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.
The following slideshow illustrates the effect at varying degrees, depending on the orientation of the filters with respect to each other.
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 no aligned; some of them oscillate up and down, some move left and right and all directions in between. Unless we are looking at polarized light that is. Polarized light waves are all oscillating parallel to each other, they all share one plane. To polarized 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.
By the way: 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 diffuse, unpolarized light again. Except for the specular component of the reflection, which is still polarized and can therefore be cancelled out by employing a secondary polarizer (CPL) filter in front of our lens, which 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:
Stay tuned if you’d like to learn more about birefringence and how to create colourful images with it!
About the Author
Maximilian Simson is a macro photographer based in London, Ontario. You can find more of his work and creative ideas on his website and YouTube channel, and follow him on Instagram and Facebook. This article was also published here and shared with permission.