About Marked Playing Cards Recognition
There are many ways that playing cards can be marked, but the basic idea is to identify the value of the card and even its suit. There are a number of different methods that cheats can use to do this, from very subtle ways (such as colouring in or scratching off small areas on the back of a card) to more advanced methods such as luminous or juice marking.
Block-out work involves adding ink to certain details on a card, for example making a flower petal narrower or blocking out a bird on the back design. This sort of mark is difficult to detect, even by experienced players.
More recent science and technology have enabled a number of more sophisticated techniques for marking cards, including shade systems which allow the marks to be read close-up but require special contact lenses or gimmicked sunglasses. There are also luminous marked decks that use invisible ink which can be detected with special filtered glasses.
There are even some techniques that make the marks on a card appear as if they are inked on the face of the card, but are actually printed on the backs in an infrared invisible ink. This allows the marks to be scanned and read using a special poker scanner. Usually these cards are screened for any signs of contamination before they are sold to consumers, which is important because the chemicals used in these types of decks can be very toxic.