Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 73)

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The use of a standard lam p either as a calibration source or as an irradiance source for use with a
detector standard, requires precision measurements and optical alignm ent. A specially designed calibration
assem bly or an optical bench is essential to obtain a high quality calibration. The following m ust be
taken into consideration:
(1) The distance between the lam p filam ent and the first instrum ent optic m ust be precisely
m easured to determ ine the spectral irradiance at the instrum ent.
(2) The source m ust be far enough from the instrum ent so that the lam p filam ent behaves as
a point source, rather than a line source.
(3) T his distance between the lam p and the instrum ent m ust be great enough to ensure that
the FOV of the instrum ent exceeds the angular field of the lam p (the source m ust underfill
the lim iting aperture).
(4) The instrum ent m ust be perpendicular to the beam .
(5) The com bination of the lam p intensity and the integration tim e used by the detector m ust
be such that the signal is representative of the operating conditions of the instrum ent. The
signal m ust be significantly greater than the noise level of the detector.
Standard lam ps are provided with a calibration certificate indicating the distance at which the spectral
irradiance was m easured and the current setting used. The distance between the lam p and the radiom eter
being calibrated can be altered and the inverse square law used to determ ine the irradiance at the
instrum ent. The current m ust be m aintained to better than 0.01% as changes in current produce nonlinear
changes in spectral irradiance.
The calibration certificate of the standard lam p only provides the lam p output at specific wavelengths.
Interpolation m ust be used to determ ine the lam p output at interm ediary wavelengths. Changes in
the lam p output with wavelength can be significant, ranging from approxim ately 4% per nm at 300
nm to less than 0.25% per nm at 750 nm . Interpolation should be done through the fitting of a blackbody
curve to the lam p responsivity function.
The calibration of a radiom eter using interference filters m ust consider both the changing lam p output
and the transm ission function of the filter being calibrated. T hus, the output of the radiom eter for a
particular filter is the convolution of the lam p’s spectral irradiance and the filter function:
N
where V = the output voltage of the radiom eter at nom inal wavelength 8
8
E = the standard lam p irradiance at wavelength 8
8
Ft = the transm ission of the interference filter at 8
1 2
8 , 8 = the wavelengths of the filter where the transm ission is less than 10 -4
From the equation above, it can be seen that the calibration of filter radiom eters using standard lam ps
is unlikely to provide uncertainties sm all enough to calculate AOD to better than 0.01. The use of standard
detectors m ay m ake this goal achievable.
7.5 Maintenance
Daily m aintenance procedures for solar pointing spectral radiom eters are sim ilar to the care of norm al
incidence pyrheliom eters. The window of the instrum ent should be cleaned using the sam e principles
found in Section 6.2 (1) (ii). The cleaning should be at a m inim um daily and take place after sunrise,
and preferably before the solar elevation exceeds 8°. The window should also be inspected and cleaned
as necessary after precipitation events.