The fundamental principle behind a pyroelectric detector lies in the pyroelectric effect. This effect occurs in certain crystalline materials that possess a natural, or “spontaneous,” electric polarization. This means that even in the absence of an external electric field, the material has an inherent alignment of electric dipoles, resulting in a net electric dipole moment per unit volume.
Crucially, the magnitude of this spontaneous polarization is temperature-dependent. When the temperature of the pyroelectric material changes, the degree of internal polarization changes accordingly. This change in polarization manifests as an accumulation of electric charge on the opposing surfaces of the crystal, perpendicular to the axis of polarization.
If electrodes are placed on these surfaces, the change in surface charge induces a measurable voltage across them or, if connected in a circuit, causes a current to flow. The key aspect is that the sensor responds to the rate of change of temperature (dT/dt), not the absolute temperature itself. When infrared radiation strikes the sensor, it heats the pyroelectric material. If the IR source is moving or fluctuating, it causes continuous temperature changes, leading to a continuous electrical signal (usually alternating). If the temperature stabilizes, the polarization reaches a new equilibrium, and the charge dissipates (often through internal leakage or the external circuit), causing the signal to disappear. This is why pyroelectric detectors are excellent for detecting moving warm objects but not for measuring constant temperatures accurately.