Cross Polarization: What It Is and Why It Matters


Cross polarization is a technique that uses two polarizing filters – one on the light source and on e on the camera lens – to get rid of unwanted specular reflections.

This article is part one of a two-part series explaining cross-polarization and birefringence.

Understanding Polarized Light

So let’s have quick look at the science of it: light is a form of electromagnetic radiation, meaning that it consists of waves, oscillating perpendicular to its direction. But those waves are not aligned; some of them oscillate up and down, some move left and right, and still others all directions in between.

That is, of course, unless we are looking at polarized light.

Polarized light waves are all oscillating parallel to each other, meaning they all share one plane. To polarize unpolarized light, we can use a circular or linear polarizer, which only lets the light of one certain plane pass while light waves that are oscillating in a different direction we’ll be reflected.

Two polarizers that are aligned perpendicular to each other don’t let any light pass.

This is the same principle that variable ND (neutral density) filters use to block out varying amounts of light. Such filters consist of polarizing filters, one of which is stationary, while the other one can be rotated against it, which gradually blocks out more and more light until the two filters are aligned perpendicular to each other and effectively block out all light.

This gets really interesting when we are directing our polarized light source onto our subject. As the polarized light hits the surface of that subject it becomes reflected and most of it turns into diffused, unpolarized light again except for the specular component of the reflection, which is still polarized and can therefore be canceled out by employing a second polarizer (CPL) filter in front of our lens.

The result leaves us with a very clean looking image.

You can try this out at home, even if you only have one CPL filter in your camera bag; due to the way that LCD displays work, they emit polarized light by their very nature, so all you need to do is to load an empty word document or a white wallpaper and you’ll be able to see the effect of cross-polarization right away.

Even though this is a very useful technique to have in your toolbox, it isn’t the only interesting application of cross-polarization.

It can also be employed to create images like this:

In part two, we’ll discuss more about birefringence and how to create colorful images utilizing it.

About the author: Maximilian Simson is a photographer and artist based in London, Ontario. The opinoins expressed in this article are solely those of the author. You can find more of Simson’s work on his website and Facebook. This article was also published here.

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