Calcite is capable of acting as a polarizing light filter. The core element is the polarizing light filter. Wood's glass was developed as a light filter used in communications during World War I. This involves placing a "CT" light filter in front of a light source. Indium tin oxide is used as a light filter in low-pressure sodium-vapor lamps. It also features a blue light filter for low light night reading. In Shakespearean-era theater, red wine was used in a glass container as a light filter. For example, a blue light filter appears blue because it absorbs red and green wavelengths.