Liquid crystal tunable filter

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LCTFs circa 2014 with integrated circuitry for control and power (left), or an older model with a separate electronics controller box and thick, shielded cable (right).

A liquid crystal tunable filter (LCTF) is an optical filter that uses electronically controlled

wave plates
are replaced by switchable liquid crystal wave plates.

Optical systems

LCTFs enable high image quality and allowing relatively easy integration with regard to optical system design and software control. However, they emit lower peak transmission values in comparison with conventional fixed-wavelength optical filters due to the use of multiple polarizing elements. This can be mitigated in some instances by using wider

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 elements. Higher temperatures can decrease the transition time for the molecules of the liquid crystal material to align themselves and for the filter to tune to a particular wavelength. Lower temperatures increase the viscosity of the liquid crystal material and increase the tuning time of the filter from one wavelength to another.

Recent advances in miniaturized electronic driver circuitry have reduced the size requirement of LCTF enclosures without sacrificing large working aperture sizes. In addition, new materials have allowed the effective wavelength range to be extended to 2450 nm.[2]

Imaging

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.[3][4][5] Multiple LCTFs in separate imaging paths can be used in optical designs when the required wavelength range exceeds the capabilities of a single filter, such as in astronomy applications.[6]

LCTFs have been utilized for aerospace imaging.

photobleach
some of the materials used to construct the filters.

Acousto optic tunable filter

Another type of solid-state tunable filter is the acousto-optic tunable filter (AOTF), based on the principles of 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 that can have working aperture sizes up to 35mm and can be placed into positions where light rays travel through the filter at angles of over 7 degrees from the normal.[9][10]

See also

References

  1. hdl:1854/LU-1265564
    .
  2. ^ "Liquid Crystal Tunable Bandpass Filters". www.thorlabs.com. Retrieved 2022-04-01.
  3. ^ Peng, Yankun & Lu, Renfu. "An LCTF-Based Multispectral Imaging System for Estimation of Apple Fruit Firmness: Part II: Selection of Optimal Wavelengths and Development of Prediction Models". United States Department of Agriculture. Retrieved 2010-07-06.
  4. S2CID 93578607
    . Retrieved 2010-07-06.
  5. ^
    doi:10.2322/tstj.7.Tn_7. Retrieved 2010-07-06.[dead link
    ]
  6. Bibcode:2006amos.confE..21J
    . Retrieved 2013-05-30.
  7. ^ Michael P. Doherty; Susan M. Motil; John H. Snead & Diane C. Malarik. "Microscope-Based Fluid Physics Experiments in the Fluids and Combustion Facility on ISS" (PDF). NASA/TM—2000-210248. Archived from the original (PDF) on October 14, 2006. Retrieved 2010-07-06.
  8. ^ Richard M. Levenson; David T. Lynch; Hisataka Kobayashi; Joseph M. Backer; Marina V. Backer. "Multiplexing with Multispectral Imaging: From Mice to Microscopy" (PDF). ILAR Journal partially supported by a Bioengineering Re- search Grant (1RO1 CA108468-01) and by the SBIR mechanism (1R44 CA88684), both through the National Institutes of Health. Archived from the original (PDF) on July 18, 2011. Retrieved 2010-07-06.
  9. S2CID 56118629
    . Retrieved 2010-07-06.
  10. S2CID 120372420
    .
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