Chromotropic security fibres are fibres which have one colour in daylight and are then visible in the security paper, but have a second colour of fluorescence under UV lights.
Those fibres are usual in security documents and offer a third level of security, more sophisticated than fibres which are visible and fluorescent in the same colour.
Examples of those fibres are:
- visible blue in daylight and fluo green or red under UV light,
- visible green in daylight and fluo yellow or red under UV light,
- visible yellow in daylight and fluo blue or green or red under UV lights,
- visible orange in daylight and fluo green or red under UV light,
- visible red in daylight and fluo blue or green under UV light,
- visible violet in daylight and fluo blue or red under UV light.