20
VSC
®
: Diffraction
VSC
®
: UV fluorescence
Excitation lig
(UV)
Excitation light
(UV)
Additional Functions
Angular contrast
Light may be reflected or diffracted by the topology or microstructure of a security feature only at particular angles.
Retro-reflection
Retro-reflective security features become visible when a semi-silvered mirror is used to illuminate the document coaxially
with the viewing direction.
Variations in surface relief (e.g. embossing or indentations) can be revealed by the shadowing produced by oblique
illumination.
Diffraction
Holograms and other diffractive optically variable devices (DOVD) may reveal different detail according to the particular angle
of illumination.
Fluorescence
Fluorescence examination is a widely applied forensic inspection technique for generating visual contrast from a variety of trace
materials when normal reflectance or transmittance variations are small. The effect was named by Sir George Stokes (1819 -
1903) after the mineral fluorspar (calcium fluoride, CaF
2
), in which the phenomenon was first observed. The process involves
absorption by a feature of particular wavelengths from the illuminating excitation source and re-emission of fluorescence light of
a different wavelength. Most commonly, the fluorescence light has undergone a Stokes shift to a longer wavelength (i.e. a colour
shift away from the excitation light, towards the red end of the spectrum). In general, the spectrum of the excitation light must
always be chosen to match suitable absorption bands in the feature.
UV fluorescence
UV Fluorescence can be stimulated from both trace materials and certain security features using a suitable UV source.
VSC
®
: Retro-reflection
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