Cross polarising effect is where you use a pair of polarising filters - one on the camera and one on the light source (behind the subject) to show stress in plastic which appears as wonderful multicoloured pattern. It used to be quite difficult as the filter needed for the backlight needed to be large and polarising gels are not only hard to find but also quite expensive. But there is a modern day solution and it's staring right at you! Your LCD computer monitor. Yes, this can be used as the light source and even though you didn't realise - it has the polariser built in.
You'll need to use a camera with a polarising filter either attached or positioned in front of the lens. Of course, you'll need a monitor and a steady support will also be useful. A tripod is ideal but it could be something as simple as a pile of books just so long as the camera is set to the correct height.
Some clear hard plastic items - filter cases, plastic cutlery, plastic glasses, cassette and cd cases or geometry sets are a few options - will also be needed as well as something to hold the plastic item steady in front of the computer monitor. Where possible ensure your items are scratch-free as these can spoil the effect.
Step 1. Turn the monitor on and remove any desktop clutter and change the screen saver to a neutral colour. The other option is to create a plain mid grey image about 800x600 pixels and call it grey.jpb. Drag this to the web browser to display it and then hit the f11 key to display the browser full frame without menus. And then hold down the ctrl key and tap the + key until the grey image fills the monitor window.
Step 2. Position the plastic item in front and in the centre of the monitor. A clamp is a useful tool for this but really, whatever you use just has to get the area you want to photograph in the centre of the monitor.
Step 3. Attach the polarising filter to your camera and position the camera in front of the plastic so all you see is an area of the plastic with the grey image totally filling the background. If you can't get close with your camera use the lens at its telephoto setting to fill the frame and avoid edges of the monitor showing. If you still can't get close enough you will have to crop off areas you don't want later.
Step 4. Rotate the polarising filter and watch the colours of the plastic become stronger and the grey go darker. If you have it set up correctly the grey monitor will go totally black, and the colours in the plastic will be brilliant.
You can use any exposure mode and shooting at the metered setting should be fine. Under expose a little if you want even more dramatic colours.
For an extra level of abstractness bring a second item even closer to the lens to make the whole image have a multicoloured filter. By experimenting you can often find undiscovered gems.