilized successfully for a variety of purposes:
(1) If a single wavelength is well-calibrated, the ratio method can be used to successfully transfer
the calibration information to other wavelengths on the same instrument.
(2) If well-known extraterrestrial constants are known for a pair of wavelengths, the ratio technique
will provide a means of checking the stability of the filters. By combining a variety of filter
pairs, those filters with changing responsivity can be determined.
(3) The transfer of the calibration from one radiometer to a second can be improved through
the use of the ratio-Langley technique. The transfer of the extraterrestrial constant from one
instrument to another cannot be accomplished using simple procedures, except when the
radiometers have identical optics and the centre wavelengths and passbands are perfectly
matched. While modern manufacturing techniques used in the construction of commercially
available instruments provide the precision necessary to reproduce the optical geometry,
the passband and wavelength similarity of interference filters is unlikely. Rearranging the
above equation and again removing 8 and now t for clarity, it can be seen that:
Only if the instruments are identical does the )J term reduce to zero, so that the calibration transfer
is no longer airmass dependent.
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