Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 69)

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(2) A series of 20 or m ore ‘Langley’ type calibrations at a high transm ission site over a period
of three m onths or less.
(3) An absolute calibration of the radiom eter using a set of calibrated lam ps traceable to a national
standards’ laboratory.
If m ore than one spectral radiom eter is em ployed in a network of stations operated by the sam e agency,
the W orking Group suggests that a working standard radiom eter, calibrated using one of the m ethods
described above, be em ployed for the calibration of the working spectral radiom eters through direct
com parison using the ratio-Langley approach. This m ethodology requires that the instrum ents being
calibrated are identical to the standard instrum ent. Slight differences in the central wavelength and
the passband are acceptable at longer wavelengths, but can increase uncertainty dram atically in regions
of absorption bands or at wavelengths where Rayleigh scatter is significant. Ideally, the filters used
in the instrum ents will have been procured at the sam e tim e and from the sam e production batch to
ensure spectral m atching.
Calibrations should be m ade annually. On-site m ethods, for regions with high atm ospheric transm ittance,
can be used to m aintain the instrum ent calibration without rem oving the instrum ent from service.
7.4.2 On-site ‘Langley’ Style Procedures
The m easurem ent of atm ospheric transm ission is a relative m easure, so the absolute top-of-the-
atm osphere spectral flux need not be known. The m ethod of Langley calibrations is based on the Bouguer-
Lam bert-Beer law, which describes the reduction of m onochrom atic radiation through a m edium as
a function of the extinction in the m edium and the source intensity. For ideal atm ospheric conditions
this can be expressed as:
where = spectral intensity at the surface
= spectral intensity at the top of the atm osphere
= optical depth
= the optical airm ass
Assum ing the passband that represents a wavelength is relatively sm all so that the assum ption of
m onochrom atic radiation is valid, the radiom eter output signal (V) can be substituted for the intensity,
0
and a radiom eter output for the top-of-the-atm osphere (V ) can be determ ined by extrapolating a series
of observations at different airm ass values during conditions where the atm ospheric turbidity rem ains
constant. Mathem atically, this can be easily accom plished through m aking the equation linear by taking
the logarithm of both sides:
Observations obtained during stable conditions can be analysed using a least-squares regression
between airm ass and the logarithm of the radiom eter output signal. The zero-intercept is the logarithm
of the signal that would be observed at the top-of-the-atm osphere.
0
Although easy to com pute, the actual evaluation of V is difficult because the atm osphere is seldom
stable over the airm ass range needed to obtain the num ber of observations required to calculate the
0
intercept value. The task becom es m ore difficult when a large num ber of V values m ust be obtained
over a short tim e period.
To overcom e this problem , a variety of techniques have been developed, two of which are described
following Section 7.4.2.1.