The visible spectrum--the colours of light we can see with the naked eye--are just a small wedge of the full range of the electromagnetic spectrum. Just beyond their borders on either side are colours that other animals can see but that are invisible to humans without special equipment. Light waves just a little too long for us to see are in the ultraviolet range. A number of common materials fluoresce when illuminated under UV, or so-called black light.
One of the easiest fluorescent dyes which can be used as a UV ink is a mixture of the Indian spice turmeric, mixed with rubbing alcohol. Turmeric is the powdered root commonly used in curry powder and also the ingredient that gives yellow mustard its bright yellow colour. The ink, which is visibly yellow in normal light, is made by mixing a teaspoon of turmeric with an ounce or two of isopropyl alcohol, adding a teaspoon at a time stirring vigorously until it flows easily. Under UV light it glows bright yellow-green.
Household bleach and colour-safe bleach alternatives can also be used as invisible inks that will show up under black light
Ink From Medicine
Another, somewhat less common ingredient for making UV ink at home, but one that is invisible under normal light, is quinine--the ingredient in tonic water that gives it its tart-bitter flavour. Quinine sulphate tablets are commonly prescribed for treating malaria, among its medicinal uses. Quinine can be purchased from chemical supply houses as well. To make ink from the tablets, grind three tablets containing at least 200 mg of the drug for each ounce of ink, dissolving the powder in water. Carefully add sulphuric acid, a few drops at a time, until the ink stops fluorescing brighter.
Professional Grade Ink
A UV ink that is invisible under normal light can be made using an optical brightener--an ingredient used in laundry detergents to make coloured fabrics appear brighter. One common one is stilbene bistriazine derivative, sold commercially as Leucophor BSB Liquid. It is used as a brightening agent for paper, plastics and other textiles as well. Leucophor is available from industrial chemical suppliers, including Euro-Dragon GmbH, Zhejiang Hongda Chemicals Co. Ltd., Clariant International AG and others. The recipe is 1.5 parts Leucophor powder to 98.5 parts distilled water. The ink glows bright blue in black light.