There are some particular crystals showing change in refractive indices directly proportional to applied electric field. The phenomenon is called linear electro-optic effect or Pockel effect. Electrically induced birefringence causes plane polarized light propagating through the crystal to be resolved into two orthogonal vectors with change in retardation between them proportional to the magnitude of electric field (applied voltage).

The Pockel cell consists of examined LiNO₃ crystal placed in an insulated container with two electrodes attached to supply necessary electric field. The cell is designed to supply a very uniform electric field along the crystal when a potential difference is applied. 650nm diode laser is used as the source.