| Optical imaging methods for moderately turbid media |
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Imaging methods applicable to moderately turbid media (see Optical imaging in turbid media: Introduction) are based on allowing only or nearly "straight-line" photons, also referred to as ballistic and snake photons respectively. This is frequently referred to as range-gating, i.e. viewing only those photons that came along the shortest routes from the vicinity of the object as defined by a narrow distance range about the object location. These methods include range-gating by:
- time-gating
This method is widely used for imaging in turbid water (for example, Fournier GR et al 1993). Time-gating is typically achieved by using a short pulse of light (several nanoseconds) and an image intensifier. The image intensifier is turned on during a time interval (that defines the range) whose start is delayed from the time the light pulse is launched into a turbid medium by a time interval corresponding to the straight-line travel time on the path illuminator-object-viewer. Zevallos ME et al (2005) found that light pulses and gate widths that are much shorter (~100 ps) enable a perceivable improvement of the image contrast in laboratory, small-spatial scale backscattering-mode trial. The short pulses and gates simply offer a better discrimination of a small signal due to ballistic photons backscattered by the object against a large background diffusely backscattered by the medium.
A time-gating method for micrometer distances in turbid media, based on the use of an optical correlator for timing of the arrival of photons, has been proposed some time ago for tissue (for example, Fujimoto JG et al 1986).
A variation of the time-gating imaging is the amplitude-modulated imaging (see amplitude-modulated lidar)
- collimation (angular domain imaging)
This method applies to a see-through configuration of the imaging system, where an illuminator and a viewer are on the opposite sides of the turbid medium. Both the illumination beam and the viewer field-of-view are well collimated and aligned to each other, so that the viewer accepts mostly the ballistic photons (for example, Chapman GH et al 2003).
- spatial filtering (Leith E et al 1999)
- synchronous scanning
Also referred to as line scanning, this method relies on viewing only a small spot at the object's surface at a time. That spot is illuminated by a narrow beam of light. Both the field of view (FOV) and the illumination beam scan the object's surface synchronously, hence the name of this method. In underwater application, the scan in the direction orthogonal to the scan line is typically provided by the movement of the platform carring the imaging system. The illumination beam (laser) and the viewer are usually separated by a distance to minmimize the overlap of the FOV cone and the laser beam and to enable determination of the distance to the object's surface spot via triangulation.
- using polarized light
The polarization-based methods rely on the fact that the polarization state of light travelling in turbid medium is preserved for a very short time following the arrival of the first ballistic photon (~100 ps, Demos SG and Alfano 1996).
- coherence gating, also referred to as optical coherence tomography or white light interferometry
This method (for example, Huang D et al 1991) rejects non-ballistic photons by relying on a very rapid loss of coherence of light scattered in a turbid medium.
