About 17 percent of American adults (36 million people) suffer from some form of hearing loss, according to the National Institutes of Health. Yet only one in five people who could benefit from using a hearing aid wear one. A big part of the problem is that "hearing aids often just don't fit well enough, and are either uncomfortable or don't perform well enough because of the poor fit," says David Copithorne, a technology consultant familiar with the hearing-aid industry and a wearer of hearing aids himself.

A new digital scanning technique developed at MIT could offer a much better fit for future hearing aids. Developed by Doug Hart, a mechanical-engineering professor at MIT, the approach uses the absorption and emission spectra of light to capture a very accurate 3-D picture of the inner ear.

The average hearing aid costs $1,500, but prices can go up to $5,000 each. Creating each aid typically involves squirting into the ear silicon-based goop that hardens to create a mold for the aid. The process is imperfect, though: molds can deform or even damage the ear during extraction, and if the resulting hearing aid doesn't fit perfectly, it can lead to irritation, scratching, or infection. This can also decrease the sound quality for the wearer. Fitting aids well and quickly has been "a real bottleneck in the industry," says Copithorne.

Hart developed the new scanning technique "completely by accident" while experimenting with emission reabsorption laser induced fluorescence (ERLIF) as a way to measure the film thickness of engine oils, in order to understand oil consumption and engine wear. In the process, he figured out that he was getting very accurate 3-D measurements of the films. "It's so accurate," he says, "you can measure anything in 3-D."

ERLIF works on the principle that light is scattered differently depending on the depth of a liquid. Hart uses a fiber-optic camera inserted into the ear and wrapped by a liquid-filled balloon that expands to conform to the ear's shape. Measuring the light absorption of dyes in both the liquid and the balloon yields an exact 3-D picture of the ear's shape and dimensions.

ERLIF is "a way of analyzing a light path from fluorescence," says Davide Marini, a research fellow at Children's Hospital Boston who worked with Hart on the technique.

The camera's fast imaging rate means that it can even measure how the ear canal changes shape as a patient chews or talks, and how it expands due to pressure--qualities that differ for every person, with some ears softer or more resilient than others. Silicon molds, on the other hand, typically require a patient to sit with her mouth hanging open for 10 minutes while the goop sets, says Hart.

Copithorne says that the infrastructure is already in place for making molds from digital scans. Audiologists (who fit hearing aids) are increasingly choosing to scan the molds that they make, rather than hand-tooling a shell for their manufacturers. The next step for Hart's team is to test its scanning technique with audiologists and make actual hearing aids. The team expects to work out the last technical issues this summer.

While Hart hasn't set a price for his process, he says that the concept is "simple, robust, and inexpensive." His group has been talking with some of the major hearing-aid manufacturers, as well as with the U.S. Navy: hearing loss is a problem not just for pilots, but also for deck crews on aircraft carriers and gunships.