An Introduction to Polarizing Film – Functions & Applications
Polarizing film is a layer often added to optical filters with anti-reflection coatings, as well as a range of other direct applications in optics, film and photography, and automotive.
When light passes through the filter, the polarizing film only allows the transmission of light waves with a specific polarization, while blocking unwanted polarizations.
The result is a polarized output with less glare, fewer unwanted reflections, and the ability to control the visibility of the output using polarized glasses.
In this introduction to polarizing film, we will look in more detail at the mechanism by which these filters work and why they are so important across a variety of industries.
What is Polarizing Film?
Polarizing film is a thin, flexible polymer such as polyvinyl alcohol (PVA) that exhibits polarizing properties when added to a laminated optical filter.
Some other materials also have polarizing properties, including some prisms, and light can also be polarized by reflection from a surface. In this guide, we are focusing on thin-film polarizers specifically.
Function of Polarizing Films
Light travels as a transverse wave. In simplified terms, you can think of a beam of light as having a component of vibration perpendicular to its direction of travel.
Polarized light has this side-to-side vibration aligned to a single plane, as though the beam has been ‘flattened’.
When this beam passes through a polarizing film, filter or lens, it is only transmitted if it aligns with the polarization of the lens, like an envelope through a letterbox. If you rotate the lens so that it is aligned perpendicular to the beam’s polarization, no light will pass through.
Types of Polarizing Films
Thin-film polarizers can be used to create several different types of polarizing films:
Linear Polarizers
Linear polarizers work as described above to create an output with polarization on a single plane. They may be absorptive polarizers, which absorb the unwanted component of the incident beam.
When using absorptive polarizers, the non-aligned component of the light is removed, leading to an output less bright than the incident light beam.
Alternatively, beam-splitting linear polarizers can be used to create two outputs with opposing polarization, allowing for applications where the removed component is retained.
Circular Polarizers
Circular polarizers create an output where the direction of polarization rotates as it travels along the direction of the light beam, similar to the motion of a sycamore seed as it falls from the tree.
By creating circularly polarized light in opposing directions (clockwise and anticlockwise) it is possible to transmit separate signals.
This is used in 3D cinema, where each lens of a pair of 3D glasses is layered with a polarizing film and the image to each eye is circularly polarized in a CW or ACW direction.
Material Composition
One of the most common materials used in the manufacture of polarizing films is polyvinyl alcohol (PVA).
This is stretched to align the polymer strands, and then dyed using iodine to improve the polarizing efficiency. The film is then bathed in an aqueous solution of boric acid at a specific temperature to strengthen it.
Polarizing films can be manufactured in a variety of ways using principles of interference, scattering and diffraction, but the process described above has been in use since the 1850s.
How Polarizing Film Works
We have already looked at the scientific mechanism by which polarizing film works. In practical terms, polarizing optical filters serve two main purposes:
- To polarize unpolarized light
- To transmit or block polarized light
The first of these purposes is quite straightforward: the filter works by transmitting only the desirable plane of polarization while eliminating or reflecting the parts of the signal not aligned with this plane.
The second is more complicated. It’s often used to reduce glare e.g. polarized sunglasses lenses, which reduce the glare of sunlight from a wet or smooth road surface while driving.
How to See Polarized Lenses in Action
When polarized light passes through a polarized filter, the amount of light transmitted depends on the angle between the two planes of polarization.
You can see this by holding two pairs of polarized sunglasses over one another. With both pairs placed horizontally (in the normal wearing position) light should pass through both lenses.
If you rotate one pair such that its lens is at 90° (i.e. the frames are ‘vertical’) and then look through both pairs of glasses, you should see very little light. This is because the front pair is polarizing the light in a specific plane, which the rear pair is then filtering out.
Applications of Polarizing Film
There are many applications of polarizing film in industry, research and in a wide variety of consumer products. It’s likely you have several polarizing screens, lenses or filters in your home.
Photography
Polarizing films are used in film and photography to improve image quality. Without polarization, images can look unnaturally bright and washed out due to excess glare from the ground, rocks, trees, water and even human skin.
Adding a polarizing filter to the camera lens gives richer colour tones, even after editing. Polarized lenses can also reduce the visibility of reflections in glass and metallic surfaces, as reflected light is naturally polarized and the lens can largely block it.
LCD Screens
Liquid crystal displays, better known as LCD screens, contain two polarizing films with the liquid crystals sandwiched between them, and a bright backlight.
The polarizing layers ensure that light is only visible when the relevant liquid crystals are illuminated (to put it another way, the polarizing film determines the brightness and contrast of the image on the screen).
Without polarizing films, LCD screens would have very poor contrast, and the image would either be uniformly very bright or uniformly very dark.
Automotive Industry
Polarizing films can be used on automobile windows and instrument displays to reduce glare and improve at-a-glance visibility during driving.
This is not only useful in car manufacture and after-market window tints, but also in aviation to protect against glare on aeroplane and helicopter windscreens.
Optical Instruments
Polarized lenses are used across the optical instruments industry, ensuring good visibility of samples under a microscope and good signal quality in laser systems.
Envin Scientific manufacture bespoke thin-film optical filters with anti-reflection coatings and polarizing film layers, to suit the very specific needs of the optical instruments sector.
Medical Imaging
Again, polarizing filters can enhance the quality of medical imaging for better diagnostics, improved patient care and better results from clinical research.
Customers with a specific imaging application in mind should contact Envin Scientific to discuss our bespoke optical filter service.
Conclusion
There is much more to say, but hopefully this guide has covered the essentials of an introduction to polarizing films, so that you know what you are looking at next time you encounter a polarized lens or filter.
We will look in more detail at specific types of polarizing films in a future guide, covering dichroic films, wire grid polarizers and emerging nanoparticle polarizers.
If you have any questions about Envin Scientific bespoke optical filters, anti-reflection lenses or polarizing films, please get in touch and we will be happy to help.
