Tuesday, May 7, 2013

Taking advantage of polarization

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.


Anonymous said...

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.

Anonymous said...

I use polarization on nearly every color application I look at.

Anonymous said...

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