f17r

[Bernd]

Spectralon is known to attract dirt due to its porosity. Some mottling is normal but the dark spots are clearly dirt. I don't think this has any significant impact though.

I agree that it is highly unlikely the ink consistently has exactly the same reflective properties as vellum (+underlying paint) in IR. Therefore I think we can safely assume the ink becomes transparent in IR.

I think trying to replicate the spectral properties of the ink would make a nice master's thesis. I wonder why this was never done considering the MSI images are over 10 years old. At least testing a few common contemporary inks for their properties in UV and IR shouldn't be too hard.