Photon Diffusion Tomography for Imaging through Turbid Water

Under a contract from the Office of Naval Research (ONR), SRI performed experimental and modeling investigations with the objective of developing an understanding of the particular aspects of photon diffusion techniques and photon diffusion tomography that are significant for object recognition and object location in turbid media such as coastal or bay waters.

The dominant limitation for imaging through turbid water is light scattering, which blurs or obscures the image of an object. The attenuation of light in a turbid medium may be expressed by the equation

I = I₀ exp( - ( a + b ) L )

where I₀ is the incident light intensity, I is the intensity after a path length L, and a and b are the absorption and scattering coefficients, respectively. The amount of attenuation can be represented by the number of effective attenuation lengths, (a+b)L. Conventional underwater imaging using a light source and camera can image to about 2 attenuation lengths. More sophisticated techniques such as synchro-scan and range gating can operate over about 5 attenuation lengths.

Our research aims to build on and extend techniques now under development for medical applications to achieve imaging ranges up to 20-40 attenuation lengths. One approach uses a time gate to detect only the first arriving photons through the imaged region. These photons have been scattered the least and carry the most information. A second approach, that also works well in reflected light, uses an amplitude-modulated light source and phase-sensitive detection. Modulated light appears to diffuse through the scattering medium as waves, called photon density waves, that are described by conventional wave laws for wavelength, velocity, refraction, etc., governed by the scattering coefficient of the medium. For imaging with these waves, scattering becomes an enabling mechanism rather than an impediment. The photon diffusion tomography problem can be greatly simplified by applying a diffraction reconstruction algorithm to measurements performed using photon density waves.

Principal Investigators

Gregory W. Faris, E-mail: <gregory.faris@sri.com>

Representative Publications

G.W. Faris and R.L. Byer, Science, 238, 1700 (1987).
R.A. Maffione, D.R. Dana, and J.M. Voss, in Underwater Light Measurements, Proc. SPIE 2048 (1993).
R.A. Maffione, D.R. Dana, and R.C. Honey, Appl. Opt. 32 (1993).
G.W. Faris and M. Banks, in Advances in Optical Imaging and Photon Migration (Opt. Soc. Am., Washington DC, 1994).
C.D. Mobley, in Handbook of Optics, M. Bass. Ed. (McGraw Hill, New York, 1995).
P.M. Eckman and G.W. Faris, Joint Opt. Soc. Am. Annual Meeting - 11th Interdisciplinary Laser Science Conference (Portland, OR, 10-15 September 1995).

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URL: http://www-mpl.sri.com/projects/pyu6307.html