Special Optical Coatings
Precision optical components can be fabricated from many different materials, from crown or ordinary window glass to fused quartz to sapphire to mirrors, to meet the requirements of specialized optical applications. Sometimes, however, there is no single material capable of performing the desired function, so designers must turn to special optical coatings that enhance the capability of the substrate to which they are applied. Here are just a few examples.
These special optical coatings can be applied, usually as a single layer, to the surface of a lens or other optical device to reduce reflection, increasing the efficiency of the system by reducing loss of light. In complex systems such as telescopes, for example, the reduction in reflections also improves the contrast of the image by eliminating stray light from sources other than the intended source.
In other applications, the primary benefit is the elimination of the reflection itself, such as the potentially dangerous glint from a covert viewer’s binoculars or telescopic sight. Anti-reflective coatings are also used in printed circuit fabrication to help reduce image distortions associated with reflections off the surface of the substrate during microelectronic photolithography.
Narrowband “V” coatings
Generally the simplest of all multilayer anti-reflective systems, these special optical coatings are designed to reflect only a certain narrow range of wavelengths, or even a single wavelength, while transmitting all others. Commonly referred to as “V” optical coatings because the curve produced by plotting reluctance versus wavelength approximates the shape of a V with a rounded bottom, they serve to optimize the transfer of energy over a very narrow range while preventing energy transfer over wavelengths outside that range.
Common applications include fluorescence microscopy, where incident light of a single wavelength can be used to excite a specific type of molecular bond, usually within the molecular structure of a biological organism. The bond, in turn, then emits light of a different wavelength as it relaxes, in a process called “fluorescence,” making otherwise invisible details of the molecular structure visible to an observer through the lens of a microscope. These special optical coatings can also be used in single-frequency lasers designed for highly specialized applications.
Broadband AR (BBAR) Special Optical Coatings
Optimizing the transfer of energy at a single wavelength, or even over a very narrow range of wavelengths, is relatively simple. However, as the required bandwidth, or range of wavelengths, increases, the task becomes more difficult. More optical coatings layers, applied in the form of a stack, are required in order to maintain optimized energy transfer across the whole band. Plotting reluctance versus wavelength of these coatings generally produces a wider curve with a flat bottom as opposed to aV-shaped curve with a rounded bottom.
Simple high-reflection coatings function essentially as mirrors, reflecting virtually all incident light and transmitting almost none. Like narrowband and broadband coatings, they can be selected to reflect only certain wavelengths while transmitting others.
These types of special optical coatings can be divided into two separate categories – cold mirrors, which reflect visible but not infrared light, andhot mirrors, which reflect both visible and infrared.
Beam Splitter Coatings
A beam splitter is an optical device that splits a beam of light in two. In its most common form, a cube, it is made from two triangular glass prisms that are glued together at their base. Each beam splitter is designed to be used with only one specific wavelength or a specific range of wavelengths of incident light. One of the prism faces is coated before cementing to allow half of the light entering the cube through one face to be transmitted and the other half to be reflected, splitting the beam in two.
Another design is the use of a “half-silvered” mirror, a sheet of glass or plastic with a transparently thin coating of metal, usually aluminum deposited from aluminum vapor. The thickness of the deposit is controlled so that half of the light incident at a 45-degree angle and not absorbed by the coating is transmitted and the remainder is reflected.
Instead of a metallic coating, a dichroic optical coatings, capable of a beam splitter of visible light into distinct beams of different wavelengths, or colors, may be used. Such a device is called a dichroic mirror. Depending on its characteristics, the ratio of reflection to transmission will vary as a function of the wavelength of the incident light.
A third version of the beam splitter is a dichroic mirrored prism assembly that uses dichroic optical coatings to divide an incoming light beam into a number of spectrally distinct output beams. Such a device was used in three-pickup-tube color television cameras and the three-strip Technicolor movie camera. It is currently used in modern three-CCD cameras. An optically similar system is used in reverse as a beam-combiner in three-LCD projectors, in which light from three separate monochrome LCD displays is combined into a single full-color image for projection.