Liquid crystal display (LCD) modules are used at the component level in place of less efficient displays such as cathode ray tubes (CRTs). These modules do not include a housing and must be incorporated into a larger instrument or system. Liquid crystal displays (LCD) belong to the class of nonemissive technologies, or displays that do not generate light of their own but control the transmission or reflection of an external light source. A liquid crystal display utilizes two sheets of polarizing material with a liquid crystal solution between them. When an electric current is passed through the liquid it causes the crystals to align so that light cannot pass through them. Each crystal acts as a shutter, either allowing light to pass through or blocking the light.  LCDs alter the optical path of light when an electric field is placed across the liquid crystal material.