There has been considerable interest in recent years in the development of electronically tunable optical spectral filters. The applications for these filters range from rapid -scan spectrometry to multispectral imaging in remote sensor systems. The requirements on the characteristics of these filters include: moderate spectral resolution and high out -of -band rejection large field of view large optical aperture and high optical transmission electronic tunability with wide tuning range and rapid tuning rate.Practical realization of tunable spectral filters that meet these requirements is an extremely difficult task and may take many years. There has been, however, significant progress in recent years in the tunable filter technology. This special issue of Optical Engineering is devoted to this timely subject. The five papers gathered in this issue are intended to provide a good introduction to the theory and practice of tunable spectral filters and to give the reader a feeling for the present state of the art.The basic approach to the realization of tunable spectral filters has been the application of suitable spectral tuning techniques to existing fixed filter structures. The mechanisms of tuning include electromechanical, acousto -optical, and electro-optical. The two most useful forms of filter structures are Fabry -Perot filters and traveling wave birefringent filters.In the first paper of this issue, Atherton and co-workers present a theoretical discussion on the operating principles and performance limitations of Fabry -Perot filters. Methods of (electromechanical) tuning are discussed. Present state of the art of tunable Fabry -Perot filters is described.In the second paper, Title and Rosenberg review the principles and capabilities of birefringent filters. In this type of filter structure, two co-directionally propagating waves of different polarization are caused to interfere through periodic modulation of birefringence. Three types of birefringent filters are described: Lyot, partial polarizing, and Solc. Tuning methods for the birefringent filters are presented, followed by the discussion of the important issues of field of view and throughput and a brief summary of the state of the art in birefringent filters.The first two papers discuss two filter structures: the Fabry -Perot and birefringent filters, respectively. In both cases the tuning mechanism is achieved electromechanically. Strictly speaking, the tuning is mechanical in nature. These filters are thus susceptible to difficulties commonly associated with mechanically tuned devices, such as fabrication and temperature control. True electronically tuned optical spec-tral filters with no moving parts have been demonstrated using acousto -optical and electro-optical tuning techniques. Such filters are described in the remaining three papers.In the third paper, I review the theory and practice of acousto -optic tunable filters (AOTF). The operating principle of this type of filter is similar to the Solc filter using an acousto -optic effect. The pe...

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