When contact lenses work really well, you don’t notice they are there. But after too many hours of wear, the lenses and your eyes dry out, causing irritation that might make contacts more of a pain than a convenience.

Researchers at Stanford University hope to create a more comfortable contact lens. In a study published in the journal Soft Matter, the team is examining how tears shield the eyes from discomfort. They are also working on a machine that may produce better-designed contact lenses.

The researchers suspected that most of the discomfort occurs when the tear film, a wet coating on the surface of the eye, breaks up. They found that the lipid layer, an oily coating on the surface of the tear film, protects the eye’s surface in two ways  —  through strength and liquid retention.

The lipid layer also prevents evaporation of the tear film. At about 95 degrees Fahrenheit, the eye is usually warmer than the air in the surrounding environment. The eye continually heats the tear film, which then evaporates.

Imitating the lipid layer in contact construction could help millions of people avoid ocular discomfort. The Stanford team’s machine mimics the eye’s surface and accurately reproduces a tear on the surface of a contact lens. The researchers then measure how quickly that tear film breaks up.

The machine also allows the team to measure factors affecting the tear film — including temperature, humidity and a variety of substances.

Dry eye and other symptoms often prompt half of the 30 million contact-lens wearers in the United States to switch to glasses. For them, the prospect of a better contact lens is something to keep an eye on.