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Blacklight ink or blacklight-reactive ink is ink that glows under a black light, a source of light whose wavelengths are primarily in the ultraviolet. The paint may or may not be colorful under ordinary light. It is also known as luminous paint or fluorescent paint.
Blacklight paints can be mixed with similar shades of normal pigments, ‘brightening’ them when viewed in sunlight.
The word ‘dayglo’ has become a genericized trademark, as it is used as an ordinary noun while ‘Dayglo’ is a registered trademark of the DayGlo Color Corporation.
The invention of fluorescent paints is attributed to Robert Switzer, who was confined to a dark room after a fall, and his brother Joseph, who was a chemistry major at UC Berkeley, in 1934. They took a black light into the storeroom of their father's drugstore looking for naturally fluorescent organic compounds and from that developed paints.
Blacklight paints and inks are commonly used in the production of blacklight posters. Under daylight, the ultraviolet light ordinarily present makes the colors especially vivid. Under blacklight (with little or no visible light present), the effect produced can be psychedelic. The inks are normally highly sensitive to direct sunlight and other powerful light sources. The fluorescent dyes cause a chemical reaction when exposed to high intensity light sources (HILS) and the visual result is a fading in the colors of the inks. With paper, significant visible change in the color saturation can typically be observed within 45 minutes to one hour of exposure to the HILS. To date, there is no absolute method to prevent this phenomenon, although certain laminations, lacquer coatings and glass or plastic protective sheets can effectively slow the fading characteristics of the dyes.
Other common usage of the blacklight inks is in security features of money notes, various certificates printed on paper, meal coupons, tickets and similar things that represent a value (monetary or otherwise). The blacklight printed figures used for this purpose are usually invisible under normal lighting, even when they are exposed to direct sunlight (which contains ultraviolet light) but they show up glowing when exposed to blacklight source. This defeats simple and inexpensive attempts to counterfeit them by scanning the original using a high resolution scanner and printing them using an inexpensive high resolution printer (most if not all inexpensive printers do not allow using blacklight inks for printing) and no special equipment is needed to verify the presence and correctness of this feature (an inexpensive blacklight source being all that is required). Some coupons and tickets use colorful blacklight inks.
On many German locomotives the control panel labels were printed with blacklight paint and a blacklight source was provided in the cab. This left the driver with full night-vision while still enabling him to distinguish between the different switches and levers to operate his locomotive.
Blacklight paints are sometimes used in the scenery of amusement park dark rides: a blacklight illuminates the vivid colors of the scenery, while the vehicle and other passengers remain dimly lit or barely visible. This can enhance the effect of being in a fantasy world.
Some high-visibility clothing worn in survival and rough weather situations sometimes have dayglo strips and patches for conspicuity.
- Robert Switzer, Co-Inventor Of Day-Glo Paint, Dies at 83, NY Times, 1997-08-29.