LCD Monitor as a Polarimetric Stress Viewer

LCD monitors have polarizing filters that make great, large aperture polarized light sources for viewing the stress patterns in transparent plastic.  All you need is an LCD monitor, a pair of polarized sunglasses or a polarizing filter for your camera, any transparent piece of plastic, and some way to generate a white screen.

Making the white screen:  download a free copy of irfanview here, install it, and start it up.  Under the Image menu, select "create new".  Enter the image width and height in pixels to match the resolution of your monitor (usually 1280x1024).  Set the background color to white.  Click OK and you should now see a white screen with the irfanview menus at the top and bottom of the screen.  Press the "enter" key on your keyboard to turn off the menus and you'll have full screen whiteness.  You can save the image if you want for future use, or just regenerate it each time you need it.

If you look at the screen with polarized sunglasses, you should see the white screen.  If you tilt your head to either side the screen will get darker.  Hold a piece of transparent plastic up in front of you with the white monitor screen behind the plastic.  You should see the stress pattern of light and dark and colored bands running throughout the plastic.

If you have a polarized filter for your camera, you can photograph the stress pattern.  Try rotating the filter while looking at the white LCD screen until the image of the screen goes black.  Now hold up a piece of plastic between the screen and the filter and you'll see beautifully illuminated color bands showing the stresses in the plastic.  You can photograph the pattern if you want, just as easily as viewing it.

The picture here is a CDROM blank disc, the type they put at the top and bottom of the stacks that come in the spindle-type packages.  The dark bands and colors indicate areas where there are residual stresses in the plastic left over from the molding process.

More experiments:

Try squeezing a piece of transparent lucite with a C clamp and view the stress bands.

Flex a clear CDROM blank (like the one in the picture) while observing it.

 

See this page to learn a little more about what is happening here.

You can find even more information by searching for the topic "photoelasticity".

7/4/2006

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