57
(2) A series of 20 or more ‘Langley’ type calibrations at a high transmission site over a period
of three months or less.
(3) An absolute calibration of the radiometer using a set of calibrated lamps traceable to a national
standards’ laboratory.
If more than one spectral radiometer is employed in a network of stations operated by the sam e agency,
the Working Group suggests that a working standard radiometer, calibrated using one of the methods
described above, be employed for the calibration of the working spectral radiometers through direct
comparison using the ratio-Langley approach. This methodology requires that the instruments being
calibrated are identical to the standard instrument. Slight differences in the central wavelength and
the passband are acceptable at longer wavelengths, but can increase uncertainty dramatically in regions
of absorption bands or at wavelengths where Rayleigh scatter is significant. Ideally, the filters used
in the instruments will have been procured at the same time and from the same production batch to
ensure spectral matching.
Calibrations should be made annually. On-site methods, for regions with high atmospheric transmittance,
can be used to maintain the instrument calibration without removing the instrument from service.
7.4.2 On-site ‘Langley’ Style Procedures
The measurement of atmospheric transmission is a relative measure, so the absolute top-of-the-
atmosphere spectral flux need not be known. The method of Langley calibrations is based on the Bouguer-
Lambert-Beer law, which describes the reduction of monochromatic radiation through a medium as
a function of the extinction in the medium and the source intensity. For ideal atmospheric conditions
this can be expressed as:
where = spectral intensity at the surface
= spectral intensity at the top of the atmosphere
= optical depth
= the optical airmass
Assuming the passband that represents a wavelength is relatively small so that the assumption of
monochromatic radiation is valid, the radiom eter output signal (V) can be substituted for the intensity,
0
and a radiometer output for the top-of-the-atmosphere (V ) can be determined by extrapolating a series
of observations at different airmass values during conditions where the atmospheric turbidity remains
constant. Mathematically, this can be easily accomplished through making the equation linear by taking
the logarithm of both sides:
Observations obtained during stable conditions can be analysed using a least-squares regression
between airmass and the logarithm of the radiometer output signal. The zero-intercept is the logarithm
of the signal that would be observed at the top-of-the-atmosphere.
0
Although easy to compute, the actual evaluation of V is difficult because the atmosphere is seldom
stable over the airmass range needed to obtain the number of observations required to calculate the
0
intercept value. The task becomes more difficult when a large number of V values must be obtained
over a short time period.
To overcome this problem, a variety of techniques have been developed, two of which are described
following Section 7.4.2.1.
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