Anti-Reflective Coatings

Prepared by: B. Serhat Cengiz

Antireflective or anti-reflective (AR) coating is a type of optical coating applied to the surface of lenses and other optical elements to reduce reflection. In typical imaging systems, it increases efficiency as less light is lost. In complex systems such as telescopes, the reduction in reflections increases the contrast of the image by eliminating stray light. This is particularly important in planetary astronomy. In other applications, the primary benefit is the elimination of the reflection itself, such as a coating on spectacle lenses that makes the user's eye more visible to others, or a coating to reduce the glint of a hidden viewer's binoculars or telescopic image. Many coatings consist of transparent thin-film structures with successive layers of opposite refractive index. Layer thicknesses are chosen to produce destructive interference in beams reflected from the interfaces and constructive interference in corresponding transmitted beams. This makes the structure's performance vary with wavelength and angle of incidence, so color effects are usually visible at oblique angles. When designing or ordering these coatings, a wavelength range must be specified, but good performance can be achieved over a relatively wide frequency range, typically an IR, visible or UV option is offered.

Applications

Antireflective coatings are used in a wide variety of applications where light passes through an optical surface and low loss or low reflection is desired. Examples include glare-reducing coatings on corrective lenses and camera lens elements and antireflective coatings on solar cells.

Antireflective coated glass shown at 45° and 0° angles.

Corrective Lenses

Opticians may recommend "antireflective lenses" because reduced reflection improves the aesthetic appearance of lenses. Such lenses are often said to reduce glare, but the reduction is minimal. Eliminating reflections allows slightly more light to pass through, providing a slight increase in contrast and visual acuity. Antireflective ophthalmic lenses should not be confused with polarized lenses that reduce the visible glare of sunlight reflected from surfaces such as sand, water and roads (through absorption). The term "antireflective" refers to reflections from the lens surface, not to the source of light from the lens itself. Many antireflective lenses include an additional coating that repels water and grease and makes them easier to keep clean. Anti-reflective coatings are particularly suitable for high-index lenses, because these, without coating, reflect more light than a lower-power objective (as a result of Fresnel equations). Coating high-index lenses is usually easier and less expensive. Antireflective coatings are commonly used in microelectronic photolithography to help reduce image distortions associated with reflections at the substrate surface. Different types of antireflective coatings are applied before or after photoresist and help reduce standing waves, thin-film mixing and specular reflections.

References • Hecht, E. (1987). Optics (2nd ed.). Addison–Wesley. • "Thin-film spatial filters," (PDF). Retrieved 2007-05-30. • "Market Report: Global Optical Coatings Market". Acmite Market Intelligence.