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.
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.
- 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.
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).
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 if using film rather than a digital camera.
Routinely I use 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. For digital again it is best to use the lowest ASA i.e for most cameras 100ASA.
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 also good to take a control photograph under normal light as above showing normal and fluorescent photographs of a stalagmite.