Anti-Reflection Coatings

All uncoated optical elements exhibit some reflection at each air-to-surface interface. The amount of reflected energy is a function of the index of refraction (ns) of the optic. In many applications, this reflected energy is not a significant factor affecting the performance of the optical system. However, in optical systems where image quality must be maintained or where limited energy must be conserved, the reflection loss from each element becomes significant.

As an integral part of our Optical Filter manufacturing technology, we have developed a series of damage-resistant anti-reflection coatings for use in applications where surface reflection from all elements must be minimized. Our anti-reflection (AR) coatings meet or exceed the durability requirements of MIL-C-675 and MIL-C-48497 and are manufactured to withstand energy levels typically found in laser-based analytical instrumentation. Coatings for special applications where high-power lasers are required are also available.

Assessing the need for AR Coatings

Determining whether AR coatings are required for a particular element or application is dependent upon several factors. Many optical elements, filters in particular, are manufactured with external coatings. These coatings are typically matched to air, thereby performing the function of an AR coating. Optical elements being used in an imaging system may benefit from the addition of an AR coating if the surfaces are not already coated, to minimize ghost images or glare at the focal plane. Similarly, elements being used in a complex system or a system that will be processing a very weak signal could also benefit from the addition of AR coatings.

Although the designs of anti-reflection coatings are as diverse as their applications, they generally fall into three basic categories:

Anti-Reflection Coating Categories

SLAR - Single Layer Anti-Reflection Coatings

Single Layer AR coatings are typically used when there are not many optical elements in a system and preservation of initial light levels is not a prime consideration. In narrowband optical filter construction, they can be used to increase the transmission of the filter if the external surfaces of the filter are not already coated and matched to air as part of the filter design. These coatings are characterized by a relatively broad spectrum of moderate anti-reflection, and relative insensitivity to off-normal incident light.

BBAR - Broad Band Anti-Reflection Coatings

Broad Band AR coatings are composed of many layers of dielectric materials and are typically used in systems that require the maximum efficiency of transmitted light over a broad spectrum of wavelengths. These coatings are somewhat more sensitive to off-normal incident light than are SLAR coatings.

NVAR - Narrow Band (v-Coat) Anti-Reflection Coating

V-Coat anti-reflection coatings take their name from the shape of the reflectance curves resulting from these coatings - a narrowly defined V-shape. Coatings of this type result in the least amount of reflection from a surface. The trade-off for the extreme anti-reflection efficiency is a reduction in the bandwidth of extreme low reflection. These coatings are commonly used for laser applications.