Filter

1. Filters are used to regulate the wavelength of the radiation and they are placed in front of the lens. Absorption filters are common (absorption = light being "absorbed" into matter). For example, a yellow filter absorbs the blue color, letting green and red colors through (with lights, green + red = yellow)
   
   
2. Filter is a device that lets through only a part of the information, matter or energy received.

Cf. Optics