Environmental Engineering Reference
In-Depth Information
Calibration . Radiometric calibration requires the spectral characterisation of the
system's responsivity. This is facilitated by means of a standard source whose output is
well defined 11 . The primary or reference standard is a blackbody. Its spectroradiometric
quantities can be calculated from Plank's radiation law. Only national laboratories (like
National Institute of Standards and Technology, United States; National Physics
Laboratory, United Kingdom; Physikalisch-Technische Bundesanstalt, Germany etc. )
maintain blackbody standards. They derive from the blackbody standard secondary
standards, usually 1000 W quartz halogen lamps (FEL lamps). Tertiary standards (also
FEL lamps) are often transferred to certified suppliers by which the working standards
(e.g. FEL lamp or 100 W quartz halogen lamps) are calibrated. Thus the
laboratory/working calibration lamp is a forth generation standard (Fig. 9).
national metrological institutes
(NIST, NPL, PTB, etc.)
I primary standard
(black body radiator)
I secondary standard
I tertiary standard
certified suppliers
FEL lamp 1000 W
(quartz halogen lamp 100 W)
I working standard
Figure 9. Hierarchy of standards (see text for abbreviations).
The spectral irradiance E O (O) which is specified by the supplier is defined at a
distinct current rating and at a distinct distance from the lamp, these operating
parameters have to be maintained as exact as possible. As the lamps will age
specifications will hold only for a certain burning time, e.g. 20-50h. After a warm up
time of at least 15 minutes the input optics is placed at the given distance, a spectral
scan is started and the photosignal I(O) is recorded. The spectral responsivity H(O) is
then calculated as
H(O) = I(O) / E O (O)dO
The spectral irradiance of an unknown source is thus obtained by dividing the measured
photocurrent by the spectral responsivitiy.
Small changes in the moving parts of a scanning monochromator may affect the
position of the spectrum on the exit slit. Therefore, regular checking of the wavelength
setting is required. Wavelength calibration is usually performed with a low pressure
discharge lamp (e.g. mercury lamp), which emits narrow spectral lines whose
wavelength positions are well defined. An alternative method is the evaluation of the
positions of Fraunhofer absorption lines in a solar spectrum. Especially the position of
the cadmium line pair (393.37 nm and 396.85 nm) is a good indicator for the UV
wavelength performance of the instrument. Narrow spectral lines or laser lines are used
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