ameis33
Member 
What's in a name? That which we call a rose, by any other name would smell as sweet
Posts: 546
What I collect: Poland and Italy Republic
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Post by ameis33 on Jan 25, 2023 8:26:55 GMT
In a catalog i've read something like "UV-254 violet". I understand that viewing the stamp with a UV light 254 nm wave lenght the ink should show a violet color... But the 254 nm which kind of UV light is? Is perhaps the one used to see the phosphorescence i.e. in the Machins?
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khj
Member
Posts: 1,524 
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Post by khj on Jan 25, 2023 9:10:21 GMT
Can you cite a specific example?
254nm refers to the wavelength of the UV (ultra-violet) light used to illuminate the stamp to view the tagging or photoluminescent ink. The other commonly used wavelength is centered near longer UV wavelength of 365nm. While this can be used to detect some tagging, it also tends to cause a lot of paper to luminesce/phosphoresce as well. In the US, 99.9% of stamp tagging of water-activated stamps is detected using the relatively short-wavelength 254nm UV light. Those 2 wavelengths are typically chosen because anything longer, the taggant glow will be masked by the overall paper glow. In general, best results are achieved when you use a lamp that is "filtered" to remove those longer wavelengths. That's why you can't simply go out and buy a simple UV light bulb from the local store -- it's got too strong of a "tail" into the long wavelength UV range.
For British stamps, it will depend on the stamp, whether 254nm or 365nm works better. In some cases, such as some of the earlier experimental phosphors, neither results in strong phosphorescence, but still better than nothing.
The color in which the stamp "glows" ranges from green, red/orange, bright/dull white, pale yellow... and depends on the taggant/paper.
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angore
Member
Posts: 5,696
What I collect: WW, focus on British Empire
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Post by angore on Jan 25, 2023 11:05:23 GMT
It is good to have both shortwave and longwave lamps.
(Corrected - I originally post software rather than shortwave).
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angore
Member
Posts: 5,696
What I collect: WW, focus on British Empire
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Post by angore on Jan 26, 2023 11:36:11 GMT
I will add a note. The shortwave works better with stamps that used fluorescent paper. The paper can mask some of the phosphor aspects on some stamps. This is a table from Deegam Machin Handbook (copyright use under fair use for educational purposes) for Great Britain Machin issues. It shows how lamps can be used to study or identify varieties and dual nature of coatings and then toss in paper aspects. Now, for US stamps, I rarely see an afterglow since it is so short but obviously good enough for mail handling. Some Machins had tagging materials that had flourescent prosperities as well as phosphor properties so not really proper to call tagging just phosphor. It is a mixture. Some early phosphor materials lose their properties over time so do not respond. Soaking can remove/impair them,. One can try just looking for residual binder (part of the coating) left on stamp.  |
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