by Lin Yangchen

This composite image shows the stamp as seen in daylight (right half) and under longwave ultraviolet irradiation (left). The UV excitation source is an array of indium gallium nitride light-emitting diodes with peak excitation wavelength 365 nm. A Baader Venus filter eliminates visible light contamination from the UV source. Planarity of the stamp was guaranteed by placing the specimen under an optically polished fused silica plate, which transmits ultraviolet, visible and infrared radiation. The plate has a coating that reduces surface reflections to 1.5% or less.

UV source with Baader U2 Venus narrow-bandpass (320—390 nm) interference filter.

In laser-induced ultraviolet fluorescence spectroscopy, stamps were spot-illuminated with a 325 nm helium-cadmium ultraviolet laser (Kimmon IK Series) through a Thorlabs UV (325–500 nm) 40× microscope objective, with a resultant power of 5.16 μW.

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