Not all black/etc pens go clear under infrared photography, but here are the ones in my collection that do. With some on the side that have plastics that don't go clear at all that use plastic from around the same time period as the rest.
The oldest one in this grouping is the Sheaffer Triumph (1940s), and only the cap plastic goes clear on that, but not the barrel plastic. The Pilots and Montblancs are mostly from the 1960s to 1970s.
Five Views (4 on a Modified Camera, 5th reference on non-modified)
Infrared Only (850nm and up shown)
Infrared + Red Visible light (695nm and up)
Ultraviolet Only (no light above 400nm)
Full Spectrum (no blocking of Ultraviolet, Visible, or Infrared)
Visible Light Only (off a normal unmodified camera)
Some macro shots at 850nm+
Pilot Super 200 w/ 14K "Coarse" nib, can see the May 1966 date code under the plastic, tail end of the Pilot Super 250 above showing the switch filler.
Pilot Super 250, 14K Falcon, Montblanc 320, 14K EF, Montblanc 225, 14K XXF
Montblanc 221 (no transparency under IR), Montblanc 14 (faint translucence), Montblanc 225 (complete transparency thru grip and feed)
Sheaffer Snorkel Valiant and Statesman, The main plastic has no impact on IR light, but the grips do. Had I remembered to get a closer shot looking below, you could make out the snorkel tube when it's retracted.
Pelikan 400NN cap, and Sheaffer Triumph (1940s) Cap, the Body of the Sheaffer Triumph doesn't have any effect from viewing under IR. The cap seems to be a different black plastic than the body.
Pelikan 400NN, 14K Semiflex EF
Parker 45 w/ Accountant and XF Nib Unit , no impact at all, included because someone on reddit wanted to see if it would work.
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