A special lens allows you to see through cards. These lenses are tinted with a darkening purple filter in the center of the lens. The lenses come in a range of tint sizes that will fit different eye colors. They appear normal to others, but they allow you see the luminous markings on the backs the invisible marked cards deck.
The card reading systems uses high-resolution imaging to detect small marks or problems on cards, and to register their rank and suit. The card image is captured preferably at a single card corner. The use of polarized or non-polarized lights can be used to enhance contrast and identify pinholes and scratches.
Use a lens with excellent coatings or a filter to reduce the purple fringing. A UV filter is essential if you work in an environment with a lot UV sources, such as light bulbs. You can avoid this issue if you avoid scenes with a lot of contrast and stop down to a smaller opening. If you cannot avoid this issue, there are several easy ways to fix your images in a program like Adobe Lightroom. There is a setting in the Lens Correction section of the Develop tab that can correct this issue, but it may also remove other parts of your photo.