Practical aspects of thin film coating technology are reviewed. These include vacuum requirements, evaporation sources, substrate and thin film materials, and film thickness monitoring and uniformity. The influence of these practicalities on the implementation of coating designs is discussed.
A general overview is given, starting with the classification of coatings into six categories. Each of the categories is then treated by the vector model approach to show how it works. Performance, bandwidth and thickness tolerances are seen to be straightforward results of design approaches.
A conventional recording spectroradiometer can be used in several ways to find the spectral reflectance and transmittance of an optical filter. Care must be taken to compensate for systematic errors introduced by the partial polarization and angular divergence of the spectroradiometer beam. Corrections must also be made for the instrumental bandwidth, spurious reflections, beam walk-off and collimation.
The ultimate performance of optical interference filters is often limited by absorption and scattering losses in the coating materials. A basic understanding of these loss mechanisms is necessary to place realistic specifications on a given optical coating. This paper reviews the theories of absorption and scattering as they relate to optical thin films and stresses those aspects most relevant to the optical systems design engineer. Several methods are currently used to measure the absorption and scattering characteristics of substrates and coating materials. These techniques are discussed in sufficient detail to aid in the determination of which method is applicable to a particular problem.
This paper describes various types of anti-reflection coatings, how they work, and their use in optical systems. Examples are given of design types available for the visible spectrum. The use of anti-reflection coatings at non-normal incidence is discussed and practical limitations considered. A brief review of the state-of-the-art in wideband anti-reflection coatings is presented.
There are an increasing number of applications requiring the isolation of spectral bands ranging in width from less than 0.1 nm to greater than 50 nm. In general the interference filter is a simple device for performing this isolation function. Manufacturers providing these filters offer a broad choice of band position, band width, band shape, degree of off-band rejection and free filter range. It is the purpose of this paper, following an outline of the theory, to present the optical characteristics of visible and ultraviolet bandpass filters. Starting with the narrowest single cavity filter, the discussion progresses to multi-cavity designs with their improved passband shapes. Additional elements necessary to control the free filter range and the degree of rejection at wavelengths outside the passband are also considered. The presentation places emphasis on the performance of filters satisfying typical requirements such as the detection of an emission line in the presence of a strong continuum.
This paper addresses the following aspects of 2-15 μm infrared coating technology: theoretical design and analysis, properties of film and substrate materials, spectrophotometric measurement techniques, and examples of principle coating designs. Theoretical investigations are carried out using a self-optimizing multilayer thin film design program as well as classical Herpin design techniques. Optimization examples are given to illustrate the results. It is shown how the computer can be used to predict the degree of difficulty in manufacturing some types of infrared coating designs. Examples are given showing how the performance of bandpass filter designs is influenced by the f-number of the incident light beam and absorption in the film layers. Our principle instrument for infrared measurements is the Perkin-Elmer 180 Spectrophotometer, and we outline the techniques used for low temperature measurements, specular reflectance measurements, polarization measurements, and ATR measurements of our evaporated coatings. Finally, a representative selection of coatings spanning the 2-15 μm range is presented and discussed, including narrow and broadband anti-reflection coatings, bandpass filters, short and long wave pass filters, laser reflectors, and black mirrors.
The design of thin film polarizing devices is reviewed with emphasis on biprism and front surface polarizers. Particular applications of each type are outlined. Information is provided on the angular res-ponse characteristics of the devices and potential systems applications are discussed.
A design approach is discussed for color separation in an hypothetical three color TV camera with specified colorimetry characteristics. Some possible optical layouts to achieve this separation are presented together with their relative advantages and disadvantages. The types of multilayer interference filter designs which would be used in a system of this type are presented and the ways in which filter characteristics may be changed are discussed. A graphical presentation is made of the polarization effects and color shifts associated with different types of optical separation systems and with different f-numbers. Consideration is given to some of the subtleties which must be taken into account in an actual system which are not covered in this simplified model.
The visual circular variable filter (VCVF) is introduced. A typical filter is described with layer thickness that varies linearly along a circular path. Typical spectral results are presented. Bandwidths of 10 to 50 nm and peak transmittances of 15 to 50 percent are possible. Manufacturing problems are considered, and a technique is described that utilizes two rotating masks between the evaporation source and the substrate to produce layer thickness variations. Uses and advantages of VCVF's are also presented. Typical applications include instruments such as colorimeters, spectrometers, and water pollution analyzers.
For many technological applications filters with rather complicated and irregular spectral transmittance curves are required. A number of numerical approaches to the design of all-dielectric interference filters with such characteristics have been described in the past. A brief outline is given of two different automatic thin film synthesis programs developed for this purpose by the author. With these it is possible to design filters with any desired spectral transmittance curve, providing that the range over which the latter is designed is not excessive. The resulting filter designs call for thin films whose refractive indexes are intermediate to those of known materials, and whose thicknesses may assume any value. A brief des-cription of the apparatus and procedures suitable for the experimental realization of such filters is given. The results obtained for a number of prototype filters are discussed.
This paper discusses a novel method for including a 100 Å bandpass filter in an f/l optical system. The system is a gimbaled optical system comprising a video tracker and a laser spot tracker operating at f/l. Conventional spectral beam splitting of these two trackers would be done with a dichroic mirror, resulting in the laser narrow bandpass filter being placed in the f/l converging beam. Because of angle shift characteristics of inter-ference filters, this would require a minimum half power bandwidth of no less than 350 Å. The author shows that a novel filter which is both a bandpass at the laser wavelength and a broad band transmitter at the vidicon response wavelengths can be placed in the entrance pupil of the system. This reduces the incidence angles to a minimum and allows half power bandwidths of less than 100 Å.
The renewed interest in solar energy as a partial solution to the problem of supplying the future energy needs of the United States and the world has been sparked, in part, by advances in optical coating technology. In particular, thermal and direct conversion approaches are strongly dependent on coatings for optimum operational characteristics. Owing to the dilute nature of solar energy available on the earth's surface, the required collecting area is likely to be very large. The variety of solar energy conversion methods and applications currently being considered is reviewed. The role and rationale for coatings in the systems lead to coating requirements. A hypothetical model of solar energy implementation is presented, leading to a forecast of future requirements for the solar coating industry. Comparisons are made between 1973 production and future requirements.
Whether intended for use in laboratory optical systems, military, or space applications, most coated optical elements must adhere to some level of quality assurance standards or specifications. The parameters to be specified and test procedures for those parameters are presented. The emphasis is on spectral, environmental, and durability specifications. Military standards and specifications which apply to coating quality levels are discussed. Topics covered include the types of equipment and facilities required to evaluate coating performance spectrally and environmentally. Also included is a brief discussion of special requirements which may be devised for special coating applications and the associated special test procedures necessary. Finally, the problems associated with applying the military specifications and tests designed for visual optics to current state-of-the-art infrared optical components are presented. The need for new requirements and test procedures is discussed.
Requirements for new interference coating that will be needed in the next five years are presented. The emphasis is on coatings and filters for systems now being desgined but for which the manufacturing technology is not available. The topics covered are wide-band anti-reflection coatings, bandpass filters, beamsplitters, wide-band protected mirrors, high efficiency mirrors, dark mirrors, and coatings deposited directly upon detectors. In each case the requirements envisioned at present for transmittance, reflectance, absorptance, and environmental resistance are preliminary.
Optically flat pellicles of parylene C (a chlorine-substituted member of the family of polymers based on paraxylylene) are attacked on both sides when exposed to fluorine vapor. The fluorinated surface layers so formed have higher specific gravity, lower refractive index, and significantly lower specific refraction than the original material. The demarcations between the two fluorinated surface layers and virgin parylene C are sharp as evidenced by the pronounced modulation of the channelled spectrum in transmission of the thin films. The interference patterns produced by the partially-fluorinated parylene films are shown to be consistent with theory for a symmetrical low-high-low multilayer structure. Measurements of the wavenumbers and relative amplitudes of the successive loops and nodes of the interference spectra (over the range 0.3 to 30 μm) on pellicles at various stages of fluorination have been used to derive the optical properties of the surface layers and to monitor the progress of the chemical reaction.
The need for protective coatings on critical optical surfaces, such as halide crystal windows or lenses used in spectroscopy, has long been recognized. Many widely used halide materials are extremely moisture sensitive, such as sodium chloride and cesium iodide, and a number of approaches have been taken to protect such optics. These approaches, often crude, have not always been reliable or particularly convenient. It has been demonstrated that thin, one micron, organic coatings produced by polymerization of flourinated monomers in low temperature gas discharge (plasma) exhibit very high degrees of moisture resistence, e.g., hundreds of hours protection for cesium iodide vs. minutes before degradation sets in for untreated surfaces. Moreover, the index of refraction of these coatings is intermediate between that of the halide substrate and air, a condition for anti-reflection, another desirable property of optical coatings. Thus, the organic coatings not only offer protection, but improved transmittance as well. Further, the polymer coating is non-absorbing over the range 0.4 to 40 μ with an exception at 8.0 μ, the expected absorption for C-F bonds.