4. Test Setup for Aligning Linear Polarizing Film

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As a scientist, I don’t like the idea of aligning the orientation of the polarizing film by eye. Being highly subjective, it seems both too inaccurate and too imprecise.

I decided to create an apparatus for aligning the film and detecting the significance of minor misalignment. The quick answer is that aligning polarizing film by eye is just fine. You can be off by as much as ±5.74 degrees and still get 99% of the maximum filtering capabilities.

The next couple of pages describe how to make the polarizer alignment and test apparatus, as well as the test results. Even though it is not necessary for you to recreate this device, many of the techniques and features are applicable to robots.

Test equipment setup for linear polarizing film.

Test equipment setup for linear polarizing film.

The test equipment setup consists of:

  1. A light source (regulated or AC powered is preferable)
  2. Polarized sunglasses lying horizontally
  3. Rotatable wheel with test film attached
  4. An opaque box to block ambient light with a photosensor inside the box
  5. A multimeter to measure the output of the photosensor

The photosensor is connected to either a digital voltmeter or an oscilloscope, in order to measure the change in light intensity as the test film is rotated. When the voltage or resistance (depending on your type of photosensor) reaches its highest point, the test film is aligned with the polarized sunglasses, which means it is vertically oriented.

It is important that the light source be stable, since a change in light output will affect the voltage on the light-detection sensor. The light output of a battery-powered flashlight gradually drops as the battery is consumed. This would make it difficult to precisely determine the peak light intensity, and therefore the alignment might be off.

So, instead of a battery-powered flashlight, use a regulated light source, such as an ordinary AC-powered lamp or LEDs connected to a DC voltage-regulator circuit. True professionals even wait a while (15 minutes) after the light source is turned on for it to reach a stable light output.

The Polarizing Film Test Wheel

The polarizer test wheel consists of three pieces. After being installed in place, the three pieces will be fastened to each other with screws.

Test equipment rotary wheel consisting of a (1) face, (2) bearing, and (3) back.

Test equipment rotary wheel consisting of a (1) face, (2) bearing, and (3) back.

The face ring is outside of the box. The polarizing filter to be tested will be taped over the center hole of the face. All three rings have approximately the same size center hole to allow the polarized light to pass into the box.

The bearing ring is sandwiched between the face ring and the back ring. The bearing ring rests loosely in the side wall of the box. When the face is rotated, the bearing rotates inside the wall of the box.

View inside the test box of how the rings fit together.

View inside the test box of how the rings fit together.

The back ring must be slightly larger than the bearing ring to prevent the whole thing from falling out of the hole in the side of the box.

The next page describes how the rings are machined. After that, we'll look at the top of the box and the test results.