Due to Fresnel reflection, when light passes from air through an uncoated glass substrate, approximately 4% of the light will be reflected at each interface. This results in a total transmittance of only 92% of the incident light, which can be extremely harmful in many applications. Excessive reflected laser light reduces flux and can lead to laser-induced damage. Transmittance-enhancing (AR) films are applied to optical surfaces to improve the flux of the system and reduce the harm caused by reflections propagating backward in the system and creating ghosting. AR coatings are particularly important for systems containing multiple transmitting optical elements. Many low-light systems use transmission-enhancing film optics to allow efficient use of light.
AR coatings are designed so that the relative phase shift is between the beam reflections at the upper and lower boundaries of the film at 180 degrees. Destructive interference occurs between the two reflected beams and it cancels them out before they leave the surface. The optical thickness of the optical coating must be an odd multiple of λ/4, where λ is the design wavelength or peak performance wavelength optimized to achieve the required path difference between the reflected beams at λ/2 wavelengths. Achieving this difference will result in an offset of the beam. The refractive index of the film (nf) required to completely cancel the reflected beam can be found using the refractive index of the incident medium (n0) and the substrate (ns).
