Fluorescent Light vs. a Light that Excites Fluorescence

People often refer to lights for bringing out (exciting) fluorescence – whether for night diving or in the lab or field – as ‘fluorescent lights’. While there is no harm in that, they aren’t, and we try to be technically correct.

‘Fluorescent lights’ are what you commonly find in office buildings. They are called that because they emit their white light by means of fluorescence. The tubes are filled with a low pressure mercury vapor. The electronics that drive the lamp excite the mercury atoms, causing them to emit electromagnetic radiation with maximum emission at specific wavelengths. The strongest emission is at 254nm, fairly far down in the ultraviolet, with additional lines at 365 (longwave UV), 405 (violet), 436 (blue), and 546 (green) nm, plus additional weaker lines. It would be a bad thing if the 254nm light were allowed to escape the lamp since that wavelength can cause skin damage, cataracts, and other problems! The tube is coated with a phosphor that absorbs the UV light and transforms it to a broad range of wavelengths by fluorescence. By varying the chemical composition of the phosphor coating, manufacturers can adjust the spectral output to make ‘warm white’, ‘cool white’, and other color variations. [Side note – some fluorescent tubes are specifically manufactured to emit the 254nm light, and these are used for sterilizing water and various surfaces.]

The lights for seeing fluorescence do not operate by fluorescence like your office lights – they are designed to cause (excite) other things to fluoresce. For the flashlights and other steady illuminators we use high intensity light-emitting diodes (LEDs). For electronic flash units we provide custom filters that only allow the desired fluorescence excitation wavelengths to pass. A proper terminology for these lights would be ‘fluorescence excitation light’ or ‘fluorescence exciter’, but that’s a mouthful.