When the invisible ink written on the cards, it cannot be seen by naked eyes, but can be seen by IR camera lens or contact lenses. This ink if you use for IR camera, then any cards is suitable, but if you use for contact lenses, then the red cards is OK and better under yellow light.
We also sell invisible ink for printer. We offer ink and technology and machine to help you to mark the cards for marked cards contact lenses. Also, we can offer you many kinds of latest ink for different contact lenses. Do you want to see the invisible ink on back cards? Please tell us which kind of contact lenses you want!