Spectral Imaging: Methods, Design, and Applications - Biomedical Optical Imaging Technologies: Design and Applications - page 128

Nematic liquid crystal material is generally used in each LCTF tuning element.
This material can be accurately and predictably controlled with relatively low
current. Image quality is extremely high, and the field of view is relatively wide.
The liquid crystal material in LCTF designs needs a few milliseconds to respond to
changes in the electrical current.

AOTF

An acousto-optical tunable filter (AOTF) is typically constructed by an anisotropic
birefringent crystal such as Tellurium dioxide (TeO 2 ). Piezoelectric transducers are
bonded to the crystal, and by applying high-frequency voltage to the transducers,
an acoustical wave is generated in the crystal (Fig. 4.9 ). Due to the birefringent
response to the acoustic wave, the crystal forms a grating that diffracts polarized
light according to the periodicity of the waves. Therefore, by selecting different
frequencies, specific wavelengths can be selected one at a time. For measuring a
full spectral image, each spectral bandwidth is measured one at a time. Just as with
any grating, the AOTF also transmits the first-order non-diffracted beam that has to
be blocked. By changing the amplitude of the voltage applied to the transducers, the
transmitted intensity can also be controlled.
The AOTF can reach transmission efficiencies of 35 % and change the transmit-
ted wavelengths in 10-100 s. It can accept collimated beams with an acceptance
angle of ˙ 5 ı and therefore can be used for imaging where each collimated beam
that enters at a different angle corresponds to a different point in the image.
The system has the advantage that the spectral resolution can be tuned in a
relatively large range by applying multiple frequencies to the device. In principle,
adding another frequency can be used to double the spectral bandwidth (simply
by selecting two frequencies that transmit two wavelengths that are close to each
other so that their FWHM overlaps. Systems using up to 16 different frequencies
are available, so that the resolution can be selected from the narrowest one up to a
multiplication of 16 times, or practically a variable spectral resolution of 1-16 nm. It
is useful in cases where several applications require each a different spectral range.

4.4.2.2 Novel Color Filter Methods

As color filters are abundant in many types of applications, new and novel filter-
based methods immerge rapidly. The following method, developed recently, is
based on surface plasmon nanostructures constructed of two thin metal films spaced
by a subwavelength insulator [ 27 ]. The nanostructures act as surface plasmonic
resonators (SPRs). SPRs are essentially charge density waves generated by the
coupling of light to the collective oscillation of electrons on the metal surface. With
today's nanofabrication capabilities, such arrays can be manufactured with variable
density so that light traversing through them disperses respectively to its density,
thus white light is dispersed to a wide wavelength range (visible and other spectra