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.