What does “Cross Polarization” mean?
Cross polarization is a technique that allows us to virtually eliminate specular highlights from images by using polarized light, but it also holds a lot of creative potential, as it can be used to bring out eye-catching rainbow gradients in subjects such as plastics, ice and certain crystals.
In the industry this technique is used e.g. to locate areas of stress in plastic, but it also holds a lot of potential for interesting macro images:
The image illustrates the molecular structure of some thin ice. Which colours these structures take on depends on the orientation of the polarizer in front of your lens. The photo was created by freezing water on a polarizing filter, which I then placed on a flat and even light source (in this case a repurposed notebook).
The two polarizing filters in the set-up above are oriented in opposing directions. This would cancel out all light as
The CPL filter under the ice polarizes the entering light; as the light enters the ice it gets doubly refracted, which causes the original light ray to split into two rays that travel into different, according to the refractive index. Both rays are still polarized. As one wave gets retarded with respect to the other, interference occurs between the waves as they pass through the second polarizer.
This phenomenon is called birefringence and it is responsible for the gradients of colour in the images above.
Which colours these gradients take on depends on the orientation of the polarizers in relation to each other:
Using only one Polarizer
To experiment with this without having to purchase an additional polarizer you can simple use your computer screen instead. 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 can start exploring the effects of birefringence at home.
Just try to hold a CPL filter in front of your computer screen and you will see how it’s blocking out varying amounts of light, depending on the angle of the filter:
Conversely this means, that you can start exploring the effects of birefringence by using a computer screen for your light source and just a single CPL on your camera lens.
Experiment with different sets and subjects
But not only translucent subjects such as plastics, ice or crystals are capable of producing fascinating colours under cross polarization; even the human iris will create interesting gradients of colour when illuminated with polarized light and viewed with a secondary polarizer. This works best for blue eyes and has a much less noticeable effect on brown irises.
Even though I have a basic idea as of what the reason of this phenomenon might be, I am admittedly not quite certain of the physical process that causes these colours to show, so if you can explain the science please share your knowledge and I’ll add it to this post.
To have a look at my set-up that I used to capture the iris image above, please have a look here:
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.