hiddent IR message - can anyone test this?
I am just wondering if this particular idea works.
Just get a mirror(or maybe even better a retro reflective surface for camera flashes) and using a black marker... write a message on it
After that Place a infrared passing filter on top of the message... it shall look black
Now take a photo of it with flash inside a dark room... and also a photo of the object in a very sunny day outside. (keep note to take it from different angles.)
Once you done that... tell us if it works! :)
Another idea to see if it works is to replace the mirror with white paper... perhaps it may better scatter the sunlight for daytime use.