US Patent 8136690 Sensing the amount of liquid in a vessel

A vessel comprising a fluid-confining surface, a basal surface disposed below the fluid-confining surface, and a light guide. The light guide includes a first window partly defining the basal surface and a second window partly defining the fluid-confining surface, where the second window is disposed above a lowest region of the fluid-confining surface. In other embodiments, an isolating structure is provided that substantially surrounds the light guide between the first and second windows.