This pyrheliometer has a rectangular aperture, two manganin-strip sensors (20.0 mm × 2.0 mm ×
0.02 mm) and several diaphragms to let only direct sunlight reach the sensor. The diaphragms are the
same as those in the silver-disk pyrheliometer in Figure 7.7 and in the thermoelectric pyrheliometer in
Figure 7.8. The sensor surface is painted optical black and has uniform absorption characteristics for
short-wave radiation. A copper-constantan thermocouple is attached to the rear of each sensor strip, and
the thermocouple is connected to a galvanometer. The sensor strips also work as electric resistors and
generate heat when a current flows across them (see the principle drawing in Figure 7.6 (b)).