Sunday, 3 June 2018

Polarising Filters

Using polarized light filters to show stress works on the principle that stressed glass rotates the polarisation direction of the light as it comes through the glass. 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 rotated slightly 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 amount of light rotation depends on the stress direction, magnitude and light path length. The greater the intensity of the glow, the greater the stress the glass is exhibiting.   The amount light visible through the filters is wavelength dependent, as the filter transmits light with a particular polarisation direction. If there is large stress, different colours will be visible. 

This example shows extreme stress by the rainbow effect of light rotated in multiple directions

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.

revised June 2018


  1. Dear Stephen,
    The explanation "stressed glass bends the light as it comes through the glass" is physically completely wrong. Stressed glass does not bend the direction of a light ray any differently from unstressed glass. It rotates the polarisation direction depending on the stress direction, magnitude and light path length. The amount is wavelength dependent, the filter transmits light with a particular polarisation direction, hence the different colours.
    It may be necessary to simplify the explanation, but the physical facts should be correct.
    Best wishes,
    Jerry (glasspusher)
    PS see for some notes on birefringence

  2. Would the following be a good filter to purchase two of per your instructions?

    Rosco Cinegel #3114: Tough UV Filter - 20” x 24” Sheet
    The material allows less than 10% transmission below 390 nanometers. Mired Shift +8. Absorbs excess UV output of standard fluorescent bulbs and HMI with minimal color temperature shift. Deep-dyed base. (Transmission = 93%). (FStop: -0.1s)

    NOTE: A black light filter passes UV. Rosco's filter absorbs UV.

    1. These sound very technical to me. I bought simple polarised film to be used in lighting rigs for theatre. In this area, simple and cheap seem the best. These are often referred to as "gels" in the photographic and theatrical circles.