Chromotropic stitching threads are threads which have one colour in daylight and a second colour under UV light.
Those threads are usual in security books and offer a third level of security, more sophisticated than threads which are visible and fluorescent.
Examples of those threads 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 light,
- 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.