band pass filter main function is to make the pass of light of a specific wavelength, let the other wavelengths of the light reflection ( or attenuation) of optical components. Half wave width is usually controlled more than 20nm, ultraviolet, visible light, near infrared wavelength, far infrared wave band . My company's narrow bandpass filter using multilayer epidural by ion assisted deposition of nanometer materials and high vacuum evaporation coating, good compactness, high imaging definition.