Coating is a bandpass filter that allows light to pass through a specific wavelength of light reflection and let the other band (or attenuation) of the optical element. Bandpass filter work area may be ultraviolet band, visible light, near-infrared light, far infrared light, depending on your range of application. And generally through cut-off filter, or a color filter as compared to the bandpass filter design and production are more difficult, mainly because the position of the spectral peaks at both ends require more precise control. The control points of the position, of course a little more difficult than high up. Bandpass filter is a relatively narrow bandwidth, short-wave and long-wave significantly cut glass, its spectral characteristics of the maximum transmittance (TM) and wavelength (λM) position, the short cut-off wavelength, long-wave cut-off wavelength and half width to represent. The half of the wavelength λ1 corresponding to the maximum transmittance and the difference λ2 (λ1-λ1) nm called half width FWHM