Liquid crystal tunable filter
Encyclopedia
Liquid crystal tunable filters (LCTFs) are solid-state optical filters that use electronically controlled liquid crystal
(LC) elements to transmit a selectable wavelength of light and exclude others. 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 in the few dozen millisecond range.
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.
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.
LCTFs have been utilized for space and airborne imaging. 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). LCTFs can have an extremely 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.
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
(LC) elements to transmit a selectable wavelength of light and exclude others. 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
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
from 400 to the current limit of 2450 nm. The tuning speed of LCTFs varies by manufacturer and design, but is generally in the few dozen millisecond range.
LCTFs are often used in multispectral imaging or hyperspectral imaging
Hyperspectral imaging
Hyperspectral imaging collects and processes information from across the electromagnetic spectrum. Much as the human eye sees visible light in three bands , spectral imaging divides the spectrum into many more bands...
systems because of their high image quality and rapid tuning over a broad spectral range.
Another type of solid-state tunable filter is the Acousto Optic Tunable Filter (AOTF), based on the principles of the acousto-optic modulator
Acousto-optic modulator
An acousto-optic modulator , also called a Bragg cell, uses the acousto-optic effect to diffract and shift the frequency of light using sound waves . They are used in lasers for Q-switching, telecommunications for signal modulation, and in spectroscopy for frequency control. A piezoelectric...
. 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.
LCTFs have been utilized for space and airborne imaging. 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). LCTFs can have an extremely 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
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
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.
Applications
- Forensic science
- Machine visionMachine visionMachine vision is the process of applying a range of technologies and methods to provide imaging-based automatic inspection, process control and robot guidance in industrial applications. While the scope of MV is broad and a comprehensive definition is difficult to distil, a "generally accepted...
- Optical microscopy
- Preclinical imagingPreclinical imagingPreclinical imaging is the visualization of animals for research purposes, such as drug development. Imaging modalities have long been crucial to the researcher in observing changes, either at the organ, tissue, cell, or molecular level, in animals responding to physiological or environmental changes...
- Remote sensingRemote sensingRemote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
External links
Informative sites or links- Olympus Microscopy Resource Center (a web resource for researchers who use microscopy)