Didymium Glass Filters are usually sold as "color enhancers" or "color intensifiers." They seek to highlight certain colors without upsetting overall white balance. For example, red intensifiers are used to punch up the reds in photographs of fall foliage,
red rock country and sunsets. Althought the actual effect may vary from film to film, it can be estimated by viewing the scene through the filter. The estimate of the amount of color cast that any neutral or light shades will pick up in the finished image is given by the magenta color of the filter itself.
Didymium may sound like a chemical compound to an ordinary Joe, but the actually it's just a mixture of two rare earths—praseodymium and neodymium. This type of filters are produced in very thin glass sections with a thickness of between one and two millimeters, because the amount of light absorbed by the filter is dependent upon filter thickness. Artifacts such as colored backgrounds and degradation of other stain colors are introduced by using thicker
didymium filters. With these elements, the glass acquires a very spiky absorption spectrum chemically tunable between near UV and near IR wavelengths. Technically, nothing is being "intensified" here. The target color is just increased in apparent saturation by surgically attenuating adjacent bands, hopefully without upsetting the overall color balance too badly.
