Tuesday, August 9, 2011

Understanding polarization

From time to time, as you browse the literature put out by makers of machine vision lighting, you’ll come across references to polarization. As a well-schooled engineer you’ll know that a polarizing filter is the optical equivalent of a venetian blind, only allowing light whose electric field oscillates in a particular plane to pass, but do you really understand the how and why?

I believe that making use of a technology or physical effect depends on understanding the basic principles, so today I’d like to share a few links to pages that deal with polarization.

Let’s start by talking about what polarization is. Essentially, it’s a result of reflection from a surface. Yes, angle of incidence equals angle of reflection, but the oscillating electric field makes things a bit more complicated than that. To put it crudely, when light reflects off a surface towards a camera, the light entering the lens all has the same polarization. A great explanation is provided by Professor Orzel of Union College, Schenectady, NY under the heading of “Basic Concepts: Polarization of Light”.

Then, when you’ve understood that article, take a look at his follow-up, “Why Do Polarized Sunglasses Work?” which actually gets into the nitty-gritty of what causes polarization.

From there it’s time to think about how to take advantage of polarization and that’s where I’m going to send you to a short article on the Edmund Optics website called, “Successful Light Polarization Techniques”. This has some photos showing how polarizing filters can clean up an image, making it more suitable for processing.

One point that I didn’t see mentioned anywhere though concerns aperture and exposure settings. Only allowing light of a certain polarization to reach the sensor means that the total number of incident photons is reduced. In other words, the image will get darker. To compensate for this you’ll have to open the lens aperture, increase the exposure time, or just throw more light at your subject.

Now, that’s my brief polarization primer over; let’s turn it over to you my dear reader. How about sharing some examples of clever or intelligent uses of polarization?

1 comment:

Anonymous said...

I used polarization to inspect for the closure of bread wrappers.

The plastic bags which hold bread or buns were going to be presented to a vision system to insure that a tie had been affixed to the opening. I was told that the open end of the bag would be splayed out but could be pushed to one side or the other. The bags would be transparent. I used a backlight with a polarizer and the analyzer on the lens with the bag opening between the light and lens.

With the analyzer adjusted appropriately the background appeared dark but when the light hit the plastic of the bag it became depolarized. So when it went through the analyzer it appeared much lighter than the background. The result was a profile of the bag that appeared light against a dark background. With some image filtering I was able to create a binarized image which could be easily inspected for contour.