Introduction
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A comprehensive survey of all optical components currently in use would reveal that the vast majority are made of various types of glass. This survey would also reveal that a majority of these optics are coated with thin layers of material(s) different from the substrate. The purpose of these coatings is to modify the reflection and transmission properties at the surface of the optical element. Whenever light passes from one medium into a medium of different optical properties (most notably refractive index), part of the light (between 0% and 100%) is reflected and part of the light (between 100% and 0%) is transmitted. The intensity ratio of reflected and transmitted components is primarily a function of the difference in refractive index and the angle of incidence. For many uncoated optical glasses, reflected light typically represents a few percent of incident radiation. For designs using more than a few components, losses in transmitted light level can accumulate rapidly. More important are corresponding losses in image contrast or modulation caused by weakly reflected ghost images superimposed on the desired image. Such unwanted images are often defocused beyond recognition so that contrast reduction (rather than image confusion) is their primary effect. Applications generally require that the reflected portion of incident light approach 0% for transmitting optics (lenses) and 100% for reflective optics (mirrors), or is at some fixed intermediate value for partial reflectors (beamsplitters). The only applications that do not require coated optics involve transmitting optics in which only a few surfaces are in the optical path, where transmission inefficiencies may be tolerable. In principle, the surface of any optical element can be coated with thin layers of various materials (called thin films) in order to ensure the desired reflection/transmission ratio. Unfortunately, with the exception of simple metallic coatings, this ratio depends on the nature of the material from which the optic is fabricated, as well as the wavelength and angle of incidence. There is also a polarization dependence to this ratio when the angle of incidence is not 0 degrees. A multilayer coating (sometimes more than 100 individual layers) can optimize the reflection/transmission ratio for several sets of conditions (wavelength and angle of incidence) or optimize it over a particular range of conditions. Melles Griot is the leading supplier of precision simple optics. Because optics for most applications require a coating of some sort, it would not have been possible to achieve this market-leading position without our extensive knowledge of thin-film coatings. With the state-of-the-art coating department located in Carlsbad, California, as well as other coating facilities in Japan; Rochester, New York; and the British Isles, Melles Griot is able not only to coat large volumes of catalog and special optics, but also to develop and evaluate new coatings for special customer requirements. With new and expanded coating capabilities, Melles Griot now offers the same high-quality coatings as a separate service to customers wishing to supply their own substrates. As with any special or OEM order, please contact Melles Griot to discuss your requirements with one of our qualified applications engineers. Today, dielectric coatings are remarkably hard and durable. With proper care and handling, they can have a long life. In fact, the surface of many high-index glasses that are prone to staining can be protected with a durable antireflection coating. Several factors influence coating durability. Coating designs should be optimized for minimal overall thickness to reduce mechanical stress. The most resilient materials should be used. Great care should be taken in coating fabrication to ensure high-quality, nongranular, even layers. Although we cannot prevent accidental abuse of coated optics, Melles Griot concentrates on these other factors to produce coatings that are as durable as possible. Although the Melles Griot optical-coating departments have many years of experience in designing and fabricating various types of dielectric and metallic coatings, the science of thin films is still developing rapidly. Melles Griot monitors and incorporates new technology so that we are always able to offer the most advanced coatings available. The Melles Griot range of coatings currently includes antireflection, metallic reflectors, all-dielectric reflectors, hybrid reflectors, partial reflectors (beamsplitters), and filters for monochromatic, dichroic, or broadband applications. Many of the coatings can be applied to the simple optics available from the Melles Griot catalog; some coatings can be applied only to a specific range of products; and some of the coatings are supplied only as an integral part of a specific product (e.g., cube beamsplitters). If you require a special coating not described in this section, please contact a Melles Griot applications engineer to discuss our special coating design services. |
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