Polar crystals display temporary voltage and consequently an external current when exposed to a temperature gradient.
The setup for measurement of the pyroelectric current is by a periodic temperature change technique with 5.3W/cm2 heat flux produced by an 800mW TTL-modulated laser operating at λ=1.38μm wavelengths. The temperature oscillation amplitude at the surface of the crystal did not exceed ±3.5K. The measurements were performed in a Faraday chamber with a window for the laser beam and light absorbing. When the laser is on the rapid change of temperature leads to the formation of electric current and when the laser is off there is no current:
Fig  I. Lubomirsky, O. Stafsudd, Rev Sci Instrum 2012, 83.