61
The use of a standard lamp either as a calibration source or as an irradiance source for use with a
detector standard, requires precision measurements and optical alignment. A specially designed calibration
assembly or an optical bench is essential to obtain a high quality calibration. The following must be
taken into consideration:
(1) The distance between the lamp filament and the first instrument optic must be precisely
measured to determine the spectral irradiance at the instrument.
(2) The source must be far enough from the instrument so that the lamp filament behaves as
a point source, rather than a line source.
(3) This distance between the lamp and the instrument must be great enough to ensure that
the FOV of the instrument exceeds the angular field of the lamp (the source must underfill
the limiting aperture).
(4) The instrument must be perpendicular to the beam.
(5) The combination of the lamp intensity and the integration time used by the detector must
be such that the signal is representative of the operating conditions of the instrument. The
signal must be significantly greater than the noise level of the detector.
Standard lamps are provided with a calibration certificate indicating the distance at which the spectral
irradiance was measured and the current setting used. The distance between the lamp and the radiometer
being calibrated can be altered and the inverse square law used to determine the irradiance at the
instrument. The current must be maintained to better than 0.01% as changes in current produce nonlinear
changes in spectral irradiance.
The calibration certificate of the standard lamp only provides the lamp output at specific wavelengths.
Interpolation m ust be used to determine the lam p output at intermediary wavelengths. Changes in
the lamp output with wavelength can be significant, ranging from approximately 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 lamp responsivity function.
The calibration of a radiometer using interference filters must consider both the changing lamp output
and the transmission function of the filter being calibrated. Thus, the output of the radiometer for a
particular filter is the convolution of the lamp’s spectral irradiance and the filter function:
N
where V = the output voltage of the radiometer at nominal wavelength 8
8
E = the standard lamp irradiance at wavelength 8
8
Ft = the transmission of the interference filter at 8
12
8, 8 = the wavelengths of the filter where the transmission is less than 10
-4
From the equation above, it can be seen that the calibration of filter radiometers using standard lamps
is unlikely to provide uncertainties small enough to calculate AOD to better than 0.01. The use of standard
detectors may make this goal achievable.
7.5 Maintenance
Daily maintenance procedures for solar pointing spectral radiometers are similar to the care of normal
incidence pyrheliometers. The window of the instrument should be cleaned using the same principles
found in Section 6.2 (1) (ii). The cleaning should be at a minimum 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.
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