Optical sensing in diffuse media

• Main Author: Reynolds, Jeffery Scott
• Other Authors: Webb, Kevin J.
• Format: Text
• Language: English
• Published: Purdue University 1996
• Subjects: Electrical engineering|Optics; Sea ice
• Online Access: https://docs.lib.purdue.edu/dissertations/AAI9638225

The use of optical signals to probe and image in diffuse media has many applications ranging from medical imaging to the remote sensing of sea ice. There are two fundamental approaches to analyze optical scattering. First, an electromagnetic analysis can be performed, usually numerically. However, in highly scattering media like tissue, the electromagnetic approach quickly becomes intractable. A second approach, utilizing transport theory, treats the propagation of light as a diffusion of particles (photons) through the medium. The resulting transport equation can be simplified by the diffusion approximation and solved analytically for special cases or by various numerical methods. We have made frequency domain photon migration measurements in both biological and ice models utilizing a near infrared optical signal modulated at RF frequencies. Although the coherence of the optical carrier is quickly lost in highly scattering media, the modulation maintains its coherence. By detecting the magnitude and phase of the reflected or transmitted modulation signal, we are able to localize changes in absorption or scattering parameters. We have compared these measurements to 2D and 3D finite difference time and frequency domain solutions of the diffusion equation. Various source and detector configurations were evaluated numerically and experimentally. We extended the frequency domain numerical simulations to include reradiation for both fluorescent and Raman emissions and compared these results to experiments. In addition, we used Raman spectroscopy to make quantitative measurements of the concentration of acetonitrile dispersed in a highly scattering medium.