handbook of modern sensors

References 441
housing assures wide angles of view in both horizontal and vertical planes (Fig.
14.30C). The device needs high voltage for operation, and under normal conditions,
it is not electrically conductive. Upon being exposed to a flame, the high-energy
UV photons strike the cathode, releasing free electrons to the gas-filled tube interior.
Gas atoms receive an energy burst from the emitted electrons, which results in gas
luminescence in the UV spectral range. This, in turn, cause more electrons to be
emitted, which cause more UV luminescence. Thus, the element develops a fast
avalanche-type electron multiplication, making the anode–cathode region electrically
conductive. Hence, upon being exposed to a gas flame, the element works as a current
switch, producing a strong positive voltage spike at its output (Fig. 14.30B). It follows
from the above description that the element generates UV radiation in response to
flame detection. Albeit being of a low intensity, the UV does not present harm to
people; however, it may lead to cross-talk between similar neighboring sensors.
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