This 6.3" Round UV Pass Double Coat Filter from Rosco is a dichroic filter that blocks visible (normal) light and allows only UV-A light to pass through for a true "black light" effect. This type of light is often used in fluorescent tubes in entertainment venues. This filter drops into a holder that fits a fixture appropriate to its size. The coating on each side of the glass (double coating) makes it a virtually pure UV source.
Besides entertainment venue use, UV-A emitting sources are used in forensics and other scientific fields, particularly in observing fluorescence – the colored glow that many substances emit when exposed to UV. These filters are often employed for diagnostic and therapeutic uses in medicine to detect substances tagged with fluorescent dyes, for rock-hunting, for detecting counterfeit money, and for curing plastic resins.