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Light, being a transverse electromagnetic wave, has intensity, wavelength (color), phase and polarization attributes. Imaging-system engineers generally concentrate most of their efforts on capturing intensity information, and, less often, on utilizing color information. They often, however, forget that polarization can be either a useful ally or a dangerous enemy.
If controlled, polarization effects can provide important information about the object under inspection that may not be detected any other way. If uncontrolled, they can wipe out the very information your imaging system is meant to capture.
Figure 1 shows the phenomenon called “linear polarization,” which is the simplest of polarization effects. It arises when an object passes light polarized along one direction and blocks light polarized at right angles to that direction. The exiting beam then predominantly contains light polarized parallel to the selected direction.
There are a number of ways to create this polarization-filter effect. Certain anisotropic materials, for example, naturally polarize light passing through them. Linear polarization also appears when light strikes a specularly reflective non-metallic surface at an angle. When that happens, photons polarized parallel to the surface are preferentially reflected. The reflected beam thus picks up some “horizontal” polarization.
For inspection applications, we want to view the underlying material, so the specularly reflected light can be in the way. Since it is partially polarized, you can, as Figure 2 shows, eliminate it by covering the camera lens with a polarizing filter oriented to block the specularly reflected light.