Biomedical Engineering Reference
In-Depth Information
Fig. 4.8 Principle of the multiple-stage liquid crystal elements is demonstrated by showing the
transmission through each stage. Each unit provides a cosine-like transmission pattern with a
different frequency. Nevertheless, all the transmission curves are centered at the same selected
wavelength (550 nm in this example) and the output is provided by multiplying the whole set of
transmission bands. The result is shown at the bottom . By changing the voltage applied to the
liquid crystal, the central wavelength can be changed in the whole range of max D 2 min
(Image courtesy of Cambridge Research & Instrumentation, Inc. (CRi), now part of Caliper)
Fig. 4.9 An acousto-optical tunable filter (AOTF) used for spectral imaging. By inducing acoustic
waves in a crystal that has an index of refraction that is sensitive to the material density, a
grating is formed. It deflects the light according to the periodicity so that a single wavelength
is transmitted. The zero-order transmitted light is blocked, so that by changing the frequency, a
different wavelength can be selected
near-infrared light, but longer-wavelength light (e.g., in the form of thermal energy)
can damage the filters. UV light can also cause irreparable damage. Many detectors,
such as CCD or CMOS sensors, are not particularly sensitive to UV or thermal
energy, so it is good practice to utilize heat-absorbing glass, a long-pass filter, or to
check the temperature of the LCTF optics to make sure that the filter is not being
subjected to excessive energy.
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