Making a Polariscope or stress detector

What it does

A polariscope uses light polarising filters to manipulate light to simplify the detection of stress in glass.

‘Stressometer’ is the name given to one form of polariscope designed by the Bullseye people. They are sometimes available commercially, but failing that can be made from readily available materials.

What you need

The one shown in the following images was made from an Eveready ‘Red Alert’ model 736 torch.

At time of writing the Arlec ’White Pointer Lantern’ WPL300, as well as one bearing the Duracell brand also appear to be suitable.

These torches use a square 6 volt battery and have a square body so that they can be stood with the light beam pointing vertically upwards. They also have a round screw-on bezel. Avoid torches with ‘hoods’ or angled bodies.

Other materials required:

• Some ‘duct’ or similar adhesive tape.
• Piece of 2mm float glass or a circle and four squares as detailed in steps 2 and 9 below. 2mm float is sometimes called ‘picture glass’ as it is widely used in picture frames. Available from a glass merchant or picture framer:
• Piece of polarising filter. A suitable product is ‘Rosco linear polarising filter’. Product identifier is 07300. Available in various sheet sizes from Rosco dealers, including Borge Andersen & Associates in Melbourne.A convenient sheet size for this application is 480mm x 100mm.  

Making the stress detector

Fig 31-01x

1. Dismantle the torch and lay out the parts. Separate the bezel, lens, white flexible plastic clamp ring, noting the correct position of the clamp ring.

2. Identify the front plastic ‘lens’ or ‘glass’. The item from the Eveready torch is curved and must be replaced by a circle of 2mm glass the same outside diameter as the lens. Dispose of the plastic lens.
   

3. Cut a disc from the polarising filter the same diameter as the glass in 2.  This material is rolled up when supplied and is springy and difficult to flatten out. 

4. The parts are now ready to assemble and are shown in Fig 1. Assemble globe into switch bracket and fit switch bracket to reflector. 

Fig 31-2

5. Place filter disc against glass disc and offer up to the reflector so that the filter is between glass and reflector, as in Fig 2.
   

Fig 31-3

6. Fit clamp ring as in Fig 3.
   

7. Insert the battery into the torch body and then fit the lens and reflector assembly.
   

8. Screw the red bezel over the top to hold all in place.
   

9. Cut four squares of 2mm glass 100mm x100mm.
   

10. Cut two squares of polarising filter slightly smaller than the glass pieces.

Fig 31-4

11. Thoroughly clean glass. Sandwich one piece of polarising filter between two squares of glass and tape the edges to hold them securely together, as in Fig 4. Repeat with the remaining pieces.

For how to use it see ‘26.Compatability’.