600 HANDBOOK OF PHOTOGRAPHY 



Fluorescent-light Method. — The same arrangement of the lights can be used as in 

 the previous method, but it is essential that the ultraviolet transmitting filter be used 

 in front of them. A filter must be used on the lens to absorb the reflected-ultraviolet 

 and transmit the visible fluorescence radiation freely. The arrangement shown in 

 Fig. 1 is satisfactory, using a single Uviarc burner in conjunction with the Corning 

 No. 586 filter and the Wratten No. 2A filter on the lens. In the conditions depicted, 

 the exposure will be of the order of 20 min. at//4.5, using the Wratten Panchromatic 

 Process plate. 



Applications of Ultraviolet Photography. Criminology. — The use of ultraviolet 

 photography in detecting alterations in documents, in the examination of watermarks 

 and postage stamps, and in revealing invisible writing will be dealt with in the section 

 on Documentary'- Photography. One of the most important applications of ultraviolet 

 documentary photography in the field of criminology is in the detection of forgery. 

 Forged bank notes have been detected by the fluorescence of the papers but particu- 

 larly by that of the printing inks, which may be of a different nature and so fluoresce 

 differently. Alterations in the inks of written or typewritten documents may be 

 detected by the fluorescence method, although it is not always reliable for identifying 

 a particular ink. Writing may be deciphered on stained and worn documents and 

 specific inks detected on used blotting paper if they have characteristic fluorescence. 

 Textile fibers, dyes, stains, and markings may be compared by their fluorescence. 

 Stains of blood, urine, seminal fluid, and manure on clothes or other objects, and 

 vegetable particles, organic chemicals, pigments, grains of mineral origin, and other 

 clues have been identified or differentiated by fluorescence photography. Finger- 

 prints cannot usually be identified directly, but they are revealed quite clearlj^ if 

 dusted with a fluorescent powder, such as anthracene, before photographing. The 

 method shows some promise for the detection of drugs, etc., in body fluids. Adhesives 

 and sealing wax which may appear identical to the eye may show differing fluorescence, 

 so that tampering with letters and packages may be detectable. Its importance in 

 the determination of the authenticity of works of art will be treated in a later section. 



Documentary Photography. — One of the most important applications of ultraviolet 

 photography is in the examination of documents, particularly those which have been 

 altered by age, or wear, chemical or mechanical erasure, substitution, and overwriting, 

 and in the detection of secret writing. The fluorescence method is the more useful 

 because paper, parchment, pigments, etc., fluoresce when irradiated with ultraviolet. 

 The extent of the fluorescence varies with the nature of the material and the treatment 

 which it has undergone. One of the earliest and most successful applications of the 

 method was in the deciphering of the original writing on palimpsests. These are 

 manuscripts, usually on vellum, from which the original writing has been mechanically 

 erased by fine pumice, in order that the clear surfaces so obtained may be used again 

 by another scribe. Invisible traces of ink or pigment left embedded in the vellum maj^ 

 fluoresce to a less or greater extent than the vellum itself. Usually it is less, and if 

 a fluorescent-light ultraviolet photograph is made, the original writing will be revealed 

 in the negative as lighter than the background. The method may be applied to other 

 documents in which erasures have left invisible traces of the original writing. Era- 

 sures due to wear may be photographed iij the same manner. 



Chemical erasures may often be successfully revealed by the fluorescent-light 

 method. Ink eradicators are frequently used by forgers to remove original writing for 

 which other writing or typewriting is substituted. Bj^ visible light the rewriting 

 alone is usually seen, while ultraviolet photograph}'- will often disclose the original 

 writing. In many cases it provides the only reliable test. Secret writing by so-called 

 "invisible inks" may be similarly revealed. In general, where the paper has been 

 chemically treated locally for any purpose, it is probable that the areas of change can 



