Portable imaging polarimeters

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Abstract
The natural light field has long been known to be partly linearly polarized, and it has been suggested that various objects reflect light that is polarized at a specific orientation. Although humans use polarized light, our inability to see it limits our study and understanding of its distribution in nature and of the information it carries. By placing two twisted nematic liquid crystals and a fixed polarizing filter in series in front of a CCD camera, we constructed a portable polarimeter that analyses the linear polarization characteristics of a full image on a single pixel basis. Two configurations are presented: an autonomous sensor that uses a small camcorder for recording images that are analyzed at a later stage; and an online sensor that uses a digital camera connected to a personal computer which controls and analyses the information. Field measurements reveal possible usage for analyzing spatial orientation of objects or breaking color camouflage. Our new polarimeters provide an opportunity to inspect and understand an aspect of the visual world, currently obscure to our eyes.