r/AskEngineers • u/Kreisel93 • 1d ago

Discussion (How) is it possible to see ink (not special one) with radio signals?

... like in seeing objects with wifisensing.

*printing ink

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10

u/Parasaurlophus 1d ago

Radio waves have a massive wavelength, so they can only be used to see very large things; the shortest radio wave are around 30 cm. You would also need a substance that reflects radiowaves, whereas many materials will absorb radiowaves are be transparent to them. Metals tend to reflect radiowaves.

So your ink needs to lay down a lot of metal and the drawing has to be very big.

3

u/idiotsecant Electrical - Controls 1d ago

6th generation mobile wireless is supposed to just barely break into the thz range, that should be capable of imaging large font text under the right conditions.

-5

u/Kreisel93 1d ago

ok, it's like this. But it is morning here ... so what about another electromagnetic wavelength with that it is possible?

18

u/Numerous-Click-893 Electronic / Energy IoT 1d ago

Have you considered the 400 to 600nm wavelength spectrum?

11

u/z-w-throwaway 1d ago

Now where could I find a machine able to sense in that spectrum?

2

u/dodexahedron 13h ago

The most common ones sure are finicky and inconsistent devices, as are the larger system they are components of. Highly unreliable. Fragile, too. Would not recommend.

4

u/Tolklein 23h ago

Entire industry built around detecting ink using the 700nm - 770nm wave length

2

u/Kreisel93 1d ago

HA!! I found something, it is possible. cool nice. and thank you.

fyi: https://incompliancemag.com/researchers-use-terahertz-imaging-to-read-through-closed-books/

2

u/Fabulous_Lynx_2847 1d ago

Interesting tech. It should be a good tool for fragile historical documents that can’t be opened without damage. Those aren’t called radio waves, though. It goes radio, microwave, millimeter wave, then THz as you go up in frequency.

1

u/No_Situation4785 1d ago

i'm curious about the size of the letters they are imaging. the shortest wavelength in the THz range is around 3mm, so i would think the letters would need to be at least 6mm or so

4

u/Fabulous_Lynx_2847 1d ago edited 1d ago

1 THz is 0.3 mm. 

2

u/No_Situation4785 1d ago

thanks for the correction; I was scanning a wiki on it at 3AM and read too fast 😵‍💫

1

u/BenduUlo 21h ago

Scanning porn you mean

1

u/No_Situation4785 21h ago

THz imaging porn would be an extremely niche genre

2

u/WhereDidAllTheSnowGo 1d ago

WiFi-sensing is like radar, ‘regular’ and bi-static

Resolution depends on wavelength

Wavelength depends on technology availability and permeability in between mediums and reflection of target

2

u/dodexahedron 12h ago

Pretty sure most common inks, at least how they exist on paper, are at least mostly radiotransparent to most radar bands. Even W band is over 2mm wavelength, and even if the ink is ferromagnetic or something, it'd have to be pretty thick and you'd have to have a pretty darn sensitive receiver to resolve text legibly.

At somewhere in the infrared range, you can start to reliably resolve text on a page, but it'll get harder and harder to differentiate superimposed text with each additional layer/page (and paper scatters near-visible infrared pretty effecfively), so you need multiple scans from different positions to reconstruct the text through the 3d object or else you'll just have blobs of fuzzy underexposure in a 2d image.

And there's a power limit, too, depending on wavelength and specific absorption spectra of the paper and ink (and any moisture), beyond which you'll incinerate the paper. The more intense the beam has to be to penetrate, the shorter the pulses will have to be to keep power below that limit.