|Polarised Light. Sunlight and almost every other form of natural and artificial illumination produces light waves which vibrate in all planes. If the light waves are passed through a polarising filter the waves are restricted to a single plane. If two polarising filters are placed in the light path, with the plane of polarisation at right angles to each other, then all light is blocked. The concept of using two polarisers oriented at right angles with respect to each other is commonly termed crossed polarisation.|
When some types of transparent objects, such as certain plastics or substances of a crystalline nature, are placed between the filters, the plane of polarisation gets rotated at different angles depending on the wavelength, thus producing different colours.
The first three images are of thin mica washers, as used for electrical insulation between a power semiconductor and a metal heat sink, and displays the crystalline structure. The next four are of drawing instruments and display stress points within the plastic.
Dichroic filters. A dichroic filter (thin-film filter or interference filter) is a very accurate colour filter used to selectively pass light of a small range of colours while reflecting other colours. Dichroic filters can filter light from a white light source to produce light that is highly saturated in colour. The three filters in my images were taken from a photographic enlarger colour head, where filters of the three subtractive complimentary colours (yellow, magenta and cyan) are used to control the colour balance of the light source.