Passive optical detection of a vibrating surface

A new light-intensity modulation technique arising from a changing surface normal can measure the physical properties of a vibrating surface. Surfaces vibrate for a variety of reasons. In some cases these vibrations are useful, like in a speaker cone, and in others they are undesirable, like when a refrigerator door vibrates from the compressor. The ability to optically image such vibrations may lead to a better speaker or a quieter refrigerator. We have developed a passive optical technique to measure physical prop-erties of a vibrating surface using the detection of light intensity modulation in light scattered from that surface.1–5 The inten-sity modulation arises from a changing surface normal and can be used to produce an image of this changing normal without touching the surface and altering its state. The approach has the advantage of being able to image any arbitrarily sized objec