How is IR ink able to be seen with specialized filters?

[Gerome]

TL;DR Summary

Different studies on invisible infrared inks, naked eye detection and what allows us to see this from the naked eye.

I stumbled upon an old thread from 2014 about invisible IR ink and have been looking into the same thing very recently.
https://www.physicsforums.com/threads/does-infrared-ink-exist.744888/

So far on my research I have found an IR ink that absorbs Infrared. https://www.sciencedirect.com/science/article/abs/pii/014372089400068D
and further exploring this I have also found a few other papers on the detection method of this and how it can be seen to the naked eye purely just using filters on glasses or contact lenses. http://iis.sejong.ac.kr/publications/2014-FSI-02.pdf This is a quote from this paper: "On the other hand, IR-active invisible ink patterns become visible with infrared transmitting filters, which pass the light of wave length longer than the red light spectrum. Infrared filters come in various forms of eyewear, such as contact lens or eyeglasses.". The filter that seems to be commonly used to see the ink is a dark violet red filter. Unfortunately this chemical (SiNc) seems to mainly be used for gambling cheats for marked cards.

From my understanding IR light is invisible to the naked eye and no matter what kind of filter is used it cannot be seen without the use of specialized cameras that can detect IR light.
My question on this matter is how it it possible for it to be seen using just some glasses or contacts with a filter on it and what is the light spectrum being seen. Can this filter be used as a form of night vision if you wore the glasses and shined a really bright IR flashlight or if not why not?

I have not found any papers on the specific nm of light passing through the filter that makes it able to be seen by the naked eye but have found a video demonstrating the effect on marked cards. Video demonstrating filter used to see IR ink.
My other question about the matter is why do the markings with the ink produce lighter characters if what is happening is the absorbance of of IR light.

Off note :I have also managed to find the manufacturer of the ink and am planning to investigate further by mass spec at my university and further understand what is going on.

 

[berkeman]

I learned a different way of sending messages via hidden "ink" during my early days in martial arts. If you are interested, send me a PM. It is much more practical if you are being held as a prisoner but are allowed to send written letters out...

(Or are you just interested in the Physics/optics of IR inks? If so, nevermind...)

 

[nightvidcole]

Most likely if the ink is designed to absorb in the near infrared, then there will be some absorption in the far red wavelengths also (700-800 nm). These wavelengths of light can be seen as a dim red light if you filter out the rest of the visible spectrum and have bright illumination.

 

[Gerome]

Most likely if the ink is designed to absorb in the near infrared, then there will be some absorption in the far red wavelengths also (700-800 nm). These wavelengths of light can be seen as a dim red light if you filter out the rest of the visible spectrum and have bright illumination.

Yeah they absorb infrared from 710 - 880nm depending on the ink you get. My other question is why does a purple red filter for this as from my understanding purple is closer to the wavelength of UV.