A contamination detection apparatus is provided for measuring an amount of contamination on an optically transmitting element or pane. The optically transmitting element is capable of transmitting electromagnetic radiation. The contamination detection apparatus includes a contamination detection (CD) radiation source for providing a source intensity of radiation. The source intensity or radiation passes through the optically transmitting element at least one time. A reference detector is also provided for detecting the source intensity of radiation, and the reference detector has a reference output level that is representative of the source intensity.