Campbell Scientific NR-LITE2 Net Radiometer - Manual (Page 15)

NR-LITE2 Net Radiometer
FIGURE 5-1. Electrical circuit for the NR-LITE2 Net Radiometer
5.2 Spectral Properties
The upwards facing sensor is calibrated for solar radiation wavelengths. The
following assumptions are made:
• The downwards facing sensor has the same sensitivity. However, since
the two sensors may not be perfectly symmetrical, this assumption may
not always be true, but differences are small.
• The NR-LITE2’s sensitivity is the same for both solar and infrared
radiation.
5.3 Directional/Cosine Response
The measurement of the radiation falling on a surface (also known as
irradiance or radiative flux) is based on two assumptions:
1. The sensor surface is spectrally black — that it absorbs all radiation from
all wavelengths (Section 5.2, Spectral Properties).
2. That it has a true field of view of 180°.
These two properties, taken together, with which the net radiometer needs to
comply, are generally known as the cosine response.
A perfect cosine response will show maximum sensitivity at an angle of
incidence of zero degrees (perpendicular to the sensor surface) and zero
sensitivity at an angle of incidence of 90 degrees (radiation passing over the
sensor surface). At any angle between 0 and 90 degrees the sensitivity should
be proportional to the cosine of the angle of incidence.
FIGURE 5-2 shows the behavior of a typical net radiometer. The vertical axis
shows the deviation from ideal behavior, expressed in percentage deviation
from the ideal value.
9