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This review is divided into seven sections entitled "Introduction", "Basic theoretical framework", "Readily available materials", "Applications", "Performance and limitations of narrow band interference filters", "Problems in the preparation of optical coatings" and "Probable future developments". In the introductory section the scope of the review is presented and a survey is given of already published reviews which are relevant to the subject. A brief history of the subject is also presented and the reasons for continuing interest in it are discussed. The introduction is concluded by mention of problems which are currently engaging the attention of workers in the field. In the next section the matrix theory relating the optical properties of single or multilayer dielectric thin films to the basic film parameters (refractive index and thickness) is developed with particular reference to the assumptions that are made in establishing this theoretical framework. It is emphasized that the conditions implied in these assumptions are not always satisfied in practice, and that differences between observed and predicted optical performance are therefore to be expected. In addition, the basic matrix formulation is used to derive other useful expressions for the optical properties of multilayers, e.g. the reiterative amplitude reflectance formula usually associated with multiple-beam calculations, the generalized Airy formula associated with the `two effective interfaces' approach, and the method of equivalent layers for symmetrical systems. The calculation of Poynting flux and the concept of `potential transmittance' are also discussed. The section on readily available materials consists basically of a table listing the relevant physical properties of commercially available materials suitable for the vacuum deposition of dielectric thin films. A brief description of such practical detail is necessary as a prelude to the following section. In the section on applications, details are given of the design and application of filters consisting of single or multilayer dielectric thin films. The term `filters' is used here in its widest sense and the term includes antireflection coatings, mirrors, edge filters, beam splitters and narrow band filters. Magneto-optical applications of dielectric films are also described briefly. The fifth section is devoted to a more detailed examination of the properties of narrow band interference filters, specifically their performance and limitations. This choice of emphasis is made for two reasons. Firstly, the advantage of narrow band interference filters over other means of spectral selection such as prism or grating monochromators is marked and can be discussed on a quantitative basis. Secondly, the structural `defects' of layers which afflict all thin film devices produce the most easily measurable effects in narrow band filters. These effects are discussed under the subheading "Limitations". The section on problems in the preparation of optical coatings is concerned with the precise control of the optical thickness of individual films, since this is usually crucial to the performance of the complete filter. This subject is divided into two separate parts, one concerned with the uniformity of thickness of the film over a relatively large substrate area, and the other with the control of optical thickness during the deposition of the film, commonly referred to as `thickness monitoring'. In both cases the limitations of the available techniques are emphasized. The review is concluded with comments on probable future developments of the subject. |
