Detection in optical measurement is a very important step in the measurement process. A detector converts incident optical energy into electrical energy, which is then measured. When the reverse bias is applied, the junction capacitance is reduced, and the junction stays nonconductive. The reverse-bias junction, however, can conduct current when free carriers are introduced into it. Detectors based on the photo-excitation or photo-emission of electrons is by far the most sensitive and provides the highest performance. The electric field modulates the refractive index through the electro-optic effect and creates a volume phase grating. These are real-time recording materials; the records are stable, since the charges are bound to the localized traps. A thermoplastic is a multilayer structure having a substrate coated with a conducting layer, a photoconductor layer, and a thermoplastic layer. The thermoplastics for holographic recording combine the advantages of high sensitivity and resolution, dry and nearly instantaneous in situ development, erasability, and high readout efficiency.