Many readers, especially students and researchers, avoid highlighting physical books because permanent marks reduce resale value or violate library policies. Digital highlighting lacks the satisfying physical interaction of marking pages directly. There may be an opportunity for a solution that offers temporary, selective highlighting while keeping documents pristine for others.

A Highlighter That Only You Can See

One approach could be a specialized highlighter pen with ink that's invisible under normal light but becomes visible when viewed through companion glasses. The system might use UV-reactive ink combined with glasses that filter light to reveal the highlights. Users could mark up documents freely, knowing the marks won't be visible to others without the glasses. This preserves documents while maintaining the tactile experience of physical highlighting. Different ink colors could be developed for color-coding, and the glasses could be designed as lightweight, stylish accessories or clip-ons for existing eyewear.

Potential Applications and Advantages

This could be particularly useful for:

• Students annotating expensive textbooks they plan to resell
• Researchers working with rare or shared reference materials
• Professionals who need to highlight sensitive documents

Compared to existing options like UV highlighters or digital apps, this approach would offer better privacy (your highlights aren't visible to others with UV lights) while keeping the physical interaction readers prefer. Early versions could use simple UV technology, with potential to develop more advanced light-filtering systems over time.

Implementation Possibilities

A minimum viable product might include one highlighter color with basic glasses. User feedback could guide refinements like comfort, ink durability, and additional colors. The business model could follow the razor-and-blades approach, with ongoing revenue from ink refills and replacement glasses. Partnering with textbook publishers or libraries could help drive adoption.

Testing key assumptions - like whether users will consistently wear the glasses - would be important before large-scale production. If successful, this could create a new category of stationery that bridges physical and digital reading experiences.