Thermochromic Ink is ink that changes color when exposed to heat and then changes back to its original color when cooled.
This Thermochromic Ink feature is effective primarily in document-present situations where the original document can be examined by a trained professional or scanned with a specialized 3-illumination ID scanner and analyzed with adequate forgery detection technology.
Usability in online and mobile onboarding situations is challenging as the examination is made not on the physical document but on a photograph or scanned image of it.
Ref.: https://www.perfectcloud.io/resources/glossary/