A common PET bottle as visible with (right side) and without (left side) polarising filtering.  At right side, cellophane in the stripes around the bottle rotates the light polarization vector (   C. López-Mariscal/US Naval Research Labs./2014)


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Polariser filters were first developed long time ago.  They became rapidly a standard for nearly all Military and Scientific applications, but not in the Civil Industrial applications like those of the Food and Beverage Industry.    Why polarised light ?     The answer lies in the solution of a key problem.   One affecting all imaging, included the human visual imaging, is that most sources of light are classified as incoherent and unpolarized (or only “partially polarized”) because they consist of a random mixture of waves having different:

  1.  spatial characteristics, 
  2.  frequencies, 
  3.  phases, 
  4.  polarization states.   

Photons later superimposing themselves in the photo-detectors giving rise to destructive interference.    

 Constructive and destructive interferences fringes markedly visible in this thin plastic tape, illuminated and observed with polarised light (  Katy Wood/Maryland Institute College of Art/ 2014)

As an example, imagine to illuminate a photo-detector with 1 joule of radiant energy, later discovering that the Signal at the detector outfeed shall be represented by only a small fraction of the non-negligible amount of energy which entered the detector.  Semitransparent and transparent foreign bodies are much more difficult to detect than opaque and yet this basic observation pave the way to special measures when trying to detect them.   Plastic foils like those folding packs of cigarettes, are often encountered in the base of RGBs (Returnable Glass Bottles).   Plastic foils rotate in a fixed direction the spin angle of the polarized light.   Then, to detect plastic foils, light is forced to pass thru an additional delay filter, whose delay is related to the angle with whom all plastics rotate the plane of polarization.    Finally, a separate base camera detects these images.   To detect the truly difficult completely transparent cigarette plastic foils, it is not as efficient as the Inner Sidewall inspection.

  The impressive increase of the Signal-to-Noise ratio felt when circularly polarised light is emitted  through a polariser filter and later detected again filtered by a second polariser, depends by a non-classic quantum feature of the photon named “spin”.  In the example depicted here, shown left-handed clockwise circularly polarised light, propagating along the z axis. Integrating several of these waves in a photo-detector let them interfere destructively.  Meaning less photons detected with the same spin and phase angle, each one of them contributing to form the Signal


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