US Patent 7189960 Optical moisture sensor

A soil moisture sensor has a cylindrical body of transparent cyclic olefin polymer (COC). A pair of axially spaced cavities tapered toward each other are formed in the body. A light source is placed in one of the cavities, and a light sensor is placed in the other. The walls of the cavities are so curved that divergent light rays from the light source are refracted at the cavity-body interface into parallelism, and that reflected parallel rays are refracted at the body-cavity interface so as to focus on the light sensor. The parallel rays coming from the light source are reflected or refracted at the outer surface of the body, depending upon whether the ambient environment of the sensor is dry or wet.