Wednesday, 29 July 2009

Polarizing Filters

Using polarized light to show stress works on the principle that stressed glass bends the light as it comes through the glass [but see the first comment below]. As polarized light filters placed at right angles do not allow any light through, only unstressed glass will continue to appear dark. If there is stress the light is bent and becomes visible through the filters.

You can buy stress testing kits that incorporate a light source. You can also make your own. You need polarizing lighting gels. These come in sheets and are available from theatrical lighting sources. You will need to frame these in stiff card to keep them flat.

You use them over a light source. Place one filter down above the light source. Place the piece to be tested on top. Then orient the top filter so that the minimum amount of light shows through the filters. Any stress will show up as a light source. The greater the intensity of the glow, the greater the stress the glass is exhibiting.

Note that the surface through which the light comes should be rigid, as any deformation of the surface will give a false reading.  The light filters through the slight curve and gives a stress reading, which may not be true at all.  Thus a firm flat surface is required, especially if you have a large light table for your light source.

Also note that the filters are normally on plastic sheets and easily scratched, so the glass should always be lifted and placed, rather than slid, to a new position.

A description of the compatibility test can be seen here.