Sept. 10, 2011:  Strain testing in the digital age

With a lot of talk lately about testing various blanks for strain, striae, homogeneity, etc., I started thinking.

This test requires a source of reasonably evenly distributed polarized light.  Typically, a source of diffused white light is covered with a sheet of linear polarizing material.  The light that passes through is then linearly polarized.  That light then passes through the glass being tested, and the light is viewed through a polarizer that is aligned perpendicular to the first one that covers the light source.

It is important to note that we're talking about linear polarizers here, not circular or elliptical.

Strain within the glass changes the direction of polarization of the light as it passes through.  If there is little or no strain, the polarized light is not affected, and when viewed through the second polarizer the glass appears to be "evenly dark".  If there is strain, the polarization of the ligh is rotated.  This means that the polarization is no longer perpendicular to the second polarizer, and it does not attenuate it as strongly.  So, that area will appear lighter than other parts of the glass.

It's simpler than it sounds - put linearly polarized light through glass, and look through another polarizer with your eye as you rotate it.  If the mirror darkens unevenly, there is strain in the glass.

The only drawback to this method is that the amount of strain can't be quantified by this method.  However, experiece provides some guidelines for what strain might cause problems later on during optical work, so it is still a very useful test.

The main barrier to people being able to do this test themselves seems to be the lack of a large linear polarizing sheet and a suitable light source.  This has become a lot easier to come by in recent years since the popularity of LCD TVs, computer monitors, and even tablet PCs has exploded.

LCD screens produce polarized light because polarization is used to turn on and off the pixels in these types of screens.  First, the light passes through a linear polarizer within the screen, then the pixels themselves act like tiny polarizers - depending on the state of the polarization, the light either passes through, is attenuated, or is almost completely blocked (as with crossed polarizers).

So, if we display a white screen on an LCD device, the white light we see is linearly polarized.  We can place a piece of glass in front of it, and then rotate a small piece of linear polarizer material in front of the eye or a camera and observe the results.

So, today I created an image of a white rectangle, uploaded it to my iPad, put a known strained piece of glass on top of it, and then set a linear polarizer on top of that.  I photographed the results, as seen below.

The first image shows the polarizer rotated parallel to the polarizer in the iPad:

Polarizers parallel

The second shows the "crossed" polarizer condition, where the polarizer is rotated perpendicular to the polarizer in the iPad:

Polarizers crossed, or perpendicular

The results are obvious - strain patterns are easily observed.  This glass is seriously "posessed" and I'm only keeping it around as an example of bad strain.

Note that the second polarizer, sitting in the black matteboard frame on top of the glass, could be much smaller, and only has to be big enough to cover your eye or your camera lens.

This test works for all the LCD devices that I know about, including big TVs.  This means I can now get a nice TV for the shop and be completely justified in calling it a business expense!

To see how the test is typically done, you can check out this page.

To see more strain tests to help you figure out if your mirror has a little or a lot of strain, see this article.

Please check back for future installements of "In the Shop".

Mike Lockwood
Lockwood Custom Optics

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