Ultraviolet light is directed onto the specimen by Woods Lamp so the fluorescence is visible or using an ultraviolet transmission filter over a flashgun termed the excitor. The barrier filter is an ultraviolet cut-off filter for example 2E.

The ultraviolet light, blacklight, causes the substance to emit ultraviolet light. At the same time some of the ultraviolet light rather than being absorbed (to cause fluorescence) is reflected so the barrier filter is used to prevent this reflected ultraviolet light from reaching the film so only the light emitted is recorded.

Compare the two pictures below with and without a barrier filter. The one on the right without the barier filter is more blue overall and the poppy seeds seem to be purple from the reflected ultraviolet.

Seeded bread - Left without barrier filter, Right with barrier fitler 2B.

Effect of using different UV cut-off filters

By changing the ultraviolet cut-off filter to one which cuts off more ultraviolet light it is possible to enhance the quality and definition of the colour image produced. It is important to make sure your filters do not fluoresce under UV as this reduces image quality. Kodak colour compensating filters can occasionally improve the purity of colours CC20Y + light balancing 81EF (Wratten 81EF is brownish used to lower colour temperature).

1A Skylight - Not recommended

2B Useful for cutting off excessive UV from flash and sky easy to obtain for UV fluorescence work.

2E Usual barrier filter for fluorescence work.

3, 6,9 or 12 Increasing cut-off deeper yellow can however cut-off some blue fluorescence.

Exposure times are in the order of seconds 1 - 10 seconds, f/5.6 - f/8, but may be longer with faint fluorescence so would need to allow for the reciprocity characteristics of the film.

Routinely I have used Velvia 50 ASA slide film for greater colour saturation but to begin with it is sensible to use a 400 ASA colour negative film which has a greater exposure latitude. Daylight film is the preferred choice but tungsten balanced film can also be used but it is more sensitive to blue light so gives a profound blue cast in the presence of any stray UV.

A digital camera can also be used but should be set on daylight rather than auto white balance. If doing comparative work it is important to take a control photograph under normal light the images to the left show normal and fluorescent photographs of a stalagmite.

What fluoresces?

Forensic uses include drugs e.g. Lysergic Acid (LSD) which can be detected by absorption and excretion of quite small quantities. Mineral oils fluoresce differently from vegetable oils e.g. compare mineral oil, linseed oil, paraffin, vaseline etc. using oil spots on paper. Precious stones and pearls fluoresce differently depending on their origins. Wool can be distinguished from cotton and silk.