Home | Survey | Topics | Index | References | Dictionary | Contributing | Gallery | Community
| Optical imaging in turbid media: Introduction | Prev topic | Next topic |
An optical image of an object is built by photons travelling along straight lines after they interacted with the object. In turbid media, due to scattering of light, few photons travel along straight lines. Thus, a photon that interacted with a detail of an object immersed in turbid medium (TM) may come from a randomly-chosen direction other than the direction from the observer to that detail. This blurs the image of the object. In addtion, also due to scattering of light, photons that did not interact with the object may emerge from TM at locations and directions that correspond to those subtended by the object. This latter effect contributes a featureless glow to the image. Such glow is especially apparent in the reflection configuration of an imaging system, where the illuminator and viewer are on the same side of the object. In this configuration, the backscattered light that did not interact with the object can easily dominate the image.
A number of methods have evolved for improving quality of imaging of objects in turbid media. Optical imaging methods for moderately turbid media (such as natural waters). These methods 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.
As the optical thickness of a medium increases, the chances of a photon not experiencing a scattering event decrease very rapidly. This places limits on the application of the range-gated principle in turbid media, because there simply may be not enough straight-line photons to detect. For very turbid dispersions, experimental results obtained with polystyrene latex indicate that a substantial fraction of photons follow the "straight-line" route for optical thicknesses as large as 20 (for example, Zaccanti et al 1992). In other types of very turbid media, such as tissue - which superpose spatially-diffuse variations of the refractive index on localized changes of the index that are characteristic of dispersions, the limit is believed to be on the order of 10 mm, i.e. an optical thickness of ~10 (for example, Hebden et al 1995). For this reason optical imaging methods applicable to very turbid media (such as tissue), where virtually all photons are scattered many times, utilize a different approach (see Optical imaging methods for very turbid media).
| CITATION: Jonasz M. 2006. Optical imaging in turbid media: Introduction (www.tpdsci.com/Tpc/ImgTM.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 17-Jan-2006 Modified: 03-Jul-2006 Reviewed: PENDING |
| Copyright 2005-2008 MJC Optical Technology. All rights reserved. | Terms of use | Menu |