Portable imaging polarized light analyzer

Many animals, both marine and terrestrial, are sensitive to the orientation of the e-vector of partially linearly polarized light (PLPL). This sensitivity is used for navigation, spatial orientation, and detection of large bodies of water. However, it is not clear what other information animals may receive from polarized light. Natural light fields, both in the sky and underwater, are known to be partially polarized. Additionally, natural objects reflect light that is polarized at specific orientations. Sensors capable of measuring the characteristics of PLPL, namely partial polarization and orientation, throughout an image are not yet available. By placing 2 twisted nematic liquid crystals (TNLCs) and a fixed polarizing filter in series in front of a video camera, and by controlling the angles of rotation of the orientation of polarization produced by the TNLCs, we are able to fully analyze PLPL throughout a full image on a single pixel basis. As a recording device we use a small camcorder. The sensor can be operated autonomously, with the images analyzed at a later stage, or it can be connected (in a future phase) via a frame grabber to a personal computer which analyzes the information online. The analyzed image can be presented as a false color image, where hue represents orientation of polarization and saturation represents partial polarization. Field measurements confirm that PLPL is a characteristic distributed both under water and on land. Marine background light is strongly horizontally polarized. Light reflected from leaves is polarized mainly according to their spatial orientation. Differences between PLPL reflected from objects or animals and their background can be used to enhance contrast and break color camouflage. Our sensor presents a new approach for answering questions related to the ecology of vision and is a new tool for remote sensing.