Macro-scale biological processes, such as the spread of virus infections, can be followed using GFP labeling.  In the past, mutagenic ultra violet light (UV) has been used to illuminate living organisms  (e.g., see) to detect and photograph the GFP expression.  Recently, a technique using non-mutagenic LED lights have been developed for macro-photography.  The technique uses an epifluorescence camera attachment based on the same principle used in the construction of epifluorescence microscopes.