Liquid crystal tunable filters (LCTFs) are optical filters that use electronically controlled liquid crystal (LC) elements to transmit a selectable wavelength of light and exclude others. Often, the basic working principle is based on the Lyot filter but many other designs are used[1]. The main difference with the original Lyot filter is that the fixed wave plates are replaced by switchable liquid crystal wave plates.
LCTFs are known for very high image quality and relatively easy integration with regard to optical system design and software control but relatively low peak transmission values due to the use of multiple polarizing elements. This can be mitigated in some instances by using wider bandpass designs, since a wider bandpass results in more light traveling through the filter. Some LCTFs are limited to a small number of fixed wavelengths such as the red, green, and blue (RGB) colors while others can be tuned in small increments over a wide range of wavelengths such as the visible or near-infrared spectrum from 400 to the current limit of 2450 nm. The tuning speed of LCTFs varies by manufacturer and design, but is generally several tens of milliseconds, mainly determined by the switching speed of the liquid crystal devices.
LCTFs are often used in multispectral imaging or hyperspectral imaging systems because of their high image quality and rapid tuning over a broad spectral range.[2][3][4]
Another type of solid-state tunable filter is the Acousto Optic Tunable Filter (AOTF), based on the principles of the acousto-optic modulator. In comparison to LCTFs, AOTFs enjoy a much faster tuning speed (microseconds versus milliseconds) and broader wavelength ranges. However, since they rely on the acousto-optic effect of sound waves to diffract and shift the frequency of light, imaging quality is comparatively poor, and the optical design requirements are more stringent. Indeed, LCTFs are capable of diffraction-limited imaging onto high-resolution imaging sensors. AOTFs have smaller apertures and have narrower angle-of-acceptance specifications compared with LCTFs which can have working aperture sizes exceeding 35 mm and can be placed into positions where light rays travel through the filter at angles up to 15 degrees from the normal.[5][6]
LCTFs have been utilized for space and airborne imaging.[7][8] Their light weight and low power requirements make them good candidates for remote-sensing applications. They can be found integrated into compact but high-performance scientific digital imaging cameras as well as industrial- and military-grade instruments (multispectral and high-resolution color imaging systems).[9] LCTFs can have a long lifespan, usually many years. Environmental factors that can ‘age’ filters are extended exposure to high heat and humidity, thermal and/or mechanical shock (most, but not all, LCTFs utilize glass as the principal base material), and long-term exposure to high photonic energy such as ultraviolet light which can photobleach some of the materials used to construct the filters.
Recent advances in miniaturized electronic driver circuitry have reduced the size requirement of LCTF enclosures without sacrificing large working aperture sizes.
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