Here we see a photobombing trick, used by Harley Davidson.
They achieve to incrust a logo on a photo you take. The logo is invisible with naked eyes and only revealed on the photo you take.
How does it work?
The camera in a smartphone is sensitive to wavelengths of light that the human eye cannot see. Have a look at this picture from this article:
This is actually a Canon EOS 40D SLR, but the idea is the same. The human eye can't see any wavelengths longer than about 700nm, but the graph shows the camera can detect light out to 1000nm.
So if you project your message using light with a wavelength of, for example, 900nm your eye won't be able to see the message but the camera will, and it will appear on the photo.