It is these materials that Lockheed is replacing with liquids. Since 1985 they have acquired data on more than 400 types of optical liquids that transmit well in the visible. Of those maybe 10 or 12 are outstanding. They possess the abnormal dispersion characteristic to a degree never found in any solid material, and can replace solid materials now used in high performance lenses economically. By using these liquids in combination with common optical glasses, performance can be better with lower material cost.
Elements made with abnormal dispersion liquids offer additional benefits to lens makers.
When a liquid is contained between two glass elements, it contacts the surface of the adjoining glass elements and closely matches the refractive index of the glass, therefore the internal surfaces that are wetted by these liquids almost do not have to be polished. Any small surface imperfections seem to disappear, and it appears you're looking through a single piece of glass. Internal surfaces do not require antireflection coatings as they would in a lens with an air-glass interface. The result is a higher contract image because of much less scatter and veiling glare due to internal reflections. Higher contrast also means higher resolution.