The
math of polarization gets pretty hairy, but you don’t need to know
it to put the technique to work. Just take a look at these two
images.
These
show the same bar code label fixed to the same piece of bright steel
and were acquired with the same camera, lens and
light.
In both cases there was a polarizing filter over the lens, but the
image on the left also had a polarizer over the light. In other words
it was throwing polarized light on the target.
The
polarizing filter was turned at 90 degrees to that over the light, so
only light that changed polarization when it bounced off the target
could reach the camera’s sensor. That’s why the metal looks
black: no light is returned from this region.
Usually
light becomes polarized when it reflects off a surface, like water,
but that doesn’t happen when metal is the reflecting surface. In
the example above the label was paper and so changed the polarity of
the light, which was then captured by the sensor.
I
don’t pretend to follow all the math of polarization, but I think
this effect is pretty cool. Expect to see it in an application near
you very soon.
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3 comments:
I just happened to read this note by Smart Vision Lights about polarizing filters: http://www.smartvisionlights.com/wp-content/uploads/pdf/Linear_PolarizerR7.pdf
62% of energy converts to heat with the filter, so you need to be careful to not to fry your LEDs.
I use polarization on nearly every color application I look at.
I use it a lot, for instance OCR on plastic barrels.
Its like wearing two pairs of sunglasses though so if you need a fast inspection and deep depth of field, powerful LED lighting is recommended
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