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3. Aligning Linear Polarizing Film

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When your polarizing film arrives, you'll be faced with the inevitable question, “Which way is vertical or which way is horizontal?”

Rough Calibration with Polarized Sunglasses

There is a simple technique for roughly aligning polarizing-film relative to the surface of the Earth. The technique relies on the fact that a portion of non-polarized light becomes polarized when reflected off of certain types of surfaces. This is known as the Brewster effect. Reflective polarization occurs most significantly at a particular angle (Brewster angle) on a flat, reflective, non-metallic material.

An everyday example is sunlight reflecting off of calm water or roadways. Much of that annoying glare is horizontally-polarized light.

To reduce glare, polarized sunglasses include vertically-polarized lenses to block the naturally horizontally-polarized glare. This means you can use a pair of commonly-available polarized glasses as the reference for aligning your polarizing film.

Look through sunglasses while rotating a piece of polarizing film in front of the lens. If there is no change in brightness, those sunglasses aren’t polarized. However, if there is a change in intensity, the brightest point occurs when the polarizing film is vertically aligned, and the darkest point occurs when the polarizing film is horizontally aligned.

Calibration of Polarized Film through Natural Sources

If you don’t own a pair of polarized sunglasses, and if you’re too shy to perform this experiment at the sunglass rack at your local store, you can reference Mother Nature directly.

Natural sources of polarized light. Left: Vertically-oriented glare from glass. Right: Horizontally-oriented glare from leaves and roadways.

Natural sources of polarized light. Left: Vertically-oriented glare from glass. Right: Horizontally-oriented glare from leaves and roadways.

On a sunny day, walk around looking at trees with flat leaves (from afar, not close-up), bodies of water, or flat roads through the polarizing film. This trick works best when the sun is at a 37 degree angle in the sky (Brewster angle of water) relative to the ground.

Try rotating the polarizing film to see if the glare increases or decreases. At some point, you'll locate something horizontal (parallel to the ground) that obviously changes brightness as you rotate the polarizing film. When the brightness/intensity is greatest, the polarizing film is horizontally oriented (compare items #1 and #3 in the photograph above).

You can also reference the reflective glare of sunlit vertical glass, such as building windows or framed pictures. The reflective polarization is greatest at a 33 degree angle (Brewster angle of glass) relative to the plane of the glass. When the reflective glare disappears (you can see through the glass), the polarizing film is horizontally oriented (compare items #2 and #4 in the photograph above).

Sidebar: Telling the Difference Between Circular and Linear Polarizing Filters

Circular-polarizing filters are more expensive than linear-polarizing filters because circular-polarizing filters consist of a linear-polarizing film plus another material that twists the light. You can tell the difference between a circular polarizing filter and a linear polarizing filter by looking at polarized reflections/glare through one side of the filter and then the other side. If rotating the filter only has an effect when looking through one side of the filter, it’s a circular polarizing filter.

Why? When the twisting material in the circular polarizing filter is facing out, it alters the orientation of the reflected polarized light before it reaches the linear polarizer in the filter. The twist eliminates the polarization of the reflection, which was what we had been relying on to distinguish orientation.

Remember, you only want to use a linear polarizing filter for the beacon and robot sensors. However, if you’re really desperate, you can use a circular polarizing filter so long as the linear portion faces out.

Is there a more scientific method to align the polarizing filter for optimal performance? Yes! Let’s see how...