Cross Polarisation
Some Theory
Some Theory
1. Ordinary light can be regarded as travelling in waves that vibrate in all directions at right angles to the direction along which the light is travelling.
2. Certain crystals will only allow electro-magnetic waves to pass through them if they are vibrating in one particular plane.
3. So, any light that passes through the crystal, emerges vibrating in the one plane along which the crystal is transparent.
4. Light which thus has its vibration restricted to a single plane is said to be plane polarised.
5. In this technique the object is placed between two polarising filters. By crossing one of the filters (in our case the one on the camera lens) we would normally stop all light from coming through.
6. If however the object affects the polarisation of the light it will become visible from behind the filters. Here the photo-elastic stresses in the clear plastic materials, which do affect such polarisation, become visible and it is these we are trying to record.
2. Certain crystals will only allow electro-magnetic waves to pass through them if they are vibrating in one particular plane.
3. So, any light that passes through the crystal, emerges vibrating in the one plane along which the crystal is transparent.
4. Light which thus has its vibration restricted to a single plane is said to be plane polarised.
5. In this technique the object is placed between two polarising filters. By crossing one of the filters (in our case the one on the camera lens) we would normally stop all light from coming through.
6. If however the object affects the polarisation of the light it will become visible from behind the filters. Here the photo-elastic stresses in the clear plastic materials, which do affect such polarisation, become visible and it is these we are trying to record.
The Practice
1. I started off with a light box. Normally used for viewing transparencies or negatives, you can obtain one for £29.99 from www.digitaldepot.co.uk. You could of course use a sheet of glass lit fron beneath with a diffused light but the big advantage of a lightbox is that its built in light source is daylight balanced.
2. Place a polarising filter, or better still if you can afford it, a sheet of polarising material (available as A4 or 10"x8") on top of the light box.
3. On top of the filter, place the plastic objects to be photographed.
4. With the second polariser fitted on the camera lens you will notice that as you rotate this filter, the cross polarisation effect will become visible. At a certain point in the rotation you will notice that the white light coming from the light box will go black, proving point 5 above.
5. At the same time though the stressed portions of the plastic objects will become visible as a rainbow of colour patterns.
6. Now is the time to shoot. I find that using manual exposure works best, with a stop or so of underexposure dialled in. This makes the blacks really black and the rainbow colours really rich and vibrant. Autofocus generally works well but if your system begins to hunt for a focus point you can always switch to manual focus instead. Because the lightbox delivers light the equivalent of daylight you will find that Auto White Balance will do just fine but you may need to switch to the tungsten setting if you are using the sheet of glass and artificial lighting set up.
2. Place a polarising filter, or better still if you can afford it, a sheet of polarising material (available as A4 or 10"x8") on top of the light box.
3. On top of the filter, place the plastic objects to be photographed.
4. With the second polariser fitted on the camera lens you will notice that as you rotate this filter, the cross polarisation effect will become visible. At a certain point in the rotation you will notice that the white light coming from the light box will go black, proving point 5 above.
5. At the same time though the stressed portions of the plastic objects will become visible as a rainbow of colour patterns.
6. Now is the time to shoot. I find that using manual exposure works best, with a stop or so of underexposure dialled in. This makes the blacks really black and the rainbow colours really rich and vibrant. Autofocus generally works well but if your system begins to hunt for a focus point you can always switch to manual focus instead. Because the lightbox delivers light the equivalent of daylight you will find that Auto White Balance will do just fine but you may need to switch to the tungsten setting if you are using the sheet of glass and artificial lighting set up.
Needs
One article made of clear extruded plastic, two regular polarising filters or one regular and one sheet of polarising material, one lightbox, one camera and a little bit of inspiration.
