US Patent 7581442 Optically monitoring fullness of fluid container

A relative fullness of a fluid container is monitored by directing reference light at a bottom surface of the fluid container. The fluid container is designed so that the amount or pattern of reference light reflected from the fluid container varies in relation to the relative fullness of the fluid container. Accordingly, the relative amount or pattern of reference light reflected from the fluid container can be monitored and used to determine the relative fullness of the fluid container.