Invisible Ink - Application and Use

Application and Use

Invisible ink is applied to a writing surface with a specialty purpose stylus, stamp, fountain pen, toothpick, or even a finger dipped in the liquid. Once dry, the written surface should appear blank, with a similar texture and reflectivity as the surrounding surface. With letters, a cover message should be written over the invisible message, as a blank sheet of paper might arouse suspicion that an invisible message is present. This is best done with a ballpoint pen, since fountain pen ink may "run" when it crosses a line of invisible ink, thus betraying the presence of invisible ink. Invisible ink should not be used on ruled paper as it may similarly alter or streak the colour of the lines.

The ink is later made visible by different methods according to the type of invisible ink used. The ink may be developed by heat or by application of an appropriate chemical, or it may be made visible by viewing under ultraviolet light. Inks which are developed by a chemical reaction may depend on an acid-base reaction (like litmus paper), reactions similar to the blueprint process, or any of hundreds of others. Developer fluids may be applied using a spray bottle, but some developers are in the form of vapours, e.g. ammonia fumes used to develop phenolphthalein ink.

One can obtain toy invisible ink pens which have two tips—one tip for invisible ink writing, and another tip for developing the ink. Also, invisible ink is sometimes used to print parts of pictures or text in books for children to play with, particularly while they are travelling. A "decoder pen" is included with these books and children may rub this pen over invisible parts of texts or pictures, thus revealing answers to questions printed in regular ink or completing missing parts of pictures.

Security marker pens or UV Markers with fluorescent ink that glows when illuminated with a UV light may be used to invisibly mark valuable household items in case of burglary. They may be especially formulated for writing on non-porous surfaces such as glass, plastics, etc. The inks are applied and then identified using a black light or other UV light source. The owner of a recovered, stolen item which has been marked in this way can be traced simply by using an ultraviolet lamp. Security marker pens can be obtained commercially and are widely used as a crime countermeasure.

Some commercially available invisible inks glow very brightly, in a variety of colours, under UV light. This makes them suitable for use in readmissions such as hand stamping.

There is a commercially available red invisible ink which is only invisible when applied to certain types of surfaces, but visible on others.

Some vendors now offer invisible ink for use in computer inkjet printers. Such inks are usually visible under ultraviolet light. Typical uses include printing information on business forms for use by the form processor, without cluttering up the visible contents of the form. For example, some United States Postal Service mail sorting stations use UV-visible ink to print bar codes on mailed envelopes giving routing information for use by mail handling equipment further down the line before delivery.

Very rarely, invisible ink has been used in art. It is usually developed, though not always. There are artists who use the effect in conjunction with invisible and other reactive inks and paints to create a variety of effects when used in conjunction with UV lights.

An E2E voting system called Scantegrity II uses invisible ink to enable the voter to obtain a confirmation code only for the voted selection.