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Comment by nomel | Hacker News Reader

It's lens + sensor stackup reflection. Camera sensors are much more reflective than a lens.

Yep, the combination is highly reflective. I cited another work (by a physics research group) in a comment that modelled and tested this effect comprehensively.

Would you speculate if anti reflective glass on the outside of the camera and a layer of one way tint on the inside that would allow light through but prevent it from reflective back out? I’m just thinking what we will expect in the future to sell the “undetectable spy camera”.

That's a great question. I'm not totally sure if this is possible. If it was, I think one-way mirrors would use this technology. For now, it seems like they only work because they only let 50% of the light through + there's a brightness differential between both sides. Perhaps someone more informed about this could chime in.

Yeah, there's no magic material that can violate the second law of thermodynamics, because it's the second law of thermodynamics. One-way mirrors are just partial mirrors with less light on one side than the other.

There's no such thing as a "one way tint".

https://en.wikipedia.org/wiki/One-way_mirror#Principle_of_op...

This is a simple implementation of a one way mirror.

Metamaterials have a chance at at: https://www.nist.gov/news-events/news/2014/07/new-nist-metam...

Then they'd have a signature of reflecting less than surrounding. In the long long run, probably more robust to detect the entropy generated by computation in a physical area (i.e. heat) and not the optics.

Agreed. I expect however that any easily available thermal sensing, especially on smartphones (see FLIR, etc), could be easily defeated with a strong adversary doing good thermal management.

As you say though, extra heat is a physical guarantee, so maybe a smarter technique exists to separate signal from noise in the thermal domain that I don't know of yet.

So would something like anti reflective glass for light coming into the camera then another layer of like tint that will stop light from reflecting back out if the camera? Ya I know that doesn’t exist but That is because they were not trying to do undetected as hard before. Just thinking of counter surveillance techniques.

> Just thinking of counter surveillance techniques.

Counter-counter surveillance techniques, IIUC.

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