Sunglasses

To reveal the hidden image in the frame, players wore special glasses that made two types of tiles visible. These glasses, like 3D glasses, and the tiles are made of plastic sheets called polarizers. To understand this, one must first grasp polarization. 

Light can behave like a wave, with oscillations in a specific plane — like a rope moving up and down. A polarizer acts as a filter, allowing only oscillations in one direction to pass. Using the rope analogy: if the wave passes between two sticks, only the movement aligned with them remains, while other oscillations are blocked.  

In the puzzle, this principle enabled the hidden image to appear. Each square "pixel" of the picture was made of a polarized filter, rotated to have either vertical or horizontal polarization. They were identical for the naked eye, but when looking through polarizing glasses, some squares appeared brighter or darker, depending on whether they were aligned with the glasses.

You can make your own hidden message, the description is given here.

• Polarisation: Wikipedia 
• Image source: schoolphysics.co.uk