This type of invisible ink can be seen through glasses and can be used to create secret messages on paper. This type can be seen using special sunglasses or a laser light that emits specific wavelengths of light. This ink is often used by spies to convey secret messages that cannot be read by the naked eye.
There are several types of invisible inks, including heat activated ink, light activated ink and chemically activated ink. Heat-activated ink can be made using substances that glow when heated, like laundry detergents and human saliva. Chemical-activated ink is made by using an invisible solution that changes color when exposed to certain chemicals, such as methyl ethyl ketone or nitrocellulose. Light-activated ink is made by using invisible ink that is only visible when illuminated by a particular wavelength of light, such as ultraviolet or infrared.
The longevity and quality of invisible ink depends on the ink type, the storage conditions and the quality of the ink. Inks of higher quality can last for many years when stored properly. Low-quality invisibles inks, on the other hand, may only last several days. Some invisible inks only appear when exposed to certain wavelengths of light such as infrared or UV, while others can only be viewed when viewed with a filter that blocks the wavelength of light.