The need for more and more secure documents, especially banknotes, ID papers, tax banderoles, security labels and product marks, stimulates the growth of the invisible ink industry. Criminal activities related to document forgery also contribute to the expansion of the invisible ink market. A nontoxic luminous security ink that emits visible light upon being exposed to ultraviolet (UV) light is now available to combat such crimes.
There is a wide range of substances that can be used as invisible inks to write messages. Students will need to investigate a few of these materials to find out how changing the conditions under which they are used affects their visibility.
Amongst the most popular invisible inks are organic substances that can be rendered visible by heating them. Usually, heat is applied by placing the paper on a radiator or using a hair dryer. Other methods include placing the paper in a pan of hot water or applying an acidic solution. Alternatively, some invisible inks contain a special fluorophore that only becomes visible when it is illuminated with far red/infrared (FR/IR) or UV sensitive light. These inks are often found on cheques and security paper. IR1 invisible ink can also be used to mark items that need to be marked in order to control the process of production; for example, glue presence during the production of paper bags. In this case the IR fluid is mixed in very small quantities with the glue to mark its presence. The infrared marking can then be detected automatically by a laser sensor.