Cross-polarized light microscopy (CPLM) is effective for early detection of bivalve larvae or veligers in plankton. The crystalline structure in their tiny mussel shells produces a distinctive “maltese cross” (Figure 1) under a cross-polarized light field. Roughly, on the compound light microscope, the light passes up from the source in the base through the condenser, stage, through the sample, into the objective lens up through the microscope body, through the eyepiece and into our eyes (Figure 2). During CPLM, the sample is placed between two rotatable polarized filters (think sunglass lens), and the two polar filters are oriented perpendicular to each other, essentially blocking the visible light from reaching the eyepiece unless the light is reoriented when passing through the sample. Cross-polarized light microscopy looks like a starry night sky, a dark background with bright objects. There are many things that produce birefringence and appear as bright objects in a dark background including inorganic particles, plastics, and other organisms. Cross-polarized light is used to detect suspect specimens, and identification is based on morphology often visible using brightfield or a reduced degree of polarization.
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