  The application of precision interferometers is generally restricted to
expensive and smooth high-quality surfaces. Here, we offer a route to ultimate
miniaturization of interferometer by integrating beam splitter, reference
mirror and light collector into a single optical element, an interference lens
(iLens), which produces stable high-contrast fringes from in situ surface of
paper, wood, plastic, rubber, unpolished metal, human skin, etc. A
self-referencing real-time precision of sub-20 picometer (~ lambda/30000) is
demonstrated with simple intensity detection under ambient conditions. The
principle of iLens interferometry has been exploited to build a variety of
compact devices, such as a paper-based optical pico-balance, having 1000 times
higher sensitivity and speed, when compared with a high-end seven-digit
electronic balance. Furthermore, we used cloth, paper, polymer-films to readily
construct broadband acoustic sensors possessing matched or higher sensitivity
when compared with piezo and electromagnetic sensors. Our work opens path for
affordable yet ultra-precise frugal photonic devices and universal
micro-interferometers for imaging.
