Biomedical Engineering Reference
In-Depth Information
Fig. 6.9 The basic geometry for beam scanning. The aperture stop is situated in the back focal
plane of the tube lens and front focal plane of the objective. The scan mirror is placed in the front
focal plane of the scan lens. The light from the sample returns through the optical system and is
descanned by the scan mirror
Fig. 6.10 For two-dimensional x-y scanning, the scan mirrors can be ( a ) close-coupled or ( b )
arranged using a telecentric relay system
dimensional x-y scanning, a single mirror is sometimes used, but if separate
mirrors are used, they must be either close-coupled (Fig. 6.10 a) or arranged using
a telecentric relay system (Fig. 6.10 b). In practice, the close-coupled arrangement
usually works adequately and avoids the extra complication and light loss of the
relay lens system.
Scan speed can be increased by using acousto-optic scanners, but as they are
diffractive devices, they suffer problems when used in fluorescence imaging, as
the wavelength of the fluorescent light is longer than that of the excitation, so
they cannot be used to descan fluorescence signals because of the Stokes shift.
Various other beam-scanning designs have been employed to improve scan speed. In
Fig. 6.11 a, a scanning spot is deflected over the specimen, using a TV-rate acousto-
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