X-ray glasses to see infrared marked cards
X-ray Glasses can see through some type of materials making it possible to see through marked playing cards that in the same condition cannot be seen by the naked eye.
The theory behind it is simple. Under normal light, the visible and infrared lights can pass through some type of material covering an object and are reflected by the object's surface.
The reflected visible light is too strong and saturated to see. Therefore the covered marks on playing cards can not be visible using naked eyes. However, if the reflected visible light is filtered out and only the reflected infrared and the required light is captured using special made sensitive cameras inside the Glasses, the infrared markings hidden on poker cards will be visible.
This is the working theory of X-ray glasses, infrared contact lenses and invisible ink marked cards.