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Retro-Reflection Laser System |
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System operation is described below:
1. A laser spot is focused on the
moving web or sheet, and swept
(scanned) across the width of the
product. This spot is swept from one
edge of the product to the other
(constituting one “scan”), via a rotating
polygon. The laser spot size and scan
rate are set to provide a minimum of
100% coverage at the highest line
speed.
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2. For transparent materials (glass,
PET, photoresist, window film, etc.), the
laser light passes through the material
and meets the retro-reflector. The retro-
reflector reflects the light back along the
path of incidence, returning the light to
the scanner. The retro-reflector is
composed of millions of small glass
spheres; each sphere contributes to the
large cone of return light. The laser
scanner becomes the laser light
collector. |
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Click to enlarge
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Click to enlarge
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3. The return “bundle” of light is
descanned after being reflected off the
polygon and separated into two or more
light “bundles”. Each bundle of light is
then focused to a point onto a
photomultiplier (PMT), creating bright
field, dark field, distortion and other
optical channels. These laser spots
mimic the laser spot generated by the
product, on the retro-reflectors. All
optical effects, exhibited on the retro-
reflector, produce identical images
inside the scanner. If the defect absorbs light, the transmitted/reflected
energy is reduced, reducing the amount
of return light, hence reducing the
intensity of the focused light spots.
Defects which change the beam
direction (refractive index changes)
cause the spots to shift. Scattering
defects, like scratches, cause the beam
profile to change (light scatters from the
edges).
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