Brightness waves in cloudy liquids.

Deflected by a host of microscopic globules of fat and protein, light traveling through a glass of milk constantly changes direction. Such light scattering makes it difficult to view objects completely immersed in the liquid.

Until recently, researchers tended to look upon light scattering in liquids as a nuisance. But this attitude has changed with the growing realization that even the randomly scattered, or diffuse, light emerging from a cloudy liquid carries recoverable information about what's inside the liquid (SN: 4/20/91, p.248).

Now researchers have uncovered a new scattered-light phenomenon that shows potential as the basis for a medical imaging technique. Physicist Arjun G. Yodh and his collaborators at the University of Pennsylvania in Philadelphia report their findings in the Nov. 2 PHYSICAL REVIEW LETTERS.

The researchers achieve this effect by sending a modulated laser beam, which rises and falls in brightness at a certain rate, through a cloudy liquid. The resulting chain of dark and light patches, produced by the collective effects of randomly scattered photons of light, behaves like an ordinary wave with a characteristic wavelength as it travels through the liquid. That wavelength depends on the modulation frequency and the type of liquid involved.

"It's really rather surprising that you can generate this kind of wave in a diffusive system," Yodh says.

The researchers have demonstrated experimentally that a brightness wave can be refracted. In other words, when the wave passes at an oblique angle from one cloudy liquid to another -- for example, from whole milk into skim milk -- its direction changes in accordance with the same laws that govern the passage of sound or light waves from one medium into another. In this case, however, it's not the 816-nanometer wavelength of the incoming laser light but the much longer, 10-centimeter wavelength of the resulting brightness wave that's important.

"We also showed that if you have a curved boundary, you can focus these brightness wave fronts," Yodh remarks.

To find out what information they can glean about what's inside a milky liquid, the researchers are now studying how brightness waves are affected by lumps and other obstructions in the liquid. "In practical biomedical scenarios, we will be looking for distortions of these wave fronts as a result of absorptive and dispersive inhomogeneities within objects such as the human breast," the researchers note.

Such an imaging technique would represent a potentially attractive alternative to an approach based on the capture of only those few photons that manage to get through a cloudy liquid without being scattered widely (SN: 5/25/91, p.325).