User:Sterenborg/Differential Path-length Spectroscopy

Differential Pathlength Spectroscopy
Differential Pathlength Spectroscopy or DPS is a fiber-optic measurement technique for quantitative measurements in optically turbid media.

In classical spectrophotometry, light absorption is measured in a clear solution using a cuvette and a parallel beam of light. According to the law of Beer-Lambert, the light absorption depends on the concentration of the absorber in the liquid and the optical pathlength of the light through the solution, which is determined by the dimensions of the cuvette. In optically scattering media such as milk the light no longer travels in a straight line, but is scattered many times. In this case the optical pathlength and the sampling volume depend strongly on both the scattering and absorption properties of the medium, as well as on the geometry of the measurement setup.

Several different methods for quantitative optical spectroscopy in turbid media have been developed. Most are based on diffusion theory, either using time resolved, frequency domain or spatially resolved measurements. In highly inhomogeneous media the accuracy of these measurements may be compromised by the variable photon pathlength. Photons from different wavelength regions will encounter different optical properties and will travel different paths. Hence different wavelength bands will sample different volumes of the medium.

Basic Principle
The basic principle behind DPS is to select photons with a predetermined optical pathlength using a specific measurement geometry. The fiber optic geometry by which this is done is depicted in figure 1. DPS fundamentally uses 2 optical fibers placed in contact with the medium. Both optical fibers are used to detect light from the medium, while one of these fibers is also used to launch light into the medium. To obtain the DPS signal, the two detected signals are then subtracted. Photons that have traveled a large distance from the source fiber have a roughly equal chance of being detected by the c-or the dc-fiber. On the other hand, photons that have just emerged from the source fiber have a much higher chance of being detected by the dc-fiber than by the c-fiber. As a result, the differential signal relies heavily on photons that turned back to the dc-fiber after just a few scatter events and much less on the photons that have scattered multiple times and wandered off deeper into the medium.

Main properties
Main property of