Ultraviolet (UV) light (invisible light) is used in theatre for underwater scenes, flying scenes and other "fantastic" scenes where visible light might reveal too much of the magic. The safe type of UV light used is called "Black light". It causes certain pigments in fluorescent paint to glow brightly. These pigments can also be made to glow by using Congo Blue gel on standard incandescent lanterns. Although this light source is not UV, it has a similar effect on the pigments. Fabrics that have been washed with some detergents (with "brightening agents") will also glow bright under UV light, as will some paper. Some pigments are designed to glow under normal daylight ("dayglo" colours - bright orange, yellow and green are the most common) - the pigment in fluorescent pens is dayglo.

UV light can be produced by a blacklight fluorescent tube (appear to be dark purple, but that's the colour of the filter glass on the tube) or a UV Cannon which uses a large discharge lamp in a similar way. There is very limited UV light given off by an incandescent (dimmable) theatre lantern. Some moving lights include UV filters as the discharge lamps they contain give off a high degree of UV light.

DEFINITIONS:
Chambers Dictionary:
Fluorescence, the property of some substances of emitting, when exposed to radiation, rays of a greater wavelength than those received.

Shorter Oxford English Dictionary:
Fluorescence, the coloured luminosity produced to some bodied by the direct action of light, especially of the violet and ultra-violet rays.

Fluorescence: the emission of radiation that occurs when a molecule in an excited electronic state returns to the ground state. It involves excitation followed by emission.
(I. D. Campbell, R.A. Dwek: Biological Spectroscopy)

LINKS:

• Blue light fluorescence Site about underwater photography using blue light fluorescence in preference to UV light.
• Nocturn UV Effects (USA)
• UVFX (USA)
• Wildfire FX (USA)
• Glowbug Luminescent Colours (Ely, Cambs, UK)

IN DEPTH: (thanks to Charles Kaiser)

1. Å = Angstroms - 1 Angstrom Unit is equal to 1 hundred millionth of a centimetre or 1E-10 Metres.

2. 1 nanometre (1E-09 Metres) = 10 Å

3. The visible wavelengths of light are between 4000 - 7000 Å
(NB: The below figures are VERY general)

• Violet to Blue 4000 - 4500 Å
• Blue to Blue-Green 4500 - 5000 Å
• Blue-Green to Green 5000 - 5500 Å
• Green to Yellow 5500 - 5700 Å
• Yellow to Orange 5700 - 6000 Å
• Orange to Red 6000 - 6500 Å
• Red to Infrared 6500 - 7000 Å
  

4. The Ultraviolet wavelengths of light are between 100 - 4000 Å

• UVA 3200 - 4000 Å
• UVB 2900 - 3200 Å
• UVC 2000 - 2900 Å

A "blacklight" tube emits UV in the range of 2537Å which its phosphor re-emits at around 3650Å which will pass easily through the glass of the tube. About 2% of the total input power of these lamps can be emitted in the visible 4000 - 7000Å band. Most fluorescent dyes, inks and pigments will emit well into the violet to blue (4000 - 4500 Å) range of visible light. Most theatre lighting fixtures will also emit quite a bit of light in the range of 3500 Å to 4000 Å. Congo Blue gels pass a very high percentage of light in this range and the resultant UV emissions will cause a wide range of fluorescent dyes, inks and pigments to happily fluoresce.