The measurement of field quantities, that is, flux, flux density, radiance, and intensity, involves the use of a radiometer, an instrument that measures the amount of flux that gets to the detector. Such a measurement requires an appropriate instrument and its calibration. In this chapter we assume that the radiometer can measure the flux with sufficient accuracy.
12.1 Measurement of Power
This is the basic measurement. The power on the detector creates a certain electrical signal, and via calibration this represents the power received by the radiometer. Power, normalized to the detector response and spectral band, can be measured by the ESRs described above, by the self-calibrated detectors described above, and by both chopper radiometers and dc radiometers described later.
12.2 Measurement of Incidance
Incidance is a flux density. If the radiometer can measure power, it can measure flux density if the area of the entrance aperture is known to sufficient accuracy. Simple division does the job. Of course, it will be the incidance averaged over the area of the entrance aperture. The measurements must be just like the calibration, and the calibration requirements and procedures are described in this chapter.
12.3 Measurement of Exitance
Exitance is a source property. It can be inferred from the measurement of power. If the power on the detector is known (by calibration and measurement), then the power from the source can be calculated with certain assumptions. The main assumptions are that the distance and size of the source are known and so is the transmission of the intervening atmosphere. If the atmosphere is not known, many investigators cite an apparent radiant exitance. In this case, apparent means âwithout correction for atmospheric losses.â This can often be very important. One such case involved understanding the emission from an ICBM. Of course, the rocket had to be measured from a distance, and it radiates copiously in the carbon dioxide band. But the atmosphere absorbs intensely in the carbon dioxide band. The apparent radiant exitance had little relation to the real values.
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